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

• DOI: 10.1016/j.nanoen.2017.10.025
• Publication Date: 2017-10-13T05:16:02Z
• Authors: Chun Wang; Faxing Wang; Zaichun Liu; Yujuan Zhao; Yong Liu; Qin Yue; Hongwei Zhu; Yonghui Deng; Yuping Wu; Dongyuan Zhao
• Keywords: Carbon fibers; Pseudocapacitance

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.