Probing the Origin of Enhanced Stability of “AlPO4” Nanoparticle Coated LiCoO2 during Cycling to High Voltages: Combined XRD and XPS Studies

Abstract

“AlPO 4 ”-coated LiCoO 2 was shown to exhibit markedly improved capacity retention and reduced impedance growth relative to bare “LiCoO 2 ” upon cycling to 4.7 V. Scanning electron microscopy imaging showed that the surfaces of the cycled bare “LiCoO 2 ” particles remained very smooth whereas there were many newly formed patches distributed on the surfaces of the cycled coated particles. X-ray powder diffraction analyses revealed that select peak broadening was observed for cycled bare electrodes suggesting that structural damage to Li x CoO 2 was introduced upon cycling. In contrast, no apparent structural changes to Li x CoO 2 were found for cycled coated electrodes. Pristine and cycled bare and “AlPO 4 ”-coated LiCoO 2 electrodes were studied by X-ray photoelectron spectroscopy. No significant change was detected in the surface chemistry of Co for cycled bare electrodes, but surface LiF and Li x PF y O z components were found to considerably increase during cycling, which led to partial surface coverage of Li x CoO 2 . A very small amount of Co-containing oxyfluoride species was detected on the cycled bare electrodes while considerable amounts of Co-containing and Al-containing fluorides and/or oxyfluorides and species such as PF x (OH) y were found on the cycled coated electrodes, which completely covered the surfaces of the Li x CoO 2 particles. The mechanism responsible for the enhanced cycling stability and reduced impedance of coated relative to bare electrodes is discussed in detail.