FIGURE 3 SEM images (A-C) and EDX mapping results (D) of NMC
material calcined at 650 oC, SEM images (E-G) and EDX
mapping results (H) of NMC material calcined at 800oC.
The electrochemical properties characterization of the materials is
presented hereafter. CV curves of the synthesized samples are given in
Figure 4A. Typically, the CV curves show many peak couples corresponding
to several successive phase transitions and structure evolution during
sodiation/desodiation. For the voltage range 1.5 to 4.5 V, the CV curves
could be divided into three regions at about 2.7 and 4.3 V knots. The
first and the second regions have peaks that mainly contributed to
Mn4+/Mn3+ and
Co4+/Co3+ redox couples,
respectively, while the last one contributed by
Co4+/Co3+ redox reaction and
decomposition of the electrolyte when being charged up to 4.5 V.
Additionally, the CV curve of the sample calcined at 900oC is smoother, and the peak couples are rather more
reversible. It could be noticed that the CV curve of NMC-800 is a
superposition of the others because both P3 and P2 phases exist. In
general, the CV feature of P-type layered NMC is almost the same
regardless of the difference in the crystal lattice, probably because
they have the same redox species and composition.