3.1. Effects of the preparation parameters on NaNMF
Orthogonal experimental design is an economical and time-saving technology for process parameter optimization. In this study, the three factors, reaction temperature, heating rate and soaking time were selected to analyze their influence on the performance of Na2/3Ni1/3Mn1/3Fe1/3O2 in the battery. Table 2 lists the specific parameters of each experiment, and the results are shown in Fig. 1. For the temperature factor, the specific capacity increases as the temperature rises. From 700 oC and 800 oC, the specific capacity does not change significantly with increasing temperature, but when the temperature rises to 900 oC, the specific capacity of the discharge changes greatly. Compared with the specific capacity of 800 oC, there is a significant increase, so the optimal calcination temperature of orthogonal experiment is 900 oC. As for the heating rate, as the heating rate increases, the change in discharge specific capacity first increases and then decreases. The discharge specific capacity is the highest when the heating rate is 4°C min− 1. According to Table 2, the range of heating rate is only 3.2, indicating that the influence of heating rate on discharge capacity is very small and can be ignored. Therefore, the optimal heating rate is 5 oC min− 1 based on the principle of improving efficiency. The trend chart of the soaking time is similar to that of the calcination temperature. With the increase of the soaking time, the discharge capacity of the first cycle also increases. It can be seen from the trend chart that when the soaking time is 12 h, the discharge capacity has a relatively large increase, so the best soaking time parameter is 12 h. The experimental results show that reaction temperature is the most important factor, followed by reaction time and soaking rate. Therefore, subsequent research focused on the effect of reaction temperature, while those of the calcination time and heating rate were not further investigated. Apparently, the synthetic condition of Na2/3Ni1/3Mn1/3Fe1/3O2 was as follows: heating rate 5 oC min− 1 and soaking time 12 h.
Table 1
Orthogonal experimental factors and levels.
Level
|
A
|
B
|
C
|
Temperature (oC)
|
Heating rate (oC min− 1)
|
Soaking time (h)
|
1
|
700
|
3
|
4
|
2
|
800
|
4
|
8
|
3
|
900
|
5
|
12
|
Table 2
Orthogonal experimental design.
Level
|
Temperature (oC)
|
Heating rate (oC/min)
|
Soaking time (h)
|
Specific capacity (mAh g− 1)
|
1
|
700
|
5
|
4
|
73.6
|
2
|
700
|
3
|
8
|
79.6
|
3
|
700
|
4
|
12
|
84.1
|
4
|
800
|
5
|
12
|
77.1
|
5
|
800
|
3
|
4
|
60.1
|
6
|
800
|
4
|
8
|
63.8
|
7
|
900
|
5
|
8
|
60.8
|
8
|
900
|
3
|
12
|
68.7
|
9
|
900
|
4
|
4
|
70.1
|
K1a
|
79.100
|
70.467
|
68.667
|
67.933
|
K2b
|
66.967
|
69.450
|
75.583
|
68.033
|
K3c
|
66.500
|
72.650
|
68.317
|
76.600
|
Rd
|
12.600
|
3.200
|
7.266
|
8.667
|
aStand for the corresponding mean value of the specific capacity at different temperatures; |
b Stand for the corresponding mean value of the specific capacity at different heating rate; |
c Stand for the corresponding mean value of the specific capacity at different calcination time; |
d Refers to the extreme difference, which can reflect the importance of the effective factor. |
3.2. Optimize reaction temperature
In order to optimize the experimental conditions, the calcination temperature is further discussed. According to the orthogonal test design, the calcination temperature is the main factor affecting the calcination effect. The samples were synthesized at 5 oC min− 1 heating rate, and the soaking time was 12 h.
