As is known, the characteristics of electrolyte play a significant role in lithium-ion batteries in terms of high conductivity, good compatibility with electrode material and safety. Because of the volatility and flammability of the traditional organic electrolytes, the safety risk has been a big concern for the batteries, especially when it is used in hybrid electric vehicles (HEVs) and electric vehicles (EVs). On the other hand, the room temperature ionic liquids (RTILs) usually are low volatile, non-flammable liquids with wide electrochemical window that benefit to use as electrolytes in high-safety lithium battery. Since the chemical and physical properties are strongly related to combination of the cations and anions composed, unique ionic liquids may be designed for the particular application. Recently, the RTILs formed by imidazolium, pyrrolidinium, piperidinium cations and hexafluorophosphate (PF₆^-), tetrafluoroborate (BF₄^-), bis(trifluoromethanesulfonyl)imide (TFSI^-) anions have been frequently used as the electrolytes for lithium-ion batteries. However, as reported, the organic solvents i.e. ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC) and diethyl carbonate (DEC), which are used for the traditional liquid electrolytes, have often been mixed with the RTIL for reducing the viscosity to increase the conductivity. Yet, the thermal stability can’t be improved essentially. In this study, a solvent-free ternary RTIL-based electrolyte system has been constructed by the RTILs, 1-butyl-3-methylimidazolium tetrafluoroborate (BMImBF₄), 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR₁₄TFSI) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI). Three different electrolytes with various compositions are prepared and characterized by FTIR, thermogravimetric analysis, differential scanning calorimetry, linear sweep voltammetry, conductivity meter and rheometer. The results show that all three electrolytes are thermally stable up to about 350℃ and have wide electrochemical windows (~5V). Besides, the lowest viscosity and the highest conductivity obtained were 66.2 cP and 2.76 mS, respectively. The highest discharge capacity of the LiFePO₄/electrolytes/Li half-cell obtained in the voltage range of 2.5–4.3 V at 0.2C and 55℃ is 158 mAh g^-1. This study reveals that the RTIL-based ternary electrolyte is a promising electrolyte candidate for the high-safety lithium-ion battery.

Acknowledgements

The authors gratefully acknowledge the Ministry of Science and Technology, Taiwan, R. O. C. and Chung Yuan Christian University for supporting this research work.