Room Temperature Ionic Liquid Electrolytes Based on Azepanium Imide Salts for Lithium Batteries

Ionic liquids based on the imide salts of the dialkyl azepanium cation, AZ (R-1, R-2) were synthesized, characterized, and evaluated as electrolytes in lithium batteries. Nine salts were synthesized with variation in the length of the two alkyl groups [R-1, R-2 = methyl (1), ethyl (2), propyl (3) and butyl (4)]. It was found that the asymmetric ones with R-2-R-1 >= 2 are liquids at ambient temperature. The three room-temperature ionic liquids (AZ13-TFSI, AZ14-TFSI, AZ24-TFSI) showed, in the neat form, ionic conductivities in the range of 0.1-0.6 mS/cm, viscosities of 260-350 cP and an electrochemical window of 5.5-6 V. It was found that mixing the ionic liquids with ethylene carbonate gave electrolytes with lower viscosities, increased conductivities and improved electrochemical cathodic stability. Electrolytes based on the ILs were evaluated in lithium half cells with different chemistries; Li/LiFePO4, Li/Li4Ti5O12, Li/LiCoO2; and in the case of AZ14-TFSI a Li/LiFePO4 cell, gave discharge capacities reaching 125 mAh.g(-1) over 100 cycles with 98% coulombic efficiencies. (C) 2011 The Electrochemical Society. [DOI: 10.1149/2.102202jes] All rights reserved.