Correlation of Metal Ion Coordination with Mechanical Properties and Ionic Conductivity in Homogeneous PEG-Based Network Solid Electrolytes

Abstract

Understanding the relationship between metal ion coordination within polymer networks and macroscopic properties is a key factor in designing polymer electrolytes. In this study, homogeneous tetra/linear-PEG solid electrolytes were prepared by combining tetrafunctional and linear poly(ethylene glycol)s (tetra-PEG and linear-PEG, respectively) and used as a model system to systematically investigate the effects of polymer network mesh size and metal ion species (Li+, Na+, and Mg2+) on solid electrolyte properties, particularly mechanical properties and ionic conductivity. The resulting tetra/linear-PEG-based electrolytes exhibited significant dependence of both mechanical properties and ionic conductivity on the mesh size. The dependence on metal ion species revealed that Young’s modulus increased in the order Li < Na < Mg, whereas the molar ionic conductivity decreased in the order Li ≫ Mg > Na. Raman spectroscopy showed that the coordination states of bis(trifluoromethanesulfonyl)amide (TFSA) anions strongly depend on the metal ion species: free TFSA and contact ion pairs (CIPs) coexist in the Li system, whereas CIPs and ionic aggregates (AGGs) dominate in the Na and Mg systems. High-energy X-ray total scattering combined with molecular dynamics simulations further revealed that PEG and TFSA anions (both coordinated via O atoms) competitively coordinate to the metal ions, and that Mg2+ exhibits a relative preference for coordination with PEG over TFSA among the examined ions. This preferential Mg2+–PEG interaction suggests the formation of transient interchain bridging points, which may contribute to the high mechanical strength observed in the Mg system. The Li salt-containing tetra/linear-PEG electrolyte with optimized mesh size exhibited reversible Li deposition/dissolution behavior, suggesting its potential as a solid electrolyte for battery systems.