A polyanionic positive electrode active material, and a preparation method and application thereof

Abstract

The present application discloses a kind of polyanion positive electrode active material, the positive electrode active material is the spherical secondary particle with porous structure inside formed by primary particle, the diameter D of internal pore structure ≤800nm, the length of the primary particle is 300-1000nm, the diameter is 50-400nm, the chemical formula of the positive electrode active material is Na x Fe y M z P 4‑a O 15‑b ,3

Description

Technical Field

[0001] This application belongs to the technical field of secondary batteries, and specifically relates to a polyanion cathode active material and a preparation method thereof. Further, this application also relates to a secondary battery and an electrical device. Background Art

[0002] With the widespread popularity of new energy vehicles, the rapid development of lithium-ion batteries has been promoted. However, the reserves of lithium resources are scarce and unevenly distributed geographically, which cannot meet the needs of the electric vehicle field, let alone the cheap requirements for large-scale energy storage. The working principle of sodium-ion batteries is similar to that of lithium-ion batteries. Sodium resources are abundant in the earth's crust, cheap, and have more advantages than lithium-ion batteries in terms of low-temperature performance, safety, etc.

[0003] Sodium ion

[0004] The cathode materials of sodium-ion batteries include transition metal oxides, Prussian blue and its analogues, and polyanion materials. Among them, the polyanion cathode materials for sodium-ion batteries have received extensive attention due to their stable framework structure and excellent electrochemical performance. However, at present, the polyanion cathode materials for sodium-ion batteries still have problems such as insufficient strength and low tap density, and need to be further improved to better exert their capacity and cycle performance. Summary of the Invention

[0005] This application aims to solve at least one of the technical problems in the related art. For this reason, the embodiments of this application propose a polyanion cathode active material, which has a spherical structure with loose interior and an appropriate ratio of P and O vacancies. It can not only improve the tap density of the material, enhance the material strength, and enable the cathode material to better exert its capacity and cycle performance, but also adjust the friction coefficient on the surface of the material to make it have better fluidity, thereby improving the processing performance of the cathode material.

[0006] The embodiments of this application provide a polyanion cathode active material. The cathode active material is spherical secondary particles with a porous structure inside formed by primary particles. The diameter D of the internal pore structure is ≤ 800 nm. The length of the primary particles is 300 - 1000 nm, and the diameter is 50 - 400 nm. The chemical formula of the cathode active material is Na x Fe y M z P 4-a O 15-b , 3 < x ≤ 4.2, 2 < y ≤ 3, 0 ≤ z ≤ 1.0, 0 ≤ a ≤ 0.4, 0 ≤ b ≤ 0.4. M is selected from at least one element of Ni, Mn, Al, Cu, Mg, Co, Ti, and 0 < O vacancy rate n(O) ≤ 5%, 0 < P vacancy rate n(P) ≤ 5%.

[0007] The advantages and technical effects of the polyanionic positive electrode active material in this application are as follows: 1. The positive electrode active material in this application is a porous spherical secondary particle with a cross-section resembling a "dragon fruit" structure. The irregular pore structure inside is like the seeds, and the primary particles are connected to form the pulp. This loose spherical structure is more conducive to electrolyte wetting. By controlling the size of the internal pore structure and the size of the primary particles, the positive electrode active material can better exert its capacity. In addition, the spherical porous structure can improve the elastic modulus of the positive electrode active material, thereby increasing its compaction density. 2. In this application embodiment, by controlling the vacancy rate of O and P, the distribution of electron vacancies in the material's crystal structure can be adjusted, thereby improving the electronic conductivity and effectively improving the material's capacity and cycle performance. 3. The positive electrode active material in this application embodiment has good fluidity, which is beneficial to the homogenization process in subsequent cell manufacturing and improves processing performance.

[0008] In some embodiments, the positive electrode active material has a carbon coating layer.

[0009] In some embodiments, the O vacancy rate n(O) and the P vacancy rate n(P) satisfy the following relationship: k=(n(O)+n(P)) / ((V1-V2)*(e(Pd-Td))), 0.05≤k≤0.3, where V1 is the total volume of macropores in the positive electrode active material, V2 is the total volume of micropores in the positive electrode active material, e is the compressibility coefficient of the positive electrode active material, Pd is the compaction density of the positive electrode active material, and Td is the tap density of the positive electrode active material.

[0010] In some embodiments, the compressibility coefficient e of the positive electrode active material is 10-60%.

[0011] In some embodiments, the flowability of the positive electrode active material is 10-80 s / 50 g.

[0012] In some embodiments, the positive electrode active material has a carbon coating layer.

[0013] This application also provides a method for preparing a polyanionic positive electrode active material, comprising the following steps:

[0014] a. Under an inert atmosphere, the oxidant, iron source, and phosphorus source are dissolved and mixed in water, heated and stirred, then centrifuged and dried to obtain an intermediate.

[0015] b. The intermediate, sodium source, dopant containing element M, carbon source and water are mixed in an inert atmosphere and subjected to the first sand milling treatment;

[0016] c. The mixture after sand milling in step b is fed together with the organic acid solution into an atomizing plate for the first spray drying treatment.

[0017] d. The mixture after spray drying in step c is subjected to a first sintering treatment in an oxygen-free atmosphere;

[0018] e. After cooling the material obtained from the sintering process in step d, mix it with a carbonyl compound and perform a second sand milling process.

[0019] f. The mixture after sand milling in step e is subjected to a second spray drying treatment, followed by a second sintering treatment under an oxygen-free atmosphere to obtain a polyanionic positive electrode active material.

