Non-flammable electrolyte mixtures of ringed ammonium-based ionic liquids and ethylene carbonate for high voltage Li-ion batteries

Three ionic liquids based on ringed ammonium cations with different ring sizes [seven: azepanium; six: piperidinium; five: pyrrolidinium] and imide anion mixed with ethylene carbonate, EC, have been evaluated as electrolytes in high voltage lithium-ion batteries. It is found that mixing the ionic liquids with ethylene carbonate gives electrolyte mixtures with lower viscosities, increased conductivities and improved electrochemical cathodic stability than the neat ionic liquids. The flammability and thermal stability of the electrolytes have been studied by open-cup flash point measurements and thermogravimetric analysis combined with Infrared and Mass Spectroscopy. It is found that the neat ionic liquids have good thermal stability above 300\ub0C and no flammability compared to carbonate-based electrolyte but upon mixing with EC the mixture decomposes at temperature below 130\ub0C but stays non-flammable. The electrolyte mixtures have been evaluated in Li/LiMn\u2081.\u2085Ni\u2080.\u2085O\u2084 (LMNO) and graphite/Li half cells and in the presence of fluoroethylene carbonate additive. Discharge capacities reaching 112\u2013117 mAh.g\u207b\ub9 and 280\u2013293 mAh.g\u207b\ub9 with coulombic efficiencies of 95% and 99% were obtained for graphite and LMNO, respectively, over 100 cycles at C/12.Peer reviewed: YesNRC publication: Ye