Understanding the Interfacial Processes at Silicon–Copper Electrodes in Ionic Liquid Battery Electrolyte

The interfacial processes at an amorphous silicon–copper (Si–Cu) electrode in the ionic liquid electrolyte of 1 M lithium bis(trifluoromethanesulfonyl)imide/1-methyl-1-propylpyrrolidinium bis(trifluoromethylsulfonyl)imide (LiTFSI/Py1,3TFSI) during initial charge–discharge cycle are studied by characterizing the solid electrolyte interphase (SEI) composition at different states of charge using ex situ attenuated total reflection FTIR spectroscopy combined with X-ray photoelectron spectroscopy. The analyses data reveal that in the very early stage of charge (1.5 V vs Li/Li+), alkylated Si and ester-containing species first form by the reductive decomposition of Py1,3 ion, and LiF salt and Si–F bond-containing compound first form by the decomposition of TFSI anion, respectively. TFSI decomposition is observed to begin with the C–F cleavage, which was proposed as the cleavage of N–S bond in the previous reports. Charging to lower voltage thickens the SEI layer, but lithiation of silicon results in damage or destabilization of the existing SEI probably due to changes in structural volume together with particle morphology. The SEI layer is however reversibly rebuilt in the course of delithiation. The data provide a basic understanding of the SEI formation mechanism on the silicon-based anodes in ionic liquid electrolyte for less flammable batteries.