Halide solid electrolytes: composition tuning, structural design, and performance optimization for all-solid-state lithium batteries

Abstract

Halide solid electrolytes have become key candidate materials for all-solid-state lithium batteries (ASSLBs) due to their high ionic conductivity, wide electrochemical window, and good compatibility with high-voltage cathodes. This paper reviews their material chemistry, crystal structure, synthesis, and characterization methods, with a focus on analyzing the composition–structure–performance relationship, and explores the effects of halogen types and their mixtures on ion transport and stability. Meanwhile, it analyzes the challenges at the cathode/anode interface and summarizes strategies such as element doping, interface engineering, and mechanical performance and moisture resistance optimization. Despite significant progress in the laboratory, practical applications are still constrained by issues such as high cost, moisture sensitivity, and unstable interfaces with lithium metal. In the future, efforts should be focused on low-cost alternatives, improved moisture resistance, and multi-functional interface design, combined with high-throughput computing and advanced characterization, to accelerate the industrialization process in high-energy-density ASSLBs.

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1 Introduction

With the increasing demand for high-energy density and safe energy storage, all-solid-state lithium batteries (ASSLBs) have received extensive attention due to their high safety and potential for high energy density.^1,2 Liquid organic electrolytes used in traditional lithium-ion batteries have safety hazards such as flammability and leakage, and their electrochemical windows are limited, which restricts the application of high-voltage cathode materials.^3,4 In contrast, ASSLBs use non-flammable inorganic solid electrolytes, which significantly enhance safety and are expected to achieve higher energy density.⁵ Among various solid electrolytes, halogen-based solid electrolytes stand out due to their high ionic conductivity (such as over 10⁻² S cm⁻¹), wide electrochemical window (up to over 4 V), and good compatibility with high-voltage cathode materials.⁶ In recent years, representative halides like Li₃YCl₆ and Li₃InCl₆ have demonstrated outstanding comprehensive performance.⁶ Halide solid electrolytes (HSEs) have become auspicious materials due to their high ionic conductivity, excellent oxidation stability, and compatibility with high-voltage cathode materials.⁷ Their conductivity and stability can be further optimized through strategies, such as cation/anion substitution, amorphization, and high-entropy design.⁸ Specifically, halide electrolytes, especially chloride and bromide systems, offer a wider electrochemical window than sulfides and oxides due to the high electronegativity of their single halogen anions.⁹ For instance, materials such as Li₃InCl₆ and Li₃YCl₆ can have ionic conductivity exceeding 10⁻³ S cm⁻¹ at room temperature. They can be directly compatible with 4 V-class cathode materials without the need for additional coating protection.¹⁰ However, the sensitivity of HSEs to humidity and their strong reactivity with lithium metal anodes still need to be addressed through interface engineering and material modification.¹¹ Although they have significant advantages in electrochemical stability and electrode compatibility, they still face multiple challenges in practical applications. This includes issues such as interfacial reactions with electrode materials,¹² hydrolysis sensitivity in air, and compatibility with lithium metal anodes.¹³ Additionally, industrial applications are also limited by key factors such as interfacial stability, stability under atmospheric conditions, large-scale synthesis, and cost control.¹⁴

Among the diverse classes of solid electrolytes, halide-based materials have emerged as promising candidates owing to their excellent combination of ionic conductivity, stability, and processability. Oxide electrolytes (such as Li₇La₃Zr₂O₁₂ and Li_0.5La_0.5TiO₃) have the benefits of high electrochemical stability, moderate air chemical stability, and wide electrochemical windows (e.g., 0–6 V vs. Li⁺/Li). However, oxide electrolytes are highly rigid and brittle, resulting in poor interface contact with the electrode and high interface resistance. Integration into ASSLBs typically requires a harmful high-temperature sintering process or mixing with polymer electrolytes,¹⁵ and they often exhibit high bulk electronic conductivity, which may promote the formation and growth of lithium dendrites at the Li/SE interface.¹⁵ The main advantages of sulfide electrolytes (such as Li₁₀GeP₂S₁₂ and Li₆PS₅Cl) lie in their extremely high ionic conductivity (up to 10⁻³–10⁻² S cm⁻¹) and excellent mechanical deformability (low Young's modulus), which enables them to achieve close contact through cold pressing, and they are easy to process.¹⁵ However, they are sensitive to air and moisture. It will cause structural degradation and a decrease in ionic conductivity due to hydrolysis and release toxic H₂S gas, posing strict requirements for the production environment.¹⁵ Meanwhile, their intrinsic electrochemical stability window is narrow (about 1.7–2.1 V vs. Li⁺/Li), and harmful side reactions will occur when they come into direct contact with high-voltage cathode materials. Moreover, a space charge layer is prone to form between them and the oxide cathode, resulting in a large interfacial resistance and poor cycling performance.¹⁵ In terms of lithium dendrite inhibition, sulfide electrolytes are one of the most vulnerable types.¹⁶ Polymer electrolytes have advantages such as good flexibility, easy processability, light weight, and good interfacial contact with electrodes.¹⁷ They usually exhibit better mechanical flexibility and strength than inorganic electrolytes, thereby enhancing safety.^15,16 However, their most significant drawback is that they have a very low ionic conductivity and a low migration number at room temperature. This will lead to problems such as lithium dendrite formation and low electrochemical stability, while being relatively expensive and facing significant manufacturing challenges.^15,16 In contrast, halide electrolytes (such as Li₃YCl₆ and Li₃InCl₆) combine the advantages of other electrolytes to a certain extent. They feature a high ionic conductivity (approximately 10⁻³ S cm⁻¹), a wide electrochemical window (up to 0.6–4.2 V vs. Li⁺/Li for chlorides), and excellent compatibility with oxide cathode materials. This is attributed to the high oxidation stability brought about by the high electronegativity of halogen anions.¹⁸ Their mechanical properties lie between those of oxides and sulfides, and they have good deformability, making them suitable for cold pressing processing.¹⁵ Compared with sulfides, halide electrolytes exhibit superior structural recovery and cycling stability and are relatively less sensitive to lithium dendrite growth.¹⁶ However, halide electrolytes are not perfect. Their reduction potential is mainly determined by metal cations and is usually high, resulting in poor compatibility with lithium metal anodes and degradation at the interface, which limits their application at low potentials.¹⁹ Moreover, when the working voltage exceeds approximately 4.3 V, some halide electrolytes will still decompose.¹⁸

Halide solid electrolytes have attracted much attention in the field of ASSLBs due to their high ionic conductivity and excellent electrochemical stability. Many studies have systematically reviewed them. For instance, Xu et al.²⁰ systematically explored the application of this type of electrolyte in batteries, with a focus on analyzing the kinetic mechanism of its ion transport, including the influence of multiple factors such as the crystal structure, cation radius and arrangement, vacancy concentration, disordered structure, and anion framework on lithium-ion conductivity, and revealed its failure mechanism during battery operation. Zhang et al.²¹ systematically reviewed the latest progress in halide solid electrolytes in chemistry, electrochemistry, and interface stability and evaluated their application potential and challenges. Wu et al.²² focused on the application prospects of halide electrolytes in next-generation ASSLBs and particularly reviewed the research trajectory in terms of crystal structure, ion transport mechanism, synthesis methods, and modification strategies since the breakthrough of materials such as Li₃YCl₆ in 2018. Tan et al.²³ systematically reviewed the interface challenges in halogen-based ASSLBs, particularly exploring the advantages of this type of electrolyte in terms of high ionic conductivity and oxidation stability, as well as its incompatibility with high-voltage cathodes and lithium metal anodes in terms of chemistry, electrochemistry, and mechanics. Huang et al.²⁴ started from material classification and systematically reviewed the research progress and modification strategies of halide solid electrolytes. First, they classified them according to central elements and analyzed their performance characteristics. Then, they summarized key modification methods such as chemical doping, interface modification, and composite electrolyte design. Nie et al.²⁵ systematically reviewed the development history, research progress, and existing challenges of halogen-based solid electrolytes in lithium metal solid-state batteries, with a focus on elaborating their advantages and problems in terms of structural characteristics, ionic conductivity, synthesis methods, and chemical and electrochemical stability. Li et al.²⁶ systematically summarized the research progress in halide solid electrolytes in ASSLBs, covering their development history, structural types, synthesis methods, stability, and application potential in energy storage systems. They pointed out that they possess high ionic conductivity, wide electrochemical windows, and good compatibility with oxide cathodes. At the same time, the advantages of the aqueous synthesis method and the challenges in the stability of lithium metal anodes were emphasized. Kwak et al.²⁷ comprehensively explored the application of halide superionic conductors in ASSLBs, systematically summarized their ion diffusion mechanism, crystal structure design principles, synthesis methods, and influence on ionic conductivity, evaluated their compatibility and interface stability with high-voltage cathodes, and analyzed practical challenges such as air sensitivity, cost, and electrode sheet manufacturing process. And future directions such as fluorine substitution, development of low-cost materials, and mixed electrolyte systems were proposed. Tuo et al.²⁸ systematically reviewed the research progress in emerging halide solid electrolytes, with a focus on summarizing their development history, synthesis methods, and strategies for enhancing ionic conductivity, chemical stability, and battery application performance. They emphasized the role of in situ characterization techniques in revealing the structural evolution and interfacial reactions of electrolytes. The compatibility with high-voltage cathodes and lithium metal anodes and the challenges and prospects in electrode preparation and cost control were discussed.

Compared with the above reviews, this review offers more systematic and multi-dimensional unique insights. In the field of materials chemistry and structural classification, crystal structure classification based on anion sublattice types such as hexagonal close-packed and cubic close-packed was elaborated in detail, and the regulatory effects of cation vacancies and anion engineering on ionic conductivity were revealed.²⁹ In terms of synthetic methods, a comprehensive comparison was made of the advantages and disadvantages of solid-state, gas-phase, and liquid-phase methods, highlighting the potential of green synthetic strategies such as solvent-assisted methods and their direct impact on ionic conductivity.²⁸ In the ion transport mechanism, the mechanisms of vacancies, gaps, and cooperative migration were deeply analyzed, emphasizing the quantitative influence of halogen types and their mixtures on the migration energy barrier and conductivity.³⁰ In terms of performance optimization, specific strategies such as defect engineering, element doping like fluorine doping, and interface layer design were proposed to enhance electrochemical stability, mechanical properties, and air-moisture stability.³¹ The concept of “dynamic stability window” was particularly introduced to explain the difference between theoretical and experimental stability windows.³² In terms of interface engineering, the challenges and countermeasures of cathode and anode interfaces were discussed in detail, such as the adoption of sulfide buffer layers, alloy anodes, and other strategies.³³ Looking ahead, it emphasizes accelerating material design through high-throughput computing and advanced characterization techniques and exploring the application prospects of multivalent systems such as sodium ions.³⁴ These insights collectively construct a comprehensive perspective from material design to practical application, expanding the coverage of existing reviews. Overall, this review systematically explores the material chemistry basis, structure–performance relationship, synthesis methods, and interface stability of halide solid electrolytes, deeply analyzes the current challenges,²⁶ and provides theoretical support and technical directions for further development to promote their practical application in high-performance ASSLBs.³⁵Fig. 1 shows the key progress in the research and application of halide solid electrolytes since 2018.

Fig. 1 Important events related to halide solid electrolytes since 2018.

2 Material chemistry and structural classification

2.1 Chemical composition and taxonomy

Halide solid electrolytes can be classified based on the type of cation or anion in their chemical composition. The classification of cationic groups mainly includes rare earth groups (such as Li₃YX₆ and Li₃InCl₆), zirconium groups (such as Li₂ZrCl₆ and Li₃ZrCl₆), and titanium groups (such as Li₂TiCl₆).²⁹ The classification of anionic groups includes chlorides, bromides, iodides, and mixed halogen compounds (such as Li₂ZrCl₄Br₂ and Li₃YCl₅Br).^29,36

In the material design of halide solid electrolytes, cation substitution is a common strategy. For instance, the crystal structure of zirconium-based chlorides can be regulated by doping with elements such as Sc, Y, Yb, Er, Fe, V, Cr, Mg, and In, thereby enhancing their ionic conductivity.³⁷ However, when developing such electrolytes, cost is an important consideration. The prices of different halide compounds vary significantly. For instance, the prices of InF₃ and InI₃ are as high as $62.55 per gram and $69.51 per gram, respectively, while those of AlCl₃ and SnCl₄ are only $0.56 per gram and $0.08 per gram.³⁸ At present, most of the commonly used halide electrolytes rely on rare earth or precious metal elements, such as yttrium, erbium, scandium, indium, etc. The abundance of these elements in the earth's crust is generally low (for example, indium is 0.25 ppm and yttrium is 33 ppm), which leads to the batch synthesis price of their corresponding chlorides exceeding $1000 per kilogram. Therefore, in terms of cost-effectiveness, they are still not ideal.³⁹ Although introducing metals such as yttrium into the chloride framework can enhance ionic conductivity, the price of raw materials like YCl₃ is as high as $320.3 per kilogram, significantly weakening the cost advantage of halide electrolytes and pushing them far beyond the acceptable upper limit of $50 per kilogram. Except for zirconium and aluminium, the abundance of other metal elements used for synthesis is all below 100 ppm, further limiting their cost competitiveness.⁴⁰ In contrast, using zirconium as the central metal has a distinct cost advantage. The crustal abundance of zirconium is relatively high (165–180 ppm), about five times that of yttrium. The bulk price of ZrCl₄ is approximately $8.1 per kilogram, making the volume cost of Li₂ZrCl₆ approximately $27 per liter, which is about 21 times cheaper than Li₆PS₅Cl.²⁷ The crustal abundance of indium is only 25 ppm, and its global proven reserves are limited (about 20 000 tons). Moreover, it is widely demanded in high-tech industries. If it is used in electric vehicle batteries, its price may increase rapidly.⁴¹ In the synthesis of halide electrolytes, the cost of LiCl precursors is relatively low ($ 5.88 per kilogram), but most central metals, such as yttrium and indium, are rare earth elements with low crustal abundance (Y at 33 ppm and In at 25 ppm), which leads to a relatively high price of their chloride precursors. The high zirconium abundance (165 ppm) makes the bulk price of ZrCl₄ significantly lower than that of other chloride precursors, which helps achieve cost-effective halide superionic conductors.²⁸ In practical applications, it is necessary to reasonably balance the improvement effect of cationic doping on ionic conductivity and the cost of raw materials it introduces.

On the other hand, anion engineering is also widely used, such as halogen substitution (X = F, Br, I) in Li₃InCl₆ to form Li₃InCl_6−xX_x, to study its structure and changes in ionic conductivity.⁴² It is worth noting that different halogens have a significant impact on the ionic conductivity and electrochemical stability of materials. Generally speaking, bromides have higher ionic conductivity than chlorides, but their stability towards lithium metal is poor. Fluorides exhibit a wider electrochemical stability window (up to 6 V or more), but have lower room temperature ionic conductivity.⁴³ By using mixed anion strategies such as Li₃Y(Br₃Cl₃), high ionic conductivity and good electrochemical performance can be achieved to a certain extent.⁴³

Halide solid electrolytes exhibit more balanced performance in terms of ionic conductivity, electrochemical window, and moisture resistance compared to oxides and sulfides. Fig. 2a shows the elements and their ionic radii suitable for HSEs in the periodic table, highlighting that the chloride ion (Cl⁻) is the most preferred halide anion due to its moderate ionic radius and electronegativity, while Sc, Y, and lanthanide elements are considered the most promising metal cations due to their matching electronegativity with Cl⁻. In addition, its performance can be further optimized through strategies such as double halogen, equivalent, and heterovalent cation substitution.³⁵ Halide electrolytes mainly include four categories: chlorides, bromides, fluorides, and iodides. Among them, chlorides and bromides represented by Li₃YCl₆, Li₃YBr₆, and Li₃InCl₆ have become research hotspots due to their high ionic conductivity (up to 0.5–3 mS cm⁻¹ at room temperature), wide electrochemical window (>4 V), and good air stability. Fig. 2b systematically presents the classification framework of halide electrolytes in the form of a tree diagram, further subdivided by central metal elements (such as Y, In, Sc, Zr, etc.) and cost levels, highlighting the research progress in halide electrolytes in chemical diversity and structural regulation.⁴¹

Fig. 2 (a) Elements and their ionic radii applicable to halide solid electrolytes in the periodic table. Reproduced with permission.³⁵ Copyright 2023 Royal Society of Chemistry. (b) Classification tree diagram of halide electrolytes. Reproduced with permission.⁴¹ Copyright 2022 Elsevier B.V.

2.2 Crystallographic fundamentals

From the perspective of crystal structure, halide electrolytes can be further classified based on their anionic sublattice types, commonly including hexagonal close-packed (HCP) and cubic close-packed (CCP) configurations.²² For example, Li₃YCl₆ and Li₃ErCl₆ belong to the HCP-type, while Li₃InCl₆ and Li₂Sc_2/3Cl₄ belong to the CCP-type structure.⁴⁴ The octahedron, as a fundamental structural unit, has a significant impact on ion migration pathways and energy barriers due to its connection mode (common or coplanar).⁴⁵ In addition, there is a type of non-dense packing halide conductor with a more open skeleton structure, such as LaCl₃-type materials, which provides one-dimensional channels for ion migration.⁴⁶

Representative structures include spinel, trigonal, orthogonal, and layered structures.⁴⁷ Halide SEs containing divalent metal elements typically have olivine, spinel, or Suzuki type structures, with spinel structures exhibiting the best ionic conductivity.⁴⁷ For example, Li₂ZnCl₄ has an orthorhombic olivine phase at high temperatures, and Li⁺ migrates through tetrahedral gaps within the ab plane and octahedral gaps along the c-axis.⁴⁷ The choice of crystal structure is closely related to the type of anion packing: the trigonal and orthogonal structures are based on the HCP anion arrangement, while the monoclinic structure is based on the CCP anion arrangement.⁴⁸ Electrolytes containing trivalent metal elements, such as Li₃YCl₆, typically have a cubic structure with HCP-type anion arrangement (space group P3̄m1) or a monoclinic structure with CCP-type anion arrangement (space group C2/m).³⁵ The cation arrangement and vacancy distribution in these structures have a decisive impact on the migration pathways and energy landscapes of lithium ions.³¹

Fig. 3 shows the stacking mode of anions in halide solid electrolytes and their influence on the migration path of lithium ions. Fig. 3a, c, b and d depict the cubic close packing (CCP) and hexagonal close packing (HCP) arrangement of halide anions, respectively. Fig. 3a shows the cubic (Fd3̄m) and monoclinic (C2/m) crystal structures with CCP anion arrangement. Fig. 3b shows the triangular (P3̄m1) and orthogonal (Pnma) crystal structures with HCP anion arrangement, respectively. Furthermore, Fig. 3c and d demonstrate the crystal structures and lithium ion probability density distributions of Li₃YBr₆ (CCP-type) and Li₃YCl₆ (HCP-type) through AIMD simulations. Fig. 3c illustrates the three-dimensional isotropic migration of lithium ions through the octahedral tetrahedral octahedral pathway in the CCP structure. Fig. 3d shows that in the HCP structure, lithium ions undergo one-dimensional rapid diffusion along the c-axis and form an anisotropic three-dimensional migration network through tetrahedral site connections. These structural features directly affect the ion conductivity and transport mechanism of materials, providing an important crystallographic basis for understanding and designing high-performance halide electrolytes.²⁵

Fig. 3 (a) (left) Schematic diagram of cubic close-packed (CCP) arrangement of halide anions. (right) A cubic crystal structure with CCP anion arrangement (Fd3̄m symmetry) and a monoclinic crystal structure (C2/m symmetry). Reproduced with permission.²⁶ Copyright 2020 Royal Society of Chemistry. (b) (left) Schematic diagram of a hexagonal close-packed (HCP) arrangement of halide anions. (right) Triangular crystal structure with HCP anion arrangement (P3̄m1 symmetry) and an orthogonal crystal structure (Pnma symmetry). Reproduced with permission.⁴⁹ Copyright 2020 American Chemical Society. (c) (left) Crystal structure and lithium ion probability density distribution (CCP-type) of Li₃YBr₆. The three-dimensional migration mechanism of lithium ions through the octahedral tetrahedral octahedral pathway in (right) Li₃YBr₆.⁵⁰ (d) (left) Crystal structure and lithium ion probability density distribution (HCP-type) of Li₃YCl₆. The one-dimensional diffusion of lithium ions along the c-axis and their migration path through tetrahedral connections on the ab plane in Li₃YCl₆ (right). (c and d) Reproduced with permission.⁵⁰ Copyright 2019 Wiley-VCH Verlag GmbH.

