Reversible Plating and Stripping of Sodium at Inert Electrodes in Room Temperature Chloroaluminate Molten Salts

Abstract

Sodium has been suggested as a possible anode in high energy-density batteries using room temperature chloroaluminate molten salt electrolytes, but it cannot be used directly in typical melts because the reduction of Na § falls beyond the negative voltage limit. When a neutral melt of 1-methyl-3-ethylimidazolium chloride and aluminum chloride (MEIC/AICI~) is buffered with NaC1, and excess protons (1-methyl-3-ethylimidazolium chloride/HC1) are added, the negative voltage limit is extended to -2.4 V (vs. an Al/N = 0.6 melt reference electrode) and the reversible plating and stripping of sodium is observed. Compositional data from scanning electron microscopy and energy dispersive spectra (SEM/EDS) verify that a layer of sodium is deposited on the surface of Pt and W electrodes. MEI § appears to be reduced at nearly the same potential as sodium and probably forms a protective layer on top of the sodium. These plated species are fairly stable on the electrode surface as judged by a constant rest potential over several hours, but can easily be stripped offthe electrode by a positive potential scan. These characteristics suggest that sodium is a good candidate for the anode in a rechargeable battery.The development of high energy-density batteries is important for applications in space, for electric vehicles, and for other uses where weight must be minimized. One approach is to combine the use of room temperature molten salt electrolytes and electrodes with high standard potentials. Whereas conventional batteries use aqueous salt solutions for the conducting medium, molten salts provide an ionic medium for conduction without the weight and reactivity of water. Until recently, the use of molten salt electrolytes required high temperatures (often 200 to 400~ to form the melts and keep them liquid. Now room temperature molten salts are being evaluated as possible electrolytes for such applications. One type of chloroaluminate melt is made by mixing the organic salt, 1-methyl-3-ethylimidazolium chloride (MEIC) with aluminum chloride (1). Such MEIC/AIC13 melts are stable liquids at and below room temperature over a range of stoichiometries. It has recently been shown that when Lewis neutral melts are buffered by adding an alkali metal salt, such as NaC1 or LiC1, a voltage window of greater than 4 V can be achieved (2). The buffering action prevents Lewis acids or bases from producing A12CI{ or Cl-, respectively, which would significantly decrease the voltage window of the melt.Given the availability of a wide voltage window, our work is now focusing on identifying active metal electrodes for use in such systems to produce high voltages. Because of their high standard potentials and low atomic weights the alkali metals are attractive as possible battery anodes. Also, their reactivity with water is not a concern because of the nonaqueous environment of the melts. Several groups have studied the reduction of Li § Na § or K + in MEIC/A1C13 melts. Lipsztajn and Osteryoung (3) observed the plating and stripping ofLi § on tungsten...

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“…The use of ionic liquids as a substitute for aqueous electrolytes is thus of interest and worthy of study. Electrodeposition of Al on copper, platinum and tungsten, involving chloroaluminate ionic liquids, has been reported by several investigators [7][8][9][10]. The co-deposition of Al with other metallic components has also been explored [11][12][13].…”

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