Lithium Salt Effects on Silicon Electrode Performance and Solid Electrolyte Interphase (SEI) Structure, Role of Solution Structure on SEI Formation

Silicon electrodes were cycled with electrolytes containing different salts to investigate the effect of salt on the electrochemical performance and SEI structure. Comparable capacity retention were observed for the 1.2 M LiPF 6 , LiTFSI and LiClO 4 electrolytes in ethylene carbonate (EC):dimethyl carbonate (DEC), 1:1, but severe fading was observed for the 1.2 M LiBF 4 electrolyte. The differential capacity plots and EIS analysis reveals that failure of the 1.2 M LiBF 4 electrolyte is attributed to large surface resistance and increasing polarization upon cycling. However, when LiBF 4 was added as an electrolyte additive (10% LiBF 4 and 90% LiPF 6 ), the capacity retention and Coulombic efficiency were improved. The SEI was analyzed by FTIR and XPS for each electrolyte. Both spectroscopic methods suggest that the main components of the SEI are lithium ethylene dicarbonate (LEDC) and Li 2 CO 3 in the 1.2 M LiPF 6 , LiTFSI and LiClO 4 electrolytes, while an inorganic-rich SEI, composed of LiF and borates, was generated for both the 1.2 M LiBF 4 electrolyte and the 10% LiBF 4 electrolyte. The chemical composition of the SEIs and corresponding electrochemical performance of the Si electrodes were strongly correlated with electrolyte solution structure.