High-energy lithium-ion hybrid supercapacitors composed of hierarchical urchin-like WO₃/C anodes and MOF-derived polyhedral hollow carbon cathodes

A lithium-ion hybrid supercapacitor (Li-HSC) comprising a Li-ion battery type anode and an electrochemical double layer capacitance (EDLC) type cathode has attracted much interest because it accomplishes a large energy density without compromising the power density. In this work, hierarchical carbon coated WO₃ (WO₃/C) with a unique mesoporous structure and metal-organic framework derived nitrogen-doped carbon hollow polyhedra (MOF-NC) are prepared and adopted as the anode and the cathode for Li-HSCs. The hierarchical mesoporous WO₃/C microspheres assembled by radially oriented WO₃/C nanorods along the (001) plane enable effective Li⁺ insertion, thus exhibit high capacity, excellent rate performance and a long cycling life due to their high Li⁺ conductivity, electronic conductivity and structural robustness. The WO₃/C structure shows a reversible specific capacity of 508 mA h g^-1 at a 0.1 C rate (1 C = 696 mA h g^-1) after 160 discharging-charging cycles with excellent rate capability. The MOF-NC achieved the specific capacity of 269.9 F g^-1 at a current density of 0.2 A g^-1. At a high current density of 6 A g^-1, 92.4% of the initial capacity could be retained after 2000 discharging-charging cycles, suggesting excellent cycle stability. The Li-HSC comprising a WO₃/C anode and a MOF-NC cathode boasts a large energy density of 159.97 W h kg^-1 at a power density of 173.6 W kg^-1 and 88.3% of the capacity is retained at a current density of 5 A g^-1 after 3000 charging-discharging cycles, which are better than those previously reported for Li-HSCs. The high energy and power densities of the Li-HSCs of WO₃/C//MOF-NC render large potential in energy storage.