2.3 Composition–structure–property interplay

In halide solid electrolytes, changes in composition directly affect their crystal structure, which in turn determines key properties such as ion conductivity and stability. For example, in the Li–M–X (where M is a metal and X is a halogen) system, the transformation of crystal structure can be achieved by adjusting the type and proportion of halogen anions, thereby optimizing the ion transport pathway. The concentration of cation vacancies is also a key factor affecting ion transport. In the Li_xScCl_3+x system, Li₃ScCl₆ exhibits the highest room temperature ionic conductivity (3.02 × 10⁻³ S cm⁻¹) due to its appropriate balance between the Li⁺ carrier concentration and vacancy concentration.⁵¹ A similar compositional regulation strategy also applies to sodium ion conductors; for example, introducing vacancies in Na_3−xEr_1−xZr_xCl₆, which can increase the ion conductivity to 0.035 mS cm⁻¹.⁵²

Oxygen doping is another effective means of regulating structures. Introducing oxygen into Li₂ZrCl₆ can induce a transition from the triangular phase to the monoclinic phase, resulting in a high ionic conductivity of 2.42 mS cm⁻¹.⁵³ In addition, halogen mixtures (such as Br/Cl) can not only induce phase transitions, but also optimize the local chemical environment of Li⁺, reduce grain boundary resistance, and synergistically enhance bulk ion conductivity.⁵⁴ In the Li₃YCl_6−xBr_x series materials, as the Br content increases, the crystal structure changes from a triangular phase (P3̄m1) to a monoclinic phase (C2/m), and the ionic conductivity significantly increases, reaching up to 5.4 mS cm⁻¹ (30 °C).⁵⁵

Yu et al.⁵⁶ systematically screened lithium metal halide solid electrolytes based on low-cost and sustainable metals such as Mg, Zn, Al, etc., through first principles calculations. Fig. 4a shows the multi-step screening process from composition design to structure prediction and performance evaluation, including initial screening based on the octahedral factor, USPEX structure optimization, HSE06 bandgap calculation, phonon spectrum dynamic stability verification, and phase diagram and formation energy analysis. Finally, candidate materials with high ionic conductivity, wide electrochemical window, and good mechanical properties, such as Li₂MgCl₄, LiZnCl₃, and Li₃AlCl₆, were screened, revealing the intrinsic relationship between composition, structure, and performance.⁵⁶ By regulating the radius and valence state of central metal ions (such as Zr⁴⁺, In³⁺, etc.), the crystal structure type of halide solid electrolytes (such as HCP triangular or CCP mononuclear) can be changed, thereby significantly affecting Li⁺ migration channels and ion conductivity. Fig. 4b specifically illustrates the dependence of the phase evolution of halide SEs on the central metal/halide ion radius ratio (rM/X), indicating that different r_M/X values correspond to different dense packing anion arrangements (such as HCP or CCP), which is a key basis for understanding the relationship between structural regulation and performance optimization.²⁷ Labalde et al.⁵⁷ systematically studied the interaction between composition, structure, and properties of the Na_3−2xIn_1−xTa_xCl₆ halide solid electrolyte system, with a focus on revealing how the strategy of introducing Ta⁵⁺ to replace In³⁺ to regulate Na⁺ vacancies affects the crystal structure, microstructure, and ion transport properties of materials. Fig. 4c visually illustrates the synergistic effect of Na⁺ vacancy content and arrangement on crystal structure type, microstructure orderliness, and ion conductivity, indicating that the optimal ion transport performance is not solely determined by the number of vacancies, but depends on the ordered arrangement and moderate vacancy concentration of vacancies, as well as the coupled microstructure characteristics.⁵⁷Fig. 4d reveals the direct effects of different halogen components (Cl/Br/I) on the crystal structure arrangement and lithium ion transport channels. Fig. 4d shows that as the radius of halide ions increases, the structure transitions from a tightly packed structure to an open framework, thereby significantly improving the ion conductivity. This structural evolution is directly related to the reduction of the lithium ion migration energy barrier, providing key insights for the rational design of high-performance solid-state electrolytes.⁵⁸ The ion transport performance of halide solid electrolytes strongly depends on the crystal structure type (such as the monoclinic phase, orthorhombic phase, and trigonal phase) and the anion/cation composition. Fig. 4e reveals the influence of polarization effects between different metal cations and halide anions on structural stability through the key parameter of “cation polarization factor”, which determines the ion migration path and conductivity level. In addition, element substitution (such as Zr, In, F, etc.) can regulate the structural order, vacancy concentration, and interface stability, thereby optimizing the overall electrochemical performance and promoting the application of halide solid electrolytes in practical all-solid-state batteries.⁴⁷

Fig. 4 (a) A multi-step first principles screening process diagram for halide solid electrolytes, covering structural stability, bandgap, kinetic stability, and phase diagram evaluation. Reproduced with permission.⁵⁶ Copyright 2021 Royal Society of Chemistry. (b) The relationship between the phase evolution of halide superconductors and the radius ratio of the central metal to halide ions. Reproduced with permission.²⁷ Copyright 2022, American Chemical Society. (c) Schematic diagram of the coupling effect of Na⁺ vacancy content and arrangement on the crystal structure, microstructure, and ionic conductivity in Na_3−2xIn_1−xTa_xCl₆. Reproduced with permission.⁵⁷ Copyright 2024 Royal Society of Chemistry. (d) The correlation mechanism between structural evolution caused by changes in halogen composition and ionic conductivity. Reproduced with permission.⁵⁸ 2024 Wiley-VCH GmbH. (e) The relationship between the cationic polarization factor and preference for the halide electrolyte structure. Reproduced with permission.⁴⁷ Copyright 2025, American Chemical Society.

3 Synthesis and characterization methods

3.1 Synthesis approaches

The synthesis method of halide solid electrolytes (HSEs) has a significant impact on their crystal structure, ionic conductivity, and final battery performance. At present, the mainstream synthesis methods can be divided into three categories: the solid-state method, gas-phase method, and liquid-phase method.²⁸ The solid-state reaction method is the earliest and most commonly used synthesis method for halide solid electrolytes, mainly including mechanical ball milling, annealing after ball milling, and high-temperature solid-state sintering.²⁸ The operating conditions and process parameters of these methods have a significant impact on the crystal structure, crystallinity, and ionic conductivity of HSEs.²⁸ For example, high-energy ball milling provides reaction activation energy through mechanical energy, enabling the preparation of target products at lower temperatures and introducing structural defects to promote ion transport.⁵⁹ Although high-temperature solid-state sintering can easily obtain thermodynamically stable phases, it has high energy consumption and a long time consumption and needs to be carried out under an inert atmosphere or vacuum sealing conditions, which limits its large-scale application.³⁵

The vapor phase method mainly includes chemical vapor deposition (CVD) and atomic layer deposition, which are suitable for preparing electrolytes in thin film form.²⁵ CVD relies on low kinetic energy particles for deposition and has multiple chemical pathways.⁴⁸ This type of method can obtain thin films with high purity and controllable crystallinity, but it has high equipment costs and complex processes and may produce harmful by-products.⁶⁰

Solvent-assisted synthesis (also known as wet chemistry) includes water-mediated synthesis, ammonia-assisted synthesis, anti-solvent crystallization, and solvothermal synthesis.²⁸ This type of method has advantages such as low energy consumption, short cycle time, and easy scalability, making it particularly suitable for large-scale production.⁵⁹ For example, the synthesis of Li₃InCl₆ through aqueous solution reactions can result in an ion conductivity of up to 2.04 mS cm⁻¹, and its moisture resistance has also been improved.⁴⁸ Ammonia-assisted hydrolysis combined with the freeze-drying method (AAH-FD) has also been used to synthesize Li₃YCl₆, exhibiting excellent particle uniformity and ionic conductivity.⁶¹

In addition, new methods such as microwave-assisted synthesis and vacuum evaporation-assisted reactions are gradually being explored to shorten reaction time, reduce energy consumption, and improve product purity.^62,63 Overall, the selection of synthesis methods should take into account factors such as ion conductivity, phase purity, air stability, and process feasibility.

Fig. 5a shows a schematic diagram of the process flow for directly mechanically mixing raw materials through ball milling and sintering to obtain Li₃InCl₆ electrolyte.⁶⁴ Yang et al.⁶⁵ synthesized a high ionic conductivity halide solid electrolyte Li₂ZrCl₆ using an optimized two-step ball milling method. Fig. 5b shows a schematic diagram of the synthesis strategy. In the first step, a low ball-to-material ratio is used for uniform mixing, and in the second step, the structure is further optimized by adjusting the rotation speed and ball-to-material ratio, ultimately obtaining an electrolyte with a network fiber morphology. Its room temperature ion conductivity is increased to nearly 1 × 10⁻³ S cm⁻¹, which is superior to that in the traditional single step ball milling method and demonstrates good potential for solid-state battery applications.⁶⁵ Qiu et al.⁶¹ developed a scalable synthesis method using ammonia-assisted hydrolysis combined with freeze-drying (AAH-FD) for the preparation of halide solid electrolytes (such as Li₃YCl₆) as an alternative to traditional high-energy ball milling methods. Fig. 5c shows the process flow of this method. First, LiCl, NH₄Cl, and YCl₃·6H₂O are dissolved in water, and the precursor is obtained by rapid freezing and vacuum freeze-drying with liquid nitrogen. Then, the final electrolyte material is obtained by annealing treatment. This method effectively inhibits hydrolysis, avoids particle aggregation, and can obtain Pnma structured electrolytes with a smaller particle size and more uniform distribution, significantly improving ion conductivity and battery performance.⁶¹ Huang et al.⁵ synthesized halide electrolyte Li₃InCl₆ using a solvent-mediated method and compared the effects of water, ethanol, and their mixed solvents on the material structure and properties. As shown in Fig. 5d, the synthesis process involves dissolving InCl₃ and LiCl in different solvents in stoichiometric ratios, followed by drying and heat treatment to obtain the final product. Research has shown that ethanol as a solvent can induce lattice expansion and (131) preferred orientation of crystal planes, forming more vacancies and three-dimensional lithium ion migration channels, resulting in the electrolyte exhibiting the highest ion conductivity (1.06 mS cm⁻¹) and the lowest migration energy barrier (0.272 eV), which is suitable for high-performance all-solid-state lithium metal batteries.⁵ Ma et al.⁶⁴ synthesized halide solid electrolyte Li₃InCl₆ using freeze-drying technology, as shown in Fig. 5e (top). This method rapidly froze the solution and removed the solvent through vacuum sublimation, effectively avoiding particle collision and high-temperature agglomeration problems caused by thermal evaporation in traditional hydrolysis methods (Fig. 5e-bottom). They successfully obtained electrolyte particles with a smaller particle size (80% less than 200 nm) and more uniform distribution, significantly improving the ion transport capacity and interface contact performance of cathode composite materials in ASSLBs.⁶⁴Fig. 5f (top) shows the ammonium-assisted route (using NH₄Cl as a coordinating agent to avoid hydrolysis), and Fig. 5f (bottom) shows the HBr-assisted vacuum evaporation method, which provides a new approach for large-scale preparation of humidity-stable halide electrolytes such as Li₃YCl₆ and Li₃HoBr₆.²⁷

Fig. 5 (a) Schematic diagram of the process flow for synthesizing Li₃InCl₆ by the ball milling method.⁶⁴ (b) Schematic diagram of the two-step ball milling method for preparing Li₂ZrCl₆ electrolyte. Reproduced with permission.⁶⁵ Copyright 2024, American Chemical Society. (c) Schematic diagram of the ammonia-assisted hydrolysis and freeze-drying synthesis process of Li₃YCl₆ and its application in ASSLBs. Reproduced with permission.⁶¹ Copyright 2025, American Chemical Society. (d) Schematic diagram of the synthesis process of Li₃InCl₆ electrolyte in water, ethanol, and their mixed solvents. Reproduced with permission.⁵ Copyright 2024 American Chemical Society. (e) Schematic diagram of the process flow for preparing Li₃InCl₆ solid electrolyte by freeze-drying and hydrolysis methods. (a and e) Reproduced with permission.⁶⁴ Copyright 2023 Royal Society of Chemistry. (f) (Above) Schematic diagram of ammonium-assisted synthesis of halide solid electrolyte. (Below) Schematic diagram of the process for synthesizing Li₃HoBr₆ using the HBr-assisted vacuum evaporation method. Reproduced with permission.²⁷ Copyright 2022 American Chemical Society.

3.2 Characterization techniques

The comprehensive characterization of halide solid electrolytes (HSEs) is crucial for understanding their structure–performance relationship and evaluating their electrochemical behavior and interfacial stability. Structural analysis typically utilizes techniques such as X-ray diffraction (XRD), transmission electron microscopy (TEM), and neutron diffraction. For example, the XRD pattern of Li₃YCl₆ can be calibrated as a cubic, P-3m1 space group, and scanning electron microscopy (SEM) images show that it is composed of micrometer-sized particles.⁶⁶ Neutron diffraction is highly sensitive to the distribution of light elements such as lithium, providing atomic displacement parameters and lithium occupancy information, which is particularly important for understanding ion transport mechanisms.⁶⁷

Interface stability analysis is crucial for ASSLBs. In situ X-ray photoelectron spectroscopy (XPS) and SEM are commonly used to study the interfacial chemical reactions and morphological evolution between electrolytes and electrodes. Research has shown that the impedance at the interface between LiCoO₂ and LiPON is due to chemical changes between the electrolyte and the electrode, rather than space charge effects. Similar methods are also applicable to halide solid electrolyte systems.²⁵ In addition, time-of-flight secondary ion mass spectrometry (ToF-SIMS) and synchrotron X-ray absorption spectroscopy can reveal the chemical species of interface phases (such as CEI layers), providing rich information for understanding interface reactions.²³

Fig. 6a (top) captured the hydrophilicity and phase transition process of Li₃InCl₆ in humid air in real-time by operating an optical microscope and Raman spectroscopy. Fig. 6a (bottom) shows a schematic diagram and synchrotron X-ray diffraction (SXRD) spectrum of monitoring the dynamic formation of hydrolysis products using an SXRD device. The SXRD spectrum reveals the degradation pathway of Li₃InCl₆, gradually transforming into hydrates Li₃InCl₆·xH₂O and LiCl in humid environments.²⁸Fig. 6b reveals the structural evolution process of Li₃YCl₆ during the heating process from the β-metastable state to the α-phase through in situ synchrotron radiation XRD. It also shows the mass fractions of each phase with temperature changes, indicating that the β-phase has higher ionic conductivity in the 450–600 K range, but its transition to the α-phase is irreversible. These in situ characterization techniques provide key experimental evidence for understanding the dynamic structural evolution and stability mechanism of halide electrolytes.²⁸Fig. 6c shows the decomposition behavior of halide electrolytes (such as Li₃InCl₆) in contact with lithium metal through in situ XPS analysis, confirming the formation of an electron-ion mixed conductive interface, leading to continuous degradation and battery short circuit.²⁵Fig. 6d shows the phase transition behavior of α-Li₂ZrCl₆ to β-Li₂ZrCl₆ during the heating process using the in situ neutron powder diffraction technique, revealing the correlation between the decrease in ionic conductivity and the disappearance of non-periodic defect structures.²⁸Fig. 6e visually compares the distribution differences of rare earth cations in Li₃YbCl₆ and Zr-doped Li_2.7Yb_0.7Zr_0.3Cl₆ through powder neutron diffraction refinement and simulated distribution function images, revealing how the structural disorder caused by heterovalent cation substitution reduces the lithium ion migration energy barrier and thereby enhances the ionic conductivity.⁵⁹Fig. 6f shows that as the bromine content increases, the ⁶Li NMR spectrum peak shifts towards lower fields, indicating an enhanced electronic environment around lithium ions, a decrease in the average negative charge of 4d sites, and a weakening of the electrostatic interaction with lithium, which is beneficial for lithium cage expansion and ion conductivity improvement, reaching up to 8.55 mS cm⁻¹.⁶⁸

Fig. 6 (a) (Top) In situ optical microscopy image and Raman spectroscopy changes of Li₃InCl₆ after exposure to humid air. (Below) Schematic diagram of the synchrotron radiation X-ray diffraction experiment, as well as the synchrotron radiation X-ray diffraction pattern of Li₃InCl₆ exposed to 30% humidity air for 120 minutes.²⁸ (b) In situ synchrotron radiation XRD patterns and temperature-dependent curves of mass fractions of various phases during the heating process of a LiCl and YCl₃ mixture.²⁸ (c) In situ XPS analysis showing that Li₃InCl₆ was reduced to In⁰ upon contact with lithium metal, forming a mixed conductive interface. Reproduced with permission.²⁵ Copyright 2023 John Wiley and Sons. (d) The two-dimensional intensity spectrum of in situ neutron powder diffraction of α-Li₂ZrCl₆ during the heating process showing its phase transition towards β-Li₂ZrCl₆. (a, b and d) Reproduced with permission.²⁸ Copyright 2023 Springer Nature. (e) Comparison of the distribution of Yb/Zr atoms in Li₃YbCl₆ and Li_2.7Yb_0.7Zr_0.3Cl₆ and simulation of the distribution function image. Reproduced with permission.⁵⁹ Copyright 2025 Elsevier B.V. (f) (left) Li NMR spectra of Li_6−xPS_5−xBr_1+x at different bromine contents; (right) the chemical shift of ⁶Li shifts towards lower fields with increasing bromine content. Reproduced with permission.⁶⁸ Copyright 2023 American Chemical Society.

