A Comprehensive Review of Sulfide Solid-State Electrolytes: Properties, Synthesis, Applications, and Challenges

Abstract

Traditional lithium-ion batteries (LIBs) utilize liquid electrolytes, which pose significant safety risks. To address these concerns and enhance energy density, all-solid-state batteries (ASSBs) have emerged as a safer and more efficient alternative to conventional liquid electrolyte-based systems. ASSBs offer notable advantages, including higher energy density and improved safety, driving growing interest from both industry and academia. A key component in all-solid-state battery (ASSB) development is the solid-state electrolyte (SSE), which plays a crucial role in determining the overall performance and safety of these batteries. Sulfide SSEs are characterized by distinctive attributes, including notably high ionic conductivity and remarkably low interfacial resistance with lithium metal anodes, which renders them particularly advantageous for advancing ASSB technology. This paper systematically examines sulfide-based SSEs, with particular emphasis on their underlying physicochemical properties, structural characteristics, and essential functional attributes relevant to ASSB applications. Additionally, we explore preparation methods for sulfide SSEs and analyze their potential applications in next-generation ASSBs. Considering current challenges (e.g., interfacial instability or air sensitivity) we summarize strategies to address these obstacles, aiming to facilitate their integration into future energy storage systems.

1. Introduction

With the rapid advancement of secure, flexible, and scalable electronic devices, the development of efficient energy storage systems to support these innovations has become paramount [1,2]. Among various energy storage solutions, lithium-ion batteries (LIBs) have emerged as a leading technology due to their high energy density and long cycle life. However, traditional LIBs employing liquid electrolytes pose significant risks associated with volatilization and leakage, leading to safety concerns and necessitating complex packaging measures. In response to these challenges, all-solid-state batteries (ASSBs) have attracted increasing attention over the past few decades [3,4,5,6,7]. Figure 1 illustrates the key advancements in sulfide electrolytes in recent years. A key component of ASSBs is the solid-state electrolyte (SSE), which offers a promising alternative by effectively addressing the inherent issues of liquid electrolytes, such as flammability and instability. SSEs enhance safety and stability while enabling improvements in the performance and reliability of next-generation energy storage systems.

Currently, ASSBs can be classified based on the type of SSE employed, with the primary categories being polymer-based, oxide-based, sulfide-based, and halide-based ASSBs [8,9]. Among these, sulfide-based ASSBs exhibit considerable promise for commercialization due to their superior ionic conductivity, favorable mechanical properties, thermal stability, and moderate production costs [10,11,12,13,14,15,16,17,18,19]. Halide-based SSEs also show potential due to their unique ionic transport mechanisms and compatibility with diverse electrode materials [20,21]. Collectively, these advancements position ASSBs as a highly promising solution for meeting the evolving demands of next-generation energy storage applications. Their ability to improve safety, stability, and performance aligns with the requirements of emerging technologies, solidifying their status as a central focus of research and development in the field [22].

Figure 1. Significant developmental milestones of sulfide electrolytes over the past 20 years [12,23,24,25,26,27,28,29,30,31,32,33,34].

Figure 1. Significant developmental milestones of sulfide electrolytes over the past 20 years [12,23,24,25,26,27,28,29,30,31,32,33,34].

1.1. Background of SSEs

With the rapid advancement of the global economy, the demand for energy has surged. The accelerated consumption of fossil fuels exacerbates energy supply strain and contributes to environmental pollution and climate change. To achieve sustainable and low-carbon development goals, promoting new energy technologies—such as photovoltaic and wind power—is imperative. These technologies are critical for mitigating environmental degradation and global warming. However, their inherent challenges, including temporal variability and instability in energy output, necessitate complementary solutions. Developing efficient, clean, and renewable energy sources, alongside scalable energy storage technologies, is vital for enhancing national productivity and quality of life while reducing environmental harm.

LIBs, commercialized in the 1990s, dominate the electronics market due to their high operating voltage, large specific energy, long cycle life, and low environmental impact [35]. Innovations in electrolyte materials have been pivotal in advancing LIB performance [36]. Early LIBs relied on liquid organic electrolytes with carbonate solvents, which provided sufficient ionic conductivity for commercialization. Subsequent improvements, particularly through the introduction of functional additives, have focused on increasing energy density, reducing internal resistance, and extending cycle life. Lithium metal anodes, with their high theoretical specific capacity (3860 mAh/g), low potential (−3.04 V vs. standard hydrogen electrode), and low density (0.53 g/cm³), are ideal for replacing graphite anodes, potentially doubling battery specific energy from 200 Wh/kg to 400 Wh/kg [37]. Novel solvents (e.g., ethers, nitriles, and sulfones) and advanced electrolyte designs (e.g., high-concentration, localized high-concentration, and weakly solvated electrolytes) have further enhanced electrochemical performance, safety, and stability [38]. Nevertheless, the flammability, leakage, and poor temperature stability of these electrolytes remain unresolved safety concerns [39].

ASSBs, exhibiting superior energy density, enhanced safety profiles, and extended cycle durability, have emerged as leading candidates for next-generation electrochemical energy storage systems [40]. The replacement of liquid electrolytes with SSEs in ASSBs effectively addresses electrolyte leakage and thermal runaway risks while simultaneously mitigating lithium dendrite formation through enhanced interfacial stability [41]. The core of ASSB technology lies in SSE development, including: oxides (e.g., NASICON SSE: Li_1.3Al_0.3Ti_1.7(PO₄)₃ [42,43], garnet electrolyte: Li₇La₃Zr₂O₁₂ [44]); polymers (e.g., polyethylene oxide (PEO) [45], polyvinylidene fluoride (PVDF) [46]); sulfides (e.g., lithium thiophosphate: Li₇P₃S₁₁ [47], Li₁₀GeP₂S₁₂(LGPS) [48]); halides (e.g., Li₃InCl₆ [49], Li₃YCl₆ [50], Li₃HoCl₆ [51]).

SSEs must exhibit high ionic conductivity, wide electrochemical windows, robust mechanical properties, and compatibility with electrode materials. Among these characteristics, mechanical robustness and ionic conductivity directly determine the overall battery performance [52].

Polymer SSEs exhibit favorable mechanical properties and effectively suppress lithium dendrite formation [53]. However, their narrow electrochemical window (e.g., the electrochemical window of PEO < 3.9 V) render them susceptible to oxidation when paired with high-voltage cathodes. For instance, severe electrochemical decomposition occurs in systems utilizing cathodes such as LiCoO₂ (LCO) [54,55], LiNi_xCoγMn_vO₂ (NCM) [23], or LiNi_xCoγAl_vO₂ (NCA) [23], leading to rapid performance degradation [56]. Additionally, polymer SSEs demonstrate inadequate thermal stability: exposure to temperatures exceeding 400 °C triggers decomposition and combustion, posing critical safety risks [57].

Oxide SSEs are favored for their chemical stability, thermal resilience, and versatility, making them a preferred choice for industrial applications. However, high interfacial impedance—stemming from poor electrode–electrolyte wettability due to the rigid and non-fluid nature of oxides—remains a critical barrier to their adoption in ASSBs [58,59]. Approaches to address this issue include interfacial modification layers and densification techniques, such as spark plasma sintering or hot pressing. Furthermore, oxide SSEs require sintering temperatures exceeding 1200 °C, which can induce lithium loss and degrade battery performance [60].

