2019 Volume 87 Issue 4 Pages 234-241
Different oxide-based surface coatings were applied to the LiCoO2 positive electrode material to improve its interfacial Li-ion transfer and current rate performance. Zr-oxide and Al-oxide were selected as coating materials, as the former hardly forms a solid-solution with LiCoO2 while the latter readily does so. Zr-oxide-coated LiCoO2 showed higher current rate performance and lower charge transfer resistance (Rct) compared to bare and Al-oxide-coated LiCoO2. The difference in current rate performance was quantitatively explained by the different Rct. The activation energy (Ea) for the charge transfer reaction was approximately 10 kJ mol−1 lower for Zr-oxide-coated LiCoO2 than the other two. The lower Ea of the former, which suggests lower activation barriers for the elementary processes of Li-ion transfer at the electrode/electrolyte interface, lowered the Rct and increased the current rate performance. The Rct of Zr-oxide-coated LiCoO2 decreased from the 2nd to 4th cycle, while for Al-oxide-coated LiCoO2 it remained almost constant during the early cycles. The formation of the new electrode/electrolyte interface during the early cycles that depends on the solubility of cations in the oxide coatings into LiCoO2 contributes to the different Rct change and Ea values.