FIGURE 6 XRD patterns of the electrodes obtained (A) before and
(B) after cycling. EIS spectra of the electrode (C) NMC-650, (D) NMC-800
and (E) NMC-900 obtained at the 1st and the
5th cycles
The kinetic of sodium ion intercalation/deintercalation in the structure
that correlates to the performance of the electrode could be evaluated
by CV conducted at various scan rates. Figure 7A-C shows the CV curves
of the electrodes at the scan rate varies from 10 to 250
µV.s-1. Several redox peaks on the CV could be
assigned to Mn4+/Mn3+ and
Co4+/Co3+ redox couples together
with phase transition and evolutions during sodium
intercalation/deintercalation 5,33. As one can see,
with the scan rate increases, the redox peaks’ intensity increases but
the CV profiles are almost unchanged, indicating high redox
reversibility and coinciding to the prominent rate capability of the
electrodes.
The plot of peak current intensity versus square root of scan rate was
used to check if the redox reaction is diffusion controlled34. The peak intensity of the oxidation peak and
reduction peak located at around 4 V are plotted versus the square root
of the scan rate and illustrated in Figure 7D, for instance. The fitted
lines indicate that the linear relationship is satisfied, so the forward
and reverse reactions are diffusion controlled. The feature is also
recognized using the fitting way with other peaks in the CV profiles.
Therefore, the diffusion coefficients can be calculated according to the
Randles-Sevcik equation 34:
\(I_{p}\ =\ (2.69\ \times\ 10^{5})n^{2/3}AC_{o}D^{1/2}v^{1/2}\)(1)
Where Ip is the peak current (A), n is the number
of exchange electron, A is the electrode area, D is
Na+ diffusion coefficient, C ois the initial concentration of Na+ ion.
The calculated D values of oxidation peaks and reduction peaks
observed in CV profiles are plotted versus the peak position presented
in Figure 7E and Figure 7F, respectively. From the D values, one
could elucidate that NMC-900 possesses the lowest diffusion
coefficients, which is around 10-12.8 –
10-12.0 cm2.s-1,
so the migration of Na+ ions is not as favorable
comparing to the others. Meanwhile, the D values of the samples
NMC-800 and NMC-650 are higher, which is around
10-12.2 – 10-11.4cm2.s-1, so the migration of
Na+ ion is probably more favorable by the integration
of P3/P2 bi-phase than the P2 phase solely. The diffusion coefficient of
the P3/P2 bi-phase NMC-800 pretty coincided with the previous study35. To explain the notable higher diffusivity of P3/P2
bi-phase, one could agree that the diffusion of Na+ion in the P3 structure is higher than P2 and O3 in some cases25,27. Furthermore, for the sample NMC-800 that
exhibits P3/P2 bi-phase integration, the P2 component probably maintains
the structure but is electrochemically inactive, so the NMC-800
performance is not the best of the three, as expected.
Hence, the P-type layered structures exist within 650 - 900oC, given the high thermal stability of the P-phase in
the Na-Mn-Co system. The materials exhibit structural preservation after
a hundred cycles, so the structural and electrode interphase properties
play an essential role in the cycling performance. The two-phase
intergrowth cathodes were reported to improve the rate capability and
specific capacity contributed by the synergic effect of both components24,34,36–38. In this case, the P3/P2 integration in
NMC-800 is beneficial to capacity retention, but the capacity is lower
than the single-phase components, which could be the occurrence of an
unexpected tracking fault and/or cation mixing in micro/nanostructures
that need to be thoroughly examed. This structural fault might block the
sodium ion diffusion and hinder the electrochemical activity of the
phase components in the multiphase composite electrodes. Additionally,
the results demonstrated that the P3 phase could have a facile
environment for sodium ion conductivity, but poor capacity retention
compared to the P2 phase due to less structure stability.