Sulfide SSEs stand out due to their ultrahigh ionic conductivity (>10⁻³ S·cm⁻¹) and machinability, enabling seamless integration into ASSBs [61]. Their advantages include enhanced ionic conductivity—the larger atomic radius and lower electronegativity of sulfur reduce Li⁺ binding energy, facilitating ion migration, and superior mechanical properties—ductility and low hardness improve interfacial contact with electrodes [62,63,64].

Despite their advantages, sulfide-based solid-state electrolytes (SSEs) still face significant challenges, including air sensitivity, interfacial instability (both electrochemical and thermomechanical), and scalability issues, all of which impede their commercial adoption. Overcoming these obstacles necessitates a comprehensive understanding of their structural properties, ion transport mechanisms, and failure modes, coupled with the development of innovative approaches for stabilizing interfaces [65,66].

This review systematically examines the characteristics, synthesis methods, applications, and challenges of sulfide SSEs, and summarizes future directions for sulfide-based ASSBs. Sulfide SSE synthesis is typically employed to prevent moisture-induced degradation [67]. Liquid-phase synthesis, utilizing organic solvents to produce fine particles, offers scalability and cost-effectiveness compared to high-temperature solid-state reactions [68,69].

Sulfide SSEs have gained significant research interest owing to their ultrahigh ionic conductivity [67]. However, challenges such as interfacial instability, electrode architecture optimization, and scalable manufacturing processes hinder their practical applications [70] Recent advancements in material science and engineering have positioned sulfide SSEs as critical enablers for next-generation ASSBs. Their remarkable ionic conductivity, with intrinsic electrochemical stability and robust safety profiles, make them critical for developing high-energy-density and inherently safe battery systems. Consequently, sulfide-based ASSBs are poised for transformative applications in portable electronics and grid-scale energy storage, where energy density and operational safety are crucial.

1.2. Significance of Research on Sulfide SSEs

Sulfide SSEs are regarded as an essential technology for ASSBs due to their exceptional ionic conductivity, mechanical ductility, and stable interfacial compatibility with electrodes [71]. Notably, lithium-ion conductors such as LGPS exhibit ultrahigh ionic conductivities (>10⁻² S·cm⁻¹ at 25 °C) [57], approaching those of liquid electrolytes. Solid Power Semiconductor and SVOLT Energy Technology Co., Ltd. (Changzhou, China) have successfully developed 20 Ah sulfide-based ASSBs. Meanwhile, Mitsui Mining & Smelting Co., Ltd. (1-11-1, Oosaki, Shinagawa-ku, Tokyo, Japan) and Pohang Iron and Steel Co., Ltd. (Pohang, Republic of Korea) have established pilot production lines for sulfide-based solid-state electrolytes (SSEs). Additionally, Solid Power (Louisville, CO, USA), Samsung Electronics (Shenzhen, China), and Nissan Motor Co., Ltd. (1-1-1, Takashima, Nishi-ku, Yokohama, Japan) have initiated the construction of pilot production lines for sulfide-based ASSBs [72].

As shown in Figure 2, despite these achievements, the full-scale commercialization of sulfide-based ASSBs remains hindered by critical challenges: material instability, electrode limitations, interfacial failure, and composite electrode optimization.

Material instability: Sulfide SSEs suffer from poor air stability and narrow electrochemical stability windows (ESWs) [73,74]. For instance, LGPS demonstrates high ionic conductivity but reacts aggressively with lithium metal anodes. Wet-chemical synthesis and particle size optimization (e.g., via ball milling) are being explored to enhance air stability and ionic transport [74].

Electrode limitations: High-voltage cathodes (e.g., Ni-rich NCM, and Li-rich layered oxides) and silicon/lithium anodes enable high energy density but face structural degradation during cycling [54,56]. Microstructure engineering and doping strategies are critical to mitigate mechanical fracture [56].

Interfacial failure: Space charge layer (SCL) formation, interfacial side reactions, and mechanical stress disrupt Li⁺ transport at electrode–electrolyte interfaces. Buffer layers (e.g., cathode coatings and artificial SEI layers) and hybrid electrolyte designs show promise, yet achieving uniformity at scale remains challenging [75,76].

Composite electrode optimization: Inhomogeneous distributions of active materials, SSEs, and conductive additives in composite electrodes impede ion/electron transport. Mechanical failures (e.g., particle cracking and pore formation) exacerbate dendrite growth. Applied pressure during cell assembly alleviates these issues, but optimal pressure parameters require further study [77,78].

Addressing these challenges—material instability, interfacial failure, electrode design limitations, and scalable manufacturing—will unlock the full potential of sulfide-based ASSBs. With continued innovation, these systems are poised to revolutionize energy storage for electric vehicles, portable electronics, and grid-scale applications, offering superior energy density and safety [79].

2. Properties of Sulfide SSEs

2.1. Types and Chemical Compositions of Sulfide SSEs

Sulfide SSEs exhibit exceptional lithium-ion conductivity and mechanical properties, surpassing polymer-based and oxide-based SSEs [80]. Based on crystallinity, sulfide SSEs are categorized into three classes: amorphous (glassy), glass–ceramic, and crystalline [81].

2.1.1. Glassy Sulfide SSEs

Glassy sulfide SSEs are typically synthesized by melting stoichiometric mixtures of Li₂S and P₂S₅ at temperatures exceeding 900 °C, followed by rapid quenching to inhibit crystallization. The absence of grain boundaries in amorphous structures eliminates interfacial resistance, enabling isotropic Li⁺ transport. The binary system xLi₂S·(100 − x)P₂S₅ (40 ≤ x ≤ 80) is the most studied, with ionic conductivity following an Arrhenius temperature dependence, indicating thermally activated Li⁺ migration [82].

Strategies to enhance conductivity include [83] high-energy ball milling to induce disordered structures by bond disruption; melt quenching for rapid glassy-phase formation that enhances atomic mobility; and anion doping (e.g., O²⁻ or Cl⁻ for S²⁻) to optimize Li⁺ pathways via lattice modification. These concerted multiscale engineering strategies address ion transport barriers across atomic and microstructural scales.

2.1.2. Glass–Ceramic Sulfide SSEs

When glassy sulfide electrolytes undergo complete crystallization, they typically exhibit reduced ionic conductivity. This is due to structural reorganization during crystallization, which disrupts ion transport pathways. However, certain glass–ceramic electrolytes demonstrate higher ionic conductivity than their fully amorphous or crystalline counterparts, rendering them particularly advantageous in specific applications.

The high-temperature crystallization process enables the transformation of glassy sulfides into glass–ceramic electrolytes by partially retaining amorphous domains while introducing crystalline phases [84]. This dual-phase structure–combining amorphous and crystalline regions–alters atomic arrangements and chemical bonding at the microscale, critically influencing ionic conduction [85].

