Imidazolium-based ionic liquids as electrolyte additives for high-voltage Li-ion batteries

Functionalized ionic liquid 1-cyanopropy-1-3-methylimidazoliumbis(trifluoromethanesulfonyl)imide (CPMIMTFSI) was synthesized as an electrolyte additive for high-voltage Li-ion batteries to improve the voltage resistance of carbonate-based electrolytes. The electrochemical behavior of LiNi0.5Mn1.5O4/Li cells was studied by charge-discharge testing, electrochemical impedance spectroscopy, and cyclic voltammetry. The surface morphology of LiNi0.5Mn1.5O4 cathodes was studied by scanning electron microscopy (SEM), and the results showed that when 15 vol% CPMIMTFSI was added to the ethylene carbonate (EC)/dimethyl carbonate (DMC) electrolyte, the highest discharge capacity of LiNi0.5Mn1.5O4/Li cells was 133.09 mAh g(-1) at 0.2 C, which changed to 109.91 mAh g(-1) at a high rate of 5 C. This value was 92.6% higher than that in EC/DMC electrolyte at 5 C. In addition, the discharge capacity retention rate of LiNi0.5Mn1.5O4/Li cell with 15 vol% CPMIMTFSI reached 95% after 50 cycles at 0.2 C, which was nearly 10% higher than that with EC/DMC electrolyte. The results of SEM and X-ray photoelectron spectroscopy showed that a uniform and dense electrolyte interface film was attached to the surface of LiNi0.5Mn1.5O4 cathode after adding CPMIMTFSI to the carbonate-based electrolyte.