Electrochemical performance of hybrid supercapacitor device based on Mn-doped LiFePO4/C and imidazolium ionic liquid electrolyte

Abstract

The electrochemical performance of lithium-ion hybrid supercapacitors (Li-HSCs) build with nanostructured LiMnxFe1-xPO4/C in an ionic liquid electrolyte system was analyzed to determine the effects of Mn doping on olivine structure. The construction of hybrid cells using ionic liquids is strategic due to their low vapor pressure, which inhibits the evolution of gas and leakage in the cell, and non-flammability. The Li-HSCs cells were assembled using as electrolyte an ionic liquid system consisting of lithium bis(trifluorosulfonyl)imide (LiTFSI) and 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonamide) (EMITFSI) at a concentration of 1 mol L−1; a carbon with mesopore structure (MESO) as capacitive electrode; and Mn-doped LiFePO4 olivines, i.e., LiMnxFe1-xPO4 (x = 0.01 ≤ x ≤ 0.10), as positive electrodes. The olivines were synthesized using a solvothermal method, resulting in pure nanostructured materials. Subsequently, a thin carbon layer was added from a carbon source and calcinated at 550 °C, as confirmed by XRD, FTIR, EDS, and TEM. The electrochemical characterization of LiMnxFe1-xPO4 active materials was performed using a standard lithium cell (3-electrode cell). The LiMn0.05Fe0.95PO4 presented the best properties. The electrochemical performance of Li-HSCs constructed using all LiMnxFe1-xPO4 (x = 0.01 ≤ x ≤ 0.10) based cathodes was then evaluated. The Li-HSC built with LiMn0.05Fe0.95PO4 showed the highest capacitance, power, and energy densities among all the evaluated cells, as also verified by the electrochemical characterization of the standard lithium cells. This cell was able to deliver an energy density of 43.1 W h kg−1 at 25.9 W kg−1 (power density), which was higher than the device constructed with the un-doped olivine, LiFePO4, which delivers 35.9 W h kg−1 at 23.2 W kg−1, as well as higher than other LiMnxFe1-xPO4 based cells. Additionally, the Li-HSC constructed with LiMn0.05Fe0.95PO4 showed 67 % of energy retention after 1000 cycles, a superior performance compared to Li-HSC prepared with the un-doped olivine, which retained 54 % of its initial energy after the same cycling process.