Self-standing ultrathin NiCo2S4@carbon nanotubes and carbon nanotubes hybrid films as battery-type electrodes for advanced flexible supercapacitors

An electrode with flexible construction, high capacity, and battery-type Faradaic mechanism is highly required for emerging wearable and portable electronics. A class of materials with favorable architectures that store energy through the Faradaic mechanism is a fertile ground for breaking through the energy density limit of an electric double-layer-based supercapacitor. Herein, a typical battery-type electrode of NiCo2S4 composited with carbon nanotubes (CNT) has been developed via a facile two-step solution reaction. The NiCo2S4@CNT composite exhibits good energy storage ability with an ultrahigh specific capacitance of 1,123 F g−1 at 0.5 A g−1 and superior stability. After being shaped into films via alternately stacking with pure CNT films, lightweight, flexible, and self-standing NiCo2S4@CNT/CNT hybrid film electrodes are fabricated, which exhibit a high volumetric capacitance of about 443 F cm−3. All-solid-state asymmetric supercapacitors with high flexibility are assembled using the hybrid film as cathode, treated carbon cloth as anode, and polyvinyl alcohol/KOH gel as solid electrolyte. Counting the weight of the two electrode films, a high energy density of 11.7 Wh kg−1 at 1.32 kW kg−1 with good cycling stability (66% of the initial capacitance after 25,000 cycles) is accomplished, suggesting promising potential for practical application in load-leveling wearable equipment.