Carbon nanofibers films are typical flexible electrode in the field of energy storage, but their application in Zinc-ion hybrid capacitors (ZIHCs) is limited by the low energy density due to the lack of active adsorption sites. In this work, an in-situ exfoliation strategy is reported to modulate the chemisorption sites of carbon nanofibers by high pyridine/pyrrole nitrogen doping and carbonyl functionalization. The experimental results and theoretical calculations indicate that the highly electronegative pyridine/pyrrole nitrogen dopants can not only greatly reduce the binding energy between carbonyl group and Zn2+ by inducing charge delocalization of the carbonyl group, but also promote the adsorption of Zn2+ by bonding with the carbonyl group to form N–Zn–O bond. Benefit from the multiple highly active chemisorption sites generated by the synergy between carbonyl groups and pyridine/pyrrole nitrogen atoms, the resulting carbon nanofibers film cathode displays a high energy density, an ultralong-term lifespan, and excellent capacity reservation under commercial mass loading (14.45 mg cm‒2). Particularly, the cathodes can also operate stably in flexible or quasi-solid devices, indicating its application potential in flexible electronic products. This work established a universal method to solve the bottleneck problem of insufficient active adsorption sites of carbon-based ZIHCs.Imoproved should be changed into Improved.
With the emergence of wearable electronics, flexible energy storage materials have been extensively studied in recent years. However, most studies focus on improving the electrochemical properties, ignoring the flexible mechanism and structure design for flexible electrode materials with high rate capacities and long-time stability. In this study, porous, kinked, and entangled network structures are designed for highly flexible fiber films. Based on theoretical analysis and finite element simulation, the bending degree of the porous structure (30% porosity) increased by 192% at the micro-level. An appropriate increase in kinking degree at the meso-level and contact points in entanglement network at the macro-level are beneficial for the flexibility of fiber films. Therefore, a porous and entangled network of sulfur-/nitrogen-co-doped kinked carbon nanofibers (S/N-KCNFs) is synthesized. The nanofiber films synthesized from melamine as nitrogen sources and segmented vulcanization exhibited a porous, kinked, and entangled network structure, and the stretching degree increased several times. The flexible S/N-KCNFs anode delivered a higher rate performance of 270 mAh g−1 at a current density of 2000 mA g−1 and a higher capacity retention rate of 93.3% after 2000 cycles. Moreover, the foldable pouch cell assembled by potassium-ion hybrid supercapacitor operated safely at large-angle bending and showed long-time stability of 88% capacity retention after 4000 cycles. This study provides a new idea and strategy for the flexible structure design of high-performance potassium-ion storage materials.
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