The Complex Crystal Chemistry of Niobium Tungsten Oxides

Niobium tungsten oxides are currently intensively studied because of their potential use as high-performance anode materials in lithium ion batteries, showing fast ion exchange and high cycling stability. Such properties originate from a varied structural chemistry in the pseudobinary system Nb2O5/WO3, which is based upon multifaceted octahedral frameworks derived from the ReO3 type. Different structure types like the block phases and the tetragonal tungsten bronze derivatives are realized to accommodate oxygen:metal ratios in the range 2.5 <= O/Sigma M <= 3.0 (M = Nb,W). This review starts with an overview about the synthesis of the existing phases and their structures. The building principles of structures occurring in this system are described. In a given phase, the substitution of Nb5+ by equimolar amounts of various tetravalent metals and W6+ gives rise to isostructural solid solution series. The option to oxidize reduced phases at different temperatures introduces further complexity and generates unprecedented structural variants. Possibilities and limitations of X-ray diffraction and various electron microscopy methods for a comprehensive structural characterization of pure phases and less-ordered arrangements are discussed in a historical context. Finally, the outstanding electrochemical performance and thermoelectric properties of niobium tungsten oxides are addressed and perspectives for future applications discussed.

Automated summary

This review summarizes the synthesis and structures of niobium tungsten oxides, and discusses their electrochemical and thermoelectric performance and potential applications.