Sodium ion single crystal cathode material and preparation method thereof

Abstract

The application discloses a sodium ion single crystal positive electrode material and a preparation method thereof. The method uses a transition metal source compound as raw material, uses a NaOH solution as a precipitating agent, and adopts a co-precipitation method to obtain a transition metal oxide precursor; the prepared precursor is mechanically mixed with a lithium source in proportion, and then sintering is performed to obtain a positive electrode material; the sintered positive electrode material is mixed with a carbon source completely, and then the material is placed in a sintering furnace for sintering to obtain a sodium ion single crystal positive electrode material; the chemical general formula of the sodium ion single crystal positive electrode material is Na x Li y Ni u Mn v O2; wherein 0.54<=x<=0.7, y=0.1; 0.25<=u<=0.35; 0.65<=v<=0.75. Through a low-cost modification method such as doping and coating, a positive electrode material with excellent performance is obtained, the obtained material has high electrical conductivity, the capacity is increased by 20-30 mAh.g-1, the stability is obviously enhanced, and the cycle performance of the material is improved.

Description

TECHNICAL FIELD

[0001] The application is a kind of sodium ion single crystal positive electrode material and its preparation method, belonging to the technical field of sodium ion battery positive electrode material preparation. BACKGROUND

[0002] Sodium ion battery has become another emerging energy storage solution after lithium ion battery, which has attracted widespread attention due to its advantages of low cost, high capacity and high energy density. Layered manganese-rich oxides as a kind of high energy density sodium ion battery cathode material has the advantages of high rate performance, long cycle life and high energy density, and is the current application hotspot and is concerned. However, the current electrochemical stability still has problems. Layered sodium ion battery cathode material is easy to cause phase change during sodium ion deintercalation, resulting in poor cycle performance and weakened stability. It can be clearly seen that the existing sodium ion cathode material has not been mass-produced, the main reason is that the electrochemical performance such as capacity and cycle stability is poor. The reasons are as follows: first, the nickel and manganese content or proportion in the cathode material has a great influence on the performance; second, the existing way of doping is mainly noble metal doping, which increases the cost and reduces the specific capacity; third, the current coating mainly uses compounds with poor conductivity such as metal oxide to coat, and the coating layer is thick, which increases the cost and reduces the mass specific capacity. In summary, people still need to improve the sodium ion cathode material.

[0003] In order to improve the electrochemical performance of layered manganese-rich oxide sodium ion cathode material, enterprises and relevant college and other scientific and technological personnel have carried out a lot of work from the aspects of morphology, structure, element change, such as preparation method to improve the morphology, chemical element doping and surface modification to improve the structure stability, and have made significant progress. However, the layered manganese-rich oxide sodium ion cathode material is still difficult to meet the current application requirements in terms of capacity and charge-discharge cycle stability and other electrochemical aspects. Therefore, it is of practical significance to research and develop high-performance layered manganese-rich oxide sodium ion cathode material to promote the development of sodium ion. SUMMARY

[0004] The purpose of the present application is to solve the problems existing in the prior art, and to provide a sodium ion single crystal positive electrode material and its preparation method. In the method, lithium-doped sodium ion battery layered cathode material containing impurity atoms is obtained by pre-sintering and subsequent high-temperature sintering, and then macromolecular organic matter such as glucose and sucrose is mixed with the layered cathode material by mechanical mixing, and a thin-layer structure carbon coating layer is formed on the surface of the cathode material by high-temperature sintering again. Lithium ions as a structure supporting layer can provide a certain capacity and maintain the stability of the structure. As a protective layer, structural carbon can provide physical barrier on the surface of the cathode, protect the surface and inhibit the occurrence of side reactions, which can improve the structural stability of the material.

[0005] To achieve the above object, the specific technical scheme of the present application is as follows:

[0006] A preparation method of a sodium ion single crystal positive electrode material, comprising the following steps:

[0007] 1) A transition metal source compound is used as a raw material, a NaOH solution is used as a precipitating agent, and a co-precipitation method is used to obtain a transition metal oxide precursor;

[0008] 2) The precursor prepared in step 1) is mechanically mixed with a lithium source in a certain proportion, and then sintered to obtain a positive electrode material Na x Li y Ni u Mn v O2;

[0009] 3) The positive electrode material Na x Li y Ni u Mn v O2 sintered in step 2) is mixed with a carbon source, and then sintered in a sintering furnace to obtain a sodium ion single crystal positive electrode material; the chemical formula of the sodium ion single crystal positive electrode material is Na x Li y Ni u Mn v O2; wherein 0.54≤x≤0.7, y=0.1; 0.25≤u≤0.35; 0.65≤v≤0.75.

