Influence of Conductive Agent in C/Si-Composite Electrodes for High-Energy Lithium Ion Batteries

Abstract

Lithium ion batteries outperform other energy storage technologies in the combination of specific power and specific energy [1]. However, continuous improvement is necessary regarding the ever-increasing demands in terms of high energy densities, long cycle life and improved safety properties. One strategy to increase the specific energy density and decrease the overall cost of lithium ion batteries is the partial substitution of graphite with silicon in the negative electrode. Silicon forms alloys with lithium and offers an extremely high theoretical capacity (3579 mAh g-1) [2, 3]. However, the lithiation and delithiation process of silicon generates mechanical stress due to particle swelling and contraction (∆V ≥ 300%). This stress eventually causes mechanical fracture and pulverization, which in turn leads to active material loss. Inactive materials and the use of a composite of active materials, silicon and graphite (C/Si), can help to mitigate these degradation effects. The binder ensures adhesion and cohesion of and within the electrode and its different components. Conductive agents improve the electrical conductivity and provide inter-particle as well as particle-current collector contacts. Therefore, the electrode and cell design can largely influence the electrochemical performance and cycle life. Different applications require different ratios of active material, conductive agent and binder. Especially considering the low electrical conductivity of silicon [4], the use of conductive carbon is inevitable to increase electrical conductivity within the composite electrode. However, with C/Si electrodes, the necessity of conductive agent is questionable, since the graphite present in the composite might be able to provide sufficient electrical conductivity. Furthermore, an excessive amount of inactive material reduces the energy density of the electrode and has to be optimized. This study focuses on the impact of conductive agent in high energy C/Si composite electrodes. To ensure a high loading and high energy density, the amount of inactive materials were optimized. For that, electrodes were prepared with different amounts of conductive agent and electrochemically tested in coin cells (charge/discharge cycle). Internationl Energy Agency Technology Roadmap Electric and plug-in hybridelectric devices: Technical report 2011.Sharma, R. A. and Seefurth R. N. J. Electrochem. Soc. 1976 volume 123 , 1763-1768. 1976, 123 (12), 1763.Boukamp, B.A., et al. J Electrochem Soc 1981, 128, 725-729Pollak, E.,et al. J. Phys. Chem. C 2007, 111, 30, 11437-11444