Within glass–ceramic sulfide electrolyte research, the Li₂S-P₂S₅ system remains predominant due to its tunable ionic conductivity and chemical stability [86]. Annealing temperature plays an important role: higher temperatures accelerate ion migration and lattice vibrations, thereby enhancing conductivity [87]. For example, J. Kim et al. annealed 78Li₂S-22P₂S₅ glass at 160 °C and 260 °C, respectively [88]. At 160 °C, microcrystalline nucleation occurred, yielding a room-temperature conductivity of 4.5 × 10⁻⁴ S·cm⁻¹ [88]. Subsequent annealing at 260 °C promoted grain growth, forming a thio-LISICON phase with conductivity reaching 8.5 × 10⁻⁴ S·cm⁻¹ [88]. These results demonstrate that controlled crystallization optimizes microstructural and macroscopic properties, balancing amorphous flexibility with crystalline ionic pathways.

2.1.3. Crystalline Sulfide SSEs

Among crystalline sulfide SSEs, thio-LISICON and argyrodite electrolytes have emerged as prominent candidates due to their unique ionic conduction mechanisms [89]. The thio-LISICON family, derived by substituting sulfur for oxygen in oxide LISICON structures (e.g., γ-Li₃PO₄), adopts the general formula Li_4−xA_1−γB_γS₄ (A = Si, Ge; B = Zn, Al, P) [90]. For instance, the interaction between Li₆PS₅Cl(LSPC) and polar PVDF-TrFE ensures a high lithium-ion conductivity (≈1.2 ms/cm) at room temperature and good mechanical ductility of the composite electrolyte membrane [91]. However, its poor compatibility with lithium metal and the high cost of germanium limits practical applications. To address this, SnS₂ and SiS₂ substitutions have been explored. Y. Kato et al. synthesized Li_9.54Si_1.74P_1.44S_11.7Cl_0.3 (25 ms/cm) via solid-state methods, and it exhibited twice the conductivity of LGPS and superior cycling stability [23]. In ASSBs employing LiNi_0.83Mn_0.06Co_0.11O₂ (NMC811) cathodes, this SSE maintains 92% capacity retention after 1000 cycles at 1.0 mA·cm⁻², even under high humidity [92].

Argyrodite is named after the mineral Ag₈GeS₆, whose high ionic conductivity in silver and germanium ions inspired its adaptation as a structural template for designing fast lithium-ion conductors. Among argyrodite-type electrolytes, Li₆PS₅X (X is Cl, Br, I, etc.) has garnered the most extensive research attention due to its superior ionic transport properties [93]. H. J. Deiseroth et al. pioneered the discovery of Li₆PS₅X, a lithium-ion conductor derived from the Ag₈GeS₆ framework, which stabilizes the high-temperature phase of Li₇PS₆ at room temperature [24]. The crystal structure of Li₆PS₅X (cubic F-43 m space group) features halogen atoms (X) at cube vertices and face centers, PS₄ tetrahedra at edge midpoints, and lithium ions dynamically occupying 18 sites, forming a 3D conduction network [94].

S. J. Liu et al. developed a flexible composite SSE membrane through a sequential process of electrospinning, solution perfusion, and hot pressing. The resulting membrane, with a thickness of 30–40 μm, integrates LSPC particles within a polar PVDF-TrFE [95]. Strong interfacial interactions between the polar PVDF-TrFE framework and LSPC endow the composite with high mechanical ductility and a room-temperature Li⁺ conductivity of 1.2 × 10⁻³ S·cm⁻¹ [95]. When incorporated into a button-type ASSB configuration—featuring an LiNi_0.8Co_0.1Mn_0.1O₂ (NCM811) cathode and lithium–indium alloy anode—the membrane demonstrated exceptional cycling stability. At a current density of 1.0 mA/cm² within a voltage window of 2.5–4.3 V, the ASSB retained 92% capacity after 1000 cycles and 71% after 20,000 cycles, indicating its potential for long-term applications [91].

2.2. Physical and Chemical Properties

2.2.1. Ionic Conductivity

The ionic conductivity of an SSE is a key determinant of Li⁺ ion mobility and serves as a critical performance metric for evaluating its suitability in practical applications. Sulfide SSEs exhibit superior ionic conductivity compared to oxide and polymer counterparts due to sulfur’s larger atomic radius, lower electronegativity, and reduced Li⁺ binding energy. For instance, the ionic conductivity of LGPS is 1.2 × 10⁻² S·cm⁻¹, and that of Li_9.54Si_1.74P_1.44S_11.7Cl_0.3 is 2.5 × 10⁻² S·cm⁻¹ [23]. High ionic conductivity can reduce the internal resistance of the battery, and improve the electrochemical properties [96].

Sulfide SSEs are typically prepared by melting stoichiometric mixtures (e.g., Li₂S and P₂S₅) at high temperatures (>900 °C) under argon, followed by rapid quenching to form amorphous phases. For glass–ceramic electrolytes, controlled crystallization is subsequently induced [97].

The melt synthesis of sulfide solid-state electrolytes typically follows a three-stage procedure to ensure reliable ionic conductivity measurements [98]. Initially, precursor powders with controlled stoichiometry are prepared through either pre-synthesized spheroidal particles or mechanically milled raw materials. These homogenized mixtures are subsequently compressed under controlled pressure (typically 300–500 MPa) to form densified pellets with minimized porosity. The final consolidation involves encapsulating the compacted specimens in vacuum-sealed quartz ampoules under argon atmosphere, a critical step for preventing sulfur volatilization and oxide contamination during high-temperature processing. This standardized protocol enables a reproducible fabrication of SSE samples suitable for electrochemical characterization [98].

After calcination at high temperature, the mixture is rapidly cooled to achieve rapid quenching, thus obtaining a glassy sulfide electrolyte. If it is necessary to prepare a glass–ceramic electrolyte, it can be achieved by further crystallization. F. Mizuno et al. demonstrated that increasing the melting temperature from 750 °C to 900 °C enhances the formation of P₂S₆⁴⁻ units in 70Li₂S-30P₂S₅ (Li₇P₃S₁₁), significantly boosting ionic conductivity [99]. Y. Seino et al. further optimized this process through hot pressing, achieving 1.7 × 10⁻² S·cm⁻¹ by reducing grain boundary resistance [25].

Glassy sulfide electrolytes exhibit intricate local structures influenced by Li₂S content [36]. Dietrich et al. demonstrated that the composition of xLi₂S·(100 − x)P₂S₅ glass varies with the Li₂S content [100]. When the Li₂S content is low (x ≤ 60), the glassy sulfide electrolyte is mainly composed of corner-sharing tetrahedra P₂S₇⁴⁻ units, each containing one bridging S atom and three terminal S atoms. Glassy sulfide electrolytes with higher Li₂S content (x ≥ 70) contain more isolated tetrahedral PS₄³⁻ units, where all S atoms are terminal [100]. Among all glassy sulfide electrolytes, 75Li₂S·25P₂S₅ has the highest ionic conductivity at room temperature [36].

2.2.2. Mechanical Strength

Lithium dendrite penetration remains a critical challenge at the lithium metal–SSE interface, despite the higher mechanical strength of sulfide SSEs compared to lithium metal [100]. Dendrites tend to nucleate preferentially at material defects, such as grain boundaries and cracks, and can propagate even in amorphous sulfide electrolytes that lack crystalline interfaces [101,102]. For instance, M. Nagao et al. observed lithium dendrite growth along grain boundaries in polycrystalline SSEs and within bulk regions of glassy sulfides [102].

