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/LiFePO 4 lithium-ion cells at 80   °C. The electrochemical performances of the Li/natural graphite, Li/LiFePO 4 , and natural graphite/LiFePO 4 cells using this molten salt electrolyte have been evaluated, in terms of cycling performances and electrochemical impedance spectra (EIS) at 80   °C. Both the Li/natural graphite and Li/LiFePO 4 cells show good cycling performances. The initial specific capacities after conditioning for the natural graphite and LiFePO 4 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   °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/LiFePO 4 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   °C. The variations of cycling performances of these cells can be well correlated with their impedance evolution with cycling.