A sodium-ion battery positive electrode material and a preparation method and application thereof

Abstract

The application belongs to the technical field of new energy materials, and particularly relates to a sodium ion battery positive electrode material and a preparation method and application thereof. The sodium ion battery positive electrode material is a P2 phase high-entropy layered transition metal oxide, and the preparation method comprises the following steps: (1) uniformly dispersing acetate in deionized water or ultrapure water to obtain solution A; (2) dissolving oxalic acid dihydrate in deionized water or ultrapure water to obtain solution B; (3) slowly adding solution A to solution B under the condition of heating and stirring, and heating and stirring in a water bath until evaporation; (4) vacuum drying the evaporated sample, and grinding to obtain a precursor powder; and (5) calcining the precursor powder, and then naturally cooling to room temperature to obtain the sodium ion battery positive electrode material. The preparation method is simple in operation, effectively solves the problems of structural instability of the sodium ion battery positive electrode material in the charge-discharge cycle process and poor cycle and rate performance, and has a wide application prospect.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of new energy materials, and particularly relates to a sodium ion battery positive electrode material and a preparation method and application thereof. BACKGROUND

[0002] With the rapid development of portable electronic products and electric vehicles, and the transformation of fossil fuels to renewable energy such as solar energy, wind energy, and tidal energy, people urgently need high-efficiency, stable, and low-cost rechargeable batteries. In recent years, lithium ion batteries have been rapidly developed due to their high energy density, high voltage, and long cycle life. However, with the growing market demand, the mass production and use of lithium ion batteries have led to a sharp rise in the price of lithium resources. More importantly, lithium batteries have poor safety and are dangerous in the process of use. At present, people have begun to study other low-cost, high-energy-density, and long-cycle-life rechargeable secondary batteries to replace lithium ion batteries. Since sodium has similar physical and chemical properties to lithium and high abundance in the earth's crust, sodium ion batteries are considered as an ideal low-cost alternative to lithium ion batteries, and have great application potential in smart grids, electric vehicles, and large-scale energy storage systems.

[0003] The electrochemical performance of the sodium ion battery positive electrode material, including the rate performance and cycle stability, determines the development potential of the sodium ion battery. At the same time, the cost of the positive electrode material affects the overall cost of the sodium ion battery. Therefore, improving and enhancing the performance of the positive electrode material is the key to studying the sodium ion battery. The positive electrode material of the sodium ion battery mainly includes the following categories: oxide materials, including transition metal layered oxides, such as tunnel type Na 0.44 MnO2, P2 phase Na 0.67 MnO2, and O3 phase NaMnO2 with a sodium content of more than 0.8; polyanion materials, such as phosphates, fluorophosphates, pyrophosphates, and sulfates; Prussian blue materials, such as cyanides formed by coordination of transition metals and Na; and sodium-containing organic compound materials. The tunnel type oxide positive electrode material can maintain structural stability in air and water, and is easy to store and transport. However, the sodium content of the material is limited, and the capacity is extremely low. The polyanion type positive electrode material has excellent cycle performance, and the industrial production technology of the material, such as Na3V2(PO4)3, can directly follow the production process of LiFePO4 of the lithium ion battery, so the material has good industrializability. However, the energy density of the material is not high, and the material contains toxic element V, which is not in line with the original intention of developing green sodium ion batteries. The Prussian blue material is CN -The complex formed with transition metal ions has a three-dimensional open pore structure, which can realize the rapid de-intercalation of sodium ions, and usually has good rate performance. However, the preparation process of such materials will produce toxic cyanide, and the content of crystallization water is difficult to control, and the dehydration is difficult, which has a great impact on the battery performance. The organic compound materials mainly have the disadvantages of low working voltage and low volume energy density, and have not been widely applied.

