Electrorefining reaction of sodium in sodium-bis(trifluoremethylsulfonyl)imide and tetraethylammonium-bis(trifluoremethylsulfonyl)imide mixture ionic liquid

To develop a electrorefining process of sodium for the recycling of used sodium–sulfur secondary battery, a non-aqueous electrolytic melt was investigated as a candidate for the process. A mixed ionic liquid of NaTFSI (sodium-bis(trifluoremethylsulfonyl)imide)–TEATFSI (tetraethylammonium-bis(trifluoremethylsulfonyl)imide) was selected for the electrolyte, since it has a wide potential window and a weak reactivity with metallic sodium. From AC impedance measurements, the maximum electric conductivity of 36 mS cm−1 was found for a concentration of 20 mol% NaTFSI–TEATFSI at 433 K. The cathodic deposition of liquid sodium, 99.99% pure, was obtained on a glassy carbon electrode by constant current electrolysis. The calcium concentration in the sodium was decreased from 500 to 12 ppm by the electrorefining. A cathodic current efficiency at 88% was achieved in these electrorefining experiments.