Spectroscopic Techniques and DFT Calculations to Highlight the Effect of Fe3+ on the Properties of FeNb11O29, Anode Material for Lithium-Ion Batteries

Niobium oxides have a prominent role in many technological applications, including electrochemical energy storage where they are employed as anodes for lithium- and sodium-ion batteries, hybrid supercapacitors, electrocatalysts in vanadium redox flow batteries, and electrochemical support in fuel cells. Owing to the great complexity of the available coordination environment, they can host other transition metals without phase changes. In this paper, multiple techniques are employed, for the first time, to study the influence of iron cations on the structural and functional properties of monoclinic and orthorhombic FeNb11O29. The element-selectivity of X-ray absorption spectroscopy and Mossbauer spectroscopy demonstrated the disorder of Fe3+ cations over the six octahedral sites of FeNb11O29. Ab-initio Density Functional Theory calculations were used in tandem with spectroscopic techniques to confirm the cationic disorder of the material and link its implications to the conductivity investigated with electrochemical impedance spectroscopy. The paramagnetism given by Fe3+ was confirmed with muon spin relaxation spectroscopy. The differences between FeNb11O29 and other analogous niobium-based compounds were rationalized and discussed, in view of their electrochemical applications.