Composite sodium ion oxide single-crystal positive electrode material as well as preparation method and application thereof

Abstract

The invention discloses a composite sodium ion oxide single-crystal positive electrode material as well as a preparation method and application thereof, and belongs to the technical field of battery materials. The prepared positive electrode material is of a core-shell structure, the chemical formula of the positive electrode material is Na Mn < x > Mg < y > M < z > NO2, x is more than or equal to 0.5 and less than or equal to 1, y is more than or equal to 0 and less than or equal to 0.5, M is a doping element, z is more than or equal to 0.005 and less than or equal to 0.11, and N is a shell element. According to the preparation method, a precursor does not need to be synthesized, metal or salts thereof are directly used as raw materials, in addition, primary sintering, wet coating and secondary sintering are completed in the same device, so that the production time is shortened by more than 20 times, and the processing cost of the positive electrode material is reduced by more than 60%; in addition, wet coating and a microwave or infrared sintering process are combined, the prepared positive electrode material can form a high-pressure solid core-shell composite structure, and the particle strength of the positive electrode material is improved, so that the electrochemical performance of the positive electrode material is improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of battery materials, and particularly relates to a composite sodium ion oxide single crystal cathode material, a preparation method thereof, and an application thereof. Background Art

[0002] Driven by the global energy structure transformation and green and low-carbon development, the new energy vehicle industry has become an important part of the global strategic emerging industries. As the core component of new energy vehicles, the continuous innovation of power battery technology has become the key to the industry's development. In the current mainstream power battery systems, lithium-ion batteries occupy the market leading position with their high energy density, long cycle life, and mature technical routes, and are widely used in fields such as electric vehicles, consumer electronics, and energy storage systems. However, problems such as uneven geographical distribution of lithium resources, rising costs, and performance degradation in low-temperature environments have prompted the academic and industrial circles to actively explore diversified battery technology routes. Under this background, differentiated technical paths such as sodium-ion batteries, hydrogen fuel cells, and lead-acid batteries have gradually formed a pattern of coordinated development.

[0003] Among them, sodium-ion batteries have attracted much attention due to their unique resource endowment and performance characteristics. This battery system realizes energy storage based on the reversible migration of sodium ions between the positive and negative electrodes. Its electrochemical principle is similar to that of lithium-ion batteries, but the abundance of sodium in the earth's crust (2.74%) is significantly higher than that of lithium (0.0065%), having a natural cost advantage. At the technical level, sodium-ion batteries exhibit excellent low-temperature tolerance (can operate stably in an environment of -40°C), intrinsic safety characteristics, and deep discharge storage capabilities, and at the same time are superior to lithium-ion batteries in terms of rate charge and discharge performance technical indicators. This makes sodium-ion batteries show significant competitiveness in scenarios such as large-scale energy storage power stations, low-speed electric vehicles, backup power systems, and distributed energy networks. Notably, with the optimization of the material system and the emergence of the industrial chain scale effect, the cost per kilowatt-hour of sodium-ion batteries is expected to be further reduced, and there will be a clear industrialization trend to replace traditional lead-acid batteries in fields such as electric bicycles and micro-electric vehicles, thus constructing a diversified battery technology ecosystem complementary to lithium-ion batteries.

[0004] At present, due to the high cost of the layered oxide single crystal cathode material for sodium-ion batteries, the cost of the entire battery cell is high, resulting in slow market promotion of sodium-ion batteries. However, one way to solve the high cost of the layered oxide single crystal cathode material is to reduce the processing cost of the material, because the processing cost accounts for about 30-40% of the cathode material cost. Generally, the preparation of the layered oxide single crystal cathode material is to first synthesize a hydroxide precursor, then mix it with a sodium source and sinter it, and then perform post-treatment to obtain the final product. The synthesis of the hydroxide precursor not only has a long cycle, but also generates waste water, waste gas and waste residue. On the other hand, the particle strength of the cathode material not only determines the compaction density of the battery electrode sheet, but also affects the battery cycle performance. Because during long-term cycling, the low-strength sodium-ion battery cathode material particles are continuously compressed / expanded by the insertion / extraction of sodium ions during the charge / discharge cycle, resulting in cracks and fragmentation of the particles. Therefore, reducing the preparation cost of the layered oxide single crystal cathode material for sodium-ion batteries and improving the particle strength of the sodium-ion battery cathode material are key technical problems. Summary of the Invention

[0005] In view of the above-mentioned prior art, the present invention discloses a composite sodium-ion oxide single crystal cathode material, its preparation method and application, to solve the technical problems of high cost of the sodium-ion battery cathode material in the prior art, resulting in expensive battery prices, and low particle strength of the sodium-ion battery cathode material, resulting in weak electrochemical performance.

