Positive electrode active material, positive electrode sheet, battery, and electric device

Abstract

The present application relates to the technical field of electrochemistry. The present application relates to a positive electrode active material, a positive electrode sheet, a battery, and an electric device. The positive electrode active material comprises secondary particles formed by agglomeration of primary particles; the primary particles comprise a carbon-coated iron-based phosphate; and the secondary particles have an internal average porosity of less than or equal to 15%.

Description

A positive electrode active material, a positive electrode sheet, a battery, and an electrical device.

[0001] This application claims priority to Chinese Patent Application No. 202510068544.X, filed on January 15, 2025, entitled "A Positive Electrode Active Material, Positive Electrode Sheet, Battery and Electrical Device", the entire contents of which are incorporated herein by reference. Technical Field

[0002] This application relates to a positive electrode active material, a positive electrode sheet, a battery, and an electrical device, belonging to the field of electrochemical technology. Background Technology

[0003] With the rapid development of the new energy vehicle industry, the demand for lithium batteries is also increasing. However, global lithium resources are limited and unevenly distributed. Therefore, the development of new, high-performance, and low-cost alternative energy storage devices has attracted much attention. Sodium-ion batteries have similar working principles to lithium-ion batteries, and sodium resources are far more abundant in the Earth's crust than lithium resources. Therefore, sodium-ion batteries are considered a potential alternative to lithium-ion batteries.

[0004] The cathode active material is a key factor affecting the performance of sodium-ion batteries. Iron-based phosphate materials are rich in phosphorus and iron, have abundant ion diffusion channels, and have long cycle life, making them one of the most promising cathode active materials in the energy storage field of sodium batteries.

[0005] However, the compaction density of cathode sheets currently prepared based on iron-based phosphate cathode materials is not high, resulting in low energy density of sodium-ion cells, which severely limits their use in many application scenarios. Summary of the Invention

[0006] In view of this, this application provides a positive electrode active material, wherein the internal porosity of the secondary particles of the positive electrode active material is less than or equal to 15%, which can achieve a high solid density of the positive electrode sheet, thereby improving the energy density of the battery cell.

[0007] This application also provides a method for preparing the above-mentioned positive electrode active material. The preparation method is simple and can successfully prepare the above-mentioned positive electrode active material.

[0008] This application also provides a positive electrode sheet comprising the above-mentioned positive electrode active material, thereby enabling the positive electrode sheet to achieve a higher ultimate compaction density.

[0009] This application also provides a battery that includes the above-mentioned positive electrode, and therefore the battery has a high energy density.

[0010] This application also provides a battery pack comprising the aforementioned battery, which thus has a high energy density and can provide more energy in a smaller volume and weight, making it suitable for use in portable electronic devices (such as smartphones and laptops).

[0011] This application also provides an electrical device that includes the aforementioned battery or battery pack, thus possessing the advantages of being lightweight and having a long battery life.

[0012] In detail, in a first aspect, this application provides a positive electrode active material comprising secondary particles formed by the stacking of primary particles, an iron-based phosphate, and a carbon material located on at least a portion of the surface of the iron-based phosphate; the internal average porosity of the secondary particles is less than or equal to 15%.

[0013] Furthermore, the internal average porosity of the secondary particles is greater than or equal to 7%.

[0014] Furthermore, the average particle size of the primary particles is less than or equal to 120 nm.

[0015] Furthermore, the average particle size of the primary particles is 75nm-100nm.

[0016] Furthermore, the average pore size inside the secondary particles is less than or equal to 100 nm.

[0017] Furthermore, the iron-based phosphate includes sodium iron-based phosphate.

[0018] Furthermore, the iron-based sodium phosphate salt includes at least one of sodium pyrophosphate, sodium iron phosphate, and sodium pyrophosphate.

[0019] Secondly, this application provides a method for preparing the above-mentioned positive electrode active material, comprising the following steps:

[0020] The positive electrode active material is obtained by grinding, spray drying, and sintering a mixed slurry comprising a phosphorus iron precursor, a metal ion source, a carbon source, and a solvent; wherein the mixed slurry comprises solid particles, the particle size of which after grinding is less than or equal to 300 nm, and the phosphorus iron precursor satisfies at least one of the following conditions:

[0021] 1) The crystallinity of the phosphorus-iron precursor is less than or equal to 30%;

[0022] 2) The water of crystallization content of the phosphorus-iron precursor is greater than or equal to 10%;

[0023] 3) The specific area of ​​the phosphorus-iron precursor is greater than or equal to 15 m². 2 / g;

[0024] 4) The tap density of the phosphorus-iron precursor is greater than or equal to 1 g / cm³. 3 .

[0025] Furthermore, the mixed slurry also contains a phosphorus source;

[0026] And / or, in the mixed slurry, the molar ratio of phosphorus, metal ions and iron is 4:(3.8-4.2):(2.8-3.2).

[0027] Furthermore, the particle size of the ground solid particles is 50nm-300nm.

[0028] Furthermore, the sintering process includes the following steps:

[0029] Heat to 450℃-600℃ at a heating rate of 5-20℃ / min, and hold for 5-30 hours.

[0030] Thirdly, this application provides a positive electrode sheet, comprising the positive electrode active material described in the first aspect.

[0031] Furthermore, the compaction density of the positive electrode sheet is greater than 2.05 g / cm³. 3 .