On the other hand, the interface between the SSE and lithium metal experiences significant strain and uneven current density during cycling, primarily due to volume expansion and non-uniform lithium deposition/dissolution. These issues are largely attributed to poor interfacial contact. Regions with localized high current density are known to facilitate the nucleation and growth of lithium dendrites [103]. The study of lithium electrodeposition on the single crystal Li₆La₃ZrTaO₁₂ garnet shows that to reduce the penetration of lithium through the brittle electrolyte, the interface defects between the SSE and the lithium metal should be minimized [104].

In ASSBs, the volume changes of the lithium metal anode during Li deposition and dissolution induce significant structural stress at the electrode–electrolyte interface. Therefore, the design of the SSE–electrode interface must account for and endure cyclic mechanical stresses during battery operation [105]. Electrochemical reactions in ASSBs occur at the solid–solid interface between the electrolyte and electrode, requiring specialized engineering to achieve optimal interfacial contact and efficient ion–electron transport [106]. To engineer these challenges, it is essential to reduce interfacial impedance by minimizing resistance caused by physical contact defects and poor interfacial adhesion [90]. Additionally, the mechanical compatibility of sulfide-based SSEs must be ensured, leveraging their lower elastic modulus (e.g., (18.5 ± 0.9) GPa for 70Li₂S-30P₂S₅) to achieve compliant contact and accommodate interfacial stress mismatches [107].

However, sulfide SSEs face trade-offs between flexibility and brittleness. Their fracture toughness (0.23 ± 0.04 MPa·m^1/2) and hardness (1.9 ± 0.2 GPa) are considerably lower than those of oxide-based electrolytes, rendering them prone to defect propagation during cycling [107]. Resolving these issues demands a deeper understanding of sulfide SSE mechanics to balance ionic conductivity, interfacial stability, and mechanical robustness [108,109,110].

2.2.3. Thermal and Electrochemical Stability

Most highly conductive sulfide SSEs (e.g., LGPS, Li₇P₃S₁₁, and Li_9.54Si_1.74P_1.44S_11.7Cl_0.3) exhibit thermodynamic instability when in contact with lithium metal. Wenzel et al. classified the interfacial stability into three categories [111].

Thermodynamically stable interfaces: SSE and Li metal coexist in equilibrium without reactive decomposition [112]. Unstable interfaces with mixed conduction: electron/ion-conducting interphases form, enabling continuous electrolyte degradation and rapid capacity fade [113]. Passivating interfaces: electrically insulating but ion-conductive layers (analogous to liquid electrolyte-derived SEI) prevent further side reactions [111].

The narrow ESW of sulfide SSE limits compatibility with conventional electrodes operating beyond their stability limits [114]. To achieve stable cycling in ASSBs, engineered interphases must be designed to fulfill multiple functions simultaneously: passivation to suppress electrolyte decomposition; high ionic conductivity to minimize interfacial impedance; and electron insulation to prevent undesirable side reactions. For instance, electron-insulating interphases (e.g., Li₃P and Li₂S) can effectively passivate sulfide-based SSEs, thereby preserving cycling stability. Conversely, mixed-conductor interphases, which facilitate concurrent ion and electron transport, may drive thermodynamically favored electrolyte decomposition, leading to progressive interface thickening, increased impedance, and accelerated performance degradation [115,116,117,118,119,120,121,122,123].

Interface engineering strategies primarily utilize buffer layers (e.g., LiNbO₃ and Li₃PO₄) inserted between electrodes and sulfide-based SSEs to address interfacial challenges. These buffer layers reduce interfacial resistance by improving physical contact, inhibit electron transfer to prevent detrimental side reactions, and enhance air stability through moisture and oxygen shielding, thereby maintaining the structural and chemical integrity of sulfide SSEs [36]. Such advancements highlight the important role of tailored interfacial design in enabling robust sulfide-based all-solid-state lithium batteries with extended cycle life and operational reliability [117].

2.3. Ionic Conduction Mechanism of Sulfide SSEs

2.3.1. Thio-LISICON Mechanism

Thio-LISICON structures, first identified in Li₂S-GeS₂, Li₂S-GeS₂-Ga₂S₃, and Li₂S-GeS₂-ZnS systems, are derived from the γ-Li₃PO₄-type oxide LISICON framework by substituting oxygen with sulfur. The lower electronegativity and larger ionic radius of sulfur reduce Li⁺ binding energy, widening ion migration channels and enabling higher ionic conductivity compared to oxide analogues [118].

These materials adopt an orthorhombic Pnma crystal structure, where S^2- ions form hexagonally close-packed arrays. Heavy metal cations (e.g., Ge⁴⁺ and Zn²⁺) occupy tetrahedral sites, while Li⁺ ions reside in disordered octahedral interstices. Representative compounds include Li₄GeS₄ (2.0 × 10⁻⁷ S·cm⁻¹) and Li₄SnS₄ (7.0 × 10⁻⁵ S·cm⁻¹), though their intrinsic conductivity remains insufficient for practical applications [119].

Aliovalent cationic doping (e.g., P⁵⁺ or Al³⁺ substitution for Si⁴⁺ in Li₄SiS₄) enhances ionic conductivity by introducing Li vacancies or interstitials, thereby optimizing percolative ion transport pathways. For instance, Li_3.4Si_0.4P_0.6S₄: P⁵⁺ doping creates Li vacancies, elevating to 6.7 × 10⁻⁴ S·cm⁻¹ at 27 °C [124]. (Li_4.8Si_0.2Al_0.8S₄: Al³⁺ substitution generates Li interstitials, elevating to 2.3 × 10⁻⁷ S·cm⁻¹) [124].

2.3.2. Structure Mechanism of Li_11−xM_2−xP_1+xS₁₂ (M = Ge, Sn, Si)

The Li_11−xM_2−xP_1+xS₁₂ (M = Ge, Sn, Si) exemplifies how atomic-scale structural engineering enhances ionic transport. In LGPS, the crystal framework comprises interconnected chains of (Ge_0.5P_0.5) S₄ tetrahedra and LiS₆ octahedra, forming 1D Li⁺ diffusion channels along the c-axis [123]. These channels are bridged by LiS₄ tetrahedra, creating a hierarchical conduction network. The density functional theory (DFT) calculations reveal ultra-low activation energies for Li⁺ migration: 0.17 eV for interplanar hopping between ab-plane and tetrahedral sites, 0.18 eV for intra-channel (c-axis) migration, and 0.37 eV for pure ab-plane diffusion [123]. This anisotropic energy landscape enables rapid Li⁺ transport, even with partial channel obstruction, by leveraging low-barrier interplanar pathways, thus underpinning the exceptional ionic conductivity of LGPS [123].

The anionic substitution of S with O in Li₁₀MP₂O₁₂ (M = Ge or Si) enhances redox stability but compromises ionic conductivity due to tightened Li⁺ migration channels, raising activation energy to 0.36 eV and reducing to 3.0 × 10⁻⁵ S·cm⁻¹ [125]. However, partial O substitution (0 ≤ x<0.9) retains the parent P42/nmc space group while improving electrochemical resilience [125]. For instance, Li_9.42Si_1.02P_2.1S_9.96O_2.04 maintains a moderate 3.2 × 10⁻⁴ S·cm⁻¹ alongside enhanced stability, demonstrating the viability of controlled anionic doping to balance ionic transport and interfacial durability in sulfide SSEs [126].

