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.