Thermal diffusion of correlated Li-ions in graphite: A hybrid quantum¬タモclassical simulation study

Diffusion of Li-ions in graphite is an essential elementary process in the current lithium-ion battery. The C-layers of graphite deform with Li due to relatively large size of Li-ion, act to confine the Li-ions, and thereby creates correlation between them. We address theoretically the thermal diffusivity of such correlated Li-ions in graphite by the hybrid quantum–classical simulation method. In this method, the quantum-region composed of the Li-ions and surrounding C atoms is treated by the density-functional theory, while it is embedded dynamically in the total system described with an empirical inter-atomic interaction potential. We thereby take into account the long-ranged deformation field in graphite in simulating the Li-ion dynamics. Two kinds of settings of Li-ions are considered for the simulation runs at temperature 443 K: (i) seven Li-ions are inserted in the same inter-layer space of the C-layers to study their intra-plane correlation, and (ii) additional seven Li-ions are inserted in the neighboring space (i.e., fourteen Li-ions totally, 7 Li-ions in upper and 7 Li-ions in lower spaces) to study their inter-plane correlation. As for (i), the Li-ions, concentrated initially with inter-ion distances of 2:5—4:2 Å, scatter due to their mutual Coulomb repulsion. After about 1 ps, the Li-ions and surrounding C atoms thermalize well with deformed C-layers creating a cage of radius about 13:5 Å for 7 Li-ions. Diffusivity of Li-ions inside the cage is much higher than that of the cage itself. The long-time diffusion constant of the cage is the same order as that of an isolated Li-ion in graphite. As for (ii), the Li-ions, concentrated initially in the upper and lower inter-layer spaces of the C-layer, firstly form domains, and then the domains repel each other horizontally. The result is in accord with the experimental finding that the Li-rich and Li-poor planes stack in an alternating sequence in graphite. 2015 Elsevier B.V. All rights reserved.