3.1 Inductively coupled plasma-atomic emission spectroscopy (ICP-AES) and XRD measurements of the cathode materials
Na0.8NixMn1−xO2 (x = 0.0, 0.1, 0.2, 0.3) were synthesized using a simple, one-step solid phase method, and their phase composition and crystal structure were analyzed. ICP-AES analysis confirmed that the actual proportion of elements in the sample was consistent with the expected proportion (Table 1). The peak positions of the four samples (Fig. 1a) are consistent with the standard card (JCPDS No.54–0894, space group P63/mmc), and no other impurity peaks were observed, indicating that the P2-type materials have high purity. No significant differences in the peak position of the (002) plane of NNM-8010, NNM-819, and NNM-828 were observed (Fig. 1b). In contrast, the peak position of the (002) plane of NNM-837 shifted to a higher angle, indicating a reduction in its lattice parameter c. This result is consistent with the results of refinement shown in Figs. 2a-d and Table 2 and is not conducive to the diffusion of Na+ ions 30. Furthermore, an increase in the nickel content shifted the (012) peaks of all the samples to a lower angle, with NNM-819 exhibiting the largest shift, indicating an increase in the (a, b) lattice parameter 30. This result is consistent with the results of refinement shown in Table 2. However, this shift is conducive to the transmission of Na+ and, hence, favorable for high electrochemical performance (Fig. 6a).
Table 1
Chemical components of Na0.8NixMn1−xO2 (x = 0.0, 0.1, 0.2, and 0.3)
Samples | Na | Mn | Ni |
NNM-8010 | 0.787 | 0.990 | - |
NNM-819 | 0.781 | 0.890 | 0.101 |
NNM-828 | 0.792 | 0.791 | 0.198 |
NNM-837 | 0.796 | 0.695 | 0.294 |
Table 2
Lattice parameters for NNM-8010, NNM-819, NNM-828, and NNM-837
Samples | a, b (Å) | c (Å) | c/a | V (Å3) |
NNM-8010 | 2.886631 | 11.313933 | 3.91937 | 81.644 |
NNM-819 | 2.890525 | 11.322013 | 3.90737 | 81.923 |
NNM-828 | 2.889497 | 11.293011 | 3.90829 | 81.317 |
NNM-837 | 2.883126 | 11.234961 | 3.91739 | 80.878 |
To explore the influence of the Mn/Ni ratio on the material morphology, SEM and EDS analyses were performed on the samples. The secondary particles of all the samples showed a sphere-like structure accompanied by obvious accumulation, with an average particle size of 2–5 µm (Figs. 3a-d). Compared with the surfaces of the other samples, the surface of NNM-819 was relatively smooth, indicating the formation of less resistive substances; this result was later confirmed via electron impedance spectroscopy (EIS, Fig. 8). The EDS spectrum of NNM-819 revealed that Na, Ni, Mn, and O were uniformly distributed on the surface of the material (Fig. 3e).
High-resolution TEM (HR-TEM) and the corresponding fast Fourier transform (FFT) were used to observe the microstructural changes in NNM-8010, NNM-819, and NNM-828 (Figs. 4b, f, and j). All samples have clear lattice fringes, suggesting a good, layered structure. No significant difference was observed in the lattice spacing (d = 0.597 nm) of NNM-819 and NNM-828, which is slightly wider than that of the original NNM-8010 (d = 0.558 nm). The lattice fringes correspond to the (002) crystal face of the hexagonal crystal system (P63/mmc). Moreover, the introduction of a certain amount of nickel is conducive to improving the diffusion kinetics of Na+.
XPS analysis was conducted to clarify the changes in the chemical valence states on the sample surfaces and the binding energy of the elements (Fig. 5). The Mn2p1/2 and Mn2p3/2 binding energies of NNM-819 and NNM-828 were shifted by + 0.64 and + 0.56 eV, respectively, compared to those of NNM-8010 (Fig. 5a). Additionally, the relative content of Mn3+ and Mn4+ in the XPS spectra (Table 3) indicates that the proportion of Mn4+ increases with increasing Ni content. The structural stability of the material is seriously damaged due to the Jahn–Teller effect of Mn3 + 31. However, the introduction of different amounts of Ni into the Mn sites effectively reduced the Mn3+ content in the sample from 23.55–17.11% (Table 3). Nevertheless, the increase in the valence state of Mn will decrease its electrochemical reactivity 31, thus reducing the specific capacity of the material. This result was later confirmed with electrochemical analyses (Fig. 6). No significant shift was observed for the Ni2p peak (Fig. 5b).
