High Capacity Si-based Film Anode For Lithium Ion Batteries

Si has been proposed as a promising anode material for the next generation high energy lithium ion batteries due to its highest specific capacity. We investigated the structure characteristics and electrochemical performance of Si film anode prepared by magnetron sputtering. To further improve the electrochemical performance of Si film, the Si-Al and Si-Ni films were fabricated by co-sputtering technique.Si film was amorphous and exhibited excellent electrochemical performance. The 300 ran Si film showed a reversible capacity of 3134.4 mAh/g and the initial coulomb efficiency of 87.1%. The capacity retention of Si film was 61.3% after 500 cycles at 0.5C rate. Si film exhibited good rate performance. The discharge capacity at 10C rate was 41.4% of the discharge capacity at 0.5C rate.The formation mechanism of SEI layer on the surface of Si film anode was studied. The major components of SEI layer of Si film anode were ROCO₂Li, Li₂CO₃, LiF and polycarbonate. It was also found that a small quantity of silicon oxide was formed in the SEI layer due to the reaction of lithiated silicon with permeated electrolyte. The effect of electrolyte additive of vinylene carbonate (VC) on the SEI layer properties was investigated in details. The perfect SEI layer was formed in VC-containing electrolyte and no LiF crystals appeared on the SEI layer after the first cycle. And the impedance of SEI layer kept almost invariant upon cycling. Especially, the cycling performance of Si film was greatly improved by the presence of VC in electrolyte. The capacity retention of Si film increased from 25.6% in VC-free electrolyte to 78.5% in VC-containing electrolyte after 200 cycles.To enhance the conductivity and decrease volumetric change of Si film, Si-Al and Si-Ni films were prepared by co-sputtering technique. The capacity of Si-Al film was lower than that of Si film with the similar thickness, but the cycling performance and rate performance were better than that of Si film. The capacity and cycling performance of Si-Ni film were lower than that of Si film. However, the Si-Ni film showed excellent rate performance. The diffusion coefficient of lithium ion is a key factor to decide the electrochemical kinetic property of Si-based film. The lithium ion diffusion coefficient of Si-based film was measured by cyclic voltammetry. The diffusion coefficient of lithium ion is 1.815×10^-9 cm/s in Si film, 2.38×10^-7 cm/s in Si-Al film and 1.79×10^-8 cm/s in Si-Ni film.