With the rapid development of the new energy vehicle industry, the market has imposed higher demands on energy storage devices. Lithium-ion batteries (LIBs), which are increasingly mature and commercially available, play a significant role in large-scale energy storage due to their high energy density. However, traditional LIBs, which use liquid organic electrolytes, are prone to issues such as combustion and leakage. To address these safety concerns, researchers have proposed all-solid-state lithium batteries (ASSBs) that replace liquid electrolytes with inorganic solid electrolytes. This approach opens new avenues for fundamentally enhancing the safety of lithium batteries. Compared to conventional liquid LIBs, all-solid-state batteries offer two primary advantages: high safety and high energy density. Traditional LIBs employ organic electrolytes that generate heat during prolonged operation. When lithium metal is used as the anode, lithium dendrites can easily form and pierce the separator, leading to short circuits and heat generation. In contrast, solid electrolytes possess a certain elastic modulus, which helps suppress lithium dendrite growth, thereby ensuring the intrinsic safety of all-solid-state batteries.
Currently, the most studied inorganic solid electrolytes can be categorized into three types: oxide solid electrolytes, sulfide solid electrolytes, and polymer solid electrolytes. Among these, sulfide solid electrolytes are considered one of the most promising due to their high ionic conductivity and suitable mechanical properties. Despite significant progress in sulfide electrolyte research over recent decades, their industrial application still faces challenges. For instance: (1) sulfide solid electrolytes have a narrow electrochemical window, leading to poor electrochemical compatibility with electrodes; (2) sulfide electrolytes exhibit poor chemical stability in humid air; and (3) toxic gases may be generated during preparation, and side reactions can occur at the electrode-electrolyte interfaces. To mitigate these issues, researchers have conducted various studies. Strategies to reduce these negative effects and optimize the performance of sulfide solid electrolytes include: (1) employing different preparation methods to enhance stability and electrochemical compatibility with electrodes; and (2) introducing element doping to appropriately modify sulfide solid electrolytes, reducing oxygen erosion and maintaining the electrochemical performance of all-solid-state lithium batteries. This article primarily discusses the types and preparation methods of sulfide solid electrolytes and proposes corresponding solutions to common problems.
Sulfide solid electrolytes can be broadly classified into three categories based on their crystalline forms: glassy, glass-ceramic, and crystalline. Through extensive research efforts, the ionic conductivity of sulfide solid electrolytes has reached or even surpassed that of liquid organic electrolytes. Table 1 summarizes the ionic conductivities and activation energies corresponding to different crystalline forms of electrolytes.
| Electrolyte | Morphology | Room Temperature Ionic Conductivity / (S·cm⁻¹) | Activation Energy / eV |
|---|---|---|---|
| Li₃PS₄ | Glassy | 2.0 × 10⁻⁴ | 0.35 |
| 0.4Li–0.6LiSnS₄ | Glassy | 4.1 × 10⁻⁴ | 0.43 |
| Li₂S–P₂S₅–LiI | Glassy | 1.0 × 10⁻³ | 0.30 |