4 Ion transport properties and mechanisms

4.1 Key performance metrics

The core performance indicators of halide solid electrolytes mainly include ion conductivity, activation energy, and structural stability. Ionic conductivity is a key parameter for measuring the migration ability of lithium ions in electrolytes, usually measured by electrochemical impedance spectroscopy (EIS), and the activation energy is calculated using the Arrhenius formula.⁶⁹ For example, in the Li₃MCl₆ series, Li₃AlCl₆ exhibits the highest lithium-ion conductivity (158 mS cm⁻¹) and the lowest activation energy (0.124 eV), while Li₃ScCl₆ has a lower conductivity (1.06 mS cm⁻¹) and a higher activation energy (0.29 eV).⁶⁹ Halogen substitution is an effective strategy for regulating ion conductivity. For example, partially replacing Cl with Br in Li₃YCl₆ can significantly improve ion conductivity, but may reduce its high-pressure stability.⁷⁰ Crystal orientation also has a significant impact on ion conductivity. For example, the (010) oriented Li₃YBr₆ structure exhibits nearly 14 times higher ion conductivity than disordered Li₃YBr₆, and the activation energy is reduced by nearly half.⁷¹ In addition, the contributions of grain boundary resistance and bulk resistance also need to be evaluated separately. For example, in Li₃YBr₆, grain boundary resistance may be one order of magnitude higher than bulk resistance, significantly affecting the total ion conductivity.⁷²

Yan et al.⁷³ studied improvement in the performance of halide solid-state electrolyte Li₂ZrCl₆ (LZC) through an AI-F co-doping strategy, aiming to enhance its ionic conductivity, electrochemical window, and interface stability for all-solid-state lithium batteries. The activation energy data in Fig. 7a show that the minimum activation energy of LZC-0.1AlF₃ is 0.2904 eV, indicating that moderate AI-F doping effectively reduces the lithium-ion migration energy barrier, thereby enhancing ion conduction and improving battery cycle performance, while reducing interfacial side reactions and lithium dendrite growth.⁷³Fig. 7b compares the ionic conductivities of various electrolytes, showing that some halide electrolytes, such as Li₃InCl₆, can reach 2.04 mS cm⁻¹, while Li_2.99Ba_0.005OCl glass electrolytes can even reach 25 mS cm⁻¹, significantly superior to certain oxide and sulfide systems. Halide electrolytes typically have a low activation energy and a high ion migration number, but their diffusion coefficient and long-term stability still need to be optimized, especially in terms of compatibility with metal anodes, which still face challenges.⁴⁸Fig. 7c compares the thermodynamic intrinsic electrochemical stability windows of different anionic chemical systems (fluorine, chlorine, bromine, and iodine) through calculations, highlighting the high oxidation stability of chlorides at approximately 4 V.²⁷Fig. 7d compares the water absorption behavior of Li₃InCl₆ and Li₃YCl₆ in humid air, showing that Li₃InCl₆ absorbs water faster but with a lower saturation amount, while Li₃YCl₆ absorbs water slower but with a higher total amount, reflecting the influence of different cations on humidity stability, which in turn affects their ion transport properties and interface stability.⁹ Halide solid electrolytes have attracted much attention due to their high ionic conductivity (such as 7.2 × 10⁻³ S cm⁻¹ for Li₃Y(Br₃Cl₃)), low activation energy (such as 0.34 eV for Li₂Sc₂/₃Cl₄), and excellent diffusion mechanism (such as the three-dimensional oct–tet–oct pathway in the CCP structure). Fig. 7e reveals the trade-off between humidity stability and electrochemical reduction stability of 14 lithium-containing ternary chlorides by comparing their hydrolysis and reduction reaction energies. Among them, In³⁺-based electrolytes exhibit the best humidity stability, but most halides still face the challenge of insufficient reduction stability, which affects their compatibility with lithium metal anodes.²⁵ Wang et al.⁷⁴ optimized the Li₃InCl₆ halide solid electrolyte by Hf doping and achieved the highest ion conductivity of 1.28 mS cm⁻¹ at x = 0.3. The activation energy was reduced, the lithium ion migration path was optimized, and the diffusion ability was enhanced. Fig. 7f shows that the intensity of the oxidation–reduction peak of Li₃InCl₆ gradually decreases and tends to stabilize in the first five cycles, indicating that its electrochemical interface gradually activates and reaches a stable state during cycling, demonstrating good cycle reversibility.⁷⁴ Chen et al.⁷⁵ identified three novel solid-state electrolyte materials (Li₄ZrF₈, Li₃ErBr₆, and Li₂ZnI₄) from 3119 halides through high-throughput screening. Their room temperature ion conductivities reached 4.93 × 10⁻⁴, 4.62 × 10⁻⁴, and 1.79 × 10⁻⁴ S cm⁻¹, respectively, with activation energies ranging from 0.331–0.372 eV and diffusion coefficients in the order of 10⁻⁹ cm² s⁻¹. Fig. 7g further compares the interfacial reaction energies between these halide electrolytes and various cathode materials in the form of heat maps, indicating that they have good chemical stability, especially when matched with cathodes such as LiCoO₂ and LiFePO₄, with higher reaction energies and better interfacial compatibility than traditional sulfide and oxide electrolytes.⁷⁵ Shi et al.⁷⁶ systematically studied the rare earth (Y, Gd, Tb, Ho, and Er) bromide solid electrolyte Li₃REBr₆ and found that Li₃GdBr₆ (LGdB) exhibited optimal deformability due to the largest Gd³⁺ ion radius, longest Gd Br bond, and softest lattice. Its ion conductivity reached 1.4 mS cm⁻¹, activation energy was 0.28 eV, lithium ion diffusion coefficient was 10⁻⁹–10⁻¹³ cm² s⁻¹, and was compatible with the Li–In anode. The assembled solid-state battery exhibited excellent fast charging and cycling performance after 6000 stable cycles at 10 °C (Fig. 7h).⁷⁶

Fig. 7 (a) The activation energy of Li_2+xZr_1−xAl_xCl_6−3xF_3x. Reproduced with permission.⁷³ 2024 Elsevier B.V. (b) Comparison of ion conductivity of various solid electrolytes, including halide and oxyhalide electrolytes. Reproduced with permission.⁴⁸ Copyright 2024 Royal Society of Chemistry. (c) Comparison of thermodynamic intrinsic electrochemical windows of various lithium-based ternary compounds (fluoride, chloride, bromide, and iodide). Reproduced with permission.²⁷ Copyright 2022 American Chemical Society. (d) Normalized water absorption mass over time curves of Li₃InCl₆ and Li₃YCl₆ in humid air. Reproduced with permission.⁹ Copyright 2024 Elsevier B.V. (e) The hydrolysis reaction energy and reduction reaction energy of 14 lithium-containing ternary chlorides. Reproduced with permission.²⁵ Copyright 2023 John Wiley and Sons. (f) The cyclic voltammetry curve of Li₃InCl₆ in the first five cycles showing that the intensity of the oxidation–reduction peak gradually decreases and tends to stabilize. Reproduced with permission.⁷⁴ Copyright 2023 American Chemical Society. (g) The maximum reaction energy heatmap between halide solid electrolytes and typical cathode materials. Reproduced with permission.⁷⁵ Copyright 2024 Elsevier B.V. (h) The ion conductivity of LGdB solid-state electrolyte reaches 1.4 mS cm⁻¹, enabling the battery to cycle stably for 6000 times at 10 C. Reproduced with permission.⁷⁶ Copyright 2025 American Chemical Society.

4.2 Primary transport mechanisms

In halide solid electrolytes, ion migration is mainly achieved through three basic mechanisms: the vacancy mechanism, gap mechanism, and synergistic migration mechanism.³⁰ Cationic vacancies or interstitial ions are usually charged mobile species. Vacancy diffusion refers to the migration of adjacent ions into vacancies. Direct gap migration occurs between adjacent gap positions. The collaborative (or interstitial) mechanism involves the migration of interstitial ions pushing neighboring lattice ions into adjacent sites. Vacancy and direct gap migration are considered routine jumping processes, while cooperative migration is a highly correlated process in which rapid ion transport is influenced by the coordinated movement of multiple ions and may involve mechanisms involving correlated rotational motion.³⁰

Cationic disorder includes positional disorder and substitution disorder.⁴⁷ Positional disorder refers to the deviation of ions from crystallographic positions, which helps form new ion transport pathways. For example, in Li₂Sc₂/₃Cl₄ synthesized by Zhou et al., additional Li⁺ migrated to new sites, expanding the ion diffusion pathway. Substituting disorder refers to changes in ion arrangement at sites, including changes in Li⁺ site occupancy, which helps smooth the energy landscape and reduce ion diffusion barriers.⁴⁷ In typical halide solid electrolytes containing group III elements (such as Li₃MX₆), the theoretical intrinsic vacancy content in the octahedral sites is as high as 33%.²⁶ People believe that a much higher vacancy content is another fundamental parameter that further promotes the rapid migration of Li⁺ in solid electrolytes.²⁶

Jeon et al.⁷⁷ revealed the efficient transport mechanism of lithium ions in Li₃Y(Br₃Cl₃) mixed halide solid electrolytes through first principles calculations. The high ion conductivity (22.3 mS cm⁻¹) is mainly due to the interlayer synergistic diffusion mechanism. Fig. 8a compares the energy barriers of synergistic diffusion and vacancy diffusion. Synergistic diffusion (0.23 eV) is much lower than vacancy diffusion (0.56 eV) because when two Li⁺ migrate simultaneously, there is always one Li⁺ located near the 4g site, stabilizing the surrounding Br⁻/Cl⁻ anions, reducing electrostatic repulsion, and achieving rapid three-dimensional ion transport.⁷⁷ In Fig. 8b, the influence mechanism of different charge carriers and vacancy concentrations on ion transport behavior in rare earth HSEs is shown, indicating that optimal ion conductivity can be achieved when the Li⁺ concentration matches the vacancy concentration. This emphasizes that vacancy-assisted lithium ion migration is one of the main transport mechanisms in halide electrolytes.⁵⁹Fig. 8c reveals three main ion migration mechanisms at the atomic scale: the vacancy mechanism, gap mechanism, and gap sub mechanism and demonstrates the hindering effects of grain boundaries and space charge layers on ion transport. Halide solid electrolytes have a wide voltage window and good oxidation stability, but their room temperature conductivity is generally low. Therefore, it is necessary to optimize the structure and defect concentration through strategies such as Zr⁴⁺ doping and anion mixing to improve ion conductivity and interface stability.⁷⁸ Flores et al.⁷⁹ synthesized the LiAlX₄ (X = Cl, Br, I) series of solid electrolytes using mechanochemical methods and analyzed their lithium ion migration mechanism using neutron and synchrotron radiation diffraction combined with BVSE (Bond Valence Site Energy) calculation. Fig. 8d shows through BVSE barrier simulation that Li⁺ migration is mainly achieved through transitions from octahedral lattice sites to tetrahedral interstitial sites (such as the “i7” site in LiAlI₄), and the pathway is regulated by the synergistic movement of halogen polarizability and [AlX₄]⁻ complex anions, indicating that the ion conductivity of this type of halide electrolyte is closely related to anion dynamics.⁷⁹ Luo et al.⁴⁶ compared the ion transport mechanisms in non-tightly packed and tightly packed halide electrolytes and found that non-tightly packed frameworks (such as UCl₃-type structures) have larger diffusion channels and more significant site distortions, thereby reducing the migration energy barrier. Fig. 8e shows through the radial distribution function that in non-tightly packed structures, the distance between migrating lithium ions and the nearest fixed cation is larger (such as 4.4 Å in Li₃La₃Cl₁₆, while only 3.8 Å in Li₃YCl₆), reducing electrostatic repulsion and promoting three-dimensional fast ion conduction.⁴⁶ Chun et al.⁸⁰ revealed the transport mechanism of lithium ions in halide spinel electrolyte Li₂Sc₂/₃X₄ (X = Cl, Br, I) through first principles calculations and molecular dynamics simulations. Unlike traditional predictions based on single ion hopping or polarization effects, Li₂Sc₂/₃Cl₄ exhibits excellent ionic conductivity, with its core mechanism being the synergistic migration behavior of lithium ions through octahedral and tetrahedral channels shared by the surface. Fig. 8f specifically illustrates this collaborative migration pathway, where two lithium ions jump along the channel in sequence, forming highly correlated collective motion, significantly reducing the diffusion energy barrier and achieving conductivity at the level of a superconductor.⁸⁰

Fig. 8 (a) Energy barrier comparison and atomic configuration evolution of interlayer lithium ion synergistic diffusion and vacancy diffusion in Li₃Y(Br₃Cl₃). Reproduced with permission.⁷⁷ Copyright 2023 Royal Society of Chemistry. (b) Schematic diagram of ion transport scenarios in rare earth HSEs at different charge carrier and vacancy concentrations. Reproduced with permission.⁵⁹ Copyright 2025 Elsevier B.V. (c) The diffusion mechanisms of vacancies, gaps, and interstitial ions at the atomic scale, as well as the effects of grain boundaries and space charge layers on ion transport. Reproduced with permission.⁷⁸ Copyright 2023 Royal Society of Chemistry. (d) Schematic diagram of the lithium ion migration path and energy barrier based on BVSE analysis in LiAlI₄. Reproduced with permission.⁷⁹ Copyright 2021 American Chemical Society. (e) In the non-tightly packed structure of halides, the distance between lithium ions and fixed cations is greater, reducing electrostatic repulsion and facilitating ion migration. Reproduced with permission.⁴⁶ Copyright 2024 Wiley-VCH GmbH. (f) Schematic diagram of the synergistic migration path of lithium ions sharing octahedral tetrahedral channels along the surface in halide spinel. Reproduced with permission.⁸⁰ Copyright 2021 Royal Society of Chemistry.

4.3 Effect of halogen identity and mixing on ion transport

The selection of halogen anions has a significant impact on the ion transport performance of halide solid electrolytes. In the Li_7−x−γ(PS₄)(S_2−x−γCl_xBr_γ) argon–silver ore structure, the types and proportions of halogens regulate the lithium cage structure and the length of the lithium ion conduction path between the cages, thereby affecting the ionic conductivity. Research shows that Li_5.4(PS₄)(S_0.4Cl_1.0Br_0.6) and Li_5.4(PS₄)(S_0.4Cl_0.6Br_1.0) have an ionic conductivity of about 11.6 S cm⁻¹, which is one of the highest in the current report.⁸¹

Through first-principles molecular dynamics simulations, it was found that among the Li₃YX₆ (X = Cl, Br, I) series, Li₃YCl₆ has the lowest activation energy and the highest ionic conductivity. Its outstanding performance is attributed to the Li ion diffusion path of octahedron–octahedron and the weak Coulomb force between Li and Cl ions.³⁵ Halogen mixing strategies are also widely applied to optimize ion transport performance. In lithium antiperovskite materials, the mixed phase of Li₃OCl_0.5Br_0.5 exhibits a higher ionic conductivity (1.94 × 10⁻³ S cm⁻¹) than pure Li₃OCl or Li₃OBr. The reason is that the co-introduction of Cl and Br generates a larger unit cell and reduces octahedral tilt, thereby forming a looser Li⁺ migration channel.⁸² In sodium-ion conductors, halogen doping has also been proven to be an effective method for introducing sodium vacancies and enhancing ionic conductivity. For instance, doping halogen atoms (F, Cl, and Br) in t-Na₃PS₄ can significantly enhance the conductivity of Na⁺. Among them, t-Na₃PS₄ doped with Br⁻ exhibits the smallest activation energy and the highest conductivity of Na⁺ due to halogen substitution defects and the lower binding energy of Na⁺ vacancy defects.⁸³

In addition, the polarizability of halogen anions also has a significant impact on ion migration. More polarizable anions (such as I⁻) can weaken the coulombic interaction with Li⁺, thereby reducing the migration energy barrier and enhancing the ionic conductivity. In the Li_7.5B₁₀S₁₈X_1.5 (X = Cl, Br, I) structure, as X changes from Cl to I, the activation energy decreases from 0.36 eV to 0.30 eV and the room-temperature ionic conductivity increases from 0.5 mS cm⁻¹ to 1.4 mS cm⁻¹.⁸⁴ The types of halogens and their mixing strategies significantly affect the ion transport behavior of halide solid electrolytes by regulating multiple factors such as lattice parameters, vacancy concentrations, anion disorder, migration energy barriers, and lattice dynamics.

Plass et al.⁸⁵ systematically investigated the effects of halogen identity (Br/I) and its mixtures on the ion transport behavior in layered halide solid electrolytes Li₃HoBr_6−xI_x. Through structural characterization and electrochemical impedance spectroscopy analysis, it was found that as the iodine content increases, lattice expansion and softening are beneficial for lithium ion migration, thereby improving ion conductivity and reducing activation energy. However, excessively high iodine content (x > 3.5) significantly enhances the cation disorder within the layer (Ho³⁺ and Li⁺ site exchange), introducing electrostatic repulsion and blocking the lithium ion migration channel. Fig. 9a and b visually demonstrate the trend of ion conductivity and activation energy with iodine content in the solid solution series, achieving optimal performance at x ≈ 3. The room temperature ion conductivity is 2.7 × 10⁻³ S cm⁻¹, and the activation energy is 0.18 eV, indicating that halogen mixing can optimize ion transport performance by regulating lattice stiffness and cation ordering.⁸⁵ Fichtner et al.⁸⁶ systematically investigated the effect of fluorine partially replacing chlorine on the ion transport performance and interfacial stability of halide solid electrolyte Li₂ZrCl₆. Research has shown that as the fluorine content (x in Li₂ZrCl_6−xF_x) increases, the ionic conductivity gradually decreases. The unsubstituted sample (x = 0) has the highest room temperature conductivity (2.8 × 10⁻⁴ S cm⁻¹), while the activation energy increases from the lowest value of 0.22 eV (x = 0) (Fig. 9c and d). This trend is attributed to the smaller radius and stronger electronegativity of fluoride ions, which leads to lattice contraction and stronger Li–F bonding, thereby increasing the migration energy barrier of lithium ions. Although fluorine substitution sacrifices some bulk ion conductivity, it promotes the formation of a fluorine-rich (LiF) passivation layer at the interface with lithium metal, greatly enhancing interface stability and enabling stable cycling for over 800 hours in symmetric batteries. Fichtner's work reveals the regulatory effect of halogen identity (Cl and F) on material properties, indicating that an optimized balance between ionic conductivity and interfacial stability can be achieved through halogen mixing.⁸⁶

Fig. 9 (a) Arrhenius plots of ion conductivity of Li₃HoBr_6−xI_x with different iodine contents.⁸⁵ (b) The relationship between ion conductivity and activation energy with iodine content x. (a and b) Reproduced with permission.⁸⁵ Copyright 2022 American Chemical Society. (c) The Arrhenius diagram of the ion conductivity of Li₂ZrCl_6−xF_x (0 ≤ x ≤ 1.2) electrolyte at different temperatures.⁸⁶ (d) The relationship between room temperature ionic conductivity and activation energy of Li₂ZrCl_6−xF_x with fluorine content x. (c and d) Reproduced with permission.⁸⁶ Copyright 2023 American Chemical Society.

