Structural Effects of Solvents on Li-Ion-Hopping Conduction in Highly Concentrated LiBF₄/Sulfone Solutions

Li-ion-hopping conduction is known to occur in certain highly concentrated electrolytes, and this conduction mode is effective for achieving lithium batteries with high rate capabilities. Herein, we investigated the effects of the solvent structure on the hopping conduction of Li ions in highly concentrated LiBF₄/sulfone electrolytes. Raman spectroscopy revealed that a Li⁺ ion forms complexes with sulfone and anions, and contact ion pairs and ionic aggregates are formed in the highly concentrated electrolytes. Li⁺ exchanges ligands (sulfone and BF₄^-) rapidly to produce unusual hopping conduction in highly concentrated electrolytes. The structure of the solvent significantly influences the hopping conduction process. We measured the self-diffusion coefficients of Li⁺ (D_Li), anions (D_anion), and sulfone solvents (D_sol) in electrolytes. The ratio of the self-diffusion coefficients (D_Li/D_sol) tended to be higher for cyclic sulfones (sulfolane and 3-methylsulfolane) than for acyclic sulfones, which suggests that cyclic sulfone molecules facilitate Li-ion hopping. The hopping conduction increases the Li⁺-transference number ($$) under anion-blocking conditions, and $$ of [LiBF₄]/[cyclic sulfone] = 1/2 is as high as 0.8.