Synthetic Variations of Hollow Graphene Nanoshells for Li-Ion Battery Anode

Abstract

The pyrolysis of cellulose produces bio-fuels, a net carbon neutral energy source, and bio-char that can be used as a soil enhancer. The economic feasibility of the production of bio-fuel from fast growing plants that can grow rapidly on minimally viable land, such as switch grass, would be greatly enhanced by producing higher added value products from the bio-char. Pyrolyzing bio-char in the presence of metal halide catalysts with a CO2laser for a few seconds results in the production of hollow graphene nanoshells (HGNS), a carbon negative material, which can be used in place of graphite active materials in Li-ion battery anodes with remarkably higher charge rate capability and low temperature performance in relatively inexpensive electrolytes. Graphite is the most widely used anode active material in commercial Li-ion batteries. With a standard reduction potential of -2.9 V vs SHE and theoretical gravimetric capacity of 372 mAh/g (at LiC6) it provides 2 to 3 times the energy density compared to aqueous battery chemistries. The challenge remains to charge graphite anodes at the very high rates required for applications such as electric vehicles while maintaining its capacity over thousands of cycles. Shortening the Li-ion diffusion distance in graphite crystallites allows one to increase the charging rate, however, small graphite particles have poor cycle life. We utilize hollow graphene nanoshells that have orders of magnitude shorter diffusion distances than standard graphite as an anode active material. These HGNS are made up of concentric multilayer graphene shells ~50 nm in diameter that intercalate Li+ in manner analogous to graphite but with remarkably higher charge rate capability (up to 22% charge in 7.2 s) and low temperature performance in relatively inexpensive electrolytes such as PC. Unfortunately, the storage capacity for HGNS is lower than graphite (~220 mAh/g vs. ~330 mAh/g practical), which motivated us to investigate synthetic variation to improve HGNS. Here we will present the results of our studies, finding that simple, inexpensive modifications to the synthetic procedures can result in HGNS with 40% greater reversible capacity (320 mAh/g) with no capacity loss after 100 cycles and a Coulombic efficiency of 99.9%. This new form of carbon is a very promising anode active material that could enable Li-ion batteries with dramatically faster charging rates, longer cycle lives and much better low temperature performance than standard graphite anodes. Furthermore, adoption of this carbon negative material in place of synthetic graphite could have a significant impact on global climate change, both directly by CO2 sequestration and indirectly by improving the economic viability of bio-fuels. Figure 1