[0010] As a better embodiment in the present application, the transition metal source compound in step 1) includes a nickel source compound and a manganese source compound; wherein the nickel source compound is nickel acetate, nickel nitrate or nickel sulfate, etc.; the manganese source compound is manganese nitrate, manganese sulfate or manganese acetate, etc.

[0011] As a better embodiment in the present application, the molar ratio of nickel to manganese in the transition metal source compound is 1:3-1:1.86, and can be specifically 1:1.86, 1:1.9, 1:1.95, 1:2, 1:2.1, 1:2.2, 1:2.3, 1:2.4, 1:2.5, 1:2.6, 1:2.7, 1:2.8, 1:2.9, 1:3, etc.

[0012] As a better embodiment in the present application, the molar ratio of sodium to transition metal in step 1) is 0.45-0.95:1, and can be specifically 0.45:1, 0.5:1, 0.55:1, 0.6:1, 0.65:1, 0.7:1, 0.75:1, 0.8:1, 0.85:1, 0.9:1, 0.95:1, etc.

[0013] As a better embodiment in the present application, the lithium source in step 2) is lithium carbonate or lithium hydroxide; the molar ratio of the precursor to lithium carbonate is 1:0.54 to 1:0.7, and specifically can be 1:0.54, 1:0.56, 1:0.58, 1:0.6, 1:0.62, 1:0.64, 1:0.68, 1:0.7, etc.; the molar ratio of the precursor to lithium hydroxide is 1:0.08 to 1:0.12, and specifically can be 1:0.08, 1:0.09, 1:0.1, 1:0.11, 1:0.12, etc.

[0014] As a better embodiment in the present application, the ratio of the precursor to lithium carbonate, lithium hydroxide is 0.45≤x≤0.75, 0

[0015] As a better embodiment in the present application, the sintering step in step 2) is: first, pre-sintering in a high-temperature sintering furnace for 2-3 h (specifically, 2 h, 2.5 h, 3 h, etc.), and the pre-sintering temperature is 350-480°C (specifically, 350°C, 360°C, 370°C, 380°C, 390°C, 400°C, 410°C, 420°C, 430°C, 440°C, 450°C, 460°C, 470°C, 480°C, etc.), and after cooling to room temperature, the sample is taken out and mechanically ground. After grinding, the particles are uniform, and the particle size is 4-8 μm (specifically, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, etc.); then continue the second step of high-temperature sintering, and the sintering temperature of the second step is 850-1000°C (specifically, 850°C, 860°C, 870°C, 880°C, 890°C, 900°C, 910°C, 920°C, 930°C, 940°C, 950°C, 960°C, 970°C, 980°C, 990°C, 1000°C, etc.), and the time is 6-8 h (specifically, 6 h, 7 h, 8 h, etc.).

[0016] As a better embodiment in the present application, the carbon source in step 3) is sucrose or glucose; the positive electrode material Na x Li y Ni u Mn v O2 after sintering is mixed with a carbon source, and the carbon source accounts for 1.7-3.3 wt.% of the total mass of the positive electrode material, and specifically can be 1.7 wt.%, 1.8 wt.%, 1.9 wt.%, 2.0 wt.%, 2.1 wt.%, 2.2 wt.%, 2.3 wt.%, 2.4 wt.%, 2.5 wt.%, 2.6 wt.%, 2.7 wt.%, 2.8 wt.%, 2.9 wt.%, 3.0 wt.%, 3.1 wt.%, 3.2 wt.%, 3.3 wt.%, etc.

[0017] As a preferred embodiment in the present application, the sintering in step 3) is as follows: the material is placed in a sintering furnace, and then sintered at a high temperature after argon gas is introduced, the sintering temperature is 400-550°C (specifically, 400°C, 410°C, 420°C, 430°C, 440°C, 450°C, 460°C, 470°C, 480°C, 490°C, 500°C, 510°C, 520°C, 530°C, 540°C, 550°C, etc.), and the time is 2-3h (specifically, 2h, 2.5h, 3h, etc.).