[0004] Layered transition metal oxides have become a promising candidate material due to their high theoretical capacity, simple structure and easy synthesis. According to the coordination environment of sodium polyhedron (prismatic and octahedral) and the stacking order of oxygen layer, the layered oxides are usually divided into O2, O3, P2 and P3 phase materials, and researchers have studied O3 and P2 phase materials most widely. The O3 phase layered oxide has high sodium ion content and high theoretical specific capacity. However, the O3 phase material has complex phase transition during charging and discharging, which causes serious damage to the structure of the positive electrode material during the electrochemical cycle. Compared with the O3 phase material, the P2 phase positive electrode material has an open prismatic diffusion channel in the sodium layer, and the diffusion speed of sodium ions is faster, thereby resulting in higher rate performance. However, due to the easy occurrence of harmful phase transition during charging, its low cycle stability is still the main problem for further application. Therefore, it has great practical significance to develop a sodium ion battery positive electrode material with high energy density and excellent cycle stability for the development of low-cost sodium ion batteries. SUMMARY

[0005] In view of the problems of the prior art sodium ion battery positive electrode material that the structure is unstable during the charging and discharging cycle, the capacity attenuation is serious, and the rate performance is poor, the present application provides a layered positive electrode material for sodium ion batteries and a preparation method and application thereof.

[0006] As one of the most common and important defects in transition metal oxides, oxygen vacancies can effectively regulate the electronic and crystal structure of the material. Oxygen vacancies can have a profound impact on the physical and chemical properties of transition metal oxide cathode materials, greatly affecting the performance indicators of sodium-ion batteries. Specifically, oxygen vacancies can adjust the electronic structure, charge capacity, electrical conductivity, cation diffusion, surface structure, and structural stability of oxide-based cathode materials, thereby making a significant contribution to the cycle life and performance of sodium-ion batteries. Therefore, the introduction of oxygen vacancies is an effective strategy to improve the electrochemical performance of sodium-ion battery cathode materials, which will facilitate sodium-ion diffusion, reduce charge transport resistance, and thus improve the capacity and rate performance of sodium-ion batteries. In the high-entropy layered cathode material of the application, lithium is designed to trigger the reaction characteristics of oxygen by generating non-bonding oxygen. By introducing Mg / Zn element doping, the binding ability to oxygen is further weakened, and oxygen participates in redox and is released in the first electrochemical reaction, leaving appropriate oxygen vacancies in the bulk phase, which greatly simplifies the operation difficulty and cost of introducing oxygen vacancies in the cathode material.

[0007] In order to achieve the above-mentioned purpose, the technical scheme of the present application is as follows:

[0008] In one aspect, the present application provides a sodium-ion battery cathode material, which is a P2 phase high-entropy layered transition metal oxide, and the chemical composition is Na a Li b Mg c Zn d Cu 0.05 Ni 0.18 Mn 0.67 O2, 0.70≤a≤0.80, 0<b≤0.1, 0≤c≤0.1, 0≤d≤0.1.

[0009] In another aspect, the present application provides a preparation method of the above-mentioned sodium-ion battery cathode material, which comprises the following steps:

[0010] (1) uniformly disperse acetate in deionized water or ultrapure water to obtain solution A;

[0011] (2) dissolve oxalic acid dihydrate in deionized water or ultrapure water to obtain solution B;

[0012] (3) under the condition of water bath heating and stirring, slowly drop solution A into solution B, continue water bath heating and stirring until evaporation to dryness;

[0013] (4) vacuum dry the evaporated sample, and grind to obtain a precursor powder;

[0014] (5) calcine the precursor powder, and then naturally cool to room temperature to obtain a sodium-ion battery cathode material.

[0015] Further, in step (1), the acetate raw material is sodium acetate, lithium acetate, magnesium acetate tetrahydrate, zinc acetate dihydrate, copper acetate monohydrate, nickel acetate tetrahydrate, and manganese acetate tetrahydrate.

[0016] Further, in step (1), the molar ratio of sodium acetate, lithium acetate, magnesium acetate tetrahydrate, zinc acetate dihydrate, copper acetate monohydrate, nickel acetate tetrahydrate, and manganese acetate tetrahydrate is (0.70-0.80):(0-0.1):(0-0.1):(0-0.1):(0-0.1):0.05:0.18:0.67, and the molar amount of lithium acetate is not 0.

