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

Abstract

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.