Ionic Liquid - Glyme Mixtures to Modify Solvation Chemistry, Electrochemical and Physiochemical Properties in Lithium Containing Electrolytes

Abstract

Pyrrolidinium-based ionic liquids are an intriguing material for lithium-based battery electrolytes due to their inherit non-flammability and large electrochemical windows. Poor lithium-ion transport in ionic liquid-based electrolytes hinder the effectiveness of these electrolytes. Solvate ionic liquids were introduced as a subclass of ionic liquids consisting of high concentrations of lithium salts and glymes. For example, the solvate ionic liquid Li(G4)TFSI is an equimolar ratio of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and tetraethylene glycol dimethyl ether (G4). In this work, pyrrolidinium-based ionic liquids containing short polyether side chains, structurally analogous to glymes, are mixed with equimolar amounts of LiTFSI and varying portions of G4. These tertiary mixtures were evaluated based on the ratio of solvating oxygen to lithium ion ([O]/[Li+]) present in the mixture. Trends in the oxidative stability, conductivity and lithium transference number were evaluated from mixtures of [O]/[Li+] between 5 to 8. Oxidative stability of tertiary mixtures with [O]/[Li+] = 5 showed improved oxidative stability compared to the binary Li(G4)TFSI, while [O]/[Li+] > 5 had diminished oxidative stability. DSC thermal analysis between -85°C to +120°C showed the tertiary mixtures helped suppress the glass transition temperature of Li(G4)TFSI to lower temperatures. Changes in the solvation structures were evaluate using spectroscopic analysis, including Raman spectroscopy. The changes in the solvation chemistry were correlated to the physiochemical and electrochemical properties.