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 Li10Si0.3PS6.7Cl1.8 electrolytes and LiNi0.8Mn0.1Co0.1O2 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/Li10Si0.3PS6.7Cl1.8 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. Here, authors use Cryo-FIB and Cryo-TEM to reveal the atomic structures of the sulfide electrolyte/Si electrode interfaces, showing that the continuous lithium-ion consumption during interfacial reaction rather than interface impedance leads to capacity fade and battery failure.