In recent years, the rapid development of new energy vehicles has imposed higher demands on energy storage devices. Lithium-ion batteries (LIBs), which have become increasingly commercialized due to their high energy density, play a crucial role in large-scale energy storage. However, traditional LIBs employ liquid organic electrolytes, which can lead to safety issues such as combustion and leakage. To address these challenges, researchers have proposed all-solid-state lithium batteries (ASSLBs) that utilize inorganic solid electrolytes instead of liquid electrolytes, opening new avenues for enhancing battery safety. Compared to conventional liquid LIBs, all-solid-state batteries offer significant advantages, including superior safety and higher energy density. The organic electrolytes in traditional LIBs generate heat during prolonged operation, and when lithium metal is used as the anode, dendrite formation can puncture the separator, causing short circuits and thermal runaway. Solid electrolytes, with their inherent elastic modulus, can suppress lithium dendrite growth, thereby fundamentally improving the safety of all-solid-state batteries.
Among the most studied inorganic solid electrolytes are oxide-based, sulfide-based, and polymer-based types. Sulfide solid electrolytes are particularly promising due to their high ionic conductivity and suitable mechanical properties. Despite significant progress over the past few decades, the industrial application of sulfide electrolytes faces several challenges. These include a narrow electrochemical window leading to poor compatibility with electrodes, poor chemical stability in humid air, and the generation of toxic gases during preparation. Additionally, interfacial side reactions between the electrolyte and electrodes remain a concern. To mitigate these issues, strategies such as optimizing preparation methods and element doping have been explored. These approaches aim to enhance stability, improve electrochemical compatibility, and maintain the performance of all-solid-state lithium batteries. This review comprehensively discusses the types of sulfide solid electrolytes, their synthesis methods, and modifications to address common problems like air stability and ionic conductivity.
Sulfide solid electrolytes can be broadly classified into three categories based on their crystalline forms: glassy, glass-ceramic, and crystalline. Extensive research has enabled sulfide electrolytes to achieve ionic conductivities comparable to or even exceeding those of liquid organic electrolytes. The ionic conductivity and activation energy of various sulfide electrolytes are summarized in Table 1. Glassy sulfide electrolytes exhibit isotropic conduction pathways and easily eliminated grain boundary resistance, often resulting in higher ionic conductivities than their crystalline counterparts. For instance, studies on the xLi₂S–(100−x)P₂S₅ binary system have shown that a single glass phase forms when x ranges from 0.4 to 0.8. The local structure of thiophosphate units, such as the transition from dominant P₂S₇⁴⁻ ditetrahedra to PS₄³⁻ monotetrahedra with increasing Li₂S content, significantly influences ionic conductivity. Glass-ceramic sulfide electrolytes, composed of amorphous and crystalline phases, are typically prepared by heat-treating glassy electrolytes. In the Li₂S–P₂S₅ system, different crystalline phases form depending on the Li₂S content. For example, hot-pressing 70Li₂S–30P₂S₅ glass-ceramic eliminates grain boundaries, achieving an ionic conductivity of 1.7 × 10⁻² S·cm⁻¹. Crystalline sulfide electrolytes include thio-LISICON and argyrodite types. Thio-LISICON structures, derived from γ-Li₃PO₄ by substituting O with S, benefit from sulfur’s lower electronegativity and larger radius, which reduce Li⁺ binding energy and widen migration channels. Heterovalent doping, such as replacing Ge⁴⁺ with P⁵⁺ in Li₄GeS₄ to form Li₄−ₓGe₁−ₓPₓS₄, introduces lithium vacancies, enhancing conductivity. Argyrodite electrolytes, like Li₆PS₅X (X = Cl, Br, I), exhibit complex Li⁺ migration processes, including inter-cage jumps that dominate long-range transport. The ionic conductivities of Li₆PS₅Cl, Li₆PS₅Br, and Li₆PS₅I are 1.9 × 10⁻³, 6.8 × 10⁻⁴, and 4.6 × 10⁻⁷ S·cm⁻¹, respectively, influenced by the disorder between Wyckoff sites.