Halogen substitution (e.g., Cl⁻ and I⁻) in sulfide electrolytes enhances Li⁺ mobility by reducing anion–Li⁺ electrostatic interactions and expanding ionic diffusion channels. In Li_9.54Si_1.74P_1.44S_11.7Cl_0.3, Cl⁻ incorporation facilitates three-dimensional (3D) Li⁺ diffusion through 16h/8f/4c interstitial sites, achieving 2.5 × 10⁻² S·cm⁻¹ at room temperature [127]. Iodine’s larger ionic radius and lower electronegativity further widen channels, enabling lower migration barriers. This is exemplified by Li_9.54Si_1.74P_1.44S_11.7Cl_0.3, which achieves an ionic conductivity of 1.35 × 10⁻³ S·cm⁻¹ at room temperature through iodine doping, demonstrating that halogen size and electronic properties critically govern channel dimensions and Li⁺ transport efficiency in sulfide SSEs [128].

2.3.3. Ionic Conduction Mechanism in Li₆PS₅X (X = Cl, Br and I) Argyrodites

The Li₇PS₆ system exists in two phases: a high-temperature phase (HT-Li₇PS₆, 3.0 × 10⁻⁵ S·cm⁻¹) and a low-temperature phase (LT-Li₇PS₆, 1.6 × 10⁻⁶ S·cm⁻¹), both exhibiting limited ionic conductivity. In contrast, the Li₆PS₅X (X = Cl, Br, I) argyrodite family adopts a cubic F-43 m structure, where S²⁻ and X⁻ anions occupy face-centered 4a/4d sites, while PS₄³⁻ tetrahedra reside at octahedral positions (P at the 4b positions) [129]. Sulfur anions fully occupy 16e sites, whereas S²⁻ and X⁻ share 4a/4b sites, creating a dynamically disordered Li⁺ migration network [129].

The Li⁺ transport dynamics in Li₆PS₅X involve three cooperative mechanisms: short-range intra-cage hopping between adjacent 48 h sites; inter-cage transitions within tetrahedral subunits; and long-range inter-cage diffusion across adjacent units. The rate-limiting step is typically governed by the slowest process, which is often long-range diffusion. The introduction of Li⁺ vacancies through halogen substitution (e.g., Cl⁻ replacing S²⁻ in Li₇PS₆) significantly enhances ionic conductivity. For example, Li₆PS₅Cl and Li₆PS₅Br achieve conductivities of ~10⁻² S·cm⁻¹ at 25 °C, whereas Li₆PS₅I exhibits a lower conductivity of ~10⁻⁵ S·cm⁻¹ due to the larger ionic radius of iodide, which induces lattice distortion and steric hindrance to Li⁺ migration [101].

Doping strategies significantly enhance the ionic conductivity of sulfide SSEs by tailoring their atomic-scale structure and defect chemistry. The partial substitution of S²⁻ and I⁻ in Li_7−xPS_6−xI_x introduces interstitial sites that enable 3D Li⁺ percolation, achieving optimal conductivity at 0.75 ≤ x ≤ 0.95, the Li⁺ due to a balanced vacancy–interstitial distribution [128]. Similarly, Si⁴⁺ doping in Li_6+xP_1−xSi_xS₅Br (x = 0.35–0.5) expands Li⁺ migration channels through lattice parameter enlargement and carrier concentration elevation, tripling the ionic conductivity compared to undoped Li₆PS₅Br. This performance enhancement is predominantly governed by reduced activation barriers and enhanced Li⁺ mobility across widened diffusion pathways [86]. These doping strategies underscore the intricate relationship between cationic/anionic substitution, lattice dynamics, and ionic transport efficiency in sulfide-based electrolytes [86].

3. Synthesis Methods for Sulfide SSEs

The synthesis of sulfide-based SSEs is typically carried out under inert atmospheres to prevent moisture-induced degradation, and the resulting materials are categorized into glassy, glass–ceramic, and crystalline states based on their structural order [130,131,132,133]. Key synthesis routes include mechanical ball milling, high-temperature solid-state reactions, and liquid-phase synthesis, each offering distinct advantages in optimizing ionic conductivity and interfacial compatibility [134].

3.1. Mechanical Ball Milling

Mechanical ball milling involves blending stoichiometric precursors (e.g., Li₂S, P₂S₅, and LiCl) in a ball mill with grinding balls, followed by controlled milling to induce amorphous-to-crystalline phase transitions. For instance, LPSC argyrodite was synthesized by milling at 300 rpm for 8 h, achieving 4.049 × 10⁻³ S·cm⁻¹ after sintering at 500 °C [135,136,137]. Prolonged milling enhances particle refinement and amorphous phase formation, while excessive sintering temperatures (>500 °C) risk crystal structure degradation.

Liu et al. demonstrated that 70Li₂S − (30 − x)P₂S₅₋xCe₂S₃ (x = 0, 0.5, 1, 2, 3) glass–ceramic, prepared via high-energy ball milling and heat treatment, exhibits 1.52 × 10⁻³ S·cm⁻¹ at 25–40 °C higher than undoped 70Li₂S-29P₂S₅-Ce₂S₃. ASSBs incorporating this electrolyte delivered a 105.3 mAh·g⁻¹ initial capacity at 0.1 °C, retaining 87.2% after 50 cycles [138].

3.2. High-Temperature Solid-State Reaction

Solid-state reactions at elevated temperatures (e.g., 750 °C to 900 °C) promote the formation of conductive phases like Li₇P₃S₁₁. Mizuno et al. showed that increasing the melting temperature from 750 °C to 900 °C enhances P₂S₆⁴⁻ unit formation, boosting ionic conductivity by 50% due to optimized Li⁺ percolation pathways [139,140,141].

3.3. Liquid-Phase Synthesis

Liquid-phase synthesis dissolves metal salts (e.g., Li₂S and P₂S₅) in solvents like tetrahydrofuran (THF), followed by precipitation or evaporation to yield nanostructured SSEs. Liu et al. synthesized β-Li₃PS₄-containing glass–ceramic via THF-mediated routes, achieving 1 × 10⁻⁴ S·cm⁻¹ at room temperature after crystallization [142,143,144,145,146,147,148].

4. Synthesis and Application Strategies for Sulfide SSEs

Sulfide SSEs have emerged as a cornerstone technology for next-generation ASSBs, driven by their ultrahigh ionic conductivity (>1 × 10⁻³ S·cm⁻¹) and exceptional mechanical flexibility. These attributes enable seamless integration with high-capacity electrodes like silicon and lithium metal, addressing key challenges in energy density (>400 Wh/kg) and safety for applications in electric vehicles and portable electronics [149,150,151,152,153,154].

The structural and ionic advantages of sulfide-based solid-state electrolytes (SSEs) arise from the replacement of oxygen with sulfur in oxide-derived frameworks. The lower electronegativity of sulfur (2.58 compared to 3.44 for oxygen) weakens the interaction between Li⁺ and anions, while its larger ionic radius (1.84 Å versus 1.40 Å for O²⁻) expands the Li⁺ migration pathways. These combined effects facilitate ultrafast ionic transport [12,24]. These properties have driven landmark advancements, such as the 2011 synthesis of LGPS with a room-temperature conductivity of 1.2 × 10⁻² S·cm⁻¹, and the 2016 development of Li_9.54Si_1.74P_1.44S_11.7Cl, which achieved 2.5 × 10⁻² S·cm⁻¹—performance rivaling conventional liquid electrolytes [24].