[0032] Fourthly, this application provides a battery including a negative electrode and a positive electrode as described in the third aspect.

[0033] Fifthly, this application provides a battery pack including the battery described in the fourth aspect.

[0034] Sixthly, this application provides an electrical device including the battery described in the fourth aspect or the battery pack described in the fifth aspect.

[0035] The secondary particles of the positive electrode active material provided in this application have an average porosity of less than or equal to 15%. This means that there are fewer gaps between the primary particles and the secondary particles themselves are relatively dense. During the compaction process, the deformation of the positive electrode active material is small, which helps to maintain the structural integrity of the positive electrode active material and thus ensures the effective conduction of ions inside the material. Furthermore, due to the low porosity inside the secondary particles, a positive electrode sheet with a higher compaction density can be prepared through compaction. This is beneficial to improving the electronic conductivity of the positive electrode sheet and thus effectively improving the energy density of the battery cell. Attached Figure Description

[0036] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with this application and, together with the description, serve to explain the principles of this application.

[0037] Figure 1 shows the XRD diffraction pattern of the phosphorus-iron precursor in Example 1;

[0038] Figure 2 shows the XRD diffraction pattern of the positive electrode active material in Example 1;

[0039] Figure 3 is a high-magnification TEM image of the positive electrode active material in Example 1;

[0040] Figure 4 is a low-magnification SEM image of the positive electrode active material of Example 1;

[0041] Figure 5 is a high-magnification SEM image of the positive electrode active material of Example 1;

[0042] Figure 6 is a cross-sectional SEM image of the positive electrode active material of Example 1;

[0043] Figure 7 shows the positive electrode sheet prepared using the positive electrode active material of Example 1 at 2.35 g / cm³. 3 Folded image under compacted density;

[0044] Figure 8 is a graph showing the first charge-discharge capacity of the battery prepared using the positive electrode active material of Example 1.

[0045] Figure 9 shows the XRD diffraction pattern of the phosphorus-iron precursor in Comparative Example 1.

[0046] Figure 10 shows a high-magnification SEM image of the positive electrode active material of Comparative Example 1;

[0047] Figure 11 shows a cross-sectional SEM image of the positive electrode active material in Comparative Example 1;

[0048] Figure 12 shows the positive electrode sheet prepared using the positive electrode active material of Comparative Example 1 at 1.95 g / cm³. 3 Folded image under compacted density;

[0049] Figure 13 shows a high-magnification SEM image of the positive electrode active material of Comparative Example 2;

[0050] Figure 14 shows a cross-sectional SEM image of the positive electrode active material in Comparative Example 2;

[0051] Figure 15 shows a high-magnification SEM image of the positive electrode active material of Comparative Example 3;

[0052] Figure 16 shows a cross-sectional SEM image of the positive electrode active material of Comparative Example 3. Detailed Implementation

[0053] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0054] The pores within the secondary particles of the positive electrode active material are relatively stable and difficult to further compress during rolling. High porosity within the secondary particles hinders the improvement of the compaction density of the corresponding positive electrode sheet. However, the porosity of most current iron-based phosphate positive electrode active materials' secondary particles is >30%. Therefore, reducing the internal porosity of iron-based phosphate positive electrode active materials is one of the advantageous ways to improve the compaction density of the positive electrode sheet. Based on this, this application provides the following technical solution:

[0055] In a first aspect, this application provides a positive electrode active material, comprising secondary particles formed by the stacking of primary particles, an iron-based phosphate, and a carbon material located on at least a portion of the surface of the iron-based phosphate; the internal average porosity of the secondary particles is less than or equal to 15%.

[0056] The internal average porosity of the secondary particles mentioned above refers to the average porosity of any cross-section of the secondary particles. The testing method is as follows: use SEM (Scanning Electron Microscope) to test any cross-section of the positive electrode active material, adjust the contrast of the obtained image to the highest level so that the pores appear black, and use ImageJ software to count the area ratio of the black area in the whole image to calculate the proportion of pores in the image. Take 10-20 samples and measure them according to the above method. Finally, take the average value to obtain the internal average porosity of the secondary particles.

[0057] In this application, the secondary particles of the positive electrode active material have an average porosity of less than or equal to 15%, which can be used to prepare a positive electrode sheet with higher compaction density through compaction, thereby effectively improving the energy density of the battery cell.

[0058] In one specific embodiment, the internal average porosity of the secondary particles is greater than or equal to 7%.

[0059] The secondary particles mentioned above can not only be used to prepare high-density positive electrode sheets, but also ensure the normal penetration of electrolyte and ion conduction pathways, thereby facilitating rapid ion conduction.

[0060] In some embodiments, the secondary particles are spherical or near-spherical particles.

[0061] In some embodiments, the internal average porosity of the secondary particles is any value or a range of any two of 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, etc.

[0062] In one specific embodiment, the average particle size of the primary particles is less than or equal to 120 nm.

[0063] The method for testing the average particle size of the primary particles is as follows: The surface of the positive electrode active material is characterized under a scanning electron microscope (JEOL SEM-200201 scanning electron microscope) to obtain the morphology of the positive electrode active material. The size of the primary particles in the image is randomly measured and counted using the testing software built into the SEM instrument or Nano Measurer software until the counted particle data is greater than or equal to 500. The average value of the counted data is then taken to obtain the average particle size of the primary particles.

