A bis(fluorosulfonyl)imide based ionic liquid as safe and efficient electrolyte for Si/Sn-Ni/C/Al composite anode

Abstract

The composite material Si0.26Ni0.13Sn0.15Al0.04C0.42 (Si/Sn-Ni/C/Al) exhibits a high reversible capacity (>600mAhg−1) at a low potential and can be cycled for more than 100 cycles. Moreover fast rates of charge and discharge can be achieved without any significant loss in capacity. In order to optimize the performances of this material and especially to obtain larger and stable cycling results, the use of room temperature ionic liquids (RTIL) as electrolyte is investigated. RTIL selected for this purpose contain the FSI (bis(fluorosulfonyl)imide) or TFSI (bis(trifluoromethylsulfonyl)imide) anions and pyrrolidium (methyl-propylpyrrolidinium, PYR13 or methyl-butylpyrrolidinium, PYR14) and ethyl-methylimidazolium (EMI) as cations. Charge/discharge cycling tests of Li|Si/Sn-Ni/C/Al half-cells containing RTIL based on the FSI or TFSI anions have been performed. Results revealed significantly higher capacities and improved capacity retention (73% of the discharge capacity after 100 cycles at a C/5 rate) for cells containing FSI anions as components of the electrolyte. The choice of the RTIL cation has also an impact on the cycling stability and mean capacity of the cell. Using the EMIFSI ionic liquid and 1molL−1 of LiFSI, the mean capacity during 100 cycles at C/5 rate (635mAhg−1) is higher than with [PYR13][FSI] (579mAhg−1) or [PYR14][FSI] (550mAhg−1) ionic liquids. For comparison, the capacity of a half-cell containing a classical alkylcarbonate mixture with 1molL−1 of LiFSI drops to 206mAhg−1 after 20 cycles. It is concluded that the Si/Sn-Ni/C/Al/electrolyte interface is more stable in the presence of FSI anions and especially in the presence of the [EMI][FSI] ionic liquid.