Degradation Mechanism of Si-C Anode and Effect of Fluoroethylene Carbonate

The charging/discharging behaviors of a Si-C anode for lithium-ion batteries were investigated in the electrolytes with and without adding Fluoroethylene Carbonate (FEC). The compositions and morphologies of the Si-C anodes before and after charging/discharging were characterized with spectral techniques. And the reduction activity of the electrolyte compositions were compared with theoretical calculations. It is found that the Si-C anode in the electrolyte without FEC presents a low initial coulombic efficiency and a fast capacity degradation. This is because the SEI formed on the anode in the electrolyte without FEC is unstable and will be broken when the anode is inflated after inserting lithium ions, leading to the continuous decompostion of the electrolyte. In contrast, a stable SEI can be constructed by FEC because of its prefential reduction activity compared with other components of the electrolyte. The SEI constructed by FEC is mainly composed of F-containing polymer, which can suppress the electrolyte decomposition and buffer the volume change of the anode during cycling, leading to a significatnly improved initial coulombic efficiency and cyclic stability of the anode in the electrolyte containing electrolyte. The initial coulombic efficiency and the capacity retenition after 60 cycles at 0.2C can be improved from 83% to 86% and 28% to 75%, respectively, by adding 5% and 10% FEC.