Direct Evidence for Li Ion Hopping Conduction in Highly Concentrated Sulfolane-Based Liquid Electrolytes

We demonstrate that
Li+ hopping conduction, which cannot
be explained by conventional models i.e., Onsager’s theory
and Stokes’ law, emerges in highly concentrated liquid electrolytes
composed of LiBF4 and sulfolane (SL). Self-diffusion coefficients
of Li+ (DLi), BF4– (DBF4),
and SL (DSL) were measured with pulsed-field
gradient NMR. In the concentrated electrolytes with molar ratios of
SL/LiBF4 ≤ 3, the ratios DSL/DLi and DBF4/DLi become lower
than 1, suggesting faster diffusion of Li+ than SL and
BF4–, and thus the evolution of Li+ hopping conduction. X-ray crystallographic analysis of the
LiBF4/SL (1:1) solvate revealed that the two oxygen atoms
of the sulfone group are involved in the bridging coordination of
two different Li+ ions. In addition, the BF4– anion also participates in the bridging coordination
of Li+. The Raman spectra of the highly concentrated LiBF4–SL solution suggested that Li+ ions are
bridged by SL and BF4– even in the liquid
state. Moreover, detailed investigation along with molecular dynamics
simulations suggests that Li+ exchanges ligands (SL and
BF4–) dynamically in the highly concentrated
electrolytes, and Li+ hops from one coordination site to
another. The spatial proximity of coordination sites, along with the
possible domain structure, is assumed to enable Li+ hopping
conduction. Finally, we demonstrate that Li+ hopping suppresses
concentration polarization in Li batteries, leading to increased limiting
current density and improved rate capability compared to the conventional
concentration electrolyte. Identification and rationalization of Li+ ion hopping in concentrated SL electrolytes is expected to
trigger a new paradigm of understanding for such unconventional electrolyte
systems.