[0020] The advantages and technical effects of the preparation method of the polyanionic positive electrode active material in this application are as follows: 1. In the method of this application, an oxidant is first mixed with an iron source and a phosphorus source, so that the iron ions in the iron source are oxidized to Fe. 3+ 1. The valence state is more conducive to co-precipitation with phosphorus source to obtain intermediate iron phosphate mixture; 2. In the method of this application embodiment, the mixture after the first sand milling treatment and organic acid are simultaneously subjected to the first spray drying treatment in the atomizing disk. With the help of the small bubbles generated by the neutralization of organic acid and alkaline sodium source, the material forms a porous structure under the high speed rotation of the atomizing disk, and the introduced organic acid can also act as a carbon source to coat the surface of the positive electrode active material to form a carbon coating layer; 3. In this application embodiment, after the first sintering treatment, carbonyl compound is introduced and sand milled together with the sintered material, which is beneficial to provide a reducing atmosphere in the subsequent second sintering treatment process and promote the synthesis of polyanionic positive electrode active material; 4. The positive electrode active material prepared by the method of this application embodiment is a spherical secondary particle with a porous internal structure formed by primary particles. It not only has high strength, but also high capacity and excellent cycle performance, while having good fluidity, which improves the compressibility coefficient and density of the material.

[0021] In some embodiments, in step a, the oxidant includes at least one of sodium peroxide, hydrogen peroxide, potassium peroxide, potassium permanganate, and sodium permanganate; the iron source includes at least one of iron powder, ferric citrate, ferrous citrate, ferric nitrate, ferrous nitrate, ferric sulfate, ferrous sulfate, ferrous oxide, ferric oxalate, ferrous oxalate, ferric acetate, ferric phosphate, and ferric pyrophosphate; and the phosphorus source includes at least one of sodium dihydrogen phosphate, sodium phosphate, sodium monohydrogen phosphate, phosphoric acid, ammonium dihydrogen phosphate, triammonium phosphate, pyrophosphate, sodium pyrophosphate, and sodium dihydrogen pyrophosphate.

[0022] In some embodiments, in step b, the sodium source includes at least one of sodium carbonate, sodium hydroxide, sodium acetate, sodium oxalate, sodium sulfate, sodium bicarbonate, sodium citrate, and sodium pyrophosphate; the dopant containing element M includes at least one of metal oxide, metal hydroxide, and metal carbonate; optionally, the dopant containing element M includes at least one of magnesium oxide, magnesium hydroxide, aluminum oxide, aluminum hydroxide, copper oxide, titanium oxide, nickel oxide, nickel sulfate, nickel hydroxide, nickel oxalate, nickel phosphate, cobalt hydroxide, manganese oxide, manganese sulfate, calcium carbonate, and manganese oxalate; the carbon source includes at least one of dopamine, soluble starch, polyethylene glycol, sucrose, or glucose.

[0023] In some embodiments, in step c, the organic acid includes at least one of formic acid, acetic acid, oxalic acid, and citric acid.

[0024] In some embodiments, in step e, the carbonyl compound includes at least one of acetophenone, methyl ethyl ketone, propionaldehyde, and acrolein.

[0025] In some embodiments, in step a, the heating temperature is 40-90°C.

[0026] In some embodiments, in step b, the time for the first sand milling treatment is 50-150 min; the solid content of the mixture after the first sand milling treatment is 20%-60 wt%.

[0027] In some embodiments, in step c, the rotation speed of the atomizing disc in the first spray drying process is 10,000-15,000 rpm / min; and the moisture content of the mixture after the first spray drying process is 2-10%.

[0028] In some embodiments, in step d, the temperature of the first sintering treatment is 400-600°C and the time is 2-10 hours.

[0029] In some embodiments, in step e, the second sanding process takes 1-4 hours.

[0030] In some embodiments, in step f, the rotation speed of the atomizing disc in the second spray drying process is 20,000-25,000 rpm / min; the temperature of the second sintering process is 500-700℃, and the time is 5-20h.

[0031] In some embodiments, in steps a and / or b, the inert atmosphere includes at least one of nitrogen or argon.

[0032] In some embodiments, in steps d and / or f, the oxygen-free atmosphere includes at least one of nitrogen, argon, and hydrogen, or is a vacuum state.

[0033] In some embodiments, in step c, the molar ratio of sodium source to organic acid solution in the milled mixture is (0.9:1)-(1.5:1).

[0034] In some embodiments, in step e, the mass ratio of the material obtained by the sintering treatment to the carbonyl compound is (10:1)-(50:1).

[0035] This application also provides a secondary battery, which includes the polyanion cathode active material of this application. The secondary battery of this application possesses all the advantages of the polyanion cathode active material of this application, and will not be elaborated further here.

[0036] This application also provides an electrical device that includes the secondary battery described in this application. The electrical device of this application possesses all the advantages of the secondary battery described in this application, which will not be elaborated further here. Attached Figure Description

[0037] Figure 1 This is a SEM image of the polyanionic positive electrode active material prepared in Example 1. Detailed Implementation

[0038] The embodiments of this application are described in detail below. These embodiments are exemplary and intended to explain this application, and should not be construed as limiting this application.

[0039] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains; the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the application; the terms “comprising” and “having”, and any variations thereof, in the specification and claims of this application are intended to cover non-exclusive inclusion.

[0040] In the description of the embodiments of this application, technical terms such as "first" and "second" are used only to distinguish different objects and should not be construed as indicating or implying relative importance or implicitly specifying the number, specific order, or primary and secondary relationship of the indicated technical features. In the description of the embodiments of this application, "multiple" means two or more, unless otherwise explicitly defined.