[0017] Further, the chemical formula of the sodium ion battery positive electrode material is Na 0.76 Li 0.1 Cu 0.05 Ni 0.18 Mn 0.67 O2, the molar ratio of sodium acetate, lithium acetate, copper acetate monohydrate, nickel acetate tetrahydrate, and manganese acetate tetrahydrate is 0.76:0.1:0.05:0.18:0.67;

[0018] The chemical formula of the sodium ion battery positive electrode material is Na 0.71 Li 0.05 Mg 0.05 Cu 0.05 Ni 0.18 Mn 0.67 O2, the molar ratio of sodium acetate, lithium acetate, magnesium acetate tetrahydrate, copper acetate monohydrate, nickel acetate tetrahydrate, and manganese acetate tetrahydrate is 0.71:0.05:0.05:0.05:0.18:0.67;

[0019] The chemical formula of the sodium ion battery positive electrode material is Na 0.71 Li 0.05 Zn 0.05 Cu 0.05 Ni 0.18 Mn 0.67 O2, the molar ratio of sodium acetate, lithium acetate, zinc acetate dihydrate, copper acetate monohydrate, nickel acetate tetrahydrate, and manganese acetate tetrahydrate is 0.71:0.05:0.05:0.05:0.18:0.67.

[0020] Further, in step (1), the concentration of solution A is 1-2 mol / L.

[0021] Further, in step (2), the ratio of the molar amount of dihydrate oxalic acid to the total molar amount of metal cations is (1-2):1; the concentration of solution B is 1-2 mol / L.

[0022] Further, in step (3), the temperature of the water bath heating is 60-90 DEG C.

[0023] Further, in step (4), the vacuum drying temperature is 80-120 DEG C.

[0024] Further, in step (5), the calcination atmosphere is air or oxygen, and the calcination conditions are: heating at 350-450 DEG C for 2-5 h, and then calcining at 850-1000 DEG C for 10-15 h, with a heating rate of 2-5 DEG C / min.

[0025] The application further provides use of the above positive electrode material in a sodium ion battery.

[0026] The application has the following beneficial effects:

[0027] 1. The acetate transition metal precursor precipitate is prepared by co-precipitation, and the P2 phase high-entropy layered transition metal oxide positive electrode material for sodium ion batteries is obtained by annealing and calcining under a certain atmosphere, which is low in cost, simple in preparation method and can be expanded on a large scale.

[0028] 2. The lithium in the high-entropy layered positive electrode material is designed to trigger the reaction characteristics of oxygen by generating non-bonding state oxygen, and the lithium doping significantly improves the stability of the positive electrode material, which can resist phase changes caused by humidity and sliding between transition layers; the introduction of Mg / Zn element doping further weakens the binding ability to oxygen, and oxygen participates in redox and is released in the first electrochemical reaction, and then leaves appropriate oxygen vacancies in the bulk phase, which greatly simplifies the operation difficulty and cost of introducing oxygen vacancies in the positive electrode material. At the same time, Cu / Ni / Mn elements as the basic structure can undergo oxidation-reduction reaction in the charging and discharging cycle, and the electrons in the oxygen vacancy are transferred to the active transition metal center in the charging and discharging process, reducing the average valence of Cu / Ni / Mn, thereby significantly improving the specific capacity of the P2 phase positive electrode material. After accumulating enough oxygen vacancies in the first cycle, the degree of redox reaction of oxygen is sharply reduced in the subsequent charging and discharging cycle, avoiding further loss of lattice oxygen and irreversible structural degradation.

[0029] In summary, the positive electrode material triggers the oxidation-reduction reaction of oxygen in the first charging and discharging process, generates oxygen vacancies in situ through anion electrochemical induction, and the electrons are transferred to Cu / Ni / Mn and reduce their average valence, thereby improving the discharge specific capacity and structural reversibility of the sodium ion oxide positive electrode material. BRIEF DESCRIPTION OF DRAWINGS

[0030] Figure 1X-ray diffraction (XRD) pattern of the sodium-ion battery cathode material prepared for Example 1, Example 2 and Example 3;

[0031] Figure 2 Scanning electron microscope (SEM) image of the sodium-ion battery cathode material prepared for Example 1;

[0032] Figure 3 Scanning electron microscope (SEM) image of the sodium-ion battery cathode material prepared for Example 2;

[0033] Figure 4 Scanning electron microscope (SEM) image of the sodium-ion battery cathode material prepared for Example 3;

