High-Temperature Performance of Selected Ionic Liquids as Electrolytes for Silicon Anodes in Li-ion Batteries

Abstract

Ionic liquids, like bis(fluorosulfonyl)imide (FSI) ionic liquids, are promising solvents for lithium ion batteries due to their high thermal stability and wide electrochemical window. Here, electrolytes based on LiFSI and ionic liquids of different cations; pyrrolidinium (PYR13+), imidazolium (EMI+) and phosphonium (P111i4+), and FSI and bis(trifluoromethanesulfonyl)imide (TFSI) anions, have been cycled together with silicon anodes and LiFePO4 cathodes at 60 °C at rates up to 2 C. The thermal stability was determined through differential scanning calorimetry (DSC). Ionic conductivities were measured in the temperature range −20 °C to 80 °C and the lithium ion mobilities, relative to standard carbonate electrolytes, were determined at room temperature and 60 °C. Significant differences in the temperature dependence of the transport properties were revealed, with a considerable increase in the lithium ion mobility from 20 °C to 60 °C for electrolytes based on EMIFSI and P111i4FSI. The best rate performance at 60 °C was demonstrated for the P111i4FSI based electrolyte, with discharge capacities around 1000 mAh/gSi at a 2 C rate. The improved performance was attributed to a combination of the improved lithium ion mobility and formation of a more conductive solid electrolyte interphase (SEI) for this electrolyte.