(65e) Li Adsorption and Intercalation in Graphene, Graphite and Carbon Onions By Reaxff Reactive Force Field

A ReaxFF reactive force field has been developed to describe Li/C systems. The ReaxFF force-field parameters for Li-C systems were optimized against a density-functional theory (DFT) (with van der Waals corrections) based training set, containing a collection of data (energies, geometries, charges etc.) related to adsorption and diffusion of Li on graphene and in graphite. ReaxFF reproduces accurately the adsorption energies for Li on hollow, bridge and top sites on pristine graphene, formation energy of stage I compounds in graphite and intralayer Li migration barriers in graphite at separations of 3.35 Å and 3.7 Å. To validate this force field we have compared it against DFT calculations for Li diffusion in the direction perpendicular to the graphene sheet for graphene with single and double-vacancy. We performed grand canonical Monte Carlo simulations to obtain ground states for Li intercalation in graphite and find that ReaxFF describes the in-plane Li ordering and interlayer separations for stage I and II compounds in good agreement with previous DFT studies and experiment. The voltage profile for the Li-graphite system as a function of Li concentration obtained by ReaxFF matches experiment and DFT data. We study Li intercalation in presence of vacancy defects in graphite and find that the presence of vacancy defects increase the ratio of Li/C and shifts the voltage profile downwards, both in proportion to the number of vacancy defects. Li intercalation and diffusion kinetics in Carbon Onions was investigated for potential applications in batteries and electrochemical capacitors and we report a different loading behavior/voltage profile for these Carbon Onions as compared to graphitic systems. The Voltage vs Li concentration for Carbon Onions has a ‘saw tooth’ shape whereas the voltage profile for graphitic systems consists of plateaus.