Recently, high purity metallic Na is used in both large scale secondary batteries and in the flux for crystal growth of gallium nitride (GaN), increasing the demand for high purity Na. We have studied the electrorefining process using hydrophobic ionic liquids1,2). In electrorefining, Ca and K in metallic Na dissolve into the electrolyte, and there is preferential reduction reaction of Na ions in the electrolyte. Further, we have identified an exchange reaction between Ca or K in metallic Na and Na ions in the electrolyte both before and after the electrorefining. In this study, the effect of exchange reaction is quantitatively investigated. Accumulation of Ca ions and K ions in the electrolyte may result in electrodeposition with Na. In this study, we control the simulated conditions of the accumulation of Ca and K in the ionic liquid by repetition of the exchange reaction, and evaluate the purity of the generated metallic Na.
Experimental
Two solid powder, NaTFSI (sodium-bis(trifluoromethanesulfonyl)imide) and TEATFSI (tetraethylammonium-bis(trifluoromethanesulfonyl)imide) were mixed at a molar ratio of 1 : 4 in an Ar filled glove box. In the exchange reaction experiment, metallic Na (99.95 %, Ca:340 ppm, K:120 ppm, Aldrich) was floated on the ionic liquid mixture, at temperatures of 120 or 160 °C, and with a maximum reaction time of 720 min. After the experiments, a small amount of metallic Na was sampled and dissolved in pure water. The concentrations of Ca and K in the metallic Na were analyzed by ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectroscopy).
For accumulating impurities in the ionic liquid, 150 g of NaTFSI - TEATFSI (1:4) mixed ionic liquid melted at 160 °C was used, and 10 g of metallic Na (99.95 %, Ca : 330 ppm, K : 220 ppm, Aldrich) was floated on this ionic liquid and Ca and K were introduced into the ionic liquid by maintaining the exchange reaction for 60 min. A part of the floated metallic Na and ionic liquid were sampled after the exchange reaction. The samples were dissolved in pure water or ethanol. Further, 10 g of other new metallic Na was replaced in the previously used metallic Na on the same ionic liquid. The Ca and K contents in the ionic liquid were increased by repeating the experiment of 20 times. Concentrations of Ca and K in the prepared 20 aqueous solutions were analyzed by ICP-AES.
Results and discussion
The relationship between the reaction time and the Ca or K concentrations at 120 °C using the NaTFSI-TEATFSI mixed ionic liquid was investigated. The Ca and K concentrations rapidly decrease up to about 120 min of exchange reaction and thereafter decrease more gradually. A similar behavior was also observed in the experiment at 160 °C, however the duration of the rapid decrease was shorter than at 120 °C. Further, the Ca and K concentrations in the metallic Na obtained at 160 °C by the exchange reaction at 720 min were lower than those at 120 °C. The minimum concentration in the purifyed Na was 4 ppm for Ca and 11 ppm for K. The reason is that the reaction rate increases at the higher temperature, as well as that the ions move more easily when increasing the electric conductivity of the ionic liquid, and the convection becomes faster as the viscosity decreases. Therefore, it is possible to remove more Ca and K by conducting the exchange reaction at 160 °C for a longer period, and metallic Na with an initial purity of 99.95 % can be purified to 99.99 %.
After repeating the exchange reaction 20 times, the concentration of Ca in the ionic liquid was close to 0 ppm, and K was accumulated at a concentration lower than the expected accumulated concentration. Precipitate containing Ca and K was identified in the ionic liquid after the experiments. Since the concentrations of Ca and K in all metallic Na after the exchange reaction became lower than the initial concentration, it may be assumed that the Ca and K components dissolved into the ionic liquid from the metallic Na and may have formed this precipitate. At a reaction time of 480 min, 99.99 % of metallic Na could be purified even for the ionic liquid after 20 exchange reaction cycles.
References
1) Mikito Ueda, Kazuya Honda, Toshiaki Ohtsuka, Electrochimica Acta 100, 265 (2013).
2) Mikito Ueda, Ryuichi Inaba, Hisayoshi Matsushima, Toshiaki Ohtsuka, ECS Electrochemistry Letters, 4, E1 (2015).