An investigation of the improvement in energy storage performance of Na2/3Mn1/2Fe1/2O2 by systematic Al-substitution

We successfully fabricated Na2/3Mn1/2Fe1/2−xAlxO2, where x = 0, 0.01, … 0.10, by a modified solid-state reaction technique. The structural properties of the Al-substituted samples were investigated by x-ray diffraction (XRD), scanning electron microscope (SEM), Fourier transform infrared spectroscopy and x-ray absorption fine structure (XAFS) measurements. It was found that there were no impurity phases in the XRD patterns of the samples and they fit the P63/mmc symmetry. The Al substitution in Na2/3Mn1/2Fe1/2O2 causes a decrease in the a-lattice parameter, but the c-parameter starts to increase after a certain substitution value of Al. We suggest that a certain proportion of Al in the samples triggers the change of the spin configuration of the Fe ions, and it may cause an increase in the lattice parameters. The size of the grains was found to be less than 0.9 µm, from SEM images for all samples. The valence states of the substituted samples as well as the local structure around Fe and Mn were investigated by means of XAFS measurements. The highest capacity for the first cycle was obtained as 134.3 mAh/g for x = 0.07, and the best capacity fade was found to be 0.23 for x = 0.08 substitution. So, the highest performance of the Al-substituted cells was found when 0.08 ≥ x ≥ 0.06. The environmental temperature effects on the battery cells were determined at 10 oC, room temperature and 50 oC, and it was found that the temperature plays a crucial role in the Na-ion batteries.