Temperature effects on the electrochemical behavior of spinel LiMn2O4 in quaternary ammonium-based ionic liquid electrolyte

Temperature dependence of the physiochemical characteristics of a room-temperature ionic liquid consisting of trimethylhexylammonium (TMHA) cation and bis(trifluoromethane) sulforylimide (TFSI) anion containing different concentrations of LiTFSI salt was examined. Electrochemical properties of a spinel LiMn2O4 electrode in 1 M LiTFSI/TMHA-TFSI ionic electrolyte were investigated at different temperatures by using cyclic voltarnmetry, galvanostatic measurements, and electrochemical impedance spectroscopy. The Li/ionic el-ectrolyte/LiMn2O4 cell exhibited satisfactory electrochemical properties with a discharge capacity of 108.2 mA h/g and 91.4% coulombic efficiency in the first cycle under room temperature. At decreased temperature, reversible capacity of the cell could not attain a satisfactory value due to the high internal resistance of the cell and the large activation energy for lithium ion transfer through the electrode/electrolyte interface. Anodic electrolyte oxidation results in the decrease of coulombic efficiency with increasing temperature. Irreversible structural conversion of the spinel LiMn2O4 in the ionic electrolyte, possibly associated with the formation of TMHA intercalated compounds and/or Jahn-Teller distortion, was considered to be responsible for the electrochemical decay with increasing cycles.