Molecular-level engineering of gel polymer electrolytes in sodium-ion batteries: a comprehensive computational study

Sodium-ion batteries (SIBs) represent promising low-cost alternatives to lithium-ion batteries (LIBs). However, critical challenges in electrolyte design - particularly in achieving both high ionic conductivity and cation transference number - must be addressed. To create a flexible and conductive polymer, we designed three novel PSO-TFSI-based polyanionic polymers with strategically positioned electron-withdrawing groups (-CF₃ and -NO₂) at the 2,6-positions of the benzene ring (PSO-BTFM-TFSI, PSO-NTFM-TFSI, and PSO-DN-TFSI). Using quantum mechanical calculations, we systematically evaluated the cation-anion binding energies and solvent interactions, and identified EC, DMSO, and DEC as optimal candidates. Molecular dynamics simulations further identified PSO-BTFM-TFSI in DEC solvent as the top performer in terms of ionic conductivity, prompting its selection for further optimization. Subsequent optimization yielded an optimal ternary solvent ratio (EC : DEC : DMSO = 0.15 : 0.05 : 0.80) with the PSO-BTFM-TFSI polymer, achieving an ionic conductivity of 2.68 ± 0.26 mS cm^-1 and 0.713 ± 0.077 cation transference number at 70 °C. This system also maintained moderate viscosity (8.84 ± 1.65 cP). We also delved deep into the diffusion mechanisms of both ionic species across the ternary solvent phase diagrams. Our work demonstrates a comprehensive design strategy for high-performance gel polymer electrolytes through targeted molecular engineering and multiscale computational screening.