Relationship between the Properties and Cycle Life of Si/C Composites as Performance-Enhancing Additives to Graphite Electrodes for Li-Ion Batteries

Abstract

With its high theoretical specific charge, silicon is a promising candidate as electrode additive to enhance specific charge of graphite electrodes for high-energy-density Li-ion batteries. We prepared Si/C composites by a two-step procedure: the ballmilling of silicon nanoparticles with a carbon precursor for homogenization, followed by a carbonization step. The effects of the carbon source and the carbonization parameters on the physical and electrochemical composite properties were identified. Longer cycle life was reached for graphite electrodes containing Si/C composites than with silicon nanoparticles simply mixed with carbon black, and the extent of the improvement was dependent on the physical properties of Si/C composite. A carbon host with a larger pore volume — obtained using sucrose precursor, especially at lower heat-treatment temperatures, — enabled a more efficient buffering of the silicon volume changes. However, this did not define good cycling stability. The electrochemical performance was found, instead, being significantly affected by the contact between the silicon nanoparticles and the carbon matrix, and by the structure of the latter. The stacking and in-plane ordering of the graphitic domains in the amorphous carbon — tuned by the precursor nature and the heat-treatment temperature — were crucial for effectiveness of lithiation/delithiation processes.