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

Abstract

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 113mAhg−1 (at 0.1 C) is found for LiFePO4 in the IL with 0.5M 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 1M LiTFSI-doped IL electrolyte (the optimum concentration at 50°C), a capacity of 140mAhg−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 1M LiTFSI-doped BMP–TFSI IL electrolyte.