[0041] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0042] The "range" disclosed in this application is defined by a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of a particular range. Ranges defined in this way can include or exclude endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a specific parameter, it is expected that ranges of 60-110 and 80-120 are also included. Furthermore, if minimum range values ​​of 1 and 2 are listed, and if maximum range values ​​of 3, 4, and 5 are listed, then the following ranges are all expected: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In this application, unless otherwise stated, the numerical range "ab" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0-5" have been listed in this article; "0-5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is stated as an integer ≥2, it is equivalent to disclosing that the parameter is, for example, an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0043] In the description of the embodiments in this application, the term "and / or" is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, and B existing alone. Additionally, the character " / " in this document generally indicates that the preceding and following related objects have an "or" relationship.

[0044] In the description of the embodiments of this application, the term "multiple" refers to two or more (including two), similarly, "multiple sets" refers to two or more (including two sets), and "multiple pieces" refers to two or more (including two pieces).

[0045] In this application, terms such as "one embodiment", "some embodiments", "example", "specific example", or "some examples" mean that the specific features, structures, materials, or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of this application. In this specification, the schematic descriptions of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials, or characteristics described can be combined in a suitable manner in any one or more embodiments or examples. In addition, without contradiction, those skilled in the art can combine and combine the different embodiments or examples described in this specification and the features of different embodiments or examples.

[0046] An embodiment of this application provides a polyanionic cathode active material. The cathode active material is spherical secondary particles with a porous structure inside formed by primary particles. The diameter D of the internal pore structure is ≤ 800 nm. The length of the primary particles is 300 - 1000 nm, and the diameter is 50 - 400 nm. The chemical formula of the cathode active material is Na x Fe y M z P 4-a O 15-b , 3 < x ≤ 4.2, 2 < y ≤ 3, 0 ≤ z ≤ 1.0, 0 ≤ a ≤ 0.4, 0 ≤ b ≤ 0.4. M is selected from at least one element of Ni, Mn, Al, Cu, Mg, Co, Ti, and 0 < oxygen vacancy rate n(O) ≤ 5%, 0 < phosphorus vacancy rate n(P) ≤ 5%.

[0047] The polyanionic cathode active material of the embodiment of this application is spherical secondary particles with pores inside. Its cross-section presents a "dragon fruit" structure. The irregular internal pore structure is the fruit seeds, and the primary particles are connected to form the pulp. This internally loose spherical structure is more conducive to the infiltration of the electrolyte. At the same time, by controlling the size of the internal pore structure and the size of the primary particles, the cathode active material can better exert its capacity. In addition, the spherical porous structure can increase the elastic modulus of the cathode active material, thereby increasing its tap density. In the embodiment of this application, by controlling the vacancy rates of O and P, the distribution of electron vacancies in the crystal structure of the material can be adjusted, thereby increasing the electron conductivity and effectively improving the capacity and cycle performance of the material. The cathode active material of the embodiment of this application has good fluidity, which is beneficial to the homogenization process in the subsequent production of battery cells and improves the processing performance.

[0048] In some embodiments, the cathode active material has a carbon coating layer.

[0049] In some embodiments, the O vacancy rate n(O) and the P vacancy rate n(P) satisfy the following relationship: k = (n(O) + n(P)) / ((V1 - V2) * (e(Pd - Td))), 0.05 ≤ k ≤ 0.3, where V1 is the total macropore volume of the positive electrode active material, V2 is the total micropore volume of the positive electrode active material, e is the compressibility coefficient of the positive electrode active material, Pd is the compaction density of the positive electrode active material, and Td is the tap density of the positive electrode active material. In the embodiments of this application, by controlling the O vacancy rate and P vacancy rate of the positive electrode active material to satisfy the above relationship, it is beneficial to improve the uniformity of the pore distribution of the material, stabilize the material structure, thereby balancing the compaction density and capacity, and improving the performance of the material; at the same time, appropriate vacancies will increase the sodium ion diffusion rate and stabilize the capacity. If k is too large, the vacancy rate ratio increases, which is not conducive to the stability of the structure, and thus not conducive to the improvement of cycle performance; if k is too small, the vacancy rate ratio decreases, which is not conducive to the transport of sodium ions, and thus not conducive to the improvement of capacity.

[0050] In some embodiments, the compressibility coefficient e of the positive electrode active material is 10-60%. The compressibility coefficient in this embodiment is calculated using the loose packing density and tapped density, as follows:

[0051] The compressibility factor e = (tap density - loose density) / tap density, where loose density refers to the bulk density measured after the powder is freely filled into a standard container under test conditions, that is, the mass per unit volume when the powder is loosely packed. The test method used in this application is GB / T 1479.1-2011.

[0052] The compressibility coefficient of the positive electrode active material in this application embodiment is controlled within a suitable range, which can improve the compaction density of the material.

[0053] In some embodiments, the flowability of the positive electrode active material is 10-80 s / 50g. The positive electrode active material of this application embodiment has good flowability, resulting in a high compressibility coefficient and density. If the flowability is too poor, it can lead to electrode breakage during the preparation of the battery cell electrode.

[0054] This application also provides a method for preparing a polyanionic positive electrode active material, comprising the following steps:

[0055] a. Under an inert atmosphere, the oxidant, iron source, and phosphorus source are dissolved and mixed in water, heated and stirred, then centrifuged and dried to obtain an intermediate.

[0056] b. The intermediate, sodium source, dopant containing element M, carbon source and water are mixed in an inert atmosphere and subjected to the first sand milling treatment;

[0057] c. The mixture after sand milling in step b is fed together with the organic acid solution into an atomizing plate for the first spray drying treatment.