| Li₇P₃S₁₁ | Glass-Ceramic | 1.7 × 10⁻² | 0.18 |
| Li₇P₂.₉Sb₀.₁S₁₀.₇₅O₀.₂₅ | Glass-Ceramic | 1.6 × 10⁻³ | 0.28 |
| Li₅.₆PS₄.₆I₁.₄ | Glass-Ceramic | 2.0 × 10⁻³ | 0.31 |
| Li₆.₅In₀.₂₅P₀.₇₅S₅I | Glass-Ceramic | 1.0 × 10⁻³ | 0.28 |
| Li₇Ag₀.₁P₃S₁₁I₀.₁ | Crystalline | 1.3 × 10⁻³ | 0.23 |
| Li₉.₆P₃S₁₂ | Crystalline | 1.2 × 10⁻³ | 0.26 |
| Li₄SnS₄ | Crystalline | 7.0 × 10⁻⁵ | 0.41 |
| Li₃.₃₃₄Ge₀.₃₃₄As₀.₆₆₆S₄ | Crystalline | 1.1 × 10⁻³ | 0.17 |
| Li₃.₂₅Ge₀.₂₅P₀.₇₅S₄ | Crystalline | 2.2 × 10⁻³ | 0.21 |
| Li₁₀GeP₂S₁₂ | Crystalline | 1.2 × 10⁻² | 0.25 |
| Li₁₀SnP₂S₁₂ | Crystalline | 4.0 × 10⁻³ | 0.27 |
| Li₁₀SiP₂S₁₂ | Crystalline | 2.0 × 10⁻³ | 0.30 |
| Li₁₀Si₀.₃Sn₀.₇P₂S₁₂ | Crystalline | 8.0 × 10⁻³ | 0.29 |
| Li₁₀GeP₀.₉₂₅Sb₀.₀₇₅S₁₂ | Crystalline | 1.7 × 10⁻² | 0.27 |
| Li₅.₅PS₄.₅Cl₁.₅ | Crystalline | 9.4 × 10⁻³ | 0.29 |
| Li₆.₇Si₀.₇Sb₀.₃S₅I | Crystalline | 1.1 × 10⁻² | 0.26 |
| Li₅.₃PS₄.₃ClBr₀.₇ | Crystalline | 2.4 × 10⁻² | 0.15 |
| Li₆.₂₅PS₄.₇₅N₀.₂₅Cl | Crystalline | 1.5 × 10⁻³ | 0.26 |
| Li₆.₃P₀.₇Sn₀.₃S₄.₄O₀.₆I | Crystalline | 2.3 × 10⁻⁴ | 0.32 |
Types of Sulfide Solid Electrolytes
Sulfide solid electrolytes are categorized into glassy, glass-ceramic, and crystalline types based on their structural morphology. The ionic conductivity in these materials is governed by the Arrhenius equation: $$\sigma = A \exp\left(-\frac{E_a}{kT}\right)$$ where $\sigma$ is the ionic conductivity, $A$ is the pre-exponential factor, $E_a$ is the activation energy, $k$ is Boltzmann’s constant, and $T$ is the temperature. This relationship highlights the importance of low activation energies for high conductivity in solid state batteries.
Glassy Sulfide Electrolytes
Glassy sulfide electrolytes exhibit isotropic conduction pathways, and their grain boundary resistance is easily eliminated. Consequently, glassy sulfide electrolytes are generally considered to have higher ionic conductivity than their crystalline counterparts. Studies on the xLi₂S–(100−x)P₂S₅ binary system with varying Li₂S fractions have shown that a single glass phase forms when x ranges from 0.4 to 0.8. Furthermore, the relationship between the local structure of thiophosphate structural units and ionic conductivity has been elucidated. As the Li₂S content increases, a continuous transition from dominant double tetrahedra P₂S₇⁴⁻ to single tetrahedra PS₄³⁻ is observed. This structural evolution significantly impacts the ionic transport properties, making glassy sulfides crucial for advancing solid state battery technology.
Glass-Ceramic Sulfide Electrolytes
Glass-ceramic sulfide electrolytes consist of both amorphous and crystalline phases and are typically prepared by heat-treating glassy electrolytes. The type of crystals formed depends on the composition of the glassy electrolyte and the heat treatment conditions. Current research on glass-ceramic electrolytes primarily focuses on the Li₂S–P₂S₅ system. In xLi₂S–(100−x)P₂S₅ glassy electrolyte materials, different crystalline phases emerge based on the value of x. To enhance the conductivity of glass-ceramic electrolytes, hot-pressing techniques have been applied to 70Li₂S–30P₂S₅ glass-ceramics to eliminate grain boundaries, resulting in an ionic conductivity of 1.7 × 10⁻² S·cm⁻¹. This improvement is vital for developing high-performance all-solid-state batteries.
Crystalline Sulfide Electrolytes
Crystalline sulfide electrolytes include thio-LISICON electrolytes and argyrodite electrolytes.
