Comparison of LiTDI and LiPDI salts and influence of their perfluoroalkyl side-chain on association and electrochemical properties in triglyme

Abstract

For the first time, the effect of minor structural changes to the electrolyte salt on solution properties is investigated experimentally. It was achieved by decomposition of the overall changes into individual components. It allowed to obtain information on the contradicting effects influence on the final result. This study is focused on comparison of two lithium salts: lithium 4,5-dicyano-2-(trifluoromethyl) imidazolide (LiTDI) and lithium 4,5-dicyano-2-(pentafluoroethyl) imidazolide (LiPDI). LiTDI is a very promising salt for lithium-ion battery application. PDI− anion differs from TDI− only in length of perfluorinated alkyl chain. Triethylene glycol dimethyl ether (triglyme) solutions of both salts in a wide range of concentrations were prepared. Triglyme was chosen as a solvent due to number of oxygen atoms which allows for fulfilment of lithium cation coordination sphere. Use of such similar salts in a model system allows us to find the correlation between salt structure and properties of electrolyte. Conductivity, viscosity, lithium transference number, thermal properties and FTIR spectra were measured for all solutions. Ionic fractions were also estimated by Fuoss-Kraus formalism. Obtained results showed that electrochemical properties of electrolyte are result of several opposing factors. Transference numbers are mostly dependent on association. We have also observed interesting correlation between thermal properties and conductivity.