Synthesis of the electrode materials of sodiumion storage devices from sustainable precursors via green methods is highly desirable. In this work, we fabricated a unique N, O dual-doped biocarbon nanosheet with hierarchical porosity by direct pyrolysis of low-cost cuttlebones and simple air oxidation activation (AOA) technique. With prolonging AOA time, thickness of the carbon sheets could be reduced controllably (from 35 to 5 nm), which may lead to tunable preparation of carbon nanosheets with a certain thickness. Besides, an unexpected increase in N-doping amount from 7.5 to 13.9 atom % was observed after AOA, demonstrating the unique role of AOA in tuning the doped heteroatoms of carbon matrix. This was also the first example of increasing N-doping content in carbons by treatment in air. More importantly, by optimizing the thickness of carbon sheets and heteroatom doping via AOA, superior sodium capacity−cycling retention−rate capability combinations were achieved. Specifically, a current state-of-the-art Na + storage capacity of 640 mAh g −1 was obtained, which was comparable with the lithium-ion storage in carbon materials. Even after charging/discharging at large current densities (2 and 10 A g −1 ) for 10 000 cycles, the as-obtained samples still retained the capacities of 270 and 138 mAh g −1 , respectively, with more than 90% retention. The assembled sodium-ion capacitors also delivered a high integrated energy− power density (36 kW h kg −1 at an ultrahigh power density of 53 000 W kg −1 ) and good cycling stability (90.5% of capacitance retention after 8000 cycles at 5 A g −1 ).

A naturally fibrous carbon foam is fabricated by employing fish bones as raw materials. Differing to carbon foams in previous reports, numerous unwoven and crosslinked carbon nanofibers were found to construct the whole foam, exhibiting a hierarchical porosity with interconnected channels and multisize pores. In the meantime, on the basis of the rich organic−inorganic components of fish bones, an N-S-P-O codoping was achieved in carbon foams. Benefiting from the synergistic effects of hierarchically porous fibrous foam and multiple heteroatom doping, the as-obtained samples were employed both as anode and cathode materials in lithium ion hybrid supercapacitors (LIHC), which delivered a superior energy density of 131 Wh kg −1 in 0−4.0 V. Significantly, at an impressive power density of 62000 W kg −1 , the energy density of this device can still reach an ultrahigh energy density of 72 Wh kg −1 , which presents a state of the art supercapacitors. More importantly, continuously being charged/discharged at a high current density of 5 A g −1 for 20000 cycles, the LIHC device can still retain a high energy density of 79 Wh kg −1 with 82% retention, demonstrating the promising prospect of heteroatom doped fibrous carbon foam in high-performance hybrid supercapacitors.

A new strategy is proposed for a non-carbon film coating to improve the lithium storage of a porous biomass-derived carbon matrix.