Charge Scaling in Classical Force Fields for Lithium\nIons in Polymers

Polymer electrolytes\nare of interest for applications in energy\nstorage. Molecular simulations of ion transport in polymer electrolytes\nhave been widely used to study the conductivity in these materials.\nSuch simulations have generally relied on classical force fields.\nA peculiar feature of such force fields has been that in the particular\ncase of lithium ions (Li<sup>+</sup>), their charge must be scaled\ndown by approximately 20% to achieve agreement with experimental measurements\nof ion diffusivity. In this work, we present first-principles calculations\nthat serve to justify the charge-scaling factor and van der Waals\ninteraction parameters for Li<sup>+</sup> diffusion in poly(ethylene\nglycol) (PEO) with bistriflimide (TFSI<sup>–</sup>) counterions.\nOur results indicate that a scaling factor of 0.79 provides good agreement\nwith DFT calculations over a relatively wide range of Li<sup>+</sup> concentrations and temperatures, consistent with past reports where\nthat factor was adjusted by trial and error. We also show that such\na scaling factor leads to diffusivities that are in quantitative agreement\nwith experimental measurements.