Effect of imidazolium-based ionic liquid as electrolyte additive on electrochemical performance of 18650 cylindrical Li-ion batteries at room and 60 °C temperatures

To optimize the electrochemical performance and greater safety of Li-ion batteries (LIBs) in order to meet the market demands, an imidazolium-based ionic liquid (IL), namely 1-butyl-3-methyl imidazolium hexafluorophosphate ([BMIm][PF₆]) as a commercially available and most widely studied IL, was synthesized and applied as electrolyte additive in commercial 18650 LIBs. The different compositions of mixed electrolyte were prepared by adding 0–5 wt% of IL to the organic electrolyte. The electrochemical measurements were done using galvanostatic charge and discharge and electrochemical impedance spectroscopy, EIS, analyses to obtain the optimal percentage of additive resulting in highest cycleability and discharge capacity as two main criteria of Li-ion batteries. The highest electrochemical performance was obtained at room temperature for the cell containing 3 wt% IL compared to other IL percentages. Moreover, the cell with 3 wt% IL over 50 cycles at 60 °C showed no capacity fade while the blank electrolyte looses more than 18% of its capacity in this condition. Based on EIS findings, the diffusion of Li ions in the electrode is facilitated by the presence of IL additive at 60 °C due to formation optimal SEI layer and a noticeable reduction of charge transfer resistance contribution. The surface characterization was done through IR and SEM analyses, which confirmed that the electrolyte decomposition is diminished in the presence of IL at elevated temperature. To sum, a considerable electrochemical improvement at a relatively high temperature in the presence of a small amount of IL in commercial 18650 LIBs seems to be feasible and economically favorable.