Plating and stripping of sodium from a room temperature 1-methyl-3-propylimidazolium chloride melt

Room temperature molten salts consisting of 1-methyl-3-propylimidazolium chloride and aluminum chloride have been examined as possible electrolytes for a room temperature design of the sodium/metal chloride battery; however, the coulombic efficiency of the sodium couple is less than 95%. This work examines the reduction and oxidation efficiency of the sodium couple from a 1-methyl-3-propylimidazolium chloride/aluminum chloride neutral melt. Most of the work was performed on a tungsten substrate using cyclic voltammetry. The coulombic efficiency of the sodium couple was improved by treating the melt with gaseous HCl using a closed electrochemical cell which allowed for quantification of the effect of HCl on the electrochemical behavior of sodium. Thionyl chloride was also found to induce sodium plating and stripping in 1-methyl-3-propylimidazolium chloride/aluminum chloride melts. Optical microscopy was used to examine the surface of the tungsten electrode during sodium deposition, open-circuit periods, and sodium stripping. In comparison to the stability of sodium in two other imidazolium melts, (1,2-dimethyl-3-propylimidazolium chloride and 1-methyl-2-ethylimidazolium chloride) the 1-methyl-3-propylimidazolium chloride system was found to have the widest stability window.