Vinyl-functionalized imidazolium ionic liquids as new electrolyte additives for high-voltage Li-ion batteries

Four functionalized ionic liquids based on imidazolium cations with vinyl or alllyl group and TFSI− anion were synthesized as electrolyte additives for high-voltage Li-ion battery to stabilize carbonate-based electrolytes on the surface of 5 V class cathode materials. The electrochemical behaviors and surface morphology of LiNi0.5Mn1.5O4 cathode had been investigated by cyclic voltammetry, charge–discharge test, X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM), respectively. Cycle life and rate performance of the Li/LiNi0.5Mn1.5O4 cells containing 1.2 M LiPF6 in ethylene carbonate/ethyl methyl carbonate can be improved by adding 1-allyl-3-vinyl imidazolium bis(trifluoromethanesulphonyl)imide ([AVIm][TFSI]). The addition of 3 wt.% [AVIm][TFSI] results in high discharge capacity of above 130 mAh g−1. Surface analysis of the cathode material (XPS and SEM) suggested that a stable and compact polymer film was formed on the LiNi0.5Mn1.5O4 cathode by electroinitiated polymerization of imidazolium cation with vinyl and allyl group.