Electrochemical performance of rechargeable Li/LiFePO4 cells with ionic liquid electrolyte: Effects of Li salt at 25 °C and 50 °C

Lithium hexafluorophosphate (LiPF6) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) are used as Li salts in butylmethylpyrrolidinium-bis(trifluoromethanesulfonyl)imide (BMP-TFSI) ionic liquid (IL) electrolyte for Li/LiFePO4 cells. This kind of IL electrolyte shows high thermal stability (>400 °C) and non-flammability, and is thus ideal for high-safety applications. At 25 °C, a maximum capacity of 113 mAh g−1 (at 0.1 C) is found for LiFePO4 in the IL with 0.5 M LiTFSI. An excessive LiTFSI concentration leads to a capacity decrease due to reduced electrolyte ionic conductivity. At 50 °C, the measured capacity and rate capability are significantly improved compared to those at 25 °C. With 1 M LiTFSI-doped IL electrolyte (the optimum concentration at 50 °C), a capacity of 140 mAh g−1 is found at 0.1 C and 45% of the capacity can be retained when the rate increases to 5 C, values which are comparable to those found in a traditional organic electrolyte. In the IL electrolyte, the LiFePO4 electrode shows better cyclic stability at 50 °C than it does at 25 °C; this trend is opposite to that found in the organic electrolyte. At 50 °C, there is negligible capacity loss of LiFePO4 after 100 charge-discharge cycles in 1 M LiTFSI-doped BMP-TFSI IL electrolyte.