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.