(Invited) Electrorefining of Na in Hydrophobic Ionic Liquids

Introduction    
    The sodium-sulfur secondary battery (Na-S battery) has several advantages such as high energy density, low material cost, long lifetime, and maintenance free^1-2) and commercial scale of the battery had sold since 2003. ³⁾
    Recently, the battery has been applied to load leveling of electric power, emergency power supply for interruption in factory, and storage device of power generation from solar or wind energy. 
　Lifetime of the battery is about 15 years and large amount of the used batteries will be discarded in near future. In the used battery, metallic sodium and sodium polysulfide remain in inside of the battery. Since metallic sodium is an active metal, effective recycle of metallic sodium is required.  We have proposed electrorefining process of metallic sodium from the used Na-S battery.⁴⁾ We investigated propylene carbonate as an electrolytic melt in previous study. From the results, it was found that temperature of upper limit in propylene carbonate was 373 K. In this paper, to survey the electrolytic melt that can be used above 373 K, impedance measurement, voltammogram measurement, and constant current electrolysis were carried out in NaTFSI-TEATFSI ionic liquids and NaTFSI-TBATFSI ionic liquids
Experimental
　NaTFSI (sodium-bis(trifluoremethylsulfonyl)imide) was prepared by HTFSI (bis(trifluoremethylsulfonyl) imide, 99% pure, Kanto chemical) and NaOH(99.99% pure, Merk). TEATFSI (Tetraethylammonium bis (trifluoremethylsulfonyl) imide, 99% pure, Kishida chemical) and TBATFSI (Tetrabuthylammonium bis (trifluoremethylsulfonyl) imide, 97% pure, Iolitech) were used as received.  Electrochemical cell was made of Pyrex glass cylinder. 10-30mol%NaTFSI-TEATFSI or 10-30mol%NaTFSI-TBATFSImixture melt of 30 ml were kept at 433 K. Glassy carbon (GC, Tokai carbon, GC-20) plates were used for working and counter electrode. Pt (99.99% pure, Tanaka Kikinzoku Kogyo) wire was used as a quasi reference electrode. Impedance measurement was carried out with amplitude of 15 mV and frequency from 1 Hz to 20 kHz. Electrical conductivity was calculated from the resistance value at a high frequency limit in impedance measurement.
    In voltammogram measurement, metallic sodium (Aldrich, 99.9% pure, calcium content : 500 ppm) was used as a counter electrode. The electrochemical measurement was carried out in a glove box under pure argon gas atmosphere.
   Electrolysis was done by constant current of 1-10 mAcm^-2in 20 mol% NaTFSI-TEATFSI melt or 20 mol% NaTFSI-TBATFSI melt and the electrodeposit was analyzed by ICP atomic emission spectroscopy (Thermo i-CAP6000). 

Results and Discussion
    The conductivity of electrolytic melts were determined by AC impedance measurement in 10 - 30 mol% NaTFSI-TEATFSI. The maximum conductivity was attained to 36 mScm^-1 at 20 mol% NaTFSI-TEATFSI. On the other hand, maximum conductivity was 16 mScm^-1 at 20 mol% NaTFSI-TBATFSI. From the result, we selected 20 mol% NaTFSI melt was a suitable concentration and the following experiment was made in the melt.
     In voltammogram on GC electrode in 20 mol% NaTFSI-TEATFSI, a cathodic current was observed from -1.0 V and the current increased until -3.0 V. When the potential is reversed at -3.0 V, anodic current is observed from -2.2 to 0.5 V. It is considered that the cathodic and anodic currents correspond to electrodeposition and electrodissolution of sodium.
     From ICP analysis, the electrodeposit formed by constant current electrolysis was found to contain sodium of 99.99 % pure and 12 ppm calcium as impurity. The calcium content was seen to be decreased to about one fortieth by the electrorefining process. 

References
1) J. L. Sudworth and A. R. Tilley, The Sodium Sulfur Battery, Chapman and Hall, (1985).
2) R. Okuyama, H. Nakashima, T. Sano, and E. Nomura, J. Power Sources, 93, 50 (2001).
3) T. Oshima, M. Kajita, A. Okuno, Int. J. Appl. Ceram. Technol., 1, 269(2004).
4) M. Ueda, H. Hayashi, and T. Ohtsuka, J. Japan Inst. Metals, 73, 691(2009).

Acknowledgements

    We thank New Energy and Industrial Technology Development Organization (NEDO) and Hokkaido Bureau of Economy, Trade and Industry for supporting this research.