[0018] The preparation steps of the transition metal nickel-manganese oxide precursor are as follows:

[0019] S1: transition metal salt compounds of nickel and manganese are weighed, and then dissolved in deionized water until clear and transparent, to obtain a nickel-manganese transition metal salt solution with a concentration of 2-4mol / L.

[0020] S2: a certain concentration of NaOH is used as a precipitant, and then uniformly added dropwise into the above nickel-manganese transition metal salt solution, and the pH is controlled at 10-11.5 (specifically, 10, 10.5, 11, 11.5, etc.) and the temperature is controlled at 50-60°C (specifically, 50°C, 51°C, 52°C, 53°C, 54°C, 55°C, 56°C, 57°C, 58°C, 59°C, 60°C, etc.), and after the solution is reacted for 1-2h, the product is filtered and dried to obtain a transition metal nickel-manganese oxide precursor.

[0021] NaOH is used as a precipitant to control the pH to be 10-11.5, so that the precipitation can be uniformly formed, and the integrity of the subsequent single crystal morphology is ensured. The temperature of 50-60°C can significantly affect the growth rate of the single crystal. At the same time, the precipitation reaction time is controlled to be 1-2h, so that the particle size of the single crystal is moderate, and the optimal value of the material performance is achieved.

[0022] Another object of the present application is to protect the sodium ion single crystal positive electrode material obtained by any one of the above method steps or the combination of the steps. The present application can obtain a sodium ion single crystal positive electrode material with excellent performance, and the method can significantly improve the specific capacity and cycle performance of the material.

[0023] In the sodium ion single crystal positive electrode material, when the sodium content (x) is too high, the material is extremely easy to absorb moisture and not easy to store, and when the sodium content is too low, the capacity and other electrochemical performances are significantly reduced. When the lithium atom content y=0.1, the material has excellent cycle stability, because the lithium ion can stabilize the interaction between the oxygen atoms in the transition metal layers, so that the structure is more stable. The nickel-manganese ratio (u:v) is better for the crystallinity of the material, so that the material has more excellent electrochemical performance.

[0024] Compared with the prior art, the present application has the following beneficial effects:

[0025] (I) The nickel and manganese contents or proportions in the positive electrode material have a greater influence on the performance, so the nickel and manganese proportion used in the present application is 0.25<=Ni u <=0.35; 0.65<=Mn v <=0.75, which is different from the existing layered structure.

[0026] (II) The existing doping is mostly noble metal doping, which increases the cost and reduces the specific capacity. The present application adopts lithium heteroatom doping and mixed sintering with metal oxides, uses the low melting point of lithium compounds, and adopts pre-sintering to make the crystal particles more uniform.

[0027] (III) The current coating mostly uses compounds with poor conductivity such as metal oxides, and the coating layer is thick, which increases the cost and reduces the mass specific capacity. The carbon coating layer used in the present application can significantly increase the electrical conductivity, which is beneficial to the capacity development and improves the cycle stability; the stability of the chemical properties can provide protection for the positive electrode; the raw material of the carbon material is sucrose or glucose, which is abundant in source and can reduce the synthesis cost.

[0028] (IV) The blending of sodium salt and lithium salt with the precursor enables the sodium / lithium and the precursor to be fully mixed, which reduces the cost of initial mixing; the doping site of lithium ions is located in the sodium ion layer, which helps the stability of the structure and has an important role in the capacity development of the sodium ion layered positive electrode.

[0029] (V) The low-cost modification methods such as doping and coating obtain a positive electrode material with excellent performance, and the obtained material has high electrical conductivity, capacity improvement of 20-30 mAh·g-1, and significantly enhanced stability, which improves the cycle performance of the material. BRIEF DESCRIPTION OF DRAWINGS

[0030] Figure 1 SEM of the sample of Example 1

[0031] Figure 2 SEM of the sample of Example 2

[0032] Figure 3 SEM of the sample of Example 3

[0033] Figure 4 SEM of the sample of Comparative Example 1

[0034] Figure 5 SEM of the sample of Comparative Example 2

[0035] Figure 6 Cycle stability of the sodium ion single crystal positive electrode material prepared by Examples 1-3 and Comparative Examples 1-2

