Na2FePO4F/multi-walled carbon nanotubes for lithium-ion batteries: Operando Mössbauer study of spray-dried composites

•Na2FePO4F/CNT powders were prepared by spray-drying and calcination at 600 °C in Ar.•Spray-drying process was found to result in an excellent dispersion of the CNTs.•Some Fe(III) and Fe2O3 were detected by Mössbauer spectroscopy in calcined powder.•In situ Mössbauer proves that all Fe(III) is reduced to Fe(II) during discharge.•The Na2FePO4F/CNT powder shows a good discharge capacity of 104 mA h g−1 at C/10 rate.

In order to favor electronic conductivity in sodium iron fluorophosphate electrodes for lithium- or sodium-ion batteries, composites of Na2FePO4F with multi-walled carbon nanotubes (CNTs) were prepared by pilot-scale spray drying. Addition of multi-walled CNTs in the solution results in an excellent dispersion of the CNTs within the volume of Na2FePO4F and not only at the surface of the particles. Following a heat treatment at 600 °C in argon in order to reach crystallization, X-ray diffraction and ex situ Mössbauer spectroscopy revealed the presence of significant amounts of Fe(III) and maghemite (γ-Fe2O3) in the powder. However, Na2FePO4F/CNTs composites exhibit good electrochemical performance when cycling against lithium, with a discharge capacity of 104 mA h g−1 at C/10 rate and 90 mA h g−1 at 1C rate. Therefore, operando57Fe transmission Mössbauer spectroscopy analyses were carried out in order to investigate the evolution of the iron oxidation state during cycling. During the first discharge, all the Fe(III) is reduced to Fe(II), explaining the good electrochemical performance.