Effect of Concentration of Lithium Bis(fluorosulfonyl)imide on the Performance of Silicon Anodes for Li-Ion Batteries

The conventional electrolytes for Li-ion batteries are based on the LiPF6 salt and carbonate solvents. Due to challenges with the stability, alternative salts are sought, and lithium bis(fluorosulfonyl)imide (LiFSI) is an interesting candidate. In this work, we investigate the performance of concentrated electrolytes based on LiFSI (range 1-10 M) and carbonate solvents, in combination with low-cost, micron-sized silicon anodes. LiFSI has an excellent solubility, and by use of concentrated electrolytes, corrosion of the aluminium current collector on the cathode side can be avoided, which is otherwise a challenge. The 5 M LiFSI electrolyte (molar salt to solvent ratio of 1:2.5) shows a similar ohmic resistance and rate performance as the 1 M LiFSI electrolyte. The solid electrolyte interphase formed in 5 M LiFSI is thin and dominated by inorganic compounds, in particular LiF. For long-term galvanostatic cycling with a lower cut-off potential of 50 mV, the 1 M LiFSI electrolyte shows the best stability. However, by limiting the lithiation, and thus the expansion of the silicon by increasing the cut-off voltage to 120 mV, the cycling performance is similar for all electrolytes and electrodes deliver >1000 mAh/g for more than 300 cycles.