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.