Introduction Energy storage technology plays a vital role in advanced electronic and power systems. Ceramic dielectric capacitors show a great potential in electronics, pulse power, and defibrillators due to their ultrahigh power density, fast charging/discharging speed, and superior reliability. Increasing the energy storage density of dielectric capacitors will boost the development of powder devices towards miniaturization, lightweight, and low cost. It is thus crucial to improve the energy storage density of dielectric capacitor. In general, high energy storage properties (ESP) are closely related to a larger maximum polarization (Pm), a smaller remnant polarization (Pr), and a larger breakdown strength (Eb). The recoverable energy storage density (Wrec) and energy storage efficiency (η) are the crucial parameters to evaluate their ESP. This paper designed and prepared a series of (Bi0.5Na0.5)0.65(Sr1-xCax)0.35Ti0.96(Al0.5Nb0.5)0.04O3 (BNST-xC) ceramics (x=0.05, 0.10, 0.15, 0.20). The effect of relaxation characteristics on the energy storage characteristics of BNT ferroelectric ceramics was investigated. Methods The BNST-xC ceramics were fabricated via conventional solid-state reaction and subsequent tape-casting. The oxides and carbonates as raw materials were weighed according to a stoichiometric ratio and ground in a planetary mill with a nylon grinding chamber for 12 h. After drying, the powders were calcined at 850 ℃ for 6 h and then ground for 12 h. Subsequently, the synthesized BNST-xC powders were mixed with glycerol trioleate, ethanol absolute, butanone, dibutyl phthalate, polyvinyl butyral, and polyethylene glycol to obtain a slurry. The uniformly mixed slurry was tape-casted on the surface of vacuum adsorbed PET release film using a model MSK-AFA-IV automatic thick film coater at a casting speed of 0.6 cm/s. After stacking and pressing, the films were placed in a model PTC LT08001 isostatic pressing machine (EASEN) and pressed at 75 ℃. Finally, the green samples were firstly heated at 600 ℃ for 8 h to remove the organics, and then sintered at 1 180-1 220 ℃for 2 h.The phase composition of ceramic was characterized by a model D/max 2550V X-ray diffractometer (XRD, Rigaku Co., Japan). The grain size of the sample was determined by a model TM4000 desktop scanning electron microscope (SEM, Hitachi Co., Japan). The Raman spectra were collected by a model Jobin Yvon HR800 Raman scattering spectrometer (Horiba Co., France) under 532 nm laser excitation. The hysteresis loop was determined by a model Premier II and radiant ferroelectric analyzer (Precision Co., USA) at 10 Hz; The dielectric performance and frequency dependence were analyzed by a model Agilent E4980A LCR meter and heating equipment (Tongguo Technology Co., China). A specially designed RLC circuit was used to test the charging and discharging performance of the capacitor.Results and discussion From the analysis of phase composition, Sr and Ca ions at a position A, and Al and Nb ions at a position B completely enter the interior of the lattice, forming a solid solution. The improvement of grain size by B-site (Al0.5Nb0.5)4+ ion doping is beneficial to enhancing the breakdown field strength. The average grain size of the ceramic gradually increases from 1.36 μm to 1.09 μm as the Ca doping content increases. From the Raman spectroscopy analysis, the substitution of A-site ions increases an overall local structural disorder. Furthermore, the transformation of their internal phase structure can be investigated based on the dielectric temperature spectra of BNST-xC ceramics. The substitution of Ca causes Ts and Tm moving towards a high-temperature region. Also, the dispersion of the peak shape in the dielectric temperature spectrum further proves that the relaxation degree of the overall material system continuously increases with the increase of Ca ion doping content. To more accurately demonstrate the law of material relaxation degree, the Curie-Weiss law is used to calculate the relaxation coefficient of the dielectric temperature spectrum. The flipping of the internal domain structure at the electric field was tested by PFM, further showing the contribution of Ca ions to the enhancement of relaxation properties.Conclusions A series of BNST-xC ceramics were prepared by a solid-state reaction method. The relationship between the relaxation characteristics and energy storage characteristics of BNT based ceramics was analyzed via adjusting the ratio of Sr and Ca ions. The optimal energy storage performance (i.e., Eb of 630 kV/cm, and Wrec of 6.4 J/cm3) was achieved as x=0.15. The energy storage efficiency reached 85%. At Eb of 300 kV/cm, the ceramic exhibited a good stability at different temperatures and frequencies as well as a high power density and an operating current density. It was indicated that BNT based relaxor ferroelectrics could have a promising application potential.

Introduction Dielectric capacitors are used in pulse power systems due to their advantages of high energy storage efficiency, long storage lifetime, and fast charge/discharge speed. Among the major dielectric materials, ceramic-based dielectrics have a superior temperature stability and a longer lifetime rather than polymer-based dielectrics. Dielectric energy storage ceramics have attracted recent attention. Among various dielectric energy storage ceramics, SrTiO3 ceramic with a linear dielectric behavior at room temperature is a promising dielectric energy storage material with a high breakdown strength and a low dielectric loss. However, SrTiO3 ceramic has a relatively low dielectric constant and lacks a spontaneous polarization, resulting in a limited energy storage density. Therefore, SrTiO3 ceramic cannot meet requirements for specific energy storage application. Improving the polarization strength while maintaining the high breakdown strength through effective structural adjustment can be an effective method to enhance its energy storage performance. In this paper, we fabricated high-performance (Sr0.42Na0.15Bi0.29Ca0.07□0.07)TiO3 (SNBCT) energy storage ceramics with a small amount of ZnO as doping species. In addition, the effect of ZnO doping on the structure and energy storage properties of the ceramics were also investigated.Methods SNBCT-x%ZnO energy storage ceramics were prepared by a conventional ceramic preparation process. The raw materials were weighed and then mixed in alcohol by grinding in a ball mill for 24 h. The slurry was dried and calcined at 950 ℃ for 4 h to obtain the powders. After further grinding for 24 h, the powders were mixed with polyvinyl alcohol (PVA) binder solution and then pressed into pellets with 10 mm in diameter and 0.5 mm in thickness. After burning off the PVA, the disk samples were sintered at 1 140 ℃ for 2 h. The samples used for dielectric performance tests were polished and coated with silver electrodes, and the samples used for energy storage and charge/discharge performance testing were thinned to a thickness of 0.1 mm, and gold electrodes with a diameter of 2 mm were sputtered on their surfaces.The structure of the ceramic was analyzed by a model D8 advance X-ray diffractometer (Bruker Co., Germany). The microstructure of the ceramics was determined by a model Merlin compact field emission scanning electron microscope (Carl Zeiss Co., Germany). The ferroelectric properties of the ceramics were analyzed by a model TF Analyzer 3000 ferroelectric analyzer (aixACCT Co., Germany). The temperature-dependent dielectric properties were characterized by a model Wayne Kerr 6500B impedance analyzer (WayneKerr Co., UK). The charge-discharge characteristics were analyzed by a model CFD-003 plus pulsed charge-discharge system (Tongguo Technology Co., China).Results and discussion ZnO-modified SNBCT samples show a perovskite structure with a cubic phase. A trace of impurity peak appears in the XRD pattern as ZnO doping amount increases to 1.0%, corresponding to a small amount of the secondary Bi2Ti2O7 phase (JCPDS 032-0118). All the ceramics show a uniform and dense microstructure. The ZnO doping has little influence on the microstructure of the ceramics as the average grain size in these compositions changes slightly.For the electrical properties, the doping of ZnO weakens the ferroelectric properties of SNBCT ceramics, which is manifested by a decreased maximum polarization strength Pmax. In addition, the doping of ZnO improves the breakdown strength, thus enhancing the energy storage performance of SNBCT ceramics. For the sample doped with 0.5% ZnO, a high recoverable energy-storage density (Wrec) of 5.16 J/cm3 and a high energy-storage efficiency (η) of 88.2% in a large breakdown electric field of 468 kV/cm are obtained. Compared with the SrTiO3-based energy storage ceramics, a great energy storage performance of the studied sample is obtained. A simulation model containing grains and grain boundaries is proposed to explain the enhanced breakdown strength, and the results show that fine grains with many grain boundaries hinder the propagation of breakdown cracks. For the charge-discharge performance, the sample doped with 0.5% ZnO shows a discharge energy density (Wd) of 2.61 J/cm3 and a discharge rate (t0.9) of 227 ns in an electric field of 280 kV/cm. Conclusions All the samples exhibited a perovskite structure dominated by a cubic phase, and provided a slim P-E loops. ZnO doping effectively improved the breakdown strength of the SNBCT matrix, thus enhancing the energy storage performance. The effective energy storage density (Wrec) of the sample reached 5.16 J/cm3 and the energy storage efficiency () was 88.2% in the electric field of 468 kV/cm for the sample doped with 0.5% ZnO. A simulation model containing grains and grain boundaries was established and the extension of breakdown cracks in an external electric field was simulated, which explained the enhanced breakdown strength. In addition, the sample doped with 0.5% ZnO had a good charge-discharge performance, showing a discharge energy density (Wd) of 2.61 J/cm3 and a discharge rate (t0.9) of 227 ns in an electric field of 280 kV/cm. Meanwhile, the sample exhibited a good temperature stability. These results indicated that the SNBCT-x%ZnO ceramics could be promising materials for pulse power capacitor applications.

Introduction Ceramic-based dielectric capacitors with a high power density are one of the most prospective energy-storage components, compared with Li-ion batteries and super-capacitors. Unfortunately, the energy-storage density of ceramic-based dielectric capacitors is one or two orders of magnitude lower than that of the batteries, which is restricted by the property of dielectric materials. Therefore, developing reliable and high-performance dielectric ceramic is a challenge in the field of dielectric capacitors. Among dielectric ceramics, antiferroelectic (AFE) material is one of the most promising candidates for energy-storage devices since they show a high maximum polarization (Pmax), low remnant polarization (Pr), and a regulated AFE-ferroelectric (FE) phase transition electric field. Recent efforts are made to enhance energy-storage properties of ceramic by component design, ion-doping and material form construct. However, a superior η is always gained at the cost of a high Wrec, becoming a challenge for synergistically achieving a ultrahigh Wrec and a superior η.Methods High-purity Pb3O4 (≥95.0%, Sinopharm Chemical Reagent Co., Ltd., China), La2O3 (≥99.9%, Sinopharm Chemical Reagent Co., Ltd., China), Nd2O3 (>99.99%, Aladdin Biochemical Technology Co., Ltd., China), ZrO2 (≥99.0%, Sinopharm Chemical Reagent Co., Ltd., China), and SnO2 (≥99.5%, Sinopharm Chemical Reagent Co., Ltd.) were used as raw materials. Pb0.98-1.5xLa0.02Ndx(Zr0.60Sn0.40)0.995O3 (PL100xNZS) was selected as a basic system. Nd3+ with a small ionic radius (i.e., 1.29 ?) was introduced in PLZS to replace Pb2+ (i.e., 1.49 ?) for enhancing electric filed-induced EFE-AFE. A Pb0.98-1.5xLa0.02Ndx(Zr0.60Sn0.40)0.995O3 component was designed and the ceramics were fabricated by a tape-casting method.The crystallinity, phase structure and microstructure of PL100xNZS ceramics were analyzed by an X-ray diffractometer (Rigaku Co., Ltd., Japan) and a model TESCAN-MIRA3 field-emission scanning electron microscope (TESCAN Co., Czech). The dielectric properties of the PL100xNZS ceramics were measured by a computer-controlled Agilent E4980A LCR analyzer. The displace-electric field (D-E) hysteresis loops and current density-electric field curve were determined by a ferroelectric measurement system (Radiant Technologies Co., USA). The pulsed charge-discharge behaviors were determined by a model CFD-001 resistance-inductance- capacitance (R-L-C) electric circuit (Guoguo Technology Co., China). Results and discussion The XRD patterns show that all the samples exhibit a perovskite structure with an orthorhombic phase. The SEM images show that a dense and compact microstructure appears in PL100xNZS ceramics. The average grain size is decreased by doping of Nd3+. The dielectric behavior of PL100xNZS ceramics as a function of temperature measured at 10 kHz and 25-400 ℃ indicates that there are three stages of dielectric constant with the increase of temperature, i.e., the rising, the plateau and the falling regions, which correspond to the AFE, multicell cubic (MCC) and single-cell cubic (PEC) phases. The maximum polarization decreases at the same electric field, and the breakdown strength (BDS) increases with the increase of Nd3+ content due to the decreased grain size. The antiferroelectric-ferroelectric phase transition electric field (EAFE-FE) increases as the Nd3+ content increases, because of a stable antiferroelectric phase. The enhanced BDS and increased EAFE-FE imply a high energy-storage density. The recoverable energy-storage density and efficiency are critical parameters to characterize the material performance. Herein, the PL4NZS ceramic can be used as a promising energy-storage material because it exhibits a linear-like D-E loop with a large Pmax (i.e., 60 μC/cm2) and a high BDS (i.e., 490 kV/cm). A large energy storage density of 14.8 J/cm3 is obtained in PL4NZS ceramic with a high efficiency of 85%. The calculated values of Wrec and η fluctuate slightly in the range of 6.18-4.88 J/cm3 with increasing the frequency. The discharge waveforms of PL4NZS ceramic at different electric fields are investigated at room temperature with an overdamped R-L-C electric circuit. The maximum value of Wdis is 4.4 J/cm3 at 300 kV/cm, and the t0.9 is 28 ns. A large CDmax of 3 360 A/cm2 and a high PDmax of 504 MW/cm3 are achieved at 300 kV/cm in PL4NZS ceramic, indicating that the PL4NZS ceramic has a great competitiveness in high power system applications. Conclusions The high energy-storage density and efficiency were realized in designed PL100xNZS ceramics fabricated by a tape-casting technique. The effects of Nd3+ doping on the microstructure, dielectric properties, ferroelectric properties and energy storage properties of the ceramics were investigated. The Wrec was 14.8 J/cm3 with a giant η of 85% at 490 kV/cm for PL4NZS ceramic. As the temperature increases, Wrec and η of PL4NZS ceramic increased slightly. Moreover, a high Wdis of 4.4 J/cm3, a large CDmax of 3 360 A/cm2 and a high PDmax of 504 MW/cm3 were obtained in PL4NZS ceramic. It was indicated that PL4NZS ceramic could be used as a prospective dielectric material for energy-storage applications.

