Cycle Performance of Lithium-Ion Batteries Containing Ionic Liquids with Improved Reduction Stability

The high-temperature cycling performance of lithium-ion batteries containing a new series of ionic liquids based on an aliphatic quaternary ammonium cation and a bis(fluorosulfonyl)amide anion were investigated, by conducting charge-discharge cycling tests of graphite / LiFePO4 cells. The new series of ionic liquids were 1,3-dimethyl-1-propylpiperidinium bis(fluorosulfonyl)amide (3mPP13-FSA) and 7-methyl-5-azoniaspiro[4,5]decane bis(fluorosulfonyl)amide (7mAS45-FSA), which were obtained by introducing methyl groups to known compounds. The relationship between a change in electronic structure and reduction stability was researched. Introducing methyl group improved reduction stability as electron-donating ability. Further, melting points were lowered owing to the asymmetric structures of the cations. The electrochemical properties of these ionic liquids were evaluated by linear sweep voltammetry. The cycle performance of graphite/LiFePO4 cells containing these ionic liquids for the electrolytes was improved compared with that of cells containing organic electrolytes based on LiPF6/ECDEC, and the capacity retention ratios were more than 50% at the 500th cycle at 60°C.