FIGURE 2 Rietveld refinement results of the NMC materials obtained at various calcined temperatures (A) NMC-650, (B) NMC-800 and (C) NMC-900, (D) Raman spectra of the synthesized materials. Inset of (A) and (C) shows the schematic of crystal structures of P3 and P2-type layered oxide
Rietveld refinement results for structure analysis of the materials shown in Fig 2A-C reveal a good match between the simulation and the experiment patterns with a depleted different curve. The reasonably low Rwp (11.23, 9.38, and 6.64% for the calcinated temperature of 650, 800, and 900 oC, respectively) shows that the higher temperature exhibits the less mismatch due to higher crystallinity. In addition, it is observed that almost all the samples are high purity because of the absence of unidentified peaks. The Rietveld refinement results also indicated that the percentage of the P2 phase is higher a bit compared to the P3 phase when calcined at 800 oC. Table 1 lists the lattice parameters of different phases in the materials. The calculated results of a, c axes and the volume V do not possess significant differences in the same phases.
The Raman spectra for the three samples with different temperatures shown in Figure 2D are similar to that obtained of Na2/3Co2/3Mn2/3O230, but peaks shift forward due to the difference in sodium concentration and element composition. The spectra present two peaks at 630 and 581 cm-1, which are attributed to the deformation and symmetric stretching vibrations (A1g and Eg, respectively) of metal-oxygen (Mn/Co–O) bonds of Mn/CoO6 octahedral in layered Na-metal transition oxides30,31. The peaks at 488 and around 800 cm-1 could be explained by impurity (other phase structure, metal oxides) or sublattice. It is induced that the P3 and P2 layered structure of NMC material is probably designable by Raman spectra because of the similarity between the spectra of the sample calcined at 650 and 800 oC. Besides, the peak splitting at 488 cm-1 and the peak shifting from 800 cm-1 to high wavelength were seen in the spectra of the 900 oC sample, which is worth to differ the P2 phase from the P3 phase.
TABLE 1 Crystallographic parameters of NaxMn0.5Co0.5O2materials prepared at different calcination temperatures of 650, 800 and 900 oC