Understanding the effect of polarizability on Li⁺ cation transport in PEO electrolytes by using molecular dynamics simulation

Nonpolarizable force fields have been employed successfully to study a wide range of electrolyte systems of low salt concentrations. These force fields provided important insights into the structure of ionic systems. However, they showed much slower dynamics compared to experiments. Despite the expensive computational cost, therefore, it is important to consider polarizability. In this study, we aim to demonstrate the importance of polarizability in the computational simulations for ion dynamics by comparing the diffusion of Li⁺ cations. We perform all-atom molecular dynamics simulations with APPLE&P force field, and turn on and off the polarizability. We employ PEO polymer electrolytes composed of 10 chains of PEO of 54 EO monomers and 54 LiTFSI salts. We find that the coordination numbers of Li⁺ cations and PEO chains vary depending on the presence of polarizability of each component. If polarizability is properly applied, the coordination number ranges from 5 to 6. With polarization ignored, the coordination number decreases significantly. In addition to these structure differences, the diffusion of Li⁺ cations is much faster with polarizability. Interesting is that while the diffusion of Li⁺ cations becomes slower with polarizability, cooperative motion of Li⁺ cations and PEO leads to the faster diffusion of Li⁺ cations. This indicates that the diffusion of Li⁺ cations is not only affected by the polarizability but also by the structures.