Mixtures of unsaturated imidazolium based ionic liquid and organic carbonate as electrolyte for Li-ion batteries

Room temperature ionic liquid (RTIL) 1-allyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (AMIMTFSI) was obtained with high conductivity (7.8 mS/cm) and a wide electrochemical window (1–5.8 V). The AMIMTFSI based electrolytes with PC have been investigated by comparison with the ionic liquid 1-propyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (PMIMTFSI) based electrolytes. Raman observations revealed AMIMTFSI-based electrolytes achieved lower degree of [Li(TFSI)2]− anionic clusters and more free Li+ charge carriers than PMIMTFSI-based electrolyte. LiFePO4 cathode with AMIMTFSI-PC (50 wt%)–1 M LiTFSI electrolyte showed the best performance in the rate charge–discharge among the binary and ternary ionic liquid based electrolytes. The first reversible capacity of 159 mAh g−1 and coulombic efficiency of 97.2% at C/20 were obtained with AMIMTFSI-PC (50 wt%)–1 M LiTFSI electrolyte compared to 144.2 mAh g−1 and 94.5% respectively with PMIMTFSI-PC (50 wt%)–1 M LiTFSI electrolyte. EIS results indicated that a stable protective layer was formed on the electrode surface in AMIMTFSI-based electrolyte. The layer helped to suppress the solvent decomposition and improve the cycling performance of Li-ion batteries. The Li/LiFePO4 cell showed interfacial stability and stable discharge capacities with a retention of 97.4% (151 mAh g−1) after 100 cycles at 0.1 C rate in AMIMTFSI-PC (50 wt%)–1 M LiTFSI electrolyte.