Synthesis of Na2FePO4F by spray drying in order to optimise the composition and morphology for Na-ion batteries applications

Li-ion batteries are well-developed for numerous applications but the limited resources and expensive price of lithium urge the necessity to find a viable alternative to lithium. Na-ion batteries are considered as a good alternative to Li-ion batteries due to the abundance and well-distributed presence of sodium on earth crust. It is then necessary to develop new electrode material for sodium batteries, with low cost, high energy density and long cycling life.
This work presents the synthesis of Na2FePO4F by spray-drying method. In order to compensate the low conductivity of phosphate based materials, carbon nanotubes (CNT) are added during the preparation of the precursor solution. The solution/suspension is spray-dried to obtain a composite precursor and after calcination the desired phase. The spray-drying technique is appropriate to variate the morphology and to control it by modifying the injection parameters (Pressure, temperature, flow rate, injection nozzle). The addition of carbon nanotubes lead to an undesired reduction of the iron to the metallic state. To resolve this problem, the concentration of carbon was modified until no metallic iron remained in the composition. In the present work, two methodology of injection are compared to study their influence on the phase purity, size and morphology of the particles as well as their electrochemical properties.
X-ray diffraction is used to verify the crystallinity and the purity of the phase obtained. The results show that a pure phase is obtained in optimized condition and samples with low amount of impurities like Fe2O3 are obianed when no optimization is done. The oxidation state of Fe in the samples, the nature of the impurities and the quantities are confirmed by Mossbauer spectroscopy. During calcination, we observed a phenomenon of carbothermal reduction which was evidenced by Mossbauer spectroscopy (Figure 1). SEM analysis shows the morphology of the obtained particles with a size range between 1,5𝜇m and 8𝜇m using bi-fluid and rotary nozzles, respectively.

The electrochemical performances are tested by galvanostatic cycling in sodium half cells and show significant improvement with the optimization of morphology and carbon content.