Sulfide-based SSEs effectively address the 300% volume expansion of silicon anodes through several mechanisms: rapid Li⁺ diffusion (D ~ 10⁻⁸ cm²·s⁻¹), which homogenizes electrochemical reactions [72]; elastic buffering (Young’s modulus: 18.5 GPa), which reduces interfacial stress [155]; and the application of external pressure (1–10 MPa), which maintains electrode–electrolyte contact and lowers impedance by up to 50% [156,157].

The higher lithiation potential of silicon (0.4 V vs. Li⁺/Li) compared to graphite (0.1 V) further inhibits electrolyte decomposition, enabling stable operation at voltages up to 4.3 V [158,159,160]. Prototype ASSBs incorporating silicon anodes and sulfide-based SSEs achieve energy densities exceeding 300 Wh/kg, even with electrolyte layers as thin as 50 μm [161].

The development of Li₇P₃S_7.5O_3.5 (LPSO) represents a significant advancement in cost-effective sulfide-based SSEs, as illustrated in Figure 3a,b. This material combines an ultra-low density (1.70 g·cm⁻³, 32% lower than halides), competitive raw material costs ($14.42/kg, less than 8% of conventional sulfide SSEs), and robust electrochemical performance [158,159,160]. Synthesized from low-cost hydrated lithium hydroxide and phosphorus sulfide, LPSO retains 89.29% capacity after 200 cycles at 60 °C in Figure 3c,d. As shown in Figure 3e, the symmetrical battery composed of LPSO and lithium metal can achieve a stable cycle of more than 4200 h at room temperature, and its production cost aligns with industrial targets (<$50/kg), positioning it as a frontrunner for scalable, high-performance ASSBs [161].

Sc₂O₃ doping (4 mol%) elevates ionic conductivity to 3.17 × 10⁻² S·cm⁻¹ at room temperature while enabling 300 h stable Li–Li symmetric cell cycling at 0.1 mA/cm² [162,163,164]. Thin-film processing via roll pressing (e.g., Li_5.4PS_4.4C_l1.6/PTFE) yields 30 μm membranes with 8.4 mS·cm⁻¹, delivering a 135.3 mAh·g⁻¹ (1.4 mAh·cm⁻²) capacity and 80.2% retention over 150 cycles in NCM-based ASSBs [165].

Despite significant advancements in air stability (achieved through rare-earth doping) and interfacial passivation (enabled by artificial SEI layers), sulfide-based SSEs still face several challenges: enhanced moisture resistance, requiring handling protocols under relative humidity (RH) < 1%; improved compatibility with lithium metal, targeting critical current densities exceeding 1 mA·cm⁻²; and scalable manufacturing processes capable of producing films thinner than 50 μm to enable energy densities surpassing 400 Wh/kg [166]. Addressing these interconnected challenges, which encompass material stability, interfacial kinetics, and industrial process engineering, will play a decisive role in determining the viability of sulfide-based SSEs in next-generation ASSBs [167,168].

5. Challenges and Solutions for Sulfide SSEs

Sulfide-based SSEs have become key materials for advancing ASSBs due to their superior ionic conductivity and mechanical flexibility compared to oxide and halide alternatives. However, their practical application is hindered by significant challenges, such as interfacial instability, air sensitivity, and high production costs. Overcoming these obstacles demands a thorough understanding of material properties, interfacial behavior, and scalable synthesis approaches.

5.1. Interface Stability of Sulfide SSEs

Sulfide-based SSEs are critical for enabling high-performance ASSBs. However, their practical application is limited by interfacial instability caused by inherent material constraints. The narrow electrochemical stability window (ESW) of sulfide SSEs (1.6–2.3 V vs. Li⁺/Li) triggers detrimental side reactions at high-voltage cathode interfaces, such as LiCoO₂ (LCO), leading to the formation of resistive space charge layers (SCLs), as illustrated in Figure 4 [54,55]. These layers form as a result of lithium-ion depletion within the sulfide-based SSE and simultaneous enrichment at the cathode during cycling, driven by gradients in chemical potential. For instance, LSPC exhibits a room-temperature conductivity of 2.5 × 10⁻² S·cm⁻¹ at room temperature, but its utility is curtailed by interfacial reactions that increase impedance and degrade cycle stability [57].

The formation and evolution of SCLs are further illustrated in Figure 5, and as shown in Figure 5a, similar to the charging voltage curve of a capacitor. At the onset of cell polarization, lithium ions migrate from the sulfide SSE to the anode, while cathode delithiation is delayed until the interfacial potential exceeds the cathode’s redox threshold in Figure 5b–d. This asymmetry thickens the Li⁺-depleted region in the SSE, amplifying interfacial resistance. Even after cathode delithiation compensates for the depletion layer in Figure 5d, residual impedance persists, underscoring the need for mitigation strategies such as buffer layers or electric field modulation [169,170,171,172,173,174,175,176,177].

As shown in Figure 6a,b, interfacial side reactions exacerbate degradation. At the cathode, sulfide SSEs oxidize to form low-conductivity phases (e.g., sulfates and polysulfides), while anode-side reduction generates electronically conductive interphases that accelerate electrolyte decomposition.

These reactions release gases like SO₂ and create mixed ion–electron-conductive interfaces, further destabilizing ASSBs [180,181,182,183,184]. Coating strategies, such as the 6.3 nm LiNbO₃ layer on NCM811 cathodes in Figure 7a,b, physically isolate the cathode from the SSE, reducing interfacial impedance by 50% and enhancing cycling stability [178]. Ideal coatings must balance ionic conductivity, electronic insulation, and mechanical resilience to accommodate volume changes during lithiation/delithiation [178].

As illustrated in Figure 8, the electrochemical stability limitations of sulfide-based SSEs stem from their narrow ESW compared to other electrolyte systems. The instability of SSEs under extreme lithium chemical potentials, coupled with their inherent reactivity with electrode materials, necessitates both surface and bulk modifications. For instance, coatings such as Li₃PO₄ or LiTi₂(PO₄)₃ on NCM cathodes effectively suppress decomposition, while bulk doping strategies improve stability without sacrificing ionic conductivity [185,186,187].

A systematic screening of 41 coating materials identified oxides, such as Li₅TaO₅, as promising candidates for interfacial passivation, as demonstrated in Figure 9 [187].

Mechanical instability at solid–solid interfaces stems from the inherent rigidity of sulfide-based SSEs, which are unable to accommodate the significant volume changes associated with silicon anodes (up to 300% expansion) or lithium metal deposition [72]. As shown in Figure 10, contact loss at the anode/SSE interface localizes current density, promoting dendritic Li penetration even in grain-boundary-free glassy electrolytes [178,179]. Strategies to mitigate this include using single-crystal Li₆La₃ZrTaO₁₂ garnet electrolytes to eliminate grain boundaries and exerting external pressure (1–10 MPa) to maintain interfacial contact [188,189,190,191,192,193].

