Hard carbons (HCs) possess high lithium/sodium storage capacities, which however suffer from low electric conductivity and poor ion diffusion kinetics. An efficient structure design with appropriate heteroatoms doping and optimized graphitic/defective degree is highly desired to tackle these problems. This work reports a new design of N-doped HC nanoshells (N-GCNs) with homogeneous defective nanographite domains, fabricated through the prechelation between Ni 2+ and chitosan and subsequent catalyst confined graphitization. The as-prepared N-GCNs deliver a high reversible lithium storage capacity of 1253 mA h g −1 , with outstanding rate performance (175 mA h g −1 at a high rate of 20 A g −1 ) and good cycling stability, which outperforms most state-of-the-art HCs. Meanwhile, a high reversible sodium storage capacity of 325 mA h g −1 is also obtained, which stabilizes at 174 mA h g −1 after 200 cycles. Density functional theory calculations are performed to uncover the coupling effect between heteroatom-doping and the defective nanographitic domains down to the atomic scale. The in situ Raman analysis reveals the "adsorption mechanism" for sodium storage and the "adsorptionintercalation mechanism" for lithium storage of N-GCNs.

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Supercapacitors or ultracapacitors are considered as one of the potential candidates in the domain of energy storage devices for the forthcoming generations. These devices have earned their significance in numerous applications, viz., to power hybrid electric/electric vehicles and other power and electronic systems which require electrical energy for their operation. Supercapacitors are the most versatile devices which are most widely used for delivery of electrical energy in short time and in arenas which demand long shelf life. Therefore, the development of supercapacitors has huge market requirements, and long-term progress is needed for their successful advancement and commercialization. Meanwhile, supercapacitors are also facing challenges such as technical problems, establishing electrical parameter models, consistency testing, and establishing industrial standards. In this paper, the above challenges and the future development opportunities of supercapacitors are introduced in detail. This perspective will provide corresponding guidance and new directions for the development of supercapacitors.

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