5 Performance optimization and stability enhancement

5.1 Ionic conductivity

In the crystal structure, the migration of lithium ions mainly occurs through the vacancy mechanism, gap mechanism, and gap substitution exchange mechanism.⁸⁷ For example, in halides with CCP (cubic close packing) anion arrangement, Li⁺ migrates between octahedral sites through tetrahedral interstitial sites (oct–tet–oct), while in HCP (hexagonal close packing) arrangement, Li⁺ undergoes anisotropic one-dimensional migration through vacancies between octahedral sites shared by the faces.⁵⁸ By regulating the anion sublattice type (CCP or HCP) and cation/vacancy arrangement, the ion transport pathway can be optimized.⁵⁸ The strategy for improving the ion conductivity of halide solid electrolytes mainly revolves around defect engineering and structural design. By engineering point defects such as vacancies and interstitial sites and by modulating anion disorder, the lithium-ion migration capability can be significantly enhanced.⁸⁷ For example, in the Li₃MX₆ structure, doping with M³⁺ will remove three Li⁺ and introduce two octahedral interstitial vacancies, accounting for about 33.3% of the vacancies, which are crucial for high ion conductivity.⁸⁸ In addition, heterovalent substitution (such as partially replacing Y³⁺ with Zr⁴⁺) can introduce a large number of vacancies, further improving ion conductivity.⁸⁸

The material synthesis method also has a significant impact on ion conductivity. Mechanical ball milling can introduce disordered cation sites and abundant defect structures, such as Li₃YCl₆, exhibiting partially disordered cation arrangement and LiCl₆⁵⁻ octahedral distortion through mechanical synthesis, thereby significantly enhancing ion transport.²⁸ However, such metastable defect structures are easily eliminated under low-temperature heat treatment (such as 60 °C), resulting in a decrease in ionic conductivity.²⁸ In addition, anion engineering is also an effective strategy for improving ion conductivity. For example, replacing chlorine with oxygen in Li₂ZrCl₆ forms Li_3.1ZrCl_4.9O_1.1, which can increase the ion conductivity from 0.33 mS cm⁻¹ to 1.3 mS cm⁻¹.³¹ This is mainly attributed to the energy stabilization effect of interstitial sites, rather than simply the CCP anion sublattice.³¹

Table 1 summarizes the ion conductivity data of various halide solid electrolytes (mainly including chlorides, bromides, fluorides, and iodides) at room temperature or specific temperatures. The table covers multi-element composite halides mainly composed of lithium (such as Li₂ZrCl₆, Li₃YCl₆, LiTaCl₆, etc.) and some sodium-based and rare-earth-doped systems. Some materials, such as LiNbOCl₄ and LiTaCl₆, exhibit better conductivity than typical oxide solid electrolytes (>10 mS cm⁻¹), demonstrating their potential for application in ASSLBs.

Table 1 Comparison of ionic conductivity of different halide solid electrolytes (unit: mS cm⁻¹)

Electrolyte type	Ionic conductivity	Ref.	Electrolyte type	Ionic conductivity	Ref.
2LiCl-0.7AlF3-0.3GaF3	5.4	89	Li3InCl4.8F1.2	0.51	90
2LiCl-GaF3	3.6	91	Li3InCl6	0.2 at 300 °C	79
2LiF:Li2TiF6	2.5 × 10^−3	92	Li3InCl6	1.49	93
Li0.388Ta0.238La0.475Cl3	3.02	94	Li3InCl6	2.04	95
Li0.447Ta0.179Zr0.059La0.475Cl3	5.17	46	Li3LaI6	1.23 (calculate)	96
Li0.8Zr0.25La0.5Cl2.7O0.3	0.75	97	Li3Sc2/3Cl4	1.5	98
Li1.25La0.58Nb2O6F	3.9	99	Li3ScCl6	3.02	51
Li1.25La0.58Ta2O6F	1.7	99	Li3ScCl6	3	100
Li1.4MgCl3.4	8.69 × 10^−3	54	Li3Ta2O2FCl9	2.3	101
Li2.31Y0.98Nb0.02Cl5.31	1	102	Li3TbBr6	1.7	103
Li2.5Lu0.5Zr0.5Cl6	1.5	104	Li3YBr6	1.7	105
Li2.5ZrCl5F0.5O0.5	1.17	106	Li3YBr6	3.31	107
Li2.61Y1.13Cl6	0.47	108	Li3YBr6	0.72	105
Li2.6In0.8Ta0.2Cl6	4.47	109	Li3YCl5F	0.22	110
Li2.6Sc0.6Hf0.4Cl6	1.33	111	Li3YCl6	0.51	105
Li2.6Sc0.6Zr0.4Cl6	1.61	111	Li3YCl6	0.51	105
Li2.6Yb0.6Hf0.4Cl6	1.5	112	Li3YCl6	3.07 at 450 °C (calculate)	28
Li2.73Ho1.09Cl6	1.3	113	Li3ZrCl4O1.5	1.35	114
Li2.75Y0.16Er0.16Yb0.16In0.25Zr0.25Cl6	1.171	115	Li4YI7	1.04	116
Li2.7Yb0.7Zr0.3Cl6	1.1	117	LiAlBr4	0.033	118
Li2HfCl6	∼0.5 (calculate)	119	LiAlCl4	1.2 × 10^−3	120
Li2TiCl6	0.115	96	LiAlI4	0.012	118
Li2TiCl6	1.04 at 300 °C	96	Li-LaCeZrHfTa-Cl	1.8	121
Li2ZrCl5.2F0.8	0.2	122	LiNbCl6	12.192 (calculate)	123
Li2ZrCl5.6F0.4	0.321	124	LiNbOCl4	10.4	125
Li2ZrCl6	0.81	126	LiTaCl6	7.16	127
Li2ZrCl6	5.81 × 10^−3 at 350 °C	126	LiTaCl6	10.95	128
Li2ZrF5Cl1	5.5 × 10^−4	43	LiTaCl6	7.16	127
Li3ErBr6	1	129	LiTaCl6	11 (calculate)	130
Li3ErCl6	3 (calculate)	26	LiTaOCl4	12.4	125
Li3ErI6	0.65	131	Na0.7La0.7Zr0.3Cl4	0.29	132
Li3HoBr3I3	2.7	85	NaTaCl6	4	133
Li3HoBr6	1.1	134	SmCl3·0.5LiCl	0.1	135
Li3HoBr6	1.25	136

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Tang et al.¹³⁷ proposed a non-isomolar substitution strategy and designed a halide solid electrolyte (Li₂Zr_0.75Ta_0.2Cl₆) with co-enrichment of lithium and vacancies, which had an ionic conductivity as high as 1.74 mS cm⁻¹ (30 °C), significantly higher than that of the traditional isomolar substituted Li_1.6Zr_0.6Ta_0.4Cl₆ (0.96 mS cm⁻¹), as shown in Fig. 10a. This strategy optimizes the lithium-ion migration path by replacing more Zr⁴⁺ with less Ta⁵⁺ to generate Zr vacancies while maintaining the lithium content. The all-solid-state battery based on this electrolyte, when paired with a high-voltage nickel-rich cathode (such as scNCM811), demonstrates excellent long-cycle stability, with a capacity retention rate of 80% after 7800 cycles at 5 C.¹³⁷ Li et al.¹³⁸ systematically investigated the effects of doping ions with three different valence states, Zr⁴⁺, Ta⁵⁺, and W⁶⁺, on the crystal structure and lithium ion diffusion behavior of Li₃YCl₆ (Fig. 10b). The study found that Zr⁴⁺ doping achieved rapid lithium ion diffusion on both the (001) and (002) crystal planes by regulating the octahedral volume and tetrahedral gaps, while Ta⁵⁺ and W⁶⁺ produced enhancement or inhibition effects on specific crystal planes, respectively. Experimental results combined with structural analysis and theoretical calculations revealed that the Zr⁴⁺ doped sample (LYZrC-0.04) achieved an optimal ion conductivity of 0.437 mS cm⁻¹ at room temperature and exhibited excellent full cell performance.¹³⁸ Huang et al.⁵⁴ investigated the correlation between the structure and ionic conductivity of Li_xMgCl_2+x (2 ≥ x ≥ 1) halide spinel solid electrolytes through neutron diffraction and Rietveld refinement analysis. The study found that the ion conductivity showed a non-monotonic trend with the change of the x value and reached its highest value at x = 1.4 (8.69 × 10⁻⁶ S cm⁻¹), mainly due to the synergistic effects of crystal parameters such as cell parameters, Li⁺ vacancies (located at the 8a site), Debye–Waller factor, and Li–Cl bond length on Li⁺ migration behavior at different x value stages (Fig. 10c). In the initial stage (x = 2–1.6), the increase in the Li–Cl bond length and Debye–Waller factor promoted the improvement of conductivity. The dominant effect of increasing vacancy concentration and Debye–Waller factor in the intermediate stage (x = 1.6–1.4) overcomes the negative effects of cell shrinkage and bond length shortening. At the final stage (x = 1.4–1), all parameters are unfavorable for ion transport. The calculation of bond valence site energy (BVSE) indicates that Li⁺ mainly migrates through the three-dimensional Li1–Li1 pathway.⁵⁴ Fu et al.¹³⁹ investigated the crucial role of covalent chemical bonds in halide solid electrolytes in the superionic transition (SIT) and ionic conductivity. Using Li₃InBr₆ as a model, they revealed that the SIT does not stem from crystal phase transitions but is driven by the transformation of thermally induced bonding properties from ionic to covalent. By extending to the Li₃LnBr₆ (Ln = Gd, Tb, Ho, Tm, Lu) series, the study found that the SIT phenomenon is widespread in this type of material. Among them, Fig. 10d (left) shows the Arrhenius diagram of the ionic conductivity of Li₃LnBr₆ with temperature, and all compounds show the transformation of activation energy, confirming the wide existence of the SIT. Fig. 10d (right) summarizes the Li⁺ conductivity of this series of electrolytes at 25 °C and 75 °C, with Li₃GdBr₆ reaching its peak value of 5.2 mS cm⁻¹ at 298 K. These findings offer a new perspective for designing halide solid-state electrolytes with high ionic conductivity and have been successfully applied to all-solid-state lithium-ion batteries, demonstrating excellent low-temperature and high-capacity performance.¹³⁹ Qiu et al.⁶¹ developed an ammonia-assisted hydrolysis combined with freeze-drying (AAH-FD) method, which significantly improved the ionic conductivity of halide electrolyte Li₃YCl₆. By optimizing the synthesis process, Li₃YCl₆ prepared by the AAH-FD method has an orthogonal Pnma structure and smaller and uniform particle size, effectively reducing grain boundary resistance and the lithium ion migration energy barrier and achieving a room temperature ion conductivity of 0.50 mS cm⁻¹, which is significantly better than that of the traditional ball milling method. Fig. 10e visually presents the differences in conductivity and activation energy of samples prepared by different methods, highlighting the advantages of AAH-FD in improving ion transport performance.⁶¹ Jeon et al.¹⁴⁰ revealed the key role of high valence cations (such as Sc³⁺) in enhancing the lithium-ion conductivity of cubic spinel structured halide solid electrolytes (such as Li₃Sc₂/₃Cl₄) through first principles calculations. Research has found that lithium ions in Li₃Sc₂/₃Cl₄ tend to be randomly distributed at the 8a, 16c, and 16d sites and achieve rapid migration through a single ion diffusion mechanism (Fig. 10f). Its room temperature conductivity is as high as 1.36 mS cm⁻¹, which is much better than that of similar structures of Li₃MgCl₄ (5.3 × 10⁻⁴ mS cm⁻¹). This significant difference is mainly due to the fact that Sc³⁺ can reduce the blockage of lithium ion diffusion pathways compared to Mg²⁺, especially the concentration of multivalent cations at the 16d site, which is reduced, providing a more continuous three-dimensional lithium ion transport channel. Jeon's work provides a new approach for designing high-conductivity halide electrolytes by introducing high-valence cations.¹⁴⁰

Fig. 10 (a) Comparison of lithium-ion diffusion paths with non-isomolar substitution and isomolar substitution, as well as the variation trend of ionic conductivity with component x. Reproduced with permission.¹³⁷ 2025 Wiley-VCH GmbH. (b) Zr⁴⁺ doping regulates the lattice gap of Li₃YCl₆, achieving rapid Li⁺ diffusion on the (001)/(002) crystal plane. Reproduced with permission.¹³⁸ Copyright 2025 American Chemical Society. (c) Neutron diffraction and Rietveld refinement reveal that the Li_xMgCl_2+x structural parameters synergistically regulate conductivity, reaching a peak of 8.69 × 10⁻⁶ S cm⁻¹ at x = 1.4. Reproduced with permission.⁵⁴ Copyright 2024 American Chemical Society. (d) (left) Arrhenius plot of ionic conductivity of Li₃LnBr₆ (Ln = Gd, Tb, Ho, Tm, Lu) with temperature; (right) summary of Li⁺ conductivity of Li₃LnBr₆ SEs at 25 °C and 75 °C. Reproduced with permission.¹³⁹ 2025 Wiley-VCH GmbH. (e) Li₃YCl₆ prepared by the AAH-FD method has higher room temperature ionic conductivity and lower activation energy compared to ball milling and annealing methods. Reproduced with permission.⁶¹ Copyright 2025 American Chemical Society. (f) The three-dimensional diffusion path and energy distribution of lithium ions through 8a–16c–16d sites in Li₃Sc₂/₃Cl₄. Reproduced with permission.¹⁴⁰ Copyright 2024 American Chemical Society.

5.2 Electrochemical stability

Although halide solid electrolytes such as Li₃YCl₆ and Li₃InCl₆ have high ionic conductivity, they still face challenges in terms of electrochemical stability in practical battery applications, especially in terms of interface side reactions when in contact with high-voltage cathodes and lithium metal anodes.^30,141 Research has shown that the actual electrochemical stability window of halide electrolytes is usually wider than the theoretical window calculated based on thermodynamic decomposition products, because their decomposition process often occurs through the intermediate state of (de)lithiation, rather than directly decomposing into the most stable product, thus delaying the decomposition process kinetically.³⁰

Element doping is an effective strategy to improve the electrochemical stability of halide electrolytes. For example, introducing fluorine doping into Li₃YBr₆ can in situ form a fluoride-rich interfacial layer (Li₃YBr_5.7F_0.3) during cycling, significantly reducing interfacial resistance and improving its compatibility with lithium metal.³⁸ Similarly, introducing Zr⁴⁺ doping into Li₃InCl₆ can reduce its reactivity with lithium metal, while co-doping with Al and F can promote the formation of Li–Al alloy and fluoride interfaces, further stabilizing the interface and inhibiting dendrite growth.²³

Building an artificial interface layer is also an important way to improve interface stability. Coating Li₃OCl on the surface of Li₃YCl₆ can effectively improve its interfacial stability with lithium metal.²³ By forming a passivation layer rich in LiF, electron transfer can be effectively blocked and the growth of lithium dendrites can be suppressed, thereby achieving dendrite-free deposition and a stable electrode/electrolyte interface.³⁸ In practical battery systems, halide electrolytes are often combined with sulfide electrolytes to balance high ion conductivity and interfacial stability. However, the interfacial chemical stability between the two is still a matter of concern, and it is necessary to suppress interfacial side reactions through reasonable material selection and doping strategies.¹⁴²

Samanta et al.¹⁴³ systematically compared the chemical compatibility of halide solid electrolytes (HSEs) with different central metals and sulfide solid electrolytes Li₆PS₅Cl, revealing their key impact on the electrochemical stability of the double-layer structure of ASSLBs. Fig. 11a shows that the capacity retention rate of the battery using Li₃YCl₆ is significantly higher than that of the Li₃InCl₆ system, indicating serious interface side reactions between the latter and sulfide electrolytes, leading to impedance growth and cycling degradation. This highlights the importance of HSEs based on a reasonable selection of central metal ion electronegativity for optimizing interface stability and battery performance.¹⁴³ Wang et al.¹⁴⁴ synthesized Li₂ZrCl_6−xI_x halide solid electrolytes by partially replacing chlorine with iodine. By utilizing the high polarization of iodine to enhance its covalent bond with the central cation, the reduction stability of lithium metal was significantly improved. The Li₂ZrCl₄I₂-based symmetric battery was stably cycled for over 6000 hours at 0.2 mA cm⁻² (Fig. 11b), with a critical current density of 6 mA cm⁻². XPS and ToF-SIMS analysis showed that a passivation layer composed of LiI and LiCl was formed at the interface, effectively suppressing the continuous decomposition of the electrolyte and optimizing the electrochemical stability of the interface.¹⁴⁴ Tuo et al.³⁹ optimized the electrochemical stability of halide solid electrolyte Li_2+xHf_1−xFe_xCl₆ by doping low-cost, crustal-abundant Fe³⁺ with heterovalent doping, resulting in an increase in ion conductivity to 0.91 mS cm⁻¹ at 30 °C and a significant enhancement of antioxidant capacity (>4 V vs. Li⁺/Li). As shown in Fig. 11c, the ASSLB using this electrolyte exhibits higher initial discharge capacity (113.4 mAh g⁻¹) and coulombic efficiency (91.2%) at 0.1 C, significantly better than the undoped system. Structural analysis and theoretical calculations indicate that Fe doping induces lattice distortion and lithium ion redistribution, forming three-dimensional continuous migration channels, while improving compatibility with uncoated LiCoO₂ cathodes, achieving excellent electrochemical performance of ASSLBs.³⁹ Xue et al.⁴⁴ significantly improved the ionic conductivity (up to 9 mS cm⁻¹ at 30 °C and 0.59 mS cm⁻¹ at −35 °C) and electrochemical stability of halide electrolytes by developing a novel Li-M-X₅ oxyhalide solid electrolyte Li_3xTaO_3xCl_5−x. The introduction of oxygen reduces the coordination number of Ta–O/Ta–C and induces polyhedral distortion, broadening the Li⁺ migration pathway. The linear sweep voltammetry test showed that its oxidation decomposition voltage reached 4.0 V vs. Li⁺/Li, which is suitable for high voltage systems. The ASSLB maintains 100% capacity after 3200 cycles at a 4C rate (Fig. 11d), demonstrating excellent interface stability and cycling durability.⁴⁴ Yanagihara et al.¹⁴⁵ systematically evaluated the electrochemical stability and compatibility of three halide solid electrolytes (Li₃InCl₆, Li₃YCl₆, and Li₃YBr₆) in the cathode of solid-state lithium sulfur batteries and found that although halides have high oxidation stability, their reduction stability and chemical compatibility with sulfur active materials are still key limiting factors. Fig. 11e shows that Li₃YBr₆ exhibits optimal cycling performance in the range of 0.6–2.6 V, maintaining a capacity of 1100 mAh g⁻¹ (based on sulfur mass) for 20 cycles, while Li₃InCl₆ and Li₃YCl₆ rapidly decay due to reduction decomposition or reaction with Li₂S to form insulating phases (such as LiYS₂), indicating that anion selection (Br⁻vs. Cl⁻) has a decisive impact on interface stability and electrochemical performance.¹⁴⁵ Ye et al.¹⁰⁹ optimized the electrochemical stability of halide solid electrolytes through a doping strategy of high-valent elements, with a focus on developing Li materials. The ionic conductivity of Li_2.6In_0.8Ta_0.2Cl₆ materials reached 4.47 mS cm⁻¹ at 30 °C, and the oxidation initiation potential was as high as 5.13 V. Fig. 11f shows that its structure is stable, the lithium-ion migration energy barrier is low, and it has good compatibility with high-voltage cathodes (such as LiCoO₂), enabling the ASSLB to achieve over 1400 cycles at 4.6 V, with a capacity retention rate of 70%, significantly enhancing the application potential of halide electrolytes at high voltages.¹⁰⁹

Fig. 11 (a) Comparison of cycle capacity retention rates of double-layer batteries composed of Li₃YCl₆ and Li₃InCl₆ with Li₆PS₅Cl, respectively. Reproduced with permission.¹⁴³ Copyright 2024 American Chemical Society. (b) Iodine substitution enhances the stability of Li₂ZrCl₆, achieving over 6000 hours of stable cycling. Reproduced with permission.¹⁴⁴ Copyright 2025 American Chemical Society. (c) Fe³⁺ doping enhances the conductivity and oxidation resistance of halide solid electrolytes, optimizing the performance of ASSLBs. Reproduced with permission.³⁹ Copyright 2024 American Chemical Society. (d) Linear scanning voltammetry curve of Li_1.2TaO_1.2Cl_3.8; oxidation decomposition voltage of 4.0 V vs. Li⁺/Li. Reproduced with permission.⁴⁴ Copyright 2025 American Chemical Society. (e) Comparison of the cycling performance of three halide electrolytes in solid-state lithium sulfur batteries; Li₃YBr₆ showed the optimal capacity retention rate. Reproduced with permission.¹⁴⁵ Copyright 2024 American Chemical Society. (f) High-priced element doping enhances the performance of halide electrolytes, enabling solid-state batteries to achieve long cycle life. Reproduced with permission.¹⁰⁹ Copyright 2024 American Chemical Society.

