Latest ArticlesThe 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.
The 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.
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., 3860 mA·h/g) and a minimum electrochemical potential (i.e., −3.04 V 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.
One 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.
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 μm-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 μm-thick multilayer ceramic exhibits an ultra-high critical current density of 100 mA·cm-2. A 12 μm-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.
Although 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.
Garnet-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.
High-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.
Future 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.
Solid-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.
Although 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.
The 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.
LRMO 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.
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