ELECTROCHEMISTRY OF ROOM-TEMPERATURE CHLOROALUMINATE MOLTEN-SALTS AT GRAPHITIC AND NONGRAPHITIC ELECTRODES

The electrochemistry of unbuffered and buffered neutral AlCl3-EMIC-MCl (EMIC = 1-ethyl-3-methylimidazolium chloride and MCl = LiCl, NaCl or KCl) room-temperature molten salts was studied at graphitic and nongra phitic electrodes. In the case of the unbuffered 1:1 AlCl3:EMIC molten salt, the organic cation reductive intercalation at about -1.6V and t he AlCl4- anion oxidative intercalation at about +1.8 V were evaluated at porous graphite electrodes. It was determined that the instability of the organic cation in the graphite lattice limits the performance of a dual intercalating molten electrolyte (DIME) cell based on this e lectrolyte. In buffered neutral 1.1:1.0:0.1 AlCl3:EMIC:MCl (MCl = LiCl , NaCl and KCl) molten salts, the organic cation was intercalated into porous and nonporous graphite electrodes with similar cycling efficie ncies as the unbuffered 1:1 melt; however, additional nonintercalating processes were also found to occur between -1 and -1.6V in the LiCl a nd NaCl systems. A black electrodeposit, formed at -1.4V in the LiCl b uffered neutral melt, was analysed with X-ray photoelectron spectrosco py and X-ray diffraction and was found to be composed of LiCl, metalli c phases containing lithium and aluminium, and an alumina phase formed from reaction with the atmosphere. A similar film appears to form in the NaCl buffered neutral melt, but at a much slower rate. These films are believed to form by reduction of the AlCl4- anion, a process prom oted by decreasing the ionic radius of the alkali metal cation in the molten salt. The partially insulating films may limit the usefulness o f the LiCl and NaCl buffered neutral melts as electrolytes for recharg eable graphite intercalation anodes and may interfere with other elect rochemical processes occurring negative of -1V.