Highly Concentrated NaN(SO₂F)₂/3-Methylsulfolane Electrolyte Solution Showing High Na-Ion Transference Number under Anion-Blocking Conditions

Abstract

The performance of a sodium-ion (Na) battery is significantly influenced by its electrolyte characteristics. In particular, the transport properties of the electrolyte have considerable effects on the discharge rate capability. During discharging of a Na battery at high current densities, a concentration gradient of Na salt develops because both cations and anions are mobile in the liquid electrolyte. Concentration polarization can be suppressed by increasing the Na⁺ transference number (t_Na+) of the electrolyte. This study demonstrates that highly concentrated NaN(SO₂F)₂ dissolved in 3-methylsulfolane (MSL) exhibits a high t_Na+ value of >0.6 under anion-blocking conditions. Raman spectroscopy revealed that Na⁺ ions formed complexes with MSL and anions in the electrolyte. Na⁺ ions exchange ligands dynamically and move faster than the ligands, resulting in a high t_Na+. The high t_Na+ enables a high-rate discharge of the Na battery, despite the low ionic conductivity of the highly concentrated electrolyte.