Enhancing the diffusivity of the Li ion in a Li-graphite intercalation compound that was used as a negative electrode in the Li-ion rechargeable battery, was important in improving both the recharging speed and power of the battery. In the compound, the Li ion created a long-range stress field around itself by expanding the interlayer spacing of graphite. A hybrid quantum-classical simulation code was proposed which included the external electric field, in addition to the long-range stress field, via first-principles simulation. In the hybrid code, the quantum region selected adaptively around the Li ion was treated by using the real-space density-functional theory for electrons. The rest of the system was described with an empirical interatomic potential that included the term relating to the dispersion force between the C atoms in different layers. Hybrid simulation runs for Li dynamics in graphite were performed at 423K under various settings of the amplitude and frequency of alternating electric fields perpendicular to C-layers. It was found that the in-plane diffusivity of the Li ion was enhanced significantly by the electric field if the amplitude was larger than 0.2V/Å within its order and the frequency was as high as 1.7THz. The microscopic mechanisms of the enhancement were explained.

Enhanced Thermal Diffusion of Li in Graphite by Alternating Vertical Electric Field: a Hybrid Quantum-Classical Simulation Study. Ohba, N., Ogata, S., Tamura, T., Kobayashi, R., Yamakawa, S., Asahi, R.: Journal of the Physical Society of Japan, 2012, 81[2], 023601