Improving Structural and Chemical Stability of O3-Type Sodium Layered Oxide Cathode Via Fluorination

Abstract

A spherical O3-type layered oxide cathode, composed of compactly‐packed nanosized primary particles, is synthesized by the coprecipitation method so that the high tap density of the cathode ensures increased volumetric energy density for energy storage applications. However, drastic volume changes in the deeply charged states contribute to structural degradation, by inducing mechanical stress and the eventual disintegration of the cathode particles by the formation of microcrack. The microcrack traversing the entire secondary particle compromise the mechanical integrity of the cathode and accelerate electrolyte infiltration into the particle interior, causing the subsequent degradation of the exposed internal surfaces. In this study, we suggested a promising fluorination strategy to extend the cycle life of the O3‐type Na[Ni0.5Mn0.5]O2 cathode by improving their mechanical integrity and protecting their surfaces against electrolyte attack. Fluorination not only inhibits the microcracking of cathode secondary particles but also suppresses surface degradation, i.e., the formation of an electrochemically inactive NiO-like rock salt phase and dissolution of transition metals into electrolyte solution.