Investigation of Interfacial Changes on Grain Boundaries of Li(Ni 0.5 Co 0.2 Mn 0.3 )O 2 in the Initial Overcharge Process

Abstract The interfacial changes and Li ions kinetics on grain boundaries of cathode are still not very clear, especially in the initial overcharge process. Herein, interfacial changes and electrochemical kinetics of Li ions on grain boundaries of Li(Ni 0.5 Co 0.2 Mn 0.3 )O 2 (NCM523) are investigated in the initial overcharge to 4.9 V. The mechanism of electrochemical process and interfacial changes of NCM523 is proposed in this study. Two triggering potentials are detected: 4.4 V, indicating oxidation of lattice oxygen; 4.7 V, meaning electrochemical oxidation of electrolyte. Combining with in situ electrochemical impedance spectroscopy results and transmission electron microscope characterization, oxidation of lattice oxygen can be related to the formation of rock salt phase, hampering lithium ions migration from one primary particle to another when charging to higher than 4.4 V. When potential rises to 4.7 V, electrolyte begins to be electrochemical oxidized, causing severe capacity degradation due to excessive consumption of lithium ions. All these factors synergistically induce the capacity degradation of NCM523 during the overcharge cycling performance. The upper limiting voltage of NCM523 should be no higher than 4.4 V to prolong its life cycle.