Chemical States of Overcharged LiCoO2 Particle Surfaces and Interiors Observed Using Electron Energy-Loss Spectroscopy

Deterioration mechanisms of LiCoO2 electrode materials for lithium ion batteries remain unclear. Using electron energy-loss spectroscopy and transmission electron microscopy, this study investigated chemical states of LiCoO2 particles on first overcharging. We present a scheme for quantification of the Li/Co atomic ratio. Using quantitative Li mapping and comprehensive probing of Li–K, Co–M2,3, Co–L3, and O–K edges, we observed that overcharging causes the progression of Co3+/Co2+ reduction with oxygen extraction from the particle surface to the interior. A gradual change in the chemical composition at and around the particle surfaces after charging of 60% revealed the presence of Co3O4-like and CoO-like phases at surface regions. We also observed nanocracks with deficient Li ions. These results are key factors affecting degradation on overcharging.