The global energy transition toward sustainable and green energy systems have intensified a demand for advanced electrochemical energy storage technologies. Lithium-ion batteries (LIBs), while being dominant in portable electronics and electric vehicles, face some challenges, including safety risks from flammable liquid electrolytes and limited energy density. These limitations hinder their application in emerging fields such as electric aviation, high-performance drones, and long-range electric vehicles. Recent incidents of LIB thermal runaway and combustion further highlight an urgency for safe and high-energy-density alternatives. In this context, all-solid-state lithium metal batteries (ASSLMBs) emerge as a promising next-generation energy storage solution, offering enhanced safety and superior energy density.ASSLMBs replace volatile liquid electrolytes with solid-state electrolytes (SSEs), thereby eliminating combustion risks and improving thermal stability. Also, the use of lithium metal anodes with an ultrahigh theoretical capacity (i.e., 3860mA·h/g) and a minimum electrochemical potential (i.e., −3.04V vs. SHE) significantly boosts energy density, compared to conventional graphite anodes. These advantages make ASSLMBs a focal point of research and development. However, despite their theoretical potential, ASSLMBs face some practical challenges, particularly the electrochemo-mechanical issues induced by volume expansion during charge and discharge cycles.Volume changes in ASSLMBs stem from the expansion and contraction of electrode materials during lithium (de)intercalation. Cathode materials, such as lithium-rich manganese-based oxides, typically exhibit volume changes of 2%–10%. In contrast, lithium metal anodes can experience extreme volume expansion of up to 1000% when operating at a low negative-to-positive capacity ratio (i.e., N/P=1.1). In high-energy-density configurations, such as ASSLMBs targeting about 600 W·h/kg, the overall cell-level volume change can reach 18%. Unlike conventional LIBs that use liquid electrolytes to accommodate strain, the rigid solid-solid interfaces in ASSLMBs cannot self-adjust to such deformations. This rigidity leads to significant mechanical stresses, including stack-level stresses in the MPa range and localized stress concentrations in the GPa range, caused by heterogeneous current distribution, lithium dendrite growth, and particle cracking. These stresses can result in interfacial delamination, active material fracture, and ion transport blockages, severely degrading the battery performance and cycle life.This review provides a comprehensive analysis of the electrochemo–mechanical challenges in all-solid-state lithium metal batteries (ASSLMBs) and explores some strategies to mitigate these issues. First, we examine the fundamental mechanisms of mechanical-electrochemical coupling, emphasizing a relationship between material properties (i.e., modulus, fracture toughness), operational parameters (i.e., current density, pressure), and stress generation. Advanced characterization techniques, such as in-situ stress sensors, X-ray computed tomography, and finite element modeling, are employed to reveal multi-scale stress evolution and its impact on the battery performance. These tools enable to visualize and quantify stress distribution in micro- and macro-scale, providing insights into the dynamic interplay between mechanical and electrochemical processes. Second, we analyze the consequences of volumetric strain on key performance metrics, including interface stability, lithium deposition behavior, and cathode degradation.To address these challenges, we evaluate strategic approaches such as material innovation, interface engineering, and structural optimization. Material innovation focuses on designing SSEs with balanced ionic conductivity and mechanical compliance. Strain-tolerant cathode architectures, including single-crystal particles and composite electrodes with buffer matrices, are also explored to mitigate volume changes. Interface engineering involves introducing functional interlayers, such as Li3N coatings, to enhance adhesion and redistribute interfacial stresses. Artificial solid-electrolyte interphases (SEI) with self-healing properties are highlighted for stabilizing lithium anodes and preventing dendrite formation. Structural optimization explores cell-level designs, including pre-stress mechanisms and gradient porosity electrodes, to mitigate strain accumulation and improve the overall battery performance. Pressure management strategies for stack assemblies are also discussed to balance contact maintenance with stress relaxation. ASSLMBs with enhanced mechanical stability, improved electrochemical performance and extended cycle life are developed via integrating these approaches, paving a way for their practical application in next-generation energy storage systems.Finally, we outline future research directions, emphasizing a need for multi-physics models that integrate electrochemical, thermal, and mechanical dynamics across scales. Advanced manufacturing techniques and novel diagnostic tools for real-time stress monitoring are identified as critical enablers for advancing ASSLMB technology. In addition, we also analyze the existing technical challenges and potential solutions, which can provide theoretical support and practical guidance.Summary and prospectsOne of the core challenges faced by all-solid-state lithium metal batteries during their development is a mechanical-electrochemical coupling effect caused by volumetric changes. The volumetric changes of active materials during charge and discharge, especially under high energy density conditions, can significantly negatively impact the structural stability and electrochemical performance of the battery, leading to some problems such as interface delamination and crack formation. These problems severely limit the cycle life and high-rate performance of all-solid-state lithium metal batteries, becoming a major obstacle to their practical application. To address these challenges, some strategies such as optimizing cell module structures, material modification, and structural regulation have been proposed. Future research should focus on four key directions, i.e., exploring the mechanical-electrochemical coupling mechanism using advanced experimental techniques and multiscale modeling to better understand the interactions between stress, ion transport, and electrochemical reactions, developing advanced characterization and testing methods to cope with the complexity of mechanical-electrochemical coupling mechanism, developing volumetric expansion suppression strategies for practical applications (i.e., the development of zero-strain electrode materials and interface engineering designs) to mitigate the adverse effects of volumetric changes during cell cycling while reducing the high stack pressure demands of all-solid-state lithium metal batteries, and leveraging machine learning and artificial intelligence to accelerate the discovery of high-performance materials and optimize the battery structure design.

IntroductionLi10GeP2S12 solid electrolyte is regarded as a promising candidate for all-solid-state lithium batteries due to its ultra-high ionic conductivity and low grain boundary resistance. However, its instability against moisture and incompatibility with lithium metal impede its application. Elemental substitution or doping is commonly employed to improve the overall properties of solid electrolytes. Based on hard-soft acid-base theory, partially substituting S with O can form a more stable structure, thereby inhibiting hydrolysis and structural damage upon exposure to moist air. However, excessive O doping causes a reduction of ionic conductivity. The introduction of rare-earth elements, such as scandium (Sc) with larger ionic radius, can enlarge the lattice volume and mitigate the adverse effects of O doping. Meanwhile, Sc-containing compounds generated in the interface between solid electrolyte and lithium metal is beneficial to suppress the interfacial side reactions. In this work, Sc and O co-doped Li10GeP2S12-based electrolytes were synthesized. The moisture stability, interface between the electrolyte and lithium metal and electrochemical performances of all-solid-state lithium batteries were significantly improved.MethodsLi10+0.5xGe1–xScxP2S12–1.5xO1.5x(x=0, 4%, 8%, 12%, 16%, in mole) solid electrolytes were prepared by ball-milling and subsequent high-temperature sintering. The structure of obtained solid electrolytes was analyzed by a model AXS D8 Advance X-ray diffractometer (Bruker Co., Germany) with Cu K radiation in the angular range of 10°–80°. The Raman spectra were recorded by a model inVia-reflex Raman spectrophotometer (Renishaw Co., UK) with an excitation wavelength of 532 nm. The morphology and elemental distribution of the solid electrolyte particles were determined by a model Regulus-8230 scanning electron microscope (SEM, Hitachi Co., Japan) with an energy-dispersive X-ray spectroscope. The ionic conductivity of the solid electrolytes was measured by an electrochemical impedance spectroscope and the electronic conductivity was tested through direct current polarization at 0.5 V by a model 1470E electrochemical workstation (Solartron Co., UK). The air stability of the solid electrolytes was evaluated through the amount of H2S gas released in a confined environment with approximately 40% relative humidity by a model GX-2009 H2S gas sensor (Riken Keiki Co., Ltd., Japan).Symmetric batteries were fabricated with two lithium foils attached to both sides of the pelletized solid electrolytes. The critical current density was conducted through galvanostatic cycling at step-increased current densities using a model Land-CT2001A battery test system (Wuhan Rambo Testing Equipment Co., Ltd., China). To assemble LiCoO2⊥ solid electrolyte/Li all-solid state lithium batteries, composite cathode was prepared by mixing LiCoO2 and the electrolyte powder in a mass ratio of 70:30. The composite cathode was spread on to one side of the solid electrolyte tablet uniformly and pressed at 360 MPa. The lithium foil was placed on the another side of solid electrolyte to serve as an anode. The charge and discharge measurements of all-solid-state batteries were conducted at room temperature in a voltage range of 3.0–4.2 V using a model Land-CT2001A battery test system.Results and discussionSc and O co-doped Li10GeP2S12 solid electrolytes are synthesized through ball-milling and subsequent high-temperature sintering at 620 ℃. The XRD patterns and Raman spectra indicate that the optimal doping concentration is 8%, at which the lattice volume is enlarged and no heterogeneous phase is detected. The ionic conductivity of optimized Li10.04Ge0.92Sc0.08P2S11.88O0.12 solid electrolytes sintered at 620 ℃ has a high ionic conductivity of 5.85 mS·cm–1. In addition, it also exhibits a decreased electronic conductivity from 2.93×10–8 S·cm–1 to 1.65×10–8 S·cm–1 with a low activation energy of 0.20 eV. After 120-min exposure in a confined environment with a relative humidity of approximately 40%, Li10GeP2S12 undergoes irreversible hydrolysis and releases 0.59 cm3·g-1 of H2S gas. In contrast, Li10.04Ge0.92Sc0.08P2S11.88O0.12 produces only 0.24 cm3·g–1 of H2S gas under the same conditions and retains an ionic conductivity of 1.21 mS·cm–1, which is one order of magnitude greater than that of Li10GeP2S12. Annealing can restore up to 70.77% of its original ionic conductivity, which is attributed to Sc and O doping that forms a more stable structure, thereby inhibiting the hydrolysis reaction.Li10.04Ge0.92Sc0.08P2S11.88O0.12-based symmetric cell achieves a significant increase in critical current density from 1.0 mA·cm–2 to 2.4 mA·cm–2. After undergoing constant-current cycling for 800 h at a current density of 0.1 mA·cm–2, the polarization voltage of Li\ Li10.04Ge0.92Sc0.08P2S11.88O0.12\Li maintains at ±0.5 V, demonstrating an effective suppression of the side reaction between electrolyte and lithium metal. LiCoO2\Li10.04Ge0.92Sc0.08P2S11.88O0.12 \Li all-solid-state lithium battery exhibits superior cycling stability and rate performance, with an initial discharge capacity of 128.45 mA·h·g–1 and a capacity retention of 87.5% after 100 cycles at 0.1 C. Furthermore, the capacity retention remains at 81.7% after 500 cycles at 1 C.ConclusionsSc and O co-doped Li10.04Ge0.92Sc0.08P2S11.88O0.12 solid electrolyte sintered at 620 ℃ had an optimal ionic conductivity of 5.85 mS·cm–1. The humid air stability was improved after partially substituting S with hard base O. LiCoO2 | Li all-solid-state lithium battery assembled with Li10.04Ge0.92Sc0.08P2S11.88O0.12 exhibited enhanced long-term cyclic performance and rate capability. This work demonstrated a significant potential of Sc and O co-doping in enhancing both the structural stability and electrochemical performance of Li10GeP2S12-based solid electrolytes, making them promising candidates for all-solid-state lithium batteries.