[0036] Figure 7XRD pattern of sodium-ion single crystal cathode material prepared for Examples 1-3 and Comparative Examples 1-2

[0037] Figure 8 SEM for Comparative Example 3 sample

[0038] Figure 9 SEM for Comparative Example 4 sample

[0039] Figure 10 SEM for Comparative Example 5 sample

[0040] Figure 11 XRD for Comparative Examples 3-5 samples

[0041] Figure 12 Cycle performance graph for Comparative Examples 3-5 samples DETAILED DESCRIPTION

[0042] In order to make the objects, technical solutions and advantages of the present application clearer, the present application will be further described in detail below with reference to the embodiments. The specific embodiments described herein are only used to explain the present application and not intended to limit the present application.

[0043] In the present application, the unspecified proportion is mass ratio, and the % is mass percentage.

[0044] Example 1

[0045] 1) Preparation of transition metal oxide precursor: 13.91 g of NiSO4·7H2O and 25.11 g of MnSO4·H2O were dissolved in 100 ml of deionized water to obtain a clear transparent solution. 350 ml of 0.5 mol / L NaOH was used as a precipitant and was uniformly added dropwise to the transition metal salt solution, and the pH was controlled at 10.5 and the temperature was 55°C for 1 h. The reaction was filtered and dried to obtain the transition metal oxide precursor;

[0046] 2) Preparation of cathode material Na 0.54 Li 0.1 Ni 0.25 Mn 0.75 O2: 20 g of the transition metal oxide precursor prepared in step 1) was mixed with 5.67 g of Na2CO3 and 0.73 g of Li2CO3, and then was loaded into a porcelain boat and placed in a muffle furnace. Then it was pre-sintered at a temperature of 350°C for 180 min, and after cooling to room temperature, the sample was taken out and mechanically ground, and the particle size was 3-5 μm. Then secondary sintering was carried out; the secondary sintering program was set to a temperature increasing rate of 5°C / min, from room temperature to 850°C and kept at this temperature for 480 min, and after cooling, the cathode material Na 0.54 Li 0.1 Ni 0.25Mn 0.75 O2;

[0047] 3) Preparation of sodium-ion single-crystal cathode material: the sintered cathode material in step 2) is mechanically mixed with glucose (the addition amount is 2% of the mass of the cathode material) uniformly, then loaded into a porcelain boat and placed in a tube furnace, and the sample is calcined at 500℃ for 6h under nitrogen atmosphere. Naturally cooled to room temperature to obtain sodium-ion single-crystal cathode material Na 0.54 Li 0.1 Ni 0.25 Mn 0.75 O2@C.

[0048] As Figure 1 shown, the particle size of the cathode sample of this example is moderate, the particles are not agglomerated and no binding phenomenon occurs, and they are all polyhedral in morphology.

[0049] Example 2:

[0050] 1) Transition metal oxide precursor: 13.56g NiSO4·7H2O and 24.48g MnSO4·H2O were dissolved in 100ml deionized water to obtain a clear transparent solution. 400ml of 0.5mol / L NaOH was uniformly added to the transition metal salt solution as a precipitant, and the pH was controlled at 11 and the temperature was 55℃ for 2h. The reaction was filtered and dried to obtain a transition metal oxide precursor;

[0051] 2) Preparation of cathode material Na 0.64 Li 0.1 Ni 0.32 Mn 0.68 O2: 20g of the transition metal oxide precursor prepared in step 1) was mixed with 5.53g of Na2CO3 and 0.71g of Li2CO3, then loaded into a porcelain boat and placed in a muffle furnace, and then pre-sintered at 420℃ for 150min. After cooling to room temperature, the sample was taken out and mechanically ground, and the particle size was 3-5μm. Then secondary sintering was carried out; the temperature rising rate was set to 5℃ / min in the secondary sintering program, from room temperature to 900℃ and kept at this temperature for 420min, and after cooling, the cathode material Na 0.64 Li 0.1 Ni 0.32 Mn 0.68 O2;

[0052] 3) Sodium-ion single-crystal cathode material: the sintered cathode material in step 2) is mechanically mixed with sucrose (the addition amount is 3.3% of the mass of the cathode material) uniformly, then loaded into a porcelain boat and placed in a tube furnace, and the sample is calcined at 500℃ for 6h under nitrogen atmosphere. Naturally cooled to room temperature to obtain sodium-ion single-crystal cathode material Na 0.64 Li0.1 Ni 0.32 Mn 0.68 O2@C。

[0053] As Figure 2 shown, the particle size of the positive electrode sample of this example is moderate, the particles are agglomerated, no binding phenomenon occurs, and all are polyhedral morphology.