Introduction Pulse power technology has important applications in national defense industry. Dielectric capacitor is one of the energy storage components of pulse power devices. Ceramic dielectric capacitors have some advantages like fast charging- discharging speed, good electromechanical performance, anti-aging, and resistance to extreme conditions, which can be used in dielectric energy storage. The existing energy storage properties of BaTiO3-based dielectric ceramics are generally low at a high electric field, and lead-based ceramic capacitors are increasingly limited because of the presence of lead as a highly toxic element. It is thus necessary for pulse power technology to develop lead-free ceramic capacitors with superior comprehensive energy storage properties. NaNbO3 (NN) ceramics have a rich structural phase transition and a low theoretical density, which makes their electrical properties highly adjustable and is conducive to the development of lightweight capacitors. Recent work on relaxor ferroelectric, anti-ferroelectric and relaxor antiferroelectric NaNbO3-based ceramics are performed. Compared with the typical lead-free antiferroelectric AgNbO3, NN as one of of lead-free antiferroelectric materials has some advantages like low theoretical density (i.e., 4.55 g/cm3), high polarization intensity (i.e., 40 μC/cm2), simple preparation process, and relatively low raw material cost. Developing NN-based relaxor antiferroelectric ceramics with superior comprehensive energy storage properties is still a challenge because of the contradiction between polarization and breakdown strength. Therefore, exploring a relationship between the structure and energy storage properties of NN-based relaxor antiferroelectric ceramics is still a research aspect for NN-based ceramics. In this paper, a strategy of reducing tolerance factor was adopted to stabilize the antiferroelectric phase of NN-based ceramics. 10% (in mole fraction) BiFeO3(BF) with a low tolerance factor (i.e., 0.954 3) was incorporated into pure NN as a second end member, and the antiferroelectric R phase (Pbnm) of NN ceramics was stabilized at room temperature. The third end member Sr(Ti0.85Zr0.15)O3(STZ) was introduced to further improve the relaxor characteristic of the ceramics, and then the relaxor antiferroelectric phase was constructed in NN-based ceramics. In addition, the phase structure, microstructure, energy storage properties, stability and charging-discharging performance of the ceramics were also analyzed.Methods The chemical composition of the ceramics was NaNbO3, 0.9NaNbO3-0.1BiFeO3, and (1-x)[0.9 NaNbO3-0.1BiFeO3]- xSr(Ti0.85Zr0.15)O3, where x=0.05, 0.10, 0.15. All the ceramics were prepared by a conventional solid phase method. The ceramics were prepared via sintering at 1 230?1 260 ℃ for 2 h. The crystal structure of ceramics was analyzed by a model DX-2700BH X-ray diffractometer (Haoyuan Instrument Co., China) and a model Lab-Ram HR800 spectrometer (HORIBA Co., France) with a laser wavelength of 532 nm. The in-situ temperature test for Raman spectra was performed on a model Linkam THMS600 temperature control platform. The sample used for in-situ electric field Raman spectroscopy test was plated with an electrode by an ion sputtering method. The sputtering current was set to 6 mA and the sputtering time to 2 min to ensure that the sample had both electrical conductivity and the penetration in Raman spectroscopic laser. The natural surface morphology of the samples was determined by a scanning electron microscope, and the grain size of the ceramics was determined using a software named Nano Measurer. After the sample was thinned, the domain structure of the ceramics was characterized by a model JEM-2100F field emission transmission electron microscope. The sample was polished to 0.5 mm thickness, cleaned, and coated with silver electrodes on the upper and lower surfaces of the sample. The dielectric properties of the ceramics were measured at -120 - 450 ℃ by a model E4980A precision LCR bridge (Agilent Co., USA). The hysteresis loops of the ceramics with the thickness of 150 μm were tested by a model Precision Premier II ferroelectric tester (Radiant Co., USA).Results and discussion Based on the XRD patterns, NN, NN-BF and NN-BF-xSTZ ceramics are a pure perovskite structure. NN ceramics show diffraction peaks for phases (110) and (200), manifesting that they are antiferroelectric orthogonal P(Pbcm) phases at room temperature. The orthogonal phase features disappear, and the diffraction peaks of phases (110) and (200) change from split state to single peak with the addition of BF and STZ, indicating that the addition of BF and STZ improves the symmetry of the ceramic and the ceramic transforms into a pseudo-cubic phase. NN ceramics have a dielectric anomaly peak at 400 ℃, which represents a transition from the antiferroelectric P phase to the antiferroelectric R phase. For NN-BF and NN-BF-xSTZ ceramics, the P-R phase transition peak disappears, and a diffuse dielectric anomaly peak appears at -100 ℃. Compared with NN ceramics, the room temperature dielectric constant of NN-BF ceramics increases, which is mainly due to the presence of anti-ferroelectric R phase in the ceramics. The in-situ XRD and Raman measurements show that the symmetry of the local structure of NN-BF-10STZ ceramics has a good stability. After BF and STZ doping, the grain size of the ceramics is reduced and the grain distribution uniformity is improved. The breakdown strength of ceramics can be enhanced due to the effect of decreasing grain size and improving distribution uniformity. STZ doping reduces the size of antiferroelectric domains. They can respond quickly to an external electric field due to the high activity and low energy barrier of nano-domains, thus reducing the remanent polarization, which is conducive to improving the energy storage properties. The transformation from antiferroelectric phase to ferroelectric phase of NN ceramics only occurs in the presence of the electric field. After the introduction of BF, the antiferroelectric R phase of the ceramics is stabilized, and the P-E curve shows a double hysteresis loop. The relaxation characteristics of the ceramics are further enhanced with the introduction of STZ. The P-E loops of NN-BF-10STZ ceramics become more elongated, and the polarization hysteresis is suppressed. The NN-BF-10STZ achieves the optimal energy storage properties (i.e., Wrec of 5.22 J/cm3 and η of 83.92%). Also, NN-BF-10STZ ceramics have a superior stability at 25?120 ℃ and 10?150 Hz. Conclusions Pure NN ceramics exhibited a typical square hysteresis loop with low energy storage density and efficiency (i.e., Wrec of 0.14 J/cm3, and η of 6.69%). After the introduction of the second component of BF, the ceramics had a double hysteresis loop like antiferroelectric, and the energy storage characteristics (i.e., Wrec~3.55 J/cm3, η~70.61%) were optimized. Furthermore, the introduction of STZ enhanced the relaxation of the ceramics, and reduced the polarization hysteresis of the hysteresis loop. The energy storage properties were optimized (i.e., Wrec~5.22 J/cm3, η~83.92%). 0.9[0.9NaNbO3-0.1BiFeO3]-0.1Sr(Ti0.85Zr0.15)O3 ceramics with the optimal energy storage performance had a stability at different temperatures and frequencies, which could be used as a promising dielectric energy storage material with application prospects.

Introduction As one of the important electronic components in pulsed power devices, dielectric capacitors have wide applications in hybrid vehicles, and distributed power system. However, a relatively low energy density affects their application in portable and light-weight devices. The dielectric capacitors store and release energy in form of electrostatic polarization and depolarization of dielectric materials. Hence, achieving the maximum polarization (Pmax), small remanent polarization (Pr) and large electric breakdown strength (Eb) in dielectric materials is an effective approach to improve the energy storage properties. The antiferroelectric ceramics are one of the most prospect materials for energy storage due to their unique antiferroelectric-ferroelectric (AFE-FE) phase transition during charging and discharging process. As one of the classic antiferroelectric ceramics, Pb(Zr,Ti)O3 antiferroelectric ceramics are widely investigated. However, the content of Ti should be lower than 10% to stabilize the antiferroelectric phase, leading to the narrow windows for compositional regulation. In recent years, Pb(Zr,Sn)O3 antiferroelectric ceramics become popular because of the electric field induced multistage phase transition. In this paper, (Pb0.97La0.02)(Zr0.6Sn0.4)O3 (PLZS) antiferroelectric ceramic was synthesized via solid-state sintering at different temperatures. In addition, the crystal structures, microstructures, dielectric properties and energy storage properties of the ceramic were also investigated.Methods PbO, La2O3, ZrO2 and SnO2 were used as raw materials, and weighted and mixed according to the stoichiometry. The mixed materials with alcohol were ground in a ball mill for 12 h. Afterwards, the suspension was dried, the powders were calcined at 900 ℃ for 3 h, and then ground with alcohol for another 12 h. The dried powders with polyvinyl alcohol were pressed into disks with the diameter of 10 mm and the thickness of 1 mm. Finally, the green disks were sintered at 1 200-1 250 ℃ for 2 h to obtain the densified ceramics.The crystal structures of ceramics were determined by X-ray diffraction (XRD). The surface microstructure of ceramics was characterized by scanning electron microscopy (SEM). For the measurement of dielectric properties, the ceramics were polished and covered with a silver paste, which was dried at 550 ℃ for 10 min. The dielectric properties were analyzed by a dielectric test system at room temperature to 450 ℃, and the P-E loops of ceramics were determined by a ferroelectric analyzer.Results and discussion All the prepared ceramics show an orthorhombic perovskite structure without secondary phase. Moreover, some weak diffractions appear due to an antiparallel aligned adjacent Pb2+ in antiferroelectric ceramics. The microstructure of ceramics becomes denser with decreasing holes and increasing grain size as sintering temperature increases, which is beneficial to enhancing Eb. All the samples show three dielectric anomalies in a temperature-dependent dielectric constant (εr), indicating a successive phase transition between orthorhombic antiferroelectric phase (AFEO), tetragonal antiferroelectric phase (AFET), multicell cubic phase (MCC) and cubic paraelectric phase (PEC) as the temperature increases. In addition, the dielectric loss (tanδ) for all the samples is quite low (<0.2%) at a low temperature, which is also helpful to enhance Eb. All the samples sintered at different temperatures show double P-E loops and I-E curves, having the typical antiferroelectric properties. Furthermore, the polarization increases slowly at a relatively low electric field, and increases quickly at a moderate electric field due to the electric field induced AFE-FE phase transition., the polarization increases more quickly with further increasing electric field because of the electric field induced FE-FE phase transition. Hence, the corresponding I-E curves exhibit electric field induced current peaks. As a result, the PLZS ceramic sintered at 1 225 ℃ has the optimal energy storage properties (i.e., Wrec of 13.3 J/cm3 and η of 83.6%) at 480 kV/cm, indicating great potential applications in pulsed power devices. Note that a large Eb plays an important role in achieving superior energy storage properties, and a large Eb can be attributed to dense microstructure, small thickness and electrode area, moderate εr and low tanδ.Conclusions PLZS antiferroelectric ceramics were fabricated via solid-state reaction. The crystal structure, microstructure morphologies, antiferroelectric properties and energy storage behavior of PLZS ceramics sintered at different temperatures were investigated. All the samples exhibited an orthorhombic antiferroelectric phase. Also, the density of prepared ceramics improved, and grain size increased with increasing sintering temperature. Therefore, the sample sintered at 1 225 ℃ achieved the optimum energy storage properties (i.e., recoverable energy density of 13.3 J/cm3 and efficiency of 83.6%).

Introduction Dielectric materials have critical applications in many fields. Compared with electrochemical energy storage batteries, ceramic capacitors show outstanding competitive power performance in actual energy storage applications, such as diesel engine starters, camera flashlights, spacecraft, pulsed power weapons, and medical devices due to their ultra-fast charge/discharge capability and high-power density. However, with the development of electronic device integration and miniaturization, capacitors are required to have a high effective energy storage density (Wrec) under low electric field. The existing dielectric ceramic capacitors are difficult to meet the corresponding requirements, so it is urgent and significant to develop dielectric ceramic capacitors that can obtain high energy storage density under low voltage. In recent years, the co-design strategy has been used as a typical and effective method to enhance the energy storage performance of BNT, which is achieved by adding different kinds of ABO3 perovskites and various doping ions. Co-design strategy improves energy storage properties by compositional design to induce the formation of polar nanoregions (PNRs) as a result of A/B site ion disordering or structural phase transitions. In this paper, KNN was added to BNBST ceramics to modify the internal crystal structure, and the effect of KNN on the phase structure, microstructure, dielectric properties, energy storage properties, ferroelectric stability and fast charge-discharge characteristics of BNBST ceramics were investigated.Methods As we know, high Pmax, low Pr and high breakdown strengthen Eb are required for dielectric ceramics to achieve high Wrec. To obtain high energy storage density under a low electric field, we chose Ba0.105Na0.325Bi0.325Sr0.245TiO3 (BNBST) with large Pmax and high Curie temperature as the research object, the co-design strategy with the introduction of K0.5Na0.5NbO3 was adopted to further destroy the ordered arrangement of A/B position ions, reduce their residual polarization intensity Pr, and optimize their electric polarization behavior. Using solid-state method prepared Ba0.105Na0.325Bi0.325Sr0.245TiO3+x%K0.5Na0.5NbO3 (BNBST-x% KNN, mole fraction, x=0, 2, 4, 6, 8, 10, 12) ceramic samples. Bi2O3 (99.999%), K2CO3 (99%), BaCO3 (99%), SrCO3 (99%), Na2CO3 (99.8%), TiO2 (99%), Nb2O5 (99.95%) were used as synthetic raw materials to prepare samples according to the following process: 1) Accurately weigh the ingredients according to the corresponding stoichiometric ratio, put them in the nylon ball mill tank for 24 h, dry, grind, and pass 80 mesh sieve; 2) Pre-burning after pressing into large sections, heating up to 850 ℃ with a heating rate of 5 ℃/min and holding for 3 h to exclude CO2 and pre-synthesize the powder; 3) The large pieces are ground into powder, passed through an 80-mesh sieve, ball milling again for 24 h, drying, adding 5% concentration of PVA adhesive for granulation, pressing into a small disc with thickness of about 1 mm, diameter of about 13 mm (pressure (120±10) MPa); 4) The small disc was heated to 650 ℃ for 3 h to remove organic matter, and then heated to 1 240 ℃ for 2 h to sintering; 5) After polished and silvered, heating up 30 min at 650 ℃, and then various electrical properties are tested.Results and discussion All BNBST-x%KNN ceramics have a single perovskite pseudo-cubic phase structure, and no second phase is generated. The grain distribution of all ceramics is uniform and the density is good. When x=6, the optimized energy storage characteristics are obtained only at the low electric field of 140 kV/cm with Wrec=1.8 J/cm3 and =86%. Whatmore, BNBST-6% KNN ceramics have TCC=±15% high dielectric constant (εr=3 128@125 ℃) and excellent temperature, frequency, cycle stability and fast charge-discharge characteristics in the temperature range of -8-215 ℃. The results show that BNBST-6%KNN ceramics can be used as a candidate material for pulsed power capacitors, and has obvious advantages in low electric field application.Conclusions The main innovation points of this paper are summarized as following. A collaborative design strategy with the introduction of K0.5Na0.5NbO3 was used to further destroy the ordered arrangement of A/B ions and promote the formation of polar nanodomains (PNRs), under a certain electric field, PNRs can be transformed into long-range ordered ferroelectric domains, resulting in larger Pmax. When the electric field is removed, the ferroelectric domains formed by PNRs will quickly break back to the initial state, resulting in smaller Pr. More importantly, “premature saturation” is delayed due to the appearance of PNRs, which can obtain hight larger Pmax at low electric field, resulting in a high Wrec=1.8 J/cm3 at the low electric field of 140 kV/cm.

Introduction With the development of power electronic systems, energy storage capacitors have the advantages of large discharge power, fast charging and discharging speed and stable performance, and play an important role in power systems, electronic devices, pulse power supplies, etc.. They are widely used in civilian and military fields. Compared with fuel cells, the energy storage capacitors do not need to convert the Gibbs free energy of the chemical energy of the fuel into electrical energy through electrochemical reactions, but store the charge by pressurizing between the two plates of the capacitor with higher safety and reliability as well as environmentally friendness. The existing pulsed dielectric materials are mainly divided into ceramics, glass ceramics, thin films and other composite materials. Glass ceramics have energy storage advantages, compared with other materials. Increasing the dielectric constant of dielectric materials plays a crucial role in increasing the energy density.Methods A glass with the composition of 13K2O-21SrO-32Nb2O5-5B2O3-4Al2O3-25SiO2-xBi2O3 (x=0.0%, 1.0%, 2.0%, and 4.0%, in mole) was produced by a conventional melt annealing process. First, 40 g of raw materials were weighed and ground in a polypropylene ball mill in ethanol for 12 h. The resulting mixture was dried at 100 ℃ and subsequently melted in a corundum crucible in a resistance furnace at 1 500 ℃ for 2 h. The molten liquid was pressed into a sheet on a preheated copper plate and then annealed at 500 ℃ for 6 h to eliminate residual internal stresses. Finally, a niobate glass was obtained. The glass slices were crystallized at 850 ℃ and 950 ℃ for 2 h to obtain glass ceramics. The glass ceramic samples were ground, polished, and plated with electrodes for the structural and performance tests.Results and discussion The effect of Bi2O3 concentrations (x=0.0%, 1.0%, 2.0%, and 4.0% in mole) on the phase evolution, microstructure, dielectric and energy storage properties of K2O-SrO-Nb2O5-B2O3-Al2O3-SiO2 glass ceramics was investigated by a controlled crystallization process. Based on the XRD analysis, KSr2Nb5O15 is a main precipitated crystalline phase, and the crystallization promotes due to the addition of bismuth oxide. The A-site ions are adjusted by Bi3+ with a smaller radius (i.e., 1.03 ?) into the crystal lattice instead of Sr2+ with a larger radius (i.e., 1.18 ?), thus reducing the unit cell volume and increasing the lattice distortion. From the Raman spectra, the change of A-site occupancy leads to the distortion of the structure and enhances the spontaneous polarization. The microstructure shows that the grains are dense and evenly distributed, and the grain size is fine. The dielectric temperature spectroscopy indicates that the relaxation characteristics correspond to the polar nanoregion of the microstructure, and the DC breakdown strength firstly increases and then decreases, which is consistent with the change of the dielectric constant. The polarized electric field curve is slender with a large spontaneous polarization and a small residual polarization, improving the energy storage efficiency. The charge-discharge curve shows that the energy density firstly increases and then decreases with the addition of bismuth oxide, and the glass-ceramic sample B2-9 obtains the maximum energy density, discharge power density and fast discharge time, having a great application prospect in the field of dielectric energy storage.Conclusions Strontium niobate potassium glass ceramics were prepared by a conventional melting method and a controlled crystallization technology. At a crystallization temperature of 950 ℃, the unit cell volume of the crystalline phase decreased from 611.83 ?3 to 610.19 ?3 with the increase of Bi2O3 doping content. The grain size of the microstructure firstly decreased and then increased, and the optimal grain size of glass ceramic sample B2-9 was 57.72 nm. The dielectric and energy storage properties also firstly increased and then decreased, and the maximum dielectric constant of glass-ceramic sample B2-9 was 342 times greater than that of glass-ceramic sample B2-8, which was 1.9 times greater than that of glass-ceramic sample B2-8. According to the P-E curve, the glass ceramic sample B2-9 had the optimum performance (i.e., Pmax=9.33 μC/cm2, Pr=0.73 μC/cm2, Wrec=1.27 J/cm3, Wtotal=1.55 J/cm3, η=82% at 350 kV/cm). The energy density of the glass ceramic sample B2-9 was 1.33 J/cm3 at 400 kV/cm, the maximum discharge power density was 124.13 MW/cm3, and the fastest discharge time was 22 ns. The breakdown strength firstly increased and then decreased, which were 742, 886, 923 kV/cm and 622 kV/cm, respectively. Glass-ceramic materials could have great application prospects in the field of dielectric energy storage.