All-solid-state batteries (ASSBs) emerge as a promising next-generation energy storage technology due to their potential for high energy density and enhanced safety. Among the various types of solid electrolytes, sulfide solid electrolytes have attracted much attention due to their high ionic conductivity and excellent mechanical properties. Despite these advantages, the development and industrialization of sulfide-based ASSBs face numerous scientific and engineering challenges. This review focuses on the fundamental scientific issues and engineering difficulties associated with sulfide-based ASSBs, and proposes future directions and recommendations to advance their development and commercialization.The rapid growth of electric vehicles, consumer electronics, and energy storage systems has driven the development of lithium-ion batteries. However, conventional lithium-ion batteries with a high energy density pose safety risks due to the use of flammable liquid electrolytes. ASSBs, which replace liquid electrolytes with solid electrolytes, offer a safer alternative with a potential for higher energy density. Sulfide solid electrolytes, in particular, exhibit ionic conductivities, compared to those of liquid electrolytes, making them a leading candidate for ASSBs. Despite significant progress, several challenges remain. Sulfide solid electrolytes face issues related to electrochemical stability, humidity sensitivity, and thermal stability. The interfaces between sulfide electrolytes and electrodes (i.e., both cathode and anode) are critical for the battery performance, but are prone to poor contact, chemical reactions, and lithium dendrite growth. These issues hinder the practical application of sulfide-based ASSBs.The stability of sulfide solid electrolytes is crucial for the performance and safety of ASSBs. Electrochemical stability ensures that the electrolyte can operate in a wide voltage range without decomposing. However, sulfide electrolytes often exhibit narrow electrochemical windows, leading to decomposition at high voltages. Humidity stability is another concern, as sulfide electrolytes tend to react with moisture, producing toxic H2S gas. Thermal stability is also critical, as sulfide electrolytes can undergo thermal decomposition, increasing the risk of thermal runaway. The interfaces between sulfide electrolytes and electrodes are a major bottleneck in ASSBs development. The solid-solid contact between the electrolyte and electrodes often leads to a poor ion transport and an increased interfacial resistance. In addition, chemical reactions at the interface can also degrade battery performance over time. For instance, the interface between sulfide electrolytes and high-voltage cathodes can lead to the formation of high resistance interface layers, reducing battery efficiency. The compatibility of sulfide electrolytes with different anode materials, such as graphite, silicon, and lithium metal, is another critical issue. Graphite anodes suffer from lithium plating at high currents. Silicon anodes offer a high capacity but experience significant volume changes during cycling, leading to mechanical instability. Lithium metal anodes with their high theoretical capacity are prone to dendrite growth and interfacial reactions with sulfide electrolytes.The large-scale production of sulfide electrolytes is essential for the commercialization of ASSBs. However, the synthesis of sulfide electrolytes is complex and requires a precise control of reaction conditions. High-temperature solid-state methods and liquid-phase synthesis are the two main approaches, each with its own advantages and challenges. Cost control is also a significant factor, as the raw materials for sulfide electrolytes, particularly Li2S, are expensive. The fabrication of thin, uniform, and mechanically robust electrolyte membranes is crucial for ASSB performance. Wet and dry processing methods are commonly used, but each has limitations. Wet processing can lead to solvent-induced degradation of the electrolyte, while dry processing faces challenges in achieving uniform mixing and thin film formation. The assembly of ASSBs involves stacking multiple layers of electrodes and electrolyte membranes. Ensuring a good contact between these layers is critical for battery performance. However, the solid nature of the components makes it challenging to achieve uniform pressure distribution during stacking, leading to increased interfacial resistance.Summary and ProspectsThe development of sulfide-based ASSBs holds a great promise for the future of energy storage. However, significant challenges remain in terms of material stability, interface engineering, and large-scale production. Future research should focus on, i.e., 1) material innovation: Developing new sulfide electrolytes with improved stability and compatibility with high-voltage cathodes and lithium metal anodes, 2) interface engineering: Optimizing the interfaces between sulfide electrolytes and electrodes to enhance ion transport and reduce interfacial resistance, 3) process optimization: Improving the scalability and cost-effectiveness of sulfide electrolyte production and battery assembly processes, 4) battery design: The future design of all-solid-state batteries should focus on optimizing the internal structure via pairing high-performance cathode and anode materials with sulfide electrolytes to enhance energy density, power density, and safety, while improving thermal management and structural stability to extend lifespan and operational efficiency, and 5) standardization and collaboration: Establishing industry standards and fostering collaboration between academia, industry, and policymakers to accelerate the commercialization of ASSBs.Sulfide-based ASSBs can achieve their full potential, offering high energy density, enhanced safety, and long cycle life for different applications from electric vehicles to grid storage.

The advancement of the global new energy revolution has placed stringent demands on energy storage technologies, emphasizing the need for high-energy-density and high-power electrochemical devices to enable the grid-scale and public infrastructure energy storage. All-solid-state batteries (ASSBs) exploiting solid ionic conductors as electrolytes offer advantages of high-energy-density, enhanced safety performance and long cycle life, which are considered as a crucial direction for the development of next-generation electrochemical energy storage. Designing and synthesizing solid-state electrolytes (SSEs) with ultrafast ionic conductivity, wide electrochemical windows and excellent mechanical deformability plays a crucial role to realizing the practical application of ASSBs.To date, various types of inorganic solid electrolytes including oxides and sulfides are extensively investigated, but none of them are successfully combined the advantages of excellent interfacial wettability and outstanding oxidation stability. Oxyhalide SSEs are among the most promising solid electrolytes for the commercialization of ASSBs due to their advantages like good compatibility with high-voltage cathodes, excellent mechanical deformability and remarkable ionic conductivity compared to that of liquid electrolytes. Despite oxyhalide electrolytes use in ASSBs, challenges remain in their practical application, including the scalable preparation, enhancement of humid tolerance and improved compatibility with electrode materials.In this review, a critical overview of the development, synthesis, ionic conduction mechanisms and challenges of oxyhalide electrolytes is given. Different synthesis routes of oxyhalide electrolytes including the promising hydrate-assisted synthesis method are summarized in detail. Furthermore, the framework structures of various types of oxyhalide electrolytes are analyzed, and the ionic conduction mechanisms in crystalline and amorphous oxyhalide electrolytes are elucidated. The interfacial compatibility of oxyhalide electrolytes with lithium metal anodes and various cathode materials is highlighted, and the latest progress of oxyhalide electrolytes in low-temperature ASSBs is also represented. Finally, future perspectives on designing high-performance oxyhalide electrolytes and their practical applications in ASSBs are provided, aiming to offer guidance for the advancement of oxyhalide-based ASSBs in energy conversion and storage.Summary and prospectsMetal oxyhalides ionic conductors represent a highly promising class of inorganic solid electrolytes for realizing the commercialization of high-performance ASSBs due to their outstanding electrochemical stability, excellent mechanical deformability, low-cost preparation and high ionic conductivity, compared to that of liquid electrolytes. Despite certain progress is made for the oxyhalide electrolytes in improving ionic conductivity, hydrate-assisted synthesis and low temperature application in ASSBs, some challenges remain for their practical application, including large-scale preparation, improving of moisture instability and enhancing compatibility with electrode materials. The attractive future research directions and prospects are outlined as follows, i.e., (1) Scale-up preparation: The wet-chemistry synthesis offers several advantages for the preparation of oxyhalide electrolytes, including scalable production, effective size control and shortened reaction time. However, reports on synthesizing oxyhalide electrolytes employing the wet-chemistry method remain scarce. By introducing auxiliary agents into the aqueous solution as a coordination agent to preferentially react with metal halides to form stable intermediates, offering a crucial guidance for the development of wet-chemistry synthesis of oxyhalide electrolytes, (2) Revealing ion-conduction mechanism: Advanced characterization techniques, such as synchrotron X-ray diffraction (SXRD), neutron powder diffraction (NPD), X-ray absorption near-edge structure (XANES) and solid-state nuclear magnetic resonance (SSNMR), can be preferentially utilized to deeply analyze the local structure of oxyhalide electrolytes, and combined with simulation and computational modeling to establish the structure-performance relationships of oxyhalide electrolytes, effectively promoting the design and synthesis of next-generation ultrafast ionic conductive oxyhalide electrolytes, (3) Enhancement of chemical stability: The chemical stability of oxyhalide electrolytes exerts a crucial influence on the implementation in their entire life from preparation, storage and transportation to application. An in-depth understanding of the degradation mechanisms of oxyhalide electrolytes when exposed to humid air and polar solvents can provide a significant theoretical guidance for the development of highly moisture-resistant oxyhalides and solvents with a good compatibility towards oxyhalide electrolytes, (4) Improving electrode-electrolyte interface compatibility. The development of highly ionically conductive oxyhalides with a superior reduction stability that are compatible with alkali metal anodes, and the construction of stable passivation layers at cathode-oxyhalide interfaces to suppress side reactions can offer crucial pathways for achieving high-energy-density ASSBs with oxyhalide electrolytes, and (5) Fabrication of electrolyte/electrode film: Tape casting is considered as a scalable strategy for the preparation of ultra-thin electrolyte membrane and electrode sheet. The screening of solvents and binders compatible with oxyhalides and the optimization of slurry component content can be the promising research directions in future due to the limited compatibility of oxyhalides with conventional polar solvents that are similar to those used in halide electrolytes. In addition, the solvent-free process for preparing self-supporting oxyhalide electrolyte membranes and sheet electrodes presents advantages such as simplified processes and reduced environmental pollution, providing a viable pathway for large-scale applications of oxyhalide electrolytes. The unique dual-anion chemistry of oxyhalide electrolytes grants them outstanding comprehensive performance, positioning them as one of the most promising solid electrolytes for the commercialization of ASSBs. It is foreseeable that ASSBs utilizing oxyhalide electrolytes will unlock new opportunities for the storage and conversion of renewable energy in future.

With the increasing global demand for electric vehicles and large-scale energy storage systems, solid-state batteries (SSBs) have emerged as a promising alternative to conventional lithium-ion batteries due to their enhanced safety and energy density. The rigid nature of solid-state electrolytes (SSEs) allows SSBs to avoid flammable liquid electrolytes and minimize the risk of thermal runaway. However, the complexity of solid–solid interfaces, the growth of lithium dendrites, and poor interfacial contact during battery operation pose significant challenges to their performance and longevity. Understanding these internal phenomena in real time thus becomes a critical research priority.Ultrasound and optical fiber sensing technologies are increasingly recognized as effective tools for in-situ, non-destructive, and real-time monitoring of internal battery behaviors. Compared with conventional electrochemical and spectroscopic methods, these techniques provide unique advantages in terms of spatial resolution, penetration depth, and multiparameter detection. This review outlines the fundamental principles, technological advancements, and practical applications of both ultrasound and optical fiber sensors in the context of solid-state batteries.Ultrasound technology utilizes high-frequency mechanical waves to analyze internal structural changes. Some parameters such as acoustic impedance, attenuation, and time-of-flight (ToF) provide valuable insights into material density, elastic modulus, and crack formation. These features allow ultrasound to dynamically evaluate gas evolution, pore formation, and interfacial degradation within batteries. For instance, customized ultrasonic imaging systems are developed to detect gas generation rates and interfacial contact loss in SSEs, enabling accurate assessments of degradation mechanisms. Research from our group indicates that ultrasound can effectively distinguish between chemical passivation and physical delamination at the electrode–electrolyte interface and track the impact of polymer cross-linking on interface uniformity during in-situ polymerization.Optical fiber sensing, especially fiber Bragg grating (FBG) technology, offers a high sensitivity to strain and temperature, making it well-suited for harsh battery environments. Embedded within cells, FBG sensors enable real-time monitoring of stress evolution and thermal distribution across electrodes and interfaces. Their immunity to electromagnetic interference, compact form factor, and low cost enhance their applicability in sealed battery systems. FBG sensors can effectively decouple strain and temperature responses via analyzing wavelength shifts induced by mechanical or thermal stimuli. Advanced designs, such as micro-FBGs with reduced diameters, further improve integration with battery components, minimizing interference with electrochemical performance. Beyond mechanical and interfacial diagnostics, these sensing technologies also allow for monitoring of key physicochemical parameters like internal temperature gradients, refractive index changes, and byproduct formation through distributed optical networks and acoustic mapping. Recent developments on lab-on-fiber platforms, which integrate Raman scattering and near-infrared spectroscopy with structural sensing, offer a multimodal approach for revealing degradation mechanisms and supporting real-time diagnostics in solid-state batteries.As emerging in-situ monitoring technologies, ultrasound and optical fiber sensing have significant advantages over conventional battery characterization methods. Ultrasound provides a non-destructive, efficient, and cost-effective means to sensitively detect internal pore evolution, gas generation, and electrolyte wetting, addressing limitations in penetration depth and response speed of conventional techniques. Optical fiber sensing enables real-time monitoring of structural phase transitions, ion migration, and stress distribution within batteries, with intense electromagnetic immunity, low cost, and flexible integration-particularly suited for long-term in-situ monitoring of sealed battery systems. While many reviews focus on the application of these techniques in lithium-ion batteries, their use in solid-state batteries remains relatively underexplored. This review systematically represents recent development and application of ultrasound and optical fiber sensing technologies to solid-state batteries from three perspectives, i.e., interfacial behavior, mechanical properties, and physicochemical characteristics. It also summarizes relevant work conducted by our research team and discusses future integration with intelligent battery systems, highlighting key opportunities and remaining challenges.Summary and prospectsUltrasound and optical fiber sensing technologies are poised to play a transformative role in the next generation of battery diagnostics. Their unique ability to perform high-resolution, real-time, and multi-physics monitoring of internal battery phenomena represents a significant advancement over conventional diagnostic approaches. These sensing techniques offer some possibilities for tracking complex degradation mechanisms and enabling proactive battery management strategies. Despite the notable progress achieved in recent years, several critical technical challenges persist. First, signal decoupling in complex environments, in which multiple physical processes such as gas evolution, crack propagation, and stress accumulation occur simultaneously, remains a considerable obstacle. Distinguishing these overlapping signals requires advanced data processing and modeling techniques. Second, the development on integrated, multi-sensor platforms that combine ultrasound, fiber–optic, and electrochemical sensors is still at a nascent stage. Such hybrid systems have a potential to provide more comprehensive and cross-validated data but require further innovation in sensor compatibility and system integration. Third, translating lab-scale sensing systems into practical applications for large-format battery packs demands further progress in miniaturization, cost-effectiveness, and long-term operational stability under dynamic conditions. Future research efforts should focus on three pivotal areas. At the interface level, there is a pressing need to develop multi-parameter sensing systems capable of simultaneously capturing interfacial gas release, contact resistance changes, and local stress distribution. At the mechanical level, high-resolution ultrasonic imaging combined with distributed optical fiber networks can enable real-time, spatially resolved tracking of crack initiation and propagation from micro-scale to macro-scale. At the physicochemical level, integrating spectroscopic methods such as Raman or near-infrared spectroscopy-with ultrasound and optical fiber sensing can unlock deeper insights into phase transitions, compositional changes, and side reactions. Solid-state batteries can achieve real-time diagnostics and early fault detection via utilizing advanced sensing data intelligence and material innovations, accelerating their path to reliable commercial deployment.

