Molten salt of lithium bis(fluorosulfonyl)imide (LiFSI)-potassium bis(fluorosulfonyl)imide (KFSI) as electrolyte for the natural graphite/LiFePO4 lithium-ion cell

The binary eutectic mixture of lithium bis(fluorosulfonyl)imide (LiFSI) and potassium bis(fluorosulfonyl)imide (KFSI) with a molar ratio of x(LiFSI): x(KFSI) = 0.41: 0.59 has been investigated as molten salt electrolyte for natural graphite/LiFePO4 lithium-ion cells at 80 degrees C. The electrochemical performances of the Li/natural graphite, Li/LiFePO4, and natural graphite/LiFePO4 cells using this molten salt electrolyte have been evaluated, in terms of cycling performances and electrochemical impedance spectra (EIS) at 80 degrees C. Both the Li/natural graphite and Li/LiFePO4 cells show good cycling performances. The initial specific capacities after conditioning for the natural graphite and LiFePO4 electrodes are 353 and 150 mAh g(-1), respectively, and the corresponding capacity retention rates are 98% and 97%, respectively, after 100 cycles at 80 degrees C, except for a large irreversible capacity (162 mAh g(-1)) observed for the natural graphite at the first cycle for forming solid electrolyte interface (SEI) film. The natural graphite/LiFePO4 lithium-ion cell using this molten salt electrolyte shows a low capacity in the range of 71-86 mAh g(-1), due to the large irreversible capacity (71 mAh g(-1)) at the first cycle, but it shows a good cycleability after the first cycle, and provides a specific capacity of 71 mAh g(-1) after 100 cycles at 80 degrees C. The variations of cycling performances of these cells can be well correlated with their impedance evolution with cycling. (C) 2014 Elsevier Ltd. All rights reserved.