Filling and unfilling carbon capsules with transition metal oxide nanoparticles for Li-ion hybrid supercapacitors: towards hundred grade energy density

Li-ion hybrid supercapacitors (Li-HECs) facilitate effective combination of the advantages of supercapacitors and Li-ion batteries (LIBs). However, challenges remain in designing and preparing suitable anode and cathode materials, which often require tedious and expensive procedures. Herein, we demonstrated that hollow N-doped carbon capsules (HNC) with and without a Fe3O4 nanoparticle core can respectively function as the anode and the cathode in very-high-performance Li-HECs. The Fe3O4@NC anode exhibited a high reversible specific capacity exceeding 1530 mA h g(-1) at 100 mA g(-1) and excellent rate capability (45% capacity retention from 0.1 to 5 A g(-1)) and cycle stability (>97% retention after 100 cycles). Moreover, high rate performance was achieved in a full-cell using the HNC cathode. By combining the respective structural advantages of the components, the hybrid device with Fe3O4@NC//HNC exhibited a remarkable energy density of 185 W h kg(-1) at a power density of 39 W kg(-1). The hybrid device furnished a battery-inaccessible power density of 28 kW kg(-1) with rapid charging/discharging within 9 s at an energy density of 95 W h kg(-1).