Molecular Dynamics Simulation Study of PEO-based Polymer Electrolytes in Aqueous Solution

Abstract

Abstract

Lithium polymer batteries (LPB) using polyethylene oxide (PEO) polymer host are one of the most popularly used rechargeable batteries. A major factor limiting their performance is low ionic conductivity at room temperature due to the high crystallinity of the polymer matrix. The effort to understand ion transport mechanisms and to improve the conductivity led to experimental and simulation studies of various polymer matrix systems. Recent experimental studies have shown that ternary mixtures of polymer electrolytes with water can provide even more attractive properties than binary polymer-salt systems. In this work we use molecular dynamics simulations to investigate the microscopic structure and ion diffusion mechanisms of ternary PEO/LiI/water mixtures. To accurately describe interactions between all components in the mixture, we used a many-body polarizable forcefield, a part of which we developed on the basis of ab initio calculations. Our simulations predict that at higher water concentrations (water to ether oxygen ratio ~5) lithium ions will be surrounded almost exclusively by water molecules, which is in a good agreement with available neutron scattering data. However, it was also found that despite the relatively weaker interaction between ions and individual ether oxygens, PEO can still compete with water due to the stabilizing chelate effect, which is particularly pronounced at lower water concentrations (water to ether oxygen ratio < 1) with lithium ions surrounded mostly by ether oxygens. We found that at all concentrations water increases conductivity due to (i) the changes in the structure of PEO without a direct contact with Li+ ions, (ii) formation of mobile Li complexes with water, and (iii) separation of Li+ ions from heavy I- counterions. Further increases in ion conductivity can be potentially achieved by tuning the system to the most ‘diffusion efficient’ structures by the optimization of water content, polymer chain length, or ion content.