[0064] Primary particles of the aforementioned size can form a denser particle packing, which helps to further ensure the low porosity inside the secondary particles.

[0065] To further ensure the ion diffusion path, in one specific embodiment, the average particle size of the primary particles is 75nm-100nm.

[0066] For example, the average particle size of a primary particle is any value or a range of any two of the following: 75nm, 78nm, 80nm, 82nm, 85nm, 88nm, 90nm, 92nm, 95nm, 97nm, 99nm, 100nm, etc.

[0067] In one specific embodiment, the average pore size inside the secondary particles is less than or equal to 100 nm.

[0068] The method for testing the average pore size inside the secondary particles is as follows: Use SEM (Scanning Electron Microscope) to test any cross section of the positive electrode active material, adjust the contrast of the obtained image to the highest level so that the pores appear black, and then measure the maximum size of each pore. If more than 500 pores are randomly measured, the average value of the statistical pore data can be obtained to obtain the average pore size inside the secondary particles.

[0069] As mentioned above, the secondary particles not only have low porosity, but also small pore size, which can further increase the compaction density of the positive electrode sheet, thereby further improving the energy density of the battery cell.

[0070] In some embodiments, the average pore size inside the secondary particles is any value or a range of any two of the following: 20nm, 25nm, 30nm, 35nm, 40nm, 45nm, 55nm, 60nm, 65nm, 70nm, 75nm, 80nm, 85nm, 90nm, 95nm, 100nm, etc.

[0071] In one specific embodiment, the iron-based phosphate includes sodium iron-based phosphate.

[0072] In one specific embodiment, the iron-based sodium phosphate salt includes at least one of sodium pyrophosphate, sodium iron phosphate, and sodium pyrophosphate.

[0073] As mentioned above, iron-based phosphates exhibit good structural stability, making it easier to achieve long cycle life. In some embodiments, the chemical composition of sodium ferric pyrophosphate includes Na2FeP2O7, the chemical composition of sodium iron phosphate includes NaFePO4, and the chemical composition of sodium ferric pyrophosphate includes Na4Fe3(PO4)2P2O7 and / or Na 4.5 Fe 3.5 (PO4) 2.5 P2O7.

[0074] In one specific embodiment, the positive electrode active material is prepared by a method comprising the following processes:

[0075] A positive electrode active material is obtained by grinding, spray drying, and sintering a mixed slurry comprising a phosphorus iron precursor, a metal ion source, a carbon source, and a solvent; wherein the mixed slurry comprises solid particles, the particle size of which is less than or equal to 300 nm after grinding, and the phosphorus iron precursor meets at least one of the following conditions:

[0076] 1) The crystallinity of the phosphorus-iron precursor is less than or equal to 30%;

[0077] 2) The water of crystallization content of the phosphorus-iron precursor is greater than or equal to 10%;

[0078] 3) The specific area of ​​the phosphorus-iron precursor is greater than or equal to 15 m². 2 / g;

[0079] 4) The tap density of the phosphorus-iron precursor is greater than or equal to 1 g / cm³. 3 .

[0080] Secondly, this application provides a method for preparing the above-mentioned positive electrode active material, comprising the following steps:

[0081] A positive electrode active material is obtained by grinding, spray drying, and sintering a mixed slurry comprising a phosphorus iron precursor, a metal ion source, a carbon source, and a solvent; wherein the mixed slurry comprises solid particles, the particle size of which is less than or equal to 300 nm after grinding, and the phosphorus iron precursor meets at least one of the following conditions:

[0082] 1) The crystallinity of the phosphorus-iron precursor is less than or equal to 30%;

[0083] 2) The water of crystallization content of the phosphorus-iron precursor is greater than or equal to 10%;

[0084] 3) The specific area of ​​the phosphorus-iron precursor is greater than or equal to 15 m². 2 / g;

[0085] 4) The tap density of the phosphorus-iron precursor is greater than or equal to 1 g / cm³. 3 .

[0086] The above preparation method, by controlling the particle size of the solid particles in the mixed slurry after grinding and the parameters of the phosphorus-iron precursor, can make the material have the characteristics of high compatibility reaction activity during the sintering process, thereby making the primary particles tightly packed into secondary particles with an internal porosity of less than or equal to 15%. In detail: The solid particles in the finely ground slurry with a particle size less than or equal to 300 nm effectively increase the reaction area and sites, promoting the primary particle fusion reaction. This further limits the phosphorus-iron precursor to a low-crystallinity crystal structure (<30%) or an amorphous crystal structure, which has a higher Gibbs free energy compared to a highly crystalline structure, making it thermodynamically easier to react. Simultaneously, the primary particles fuse, thus reducing the internal porosity of the secondary particles. And / or, the phosphorus-iron precursor has a water of crystallization content >10%. During heating, the release of water of crystallization causes lattice distortion and defects in the precursor material. Therefore, the higher the water of crystallization content of the phosphorus-iron precursor, the easier the material reacts, the easier the primary particles fuse, and the fewer the internal pores of the secondary particles. And / or, the phosphorus-iron precursor has a high specific surface area (>15 m²). 2 The precursor with a surface area of ​​1 g / cm³ has a higher probability of undergoing a compatibility reaction and can effectively promote the occurrence of the compatibility reaction; and / or, it has a high tap density (>1 g / cm³). 3 The phosphorus-iron precursor has a low internal porosity, which promotes the fusion reaction of primary particles, thereby reducing the internal porosity of secondary particles.

