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.