[0058] d. The mixture after spray drying in step c is subjected to a first sintering treatment in an oxygen-free atmosphere;

[0059] e. After cooling the material obtained from the sintering process in step d, mix it with a carbonyl compound and perform a second sand milling process.

[0060] f. The mixture after sand milling in step e is subjected to a second spray drying treatment, followed by a second sintering treatment under an oxygen-free atmosphere to obtain a polyanionic positive electrode active material.

[0061] In the preparation method of the polyanionic positive electrode active material in this application embodiment, an oxidant is first mixed with an iron source and a phosphorus source, so that the iron ions in the iron source are oxidized to Fe. 3+ The valence state is more conducive to co-precipitation with phosphorus source to obtain intermediate iron phosphate mixture; in the method of this application embodiment, the mixture after the first sand milling treatment and organic acid are simultaneously subjected to the first spray drying treatment in the atomizing disk. With the help of the small bubbles generated by the neutralization of organic acid and alkaline sodium source, the material forms a porous structure under the high speed rotation of the atomizing disk, and the introduced organic acid can also act as a carbon source to coat the surface of the positive electrode active material to form a carbon coating layer; in this application embodiment, after the first sintering treatment, carbonyl compound is introduced and sand milled together with the sintered material, which is beneficial to provide a reducing atmosphere in the subsequent second sintering treatment process and promote the synthesis of polyanionic positive electrode active material; the positive electrode active material prepared by the method of this application embodiment is a spherical secondary particle with a porous internal structure formed by primary particles. It not only has high strength, but also high capacity and excellent cycle performance, while having good flowability, which improves the compressibility coefficient and density of the material.

[0062] In some embodiments, in step a, the oxidant includes at least one of sodium peroxide, hydrogen peroxide, potassium peroxide, potassium permanganate, and sodium permanganate; the iron source includes at least one of iron powder, ferric citrate, ferrous citrate, ferric nitrate, ferrous nitrate, ferric sulfate, ferrous sulfate, ferrous oxide, ferric oxalate, ferrous oxalate, ferric acetate, ferric phosphate, and ferric pyrophosphate; and the phosphorus source includes at least one of sodium dihydrogen phosphate, sodium phosphate, sodium monohydrogen phosphate, phosphoric acid, ammonium dihydrogen phosphate, triammonium phosphate, pyrophosphate, sodium pyrophosphate, and sodium dihydrogen pyrophosphate.

[0063] In some embodiments, in step b, the sodium source includes at least one of sodium carbonate, sodium hydroxide, sodium acetate, sodium oxalate, sodium sulfate, sodium bicarbonate, sodium citrate, and sodium pyrophosphate; the dopant containing element M includes at least one of metal oxide, metal hydroxide, and metal carbonate; optionally, the dopant containing element M includes at least one of magnesium oxide, magnesium hydroxide, aluminum oxide, aluminum hydroxide, copper oxide, titanium oxide, nickel oxide, nickel sulfate, nickel hydroxide, nickel oxalate, nickel phosphate, cobalt hydroxide, manganese oxide, manganese sulfate, and manganese oxalate; the carbon source includes at least one of dopamine, soluble starch, polyethylene glycol, sucrose, or glucose.

[0064] In the embodiments of the present invention, there are no particular limitations on the oxidant, iron source, phosphoric acid, sodium source, dopant containing M, and carbon source; all commonly used existing technologies can be applied to the methods of this application.

[0065] In some embodiments, in step c, the organic acid includes at least one selected from formic acid, acetic acid, oxalic acid, and citric acid. In the embodiments of this application, the use of a selected organic acid is beneficial for the formation of an internal pore structure in the material.

[0066] In some embodiments, in step e, the carbonyl compound includes at least one selected from acetophenone, methyl ethyl ketone, propionaldehyde, and acrolein. In the method of this application embodiment, using a selected carbonyl compound can provide a reducing atmosphere, which is more conducive to the synthesis of sodium iron pyrophosphate material.

[0067] In some embodiments, in step a, the heating temperature is 40-90°C.

[0068] In some embodiments, in step b, the time for the first sand milling treatment is 50-150 min; the solid content of the mixture after the first sand milling treatment is 20%-60 wt%.

[0069] In some embodiments, in step c, the atomizing disc rotation speed is 10000-15000 rpm / min during the first spray drying process; the moisture content of the mixture after the first spray drying process is 2-10%. In this embodiment, controlling the rotation speed of the atomizing disc during the first spray drying process is beneficial for the formation of a suitable pore structure within the positive electrode active material, thereby improving the performance of the positive electrode active material. If the atomizing disc rotation speed is too high, the sphericity of the material will deteriorate, tending to form a disc shape, which is not conducive to improving the compaction density. If the atomizing disc rotation speed is too low, oversized particles will appear, which is also not conducive to improving the compaction density.

[0070] In some embodiments, in step d, the temperature of the first sintering treatment is 400-600°C and the time is 2-10 hours.

[0071] In some embodiments, in step e, the second sanding process takes 1-4 hours.

[0072] In some embodiments, in step f, the rotation speed of the atomizing disc in the second spray drying process is 20,000-25,000 rpm / min; the temperature of the second sintering process is 500-700℃, and the time is 5-20 h. In this embodiment, by controlling the rotation speed of the atomizing disc in the second spray drying process, the particle size of the positive electrode active material can be controlled. If the rotation speed is too low, oversized particles are likely to appear, which is not conducive to improving the compaction density. If the rotation speed is too high, fine powder with a small particle size will appear, resulting in an excessively high electrochemical active area, which can easily lead to gas generation in the battery cell.

[0073] In some embodiments, in steps a and / or b, the inert atmosphere includes at least one of nitrogen or argon.