Introduction The common energy storage devices mainly include supercapacitors, lithium-ion batteries, fuel cells, and ceramic capacitors. Compared to other energy storage devices, ceramic capacitors have attracted much attention due to their ultra-high charge-discharge rates and power density. Antiferroelectric ceramics exhibit a large maximum polarization and close-to-zero residual polarization, displaying characteristics of a double hysteresis loop. Also, antiferroelectric ceramics have an advantage of low dielectric loss, making them one of the optimal energy storage materials. Lead-free NaNbO3-based antiferroelectric ceramics become popular due to the environmental concern and cost reduction. However, NaNbO3-based ceramics can transform from an antiferroelectric phase to a ferroelectric phase at a certain applied electric field. Even after removing the applied electric field, some ferroelectric phase remains, increasing the residual polarization strength and affecting the formation of the double hysteresis loop at room temperature. Furthermore, a transition between antiferroelectric phase and ferroelectric phase caused by the complex phase structure results in a loss of most energy in the form of heat, reducing the energy storage efficiency (η). To further regulate the energy storage characteristics of NN-based antiferroelectric ceramics, this paper introduced Bi3+ and Fe3+ into the NaNbO3 matrix to enhance the stability of the antiferroelectric phase and the relaxor behavior of the ferroelectric phase, resulting in a relaxor antiferroelectric ceramic. Different molar fractions of Sm2O3 were doped into the NN-BF ceramics with Sm3+ solid solution into the NaNbO3 lattice. In addition, the influence of doping amount of Sm3+ on the phase structure, microstructure, dielectric properties, and energy storage performance of NN-BF ceramics was also investigated.Methods Different Sm3+ doping amounts of (1-x)[0.9NaNbO3-0.1BiFeO3]-xSm2O3 (x=0.01, 0.02, 0.03, 0.04) relaxor antiferroelectric ceramics were prepared via conventional solid-state reaction. The ceramic green bodies with a diameter of 10 mm and a thickness of 1 mm were prepared. They were sintered in a furnace at a rate of 3 ℃/min at 1,150 ℃ for 3 h, and then cooled naturally to form ceramics. The ceramic samples were polished to a thickness of 0.1 mm, and silver electrodes with a radius of 2 mm were screen-printed on the upper and lower surfaces of the ceramic samples. The samples were heated at 500 ℃ for 1 h to connect the ceramics and electrodes tightly. The phase structures of different ceramic samples were analyzed by an X-ray diffractometer. The microstructure and elemental distribution of the ceramic samples were analyzed by a field emission scanning electron microscope. The dielectric loss and dielectric constant of the ceramic samples at different temperatures (i.e., 25~300 ℃) and frequencies (i.e., 1, 10, 100, 500, 1 000 kHz) were tested by an LCR meter. The electric hysteresis loops of the ceramic samples were measured at room temperature and 10 Hz by a ferroelectric tester, and the energy density and energy storage efficiency of the ceramic samples were calculated based on these measurements. The XPS spectra of Fe 2p in ceramic samples with different Sm3+ doping amounts (x=0.01, 0.02, and 0.03) were determined by an X-ray photoelectron spectrometer.Results and discussion The XRD patterns show that the ceramic with the Sm3+ doping amount (x) of 0.01 or 0.02 has a homogeneous perovskite structure without the second phase. Bi3+, Fe3+, and Sm3+ are all solid-solutions into the NaNbO3 lattice. When the Sm3+ doping amount (x) is 0.02, Sm3+ occupies the position of Na+ in the NaNbO3 lattice. The lattice energy increases after solid solution since the ionic radius of Sm3+ is greater than that of Na+, inhibiting the growth of grains. Consequently, the microstructure of the ceramic becomes denser, with smaller and more uniformly distributed grains. The larger dielectric constant reduces the breakdown field strength of the ceramic, while the smaller dielectric constant decreases the polarization of the ceramic. Therefore, a moderate dielectric constant is conducive to achieving a larger energy storage density for the ceramic composition when the Sm3+ doping amount (x) is 0.02. The hysteresis loop of the NN-BF ceramic is relatively slender, resembling a “waist-like” shape typical of relaxor antiferroelectric. This is because the doping of Bi3+ and Fe3+ enhances the stability of the antiferroelectric phase and the relaxor behavior of the ferroelectric phase. The XPS spectra of Fe 2p indicate that an appropriate doping of Sm3+ can stabilize the valence state of Fe3+, further stabilizing the antiferroelectric phase of NN-BF and reducing the generation of oxygen vacancies in the ceramic. This leads to an increase in the breakdown field strength.Bi3+ in the A-site of the 0.9NaNbO3-0.1BiFeO3 lattice is gradually replaced by Sm3+, resulting in the appearance of the second phase Bi2O3. The low-melting-point Bi2O3 plays a role in promoting sintering during the firing process, thereby facilitating the grain growth. However, the second phase Bi2O3 is not resistant to breakdown, leading to a decrease in the breakdown field strength of the ceramic.Conclusions The stability of the antiferroelectric phase and the relaxor behavior of the ferroelectric phase in NN-BF ceramics with the Sm3+doping amount of 0.02 were enhanced. An appropriate doping of Sm3+ could stabilize the valence state of Fe3+, further stabilizing the antiferroelectric phase of NN-BF ceramics. In an applied electric field of 300 kV/cm, the maximum polarization (Pmax) of NN-BF ceramics (i.e.,17.83 μC/cm2), the minimum residual polarization (Pr) (i.e., 2.87 μC/cm2), and the maximized Pmax-Pr (i.e.,14.96 μC/cm2) were obtained. The energy density and energy storage efficiency of NN-BF ceramics both reached their maximum values (i.e., 3.67 J/cm3 and 71.63%), respectively. The breakdown field strength of the ceramic was calculated to be 438 kV/cm through the Weibull distribution. These results indicated that NN-BF relaxor antiferroelectric ceramics with an appropriate Sm3+ doping could have promising potential applications in energy storage.

Introduction Polymer dielectrics play a crucial role in contemporary electronics because of their flexibility, low cost, high operating voltage, corrosion resistance, self-healing, etc., which are widely used in oil and gas exploration, and electric vehicles at operating temperatures (>105 ℃). However, polymers necessitate the maintenance of a substantial volume during usage due to their low dielectric constant, thereby resulting in a diminished energy storage density. Furthermore, polymers demonstrate a relatively low thermal conductivity, and are susceptible to failure when exposed to high-temperature conditions. To function effectively in high-temperature conditions exceeding 105 ℃, an extra refrigeration system must be incorporated into an electronic power system. This undoubtedly results in an increase in both energy consumption and overall weight of the system. Consequently, developing a new generation of polymer-based film capacitors with superior high-temperature energy storage capabilities becomes a necessity. In this paper, we utilized a multidimensional synergistic concept to comprehensively improve the high-temperature energy storage performance of PI-based polymer dielectrics via combining the electrical and thermal advantages of inorganic materials and polyimide (PI) in different dimensions.Methods (BNNS-TiO2-BNNS)/PI sandwich-structured composite films (denoted as BTB/PI film) were prepared by a casting method based on a concept of multidimensional synergy. Titanium dioxide (TiO2) with a high dielectric constant was prepared into a one-dimensional nanofiber structure by an electrostatic spinning method with PI as an interlayer of BTB/PI film. Electrically insulating and thermally conductive boron nitride was prepared into a two-dimensional nanosheet structure by a liquid phase exfoliation method with PI as an insulating and thermally conductive outer layer of the BTB/PI film. The frequency-/temperature- dependence of the films were examined by a model LRS-003 high and low temperature cooling and heating system and a model E4980 precision LCR meter. The PE loops for the films were determined by a linked test rig consisting of a model ZJ-6A quasi-static d33 tester, a model MODEL 610C Trek HV amplifier and a model BWT-001 variable temperature test rig at 25 ℃ and 200 ℃, respectively. Results and discussion The (BNNS-TiO2-BNNS)/PI sandwich composite films with the thicknesses of 22-25 μm prepared are dense and uniform, without obvious holes and obvious delamination, having superior dielectric properties, breakdown strength and high-temperature energy storage properties. The results by frequency-/temperature-dependent tests indicate that the dielectric constant of BTB/PI films increases with the increase of BNNS content. Compared with the dielectric constant of pure PI film at 1kHz (i.e., 3.12), BTB/PI-3, BTB/PI-5, 2BTB/PI-7 and BTB/PI-10 improve to 4.20, 4.37, 4.67 and 4.92, respectively. The dielectric loss of the composite film at 103-106 Hz and 25-200 ℃ is <0.025. From the breakdown strength of the material, the carriers inside the composite film have greater energy and concentration at high temperatures, making the PI chain more vulnerable to attack and breakage. The Eb of pure PI film thus decreases from 380 MV/m to 337 MV/m and the β reduces from 14.60 to 8.04 when the temperature increases from room temperature to 200 ℃. However, the Eb and β of BTB/PI-5 and BTB/PI-7 are higher than those of the pure PI films at the tested temperature. At an optimal BNNS volume ratio, BTB/PI-7 achieves the maximum Eb and β at 25, 100, 150 ℃, and 200 ℃. This can be attributed to the incorporation of wide-band, high-thermal-conductivity BNNS, preventing the composite films from experiencing breakdown caused by electrical or thermal runaway. At 25, 100, 150 ℃ and 200 ℃, the energy storage performance of BTB/PI film is higher than that of pure PI film. At an optimum BNNS volume ratio, BTB/PI-7 film achieves the maximum energy density and maintains a stable high efficiency in the tested temperature range. The energy density and efficiency of BTB/PI-7 film are 4.61 J/cm3 and 88.75% (at 465 MV/m) at 25 ℃, 3.95 J/cm3 and 88.08% (at 450 MV/m) at 100 ℃, 3.65 J/cm3 and 88.53% (at 440 MV/m) at 150 ℃, 3.04 J/cm3 and 88.00% (at 380 MV/m) at 200 ℃. Conclusions The polymer-based composite films were obtained to enhance the high temperature resistant energy storage performance based on the multidimensional synergistic design. The BTB/PI film exhibited stable frequency/temperature-dependence with a dielectric loss of lower than 0.025 at 103-106 Hz and 25-200 ℃. At 200 ℃, BTB/PI-7 achieved a high energy density of 3.04 J/cm3. Moreover, the energy efficiency of BTB/PI-7 was higher than 88% at 25, 100, 150 ℃ and 200 ℃. It was demonstrated that different materials with advantages (i.e., high polarization, great breakdown strength, and high temperature resistance) were obtained via incorporating various dimensional fillers into the polymer and implementing rational structural design. Consequently, the high temperature-resistant energy storage performance of PI-based composite films could be significantly enhanced.

Introduction With the increasingly severe environmental pollution and global warming, developing green and sustainable energy storage devices with high power density, energy storage density, and good stability becomes a research hotspot. Compared with batteries and electrochemical capacitors, lead-free ceramic dielectric capacitors exhibit eco-friendly, high power density, fast charge/discharge speed and excellent reliability, which are promising candidates for advanced pulse power systems. In general, the energy storage density of ceramic dielectric capacitors can be evaluated via measuring the polarization versus electric field hysteresis loops (i.e., P-E loops). Large maximum polarization (Pmax), small remnant polarization (Pr) and high breakdown strength (Eb) are critical parameters to achieve a high recoverable energy storage density (Wrec) and an energy storage efficiency (η). Although some previous work reported a higher energy storage density at higher electric field (>200 kV/cm), the high electric field could restrict the application in some fields. It is thus crucial to develop ceramic dielectric capacitors with a superior energy storage performance at a lower electric field (<200 kV/cm). In this paper, we designed and prepared a series of (1-x)(0.94Bi0.55Na0.45TiO3-0.06BaTiO3)- xSr0.7Bi0.2TiO3 (x=0, 0.10, 0.20, 0.30, 0.40) lead-free relaxor ferroelectric ceramics to achieve a high energy storage performance and a superior stability at a low electric filed. In addition, the energy storage properties were also investigated.Methods TiO2 (98%), SrCO3 (99%), Bi2O3 (99%), Na2CO3 (99.8%) and BaCO3 (99%) were used as raw materials for the synthesis of (1-x)BNBT-xSBT lead-free ceramics. According to the stoichiometric ratio, these raw materials were weighed. The materials were mixed with alcohol and ground in nylon jars for 12 h. After drying, the powders were calcined at 950 ℃ for 4 h. Subsequently, the calcined powders with alcohol were further ground in nylon jars for 12 h.Afterwards, the obtained powders were mixed with binders and pressed uniaxially into disk-shaped samples with 1 mm in thickness and 10 mm in diameter. Finally, the samples were sintered at 1 150-1 250 ℃ for 2 h after removing the binders at 600 ℃ for 8 h.The phase structure of the ceramics was analyzed by a model D8-Advance X-ray diffractometer (XRD, Germany). The Raman spectra were collected by a model Jobin-Yvon HR800 Raman spectroscope (Horiba Co., France). The microstructure of the ceramics was determined by a model APERO HIVAC scanning electron microscope (SEM, USA). In order to test the electrical properties of the (1-x)BNBT-xSBT ceramics, the silver paste was coated on the surface of the samples and fired at 580 ℃ for 10 min to form silver electrodes. The temperature-dependent dielectric constant and loss were measured by a model Agilent E4980A impedance analyzer. The ferroelectric performance was determined by a model Precision Premier II ferroelectric analyzer (Radiant Co., USA) with 2 mm in electrode diameter.Results and discussion The microstructure analysis indicates that all the ceramics samples have a slight porosity. Although some secondary phases appear in the ceramics due to the non-stoichiometric ratio of Bi and Na elements, the ceramics maintain a main perovskite structure. The difference of peak intensity between v2, v3 and v4, v5 reduces gradually with increasing the SBT content due to the enhancement of structural symmetry. Meanwhile, the peaks of dielectric constant shift towards a lower temperature along with the improvement of temperature stability of dielectric constant as the SBT content increases. At 25-343.9 ℃, the variation of dielectric constant is less than ±10% for (1-x)BNBT-xSBT ceramics when x=0.40. At 10 Hz and 120 kV/cm, the Pmax decreases with increasing the SBT content, but the P-E loops become slimmer, which is beneficial to achieving a higher energy storage efficiency. Meanwhile, four peaks appear in current versus electric field curves (I-E curves) and the current peak shifts towards zero electric field along with the decrement of current intensity when the SBT content increases, indicating the enhancement of response speed for domains after removing the external voltage. The ceramic with the composition of x=0.30 exhibits slim P-E loops at different electric fields and a recoverable energy storage density of 2.16 J/cm3 along with a high energy storage efficiency of 90% at a low electric field of 192 kV/cm. In addition, the energy storage performance shows superior frequency and cycle stability. The variation of recoverable energy storage density and energy storage efficiency is less than ±8% within 1-200 Hz and 1-105 cycles.Conclusions (1-x)BNBT-xSBT lead-free ceramics were prepared by a solid-state reaction method. The effect of SBT content on the microstructure, dielectric, ferroelectric, energy storage properties and stability of (1-x)BNBT-xSBT ceramics were investigated. The temperature stability of dielectric constant was improved and the variation of dielectric constant was less than ±10% at 25-343.9 ℃ for the composition of 0.60BNBT-0.40SBT. The superior energy storage performance with a recoverable energy storage density of 2.16 J/cm3 and an energy storage efficiency of 90% was achieved at a low electric field (<200 kV/cm) and an optimized SBT content. In addition, the energy storage performance of (1-x)BNBT-xSBT ceramics showed a superior frequency and cycle stability. This study demonstrated that the addition of SBT could be beneficial to optimizing the energy storage performance of BNT-based lead-free ceramics and the (1-x)BNBT-xSBT ceramics could be promising candidates for advanced pulse power systems application.

Introduction The pulse power capacitors can be used in military, medical, and aerospace fields due to their high power density and rapid charge-discharge capabilities. Despite these advantages, limited energy storage capacity and efficiency become challenges in achieving miniaturization and high-power performance. Among dielectric energy storage materials, antiferroelectric materials are regarded as promising for high-performance pulse power capacitors due to their distinctive double hysteresis loops, lower residual polarization, and higher energy storage efficiency. However, the breakdown strength decreases due to non-uniform electric field when the resistance of ceramic grain is equal to that of grain boundary, resulting in a decrease of energy storage density. Therefore, the enhancement of grain boundary impedance plays a key role in improving the dielectric breakdown strength of the system. In this paper, (Pb(1-1.5x)Tmx)(Zr0.55Sn0.44Ti0.01)O3 (x=0.00, 0.02, 0.04, 0.06) antiferroelectric ceramics were prepared via tape casting. The strategy of grain boundary impedance was constructed via enhancing the grain boundary quantity. Methods Powders of Pb3O4, Tm3O2, ZrO2, SnO2, and TiO2 as raw materials were precisely weighed according to their chemical formulae after drying. The powders were mixed with alcohol and then ground in a ball mill for 24 h. Afterwards, the mixture was dried and pre-sintered at 850 ℃, and further ground in a ball mill for 24 h. The ground powders were mixed with toluene ethanol as a solvent, polyvinyl alcohol as an adhesive and butyl phthalate as a plasticizer. After 24 h milling, the mixture underwent degassing for 40 min, resulting in a slurry with an optimal fluidity. A flexible film with a thickness of 0.02 mm was produced via casting at a linear speed of 40 cm/min, and dried for 6 h. This film was then compressed into a block with the sizes of 10.0 mm×10.0 mm×0.5 mm. The samples were sintered in a muffle furnace via increasing the temperature from room temperature to 600 ℃ for 12 h to firstly remove the adhesive and then at 1 300 ℃. The sintered samples were cooled to room temperature.The crystal structure of PTZST ceramics at room temperature was determined by X-ray diffraction . The ion vibration mode and the degree of disorder in the local structure were characterized by Raman spectroscopy. The surface morphology and grain size of the ceramic samples were analyzed by scanning electron microscopy. The particle sizes were determined by a software named Nano Measure. The dielectric temperature spectra were analyzed. The hysteresis curves were measured at various electric fields and temperatures as well as at a constant electric field intensity of 10 Hz, by a ferroelectric testing system. The test samples had a thickness of 0.1 mm and an electrode diameter of 2 mm. The impedance data were recorded by a high-temperature dielectric test system and subsequently analyzed by a software named ZView to evaluate the electrical performance of the samples.Results and discussion All the PTZST ceramics exhibit a pronounced perovskite structure without elemental distortion, as evidenced by X-ray diffraction analysis. The orthogonal antiferroelectric phase of these ceramics is confirmed through superlattice diffraction and component selection analysis. The scanning electron microscopy images reveal that the grain size of the ceramics decreases with increasing Tm3+ content. This phenomenon is attributed to the formation of lead ion vacancies induced by the substitution of high valence Tm3+ for Pb2+, subsequently inhibiting oxygen vacancy formation and grain growth. Moreover, this modification increases the number of grain boundaries, enhancing the effectiveness of the grain boundary impedance strategy. However, excessive Tm3+ doping deteriorates the sample properties. The analysis of the P-E loops indicates a change in the energy storage density of PTZST ceramics. The PT4ZST ceramics demonstrate the maximum energy storage density (i.e., Wrec=9.37 J/cm3) and efficiency (i.e., η=77%). There is little variation in energy storage density at 30-120 ℃.The impedance spectrum transformation from two semicircles to a single semicircle indicates the effective implementation of the grain boundary impedance strategy. This alteration enhances the dielectric breakdown strength of PTZST ceramics, thereby augmenting the energy storage density.In exploring practical applications, charge-discharge tests were conducted. The discharge energy density of PT4ZST ceramics increases with increasing applied electric field, having the maximum value at a discharge energy density (i.e., Wdis=5.5 J/cm3) and a discharge rate (i.e., t0.9=199.85 ns).Conclusions The original long-range ordered structure was disrupted via doping Tm3+ in the A-site to replace Pb2+, reducing a tolerance factor and stabilizing an antiferroelectric phase. Consequently, this led to an improved dielectric breakdown and an enhanced energy storage capability. The results indicated that the PT4ZST ceramics achieved a high dielectric breakdown (i.e., Eb=490 kV/cm) and a significant energy storage density (i.e., Wrec=9.37 J/cm3). These ceramics exhibited a high energy storage efficiency (i.e., 77%) and a substantial discharge energy density (i.e., Wdis=5.5 J/cm3), coupled with a rapid discharge rate (i.e., t0.9=199.85 ns). Furthermore, the PT4ZST ceramic had a superior temperature stability at 30~120 ℃. The results indicated that the PT4ZST ceramic could be used as an effective material for high-temperature pulse power capacitors.

