Probing the Origin of Enhanced Stability of AIPO4 Nanoparticle Coated LiCoO2 during Cycling to High Voltages: Combined XRD and XPS Studies

AIPO(4)-coated LiCoO2 was shown to exhibit markedly improved capacity retention and reduced impedance growth relative to bare LiCoO2 upon cycling to 4.7 V. Scanning electron microscopy imaging showed that file surfaces of the cycled bare LiCoO2 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 LixCoO2 was introduced upon cycling. In contrast, no apparent structural changes to LixCoO2 were found for cycled coated electrodes. Pristine and cycled bare and AIPO(4)-coated LiCoO2 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 LixPFyOz components were found to considerably increase during cycling, which led to partial surface coverage of LixCoO2. A very small amount of Co-containing oxyfluoride species was detected oil the cycled bare electrodes while considerable amounts of Co-containing and Al-containing fluorides and/or oxyfluorides and species such as PFx(OH)(y) were found oil the cycled coated electrodes, which completely covered the surfaces of the LixCoO2 particles. The mechanism responsible for the enhanced cycling stability and reduced impedance of coated relative to bare electrodes is discussed in detail.