[0006] To achieve the above object, the technical solution adopted by the present invention is: to provide a composite sodium-ion oxide single crystal cathode material, the composite sodium-ion oxide single crystal cathode material has a core-shell structure, and the chemical formula is Na a Mn x Mg y M z NO2, where 0.4 < a ≤ 1.1, 0.5 ≤ x ≤ 1, 0 ≤ y ≤ 0.5, M is a doping element, 0.005 ≤ z ≤ 0.11, M is at least one of Fe, Ni, Cu, Zr, Cs, Mo, Sn, Zn, Ca, Li, Ru, K, In, Se, Si, S, B, Te, Cr, Sb, La, Gd, Nb, W and F; N is a shell element, N is at least one of Cu, Zr, Cs, Sn, Zn, Ca, Ti, W, Ru, Se, Si, B, Te, Sb, La, Gd, Nb and Al, and the shell element accounts for 0.05-1% of the overall mass of the cathode material, and the shell thickness is 2-10 nm.

[0007] The beneficial effect of the above technical solution is: The cathode material does not use expensive Ni and Cu as the main raw materials, but uses inexpensive Mn as the main raw material, reducing the raw material cost.

[0008] The present invention also discloses a preparation method of the composite sodium ion oxide single crystal cathode material, which comprises the following steps: S1: A sodium source, a manganese source, a magnesium source and a dopant containing a doping element M are placed in deionized water, nitric acid and a chelating agent are added, and after dissolution, the pH of the solution is adjusted to 4-9. After spray drying the solution, a solid powder is obtained; M is at least one of Fe, Ni, Cu, Zr, Cs, Mo, Sn, Zn, Ca, Li, Ru, K, In, Se, Si, S, B, Te, Cr, Sb, La, Gd, Nb, W and F; S2: The solid powder is subjected to a first sintering to obtain an intermediate, and then the intermediate is wet-coated with a coating agent containing a shell element N, and then subjected to a second sintering, pulverized and sieved to obtain the product; the sintering method is infrared sintering or microwave sintering; N is at least one of Cu, Zr, Cs, Sn, Zn, Ca, Ti, W, Ru, Se, Si, B, Te, Sb, La, Gd, Nb and Al.

[0009] The beneficial effects of the above technical solution are as follows: Instead of synthesizing a precursor, a metal or its salt is added to deionized water together with a sodium salt, and then nitric acid and a chelating agent are added to form a metal salt solution. Then, solid powder particles are obtained by spray pyrolysis. Then, after the solid powder is sintered once by infrared or microwave, wet coating is carried out through a nozzle and then sintered a second time by infrared or microwave to finally obtain a high-compact core-shell composite structure sodium ion battery layered oxide single crystal cathode material. Microwave heating generates microwaves inside the object to be heated, and the heat source comes from inside the object. Therefore, it can heat evenly without temperature difference, can quickly increase the temperature, improve the heating efficiency, can realize continuous production, and the comprehensive energy consumption is reduced by about 70%; Infrared heating transfers radiation to the object to be heated through infrared rays. After the radiation source is heated, its internal energy is converted into radiant energy, that is, infrared rays directly radiate from the heat source to the surface of the object to be heated in the form of electromagnetic waves and transfer heat through radiation; Due to the high penetrability of infrared heating, it can heat both the surface and the inside of the heated object simultaneously, realizing high efficiency and energy saving. Microwave or infrared heating can enable sodium ions to diffuse and migrate evenly on the surface and inside of the layered oxide single crystal cathode material particles. Due to the temperature uniformity of microwave and infrared heating and heating from the inside of the particles, the gas inside the particles can be quickly discharged, thereby reducing particle voids and grain boundaries. Therefore, it helps to form a single crystal particle morphology with high strength. At the same time, infrared or microwave heating can quickly open the oxygen-metal bonds on the surface of the coated matrix particles, making the oxygen ion bonds of the matrix more easily and tightly combined with the bonds of the coating elements, generating stronger bond energy. In addition, the combination of infrared or microwave sintering and wet coating can increase the compressive strength value of the cathode material particles.

[0010] On the basis of the above technical solution, the present invention can be further improved as follows.