5.3 Mechanical properties

The mechanical properties of halide solid electrolytes such as Li₃YCl₆ and Li₃YBr₆ are crucial for their application in ASSLBs. Research has shown that this type of material has a higher shear modulus than thiophosphate-type electrolytes, but lower than garnet-type solid electrolytes, indicating its potential in inhibiting lithium dendrite growth, adapting to electrode material volume changes, and preventing self-degradation.¹⁴⁶ Specifically, Li₃YCl₆ and Li₃ScCl₆ exhibit better ductility than other halide candidate materials, and their Poisson's ratio and Pugh index calculations indicate good mechanical compatibility.¹⁴⁶

In practical battery applications, solid electrolytes need to have sufficient deformability and ductility to adapt to the volume changes of electrode active materials during charging and discharging, thereby reducing stress accumulation at the interface and maintaining stable solid–solid contact.¹⁴⁷ Although sulfide electrolytes have high ionic conductivity and good mechanical flexibility, their oxidation stability is low, and they are sensitive to moisture.⁵⁴ In contrast, halide electrolytes such as Li₃YCl₆ and Li₃InCl₆ not only have high ionic conductivity (about 10⁻³ S cm⁻¹), but also exhibit good mechanical deformability and compatibility with high-voltage cathode materials.⁵⁴ The greater polarizability of halide anions in halide electrolytes results in greater mechanical deformability compared to oxide electrolytes, which helps maintain close contact between electrodes and electrolytes during battery cycling.⁵⁴ Although oxide electrolytes have high chemical, mechanical, and environmental stability, their inherent brittleness leads to high interfacial resistance with electrodes, while polymer electrolytes are difficult to effectively inhibit the growth of lithium dendrites due to insufficient mechanical strength.^54,148 Therefore, halide electrolytes exhibit unique advantages in mechanical properties and have become powerful candidate materials for achieving high-performance ASSLBs.

By regulating the mechanical properties of halide solid electrolytes, their clay-like deformability can be achieved. The molecular dynamics simulation in Fig. 12a shows that LiCl and GaF₃ undergo anion exchange at high temperatures, forming GaCl₃ molecular crystals (Fig. 12a-left). The EXAFS spectrum confirms the changes in the coordination environment of Ga (Fig. 12a-right). The optical image in Fig. 12b shows that the electrolyte films, such as LiAlCl_2.5O_0.75, have good flexibility. This soft property helps achieve close electrode–electrolyte interface contact at low stacking pressure, thereby enhancing battery performance.¹⁴⁹Fig. 12c compares the mechanical properties of rigid oxides and tough electrolytes, emphasizing that halide solid electrolytes have lower Young's modulus and good cold-press formability due to their softer anionic sublattices, thereby achieving higher ion conductivity without the need for high-temperature sintering, demonstrating excellent mechanical adaptability and industrial potential.¹⁴⁹

Fig. 12 (a) (left) High-temperature molecular dynamics simulation of the LiCl-GaF₃ composite material generates GaCl₃ molecules. Comparison of K-edge EXAFS of Ga in different halides.¹⁴⁹ (b) Optical images of LiAlCl_2.5O_0.75 and NaAlCl_2.5O_0.75 flexible halide electrolyte membranes.¹⁴⁹ (c) (left) Rigid inorganic solid electrolytes have limited ion transport under cold-pressure conditions, and high grain boundary impedance leads to low conductivity. Under cold-pressure, the grain boundary impedance of tough electrolytes can be neglected, thereby achieving high ionic conductivity. (right) The hardness modulus relationship diagrams of various solid electrolytes highlight the advantages of ductile materials in mechanical properties. (a–c) Reproduced with permission.¹⁴⁹ Copyright 2025 American Chemical Society.

5.4 Air-moisture stability

One of the main challenges faced by halide solid electrolytes (HSEs) in practical applications is their sensitivity to moisture in the air.¹⁶ Common halide electrolytes, such as Li₃InCl₆ and Li₃YCl₆, are highly sensitive to moisture in the air and can quickly absorb moisture and completely liquefy after exposure. For example, Li₃InCl₆ and Li₃YCl₆ transform into transparent solutions within 2 hours and 8 hours, respectively.¹⁶ Research has shown that their moisture absorption rate is positively correlated with the contact area of humid air.¹⁶

After exposure to air, Li₃InCl₆ first forms Li₃InCl₆·2H₂O crystalline hydrate, which then undergoes partial hydrolysis and decomposes into InCl₃ and LiCl. Among them, InCl₃ can be further hydrolyzed to form the intermediate phase In(OH)₃ and finally dehydrated to form In₂O₃ impurities.¹⁶ The coating strategy is considered an effective solution to improve the air stability of halide solid electrolytes. For example, by depositing powder atomic layers on the surface of Li₃InCl₆ and coating Al₂O₃, it can effectively isolate humid air, significantly improve its moisture resistance, and extend the air stability time by 4–7 times.¹⁶ Among various halides, chlorides exhibit better water stability than sulfides, and their hydrolysis reaction energy is higher, consistent with experimental results.²⁷ In chlorides, the “In” cation exhibits the best water stability, consistent with experimental reports of excellent water stability of Li₃InCl₆.²⁷

Doping strategies have also been used to improve the moisture resistance of halide solid electrolytes.^150,151 For example, F⁻doping can significantly reduce the water absorption rate of Li₃InCl₆, such that after exposure to a dew point drying chamber at −20 ± 3 °C for 5 hours, Li₃InCl₆₅₆F_0.4 can still maintain 62% ion conductivity, while undoped samples only retain 22.2%.³⁵ This improvement may be related to the stable Li–F and In–F bonds formed after F⁻ doping.³⁵

Halide solid electrolytes (HSEs, such as Li₃InCl₆) have high ionic conductivity, but are sensitive to moisture in the air and are prone to hydrolysis, leading to structural degradation and performance degradation. To improve the air stability of HSEs, surface modification strategies involving protective coatings have been proposed. For example, as shown in Fig. 13a, an Al₂O₃ layer is coated on the surface of Li₃InCl₆ using atomic layer deposition technology to effectively isolate moisture contact and significantly prolong the stability time of the material in humid environments. However, this method will slightly reduce ion conductivity and increase manufacturing costs.³⁵ Chen et al. significantly improved the stability of halide solid electrolyte Li₃InCl₆ in humid environments by using a fluorine doping strategy. As shown in Fig. 13b, at a dew point of −20 °C, the undoped sample (LIC) rapidly absorbed moisture, with a water absorption rate of 7.8 g mol⁻¹ within 12 hours. In contrast, the fluorine-doped sample (LICF_0.4) absorbed only 1.8 g mol⁻¹ within the same period of time, with the water absorption rate dropping to approximately a quarter, indicating a significant enhancement in its moisture resistance. Furthermore, the ionic conductivity test (Fig. 13c) revealed that after 5 hours of moist exposure, LICF_0.4 still maintained a relatively high conductivity of 0.85 × 10⁻³ S cm⁻¹, with a decrease much smaller than that of the undoped sample, which was 0.42 × 10⁻³ S cm⁻¹. The results show that fluorine doping effectively inhibits water-induced structural degradation and performance attenuation, providing an effective approach for the development of moisture-resistant halide solid electrolytes.¹⁵² Halide solid electrolytes (such as REHSEs) commonly suffer from strong hygroscopicity and easy decomposition when exposed to water. For example, Li₃YCl₆ can be converted into LiCl·H₂O and YCl₃·6H₂O in humid air. However, as shown in Fig. 13d, the formation of Li₃Y_1−xIn_xCl₆ (x > 0.5) through In³⁺ doping can significantly improve its air stability, and the hydrated intermediates generated in humid environments can recover their structure and maintain high ionic conductivity after heat treatment.⁵⁹ Long et al.¹¹ systematically analyzed the degradation mechanism of halide solid electrolyte Li₃YCl₆ at different humidity levels and proposed the use of the graphite phase nitride carbon (g-C₃N₄) coating strategy to improve its humidity stability, in response to the problem of easy hydrolysis of Li₃YCl₆ in air. Fig. 13e shows a schematic diagram of the preparation of uniformly coating g-C₃N₄ on the surface of Li₃YCl₆ particles by the ball milling method. The coating, as a physical barrier layer, effectively slows down water erosion, maintaining high structural stability and ionic conductivity of Li₃YCl₆ after coating in humid environments, thereby enhancing its practical application potential.¹¹ Li et al. significantly improved the air humidity stability of halide solid electrolytes by regulating the content of In³⁺ in Li₃Y_1−xIn_xCl₆. When the In content is high (x > 0.5), the material forms a hydrated intermediate phase in humid air rather than irreversible decomposition products, allowing it to recover its structure and ionic conductivity after reheating. Fig. 13f shows the Arrhenius plot of Li₃Y_0.2In_0.8Cl₆ after exposure to 3–5% humidity air and reheating, with an ion conductivity retention rate of up to 85.37%, indicating that high In content effectively enhances the humidity tolerance of the material.⁴⁹ Nie et al. significantly improved the ion conductivity of monoclinic Li₃GaF₆ by doping the particle boundary with chlorine using the low-temperature molten salt ablation method. Compared to chloride/bromide solid electrolytes that are easily affected by moisture, fluoride-based electrolytes exhibit excellent air stability due to their inherent hard alkali properties. Fig. 13g compares the electrochemical impedance spectra before and after exposure to a 35% relative humidity environment, indicating that the ion conductivity of the Li₃GaF₆ electrolyte prepared by the low-temperature molten salt ablation method did not significantly decrease. This confirms that it can still maintain structural stability and ion transport performance under conventional atmospheric conditions, overcoming the limitation of humidity sensitivity of most halide electrolytes.¹⁵³ The air/humidity stability of halide solid electrolytes is closely related to the type of central metal element. Fig. 13h shows the hydrolysis reaction energies of different chlorides (including binary M–Cl, ternary Li–M–Cl, and Na–M–Cl) through first principles calculations. It was found that sodium-based ternary chlorides (especially those containing lanthanide elements such as Tm³⁺, Er³⁺, etc.) and lithium-based ternary chlorides (containing In³⁺, Ga³⁺, etc.) have better humidity stability, with Li₃InCl₆ showing the best performance, providing a theoretical basis for their application in humid environments.²⁸

Fig. 13 (a) Schematic diagram of coating Al₂O₃ on the surface of Li₃InCl₆ by powder atomic layer deposition to enhance its air stability. Reproduced with permission.³⁵ Copyright 2023 Royal Society of Chemistry. (b) Comparison of the water uptake profiles of LIC and LICF_0.4 at a constant dew point of −20 ± 3 °C.¹⁵² (c) EIS and ionic conductivity of LIC and LICF_0.4 after 5 h of exposure to moisture. (b and c) Reproduced with permission.¹⁵² Copyright 2022 Elsevier B.V. (d) Comparison of humidity stability between Li₃YCl₆ and Li₃Y_1−xIn_xCl₆ with different In³⁺ doping ratios. Reproduced with permission.⁵⁹ Copyright 2025 Elsevier B.V. (e) Schematic diagram of coating g-C₃N₄ on the surface of Li₃YCl₆ particles by the ball milling method. Reproduced with permission.¹¹ Copyright 2023 Royal Society of Chemistry. (f) Comparison of Arrhenius plots of Li₃Y_0.2In_0.8Cl₆ after humidity exposure and reheating with the original sample. Reproduced with permission.⁴⁹ Copyright 2020 American Chemical Society. (g) Comparison of Nyquist plots before and after exposure to a 35% relative humidity environment for 24 hours for Li₃GaF₆ electrolyte prepared by the low-temperature molten salt ablation method. Reproduced with permission.¹⁵³ Copyright 2024 American Chemical Society. (h) Comparison of hydrolysis reaction energies of binary chloride, ternary lithium chloride, and ternary sodium chloride. Reproduced with permission.²⁸ Copyright 2023 Springer Nature.

5.5 Thermal stability

Inorganic solid electrolytes (ISEs) exhibit significant advantages in thermal stability, with oxide, sulfide, and halide-based ISEs having good thermal stability, in stark contrast to polymers that may melt, decompose, or undergo phase transitions at lower temperatures.¹⁵⁴ However, the thermal stability of the material itself does not represent the thermal stability of the entire battery system. The interaction between electrode materials and ISE typically exhibits different thermal behavior at the interface compared to a single component.¹⁵⁴ The interface reaction under thermal drive is often accompanied by morphological changes, and the high impedance impurities generated will further damage the performance of solid-state batteries.¹⁵⁴ In addition, the products and heat of interface reactions may further trigger a chain reaction, leading to thermal runaway behavior.¹⁵⁴ The study of interface thermal stability aims to elucidate the reaction mechanisms that lead to thermal failure and thermal runaway, with the goal of minimizing system heat generation during battery charging and discharging processes.¹⁵⁴

The interface instability between halide inorganic solid electrolytes and electrodes also affects the thermal stability of ASSLBs and may potentially lead to thermal runaway. It should be emphasized that although replacing flammable liquid electrolytes with non-flammable solid electrolytes theoretically enhances battery safety, ASSBs still face significant thermal stability challenges, for example, the intense exothermic reaction between the delithiated nickel-rich oxide cathode and the ISE.²³ These reactions are exacerbated by the reaction between oxygen released from the cathode and the electrolyte, which generates heat and may ignite the material. Halide inorganic solid electrolytes exhibit the potential to mitigate these thermal risks by participating in competitive endothermic reactions that consume oxygen, thereby reducing the likelihood of thermal runaway.²³ These reactions replaced reactive oxygen with less reactive chlorine gas, improving the overall thermal stability of the battery. However, when the halide ISE comes into direct contact with the lithium metal anode, its thermal stability is compromised, so careful interface engineering is required to prevent decomposition and ensure the safe operation of the battery. A deeper understanding of the thermal stability mechanisms is essential, along with the development of effective strategies to enhance the safety of halide ASSBs.²³

Specifically at the material level, Li₃InCl₆ (LIC) based electrolytes exhibit excellent thermal stability. Studies have shown that incorporating LiX@Mil-100 into PVDF-HFP can significantly enhance the thermal stability of the E-LiX electrolyte, achieving dimensional stability at 100 °C, which far exceeds that of commercial polypropylene diaphragms (thermal shrinkage at approximately 80 °C).¹⁵⁵ Thermogravimetric analysis (TGA) further revealed that the E-LiI electrolyte does not decompose at temperatures up to 160 °C, and the significant decomposition initiation temperature is only above 350 °C, demonstrating excellent thermal stability.¹⁵⁵

Xian et al.¹⁵⁶ synthesized Li₃In_xHo_1−xCl₆ halide solid electrolytes by partially replacing Ho⁺ with In⁺. They found that moderate doping can improve ion conductivity and optimize structural stability. Fig. 14a shows the TGA and DSC curves of Li₃In_0.3Ho_0.7Cl₆ in a nitrogen atmosphere, with a mass loss of less than 2% and no significant thermal peak, indicating that this material has good thermal stability and is suitable for ASSLBs.¹⁵⁶ Jung et al.'s¹⁵⁷ study revealed the crucial role of halide solid electrolytes (such as Li₃InCl₆) in enhancing the thermal stability of ASSLBs. It significantly inhibits oxygen release and delays the phase transition process through interface interaction with the charged NCM622 cathode material. Halide electrolytes capture oxygen species through oxidative decomposition, generating metal oxychlorides and oxides, while releasing Cl₂ instead of flammable O₂, accompanied by endothermic reactions, thereby reducing overall thermal risk. Fig. 14b compares the thermal decomposition behavior of NCM alone with the NCM/LIC composite material, highlighting the inhibition of oxygen release and the phase transition delay effect of halide electrolyte.¹⁵⁷ Chen et al.⁷⁵ identified three lithium ion solid electrolyte candidate materials with good thermal stability from 3119 halides through high-throughput screening: Li₄ZrF₈, Li₃ErBr₆, and Li₂ZnI₄. Fig. 14c shows their phase diagram and stability analysis, where the E_hull values of Li₄ZrF₈ and Li₃ErBr₆ are 18 and 2 meV per atom, respectively, indicating high thermodynamic stability, while the E_hul of Li₂ZnI₄ is 26 meV per atom, which is slightly higher but still has the potential to exist stably at room temperature.⁷⁵Fig. 14d shows the thermogravimetric analysis curve of annealed Li₃InCl₆ in dry air, with stable weight and no obvious exothermic or endothermic peaks, indicating that the material has good thermal stability and is suitable for practical applications in solid-state batteries.⁴¹ Dong et al.¹⁵⁸ successfully developed a novel Na–Zr–S–Cl sulfide chloride solid electrolyte by regulating the anion ratio of sulfur and chlorine. The thermogravimetric analysis (TGA) curve in Fig. 14e shows that the Cl-deficient (Na₂S-1.3ZrCl₄) structure remains stable before 400 °C, while the Cl-enriched (Na₂S-3.3ZrCl₄) structure begins to decompose at 150 °C, indicating that an increase in Cl content significantly reduces the thermal stability of halide solid electrolytes.¹⁵⁸ Braga et al.¹⁵⁹ reported a novel halide solid electrolyte Li₃–_2xM_xHalO (M = Mg²⁺, Ca²⁺, Ba²⁺) based on an anti-perovskite structure (Hal = Cl⁻, I⁻), which exhibits extremely high ionic conductivity (25 mS cm⁻¹, 25 °C) and a wide electrochemical window (>8 V). Fig. 14f shows by differential scanning calorimetry (DSC) that the melting peak of the hydroxide of the material disappears after heating cycles, and glass transition (T_g ≈ 136 °C) and melting peak (T_m = 269 °C) are observed, indicating that the electrolyte has good thermal stability and glass-forming ability and is suitable for high-temperature lithium battery applications.¹⁵⁹ Qiu et al.¹⁶⁰ evaluated the bulk properties and transport mechanism of Li₃MX₆ (M = Sc, Y, Er; X = Cl, Br, I) halide solid electrolytes using first principles calculations. They found that chlorides exhibit wider band gaps, better mechanical properties, and thermal stability due to significant differences in electronegativity. Fig. 14g shows that chlorides (such as Li₃YCl₆) have high lattice thermal conductivity (reaching 2.72 W (m K)⁻¹ at 300 K), which decreases with increasing temperature, indicating their better thermal management potential in ASSLBs and helping to suppress thermal runaway.¹⁶⁰ Sun et al.¹⁶¹ studied Li Fe chloride halide solid electrolytes and significantly improved their ion conductivity through a self-doping strategy, while systematically evaluating their thermal stability. The DSC curve in Fig. 14h shows that the melting points of Li₂FeCl₄ and Li₆FeCl₈ are 546 °C and 529 °C, respectively, indicating that both have uniform melting characteristics and good high-temperature thermal stability, making them suitable for ASSLB systems.¹⁶¹ Usami et al.¹⁶² studied the thermal stability and reaction behavior with atmospheric moisture of halide solid electrolytes Li₃YCl₆ and Li₃InCl₆. Fig. 14i shows that the H₂O release peak of Li₃InCl₆ appears at about 100 °C, close to the boiling point of water, indicating that its surface is mainly physically adsorbed. The H₂O release peak of Li₃YCl₆ shifts to about 140 °C, indicating that water molecules are more likely to penetrate its bulk phase. This water infiltration behavior further triggers the release of HCl at low temperatures. Fig. 14j shows through gas detection tube experiments that Li₃YCl₆ begins to release HCl below 100 °C, while Li₃InCl₆ needs to exceed 150 °C to release significantly, indicating that Li₃InCl₆ has better thermal stability and water tolerance, which is closely related to the permeation of water in its bulk phase and the substitution reaction of OH⁻ on Cl⁻.¹⁶²

