Effects of lithium salt concentration in ionic liquid electrolytes on battery performance of LiNi0.5Mn0.3Co0.2O2/graphite cells

Abstract

Ionic liquids (ILs) possess low volatility and low flammability and are promising electrolytes for thermally stable Li-ion batteries (LIBs). Among ILs, bis(fluorosulfonyl)imide (FSI-) anion-based ILs have low viscosity and high ionic conductivity and FSI-based electrolytes are useful for achieving a high power density LIB. In this study, we investigated the effects of LiFSI concentration in IL electrolytes on the performance of LIBs. We prepared electrolytes composed of 1-ethyl-3-methyl imidazolium bis(fluorosulfonyl)imide (EMImFSI) and LiFSI. The ionic conductivity of the electrolyte decreased with increasing LiFSI concentration due to an increase in viscosity; however, the Li+ transference number increased with increasing LiFSI concentration. The IL electrolyte was tested in a LiNi0.5Mn0.3Co0.2O2/graphite pouch cell. The discharge rate capability of the cell was improved by increasing the LiFSI concentration. The higher LiFSI concentration was effective in suppressing the depletion of Li+ in the vicinity of the cathode during the high current discharge. Furthermore, we demonstrated that cells with IL electrolytes can be stably operated over 500 charge-discharge cycles at 25 °C and 60 °C.