[0034] Figure 5 Cyclic voltammetry (CV) curve of the sodium-ion battery cathode material prepared for Example 1;

[0035] Figure 6 Cyclic voltammetry (CV) curve of the sodium-ion battery cathode material prepared for Example 2;

[0036] Figure 7 Cyclic voltammetry (CV) curve of the sodium-ion battery cathode material prepared for Example 3;

[0037] Figure 8 Electron paramagnetic resonance (EPR) spectrum of the sodium-ion battery cathode material prepared for Example 1;

[0038] Figure 9 Electron paramagnetic resonance (EPR) spectrum of the sodium-ion battery cathode material prepared for Example 2;

[0039] Figure 10 Electron paramagnetic resonance (EPR) spectrum of the sodium-ion battery cathode material prepared for Example 3;

[0040] Figure 11 Constant current charge-discharge curve of the sodium-ion battery cathode material prepared for Example 1;

[0041] Figure 12 Constant current charge-discharge curve of the sodium-ion battery cathode material prepared for Example 2;

[0042] Figure 13 Constant current charge-discharge curve of the sodium-ion battery cathode material prepared for Example 3;

[0043] Figure 14 Rate capability of the sodium-ion battery cathode material prepared for Example 1, Example 2 and Example 3;

[0044] Figure 15 The cycle performance of the sodium-ion battery cathode material prepared in Example 1, Example 2 and Example 3 was tested. DETAILED DESCRIPTION

[0045] The application will be further described in conjunction with the examples and the drawings, but the embodiments of the application are not limited thereto. All other examples obtained by those of ordinary skill in the art based on the examples in the application without creative labor are within the scope of protection of the application.

[0046] Unless otherwise specified, the materials used in the examples of the application can be obtained commercially or prepared according to conventional methods well known to those skilled in the art.

[0047] Example 1

[0048] A sodium-ion battery P2 phase high-entropy layered transition metal oxide cathode material has a chemical composition of Na 0.76 Li 0.1 Cu 0.05 Ni 0.18 Mn 0.67 O2, marked as P2-NLCNM, and the preparation method comprises the following steps:

[0049] (1) 3.7406 g of sodium acetate, 0.3959 g of lithium acetate, 0.5990 g of copper acetate monohydrate, 2.6875 g of nickel acetate tetrahydrate and 9.8036 g of manganese acetate tetrahydrate are uniformly dispersed in 50 mL of ultrapure water to obtain solution A;

[0050] (2) 13.3130 g of oxalic acid dihydrate is dissolved in 50 mL of ultrapure water to obtain solution B;

[0051] (3) Solution A is slowly added to solution B under the condition of 80℃ water bath heating and stirring, and the water bath heating and stirring is continued until it is evaporated to dryness;

[0052] (4) The evaporated sample is vacuum dried at 120℃, and the precursor powder is obtained after grinding;

[0053] (5) The precursor powder is placed in an air atmosphere, heated to 450℃ at a heating rate of 5℃ / min for 5h, and then calcined at a heating rate of 5℃ / min to 950℃ for 12h, and then naturally cooled to room temperature to obtain the sodium-ion battery cathode material P2-NLCNM.

[0054] Example 2

[0055] A sodium-ion battery P2 phase high-entropy layered transition metal oxide cathode material has a chemical composition of Na 0.71 Li 0.05 Mg0.05 Cu 0.05 Ni 0.18 Mn 0.67 O2, labeled as P2-NLMCNM, the preparation method comprising the following steps:

[0056] (1) uniformly dispersing 3.4945 g of sodium acetate, 0.1980 g of lithium acetate, 0.6434 g of magnesium acetate tetrahydrate, 0.5990 g of copper acetate monohydrate, 2.6875 g of nickel acetate tetrahydrate and 9.8036 g of manganese acetate tetrahydrate in 50 mL of ultrapure water to obtain solution A;

[0057] (2) dissolving 12.9348 g of oxalic acid dihydrate in 50 mL of ultrapure water to obtain solution B;

[0058] (3) slowly adding solution A to solution B under the condition of heating and stirring in a water bath at 80°C, and continuing to heat and stir in a water bath until evaporated to dryness;

[0059] (4) vacuum drying the evaporated sample at 120°C, and grinding to obtain a precursor powder;

[0060] (5) placing the precursor powder in an air atmosphere, heating to 450°C at a heating rate of 5°C / min for 5 h, and then calcining to 950°C at a heating rate of 5°C / min for 12 h, and then naturally cooling to room temperature to obtain a sodium ion battery positive electrode material P2-NLMCNM.

