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.