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

Abstract

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 ∼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 (Rct) 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 Rct of the uncoated LiCoO2 rapidly increased from the first charge. In contrast, the Rct 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 (∼100 Ω cm2). 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.