[0074] In some embodiments, in steps d and / or f, the oxygen-free atmosphere includes at least one of nitrogen, argon, and hydrogen, or is a vacuum state.

[0075] In some embodiments, in step c, the molar ratio of sodium source to organic acid solution in the milled mixture is (0.9:1)-(1.5:1). In this embodiment, by controlling the amount of sodium source and organic acid solution in the milled mixture during the first spray drying process, the pH value of the mixture after mixing with the organic acid is approximately neutral. This is beneficial not only for obtaining a positive electrode active material with suitable pore size but also for the synthesis of sodium iron pyrophosphate material. If the amount of organic acid is too large, it will be detrimental to the synthesis of sodium iron pyrophosphate material. If the amount of organic acid is too small, it will be detrimental to the formation of pores inside the particles, and instead tend to form grain boundaries, which is not conducive to electrolyte wetting and also to improving compaction density.

[0076] In some embodiments, in step e, the mass ratio of the material obtained from the sintering treatment to the carbonyl compound is (10:1)-(50:1). In this embodiment, by controlling the mass ratio of the sintered material to the carbonyl compound in the second milling treatment, it is more conducive to the synthesis of sodium iron pyrophosphate. If too much carbonyl compound is added, the carbon content in the final sodium iron pyrophosphate material will be too high, which is not conducive to improving the compaction density and volume of the material. If too little carbonyl compound is added, the reducing atmosphere during sintering will be insufficient, which is not conducive to the synthesis of sodium iron pyrophosphate.

[0077] The technical solutions in the embodiments of this application are described clearly and completely below. Obviously, the described embodiments are only a part of the embodiments of this application, and not all of them. Based on the embodiments in this application, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of this application.

[0078] Example 1

[0079] Add 30wt% H2O2 to 100L of pure water until the H2O2 concentration is 2wt%. Then add 6kg FeSO4, 1.5kg NaPO4 and 4.5kg sodium pyrophosphate. Stir at 50℃ for 30min. Then centrifuge the mixture and dry it at 120℃ to obtain the mixed intermediate.

[0080] Mix 5 kg of intermediate, 1.44 kg of Na₂CO₃, 0.02 kg of NiSO₄, and 0.83 kg of C₆H₂O. 12 O6 and 25.68 kg of water were sequentially added to the sand mill for the first sand milling process for 60 minutes.

[0081] The mixture after the first sand milling treatment was pumped into a spray dryer at a rate of 2 L / min. At the same time, 0.05 mol / L acetic acid was added at a rate of 2 L / min through another feed pipe. The molar ratio of Na2CO3 to acetic acid in the mixture was 1:1. The mixture and acetic acid were fed together into an atomizing disc for the first spray drying treatment. The rotation speed of the atomizing disc was 15000 rpm / min. The moisture content of the dried mixture was 5.2% (test method was GB / T 6283-2008, and the instrument used was Mettler).

[0082] The material after the first spray drying treatment was placed in a box furnace and sintered at 480°C under a nitrogen atmosphere for 4 hours to complete the first sintering treatment.

[0083] The sintered material is cooled and coarsely crushed, then mixed with methyl ethyl ketone at a mass ratio of 24:1, and pulverized using a mechanical pulverizer to complete the second sand milling process.

[0084] Finally, the milled mixture was placed in a box furnace and sintered at 525°C under a nitrogen atmosphere for 10 hours to complete the second sintering process. After cooling, porous polyanion cathode active material was obtained.

[0085] The chemical formula of the polyanionic positive electrode active material prepared in this embodiment is Na. 3.6 Fe 2.4 Ni 0.02 P 3.7 O 14.4 .

[0086] Figure 1The SEM image of Example 1 shows that the material has a uniform distribution of pores with a diameter of approximately 50-200 nm.

[0087] Example 2

[0088] Na2O2 was added to 100L of pure water and heated under nitrogen protection until the Na2O2 concentration reached 2.5wt%. Then, 5.6kg FeC2O4, 1.8kg NaPO4, and 4.5kg ammonium dihydrogen phosphate were added. The mixture was stirred at 50℃ for 120min. The mixture was then centrifuged and dried at 100℃ to obtain the intermediate mixture.

[0089] Mix 5 kg of intermediate, 2.76 kg of NaHCO3, 0.05 kg of Co(OH)2, and 0.86 kg of C. 12 H 22 O 11 31.7 kg of water were added to the sand mill in sequence for the first sand milling process for 90 minutes.

[0090] The mixture after the first sand milling treatment was pumped into a spray dryer at a rate of 1.5 L / min. At the same time, 0.08 mol / L formic acid was added into another feed pipe at a rate of 2 L / min. The molar ratio of NaHCO3 to formic acid in the mixture was 1:1. The mixture and formic acid were fed together into an atomizing disc for the first spray drying treatment. The rotation speed of the atomizing disc was 13,000 rpm / min. The moisture content of the dried mixture was 7.4%.

[0091] The material after the first spray drying treatment was placed in a box furnace and sintered at 600°C under an argon atmosphere for 3 hours to complete the first sintering treatment.

[0092] The sintered material is cooled and coarsely crushed, then mixed with acetophenone at a mass ratio of 43:1, and pulverized using a mechanical pulverizer to complete the second sand milling process.

[0093] Finally, the milled mixture was placed in a box furnace and sintered at 650°C under an argon atmosphere for 6 hours to complete the second sintering treatment. After cooling, porous polyanion cathode active material was obtained.

[0094] The chemical formula of the polyanionic positive electrode active material prepared in this embodiment is Na. 3.7 Fe 2.5 Co 0.03 P 3.8 O 14.6 .

