Molecular dynamics simulation of a LixMn2O4 spinel cathode material in Li-ion batteries

Abstract

In this study molecular dynamics simulations and a particle-level mathematical model were used to study the state of charge (SOC) dependent mechanical properties such as yield stress, ultimate stress and Young’s modulus of lithium manganese oxide as a cathode material in Li-ion batteries during electrochemical cycling. The molecular model was applied on a unit cell of LiMn₂O₄, containing 56 ions (8 lithium ions, 8 trivalent manganese ions, 8 tetravalent manganese ions and 32 oxygen atoms) that was replicated in 2 × 2 × 2 cubic structure. The volume changes of Li_xMn₂O₄ was investigated as a function of the SOC (0 < x < 1). MD simulations indicated that the lattice volume of Li_xMn₂O₄ varied by 6.87% in one half cycle. This large volume change was attributed to lithium compositional changes during electrochemical cycling. MD simulations showed that at low SOC values Li_xMn₂O₄ behaves as a brittle material and at high SOC values behaves as a ductile material. Furthermore, due to the existence of two phase of Li_xMn₂O₄ in the range of low SOC values, we observed that the elastic properties increase as the SOC decreases from 0.375 to 0. By employing visualization techniques it was clear that the LiMn₂O₄ fracture process is initiated by void formation in a nearby material’s surface and consequently leads to surface fracture. Using long MD simulations, mean square displacement (MSD) calculations indicated that there are three different regimes in the MSD curves: ballistic, caging and diffusive. Also the SOC-dependent Li ion diffusion coefficients were investigated and revealed that due to the greater availability of vacant sites at low SOC values the Li ion diffusion coefficient is higher than at high SOC values.