Effects of the Mn/Ni Ratio on the Battery Performance of Layered Na-Ni-Mn Oxide Cathode Materials in Sodium-ion Batteries

The development of e�cient sodium-ion batteries is essential to overcome the issue of limited lithium sources for preparing lithium-ion batteries. Layered Mn-based cathode materials have signi�cant application potential because of their simple structure and high speci�c capacities. However, sodium-ion batteries with these cathode materials demonstrate considerable voltage attenuation and phase transition during battery operation. To eliminate these issues, in this study, we investigated the effects of different Mn/Ni ratios in Na-Ni-Mn cathode materials on their structural stability and electrochemical performances. Na0.8MnO2 (NNM-8010), Na0.8Ni0.1Mn0.9O2 (NNM-819), Na0.8Ni0.2Mn0.8O2 (NNM-828), and Na0.8Ni0.3Mn0.7O2 (NNM-837) were synthesized and characterized using X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, transmission electron microscopy, and electrochemical analyses. The addition of Ni + increased the Mn oxidation state from + 3 to + 4, thus reducing the Jahn–Teller effect of Mn 3+ and stabilizing the material structure. NNM-819 exhibited the best electrochemical performance. Its initial discharge speci�c capacity was 198.5mAh g − 1 at a current density of 0.2C, and the capacity retention rate after 100 cycles was 86.9% at 0.5C. Moreover, its capacity retention rate at 1.0C high-rate cycling after 100 cycles remained high 81.9%.


Introduction
Lithium-ion batteries are widely used as energy storage devices for electronic devices and electric vehicles.However, this has resulted in a shortage and uneven distribution of lithium sources [1][2][3][4][5][6] .To overcome this issue, next-generation batteries with high energy density and low cost must be developed.
The abundance of sodium in the Earth's crust has led to renewed interest in sodium-ion batteries 7,8 .Layered transition metal oxide cathode materials with high energy densities are particularly favored [9][10][11][12] .
P2 and O3 type layered transition metal oxide electrode materials are commonly used 13,14 , where "P" and "O" indicate that Na + ions exhibit a triangular prism and octahedral, respectively, and "2" and "3" represent the number of repeated transition metal layers 15 .
However, the main issue with sodium-ion batteries is the large radius of Na + (r = 1.02Å) [16][17][18] , which prevents the removal of Na + ions from the lattice structure, as their removal negatively affects the electrochemical performance of the battery [19][20][21] .Stabilizing the structure of the material is the best way to overcome this issue.The degree of Na removal can be controlled by limiting the charge cut-off voltage, and bulk-phase doping can inhibit the structural phase transition of the material [22][23][24][25][26][27] .However, limiting the cut-off voltage will cause a capacity loss in the battery, and hence, introducing other ions into the lattice structure of the material is the preferred way to enhance the structural stability of the material 28 .Tingting Yang et al. introduced Ti + ions into the positive electrode material Na 2/3 Fe 1/3 Mn 2/3 O 2 to suppress the Jahn-Teller distortion and P2-P2' transitions of the electrode material during battery operation, while weakening the ordered and disordered changes of Na + ions and /vacancies 29 .This strategy stabilizes the material structure and considerably improves the cycle stability and electrochemical reversibility of the battery.

Cathode material preparation
Four samples with different Mn/Ni ratios (NNM-8010, NNM-819, NNM-828, and NNM-837) were synthesized via a solid-phase method.Na 2 CO 3 , NiO, and Mn 2 O 3 were weighed according to the desired stoichiometry ratio and reacted in a planetary ball mill for 10 h, with anhydrous ethanol as the dispersant.
The product was dried in a drying oven at 85 ℃ for 6 h and air-red in a tube furnace to 875 ℃ and at a heating rate of 3 ℃/min for 15 h.The nal product was then cooled to room temperature and stored in a glovebox lled with argon gas.

