Influence of the Carbon Coating on the Electrochemical Performances of Si/C Composites for Lithium Ion Batteries

Silicon, with a theoretical capacity of 3579 mAh/g, is one of the most promising anode materials as a substitute to graphite for Li-ion batteries. However, the lithiation process involves a volume expansion of 270% causing electrode fracture and the creation of an unstable SEI and fast capacity fading. Si/C composites made using petroleum pitch as carbon precursor are a good alternative to minimize these problems. An original way of synthesis has been developed by dispersing Si nanoparticles (40 nm in diameter, prepared by laser pyrolysis) in a pitch (either molten, or dissolved in a solvent) in order to obtain Si/C composites after thermal treatment. Even though pitch is a disordered carbon, after pyrolysis at 900°C, it exhibits a reasonable electronic conduction, has a reversible capacity of 280 mAh/g and acts as a buffer for the Si volume expansion. Si/C composites prepared using this method with 12 wt.% of Si present a reversible capacity of 600 mAh/g at the rate of C/5 and a coulombic efficiency at the first cycle up to 85%. We will discuss about the influence of the synthesis conditions on the texture of the composites and the resulting electrochemical performances. In order to study deeper the capacity retention, the Si/C composites were tested in full cells using NMC as the cathode. We will show that the control of the Si/C interface allows to obtain composites materials with high energy density up to 290 Wh/kg with a nice capacity retention.