Flexible Nb₂O₅ nanowires/graphene film electrode for high-performance hybrid Li-ion supercapacitors

The hybrid Li-ion electrochemical supercapacitor (Li-HSC) combining the battery-like anode with capacitive cathode is a promising energy storage device boasting large energy and power densities. Orthorhombic Nb₂O₅ is a good anode material in Li-HSCs because of its large pseudocapacitive Li-ion intercalation capacity. Herein, we report a high-performance, binder-free and flexible anode consisting of long Nb₂O₅ nanowires and graphene (L-Nb₂O₅ NWs/rGO). The paper-like L-Nb₂O₅ NWs/rGO film electrode has a large mass loading of Nb₂O₅ of 93.5 wt% as well as short solid-state ion diffusion length, and enhanced conductivity (5.1 S cm⁻¹). The hybrid L-Nb₂O₅ NWs/rGO paper electrode shows a high reversible specific capacity of 160 mA h g⁻¹ at a current density of 0.2 A g⁻¹, superior rate capability with capacitance retention of 60% when the current density increases from 0.2 to 5 A g⁻¹, as well as excellent cycle stability. The Li-HSC device based on the L-Nb₂O₅/rGO anode and the cathode of biomass-derived carbon nanosheets delivers an energy density of 106 Wh kg⁻¹ at 580 W kg⁻¹ and 32 Wh kg⁻¹ at a large power density of 14 kW kg⁻¹. Moreover, the Li-HSC device exhibits excellent cycling performance without obvious capacitance decay after 1000 cycles.