First Principle Study of Mechanical Degradation in Sulfide Solid Electrolytes—The Role of Li-Ion Concentration

All-solid-state batteries (ASSBs) utilizing sulfide solid electrolytes are considered a promising alternative to traditional lithium-ion batteries due to their high lithium-ion conductivity, modulus, and chemical compatibility with lithium metal anodes. These characteristics are anticipated to inhibit lithium penetration within the electrolyte and prevent cell short-circuiting. Nonetheless, instances of lithium penetration occurring within sulfide-based electrolytes during battery operation have been reported. One of the major reasons why this happens can be due to the mechanical degradation of solid electrolytes during battery operations. In this study, using first-principles atomistic simulations, we investigate the impact of lithium-ion concentration on the mechanical properties of sulfide-based solid electrolytes Li₆PS₅Cl (LPS). By systematically varying the concentrations of Li ions, we aim to explore how different levels of ion removal/addition influence the mechanical stability and performance of the solid electrolyte. Our analysis focuses on key mechanical properties such as elastic modulus, tensile strength, and fracture toughness. By simulating and analyzing the deformation of computational cells across various lithium concentrations, this study identifies a critical relationship between lithium concentration and the deterioration of mechanical properties. This deterioration is implicated as a potential cause of mechanical failure in ASSBs. We also perform a comparative study with the effects of electrochemical reactions of LPS by altering Li atomic concentration to provide a comprehensive understanding of the structural and mechanical implications at the solid electrolyte grain boundaries and electrode interface. The results showed that there is a significantly more reduction in the mechanical strength when there is a decrease in the Li⁺ concentration. This indicates that LPS may have internal weak spots that can lead to fracture and promote lithium metal penetration. The findings from this research contribute to understanding the root cause of ASSB failure and the optimization and design of sulfide-based solid electrolytes for advanced battery technologies, potentially leading to improved performance and longevity of next-generation energy storage systems.