[0087] It is understood that the above-mentioned mixed slurry includes liquid and solid particles that are insoluble in solvent. In some embodiments, the solid particles include ferrophosphorus precursors.

[0088] Furthermore, it can be understood that the mixed slurry is ground and the phosphorus iron precursor meets at least one of the above four conditions, and the two work together to obtain the positive electrode active material of this application. In order to further improve the compatibility reaction activity of the primary particles, in some embodiments, the phosphorus iron precursor meets at least two of the above four conditions.

[0089] For example, the particle size of the solid particles in the finely ground slurry is 50nm-300nm, more specifically any value or a range of any two of 50nm, 80nm, 100nm, 120nm, 150nm, 170nm, 200nm, 220nm, 250nm, 270nm, 300nm, etc.

[0090] For example, the water of crystallization content of the ferrophosphorus precursor is any value or a range of any two of the following: 10%, 12%, 15%, 17%, 18%, 20%, 22%, 25%.

[0091] For example, the specific area of ​​the phosphorus iron precursor is 15m². 2 / g、20m 2 / g、25m 2 / g、3m 2 / g、35m 2 / g、40m 2 Any value in / g, or a range consisting of any two of them.

[0092] For example, the tap density of the phosphorus iron precursor is 1 g / cm³. 3 1.5g / cm 3 2g / cm 3 2.5g / cm 3 3g / cm 3 3.5g / cm 3 4g / cm 3 The range of any value or any combination of both.

[0093] Optionally, the mixed slurry also contains a phosphorus source. In some embodiments, the phosphorus content of the iron-phosphorus precursor meets the requirements of the positive electrode active material. In this case, the iron-phosphorus precursor serves not only as an iron source but also as the sole phosphorus source. However, in other embodiments, the phosphorus content of the iron-phosphorus precursor is insufficient, and in this case, an additional phosphorus source needs to be added.

[0094] The molar ratio of phosphorus, metal ions, and iron can be adjusted according to the type of positive electrode active material required. In some embodiments, the molar ratio of phosphorus, metal ions, and iron in the mixed slurry is 4:(3.8-4.2):(2.8-3.2).

[0095] Optionally, the ratio of the metal ions: iron: phosphorus can be 3.8:2.8:4, 3.9:2.9:4, 4.0:2.9:4, 4.1:2.9:4, 4:3:4, 4.2:3.1:4 and 4.0:3.2:4.

[0096] In some embodiments, the aforementioned phosphorus-iron precursor is composed of phosphorus-containing anions and iron cations. The phosphorus-containing anions include, but are not limited to, phosphate, pyrophosphate, hydrogen phosphate, dihydrogen phosphate, or combinations of the aforementioned phosphorus sources. The iron cations include ferric iron or ferrous iron. The molar ratio of iron to phosphorus can be adjusted according to the specific performance of the desired positive electrode active material. For example, the molar ratio of iron to phosphorus is 0.6-1.2.

[0097] In some embodiments, the metal ion source is a sodium source, which includes, but is not limited to, one or more of the following: sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium phosphate, sodium pyrophosphate, phosphoric acid, sodium hexametaphosphate, sodium carbonate, sodium acetate, sodium sulfide, sodium chloride, and sodium sulfate. The sodium sources described above can all provide sodium element and have good solubility in water, which is beneficial to improving the purity of the product.

[0098] In some embodiments, the phosphorus source includes, but is not limited to, phosphoric acid, sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium phosphate, sodium pyrophosphate, sodium hexametaphosphate, etc.

[0099] In some embodiments, the carbon source includes, but is not limited to, one or more of the following: glucose, sucrose, polyvinylpyrrolidone, cellulose, starch, polyethylene glycol, polystyrene, polyvinyl alcohol, polyethylene, polypropylene, citric acid, and stearic acid. All of the above carbon sources are soluble carbon sources capable of forming a carbon coating layer on at least a portion of the surface of the iron-based phosphate.

[0100] Regarding the amount of carbon source used, this application does not impose any particular limitation. Technicians can adjust it according to the conductivity of the positive electrode active material. In some embodiments, the carbon source accounts for 2%-20% of the total mass of the iron source, phosphorus source, and sodium source.

[0101] In some embodiments, the solvent is water, and the amount of water depends on the desired solids content of the mixed slurry. For example, the solids content of the mixed slurry is in the range of 5% to 30%. Alternatively, the solids content of the slurry is any value or a range of any two of the following: 5%, 7%, 9%, 10%, 12%, 14%, 16%, 18%, 20%, 22%, 24%, 26%, 28%, 30%.

[0102] In some embodiments, the above-mentioned grinding process is performed to reduce the particle size of the solid particles to less than or equal to 300 nm. Specifically, the grinding process includes milling, sand milling, or ball milling, with a grinding time of 10-30 hours. Optionally, the sand milling time can be 10 hours, 13 hours, 9 hours, 10 hours, 12 hours, 14 hours, 16 hours, 18 hours, 20 hours, etc. Extending the sand milling time will reduce the particle size of the phosphorus iron precursor material.

