Evidence for Anisotropic Mechanical Behavior and Nanoscale Chemical Heterogeneity in Cycled LiCoO2

Commercial lithium-ion battery cells were cycled to various depths of discharge at various rates while the relative capacities were periodically measured. After 1000 cycles, lithium cobalt oxide (LiCoO₂) cathode material was extracted from the most severely aged cell. Nanoindentation was performed on individual LiCoO₂ particles. Fractures in these particles exhibited anisotropic behavior, which was confirmed by electron microscopy and diffraction examination indicating both intra- and inter-granular fracture occurred along {001} planes. Computation of the charge density structure for LiCoO₂ indicated that the Li-O bonds along the {001} planes require the lowest energy for cleavage, supporting the experimental findings. Atom probe tomography (APT) analysis indicated the nanoscale composition distributions within specimens from both fresh and cycled material. Among the cycled particles, nanoscale inhomogeneities in the Li content were observed. For APT specimens containing grain boundaries, accumulation of Li (up to 80 at%) on one side of the boundary was observed. Correlation of the electrochemical, mechanical, and compositional results indicates a combination of these mechanical and chemical mechanisms contributed to the measured capacity fade.