[0011] Further, in step S1, the sodium source is at least one of sodium carbonate, sodium bicarbonate, sodium hydroxide, and sodium nitrate; the manganese source and the magnesium source are at least one of oxalates, acetates, nitrates, sulfates, chlorides, oxides, hydroxides, and metal elements containing manganese or magnesium; the dopant is an M element or a compound containing M, or the dopant is at least one of iron oxide, iron hydroxide, metal nickel, nickel oxide, nickel hydroxide, metal copper, copper oxide, copper hydroxide, zirconium oxide, zirconium hydroxide, cesium oxide, metal molybdenum, molybdenum oxide, ammonium molybdate, tin oxide, metal zinc, zinc oxide, zinc hydroxide, calcium oxide, calcium hydroxide, lithium oxide, lithium carbonate, lithium hydroxide, ruthenium oxide, potassium oxide, potassium hydroxide, indium oxide, selenium oxide, silicon oxide, sulfur oxide, boric acid, boron oxide, tellurium oxide, chromium oxide, antimony oxide, lanthanum oxide, gadolinium oxide, niobium oxide, tungsten oxide, ammonium tungstate, ammonium fluoride, and sodium fluoride.

[0012] Further, the chelating agent is at least one of polyethylene glycol, ammonia water, sucrose, NTA, EGTA, EUG, EA, PEO, PEG-EA, ADH, DAAM, glycine, alanine, proline, and alginic acid.

[0013] Further, the mass ratio of deionized water, nitric acid, and the chelating agent is 100:0.1 - 1:0.015 - 0.5; the metal molar amount in the sodium source is A, and the total metal molar amount in the manganese source, magnesium source, and dopant is B, and A:B = 0.4 - 1.1:1.

[0014] Further, the spray drying temperature is 100 - 250 °C, and the spray drying time is 1 - 30 s.

[0015] Further, the first sintering temperature is 600 - 800 °C, and the sintering time is 1 - 3 h.

[0016] Further, the particle size D of the coating agent 50 < 50 nm; the coating agent is a compound containing N; or the coating agent is at least one of copper oxide, copper hydroxide, zirconium oxide, zirconium hydroxide, cesium oxide, tin oxide, zinc oxide, zinc hydroxide, calcium oxide, calcium hydroxide, titanium oxide, tungsten oxide, ammonium tungstate, ruthenium oxide, selenium oxide, silicon oxide, boric acid, boron oxide, tellurium oxide, antimony oxide, lanthanum oxide, gadolinium oxide, niobium oxide, aluminum oxide, and aluminum hydroxide.

[0017] Further, the second sintering temperature is 200 - 500 °C, and the sintering time is 1 - 3 h.

[0018] The present invention also discloses the application of this composite sodium ion oxide single crystal cathode material in the preparation of sodium ion batteries.

[0019] The beneficial effects of the present invention are: 1. Instead of using the traditional precursor process method, the present invention directly uses metals or their salts as raw materials and synthesizes a composite sodium ion oxide single crystal cathode material by a one-step method. Its processing cost is reduced by more than 50% compared with the traditional precursor process. The primary sintering, wet coating, and secondary sintering of the present invention are all completed in the same microwave or infrared rotary kiln, greatly shortening the preparation time. The sintering is completed within 2 - 6 hours for the whole process, and its preparation time is shortened by more than 20 times compared with the traditional process, and the processing cost is also reduced by more than 60%.

[0020] 2. The present invention uses wet coating, which can uniformly disperse the coating agent on the surface of the sodium battery cathode material particles. Then, through microwave or infrared drying, the coating agent can be tightly coated on the surface of the matrix particles. At the same time, microwave or infrared sintering can densify the sodium battery cathode material particles, and a cathode material with a high-compact core-shell composite structure can be formed, thereby improving the areal density of the sodium ion battery, reducing the gas generation of the sodium ion battery, and improving the cycle and storage characteristics of the sodium ion battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 SEM diagram of the sodium ion battery oxide single crystal cathode material prepared in Example 1; Figure 2 SEM diagram of the sodium ion battery oxide single crystal cathode material prepared in Comparative Example 1; Figure 3 Capacity curve diagrams of the sodium ion battery oxide single crystal cathode materials prepared in Example 1 and Comparative Example 1; Figure 4 Cycle curve diagrams of the sodium ion battery oxide single crystal cathode materials prepared in Example 1 and Comparative Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0022] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Those not specified in the embodiments are carried out according to conventional conditions or conditions recommended by the manufacturer. The reagents or instruments not specified in the manufacturer are all conventional products that can be obtained through commercial purchase.