All-solid-state batteries (ASSBs) emerge as a current research hotspot due to their high safety, extended cycle life, and elevated energy density. Understanding the intrinsic relationship between the structure and performance of solid electrolytes (SEs), along with interfacial charge transport properties, is crucial for the stable operation of ASSBs. Synchrotron radiation (SR) technology provides a significant support for analyzing the composition-structure-performance relationship in ASSBs due to its high brightness and adjustable energy. It is especially applicable to investigate the dynamic evolution of materials and interfaces, charge migration, and failure mechanisms during battery cycling.The review outlines the classification, principles, and applications of synchrotron radiation X-ray (SR-X) technologies, i.e., synchrotron X-ray diffraction (SXRD), X-ray absorption spectroscopy (XAS), synchrotron X-ray photoelectron spectroscopy (SXPS), and synchrotron X-ray microscopy (SXM). The focus is on the advantages of SR in elucidating local structures, chemical states, and microstructures.The review also represents the applications and progress of SR-X in ASSBs. Initially, it highlights progress on using SR-X to explore the composition-structure-property relationships in SEs and monitor structural evolution and charge mobility during ASSB operation. The review represents the use of SR-X in detecting SEs/interfaces, examining changes in interface structure, composition, and morphology during battery cycling, with a focus on SEs/cathode and SEs/anode interfaces.Summary and prospectsThis review represents recent advancements in the application of SR-X techniques for studying SEs and their interfaces, emphasizing their unique capabilities in resolving local structures, chemical states, coordination environments, and interfacial heterogeneities, while elucidating the failure mechanisms of ASSBs. To advance the practical implementation of ASSBs, the future development on SR-X technology should focus on three key areas, i.e.,1) Advancing in-situ SR-X techniques with higher temporal and spatial resolution. Real-time monitoring of the dynamic evolution of solid-solid interfaces during battery operation is essential for capturing transient phenomena in chemical and electrochemical reactions. An atomic-scale resolution will enable a detailed investigation of microstructural and chemical properties, particularly for sensitive elements like Li and Na. 2) Integrating SR-X with complementary techniques for multiscale analysis. Combining methods such as SXM and XAFS can provide synergistic insights into the microstructure and chemical composition of materials, offering a comprehensive understanding of both heterogeneous and average properties in ASSB components. 3) Developing in-situ electrochemical cell designs compatible with SR-X techniques. These configurations should ensure sufficient signal strength for characterizing material components and interfaces, while maintaining a typical electrochemical behavior under synchrotron beamline conditions. Advancing these aspects will unlock the full potential of SR-X techniques, driving the commercial viability of ASSBs.

All-solid-state batteries (ASSBs) emerge as a promising next-generation power battery technology due to their ability to eliminate flammable organic components in conventional liquid lithium-ion batteries via introducing solid electrolytes. This innovation significantly enhances the safety window of the battery and holds a potential to further increase the energy density of the battery when combined with high-energy electrode systems. Consequently, ASSBs are regarded as a major technological route for next-generation energy storage devices.While liquid lithium-ion batteries are developed for years, establishing a well-developed and multi-tiered testing and evaluation system (i.e., basic electrode material tests, interface behavior analysis, single-cell performance assessments, system integration/management for performance, lifespan, and safety evaluation), ASSBs differ significantly in their technological characteristics. These differences span ion transport mechanisms, interface properties, thermal stability, and environmental adaptability. Furthermore, the introduction of new materials, processes, and structures for ASSBs presents some challenges for the existing testing and evaluation systems.The existing solid-state battery technology is still in a phase of rapid development, and some technical issues and challenges for testing and evaluation are expected to emerge throughout this process. It is thus necessary to simultaneously advance new testing technologies such as in-situ detection, intelligent simulation, and data-driven approaches, alongside the establishment of conventional testing and evaluation systems. These advancements enable highly efficient testing and evaluation throughout the entire lifecycle of solid-state battery products, from design to application and post-decommissioning processing.In terms of in-situ detection, it is critical to develop real-time observation techniques with high temporal and spatial resolution to capture key dynamic processes such as interface evolution, lithium dendrite growth, and side reactions during battery operation. These techniques will provide multi-scale and multi-dimensional dynamic data. Intelligent simulation involves coupling multiphysics models (i.e., electrochemical-mechanical-thermal models) with high-performance computing to analyze and predict the battery performance and failure mechanisms under complex operating conditions. Data-driven technologies offer an innovative direction for testing and evaluation systems by leveraging big data analytics and machine learning algorithms. These technologies will enable precise predictions of unknown parameters and enhance the flexibility and adaptability of testing methods.With the integration of these emerging testing technologies, the development of ASSB technology will gain new momentum and drive breakthroughs in the field. To address the challenges for ASSBs, novel testing methodologies should be developed, including key material analysis, in-situ characterization, comprehensive battery performance evaluation, failure and hazard analysis, and simulation techniques. This review explores the fundamental technological differences between ASSBs and liquid lithium-ion batteries and systematically discusses the key ideas for establishing a testing and evaluation system tailored to ASSBs. The review also examines some key issues and technical challenges related to testing at the material, cell, and system levels. It proposes a framework for building a comprehensive testing and evaluation system specifically suited to the characteristics of ASSBs via analyzing the current state of testing technology development. The aim is to provide a theoretical foundation and practical guidance for the technological research and industrial application of all-solid-state batteries.Summary and prospectsThis review focuses on the status and challenges of testing and evaluation technologies for all-solid-state batteries (ASSBs). As ASSBs are considered as a promising alternative to conventional lithium-ion batteries due to their enhanced safety and potential for higher energy density, understanding and addressing the unique testing challenges they pose is critical. The review systematically represents the novel characteristics of ASSBs, particularly in terms of ion transport mechanisms, interfacial stability, thermal safety, and environmental adaptability. These characteristics significantly differ from those of liquid-based lithium-ion batteries, presenting new challenges in evaluating their performance, reliability and safety.The review also analyzes the limitations of existing testing and evaluation systems, which are primarily designed for liquid lithium-ion batteries, and highlights a need for new approaches to meet the specific demands of ASSBs. The discussion emphasizes the necessity of developing a comprehensive testing framework that spans all levels, from materials and interfaces to battery cells, systems, and even vehicles. This comprehensive approach is essential to properly assess ASSBs in terms of their performance, lifecycle, and safety under different environmental conditions.Furthermore, the review identifies some key challenges arising in the evolution of testing technologies, i.e., the need for more accurate in-situ analysis techniques, the integration of intelligent simulation tools, and the incorporation of data-driven approaches. To address these challenges, the review proposes directions for advancing testing methodologies, such as developing high-resolution, real-time in-situ observation tools to capture dynamic processes, utilizing multi-physics simulations to predict performance under complex conditions, and leveraging machine learning for data analysis and prediction of battery behavior. These advancements can integrate cutting-edge techniques into the testing and evaluation processes, facilitating the transition of ASSBs from laboratory research to commercial applications, and ultimately enabling the broader adoption of solid-state battery technology in various industries.

Two-dimensional (2D) ion conductors have demonstrated significant advantages in applications such as water treatment, bio-medical, catalysis, energy storage and conversion. The confinement structure of 2D ion conductors provides an ideal platform for studying ion-transport behaviors at nano/angstrom-scale. When the size of the 2D channel is less than 100 nm in at least one direction, the size confinement effect and surface charge of the 2D channel can regulate ion flow, creating nanofluidic transport. This surface-charge-controlled ion transport shows many different properties (e.g. ultrahigh ion conductivity and selectivity) from bulk solutions.The nanofluidic ion transport has been observed in biological channels (e.g. transmembrane protein on the cell) and natural layered materials (e.g. vermiculite and mica), and gained extensive research interest. To finely control the structure, research attempts have been made to synthesize artificial 2D ion conductors, which are typically achieved by "top-down" and "bottom-up" methods. These artificial 2D ion conductors are normally restacked 2D materials with extrinsic nanofluidics or lamellar crystal materials with intrinsic nanofluidics. Artificial 2D ion conductors can be categorized into three types, including inorganic (e.g. graphene), organic (e.g. covalent organic frameworks), and organic-inorganic hybrid ion conductors (metal-organic frameworks). The nanofluidics in 2D nanochannels originate from the interactions between ions and surface charge. When the spatial size of nanochannels is comparable to those of solvated ions, the ions can be partially or fully desolvated to separate solvent molecules and ions. When ions transport in the nanofluidic channels, ions with the same charge of surface charge on the inner wall can be repeled by the electrostatic force, which leads to unipolar ion transport with high flux. By contrast, ions with the opposite charge will be attracted to suppress their transport. Such ion transport within 2D nanoconfined spaces can create unusual phenomena that are distinct from bulk solution, including ion gating, ion current rectification, selective ion conduction, and ultrahigh ion flux. This review also summarizes key factors influencing nanofluidic ion transport in 2D ion conductors and strategies for regulation, which can be achieved by adjusting internal interlayer spacing and surface chemistry of 2D materials, as well as external field regulation of the confined space.Benefiting from the studies of nanofluidic mechanisms, the development of 2D ion conductors is rapid in applications where separation and conduction play critical roles. The application can be classified into three types. The first type is the separation of ions and water molecules, which is mainly used for seawater desalination and water purification to promote efficiency and low energy consumption. The second type is the separation of ions with different valences and sizes, which is crucial for ion separation and extraction of high-value elements (e.g. lithium). The third type is accelerating the ion transport of cations or anions, which is one of the keys to determining the rate capabilities in devices for energy conversion (e.g. osmotic energy conversion and photoelectric conversion) and energy storage (e.g. supercapacitors and batteries).Summary and ProspectsAlthough research on nanofluidics and applications of 2D ion conductors has made significant progress in recent years, there are still several critical challenges from theoretical to practical perspectives. The differences in the size, tortuosity and surface chemistry of parallel and perpendicular nanochannels in 2D ion conductors can lead to increased overall resistance. For instance, restacked 2D membranes usually show fast horizontal ion transport but significantly hindered ion transport across the vertically restacked layers of membranes. It is thus interesting to investigate perpendicular 2D nanofluidic channels constructed by using novel materials and fabrication methods, particularly introducing the external fields (e.g. magnetic and electrical) as an optimal fabrication strategy in the future. To enhance the consistency of nanofluidic ion flow, the connection precision of diverse 2D nanochannels may be modified by vertical pre-alignment of 2D nanosheets to minimize the tortuosity. The electrical double layer (EDL) model is critical to drive the nanofluidics. In previous studies of EDL, the ions are usually simplified as point charges. However, in such narrow nanochannels, the size of ions should not be neglected, as it could directly influence the ion distribution in the EDL and hence the interactions to regulate nanofluidics. To gain a deep insight into ion transport behaviours in 2D nanochannels, it is crucial to obtain a precise geometric structure, EDL structure as well as various surface properties, such as roughness, hydrophilicity and the distribution of surface charge. Using advanced characterization methods combined with precise theoretical calculation can provide invaluable insights into how to control nanofluidic ion transport. For instance, combining electrochemical quartz crystal microbalance with density functional theory can effectively monitor the ion transport and mass exchange processes in 2D ion conductors. In-situ Raman spectroscopy could be used to record the surface chemistry of the inner walls of 2D nanochannels during ion transport, which helps to further clarify the mechanism. Moreover, the ultrahigh ionic conductivity of 2D ion conductors presents a promising avenue for boosting new technologies, such as solid-state batteries, but issues related to the stability and compatibility of the interface between 2D ion conductors and electrodes should be addressed. Finally, scaling up the production of 2D ion conductors is vital for real-world applications in various devices.