Song et al. synthesized a series of novel glassy sulfide LBPSI (Li₂S-B₂S₃-P₂S₅-LiI) electrolyte materials for application in sulfide-based ASSBs, achieving rapid solid–solid sulfur electrochemistry and high cycling stability. This electrolyte not only serves as an ultra-ionic conductor within the sulfur cathode but also contains redox-active iodine elements that exert a surface redox-mediated role in the solid–solid sulfur conversion process, thereby facilitating the efficient oxidation of Li₂S. This redox-mediated process at the SSE surface enables reactions at the SE|Li₂S dual-phase boundaries, significantly increasing the density of active sites as illustrated in Figure 10a [22].

By adjusting the ratio of B₂S₃ and P₂S₅ glass formers, Song et al. designed a family of LBPSI glass electrolytes with high ionic conductivity (P₂S₅/(P₂S₅  +  B₂S₃) mol/mol ratio of 0.80; n(P₂S₅)/n(P₂S₅  +  B₂S₃) hereafter). As shown in Figure 10b, ¹¹B solid-state NMR spectra reveal that when the molar ratio of P₂S₅ reaches n(P₂S₅)/n(P₂S₅ + B₂S₃) = 0.17, the peak corresponding to [BS₄] structural units shifts toward higher frequencies (from −2.88 ppm to −1.01 ppm), indicating a transition from multi-bridging to single-bridging configurations in the [BS₄] units [194,195,196]. This observation demonstrates the fragmentation of the B-S network and the formation of smaller island-like local structures with reduced rigidity, which facilitates Li⁺ migration [22].

The ⁷Li NMR spectra in Figure 10c show that at n(P₂S₅)/n(P₂S₅ + B₂S₃) = 0.17, the peak corresponding to Li⁺ associated with iodine (−4.4 ppm) diminishes significantly, suggesting an improved integration of Li⁺/I⁻ ions and a reduction in isolated ion pairs [22]. Furthermore, the shoulder peaks (0.50 and 0.17 ppm) attributed to Li⁺ coordinated with non-bridging sulfur evolve into a single peak at 0.22 ppm, reflecting weakened nuclear shielding effects that promote Li⁺ mobility [197,198].

In addition to interfacial challenges, air sensitivity poses a significant barrier to the practical application of sulfide-based SSEs [199]. When exposed to humid environments, sulfide SSEs undergo hydrolysis, releasing H₂S and resulting in a decline in ionic conductivity [200]. Notably, the 75Li₂S·25P₂S₅ glass–ceramic demonstrates exceptional air stability, retaining a conductivity of 1.5 × 10⁻⁴ S·cm⁻¹ even after 7 h of air exposure [79]. Oxygen doping (e.g., LPSO) or hydrophobic coatings like g-C₃N₄ suppress H₂S generation [201]. As demonstrated in Figure 11, g-C₃N₄-coated(651-5%, 651-5%-exposure) LPSC retains 72% conductivity after air exposure, compared to 52% for uncoated samples(651, 651-exposure) [200]. Ni et al. successfully synthesized a Li_3.12P_0.94Bi_0.06S_3.91O_0.09 sulfide solid electrolyte by co-doping Li₃PS₄ with Bi and O, which exhibits enhanced air stability. Air stability tests were conducted by exposing Li_3+2xP_1−xBi_xS_4−1.5xO_1.5x electrolytes with varying doping levels to air under 50% relative humidity [93]. As shown in Figure 10c,d, undoped Li₃PS₄ readily reacts with H₂O, releasing significant amounts of H₂S gas. In contrast, Bi- and O- doped Li₃PS₄ electrolytes demonstrate suppressed reactivity with H₂O, with the release of H₂S gas progressively decreasing as doping levels increase. As shown in Figure 10b, X-ray diffraction (XRD) measurements reveal that the Li_3.12P_0.94Bi_0.06S_3.91O_0.09 electrolyte maintains structural integrity in air, whereas undoped Li₃PS₄ undergoes substantial structural degradation, as evidenced by the emergence of numerous impurity peaks after air exposure [202].

5.2. Conductivity Optimization of Sulfide SSEs

The ionic conductivity of sulfide SSEs is governed by anion disorder and lattice dynamics, which can be strategically engineered through structural modifications and doping strategies [205].

In Li₆PS₅X (X = F, Cl, Br, I) electrolytes, the degree of S²⁻/X⁻ mixing directly dictates Li⁺ migration pathways. For X = Cl and Br, the similar ionic radii of S²⁻ (1.84 Å), Cl⁻ (1.81 Å), and Br⁻ (1.96 Å) promote anion disorder, broadening Li⁺ transport channels and achieving more than 1 × 10⁻² S∙cm⁻¹ [206]. In contrast, the large size mismatch between S²⁻ and I⁻ (2.20 Å) in Li₆PS₅I induces ordered anion arrangements, reducing mixing and resulting in lower conductivity (10⁻⁵ S∙cm⁻¹) [206]. Additionally, softer lattices with enhanced bond flexibility lower Li⁺ migration energy barriers, further improving ionic transport [26,207].

Doping with aliovalent cations (e.g., Si⁴⁺, Ge⁴⁺) or anions (e.g., I⁻) enhances both ionic conductivity and electrochemical stability [27]. For instance, Si⁴⁺ doping in Li₆PS₅Br increases S²⁻/Br⁻ disorder, with Li_6.35P_0.65Si_0.35S₅Br reaching 2.4 × 10⁻³ S∙cm⁻¹ at 25 °C [86]; Ge⁴⁺ doping in Li₆PS₅I elevates S²⁻/I⁻ mixing, yielding Li_6.6P_0.4Ge_0.6S₅I with (18.4 ± 2.7) × 10⁻³ S∙cm⁻¹ [207]; and MoS₂ doping in Li₇P₃S₁₁ produces Li₇P_2.9S_10.85Mo_0.01, achieving 4.8 mS·cm⁻¹ and a widened ESW (5V vs. Li/Li⁺) [208].

These modifications not only improve Li⁺ mobility, but also enhance compatibility with lithium metal anodes, as demonstrated by Sb-doped β-Li₃PS₄ (Li₃P_0.98Sb_0.02S_3.95O_0.05, 1.08 mS·cm⁻¹) [209,210,211,212].

Quenching protocols play a critical role in modulating ionic conductivity by controlling the degree of anion disorder. As shown in Figure 12a,b, theoretical and experimental studies on Li₆PS₅Br reveal that rapid quenching amplifies S²⁻/Br⁻ mixing, increasing Li⁺–anion distances and weakening Coulombic interactions. This delocalization effect elevates conductivity from 5 × 10⁻⁴ S∙cm⁻¹ (slow cooling) to 2 × 10⁻³ S∙cm⁻¹ (quenching) [211]. Such process-dependent structural tuning highlights the critical role of synthesis kinetics in optimizing sulfide SSE performance.

5.3. Mechanical Stability of Sulfide SSEs

The mechanical stability of sulfide-based SSEs is fundamentally tied to their ionic conductivity and interfacial integrity, which are critical factors determining the cycle life and safety of ASSBs. Despite their superior ionic conductivity, sulfide SSEs encounter significant challenges for large-scale deployment, primarily due to limited electrochemical stability and mechanical degradation under operational stress conditions [212,213,214].

