Insights Into the Interfacial Degradation of High-Voltage All-Solid-State Lithium Batteries

Poly(ethylene oxide) (PEO)-based solid polymer electrolyte (SPE) is considered as a promising solid-state electrolyte for all-solid-state lithium batteries (ASSLBs). Nevertheless, the poor interfacial stability with high-voltage cathode materials (e.g., LiCoO₂) restricts its application in high energy density solid-state batteries. Herein, high-voltage stable Li₃AlF₆ protective layer is coated on the surface of LiCoO₂ particle to improve the performance and investigate the failure mechanism of PEO-based ASSLBs. The phase transition unveils that chemical redox reaction occurs between the highly reactive LiCoO₂ surface and PEO-based SPE, resulting in structure collapse of LiCoO₂, hence the poor cycle performance of PEO-based ASSLBs with LiCoO₂ at charging voltage of 4.2 V vs Li/Li⁺. By sharp contrast, no obvious structure change can be found at the surface of Li₃AlF₆-coated LiCoO₂, and the original layered phase was well retained. When the charging voltage reaches up to 4.5 V vs Li/Li⁺, the intensive electrochemical decomposition of PEO-based SPE occurs, leading to the constant increase of cell impedance and directly causing the poor performance. This work not only provides important supplement to the failure mechanism of PEO-based batteries with LiCoO₂, but also presents a universal strategy to retain structure stability of cathode-electrolyte interface in high-voltage ASSLBs.