[0061] Example 3

[0062] A sodium ion battery P2 phase high-entropy layered transition metal oxide positive electrode material, the chemical composition of which is Na 0.71 Li 0.05 Zn 0.05 Cu 0.05 Ni 0.18 Mn 0.67 O2, labeled as P2-NLZCNM, the preparation method comprising the following steps:

[0063] (1) uniformly dispersing 3.4945 g of sodium acetate, 0.1980 g of lithium acetate, 0.6585 g of zinc acetate dihydrate, 0.5990 g of copper acetate monohydrate, 2.6875 g of nickel acetate tetrahydrate and 9.8036 g of manganese acetate tetrahydrate in 50 mL of ultrapure water to obtain solution A;

[0064] (2) dissolving 12.9348 g of oxalic acid dihydrate in 50 mL of ultrapure water to obtain solution B;

[0065] (3) slowly adding solution A to solution B under the condition of heating and stirring in a water bath at 80°C, and continuing to heat and stir in a water bath until evaporated to dryness;

[0066] (4) The evaporated sample was vacuum dried at 120°C, and the precursor powder was obtained after grinding;

[0067] (5) The precursor powder was heated to 450°C at a heating rate of 5°C / min for 5h, and then calcined at 950°C at a heating rate of 5°C / min for 12h, and then naturally cooled to room temperature, to obtain the sodium-ion battery cathode material P2-NLZCNM.

[0068] Test Example 1

[0069] Half-cell assembly: The sodium-ion battery P2 phase high-entropy layered transition metal oxide cathode materials prepared in Example 1, Example 2 and Example 3 were respectively slurried and coated with NMP as the solvent, and conductive carbon Super P and PVDF at a mass ratio of 8:1:1, and then cut into 14mm-diameter electrode sheets after vacuum drying at 120°C. The metal sodium was used as the negative electrode, and the high-pressure-resistant NaClO4 electrolyte purchased from Suzhou Duoduo Chemical Technology Co., Ltd. was used to assemble the half-cell in an argon-filled glove box.

[0070] Charge-discharge test: The voltage range of the coin cell charge-discharge was 2.0-4.3V, and all the electrochemical performance tests were carried out at room temperature.

[0071] The sodium-ion battery cathode materials prepared in Example 1, Example 2 and Example 3 were subjected to X-ray diffraction (XRD) test, and the results are shown in Figure 1 . By comparison, it can be found that the positions of the (002), (004), (100), (101), (012), (103) and (104) diffraction peaks correspond to the PDF card 54-0894 one by one, so the three materials all belong to the P2 phase layered structure of the P63 / mmc space group. In addition, the main characteristic peaks of Example 1, Example 2 and Example 3 are clear and have high intensity, indicating that the crystal phase of the materials is good.

[0072] The sodium-ion battery cathode materials prepared in Example 1, Example 2 and Example 3 were subjected to scanning electron microscope (SEM) test, and the results are shown in Figure 2 , Figure 3 and Figure 4 . By comparison, it can be found that the morphologies of the three materials are all hexagonal plate-shaped particles, the particles are angular and have smooth surfaces, and have good crystallinity. The particles of the three materials are about 3μm long and 1μm thick, and have no obvious agglomeration phenomenon.

