Highly bonded T-Nb₂O₅/rGO nanohybrids for 4 V quasi-solid state asymmetric supercapacitors with improved electrochemical performance

Orthorhombic niobium pentoxide (T-Nb₂O₅)/reduced graphene oxide nanohybrids were fabricated via the hydrothermal attachment of Nb₂O₅ nanowires to dispersed graphene oxide nanosheets followed by a high-temperature phase transformation. Electrochemical measurements showed that the nanohybrid anodes possessed enhanced reversible capacity and superior cycling stability compared to those of a pristine T-Nb₂O₅ nanowire electrode. Owing to the strong bonds between graphene nanosheets and T-Nb₂O₅ nanowires, the nanohybrids achieved an initial capacity of 227 mAh·g^-1. Additionally, non-aqueous asymmetric supercapacitors (ASCs) were fabricated with the synthesized nanohybrids as the anode and activated carbon as the cathode. The 3 V Li-ion ASC with a LiPF₆-based organic electrolyte achieved an energy density of 45.1 Wh·kg^-1 at 715.2 W·kg^-1. The working potential could be further enhanced to 4 V when a polymer ionogel separator (PVDF-HFP/LiTFSI/EMIMBF₄) and formulated ionic liquid electrolyte were employed. Such a quasi-solid state ASC could operate at 60 ℃ and delivered a maximum energy density of 70 Wh·kg^-1 at 1 kW·kg^-1.