Nb-Based Mixed Oxides As Anodes for Metal-Ion Capacitors: Progress, Challenge, and Perspective

To reduce the kinetic imbalance between the anode and cathode electrodes of metal-ion capacitors (MICs), researchers have conducted intensive explorations to develop new anode materials. Niobium-based oxides (NBOs) have been established as typical anodes for MICs. Unfortunately, conventional NBOs can hardly meet the future demands for high-power applications. The niobium-based mixed oxides (NBMOs) formed by doping niobium oxides with other elements (Ti, P, V, Cr, etc.) are drawing immense interest for advanced MICs as competitive anodes. Unlike the conventional layered Nb2O5, NBMOs exhibit diverse structures (Wadsley–Roth phase, tungsten bronze structure, ABO3 perovskite structure, etc.), which renders them appealing merits including enhanced specific capacities, higher electronic/ionic conductivities, etc., for MICs. Even so, there is still extensive room for progress to further improve their electrochemical kinetics. In this Review, we systematically summarize the doping species, crystal structures, charge-storage mechanism, synthesis strategies, and recent contributions/progress of diverse NBMOs for advanced MICs toward advancing the process of practical applications. Besides, the challenges and prospects in the booming field are proposed. The review will guide future purposeful design and controllable synthesis of high-performance anodes for next-generation MICs.