During charge–discharge cycles, the volumetric expansion/contraction of active materials (e.g., silicon anodes with 300% volume change) and the inherent rigidity of sulfide SSEs induce interfacial delamination and crack propagation at the electrode–electrolyte interface [72]. Unlike liquid electrolytes, sulfide SSEs cannot dynamically adapt to these volume changes, leading to stress accumulation and the eventual mechanical failure of composite electrodes [215]. Advanced characterization techniques, such as X-ray computed tomography and transmission X-ray microscopy, enable real-time observation of crack formation and contact loss at solid–solid interfaces, while focused ion beam-SEM provides 3D microstructural insights into degradation mechanisms [216,217].

To mitigate these issues, strategies include applying external pressure (1–10 MPa) to maintain interfacial contact and suppress crack initiation; utilizing low-strain active materials (e.g., Li₄Ti₅O₁₂) to minimize volume changes; and optimizing the distribution of SSE, conductive additives, and voids within composite electrodes [218,219,220,221].

These approaches underscore the need for holistic material design to balance ionic transport and mechanical resilience in sulfide-based ASSBs.

Dealloying, the selective removal of one or more components from an alloy, is widely employed to fabricate metallic materials with tailored nano-porosity and composition. Compared to conventional materials, dealloyed metals exhibit superior porosity and tunable structural characteristics, demonstrating unique advantages in battery technologies and other advanced applications. However, the mechanical stress effects during dealloying processes remain insufficiently explored, particularly in solid-state battery systems. A critical challenge lies in effectively controlling structural evolution and electrochemical performance during alloying/dealloying cycles [222,223,224,225].

Wang et al. investigated the influence of stacking pressure on the alloying/dealloying behavior of lithium-alloyable materials (Al and Si). By analyzing porosity evolution during alloying/dealloying processes in both solid-state and liquid electrolytes (Figure 13a,b), they systematically compared silicon wafer electrodes (0.38 V and 1.0 V vs. Li/Li⁺) under varying stacking pressures (0.5 MPa, 2 MPa, 10 MPa, and 30 MPa) [73]. Their findings revealed distinct morphological transformations in dealloyed metals across electrolyte types (Figure 13c,d). Stacking pressure emerged as a decisive factor in porosity regulation: At lower pressures (2–5 MPa), indium-coated interfaces significantly enhanced interfacial contact, enabling reversible lithium storage with high areal capacity retention and cycling efficiency. This work provides critical insights into interface engineering for optimizing alloy-based anodes in energy storage systems.

5.4. Emerging Research Directions and Opportunities

The development of sulfide-based SSEs for ASSBs necessitates overcoming three key interconnected challenges: understanding ion transport mechanisms, suppressing lithium dendrite formation, and addressing multi-field coupling effects [226,227,228].

Ion transport mechanisms: Although sulfide-based SSEs demonstrate high bulk ionic conductivity, the atomic-scale dynamics of Li⁺ migration—particularly the interaction between lattice phonons and interfacial processes—remain insufficiently understood. Current limitations in real-time characterization techniques, such as operando spectroscopy, and computational models impede the optimization of ion transport pathways. The development of advanced tools, including neutron pair distribution function (PDF) analysis and machine learning-driven molecular dynamics simulations, will be essential for elucidating these mechanisms and guiding the design of improved materials [229,230,231].

Lithium dendrite growth: despite the mechanical robustness of sulfide SSEs (>1GPa hardness), lithium dendrites propagate through grain boundaries and defects, threatening cell safety [191]. Future studies must resolve the nucleation and growth kinetics of Li metal within SSEs under operational conditions (e.g., current density and pressure). Strategies such as artificial SEI layers and lattice strain engineering could homogenize Li deposition, but their long-term efficacy requires validation through coupled experimental–theoretical approaches [232].

Multi-field coupling effects: ASSBs operate under complex multi-physics coupling conditions, where localized hotspots, stress gradients, and interfacial reactions interact synergistically, leading to accelerated degradation. Establishing predictive multi-physics models that integrate these interactions is essential to decode failure mechanisms (e.g., crack propagation and impedance rise) and optimize cell designs. For instance, phase-field modeling combined with in situ X-ray tomography could map stress distributions during cycling, informing pressure management strategies [233,234,235,236].

Ultimately, bridging nanoscale material insights with macroscale battery performance will require interdisciplinary collaboration across materials science, electrochemistry, and computational modeling.

6. Conclusions

6.1. Research Status and Challenges

Sulfide SSEs present a dual challenge: enhancing ionic conductivity to maximize battery performance and improving air stability to enable cost-effective large-scale production. This review systematically examines the fundamental properties of sulfide SSEs, including their chemical composition, ionic conduction mechanisms, and physicochemical characteristics, while evaluating their applicability in ASSBs. Critical issues such as air sensitivity and interfacial side reactions are analyzed, providing a theoretical foundation for designing high-energy-density, long-cycle-life ASSBs [198].

Despite advancements in ionic conductivity, sulfide SSEs face significant barriers to commercialization, including rapid hydrolysis in humid environments and interfacial degradation during cycling. Recent progress in anion doping (e.g., oxygen incorporation) and surface coating strategies has demonstrated promising results in suppressing H₂S generation and enhancing electrochemical stability [224,225,226,227,228,229,230,231,232,233,234,235,236,237,238]. However, achieving scalable production requires the further optimization of synthesis protocols to reduce costs below $50/kg while maintaining mechanical and chemical integrity [54,168].

6.2. Future Perspectives

Realizing the full potential of sulfide-based ASSBs requires coordinated advancements across material design, manufacturing processes, characterization techniques, and system integration.

Material innovation requires compositional engineering to develop cost-effective batch production methods for sulfide SSEs and thin-film electrolytes, coupled with advanced doping strategies (e.g., halogen or oxide incorporation) to improve atmospheric stability and interfacial compatibility with high-capacity electrodes such as silicon–carbon anodes [225,226,227,228,229,230,231,232,233,234,235,236,237]. Concurrently, manufacturing processes must transition from energy-intensive synthesis to scalable techniques like roll-to-roll processing, ensuring compatibility with high-voltage cathodes (e.g., NCM811) and lithium metal anodes while reducing production costs [90]. Advanced characterization urgently demands in situ/operando methodologies to overcome material air sensitivity, exemplified by differential phase-contrast STEM for solid-concentration layer analysis and coupled in situ NMR/Raman spectroscopy for probing dynamic interfacial evolution during cycling [239]. Cell architecture optimization focuses on reducing SSE membrane thickness below 20 μm to achieve energy densities exceeding 400 Wh/kg, though mechanical robustness challenges necessitate innovative solutions like flexible polymer binders or organic–inorganic hybrid matrices [28,240]. Finally, comprehensive safety protocols must integrate multi-physics modeling frameworks that couple electrochemical, thermal, and mechanical parameters to evaluate extreme-condition behaviors (e.g., thermal runaway and mechanical stress), thereby guiding the design of durable battery systems [214].

6.3. Challenges and Prospects for Commercialization

The commercialization of sulfide-based ASSBs necessitates interdisciplinary efforts to address key challenges in material synthesis, interface engineering, and scalable production. With coordinated policy frameworks and sustained R&D investment, sulfide-based SSEs are expected to overcome existing limitations, paving the way for the widespread adoption of safe, high-energy-density ASSBs within the next decade. Such advancements will significantly contribute to global carbon neutrality goals and the development of sustainable energy storage solutions.