We used Packmol to create an initial simulation box containing 30 polymer chains together with the necessary ion concentration. For example, a 1 M PEO system corresponds to 190 Li and 190 TFSI units. This on average corresponds to having boxes of around 30000 atoms. The molality was always selected to match experiment, and if multiple values were available, the one closest to 1 was selected. After creating an initial box structure, we applied a multi-step equilibration scheme until the volume and potential energy in the NPT ensemble converge. All of the MD simulations were performed with the GROMACS simu-lation package. Details on the full equilibration scheme and MD simulations are provided in the SI section 2. After the polymer electrolyte boxes were equilibrated, we performed a 300-ns production simulation to calculate the ionic conductivities from the motion of the Li and TFSI ions. In the case of glass transition temperature calculations, we always used the aforementioned DFT charges and followed a scheme similar to Afzal et al. (details in section 2 of the SI). The Nernst-Einstein (NE conductivity) formalism calculates the ionic conductivity from the self-diffusion coefficients of the cation (Li) and anion (TFSI) by assuming uncorrelated motion between distinct particles. However, in polymeric systems, motion of the cation and anion tend to be correlated, partly through the dynamic formation of ionic clusters. This is because polymeric materials tend to have a comparatively low (to liquid electrolytes) dielectric constant and ion chain coordination is not enough to fully solvate the Li ions. To quantify the full correlation between all cations and all anions we use the Einstein formalism further expanded by Wheeler and Newman (WN conductivity). It has been previously