Phenothiazine/reduced graphene oxide composite as a pseudocapacitive cathode for lithium ion capacitors

Abstract

The specific capacity of cathode in lithium-ion capacitors is greatly lower than that of anode, resulting in the energy density of device is limited. In this work, to enhance the specific capacity of cathode, redox-active phenothiazine is introduced and a superior-performance phenothiazine/reduced graphene oxide composite cathode is synthesized under hydrothermal condition. Owing to the production of pseudocapacitance by phenothiazine, the optimized composite cathode can give rise to higher discharge specific capacity (86.6 mAh g–1 under 0.5 A g–1) than reduced graphene oxide cathode (36.2 mAh g–1), even though the lower specific surface area. In addition, outstanding cyclic performance is presented by this composite cathode with capacity retention ability of 80.4% at the end of 700 cycles in Li-half cell under 0.5 A g–1. Furthermore, when paired with a carbon tube anode, the constructed lithium-ion capacitor possesses a larger energy density (114.4–17.8 Wh kg–1) than the device (57.0–1.9 Wh kg–1) without phenothiazine in the cathode. And it is found that the introduction of phenothiazine doesn't decrease the power density of device. Additionally, this composite cathode based device has good cycling stability with capacity fade ratio as low as 12.7% even after 8000 cycles.