Low‐Temperature Molten Salt Electrolytes Based on Aralkyl Quaternary or Ternary Onium Salts

Abstract

A family of commercially available, fully substituted onium ions has been found to form low‐temperature molten salt electrolytes when combined with aluminum chloride. The key chemical feature of the group is that at least one of the substituents is an aromatic radical like phenyl or benzyl. The other substituents are preferably small aliphatic groups like methyl or ethyl. The central atom of the onium ion can be nitrogen, phosphorus, or sulfur. The onium salt studied most was trimethylphenylammonium chloride (TMPAC) which was found to have electrochemical stability as good as 1‐methyl‐3‐ethylimidazolium chloride (MEIC). The conductivity is comparable to N‐(n‐butyl) pyridinium chloride (BPC), but not as good as MEIC. Aluminum metal was found to cycle reversibly in the 67 mole percent (m/o) solution. A partial phase diagram for this system was also determined and showed a very low melting point at of −75°C. The resistivities of the various onium ion solutions correlate approximately with the size of the onium ion, while the minimum temperature occurs in the vicinity of 67 m/o for all combinations. These results indicate that anion formation reactions are similar for all of these solutions, including MEIC and BPC.