[0023] Example 1 A composite sodium ion oxide single crystal cathode material, whose chemical formula is Na 0.4~1.1 Mn 0.55 Mg 0.35 Fe 0.01 Ni 0.01 Cu 0.05 Zr 0.005 Ca 0.015 B 0.005 Li 0.005(Cu + Al + Zr + Ti)O2, and its preparation method includes the following steps: S1: According to the molar amounts of elements in the molecular formula, add Na2CO3, metallic manganese, metallic magnesium, and dopants into deionized water. The mass ratio of deionized water to solid raw materials is 2:1. The dopants are metallic Fe, NiO, CuO, ZrO2, CaO, B2O3, and Li2CO3. Then continue to add 1 wt% nitric acid (calculated based on the mass of deionized water) and 0.5 wt% chelating agent (calculated based on the mass of deionized water). The chelating agent is polyethylene glycol, ammonia water, and sucrose, and the mass ratio of polyethylene glycol, ammonia water, and sucrose in the chelating agent is 1:1:1. Stir to dissolve, and add ammonia water to the solution to adjust the pH of the solution to 4. Then spray-dry the solution at 100 °C for 30 s to obtain solid powder; S2: Conduct the first high-temperature sintering on the solid powder at the front end of the microwave rotary kiln. The sintering temperature is 800 °C, and after 3 h of sintering, an intermediate is obtained. Then, at the rear end of the microwave rotary kiln, spray wet coating treatment is carried out on the intermediate with coating agents (Cu(OH)2, Al2O3, ZrO2, and TiO2). The dosage of the coating agent is based on 1% of the overall mass of the cathode material by (Cu + Al + Zr + Ti). The molar ratio of Cu, Al, Zr, and Ti is 1:1:1:1, and the particle size of the coating agent is less than 50 nm. Then conduct the second high-temperature sintering on the coated intermediate at the front end of the microwave rotary kiln. The sintering temperature is 500 °C, and after 1 h of sintering, a coating layer with a thickness of 10 nm is formed. After pulverization and screening, a composite sodium ion oxide single crystal cathode material with a particle size of 7 μm is obtained.

[0024] Example 2 A composite sodium ion oxide single crystal cathode material, whose chemical formula is Na 0.4~1.1 Mn 0.52 Mg 0.45 Cs 0.01 Mo 0.01 5Ru 0.004 K 0.001 (Cs + Sn + Zn + W)O 1.99 F 0.01 , and its preparation method includes the following steps: S1: According to the molar amounts of elements in the molecular formula, add NaHCO3, MnO, MgO and dopants into deionized water. The mass ratio of deionized water to solid raw materials is 5:3. The dopants are Cs2O, MoO2, RuO2, K2O, NH4F. Then continue to add 1 wt% nitric acid (calculated based on the mass of deionized water) and 0.3 wt% chelating agent (calculated based on the mass of deionized water). The chelating agents are NTA, EGTA, EUG, EA, PEO and PEG-EA. The mass ratio of NTA, EGTA, EUG, EA, PEO and PEG-EA in the chelating agent is 1:1:1:1:1:1. Stir to dissolve, and add ammonia water to the solution to adjust the pH of the solution to 9. Then spray-dry the solution at 250 °C for 1 s to obtain solid powder; S2: Conduct the first high-temperature sintering on the solid powder at the front end of the microwave rotary kiln. The sintering temperature is 600 °C. After 3 h of sintering time, an intermediate is obtained. Then, at the rear end of the microwave rotary kiln, spray wet coating treatment is carried out on the intermediate with coating agents (Cs2O, SnO2, ZnO and WO3). The dosage of the coating agent is based on 0.6% of the total mass of the cathode material for (Cs + Sn + Zn + W). The molar ratio of Cs, Sn, Zn and W is 1:1:1:1. The particle size of the coating agent is less than 50 nm. Then conduct the second high-temperature sintering on the coated intermediate at the front end of the microwave rotary kiln. The sintering temperature is 300 °C. After sintering for 3 h, a coating layer with a thickness of 8 nm is formed. After pulverization and screening, a composite sodium ion oxide single crystal cathode material with a particle size of 3 μm is obtained.