Thio-LISICON Electrolytes
Thio-LISICON structures, discovered in systems like Li₂S–GeS₂, Li₂S–GeS₂–Ga₂S, and Li₂S–GeS₂–ZnS, are derived from LISICON-type γ-Li₃PO₄ electrolytes by substituting O with S. Sulfur has lower electronegativity and a larger radius than oxygen, which reduces Li⁺ binding energy and widens Li⁺ migration channels. Thus, thio-LISICON electrolytes are expected to have higher ionic conductivity than oxide electrolytes. For instance, in Li₄GeP₄ electrolytes, partial substitution of Ge⁴⁺ with P⁵⁺ yields Li₄−ₓGe₁−ₓPₓS₄. Heterovalent phosphorus doping introduces lithium vacancies in the crystal structure, significantly enhancing ionic conductivity. The crystal structure is based on the parent Li₄GeS₄ structure, and X-ray diffraction patterns indicate that Li₄−ₓGe₁−ₓPₓS₄ (0 < x < 1) electrolytes can be regarded as solid solutions between γ-Li₃PS₄ and Li₄GeS₄. This structural tuning is key to optimizing materials for solid state batteries.
Argyrodite Electrolytes
Ag₈GeS₆ was the first known solid electrolyte with an argyrodite structure, exhibiting high ionic conductivity and Ag⁺ mobility due to its highly disordered cation arrangement. In the Li₆PS₅X (X = Cl, Br, or I) system, three different transition processes occur: double jumps (Wyckoff position 48h–24g–48h), intra-cage jumps (Wyckoff position 48h–48h), and inter-cage jumps. Inter-cage jumps dominate the long-range transport of Li⁺, and the site disorder between Wyckoff 4a and 4c positions significantly influences ionic conductivity. The ionic conductivities of argyrodite-type electrolytes Li₆PS₅Cl, Li₆PS₅Br, and Li₆PS₅I are 1.9 × 10⁻³, 6.8 × 10⁻⁴, and 4.6 × 10⁻⁷ S·cm⁻¹, respectively. Understanding these mechanisms is essential for designing better solid state batteries.
Preparation Methods
Current synthesis methods for sulfide solid electrolytes mainly include melting, liquid-phase, and mechanical ball milling techniques. Each method has its advantages and limitations, influencing the scalability and performance of the resulting electrolytes for solid state batteries.
Melting Method
The melting method is one of the most traditional approaches for preparing glassy sulfides. The process involves grinding raw materials, sealing them in quartz tubes under an argon atmosphere, heating at high temperatures, and then rapidly quenching the molten sample in ice water. This procedure is challenging and prone to impurity formation. By combining melting, quenching, and hot-pressing processes, grain boundaries and pores in sulfide electrolytes can be eliminated, resulting in Li₂S–P₂S₅ glass-ceramic electrolytes with high ionic conductivity of 1.7 × 10⁻² S·cm⁻¹. The general equation for the reaction can be represented as: $$\text{Li}_2\text{S} + \text{P}_2\text{S}_5 \rightarrow \text{Glass Ceramic Electrolyte}$$ This method, while effective, requires precise control to avoid defects.
Liquid-Phase Method
The liquid-phase method for preparing sulfide electrolytes offers advantages such as high efficiency and energy savings. Raw materials can react fully in solution, producing uniform precursors, which holds promise for large-scale industrial applications. Additionally, liquid-phase methods have been used to enhance electrode/electrolyte interface contact, improving battery performance. For example, Li₇P₃S₁₁ glass-ceramic electrolytes synthesized using different organic solvents like tetrahydrofuran (THF), acetonitrile (ACN), and THF/ACN mixtures were studied. Results showed that Li₇P₃S₁₁ prepared with ACN exhibited the highest ionic conductivity (9.7 × 10⁻⁴ S·cm⁻¹) and the lowest activation energy (31.2 kJ·mol⁻¹). The liquid-phase process can be described by: $$\text{Li}_2\text{S} + \text{P}_2\text{S}_5 \xrightarrow{\text{Solvent}} \text{Li}_7\text{P}_3\text{S}_{11}$$ This method is particularly suitable for manufacturing solid state batteries due to its scalability.