Introduction Antiferroelectric ceramic and multilayer ceramic capacitors have the advantages of high energy storage density, fast discharge speed, high discharge current, etc., which can improve the energy storage density of pulse power device and effectively suppress ripple in power electronic system, etc. Unlike ferroelectrics, antiferroelectric materials adjacent to the lattice of dipoles reverse parallel, and therefore do not show a macroscopic spontaneous polarization phenomenon. In a certain external electric field, the dipoles redirect, the antiferroelectric phase converts to a ferroelectric phase, the polarization intensity increases abruptly after the withdrawal of the external electric field, and it returns to an antiferroelectric phase. This polarization/phase transition behavior can be characterized by the hysteresis loop. In the hysteresis loop, antiferroelectric material shows a double loop, and it is divided into two kinds, i.e., “Square” and “Slant” according to the different shapes of the antiferroelectric hysteresis loop. The polarization effect of the antiferroelectric capacitor gives it the better energy storage characteristics and discharge performance, thus having an advantage in practical applications. General capacitors are polarized under a certain period of time and voltage in the factory. However, changes of external conditions (i.e., temperature, voltage, time, etc.) can depolarize the antiferroelectric capacitor. Also, some antiferroelectric capacitors need to consider the effect of polarity. As a result, the practical application of antiferroelectric MLCCs is different from that of conventional Class I or Class II MLCCs. Therefore, in this paper, antiferroelectric ceramics with hysteresis loop shapes of “square” and “slant” were prepared by a conventional solid-phase reaction method to investigate the polarization effect of antiferroelectric ceramics and capacitors, and provide a guidance for the application of antiferroelectric MLCCs.Methods For the preparation of the antiferroelectric ceramic, the raw materials were weighed according to the chemical formula. The excess Pb3O4 was used to compensate for lead loss in sintering. The weighed raw materials were mixed in a mass ratio of 1.0:1.0:1.5 and ground in anhydrous alcohol by grinding in a ball mill with zirconium balls at 300 rpm for 20 h. After ball-milling, the slurry was pre-fired at 850 ℃ for 3 h. The film was formed, and heated at 1 280 ℃ for 2 h. The samples were prepared after polished with sandpaper to a certain thickness (less than or equal to 0.1 mm) and sputtered with gold electrodes. The micro-morphology of the samples was measured by a model JSM-6390A scanning electron microscope (SEM, JOEL Co., Japan). The crystalline phase composition was determined by a model D/Max-2400 X-ray diffractometer (XRD, Rigaku Co., Japan), using Cu Kα rays, with a scanning range of 20°-70°. The hysteresis loops and polarization current curves of the samples were examined by a model TF Analyzer 2000 ferroelectric test system (aix-ACCT Co., Germany). The basic principle of the test was based on the Sawyer-Tower circuit. The test frequency was 1 Hz, and the voltage waveform was a triangular wave. The pulse discharge test used a self-built platform.Results and discussion From the SEM images of the samples, the grains are uniform and have a good densification. From the XRD patterns of the samples, the prepared samples have a quadrilateral phase structure. The prepared samples have a typical double hysteresis loop, and the shapes of hysteresis loops for sample S1 and S2 are “square” and “slant”. The hysteresis loops of the samples after and before polarization indicate that for the unpolarized ceramic sample, the transition field is higher, the stored energy density and energy conversion efficiency are lower. This phenomenon is also related to the shape of the hysteresis loops, so the polarization has a certain impact on the practical application of antiferroelectric ceramics. The electric hysteresis loop nearby transition field shows that for capacitors with antiferroelectric ceramic dielectrics, the antiferroelectric capacitor is unpolarized (or is depolarized) if the operating voltage is set nearby the transition field, thus leading to a weakening of its performance. To analyze whether high temperature should have an effect on the degree of polarization of antiferroelectric ceramics, the capacitances of the antiferroelectric ceramics are tested after depolarization at different temperatures, and the pulse discharge currents of the antiferroelectric MLCCs after depolarization at different temperatures are tested. The results show that if the antiferroelectric capacitor is overheated and warmed up, and although the temperature is recovered after using, the temperature still has a certain effect on the performance of antiferroelectric capacitor, especially at a lower applied voltage. To study the effect of polarity on the antiferroelectric material, the hysteresis loops of antiferroelectric ceramics in positive and negative electric fields were tested, and the pulse discharge of antiferroelectric MLCCs in different polarity electric fields was tested. The results show that the antiferroelectric domains are preferentially oriented, and the antiferroelectric domains have a memory effect after a certain direction of the electric field polarization, preventing its entry into the opposite direction of the ferroelectric domains. And this effect is more serious on the antiferroelectric ceramics with a “square” hysteresis loop. Conclusions The antiferroelectric polarization and polarity characteristics had dominant effects on their applications. Those effects above were lower although antiferroelectric capacitors were generally made of materials with “slant” hysteresis loops. However, since antiferroelectric materials were in an electric field that was slightly higher than the EAFE-FE phase transition field, their depolarization and polarity characteristics still affected their performance. In addition, a slight attenuation of the antiferroelectric MLCCs performance still adversely affected the reliability of the whole system since some applications required an extremely high reliability. In practice, antiferroelectric MLCCs were bound to depolarize after soldering, and their performance could be greatly degraded if the applied electric filed was lower than the transition field.

Introduction Flexible and stretchable electronics have attracted much attention due to their potential applications in wearable devices, robotics, and healthcare. Among various materials, barium titanate (BTO) is a promising candidate for flexible and stretchable electronics because of its high dielectric constant, piezoelectric property, and superior thermal stability. However, the brittle nature of BTO restricts its application in flexible and stretchable devices. Polymer composites such as polydimethylsiloxane (PDMS) can be used, providing a flexibility and a stretchability for BTO-based devices. In this paper, flexible BTO/PDMS composites with a hierarchical microstructure were prepared and their dielectric performance was optimized. The hierarchical microstructure could enhance the mechanical properties, electrical conductivity, and dielectric performance of the composites. This study could provide valuable insights into the design and fabrication of flexible and stretchable electronic devices based on BTO/PDMS composites.Methods A piece of foam nickel was prepared, cleaned with acetone under ultrasound for 10 min and then washed with deionized water for several times to remove impurities on the surface. An isopropanol was used as a solvent, an appropriate amount of phenolic resin was used as a stabilizer, and an appropriate amount of aluminum nitrate nonahydrate was added to make the BTO positively charged. Little precipitation occurred after mechanical stirring and ultrasonic treatment. A uniform and stable barium titanate suspension was formed. A layer of BTO coating was deposited on the pore walls of the pretreated foam nickel by an electrophoretic deposition (EPD) process. Two copper plates were used as the poles of the electrophoretic deposition. A nickel metal foam template was fixed on a negative pole, immersing it into the EPD tank with the stable BTO suspension To prepare the samples with different BaTiO3 contents, the diameter of the BTO framework pore was controlled via adjusting the electrophoresis time. The sample was sintered in an argon atmosphere at 1 200 ℃ for 2 h, and then immersed in a 1 mol/L FeCl3 solution to completely remove the foam nickel. Finally, the sample was sintered in air at 700 ℃ for 2 h to remove carbon, forming a BTO framework. To determine the mass fraction of BTO, a barium titanate framework was weighed before and after penetration by epoxy resin. The PDMS (with curing agent) was filled into the porous structure and cured at room temperature for 24 h to obtain the BTO/PDMS dielectric composite material. Results and discussion The results indicate that when the BTO content reaches 10.16% (in mass), the relative permittivity of the composite increases up to 85 at 1 000 Hz (compared to only 2.75 for pristine PDMS). Also, the dielectric permittivity increases as the mass fraction of BTO increases from 5% to 20% (in mass). For the BTO with an ordered morphology, the dielectric permittivity value increases, improving the dielectric strength performance in specific directions. Adding PVDF slightly increases the dielectric permittivity, while reducing the difference between the dielectric properties of filler and substrate, thereby enhancing the dielectric strength performance via combining PVDF with PDMS to form a core-shell structure and optimizing the dielectric properties by a finite element method. The hierarchical microstructure of the composites plays an important role in enhancing the dielectric properties. The uniform distribution of BTO particles and the interconnected porous structure of PDMS matrix provide a continuous path for the movement of charge carriers, which contributes to a high dielectric constant and a low dielectric loss of the composites. In addition, the flexible nature of PDMS matrix also makes the BTO/PDMS composites suitable for various applications, such as flexible electronic devices, sensors, and actuators.Conclusions The BTO/PDMS flexible dielectric composites with a hierarchical microstructure were prepared by a template method and an electrophoresis deposition method with BTO as a filling material and PDMS as a matrix. The influences of filler morphology and content on the dielectric constant and dielectric strength of the composites were investigated via finite element simulation analysis. Two kinds of ordered network structures of the composites, i.e., tetrahedral network structure and honeycomb network structure, were designed. This provides a theoretical guidance for the preparation of high-quality dielectric composite materials with superior energy storage density. The main conclusions were as follows: (1) Compared to the particle-filled composites, the BTO/PDMS composites with a hierarchical microstructure have an improved dielectric constant at a low filler content. For instance, the relative dielectric constant of the sample with 10.16% BTO could reach 85 at 1 kHz, which was 31 times higher than that of pure PDMS; (2) The morphology of filler had an impact on the dielectric performance of composites. In the case of a low filler content, the composites with a hierarchical microstructure had a higher dielectric constant rather than those with a solid network structure; (3) PVDF was used to form a nuclear shell structure (PVDF@BTO/PDMS) to improve the relative dielectric constant of the composites to a limited extent and the breakdown performance of composites; (4) The orderly network structures were designed. The ordered network structures with a tetrahedral network structure and a honeycomb network structure could improve the dielectric constant of composites to a certain extent and had the anisotropic characteristics.

Introduction Global energy crisis has sparked unprecedented momentum for renewable energy sources. Energy storage systems such as electrochemical capacitors, batteries, fuel cells, and dielectric capacitors are considered as the most important technologies to address rapidly growing energy demands and environmental concerns. In comparison with batteries or fuel cells, dielectric capacitors possess high power density resulting from their faster charging/discharging characteristics, which are advantageous for power electronics and pulse power applications. The majority of materials used for fabrication of dielectric capacitors consist of linear dielectrics, ferroelectrics, relaxor ferroelectrics, and anti-ferroelectrics. Among these materials, relaxor ferroelectrics are the most popular choice for capacitor applications because of their lack of long-range order ferroelectric domains. (1-x)Pb(Mg1/3Nb2/3)O3- xPbTiO3 (PMNT) as the typical representative of relaxor ferroelectrics, which possesses small remanent polarization and low coercive electric field, enjoys promising prospects in dielectric capacitor application. Nevertheless, limited by its low Eb, the energy storage density and efficiency for pure PMNT film are not yet ideal. Moreover, it is difficult for traditional PMNT film to be bendable due to limitations of rigid substrates, which lead to descend of flexibility and cannot meet modern flexible electronic production requirements. In this paper, to overcoming the bottlenecks of inflexibility and inferior energy storage performance of the pure PMNT films, rare-earth element of Sm and promising LaNiO3 (LNO) buffer layer were introduced into PMNT thin film. To obtain extraordinary flexibility, two-dimensional mica substrate was used to prepare the modified PMNT film via a simple sol-gel method. The microstructure, dielectric, ferroelectric and energy storage properties of the flexible Pb0.99Sm0.01(Mg1/3Nb2/3)0.68Ti0.32O3 (PSMNT) film were studied systematically.Methods The PSMNT and LNO films were prepared using a sol-gel method. First, niobium ethoxide (Nb(OC2H5)5) was dissolved in 2-methoxyethanol at room temperature. And tetrabutyl titanate (Ti(OC4H9)4) was stabilized in acetylacetone. Second, lead (II) acetate trihydrate (Pb(CH3COO)23H2O) with 10% excess Pb, samarium (III) nitrate hexahydrate (Sm(NO3)36H2O), and magnesium (II) acetate tetrahydrate (Mg(CH3COO)24H2O) were dissolved together in a mixed solvent of glacial acetic acid and 2-methoxyethanol (volume ratio of 2:1) at room temperature. Then, the solutions above were mixed to obtain the PSMNT precursor. To prepare the LNO precursor, the lanthanum (III) nitrate hexahydrate (La(NO3)36H2O) and nickel (II) acetate tetrahydrate (Ni(CH3COO)24H2O) were dissolved in 2-methoxyethanol. The concentration of the PSMNT and LNO precursors were 0.2 mol·L-1 and 0.1 mol·L-1, respectively. The flexible mica substrate was separated from the bulk fluorphlogopite mica crystal by mechanical exfoliation using a razor blade in deionized water. When the thickness of the mica substrate was reduced to ~15 μm, good flexibility could be realized. The LNO precursor solution was firstly spin-coated onto the mica substrate, followed by an annealing process at 750 ℃ for 2 min. Then, the PSMNT precursor was spin-coated on the surface of the freshly prepared LNO layer and then the wet film was annealed at 700 ℃. The spin-coating and annealing process was repeated to achieve a desirable film thickness. Finally, Au top electrodes, ~200 μm in diameter, were sputtered on the film surface through a shadow mask for electrical properties measurements.Results and discussion PSMNT and PMNT thin films were directly deposited on LaNiO3 buffered mica substrate via simple one-step fabrication process. X-ray diffraction (XRD) results indicated that the crystal structure of PSMNT film was pure perovskite phase without detectable secondary phases. Scanning electron microscope (SEM) measurement revealed that the film exhibited compact microstructure with uniform crystal grain distribution, indicating that the PSMNT film can be successfully grown on the LaNiO3 buffered mica substrate. In terms of polarization characteristics, slim hysteresis loop with a large maximum polarization Pmax (~74.5 μC/cm2) and a small remnant polarization Pr (~16.4 μC/cm2) was observed in the PSMNT film under the electric field of 2 468 kV/cm, while fatted P-E loop was observed in the PMNT film. The leakage current density in the PSMNT film is almost an order of magnitude lower than the pure PMNT sample, indicating the success of the introduction of Sm in improving of electric resistance. The recoverable energy storage density for the PSMNT film capacitor at the maximum electric field was 40.7 J/cm3, and the corresponding energy-storage efficiency was 70% due to its high breakdown strength and strong relaxor dispersion. The PSMNT film capacitor also exhibits excellent stability in energy storage performance in a wide operating frequency (1-20 kHz) and broad temperature (25-120 ℃) ranges. With the assistance of a mica substrate, the all-inorganic PSMNT/LNO/mica capacitor possesses an outstanding mechanical-bending resistance without obvious deterioration in its energy storage performance even down to a bending radius of 2 mm or repeated bending for 103 cycles. Conclusions The main conclusions of this paper are summarized as following. Flexible Pb0.99Sm0.01(Mg1/3Nb2/3)0.68Ti0.32O3 (PSMNT) film was successfully deposited on LaNiO3 buffered mica substrate by a simple sol-gel method. A high recoverable energy density of 40.7 J/cm3 and efficiency of 70% were realized in the fabricated film capacitor. The energy storage performance was quite stable in a wide temperature (25-120 ℃) and frequency (1-20 kHz) ranges. Meanwhile, the designed PSMNT film capacitor exhibited superior mechanical-bending resistance since it is attached on the bendable mica substrate, and its energy storage behavior did not show obvious deterioration even under a small bending radius of R=2 mm and 1 000 mechanical bending cycles (R=4 mm). This study showed that flexible PSMNT thin film on mica platform has bright prospects in future ?exible electronics application for energy storage.