The rapid advancement of electric vehicles has increasing demands on the energy density of power batteries(i.e., 400 (W·h)/kg in 2025, and 500 (W·h)/kg in 2030). However, the existing lithium-ion batteries have their theoretical energy capacity limits primarily due to the constraints of graphite anodes. Also, the use of flammable electrolytes in lithium-ion batteries poses significant safety risks, making the development of safer and higher-energy-density batteries an urgent necessity. All-solid-state lithium batteries (ASSLBs), which replace liquid electrolytes with non-flammable solid electrolytes, offer a promising solution to these challenges. The use of solid-state electrolytes can enhance battery safety, extend service life, and enable the use of lithium metal anodes, thus increasing energy density via eliminating dendrite growth. Moreover, solid-state electrolytes allow for a wider operational temperature range, reducing a need for complex thermal management systems and further improving energy density.Solid-state electrolytes are central to the performance of ASSLBs, impacting key metrics such as power density, energy density, and cycle life. The success of ASSLBs large-scale production hinges on the selection of appropriate solid-state electrolyte materials. These electrolytes are generally classified into inorganic and organic polymer types. The inorganic group is further subdivided into oxides, sulfides, and halides. Despite significant progress in each of these types, no single electrolyte material meets all the industrial requirements for ASSLB production. The primary challenge lies in balancing high ionic conductivity at room temperature, low cost, good chemical and electrochemical stability, high thermal stability, high mechanical strength, and ease of processing.In response, composite solid electrolytes with both inorganic and polymer electrolytes emerge as a promising strategy. These materials offer improved performance characteristics via leveraging the benefits of both components. However, the challenge remains in selecting and optimizing the right inorganic and polymer combination to achieve the desired balance of properties. This review introduces a concept of SHOP-type composite solid electrolytes, where "S," "H," "O," and "P" represent sulfides, halides, oxides, and polymers, respectively. SHOP-type electrolytes can overcome the limitations of individual materials via utilizing synergistic effects between multiple components. The review also explores the potential advantages of SHOP-type composite electrolytes for the industrialization of ASSLBs, highlighting some promising development directions for future research.Summary and ProspectsThis review addresses some challenges faced by four major types of single-phase solid-state electrolytes, i.e., oxides, sulfides, halides, and polymers, in large-scale production and application. These challenges stem from the difficulty of balancing the various performance requirements for commercial feasibility. Oxides offer excellent thermal stability and electrochemical windows but suffer from low ionic conductivity and complex processing requirements. Sulfides provide high ionic conductivity but are prone to air sensitivity and narrow electrochemical windows. Halides, while offering high ionic conductivity and electrochemical stability, face high production costs and poor interface stability with lithium metal anodes. Polymers, known for their flexibility and processability, are limited by low ionic conductivity and poor high-voltage stability.A concept of SHOP-type composite solid electrolytes is proposed to address these challenges. SHOP-type electrolytes provide a balanced solution that meets the demanding requirements for ASSLBs via combining the benefits of polymer electrolytes (i.e., processing flexibility and low cost) with the advantages of inorganic electrolytes (i.e., high ionic conductivity, thermal stability, and mechanical strength). These materials offer a potential for improved safety, performance, and scalability in large-scale production, making them highly promising for future energy storage systems.Despite their numerous advantages, the development of SHOP-type electrolytes still faces several key challenges. Future research should focus on optimizing the composition and structure of composite solid electrolytes. Understanding the interactions between inorganic and polymer components will be crucial to achieving the ideal performance characteristics. Also, the compatibility of SHOP electrolytes with lithium metal anodes and high-voltage cathodes must be further explored to improve battery efficiency and longevity. Another critical area of research is the reduction of production costs, which is achieved via selecting sustainable materials and optimizing processing methods. Innovations in fabrication techniques that can lower costs and improve scalability are essential for the widespread adoption of ASSLBs.Interdisciplinary collaborations also play a vital role in advancing the development of ASSLB technologies. Researchers can overcome the existing barriers and accelerate the transition of ASSLBs from lab-scale to industrial-scale production via integrating insights from materials science, electrochemistry, and engineering. Through continuous innovation and optimization, SHOP-type composite solid electrolytes hold a great promise for enabling the widespread adoption of ASSLBs, driving forward the development of high-energy-density, safe, and long-lasting energy storage systems in the future.Some significant strides are made in the development of solid-state electrolytes, the challenges remain considerable. SHOP-type composite electrolytes with their potential for balanced performance offer a promising path toward the commercial realization of ASSLBs. With further research and development, these materials can significantly contribute to the advancement of next-generation energy storage technologies.

Compared with inorganic solid-state electrolyte, solid polymer electrolyte (SPEs) is an ideal electrolyte material for solid-state lithium metal batteries due to its high flexibility, excellent processability and good interfacial contact. The ionic conductivity, electrochemical window and interface stability of SPEs with electrodes play a crucial role in the overall performance of solid-state lithium batteries. According to the different electrochemical stability windows, this review represents research progress on two types of typical SPEs systems, i.e., low-voltage stable polyethers and high-voltage stable polyesters. Polyether-based electrolytes, which mainly outlines the problems and challenges faced by four types of polyether, such as poly(ethylene oxide) (PEO), poly(ethylene glycol monomethyl ether acrylate) (pPEGMEA), poly(1,3-oxolane) (PDOL), and polyoxymethylene (POM)-based SPEs. These electrolytes have a better compatibility with lithium metal, but a poor oxidation resistance and a low ionic conductivity at room temperature. The molecular structure design of polymers and the introduction of functional groups (—F, —CN, —C=O) can reduce the HOMO value of SPEs and improve the antioxidant ability of electrolytes. The crystallinity of PEO-SPEs can be reduced via compounding, blending and cross-linking with inorganic fillers to improve the ionic conductivity. In addition, the balanced interactions between Li+, lithium salts, ether groups, anions and polymer matrix structure also need to be considered to achieve high ionic mobility and ionic conductivity. Polyester electrolytes mainly outline the challenges and solution strategies of four types of polyester, i.e., poly(-caprolactone) (PCL), poly(trimethylene carbonate) (PTMC), poly(propylene carbonate) (PPC) and polyoxalate (POE). This type of electrolyte is a good match for high-voltage cathode materials, but is easily reduced by lithium metal. The construction of a stable solid electrolyte interface membrane (SEI) layer can improve the stability with lithium metal via introducing multifunctional groups. In addition, the development of in-situ polymerisation processes is also reviewed, considering that solid-state electrolytes suffer from a poor electrode–electrolyte interfacial contact, a low ionic conductivity and complex manufacturing processes. Therefore, this review outlines the advantages and disadvantages of the two types of polyether-based and polyester-based SPEs, and analyzes their key scientific issues as well as recent research progress of the modification methods to provide a reference for future research and practical applications.Summary and prospectsDespite important advances in solid polymer electrolytes (SPEs), polyether and polyester electrolytes and in-situ polymerisation technologies still face several challenges before they can be applied commercially. Improving ionic conductivity and electrochemical stability while maintaining good mechanical properties and interfacial compatibility are key issues in the development of SPEs. In addition, the economics and scale-up of preparation are important considerations for commercial applications, and greener and more efficient preparation processes also need to be explored. The development of SPEs in solid-state lithium-ion batteries focuses on the following aspects, i.e., 1) designing new molecular structures, exploring the constitutive relationship between molecular structure and properties, and improving the ionic conductivity and antioxidant properties of electrolytes, 2) since all electrochemical reactions occur at the interface, constructing a stable cathode-electrolyte (CEI) and SEI membrane between SPEs and cathode and anode to improve the interfacial stability and inhibit the interfacial reactions, 3) improving the ionic conductivity of SPEs through strategies such as grafting, copolymerisation and organic-inorganic composites, and 4) exploring the recycling methods of lithium salts and polymers to further reduce the manufacturing cost of SPEs.For quasi-solid state electrolytes (QSSEs) with a high ionic conductivity, it is necessary to solve the following problems, i.e., 1) to improve the molecular stability of polymers for high-voltage cathode materials and low-voltage anode materials, 2) to enhance the liquid-locking ability of polymer molecules, inhibit the interfacial side reactions and improve the safety of the battery, 3) to regulate the interactions among the components of polymer, solvent and lithium salt to optimize the solvation structure of Li+ and achieve the rapid migration of Li+, and 4) the electrolyte film thickness and electrode active substance loading must be as close as possible to or exceed the targets of commercial liquid batteries. In addition, the production of large capacity energy storage batteries (i.e., >200A·h) , results in local inhomogeneities within the cell due to the inhomogeneous heating within the cell. The influence of various factors (i.e., initiator activity, initiator dosage, and polymerisation temperature) on the polymerisation process should be investigated. The "step-by-step in-situ curing" route should be adopted, so that the polymer is cured in the battery separator, cathode and anode, and the polymer is cured in a step-by-step process. The polymer is in-situ cured at the battery diaphragm, positive electrode and negative electrode, and then laminated to assemble the solid-state battery. In summary, the synergistic development of polyether electrolyte, polyester electrolyte and in-situ polymerisation technology has an important foundation for the commercial application of lithium solid-state batteries. In the future, high-performance, low-cost SPEs, especially the development of in-situ curing, are expected to be applied in a large scale, becoming the first practical solid-state battery system.