[0073] The positive electrode materials of sodium-ion batteries prepared by Example 1, Example 2 and Example 3 were subjected to cyclic voltammetry (CV) test on CHI660E electrochemical workstation, with voltage range of 2.0-4.3V and scanning rate of 0.1mV / s, and the results are shown in Figure 5 、 Figure 6 and Figure 7 . In the CV curves of the three positive electrode materials, the peak at about 2.10V is attributed to the oxidation of Mn 3+ to Mn 4+ , the redox peaks between 2.5-4V are attributed to Ni 2+ / Ni 4+ , the redox peaks at 4.10 / 3.94V are attributed to Cu 2+ / Cu 3+ , and the redox peaks at 4.30 / 4.24V are attributed to O 2- / (O2) n- . By comparison, it can be found that the proportion of O 2- / (O2) n- in P2-NLCNM and P2-NLMCNM materials is smaller in the first charge-discharge process, while the proportion of O 2- / (O2) n- in P2-NLZCNM material is larger, which may be because the appropriate Zn doping weakens the binding ability to oxygen, so that more oxygen participates in the redox reaction and leaves enough oxygen vacancies. In addition, the oxygen redox chemistry in P2-NLZCNM gradually weakens in subsequent cycles, which means that the in-situ generation of appropriate oxygen vacancies is induced by the first cycle anion redox reaction, which stably exists in the subsequent cycles and further reduces the loss of lattice oxygen and stabilizes the crystal structure. In the charging process, Mn 3+ is oxidized to Mn 4+ , while in the discharging process, it cannot be reduced, which implies the accumulation of Mn 4+ . However, the CV curve shows that Mn 3+ is continuously oxidized to Mn 4+ , and the redox activity of Mn 3+ / Mn 4+ gradually increases, indicating that Mn 4+ is reduced to Mn 3+ in some way. In addition, compared with P2-NLCNM and P2-NLMCNM, the degree of Cu 2+ / Cu 3+ and Ni 2+ / Ni 4+ redox in P2-NLZCNM material gradually increases with the cycle, which greatly improves its rate and cycle performance.

[0074] The sodium-ion battery cathode materials prepared in Example 1, Example 2 and Example 3 were subjected to electron paramagnetic resonance (EPR) test on an A200 electron paramagnetic resonance spectrometer, with a voltage range of 2.0-4.3V and a test temperature of 110K, and the results are shown in Figure 8 、 Figure 9 and Figure 10 . By comparison, it can be found that the three materials all have a signal with a g value of 2.003 when charged to 4.15V, which is attributed to oxygen vacancies, and the oxygen vacancies exist throughout the charging and discharging process. In combination with the CV curve, it can be known that with the occurrence of oxygen redox reaction, oxygen is removed from the bulk structure and leaves oxygen vacancies, and the electron transfer reduces the average valence of Mn / Ni / Cu, thereby improving the electrochemical performance of the material. When discharged to 2.0V, the signal intensity of oxygen vacancies in P2-NLZCNM is the highest compared with P2-NLCNM and P2-NLMCNM, indicating that the concentration of oxygen vacancies in the material is higher. This may be because appropriate Zn doping weakens the binding ability to oxygen, leading to the induction of more oxygen vacancies in the electrochemical cycle, effectively adjusting the electronic structure and structural stability of the cathode material, thereby improving its electrochemical performance.

