FIGURE 7 Cyclic voltammogram scanned at various scan rates of (A) NMC55-9012, (B) NMC55-8012 and (C) NMC55-6512. (D) The plot of current intensity at a pair of oxidation/reduction peak is located at about 4 V versus the square root of the scan rate of the samples; The plot of diffusion coefficients calculated at (E) oxidation and (F) reduction peaks
CONCLUSION
In summary, NaxMn0.5Co0.5O2possessed phase transition from P3- to P2-type layered structure when calcinated at 650, and 900 oC with bi-phasic P3/P2 coexisted in medium temperature of 800 oC. The electrochemical properties examinations indicated that the higher calcinated temperature improves the crystallinity so that strongly contribute to the cycling stability and performance. For instance, the sample NMC-900 exhibited the highest specific capacity and capacity retention at high rates that were superior to the others. Meanwhile, the bi-phase P3/P2 NMC-800 delivered the lowest specific capacity, but showed the highest diffusion coefficient, contributing to the high-rate capability and structure stability during sodium intercalation/deintercalation. The poor performance of the P3 and P3/P2 phases could be resulted from its own structure instability of the P3 phase. The results demonstrated that single phases are better for NaxMn0.5Co0.5O2material and P2 phase is preferred in terms of electrochemical performance.