P2-type Na_0.67Mn_0.5-xV_xFe_0.43Ti_0.07O₂ powders for Na-ion cathodes: Ex-situ structural analysis and full-cell study

This study used a modified solid-state synthesis technique to synthesize Na0.67Mn0.5-xVxFe0.43Ti0.07O2 (x = 0.02 0.1) cathode materials. The XRD pattern shows that there are no impurity phases in the samples for x <= 0.06. The granular grain formation was observed in each sample and the largest surface area was obtained for x = 0.06 Vdoped composition. According to XPS analysis of the x = 0.06 sample, the V and Ti ions have three different valence states in the structure and the ratio of V3+/V4+/V5+ ions in the powders was calculated as 13 %/36 %/51 % and the spin splitting binding energy gaps were found as 7.1 eV for each V-ions and they affected by cycling process. The redox mechanism of the half cells was investigated at 10 degrees C and room temperature. The diffusion coefficient values of Na+ were calculated by cycling voltammetry (CV) and GITT techniques for the x = 0.06. Although the highest capacity of the half cells for the V-substituted samples was found to be 188.3 mAh/g for x = 0.02 V-doping in the cells for C/3-rate, the best capacity fade among the cells was obtained for x = 0.06 as 36.9 %. The ex-situ analysis of the electrodes after 100 cycles at the environmental temperatures of 10 degrees C, 50 degrees C, and 60 degrees C was investigated and it was found that the valence state of the elements changed by the cycling process. The artificial solid electrolyte interface (SEI) formation on the anode surface was performed by presodiation technique and the full cells were assembled using Na0.67Mn0.44V0.06Fe0.43Ti0.07O2/hard carbon architecture and the obtained first capacity values for C/3-rate were 90.1 mAh/g and 66.6 mAh/g, respectively, and the capacity value decreased with the cycling process up to 60 cycles and then gave a plateau with increasing cycle numbers up to 500 cycles.