All-solid-state sodium batteries (ASSSBs) emerge as a highly promising next-generation energy storage technology due to their inherent high safety, abundant sodium resources, and potential for cost-effective production. A central to the performance of these batteries is solid electrolyte (SSE) that has a pivotal role in determining the overall electrochemical performance. Among various SSE candidates, halide solid electrolytes (HSSEs) have attracted much attention due to their high ionic conductivity, wide electrochemical window and good deformability. These characteristics make HSSEs particularly suitable for high-energy-density applications, where conventional liquid electrolytes often fall short due to safety concerns and limited voltage windows.The crystal structures of halide SSEs are intricately linked to their ion transport mechanisms and electrochemical performance. These structures can be broadly classified based on the metal elements incorporated and the arrangement of halide ions. For instance, sodium-based halide SSEs can be categorized into three main types, i.e., those with subgroup 3 and 4 elements (i.e., Sc, Y, La–Lu, Zr, Hf), subgroup 5 elements (i.e., Nb, Ta), and subgroup 3 main group elements (i.e., Al, Ga, In). Each category exhibits distinct structural characteristics and corresponding ionic conductivities. For instance, Na3YCl6, which belongs to the subgroup 3 and 4 elements category, is reported to have an ionic conductivity of 1.4×10−7 S/cm at room temperature and a monoclinic crystal structure. In contrast, NaTaCl6 as a member of the subgroup 5 elements category has an ionic conductivity of 6.2×10−5 S/cm and a monoclinic structure. These structural differences can affect the ion transport mechanisms and overall electrochemical performance of SSEs.The synthesis methods of halide SSEs play a crucial role in determining their structural and electrochemical properties. Common synthesis methods include mechanical milling, solid-state annealing, and wet chemical synthesis. Mechanical milling, especially high-energy ball milling, can introduce structural disorder and defects, which enhance the ionic conductivity of SSEs. For instance, ball-milled NaTaCl6 (NTC) exhibits a higher ionic conductivity of 4×10−3 S/m, compared to its as-synthesized form. Solid-state annealing can improve the crystallinity and phase purity of SSEs, but it may also lead to a decrease in ionic conductivity due to the reduction of structural defects. Wet chemical synthesis offers a more scalable and energy-efficient approach, but it requires careful control of the synthesis parameters to achieve the desired crystal structure and ionic conductivity.Interface stability between SSEs and the electrodes is another critical issue that needs to be addressed for the practical application of ASSSBs. Halide SSEs generally exhibit good chemical stability and compatibility with cathode materials due to their wide electrochemical window and high oxidation resistance. However, the interface between SSEs and sodium metal anode remains a challenge. The poor electrochemical reduction stability of halide SSEs can lead to severe interfacial reactions and degradation when in direct contact with sodium metal. To mitigate this issue, various strategies are proposed, such as alloying sodium with other metals (i.e., Sn) to form a more stable interface, using protective coatings to prevent direct contact between SSEs and sodium, and employing composite electrolytes to enhance the overall stability of the interface.In addition, the scalability and reproducibility of halide SSEs are also critical factors for their practical application. The existing synthesis methods often involve complex procedures and high-energy inputs, which limit the large-scale production of these materials. It is essential for the commercialization of halide SSEs to develop cost-effective and scalable synthesis routes. Moreover, the long-term stability and reliability of ASSSBs incorporating halide SSEs need to be thoroughly evaluated under various operating conditions (i.e., temperature, cycling rate and environmental factor).Summary and ProspectsHalide solid-state electrolytes (HSSEs) are poised to revolutionize all-solid-state sodium batteries (ASSSBs) due to their exceptional ionic conductivity (i.e., often surpassing 10–3 S/cm), broad electrochemical stability windows (i.e., up to 6 V vs. Na+/Na), and mechanical flexibility, which enable dense electrode-electrolyte integration and mitigate dendrite growth. However, their practical implementation faces multifaceted challenges. Crystal structure optimization is required to balance ionic transport and thermodynamic stability, particularly in systems like Na3YCl6 or Na2ZrCl6, where lattice defects and anion/cation disorder impede performance. Synthesis methods such as mechanochemical milling or solvent-based routes need refinement to reduce impurities and ensure reproducibility. Electrode-electrolyte interface instability, driven by chemical incompatibility or volumetric changes during cycling, remains a critical bottleneck for long-term cycling. Scalable production techniques should bridge a gap between lab-scale innovations and industrial manufacturing. Future research should prioritize structure-property relationship studies using computational tools like density functional theory (DFT) to design HSSEs with tailored ion migration pathways, coupled with advanced in-situ characterization (i.e., synchrotron X-ray tomography or cryo-electron microscopy) to probe dynamic interfacial degradation mechanisms. Simultaneously, innovative material engineering strategies, such as inorganic-polymer composites (i.e., HSSE-PEO hybrids) to enhance interfacial adhesion or novel dual-phase electrolytes to suppress side reactions can address stability and conductivity trade-offs. The development of low-cost synthesis routes (i.e., aqueous precursor processing or scalable sintering) and rigorous evaluation of HSSEs under extreme temperatures, high current densities, and a prolonged cycling will be pivotal for commercialization. In addition, interface optimization via atomic-layer-deposited protective coatings or 3D nanostructured electrodes can also minimize interfacial resistance and improve charge transfer kinetics. With rapid advancements in material discovery, machine learning-driven design, and interdisciplinary collaboration, HSSEs become a threshold of practical application, having ASSSBs with unparalleled energy density (i.e., >400 Wh/kg), inherent safety, and cycle lifetimes exceeding 5000 cycles. The field trajectory indicates that resolving these challenges could have sodium-based solid-state batteries as mainstream solutions for grid storage and electric vehicles, marking a paradigm shift in sustainable energy storage.

Sulfide solid-state electrolytes (SSEs) emerge as a promising candidate for replacing conventional liquid electrolytes in all-solid-state lithium batteries (ASSLBs). With their inherent advantages of high ionic conductivity, low activation energy, and good mechanical properties, sulfide SSEs hold a great potential for achieving high safety and energy density in next-generation energy storage devices. Unfortunately, the electrolyte layers or solid-state films of sulfide-based ASSLBs typically require a thickness of greater than 100 m to prevent lithium dendrite penetration and SSEs layer fracture, leading to short circuits during battery cycling. Ultra-thin sulfide SSEs films have attracted recent attention. Compared to conventional thick films, ultra-thin sulfide SSEs films can significantly reduce the non-active material content in batteries, thereby improving energy density. They can also enhance Li+ transport efficiency, lower electrolyte surface resistance and battery internal resistance, and improve battery cycling performance and rate performance.Some binders need to be added when the thickness of the sulfide SSEs layer is less than 100 m, resulting in a conductivity of less than 1.0 mS/cm. Therefore, the development of ultra-thin SSEs films with high ionic conductivity, high strength, high toughness, and based on sulfide electrolytes is a key direction for high-energy-density sulfide ASSLBs. This review represents recent development on sulfide SSEs, focusing on material development, synthesis methods, film preparation, binder selection, densification strategies, and interface stability.This review introduces the significant advancements of sulfide SSEs in recent years. Various types of sulfide SSEs with superior properties are developed, including glass, glass-ceramics, thio-LISICON, LGPS, and argyrodite-type materials. These materials exhibit diverse structures and properties, and the ion conductivity and stability of sulfide SSEs are continuously improved through various strategies, such as component optimization, element substitution, and crystal structure engineering. Several methods exist for synthesizing sulfide SSEs, each with its advantages and disadvantages. Solid-state synthesis, mechanochemical synthesis, and liquid-phase synthesis are the most commonly used methods.Developing thin and flexible sulfide SSEs films is crucial for achieving high energy density in ASSLBs. Wet processes such as slurry casting, spin coating, and dip coating are commonly used, but can be limited by solvent compatibility and film uniformity. Dry processes like powder pressing, powder spraying, and binder fiberization offer advantages in terms of film quality and process efficiency, but require further development for large-scale production. These dry methods offer a potential for minimizing solvent effects and improving film performance via directly depositing the SSEs powder onto the substrate without solvents. The choice of binder plays a vital role in film properties. Inert binders like ethylcellulose and SBR provide good mechanical strength, but can hinder Li+ transport. Li+ conducting binders like LiTFSI and PVDF offer an improved ion conductivity, but require careful optimization of processing conditions. PTFE is a popular option for dry film fabrication due to its low percolation threshold and good mechanical properties. The selection of binder depends on the specific application requirements and the desired trade-off among ion conductivity, mechanical strength, and interface stability.Densification techniques like hot pressing and cold pressing are essential for achieving high ion conductivity and mechanical strength in sulfide SSEs films. These techniques improve the packing density and reduce the porosity of the films, leading to enhanced ion transport and mechanical properties. Also, innovative approaches like solvent-free synthesis and the use of reinforcing structures like fibers are explored to further enhance film properties and address the challenges associated with conventional wet processes. Ensuring interface stability between sulfide SSEs films and electrode materials is crucial for the battery performance. Sulfide SSEs can react with both cathode materials and lithium metal, leading to interface issues such as decreased ion conductivity, increased interface resistance, and battery capacity degradation. Strategies to address these challenges include surface modification of electrode materials, component optimization, and the use of interlayer materials like precursors of solid electrolyte interface (SEI). These approaches can improve the compatibility between sulfide SSEs and electrode materials and reduce the occurrence of interface reactions.Summary and ProspectsWhile significant progress is made in sulfide SSEs research, some challenges are related to film properties, interface stability, and large-scale production remain. Future efforts should focus on several key areas, i.e., developing new sulfide SSEs materials with higher ionic conductivity, lower activation energy, and better chemical stability through material engineering and structure optimization; optimizing particle size and morphology to achieve optimal film properties and Li+ transport efficiency, which can be achieved through controlled synthesis and particle size engineering techniques; exploring new techniques like solvent-free synthesis, solution processing, and additive manufacturing to improve film performance, uniformity, and scalability; developing strategies to improve interface stability between sulfide SSEs films and electrode materials, including the use of interlayer materials, surface modification, and SEI engineering techniques; and implementing efficient and cost-effective production processes to enable commercialization of sulfide SSEs for ASSLBs, which requires the development of automation, high-throughput equipment, and optimization of production parameters. ASSLBs have a potential to revolutionize the energy storage landscape via addressing these challenges and continuously advancing sulfide SSEs technology, paving a way for a sustainable future with cleaner and more efficient energy storage solutions. The development of high-performance sulfide SSEs films is crucial for achieving this vision and unlocking the full potential of ASSLBs in various applications, including electric vehicles, grid-scale energy storage and portable electronics.

All-solid-state batteries are expected to achieve higher energy density and provide a higher safety rather than liquid batteries. As one of the core components of the all-solid-state battery, the thickness, ion transport capacity, mechanical properties, chemical/electrochemical stability and other properties of the solid electrolyte film play a crucial role in the performance of the all-solid-state battery. This review summarizes the processing methods of the existing solid electrolyte membrane. According to the solvent usage of the electrolyte membrane processing technology, it can be divided into three types of processing technologies, i.e., wet preparation, dry preparation and dry/wet preparation. The key technical indicators of the prepared solid electrolyte membrane, such as thickness, ionic conductivity, mechanical properties, battery properties and large-scale processing potential, are introduced. In addition, some challenges faced by different preparation processes and the future development direction are also summarized and prospected.Summary and ProspectsThe development of all-solid-state batteries puts forward new requirements and challenges for the processing and manufacturing of solid electrolytes. In this review, the processing technology of solid electrolyte in a relatively comprehensive and systematic way (i.e., dry preparation, wet preparation and dry/wet preparation) is introduced. Dry preparation mainly uses ball milling, roll pressing and other technologies, the processing process does not need to introduce solvents, avoid complex and cumbersome processing, efficient and environmentally friendly, and the prepared film thickness is easier to control. Wet preparation mainly uses magnetic stirring, ultrasonic and other methods to make the slurry mixed evenly, and then volatilizes the solvent through solution pouring, casting, freeze drying and other methods to form a film. The advantages of wet preparation are mainly that the prepared solid electrolyte film is more uniform and controllable. Dry/wet preparation mainly includes non-in-situ processing technologies such as 3D printing and electrospinning, as well as in-situ polymerization technologies such as photocuring and thermal curing. Among them, 3D printing technology has a high shapeability, which is easy to design the solid state battery structure. Electrospinning is convenient in the preparation of solid electrolyte film matrix. In-situ curing can achieve good interface properties and better integrate battery processing technology. Despite the existing solid electrolyte processing technologies, further research and attempts are still needed to achieve the large-scale production and landing of solid electrolyte membranes. The following aspects are concerned:1) To achieve both film thickness and mechanical properties. Making the film thickness thin to achieve a high energy density is one of the important goals of all-solid-state batteries. However, the reduction in film thickness can make it easier for dendrites to Pierce, leading to short circuits. Therefore, the mechanical properties of the films should be improved via introducing high mechanical strength polymers, adding inorganic fillers and roller compaction.2) To reduce production costs and improve preparation efficiency. Polymer synthesis usually involves relatively complex chemical reaction processes. In addition, the battery materials have generally high requirements for anhydrous and anaerobic environments. From raw materials to the production, the process steps need to be continuously optimized to reduce production costs and improve production efficiency.3) To promote the process of film preparation from the laboratory to industrialization. A basic research in laboratory is relatively easy to achieve due to its small scale. The assembled button battery has a small area, so its consistency is relatively better controlled. However, it will inevitably encounter a variety of problems after enlarging it. Some problems in wet method such as slurry defoaming and viscosity control need to be solved. The uniformity problem in dry method as well as the adaptation of new materials to the existing processing equipment all need to be continuously verified.4) Theoretical calculation and process production are closely integrated and mutually feedback, providing a more accurate and efficient path for the development of solid electrolytes.In general, there is still a long way to go to obtain a solid electrolyte film with ultra-thin, excellent electrochemical performance, stable interface, low cost and easy mass production. The progress of the process is also inseparable from the development of novel materials and the update of production equipment. It is thus necessary to integrate material research and development, process design, production and processing interactive interconnection, and truly achieve the combination of production and learning to grasp the initiative of all-solid-state batteries as soon as possible.

