Preparation and Characterisation of Na2Fe2(MoO4)3 Solid Solution as Potential Cathode Material in Sodium-Ion Batteries

Abstract

Na2Fe2(MoO4)3 is a good cathode material for sodium storage due to its low cost, non-toxicity of iron and three open dimensional crystal framework. Na2Fe2(MoO4)3 was synthesised using the one pot sol-gel method. Na2Fe2(MoO4)3 was then doped using chromium, zinc, copper, calcium and potassium to improve electrochemical performance. A sol-gel method was used to synthesise Na2MyMxFe2-x-y(MoO4)3 (where 0 ≤ x ≤ 0.5,0 ≤ y ≤ 0.5). A weakness of the synthesis method used was that it was not viscous. The presence of lead and sulphur which should not be part of the product is due to the use of iron sulphate as a source of iron. The presence of lead seems to be due to the contamination of the cathode material. The synthesized materials were calcined at 600 °C for 3 hours to produce the desired material as shown by the elemental distribution. The synthesised material was characterised by XRD, XRF, SEM-EDS, FTIR and Raman spectroscopy. The synthesis of Na2Fe2(MoO4)3 was successful because the FTIR spectra shows the observed bands of 1674.13 cm-1 for metal oxide (Na-O),1429.75 cm-1 due to (C-O),567 cm-1 due to (Fe-O) and 814.77 cm-1 due to (Mo-O), XRF shows relative the composition of the sample is Fe= 9.543 and Mo= 44.003.The SEM micrographs reveal that the morphology consists of a smooth surface with irregular shapes and particle size is approximately 1 μm. The XRD parent and doped samples present nine characteristic peaks at 13.87, 22.05, 25.60, 29.15,34.64 and 55.62° corresponding to the crystal planes 200, 311, 212, 124,420 and 0.108. Using stoichiometric amounts of reactant, Na2Fe2(MoO4)3 was doped at the Fe site and the Na site was successfully synthesised using a one pot sol-gel synthesises method followed by calcination in the presence of air for 3 hours at 600 °C