[0075] The sodium-ion battery cathode materials prepared in Example 1, Example 2 and Example 3 were subjected to constant current charge-discharge test on a CT3002A battery test system, with a voltage range of 2.0-4.3V and a current density of 0.2C (1C=120mA / g), and the results are shown in Figure 11 、 Figure 12 and Figure 13mAh / g, which is higher than its theoretical capacity of 103.9 mAh / g, indicating that oxygen participates in the redox reaction and provides capacity during the first charge-discharge process, which is consistent with the CV test results. In addition, the discharge specific capacities of P2-NLCNM for the first cycle, the tenth cycle, the twentieth cycle, the thirtieth cycle, the fortieth cycle and the fiftieth cycle are 98.3, 101.5, 102.3, 101.3, 101.1 and 99.7 mAh / g, respectively, and the discharge specific capacity continues to increase and reaches a maximum value at the twentieth cycle, with an increase of 4.0 mAh / g, and then gradually decreases. The first charge specific capacity of P2-NLMCNM is 108.8 mAh / g, which is higher than its theoretical capacity of 104.2 mAh / g, indicating that oxygen participates in the redox reaction and provides capacity during the first charge-discharge process, which is consistent with the CV test results. In addition, the discharge specific capacities of P2-NLMCNM for the first cycle, the tenth cycle, the twentieth cycle, the thirtieth cycle, the fortieth cycle and the fiftieth cycle are 99.8, 104.8, 107.8, 107.9, 106.9 and 103.8 mAh / g, respectively, and the discharge specific capacity continues to increase and reaches a maximum value at the thirtieth cycle, with an increase of 8.1 mAh / g, and then gradually decreases. The first charge specific capacity of P2-NLZCNM is 128.0 mAh / g, which is higher than its theoretical capacity of 107.2 mAh / g, indicating that oxygen participates in the redox reaction and provides capacity during the first charge-discharge process, which is consistent with the CV test results. In addition, the discharge specific capacities of P2-NLZCNM for the first cycle, the tenth cycle, the twentieth cycle, the thirtieth cycle, the fortieth cycle and the fiftieth cycle are 116.9, 130.0, 136.5, 140.9, 144.1 and 145.9 mAh / g, respectively, and the discharge specific capacity continues to increase and reaches a maximum value at the fiftieth cycle, with an increase of 29.0 mAh / g. Compared with P2-NLCNM and P2-NLMCNM, P2-NLZCNM has the largest first charge specific capacity, indicating that the degree of participation of oxygen redox reaction is the highest, which leads to the generation of more oxygen vacancies in the bulk phase of P2-NLZCNM. More importantly, the discharge specific capacity of P2-NLZCNM continues to increase during the cycle process and reaches a maximum value at the fiftieth cycle, with a growth rate much higher than that of P2-NLCNM and P2-NLMCNM. Combined with the CV results, it is shown that the redox activity of Mn 3+ / Mn 4+ , Cu 2+ / Cu 3+ and Ni 2+ / Ni 4+ continues to increase during the cycle process, leading to the continuous increase of the discharge specific capacity.

[0076] The sodium-ion battery cathode materials prepared in Examples 1, 2, and 3 were subjected to rate performance testing on a CT3002A battery testing system. The voltage range was 2.0-4.3V, and the current densities were 0.1C, 0.2C, 0.5C, 1C, 2C, 5C, 8C, 10C, and 20C (1C = 120mA / g). The results are as follows. Figure 14 As shown, the discharge specific capacities of P2-NLCNM at 0.1C, 0.2C, 0.5C, 1C, 2C, 5C, 8C, 10C, and 20C are 109.4, 107.2, 102.1, 97.7, 92.8, 82.5, 76.3, 72.9, and 58.2 mAh / g, respectively. When the current density returns to 0.1C, its specific capacity is 114.1 mAh / g, which is 4.7 mAh / g higher than the initial discharge specific capacity. The discharge specific capacities of P2-NLMCNM at 0.1C, 0.2C, 0.5C, 1C, 2C, 5C, 8C, 10C, and 20C are 102.8, 102.9, 100.4, 97.9, 94.3, 87.4, 83.3, 80.9, and 68.6 mAh / g, respectively. When the current density returns to 0.1C, its specific capacity is 108.9 mAh / g, which is 6.1 mAh / g higher than the initial discharge specific capacity. Compared with P2-NLCNM, the charge-discharge performance of the P2-NLMCNM cathode material is improved at high current densities, indicating that the oxygen vacancies brought by Mg doping reduce the average valence state of Mn / Ni / Cu, thereby promoting the increase in discharge specific capacity. The discharge specific capacities of P2-NLZCNM at 0.1C, 0.2C, 0.5C, 1C, 2C, 5C, 8C, 10C, and 20C are 134.2, 127.0, 122.7, 117.3, 111.6, 102.6, 97.2, 94.3, and 82.6 mAh / g, respectively. When the current density returns to 0.1C, its specific capacity is 161.1 mAh / g, which is 26.9 mAh / g higher than the initial discharge specific capacity. Compared to P2-NLCNM and P2-NLMCNM, the rate performance of the P2-NLZCNM cathode material is significantly enhanced, especially when returning from the ultra-high current density of 20C to 0.1C, the discharge specific capacity is much higher than its theoretical specific capacity. Therefore, by appropriately doping Zn to weaken the binding ability of oxygen, more oxygen vacancies are generated in situ through anion electrochemical induction during the first charge-discharge cycle, and the average valence state of Mn / Ni / Cu is further reduced, thereby improving the electrochemical performance of the material.

