Microscopic investigation of crack and strain of LiCoO2 cathode cycled under high voltage

The structural stability of LiCoO2 at high voltages is especially important when enhancing the cutoff voltage used as an effective method to boost the energy density. we trace the origin of different crack formation mechanisms during fast and slow charging rates using a combination of qualitative atomic-scale differential phase-contrast scanning transmission electron microscope (STEM) images and electron energy loss spectroscopy (EELS) analysis of LiCoO2 (LCO). We demonstrate that fast charging induces large heterogenous Li flux insertion into the cathode, resulting in large strain and big crack formation; in contrast, low cycling rates trigger phase transformations that only induce micro-cracks. To clarify the mechanisms, we mapped out local strain analysis for the LCO particles and detailed phase transition routes. The comprehensive characterization results are meaningful for researchers to understand the failure mechanisms under high voltage and inspire innovative solutions to capture higher capacity out of LCO.