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.