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<inf>5.5</inf> and β-Nb<inf>2</inf>WO<inf>8</inf>, 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. Nb<inf>3</inf>WO<inf>10.5</inf> with a structure closely related to β-Nb<inf>2</inf>WO<inf>8</inf> was formed at higher Nb:W ratios. Nb:W ≥ 4 compositions result in two-phase behavior forming Nb<inf>2</inf>O<inf>5</inf> and Nb<inf>3</inf>WO<inf>10.5</inf>, while W-rich bronzes (Nb:W < 1) exhibited local domains of WO<inf>3</inf> within the NbWO<inf>5.5</inf> 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.