FIGURE 5 Electrochemical performances of NMC cathodes upon cycle tests in the range of 1.5-4.5 V at (A) 1C, (B) 2C and (C) 5C respectively
Figure 6A-B show the XRD patterns of the bare electrodes NMC-650 and NMC-800 and the electrodes at the fifth cycle. Comparing with the XRD of the bare NMC-650 and NMC-800 shown in Figure 2A and Figure 2B, respectively, it could be seen that the samples retain the original structure and phase component, revealing the good structure stability. Furthermore, the fading of high redox couples during the first several cycles seem not to be correlated with the appearance of a new phase or structure changes. Thus, the results demonstrated that P2-phase displays high structural stability. Our previous work also indicated that NMC-900 could remain the P2-type layered structure after 50 cycles of charging-discharging 21.
EIS spectra of the electrodes were also collected at the beginning of the cycling test and at the 5th cycle. The EIS spectra show a semicircle at high and medium frequencies of the charge transfer resistance and one huge semicircle at low frequency due to ion diffusion in the porous electrode 32. As one can see, the charge transfer resistance from the high and medium frequency shows different changes in the three electrodes and tend to decrease in turn NMC-650 > NMC-800 > NMC-900 after 5 cycles. In general, the change of charge transfer resistances is consistent with the cycling stability of the electrodes. Specifically, the increase of charge transfer resistance and the resulting poor performance of the NMC-650 electrode suggested that the capacity degradation is consistent with the fast impedance buildup. Oppositely, the tendency decrease of charge transfer resistances at the 5th cycle reveals an improvement in the kinetic of charge transfer at the NMC-800 and NMC-900 interfaces.