Molecular Dynamics Simulations of Li+ Desolvation in Electric Double Layer at the Lithium Ion Battery Graphitic Anode Interface

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/discharge rate and cycle life than their applications in electric devices. These properties are largely caused by the phenomenon in the interphase of electrode and electrolyte. For the battery charge/discharge rate, the process of Li+ desolvation in the interphase 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 interphase can accelerate the development of Li-ion battery. In this work, to gain an understanding of the electrolyte structure in the interphase, atomistic molecular dynamics simulations of the electrode/electrolyte interphase at a constant voltage were performed and Li+ desolvation process were observed. 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 technique. 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 8nm. 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)]. The simulation of 2ns at 298K was carried out in the constant NVT ensemble with applied voltage of constant 20V. The reason why excess voltage was applied is to get the interphase structure with EDL in short timescale simulation. We observed the dynamics of electrolyte in the interphase of graphitic anode. Then we found that EMC solvate Li+ until the end, and desolvation of EC occur by force of the own dipole facing to the electric field. It was suggested that the desolvation of EC is faster than that of EMC in the EDL. We also calculated dielectric spectrums of solvents on the surface for confirming existence of solvent’s dynamics in EDL by experiment. Figure 1