Tuning Anion Composition and Mobility to Balance Ionic Conductivity and Cation Selectivity in Solid Polymer...

Solid polymer electrolytes (SPEs) offer a promising route toward safe and high-performance electrochemical energy storage, yet a fundamental challenge in SPEs involves improving ionic conductivity while maintaining selective cation transport. The hurdle exists because ion transport is typically coupled closely to polymer segmental dynamics. Herein, a glassy single-ion-conducting polymer, poly-[lithium sulfonyl-(trifluoromethane sulfonyl)-imide methacrylate] (PLiMTFSI), in which the anions were tethered to the polymer, was blended with a flexible polymer, poly-(oligo-oxyethylene methyl ether methacrylate) (POEM), and a series of small-molecule lithium salts, in which the anions were untethered [lithium bis-(trifluoromethanesulfonyl)-imide (LiTFSI), lithium bis-(fluorosulfonyl)-imide (LiFSI), lithium trifluoromethanesulfonate (LiTf), or lithium perchlorate (LiClO₄)]. The impact of salt anion volume and tethered-to-untethered anion ratio on the ion conduction behavior and thermal properties of blend electrolytes was investigated. In some cases, conductivity could be enhanced through this ternary blend approach. For example, a POEM-based polymer blend containing a bulky salt anion (TFSI^-) and an equimolar mixture of PLiMTFSI and LiTFSI exhibited a Li⁺ conductivity (4.8 × 10^-4 S/cm) an order of magnitude higher than that of a comparable POEM/LiTFSI system (6.3 × 10^-5 S/cm) at 100 °C. This enhancement was attributed to a more than 9-fold increase in lithium transference number (0.66 in the ternary blend vs 0.07 in POEM/LiTFSI). Overall, this study highlights the potential for tuning anion composition and mobility to achieve relatively high ionic conductivities and maintain selective cation transport in SPEs, offering a pathway to enable batteries that tolerate elevated temperatures.