[0103] In some embodiments, the inlet air temperature for spray drying is 120°C-250°C. Optionally, the inlet air temperature for spray drying is any value or a range of any two of the following: 120°C, 130°C, 140°C, 150°C, 160°C, 170°C, 180°C, 190°C, 200°C, 210°C, 220°C, 230°C, 240°C, 250°C.

[0104] The above sintering process is carried out under the protection of a protective gas, which includes, but is not limited to, at least one of helium, neon, argon and nitrogen.

[0105] In some embodiments, the sintering process includes the following steps:

[0106] Heat to 450℃-600℃ at a heating rate of 5-20℃ / min, and hold for 5-30 hours.

[0107] Sintering is an essential process in the preparation of positive electrode active materials. The materials obtained after spray drying will fuse and react under the action of sintering, finally forming the finished powder of positive electrode active materials. Under the sintering conditions mentioned above, it is more conducive to the fusion of primary particles, thereby further reducing the internal porosity of secondary particles of positive electrode active materials.

[0108] Optionally, the heating rate of the above sintering treatment is any value or a range of any two of the following: 5℃ / min, 8℃ / min, 10℃ / min, 12℃ / min, 14℃ / min, 15℃ / min, 16℃ / min, 18℃ / min, 20℃ / min; the holding temperature is any value or a range of any two of the following: 450℃, 470℃, 490℃, 510℃, 530℃, 550℃, 570℃, 590℃, 600℃; and the holding time is any value or a range of any two of the following: 5h, 8h, 11h, 14h, 17h, 20h, 23h, 26h, 29h, 32h, 35h.

[0109] Thirdly, this application provides a positive electrode sheet, including the positive electrode active material of the first aspect.

[0110] It is understood that the positive electrode sheet includes a positive current collector and a positive active layer located on at least one surface of the positive current collector, and the positive active layer includes a positive active material.

[0111] In some embodiments, the above-mentioned positive electrode active layer specifically includes, in addition to the positive electrode active material, a conductive agent and a binder, wherein the ratio of the positive electrode active material, the conductive agent, and the binder by mass is 100:(1-6):(1-5); the binder is selected from one or more of styrene-butadiene rubber, polyvinylidene fluoride, polyvinylidene fluoride-trifluoroethylene, polyvinylidene fluoride-tetrafluoroethylene, polyvinylidene fluoride-hexafluoroethylene, polyvinylidene fluoride-hexafluoropropylene, styrene-acrylic emulsion, ethyl polyacrylate, polymethyl methacrylate, polybutyl methacrylate, polyvinyl alcohol, ethylene-vinyl acetate copolymer, polyvinyl acetate, and polyurethane; the conductive agent is selected from one or more of conductive carbon black, carbon nanotubes, conductive graphite, and graphene.

[0112] In other embodiments, the compaction density of the positive electrode is greater than 2.05 g / cm³. 3 .

[0113] For example, the compaction density of the positive electrode is 2.1 g / cm³. 3 2.15g / cm 3 2.2g / cm 3 2.25g / cm 3 2.3g / cm 3 2.35g / cm 3 2.4g / cm 3 2.45g / cm 3 2.5g / cm 3 A range consisting of any value in the range, or any combination of both.

[0114] Fourthly, this application provides a battery including a negative electrode and a positive electrode, as described in the third aspect.

[0115] For example, the negative electrode sheet includes a negative electrode current collector and a negative electrode active layer located on at least one surface of the negative electrode current collector. The negative electrode active layer includes a negative electrode active material, a conductive agent, a binder, and a thickener. The negative electrode active material can be one or more of the negative electrode active materials known in the art for use in batteries, such as carbon-based materials, silicon-based materials, tin-based materials, and titanium-based materials; specifically, it can be artificial graphite, natural graphite, soft carbon, hard carbon, elemental silicon, silicon oxides, silicon-carbon composites, silicon-nitrogen composites, silicon alloys, elemental tin, tin oxides, tin alloys, elemental titanium, titanium oxides, and titanium alloys. One or more of the following: the negative electrode current collector can be a conventional negative electrode current collector in the art, such as copper foil; the conductive agent includes one or more of conductive carbon black, carbon nanotubes, conductive graphite, and graphene; the binder includes one or more of polyvinylidene fluoride (PVDF), a copolymer of polyvinylidene fluoride and hexafluoropropylene, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyvinyl ether, polymethyl methacrylate, polytetrafluoroethylene, polyhexafluoropropylene, and styrene-butadiene rubber (SBR); and the thickener includes sodium carboxymethyl cellulose (CMC-Na), etc.

[0116] Optionally, the negative electrode active material in the negative electrode active layer has a mass fraction of 92%-98%, the binder has a mass fraction of 0.5%-3%, the conductive agent has a mass fraction of 0.5%-2.5%, and the thickener has a mass fraction of 0.5%-2.5%.

[0117] The battery of this application can be manufactured according to conventional methods in the art. For example, the positive electrode, separator and negative electrode can be stacked in sequence and assembled into a cell by winding or stacking process. Then, after packaging and baking, electrolyte is injected and the battery is manufactured by hot pressing and other processes.

