Peculiar diffusion behavior of AlCl4 intercalated in graphite from nanosecond-long molecular dynamics simulations*

The diffusion property of the intercalated species in the graphite materials is at the heart of the rate performance of graphite-based metal-ion secondary battery. Here we study the diffusion process of a AlCl4 molecule within graphite – a key component of a recently reported aluminum ion battery with excellent performance – via molecular dynamics (MD) simulations. Both ab-initio MD (AIMD) and semiempirical tight-binding MD simulations show that the diffusion process of the intercalated AlCl4 molecule is rather inhomogeneous, when the simulation time exceeds approximately 100 picoseconds. Specifically, during its migration in between graphene layers, the intercalated AlCl4 molecule may become stagnant occasionally, and then recovers its normal (fast) diffusion behavior after halting for a while. When this phenomenon occurs, the linear relationship of the mean squared displacement (MSD) versus the duration time is not fulfilled. We interpret this peculiar behavior as a manifestation of inadequate sampling of rare event (the stagnation of the molecule), which does not yet appear in short-time MD simulations. We further check the influence of stains present in graphite intercalated compounds (GIC) on the diffusion properties of AlCl4, and found that their presence in general slows down the diffusion of the intercalated molecule, and is detrimental to the rate performance of GIC-based battery.