Free-Standing T‑Nb₂O₅/Graphene Composite Papers with Ultrahigh Gravimetric/Volumetric Capacitance for Li-Ion Intercalation Pseudocapacitor

Free-standing electrodes with high gravimetric/volumetric capacitance will open up potential applications in miniaturized consumer electronics. Herein, we report a simple synthesis technology of free-standing orthorhombic Nb₂O₅ (T-Nb₂O₅)/graphene composite papers for Li-intercalating pseudocapacitive electrodes. Through a facile polyol-mediated solvothermal reaction, the Nb₂O₅ nanodots are homogeneously decorated onto the surface of reduced graphite oxide (rGO), which can form a homogeneous Nb₂O₅/rGO colloidal suspension that can be easily fabricated into flexible composite papers. The heat-treated T-Nb₂O₅/graphene composite papers exhibit a nanoporous layer-stacked structure with good ionic–electric conductive pathways, high T-Nb₂O₅ loading of 74.2%, and high bulk density of 1.55 g cm^–3. Such T-Nb₂O₅/graphene composite papers show a superior pseudocapacitor performance as free-standing electrodes, as evidenced by an ultrahigh gravimetric/volumetric capacitance (620.5 F g^–1 and 961.8 F cm^–3 at 1 mV s^–1) and excellent rate capability. Furthermore, an organic electrolyte-based asymmetric supercapacitor is assembled based on T-Nb₂O₅/graphene composite papers, which can deliver a high energy density of 47 W h kg^–1 and power density of 18 kW kg^–1.