The rapid expansion of lithium-ion batteries in consumer electronics, electric vehicles, and energy storage systems escalates a demand for batteries with higher energy density and enhanced safety. However, conventional lithium-ion batteries employing liquid electrolytes are increasingly constrained due to their limited electrochemical stability, susceptibility to leakage, and lithium dendrite formation, which pose risks of short circuits and thermal runaway. Solid-state batteries (SSBs) with solid electrolytes present a promising solution to these challenges, offering a potential for higher safety and energy density.Recent advancements in SSBs include the development of solid electrolytes with a high ionic conductivity, solid–solid interface optimization, and composite electrode design. Nonetheless, some issues such as lithium-ion transport barriers across phases, interfacial impedance, and the performance limitations of thick electrodes continue to impede the commercial viability of SSBs. To address these multifaceted challenges, a concept of "lithium-ion transport throughput" is proposed as a comprehensive descriptor for evaluating SSB performance. This descriptor quantifies the quantity of lithium ions transported across the electrode/electrolyte interface per unit area over time, considering some factors such as areal capacity and charge/discharge rates.This review systematically examines strategies to enhance lithium-ion transport throughput from three key perspectives, i.e., bulk ionic transport in solid electrolytes, electrode/electrolyte interface design, and synergistic ionic/electronic transport networks within electrodes. we aim to offer insights into improving the overall performance of SSBs to meet the demands for high safety and energy density in next-generation energy storage systems via integrating materials design with structural optimization. In addition, we also conduct quantitative calculations to analyze lithium-ion transport throughput in recently published high-performance SSBs, providing a detailed comparison of various systems. We illustrate how material advancements and interface designs affect overall performance via compiling representative data (i.e., areal capacity, charge/discharge rates, and resulting throughput values). These calculations highlight trends and identify strategies that can achieve significant throughput improvements, offering a robust foundation for future optimization efforts.Summary and ProspectsThe introduction of lithium-ion transport throughput offers a novel and integrated approach to assess the charging and discharging capabilities of SSBs. Lithium-ion transport throughput provides a holistic understanding of the electrochemical processes within SSBs via considering both areal capacity and current density, bridging a gap between theoretical performance and practical application. Future studies should focus on refining this descriptor and employing it as a standard for evaluating SSB performance across various material systems.The ionic conductivity of solid electrolytes remains a key property of SSB performance. Recent advancements, such as high-entropy doping, amorphous structures, and vacancy engineering, achieve significant improvements in ionic transport. For instance, high-entropy sulfide electrolytes exhibit ionic conductivities, compared to liquid electrolytes. The development of halide electrolytes shows their high voltage stability and ionic conductivities.Interface impedance is a critical barrier in SSBs. Conventional solid–solid contacts restrict ionic transport efficiency. Innovations such as mixed-conductive interlayers, magnetron sputtering techniques, and porous or functionalized interface layers demonstrate effectiveness in reducing interface resistance and enhancing long-term stability. Future research should emphasize scalable techniques for interface engineering to ensure compatibility with large-scale production.High-loading electrode designs are essential for improving energy density, but often face ionic and electronic transport challenges. Constructing electronic and ion dual-transport networks within electrodes has a potential to address these issues. Some strategies such as integrating conductive nanomaterials and designing vertically aligned structures can optimize the utilization of active materials and improve high-rate performance. The integration of lithium alloy layers in anodes also offers an approach to enhance lithium-ion transport and address volume change issues during cycling.The development of solid-state batteries (SSBs) requires a multidisciplinary approach that combines materials science, interface engineering, and structural design. One key research priority is material innovation, which involves developing solid electrolytes with higher ionic conductivities and lower costs. Another priority is to achieve long-term stability and low impedance at solid-solid interfaces, which can be accomplished through techniques such as surface coating, interfacial buffer layers, and in-situ formation of interfacial layers. Structural optimization is also crucial, as it involves designing thick electrode architectures with optimized ionic and electronic transport pathways. Finally, system-level integration is essential, as it requires attention to thermal management, mechanical integrity, and scalability. Collaborative efforts between academia and industry are vital to accelerate the transition from laboratory-scale innovations to commercial products.SSBs can overcome current limitations and be widely used in electric vehicles and grid-scale energy storage via addressing the challenges above. The ongoing development of SSB technology is crucial for promoting energy sustainability and reaching global carbon neutrality goals.

With the rapid advancement of portable electronic devices, electric vehicles, and large-scale energy storage grids, next-generation rechargeable batteries with higher energy densities and enhanced safety have attracted much attention. Solid-state lithium metal batteries with high-safety solid-state electrolytes and lithium metal at the maximum specific capacity as an anode hold a tremendous potential for the next generation of rechargeable batteries. As a critical component of solid-state lithium metal batteries, solid-state electrolytes play a pivotal role in advancing their development. The existing solid-state electrolyte materials (i.e., organic polymer electrolytes, oxide electrolytes, sulfide electrolytes, and halide electrolytes) are systematically investigated, with the oxide solid-state electrolytes standing out due to their excellent overall performance. To achieve the practical application of oxide solid-state electrolytes, substantial research efforts are directed at modifying ceramic grain boundaries to improve ionic conductivity and mechanical properties and reduce electronic conductivity. Also, research on ceramic films is equally important to achieve a high energy density.The sintering process of ceramics is a critical factor affecting material properties, especially for the improvement of grain boundary. Relative density, grain boundary strength, and ionic/electronic conductivity are dominant factors impacting the long-term stability of the battery, with relative density as the most fundamental performance of ceramic electrolyte that enables other characteristics to be manifested effectively. The regulation of the preparation process, including powder preparation method, ceramic sintering temperature, and atmosphere, can effectively enhance the ceramic relative density. The powder preparation route, in particular, has a considerable effect on the final properties of LLZO ceramic, such as grain size and size distribution, relative density, microstructure, and ionic/electronic conductivity. In the sintering, some parameters such as sintering temperature, dwelling time, pressure, and the lithium oxide atmosphere play a crucial role in promoting the densification of LLZO ceramic.Atmospheric sintering is still the most common, simple, and low-cost method to prepare LLZO ceramics although it has a longer sintering time and a higher sintering temperature, compared to the pressurized sintering. The prolonged high-temperature process leads to lithium loss and the formation of impurity phases like La2Zr2O7 in the electrolyte, which is detrimental to reducing the densification and ionic conduction behavior of the ceramics. To promote the densification and lower the sintering temperature, various kinds of sintering aids are adopted to assist the sintering process. A variety of low melting point additives, such as Li3BO3, LiO2–B2O3–SiO2–CaO–Al2O3, etc., are used as ceramic sintering aids. Subsequently, optimization of sintering processes and sintering aids are emerging, such as embedding-sintering, atmosphere-reversible control additives, lithium source-green body separation sintering, endogenous atmosphere sintering aids, etc.. LLZO ceramics sintered in the endogenous atmosphere with sintering additive of Li6Zr2O7 can realize the densities of up to 97.21% in the absence of mother powder and under atmospheric pressure.In highly densified LLZO ceramics, dendrite growth and even piercing of the ceramic sheet are still inevitable, indicating that dendrite growth is related to ceramic relative density and ionic conductivity and is also closely related to the electronic conductivity and fracture toughness of ceramics. Multifunctional additives are reported to regulate the compositions, microstructures, as well as physical and electrochemical properties at the internal grain boundaries of LLZO ceramic in addition to improving the relative density and ionic conductivity of the ceramics. Among them, (Li2O)0.733(ZrO2)0.267, Li6Zr2O7, Li2WO4, Li2CuO2, LaTiO3, etc., are considered as effective sintering additives.To realize the high energy density of solid-state lithium metal batteries, thin-filming of ceramic electrolyte has attracted recent attention. Since the reduction of the thickness of LLZO ceramic leads to a decrease in the mechanical properties of the ceramics, a balance between the thickness and strength of the electrolyte is of great significance. Realizing the thin-film of LLZO ceramic through rational structural design is crucial to achieving a high energy density of solid-state batteries. In this case, ceramic films with different structures, such as single-layer dense ceramic, multilayer ceramic, and single-layer porous ceramic, are reported. A lithium-metal battery based on an interface-optimized 74 m-thick single-layer dense LLZO ceramic has the excellent long-cycle stability of more than 800 cycles. A lithium-symmetric battery based on an optimized 115 m-thick multilayer ceramic exhibits an ultra-high critical current density of 100 mA·cm–2. A 12 m-thick single-layer porous ceramic composited with a polymer electrolyte is expected to achieve energy densities of greater than 350 W·h·kg–1. In addition, the quality of the green body (i.e., uniformity, flatness, and residual stress) and the sintering regime have a significant effect on the phase purity, ionic conductivity, surface state, and film flatness of LLZO after sintering due to the high sintering temperature of LLZO as well as the volatile properties of Li, La, Zr, and O in the composition. Therefore, some novel preparation processes to prepare ceramic films are developed in recent years. In particular, the combination of tape-casting and ultrafast-sintering has a promising application in the preparation of LLZO ceramic films.Summary and prospectsAlthough significant progress is made in the development of garnet-type solid-state electrolytes, their practical application in all-solid-state lithium batteries still faces challenges. Future research should focus on some key issues, i.e., the application of artificial intelligence and high-throughput computing in ceramic sintering additives, the development of new preparation methods for ceramic thin films, and the preparation, evaluation, and analysis of the full battery. Moreover, it is essential for the practical application of ceramic-based solid-state lithium metal batteries to simultaneously adapt testing standards, characterization techniques and failure analysis methods.

Garnet-type solid electrolytes are the most promising materials available, which attract much attention, due to their high ionic conductivity, wide electrochemical window, and stability with lithium. However, some challenges such as surface stability, lithium dendrite penetration, and cost remain critical for its industrialization. This review focuses on the crystal structure and ion conduction mechanisms of garnet-type oxide solid electrolyte (Li7La3Zr2O12, LLZO) to investigate the formation mechanisms and removal strategies for surface passivation layers, including lithium carbonate and lithium hydroxide. For the lithium dendrite penetration of LLZO, potential solutions are explored in view of the black and surface properties electrolyte. For the industrialization, this review analyzes the cost optimization strategies for garnet-type solid electrolytes, aiming to provide valuable insights for industrial advancement.Summary and prospectsGarnet-type oxide solid electrolytes hold a significant promise for solid-state batteries due to their high ionic conductivity and wide electrochemical window. However, large-scale production faces several challenges, i.e., 1) poor stability against air leading to the formation of lithium carbonate, 2) preventing the growth of lithium dendrites at the anode interface, and 3) high costs. These issues hinder the application of garnet-type all-solid-state batteries. Future development in garnet oxide solid electrolyte should focus on the following aspects. First, the surface alkalinity and the mechanism of lithium carbonate formation should be characterized accurately. It is crucial to contrude the excessive addition of lithium raw materials. While excess lithium can compensate for lithium losses during the heating and lead to lithium accumulation on the surface. Thus, a dynamic equilibrium must be established between the lithium supplement during sintering and the residual lithium on the surface. Furthermore, the formation of lithium carbonate is a dynamic process. Removing lithium carbonate is thus insufficient. Instead, it is indicated to convert lithium carbonate into a protective barrier that is both conductive to lithium ions and stable in air. Second, previous work proposed to mitigate the growth of lithium dendrites at LLZO/Li interface via incorporating the artificial solid electrolyte interphases (SEI) or buffer layers, which could facilitate a uniform electric field distribution and promote even lithium deposition at the interface. Moreover, the fabrication of highly densified LLZO represents an idea approach to alleviate the lithium dendrite growth. Third, , rare-earth elements are widely used for doping to improve the ionic conductivity, which increases the cost of raw materials. The existing low-cost element doping, such as Si, Fe, Ca and W, are developed. In addition, developing a low-carbon sintering method is also an important way to reduce the cost. All-solid-state lithium batteries show a great potential with the continuous research efforts.

