A deep study of the protection of Lithium Cobalt Oxide with polymer surface modification at 4.5 V high voltage

Abstract

Charging the cells above a conventional voltage of 4.2 V is a promising attempt to increase the energy density of Lithium Cobalt Oxide (LCO), however, the problem of crystal instability at high voltage that leading deterioration of cycle performance needs to be urgently resolved. In this work, as an effective and easy approach to improve the cycle performance and crystal stability of LCO cycling at 4.5 V high voltage, we demonstrate direct surface modification of a LCO cathode by poly [N,N-bis(2-cryano-ethyl)-acrylamide]. The results of SEM, TEM and XRD all indicate that the crystal structure of polymer coating LCO remains unchanged after cycling at 4.5 V high voltage for 60 times. Furthermore, the XPS study of valence of cobalt on the surface of LCO demonstrates that cobaltic ion of polymer coating LCO can be reduced to cobaltous ion after charging the cell. Thus, the activity of the crystal surface can be weakened, as a result, the stability is improved, leading to the performance improvement.