Solid-state pseudocapacitors based on MnO₂-nanorod-electrodes and plastic crystal incorporated gel polymer electrolyte: synergistic effect of Li-salt addition in electrolyte and morphology of electrodes

We present a novel configuration of high-performance solid-state pseudocapacitors, fabricated with symmetric MnO₂-nanorod-electrodes, prepared via chemical and hydrothermal routes, and plastic crystals-based gel polymer electrolytes (GPEs). Comparative studies are reported on capacitors employing GPEs comprising a mixture of non-ionic plastic crystal succinonitrile (SN) and organic ionic plastic crystal (OIPC) 1-ethyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (EMPTFSI), without and with Li-salt (LiTFSI), entrapped in a co-polymer poly(vinylidine fluoride-co-hexafluoropropylene) (PVdF-HFP). The MnO₂-nanorods have been characterized for their morphological/structural aspects, specific surface area, and porosity and correlated the characteristics with their capacitive performance. Clean and uniform morphology and high surface area with mesoporous character are found to be responsible factors for superior supercapacitive performance of hydrothermally derived MnO₂-nanorod-electrodes as compared to chemically derived MnO₂-nanorods. Lithium salt incorporation in GPE has been found to be another important factor to improve the pseudoapacitive performance of the cells due to facile intercalation/extraction of Li-ions through MnO₂-electrodes. The optimum performance of the pseudocapacitor cell has been observed in terms of specific capacitance (98–101 F g⁻¹), specific energy (~ 13.7 W h kg⁻¹), and maximum specific power (~ 32.6 kW kg⁻¹) as observed from charge-discharge studies, due to synergistic effect of morphology of hydrothermally derived MnO₂-nanorod-electrodes and incorporation of Li-ions in GPE. The hydrothermally derived MnO₂-nanorod-electrodes also exhibit high rate capability; however, it reduces significantly when Li-salt incorporated GPE is employed. The optimum cell exhibits almost stable cyclic performance up to ~ 3300 charge-discharge cycles after only ~ 17% fading in specific capacitance for initial few cycles.