Li2ZrF6 protective layer enabled high-voltage LiCoO2 positive electrode in sulfide all-solid-state batteries

The application of high-voltage positive electrode materials in sulfide all-solid-state lithium batteries is hindered by the limited oxidation potential of sulfide-based solid-state electrolytes (SSEs). Consequently, surface coating on positive electrode materials is widely applied to alleviate detrimental interfacial reactions. However, most coating layers also react with sulfide-based SSEs, generating electronic conductors and causing gradual interface degradation and capacity fading. To address this, we propose a Li2ZrF6 coating layer on LiCoO2, which exhibits minimal reaction with SSEs, and its decomposition products are electron-conductive-free. Furthermore, this coating layer also efficiently mitigates the layered-to-spinel/rock-salt surface structural transformation in LiCoO2. As a result, the In-Li|Li6PS5Cl | Li2ZrF6-LiCoO2 all-solid-state cell demonstrates an initial areal capacity of 5.2 mAh cm−2 and a capacity retention of 80.5% after 1500 cycles at 70 mA/g with high LiCoO2 areal mass loading (30.19 mg cm−2) and a cut-off voltage of 3.9 V (corresponding to potential of 4.5 V versus Li+/Li), at 25 °C. High-voltage positive electrodes in sulfide all-solid-state lithium batteries face challenges due to the low oxidation stability of sulfide electrolytes. Here, authors propose a Li2ZrF6 coating on LiCoO2 to suppress interfacial reactions and enhance the long-term battery cycling performance.