Molecular Dynamics Simulations of Lithium Ion Battery Anode Interface in Battery Charging Process

Abstract

The use of lithium-ion (Li-ion) batteries to power hybrid, plug-in hybrid, and electrical vehicles in recent years calls for greater battery charge rate than their applications in electric devices. These properties are largely caused by the phenomenon in the interface of electrode and electrolyte. For the battery charge rate, the process of Li+ desolvation in the interface and intercalation to graphitic anode are key factors. However, there are various hypotheses about Li+ desolvation and it has still not been clarified. Detailed understanding of the interface can accelerate the development of Li-ion battery. In this work, to gain an understanding of the Li+ desolvation in battery charging process, atomistic molecular dynamics simulations of the electrode/electrolyte interface at a constant voltage were performed. We utilize a modified version LAMMPS code implemented the novel computational approach to carry out the simulation of the electrolyte in the presence of electrodes and applied voltage. The approach for treating the model of battery interface held at a constant potential, which has necessity to calculate the nonperiodic cell in the surface normal direction, is based on effective screening medium (ESM) method [Otani and Sugino, Phys. Rev. B 73, 115407 (2006)]. The Poisson equation is solved with the help of the green’s function technic. The electrostatic potential V(z) was solved as following equation, V(z) = -∑q i|z i-z|/2ε 0 S 0, where ε 0 is the permittivity of vacuum, S 0 is electrode’s surface area, and q i is the charge of ith atom. If a potential difference V is applied across the two electrodes, then the charges are induced on each electrode surface to be V(z le) - V(z re) = V, where z le is the z of left side electrode surface and z re is right one. Prepared simulation model is the box with two edge plane of graphitic electrodes separated by an electrolyte which consists of 1M LiPF6 / ethylene carbonate (EC) / ethyl methyl carbonate (EMC) 30:70. The interelectrode spacing is 8 nm. We confirmed it is enough to describe the Electric double layer (EDL) of each electrodes with utilizing ESM-RISM method [Nishihara and Otani, Phys. Rev. B 96, 115429 (2017)]. To get the interface structure in EDL in short timescale simulation, preliminary runs for 3 ns at 298 K was carried out in the constant NVT ensemble with applied voltage of constant 20 V. After that equilibration runs for 3.8 ns at 298 K were carried out with applied voltage of constant 1 V. Finally production runs which modeling the charging process from 1 V to 4 V was carried. We discuss about the Li+ desolvation in battery charging process with observing electrolyte dynamics in EDL. Figure 1