[0025] Example 3 A composite sodium ion oxide single crystal cathode material, whose chemical formula is Na 0.4~1.1 Mn 0.65 Mg 0.25 Zn 0.020 In 0.004 Se 0.006 Si 0.05 S 0.02 (Ca + Ru + Se + B + Te)O2, and its preparation method includes the following steps: S1: According to the molar amounts of elements in the molecular formula, add NaHCO3, NaOH, Mn(OH)2, Mg(OH)2 and the dopant into deionized water. The mass ratio of deionized water to solid raw materials is 2:1. The dopant is Zn(OH)2, In2O3, SeO2, SiO2, SO2. Then continue to add 0.4 wt% nitric acid (calculated based on the mass of deionized water) and 0.015 wt% chelating agent (calculated based on the mass of deionized water). The chelating agent is PEO, PEG-EA, ADH, DAAM and glycine. The mass ratio of PEO, PEG-EA, ADH, DAAM and glycine in the chelating agent is 1:1:1:1:1. Stir to dissolve, add ammonia water to the solution to adjust the pH to 7, and then spray-dry the solution at 200 °C for 25 s to obtain solid powder; S2: Conduct the first high-temperature sintering on the solid powder at the front end of the infrared rotary kiln. The sintering temperature is 660 °C. After 2.5 h of sintering time, an intermediate is obtained. Then, use the coating agent (Ca(OH)2, RuO2, SeO2, H3BO3 and TeO2) to conduct spray wet coating treatment on the intermediate at the rear end of the infrared rotary kiln. The dosage of the coating agent is based on (Ca + Ru + Se + B + Te) accounting for 0.05% of the overall mass of the cathode material. The molar ratio of Ca, Ru, Se, B and Te is 1:1:1:1:1. The particle size of the coating agent is less than 50 nm; Then conduct the second high-temperature sintering on the coated intermediate at the front end of the infrared rotary kiln. The sintering temperature is 250 °C. After sintering for 2.5 h, a coating layer with a thickness of 2 nm is formed. After pulverization and screening, a composite sodium ion oxide single crystal cathode material with a particle size of 2.5 μm is obtained.

[0026] Example 4 A composite sodium ion oxide single crystal cathode material, whose chemical formula is Na 0.4~1.1 Mn 0.70 Mg 0.22 Sn 0.05 Cr 0.02 Te 0.005 B 0.005 (Zr + Sb + La + Gd + Al)O2, and its preparation method includes the following steps: S1: According to the molar amounts of elements in the molecular formula, add Na2CO3, metallic manganese, metallic magnesium and dopants into deionized water. The mass ratio of deionized water to solid raw materials is 2:1. The dopants are SnO2, CrO3, TeO2 and B2O3. Then continue to add 0.7 wt% nitric acid (calculated based on the mass of deionized water) and 0.25 wt% chelating agent (calculated based on the mass of deionized water). The chelating agent is alanine, proline and alginic acid, and the mass ratio of alanine, proline and alginic acid in the chelating agent is 1:1:1. Stir to dissolve, add ammonia water to the solution to adjust the pH of the solution to 5.8, and then spray-dry the solution at 180 °C for 10 s to obtain solid powder; S2: Perform the first high-temperature sintering on the solid powder at the front end of the infrared rotary kiln. The sintering temperature is 720 °C. After sintering for 1 h, an intermediate is obtained. Then, at the rear end of the infrared rotary kiln, spray wet coating treatment is carried out on the intermediate with coating agents (Zr(OH)4, Sb2O3, La2O3, Gd2O3 and Al2O3). The dosage of the coating agent is based on 0.3% of (Zr + Sb + La + Gd + Al) in the overall mass of the cathode material. The molar ratio of Zr, Sb, La, Gd and Al is 1:1:1:1:1, and the particle size of the coating agent is less than 50 nm. Then, perform the second high-temperature sintering on the coated intermediate at the front end of the infrared rotary kiln. The sintering temperature is 340 °C. After sintering for 1 h, a coating layer with a thickness of 7 nm is formed. After pulverization and screening, a composite sodium ion oxide single crystal cathode material with a particle size of 6 μm is obtained.

[0027] Example 5 A composite sodium ion oxide single crystal cathode material, whose chemical formula is Na 0.4~1.1 Mn 0.90 Mg 0.05 Sb 0.02 Zn 0.01 Gd 0.01 B 0.005 La 0.005 (B + W)O2, and its preparation method includes the following steps: S1: According to the molar amounts of elements in the molecular formula, add Na2CO3, MnO, MgO and dopants into deionized water. The mass ratio of deionized water to solid raw materials is 2:1. The dopants are Sb2O3, ZnO, Gd2O3, H3BO3, La2O3. Then continue to add 0.5 wt% nitric acid (calculated based on the mass of deionized water) and 0.25 wt% chelating agent (calculated based on the mass of deionized water). The chelating agent is ammonia water, sucrose, glycine and proline, and the mass ratio of ammonia water, sucrose, glycine and proline in the chelating agent is 1:1:1:1. Stir to dissolve, add ammonia water to the solution to adjust the pH of the solution to 6.3, and then spray-dry the solution at 175 °C for 14 s to obtain solid powder; S2: The solid powder is subjected to the first high-temperature sintering at the front end of the infrared rotary kiln. The sintering temperature is 700 °C, and after a sintering time of 2.6 h, an intermediate is obtained. Then, at the rear end of the infrared rotary kiln, a coating agent (H3BO3 and (NH4)6W7O 24 ·6H2O) is used to perform spray wet coating treatment on the intermediate. The dosage of the coating agent is based on 0.8% of the overall mass of the cathode material for (B + W), where the molar ratio of B to W is 1:1, and the particle size of the coating agent is less than 50 nm. Then, the coated intermediate is subjected to the second high-temperature sintering at the front end of the infrared rotary kiln. The sintering temperature is 280 °C, and after sintering for 3 h, a coating layer with a thickness of 4 nm is formed. After pulverization and screening, a composite sodium ion oxide single crystal cathode material with a particle size of 5.5 μm is obtained.

