Supermolecule-derived N-containing carbon spherical nanoparticles inserted in expanded graphite for capacitive Zn-ion storage

Aqueous zinc-ion hybrid capacitors (ZIHCs) are an ideal candidate for energy storage resulted from their low cost and high safety. Nonetheless, their wide applications have been hindered by the sluggish migration kinetics of Zn 2+ , low activity and low structural stability of cathodes during prolonged cycles. Herein, we report a supermolecular route by using two small molecules (melamine and cyanuric acid) inhaled in the interlayered space of expanded graphite (EG) flakes, where they are self-assembled into N-containing supermolecules. After calcination at high temperature, the supermolecules sandwiched in EG are converted to N-containing carbon spherical nanoparticles (NCSN). The insertion of NCSNs enlarges interlayered distance and the nitrogen and oxygen groups on NCSN enhance Zn 2+ adsorption, which boosts the transfer dynamics of zinc ions. Meanwhile, the in-situ formed NCSNs enhance the specific surface area of EG. Thus the fabricated ZIHCs employing the NCSN inserted EG (NCSN@EG) as the cathode deliver a high specific capacity of 355.6 F g −1 with the energy density of 142.7 Wh kg −1 and power density high as 170.2 W kg −1 in the voltage range of 0–1.7 V. In addition, the in-situ formed NCSNs strengthen the stacked two-dimensional structure of EG as bridges, and after 30,000 cycles, the capacity retention rate is as high as 87.2 %.