Consumption of Fluoroethylene Carbonate Electrolyte-Additive at the Si–Graphite Negative Electrode in Li and Li-Ion Cells

Si is a promising anode material for lithium-ion batteries owing to its high theoretical capacity; however, due to its large volume fluctuation during lithiation/delithiation, significant decay in capacity occurs during charge–discharge cycling. Therefore, the selection of appropriate lithiation/delithiation conditions is important to suppress capacity decay. In this study, the cycling performance of silicon-based composite electrodes prepared using a cross-linked polyacrylate binder was examined at different cutoff voltages, and the degradation of the electrolyte and electrodes was investigated through gas chromatography–mass spectrometry (GC-MS) and synchrotron radiation X-ray photoelectron spectroscopy. When silicon–graphite (Si–G) electrodes were examined in a Li cell (with Li metal counter electrode), a rapid decrease in discharge capacity and Coulombic efficiency was observed at delithiation cutoff voltages of >0.7 V, which was attributed to the complete consumption of the electrolyte additive, fluoroethylene carbonate (FEC). After the decrease in FEC content, the main solvents of the electrolyte, such as ethylene carbonate and dimethyl carbonate, underwent electrolyte decomposition reactions and formed co-oligomers. The GC-MS results revealed that the FEC consumption rate increased with increasing delithiation cutoff voltage. Furthermore, higher cutoff voltage leads to oxidative decomposition and elution of the surface passivation film formed at the electrode surface because of the exposure to high voltage. FEC consumption was insignificant in the Si–G//LiFePO4 Li-ion cell (without Li metal in the cell), indicating that Li metal plating/stripping also consumed the FEC additive. The findings of this study can be used to gain insights into the degradation mechanisms of Si-based electrodes and can therefore act as a basis for research and development of Si-based electrodes for lithium-ion batteries.