Cation Chemistry and Ion Density Effects on Mono- and Divalent Ion Transport in Unentangled PEO Electrolytes

Understanding how cation valency, cation size, and ion density govern ionic transport in polymer electrolytes is essential for the development of next-generation multivalent batteries. While the transport mechanisms of monovalent ions in poly(ethylene oxide) (PEO) are now well established, considerably less is known about divalent ion transport in the diffusion-controlled regime of unentangled polymer hosts. Here, we systematically investigate LiTFSI-, NaTFSI-, Mg(TFSI)2-, and Ca(TFSI)2-based electrolytes in low-molecular-weight, unentangled PEO (Mw = 0.35 kg mol–1), where ion transport is governed by polymer translational diffusion. Thermal characterization reveals that divalent cations suppress crystallinity more efficiently and induce substantially larger increases in the glass-transition temperature than their monovalent counterparts, consistent with stronger perturbation of the PEO dynamics by the divalent salts. Within each valency class, NaTFSI electrolytes exhibit higher total ionic conductivities than their LiTFSI counterparts, while Ca(TFSI)2-containing electrolytes exhibit higher ionic conductivities than the corresponding Mg(TFSI)2 systems at comparable cation-to-ethylene oxide ratios. These differences are consistent with the corresponding trends in glass-transition temperatures, suggesting that cation-dependent changes in polymer dynamics contribute to ion transport. Despite these stronger interactions, ionic conductivities exceeding 10–4 S/cm are achieved near room temperature at low ion densities, representing comparatively high total ionic conductivities among reported dry polymer electrolytes containing Mg(TFSI)2 and Ca(TFSI)2. For all electrolytes, the conductivity exhibits a characteristic nonmonotonic dependence on ion density arising from the competition between increasing ionic concentration and decreasing ionic mobility, with the conductivity maximum occurring at significantly lower ion densities for divalent salts. Finally, correlating ionic conductivity with zero-shear viscosity and introducing a charge-normalized conductivity based on the Nernst–Einstein framework collapses the transport behavior of all four electrolytes onto a common master trend. These results reveal a common macroscopic relationship between polymer friction and total ionic conductivity across mono- and divalent electrolytes and establish a unified framework for describing diffusion-controlled transport in oligomeric PEO.