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

Si is a promisinganode material for lithium-ion batteries owingto its high theoretical capacity; however, due to its large volumefluctuation during lithiation/delithiation, significant decay in capacityoccurs during charge-discharge cycling. Therefore, the selectionof appropriate lithiation/delithiation conditions is important tosuppress capacity decay. In this study, the cycling performance ofsilicon-based composite electrodes prepared using a cross-linked polyacrylatebinder was examined at different cutoff voltages, and the degradationof the electrolyte and electrodes was investigated through gas chromatography-massspectrometry (GC-MS) and synchrotron radiation X-ray photoelectronspectroscopy. When silicon-graphite (Si-G) electrodeswere examined in a Li cell (with Li metal counter electrode), a rapiddecrease in discharge capacity and Coulombic efficiency was observedat delithiation cutoff voltages of >0.7 V, which was attributedtothe complete consumption of the electrolyte additive, fluoroethylenecarbonate (FEC). After the decrease in FEC content, the main solventsof 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 increasedwith increasing delithiation cutoff voltage. Furthermore, higher cutoffvoltage leads to oxidative decomposition and elution of the surfacepassivation film formed at the electrode surface because of the exposureto high voltage. FEC consumption was insignificant in the Si-G//LiFePO4 Li-ion cell (without Li metal in the cell), indicating thatLi metal plating/stripping also consumed the FEC additive. The findingsof this study can be used to gain insights into the degradation mechanismsof Si-based electrodes and can therefore act as a basis for researchand development of Si-based electrodes for lithium-ion batteries.