Electrochemical Study of Li-Ion 18650 Cylindrical Rechargeable Cell at Elevated Temperature Using Geminal Dicationic Ionic Liquid as Electrolyte Additive

Because of desirable features, dicationic ionic liquids (DILs) are considered as suitable for applications as the electrolyte in safe Li-ion batteries (LIBs). A germinal (symmetrical) DIL (GDIL) namely 3-methyl-1-[2-(2-{2-[2-[(3-methyl-1-H-imidazol-3-ium-1-yl)ethoxy]ethoxy}-ethoxy)ethyl]-1-H-imidazol-3-ium dihexafluorophosphate is successfully synthesized. The structure and purity of the prepared GDIL are confirmed by nuclear magnetic resonance, Fourier-transform infrared spectroscopy, Karl-Fisher technique and elemental analysis. In order to investigate the role of the GDIL as electrolyte additive on the electrochemical performance of a Li-ion 18650 cylindrical rechargeable cell, the charge and discharge, and electrochemical impedance spectroscopy methods were used at 55°C and compared with the electrochemical performance of the cell containing 1-butyl-3-methylimidazolium hexafluorophosphate monocationic ionic liquid (MIL). After being continuously charged and discharged at different C-rates, the cell containing 10% V/V GDIL as electrolyte additive indicated the better electrochemical performance compared to that of the cell with 10% V/V MIL due to the higher thermal stability and some specific properties of the GDIL. In addition, after 500 charge–discharge cycles, the cell with GDIL showed 46% capacity fade in comparison of 71% capacity fade of the commercial 18650 LIBs at 55°C and 1 C-rate. Results obtained indicate promising characteristics of the GDIL, such as improved thermal characteristics of organic solvent and stabilization of solid electrolyte interface layer on electrode's surface, being suitable candidates to be used in LIBs.