Fluorine-induced reversible cation/anion redox reactions to enhance stability in Li-rich layered oxides

Abstract

Low-cost Li-rich layered oxides (LLOs) cathode materials offer excellent initial energy density due to the contribution of cation/anion redox chemistry. However, irreversible redox reactions and phase transitions lead to low initial Coulomb efficiency (ICE) and mediocre cycling stability, which hinders their practical applications. Herein, surface depth fluorine (F)-induced achieved phase transform of Li2MnO3 structure in Li1.2Mn0.64Ni0.16O2 cathodes for the first time, generating a large number of phase-like interfaces and ultrathin interfacial integrated layers, which excite the anion/cation activity and accelerate the redox kinetics. The modified material can inhibit defect generation, increase the Li+ diffusion rate, activate reversible Mn3+/Mn4+ redox reactions, enhance oxygen redox (O2−/On−) activity, and suppress adverse phase transitions. The results show that the ICE of modified electrode is improved to 88.2%, and that the capacity retention is as high as 93.1% and 100% for 200 cycles at 1 C and 2 C. Furthermore, the improved electrochemical performance is also attributed to the formation of strong transition metal-fluorine (TM-F) bonds, which inhibit the TM migration that maintains the intact layered structure during cycling. This strategy provides a new path for efficient surface structure modification of high-energy–density LLOs cathodes.