[0054] Example 3:

[0055] 1) Transition metal oxide precursor: 13.37 g of NiSO4·7H2O and 24.14 g of MnSO4·H2O were dissolved in 100 ml of deionized water to obtain a clear transparent solution. 400 ml of 0.5 mol / L NaOH was uniformly added to the transition metal salt solution as a precipitant, and the pH was controlled at 11.5 and the temperature was 55°C for 2h, and the reaction was filtered and dried to obtain a transition metal oxide precursor;

[0056] 2) Preparation of positive electrode material Na 0.7 Li 0.1 Ni 0.35 Mn 0.65 O2: 20 g of the transition metal oxide precursor prepared in step 1) was mixed with 5.45 g of Na2CO3 and 0.70 g of Li2CO3, then loaded into a porcelain boat in a muffle furnace, and pre-sintered at 480°C for 120 min, cooled to room temperature, and the sample was mechanically ground to a particle size of 3-5 μm. Then secondary sintering was carried out; the secondary sintering program was set to a heating rate of 5°C / min, from room temperature to 1000°C and kept at this temperature for 360 min, and after cooling, the positive electrode material Na 0.7 Li 0.1 Ni 0.35 Mn 0.65 O2;

[0057] 3) Sodium ion single crystal positive electrode material: the positive electrode material sintered in step 2) was mechanically mixed with glucose (added amount is 1.7% of the mass of the positive electrode material), then loaded into a porcelain boat in a tube furnace, and the sample was calcined at 500°C for 6h under nitrogen atmosphere. Naturally cooled to room temperature to obtain a sodium ion single crystal positive electrode material Na 0.7 Li 0.1 Ni 0.35 Mn 0.65 O2@C.

[0058] As Figure 3 shown, the particle size of the positive electrode sample of this example is moderate, the particles are agglomerated, no binding phenomenon occurs, and all are polyhedral morphology.

[0059] Comparative Example 1: Excessive Sodium Content

[0060] 1) Dissolve 13.27 g NiSO4·7H2O and 23.96 g MnSO4·H2O in 100 ml of deionized water to obtain a clear and transparent solution. Add 400 ml of 0.5 mol / L NaOH as a precipitant dropwise to the transition metal salt solution, controlling the pH at 11 and the reaction at 55℃ for 2 h. Filter and dry the reactants to obtain the transition metal oxide precursor.

[0061] 2) The 20g precursor prepared in step 1) was thoroughly mixed with 5.41g Na2CO3 and 0.70g Li2CO3, and then placed in a ceramic boat for pre-sintering in a muffle furnace. The pre-sintering was carried out at 460℃ for 150min. After cooling to room temperature, the sample was removed and mechanically ground to obtain a particle size of 3-5μm. Then, a second sintering was performed. The sintering program was set with a heating rate of 5℃ / min from room temperature to 800℃ and held at that temperature for 450min. After cooling, the positive electrode material Na was obtained. 0.75 Li 0.1 Ni 0.25 Mn 0.75 O2;

[0062] 3) Sodium-ion single-crystal cathode material: The sintered cathode material from step 2) was mechanically mixed with glucose (1.7% of the cathode material mass), then placed in a ceramic boat and calcined at 450℃ for 5 hours under a nitrogen atmosphere. The mixture was then allowed to cool naturally to room temperature to obtain the sodium-ion single-crystal cathode material Na. 0.75 Li 0.1 Ni 0.25 Mn 0.75 O2@C.

[0063] like Figure 4 As shown, SEM characterization revealed that the comparative cathode samples all exhibited polyhedral morphology, but particle size showed signs of fusion and agglomeration.

