Mixed-valent iron niobate (Fe_0.35Nb_0.65)₃O₆ polymorphs for lithium battery anodes

A di-rutile (Fe_0.35Nb_0.65)₃O₆, FNO-r, is prepared under mild hydrothermal conditions at 240 °C that subsequently transforms to a columbite-type, orthorhombic polymorph, FNO-o, at 900 °C under a N₂ atmosphere. The two polymorphs are structurally characterised using high-resolution powder X-ray diffraction, X-ray absorption fine structure spectroscopy, Mössbauer spectroscopy and electron microscopy. The as-made di-rutile material has a short- and long-range disordered structure with random distribution of the metal cations, whilst the heat-treated orthorhombic form has the metal cations ordered in layers. These first mixed-valent Fe²⁺/Fe³⁺ niobates show electrochemical properties dependent on their crystal structures. The FNO-r polymorph undergoes a conversion reaction upon Li insertion, which is detrimental to overall specific capacity due to irreversible formation of phase-separated particles rich in Fe and poor in Nb. In contrast, FNO-o shows promising electrochemical properties with specific capacities of 232 mAh g^-1 after 50 cycles and 193 mAh g^-1 after 100 cycles in the range of 0.1-3.0 V vs. Li/Li⁺. Rate tests show a specific capacity of 300 mAh g^-1 at 50 mA g^-1, with the best capacity recovery of the two materials. The ease of preparation of this material offers a facile route to Nb-based negative electrode materials for Li-ion batteries that are free of critical and expensive elements such as Co or Ni.