All-solid-state batteries (ASSBs) emerge as a promising next-generation energy storage technology due to their potential for high energy density and enhanced safety. Among the various types of solid electrolytes, sulfide solid electrolytes have attracted much attention due to their high ionic conductivity and excellent mechanical properties. Despite these advantages, the development and industrialization of sulfide-based ASSBs face numerous scientific and engineering challenges. This review focuses on the fundamental scientific issues and engineering difficulties associated with sulfide-based ASSBs, and proposes future directions and recommendations to advance their development and commercialization.

The rapid growth of electric vehicles, consumer electronics, and energy storage systems has driven the development of lithium-ion batteries. However, conventional lithium-ion batteries with a high energy density pose safety risks due to the use of flammable liquid electrolytes. ASSBs, which replace liquid electrolytes with solid electrolytes, offer a safer alternative with a potential for higher energy density. Sulfide solid electrolytes, in particular, exhibit ionic conductivities, compared to those of liquid electrolytes, making them a leading candidate for ASSBs. Despite significant progress, several challenges remain. Sulfide solid electrolytes face issues related to electrochemical stability, humidity sensitivity, and thermal stability. The interfaces between sulfide electrolytes and electrodes (i.e., both cathode and anode) are critical for the battery performance, but are prone to poor contact, chemical reactions, and lithium dendrite growth. These issues hinder the practical application of sulfide-based ASSBs.

The stability of sulfide solid electrolytes is crucial for the performance and safety of ASSBs. Electrochemical stability ensures that the electrolyte can operate in a wide voltage range without decomposing. However, sulfide electrolytes often exhibit narrow electrochemical windows, leading to decomposition at high voltages. Humidity stability is another concern, as sulfide electrolytes tend to react with moisture, producing toxic H₂S gas. Thermal stability is also critical, as sulfide electrolytes can undergo thermal decomposition, increasing the risk of thermal runaway. The interfaces between sulfide electrolytes and electrodes are a major bottleneck in ASSBs development. The solid-solid contact between the electrolyte and electrodes often leads to a poor ion transport and an increased interfacial resistance. In addition, chemical reactions at the interface can also degrade battery performance over time. For instance, the interface between sulfide electrolytes and high-voltage cathodes can lead to the formation of high resistance interface layers, reducing battery efficiency. The compatibility of sulfide electrolytes with different anode materials, such as graphite, silicon, and lithium metal, is another critical issue. Graphite anodes suffer from lithium plating at high currents. Silicon anodes offer a high capacity but experience significant volume changes during cycling, leading to mechanical instability. Lithium metal anodes with their high theoretical capacity are prone to dendrite growth and interfacial reactions with sulfide electrolytes.

The large-scale production of sulfide electrolytes is essential for the commercialization of ASSBs. However, the synthesis of sulfide electrolytes is complex and requires a precise control of reaction conditions. High-temperature solid-state methods and liquid-phase synthesis are the two main approaches, each with its own advantages and challenges. Cost control is also a significant factor, as the raw materials for sulfide electrolytes, particularly Li₂S, are expensive. The fabrication of thin, uniform, and mechanically robust electrolyte membranes is crucial for ASSB performance. Wet and dry processing methods are commonly used, but each has limitations. Wet processing can lead to solvent-induced degradation of the electrolyte, while dry processing faces challenges in achieving uniform mixing and thin film formation. The assembly of ASSBs involves stacking multiple layers of electrodes and electrolyte membranes. Ensuring a good contact between these layers is critical for battery performance. However, the solid nature of the components makes it challenging to achieve uniform pressure distribution during stacking, leading to increased interfacial resistance.