[0095] Example 3

[0096] Sodium permanganate was added to 100L of pure water until the sodium permanganate concentration was 3wt%. Then, 6.7kg FePO4 and 4.5kg ammonium dihydrogen phosphate were added. The mixture was stirred at 70℃ for 120min. The mixture was then centrifuged and dried at 110℃ to obtain the intermediate mixture.

[0097] 5 kg of mixed intermediate, 1.01 kg of Na2C2O4, 0.03 kg of MnSO4, 0.65 kg of polyethylene glycol and 14.58 kg of water were sequentially added to a sand mill for the first sand milling treatment for 120 min.

[0098] The mixture after the first sand milling treatment was pumped into a spray dryer at a rate of 1.2 L / min. At the same time, 0.1 mol / L of citric acid was added into another feed pipe at a rate of 1.5 L / min. The molar ratio of Na2C2O4 to citric acid in the mixture was 1:1. The mixture and citric acid were fed together into an atomizing disc for the first spray drying treatment. The rotation speed of the atomizing disc was 15000 rpm / min. The moisture content of the dried mixture was 4.1%.

[0099] The material after the first spray drying treatment was placed in a box furnace and sintered at 450℃ in an argon / hydrogen atmosphere (Ar / H2 volume ratio of 95:5) for 8 hours to complete the first sintering treatment.

[0100] The sintered material is cooled and coarsely crushed, then mixed with acrolein at a mass ratio of 17:1, and pulverized using a mechanical pulverizer to complete the second sand milling process.

[0101] Finally, the milled mixture was placed in a box furnace and sintered at 550℃ in an argon / hydrogen atmosphere (Ar / H2 volume ratio of 95:5) for 12 hours to complete the second sintering treatment. After cooling, porous polyanion cathode active material was obtained.

[0102] The chemical formula of the polyanionic positive electrode active material prepared in this embodiment is Na. 3.4 Fe 2.2 Mn 0.03 P 3.8 O 14.6 .

[0103] Example 4

[0104] Potassium permanganate was added to 100L of pure water until the potassium permanganate concentration was 3wt%. Then, 7.1kg of ferric acetate and 4.6kg of phosphoric acid were added. The mixture was stirred at 85℃ for 60min. The mixture was then centrifuged and dried at 120℃ to obtain the intermediate mixture.

[0105] 5 kg of mixed intermediate, 1.32 kg of sodium acetate, 0.01 kg of CaCO3, 0.77 kg of starch and 15.14 kg of water were sequentially added to a sand mill for the first sand milling treatment for 120 min.

[0106] The mixture after the first sand milling process was pumped into a spray dryer at a rate of 1.2 L / min. At the same time, 0.1 mol / L of oxalic acid was added into another feed pipe at a rate of 1.5 L / min. The molar ratio of sodium acetate to oxalic acid in the mixture was 1:1. The mixture and oxalic acid were fed together into an atomizing disc for the first spray drying process. The rotation speed of the atomizing disc was 15,000 rpm / min. The moisture content of the dried mixture was 3.2%.

[0107] The material after the first spray drying treatment was placed in a box furnace and sintered at 450℃ in an argon / hydrogen atmosphere (Ar / H2 volume ratio of 95:5) for 8 hours to complete the first sintering treatment.

[0108] The sintered material is cooled and coarsely crushed, then mixed with propionaldehyde at a mass ratio of 17:1, and pulverized using a mechanical pulverizer to complete the second sand milling process.

[0109] Finally, the milled mixture was placed in a box furnace and sintered at 550℃ in an argon / hydrogen atmosphere (Ar / H2 volume ratio of 95:5) for 12 hours to complete the second sintering treatment. After cooling, porous polyanion cathode active material was obtained.

[0110] The chemical formula of the polyanionic positive electrode active material prepared in this embodiment is Na. 3.5 Fe 2.3 Ca 0.01 P 3.7 O 13.9 .

[0111] Comparative Example 1

[0112] 5 kg FeC2O4·H2O, 4.44 kg NaH2PO4, 0.02 kg NiSO4, and 0.83 kg C6H 12 O6 and 25.68 kg of water were sequentially added to a sand mill and milled for 30 minutes. Then, the mixed wet material was slowly pumped into a spray dryer for spray drying. Subsequently, the dried material was placed in a box furnace and sintered at 525°C under a nitrogen atmosphere for 10 hours. After cooling, it was pulverized by an air jet mill to obtain a porous polyanionic positive electrode active material.

[0113] The chemical formula of the polyanionic positive electrode active material prepared in Comparative Example 1 is Na4Fe3P4O 15 .

[0114] Comparative Example 2

[0115] The method is the same as in Example 1, except that acetic acid is not added when the mixture after the first grinding treatment is subjected to the first spray drying treatment.

[0116] Comparative Example 3

[0117] The method is the same as in Example 1, except that methyl ethyl ketone is not added when performing the second grinding treatment on the material after the first sintering treatment.

[0118] Comparative Example 4

[0119] The method is the same as in Example 1, except that 0.2 mol / L acetic acid is added when the material after the first sintering treatment is subjected to the first spray drying treatment.

[0120] The positive electrode active materials prepared in Examples 1-4 and Comparative Examples 1-4 were subjected to performance tests, and the test results are shown in Tables 1-4.

[0121] 1. The instrument used for SEM characterization was a JSM-7900F from NEC.