Lithium batteries are widely and profoundly applied in different fields (i.e., portable electronic devices and electric vehicles) due to their high energy density and environmental friendliness. However, high-capacity electrode materials become a key to the development of the next generation of high-energy lithium batteries with the increasing demand for extended driving ranges in new energy electric vehicles. The chemical environment in these next-generation high-energy lithium batteries is complex, with intensified electrode/electrolyte interfacial reactions. Among these challenges, some issues such as the volume expansion of high-capacity anode materials and severe side reactions at the interface have a significant negative impact on the cycle life and safety of the battery. Recent studies reveal the presence of significant amounts of lithium hydride (LiH) in the anode of lithium batteries after cycling. However, there is a considerable debate regarding the existence of LiH and its underlying mechanisms. The formation and evolution of LiH, as well as its role in inducing anode failure, remain major research gaps. This review summarizes the fundamental physicochemical properties of LiH based on the existing literature and systematically represents the research work on lithium hydride in non-lithium metal anodes, lithium metal anodes, and non-lithium battery anodes. Furthermore, this review discusses the mechanisms by which LiH induces anode failure and protection, to provide a targeted guidance for the optimization and improvement of high-capacity anode materials, interfaces, and electrolytes, thus facilitating the commercialization of the next generation of high-energy lithium batteries.This review firstly introduced the fundamental physicochemical properties of LiH. As a hydride of metallic lithium, LiH is the lightest ionic compound in nature and exhibits strong alkalinity. Furthermore, this review summarizes the conventional synthesis methods for LiH and the conventional chemical reactions in which it can participate. These related chemical reactions can provide valuable insights and considerations for research on LiH in battery anodes.From the ongoing advancement in the understanding of the interfacial chemistry of battery anodes and advanced characterization techniques, the presence of LiH is confirmed in both non-lithium metal anodes (i.e., graphite, germanium, and silicon) and lithium metal anodes, which serves as a new component of the anode solid electrolyte interphase (SEI) film. However, the existing research mainly focuses on confirming the existence of LiH, while the distribution of LiH in the anode surface/interface or bulk phase, its formation and evolution mechanisms, and its effects on different anode materials remain unclear. In addition to the presence of LiH in lithium battery anodes, related hydrides (such as sodium hydride (NaH), magnesium hydride (MgH2), etc.) are also identified in non-lithium battery anodes. The formation and decomposition of these hydrides can have significant effects on the performance of the anode materials and even the overall battery performance.Summary and prospectsHigh-capacity anode materials are a preferred option for the development of the next generation of high-energy lithium batteries. However, some issues such as the volume expansion of high-capacity anode materials and severe side reactions at the interface significantly hinder their further development. The discovery of LiH on the anode provides a perspective for investigating the problems related to anode materials and interfacial failure. However, there remains considerable controversy due to the limited scope of the existing research. Firstly, most studies on the physicochemical properties of LiH focus on bulk particles (bulk-LiH), whereas what is generated at the battery anode interface is predominantly in the form of nanoparticles (nano-LiH). It is thus crucial to fully understand the nanoparticle effects of nano-LiH. Also, there is a need for in-depth studies on the formation and decomposition mechanisms of LiH on different anode materials, as well as the various effects and mechanisms by which LiH interacts with these materials. It is important to investigate the reactivity of the nano-sized lithium hydride formed at the anode with various components of the battery, as well as its correlation with battery failure phenomena. A clarification is needed to determine whether LiH accelerates battery failure or failure issues trigger the formation of LiH. Research on these issues can deepen the understanding of LiH and provide valuable insights for the study of hydrides in other battery anodes. Furthermore, the research will offer some targeted strategies for optimizing and improving high-capacity anode materials, interfaces, and electrolytes.

All-solid-state lithium batteries (ASSLBs) emerge as a pivotal direction for next-generation energy storage systems due to their high energy density, intrinsic safety, and extended cycle life. Lithium-rich manganese-based layered oxides (LLOs) with their exceptional specific capacity (i.e., >300 mA·h/g) and cost-effectiveness are regarded as a promising cathode candidate for high-energy-density ASSLBs. However, critical challenges such as poor solid-solid interfacial contact between LLOs and solid electrolytes, irreversible lattice oxygen loss, and interfacial side reactions hinder their practical implementation. This review comprehensively analyzes the structural characteristics of LLOs, the anionic oxygen redox (OAR) mechanism, and interfacial challenges in ASSLBs, while systematically summarizing modification strategies across sulfide-, halide-, oxide-, and polymer-based solid electrolyte systems.The high capacity of LLOs primarily originates from OAR, where reversible O2-/O- redox contributes to extra capacity. However, oxygen release and transition metal (TM) migration lead to voltage decay and structural degradation. To address these issues, gradient doping and surface coating are developed to stabilize lattice oxygen and suppress phase separation. Compared to polycrystalline materials, single-crystal LLOs exhibit superior mechanical stability and interfacial contact, effectively mitigating crack propagation caused by volume changes.In sulfide-based systems, the space charge layer (SCL) effect and sulfur oxidation at high voltages are the main limiting factors. Strategies such as LLOs surface sulfurization, uniform dispersion by liquid-phase mixing, and functional coatings can effectively reduce the interfacial resistance and enhance the OAR reversibility. For halide electrolytes, the introduction of carbon additives and ion-conductive coatings can establish a continuous conductive transport network. Oxide-based systems benefit from co-sintering LLO with garnet-type Li7La3Zr2O12 and Li3BO3 sintering aids to improve interfacial densification, although Mn/La interdiffusion in co-sintering requires further attention. Polymer electrolytes, especially those formed by in-situ polymerization of propane sultone-based materials, are able to form a thin and uniform cathode-electrolyte interface (CEI), thereby widening the electrochemical stability window.A critical finding across these systems is the importance of mechanical-electrochemical coupling at interfaces. "Soft-contact" interfaces with flexible ion-conductive layers are essential to accommodate volume changes. Halide electrolytes have a unique compatibility with LLOs via minimizing SCL effects, while sulfide systems demand a precise control of oxidation-prone components to prevent degradation.Summary and prospectsFuture research should prioritize advanced characterization techniques to elucidate dynamic structural evolution and interfacial degradation pathways during OAR. Material optimization, such as designing Co-free LLOs with gradient TM distribution and single-crystal morphology, can enhance intrinsic Li+/electronic conductivity and structural integrity. Innovations in electrolytes, including hybrid organic-inorganic composites or high-entropy sulfides can balance ionic conductivity (32 mS/cm) and interfacial stability. Scaling up production processes for sulfide/halide electrolytes and addressing their moisture sensitivity are also crucial steps toward commercialization. In summary, interfacial stability remains a cornerstone for high-performance LLOs-based ASSLBs. ASSLBs are poised to achieve energy densities of exceeding 1000 W·h/kg via synergizing material design, interface engineering, and mechanistic understanding, paving a way for their application in electric vehicles and grid-scale storage.

With the rapid advancement of electric vehicles, energy storage systems, and consumer electronics, a demand for high-performance lithium-ion batteries (LIBs) is escalating. The electrolyte plays a pivotal role in affecting the battery performance, safety, and longevity. Liquid electrolytes, which are widely used in commercial LIBs, offer high ionic conductivity and ease of processing. However, solid electrolyte interphase (SEI) formed in liquid electrolyte systems degrades during repeated charge–discharge cycles, increasing impedance, reducing the Coulombic efficiency, and compromising cycling stability. In contrast, solid-state electrolytes, known for their potential to enhance battery safety, face some challenges such as lower ion conductivity, poor mechanical properties, and difficulties in maintaining stable electrode interfaces under operational conditions.The electrolyte-electrode interface, particularly at anode, is crucial for lithium battery performance. Lithium dendrite growth is a primary concern as it can cause short circuits and battery failure. In liquid electrolyte systems, dendrite formation is accelerated by uneven ion flow, SEI instability, and excessive current density. Solid-state electrolytes, while promising in terms of theoretical safety, also face some challenges, i.e., slower ion conductivity at room temperature and poor interface stability with lithium metal anodes. A better understanding of these differences is crucial for advancing electrolyte design.One of the key challenges for liquid electrolytes is to control SEI formation. The chemical composition and morphology of the SEI directly impact the battery performance. A thick or uneven SEI layer increases internal resistance and slows ion flow, thereby reducing the Coulombic efficiency and cycle stability. In solid-state systems, the solid-solid interface between the electrolyte and electrode material leads to a high interfacial resistance, restricting ion migration and affecting charge-discharge efficiency. Ion transport in solid-state electrolytes is also more uneven and slower, further compromising safety. While SEI formation in liquid electrolytes can suppress dendrite growth and promote uniform lithium deposition, solid-state electrolytes tend to exhibit a higher interfacial resistance, which accelerates dendrite growth and leads to short circuits. These distinct interface properties are the main cause of the performance gap between liquid and solid-state batteries.Optimizing interface stability and ion flow uniformity in both liquid and solid-state electrolytes is essential for improving battery technology. In this review, the characteristics of the anode interface in both systems are compared, with a focus on lithium dendrite growth mechanisms. Some factors like ion conductivity, electrode-electrolyte interactions, and interface stability that affect electrochemical performance are mentioned. This review also analyzes strategies to optimize electrolyte interface stability and ion flow uniformity in both liquid-state and solid-state systems. Electrolyte design strategies based on ion flow regulation to enhance ion conductivity, reduce interfacial resistance, and ensure more uniform ion distribution are proposed. These strategies include the development of advanced electrolyte materials, interface engineering to improve ion transport, and the use of hybrid systems that combine the advantages of both liquid and solid electrolytes.Summary and prospectsSolid-state and liquid-state electrolytes exhibit significant differences, particularly in ionic conductivity and interface stability. While liquid electrolytes with their higher fluidity can quickly respond to ion flow, they face some challenges such as flammability and side reactions at the interface, which compromise battery safety. Solid-state electrolytes with their theoretically higher safety have attracted significant attention, but their practical application is limited due to the problems such as bulk inhomogeneity leading to uneven ionic conductivity and unstable electrode interfaces. Moreover, the mechanical rigidity of solid materials results in poor interface contact and stress accumulation during cycling, which further degrades battery performance and reduces cycle life. Achieving uniform ion flow is crucial for the stable cycling and optimal performance of batteries. Optimizing the ion mobility between the electrolyte and electrode is essential for improving efficiency and extending cycle life, regardless of whether the electrolyte is solid or liquid. Design strategies based on ion flow regulation can enhance electrolyte ionic conductivity via optimizing microstructure, advancing interface engineering, and developing composite electrolyte systems. These strategies improve ion mobility, reduce interface impedance, and enhance the beneficial feedback of the interface, leading to better cycling performance and stability. In addition, the rapid growth of machine learning and big data technologies also presents a transformative opportunity for novel electrolyte materials development. Moving forward, as battery technology progresses, electrolyte optimization will focus on improving the ionic conductivity of the electrolytes themselves and optimizing the entire battery system. In commercial applications, reducing failure rates to minimal levels is essential where safety standards are extremely stringent. To meet this, enhancing the ion flow and interface stability of electrolytes will be crucial for developing high-safety, long-lifetime batteries. Moreover, integrating machine learning and big data into electrolyte optimization will drive breakthroughs in battery performance, supporting the commercialization of next-generation high-performance batteries.

