Elasticity and Cooperative Ion Motion in a Polymeric Ionic Liquid Loaded with Li Salt

Solid polymer electrolytes (SPEs) possess several advantages over liquid electrolytes, such as stability and nonflammability, that can enable next-generation batteries with improved performance. However, current SPEs suffer from sluggish Li⁺ transport and poor mechanical properties. Polymeric ionic liquids (PILs) have emerged as promising electrolyte materials due to their ability to dissociate high concentrations of Li salts. High segmental motion enables fast ion transport, but it has been challenging to find materials that are both rubbery and also have high salt dissolution. We find improved transport properties and rheological behavior for a PIL with a flexible siloxane backbone (designated as PMS-ImTFSI) in the salt-in-polymer (<50 wt % salt) regime. PMS-ImTFSI exhibits a long-lived rubbery plateau in shear rheology at salt loadings up to ca. 20 wt % salt that imparts it with greater elasticity. At the same time, PMS-ImTFSI enables high Li⁺ conductivity (up to 2 × 10^-5 S/cm at 90 °C) due to beneficial ion-ion correlations. A transition from salt-in-polymer to polymer-in-salt regimes as seen consistently across rheology, NMR diffusometry, inverse Haven ratios, and X-ray scattering suggests that PILs with flexible, nonpolar backbones at low salt loading can form ion-rich domains that simultaneously exhibit high Li⁺ conductivity and robust mechanical properties. At high salt loading, the microstructure disappears, and the optimal properties are lost. These findings guide the design of advanced SPEs for next-generation batteries.