Introduction As a core energy storage device of high energy pulse power supply, dielectric energy storage materials have the characteristics of high-power density, fast charging and discharging rate, good temperature stability, and intense aging resistance. They are widely used in power and electronic systems such as hybrid electric vehicles, oil exploration, directional weapons, etc., especially in high energy pulse power technology, having irreplaceable application prospects. With the rapid development of pulse power electronic systems, higher requirements are placed on the energy storage density, discharge current and time of dielectric capacitors in pulse power systems. These advanced devices are used in scenarios such as 100 kA current or 100 kV high voltage. This becomes some challenges for the energy storage density of dielectric capacitors. In addition, a problem of low energy storage density of dielectric capacitors also makes the pulse power system too large and cumbersome to meet the requirements of practical equipment platforms for small and lightweight high-tech weapons. Dielectric energy storage materials are key materials for pulse power devices, but their low energy storage density severely limits the miniaturization of dielectric capacitors. Further improving the breakdown field strength and increasing the dielectric constant to obtain high energy storage density dielectric materials remain a major challenge. It is necessary to develop new technologies that can improve the energy density of dielectric capacitors or explore new material systems with a high energy storage density. Doping rare-earth oxides into glass ceramics can enhance their dielectric and energy storage properties. The rare-earth element La3+ has a unique 4f electron layer structure, large ion radius and a high coordination number. Incorporating La3+ into the glass ceramics has an impact on their phase structure, dielectric properties, and energy storage performance. It is thus expected that the energy storage performance of glass ceramics can be improved via doping the rare-earth element La3+.Methods Strontium barium niobate glass ceramics doped with different mole fractions of La3+ were prepared via high-temperature melting and subsequent temperature-controlled crystallization. The strontium barium niobate glass ceramic had a composition of 20SrO-20BaO-20Nb2O5-33.5SiO2-5Al2O3-1.5B2O3. Different mole fractions (i.e., 0.5%, 1.0% and 1.5%, in mole) of La3+ elements were doped into the galss ceramics. The raw materials were weighed according to the stoichiometric ratio and evenly mixed/ground in a ball mill. The mixed raw materials were sintered in a high-temperature glass furnace at 1 550 ℃ for 2.5 h. The molten material was quickly poured into a preheated metal mold to create a bulk glass. The glass was then held at 650 ℃ for 3 h to remove any remaining stresses. The transparent glass was cut into flakes with the sizes of 1.5 mm×6.0 mm×6.0 mm and heated to 1 100 ℃ for 3 h. Finally, strontium barium niobate glass ceramics with varying mole fractions of La3+ doping were prepared.The phase structure and content of La3+ doped strontium barium niobate-based glass ceramics were analyzed by a model D8 Advanced X-ray diffractometer. The crystal size and microstructure of glass ceramics were determined by a model XL30-FEG scanning electron microscope. The dielectric constant and dielectric loss of glass ceramics at different temperatures were measured by a model E4980A LCR meter. The breakdown strength of glass ceramics was evaluated by a model ET2671B breakdown resistance tester. The P-E polarization curve of glass ceramics was tested by a model Premier-II ferroelectric tester.Results and discussion The impact of doping mole fractions of La3+ on the phase structure, microstructure, and dielectric constant, loss, breakdown field strength, and energy storage performance of strontium barium niobate glass ceramics was investigated via experiments and theoretical simulation to reveal the mechanism of enhancing the energy storage performance of strontium barium niobate glass ceramics. The results show that La3+ doping can improve the crystallinity and the content of tungsten bronze structure Ba0.5Sr0.5Nb2O6 of the glass ceramics, thereby increasing the dielectric constant of strontium barium niobate glass-ceramics. At doping mole fractions of 0.5%, 1.0%, and 1.5% for La3+, the room-temperature dielectric constants of strontium barium niobate glass ceramics are 76.3, 96.3, and 88.1, respectively. These values are all greater than the dielectric constant of undoped strontium barium niobate glass ceramics (i.e., 65.7). La3+ doped strontium barium niobate glass ceramics have a superior temperature stability and a low loss. The optimum La3+ doping mole fraction can improve the microstructure and reduce the interface activation energy of glass ceramics, thus enhancing the breakdown strength of glass ceramics. When the doping mole fraction of La3+ is 1.0%, the breakdown strength of the glass ceramics reaches 1 458 kV/cm, and the dielectric constant is 96.3. The maximum energy storage density reaches 9.36 J/cm3, which is 2.25 times greater than that of undoped strontium barium niobate glass ceramics.Conclusions In the controlled doping amount of La3+ of glass ceramics, the phases and microstructure of glass ceramics could be effectively controlled. This improved the dielectric properties and breakdown strength of the glass ceramics. The increase in energy storage density of strontium barium niobate glass ceramic with an appropriate doping amount of La3+ was attributed to the increase in crystallinity and decrease in interfacial activation energy of the glass ceramics. This study could provide a reference for the development of high energy storage density glass-ceramics.

Introduction Polymer-based dielectrics with ultrahigh power density, good flexibility and easy-processing have attracted much attention in powering electronics such as capacitor, power transmission, pulsed power systems and microelectronic systems. However, the low energy density affects the application in electronics industry. Recently, introducing the ferroelectric ceramic fillers with a high dielectric constant into the polymer matrix can improve the dielectric constant of polymer composite dielectric materials. In general, the microscopy of fillers has a great influence on the energy storage performance of polymer-based dielectric capacitors. One-dimensional nanomaterials are potential to fabricating the high-performance polymer-based dielectric capacitors due to their quantum size effect, surface effect and macroscopic quantum tunneling effect. When one-dimensional nano-ferroelectric materials are introduced into the composites, a high length?diameter ratio can reduce the specific surface area and enhance the dispersion in the polymer matrix. Also, the composites can obtain better energy storage properties with a lower filler content due to its large electric dipole. In this paper, KNbO3 nanofibers with a good crystalline and a high length-diameter ratio were firstly synthesized via hydrothermal reaction. Polymer-based dielectric capacitors were then fabricated with KNbO3 nanofibers and poly(vinylidene fluoride-co-hexafluoropropylene) & polymethyl methacrylate (P(VDF-HFP)&PMMA) polymers. The recoverable energy storage density and energy storage efficiency were analyzed. Methods For the synthesis of KNbO3 nanofibers, 36 g KOH and 1 g Nb2O5 were mixed in a 60 mL beaker and then were stirred for 1 h. Afterwards, the solution was transferred to a 100 mL reactor for hydrothermal reaction at 180 ℃ for 12 h. The particles were washed using deionized water and ethanol for several times and dried at 60 ℃ for 12 h. Finally, the composite films were prepared through a solution-casting method. 0.9 g P(VDF?HFP) and 0.1 g PMMA were dissolved in 10 mL N,N-Dimethylacetamide (DMAC) at 80 ℃. KNbO3 nanofibers with different mass fractions (x=0, 2.5%, 5.0%, 7.5%，and 10.0%) were dispersed in the DMAC solution under magnetically stirring to obtain the mixture, and then the mixture was added into the P(VDF-HFP)&PMMA solutions. The composite films were prepared by a tape casting and subsequently dried at 90 oC. The thickness of the prepared films was 20 μm. Au electrodes (2 mm in diameter) were sputtered on the composite films for the coming electrical measurements.Results and discussion Based on the crystal structure of the KNbO3 nanofibers detected by XRD, the nanofibers exhibit a pure orthorhombic perovskite structure in coincidence with the standard JCPDS 32-0822. A high diffraction intensity reflects a good crystallinity of KNbO3 nanofibers. In addition, the widen diffraction peaks show the small size of KNbO3 nanofibers. From the TEM and SEM images, KNbO3 nanofibers have a uniform microscopy with a diameter of 10-20 nm and the length of ~500 nm. The distinct lattice fingers and selected area electron diffraction patterns confirm a good crystallinity of KNbO3 nanofibers. Besides, all the elements uniformly distribute for KNbO3 nanofibers. The frequency-dependent dielectric constants of composite films with x of 0, 2.5%, 5.0%, 7.5%, and 10.0% are 2.8, 5.2, 6.1, 8.2, and 11.5, respectively. KNbO3 nanofibers enhance the dielectric properties of composite films because of the intense ferroelectricity and the Maxwell-Wagner-Sillars interface effect. In addition, all the films exhibit a low dielectric loss, indicating the superior breakdown strength. The polarization intensity at the same external electric field increases with the increase of KNbO3 nanofiber fillers. Besides, the breakdown electric field reduces as the KNbO3 nanofibers increases because a severe electric field deformation nearby KNbO3 nanofiber fillers generates a high local electric field distribution. Therefore, the energy storage density firstly increases and then decreases, achieving an optimal value of 14.3 J/cm3 when 7.5% (in mass fraction) KNbO3 nanofibers are doped in the composite films. Also, the energy storage efficiency of the optimized composites is 72%. Conclusions The ferroelectric nano-materials were filled into the polymers to enhance the energy storage performance of composite capacitors. KNbO3 nanofibers prepared by a hydrothermal method exhibited a high aspect ratio and a good crystallinity. KNbO3 nanofibers were mixed with P(VDF-HFP)&PMMA polymers to prepare KNbO3/P(VDF-HFP)&PMMA composite films. The polarization strength of composite films was enhanced. Meanwhile, the breakdown strength of composite films slightly degraded. Therefore, the optimum energy storage density of 14.3 J/cm3 and efficiency of 72% were achieved when 7.5% KNbO3 nanofibers were doped in the composite films. The results indicated that KNbO3 nanofibers could effectively enhance the energy storage performance of composite capacitors.

Introduction Environmental pollution and energy shortage become more serious. How to effectively store energy and reduce energy loss is a recent challenge. Dielectric ceramic capacitors have some advantages such as long lifespan, fast charging and discharging speed, high power density, and good temperature stability. Na0.5Bi0.5TiO3 (NBT) is a ferroelectric material with an A-site composite ABO3 perovskite structure. The Curie temperature of NBT is 320 ℃, with a large maximum polarization (Pmax) (40- 50 μC/cm2) and a wide sintering temperature range. NBT is one of the hot research topics in energy storage ceramics. However, a high remanent polarization (Pr) (~38 μC/cm2) results in low recyclable energy storage density (Wrec) and efficiency (η) for NBT. Introducing a low Pr component into NBT to form a solid solution can refine the polarization-electric field (P-E), and improve the material breakdown strength (BDS) to achieve a high Wrec. In this paper, Ti4+ was replaced in Sr0.7Bi0.2TiO3 by Zr4+. The introduction of Sr0.7Bi0.2ZrO3 (SBZ) could enhance the degree of disorder in the A-site and B-site in the NBT, respectively. The ferroelectric long-range ordered structure was effectively break, further enhancing the relaxor ferroelectricity of NBT ceramics. Zr4+ was chosen instead of Ti4+ because Zr4+ was more stable chemically, and the properties of the material were further improved via introducing ionic radius difference and oxygen vacancies, leading to the lattice distortion. Also, ZrO2 with a larger band gap was conducive to improving BDS and achieving a high energy storage performance.Methods The (1-x) Na0.5Bi0.5TiO3-xSr0.7Bi0.2ZrO3 (NBT-SBZ, x=0.1, 0.2, 0.3, 0.4) ceramics were synthesized by a solid-phase method. Na2CO3 (99.8%, in mass fraction, the same below), Bi2O3 (99.9%), ZrO2 (99.0%), SrCO3 (99.0%), and TiO2 (98.0%) (National Pharmaceutical Group Chemical Reagent Co., China) were used as raw materials. The raw materials were mixed with alcohol and ground in a ball mill for 24 h. After drying, the powder was calcined at 850 ℃ for 4 h. The green body with a diameter of 12 mm was pressed by cold isostatic pressing at 200 MPa for 3 min. The green body was sintered in air at 1 090-1 150 ℃ for 4 h.The phase structure of NBT-SBZ ceramic samples was characterized by a model D8 Advance X-ray diffractometer (XRD, Bruker Co., Germany) at 20°-80°. The microstructure was determined by a model S4800 scanning electron microscope (SEM, RIGAKU Co., Japan). The dielectric properties were measured by a model 4980A precision LCR (Agilent Co., USA). The thickness of the dielectric properties sample was 0.6 mm, and a silver electrode was fired at 850 ℃. The P-E loops were tested by a model Premier II ferroelectric analyzer (Radiant Co., USA). The P-E test sample thickness was 0.1 mm and a diameter of 2.0 mm for a gold electrode. The charging and discharging characteristics were determined from a model CFD-003 charging/discharging system (Gogo Co., China). The sample thickness was 0.3 mm, and a silver electrode was fired at 850 ℃.Results and discussion The XRD pattern of NBT-SBZ ceramics indicates that all the samples exhibit a NBT pseudocubic perovskite structure. ZrO2 and Bi2Ti2O7 second phase appear as the SBZ content increases. A peak at 46.5o (200) shifts towards a lower angle as the SBZ content increases, indicating an increase in the spacing between ceramic crystal planes. The SEM images indicate that the grain size distribution of all the samples is uniform and presents a normal distribution. The average grain size decreases from 1.57 μm for the sample with x of 0.10 μm to 1.06 μm for the sample with x of 0.4 with the increase of SBZ content. The substitution of larger radius ions increases the lattice strain energy and hinders the grain boundary movement, leading to a grain refinement. The grain refinement and reduction of pores are beneficial to obtaining a larger BDS. The dielectric constant (εr) decreases from 870.5 for the sample with x of 0.1 to 668.5 for the sample with x of 0.4 with increasing the frequency due to a decrease in polarization mechanism. As the SBZ content increases, the dielectric loss (tanδ) at 1 kHz decreases from 0.041 for the sample with x of 0.100 to 0.029 for the sample with x of 0.4. All the samples exhibit obvious relaxor ferroelectric characteristics. The P-E loops also change from a typical ferroelectric shape to a thin and elongated shape of a relaxor ferroelectric, and the current density (J)-E current peak becomes rectangular, indicating that the relaxor ferroelectricity of NBT-SBZ ceramics increases with the addition of SBZ. BDS increases from 189.7 kV/cm for the sample with x of 0.1 to 298.1 kV/cm for the sample with x of 0.4, which is consistent with the decrease in average grain size and leakage current density, indicating that small grain size and dense microstructure are important reasons for achieving a high BDS. 0.8NBT-0.2SBZ ceramic with x of 0.2 at 265 kV/cm obtains Wrec of 3.15 J/cm3 and η of 76.05%. 0.8NBT-0.2SBZ ceramics exhibit a superior frequency/temperature stability in energy storage performance. CD and PD of 0.8NBT-0.2SBZ ceramics are 188.54 A/cm2 and 11.31 MW/cm3 at 120 kV/cm, with an extremely fast t0.9 of 52.6 ns. Conclusions NBT-SBZ relaxor ferroelectric ceramics were prepared via introducing SBZ to improve the relaxor ferroelectricity and energy storage properties of NBT ceramics. 0.8NBT-0.2SBZ ceramic achieved the optimum energy storage properties (i.e., Wrec=3.15 J/cm3, η=76.05%) at 265 kV/cm, as well as a superior stability of the energy storage performance at 1-100 Hz) and 20-140 ℃. The CD and PD of 0.8NBT-0.2SBZ ceramic were 188.54 A/cm2 and 11.31 MW/cm3 at 120 kV/cm, respectively, with a fast t0.9 of 52.6 ns. The 0.8NBT-0.2SBZ ceramic had a superior temperature/frequency stability, a super high power density, and extremely fast discharge time, having a great potential for application in pulse power supply equipment.

