(Invited) Electrochemical Impedance Study of Lithium Cobalt Oxide Thin Film in Some Ionic Liquids

Abstract

Aprotic ionic liquids have been investigated as the alternative electrolytes for rechargeable lithium batteries because of less inflammability, which is expected to improve safety of the batteries in case of accident. However, the battery performance using ionic liquid electrolytes has been reported to be inferior to that using conventional organic electrolytes since the viscosity of ionic liquids is generally higher than that of the organic electrolytes. The viscosity of electrolytes is considered to affect not only the mobility of lithium ion but also charge transfer at both anode and cathode reaction. Quantitative evaluation of the charge transfer rates of the active materials is often difficult in composite electrodes involving binders and conductive additives. In the present study, the charge transfer resistance of lithium cobalt oxide (LiCoO2) thin film was evaluated by electrochemical impedance measurements in different ionic liquids. LiCoO2 thin film was prepared by RF magnetron sputtering of LiCoO2 target with Ar on a gold substrate and then annealed in the air at 700°C. MPPTFSA, BMPTFSA, MOMMPTFSA, MPPFSA and MOMMPFSA (MPP+ = 1-methyl-1-propylpyrrolidinium, BMP+ = 1-butyl-1-methylpyrrolidinium, MOMMP+ = 1-methoxymethy-1-methylpyrrolidinium, TFSA– = bis(trifluoromethysulfonyl)amide and FSA–= bis(fluorosulfonyl)amide) containing 1 M LiTFSA or LiFSA were used as the electrolytes. 1 M LiTFSA / EC + DEC (1 : 1 vol%, EC = ethylene carbonate, DEC = diethyl carbonate) was also used as the electrolyte as a control. Lithium metal was used as a reference and counter electrode. An air-tight three electrode cell was assembled in a glovebox of dry Ar atmosphere. Electrochemical impedance measurements were conducted using PARTSTAT 2263 or 2273 with the ac amplitude of 5 mV-rms, and the frequency range from 2 Hz to 5 kHz at a constant potential of 4 V. Charge and discharge of the LiCoO2 thin film electrode was confirmed to be possible in the ionic liquid electrolytes by galvanostatic charge-discharge test at ±20 μA cm–2. A slightly distorted semi-circle was observed in the Nyquist plots for the ac impedance measurement of the LiCoO2 thin film electrode in each electrolyte. The charge transfer resistance was calculated by assuming a Randles-type equivalent circuit with an electrolyte resistance, constant phase element and charge transfer resistance. The charge transfer resistance decreased with an decrease in the viscosity of the electrolytes, suggesting the charge transfer process is affected by the dynamics of the electrolytes, as reported for Li+/Li reaction in an organic electrolyte[1]. The activation energy of the charge transfer resistance was larger than that of the viscosity of the electrolytes, implying the charge transfer process is related to not only the dynamics of the electrolytes but also the activation processes at the interface between the active material and the electrolytes. Reference [1] Y. Kato, T. Ishihara, Y. Uchimoto, and M. Wakihara, J. Phys. Chem. B, 108, 4794 (2004).