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 LiBF and sulfolane (SL). Self-diffusion coefficients of Li ( D), BF ( D), and SL ( D) were measured with pulsed-field gradient NMR. In the concentrated electrolytes with molar ratios of SL/LiBF ≤ 3, the ratios D/ D and D/ D become lower than 1, suggesting faster diffusion of Li than SL and BF, and thus the evolution of Li hopping conduction. X-ray crystallographic analysis of the LiBF/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 BF anion also participates in the bridging coordination of Li. The Raman spectra of the highly concentrated LiBF-SL solution suggested that Li ions are bridged by SL and BF even in the liquid state. Moreover, detailed investigation along with molecular dynamics simulations suggests that Li exchanges ligands (SL and BF) 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.