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.