Boosting Electrochemical Properties of Carbon Cloth with Hydrothermal Deposition of Cadmium-Doped MnO2 Nanostructures: Binder-Free Electrodes for High-Performance Flexible Symmetric Supercapacitors

High-areal-energy density flexible supercapacitors are promising devices for energy storage, to fulfill the growing demand for portable and wearable electronic equipment. However, because of the slower electron/ion diffusion kinetics in dense electrodes, the electrochemical performance of high-theoretical-capacitance MnO₂ is typically limited to low-mass-loading electrodes. Metal doping is presumably an efficient approach for enhancing the capacitive performance of transition metal oxides. This study reports the preparation of cadmium-doped MnO₂@carbon cloth flexible electrodes through a typical hydrothermal reaction for powerful, flexible solid-state symmetric supercapacitors. The results show that metal-doped MnO₂ demonstrates better electrochemical performance than that of pure MnO₂. Cd-doped MnO₂@CC is an outstanding conductive backbone with an interconnected porous architecture, which leads to fast transport of electrons and ions while enhancing the application of active materials. Excellent performance, with 7.72 µWh/cm² energy density at 1133.33 µW/cm² and 94% capacitance retention after 5000 cycles, was shown by the flexible symmetric supercapacitor consisting of two binder-free electrodes, namely, Cd-doped MnO₂@CC electrode as both the negative and positive electrodes, and PVA-KOH gel electrolyte. Two 2.2 V green and three 1.6 V red light-emitting diodes (LEDs) were successfully illuminated by the device at the same time, indicating its high practical application value. This research is believed to provide a new path for the preparation of economic and flexible Cd-doped MnO₂@CC electrodes with extremely high areal energy density and outstanding flexibility, highlighting their great potential for application in the rational design of high-energy portable and wearable devices for energy storage.