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.