Force field generation and molecular dynamics simulations of Li-Nafion

A new molecular dynamics force field for Nafion^® containing Li⁺ ions has been generated using Density Functional Theory calculations (B3LYP) on a Nafion side-chain, a Li⁺ ion and a H₂O molecule. The depth of the potential energy well between Li⁺ and the sulphonate group was decreased with ∼10 kcal/mol and the optimal Li-S distance 0.5 Å shorter, as compared to force fields generated without water present. Molecular dynamics simulations based on the new force field result in a self-diffusion coefficient for Li⁺ of 8.0 × 10⁻⁸ cm²/s at 353 K, which is closer to experimental result than previous simulations using force fields based on pure Nafion-cation interactions.