[0028] Example 6 A composite sodium ion oxide single crystal cathode material with the chemical formula Na 0.4~1.1 Mn 0.70 Mg 0.22 Cu 0.05 Sn 0.02 Ce 0.005 La 0.005 (Zr + Ti + Ce + P)O2, and its preparation method includes the following steps: S1: According to the molar amounts of elements in the molecular formula, Na2CO3, Mn, and dopants are added to deionized water. The mass ratio of deionized water to solid raw materials is 2:1. The dopants are CuO, SnO2, CeO2, and La2O3. Then, 0.1 wt% nitric acid (calculated based on the mass of deionized water) and 0.35 wt% chelating agent (calculated based on the mass of deionized water) are further added. The chelating agent is polyethylene glycol, ammonia water, PEG-EA, ADH, and DAAM. The mass ratio of polyethylene glycol, ammonia water, PEG-EA, ADH, and DAAM in the chelating agent is 1:1:1:1:1. Stir to dissolve, and add ammonia water to the solution to adjust the pH of the solution to 6.5. Then, the solution is spray-dried at 120 °C for 30 s to obtain a solid powder; S2: The solid powder is subjected to the first high-temperature sintering at the front end of the infrared rotary kiln. The sintering temperature is 750 °C, and after a sintering time of 1.8 h, an intermediate product is obtained. Then, at the rear end of the infrared rotary kiln, the intermediate product is subjected to spray wet coating treatment with coating agents (ZrO2, TiO2, CeO2, and NH4H2PO4). The dosage of the coating agents is based on 0.9% of the total mass of the cathode material for (Zr + Ti + Ce + P), where the molar ratio of Zr, Ti, Ce, and P is 1:1:1:1, and the particle size of the coating agents is less than 50 nm. Then, the coated intermediate product is subjected to the second high-temperature sintering at the front end of the infrared rotary kiln. The sintering temperature is 200 °C, and after sintering for 3 h, a coating layer with a thickness of 9 nm is formed. After pulverization and screening, a composite sodium ion oxide single crystal cathode material with a particle size of 6 μm is obtained.

[0029] Comparative Example 1 A composite sodium ion oxide single crystal cathode material, whose chemical formula is Na 0.4~1.1 Mn 0.55 Mg 0.35 Fe 0.01 Ni 0.01 Cu 0.05 Zr 0.005 Ca 0.015 B 0.005 Li 0.005 (Cu + Al + Zr + Ti)O2, and its preparation method includes the following steps: S1: According to the molar amounts of elements in the molecular formula, Na2CO3, MnMg(OH)2 precursor and dopants are mixed evenly. The dopants are metal FeO, NiO, CuO, ZrO2, CaO, B2O3, and Li2CO3. Then, the mixture is subjected to the first heat preservation sintering in an orbital kiln at 880 °C for 16 h, and then after pulverization and screening, an intermediate product is obtained. S2: In a coating machine, the intermediate product is subjected to coating treatment with coating agents (Cu(OH)2, Al2O3, ZrO2, and TiO2). The dosage of the coating agents is based on 1% of the total mass of the cathode material for (Cu + Al + Zr + Ti), where the molar ratio of Cu, Al, Zr, and Ti is 1:1:1:1, and the particle size of the coating agents is less than 50 nm. Then, the coated intermediate product is subjected to the second high-temperature sintering in an orbital kiln at 600 °C, and after sintering for 5 h, a coating layer with a thickness of 10 nm is formed. After pulverization and screening, a composite sodium ion oxide single crystal cathode material with a particle size of 7 μm is obtained.

[0030] Comparative Example 2 The difference between this comparative example and Example 1 is that no coating treatment is carried out during the preparation process.