The XRD patterns of P2- Na2/3Ni1/3Mn1/3Fe1/3O2 prepared at different temperatures show that all samples match JCPDS NO.54–0894. Hexagonal layered P2 phase with space group P63/mmc can be observed NNMO target. The lattice constants of P2-Na2/3Ni1/3Mn1/3Fe1/3O are a = b = 2.8922(3) Å, c = 11.1544(9) Å. However, for NaNMF-900, the presence of some peaks indicates that at lower substrate temperatures, the sample has lower crystallinity and poorer orientation. Surprisingly, as the substrate temperature rises to 950 oC, the peaks of (100) and (103) are suppressed rapidly, while only (002) and (004) peaks remain in the newly deposited films. Compared with NaNMF-900, the main peak intensities of NaNMF-950 and NaNMF-1000 were significantly enhanced and sharper, indicating that they were highly oriented and their crystallinity was greatly improved.
In order to study the effect of Fe on material properties, the oxidation state information of NaNMF-950 was studied by XPS (Fig. 3). The binding energy of the sample was calibrated by introducing the C 1s peak to 284.85 eV. The measurement spectrum of the sample shows clear peaks of C 1s, O 1s, Mn 2p, Fe 2p and Na 1s (Fig. 3a). Two main peaks were observed in the Fe 2p core level spectrum of the original NaNMF, 711.7 and 724.1 eV (Fig. 3b), which can be designated as Fe 2p3/2 and Fe 2p1/2, respectively. The results show that the oxidation state of iron is the trivalent state. Their Mn 2p spectrum shows two main peaks, Mn 2p3/2 at about 641.6 eV, and Mn 2p1/2 at 653.3 eV, which can be deconvoluted into four characteristic peaks. Those peaks appearing at the binding energy of ~ 642.7 and ~ 654.7 eV are related to Mn4+, while the binding energy peaks at ~ 642 and ~ 653 eV are related to Mn3+, revealing that Mn3+ and Mn4+ coexist. Similarly, in Fig. 3d, the five characteristic peaks in the Ni 2p spectrum correspond to the two main peaks of Ni 2p3/2 and Ni 2p1/2 at 854.3 eV and 871.9 eV, respectively, the satellite peak, which means that nickel ions are two price.
The morphological features of the sample was tested by TEM and SEM. As shown in Fig. 4, the obtained samples of NaNMF-950 are exhibit well-formed and composed of cube rock-like particles with the size distribution of ∼1 µm smooth surface. In Fig. 4 (g), EDS mapping of NNMF-950 clearly shows that Na, Ni, Mn and Fe are uniformly distributed. To obtain further information on the morphology and structure of the target product NaNMF-950, FE-TEM and HR-TEM were performed on the sample, as shown in Fig. 4d-f. Figure 4d-f with different magnifications displays a part of cube rock-like particle (bright part). The high-resolution TEM (HRTEM) image in Fig. 4f clearly shows the layered structure of this material. The (001) P2 structure has a crystal plane spacing of 0.25 nm, indicating the high crystallinity of the sample. TEM and SEM results confirmed the good crystallinity of Fe-substituted materials generally.
In order to study the influence of iron content and morphology on the performance of SIBs, the electrochemical performance of NaNMF-900, NaNMF-950 and NaNMF-1000 were examined in coin cell using temperature as reference variable. As shown in Fig. 5a,NaNMF-900, NaNMF-950 and NaNMF-1000 deliver an initial discharge specific capacity of 51.9, 72.8, 1 76.3 and 60.7 mAh g− 1 in the voltage range 2.0-4.2 V at 2.0 C, respectively. Figure 5b shows the galvanostatic charge/discharge curve of the NaNMF-950 sample at 2.0 C for the first 3 cycles. In the initial cycle, the charge capacity of the NaNMF-950 sample was 73.5 mA h g− 1, the discharge capacity was 67.8 mA h g− 1, and the Coulomb efficiency reached 91.7%. The plateau of about 3.75 V appeared in the first discharge and disappeared in the subsequent cycles, most likely due to the formation of the cathode-electrolyte interface layer, which led to the irreversible capacity in the first cycle.
