Surface chemical heterogeneous distribution in over-lithiated Li 1+x CoO 2 electrodes

In commercial Li-ion batteries, the internal short circuits or over-lithiation often cause structural transformation in electrodes and may lead to safety risks. Herein, we investigate the over-discharged mechanism of LiCoO 2 /graphite pouch cells, especially spatially resolving the morphological, surface phase, and local electronic structure of LiCoO 2 electrode. With synchrotron-based X-ray techniques and Raman mapping, together with spectroscopy simulations, we demonstrate that over-lithiation reaction is a surface effect, accompanied by Co reduction and surface structure transformation to Li 2 CoO 2 /Co 3 O 4 /CoO/Li 2 O-like phases. This surface chemical distribution variation is relevant to the depth and exposed crystalline planes of LiCoO 2 particles, and the distribution of binder/conductive additives. Theoretical calculations confirm that Li 2 CoO 2 -phase has lower electronic/ionic conductivity than LiCoO 2 -phase, further revealing the critical effect of distribution of conductive additives on the surface chemical heterogeneity evolution. Our findings on such surface phenomena are non-trivial and highlight the capability of synchrotron-based X-ray techniques for studying the spatial chemical phase heterogeneity.