A mixture of triethylphosphate and ethylene carbonate as a safe additive for ionic liquid-based electrolytes of lithium ion batteries

A binary mixture of triethylphosphate (TEP) and ethylene carbonate (EC) has been examined as a new non-flammable additive for ionic liquid-based electrolytes for lithium-ion batteries. The optimized electrolyte composition consists of 0.6 mol dm−3 (=M) LiTFSI in PP13TFSI mixed with TEP and EC in volume ratio of 80:10:10, where TFSI and PP13 denote bis(trifluoromethanesulfonyl)imide and N-methyl-N-propylpiperidinium, respectively. The ionic conductivity of PP13TFSI dissolving 0.4 M LiTFSI was improved from 8.2 × 10−4 S cm−1 to 3.5 × 10−3 S cm−1 (at 20 °C) with the addition of TEP and EC. The electrochemical behavior of 0.4 M LiTFSI/PP13TFSI with and without TEP and EC was studied by cyclic voltammetry, which showed no deteriorating effect by the addition of TEP and EC on the electrochemical window of PP13TFSI. The flammability of the electrolyte was tested by a direct flame test. The proposed ionic liquid-based electrolyte revealed significant improvements in the electrochemical charge–discharge characteristics for both graphite negative and LiMn2O4 positive electrodes.