Electrodeposition and stripping of lithium and sodium on inert electrodes in room temperature chloroaluminate molten salts

The demonstration of a stable, reversible alkali metal anode is an important step in the development of practical secondary batteries using room temperature chloroaluminate molten salts as electrolytes. Such melts are made by mixing 1-ethyl-3-methylimidazolium chloride (EMICl) with aluminum chloride, and can be Lewis buffered by adding LiCl or NaCl. It has been shown previously that protons added to a sodium chloride buffered melt as 1-ethyl-3-methylimidazolium hydrogen dichloride (ENTIHCl(2)) provide a more negative voltage window and nearly reversible deposition-stripping behavior for sodium. While EMIHCl(2) is a proven proton source and also enhances lithium deposition and stripping, its stability in the melt is limited by the rapid loss of HCl(g). We report here that triethanolamine hydrogen chloride is effective in widening the voltage window, allows the plating and stripping of both lithium and sodium, and is stable in buffered EMICl/AlCl3 melts for months. A study of various potential proton donors suggests that deprotonation of one ethanolic group of triethanolamine . HCl is responsible for the effect. A few other reagents also give this effect, but react more slowly or are more difficult to handle. Plated sodium appears to be more stable in this melt system than lithium.