[0031] Test Example 1. Structure Characterization The samples prepared in Example 1 and Comparative Example 1 were scanned by electron microscopy, and the SEM images are as follows Figures 1-2 shown. Figure 1 It is shown in that the positive electrode material particles of the sodium-ion battery in Example 1 are relatively dispersed and have a single-crystal morphology. It can be seen from Figure 2 that the conventional sintering method in Comparative Example 1 leads to agglomeration and adhesion of the positive electrode material particles of the sodium-ion battery, and a dispersed and uniform single-crystal morphology cannot be formed. Since the conventional sintering method is external heating, it will cause a temperature gradient difference, resulting in uneven heating of the positive electrode material of the sodium-ion battery and the same sodium-ion diffusion rate. And sodium ions themselves can lower the growth temperature of single crystals. Therefore, different concentrations of sodium ions will cause partial non-uniformity of the particle morphology. Microwave heating is internal heating, resulting in no temperature gradient difference inside the positive electrode material of the sodium-ion battery. Therefore, the sodium-ion diffusion rate is the same and the sodium-ion concentration distribution is uniform. Therefore, a single-crystal morphology with a complete and dispersed uniform morphology can be formed, and the synthesis time is greatly shortened.

[0032] 2. Particle strength test The particles in the samples prepared in the examples and comparative examples were subjected to a pressure test using Shimadzu MCT of Japan. The corresponding strength value when the particles were broken was calculated by the following formula, which is the pressure strength data. The results are shown in Table 1.

[0033]

[0034] where H is the compressive fracture strength, P is the applied compressive force, and r is the diameter of the particle.

[0035] Table 1

[0036] It can be seen from Table 1 that the prepared composite sodium-ion oxide single-crystal positive electrode material in the examples has more excellent anti-fracture performance.

[0037] 2. Discharge capacity and cycle performance test The positive electrode materials, conductive carbon black, and binder PVDF (polyvinylidene fluoride) prepared in the examples and comparative examples were mixed at a mass ratio of 80:10:10, and then NMP was added to make a uniform slurry, which was coated on aluminum foil. After drying and rolling, it was cut into a positive electrode sheet with a diameter of 14 mm. A sodium-ion battery was assembled with a CR2032-type button battery. The separator was glass fiber, and the electrolyte was a 1 mol / L NaPF6 solution with a solvent of EC / PC / DEC. The negative electrode was a sodium sheet. The test conditions of the sodium-ion battery were as follows: the temperature was 25 °C ± 1 °C, the charge-discharge cycle voltage range was 2.0~4.0 V, the current was 0.1 C (120 mAh / g), and the cycle test was carried out at 0.5 C charge and 1 C discharge. The results are shown in Table 2 and Figures 3-4 shown.

[0038] Table 2

[0039] As can be seen from Table 2, the sodium cathode materials prepared in Examples 1 to 6 have excellent discharge capacity and cycling performance. For example, the retention rate after 50 cycles is above 94%.

[0040] According to Figures 3-4 It can also be seen that the layered oxide single crystal cathode material for sodium ion battery in Example 1: Na 0.4~1.1 Mn 0.55 Mg 0.35 Fe 0.01 Ni 0.01 Cu 0.05 Zr 0.005 Ca 0.015 B 0.005 Li 0.005 (Cu + Al + Zr + Ti)O2 has an initial discharge capacity of 141 mAh / g at 0.1C and a retention rate after 50 cycles of 95.37%. While the traditional sintering method in Comparative Example 1 has an initial capacity of only 134 mAh / g at 0.1C and a retention rate after 50 cycles of 92.06%. In Comparative Example 2, no coating treatment was carried out, and the retention rate of the prepared material after 50 cycles was 93.37%, and the fracture strength value was 23.2 Mpa. The above comparison data shows that microwave sintering and wet coating treatment can synergistically improve the compressive strength of particles, and thus can improve the cycling performance of sodium cathode materials.

[0041] Although the specific implementation modes of the present invention have been described in detail in combination with the embodiments, it should not be construed as a limitation on the protection scope of this patent. Within the scope described in the claims, various modifications and deformations that can be made by those skilled in the art without creative labor still fall within the protection scope of this patent.

Claims

1. A composite sodium ion oxide single crystal positive electrode material, characterized in that: The composite sodium ion oxide single crystal cathode material has a core-shell structure, and its chemical formula is Na a Mn x Mg y M z NO2, where 0.4 < a ≤ 1.1, 0.5 ≤ x ≤ 1, 0 ≤ y ≤ 0.5, M is a doping element, 0.005 ≤ z ≤ 0.11, M is at least one of Fe, Ni, Cu, Zr, Cs, Mo, Sn, Zn, Ca, Li, Ru, K, In, Se, Si, S, B, Te, Cr, Sb, La, Gd, Nb, W, and F; N is a shell element, N is at least one of Cu, Zr, Cs, Sn, Zn, Ca, Ti, W, Ru, Se, Si, B, Te, Sb, La, Gd, Nb, and Al, the shell element accounts for 0.05 - 1% of the overall mass of the cathode material, and the shell thickness is 2 - 10 nm.