Introduction Lead-free dielectric capacitors have attacted recent attention due to the super power density, fast charge-discharge rate and outstanding thermal stability. However, the recoverable energy density of lead-free ferroelectric ceramic is relatively lower, compared to ceramics containing lead. The introduction of strong relaxor ferroelectrics can realize the excellent energy storage performance via decreasing remanent polarization as well as enhancing impedance. Bi0.5K0.5TiO3 (BKT) is a promising candidate for dielectric capacitor due to the unique lone pair electronic 6s2 configuration about Bi, leading to a large spontaneous polarization (i.e., P = 52 μC/cm2). Nevertheless, the energy storage properties of BKT-based ceramics are poor (i.e., Wrec < 3 J/cm3，η < 80% ) because of the large remanent polarization and negative breakdown strength. Chen et al. reported that the recoverable energy storage density reached 7.5 J/cm3 through adding the BaTiO3 and NaNbO3 into BKT, indicating that BKT-based ceramics had a substantial prospect. The long-range ferroelectric order is broken to obtain a slim hysteresis loop, which can improve the energy storage performance. Simultaneously, the uniform small grain and compact microstructure of ceramic are essential, which can upgrade the breakdown strength for ferroelectric material. Thus, reducing grain size and breaking ferroelectric long-range order to generate polar nano-regions (PNRs) could improve the BKT-based specimens energy storage properties. In this paper, a relaxor ferroelectric material of Na0.73Bi0.09NbO3 (NBN) was induced into 0.75Bi0.5K0.5TiO3-0.25BiFeO3(0.75BKT-0.25BF) to reinforce the ESP of ceramics. BF and NBN were selected to optimize ESP. The morphotropic phase boundary (MPB) of BKT-BF binary system was constructed. Methods For the synthesis of BKT-BF-xNBN (x=0, 0.1, 0.2, 0.3 and 0.4) ceramics , Bi2O3 (99.5%, in mass fraction, the same below), K2CO3 (99.5%), TiO2 (99.5%), Fe2O3 (99.5%), Na2CO3 (99.8%) and Nb2O5 (99.5%) were used as raw materials. The dried oxides were weighted and mixed according to stoichiometric ratios in a polyethylene container, and then the mixed materials with alcohol were ground in a ball mill for 12 h. After drying, the mixture was calcined at 850 ℃ for 5 h, and the powder was further ground under the same condition. Sequentially, the dried powder with 0.5% (in mass fraction) polyvinyl alcohol (PVA) was pressed into discs with a thickness of 0.8 mm and a diameter of 12 mm. Finally, the green plates were sintered at 1 000-1 035 ℃ for 3 h.Results and discussion The results show that an optimum recoverable energy storage density (Wrec) of 4.23 J/cm3 and an improved energy storage efficiency (η) of 81.2% at 380 kV/cm are achieved. The microstructure of the specimens is compact due to the refined size of grain, leading to a boosted breakdown strength (Eb). In addition, an enhanced efficiency of the specimen with x of 0.3 can be obtained, which arose from the negligible remanent polarization (Pr). Based on the analysis of dielectric relaxation, the dispersion coefficient (γ) of the BKT-BF-0.3NBN is 2.2, implying a short-range ferroelectric order, which can increase the polar nanometer regions. To better understand the reason for the decreased Pr, the first order reversal curve (FORC) distribution of ceramics was measured. The optimized composition has a distinguished dielectric thermal stability and a superior charge-discharge performance (i.e., t0.9 (the time for releasing 90% of all storage energy) < 75 ns, current density (CD) of 757.9 A/cm2 and power density (PD) of 94.7 MW/cm3), indicating a promising prospect in the applications for high-power dielectric capacitors. These results illustrate that the composition design for BKT-BF-xNBN is effective to refine the energy storage performance of ceramic.Conclusions Lead-free BKT-BF-xNBN ceramics with x of 0, 0.1, 0.2, 0.3 and 0.4 were synthesized. The introduction of Na0.73Bi0.09NbO3 improved the microstructure of ceramics, boosted the breakdown strength, enhanced the ceramics relaxtion and resulted in the appearance of PNRs. As a result, the BKT-based ceramic with x of 0.3 had the optimum properties (i.e., Wrec of 4.23 J/cm3 and η of 81.2%). Moreover, the superior dielectric thermal stability as well as charge-discharge performance (i.e., t0.9<75 ns, CD=757.9 A/cm2, PD=94.7 MW/cm3) were achieved. This work indicated that BKT-BF-0.3NBN ceramic could have a promising application potential in cutting-edge performance dielectric capacitor.

Introduction Dielectric ceramic capacitors are essential components in next-generation advanced pulse power systems due to their ultra-fast charging/discharging rates and remarkable power density. As a typical lead-free perovskite material, Sr0.7Bi0.2TiO3 (SBT) has a larger permittivity rather than SrTiO3 because Bi modification favors producing a high polarization due to the intense hybridization between the 6s2 orbitals of Bi3+ and O 2p orbitals. Moreover, it also has a superior ferroelectric relaxor behavior with a diffused maximum dielectric constant in a wide temperature range, benefiting from Bi3+ off centering and Sr site vacancies. However, SBT ceramic still has a critical challenge for improving dielectric breakdown strength (Eb). Some previous studies indicate that adding some oxides can effectively inhibit grain growth and increase Eb. In this paper, the SBT ceramics were synthesized by a conventional solid-state sintering method with ZrO2 as an additive. A part of Zr4+ was deposited on the grain boundary in the form of oxides to inhibit grain growth. The breakdown strength and the energy storage density of the ceramics with different ZrO2 contents were analyzed.Materials and method Bi2O3(99.5%, in mass. The same below), SrCO3(99.5%), TiO2(99.8%), and ZrO2(99.9%) were used as raw materials. The raw materials were weighed according to the stoichiometric ratio. The materials were mixed with anhydrous ethanol and ground in a ball mill with zirconium balls for 12 h. The slurry was dried in an oven. After pre-firing at 850 ℃ for 5 h, the powder was further ground for 24 h according to the corresponding mass ratio Sr0.7Bi0.2TiO3+xZrO2 (SBT-xZr, x= 0, 0.5%, 1.0%, 2.0%, 5.0%, in mass fraction). The dried ground powder was pressed into sheets with a thickness of 0.8 mm and a diameter of 12 mm, and then sintered at a rate of 4 ℃/min at 1 200-1 250 ℃ for 3 h, and then cooled down naturally.Results and discussion Based on the XRD patterns and mapping energy spectra, element Zr partially dissolves in the SBT lattice in the form of Zr4+, and the rest exists in the form of ZrO2 on the grain boundary and surface, showing the formation of a 0-3 type composite structure. The dielectric constant of SBT-2%Zr ceramic consistently exhibits a high value, while the low dielectric loss mitigates excess waste heat, which is beneficial to the energy storage performance. The heightened energy storage density primarily stems from the increased Eb value. With the augmentation of ZrO2 content, the Eb value of the SBT-2%Zr increases, reaching a peak of 580 kV/cm, which is 27.6% higher than that of pure SBT ceramic. Consequently, the recoverable energy storage density (Wrec) experiences an elevation from 4.07 J/cm3 for the ceramic without x to 5.57 J/cm3 for the ceramic with x of 2%, while energy efficiency (η) maintains 88.97%. However, for the ceramic with x of 5%, despite achieving a high Eb value, the introduction of excessive ZrO2 leads to a notable reduction in maximum polarization (Pmax), preventing the attainment of the maximum Wrec value. The average grain size decreases from 2.30 μm to 1.31 μm, facilitating the attainment of more highly insulated grain boundaries. Also, the band gap energy (Eg) increases from 2.94 eV for the SBT ceramic to 3.02 eV for the SBT-5%Zr ceramic. This signifies that electrons require a larger applied electric field to achieve the valence band and conduction band transition. The expansion of the impedance circle indicates a higher resistivity of the material, and the elevated activation energy (Ea) effectively restrains the transmission of charged carriers. These interrelated mechanisms are crucial factors contributing to the substantial enhancement of Eb. In addition, SBT-2%Zr ceramic also shows an excellent fatigue stability (i.e, 5×104 cycles, Wrec change <1%) and a good charge-discharge performance (i.e., discharge energy density Wd=2.15 J/cm3, current density CD=1 060.51 A/cm2 and power density PD=169.68 MW/cm3).Conclusions A lead-free 0-3 type relaxation ferroelectric ceramic based on SBT was prepared by a stable solid-state reaction method. The ceramics doped with different concentrations of ZrO2 exhibited an original crystal structure with some Zr4+ in the lattice. The remaining Zr4+ existed in the form of a second phase, predominantly appearing at the boundaries and surfaces of grains. The increase of energy storage density mainly depended on the increase of Eb, and the possible physical mechanisms (i.e., grain size, band gap, impedance and activation energy) cooperated to promote the increase of Eb. SBT-2%Zr ceramic had the optimum energy storage performance, i.e., recoverable energy storage density (Wrec=5.57 J/cm3) and energy efficiency (η=88.97%). After 5×104 cycles, SBT-2%Zr ceramic with a good performance stability (i.e., Wrec change amplitude <1%) could be used as a promising candidate material for pulse power energy storage capacitors.

Introduction Dielectric ceramic capacitors are widely used in the field of high energy pulse because of their high power density, ultra-fast charge-discharge speed and good stability. However, compared with other capacitors, the low recoverable energy storage density (Wrec) affects its development in the field of energy storage. The typical ferroelectric BaTiO3 (BT) with good dielectric and insulation properties is restricted in the development of energy storage field due to the low breakdown strength and large residual polarization. It is indicated that the combination of multi-component solid solution and ion doping induces the polar nanoregions (PNRs) and the transformation of BaTiO3 ceramics from ferroelectrics to relaxation ferroelectrics due to the reduction of Pr and the improvement of energy storage performance. In addition, obtaining simutanously the high Wrec and energy storage efficiency (η) is a challenge. In this paper, (1-x){0.88(Ba0.6Ca0.4)TiO3-0.12Bi[Zn2/3(Nb0.85Ta0.15)1/3]O3}-xCeO2 (abbreviated as BCT-BZNT-xCe, x = 0, 0.010, 0.015, 0.020, 0.025) ceramics were prepared by a conventional solid-state reaction method. The structure and energy storage properties of the ceramics with differet Ce+4 doping contents were investigated. Methods High purity BaCO3, CaCO3, TiO2, Bi2O3, ZnO, Nb2O5, Ta2O5 and CeO2 were used as raw materials. The raw materials were weighted and mixed according to stoichometric ratio. The mixed raw materials were firstly ground in a ball mill, and then dried and calcined in air at 900 ℃ for 4 h. Afterwards, the calcined powder was further ground, dried and pressed into discs with 10 mm in diameter. The discs were sintered at 1 200 ℃ for 2 h to obtain the ceramic samples. The ceramic samples were thinned and polished to 0.05 mm, and sputtered with Au electrodes for the energy storage properties.The phase structure of the samples was analyzed by a model SmartLab X-ray diffractometer (XRD, Rigaku Co., Japan) in the range of 20o-80o. The Raman spectra of the samples were analyzed by a model Renishaw inVia Laser Confocal Raman Microspectroscopy (Renishaw Co., UK). The microstructure and elemental analysis (EDS) were determined by a model Merlin Compact Field Emission Scanning Electron Microscope (FE-SEM, Zeiss Co., Germany). The dielectric properties were analyzed by a model HCT1821 High-Low Temperature Dielectric Testing System (Tongguo Technology Co., China). The unipolar P-E curves were measured by a model TF2000 ferroelectric tester (aixACCT Co., Germany) to investigate the ferroelectric performance. The pulsed performance of the samples was examined by a model CFD-003 plus high-voltage charge-discharge measuring instrument (Tongguo Technology Co., China).Results and discussion All the samples are a multiphase structure with a tetragonal phase BaTiO3 (BT) as a main phase and an orthorhombic phase CaTiO3 (CT) as a second phase. The excessive Ce4+ causes the appearance of the diffraction peak at 28.5o of 2 when x=0.025. Also, the diffraction peaks move to a low angle from the magnified (200). The minimum average grain size (AGS) of both coarse and fine grains is obtained when x=0.02. The BCT-BZNT-0.02Ce ceramic obtains a high breakdown strength. The EDS analysis indicates that the coarse grains contain more Ba, while the fine grains contain more Ca.The dielectric permittivity of samples decreases with increasing the frequency. A large valence difference betweent the A-site and B-site ions induces PNRs, having a typical relaxation behavior. Besides, the diffuseness parameter () as x=0.02 is the maximum value (i.e., 1.917), which is similar to that of the ideal relaxation ferroelectric. The unipolar P-E curves of all the samples are slender and exhibit good relaxation characteristics. The breakdown strength and maximum polarization strength (Pmax) increase with the increase of Ce content, and reach the maximum values as x=0.02. A high Wrec of 5.49 J/cm3 and a high η of 89.4% are obtained at 600 kV/cm in BCT-BZNT-0.02Ce ceramic, which is attributed to the increasse of breakdown strength and Pmax. Moreover, the superior charge-discharge performance is obtained in BCT-BZNT-0.02Ce ceramic. The ultra-high current density (i.e., CD=1 757.07 A/cm2) and power density (i.e., PD=237.20 MW/cm3) are obtained at 270 kV/cm. Besides, a discharge energy density (WD) of 1.55 J/cm3 and an ultra-fast charge-discharge speed (t0.9) of 23.8 ns are obtained simultaneously. The WD is smaller than the Wrec due to the unremarkable contribution of domain wall motion at higher frequencies from the perspective of dynamics. The BCT-BZNT-0.02Ce ceramic with ultra-high Wrec (5.49 J/cm3), η (89.4%), CD (1 757.07 A/cm2), PD (237.20 MW/cm3) and ultra-fast t0.9 (23.8 ns) can be used in pulsed capacitors.Conclusions The grains size of BCT-BZNT-xCe ceramics was refined via the introduction of rare-earth element Ce. The of the BCT-BZNT-xCe ceramics was improved, and the BCT-BZNT-0.02Ce ceramic showed a typical relaxation ferroelectric performance (i.e., =1.917). The superior energy storage performance (i.e., Wrec=5.49 J/cm3, η=89.4%) of BCT-BZNT-0.02Ce ceramic was achieved at 600 kV/cm at room temperature due to the breakdown strength and Pmax. Also, the BCT-BZNT-0.02Ce ceramic exhibited a superior charge-discharge performance (i.e., CD=1 757.07 A/cm2, PD=237.20 MW/cm3, and t0.9=23.8 ns). The BCT-BZNT-0.02Ce ceramic with the superior comprehensive properties could have a great application prospect in pulsed capacitors.

Introduction The innovative development of advanced energy storage devices is imminent due to the continuous depletion of non-renewable energy sources and the increase in energy demand. Compared with other energy storage devices, film capacitors have some advantages of high power density, low polarity, high insulation impedance and wide frequency response, which are a recent research hotspot. Meanwhile, film capacitors have the functions of bypass, decoupling, wave filtering and energy storage, etc., which are widely used in communication, electronics, aerospace, medical equipment, new energy and other industries. Recent investigation on new dielectric materials with a low dielectric loss, a high breakdown strength and a high energy density becomes popular. In this paper, poly(vinylidene fluoride-hexafluoropropylene) (P(VDF-HFP)) was modified via irradiation using a xenon heavy ion beam, and the effect of irradiation injection on the microstructure, dielectric properties and energy storage properties of the materials were investigated. The mechanism of intrinsic structure evolution by novel irradiation modification technology was discussed. Methods P(VDF-HFP) was first dissolved in N,N-Dimethylformamide (DMF) and stirred in a water bath for 24 h to obtain a homogeneous solution. The solution was then flowed onto a quartz glass substrate and cast with a spatula to form a P(VDF-HFP) liquid film. It was dried in a vacuum drying oven at 0.08 MPa and 80 ℃ for 24 h to evaporate the solvent. The films were heated at 120 ℃ for 12 h to completely remove the solvent residue. The cast film was peeled off from the glass substrate in alcohol and dried to obtain an untreated flexible polymer dielectric film. The pure film was slit and then vertically irradiated with a beam of xenon ions (129Xe27+) of a mononuclear energy of 19.5 MeV/u at an irradiation injection of 5×106, 5×107, 5×108, 5×109, and 5×1010 ions/cm2, respectively. The six irradiation gradient samples were washed, dried and stored for subsequent analysis. The crystal form and orientation of the samples with different irradiation injections were determined by X-ray diffractometer (XRD). The phase evolution of the samples was analyzed by Fourier transform infrared spectroscopy (FTIR), and the content of each phase was quantified in combination with XRD. The microstructural morphology of the films was characterized by thermal field emission scanning electron microscopy (FSEM). The electrical properties were tested by sputtering gold electrodes on the both sides of the film. The dielectric constant and dielectric loss were measured by an LCR precision meter. The hysteresis loops and breakdown strength of the films were tested by a ferroelectric analyzer.Results and discussion Irradiation produces a cross-linking structure, effectively fixing the free charge and hinder the formation of electrical pathways. Also, the cross-linking structure effectively fills the gaps on the surface caused via solvent evaporation and crystallization, which prevents the breakdown of a local high voltage. The phase αin P(VDF-HFP) decreases with increasing irradiation injection, from 62.65% for a pristine phase to 43.72% for an irradiation injection of 5×1010 ions/cm2. The polar phases β and γincrease monotonically with increasing the phase β to 19.97%, which is twice greater than that of the pristine phase, and the phase γ increases to 36.31%. The fast heavy ion irradiation will cause the polymer chains to break bonds and produce more polar dipoles, improving the dielectric constant of the material. Moreover, the defects introduced by high energy irradiation provide a larger turning space for the dipoles due to the reduction of material losses. At 1 kHz, the dielectric constant of 5×107 ions/cm2 irradiated injection samples increases from 8.43 to 9.97, and the loss decreases from 0.033 to 0.027. A suitable irradiation dose can effectively increase the breakdown strength of the material. The breakdown strength of the material increases and then decreases with the increase of irradiation dose. At 5×107 ions/cm2, the breakdown strength of the material increases from 440 MV/m to 540 MV/m for the untreated samples, which is related to the dominant phase transition of the γ-phase and the cross-linking structure generated via irradiation. The energy storage performance of this work is compared with representative P(VDF-HFP) modifiers reported perviously. Most of the reported P(VDF-HFP) modifiers have discharge energy density values of <10 J/cm3 and breakdown field strengths of < 500 MV/m. In contrast, this work achieves greater discharge energy density and breakdown strength. Conclusions The 129Xe27+ fast heavy ion irradiation modification promoted advantageous phase transitions and the formation of surface cross-linking structures, thus improving the energy storage efficiency and breakdown strength of the material. Meanwhile, the broken bonds caused via irradiation increased the dielectric constant of the material and the effective polarization. With the synergy of multiple effects, irradiation-modified polymer materials with a high breakdown field strength (i.e., 540 MV/m) and a high discharge energy density (i.e., 16.3 J/cm3) were prepared. This work indicated that irradiation modification could realize the preparation of polymer materials with a high energy storage performance, providing a theoretical and experimental basis for the development of dielectric capacitors.

