Sulfide solid electrolytes have emerged as promising candidates for next-generation solid-state batteries due to their high ionic conductivity, which often exceeds that of oxide-based solid electrolytes. Their soft mechanical properties enable better interfacial contact with electrodes compared to rigid oxides. However, their practical implementation faces significant challenges related to electrochemical stability, particularly when paired with high-voltage cathodes and lithium-metal anodes. Understanding the stability window and degradation mechanisms is critical for developing reliable sulfide-based solid-state batteries.

The electrochemical stability window defines the voltage range within which an electrolyte remains chemically inert. For sulfide solid electrolytes, this window is typically narrower than that of conventional liquid electrolytes or oxide-based solid electrolytes. Most sulfide electrolytes, such as Li7P3S11, Li10GeP2S12, and Li6PS5Cl, exhibit thermodynamic stability only within a limited range of approximately 1.7–2.5 V versus Li+/Li. Beyond these limits, decomposition reactions occur, compromising battery performance. At low potentials, reduction reactions with lithium metal lead to the formation of interfacial phases, while at high potentials, oxidative decomposition generates resistive byproducts.

When paired with high-voltage cathodes such as LiNi0.8Mn0.1Co0.1O2 (NMC811) or LiCoO2, sulfide electrolytes undergo oxidative decomposition. The thiophosphate groups (PS4^3-) in these materials are susceptible to oxidation at potentials above 2.5 V, forming sulfur, phosphates, and other electrochemically inactive species. This decomposition increases interfacial resistance and reduces cycle life. The extent of degradation depends on the cathode material's operating voltage and the sulfide electrolyte's composition. For instance, Li6PS5Br shows marginally better oxidative stability than Li6PS5Cl due to the higher electronegativity of bromine, but it still falls short of the requirements for high-voltage applications.

Lithium-metal anodes present another challenge due to the reduction instability of sulfide electrolytes. The reaction between lithium metal and sulfide electrolytes forms a passivation layer composed of Li2S, Li3P, and other reduced phases. While this layer can partially prevent further degradation, its ionic conductivity is often lower than that of the bulk electrolyte, leading to increased impedance over time. Additionally, the mechanical instability of lithium metal during cycling causes dendrite penetration, further exacerbating interfacial degradation.

Several strategies have been explored to improve the electrochemical stability of sulfide solid electrolytes. Doping with halogen elements (Cl, Br, I) or oxygen has been shown to enhance oxidative stability. For example, oxygen substitution in Li7P3S11 shifts the oxidation onset to higher potentials by modifying the electronic structure of the thiophosphate units. Similarly, incorporating elements like Ge or Sn into the crystal lattice can improve reduction stability by forming more favorable interfacial phases with lithium metal. However, excessive doping may compromise ionic conductivity, necessitating a careful balance between stability and performance.

Interface engineering is another effective approach to mitigate degradation. Coating high-voltage cathodes with oxide layers such as LiNbO3 or LiTaO3 acts as a barrier against sulfide electrolyte oxidation. These coatings prevent direct contact between the cathode and sulfide electrolyte while allowing lithium-ion transport. On the anode side, artificial interphases composed of LiF or Li3N have been demonstrated to stabilize the lithium-metal interface by suppressing dendrite growth and reducing side reactions. These interphases must be thin and ionically conductive to avoid introducing additional resistance.

Another degradation mechanism involves interfacial reactions driven by electrochemical cycling. Even within the nominal stability window, localized high potentials or lithium plating/stripping inhomogeneities can trigger parasitic reactions. For instance, the accumulation of lithium sulfides at the anode interface increases overpotential, while cathode-electrolyte interphase growth consumes active lithium. Advanced characterization techniques such as X-ray photoelectron spectroscopy and impedance spectroscopy have revealed that these processes are highly dependent on current density and operating temperature.

The mechanical properties of sulfide electrolytes also play a role in their stability. Unlike liquid electrolytes, which can flow and accommodate volume changes, solid electrolytes experience stress buildup during cycling. This stress can lead to crack formation, exposing fresh electrolyte surfaces to further degradation. Composite electrolytes incorporating polymers or inert fillers have been explored to improve mechanical resilience while maintaining high ionic conductivity. However, the trade-off between flexibility and electrochemical stability remains a challenge.

Scalability and processing conditions further influence the stability of sulfide electrolytes. Exposure to moisture during synthesis or cell assembly leads to hydrolysis, forming toxic H2S gas and degrading ionic conductivity. Strict atmospheric control is required during manufacturing, which adds complexity and cost. Developing moisture-resistant sulfide compositions, such as those incorporating hydrophobic dopants, could alleviate this issue without sacrificing performance.

In summary, sulfide solid electrolytes exhibit high ionic conductivity but suffer from limited electrochemical stability when paired with high-voltage cathodes or lithium-metal anodes. Degradation mechanisms include oxidative decomposition at the cathode and reductive interfacial reactions at the anode. Strategies such as doping, interface engineering, and composite designs offer pathways to enhance stability, though challenges remain in balancing conductivity, mechanical properties, and processing requirements. Future research should focus on optimizing these strategies while ensuring compatibility with large-scale manufacturing processes. Advances in this area could unlock the full potential of sulfide-based solid-state batteries for high-energy-density applications.