Superionicity in Ionic-Liquid-Based Electrolytes Induced by Positive Ion-Ion Correlations.

In ionic-liquid (IL)-based electrolytes, relevant for current energy storage applications, ion transport is limited by strong ion-ion correlations, generally yielding inverse Haven ratios (ionicities) of below 1. In particular, Li is transported in anionic clusters into the wrong direction of the electric field, requiring compensation by diffusive anion fluxes. Here, we present a concept to exploit ion-ion correlations in concentrated IL electrolytes beneficially by designing organic cations with a Li-coordinating chain. 1H NMR and Raman spectra show that IL cations with seven or more ether oxygens in the side chain induce Li coordination to organic cations. An unusual behavior of an inverse Haven ratio of >1 is found, suggesting an ionicity larger than that of an ideal electrolyte with uncorrelated ion motion. This superionic behavior is consistently demonstrated in both NMR transport/conductivity measurements and molecular dynamics (MD) simulations. Key to this achievement is the formation of long-lived Li-IL cation complexes, which invert the Li drift direction, yielding positive Li+ ion mobilities for the first time in a single IL-solvent-based electrolyte. Onsager correlation coefficients are derived from MD simulations and demonstrate that the main contributions to the inverse Haven ratio, which induce superionicity, arise from enhanced Li-IL cation correlations and a sign inversion of Li-anion correlation coefficients. Thus, the novel concept of coordinating cations not only corrects the unfortunate anionic drift direction of Li in ILs but even exploits strong ion correlations in the concentrated electrolyte toward superionic transport.

Cation-Assisted Lithium-Ion Transport for High-Performance PEO-based Ternary Solid Polymer Electrolytes.

N-alkyl-N-alkyl pyrrolidinium-based ionic liquids (ILs) are promising candidates as non-flammable plasticizers for lowering the operation temperature of poly(ethylene oxide) (PEO)-based solid polymer electrolytes (SPEs), but they present limitations in terms of lithium-ion transport, such as a much lower lithium transference number. Thus, a pyrrolidinium cation was prepared with an oligo(ethylene oxide) substituent with seven repeating units. We show, by a combination of experimental characterizations and simulations, that the cation's solvating properties allow faster lithium-ion transport than alkyl-substituted analogues when incorporated in SPEs. This proceeds not only by accelerating the conduction modes of PEO, but also by enabling new conduction modes linked to the solvation of lithium by a single IL cation. This, combined with favorable interfacial properties versus lithium metal, leads to significantly improved performance on lithium-metal polymer batteries.