Lithium Secondary Battery Using The Novel Ionic Liquids As The Potential Electrolytes

Safety has been recognized as one of the most important issues for lithium secondary batteries. However, conventional organic electrolytes used in commercial batteries are volatile and inflammable, contributing to the unsafe situation of a battery system. In order to improve the safety of lithium secondary batteries, it is urgent and important to develop new electrolytes. The ionic liquids have excellent characteristics, such as negligible volatility, non-flammability and high thermal stability. It becomes very important to investigate the performances of lithium secondary batteries using the ionic liquids as electrolytes or additives.In this work, three new asymmetric sulfonium-based ionic liquids (S112TFSI, S114TFSI and S116TFSI) were synthesized and characterized. The properties of the Li/LiMn2O4 cell containing the room temperature sulfonium-based ionic liquid electrolytes (S112TFSI and S114TFSI) were studied respectively including the effects of vinylene carbonate (VC) as additive on the cell performances, cycle performances at 25℃and 60℃, rate performances and safety. Electrochemical impedance spectroscopy (EIS) and X-ray diffraction (XRD) were conducted to analyze the mechanisms affecting the cell performances at different temperatures. The lithium secondary battery system, using 0.4mol L-1 LiTFSI/ S114TFSI+10wt%VC, shows good cycle performances equivalent to those of the cells using conventional electrolytes (1mol L-1 LiPF6/EC:DMC) at 0.1 C rate. Nevertheless, both the cell rate performance and the cycle performance at higher temperature are poor. And the cells containing S112TFSI and S114TFSI-based ionic liquid electrolytes demonstrate good safety according to the experiment.The asymmetrical dicationic ionic liquid, MIC5N111-TFSI2, was used in the Li/LiMn2O4 cell as an additive in the conventional electrolyte (1mol L-1 LiPF6/EC:DMC). The influences of the amount of the MIC5N111-TFSI2 on the cell performances were investigated. And the cycling tests at 0.1C reveal that the cell performance is improved when 5wt% MIC5N111-TFSI2 is added.