Investigation of an Intermediate Temperature Molten Lithium Salt Based on Fluorosulfonyl(trifluoromethylsulfonyl)amide as a Solvent-Free Lithium Battery Electrolyte

The asymmetric amide anion, such as (fluorosulfonyl) (trifluoromethylsulfonyl)amide (fTfN(-)), formed intermediate temperature molten lithium salts at 100 degrees C, which was the lowest among the lithium single molten salts composed of conventional halogenide, nitrate, perchlorate, and perfluoro anions. The molten Li[fTfN] was highly viscous (17 Pa.s at 110 degrees C, however, it was classified as a good ionic liquid by Walden plot analysis, which indicates the good-dissociation between Li+ and fTfN(-). Due to the high transport number of the lithium cation (0.94), the Li[fTfN] behaves as a single lithium-ion conducting material. The Li[fTtN] possesses an electrochemical window (5.0 V) due to the oxidative resistance of the perfluoro anion. The charge and discharge for a conventional composite positive electrode containing LiCoO2 or LiFePO4 was successful. In particular, LiFePO4 showed a high capacity and Coulombic efficiency at 0.1-5 C.