Evaluation of Selected Ionic Liquids as Electrolytes for Silicon Anodes in Li-Ion Batteries

In this work, four selected ionic liquids (ILs), in combination with lithium bis(fluorosulfonyl)imide salt (LiFSI) were explored as electrolytes for anodes made from micron-sized metallurgical grade silicon. The ionic liquids were based on the cations; pyrrolidinium (PYR13), imidazolium (EMI) and phosphonium (P-111i4) and the anions; bis(fluorosulfonyl)imide (FSI) and bis(trifluorosulfonyl)imide (TFSI). The cycling stability and rate performance were investigated in pseudo-full cells with silicon anode and commercial LiFePO4, and compared to a carbonate electrolyte. Electrolytes based on PYR13FSI and P111i4FSI exhibit a decent rate performance up to C/5 and showed stable cycling over similar to 100 cycles, maintaining a reversible capacity of >1200 mAh g(Si)-1 at cycle 100 (C/5 rate) and over 2000 mAh g(Si)-1 at C/20. These electrolytes also had the best oxidation stability (> 5.3 V vs Li/Li). Based on assessment of the limiting current density in symmetrical cells, the Li-ion mobility was determined to be slightly higher for EMIFSI, but differences in Li-ion mobility cannot account for the differences in cycling stability among these electrolytes. While the SEI formed in EMIFSI electrolyte is the most conductive, the highest coulombic efficiency was obtained for PYR13FSI, indicating that the best passivating SEI was formed in this electrolyte.