[0118] Exemplarily, the battery also includes an electrolyte. This application does not specifically limit the electrolyte; for example, an electrolyte comprising an organic solvent and an electrolyte salt can be used. The organic solvent, as a medium for transporting ions in the electrochemical reaction, can be any organic solvent known in the art for battery electrolytes, such as ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (EMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), butenyl carbonate (BC), fluoroethylene carbonate (FEC), methyl formate (MF), methyl acetate (MA), ethyl acetate (EA), propyl acetate (... At least one of PA), methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), methyl butyrate (MB), ethyl butyrate (EB), 1,4-butyrolactone (GBL), sulfolane (SF), dimethyl sulfone (MSM), methyl ethyl sulfone (EMS), and diethyl sulfone (ESE); the electrolyte salt, as the source of sodium ions, may be an electrolyte salt known in the art for use in battery electrolytes, such as at least one of sodium perchlorate, sodium hexafluorophosphate, sodium hexafluorophosphate, sodium bis(fluorosulfonyl)imide, sodium bis(fluorosulfonyl)imide, and sodium difluorooxalate borate.

[0119] Fifthly, this application provides a battery pack including the battery of the fourth aspect.

[0120] Sixthly, this application provides an electrical device, including the battery of the fourth aspect or the battery pack of the fifth aspect.

[0121] It should be noted that the aforementioned electronic devices can be any conventional device that requires electricity, such as, but not limited to, computers, electric vehicles, air conditioners, refrigerators, washing machines, microwave ovens, printers, fax machines, etc.

[0122] The present application will be further described below with reference to specific embodiments:

[0123] The following method for testing the internal average porosity of secondary particles is as follows: A scanning electron microscope is used to test any cross-section of the positive electrode active material. The contrast of the obtained image is adjusted to the highest level so that the pores appear black and the material solid appears white. The proportion of the black area in the whole image is statistically analyzed using ImageJ software to calculate the proportion of pores in the image. The sampling is repeated 20 times and the above method is used for measurement. Finally, the average value is taken to obtain the internal average porosity of the secondary particles.

[0124] The following method for testing the average particle size of primary particles is as follows: The surface of the positive electrode active material is characterized under a scanning electron microscope (JEOL SEM-200201 scanning electron microscope) to obtain the morphology of the positive electrode active material. The size of the primary particles in the image is randomly measured and counted using the testing software built into the SEM instrument or Nano Measurer software until 500 particles are counted. The average value of the counted data is then taken to obtain the average particle size of the primary particles.

[0125] The following method for testing the average pore size inside the secondary particles is as follows: Use SEM (Scanning Electron Microscope) to test any cross-section of the positive electrode active material, adjust the contrast of the obtained image to the highest level so that the pores appear black, and then measure the maximum size of each pore. Measure 500 pores randomly, and take the average value of the statistical pore data to obtain the average pore size inside the secondary particles.

[0126] The XRD pattern testing involved below is as follows: Precursor powder and finished powder are tested using a powder X-ray diffraction analyzer (BRUKER D8 ADVANCE XRD analyzer) to obtain the phase information of the precursor and sodium iron phosphate pyrophosphate composite material. The integrated intensity of all diffraction peaks in the tested XRD pattern is calculated and compared with the integrated intensity of the diffraction peaks in the corresponding standard XRD pattern. The crystallinity of the tested substance can be obtained according to the formula: Xc = Ic / I0, where Xc is the crystallinity of the sample, Ic is the integrated intensity of all diffraction peaks of the sample, and I0 is the integrated intensity of the diffraction peaks in the corresponding standard XRD pattern.

[0127] Example 1

[0128] This example provides a positive electrode active material, including spherical secondary particles formed by the stacking of primary particles. The primary particles are sodium iron phosphate pyrophosphate material and carbon material coating at least part of the surface of the sodium iron phosphate pyrophosphate material. The average particle size of the primary particles is 75.3 nm. The average porosity of the secondary particles is 7.21%, and the average pore size of the secondary particles is 53.1 nm.

[0129] Its preparation method includes the following steps:

[0130] 1) 3 mol of phosphorus-iron precursor (chemical formula Fe3(HPO4)3PO4·4.67H2O) was selected; it exhibits an amorphous structure, with a crystal structure shown in Figure 1, no obvious diffraction peaks, and a crystallinity of approximately 0%; its specific surface area is 23 m² / g. 2 / g; tap density is 1.1g / cm³ 3 1. A mixture of 13.2% water of crystallization, 2 mol sodium carbonate, and 2 mol glucose was added to 12 L of water and mixed thoroughly to obtain a first mixed slurry with a solid content of approximately 10%.

[0131] 2) The first mixed slurry was placed in a sand mill, sand milling media was added, and it was sand milled for 20 hours. The particle size (Dmax) of the solid particles in the second mixed slurry after sand milling was about 80 nm.

[0132] 3) The second mixed slurry is spray-dried at an inlet air temperature of 170°C to obtain a solid product;

[0133] 4) The solid product obtained in step 3) is placed in a nitrogen atmosphere and heated to 550°C at a heating rate of 10°C / min, and held at that temperature for 10 hours to obtain the final product positive electrode active material powder.

[0134] Figure 2 shows the X-ray diffraction pattern of the positive electrode active material synthesized in this example. As can be seen from Figure 2, the target product is a pure phase sodium iron phosphate pyrophosphate material, and no obvious diffraction peaks of impurities were found. Figure 3 is a high-magnification TEM image of the positive electrode active material synthesized in this example. As can be seen from the image, the material surface is coated with a uniform layer of amorphous carbon, indicating that the material has achieved good carbon coating. Figure 4 is a low-magnification SEM image of the positive electrode active material synthesized in this example. As can be seen from the image, the positive electrode active material mainly exhibits a spherical secondary particle morphology. Figures 5 and 6 are high-magnification SEM images of the positive electrode active material synthesized in this example. As can be seen from the images, the primary particles of the positive electrode active material have a high degree of compatibility, and the secondary particles composed of them are very dense with very low porosity.

