Temperature dependence of the electrochemical behavior of LiCoO2 in quaternary ammonium-based ionic liquid electrolyte

Temperature significantly influences the physiochemical characteristics of trimethyl-n-hexylammonium bis(trifluoromethanesulfone)imide (TMHA-TFSI) room temperature ionic liquid containing differing concentrations of LiTFSI salt. Electrochemical behavior of a LiCoO2 electrode in 1 M LiTFSI/TMHA-TFSI ionic electrolyte was investigated under different temperatures by using cyclic voltammetry, galvanostatic measurements and electrochemical impedance spectroscopy. A discharge capacity of 128.2 mAh/g and 92.1% coulombic efficiency was obtained in the first electrochemical cycle at an optimum temperature of 35 °C. Decreasing temperature resulted in lower reversible capacity whereas increasing temperature led to serious electrochemical decay with electrochemical cycles. Mechanism related to the obvious temperature dependence of the LiCoO2 electrochemical behavior in the ionic electrolyte was discussed. High internal resistance and the resultant large IR drop between the cut-off voltage are dominant factors responsible for the low reversible capacity under low temperature. At elevated temperature, irreversible structural degradation of the LiCoO2 during electrochemical process interprets the electrochemical deterioration with electrochemical cycles.