N-doped carbon hollow microspheres for metal-free quasi-solid-state full sodium-ion capacitors

N-doped carbon hollow microspheres have been synthesized by a facile interfacial sol-gel coating process using resorcinol/formaldehyde as the carbon precursor and ethylenediamine (EDA) as both the base catalyst and nitrogen precursor. They possessed uniform size of similar to 120 nm in diameter with porous shells as thin as similar to 10 nm. The BET specific surface area and pore volume were measured to be 267 m(2) g(-1) and 1.2 cm(3) g(-1), respectively. The nitrogen doping of 8.23 wt% in carbon matrix could be achieved without sacrificing the hollow spherical morphology. Density functional theory (DFT) calculation results clearly reveal that N-doping could significantly change the interaction sites and enhance the adsorption of PF6- ions towards carbon framework. Quasi-solidstate full sodium-ion capacitors employing the nanoporous disordered carbon nanoparticles and N-doped carbon hollow microspheres as the battery-type negative and supercapacitor-type positive electrodes with a Na+ -conducting gel polymer electrolyte were demonstrated. The devices exhibit a comprehensive and superior electrochemical performance in terms of ultrahigh operating voltage of 4.4 V, high energy density of 157 W h kg(-1) at 620 W kg(-1), and prolonged cycling stability over 1000 cycles with similar to 70% of capacitance retention. Such outstanding performances suggest that the quasi-solid-state full sodium-ion capacitors could be potential safe and flexible electrochemical energy storage devices in the near future.