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