Miscible Polymer Blend Electrolytes Made with High Dielectric Polyethers Optimize Conductivity and Ion Transport at Ambient Conditions

Abstract

The implementation of energy-dense lithium metal anodes in lithium batteries requires an electrolyte material that enables rapid and selective cation motion and low anion mobility. Although single-ion-conducting polymer electrolytes are selective, they typically exhibit conductivity values several orders of magnitude lower than liquid and neutral polymer electrolytes. In this report, we investigate the effects of blending single-ion-conducting polymer electrolytes (i.e., metal salts with a macromolecular polyfunctional anion) with neutral polymer hosts on the conductivity and ionic motion. With the goal of improving ionic conductivity without overly mobilizing anions, two comb-branched copolymers, with high and low ion-content poly[(lithium 3-[(trifluoromethane) sulfonamidosulfonyl]propyl methacrylate)-co-(poly(ethylene glycol methyl ether acrylate))], were blended with either poly[(ethylene oxide)-co-(allyl glycidyl ether)] or poly(cyanoethyl glycidyl ether) at various polyelectrolyte loadings. These blends are miscible over a wide range of compositions and increase the conductivity of the single-ion-conducting polyelectrolyte by up to 2 orders of magnitude. Neither the glass-transition temperature nor dielectric constant correlates strongly to conductivity in these systems. Instead, the overall Li:O ratio of the blend influences conductivity and exhibits an maximum at ca. 0.05. Finally, we investigated ion mobility through limiting current fraction of Li⁺ ions and observed fractions up to 0.92 with ca. 0.75 for the most conductive blends.