The global transition toward renewable energy and electrified transportation has intensified the demand for advanced energy storage systems with high energy density, intrinsic safety, and long-term durability. Conventional lithium-ion batteries (LIBs) in the market face critical limitations, i.e., organic liquid electrolytes are flammable, posing significant safety risks (i.e., thermal runaway and combustion), and their energy density is approaching theoretical limits (i.e., ~300 W·h·kg–1). These shortcomings hinder their applicability in emerging technologies such as electric vehicles (EVs), grid-scale storage, and high-power portable electronics. All-solid-state batteries (ASSBs), which replace liquid electrolytes with solid-state electrolytes (SSEs), offer a revolutionary solution. SSEs eliminate flammability risks, enable the use of lithium metal anodes (i.e., theoretical capacity of 3860 mA·h·g–1), and facilitate compact cell designs through bipolar stacking. Among SSE candidates, halide-based electrolytes emerge as a game-changing material class due to their unique advantages like ultrahigh ionic conductivity, high voltage compatibility, mechanical adaptability, and scalable synthesis. The urgency to develop halide SSEs is further underscored due to the growing need for safe, high-energy-density batteries in critical applications. This article provides a comprehensive and updated review of the classification, structure, synthesis methods, air stability, mechanical stability, and applications of halide electrolytes at both cathode and anode interfaces.Structural design is pivotal in developing superionic conductors as it serves as a primary determinant of ionic conductivity. Effective strategies to enhance lithium-ion conductivity include regulating the concentration and rational occupation of lithium ions and lithium vacancies within the electrolyte. Halide-based electrolytes exhibit an exceptional structural tunability, providing a foundation for designing novel electrolytes with optimized properties. Among these strategies, elemental doping stands out as one of the most common approaches to improve ionic conductivity in halide electrolytes. In recent years, high-entropy materials, characterized by their adjustable chemical compositions and unique properties, indicate widespread applications in various fields. Introducing high-entropy strategies into SSEs can effectively boost ionic conductivity through chemical disorder and local lattice distortions, while reducing reliance on rare-earth elements. In addition, amorphous SSEs have attracted much attention due to their outstanding mechanical flexibility and rapid ion transport capabilities, which arise from the decoupling of charge carriers from the supporting matrix. This review also highlights the synthesis methods for halide-based electrolytes. Simple, cost-effective, and environmentally friendly synthetic routes are crucial for advancing the industrialization of these materials. The existing approaches primarily involve solid-state, liquid-phase, and vapor-phase synthesis techniques, each of which significantly affects the resulting electrolyte structure and ionic conductivity.Air stability is crucial for SSEs, impacting their entire lifecycle from synthesis to application. Halide-based electrolytes, derived from hygroscopic halide salts, are prone to moisture-induced degradation during synthesis and storage. To enhance their air stability, some methods like surface coating and structural modification are employed. For instance, creating physical barriers, superhydrophobic membranes, or doping with In3+ can significantly improve moisture resistance. In ASSBs, inorganic SSEs must have good electrochemical properties and excellent mechanical properties. Bulk modification as a common approach to improving halide-based electrolytes performance also boosts mechanical stability. For instance, O2– doping can disrupt the crystal structure of electrolyte, making it more ductile. In addition, adjusting the particle size of halide-based electrolytes and combining them with binders to form organic-inorganic composite membranes can also enhance their mechanical properties.As a component in direct contact with the positive and negative electrodes, understanding the electrochemical stability and interfacial reactions of halide SSEs is crucial for achieving high-energy-density and long-cycle-life halide-based ASSBs. For high-voltage cathodes, halide SSEs are among the most promising. The oxidation potential of halide SSEs mainly depends on the halogen anion, especially for chlorine- and fluorine-based halides, whose high electronegativity prevents oxidation at battery operating voltages. To further enhance the cathode interface stability of halide SSEs, element doping can broaden the thermodynamic electrochemical window or form a stable cathode-electrolyte interphase (CEI) in-situ. Structural control, like reducing the SSE's Young's modulus and particle size or coating the SSE on the cathode material surface to improve solid-to-solid contact in composite cathodes, is also effective. The instability of halide SSEs with metal lithium mainly stems from the transition metals in the halide. The high potential difference between halide SSEs and lithium metal makes direct contact with lithium or lithium alloy anodes extremely unstable. This leads to high-resistance interfacial layers that impede Li+ diffusion and cause SSE degradation, triggering lithium dendrite growth. To stabilize the halide SSE/anode interface, constructing interfacial buffer layers, optimizing SSE structure, and developing lithium alloy anodes are essential strategies.Summary and ProspectsAlthough halide electrolytes demonstrate remarkable advantages, their practical implementation in ASSBs systems still faces significant challenges. To overcome these barriers, future research must prioritize the large-scale production of highly stable, cost-effective halide electrolytes with superior ionic conductivity，while exploring composite electrolyte architectures that synergistically combine the strengths of halides with complementary materials. Advanced characterization techniques should be systematically employed to elucidate structure-property relationships and interfacial dynamics, guiding the rational design of batteries with enhanced safety, ultrahigh energy density, and extended cycle life. These efforts will accelerate the commercialization of ASSBs technologies, addressing the growing demands for clean energy solutions in electric mobility, grid storage, and next-generation electronics.

Compared to conventional commercial lithium-ion batteries, all-solid-state lithium batteries (ASSLBs) with inflammable inorganic solid electrolytes (SEs) can elevate energy density via choosing the combination of high output voltage and electrodes with a high capacity (i.e., high-nickel ternary cathode and lithium metal anode) and ensure safety. It is crucial to use solid electrolytes with a good electrode compatibility and a high ion conductivity to realize stable cycling at room temperature. Chloride SEs have been developed rapidly in recent years since their preparation in 2018 due to the success of Li3YCl6-based InLi/LiCoO2 ASSLBs without extra interface modifications. This review highlights the excellent cathode compatibility of chloride SEs, which is the most attractive advantage of these SEs. This review also discusses that the easy reduction of central elements causes an unstable interface towards an anode. For the design of chloride SEs, the non-closed packing anion sublattice to enable faster ion transport with a lower migration barrier is described. Finally, this review summarizes the typical annual progress of chloride SEs in the past five years and discusses some challenges to be addressed for the future large-scale application of chloride SEs.Summary and prospectsThe advances and limitations of chloride SEs from electrode compatibility and ionic conductivity perspectives are analyzed. To better understand recent development of chloride SEs and inspire further explorations and improvements, a typical annual progress is summarized. Chloride SEs were brought back into the spotlight in 2018 due to the stable cycling of uncoated 4 V class cathode LiCoO2 enabled by Li3YCl6 at room temperature. In 2019, a facile and scalable water-mediated synthetic route based on Li3InCl6 was developed to overcome the shortcomings of traditional time-consuming mechanical methods. AIMD simulations on LiScCl3+x is carried out to systematically explore the influence of lithium content on the structure and Li+ diffusivity. Note that the above-mentioned chloride SEs (i.e., Li3YCl6, Li3InCl6 and LixScCl3+x) all use expensive and low-abundance rare-earth elements, which cause high cost and sustainability concerns. Cost-effective Li2ZrCl6 with cheap Zr as a central element is developed. As a breakthrough to the close packing anion sublattice of conventional chloride SEs in 2023, a non-close packing style for superionic conduction was realized via selecting large central cation, i.e., LaCl3-based SE, or employing partial anion replacement, i.e., LiNbOCl4. The non-close packing anion sublattice with more distorted sites for a low ion migration barrier can theoretically or experimentally realize a high room-temperature ionic conductivity of over 10 mS/cm, compared to those sulfide superionic conductors.Although a remarkable progress is made on chloride SEs, several crucial challenges have yet to be resolved. The first is still the insufficient interface stability towards the anode. LaCl3-based SE and Li3YBr5.7F0.3 can enable the cycling of all-solid-state lithium metal battery without extra interface modifications. The interface stabilization mechanisms are still not thoroughly identified. Meanwhile, the 100 cycles and capacity of around 1 mA·h/cm2 are insufficient to meet the demand of practical applications. A deeper understanding of interface evolution, accompanied by an artificial interfacial layer to enhance anode interface stability, is needed. The second is that rare and expensive elements like In, Ta, and Sc are still mostly used for high-performance chloride SEs despite Zr-based chloride SEs. It is important for sustainability perspectives and large-scale production to apply cheap and high-abundance elements like Mg, Ca, Al to construct an ion conductive framework and ensure high ionic conductivity and electrode compatibility. The third is that the atmosphere tolerance of chloride SEs needs enhancement to restrain the ionic conductivity loss during ASSLB fabrication due to their easy reaction or combination with water. It is anticipated that chloride SEs can be pushed from laboratory study towards practical applications via taking the intrinsic advantages of chloride SEs and overcoming their shortcomings.

Solid-state batteries (SSBs) are a promising next-generation secondary battery due to their potential for high energy density and enhanced safety, offering solutions to the problems inherent in conventional lithium-ion batteries (LIBs) with organic liquid electrolytes (i.e., flammability, corrosion susceptibility, and high-voltage instability). In the construction of SSBs, the selection of the cathode materials is critical to achieving a high energy density, particularly when coupled with lithium metal anodes. However, conventional cathodes often affect the energy density of SSBs due to their constrained specific capacities. It is thus crucial for achieving substantial improvements in the energy density of SSBs to develop high specific capacity cathodes. The Li-rich Mn-based layered oxide (LRMO) is a promising cathode material for SSBs for energy densities of above 600 W·h·kg–1 due to their high discharge specific capacities. Furthermore, LRMO cathodes offer some additional advantages, i.e., reduction of Co and Ni content, leverage of the abundance of Mn to achieve lower materials costs and improved safety. Their application in SSBs also mitigates the dissolution of TM-ions into the electrolyte, thereby enhancing the structural stability and capacity retention during long-term cycling progress. In addition, the resource-efficient composition of LRMO cathodes also align with environmentally friendly and sustainable development goals.This review represents that the LRMO cathode materials are characterized by a composite crystal structure comprising two key components, i.e., Li2MnO3 phase and LiMO2(M=Mn, Ni, Co) phase. Li2MnO3 phase can be considered as a superlattice-structured variant of LiMO2, formulated as Li[LixMn1-x]O2. This superlattice-structured introduces unique unhybridized O 2p states, arising from Li—O—Li configurations. These unique oxygen states enable the participation of oxygen in charge compensation processes. Consequently, the high capacity in LRMO cathodes is attributed to the synergistic contributions of both TM cations and oxygen redox reactions.LRMO cathodes, while exhibiting a distinctive biphasic structure, encounter significantly some challenges in SSBs. Specifically, the application of LRMO cathodes in SSBs is hindered by two primary issues. Firstly, the inherent incompatibility between Li2MnO3 phase and SEs interfaces results in sluggish reaction kinetics, severely restricting the activation of oxygen redox activity and consequently reducing the associated capacity contribution. Secondly, a chemical potential mismatch between SEs and LRMO cathodes drives spontaneous reactions at the composite cathode interfaces. These reactions lead to the formation of mixed ionic/electronic conductive CEI. Furthermore, irreversible oxygen escape further oxidizes the SEs interface, generating the passivation layers. These passivation layers increase interfacial impedance and imped ion transport, ultimately hindering practical advancements in SSBs technology.LRMO cathodes hold a significant promise for SSBs, as evidenced by research progress across various SEs, including sulfides, halides, polymers, and oxides. To fully realize this potential, some strategies addressing the inherent incompatibility between LRMO cathodes and SEs are crucial. These strategies encompass bulk/ interfacial structure design, nanostructured particle engineering, and the construction of stable Li+/e– transport pathways. These approaches can suppress oxygen escape, enhance the high-voltage stability of solid-solid interfaces, and ultimately stabilize oxygen redox while optimizing interfacial dynamics. Consequently, the implementation of these strategies leads to a significant enhancement in the electrochemical performance of LRMO-based SSBs.Summary and prospectsLRMO cathodes have attracted considerable attention for SSBs due to their high discharge specific capacity and energy density. Advancements in SSBs utilizing sulfide, halide, polymer, and oxide SEs demonstrate a potential of LRMO cathodes to overcome limitations currently hindering their industrial applications in liquid electrolyte systems. These limitations include gas evolution, TM dissolution, and voltage decay. However, the practical application of LRMO cathodes in SSBs faces some challenges stemming from their inherent properties, such as poor electronic conductivity attributed to their biphasic structure, sluggish interfacial charge transfer kinetics, oxygen escape, high-voltage interfacial instability, and electrochemical-mechanical degradation. Consequently, a comprehensive understanding of failure mechanisms and the development of advanced modification strategies for LRMO cathodes in SSBs are urgently needed. This necessitates several key research directions. Firstly, optimizing large-scale synthesis techniques for single-crystal LRMO cathodes is crucial, coupled with systematic investigation into their degradation mechanisms within SSBs. Such studies should elucidate the complex interplay of mechanical, electrical, and chemical coupling within SSBs. Secondly, the development of zero-strain LRMO cathodes, designed to maintain structural integrity with minimal volume changes during cycling, can effectively mitigate mechanical stress, suppress crack formation (both intergranular and intragranular), and significantly improve long-term cycling stability. Furthermore, machine learning-driven multiscale modeling offers an effective tool for the rational design of bulk/interfacial structures, facilitating superior compatibility and high-voltage stability at the solid-solid interfaces. Finally, the exploration of high-voltage-tolerant SEs specifically tailored for LRMO cathodes, alongside innovations in scalable fabrication processes for ultrathin electrolyte membranes and electrode films, is essential. The synergistic convergence of materials innovation, interfacial engineering and scalable manufacturing offers a transformative potential for realizing the full capabilities of LRMO cathodes. This convergence is crucial for advancing SSBs toward unprecedented levels of energy density, reliability and sustainability. Specifically, these combined efforts will facilitate the production of large-format batteries at the A·h-level, ultimately enabling the large-scale commercialization of SSBs incorporating LRMO cathodes.