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 ~ 120 nm in diameter with porous shells as thin as ~ 10 nm. The BET specific surface area and pore volume were measured to be 267 m² g⁻¹ and 1.2 cm³ 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-solid-state 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 ~ 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.