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.