Tremella-like N,O-codoped hierarchically porous carbon nanosheets as high-performance anode materials for high energy and ultrafast Na-ion capacitors

Tremella-like hierarchically porous carbon nanosheets (HPCNS) with a carbon interlayer distance of 0.41 nm have been simply prepared via hydroxyapatite/KOH-synergetic pyrolysis of low-cost cattle bone at 550 degrees C. The HPCNS possesses high surface heteroatom content (17.1 at% oxygen and 6.4 at% nitrogen) due to the low-temperature thermal treatment. The HPCNS that is tested in half-cell against metallic Na can deliver a high reversible capacity of 198.5 mAh g(-1) after 1000 cycles at the current density of 0.7 A g(-1), and maintains a capacity of 150.8 mAh g(-1) even after 10,000 cycles at the current density of 7 A g(-1). The further characterization shows that the superior rate performance and the unparalleled long-term cycling stability are attributed to the capacitive storage features of HPCNS and additional Na-ion storage via reversible formation and decomposition of metallic Na in HPCNS. Consequently, the Na-ion capacitor based on the HPCNS anode delivers integrated high energy and power densities (105.2 W h kg(-1) at 0.07 kW kg(-1), 32.5 W h kg(-1) at an ultrahigh power density of 60.4 kW kg(-1)), as well as good long-term cycling stability (85.8% of the capacitance retention after 4000 cycles at the current density of 3.5 A g(-1)).