Materials characterization
The phase analysis of the synthetic material was conducted using an X-ray diffractometer (MiniFlex 600), with Cu Kα radiation at a scanning range of 10 − 80° and a scanning rate of 3°/min.The XRD patterns were re ned using the GSAS software.The morphology and structure of the materials were analyzed by SEM (ZEISS type).The elements of the samples were analyzed using an energy-dispersive X-ray spectrometer (EDS).The lattice fringes and morphology of the samples were observed using TEM (FEI Tecnai F20).XPS (Thermo Scienti c K-Alpha) was used to measure the valence states of the samples and the position of the binding energy of each element.

Battery fabrication and its electrochemical characterization
The active material (80 wt%), acetylene black (10 wt%), and polyvinylidene di uoride (PVDF) (10 wt%) were mixed in N -methyl-2-pyrrolidone (NMP) to make a slurry, which was coated on aluminum foil.The foil was dried in a drying oven at 120 ℃ for 30 min, cut into 12 mm discs, and placed in a vacuum drying oven at 60 ℃ for > 8 h.These sodium sheets were used as the negative electrodes, Whatman GF/B-grade glass ber lter paper was used as the diaphragm, and NaClO4 was used as the electrolyte (1.0 M NaClO4 in EC:DEC = 1:1 vol% with 5.0% FEC).The LIR2032 half battery was assembled in a glove box lled with argon gas.The assembled battery was allowed to stand in air for > 12 h at 25 ℃ before electrochemical analyses.The current density during the cycle was 0.2C, 0.5C, and 1.0C, and the voltage range was 2.0-4.25 V.

Results and discussion
3.1 Inductively coupled plasma-atomic emission spectroscopy (ICP-AES) and XRD measurements of the cathode materials Na 0.8 Ni x Mn 1−x O 2 (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 con rmed 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 signi cant 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 re nement 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 re nement shown in Table 2.However, this shift is conducive to the transmission of Na + and, hence, favorable for high electrochemical performance (Fig. 6a).To explore the in uence 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 con rmed 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 signi cant 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 Mn2p 1/2 and Mn2p 3/2 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 Mn 3+ and Mn 4+ in the XPS spectra (Table 3) indicates that the proportion of Mn 4+ increases with increasing Ni content.The structural stability of the material is seriously damaged due to the Jahn-Teller effect of Mn 3 + 31 .However, the introduction of different amounts of Ni into the Mn sites effectively reduced the Mn 3+ 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 speci c capacity of the material.This result was later con rmed with electrochemical analyses (Fig. 6).No signi cant shift was observed for the Ni2p peak (Fig. 5b).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 speci c discharge capacities of the rst 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 speci c 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 speci c 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 speci c 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 speci c 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 speci c capacity and capacity retention rate among the samples.This result con rms 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.
To further clarify the in uence of the Mn/Ni ratio on the cyclic properties of the materials, the samples were analyzed using SEM and XRD after 100 cycles.No signi cant difference was observed in the morphology of all samples after 100 cycles (Figs.9a-d).NNM-819 showed a atter 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 .

Conclusion
We investigated the effects of different Mn/Ni ratios on the structural stability of Na-based layered transition metal oxide cathode materials.A series of Na 0.8 Ni x Mn 1−x O 2 (x = 0.0, 0.1, 0.2, and 0.3) were synthesized.XRD re nement as well as HR-TEM with corresponding FFT revealed a slight increase in the (a, b) lattice parameter owing to the shift of the (012) diffraction peak to a lower angle.The (002) crystal surface spacing of the sample increased from 0.558 to 0.597 nm, which con rmed that the introduction of Ni facilitated the diffusion of Na + ions, thereby enhancing battery performance.XPS analysis suggested that the introduction of Ni increases the valence state of Mn from + 3 to + 4, thus weakening the Jahn-Teller effect of Mn 3+ during battery operation and averting structural damage to the material.Moreover, strong Mn-O bonding is conducive to expanding the spacing of the Na layers.NNM-819 showed the highest reversible speci c capacity (198.5 mAh g − 1 ) at a current density of 0.2C in the voltage range of 2.00-4.25 V and the highest capacity retention rate (86.9%) at a current density of 0.5C.
This study might be a method to increase the energy density of sodium-ion batteries in the future.

Declarations
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Figure 4 Transmission
Figure 4