Revealing interfacial failure mechanism of silicon based all solid state batteries via cryogenic electron microscopy

Interfaces are the critical components of all-solid-state batteries, and it is generally believed that high interfacial impedance is the major culprits of battery failure. In this study, the interface impedance has been found not to be a major issue in the batteries comprising Si negative electrode, Li₁₀GeP₂S₁₂ and Li₁₀Si_0.3PS_6.7Cl_1.8 electrolytes and LiNi_0.8Mn_0.1Co_0.1O₂ positive electrode. Instead, it is the sustainable interfacial reaction that depletes the active lithium source, causing continuous capacity decay. The interphase layer at the Si/Li₁₀Si_0.3PS_6.7Cl_1.8 interface comprising nanocrystalline Li₂S dispersed in an amorphous matrix is thin (with a thickness < 200 nm) and stable, and the battery maintains a good cyclability. In contrast, the interphase layer at the Si/Li₁₀GeP₂S₁₂ interface is thick with a thickness of 10 μm. Couter-intuitively, despite the thick interfacial layer comprising mainly needle shaped Li₂S, the interfacial impedance does not increase dramatically, suggesting that interfacial impedance is not the main issue, rather, it is the chemically/electrochemically continuous reaction of negative electrode with Li₁₀GeP₂S₁₂ that consumes the active lithium source from positive electrode and causes the capacity decay. This study provides atomic-scale interface structures of sulfide based batteries, which have important implications for the design of stable interfaces for high performance batteries.