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

摘要

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 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 (ΔV≥300%) (4). This stress eventually causes mechanical fracture and pulverization, which in turn leads to active material loss and consequently to severe capacity fading. Inactive materials and the use of a composite of active materials, silicon and graphite (Si/C), 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 electronical conductivity and provide inter-particle as well as particle-current collector contacts. However, an excessive amount of inactive material reduces the energy density of the electrode. Especially for high energy applications with graphite present in the composite, the necessity of conductive agent is debatable. Conductive agents do not only enhance the electronical conductivity, but also increase the overall surface area of the electrode. This in turn might require more binder amount and hence decrease the energy density. Furthermore, different cutoff voltages during cycling can have a positive effect on the overall cell performance, since lithiation of silicon and thus the volume expansion depends on the applied voltage. Here, the impact of conductive agent and the different cutoff voltages are investigated regarding the cell performance and electrical conductivity in high-energy Si/C-composite electrodes. For that, the amount of inactive material was optimized and electrodes were cycled at different cutoff voltages in coin cells (charge/discharge cycle).