Introduction The environmentally friendly dielectric capacitors based on lead-free relaxation ferroelectrics have attracted recent attention. The material (Bi0.5Na0.5)TiO3 (BNT) exhibits an intrinsic high saturation polarization due to the hybridization between Bi 6s and O 2p orbitals, surpassing conventional lead-free relaxor ferroelectrics. However, a low breakdown strength (BDS) of BNT results in a relatively poor energy storage performance, restricting its further applications. Therefore, improving the BDS of BNT-based materials is a necessity towards achieving a high energy storage density, and one effective measure is a compositional modification. Sr0.7Bi0.2TiO3 (SBT) as an emerging relaxor ferroelectric material has a higher dielectric constant due to the ion substitution-induced local charge imbalance and dipole fluctuations in nanoscale. It is anticipated that introducing SBT as a solid solution component into BNT matrix films can enhance the BDS of the films, optimizing their energy storage performance. In this paper, the impact of SBT/BNT solid solution ratio on phase structure, microstructure, dielectric, and energy storage properties of the SBT modified BNT-based film was investigated.Methods The precursor solutions for (1-x)(Bi0.5Na0.5)TiO3-xSr0.7Bi0.2TiO3 (BNT-xSBT, x=0.30, 0.35, 0.40, 0.45, and 0.50) were prepared by a sol-gel method. The precursor solutions prepared were aged via static aging for 24 h before spin-coating. Subsequently, the films were heat-treated on a hot plate and rapidly annealed at 630 ℃ for 2 min. The film preparation process, including spin-coating, heat treatment, and rapid annealing, was repeated for 8 times to complete the film fabrication.The crystal structure of the films was analyzed by a model PANalytical X'Pert PRO X-ray diffractometer (XRD). The surface and cross-sectional morphologies of the samples were determined a model Ultra Plus scanning electron microscope (SEM). To evaluate the electrical properties of the samples, platinum electrodes with a diameter of 0.2 mm were deposited on the film surface using magnetron sputtering. The leakage current density of the films was obtained by a model 6517A electrostatic/high-resistance meter. The ferroelectric properties of the samples were tested by a model PolyK CPE1801 ferroelectric workstation. The dielectric properties of the samples were analyzed by a model 4294A precision impedance analyzer.Results and discussion All the BNT-xSBT films exhibit a single pseudocubic phase structure. The unit cell volume initially increases and then decreases as the SBT content increases, which is affected by a combined effect of Sr2+ doping at the A-site and vacancies. The films demonstrate a well-crystallinity, clear grain boundaries, good adhesion to the substrate, and a thickness of approximately 140 nm. The introduction of SBT results in a reduction in the dielectric constant of the film, improving the temperature stability of the dielectrics. At 2 000 kV/cm, the maximum polarization (Pmax) and remnant polarization (Pr) of the films both decrease as the SBT content increases, which is consistent with the dielectric performance. The incorporation of an appropriate amount of SBT increases the BDSof the films from 2 975 kV/cm for BNT-0.3SBT to 3 622 kV/cm for BNT-0.40SBT, which is consistent with the leakage current density. Therefore, the optimal composition BNT-0.40SBT film achieves a high recoverable energy storage density (Wrec) of 85.99 J/cm3 and an efficiency of 74% at the breakdown electric field (Eb). This film demonstrates some advantages, compared to other film capacitors. In addition, the BNT-0.40SBT film exhibits a superior high-frequency stability, a heat resistance, and a fatigue resistance, showing a promising potential for applications in dielectric capacitors.Conclusions All the films exhibited a well-crystallinity, having a single perovskite phase. With the introduction of SBT, the unit cell volume initially increased and the decreased, and the BDS firstly increased and then decreased. The dielectric constant and polarization intensity gradually decreased, while dielectric loss remained stable. The optimal component BNT-0.40SBT film achieved a high BDS of 3 622.25 kV/cm and a high energy storage density (Wrec) of 85.99 J/cm3. The superior energy storage density and stability indicated that BNT-0.40SBT thin films could be promising candidates for pulse power and power electronics applications in capacitors.

Introduction In recent years, relaxor ferroelectrics (RFE) ceramics are recognized as materials with a tremendous potential for energy storage applications. Their relaxor characteristics significantly contribute to enhancing the energy storage performance of ceramics. BaTiO3 (BT) as a typical lead-free ferroelectric ceramic is popular due to its immense dielectric constant and long-range ordered spontaneous polarization. This ceramic is extensively investigated and applied in dielectric capacitor materials. However, pure BT materials often exhibit a large residual polarization and a considerable energy loss due to polarization reversal upon the application of an electric field, affecting the further development of BT materials in energy storage. To optimize the energy storage performance of the original BT ceramics, effective methods such as compositional modulation and ionic doping are proposed. The objective of this paper was to disrupt an original long-range ordered ferroelectric domain structure, creating nano-sized polar nanodomains. This could accelerate the polarization response speed and enhance the relaxor behavior of ferroelectrics.Methods (1-x)BT-x(BNT-SLT) ceramics were synthesized by a solid-state reaction method. Na2CO3 (purity: 99.500%, in mass fraction, the same below), BaCO3 (purity: 99.000%), Bi2O3 (purity: 99.975%), SrCO3 (purity: 99.000%), TiO2 (purity: 99.000%) and La2O3 (purity: 99.000%) were used as raw materials. These materials were dried and mixed in stoichiometric proportions. The mixture was then ground with anhydrous ethanol in a grinding mill with zirconium dioxide balls in a mass ratio of 1.0:2.5:4.0 for 12 h. After grinding, the slurry was dried at 100 ℃ for 12 h. The dried materials were subsequently ground using an agate mortar for over half an hour. The ground materials were firstly transferred and compacted in an alumina crucible, pre-sintered in a box furnace at 850 ℃ for 2 h, and then cooled naturally to room temperature. The powder obtained from the second pre-sintering was further ground and mixed with a polyvinyl alcohol (PVA) as a binder at a mass fraction of 8%. The mixture was manually pressed into small discs with 10 mm in diameter and 0.5 mm in thickness under 10 MPa for 5 min. These discs were sintered at 1 130-1 220 ℃ for 2 h to produce BT-BNT-SLT ceramic samples. After post-sintering, the ceramic samples were polished and coated with 2 mm diameter gold electrodes for ferroelectric testing.The phase composition of the ceramics was analyzed by a model PANalytical Empyrean X-ray diffractometer (XRD), while the surface morphology was characterized by a model Regulus 8230 scanning electron microscope (SEM). The dielectric properties, such as dielectric constant and loss, were assessed by a model TH2838A dielectric performance analysis system. The ferroelectric properties were evaluated by a model Polyk TF Analyzer 2000, and the temperature in testing was controlled by a model TLRS-003 high-low temperature thermal system.Results and discussion The energy storage ceramics of (1-x)BaTiO3-x(0.6Bi0.5Na0.5TiO3-0.4Sr0.7La0.2TiO3)(x=0.20, 0.30, 0.40, 0.45, 0.50) ((1-x)BT-x(BNT-SLT)) with different compositions (i.e., x of 0.20, 0.30, 0.40, 0.45, and 0.50) were prepared to enhance the energy storage performance of BT ferroelectric ceramics. The incorporation of BNT material can strengthen the system saturation polarization through an enhanced orbital hybridization. Also, the inclusion of SLT can further increase an ionic size disparity, thereby enhancing a relaxor behavior. The system integrates a partial vacancy design, typically resulting in the formation of defect dipoles within the ceramics. These defect dipoles in an external electric field are less likely to alter their polarization direction, preserving the initial state of polarization. This mechanism inhibits a polarization reversal and increases the coercive field (Ec). Furthermore, in the absence of the external field, a generated reaction force can revert the polarization direction back to its initial state, leading to a reduction or even a nullification of the residual polarization, consequently enhancing the energy storage density.Conclusions (1-x)BT-x(BNT-SLT) ceramics were fabricated by a solid-state reaction method. The SEM images revealed that the ceramics had clear grains and great density. The dielectric spectroscopy indicated that ion doping regulated the relaxor behavior of BT-based ceramics. The results by ferroelectric tests demonstrated substantial improvements in energy storage performance of the relaxor-modulated BaTiO3-based ceramics, particularly for the 0.55BT-0.45(BNT-SLT) ceramic, obtaining an optimal energy storage performance at 280 kV/cm with a recoverable energy density (Wrec) of approximately 3.12 J/cm3 and an efficiency (η) of 93.3%. In addition, these ceramics also exhibited superior frequency stability, temperature stability, and fatigue properties. The amalgamation of these data and stability tests indicated that the relaxor-modulated BT-based ferroelectric ceramics could have a promising potential for energy storage applications.

Introduction Lead-free sodium niobate (NaNbO3) based ceramics with a superior energy storage density have attracted recent attention in high power dielectric energy storage applications. However, a pure NaNbO3 (NN) ceramic exhibits a square-like square hysteresis loop associating with a large hysteresis and a high remnant polarization at room temperature, leading to a high energy dissipation. In general, stabilizing the antiferroelectricity in NN ceramic is one of the effective measures to solve the problems above. However, despite efforts are made on this measure, the large hysteresis is still kept due to the existence of antiferroelectric- ferroelectric phase transition. The compositions in the phase boundary region usually exhibit abnormally enhanced electrical properties and relaxation characteristics, thus constructing a phase boundary region in NN through composition modulation is another promising method. Moreover, sodium element volatilizes inevitably during the high-temperature and long-time sintering process, giving rise to a poor sintering quality in NN system, which is not beneficial to achieving superior energy storage performance. In this paper, (1-x)NaNbO3-xCaTiO3 (NN-CT100x, 0.08≤x≤0.18) ceramics were prepared by a conventional solid-state method with CuO as a sintering aid. The phase structure and polymorphic phase boundary in NN-CT100x system were investigated, and the energy storage performance of compositions in the phase boundary region was analyzed.Materials and method For the synthesis of NN-CT100x powder, Na2CO3 (≥99.8%, in mass fraction, the same below), Nb2O5 (≥99.9%), CaCO3 (≥99.0%), TiO2 (≥98.0%) and CuO (≥99.0%) were used as raw materials (Shanghai Sinopharm Chemical Reagent Co., Ltd., China). The raw materials were weighed/mixed according to the stoichiometric ratio and ground with anhydrous ethanol in a ball mill with zirconia balls for 12 h. After dried and sieved, the powder was calcined at 900 ℃ for 6 h. The calcined powders were mixed with 1.5% (in mole fraction) of CuO as a sintering aid and further ground for 12 h. The ground powder was hand-pressed into discs with the diameter of 7 mm and the thickness of 1.2 mm and then further densified by cold isostatic pressing at 300 MPa for 30 min. The NN-CT100x discs were sintered at 1 050-1 250 ℃ for 2 h.Results and discussion Based on the results by X-ray diffraction, Raman spectroscopy and dielectric behavior, an ‘antiferroelectric P phase (orthorhombic, Pbma)-ferroelectric INC phase-paraelectric CT phase (orthorhombic, Pbnm)’ polymorphic phase boundary (PPB) region exists as 0.09≤x≤0.16 when CaTiO3 content increases. The polarization exhibits an enhancement in compositions within this PPB region. The optimum comprehensive energy storage performance is obtained as x=0.16, having the energy storage density (Wrec) of 3.04 J/cm3 and efficiency (?) of 84.4% at 300 kV/cm, respectively. The difference between the maximum polarization (Pm) and the remnant polarization (Pr) of the NN-CT16 ceramic increases as the electric field increases, thus obtaining an admirable energy storage performance. This composition is in the PPB region and contains a few antiferroelectric phases. These antiferroelectric phases are easily transformed into ferroelectric phases at high electric fields, which ? reduces from 92.4% to 77.0%. The NN-CT16 composition has a high Pm of 42.2 μC/cm2, a high Wrec of 6.1 J/cm3 and ? of 77% at 590 kV/cm. For the NN-CT16 ceramic, Wrec and ? both exhibit a frequency stability at 1-300 Hz, with minimal variations of <10% and <8%, respectively. Furthermore, the Wrec and ? also demonstrate temperature-insensitive characteristics at 25-160 ℃, with small variations of <10% and <5%, respectively. The discharge energy density (Wdis) of the NN-CT16 ceramic increases and reaches a plateau in a short time. The t0.9 (time at which 90% of the stored energy is released) is less than 8 μs. Moreover, the t0.9 decreases with the increasing electric field or temperature, indicating the discharge rate accelerates, which is probably ascribed to domains respond quickly and switch more easily at high electric fields or high temperatures.Conclusions Based on the phase boundary modulation strategy, the NN-CT100x (0.08≤x≤0.18) ceramics were designed and prepared via introducing CaTiO3 into NN with CuO as a sintering aid. The polarization of the compositions in the phase boundary region was enhanced. The NN-CT16 ceramic exhibited a high maximum polarization of 42.2 μC/cm2, a high energy storage density of 6.1 J/cm3 and an energy efficiency of 77% at 590 kV/cm. It also exhibited a good frequency/ temperature stability and an excellent discharge performance. This ceramic could be used as a promising dielectric energy storage material.

Introduction There exist severe problems in the society, i.e., environmental pollution, energy shortage, and climate change. These problems directly affect the living environment and have significant threats to globally sustainable development. In the past decades, some low-carbon new energy solutions are developed to address the continuously growing energy demand and mitigate the adverse impacts of climate change. Despite of the continuous emergence of new energy devices, it is still important to face the challenges, i.e., relatively low power density, safety, large volume, etc..In this paper, the dielectric materials for multilayer energy storage capacitors (MLCC) were prepared by a solid-state reaction method at a lower sintering temperature with low-cost base metal electrodes (e.g., Cu). In addition, the energy storage properties of the MLCC were also characterized.Methods (Pb0.97La0.02)(Zr0.55Sn0.41Ti0.04)O3 (PLZST)-x%?MgO-Al2O3-ZnO-B2O3-SiO2 (MAZBS, x=0.00, 0.25, 0.50, 0.75, 1.00, 1.25) ceramics were synthesized by a conventional solid-state reaction method. The fabricated ceramics were characterized by Fourie transformation infrared spectroscopy (FTIR), thermal analysis, X-ray diffraction (XRD), and scanning electron microscopy (SEM). The dielectric, ferroelectric, and energy storage properties were examined by a LCR impedance analyzer and a polarization-electric field (P-E) hysteresis loop analyzer.PbO (purity: 99.0% in mass fraction, the same below), La2O3 (99.5%), ZrO2 (99.5%), SnO2 (99.8%), and TiO2 (99.8%) were used as raw materials to synthesize PLZST. MAZBS glass powders were prepared by a water quench method. Afterwards, they were mixed with the oxides powders in a designed mass ratio, ground by high-energy ball milling for 24 h, dried at 80 ℃ for 10 h, and then sintered to form PLZST-x% MAZBS (x=0.00, 0.25, 0.50, 0.75, 1.00, 1.25) antiferroelectric ceramics. The calcined powders with 5% polyvinyl butyral (PVB) as a binder were pressed into discs with 12 mm in diameter and 1 mm in thickness at 20 MPa. Finally, the green bodies with added MAZBS glass phase were sintered at 930-1 000 ℃ for 4 h, while the sample without x was sintered at 1 200-1 300 ℃ for 2 h. For the properties test, the ceramics were firstly polished, and gold electrodes were then deposited on the both surfaces. The electrode diameter and thickness for each sample were (3.000.11) mm and (0.0550.007) mm as x=0.00, (0.820.19) mm and (0.0720.07) mm as x=0.25, (1.680.04) mm and (0.0760.007) mm as x=0.50, (2.280.33) mm and (0.0550.004) mm as x=0.75, (0.870.30) mm and (0.0660.003) mm as x=1.00, (1.900.44) mm and (0.0620.005) mm as x=1.25.Results and discussion The XRD patterns of PLZST-x% MAZBS (x=0.00, 0.25, 0.50, 0.75, 1.00, 1.25) antiferroelectric ceramics reveal a shift towards low angles for a peak (200) at 43°-45° when the glass content increases. This shift indicates a possible substitution of Ti4+ (with the radium of 0.60 ?) by Mg2+ (with the radium of 0.69 ?). Concurrently, the dielectric constant decreases with increasing the glass content due to the lower dielectric constant of glass. The P-E hysteresis loop results demonstrate that PLZST-x% MAZBS ceramics exhibit an optimum energy storage performance when x=0.75, and a good performance at different frequencies. However, a further increase in glass content leads to the entrance of metallic ions from the glass phase into the ceramic lattice, causing the defects and reduction of the dielectric breakdown strength of the ceramics.Conclusions The impact of MAZBS glass content on the energy storage properties of PLZST ceramics was investigated. It was indicated that an appropriate content of MAZBS glass phase could reduce the sintering temperature of PLZST ceramics from 1 200 ℃ to 930 ℃. The XRD patterns confirmed a perovskite structure of PLZST-x% MAZBS ceramics. The FTIR spectroscopy revealed the formation of non-bridging oxygen bonds due to the doping of Al3+, Zn2+, and Mg2+. The glass transition temperature and softening temperature of MAZBS glass were examined via the TG/DSC analysis. The dielectric analysis indicated a tetragonal antiferroelectric-ferroelectric phase transition with increasing the temperature, and the dielectric constant peak gradually decreased with the increase of glass content. The results of P-E hysteresis loop illustrated the electric field-induced antiferroelectric-ferroelectric phase transition characteristics of ceramics, with the maximum polarization achieved at the glass phase content of 0.75%. The PLZST-0.75% MAZBS ceramics had a maximum energy storage density of 6.08 J/cm3 and an efficiency of 77% at 300 kV/cm. This result indicated that PLZST-MAZBS ceramics could be developed into high-energy density ceramic capacitors with cost-effective copper inner electrodes.

