(704d) Tailoring the Assembly of Cost-Effective Milled Silicon and Exfoliated Graphene for High Capacity, High Rate-Capable Lithium-Ion Battery Anodes

Silicon as anode material has been very promising to satisfy the high energy density demands for electric vehicle (EV) applications due its extraordinary theoretical capacity. However, silicon undergoes huge volume expansion(~ 300%) during lithium-ion storage. One approach to address this issue is nanostructuring of silicon(30-50nm) to minimize the stress generated during volume expansion. Although, nanostructuring has resolved most of the issues associated with silicon anodes, these nanostructure morphologies involve multiple and expensive production steps which makes it impractical on an industrial scale. One attractive option to reduce the cost, substantially, is by using milled low-cost metallurgical Si(200-300nm) which is often the recycled from the Si scrap that is easily obtained from solar or semi-conductor industry. To accommodate the mechanical stress caused due to the volume expansion of milled silicon, graphene or reduced graphene oxide is frequently used. Graphene not only acts as a buffer against the volume expansion to preserve structural integrity of the electrode, but also increases the overall conductivity of the electrode. Most of the Graphene available commercially is made from reduction of graphene oxide by heat, chemical or plasma treatment leading to increase in cost in the final product.

In our study we are using exfoliated graphene which is produced in-house from a batch Taylor-Couette reactor set-up. The Taylor-Couette setup used in this study included the rotation of the outer cylinder while the inner cylinder is still. Graphite flakes are dispersed in an aqueous solution with a dispersant and a stabilizer followed by introducing the solution into the gap in the Taylor-Couette system. Substituting our mechanically exfoliated graphene instead of commercial reduced graphene oxide in milled silicon anodes not only greatly reduces the cost by several orders of magnitude, due to direct synthesis of graphene from graphite as opposed to reducing GO, but also makes the overall process more facile.

Several graphite pre-cursors for the Taylor-Couette system were studied which resulted in mechanically exfoliated graphene sheets with different lateral dimensions and varied level of defects. The interplay of graphite pre-cursor and electrochemical performance was probed. Our results show that exfoliated graphene can provide high capacities and stable retention over several cycles and successfully replace commercially obtained rGO. More importantly, the resultant electrodes are able to provide impressive rate capabilities of 800-1000 mAh/g at 2C. Exfoliated Taylor-Couette Graphene not only shows great potential for cost reduction but also enhanced electrochemical performance.