Fig. 14 (a) The TGA and DSC curves of Li₃In_0.3Ho_0.7Cl₆ in an N₂ atmosphere showing its good thermal stability. Reproduced with permission.¹⁵⁶ Copyright 2025 Elsevier Ltd. (b) Comparison diagram of thermal decomposition behavior of NCM and NCM/LIC composite materials in the charged state during the heating process. Reproduced with permission.¹⁵⁷ Copyright 2024 American Chemical Society. (c) Phase diagram and stability analysis of Li₄ZrF₈, Li₃ErBr₆, Li₂ZnI₄. Reproduced with permission.⁷⁵ Copyright 2024 Elsevier B.V. (d) The thermogravimetric analysis curve of annealed Li₃InCl₆ in dry air shows that its weight is stable and there are no exothermic or endothermic peaks. Reproduced with permission.⁴¹ Copyright 2022 Elsevier B.V. (e) Thermogravimetric analysis curves of solid electrolytes Na₂S-1.3ZrCl₄ and Na₂S-3.3ZrCl₄ show that Cl defect type structures exhibit greater thermal stability at high temperatures. Reproduced with permission.¹⁵⁸ Copyright 2025 Wiley-VCH GmbH. (f) The DSC curve of Li₃₋₂ × _0.005Mg_0.005ClO showing the glass transition and melting behavior. Reproduced with permission.¹⁵⁹ Copyright 2014 Royal Society of Chemistry. (g) The relationship between the lattice thermal conductivity of Li₃MX₆ (M = Sc, Y, Er; X = Cl, Br, I) and temperature. Reproduced with permission.¹⁶⁰ Copyright 2021 American Chemical Society. (h) (left) Li₂FeCl₄ exhibiting an endothermic peak at 546 °C, corresponding to its melting temperature. (right) Li₆FeCl₈ exhibiting an endothermic peak at 529 °C, indicating its melting behavior. Reproduced with permission.¹⁶¹ Copyright 2024 American Chemical Society. (i) Comparison of the temperature-dependent H₂O release rate curves between Li₃YCl₆ and Li₃InCl₆. Reproduced with permission.¹⁶² Copyright 2023 Springer Nature. (j) Comparison of the amount of HCl gas released by Li₃YCl₆ and Li₃InCl₆ at different temperatures.

5.6 High-voltage stability

Halide solid electrolytes (HSEs) typically exhibit a wider electrochemical stability window than sulfides. For example, the electrochemical windows of Li₃ErCl₆ and Li₃ScCl₆ are 0.85–4.26 V and 0.15–4.08 V, respectively, which are significantly better than those of sulfide electrolytes (such as Li₃PS₅Cl at 1.71–2.14 V) and oxide electrolytes.²⁸ It is worth noting that halide electrolytes, such as Li₃YCl_6−xBr_x and Li₂ZrCl₆, can maintain structural integrity beyond their thermodynamically predicted stability windows, thus introducing the concept of a “dynamic stability window”. This characteristic is in sharp contrast to the irreversible decomposition and formation of stable phases of sulfide electrolytes once they exceed their window.³² In halide electrolytes, the type of anion plays a decisive role in their oxidation stability. Their oxidation potential decreases in the order of F⁻ > Cl⁻ > Br⁻ > I⁻. Fluorides have the widest electrochemical stability window due to their high electronegativity, but strong Coulomb interactions result in lower ionic conductivity. Although iodide can increase ion conductivity, it seriously sacrifices high voltage stability. Chlorine and bromide achieve a good balance between ionic conductivity and electrochemical stability.⁵⁹ In experiments, cyclic voltammetry (CV) and linear sweep voltammetry (LSV) are commonly used to measure the electrochemical window of solid-state electrolytes. It should be noted that traditional CV measurements using blocking electrodes, such as stainless steel, often overestimate the stability window, while composite electrodes of electrolyte and carbon (such as SE + C, SE: solid-state electrolyte) can more sensitively detect degradation current, thereby obtaining more accurate window results.^90,163 In addition, cation substitution can also affect the electrochemical stability of halide electrolytes. For example, Zr⁴⁺ doping causes the oxidation potential of Li_2.4Y_0.4Zr_0.6Cl₆ to decrease from 3.98 V of Li₃YCl₆ to 3.82 V, thereby narrowing its stability window.¹⁶⁴

Halide solid electrolytes (especially chlorides) have attracted much attention due to their high oxidation stability, especially when matched with high-voltage cathode materials, showing good compatibility.² For example, chloride electrolytes can come into direct contact with 4 V grade cathode materials, while sulfide electrolytes typically require a protective layer (such as LiNbO) to match high-voltage oxide cathodes due to their lower oxidation stability.¹⁶⁵ Halide electrolytes such as Li₃InCl₆ and Li₃YCl₆ not only have high ionic conductivity (up to the order of 10⁻³ S cm⁻¹ at room temperature), but also have a wide electrochemical stability window (oxidation potential up to 4 V or more), making them suitable for direct composite use with high-voltage cathodes such as LiCoO₂ (LCO) and LiNi_xCo_xMn₂O₂ (NCM) without the need for additional coating protection.⁵⁹ Theoretical research and experimental results have shown that the reaction energy between halide electrolytes and typical cathodes (such as LiFePO₄, LiCoO₂, etc.) is relatively low (usually below 50 meV per atom), indicating their good chemical stability and interfacial compatibility.²³ In particular, chlorine-based electrolytes can stably cycle in actual batteries even under conditions exceeding their theoretical oxidation potential (such as 4.3 V), because the dynamic quasi-steady state interface formed between them and the cathode can suppress further reactions and promote ion transport.¹³⁷ However, some halide electrolytes (such as Li_2.5Y_0.5Zr_0.5Cl₆) may still undergo interfacial reactions when matched with high-nickel cathodes (such as NCM85) at voltages above 4.3 V, generating products like YOCl or ZrO₂, which leads to increased impedance and capacity attenuation.³⁵ In addition, the active oxygen released by lithium manganese-based cathodes (LRMs) under high pressure can oxidize halide electrolytes (such as Li₃InCl₆), causing interface failure, which usually requires surface coating (such as LiNbO₃) to enhance interface stability.¹

As the core material of ASSLBs, the electrochemical stability window of halide solid electrolytes is a key parameter for evaluating their applicability, which determines their compatibility with high-voltage cathode and anode materials. According to existing research, there are significant differences in the stability window of different halide electrolytes. Fig. 15 compares the electrochemical stability windows of different halide solid electrolytes. Table 2 provides specific numerical ranges and literature sources. For instance, the window of Li₃YBr₆ is relatively narrow (0.59–3.15 V), while chlorine-rich materials such as Li₃ScCl₆ and Li₃YCl₆ typically exhibit wider high-voltage stability (up to ∼4.3 V). The window can be further broadened through anion replacement (such as introducing F⁻); for instance, the window of Li₃InCl_4.8F_1.2 can be extended to 3–6 V or even higher (2.95–6.59 V), significantly enhancing its matching ability with high-capacity electrodes. These studies provide important guidance for the design of next generation high-performance solid-state batteries.

Fig. 15 Comparison of electrochemical stability windows of different halide solid electrolytes.

Table 2 Numerical ranges of electrochemical stability windows for various halide solid electrolytes (HSEs)

HSEs	Electrochemical stability window (V)	Ref.	HSEs	Electrochemical stability window (V)	Ref.
Li3YBr6	0.59–3.15	50	Li17Sc5Cl32	0.91–4.26	69
Li3HoBr3I3	1.5–3.3	85	Li17Sm5Cl32	0.73–4.26	69
Li3HoBr6	1.5–3.3	134	Li17Y5Cl32	0.65–4.26	69
Li3ErBr6	1.5–3.4	129	Li3AlCl6	1.59–4.26	69
Li3HoBr6	1.5–3.4	136	Li3InCl6	2.38–4.26	33
Li3TbBr6	1.5–3.4	103	Li3ScCl6	0.91–4.26	69
Na0.7La0.7Zr0.3Cl4	1.33–3.8	132	Li3SmCl6	0.73–4.26	69
Li2.61Y1.13Cl6	0.5–4	108	Li3YCl6	0.66–4.26	69
NaTaCl6	2.5–4	133	LiScCl4	0.91–4.26	69
Li2.31Y0.98Nb0.02Cl5.31	0.81–4.04	102	LiYCl4	0.65–4.26	69
2LiCl-GaF3	2.4–4.1	91	Li0.388Ta0.238La0.475Cl3	2.01–4.27	94
Li2.6Sc0.6Hf0.4Cl6	1.6–4.1	111	Li2.7Yb0.7Zr0.3Cl6	2.8–4.3	117
Li3ZrCl4O1.5	2–4.1	114	Li3ScCl6	0.9–4.3	51
Li2.6Sc0.6Zr0.4Cl6	1.3–4.2	111	LiSmCl4	0.69–4.36	69
Li3YCl6	0.62–4.21	50	LiAlCl4	1.59–4.45	69
Li2.5Lu0.5Zr0.5Cl6	1.75–4.25	104	Li2ZrCl5.2F0.8	2.15–4.5	122
Li2.73Ho1.09Cl6	0.64–4.25	113	Li3Ta2O2F1Cl9	2.6–4.5	101
Li2Sc2/3Cl4	0.91–4.25	80	Li0.8Zr0.25La0.5Cl2.7O0.3	0.59–5	97
Li2TiF6	3.5–4.25	166	Li2ZrF5Cl1	2–5	43
Li2ZrCl5.6F0.4	1.75–4.25	124	Li3InCl4.8F1.2	3–6	90
Li2ZrCl6	1.75–4.25	126	Na3AlF6	0.46–6.19	167
Li17Al5Cl32	1.59–4.26	69	Li3InCl4.8F1.2	2.95–6.59	90

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Jing et al.¹⁹ prepared a Li₃InCl₆ (LIC) modified Li₆PS₅Cl (LPSCI) asymmetric bilayer electrolyte membrane by a combination of spray coating and slurry coating, significantly improving its compatibility with high-voltage cathodes such as NCM811 and lithium-rich manganese-based oxide LRMO. The cyclic voltammetry test results in Fig. 16a indicate that compared to a single LPSCI membrane, the LIC-LPSCI bilayer structure exhibits better oxidation stability at voltages up to 4.7 V, effectively suppressing electrolyte decomposition and achieving stable matching with high-voltage LRMO cathodes.¹⁹ Wang et al.¹⁰⁴ synthesized Li_3−xLu_1−xZr_xCl₆ (LLZC) halide solid electrolytes and investigated the influence of lithium ion and vacancy concentration equilibrium on ionic conductivity and high-voltage stability. They found that when the lithium ion and vacancy concentrations were equal (x = 0.5), the highest ionic conductivity (1.5 mS cm⁻¹) and the lowest activation energy (0.285 eV) were achieved. Heterovalent substitution enhances the oxidation stability of the electrolyte, making it suitable for high-voltage all-solid-state lithium batteries. Fig. 16b (left) shows that the all-solid-state battery with LiMn₂O₄ as the positive electrode and LLZC as the electrolyte has an initial capacity of 119.4 mAh g⁻¹ at 0.1C, with a coulombic efficiency of 91.8%, demonstrating excellent interface compatibility and high voltage stability. Fig. 16b (right) further shows the charge and discharge curves of the all-solid-state pouch battery (3 × 3 cm²), with a capacity of 9.58 mAh and a coulomb efficiency of 98.4%. It can also light up LEDs and drive fans, verifying its practical application potential. This cobalt-free all-solid-state battery shows no capacity decline after 1000 cycles at 0.3C, highlighting the long-term cycling stability of halide electrolytes at high voltages.¹⁰⁴ Nazar et al.¹¹⁷ reported a novel halide solid electrolyte, Li_3−xYb_1−xZr_xCl₆, which exhibits an electrochemical oxidation stability as high as 4.3 V, significantly higher than that of sulfide electrolytes (approximately 2.7 V). Therefore, it is directly compatible with uncoated high-voltage cathode materials (such as LiCoO₂ and LiNi_0.6Mn_0.2Co_0.2O₂). Fig. 16c shows the cycling performance of ASSLBs assembled with bare LiCoO₂ and NMC622 cathodes using the electrolyte at room temperature, exhibiting good capacity retention and coulombic efficiency, demonstrating the applicability and interface stability of the halide electrolyte at high voltages.¹¹⁷ Yanagihara et al.¹⁴⁵ evaluated the electrochemical stability of halide solid electrolytes (such as Li₃InCl₆) in a high interface density carbon environment using the LSV curve shown in Fig. 16d. The results showed that halide electrolytes have high oxidation stability (Cl⁻ oxidation peak at about 3.7 V vs. In/InLi, corresponding to about 4.32 V vs. Li/Li⁺), making them compatible with high-voltage cathodes (such as LiCoO₂). However, their reduction stability is poor, especially in the typical working voltage range of lithium sulfur batteries, which is prone to decomposition and limits their practical application in certain high-voltage systems.¹⁴⁵ Liang et al. found that Li₃ScCl₆ halide solid electrolytes exhibit a wide electrochemical stability window of 0.91–4.26 V vs. Li⁺/Li through first principles calculations (Fig. 16e), indicating good compatibility with high-voltage cathode materials such as LiCoO₂, which can avoid decomposition at high voltages and are suitable for ASSLB applications.⁵¹ Park et al.¹⁰ reported the compatibility of a novel halide solid electrolyte, Li_3−xM_1−xZr_xCl₆ (M = Y, Er), with high-voltage cathode LiCoO₂ (4 V level). Fig. 16f compares the performance of ASSLBs using sulfide and halide electrolytes. An oxidation side reaction plateau occurs when sulfide is used, resulting in lower capacity and efficiency. After using halide electrolytes, there is no oxidative decomposition, exhibiting high discharge capacity (>110 mAh g⁻¹) and high initial coulombic efficiency (96.4%), proving its excellent high voltage stability without the need for cathode protective coatings.¹⁰ Luo et al.¹⁶⁸ showed through cyclic voltammetry testing in Fig. 16g that the ethanol-mediated synthesis of Li₃InCl₆ halide solid electrolyte only exhibited significant oxidation current above 4.25 V (vs. Li/Li⁺), demonstrating its good compatibility with high-voltage cathodes. The solid-state battery assembled with this electrolyte and a LiNi_0.8Co_0.1Mn_0.1O₂ (NCM811) cathode demonstrated excellent cycle stability and capacity retention rate, verifying its application potential in high-voltage all-solid-state lithium metal batteries.¹⁶⁸ Liu et al.¹⁶⁹ investigated the high-voltage compatibility of halide solid electrolyte Li₃InCl₆ (LIC) with high-nickel layered oxide cathode LiNi_0.9Mn_0.05Co_0.05O₂ (NCM90). By introducing LIC as an intermediate layer, the interface side reactions between NCM90 and sulfide electrolyte were effectively suppressed, significantly improving the cycling stability and capacity retention of ASSLBs at high voltage. The impedance analysis in Fig. 16h further indicates that the battery with the LIC layer exhibits lower interface resistance in the voltage range of 3.6–3.8 V (vs. Li In), confirming the excellent compatibility between halide electrolyte and the NCM90 high-voltage cathode.¹⁶⁹

Fig. 16 (a) LIC-LPSCI double-layer electrolyte exhibiting lower oxidation current and better electrochemical stability compared to single LPSCI at a high voltage of 4.7 V. Reproduced with permission.¹⁹ Copyright 2025 Springer Nature. (b) (left) Comparison of the initial charge–discharge curves of LiMn₂O₄ in liquid batteries and all-solid-state batteries; (right) charge–discharge curves of halide all-solid-state pouch cells. Reproduced with permission.¹⁰⁴ Copyright 2023 Royal Society of Chemistry. (c) (Above) Charge discharge curve and cycling performance of the LCO-based ASSLB in the range of 3.0–4.3 V. (Below) Charge discharge curve and cycling performance of the NMC622 ASSLB in the range of 2.8–4.3 V. Reproduced with permission.¹¹⁷ Copyright 2021 American Chemical Society. (d) (left) Schematic diagram of the battery structure used to evaluate the stability of halide electrolytes. The linear sweep voltammetry curve of Li₃InCl₆ in the SE-C composite electrode (right) shows its oxidation and reduction stability. Reproduced with permission.¹⁴⁵ Copyright 2024 American Chemical Society. (e) The thermodynamic equilibrium voltage curve and phase equilibrium relationship of Li₃ScCl₆ showing that its stable electrochemical window relative to Li⁺/Li is 0.91–4.26 V. Reproduced with permission.⁵¹ Copyright 2020 American Chemical Society. (f) LiCoO₂ batteries using halide electrolytes exhibit higher capacity and coulombic efficiency than sulfide electrolytes and have no oxidation side reactions. Reproduced with permission.¹⁰ Copyright 2020 American Chemical Society. (g) Li₃InCl₆ exhibits an oxidation current above 4.25 V, indicating its compatibility with the high-voltage NCM811 cathode. Reproduced with permission.¹⁶⁸ Copyright 2022 American Chemical Society. (h) The interface resistance of the battery with the LIC layer is significantly lower than that of the control group without the LIC layer in the voltage range of 3.6–3.8 V, indicating excellent compatibility between halide electrolyte and the NCM90 high-voltage cathode. Reproduced with permission.¹⁶⁹ Copyright 2025 Elsevier Ltd.