[0135] Example 2

[0136] This example provides a positive electrode active material, including spherical secondary particles formed by the stacking of primary particles. The primary particles are sodium iron phosphate pyrophosphate material and carbon material coating at least part of the surface of the sodium iron phosphate pyrophosphate material. The average particle size of the primary particles is 76.0 nm. The average porosity of the secondary particles is 10.35%, and the average pore size of the secondary particles is 56.3 nm.

[0137] The preparation method is basically the same as in Example 1, except that the chemical formula of the ferrophosphorus precursor is Fe3(PO4)2P2O7, the crystallinity is 76.8%, and it does not contain water of crystallization, with a BET of 35m. 2 / g, tap density is 0.9g / cm³ 3 .

[0138] Example 3

[0139] This example provides a positive electrode active material, including spherical secondary particles formed by the stacking of primary particles. The primary particles are sodium iron phosphate pyrophosphate material and carbon material coating at least part of the surface of the sodium iron phosphate pyrophosphate material. The average particle size of the primary particles is 77.2 nm. The average porosity of the secondary particles is 12.98%, and the average pore size of the secondary particles is 76.3 nm.

[0140] The preparation method is basically the same as in Example 1, except that the chemical formula of the ferrophosphorus precursor is Fe3(PO4)2P2O7, the crystallinity is 74.8%, it does not contain water of crystallization, and the BET is 6.65m. 2 / g, tap density is 1.2g / cm³ 3 .

[0141] Example 4

[0142] This example provides a positive electrode active material, including spherical secondary particles formed by the stacking of primary particles. The primary particles are sodium iron phosphate pyrophosphate and carbon material coating at least part of the surface of the sodium iron phosphate pyrophosphate. The average particle size of the primary particles is 76.5 nm. The average porosity of the secondary particles is 9.76%, and the average pore size of the secondary particles is 60.1 nm.

[0143] The preparation method is basically the same as in Example 1, except that the chemical formula of the ferrophosphorus precursor is Fe3(HPO4)3PO4, it does not contain water of crystallization, and its BET is 6.65m. 2 / g, tap density is 1.0g / cm³ 3 The XRD diffraction test results showed no obvious diffraction peaks, and the main characteristics were low crystallinity or amorphous crystal structure, with a crystallinity of about 0%.

[0144] Example 5

[0145] This example provides a positive electrode active material, including spherical secondary particles formed by the stacking of primary particles. The primary particles are sodium iron phosphate pyrophosphate material and carbon material coating at least part of the surface of the sodium iron phosphate pyrophosphate material. The average particle size of the primary particles is 75.9 nm. The average porosity of the secondary particles is 10.12%, and the average pore size of the secondary particles is 85.3 nm.

[0146] The preparation method is basically the same as in Example 1, except that: the chemical formula of the ferrophosphorus precursor is Fe3(PO4)2P2O7·5.71H2O, the water of crystallization content is 16.19%, and the BET is 6.65m. 2 / g, tap density is 1.0g / cm³ 3 XRD diffraction results showed a crystallinity of 65.3%.

[0147] Example 6

[0148] This example provides a positive electrode active material, including spherical secondary particles formed by the stacking of primary particles. The primary particles are sodium iron phosphate pyrophosphate material and carbon material coating at least part of the surface of the sodium iron phosphate pyrophosphate material. The average particle size of the primary particles is 76.8 nm. The average porosity of the secondary particles is 8.34%, and the average pore size of the secondary particles is 54.1 nm.

[0149] The preparation method is basically the same as in Example 1, except that: in step 1), 3 mol of ferric phosphorus precursor (chemical formula Fe3(HPO4)3PO4·4.67H2O, exhibiting an amorphous structure, crystallinity of approximately 0%, water of crystallization content of 13.23%, BET: 23m) is selected. 2 / g, tap density: 1.1g / cm³ 3 4 mol of sodium bicarbonate and 300 g of sucrose were added to 24 L of water and mixed evenly to obtain a mixed slurry with a solid content of about 20%.

[0150] Example 7

[0151] This example provides a positive electrode active material, including spherical secondary particles formed by the stacking of primary particles. The primary particles are sodium iron phosphate pyrophosphate material and carbon material coating at least part of the surface of the sodium iron phosphate pyrophosphate material. The average particle size of the primary particles is 79.3 nm. The average porosity of the secondary particles is 8.67%, and the average pore size of the secondary particles is 56.7 nm.

[0152] The preparation method is basically the same as in Example 1, except that: in step 1), 3 mol of ferric phosphorus precursor (chemical formula FePO4·2.57H2O, exhibiting an amorphous structure, crystallinity of approximately 0%, water of crystallization content of 23.45%, BET: 25m) is selected. 2 / g, tap density: 1.1g / cm³ 3 4 mol sodium bicarbonate, 1 mol phosphoric acid, and 300 g sucrose are added to 10 L of water and mixed evenly to obtain a mixed slurry with a solid content of approximately 20%.