Mechanical Ball Milling Method
Mechanical ball milling is a commonly used laboratory method for synthesizing sulfide electrolytes due to its relative simplicity. Raw materials undergo mixing, vitrification, and crystallization during high-energy milling. For instance, Li₆PS₅X (X = Cl, Br, or I) electrolytes prepared by mechanical ball milling achieved conductivities up to 2.7 × 10⁻⁴ S·cm⁻¹, with optimized milling times yielding Li₆PS₅I showing high ionic conductivity of 1.82 × 10⁻³ S·cm⁻¹. However, the ball milling process is often lengthy and time-consuming, increasing production costs. Moreover, stoichiometric compositions are uniform only on a small scale; at larger scales, inhomogeneity occurs as raw materials tend to agglomerate during milling, potentially introducing impurities. Thus, mechanical ball milling has broader prospects in laboratory applications rather than industrial production for solid state batteries.
Modification Studies on Sulfide Electrolytes
To address the limitations of sulfide electrolytes, various modification strategies have been developed to improve air stability and ionic conductivity, which are critical for the practical application of solid state batteries.
Improvement of Air Stability
Chemical stability in humid air is a major concern for sulfide electrolytes. Most sulfide electrolytes have poor moisture stability and produce toxic H₂S gas when exposed to humid air. Studies show that the air stability of sulfide electrolytes is closely related to their local structure, such as the P–S framework. The PS₄³⁻ structure exhibits better air stability compared to other configurations. A common strategy to enhance the air/moisture stability of sulfide electrolytes is oxygen doping, where O replaces S in the structure. For example, in Li₆PS₅Br structures, introducing O elements to replace S resulted in oxygen-doped Li₆PS₅Br showing no significant changes in XRD patterns after air exposure, whereas undoped Li₆PS₅Br quickly generated LiBr·H₂O impurities. The primary reason O doping improves air stability is that O replaces S atoms at free S²⁻ sites, and these non-bonded S²⁻ ions have lower air stability than the PS₄³⁻ structure. This approach is promising for developing durable solid state batteries.
Enhancement of Ionic Conductivity
Enhancing ionic conductivity is paramount for the performance of solid state batteries. Two main strategies are employed: introducing lithium vacancies and element doping.
Introduction of Lithium Vacancies
Introducing an appropriate amount of lithium vacancies into sulfide solid electrolytes can significantly improve ionic conductivity. For instance, replacing S²⁻ with halogens (Cl⁻ or Br⁻) in Li₇PS₆ increases lithium ion vacancies, raising the room temperature ionic conductivity from 10⁻⁶ S·cm⁻¹ to over 10⁻³ S·cm⁻¹, approaching that of liquid organic electrolytes (10⁻² S·cm⁻¹). A series of argyrodite sulfide solid electrolytes Li₆−ₓPS₅−ₓCl₁+ₓ were synthesized by varying the Cl⁻ substitution ratio for S²⁻, where x ranges from 0 to 0.6. Specifically, Li₆PS₅Cl has a room temperature ionic conductivity of 2.5 × 10⁻³ S·cm⁻¹, while Li₅.₅PS₄.₅Cl₁.₅ exhibits the highest room temperature ionic conductivity of 9.4 × 10⁻³ S·cm⁻¹. This high conductivity is attributed to increased lithium vacancies and the mixed arrangement of Cl⁻ and S²⁻. Furthermore, doping the lithium site with Ca increases lithium vacancy concentration and enhances the mixed arrangement of Cl⁻ and S²⁻, resulting in Li₅.₃₅Ca₀.₁PS₄.₅Cl₁.₅₅ electrolyte with a room temperature ionic conductivity as high as 1.02 × 10⁻² S·cm⁻¹. Similar trends were observed in bromide-containing sulfide electrolytes, where Li₅.₃PS₄.₃Br₁.₇ achieved an ionic conductivity of 1.1 × 10⁻² S·cm⁻¹. The effect of lithium vacancies can be modeled using the formula: $$\sigma \propto \exp\left(-\frac{E_v}{kT}\right)$$ where $E_v$ is the vacancy formation energy, underscoring the importance of defect engineering in solid state batteries.