In the following cycles, the charge/discharge curves almost overlap each other, indicating that the cathode material is highly reversible. Figure 5c compares the rate performance of NaNMF-900, NaNMF-950 and NaNMF-1000 particles. The NaNMF-950 sample provides the highest capacity at different current rates from 0.2 to 5 C. Specifically, for the NaNMF-950 samples, the reversible capacities obtained at 0.2 C, 0.5 C, 1.0 C, 2.0 C, and 5.0 C were 116.2, 96.8, 83.4, 72.9, and 53.1 mA h g− 1, respectively. Even at an extremely high current density of 5.0 C, a discharge capacity of 53.1 mA h g− 1 can still be achieved, which proves the high rate capability of the NaNCMA-950 sample. In addition, when the current rate returns to 0.2 C, the capacity returns to 113.3 mA h g− 1, which indicates that the NaNMF-950 cathode has high reversibility. In contrast, NaNMF-900 and NaNMF-1000 particles showed poor performance at all current rates. Figure 5d shows the cycle performance of four samples at 2.0 C, following the same trend: NaNMF-950 > NaNMF-1000 > NaNMF-900. Note that NaNCMA-900 particles provide the lowest capacity among the four samples, which means that material design and crystallinity play an important role in electrochemical performance. In the cycle test, the capacity retention rate of NaNCMA-950 sample after 60 cycles was 79.8%, which was equivalent to 81.1% of NaNMF-1000, and was higher than 72.8% of NaNCMA-900 particle sample.
3.3 NaNMFA (4-cation oxide)
The oxidation state of aluminium of NaNMFA-2 was investigated by XPS which are shown in Fig. 6. Deconvolute the XPS spectrum and use the XPSPEAK software with linear background and Lorentzian-Gaussian function for analysis. The Al 2p spectrum is shown in Fig. 6a. The Al-O bond of materials at 73.8 eV can be designated Al3+. Fe 2p, Mn 2p and Ni 2p still maintain a similar chemical valence state to the substrate NaNMF, indicating that the increase of cations did not destroy the original chemical composition structure.
Figure 7a-c shows the SEM images of the NaNMFA-2 sample. The enlarged SEM image (Fig. 7d) shows that sample have a similar sheet shape stacked together to form an overall morphology, with a particle size distribution between 1 and 10 µm. It was found that the surface of the secondary particles was clean and smooth, which was consistent with the high crystallinity. The element distribution of NaNMFA-2 (Fig. 7d) was further studied by EDS. The results show that Ni, Mn, Fe and Al are well distributed on the surface of NaNMFA-2.
In order to verify the preferred orientation of the NaNMFA-2 sample, HRTEM was used to further investigate the NaNMFA-2 (Fig. 8). Obviously, a single growth direction can be seen in Fig. 8c, d. The crystal plane spacing is 0.55 nm, which corresponds to the (002) lattice plane. These measurements indicate that the largest exposed surface is the ab plane, indicating that the preferred P2-NNMO thin film with excellent crystallinity and c-axis orientation has been successfully prepared, which is consistent with the XRD results (Fig. 1).
