Sulfide solid electrolytes have emerged as promising candidates for solid-state batteries due to their high ionic conductivity, which often exceeds that of oxide-based solid electrolytes. However, their practical implementation faces significant interfacial challenges that can degrade battery performance and longevity. These challenges primarily stem from chemical reactivity, space-charge layer formation, and mechanical contact loss, each of which requires tailored solutions to ensure stable operation.

Chemical reactivity between sulfide solid electrolytes and electrode materials is a critical issue. Many sulfide electrolytes, such as Li₇P₃S₁₁ or Li₁₀GeP₂S₁₂, are thermodynamically unstable against lithium metal and high-voltage cathode materials. When in contact with lithium metal, reduction reactions occur at the interface, leading to the formation of a resistive interphase layer. This layer increases interfacial resistance and can promote lithium dendrite growth. Similarly, at the cathode interface, oxidation reactions degrade the electrolyte, resulting in capacity fade and increased impedance over time.

To mitigate these reactions, buffer layers and surface coatings have been explored. For the anode side, thin layers of lithium nitride or lithium fluoride can act as artificial solid-electrolyte interphases, preventing direct contact between lithium and the sulfide electrolyte. These layers must be ionically conductive but electronically insulating to block electron transfer that drives decomposition. On the cathode side, coatings such as lithium niobate or aluminum oxide are applied to active materials to suppress oxidative decomposition. These coatings must be thin enough to avoid significant resistance penalties while providing sufficient chemical stability.

Space-charge layers represent another interfacial challenge. The large difference in electrochemical potential between electrodes and sulfide electrolytes drives lithium-ion depletion or accumulation at the interfaces. For instance, at the lithium metal interface, lithium-ion depletion creates a space-charge layer that impedes ion transport. This effect is exacerbated by the low dielectric constant of sulfide materials, which cannot effectively screen the electric field. The result is an increased overpotential during cycling, reducing energy efficiency.

Composite designs have been proposed to address space-charge effects. By introducing secondary phases with higher dielectric constants, such as certain oxides or polymers, the electric field screening can be improved. These composites must maintain high ionic conductivity while mitigating space-charge formation. Another approach involves gradient interfaces, where the composition is gradually varied from the electrode to the electrolyte, smoothing the potential drop and reducing ion depletion.

Mechanical contact loss during cycling is a persistent issue due to the rigid nature of sulfide electrolytes. Volume changes in electrodes, particularly lithium metal anodes, lead to interfacial delamination. This loss of contact creates voids that increase local current density, promoting dendrite formation and accelerating cell failure. Unlike liquid electrolytes, which can wet and fill gaps, solid electrolytes cannot re-establish contact once lost.

Strategies to improve mechanical contact include the use of compliant interlayers or ductile electrolyte formulations. Polymer-sulfide composites, for example, can accommodate strain better than pure sulfide ceramics, maintaining interfacial adhesion during cycling. Applying external pressure is another common method, but it adds complexity to cell design and may not be practical for all applications. Alternatively, engineered electrode architectures with built-in porosity can accommodate volume changes while preserving contact with the electrolyte.

Interfacial engineering must also account for the processing conditions of sulfide electrolytes. Many sulfides are sensitive to moisture, requiring dry-room environments for handling. This sensitivity extends to interfacial modifications, where coating processes must avoid exposing the electrolyte to reactive atmospheres. Atomic layer deposition and physical vapor deposition are preferred for applying thin interfacial layers due to their precision and compatibility with moisture-sensitive materials.

Long-term stability remains a key concern, as interfacial reactions can progress slowly over hundreds of cycles. Advanced characterization techniques, such as X-ray photoelectron spectroscopy and impedance spectroscopy, are essential for identifying degradation mechanisms. In-situ and operando studies have revealed that even seemingly stable interfaces can evolve over time, leading to gradual performance decay.

The development of sulfide solid electrolytes for solid-state batteries hinges on solving these interfacial challenges. While no single solution has yet achieved universal success, the combination of buffer layers, composite designs, and mechanical optimization shows promise. Future research must focus on scalable and cost-effective methods to implement these strategies without compromising the intrinsic advantages of sulfide electrolytes. By addressing chemical, electrochemical, and mechanical instabilities at the interfaces, the path toward commercial solid-state batteries becomes clearer.