5.7 ASSLB performance

In the performance evaluation of lithium halide solid electrolytes applied to ASSLBs, various battery configurations demonstrated excellent electrochemical performance. For instance, the Li₂S/Si full cell achieved an area capacity of 4.6 mAh cm⁻² and an energy density of 470 Wh kg⁻¹ (based on electrodes).¹⁷⁰ The all-solid-state lithium metal battery modified with a self-limiting layer (SLL) achieved a capacity retention rate of 99.2% after 100 cycles at a rate of 0.5 C and still maintained 83.5% capacity after 250 cycles at a rate of 2C.¹³ For halogen-based full cells with red phosphorus as the anode, a high area capacity of 7.65 mAh cm⁻² was achieved and the capacity retention rate was 70% after 1000 cycles.³² Bulk ASSLBs using oxygen halides as the solid electrolyte on the cathode side demonstrated outstanding rate performance, with a capacity retention rate of 80% at 5C/0.1C.¹²⁵ The corresponding full battery can provide a high reversible capacity of 200.1 mAh g⁻¹ at 4.5 V voltage, and the capacity retention rate is as high as 95.1% after 150 cycles at 0.2 C.¹⁷¹ At moderate cathode active material loading (65 and 70 wt% LiNi_0.83Mn_0.06Co_0.11O₂) and 0.1C rates, all three halide cathode electrolytes performed well, with initial discharge capacities exceeding 177 mAh g⁻¹.¹⁷² The first-week discharge specific capacity of the LiCoO₂/LiIn ASSLB using a LiIn₃Cl₆/ZrO₂ inorganic solid electrolyte film was 126.6 mAh g⁻¹ at 25 °C and 0.1 C, and the coulombic efficiency reached 93.4%.¹⁷³ The ASSLBs prepared with Li_2.7In_0.7Hf_0.3Cl₆ solid electrolyte demonstrated high discharge capacity and good cycle stability at 25 °C.⁷⁴ Haloide-based ASSLBs can operate stably at pressures as low as 0.1 MPa (for LiIn alloys) and 0.2 MPa (for lithium metal anodes), with limited capacity loss.¹⁷⁴ The ASSLBs using inorganic halide solid electrolyte and a single-crystal LiNiO₂ composite cathode have an initial discharge capacity of 205 mAh g⁻¹ at room temperature and exhibit excellent cycle performance of over 200 times.¹⁷⁵

Fig. 17a shows the performance of the self-supporting electrode prepared by the solvent-free method and the ASSLBs. The positive electrode is a LiCoO₂ composite material coated with Li₃InCl₆, the electrolyte layer is a Li₃InCl₆ and Li₆PS₅Cl composite system, and the negative electrode is a graphite and Li₆PS₅Cl composite. The initial discharge capacity of this battery reached 121.2 mAh g⁻¹ at 0.1C, with an initial coulombic efficiency of 71.8%. After 50 cycles, the capacity remained at 81.3 mAh g⁻¹, demonstrating the application potential of halide electrolytes in high-energy-density ASSLBs.²⁸Fig. 17b shows the application of the fluoride electrolyte Li₃GaF₆ in ASSLBs. The scanning electron microscope images show the nanostructure of the material. The electrochemical impedance spectrum proves that it has a high room-temperature ionic conductivity. The crystal structure diagram reveals its ion migration channels. The performance data of the Li/LiFePO₄ ASSLB based on this electrolyte, which stably cycles over 150 times at 60 °C and 1C rate, directly verify the feasibility of such halide electrolytes in achieving long-life, practical solid-state lithium-ion batteries.²⁵ Wang et al.¹⁰⁴ investigated the application of halide solid electrolyte Li_3−xLu_1−xZr_xCl₆ (LLZC) in ASSLBs, with a focus on the optimization effect of lithium ion and vacancy concentration balance on ionic conductivity. They constructed a cobalt-free ASSLB with LiMn₂O₄ as the positive electrode and Li–In alloy as the negative electrode. Fig. 17c (left) shows a schematic diagram of the battery structure, showing the stacked configuration of the LiMn₂O₄ cathode and the LLZC electrolyte, etc.Fig. 17c (middle) presents the microstructure of the battery cross-section through SEM images. Fig. 17c (right) shows the cycling stability of the ASSLB at 0.1C, and its capacity retention rate is significantly better than that of the liquid battery. This study confirmed that halide solid electrolytes have a promising application prospect in high-voltage, cobalt-free ASSLBs.¹⁰⁴ Li et al.¹⁷⁶ investigated the application of halide solid electrolytes Li_3−xSc_1−xZr_xCl₆ and Li_3−xSc_1−xHf_xCl₆ in ASSLBs. They optimized the lithium-ion migration path and vacancy concentration by replacing Sc³⁺ with Zr⁴⁺/Hf⁴⁺. It significantly enhanced the ionic conductivity (up to 2.2 mS cm⁻¹) and reduced the activation energy. Fig. 17d (left) shows a schematic diagram of the structure of an ASSLB, where the positive electrode is a high-voltage uncoated LiNi_0.90Co_0.05Mn_0.05O₂ (Ni90) or LiNi_0.8Co_0.1Mn_0.1O₂ (Ni80) and the negative electrode is a Li–In alloy. The solid electrolyte layer is Li_2.375Sc_0.375Zr_0.625Cl₆ (LSZC), and a Li₆PS₅Cl (LPSC) protective layer is introduced at the negative electrode interface to prevent electrolyte reduction. Fig. 17d (right) shows the charge–discharge capacity of the battery with Ni90 cathode at different rates, demonstrating a high initial capacity (219.4 mAh g⁻¹), excellent rate performance, and cycle stability, which proves the practical potential of this halide electrolyte in high-performance ASSLBs.¹⁷⁶ Liang et al.⁵¹ reported a novel halide solid electrolyte, Li_xScCl_3+x (particularly Li₃ScCl₆), which has a room-temperature ionic conductivity of up to 3.02 × 10⁻³ S cm⁻¹ and a wide electrochemical window of 0.9–4.3 V vs. Li⁺/Li, suitable for ASSLBs. A fully solid-state battery was constructed with LiCoO₂ as the cathode, Li₃ScCl₆ as the electrolyte, and metallic indium (In) as the anode, and its electrochemical performance was evaluated. Fig. 17e (left) shows the initial charge–discharge curve of the battery at a current density of 0.13 mA cm⁻², indicating a charging capacity of 139.7 mAh g⁻¹ and a discharging capacity of 126.2 mAh g⁻¹. The initial coulombic efficiency is 90.3%, and the peak of the cobalt redox reaction is clearly observed in the dQ/dV curve. Fig. 17e (right) shows the cycling stability of the battery after 160 cycles, with a capacity maintained at 104.5 mAh g⁻¹ and a coulombic efficiency of 99.2%, indicating that the Li₃ScCl₆ electrolyte has good interfacial compatibility and electrochemical stability with the LiCoO₂ cathode. It provides strong support for the application of halide solid electrolytes in high-performance ASSLBs.⁵¹ Nie et al.¹⁵³ significantly enhanced the ionic conductivity of the fluoride solid electrolyte Li₃GaF₆ by means of low-temperature molten salt (LiCl + 1.33AlCl₃) ablation and particle boundary doping strategies, achieving a conductivity of 5.27 × 10⁻⁵ S cm⁻¹ at room temperature and 10⁻⁴ S cm⁻¹ at 60 °C. It also features excellent air stability (able to withstand a relative humidity of 35%) and a wide electrochemical window (approximately 6 V). In ASSLBs, LiFePO₄ is used as the positive electrode, lithium metal as the negative electrode, and modified Li₃GaF₆ (LGF-MSA) as the solid electrolyte. The side reactions between the electrolyte and the lithium negative electrode are effectively suppressed through the PEO interface layer. As shown in Fig. 17f (left), the battery structure includes a PEO-modified interface design. Fig. 17f (right) further indicates that the battery maintains a capacity of 71.7% after 200 cycles at a 0.5C rate, demonstrating excellent cycling stability.¹⁵³ Tuo et al.³⁹ reported a low-cost halide solid electrolyte, Li_2+xHf_1−xFe_xCl₆, which achieved an ionic conductivity as high as 0.91 mS cm⁻¹ at 30 °C by heterovalent doping of Fe³⁺. The ASSLB constructed with this electrolyte adopts an uncoated LiCoO₂ cathode, a Li-in alloy anode, and a halide electrolyte layer. Fig. 17g (left) shows the overall structure of the battery, including the LPSC buffer layer. Fig. 17g (middle) shows that the battery based on LHFC has an initial discharge capacity of 113.4 mAh g⁻¹ and a coulombic efficiency of 91.2% at 0.1 C. Fig. 17g (right) indicates that the LHFC battery exhibits excellent reversible capacity and stability in the rate test.³⁹ Ding et al.⁵⁸ obtained the halide solid electrolyte γ-Li₃ScCl₆ for the first time through high-pressure synthesis. It has a hexagonal close-packed (HCP) anion lattice and a non-centrosymmetric structure. Compared with the α-phase at normal pressure (cubic close-packed CCP), the change in anion stacking mode is due to the increase in the cation/anion radius ratio under pressure. In the study, ASSLBs were constructed using an uncoated NCM111 cathode and a Li–In alloy anode to evaluate the electrochemical performance of γ-Li₃ScCl₆ as the solid-state electrolyte. Fig. 17h shows the overpotential variations over time of the Li–In/γ-Li₃ScCl₆/Li–In and Li–In/α-Li₃ScCl₆/Li–In symmetric cells at a current density of 0.1 mA cm⁻², among which the γ phase cells exhibit a smaller and more stable overpotential (approximately 0.32 V). The overpotential of the α phase is relatively high (about 1.54 V), indicating that the γ phase has better electrochemical reduction stability, which is beneficial for the full battery cycle performance.⁵⁸

Fig. 17 (a) Schematic diagram of the process for preparing the Li₃InCl₆@LiCoO₂ self-supporting electrode by the solvent-free method, charge–discharge curves of the Li₃InCl₆@LiCoO₂/Li₃InCl₆ + Li₆PS₅Cl/graphite@Li₆PS₅Cl ASSLB, and the cycle performance diagram of the ASSLB. Reproduced with permission.²⁸ Copyright 2023 Springer Nature. (b) Scanning electron microscope images of Li₃GaF₆ solid electrolyte; room-temperature ionic conductivity, crystal structure, and cycling performance of the Li/LiFePO₄ ASSLB. Reproduced with permission.²⁵ Copyright 2023 John Wiley and Sons. (c) (left) Schematic diagram of the ASSLB structure, (middle) SEM image of the cross-section of the ASSLB, and (right) comparison of the cycling stability of the ASSLB and the liquid battery at 0.1C. Reproduced with permission.¹⁰⁴ Copyright 2023 Royal Society of Chemistry. (d) (left) Schematic diagram of the ASSLB structure; (right) charge–discharge capacity curves of the ASSLB using a Ni90 cathode at different rates. Reproduced with permission.¹⁷⁶ Copyright 2024 American Chemical Society. (e) LiCoO₂/Li₃ScCl₆/In ASSLB at 0.13 mA cm⁻²: (left) initial charge–discharge curve and the dQ/dV curve; (right) cycle performance and coulombic efficiency. Reproduced with permission.⁵¹ Copyright 2020 American Chemical Society. (f) (left) Schematic diagram of the structure of the Li/PEO/LGF-MSA/LiFePO₄ ASSLB. (right) Cycle capacity and retention rate curve of the Li/PEO/LGF-MSA/LiFePO₄ battery at 0.5C. Reproduced with permission.¹⁵³ Copyright 2024 American Chemical Society. (g) (left) Schematic diagram of the ASSLB structure; (middle) initial charge–discharge curves of LHC and LHFC-based all-solid-state batteries at 0.1C. (right) Rate performance comparison of LHC and LHFC-based ASSLBs. Reproduced with permission.³⁹ Copyright 2024 American Chemical Society. (h) Comparison of the overpotential of Li–In/γ-Li₃ScCl₆/Li–In and Li–In/α-Li₃ScCl₆/Li–In symmetric cells at 0.1 mA cm⁻². Reproduced with permission.⁵⁸ Copyright 2024 Wiley-VCH GmbH.

6 Interface engineering

6.1 Cathode–electrolyte interface

Halide solid electrolytes (HSEs) have attracted widespread attention due to their high ionic conductivity, wide electrochemical window, and good compatibility with high-voltage cathode materials.⁸⁷ When in contact with sulfide electrolytes and oxide cathode materials (such as LiCoO₂ and NMC), interface side reactions occur, resulting in high interface resistance and element interdiffusion, leading to rapid capacity decay, in contrast to halide electrolytes (such as Li₃MX₆, M = In, Y, Sc, Er, etc., X = Cl, Br), which exhibit excellent interfacial stability with 4 V grade cathode active materials and can be used directly without any coating.⁶⁶ Their high oxidative stability is due to the strong electronegativity of halide anions.¹⁷⁷

The excellent mechanical ductility of HSEs allows for the preparation of cathode composite materials by manual grinding with cathode active materials, forming an effective ion/electron permeation network.¹⁷⁸ After cold-pressing, the composite cathode sheet exhibits a uniform coating of HSEs on the cathode active material, which is crucial for achieving efficient ion/electron mixing.¹⁷⁸ In the design of composite cathodes, it is necessary to ensure a dual permeation network of ions and electrons.³⁹ The proportion, morphology, and microstructure of composite cathodes significantly affect the chemical and mechanical properties during electrochemical cycling processes.²⁸ A simple grinding of halide electrolyte and cathode active material (CAM), followed by a powder cold-pressing operation, can obtain a composite cathode, mainly due to the excellent plasticity of HSEs, which is conducive to achieving close contact between particles.²⁸ A reasonable proportion of composite cathodes is considered one of the most critical factors in achieving a continuous ion and electron cross-linking diffusion network. A high proportion of the CAM can form a good electron permeation path, but at the same time, may lead to a low ion transport rate.²⁸ Compared with polycrystalline materials, single-crystal CAMs exhibit better thermal, high-voltage, and mechanical resistance, effectively ensuring the mechanical integrity of the CAM and suppressing contact loss in composite cathodes.²⁸

The stability of the interface can also be effectively evaluated by calculating the mutual reaction energy. The reaction energy with the delithiated cathode is a key indicator for measuring compatibility.⁵⁰ For example, the reaction energy between Li₃YCl₆ and delithiated Li_0.5CoO₂ is small (24 meV per atom), indicating good stability during voltage cycling.⁵⁰ This good compatibility enables ASSBs based on Li₃YCl₆ and Li_0.5CoO₂ cathodes to exhibit high coulombic efficiency (94.8%) in the initial cycle.⁵⁰ Tham et al.¹⁷⁹ evaluated the interfacial stability of halide solid electrolytes (such as Li₃MCl₆ and its A-site cation mixed variants) with various high-voltage cathode materials (such as NCM, LCO, LNO, etc.) using first principles computational systems. Fig. 18 compares the energy changes of 16 mixed cationic halide electrolytes and single cationic electrolytes in the interfacial reaction with the cathode,¹⁷⁹ showing that the mixed cation design (such as Li₃Lu_0.5Y_0.5Cl₆) significantly reduces the interfacial reaction energy and enhances the chemical stability. By combining the electrochemical stability window with mechanical stability analysis, the study ultimately selected two electrolyte materials with the greatest application potential: Li₃Lu_0.5Ho_0.5Cl₆ and Li₃Lu_0.5Er_0.5Cl₆. They exhibit excellent interfacial compatibility and ionic conductivity under high pressure and are suitable for ASSLBs.

Fig. 18 Interface reaction energies between 16 cation-substituted compounds and the reference material Li₃MCl₆ (M is Lu, Ho, Er, Tm, and Sc) solid electrolyte.

Li et al.⁶³ pointed out that halide solid electrolytes, such as Li₃YCl₆ and Li₃YBr₆, exhibit excellent compatibility and low interface impedance at the high-voltage cathode interface. Fig. 19a (left) shows that the initial coulombic efficiency of the full cell using halide electrolyte is over 94%, much higher than that of the sulfide electrolyte. Fig. 19a (right) further confirms that its interface impedance is significantly lower (such as Li₃YBr₆ being only 6.6 Ω cm²), indicating that halide electrolytes can effectively suppress interface side reactions and improve interface stability and battery performance.⁶³ Wang et al.¹ applied lithium manganese-based (LRM) cathodes to halide solid-state batteries (using Li₃InCl₆ electrolyte) and found that the active oxygen released by LRM at high voltage oxidizes the electrolyte, increases interface impedance, and limits battery performance. The interface reaction was effectively suppressed by the LiNbO₃ coating (Fig. 19b), resulting in a discharge capacity of 221 mAh g⁻¹ and 100-cycle stability of the battery at 0.1C. The study revealed the interface degradation mechanism and coating protection effect through EIS, XPS, TEM, and other methods.^1,180 Samanta et al.¹⁴³ compared the compatibility of halide solid electrolytes with different central metals and sulfide solid electrolytes like Li₆PS₅Cl in double-layer battery structures, with a focus on analyzing the stability of the cathode electrolyte interface. Fig. 19c illustrates that the interface stability and cycling performance of the battery using Li₃YCl₆ are excellent, while Li₃InCl₆ increases impedance and capacity decay due to interface reactions with sulfides, highlighting the key influence of metal centers in halides on interface chemical compatibility.¹⁴³ Yanagihara et al.¹⁴⁵ investigated the compatibility of three halide solid electrolytes (Li₃InCl₆, Li₃YCl₆, and Li₃YBr₆) in the cathode of solid-state lithium sulfur batteries. Although halide electrolytes have high oxidation stability, studies have shown that Li₃InCl₆ rapidly degrades within the operating voltage of the battery due to poor reduction stability. Li₃YCl₆ undergoes a chemical incompatibility reaction with the discharge product Li₂S, generating an insulating phase LiYS₂ at the interface, resulting in capacity decay (Fig. 19d). Li₃YBr₆ exhibits excellent cycling stability (maintaining 1100 mAh g⁻¹ for 20 weeks), and its interfacial stability is attributed to a lower reaction driving force and slower anion exchange kinetics. This study revealed through graph analysis that the chemical compatibility between halide electrolytes and sulfur active materials is a key factor affecting battery performance. Jing et al.¹⁹ prepared a Li₃InCl₆–Li₆PS₅Cl (LIC-LPSCI) asymmetric bilayer electrolyte membrane by combining spray coating and slurry coating, which is used to enhance the interfacial stability of high-voltage ASSLBs. Fig. 19e compares the interface changes of batteries using double-layer (LIC-LPSCI) and single-layer (LPSCI) electrolytes during cycling using in situ electrochemical impedance spectroscopy (EIS). The results showed that at high voltages above 4.5 V, the interfacial impedance of the monolayer electrolyte sharply increased, indicating severe interfacial side reactions. The halide layer in the double-layer structure effectively suppresses interface degradation, significantly improves the stability of the cathode electrolyte interface, and enables the battery to maintain low impedance and good cycling performance at high voltage.¹⁹

Fig. 19 (a) (left) Initial charge discharge curves of Li₃YCl₆ and Li₃YBr₆ based ASSLBs at 0.1C; (right) Nyquist plot of batteries assembled with different electrolytes after the first charge. Reproduced with permission.⁶³ Copyright 2025 American Chemical Society. (b) The LiNbO₃ coating inhibits the side reactions at the interface between the lithium-rich cathode and the halide electrolyte, enhancing battery performance. Reproduced with permission.¹ Copyright 2023 American Chemical Society. (c) Comparison of interface stability and battery performance differences between Li₃InCl₆ and Li₃YCl₆ in a double-layer electrolyte structure. Reproduced with permission.¹⁴³ Copyright 2024 American Chemical Society. (d) Li₃YBr₆ has become the most promising halide solid electrolyte in lithium sulfur batteries due to its optimal interfacial chemical compatibility. Reproduced with permission.¹⁴⁵ Copyright 2024 American Chemical Society. (e) The LIC-LPSCI double-layer electrolyte significantly suppresses interface impedance growth and enhances cathode electrolyte interface stability at high voltage. Reproduced with permission.¹⁹ Copyright 2025 Springer Nature.