[0064] Comparative Example 2: Excessive Sodium and Lithium Content

[0065] 1) Based on sodium-ion cathode material Na that does not contain heteroatom lithium 0.8 Li 0.2 Ni 0.25 Mn 0.75The stoichiometric ratio of transition metal ions nickel and manganese in O2 was determined by weighing the transition metal source compound. 13.04 g NiSO4·7H2O and 23.55 g MnSO4·H2O were dissolved in 100 ml deionized water to obtain a clear and transparent solution. 400 ml of 0.5 mol / L NaOH was added dropwise to the transition metal salt solution as a precipitant, and the pH was controlled at 11. The reaction was carried out at 55℃ for 1.5 h. The reactants were filtered and dried to obtain a transition metal oxide precursor. 2) 20 g of the precursor prepared in step 1) was thoroughly mixed with 5.32 g Na2CO3 and 0.67 g Li2CO3 and placed in a porcelain boat in a muffle furnace. Pre-sintering was first performed at 370℃ for 180 min. After cooling to room temperature, the sample was removed and mechanically ground. In the secondary sintering program, the heating rate was set at 5℃ / min from room temperature to 850℃ and held at that temperature for 420 min. After cooling, the sample was removed from the muffle furnace to obtain the cathode material Na. 0.8 Li 0.2 Ni 0.25 Mn 0.75 O2;

[0066] 3) The sintered cathode material from step 2) was mechanically mixed with 1.69% glucose by mass, then placed in a ceramic boat and calcined at 500°C for 6 hours under a nitrogen atmosphere. After natural cooling to room temperature, carbon-coated sodium ion single-crystal cathode material Na was obtained. 0.8 Li 0.2 Ni 0.25 Mn 0.75 O2@C.

[0067] The sodium-ion single-crystal cathode materials prepared in Examples 1-3 and the materials prepared in Comparative Examples 1-2 were subjected to electrochemical performance tests. The specific results are shown in Table 1, which lists the first charge-discharge efficiency (first efficiency) of the examples and comparative examples. The improvement in first efficiency indicates that the examples can significantly improve the performance of the cathode material. Table 2 shows the 1C charge-discharge cycle performance data (capacity retention) of the examples and comparative examples, which shows that the cycle performance of the examples is significantly improved.

[0068] like Figure 5 As shown, SEM characterization revealed that the comparative cathode samples all exhibited polyhedral morphology, but the particle size showed significant interparticle fusion and agglomeration, as well as secondary particle formation, while single-crystal formation was not obvious.

[0069] Figure 6 Charge-discharge cycle tests of the examples and comparative samples are shown. The battery charge-discharge voltages were set to 2.0-4.3V, and the charge-discharge rate was 1C (theoretical specific capacity 140mAh·g⁻¹, current density 0.45mA·cm⁻¹). Figure 6As shown, the specific capacity and capacity retention of Comparative Example 1 and Comparative Example 2 are lower than the samples of the embodiments. Meanwhile, Table 1 and Table 2 list the initial capacity and cycle data, and the performance of the samples of the embodiments is obviously improved.

[0070] The comparative examples show that the improper addition of materials can cause single crystal melting agglomeration and lead to poor performance.

[0071] Table 1. First charge-discharge specific capacity and efficiency of sodium-ion single crystal positive electrode materials prepared in different cases

[0072]

[0073] Table 2. 1C charge-discharge cycle capacity retention of materials prepared in examples and comparative examples

[0074]

[0075] Figure 7 The X-ray diffraction analysis results of the samples of the embodiments and comparative examples are shown in the figure. As can be seen from the figure, the diffraction peaks present typical hexagonal crystal system characteristic peaks, which are consistent with the ideal structure of sodium-ion positive electrode materials, indicating that the prepared materials have reasonable structure.

[0076] Comparative Example 3: consistent with Example 1, the only difference is that no pre-sintering is performed

[0077] 1) Dissolve 13.91 g of NiSO4·7H2O and 25.11 g of MnSO4·H2O in 100 ml of deionized water to obtain a clear and transparent solution. 350 ml of 0.5 mol / L NaOH solution is uniformly added to the transition metal salt solution as a precipitant, and the pH is controlled at 10.5 and the temperature is 55℃ for 1 h. The reaction is filtered and dried to obtain a transition metal oxide precursor;

[0078] 2) Mix 20 g of the precursor prepared in step 1) with 5.67 g of Na2CO3 and 0.73 g of Li2CO3, and then load them into a porcelain boat and place them in a muffle furnace for sintering. The sintering program is set to a temperature increasing rate of 5℃ / min from room temperature to 850℃ and maintained at this temperature for 480 min. After cooling, the positive electrode material Na 0.54 Li 0.1 Ni 0.25 Mn 0.75 O2.