| Electrolyte | Morphology | Ionic Conductivity at 25°C (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 |
The synthesis of sulfide solid electrolytes primarily involves three methods: melting, liquid-phase, and mechanical ball milling. The melting method is a traditional approach for preparing glassy sulfides, where raw materials are ground, sealed in quartz tubes under argon atmosphere, heated at high temperatures, and rapidly quenched in ice water. This process, though challenging and prone to impurities, can yield high-performance electrolytes when combined with hot-pressing to eliminate grain boundaries. For instance, Li₂S–P₂S₅ glass-ceramic prepared via melting and hot-pressing achieves an ionic conductivity of 1.7 × 10⁻² S·cm⁻¹. Liquid-phase methods offer advantages such as efficiency and energy savings, enabling homogeneous precursor formation and potential for large-scale industrial application. In one study, Li₇P₃S₁₁ glass-ceramic synthesized using acetonitrile (ACN) as a solvent exhibited the highest ionic conductivity (9.7 × 10⁻⁴ S·cm⁻¹) and lowest activation energy (31.2 kJ·mol⁻¹) among solvents tested. Mechanical ball milling, a common laboratory technique, involves high-energy grinding to mix, vitrify, and crystallize raw materials. For example, Li₆PS₅X (X = Cl, Br, I) electrolytes prepared by ball milling reach conductivities up to 2.7 × 10⁻⁴ S·cm⁻¹, with optimized Li₆PS₅I showing 1.82 × 10⁻³ S·cm⁻¹. However, ball milling is time-consuming and may lead to inhomogeneities in large-scale production, limiting its industrial applicability.
Improving the air stability and ionic conductivity of sulfide electrolytes is critical for advancing all-solid-state batteries. The poor moisture stability of most sulfide electrolytes results in the generation of toxic H₂S gas upon exposure to humid air. Research indicates that air stability is closely related to local structures, such as the P–S framework, with PS₄³⁻ units demonstrating better stability than other configurations. A common strategy to enhance air/water stability is oxygen doping, where O atoms replace S in the structure. For instance, O-doped Li₆PS₅Br shows negligible changes in XRD patterns after air exposure, whereas undoped Li₆PS₅Br forms LiBr·H₂O impurities. This improvement is attributed to O substitution at non-bonded S²⁻ sites, which are less stable than PS₄³⁻ structures. To boost ionic conductivity, introducing lithium vacancies and element doping are effective approaches. Halogen substitution (e.g., Cl⁻ or Br⁻ for S²⁻ in Li₇PS₆) increases lithium vacancies, raising room-temperature ionic conductivity from 10⁻⁶ S·cm⁻¹ to over 10⁻³ S·cm⁻¹. In the argyrodite system Li₆−ₓPS₅−ₓCl₁+ₓ, Li₅.₅PS₄.₅Cl₁.₅ exhibits the highest conductivity of 9.4 × 10⁻³ S·cm⁻¹ due to enhanced lithium vacancy concentration and mixed Cl⁻/S²⁻ arrangement. Further doping with Ca at Li sites in Li₅.₃Ca₀.₁PS₄.₅Cl₁.₅ achieves 1.02 × 10⁻² S·cm⁻¹. Similarly, Li₅.₃PS₄.₃Cl₁.₇ and Li₅.₃PS₄.₃Br₁.₇ show conductivities of 1.7 × 10⁻² S·cm⁻¹ and 1.1 × 10⁻² S·cm⁻¹, respectively. Element doping at Li, P, or S sites also enhances conductivity. For example, Fe²⁺ doping at Li sites in Li₇PS₆ stabilizes the high-temperature phase and increases conductivity tenfold to 1.4 × 10⁻⁴ S·cm⁻¹. Al³⁺ doping in Li₆PS₅Br yields Li₅.₄Al₀.₂PS₅Br with 2.4 × 10⁻³ S·cm⁻¹ by reducing Li⁺ diffusion distances. P-site doping with Si, Ge, Sn, or Sb expands the lattice and increases carrier concentration; Li₆.₅P₀.₅Si₀.₅S₅Br achieves 2.4 × 10⁻³ S·cm⁻¹, while Si⁴⁺ or Ge⁴⁺ doping improves thermodynamic stability by stabilizing cubic phases in argyrodite structures.
