Molten Triazolium Chloride Systems as New Aluminum Battery Electrolytes

The possibility of using molten mixtures of 1,4-dimethyl-l,2,4-triazolium chloride (DMTC) and aluminum chloride (A1C13) as secondary battery electrolytes was studied, in some cases extended by the copresence of sodium chloride. DMTC-A1CI~ mixtures demonstrated high specific conductivity in a wide temperature range. The equimolar system is most conductive and has K values between 4.02 x 10 -~ and 7.78 • 10 -2 S cm -~ in the range from -31 to 123~ respectively. The electrochemical window of DMTC-containing sodium tetrachloroaluminate melts varied in the region of 2.5 to 2.2 V (150-170~ depending on melt acidity and anode material. DMTC, being specifically adsorbed and reduced on the tungsten electrode surface, had an inhibiting effect on the aluminum reduction, but this effect was suppressed on the aluminum substrate. An electrochemical process with high current density (tens of milliamperes per square centimeter) was observed at 0.344 V on the acidic sodium tetrachloroahiminate background, involving a free triazolium radical mechanism. Molten DMTC-A1C]3 electrolytes are acceptable for battery performance and both the aluminum anode and the triazolium electrolyte can be used as active materials in the acidic DMTC-A1C13 mixtures. The development of high energy density secondary batteries, with aluminum anode as the most attractive alternative to li thium and sodium, is an important subject of research. It can be justified by low price, chemical stability, high theoretical capacity, and high energy density of the aluminum. Depending on temperature range at least two battery systems are now being developed: a moderate temperature system with sodium chloride-aluminum chloride 1-~ or 1butylpyridinium chloride-aluminum chloride 1~ molten mixtures as electrolytes, and a room temperature system with molten mixtures of a luminum chloride and organic quaternary salts, usually 1-methyl-3-ethylimidazolium chloride (MEIC) 13-17 as electrolytes, the latter system being more attractive because of the wider temperature range. Despite satisfactory cycling efficiency of the aluminum anode in the MEIC electrolytes these batteries have the following disadvantages: 16'17 (i) low allowed current densities (<1 mA cm-2); (if) necessity of an ion-exchange membrane because of different composition of the anolyte and catholyte; and (iii) limitations in choice of cathodic materials. Several attempts to extend electrochemical windows of the room-temperature molten electrolytes have been undertaken: 18-23 a molten MEIC-aluminum chloride mixture of nearly neutral composition obtained by NaC1 addition unti l saturation and further a 1.5:1 molten mixture of A1C13-1,2-dimethyl-3-propylimidazolium chloride have been used as modified electrolytes) 8'~9 In the first case sodium can be used as anode instead of aluminum (anodic extending) and in the second case a chlorine cathode has been realized, based on an intercalation process in graphite (cathodic extending). Unfortunately, good cycling behavior has not been found for the sodium anode because of a passivating layer.18 Concerning the chlorine electrode, chlorine storage capacity of graphite has not been sufficient for a suitable energy density, 19 probably because of the absence of a chlorine intercalation into graphite. Recently, aluminum-polyanil ine secondary batteries have been developed 2~ using molten acidic MEIC-A1C13 or butylpyridinium chloride-AiC13 mixtures as electrolytes. However, Giinter et al. have determined slight dissolution of polyaniline in the acidic melts and subsequent degradation of the cathodes after 20-30 cycles. 23 Therefore the development of new electrolyte systems for room-temperature aluminum secondary batteries is still an important field of investigation. It was natural after imida* Electrochemical Society Active Member. 3108 zole to study a related compound in the group of five-membered azo-heterocycles, i.e., triazole. Here, we report the conductivity and voltammetry of DMTC-A1C13 (Fig. 1) as possible secondary battery electrolytes. For the voltammetric measurements the idea of Matsunaga et al. 2~ has been used: the electrochemical behavior of DMTC-A1C13 systems in both acidic and basic regions has been studied in a diluted state in sodium tetraehloroahiminate melts. In this way, all the processes due to DMTC became more pronounced. Experimental 1-Methyl1, 2, 4-triazole, synthesized and purified as described in Ref. 25 was dissolved in dry nitromethane (200 cm3/mol). One equivalent of trimethyloxonium tetrafluoroborate 26 was added and the mixture stirred for 2 h. Fourfold dilution with dry ether, decantation, and washing with dry ether gave 93% of 1,4-dimethyl-l,2,4-triazolium tetrafluoroborate. This compound was dissolved in boiling water (0.5 cm~/g) and 1 eq. of aqueous 5M KC1 was added. Cooling to 0~ removal of the separated KBF4 by filtration, evaporation to dryness, recrystallization from ethanolether, and drying at 0.1 mm Hg over P20~ (to remove any traces of water) gave 86% of DMTC, mp 103-104~ The preparation of distilled aluminum chloride and dried NaC1 has been described previously. 27 All sample preparation and handling took place in an argon-filled glove box (Vacuum Atmospheres Inc.), with oxygen concentration 10 ppm (monitored with a Dansensor System A/S unit).