Vanadium and Niobium MXenes—Bilayered V₂O₅ Asymmetric Supercapacitors

MXenes offer high metallic conductivity and redox capacitance that are attractive for high-power, high-energy storage devices. However, they operate limitedly under high anodic potentials due to irreversible oxidation. Pairing them with oxides to design asymmetric supercapacitors may expand the voltage window and increase the energy storage capabilities. Hydrated lithium preintercalated bilayered V₂ O₅ ( δ-Li_x V₂ O₅ ·nH₂ O) is attractive for aqueous energy storage due to its high Li capacity at high potentials; however, its poor cyclability remains a challenge. To overcome its limitations and achieve a wide voltage window and excellent cyclability, it is combined with V₂ C and Nb₄ C₃ MXenes. Asymmetric supercapacitors employing lithium intercalated V₂ C (Li-V₂ C) or tetramethylammonium intercalated Nb₄ C₃ (TMA-Nb₄ C₃ ) MXenes as the negative electrode, and a δ-Li_x V₂ O₅ ·nH₂ O composite with carbon nanotubes as the positive electrode in 5 m LiCl electrolyte operate over wide voltage windows of 2 and 1.6 V, respectively. The latter shows remarkably high cyclability-capacitance retention of ≈95% after 10 000 cycles. This work highlights the importance of selecting appropriate MXenes to achieve a wide voltage window and a long cycle life in combination with oxide anodes to demonstrate the potential of MXenes beyond Ti₃ C₂ in energy storage.