Effects of fluoroethylene carbonate addition to Li-glyme solvate ionic liquids on their ionic transport properties and Si composite electrode performance

Abstract

The design of battery electrolytes with superior transport, thermal, and electrochemical properties is a key consideration in the development of high-energy-density batteries. To this end, an equimolar mixture of lithium bis(trifluoromethanesulfonyl)amide (LiTFSA) and tetraglyme (G4) is of interest because it forms a thermally and electrochemically stable solvate ionic liquid (SIL), [Li(G4)][TFSA]. In this study, fluoroethylene carbonate (FEC) was added to [Li(G4)][TFSA] in an attempt to improve transport properties of the SIL and to modify the chemical composition of the solid electrolyte interphase (SEI) formed on Si electrodes. The addition of FEC had a negligible impact on the Li ion coordination structure in [Li(G4)][TFSA] while enhancing its ionic conductivity nearly twofold. The Si electrode in [Li(G4)][TFSA] with the FEC additive showed a stable and reversible capacity in long-term cycling with high Coulombic efficiency. X-ray photoelectron spectroscopy revealed that the chemical composition of the SEI formed in [Li(G4)][TFSA] with the FEC additive was significantly different from that in the electrolyte without FEC. The FEC-addition-derived SEI most likely consists of carbonates and LiF, retarding electrolyte decomposition and, thus, improving the electrochemical performance of the Si electrode therein.