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

摘要

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 degrees C in argon in order to reach crystallization, X-ray diffraction and ex situ Mossbauer spectroscopy revealed the presence of significant amounts of Fe(III) and maghemite (gamma-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, operando Fe-57 transmission Mossbauer 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. (C) 2015 Elsevier B.V. All rights reserved.