FIGURE 4 (A) Cyclic voltammogram of NMC materials in 1.5 - 4.5
V voltage windows at a scan rate of 100 µV.s-1 . (B)
The first charge-discharge voltage profiles, (C) Evolution of capacity
and Coulombic efficiency with cycle number of the material cycled in a
voltage range of 1.5-4.5 V and (D) Rate capability in the same voltage
limit
The galvanostatic charge/discharge curves of the initial cycle are shown
in Figure 4B. All the samples show characteristic voltage profiles that
are rather alike with relatively high discharge capacities, coinciding
with the electrochemical behavior shown in CV curves. However, the
samples NMC-650 and NMC-800 exhibit high irreversible capacity in the
first cycle, resulting in low Coulombic efficiency with the value of
about 74.2% and 68.5% for NMC-650 and NMC-800, respectively. It could
be noticed that NMC-800, with a complex composition including P3 and P2
phase, presents an even higher irreversible capacity than the P3 phase
and P2 phase components. Meanwhile, the sample NMC-900 exceptionally
displays a nearly reversible charge/discharge cycle with Coulombic
efficiency of about 96.6%. In general, the P-phases with sodium
deficiency typically display high Coulombic efficiency above 100%4,8,19. Thus, the materials obtaining P2 phase with a
high sodium content of 1 like the O3 phase, gain high Coulombic
efficiency due to the excess Na+ ions that are
possible to the electrolyte.
Moreover, the redox couple Co4+/Co3+is activated when charged to a high cutoff voltage and contributed to
the large irreversible charge capacity of the samples NMC-650 and
NMC-800. For the sample NMC-900, the huge sodium loss via evaporation
due to high calcined temperature reduces the Coulombic efficiency. As a
result, the initial discharge capacity of 149.3, 106.2 and 156.9
mAh.g-1 are obtained for the sample NMC-650, NMC-800
and NMC-900 respectively, hence the Coulombic efficiency are 74.6, 68.4
and 111.4% respectively.
Figure 4C shows the capacity and Coulombic efficiency of the samples
during long-term charge-discharge. It could be seen that the sample
NMC-800 and NMC-900 attain stable cycling that slightly changes in
capacity for each cycle. The sample NMC-900 remains a capacity that
higher than the others, with a capacity retention of 76.2% after 100
cycles. On the other hand, the capacity of NMC-650 reduces gradually and
the capacity retention is about 56.2% after 100 cycles. The sample
NMC-800 has a two-phase component, but capacity retention is the best of
the three, with a value of 86.2% after 100 cycles.
The rate capability of the samples is illustrated in Figure 4D. It could
be observed that the three samples display acceptable rate capability.
Of the three, the samples NMC-650 and NMC-800 exhibit pretty good
capacity retention in which the capacities drop steadily and are
negligible when the current density increase from C/10 to 10C.
Meanwhile, the capacity of NMC-900 falls significantly at the high
current density of above 5C. However, when the current density returns
to C/10, only NMC-650 shows a sharp decrease in capacity compared to the
initial value at the same current density.
The long-term charge-discharge at higher current density is applied to
characterize the stability of electrodes (Figure 5). Generally, the
capacities of the three samples are stable during the cycle with
low-capacity decay that coincided with the rate capability test. It is
observed that the capacity retention at the high rate is better because
of shortened retention time in high voltage regions, and the
contribution of capacity from the high voltage redox couple21. NMC-650 displays insufficient capacity and
capacity retention compared to the other two in all the test conditions,
while NMC-900 performs the best. Additionally, the Coulombic efficiency
of approximately 100% is gained by all the samples at various rates.