The ionic conductivity of sulfide electrolytes can be described by the Arrhenius equation: $$ \sigma = \sigma_0 \exp\left(-\frac{E_a}{kT}\right) $$ where $\sigma$ is the ionic conductivity, $\sigma_0$ is the pre-exponential factor, $E_a$ is the activation energy, $k$ is Boltzmann’s constant, and $T$ is the temperature. This equation highlights the trade-off between conductivity and activation energy, which is influenced by factors such as lattice structure, dopant concentration, and defect chemistry. For instance, in doped electrolytes, the activation energy decreases due to enhanced Li⁺ mobility, as shown in Table 1. The relationship between dopant concentration and conductivity can be modeled using percolation theory or effective medium approximations, but empirical data often guide optimizations. Additionally, the role of interfacial resistance in all-solid-state batteries cannot be overlooked. The total resistance $R_{\text{total}}$ of a solid-state battery cell includes contributions from the electrolyte, electrodes, and interfaces: $$ R_{\text{total}} = R_{\text{elec}} + R_{\text{anode}} + R_{\text{cathode}} + R_{\text{interface}} $$ where $R_{\text{interface}}$ arises from side reactions and poor contact. Strategies like surface coatings and optimized processing aim to minimize $R_{\text{interface}}$.
In conclusion, all-solid-state lithium batteries represent a promising direction due to their safety and energy density advantages. Sulfide solid electrolytes, with their high ionic conductivities, are key enablers for these batteries. This review has covered the classification of sulfide electrolytes, mainstream synthesis methods, and modifications to enhance air stability and ionic conductivity. Key findings include: (1) Current synthesis routes for sulfide electrolytes are complex and require high conditions, necessitating the development of simple, reproducible methods suitable for large-scale production. Liquid-phase methods show particular promise for industrial application due to their efficiency and ability to produce homogeneous precursors. (2) Although sulfide electrolytes exhibit high ionic conductivities, their air and electrochemical stability lag behind other solid electrolytes. Conventional improvements involve oxygen doping or applying hard-soft acid-base theory. (3) Introducing lithium vacancies significantly increases structural disorder, leading to notable enhancements in ionic conductivity. (4) Element doping at appropriate sites also contributes to improved conductivity. Future research should focus on scalable synthesis, interface engineering, and exploring novel compositions to overcome existing barriers. The integration of sulfide electrolytes into all-solid-state batteries holds great potential for next-generation energy storage, driving advancements in electric vehicles and portable electronics. As the field progresses, multidisciplinary approaches combining materials science, electrochemistry, and engineering will be essential to realize the full potential of all-solid-state batteries.
Further considerations for optimizing sulfide electrolytes include the impact of particle size and morphology on performance. For example, smaller particles may reduce ionic transport distances but increase interfacial areas, potentially leading to more side reactions. The mechanical properties of sulfide electrolytes, such as Young’s modulus and fracture toughness, also play a role in suppressing dendrite growth. Theoretical models, like density functional theory (DFT) calculations, can predict the effects of dopants on lattice parameters and Li⁺ migration barriers. For instance, the energy barrier $\Delta E$ for Li⁺ hopping between sites can be approximated by: $$ \Delta E = E_{\text{saddle}} – E_{\text{initial}} $$ where $E_{\text{saddle}}$ and $E_{\text{initial}}$ are the energies at the transition and initial states, respectively. Lower $\Delta E$ values correlate with higher ionic conductivities, as observed in doped argyrodites. Additionally, in-situ characterization techniques, such as X-ray diffraction and nuclear magnetic resonance, provide insights into structural changes during operation. The development of composite electrolytes, combining sulfides with polymers or oxides, may offer balanced properties, though challenges in interface compatibility remain. Overall, the continued innovation in sulfide electrolytes will accelerate the commercialization of all-solid-state batteries, contributing to a sustainable energy future.