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⁺ (D_Li), BF₄ ^- (D_BF4 ), and SL (D_SL) were measured with pulsed-field gradient NMR. In the concentrated electrolytes with molar ratios of SL/LiBF₄ ≤ 3, the ratios D_SL/D_Li and D_BF4 /D_Li 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.