Ionic Conductivity in Salt-Doped Polymers: Combined Effects of Temperature and Salt Concentration

Polyethylene Oxide (PEO) doped with LiTFSI is one of the most extensively studied system for lithium battery applications using polymer electrolytes.  For a given temperature, the ionic conductivity for this system has been shown to be a nonmonotonic function of the salt concentration with a maximum at some intermediate salt concentration.  Mongcopa et al. [ACS Macro Lett. 7, 504 (2018)] suggested that this nonmonotonic behavior resulted from the competition between increased charge carrier concentration and slowing down of polymer segmental dynamics as the salt concentration increases.  They further proposed that the slow-down in the segmental mobility decreases exponentially with salt concentration.  In this work, we construct a coarse-grained molecular dynamics model based on LiTFSI doped PEO systems to examine the combined effects of temperature and salt concentration.  We find salt doping leads to significant slowing down of the polymer chain dynamics and ion diffusivity, and this slow-down shifts the glass transition temperature upward by an amount proportional to the salt concentration in the concentration range studied. The center-of-mass polymer diffusivity is shown to be well represented by a modified Vogel-Fulcher-Tamman (VFT) equation that accounts for both the temperature and salt concentration dependence. Furthermore, we find that at any temperature, the concentration dependence of the conductivity is well described by its ideal (infinite dilution) value times the ratio of the segmental mobility at the given salt concentration over the segmental mobility at zero salt.  This mobility ratio agrees with the exponential salt concentration dependence proposed by Mongcopa et al. at elevated temperatures well above the glass transition but becomes super-exponential near the glass transition.