Cation ordering in low-temperature niobium-rich NbWO bronzes: New anodes for high-rate Li-ion batteries

Abstract

Niobium tungsten oxides are gaining attention as anodes for lithium-ion batteries due to their high volumetric energy storage densities obtained at high cycling rates. Two new niobium tungsten bronze structures, NbWO_5.5 and β-Nb₂WO₈, were prepared with microwave-assisted solution-based methods at 800°C. These adopt a simple tetragonal tungsten bronze (TTB) and a √2 × √2 TTB superstructure, respectively. Nb₃WO_10.5 with a structure closely related to β-Nb₂WO₈ was formed at higher Nb:W ratios. Nb:W ≥ 4 compositions result in two-phase behavior forming Nb₂O₅ and Nb₃WO_10.5, while W-rich bronzes (Nb:W < 1) exhibited local domains of WO₃ within the NbWO_5.5 lattice. Diffraction and electron microscopy analysis revealed cation ordering in the bronzes at different length scales. The microwave synthesis method produced microporous spheres, with the high-Nb-content phases showing promising high-rate capabilities and long cycle lives, making them suitable for energy-storage applications. The microwave-assisted solution method holds potential for synthesizing complex oxide materials across diverse applications.