The effect of the coexistence of anion species in imidazolium cation-based molten salt systems

Wider application of imidazolium cation-based room temperature molten salts in batteries and electrochemical capacitors will require an improvement of their cathodic and anodic stability. In this study, the authors found that a certain combination of anions can change the anodic stability and other properties of molten salt systems. A 1:1 (in moles) mixture of 1-ethyl-3-methyl imidazolium (EMI) tetrafluoroborate and EMI bis(tetrafluoromethanesulfonyl)imide (TFSI) exhibited an anodic stability similar to that of EMITFSI, the most anodically stable single-anion molten salt among the molten salts used in this study. This anion also altered other characteristics of the salt, such as ionic conductivity and thermal stability, compared to those seen by a simple combination of each of these single-anion salts. In contrast, coexistence of the TFSI anion and trifluoromethanesulfonyl (triflate) anion provided no significant change in any characteristics of the molten salt described above. The ¹H-NMR study of these molten salts revealed that the coexistence of tetrafluoroborate anions and TFSI anions lowered the degree of hydrogen bonding from that in each individual molten salt, while the coexistence of the TFSI anion and triflate anion provided no changes in hydrogen bonding. The coexistence of anions, such as tetrafluoroborate and TFSI, is expected to change the manner of cation-anion association, which inhibits hydrogen bonding. Based on these results, the authors suggest the possibility of controlling the electrochemical stability of a molten salt by varying the manner of its cation-anion association.