Chemical States of Overcharged LiCoO<sub>2</sub> Particle
Surfaces and Interiors Observed Using Electron Energy-Loss Spectroscopy
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Abstract
Deterioration mechanisms of LiCoO<sub>2</sub> electrode materials
for lithium ion batteries remain unclear. Using electron energy-loss
spectroscopy and transmission electron microscopy, this study investigated
chemical states of LiCoO<sub>2</sub> 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<sub>2,3</sub>, Co–L<sub>3</sub>, and O–K
edges, we observed that overcharging causes the progression of Co<sup>3+</sup>/Co<sup>2+</sup> 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 Co<sub>3</sub>O<sub>4</sub>-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
