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–M_2,3, 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.