Oxygen-functionalized soft carbon nanofibers as high-performance cathode of K-ion hybrid capacitor

Facing the calling for the new generation of large-scale energy storage systems that are sustainably low cost based on earth-abundant and renewable elements, the K-ion hybrid capacitor (KIHC) constructed with both carbonaceous cathode and anode will be one of the best choices. By using oxygen-functionalized engineering, we first obtained oxygen-containing soft carbon nanofibers (ONC) cathodes which delivered a high reversible capacity of 130 mA h g−1 over 200 cycles at a current density of 50 mA g−1 within a high voltage window. Even at 5.0 A g−1, a practical capacity of 68 mA h g−1 maintained. The surface-controlled reaction domination instead of diffusion-controlled reaction domination was proposed to harvest high capacitance performance. This storage model effectively overcomes the sluggish properties of storing large-sized K-ions by a diffusion-controlled reaction in conventional cathodes in K-ion batteries (KIBs). The rational design of oxygen functionalization towards approaching more and stable active sites was highlighted. Moreover, a renewable and low-cost full KIHC was configurated by carbonaceous cathode and anode derived from a single carbon source.