In recent years, with a rapid societal development, the utilization of clean energy becomes a necessity. This demand emphasizes on energy storage equipment, prompts some research on efficient energy storage, energy loss reduction, and environmental impact mitigation. The existing prevalent energy storage devices encompass batteries, supercapacitors, and dielectric capacitors. Batteries boast a maximum energy storage density, but exhibit a comparatively lower power density. Supercapacitors strike a balance with a moderate energy storage and a power density. Dielectric capacitors, characterized by their unparalleled power density and rapid charge/discharge rate, have extensive applications in various domains such as pulsed laser weaponry, cardiac pacemakers, and energy vehicles.Thin film materials emerge as a favorable option for addressing device miniaturization and enhancing energy density due to their smaller volume and higher energy storage density compared to polymers and block ceramics. The energy storage ceramic thin film materials can be classified based on intrinsic polarization states, i.e., encompassing linear dielectric (LD), paraelectric (PE), ferroelectric (FE), relaxation ferroelectric (RFE), superparaelectric (SPE), and antiferroelectric (AFE) materials.LD exhibits notable breakdown strength (Eb) and energy storage efficiency (η). However, their low dielectric constant (r) and polarization (P) result in a diminished energy storage density (Urec). Elevating r stands as a pivotal for enhancing the Urec of LD. PE demonstrates non-linear changes in P and r with applied electric fields (E). For the removal of E, PE maintains a non-polar state without spontaneous polarization, featuring a medium P and a substantial Eb. FE exhibits a spontaneous polarization with large P and r. Nevertheless, their prominent residual polarization (Pr), dielectric loss (tan), and increased defect density contribute to a lowered Eb, constraining the application of FE in energy storage. Recent research in FE focuses on mitigating Pr and augmenting Eb. In contrast to FE, RFE typically exhibits nanodomains or polar nanodomains in their domain structure. The coupling among these domains is relatively weak, rendering them highly responsive to electric fields. RFE often displays a substantial P, a minimal Pr, and a slender P-E hysteresis loop. This characteristic facilitates the attainment of elevated Urec and η. SPE maintains a local polar order, manifesting as polarity clusters spanning only a few nanometers or cells. This arrangement further diminishes mutual coupling between domains, reducing a polarity-switching energy barrier. Losses are minimized, significantly enhancing both Urec and η. AFE features a distinctive double hysteresis loop, stemming from the parallel and opposite alignment of spontaneous polarization on adjacent lattices. This configuration results in near-zero Pr, undergoing a reversible AFE-FE phase transition under specific E, and yielding a high r and substantial P. This property enables the achievement of a heightened Urec, presenting promising applications in energy storage.In the realm of energy storage ceramic thin film materials, enhancing Pmax, Eb, and minimizing Pr are pivotal for improving energy storage performance. To achieve these objectives, diverse modification methods are used to encompass the preparation technology, component design, and interface engineering.For the effective preparation, optimization of preparation methods, alteration of annealing processes, substitution of electrode materials, aging, and the fabrication of multi-layer film capacitors (MLFC) are employed. These interventions regulate the orientation growth and oxygen vacancy content of the material, thereby enhancing the energy storage performance of the ceramic thin film capacitor. Note that the preparation of MLFC demonstrates a substantial Urec (i.e., 78.3 J/cm3) and η (i.e., 90.2%) and offers valuable insights for the commercialization of ceramic film capacitors.The composition design encompasses elements such as doping, two-phase or multiphase solid solutions, entropy control, and incorporation of nanoparticles. Specifically, element doping and the creation of two-phase or multiphase solid solutions can achieve a slender P-E hysteresis loop via transforming FE to RFE. Using Mn2+ to form defect complexes with oxygen vacancies is investigated. This reduces the oxygen vacancy content and curbs leakage current density, thereby enhancing the energy storage performance for various energy storage ceramic thin film materials. This method offers a universal approach to mitigating leakage current issues. Entropy control strategies introduce multiple elements into the system, utilizing chemical disorder caused by atomic size mismatch to reduce Pr and delay saturation polarization, thereby improving energy storage performance. Nanoparticle composite can be prepared via adding the nanoparticles into matrix material evenly, with the base materials to improve the energy storage of materials performance as a whole.Interface engineering primarily enhances Eb via manipulating material structures, incorporating effects (i.e., ‘dead-layer’ effect, electric field amplification effect, space charge effect, heterojunction effect, interface barrier effect, and interlayer coupling effect). This optimization aims to achieve a superior energy storage performance. Multilayer films in interface engineering typically involve the use of two or more component materials, allowing the strategic utilization of diverse material characteristics to maximize effectiveness. For instance, ‘dead-layer’ engineering involves the incorporation of high resistivity and low r Al2O3 layer as the top layer, enhancing the Eb of the multilayer film. In multilayer films, the interplay of various effects collectively affects their electrical properties and energy storage performance. The comprehensive consideration of these effects is crucial in the design and research of multilayer films. The energy storage performance of multilayer films can be improved, further aligning dielectric energy storage films with the practical applications.Summary and prospects Dielectric energy storage ceramic thin film materials have attracted recent attention due to their superior power density and rapid charge/discharge rate. This review summarized recent research progress on dielectric energy storage ceramic thin film materials (i.e., linear dielectric, paraelectric, ferroelectric, relaxation ferroelectric, superparaelectric, and antiferroelectric materials). This review also represented the enhanced energy storage performance via the preparation processes, component design, and interface engineering. With a diverse range of dielectric energy storage ceramic thin film materials and various methods for improving their energy storage performance, the practical applications can tailor material selection and modification approaches. To achieve the commercialization of dielectric energy storage ceramic thin films, efforts should be made for strengthening theoretical research and developing new types of ceramic thin film-based devices. This can maintain the superior energy storage performance while preparing devices, thereby laying a foundation for the application of dielectric energy storage ceramic thin film capacitors.

Among various types of energy storage devices, dielectric capacitors show unique advantages such as a high power density and a fast charge/discharge rate. The dielectric material is a key material that affects the energy density of dielectric capacitors. Dielectric capacitors complete the storage and release of energy precisely through the polarization behavior of the dielectrics. To achieve a goal of dielectric capacitors to gradually move towards lightweight, small-sized, and integration, it is necessary to further improve their energy density. Dielectric constant and breakdown field strength are two important factors affecting the energy density of dielectrics. However, improving the dielectric constant and breakdown field strength is still a great challenge.As different types of dielectrics are thoroughly investigated, polymer-based composite dielectrics show a great potential for energy storage because they have a high dielectric constant and a low dielectric loss of inorganic dielectrics, and a unique flexibility and a superior breakdown resistance of polymers. However, there are some problems with polymer-based composite dielectrics that restrict the further improvement of their energy density. It is necessary for the improvement of the dielectric constant of polymer-based composite dielectrics to add a high content of dielectric constant fillers. However, a high content of fillers is prone to agglomeration. Large polarization differences between fillers and polymer matrix result in an interfacial compatibility issue, leading to an early breakdown in composite dielectrics. It is thus difficult to effectively improve the energy density.The structural design of the fillers and the macrostructural modulation of the dielectric composites are expected to optimize the dielectric properties. The core-shell/coaxial structure of fillers can be designed to alleviate the problem of decreasing breakdown field strength due to the large difference in dielectric properties between polymers and fillers. For instance, cladding the filler surface with a material of medium dielectric constant can improve the electric field distortion. Recent studies reported the introduction of two-dimensional fillers with large aspect ratios into polymer matrices. Two-dimensional fillers have a smaller specific surface energy, which makes it easy to achieve a uniform dispersion in the polymer matrix. Two-dimensional fillers also promote the scattering of carriers, which delays the breakdown and improves the breakdown performance of composite dielectrics.For the negative correlation between breakdown field strength and dielectric constant, many macro-regulations of polymer-based composite dielectrics are also investigated based on the development of polymer-based composite dielectric macro-control measures. Sandwich structures and composite dielectric structures with up to tens of layers are developed. The dielectric layers with different filler contents are designed to regulate the spatial distribution of the electric field, restricting the local weak electric field to a low filler content layer to inhibit the breakdown process of the dielectric. A high filler content layer enhances the dielectric constant of the composite dielectric. In addition, the use of layered films with a combination of linear and nonlinear dielectrics is expected to achieve a simultaneous increase in energy density and efficiency.As capacitors are widely used in high-power energy storage scenarios such as aerospace and electric vehicles, higher requirements are placed on the thermal stability and dielectric properties of dielectrics at high temperatures. The high conduction loss of polymer-based dielectrics at high temperatures severely weakens their energy storage performance. A major strategy that can reduce the conductivity loss at high temperatures is to introduce wide bandgap inorganic fillers such as boron nitride nanosheets and alumina nanosheets into the polymer matrix. Another strategy is to apply wide bandgap inorganic coatings on the film surface to increase the potential barrier at the electrode-dielectric interface and hinder charge injection. An idea to enhance the high-temperature energy storage performance of composite dielectrics is the structural modification of polymer molecular chains such as grafting or cross-linking treatments, which introduces charge-trapping sites and hinders charge transport, thus suppressing the conductivity loss at high temperatures.Summary and prospects Micro-structural and macro-structural designs both can enhance the energy density of polymer-based dielectrics. However, there are still some issues to be further addressed, namely, the interfacial interaction mechanism between filler and polymer and the breakdown mechanism of the layered structure. The carrier dissipation and heat accumulation at high temperatures are also aspects to be explored. In summary, the investigation of the interfacial bonding and breakdown mechanism of polymer-based composite dielectrics and the enhancement of the high-temperature energy storage performance of composite dielectrics will promote the dielectric capacitors to achieve broader application prospects.

Antiferroelectric materials are considered as the most promising candidate materials for pulsed power capacitors due to their high breakdown electric field, high saturation polarization, low remanent polarization, and electrical hysteresis. The absence of deep and comprehensive research on antiferroelectric materials still hinders its application. Hence, a further research on antiferroelectric materials is of great importance to promote the development of science and technology and the progress of society. PbHfO3 (PHO) is a special antiferroelectric material with a perovskite structure. Its unique double hysteresis loop becomes a research hotspot. PHO has a high recoverable energy density and a great energy efficiency, which can be used to develop high-performance energy storage devices such as supercapacitors and pulse power capacitors. In addition, PHO antiferroelectric materials also have superior electrocaloric effects, which can achieve energy recovery and utilization at different temperatures, thus providing a novel approach for thermal energy utilization and energy conversion. The research of PHO antiferroelectric materials mainly focuses on the crystal structure, phase transition mechanism, energy storage performance (i.e., recoverable energy density, energy efficiency, charge/discharge energy density, etc.), and electrocaloric effect.There are still many controversies on the crystal structure and phase transition mechanism of PHO. The multiple-phase transition process is rather complex. At present, it is generally believed that PHO has two temperature-induced phase transitions, i.e., the orthogonal symmetry antiferroelectric phase (AFE1) transitions to the intermediate antiferroelectric phase (AFE2) at 433 K, and the AFE2 transitions to the cubic symmetry paraelectric phase at 476 K. However, little work on the intermediate antiferroelectric phase has been reported yet. Some research work was carried out in the energy storage performance of PHO. Ion displacement, lattice distortion, and grain size are the major factors affecting the energy storage performance. As a result, ion doping becomes an effective way to adjust the electrical properties of PHO, thereby improving the energy storage characteristics such as recoverable energy density and energy efficiency, as well as charge and discharge energy density. The performance can be tuned by A-site, B-site, or A/B-site doping since PHO has a perovskite structure. For instance, A-site doping with elements such as Sr, Ba, La, etc. can change the lattice structure and improve the breakdown strength of the system, thus increasing the energy storage performance. B-site doping with Sn, Zr, and Ti can also be used to enhance the antiferroelectricity of the system to achieve a high recoverable energy density. A/B co-doping is also an effective way to improve the energy storage performance of PHO. In addition, the preparation process has a great influence on the crystal structure and energy storage performance of PHO. The rolling process and cold isostatic pressing can reduce the grain size of PHO to achieve the effect of fine crystal reinforcement, which increases its breakdown strength and achieves the excellent recoverable energy density and efficiency. In addition, the electrocaloric effect of PHO-based antiferroelectric materials gives it broad application prospects in electrocaloric refrigeration. At an external electric field, PHO exhibits a polarization reversal to produce a significant thermal effect, thereby achieving efficient refrigeration. The electrocaloric refrigeration method has a higher energy utilization efficiency and a lower power consumption, compared with the conventional mechanical refrigeration method.Summary and prospects This review represented recent development and application on the antiferroelectric lead hafnate. The basic energy storage parameters, crystal structure, phase transition mechanism, ion doping modification and preparation of PbHfO3-based materials were introduced. In contrast to energy storage and refrigeration applications, PHO also has a wide range of potential applications in other fields. The unique crystal structure and phase transition mechanism of PHO enable the development of sensors and logic devices. Its antiferroelectric properties can be used to create electronic devices with a ultra-low power consumption, such as antiferroelectric transistors and memory devices. Although several important advances are made in the investigation of PHO, it still has some challenges. For instance, the multi-phase transition mechanism is still controversial, the intermediate phase is unclear, the breakdown field strength is not high enough, and the saturation polarization value needs to be further improved. It is impossible to achieve a high energy density and a high recoverable energy density simultaneously. However, the existing research shows that PHO has broad application prospects in the fields of energy storage and solid-state refrigeration due to its fast charging and discharging capacity, extremely high recoverable energy density and power density, and superior electrocaloric effect.

Dielectric capacitors are one of the most important components in electrical equipment and electronic devices, and have attracted recent attention in the fields of new energy vehicles, advanced propulsion weapons, renewable energy storage, high-voltage transmission, and medical defibrillators. Dielectric capacitors have ultra-short charge and discharge time, ultra-high power density and high operational safety, compared with electrochemical capacitors such as batteries, fuel cells and supercapacitors. However, the low energy storage density of dielectric capacitors limits their wide application. Therefore, it is necessary to develop dielectric capacitors with a high temperature and a high energy density.NaNbO3(NN)-based ceramic material is considered as one of the most potential dielectric energy storage materials because of its wide band gap, lower cost and high polarization. However, there are still some problems in the application of NN-based ceramic materials, such as low energy storage density and energy storage efficiency, and poor temperature stability. In order to optimize the energy storage performance of NN-based ceramics, phase structure regulation, micro-morphology optimization, doping modification (for A-site, B-site, A/ B-site co-doping) are used to improve the energy storage density, efficiency,and frequency/temperature stability. These optimization strategies provide opportunities for the application of NN-based ceramics.NN-based ceramics have a complex phase structure, and pure NN cannot obtain double hysteresis loops at room temperature, which is attributed to the irreversible transition between the antiferroelectric P (AFE P) and ferroelectric Q (FE Q) phases. The AFE P phase can be stabilized, and the double hysteresis loop with a reversible phase transition can be obtained, improving the energy storage density and energy storage efficiency via reducing the tolerance factor. In terms of microstructure optimization, advanced sintering process, suitable sintering additives and preparation process can further improve the energy storage performance of NN-based ceramics. The advanced sintering process can optimize the microstructure, adjust the grain size and oxygen vacancy concentration to improve the energy storage performance. For instance, compared with Na0.7Bi0.1Nb0.9Ta0.1O3 ceramic prepared via conventional sintering, the energy storage efficiency of the ceramic sample prepared via spark plasma sintering (SPS) increases from 81.1% to 90.5%. Adding suitable sintering additives can form a liquid phase, and then improve the densification process of ceramics. Also, the addition of sintering additives can reduce the sintering temperature and play the role of refining the grains, which is conducive to increasing the resistance of the ceramics and thus increasing the breakdown field. For MnO2-doped 0.95NaNbO3-0.05SrSnO3 (NN5SS) ceramic, the energy storage density can be increased by nearly 14 times, compared to undoped NN5SS. The advanced preparation process can improve the density and electrical uniformity, thus improving the energy storage performance. The viscous polymer processing (VPP) can increase the energy storage density of 0.84 NaNbO3-0.06 BiFeO3-0.1 SrTiO3 ceramics from 2.7 J/cm3 to 5.29 J/cm3, compared to conventional solid phase sintering.The energy storage performance of NN-based ceramic can be improved via doping modification. Doping modification can introduce relaxation antiferroelectric properties, showing that long-range ordered antiferroelectric domains are broken to form micron-sized or nano-sized domains, which have a rapid response to the applied electric field, reduce the polarization lag between applied or discharged electric fields, and thus obtain a superior energy storage performance. Also, the solution of the second or third component can introduce relaxation characteristics, and the TB and Tm temperature regions with nanodomains and polar nanoregions can be adjusted to room temperature in the NN-based system, showing the relaxation characteristics and improving the energy storage performance.Summary and prospects The energy storage performance can be improved to a certain extent by means of phase structure regulation, micro-morphology optimization, doping modification, etc.. However, the harsher application environment requires a higher energy storage performance (i.e., Wrec>10 J/cm3, η>90%). The future development aspects of NN-based energy storage ceramics involve 1) raw material preparation, i.e., wet chemical method (such as hydrothermal method, sol-gel method, self-spreading method, etc.) and high-energy ball milling method are used to reduce the particle size of the powder to further improve the electrical performance；2) sintering process, i.e., advanced preparation processes (such as SPS, flash sintering, cold sintering, hot press sintering, etc.) are used to improve the energy storage performance while greatly reducing energy consumption； 3) molding process, i.e., the process of rolling film, casting and cold isostatic pressing are used to further optimize the microstructure and improve the energy storage performance; 4) theoretical research, i.e., combined with advanced in-situ characterization techniques (such as in-situ high resolution TEM, micro-infrared, etc.) and theoretical calculations (such as first-principles, phase field simulation, etc.), a relationship between process-structure-performance is further explored, and new theories and mechanisms are proposed; and 5) device preparation, i.e., the device with superior energy storage performance and high stability dielectric materials needs to be developed.