Polymer‐Induced Inversion of the Li+ Drift Direction in Ionic Liquid‐Based Ternary Polymer Electrolytes

Abstract

AbstractWhile salt‐in‐ionic liquid (IL) electrolytes provide a promising alternative to organic electrolytes, they suffer from large ionic clusters impeding lithium ion transport. Here, a lithium‐coordinating polymer is added and the transition from salt‐in‐IL to IL‐based ternary polymer gel electrolytes is followed, investigating the influence of the composition on Li transport mechanisms. The electrolyte is composed of the salt lithium bis(trifluoromethanesulfonyl)amide (LiTFSA), the IL 1‐butyl‐1‐methylpyrrolidinium (Pyr14) TFSA, and a variable amount of poly(ethylene oxide) (PEO). Impedance spectroscopy, multinuclear (1H, 7Li, and 19F) diffusion NMR and electrophoretic NMR (eNMR) yield electrophoretic mobilities, transference numbers, partial conductivities, and effective charges of all ionic species. As the polymer fraction increases, the lithium ion drift direction is inverted. The sign reversal of Li+ mobility and transference number occurs at the optimum coordination of 5–6 ethylene oxide units per Li+ ion. The dominant Li+ transport mechanism shifts from a vehicular transport via anionic clusters to chain‐dominated transport mechanisms. It is concluded that the lithium transport properties of IL‐based electrolytes can be vastly improved by the addition of PEO. Tuning the composition of the ternary electrolyte reveals optimal Li+ transport properties at moderately high polymer contents as well as high salt concentrations.