Enhanced Cycleabity in Lithium Ion Batteries: Resulting from Atomic Layer Depostion of Al2O3 or TiO2 on LiCoO2 Electrodes

This study reports the preparation of Al2O3 and TiO2 coatings on the as-prepared LiCoO2 electrodes using atomic layer deposition (ALD). A thin Al2O3 ALD coating was shown to eliminate capacity fading effectively during repeated charging and discharging, whereas a TiO2 coating led to significant improvement only at high cycle numbers. An analysis of the differential capacity versus potential curves suggests that this poorer cycling performance could be related to the participation of the TiO2 thin film in the redox reaction. Graphical representation of the energy levels of the various ALD coatings on LiCoO2 during charging and discharging indicated that the redox current is impeded at the Al2O3-LiCoO2 junction, whereas electrons and holes were energetic enough to flow into the TiO2 because of the smaller band gap energy. The barrier between the valence band maxima of TiO2 and LiCoO2 expands as the charge discharge cycle number increases, eventually making TiO2 redox-inactive. These conclusions are supported by both XPS spectra and the cycle performance in the established literature references. Our results suggest that large band gap materials should be considered to be potentially useful ALD coatings on cathode materials.