Element Doping
Element doping is another effective method to enhance the ionic conductivity of sulfide solid electrolytes. Based on the structural composition of Li₇PS₆ electrolyte, doping can be performed at Li, P, and S sites. Doping heterovalent metal cations at Li sites can stabilize the high-temperature phase structure of sulfide electrolytes, improving lithium ion conductivity. For example, doping Fe²⁺ at Li sites in Li₇PS₆ structure not only stabilizes the high-temperature phase but also increases room temperature ionic conductivity by ten times, reaching 1.4 × 10⁻⁴ S·cm⁻¹. Additionally, doping a small amount of Al³⁺ at Li sites in Li₆PS₅Br successfully prepares Li₅.₄Al₀.₂PS₅Br electrolyte, which reduces the diffusion distance of lithium ions between “inter-cage” and “intra-cage” sites, resulting in a room temperature ionic conductivity of 2.4 × 10⁻³ S·cm⁻¹. For P-site doping, elements such as Si, Ge, Sn, and Sb are commonly used. For instance, Si doping at P sites in Li₆PS₅Br structure yields Li₆.₃₅P₀.₆₅Si₀.₃₅S₅Br and Li₆.₅P₀.₅Si₀.₅S₅Br electrolytes with room temperature ionic conductivities of 7 × 10⁻⁴ and 2.4 × 10⁻³ S·cm⁻¹, respectively. As Si doping content increases, the ionic conductivity improves by more than three times, primarily because Si⁴⁺ has a larger ionic radius than P⁵⁺, which not only increases carrier concentration but also expands the material’s unit cell size, facilitating lithium ion transport. Si or Ge doping also enhances the thermodynamic stability of the material; heterovalent Si⁴⁺ or Ge⁴⁺ doping at P⁵⁺ sites in Li₇PS₆ structure helps stabilize the high-temperature cubic phase of argyrodite sulfide electrolytes. The doping effect can be expressed as: $$\Delta \sigma = k \cdot C_d$$ where $\Delta \sigma$ is the change in conductivity, $k$ is a constant, and $C_d$ is the dopant concentration, highlighting the role of compositional adjustments in advancing solid state battery technology.
Conclusion
All-solid-state lithium batteries offer advantages in safety performance and energy density, making them a promising research direction. This article reviews the classification, mainstream preparation methods, and optimization modifications of sulfide solid electrolytes regarding air stability and ionic conductivity. The following conclusions are drawn:
1. The current synthesis routes for sulfide solid electrolytes are relatively complex and require high conditions. There is a need to develop simple, efficient, highly reproducible synthesis methods suitable for large-scale preparation to ensure their application in all-solid-state batteries. The liquid-phase method, with its efficiency and energy-saving benefits, allows raw materials to react fully in solution, producing uniform precursors, and holds promise for large-scale industrialization.
2. Although sulfide solid electrolytes have high ionic conductivity, their air stability and electrochemical stability lag behind other solid electrolytes. Conventional improvements in air stability can be achieved through oxygen doping or by applying the hard-soft acid-base theory.
3. The introduction of lithium vacancies significantly affects the structural disorder of sulfide solid electrolytes, leading to a notable increase in ionic conductivity.
4. In addition to introducing lithium vacancies, selecting appropriate elements for doping also helps enhance the ionic conductivity of sulfide solid electrolytes.
Future research should focus on optimizing interface compatibility, reducing production costs, and exploring novel doping strategies to overcome existing barriers. The continuous advancement in sulfide electrolyte technology will undoubtedly accelerate the commercialization of high-performance solid state batteries, contributing to safer and more efficient energy storage solutions. The integration of computational modeling, such as density functional theory (DFT) for predicting dopant effects, and advanced characterization techniques will further elucidate structure-property relationships. Ultimately, the goal is to achieve solid state batteries that outperform conventional systems in every aspect, paving the way for widespread adoption in electric vehicles and grid storage applications.