Solvation Structure and Li-Ion Transport Properties of Highly Concentrated Sulfone-Based Electrolytes

Abstract

Highly concentrated liquid electrolytes have attracted attention as electrolyte materials for high-performance batteries due to its superior performance.1 , 2 Recently, our group reported that Li+ ion diffuses faster than solvent molecules and anions in highly concentrated sulfolane (SL)-based liquid electrolytes.3 This suggests that Li+ ion hopping conduction occurs in the highly concentrated electrolytes. In the highly concentrated electrolytes, a solvent bridged structure of Li+-SL-Li+, where two oxygen atoms of sulfone group coordinate to two different Li+ ions, is formed. Li+ ions exchange the ligands (solvent and anion) dynamically in the liquids, and the Li+ ion hopping conduction is assumed to occur through the ligand exchange. We report here the phase behaviors, solvate structures, and transport properties of highly concentrated electrolytes composed of LiBF4 and sulfone solvents, such as ethyl methyl sulfone (EMS) and 3-methyl sulfolane (MSL). LiBF4 and SL form a stable solvate at a molar ratio of 1:1.3 On the other hand, the LiBF4/EMS and LiBF4/MSL tend to be glass-forming liquids when the mole fraction of LiBF4 is higher than 0.25 in the mixtures. The solvation structures in the liquids were elucidated using Raman spectroscopy. Raman spectra suggested that solvent-bridged structure of Li+-solvent-Li+ is formed in the highly concentrated electrolytes. The self-diffusion coefficients of species in the electrolytes were measured using pulsed field gradient NMR. The self-diffusion coefficient of each species decreases with increasing the LiBF4 concentration due to the increase in viscosity of the liquid. The self-diffusion coefficient ratios of Li+/solvent (D Li/D sol) and Li+/BF4 − (D Li/D anion) in the electrolytes are shown in Figure 1. The values of D Li/D sol and D Li/D anion increase with increasing Li salt concentration and become higher than 1 for all the sulfone-based electrolytes studied. Similar to the SL-based concentrated electrolytes, Li+ ion diffuses faster than solvent and anion, suggesting that Li+ hopping conduction emerges commonly in these sulfone-based highly concentrated electrolytes. Of course, in addition to the structure of solvent, anion species affects the Li+ ion transport in the electrolytes. The fundamental properties of sulfone-based electrolytes with various Li salts will be also reported. Reference Yamada, Y. et al. Unusual Stability of Acetonitrile-Based Superconcentrated Electrolytes for Fast-Charging Lithium-Ion Batteries. J. Am. Chem. Soc. 136, 5039–5046 (2014).McOwen, D. W. et al. Concentrated electrolytes: Decrypting electrolyte properties and reassessing Al corrosion mechanisms. Energy Environ. Sci. 7, 416–426 (2014).Dokko, K. et al. Direct Evidence for Li Ion Hopping Conduction in Highly Concentrated Sulfolane-Based Liquid Electrolytes. J. Phys. Chem. B 122, 10736–10745 (2018). Figure capotion Figure 1. Self-diffusion coefficient ratios of (a) Li+ ion and solvent (b) Li+ ion and anion of binary mixtures composed of LiBF4 and the sulfones． Figure 1