Electrorefining of Sodium in Sodium Bis(trifluoromethane)sulfonylimide and Tetrabutylammonium Bis(trifluoromethane)sulfonylimide Mixture Ionic Liquids for Metallic Sodium Recycling

Abstract

Metallic sodium is presently used as an intermediate (Sodium Alcoholate) in agricultural chemicals, 1 PCB decomposing agents (Sodium Dispersion), 2,3 and sodium-sulfur secondary batteries. 4,5 Presently, the sodium production has carried out only at a few countries in the world. Therefore, development of a process to circulate metallic sodium is highly desirable not only from resources recycling considerations. A process for electrowinning of sodium (Downs process) 6,7 produces metallic Na and Cl2 gas from NaCl-CaCl2-BaCl2 molten salts. The voltage of the electrolysis increases to exceed the decomposition voltage of NaCl during the electrolysis, and the electrical power consumption is known to be about 11000 kWh/t. In electrorefining to produce highly pure sodium from sodium containing impurities, the decomposition voltage is theoretically zero, and it may be assumed that the electrolysis voltage is not high. As a result it may be expected that the electric power consumption of the electrorefining process becomes less than the electrowinning process. However, no electrorefining process for sodium has been implemented on an industrial scale. In sodium-sulfur batteries where much sodium is contained, a large amount of metallic sodium remains in the batteries also in the used state. The sodium of about 400 kg is used for production of the sodium-sulfur batteries in every year. If metallic sodium is collected fromusedsodium-sulfurbattery, electrorefining ofthe sodiummaybe carried out and resources of high purity sodium could be secured. And we believe that the development of electrorefining process becomes valuable technology in fields of high purity metal production. We have proposed a sodium recycling process which involves collection of the metallic sodium from used sodium-sulfur batteries and refining of the collected metallic sodium. 8‐10 The electrorefining process of the metallic sodium from used Na-S batteries developed by us investigated organic solvents, molten salts, and ionic liquids as the electrolyte. From the results, it was found that an ionic liquid mixture of NaTFSI (sodium bis(trifluoromethane)sulfonylimide) -TBATFSI (tetrabuthylammonium bis (trifluoromethane)sulfonylimide) has a wide electrochemical potential window and that it displays low reactivity with molten metallic sodium below 473 K. This paper reports the melting point of the investigated ionic liquid mixture, its conductance, voltammogram, and the electrorefining reaction with metallic sodium by constant current electrolysis in the NaTFSI-TBATFSI ionic liquid.