Table 3
Relative content of Mn3 + and Mn4 + in the X-ray photoelectron spectra of NNM-8010, NNM-819, and NNM-828
Samples | Mn (%) |
Mn3+ | Mn4+ |
NNM-8010 | 23.55 | 76.45 |
NNM-819 | 19.22 | 80.78 |
NNM-828 | 17.11 | 82.89 |
The electrochemical analyses of NNM-8010, NNM-819, NNM-828, and NNM-837 were performed in the voltage range of 2.00–4.25 V (Fig. 6). The electrochemical performance of each sample was analyzed at different current densities (0.1C, 0.2C, 0.5C, 1.0C, 2.0C, and 3.0C; Fig. 6a). The specific discharge capacities of the first cycle of NNM-8010, NNM-819, NNM-828, and NNM-837 were 200.1, 211.3, 171.8, and 156.4mAh g− 1, respectively, and their respective specific discharge capacities were 172.0, 187.5, 156.1, and 119.3mAh g− 1 at a current density of 0.2C after a high rate cycle. The capacity retention rates of NNM-8010, NNM-819, NNM-828, and NNM-837 were 86%, 88.7%, 90.9%, and 76.3%, respectively. NNM-819 exhibited a high specific discharge capacity and good reversibility. The effects of the Mn/Ni ratio on the cyclic performance of NNM-8010, NNM-819, NNM-828, and NNM-837 were compared. The initial specific discharge capacities of NNM-819 at the current densities of 0.2C, 0.5C, and 1.0C were 198.5, 165.6, and 165.7mAh g− 1, respectively (Figs. 6b, c, and d), and these capacities were the highest among all samples. NNM-819 further exhibited the highest specific discharge capacity even after 100 cycles. Its capacity retention rates at 0.2C, 0.5C, and 1.0C were 74.2%, 86.9%, and 81.9%, respectively, indicating good electrochemical properties. In contrast, the retention rates of NNM-8010, NNM-828, and NNM-837 after 100 cycles of cycling were only 54.9%, 57.4%, and 49.6% (Fig. 6b). The capacity retention rates of NNM-8010, NNM-819, and NNM-837 in a 1.0C high-rate cycle were 71.9%, 62.3%, and 61.7%, respectively (Fig. 6d). Therefore, NNM-819 showed the best discharge specific capacity and capacity retention rate among the samples. This result confirms that a Mn/Ni ratio of NNM-819 is optimal for an electrode material.
Strengthening the Mn–O bond (Fig. 5) weakens the repulsive forces of the adjacent oxygen layers in the material, which decreases the Na+ transmission energy barrier 32, inhibiting the structural transformation of the sample during battery operation. NNM-819 has the smallest voltage attenuation and a smoother charge-discharge curve as the cycle progresses (Fig. 7), indicating that its Mn/Ni ratio effectively inhibits the phase transitions.
EIS analyses were performed on NNM-8010, NNM-819, NNM-828, and NNM-837 to investigate the influence of the Mn/Ni ratio on electrode resistance (Fig. 8). The introduction of Ni significantly reduces electrode resistance. NNM-819 showed the least electrode resistance among the samples (200 Ω), and NNM-8010 showed the highest resistance (1000 Ω). These results are consistent with those of SEM (Fig. 3) and electrochemical data (Figs. 6 and 7).
To further clarify the influence of the Mn/Ni ratio on the cyclic properties of the materials, the samples were analyzed using SEM and XRD after 100 cycles. No significant difference was observed in the morphology of all samples after 100 cycles (Figs. 9a-d). NNM-819 showed a flatter morphology. Figure 9f shows the XRD patterns of NNM-8010 and NNM-819 before and after the 100 cycles. The spectral peaks were smaller after cycling, indicating the inevitable damage to the layered structure of the material after battery operation. In addition, at ~ 78° after 100 cycles is the characteristic peak of Al 33.