Kinetics of Li-ion Transfer at the Electrode/Electrolyte Interface and Current Rate Performance of LiCoO₂ Surface-coated with Zirconium Oxide and Aluminum Oxide

Abstract

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 (R_ct) compared to bare and Al-oxide-coated LiCoO₂. The difference in current rate performance was quantitatively explained by the different R_ct. The activation energy (E_a) for the charge transfer reaction was approximately 10 kJ mol⁻¹ lower for Zr-oxide-coated LiCoO₂ than the other two. The lower E_a of the former, which suggests lower activation barriers for the elementary processes of Li-ion transfer at the electrode/electrolyte interface, lowered the R_ct and increased the current rate performance. The R_ct 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 R_ct change and E_a values.