Marine-Biomass-Derived Porous Carbon Sheets with a Tunable N-Doping Content for Superior Sodium-Ion Storage

Synthesis of the electrode materials of sodium-ion 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).