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

Surface-coated/uncoated epitaxial LiCoO2 film electrodes with (104) orientations were fabricated on SrRuO3(100)/Nb:SrTiO3(100) using pulsed laser deposition. Films with thicknesses of similar to 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 LiCoO2 electrode and the electrolyte (LiPF6, ethylene carbonate + diethyl carbonate) were investigated by electrochemical impedance spectrometry. The charge-transfer resistance (R-ct) of uncoated LiCoO2 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 LiCoO2 rapidly increased from the first charge. In contrast, the R-ct of Li2ZrO3-coated LiCoO2 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 LiCoO2 electrodes were almost equal (similar to 100 Omega cm(2)). The activation energy for charge transfer was lower for the coated LiCoO2 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. (c) 2018 The Electrochemical Society.