2. The method for preparing the composite sodium ion oxide single crystal positive electrode material according to claim 1, characterized in that: The following steps are involved: S1: placing a sodium source, a manganese source, a magnesium source and a dopant containing a doping element M in deionized water, adding nitric acid and a chelating agent, adjusting the pH of the solution to 4-9 after dissolution, and spray drying the solution to obtain a solid powder; M is at least one of Fe, Ni, Cu, Zr, Cs, Mo, Sn, Zn, Ca, Li, Ru, K, In, Se, Si, S, B, Te, Cr, Sb, La, Gd, Nb, W and F; S2: The solid powder is subjected to a first sintering to obtain an intermediate, and then the intermediate is wet-coated with a coating agent containing shell element N, and then subjected to a second sintering, and the intermediate is obtained after crushing and screening; the sintering method is infrared sintering or microwave sintering; N is at least one of Cu, Zr, Cs, Sn, Zn, Ca, Ti, W, Ru, Se, Si, B, Te, Sb, La, Gd, Nb and Al.

3. The method for preparing the composite sodium ion oxide single crystal positive electrode material according to claim 2, characterized in that: In step S1, the sodium source is at least one of sodium carbonate, sodium bicarbonate, sodium hydroxide and sodium nitrate; the manganese source and the magnesium source are at least one of oxalate, acetate, nitrate, sulfate, chloride, oxide, hydroxide and metal element containing manganese or magnesium; the dopant is a single substance M or a compound including M, or the dopant is at least one of iron oxide, iron hydroxide, nickel oxide, nickel hydroxide, copper oxide, copper hydroxide, zirconium oxide, zirconium hydroxide, cesium oxide, molybdenum oxide, ammonium molybdate, tin oxide, zinc oxide, zinc hydroxide, calcium oxide, calcium hydroxide, lithium oxide, lithium carbonate, lithium hydroxide, ruthenium oxide, potassium oxide, potassium hydroxide, indium oxide, selenium oxide, silicon oxide, sulfur oxide, boric acid, boron oxide, tellurium oxide, chromium oxide, antimony oxide, lanthanum oxide, gadolinium oxide, niobium oxide, tungsten oxide, ammonium tungstate, ammonium fluoride and sodium fluoride.

4. The method for preparing the composite sodium ion oxide single crystal positive electrode material according to claim 2, characterized in that: The chelating agent is at least one of polyethylene glycol, ammonia water, sucrose, NTA, EGTA, EUG, EA, PEO, PEG-EA, ADH, DAAM, glycine, alanine, proline and alginic acid.

5. The method for preparing the composite sodium ion oxide single crystal positive electrode material according to claim 2, characterized in that: The mass ratio of deionized water, nitric acid and chelating agent is 100:0.1~1:0.015~0.5; the molar amount of metal in the sodium source is A, the total molar amount of metal in the manganese source, magnesium source and dopant is B, and A:B=0.4~1.1:

1.

6. The method for preparing the composite sodium ion oxide single crystal positive electrode material according to claim 2, characterized in that: The spray drying temperature is 100~250℃, and the spray drying time is 1~30s.

7. The method for preparing the composite sodium ion oxide single crystal positive electrode material according to claim 2, characterized in that: The first sintering temperature is 600~800℃ and the sintering time is 1~3h.

8. The method for preparing the composite sodium ion oxide single crystal positive electrode material according to claim 2, characterized in that: Particle size of coating agent D 50 <50nm; the coating agent is a compound including N; or the coating agent is at least one of copper oxide, copper hydroxide, zirconium oxide, zirconium hydroxide, cesium oxide, tin oxide, zinc oxide, zinc hydroxide, calcium oxide, calcium hydroxide, titanium oxide, tungsten oxide, ammonium tungstate, ruthenium oxide, selenium oxide, silicon oxide, boric acid, boron oxide, tellurium oxide, antimony oxide, lanthanum oxide, gadolinium oxide, niobium oxide, aluminum oxide and aluminum hydroxide.

9. The method for preparing the composite sodium ion oxide single crystal positive electrode material according to claim 2, characterized in that: The second sintering temperature is 200~500℃ and the sintering time is 1~3h.

10. An application of a composite sodium ion oxide single crystal positive electrode material, characterized in that: The composite sodium ion oxide single crystal positive electrode material according to claim 1 is used to prepare a sodium ion battery.

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