[0122] 2. The diameter of the pore structure and the diameter and length of the primary particles were measured under high-resolution SEM after the material was argon-ion polished;

[0123] 3. The tap density characterization instrument was Dandong Aode, and the test method was GB / T 5162-2021;

[0124] 4. The instrument used for characterizing the loose bulk density was Dandong Better, and the test method was GB / T 1479.1-2011;

[0125] 5. The formula for calculating the compressibility coefficient is (tap density - loose density) / tap density;

[0126] 6. The liquidity test method is GB 1482-84;

[0127] 7. The total volume of micropores in the material was measured by the static adsorption BET method. Micropores refer to pores with a maximum pore diameter of less than 2 nm.

[0128] 8. The total volume of macropores in the material was measured by mercury intrusion porosimetry. Macropores are defined as pores with a maximum diameter greater than 50 nm.

[0129] 9. The compaction density is the powder compaction density measured under a pressure of 3t;

[0130] 10. The vacancy rate is obtained through XRD refinement;

[0131] 11. Electrochemical tests were all coin cell tests. Specifically, the positive electrode active material, Super P (conductive agent), and polyvinylidene fluoride (PVDF) (binder) were mixed at a mass ratio of 95:2:3 (PVDF was first dissolved in N-methylpyrrolidone solution at a mass ratio of 1:39) to prepare a slurry. The current collector aluminum foil was flattened and fixed on a flatbed coating machine with a coating thickness of 150 micrometers. The slurry was slowly poured onto the aluminum foil and coated evenly. The coated aluminum foil was then placed in a vacuum oven and vacuum dried at 110°C for 8 hours. The aluminum foil was then removed and placed in a roller press (rolling thickness set at 70 micrometers) and rolled 5 times to form the positive electrode sheet.

[0132] Button assembly:

[0133] With the negative electrode shell open side up, place it flat on a glass plate. Place a 16μm thick nickel foam inside the negative electrode shell. Remove the protective films from both sides of the sodium sheet and place it directly above the nickel foam. Place a 19μm thick sodium electrolyte separator directly above the sodium sheet. Use a dropper to add 8 drops of electrolyte to the separator. Place the positive electrode plate directly above the separator. Cover the entire negative electrode shell with the positive electrode shell and seal it using a sealing machine.

[0134] Electrochemical testing method: Charge and discharge tests were conducted on the rechargeable batteries using the assembled CR2032 coin cells described above. Charge and discharge test conditions: Temperature was controlled at 25℃, test voltage was 2–4V, the first cycle was 0.2C charge / discharge, the cycle test was 0.2C charge / 1C discharge, up to 200 cycles, and the rate test was 0.2C charge / 2C discharge.

[0135] Table 1

[0136]

[0137] Table 2

[0138]

[0139] Table 3

[0140]

[0141] Note: k=(n(O)+n(P)) / ((V1-V2)*(e(Pd-Td)))

[0142] Table 4

[0143] Discharge capacity mAh / g First effect 200-cycle capacity retention 2C rate capacity retention Example 1 108.3 90.5％ 99.75％ 94.63％ Example 2 106.4 91.2％ 99.72％ 93.76％ Example 3 107.5 90.3％ 99.70％ 94.11％ Example 4 107.1 91.4％ 99.68％ 93.59％ Comparative Example 1 98.3 88.4％ 89.12％ 90.10％ Comparative Example 2 99.7 89.5％ 91.34％ 91.38％ Comparative Example 3 98.9 89.1％ 91.43％ 92.57％ Comparative Example 4 101.0 90.0％ 92.18％ 91.86％

[0144] As shown in Tables 1-4, the positive electrode active materials of Examples 1-4 effectively improved the internal structure of the material by controlling the P and O vacancies, as well as the size of the internal pores and primary particles, thereby increasing the particle strength. After being prepared into electrode sheets using the positive electrode active materials of Examples 1-4, the compaction density of all sheets reached 2.1 g / cm³. 3 In addition, the positive electrode active materials of Examples 1-4 all exhibit good fluidity, and the compressibility coefficient can be controlled below 55%. The coin cells prepared using the active materials of Examples 1-4 all achieve a discharge capacity of over 105 mAh / g, a capacity retention rate of over 99.5% after 200 cycles, and a capacity retention rate of over 93% at 2C rate, demonstrating excellent electrochemical performance.

[0145] In Comparative Example 1, there are no P and O vacancies, and the pore size inside the material is very large, exceeding 2000 nm, resulting in ultra-large pores. This leads to a significant decrease in compaction density and a substantial decline in electrochemical performance. Furthermore, the active material prepared in Comparative Example 1 has very poor flowability, with a significantly higher compressibility coefficient than that in Example 1. This indicates that the active material in Comparative Example 1 has a higher coefficient of friction, which is detrimental to the slurry coating process of the battery cell and hinders processing.

[0146] In Comparative Examples 2 and 3, although suitable P and O vacancies exist in the materials, their internal pore structure and primary particle size are too large, resulting in a compaction density of only 2 g / cm³. 3 Around 100°C, the capacity, cycle performance, and rate performance all show a significant decline, while the fluidity is very poor, which is not conducive to processing.

[0147] In Comparative Example 4, there were more P and O vacancies, and the internal pore structure size increased significantly, reaching over 5000 nm, resulting in a decrease in compaction density to 1.83 g / cm³. 3 The electrochemical performance was significantly lower than that of Example 1.

[0148] Although embodiments of this application have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting this application. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of this application.

Claims

1. A polyanionic positive electrode active material, characterized in that, The positive electrode active material is spherical secondary particles with a porous structure inside formed by primary particles. The diameter D of the internal pore structure is ≤ 800 nm. The length of the primary particles is 300 - 1000 nm, and the diameter is 50 - 400 nm. The chemical formula of the positive electrode active material is Na x Fe y M z P 4-a O 15-b , 3 < x ≤ 4.2, 2 < y ≤ 3, 0 ≤ z ≤ 1.0, 0 ≤ a ≤ 0.4, 0 ≤ b ≤ 0.

