First-Principles Study of Lithiation of Microporous Carbon

Abstract

Microporous carbon is attractive material as an anode for lithium ion batteries (LIBs) because microporous carbon has a high lithium ion capacity and good ionic conductivity. Microporous carbon structures with densities of 1.5, 2.0, and 2.5 g cm-3 are investigated using density functional theory (DFT) calculations and ab inito molecular dynamics (AIMD) simulations. The most stable lithium compositions determined from the calculated formation energies are Li8.15C6 (3032 mA h g-1), Li7.48C6 (2783 mA h g-1), and Li8.08C6 (3006 mA h g-1) for MC-15, MC-20, and MC-25, respectively. This capacities are about 8 times higher than that (372 mA h g-1) for graphite, and in good agreement with the experimentally determined capacities 2950 mA h g-1 (Li7.9C6) of zeolite-templated microporous carbon. As lithiation proceeds, the microporous carbon structures with different total pore volumes tend to have similar total pore volumes, but individual pores of lithiated microporous carbon structures evolve to have different sizes. The lithium ion conductivities of fully lithiated microporous carbon structures with average pore diameters 7.0, 9.3, and 6.6 Å at T = 300 K are 4.5, 187.6, and 8.7 mS cm−1, respectively, suggesting that large individual pore size is important for fast lithium ion diffusion. The lithium ions with a Li-rich environment in the pore cavity diffuse much faster than those with a C-rich environment in the pore wall. The lithium storage site in microporous carbon is not only the pore wall surface but also the pore cavity, and the lithium ions stored in the pore cavity play a key role in the high capacity and conductivity of microporous carbon.