Flexible all solid-state niobium nitride//activated carbon lithium-ion hybrid capacitor with high volumetric power and energy densities

Flexible electrochemical energy storage devices are gaining considerable interest for use in smart portable flexible electronic devices. Herein, a (Niobium nitride) NbN// Activated carbon (AC) flexible (bendable and twistable) solid-state lithium-ion hybrid capacitor (LIHC) enabled by a thin graphite-layer-coated adhesive tape-based flexible current collector is reported. NbN is synthesized by a solvothermal treatment of the niobium pentachloride (NbCl₅) precursor and a subsequent nitridation treatment of the intermediate solvothermal product. Microscopic, spectroscopic, and X-ray studies signify the formation of porous 2D nanoflake units with a cubic crystalline phase, which aggregate to form microspheres. The NbN displays a high specific capacity of 347.73 mAh g⁻¹ at 0.2 A g⁻¹ with excellent cyclic stability (98.1% capacity retention after 10,000 cycles). The outstanding cyclic stability is clarified by the analysis of electrochemical impedance spectroscopy (EIS). The designed highly flexible LIHC with a high electrode mass loading of 280 mg cm⁻³ possesses a high capacitance of 2597 mF cm⁻³ at a current density of 60 mA cm⁻³. The device displays a high-energy density of 3.25 mWh cm⁻³, high power density of 254.2 mW cm⁻³, and excellent cyclic stability at 133 mA cm⁻³ (97.7% capacity retention after 10,000 cycles). Therefore, the designed flexible LIHC can be utilized for the development of smart flexible electronic devices.