Investigation of solid electrolyte interface (SEI) film on LiCoO2 cathode in fluoroethylene carbonate (FEC)-containing electrolyte by 2D correlation X-ray photoelectron spectroscopy (XPS)

The effects of fluoroethylene carbonate (FEC) on the electrochemical performance of the LiCoO2 cathode were investigated by galvanostatic charge–discharge testing and cyclic voltammetry (CV). It was found that FEC has a positive effect on cycling stability and also improves cell performance. We also studied solid electrolyte interface (SEI) film on the LiCoO2 cathode, using X-ray photoelectron spectroscopy (XPS) and 2D correlation spectroscopy. The 2D correlation XPS spectra showed that, initially, the polyvinylidene fluoride (PVdF) binder and electrolyte components are decomposed, after which SEI components are formed on the LiCoO2 cathode surface. In the FEC-containing electrolyte, the polycarbonate components are more abundant than in the FEC-free electrolyte. The formed carbonates in SEI film can act as Li+-conducting materials in reducing the electrode/electrolyte interfacial impedance. This hypothesis is supported by the results of an electrochemical impedance spectrum (EIS) analysis.