[0079] As Figure 8As shown in the figure, SEM characterization revealed that the unsintered cathode sample exhibited varying particle sizes, with irregularly shaped lamellar structures combining to form secondary particles. The lack of pre-sintering led to the melting and polymerization of particles, and the failure to break them up in time resulted in the continued growth of single-crystal particles and the formation of secondary particles, ultimately leading to poor electrochemical performance.

[0080] Comparative Example 4: Same as Example 2, except that no carbon coating was applied.

[0081] 1) Transition metal oxide precursor: 13.56 g NiSO4·7H2O and 24.48 g MnSO4·H2O were dissolved in 100 ml of deionized water to obtain a clear and transparent solution. 400 ml of 0.5 mol / L NaOH was added dropwise to the transition metal salt solution as a precipitant, and the pH was controlled at 11. The reaction was carried out at 55℃ for 2 h. The reactants were filtered and dried to obtain the transition metal oxide precursor.

[0082] 2) Preparation of cathode material Na 0.64 Li 0.1 Ni 0.32 Mn 0.68 O2: 20g of the transition metal oxide precursor prepared in step 1) was thoroughly mixed with 5.53g of Na2CO3 and 0.71g of Li2CO3, and then placed in a ceramic boat in a muffle furnace. The mixture was pre-sintered at 420℃ for 150min. After cooling to room temperature, the sample was removed and mechanically ground to obtain particles with a diameter of 3-5μm. A second sintering process was then performed; the second sintering program was set with a heating rate of 5℃ / min, from room temperature to 900℃ and held at that temperature for 420min. After cooling, the positive electrode material Na2CO3 was obtained. 0.64 Li 0.1 Ni 0.32 Mn 0.68 O2.

[0083] like Figure 9 As shown in the figure, SEM characterization revealed that the uncoated carbon cathode samples exhibited varying particle sizes, with irregularly shaped lamellar structures forming secondary particles. Carbon coating plays a crucial role in the interaction between single-crystal particles, contributing not only to conductivity but also significantly influencing the physical isolation between particles. The uncoated samples exhibited pronounced melt polymerization, which also resulted in poor electrochemical performance.

[0084] Comparative Example 5: Same as Example 2, except that lithium heteroatom doping was not performed.

[0085] 1) The stoichiometric ratio of transition metal ions nickel and manganese: Weigh out the transition metal source compound, dissolve 13.69 g NiSO4·7H2O and 24.71 g MnSO4·H2O in 100 ml deionized water to obtain a clear and transparent solution. Add 400 ml of 0.5 mol / L NaOH as a precipitant dropwise to the transition metal salt solution, control the pH at 10.5, and react at 55℃ for 1.5 h. Filter and dry the reactants to obtain the transition metal oxide precursor.

[0086] 2) The 20g precursor prepared in step 1) was thoroughly mixed with 5.58g Na2CO3 and placed in a ceramic boat in a muffle furnace. Pre-sintering was first performed at 420℃ for 150min. After cooling to room temperature, the sample was removed and mechanically ground. In the secondary sintering program, the heating rate was set at 5℃ / min from room temperature to 900℃ and held at that temperature for 420min. After cooling, the positive electrode material Na2CO3 was obtained. 0.64 Ni 0.25 Mn 0.75 O2;

[0087] 3) The sintered cathode material from step 2) was mechanically mixed with 1.71% glucose by mass, then placed in a ceramic boat and calcined at 500°C for 6 hours under a nitrogen atmosphere. After natural cooling to room temperature, carbon-coated sodium ion single-crystal cathode material Na was obtained. 0.64 Ni 0.25 Mn 0.75 O2@C.

[0088] like Figure 10 As shown in the figure, SEM characterization revealed that the cathode samples without lithium heteroatom doping exhibited inconsistent particle sizes and secondary particles. This is due to the different melting points during sintering; the undoped samples showed melt polymerization, leading to poor electrochemical performance.

