Kinetics and Stability of Li-Ion Transfer at the LiCoO2 (104) Plane and Electrolyte Interface

Surface-coated/uncoated epitaxial LiCoO 2 film electrodes with (104) orientations were fabricated on SrRuO 3 (100)/Nb:SrTiO 3 (100) using pulsed laser deposition. Films with thicknesses of ∼18 nm and flat surfaces with roughnesses of less than 1 nm were model systems for clarifying the kinetics of Li-ion transfer at the electrode/electrolyte interface. The Li-ion transfer characteristics at the interface between the LiCoO 2 electrode and the electrolyte (LiPF 6 , ethylene carbonate + diethyl carbonate) were investigated by electrochemical impedance spectrometry. The charge-transfer resistance ( R ct ) of uncoated LiCoO 2 increased from the early cycles when charged/discharged at 3.0–4.2 V. When charged/discharged at 3.0–4.5 V, the R ct of the uncoated LiCoO 2 rapidly increased from the first charge. In contrast, the R ct of Li 2 ZrO 3 -coated LiCoO 2 remained almost constant during the early cycles when charged/discharged either at 3.0–4.2 or at 3.0–4.5 V. The interfacial resistances of the coated and uncoated LiCoO 2 electrodes were almost equal (∼100 Ω cm 2 ). The activation energy for charge transfer was lower for the coated LiCoO 2 electrode compared to that for the uncoated electrode. The current-rate capability was significantly improved by surface coating even at high-voltage charge/discharge at 3.0–4.5 V. The charge transfer process is the rate-determining step of the charge/discharge reaction.