(139b) Silicon-Graphene Composites for Li Ion Battery Anodes

Rechargeable batteries with large storage capacity and power density have many potential applications in portable electronics, transportation, and load leveling for renewable power sources. Li-ion batteries are the most promising to capitalize on these opportunities, but new materials with much higher energy densities are required for energy-intensive applications. Silicon, with its high gravimetric and volumetric energy density, is an attractive candidate to replace graphite as the anodic material in a Li-ion battery (3579 and 372 mAh/g, respectively). Unfortunately, the intrinsic ~300% volume expansion/contraction during charge-discharge cycling causes silicon particles to fracture, diminishing electrical contact of the particles with other electrode components, resulting in poor cycling performance. An assortment of silicon structures have been developed in an attempt to stabilize the silicon particles, including the use of 3D porous particles, nanowires, thin films, mixtures with conductive carbons, and encapsulation with carbonaceous coating. Despite the variety of structures developed, these electrodes are still not satisfactory for advanced commercial applications due to poor cycling stability, cost of manufactory, or insufficient capacity improvement.

Recently, we have developed nanocomposites composed of silicon nanoparticles dispersed between graphene sheets. These freestanding composites are formed by vacuum filtration, and form a highly conductive 3D network of graphene and reconstituted graphite after thermal reduction, thereby minimizing the need for conductive additives and heavy metallic current collector as support. Electrical conductivities of ~2500 S/m are typical for these composites. It is postulated that the 3D network of graphene provides a flexible environment for the silicon nanoparticles to expand and contract while maintaining electrical contact, even with particle degradation. These composites show improved Li-ion storage capacities and cycling stability compared to admixed samples of silicon nanoparticles and carbon support. Whereas silicon nanoparticles mixed with conductive carbon and PVDF binder typically completely degrade within the first 10 cycles, our silicon-graphene composites can achieve initial capacities >2000 mAh/g and maintain capacities over 1200 mAh/g after 100 cycles. Characterization of these composites with various processing and electrochemical conditions will be presented as well as results of modifying surfaces of silicon nanoparticles to influence dispersion.