[0089] Meanwhile, to further illustrate the superiority of the material in this invention, XRD tests were performed on comparative examples 3, 4, and 5. Figure 11 ) and electrochemical performance testing ( Figure 12 The tests showed that although these three comparative examples had the same structure as the examples, their electrochemical performance did not reach the performance of the examples.

[0090] The foregoing basic examples and their further alternative examples of the present invention can be freely combined to form multiple embodiments, all of which are embodiments that can be adopted and are claimed by the present invention. In the present invention, each alternative example can be arbitrarily combined with any other basic example and alternative example. Those skilled in the art will recognize numerous combinations.

[0091] The above merely describes preferred embodiments of the present application, and is not used to limit the present application, any modification, equivalent replacement and improvement within the spirit and principle of the present application should be included in the protection scope of the present application.

Claims

1. A method for preparing a sodium-ion single-crystal cathode material, characterized in that The method comprises the following steps: 1) obtaining a transition metal oxide precursor by co-precipitation using a transition metal source compound as a raw material and NaOH solution as a precipitant; The specific steps are as follows: a certain concentration of NaOH is uniformly added dropwise into a nickel-manganese transition metal salt solution, and the pH is controlled at 10-11.5 and the temperature is controlled at 50-60℃; after the solution is reacted for 1-2h, the product is filtered and dried to obtain a transition metal nickel-manganese oxide precursor; 2) The precursor prepared in step 1) is mixed with a sodium source and a lithium source in a certain ratio, and then sintered to obtain a positive electrode material Na x Li y Ni u Mn v O2; The sintering step is as follows: first, pre-sintering in a high-temperature sintering furnace for 2-3h, the pre-sintering temperature is 350℃-480℃, and the sample is taken out after cooling to room temperature and then subjected to sufficient mechanical grinding; after grinding, the particles are uniform, and the particle size is 4-8μm; then, high-temperature sintering in the second step is continued, the sintering temperature of the second step is 850℃-1000℃, and the time is 6-8h; 3) the sintered positive electrode material Na x Li y Ni u Mn v O2 in step 2) is mixed with a carbon source completely, and then the material is placed in a sintering furnace for sintering to obtain a sodium-ion single-crystal positive electrode material; The carbon source is sucrose or glucose; the sintered positive electrode material Na x Li y Ni u Mn v O2 is mixed with a carbon source, and the carbon source accounts for 1.7 wt.%-3.3 wt.% of the total mass of the positive electrode material; The chemical general formula of the sodium ion single crystal positive electrode material is Na x Li y Ni u Mn v O2; wherein 0.54≤x≤0.7, y=0.1; 0.25≤u≤0.35; 0.65≤v≤0.

75.

2. The method for preparing the sodium-ion single-crystal cathode material as described in claim 1, characterized in that: The transition metal source compound in step 1) includes a nickel source compound and a manganese source compound; the nickel source compound is nickel acetate, nickel nitrate or nickel sulfate; and the manganese source compound is manganese nitrate, manganese sulfate or manganese acetate.

3. The method for preparing the sodium-ion single-crystal cathode material as described in claim 1, characterized in that: The molar ratio of sodium to transition metal in step 1) is 0.45-0.95:

1.

4. The method for preparing the sodium-ion single-crystal cathode material as described in claim 1, characterized in that: The lithium source in step 2) is lithium carbonate or lithium hydroxide; the molar ratio of the precursor to lithium carbonate is 1:0.54 to 1:0.7; and the molar ratio of the precursor to lithium hydroxide is 1:0.08 to 1:0.

12.

5. The method for preparing the sodium-ion single-crystal cathode material as described in claim 1, characterized in that... The sintering in step 3) is as follows: the material is placed in a sintering furnace, inert gas argon is introduced, and then sintered at high temperature, the sintering temperature is 400℃-550℃, and the time is 2-3h.

6. The method for preparing the sodium-ion single-crystal cathode material as described in claim 2, characterized in that: The molar ratio of nickel to manganese in the transition metal source compound is 1:3-1:1.

86.

7. A sodium-ion single-crystal cathode material prepared by the method of any one of claims 1-6.

8. The sodium-ion single-crystal cathode material of claim 7, wherein: The material has good specific capacity and cycle performance and can be used as a cathode material.

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