Pseudocapacitive behavior of manganese oxide in lithium-ion-doped butylmethylpyrrolidinium-dicyanamide ionic liquid investigated using in situ X-ray absorption spectroscopy

Ideal pseudocapacitive behavior of α-MnO₂ electrodes over a potential range of 3 V is found in lithium bis(trifluoromethylsulfonyl)imide (LiTFSI)-doped butylmethylpyrrolidinium-dicyanamide (BMP-DCA) ionic liquid (IL), which is non-flammable and has a decomposition temperature of as high as ∼300 C. Accordingly, this electrolyte is promising for high-energy, high-power, and high-safety supercapacitor applications. The addition of 0.01 M LiTFSI in the IL improves the oxide capacitance from 90 F g^-1 to 120 F g^-1, which is due to the incorporated Li⁺ ions promoting Mn valent state variation (between trivalent and tetravalent) during charge-discharge. However, excessive LiTFSI doping causes a capacitance decay due to reduced electrolyte ionic conductivity. In situ X-ray absorption spectroscopy is used to investigate the energy storage mechanism. A capacitance activation process of α-MnO₂ in the Li⁺-doped BMP-DCA IL is found.