Phase Behavior and Thermal Properties of Ternary Ionic Liquid-Lithium Salt (IL-IL-LiX) Electrolytes

The phase behavior and thermal properties of ternary ionic liquid lithium salt (IL-IL-LiX) mixtures intended for use as electrolytes for lithium batteries are reported here. It is shown that the addition of small amounts of N-methyl-N-propylpyrrolidinium bis(fluorosulfonyl)imide (PY13FSI) to N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PY14TFSI)-lithium salt (LiTFSI or LiPF6) mixtures greatly hinders the ability of the samples to crystallize. This results in a much improved subambient temperature conductivity for the mixtures relative to the binary IL-LiX electrolytes at low temperature. The thermal stability of the mixtures is reduced by the addition of PY13FSI but remains acceptable for Li battery applications.