Comparison of trifluoromethyl- and pentafluoroethyl-substituted dicyanoimidazolide lithium salts–their association and electrochemical properties in oligo(ethylene glycols) of different chain length

Abstract

In this paper, the effect of the length of solvent molecules on the properties of electrolytes was investigated. It was investigated in the model electrolytes consisting of either lithium 4,5-dicyano-2-(trifluoromethyl)imidazolide (LiTDI) or 4,5-dicyano-2-(pentafluoroethyl)imidazolide (LiPDI) dissolved in diglyme and tetraglyme. Such systems were chosen for several reasons. LiTDI is a promising salt for electrolytes for lithium-ion batteries, while LiPDI has a very similar structure, allowing for an assessment of the impact of a difference on electrolyte properties. The solvents were chosen on the basis of the number of oxygen atoms, which for diglyme is insufficient to fulfil lithium’s coordination sphere and is more than enough for this purpose for tetraglyme. This work is a continuation of the study for triglyme. In this work, conductivity, viscosity, lithium transference number, thermal properties and degree of ionic association (as well as solvent-ion interactions) were measured to determine the impact of individual factors. The ionic association’s degree was obtained from FTIR measurements of the samples as well as estimated with Fuoss-Kraus formalism. The conductivity of the samples for those salts depends mostly on the solvent viscosity, while the position of conductivity maxima depends mostly on the length of the solvent molecule. Transference numbers are more dependent on the ionic association and, thus, salt being used.