High Transference Number of Na Ion in Liquid-State Sulfolane Solvates of Sodium Bis(fluorosulfonyl)amide

The phase behavior, Na+ coordination structures, and transport and electrochemical properties of binary mixtures of NaN­(SO2F)2 (NaFSA) and sulfolane (SL) solvent were investigated. The NaFSA and SL form stable solvates at molar ratios [NaFSA]/[SL] = 1/3 and 1/0.5, and there is a crystallinity gap in the range 1/1.6 ≤ [NaFSA]/[SL] ≤ 1/0.8. In the crystals of both NaFSA–(SL)3 and NaFSA–(SL)0.5, the solvent-bridged Na+–SL–Na+ and anion-bridged Na+–FSA–Na+ structures are formed. In the crystallinity gap, the mixtures remain liquid at room temperature and can be regarded as molten SL solvates of NaFSA. The Raman spectra of the molten solvates suggest that the solvent- and anion-bridged structures are partially maintained even in the liquid. However, Na+ ions exchange the ligands (solvent and anion) dynamically in the liquids. This significantly affects the Na+ ion transport in the liquids under an anion-blocking condition. The Na+ ion transference numbers (tNa+) of molten solvates were estimated using symmetric Na/Na cells, and a molten solvate of [NaFSA]/[SL] = 1/1 exhibited a high tNa+ of 0.8. Charge and discharge tests of a Na/Na0.44MnO2 battery were performed with the molten solvate as the electrolyte. The rate capability of the battery was found to be significantly affected by tNa+ in the electrolyte.