6.2 Anode–electrolyte interface

Although halide solid electrolytes have high ionic conductivity and good high-pressure stability, they face severe challenges due to thermodynamic instability with lithium metal anodes.¹⁸¹ Most halide electrolytes (such as Li₃YCl₆ and Li₃InCl₆) undergo reduction reactions when in direct contact with metallic lithium, generating conductive metals (such as Y and In) and LiCl, thus forming a mixed ion-electron conductive interface phase. This interface phase lacks self-limiting ability and will continue to grow, leading to an increasing interface impedance and eventually causing battery failure.³³ For more details, the reduction reaction between Li₃MCl₆ and lithium metal follows the chemical equation: Li₃MCl₆ + 3Li → 6LiCl + M⁰ (M⁰ = In, Y). Among the reaction products, LiCl is a Li⁺ conductor, while M is an electron conductor. Therefore, the formed interface has the characteristic of mixed ion-electron conductivity, which promotes the thermodynamically favorable decomposition reaction to continue until the halide electrolyte or lithium metal is completely consumed.³⁵ For instance, the reduction and decomposition of Li₃YCl₆ at the lithium metal interface is manifested with the gradual precipitation of metallic yttrium (Y⁰) from the lithium metal side in the form of nanoclusters, which eventually bridge the electrode, resulting in a significant increase in the electronic conductivity of the interface.¹⁸² The reduction potential is significantly affected by the properties of the central metal element. For the Li₃MX₆ system, the halides of P-zone element cations (such as In and Bi) have a relatively high reduction potential (chlorides are approximately 2.3 V), indicating poor stability when in contact with lithium metal anodes. In contrast, the halides of the element cations in the d and f regions exhibit lower reduction reaction decomposition energies.¹⁸³ It can be found that in the lithium halide electrolyte system, the interface degradation mechanisms triggered by reduction reactions mainly include the formation of non-passivated interface phases, macroscopic polarization caused by continuous side reactions, chemical degradation of the electrolyte itself, and cracks and powdering on the surface of lithium metal, ultimately leading to interface contact failure.¹⁸⁴

To overcome the instability of the interface, researchers have proposed various strategies. The most commonly used method is to introduce a buffer layer between the halide electrolyte and lithium metal. For example, using sulfide electrolytes (such as Li₆PS₅Cl) as intermediate layers can effectively block electron transfer and form a kinetically stable interface, significantly improving the cycling stability of symmetric batteries.¹⁸⁵ The double-layer electrolyte structure can combine the stability of sulfide electrolytes at low potentials with the oxidation stability of halide electrolytes at high potentials, thereby expanding the electrochemical window of ASSLBs.¹⁹ Specifically, sulfide electrolytes such as Li₆PS₅Cl are prone to oxidation and decomposition at high voltages, while halide electrolytes such as Li₃InCl₆ have a higher oxidation potential but a higher reduction potential, resulting in degradation at the anode interface at low potentials. Therefore, constructing sulfide/halide bilayer electrolytes can help alleviate the interface problem.¹⁹ Chemical compatibility studies have shown that the central metal of halide electrolytes plays a key role in reactivity with Li₆PS₅Cl. Li₃InCl₆ and Li₂ZrCl₆ react highly with Li₆PS₅Cl, while Li₃YCl₆, Li₃ScCl₆, and Li₃ErCl₆ can form stable interfaces.¹⁴³ However, the interface of heterogeneous bilayer electrolytes is not always stable. Chemical incompatibility leads to a continuous increase in interface resistance during cycling. For example, an IN-S enrichment interface phase is formed between Li₃InCl₆ and sulfide electrolytes, thereby affecting cycling stability.⁴⁷ To address this issue, it is proposed to deposit a nanoscale Li₃PO₄ protective layer between sulfide and halide electrolytes to mitigate interfacial reactions and enhance compatibility.⁴⁷

In addition, fluorine doping has also been proven to be an effective strategy by introducing fluorine into halides (such as synthesizing Li₃YBr_5.7F_0.3). During the cycling process, a fluorine-rich interface layer (rich in LiF and YF_x) can be formed in situ, which is uniform and dense. This layer can effectively suppress lithium dendrite growth and interface side reactions, enabling symmetric batteries to cycle stably for more than 1000 hours at 0.75 mA cm⁻².¹⁸⁶ Using alloy anodes (such as Li–In and Li–Ag) is another common strategy. The alloy anode, due to its higher reduction potential (relative to Li⁺/Li), can avoid the reduction of halide electrolytes and form a kinetically stable interface. However, the use of alloy anodes can reduce the operating voltage and energy density of batteries, and the high cost of elements such as indium limits their large-scale application.⁸⁶

The design of the artificial solid-state electrolyte interface has also received attention. For example, by depositing artificial layers such as Li₃N or LiF, the interface composition and structure can be controlled and side reactions can be suppressed. The ideal artificial interface layer should have high ion conductivity, electronic insulation, good mechanical stability, and high adhesion to the substrate.¹⁸⁷ Theoretical research also suggests that lithium-rich halides such as Li₃OCl can serve as potential artificial SEI materials, and their interface with lithium metal can form a stable passivation layer.¹⁸⁸

Zhang et al.¹³ proposed a self-limiting layer (SLL) composed of InF₃ and Li₂ZrCl₆ to address the interface instability between halide solid electrolytes (such as Li₂ZrCl₆ and Li₂ZrCl₆) and lithium metal anodes. The self-limiting layer is a protective layer formed at the electrode/electrolyte interface of a battery, which can automatically stop further reactions after the initial reaction. It is usually composed of materials that conduct ions but insulate electrons, which can effectively suppress interfacial side reactions, dendrite growth, and impedance increase. It is one of the key interface engineering strategies for achieving high-performance ASSLBs. Fig. 20a compares the self-limiting reaction between the SLL and lithium metal (forming a LiF passivation layer to prevent sustained side reactions) with the sustained reduction reaction that occurs when Li₂ZrCl₆ directly comes into contact with lithium metal (resulting in continuous electrolyte decomposition and battery failure) through a schematic diagram, indicating that the SLL can effectively stabilize the interface and improve the cycling performance of all-solid-state lithium metal batteries.¹³ Luo et al.¹⁸⁹ proposed a rapid in situ crosslinking strategy to address the incompatibility between halide solid electrolytes (such as Li₃InCl₆) and lithium metal anodes. By reacting trimethylaluminum vapor with the terminal hydroxyl groups of polyepoxybutane (PBO), an ultra-thin (approximately 2 µm) crosslinked polymer interface (xPBO-SPE) was constructed on the surface of halide electrolytes. This interface effectively suppresses electrolyte reduction and side reactions, significantly improves the compatibility of lithium metal, enables stable cycling of symmetric batteries at 1.0 mA cm² for over 1100 hours, and achieves stable operation of ASSLBs. Fig. 20b shows the structural characterization and electrochemical stability extension of the crosslinked interface.¹⁸⁹ Wang et al.¹⁴⁴ significantly improved the interface stability between Li₂ZrCr_6−xI_x halide solid electrolytes and lithium metal anodes by using the strategy of partially replacing chlorine with iodine. Research has shown that the high polarization of I⁻ enhances the covalency of Zr–I bonds and inhibits the reduction tendency of central cations. As shown in Fig. 20c, the Li₂ZrCl₆ interface undergoes continuous electron permeation and electrolyte decomposition, while the Li₂ZrCl₄I₂ interface can form an electron-insulating passivation layer composed of LiI and LiCl, achieving self-limiting reactions and effectively preventing further interface degradation.¹⁴⁴ Morino et al.¹⁸² revealed the reduction and decomposition mechanism of halide solid electrolyte Li₃YCl₆ at the lithium metal anode interface by constructing a Li|Li₃YCl₆|Li symmetric battery and combining electrochemical impedance spectroscopy, X-ray absorption spectroscopy, and molecular dynamics simulation. Fig. 20d shows the gradual formation of metal yttrium (Y⁰) nanoclusters and LiCl at the interface, leading to a sustained decrease in impedance and ultimately triggering electronic conduction.¹⁸² Li et al.¹⁹⁰ used a dual halogen strategy to partially replace Br⁻ in Li₃HoBr₆ with Cl⁻, successfully synthesizing the Li₃HoBr_6−xCl_x series of electrolytes. This modification significantly improves the interface stability with lithium metal while maintaining the monoclinic structure and 10⁻³ S cm⁻¹ ion conductivity. The Li/Li₃HoBr₅Cl₃/Li symmetric battery achieved stable cycling for over 800 hours (0.1 mA cm⁻²) with an overpotential of only 0.26 V. Interface analysis (Fig. 20e) showed that Cl⁻ effectively suppressed the generation of interface byproduct LiBr, and the resulting interface was denser. The electrochemical impedance spectrum in Fig. 20f further confirms that the interface impedance significantly decreases after Cl⁻ doping, and no obvious new capacitance arc appears, indicating that the interface side reactions are effectively suppressed and the interface layer is more stable, thereby significantly improving the interface compatibility and electrochemical stability between the electrolyte and lithium metal.¹⁹⁰ Banerjee et al.¹⁹¹ compared the interface electrochemical mechanical behavior of three halide solid electrolytes (Li₃InCl₆, Li₂ZrCl₆, and Li₃YCl₆) with lithium metal anodes using various in situ techniques, revealing their different interface evolution mechanisms and stability differences. Fig. 20g visually compares how the chemical and electrochemical potential gradients of the three at the lithium metal interface drive different interfacial phase growth behaviors. Li₃YCl₆ forms a dense and electronically insulating interfacial layer, achieving better passivation. Li₂ZrCl₆ forms a porous and electron conductive phase interface, which continues to decompose. The severe interface reaction caused by the formation of high electron conductivity lithium indium alloy in Li₃InCl₆ indicates that the electronic conductivity and microstructure of the interface phase jointly determine the reduction stability of halide electrolytes.¹⁹¹ Most halide solid electrolytes are thermodynamically unstable and prone to sustained side reactions with lithium metal. However, Fig. 20h shows that UCl₃ halide solid electrolytes based on LaCl₃, such as LTLC, can form a gradient-distributed LiCl passivation layer in situ when in contact with lithium. This layer can not only suppress dendrite growth and alleviate interface strain, but also effectively improve interface stability through a “clearing” mechanism, thereby achieving long-term stable cycling of symmetric batteries.⁶³

Fig. 20 (a) Comparison of the self-limiting reaction mechanism at the interface between the self-limiting layer and lithium metal with the sustained side reactions of Li₂ZrCl₆ in direct contact with lithium metal. Reproduced with permission.¹³ Copyright 2025 American Chemical Society. (b) Schematic diagram of rapid crosslinking of trimethylaluminum vapor and the PBO polymer on the surface of halide solid electrolyte to form a protective interface. Reproduced with permission.¹⁸⁹ Copyright 2023 American Chemical Society. (c) Comparison of different evolution mechanisms and stability of Li/Li₂ZrCl₆ and Li/Li₂ZrCl₄I₂ interfaces after contact with lithium metal. Reproduced with permission.¹⁴⁴ Copyright 2025 American Chemical Society. (d) Li|Li₃YCl₆|Li battery reveals interface reduction and decomposition to generate metal yttrium and LiCl, leading to electronic conduction. Reproduced with permission.¹⁸² Copyright 2025 American Chemical Society. (e) By adopting a dual halogen strategy and partially replacing Br⁻ with Cl⁻, the interface stability between Li₃HoBr₆ and the lithium metal anode was significantly improved.¹⁹⁰ (f) Cl doping makes the interface impedance of Li₃HoBr₆ more stable, with little change after standing. (e and f) Reproduced with permission.¹⁹⁰ Copyright 2024 American Chemical Society. (g) Schematic diagram of the chemical and electrochemical potential gradients at the interface between Li₃YCl₆, Li₂ZrCl₆, and Li₃InCl₆ with lithium metal and the resulting growth mechanisms of different interface phases. Reproduced with permission.¹⁹¹ Copyright 2024 American Chemical Society. (h) (Above) Schematic diagram of the intermediate layer of the gradient structure formed at the Li/LTLC interface. (Below) Schematic diagram of LTLC achieving dendrite removal by forming Ta, La, and LiCl. Reproduced with permission.⁶³ Copyright 2025 American Chemical Society.

7 Conclusions and perspectives

Halide solid electrolytes (HSEs), as highly promising electrolyte materials in ASSLBs, have attracted widespread attention due to their high ionic conductivity, excellent oxidation stability, and good compatibility with high-voltage cathode materials.² In recent years, through methods such as cation/anion doping, structural regulation, and optimization of synthesis methods, the ionic conductivity of some materials has been significantly improved and some materials have even reached a level equivalent to that of liquid electrolytes (>10 mS cm⁻¹).¹²⁵ However, their commercialization still faces challenges such as high cost, sensitivity to humidity, and unstable interfaces with lithium metal anodes.¹⁹²

Halide solid electrolytes perform exceptionally well in terms of safety, featuring extremely high thermal stability (with decomposition temperatures typically exceeding 300 °C), fundamentally eliminating the risk of flammability and making them intrinsically safe materials. Furthermore, they exhibit outstanding compatibility with high-voltage cathode materials. These characteristics make them one of the ideal choices for building high-safety, high-energy-density solid-state batteries. However, halide electrolytes still face challenges in practical applications. Although they are thermodynamically unstable with the lithium metal anode, kinetic stability can be achieved by forming a passivation interface layer and improved with the aid of interface engineering technology.³³ Despite this, thermodynamic instability still leads to persistent interfacial side reactions and impedance growth, which limit their direct application in all-solid-state lithium metal batteries.³³ At present, the long-term stability of the interface between halide solid-state electrolytes and lithium metal anodes remains a key challenge in achieving high-energy-density ASSLBs.⁸³ On the other hand, most halide electrolytes (especially chlorides) are sensitive to moisture and are prone to hydrolysis in the air, increasing the cost and difficulty of the preparation, storage, and battery assembly processes.¹¹ Although surface coatings (such as g-C₃N₄) or fluorine doping can improve air stability to a certain extent, more universal and economical protection strategies still need to be developed.¹⁹³

Future research should focus on the following directions: (1) developing new and stable halide electrolyte systems, such as high entropy design (such as Li_2.2In_0.2Sc_0.2Zr_0.2Hf_0.2Ta_0.2Cl₆) and amorphous nanocrystalline composite structures is used to simultaneously achieve high ionic conductivity, wide electrochemical window, and good mechanical properties.^8,194 The high-entropy design of halide solid electrolytes greatly enhances the structural stability of the material (entropy increased stabilization effect) by introducing multiple main elements to form a high configuration entropy, effectively suppressing the phase transition and decomposition. Meanwhile, complex chemical components can promote the formation of a more stable interface layer, reduce side reactions with electrodes and element mutual diffusion, and significantly optimize interface compatibility. In addition, lattice distortion may also broaden the ion migration channels and synergistically enhance the ionic conductivity. The advantages of amorphous and nanocrystalline composite structures stem from their unique two-phase organization. Nanocrystals can pin amorphous matrices and inhibit their crystallization, while amorphous phases hinder the growth of nanocrystals, forming a synergistic stability. The two are closely combined through the atomic-level interface formed, which can efficiently transfer loads and buffer stress, thereby synergistically enhancing the comprehensive performance of the material. Through element substitution strategies, expensive or rare earth elements (such as Y, In, and Sc) are replaced with earth's abundant elements (such as Zr, Ti, and Ta) to reduce material costs and enhance sustainability.¹⁹⁵ (2) Deepening the understanding of interface reaction mechanisms and enhancing long-term cycling stability by constructing artificial interface layers (such as self-limiting layers and polymer/oxide protective layers) to suppress side reactions between electrolytes and electrodes.^13,189 A multifunctional interface layer or composite electrode structure must be designed that is stable, low impedance, and capable of suppressing dendrites with lithium metal.²³ (3) Promoting the composite application of halide electrolytes with sulfide and oxide electrolytes, leveraging their respective advantages, and constructing multi-layer electrolyte structures to simultaneously meet the compatibility requirements of high-voltage cathodes and lithium metal anodes.^196,197 (4) Developing low-cost and scalable synthesis methods (such as aqueous synthesis and mechanochemical methods), optimizing electrode/electrolyte interface structure design, and promoting the practical application of halide-based solid-state batteries from the laboratory.^198,199 Dry process technologies must be developed with more moisture-resistant compositions or scalable production to improve the air stability of materials.²⁰⁰

In terms of research methods, high-throughput computing (such as DFT) and machine learning should be combined to accelerate the design and screening of new materials,³⁴ and advanced characterization techniques (such as in situ neutron diffraction, nuclear magnetic resonance, and low-temperature transmission electron microscopy) should be used to gain a deeper understanding of ion transport and degradation mechanisms.^201,202 By precisely controlling the chemical composition of halide electrolytes (anion type, proportion, vacancy concentration, doping, etc.), their crystal structure (phase type, lattice parameters, bottleneck size, orderliness, etc.) can be directionally altered, ultimately achieving optimization of ion conductivity, migration energy barrier, interface stability, and other properties. In addition, it is necessary to explore systems beyond lithium ions, such as the potential applications of sodium ions and other multivalent ions (Mg²⁺ and Ca²⁺) in batteries.⁵²

In summary, halide solid electrolytes have shown great potential for application in ASSLBs, but their actual industrialization still relies on further optimization of the materials themselves, innovation in interface engineering, and breakthroughs in preparation processes. Ultimately, achieving the practical application of HSEs from the laboratory requires the collaborative efforts of multiple disciplines, such as chemistry, materials science, and engineering, to jointly promote their industrialization in ASSLBs.²⁰³

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Abbreviations

AAH	Ammonia-assisted hydrothermal
AIMD	Ab initio molecular dynamics
ASSLBs	All-solid-state lithium batteries
BVSE	Bond valence site energy
CAM	Cathode active material
CCP	Cubic close packing
CEI	Cathode electrolyte interphase
CV	Cyclic voltammetry
CVD	Chemical vapor deposition
DSC	Differential scanning calorimetry
EIS	Electrochemical impedance spectroscopy
EXAFS	Extended X-ray absorption fine structure
FD	Freeze drying
HCP	Hexagonal close packing
HFP	Hexafluoropropylene
HSE	Halide solid electrolyte
ISE	Inorganic solid electrolyte
LCO	LiCoO2
LIC	Li3InCl6
LNO	Lithium nickel oxide
LLZC	Li3−xLu1−xZrxCl6
LPSCI	Li6PS5Cl
LRMO	Li-rich manganese-based oxide
LSV	Linear sweep voltammetry
LTLC	LaCl3-based solid-state electrolyte
LZC	Li2ZrCl6
NCM	Lithium nickel cobalt manganese oxide
NMR	Nuclear magnetic resonance
PBO	Polyepoxybutane
PVDF	Polyvinylidene fluoride
SE	Solid electrolyte
SEI	Solid electrolyte interphase
SEM	Scanning electron microscopy
SLL	Self-limiting layer
SPE	Solid polymer electrolyte
SXRD	Synchrotron X-ray diffraction
TEM	Transmission electron microscopy
TGA	Thermogravimetric analysis
ToF-SIMS	Time-of-flight secondary ion mass spectrometry
USPEX	Universal structure predictor: evolutionary Xtallography
XPS	X-ray photoelectron spectroscopy
XRD	X-ray diffraction

Data availability

This review article did not generate any new primary data. All data discussed in this review are derived from previously published studies, which are cited within the text and listed in the reference section.

Acknowledgements

The authors would like to acknowledge the financial support from the National Natural Science Foundation of China (NSFC: 52173205) and Shanghai University of Engineering Science.

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