Preferential Ion Solvation Drives Divergent Miscibility in Polyether Blend Electrolytes

Polymer blending provides a promising route for designing high-performance solid-state electrolytes. Coarse-grained molecular dynamics simulations of binary polyether blends formed by PEO/P(2EO-MO), PEO/P(EO-MO), and P(EO-MO)/P(2EO-MO) mixed with LiTFSI salt reproduce an experimentally observed miscibility sequence. PEO/P(2EO-MO) blends phase-separate at low salt concentrations and mix at high salt concentrations. The effective Flory–Huggins interaction parameters, evaluated via the random phase approximation, transition from critical value in the phase-separated regime to negative in the high-salt miscible regime. This behavior arises from preferential PEO solvation at low salt concentrations and coordination saturation at higher concentrations, which equalizes solvation and promotes ion bridging to restore miscibility. Similar re-entrant mixing is predicted for PEO/P(EO-MO), whereas the mismatched solvation capability and the absence of coordination saturation in P(EO-MO)/P(2EO-MO) sustains phase separation. These findings establish a molecular understanding of how selective ion solvation governs blend miscibility and provide a rational basis for electrolyte design with tailored phase morphology.