High Transference Number of Na Ion in Highly Concentrated Sodium Bis(fluorosulfonyl)Amide/γ-Butyrolactone Electrolytes for Sodium Batteries

摘要

We report here the liquid structures and physicochemical properties of highly concentrated solutions of NaN(SO_2F)_2 (NaFSA) dissolved in γ-butyrolactone (GBL) for Na-ion batteries. The solvation structure of Na~+ was analyzed using Raman spectroscopy. By adding NaFSA to GBL, the Raman band of ring stretching vibration (678 cm~(-1)) of GBL shifted to higher wavenumber because the GBL coordinates to Na~+ ion. The concentration dependence of Raman spectral change of GBL suggested the solvation number of Na~+ is 5 when the excess GBL molecules exist in the solution. As increasing the concentration of NaFSA, the bound GBL increases and free GBL decreases. At higher concentration of [NaFSA]/[GBL] > 1/2, free GBL does not exist and all of the GBL molecules coordinate to the Na~+ ion. Raman spectra of FSA anion suggested that the FSA anion also participates in the coordination of Na~+ in the highly concentrated solution. Apparently, the contact ion pairs and ionic aggregates are formed extensively in the highly concentrated solution. This gives significant effects on the Na~+ ion conduction mechanism in the liquid. We estimated the transference number of Na~+ ion in the electrolytes. The transference number of Na~+ in the liquid of [NaFSA]/[GBL] = 1/1 was as high as 0.7. This surprisingly high transference number is probably due to the peculiar conduction mechanism in the ionic aggregates. Na~+ ion is considered to exchange the anions dynamically in the ionic aggregates, and this may induce the hopping conduction in the liquid. Charge and discharge tests of a hard carbon electrode were conducted in the liquid of [NaFSA]/[GBL] = 1/1, and the reversible Na~+ insertion/extraction into/from the electrode were possible with a high coulombic efficiency. The SEI derived from the FSA anion might contribute to the reversible electrochemical reaction.