The electrochemical performance was examined to NaNMFA-2 cathode versus Na metal for the half cell. Figure 9a shows the typical charge and discharge curves of NaNMFA-2 at a current density of 0.2 C in different cut-off voltage windows of 2.0-4.2 V. The initial capacity of the first charging curve is 113.7 mAh g− 1. It is worth noting that the initial coulomb efficiency is as high as 80.1%, which has a prominent advantage in sodium ion semi battery. Starting from the second cycle, the cathode showed a highly reversible capacity of 105.1 mA h g− 1, equivalent to the extraction/insertion of 0.44 Na+ in charge/discharge [57]. Stability and rate capability are two important contrast parameters of high power energy storage devices. Therefore, as shown in Fig. 9b, all electrodes were continuously tested at varying currents of 0.2 C to 5 C between 2.0 V and 4.2 V. The charge transfer rate on the interface determines the capacity retention rate at high magnification [58]. Due to the influence of the crystallinity and element distribution of the original cathode (NaNMF-1), its performance is slightly lower than many previous reports, but its electrochemical performance can be further improved by increasing the configuration entropy (adding Al element). When the applied current increases due to the insufficient volume diffusion time, the charge and discharge curves are similar and there is no obvious voltage plateau. At higher current, NaNMFA-2 electrode shows better performance than the original electrode. The discharge capacities of the NaNMFA-2 electrode were 125.6, 119.1, 110.1, 101.2 and 90.8 mAh g− 1 at 0.2, 0.5, 1.0, 2.0 and 5.0 C rates, respectively. Due to polarization, the discharge capacity of all electrodes decreases linearly with the increase of current density [59]. The stability of NaNMFA-1, NaNMFA-2 and NaNMFA-3 were tested between 2.0 and 4.2 V, as shown in Fig. 9c. The capacity of NaNMFA-2 electrode was slightly higher than 102.8 mAh g− 1, and the capacity retention rate was 89.9% after 60 cycles, which was higher than that of the other two electrodes.
The electrochemical impedance spectroscopy (EIS) of NaNMFA-1, NaNMFA-2 and NaNMFA-3 electrodes were measured as depicted in Fig. 8d. The exchange current density and impedance values are shown in Table 3. All curves are related to a high-frequency region and a low-frequency region. The high-frequency region is attributed to the ohmic resistance (Rs) at the electrode-electrolyte interface, while the semicircle in the high-frequency region is related to the charge transfer resistance (Rct) at the active material interface [60]. The oblique line in the low-frequency region is the response of the bulk Na+ diffusion of the active material, that is, the waugh impedance. The NaNMFA-2 electrode displayed the lowest charge-transfer resistance (56.1 Ω), lower than that of the pristine NaNMFA-3 (82.3 Ω) and similar NaNMFA-1 (50.3 Ω) electrodes. In addition, according to the formula (j = RT/nFRct), the higher exchange current density of the NaNMFA-2 electrode were due to the lower charge transfer resistance (see Table 3) [61].
Table 3
Electrochemical properties of NaNMFA-1, NaNMFA-2 and NaNMFA-3 materials from EIS.
Sample
|
Rs (Ω)
|
Rct (Ω)
|
i0 (mA/cm− 2)
|
NaNMFA-1
|
679.5
|
50.3
|
5.1⋅10− 3
|
NaNMFA-2
|
302.3
|
56.1
|
4.6⋅10− 3
|
NaNMFA-3
|
938.1
|
82.3
|
3.1⋅10− 4
|
The highest exchange current density (j = 5.1 × 10− 3 mAˑcm2) was obtained with a NaNMFA-1 electrode, which was about 16 times that of NaNMFA-3 (j = 4.6×10− 4 mAˑcm2), which was similar to that of NaNMFA-2 (j = 4.6×10− 3 mAˑcm2). On the other hand, Al reduces the ohmic resistance of the material, and the order of ohmic resistance is NaNMFA-3 < NaNMFA-1 < NaNMFA-2. Therefore, NaNMFA-2 can make the charge transfer on the electrode-electrolyte interface easier, thereby reducing the internal resistance of the entire battery. In addition, the experimental results are in good agreement with the recyclability and high capacity of NaNMFA-2.
Based on the above analysis, the best rate performance and cycle stability of NaNMFA-2 samples can be attributed to the following two reasons: Compared with NaNMFA-1 samples, NaNMFA-2 and NaNMFA-3 samples may have aluminum element due to the increase in configuration entropy. It has a limiting effect on the multiphase transformation, and has better rate performance and cycle stability during the working process; at the same time, in the same cationic configuration system, with the increase of the amount of aluminum (NaNMFA-3), the performance will decrease This may be caused by the excessive doping of Al at the octahedral position, which causes the Na+ diffusion path to occupy the lattice dislocations. Therefore, for NaNMFA-2, higher sodium ion transfer resistance can be expected.