High-concentration LiPF₆/sulfone electrolytes: structure, transport properties, and battery application

Non-flammable and oxidatively stable sulfones are promising electrolyte solvents for thermally stable high-voltage Li batteries. In addition, sulfolane-based high-concentration electrolytes (HCEs) show high Li⁺ ion transference numbers. However, LiPF₆ has not yet been investigated as the main salt in sulfone-based HCEs for Li batteries. In this study, we investigated the phase behaviors, solvate structures, and transport properties of binary and ternary mixtures of LiPF₆ and the following sulfone solvents: sulfolane (SL), dimethyl sulfone (DMS), ethyl methyl sulfone (EMS), and 3-methyl sulfolane (MSL). The stable crystalline solvates Li(SL)₄PF₆ and Li(DMS)_2.5PF₆ with high melting points were formed in the LiPF₆/SL and LiPF₆/DMS mixtures, respectively. In contrast, LiPF₆/EMS, LiPF₆/MSL, and LiPF₆/SL/another sulfone mixtures remained liquids over a wide temperature range. Raman spectroscopy revealed that SL and another sulfone are competitively coordinated to Li⁺ ions to dissociate LiPF₆ in the ternary mixtures. Although the ionic conductivity decreased with increasing LiPF₆ concentration due to an increase in viscosity, Li⁺ ions diffused faster than PF₆^-via exchanging ligands in the HCE [LiPF₆]/[SL]/[DMS] = 1/2/2, resulting in a higher Li ion transference number than that in conventional Li battery electrolytes.