[0077] The sodium-ion battery cathode materials prepared in Examples 1, 2, and 3 were subjected to cycle performance tests on a CT3002A battery testing system, with a voltage range of 2.0-4.3V and a current density of 0.2C (1C = 120mA / g). The results are as follows. Figure 15The discharge specific capacity of P2-NLCNM and P2-NLMCNM cathode materials both slightly increased by 4.0 and 8.1 mAh / g, respectively, in the initial cycles, and then decreased obviously in the long cycles, and the capacity retention rates were 71.4 and 69.6% after 200 cycles, respectively. Notably, the discharge specific capacity of P2-NLZCNM cathode material continuously increased, and reached the maximum value of 145.9 mAh / g at the 50th cycle, with an increase of 29.0 mAh / g. The discharge specific capacity was 118.4 mAh / g after 200 cycles, and the capacity retention rate was more than 100%. Therefore, P2-NLZCNM cathode material has excellent long cycle performance and structural stability.

[0078] The technical features of the above-described embodiments can be combined in any manner. To make the description concise, all possible combinations of the technical features in the above-described embodiments are not described, but as long as the combinations of the technical features do not contradict each other, they should be considered within the scope of the present disclosure.

[0079] The above-described embodiments only express several implementation manners of the present application, and the description is relatively specific and detailed, but it should not be understood as a limitation on the scope of the patent. It should be pointed out that, for those skilled in the art, some modifications and improvements can be made without departing from the concept of the present application, and these all belong to the protection scope of the present application. Therefore, the protection scope of the patent of the present application should be subject to the appended claims.

Claims

1. A sodium-ion battery cathode material, characterized in that, The cathode material of the sodium-ion battery is a P2 phase high-entropy layered transition metal oxide with the chemical composition Na. a Li b Mg c Zn d Cu 0.05 Ni 0.18 Mn 0.67 O2, 0.70≤a≤0.80, 0 <b≤0.1,0<c≤0.1,0<d≤0.1。 2. A method for preparing the sodium-ion battery cathode material according to claim 1, characterized in that, The method includes the following steps: (1) Disperse acetate evenly in deionized water or ultrapure water to obtain solution A; (2) Dissolve oxalic acid dihydrate in deionized water or ultrapure water to obtain solution B; (3) While the solution is heated and stirred in a water bath, slowly add solution A to solution B, and continue heating and stirring in the water bath until the solution is evaporated to dryness; (4) The evaporated sample was vacuum dried and ground to obtain precursor powder; (5) The precursor powder was calcined and then naturally cooled to room temperature to obtain sodium-ion battery cathode material.

3. The preparation method according to claim 2, characterized in that, In step (1), the acetate raw materials are sodium acetate, lithium acetate, magnesium acetate tetrahydrate, zinc acetate dihydrate, copper acetate monohydrate, nickel acetate tetrahydrate, and manganese acetate tetrahydrate.

4. The preparation method according to claim 2, characterized in that, In step (1), the molar ratio of sodium acetate, lithium acetate, magnesium acetate tetrahydrate, zinc acetate dihydrate, copper acetate monohydrate, nickel acetate tetrahydrate, and manganese acetate tetrahydrate is (0.70-0.80):(0-0.1):(0-0.1):(0-0.1):0.05:0.18:0.67, and the molar amount of lithium acetate is not 0.

5. The preparation method according to claim 2, characterized in that, In step (1), the concentration of solution A is 1-2 mol / L.

6. The preparation method according to claim 2, characterized in that, In step (2), the molar ratio of the dihydrate oxalic acid to the total molar ratio of the metal cations is (1~2):1; the concentration of solution B is 1-2 mol / L.

7. The preparation method according to claim 2, characterized in that, In step (3), the water bath heating temperature is 60~90℃.

8. The preparation method according to claim 2, characterized in that, In step (4), the vacuum drying temperature is 80~120℃.

9. The preparation method according to claim 2, characterized in that, In step (5), the calcination atmosphere is air or oxygen, and the calcination conditions are: heating at 350~450℃ for 2~5h, followed by calcination at 850~1000℃ for 10~15h; the heating rate is 2~5℃ / min.

10. The application of the cathode material according to claim 1 or the cathode material prepared by any one of claims 2-9 in a sodium-ion battery.

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