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.
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