4. M is selected from at least one element of Ni, Mn, Al, Cu, Mg, Co, Ti. And 0 < oxygen vacancy rate n(O) ≤ 5%, 0 < phosphorus vacancy rate n(P) ≤ 5%.

2. The polyanionic positive electrode active material according to claim 1, characterized in that, The positive electrode active material has a carbon coating layer.

3. The polyanionic positive electrode active material according to claim 1 or 2, characterized in that, The vacancy rates n(O) and n(P) satisfy the following relationship: k = (n(O) + n(P)) / ((V1 - V2) * (e(Pd - Td))), 0.05 ≤ k ≤ 0.

3. Where V1 is the total volume of macropores in the positive electrode active material, V2 is the total volume of micropores in the positive electrode active material, e is the compressibility coefficient of the positive electrode active material, Pd is the compaction density of the positive electrode active material, and Td is the tap density of the positive electrode active material.

4. The polyanionic positive electrode active material according to claim 3, characterized in that, The compressibility coefficient e of the positive electrode active material is 10-60%.

5. The polyanionic positive electrode active material according to claim 1, characterized in that, The flowability of the positive electrode active material is 10-80s / 50g.

6. A method for preparing a polyanionic positive electrode active material according to any one of claims 1-5, characterized in that, Includes the following steps: a. Under an inert atmosphere, the oxidant, iron source, and phosphorus source are dissolved and mixed in water, heated and stirred, then centrifuged and dried to obtain an intermediate. b. The intermediate, sodium source, dopant containing element M, carbon source and water are mixed in an inert atmosphere and subjected to the first sand milling treatment; c. The mixture after sand milling in step b is fed together with the organic acid solution into an atomizing plate for the first spray drying treatment. d. The mixture after spray drying in step c is subjected to a first sintering treatment in an oxygen-free atmosphere; e. After cooling the material obtained from the sintering process in step d, mix it with a carbonyl compound and perform a second sand milling process. f. The mixture after sand milling in step e is subjected to a second spray drying treatment, followed by a second sintering treatment under an oxygen-free atmosphere to obtain a polyanionic positive electrode active material.

7. The method for preparing the polyanionic positive electrode active material according to claim 6, characterized in that, In step a, the oxidant includes at least one of sodium peroxide, hydrogen peroxide, potassium peroxide, potassium permanganate, and sodium permanganate; the iron source includes at least one of iron powder, ferric citrate, ferrous citrate, ferric nitrate, ferrous nitrate, ferric sulfate, ferrous sulfate, ferrous oxide, ferric oxalate, ferrous oxalate, ferric acetate, ferric phosphate, and ferric pyrophosphate; the phosphorus source includes at least one of sodium dihydrogen phosphate, sodium phosphate, sodium monohydrogen phosphate, phosphoric acid, ammonium dihydrogen phosphate, triammonium phosphate, pyrophosphate, sodium pyrophosphate, and sodium dihydrogen pyrophosphate. And / or, in step b, the sodium source includes at least one of sodium carbonate, sodium hydroxide, sodium acetate, sodium oxalate, sodium sulfate, sodium bicarbonate, sodium citrate, and sodium pyrophosphate; the dopant containing element M includes at least one of metal oxide, metal hydroxide, and metal carbonate; optionally, the dopant containing element M includes at least one of magnesium oxide, magnesium hydroxide, aluminum oxide, aluminum hydroxide, copper oxide, titanium oxide, nickel oxide, nickel sulfate, nickel hydroxide, nickel oxalate, nickel phosphate, cobalt hydroxide, manganese oxide, manganese sulfate, and manganese oxalate; the carbon source includes at least one of dopamine, soluble starch, polyethylene glycol, sucrose, or glucose. And / or, in step c, the organic acid includes at least one of formic acid, acetic acid, oxalic acid, and citric acid; And / or, in step e, the carbonyl compound includes at least one of acetophenone, methyl ethyl ketone, propionaldehyde, and acrolein.

8. The method for preparing the polyanionic positive electrode active material according to claim 6, characterized in that, In step a, the heating temperature is 40-90℃; And / or, in step b, the time for the first sand milling treatment is 50-150 min; the solid content of the mixture after the first sand milling treatment is 20%-60 wt%. And / or, in step c, the atomizing disc rotation speed in the first spray drying process is 10000-15000 rpm / min; the moisture content of the mixture after the first spray drying process is 2-10%; And / or, in step d, the temperature of the first sintering treatment is 400-600℃ and the time is 2-10h; And / or, in step e, the second sand milling process takes 1-4 hours; And / or, in step f, the atomizing disc rotation speed in the second spray drying process is 20000-25000 rpm / min; the temperature of the second sintering process is 500-700℃, and the time is 5-20h; And / or, in steps a and / or b, the inert atmosphere includes at least one of nitrogen or argon; And / or, in steps d and / or f, the oxygen-free atmosphere includes at least one of nitrogen, argon, and hydrogen, or is a vacuum state.

9. The method for preparing the polyanionic positive electrode active material according to claim 6, characterized in that, In step c, the molar ratio of sodium source to organic acid solution in the milled mixture is (0.9:1)-(1.5:1); And / or, in step e, the mass ratio of the material obtained by the sintering treatment to the carbonyl compound is (10:1)-(50:1).

10. A secondary battery, characterized in that, This includes the polyanionic positive electrode active material according to any one of claims 1-5 or the polyanionic positive electrode active material prepared by the preparation method according to any one of claims 6-9.

11. An electrical appliance, characterized in that, Includes the secondary battery as described in claim 10.

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