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

Niobium tungsten oxide phases, as anodes for lithium-ion batteries, have gained considerable attention due to their high volumetric charge storage densities at high rates. Here we report the microwave-assisted solution-based synthesis and characterization of two new niobium tungsten bronze structures, NbWO_5.5 and β−Nb₂WO₈, which adopt a simple tetragonal tungsten bronze (TTB) structure and a TTB with √2×√2 superstructure, respectively. These novel TTB structures were synthesized at temperatures below 900°C for Nb:W ratios of 1–3, the latter composition (Nb₃WO_10.5) resulting in a √2×√2 TTB closely related to β−Nb₂WO₈. Nb:W ≥ 4 compositions result in two-phase behaviour forming Nb₂O₅ and Nb₃WO_10.5, while W-rich bronzes (Nb/W < 1) exhibiting local domains of WO₃ within the NbWO_5.5 lattice. Through comprehensive analysis using X-ray and neutron diffraction and scanning transmission electron microscopy - energy dispersive spectroscopy (STEM-EDS) we observed cation ordering in the Nb-rich bronzes at both short and long length scales. The microwave synthesis method results in NbWO microspheres with a unique, microporous structure, where primary particles are interconnected by amorphous NbWO bridges. Notably, these NbWO bronzes, with the highest Nb content and thus specific energy density of all known NbWO bronzes, exhibited high-rate capabilities and long cycle lives, positioning them as promising candidates for energy storage applications. Our findings underscore the potential of the microwave-assisted solution method for synthesizing complex oxide materials, with significant implications for the development of advanced functional materials across diverse applications.