[0153] Example 8

[0154] This example provides a positive electrode active material, including spherical secondary particles formed by the stacking of primary particles. The primary particles are sodium iron phosphate pyrophosphate and carbon material coating at least part of the surface of the sodium iron phosphate pyrophosphate. The average particle size of the primary particles is 103.6 nm. The average porosity of the secondary particles is 14.35%, and the average pore size of the secondary particles is 124.3 nm.

[0155] The preparation method is basically the same as in Example 1, except that: in step 2), the first mixed slurry is placed in a sand mill, sand milling media is added, and it is sand milled for 10 hours. The particle size (Dmax) of the solid particles in the second mixed slurry obtained after sand milling is about 150 nm.

[0156] Example 9

[0157] This example provides a positive electrode active material, including spherical secondary particles formed by the stacking of primary particles. The primary particles are sodium iron phosphate pyrophosphate material and carbon material coating at least part of the surface of the sodium iron phosphate pyrophosphate material. The average particle size of the primary particles is 85.3 nm. The average porosity of the secondary particles is 7.92%, and the average pore size of the secondary particles is 59.3 nm.

[0158] The preparation method is basically the same as that in Example 1, except that: in step 4), the solid product obtained in step 3) is placed in a nitrogen atmosphere and heated to 450°C at a heating rate of 5°C / min, and held for 5 hours to obtain the final positive electrode active material powder.

[0159] Example 10

[0160] This example provides a positive electrode active material, including spherical secondary particles formed by the stacking of primary particles. The primary particles are sodium iron phosphate pyrophosphate material and carbon material coating at least part of the surface of the sodium iron phosphate pyrophosphate material. The average particle size of the primary particles is 77.3 nm. The average porosity of the secondary particles is 6.89%, and the average pore size of the secondary particles is 47.9 nm.

[0161] The preparation method is basically the same as that in Example 1, except that: in step 4), the solid product obtained in step 3) is placed in a nitrogen atmosphere and heated to 600°C at a heating rate of 20°C / min, and kept at the temperature for 30 hours to obtain the final product positive electrode active material powder.

[0162] Comparative Example 1

[0163] This example provides a positive electrode active material, including spherical secondary particles formed by the stacking of primary particles. The primary particles are sodium iron phosphate pyrophosphate and carbon material coating at least part of the surface of the sodium iron phosphate pyrophosphate. The average particle size of the primary particles is 291.9 nm. The average porosity of the secondary particles is 38.92%, and the average pore size of the secondary particles is 384.5 nm.

[0164] The preparation method includes the following steps:

[0165] 1) 3 mol of iron-phosphorus precursor (chemical formula FePO4; crystal structure as shown in Figure 9; crystallinity 89%; specific surface area 5.13 m²) was selected. 2 / g; tap density is 0.9g / cm³ 3 1.5 mol sodium carbonate, 1 mol sodium dihydrogen phosphate, and 2 mol glucose were added to 12 L of water and mixed evenly to obtain a first mixed slurry with a solid content of approximately 10%.

[0166] 2) Place the above mixed slurry in a sand mill, add sand milling media, and sand mill for 20 hours. The particle size (Dmax) of the solid particles in the second mixed slurry after sand milling is about 500 nm.

[0167] 3) The second mixed slurry is spray-dried at an inlet air temperature of 170°C to obtain a solid product;

[0168] 4) The solid product obtained in step 3) is placed in a nitrogen atmosphere and heated to 550°C at a heating rate of 10°C / min, and held at that temperature for 10 hours to obtain the final product positive electrode active material powder.

[0169] Figures 10 and 11 are high-magnification SEM images and cross-sectional SEM images of the target product synthesized in this comparative example, respectively. As can be seen from the figures, the primary particles of the target product synthesized in this comparative example have a very low degree of compatibility and severe particle dispersion. The secondary particles composed of these particles are very loose and porous with a high porosity.

[0170] Comparative Example 2

[0171] This example provides a positive electrode active material, including spherical secondary particles formed by the stacking of primary particles. The primary particles are sodium iron phosphate pyrophosphate material and carbon material coating at least part of the surface of the sodium iron phosphate pyrophosphate material. The average particle size of the primary particles is 123.5 nm. The average porosity of the secondary particles is 29.88%, and the average pore size of the secondary particles is 190.3 nm.

[0172] The preparation method includes the following steps:

[0173] 1) Select 3 mol of ferric phosphorus precursor (chemical formula FePO4; crystallinity 89%; specific surface area 5.13 m²). 2 / g; tap density is 0.9g / cm³ 3 1.5 mol sodium carbonate, 1 mol sodium dihydrogen phosphate, and 2 mol glucose were added to 12 L of water and mixed evenly to obtain a first mixed slurry with a solid content of approximately 10%.

[0174] 2) Place the above mixed slurry in a sand mill, add sand milling media, and sand mill for 20 hours. The particle size (Dmax) of the solid particles in the second mixed slurry after sand milling is about 110 nm.

[0175] 3) The second mixed slurry is spray-dried at an inlet air temperature of 170°C to obtain a solid product;

[0176] 4) The solid product obtained in step 3) is placed in a nitrogen atmosphere and heated to 550°C at a heating rate of 10°C / min, and held at that temperature for 10 hours to obtain the final product positive electrode active material powder.

[0177] Figures 13 and 14 are high-magnification SEM images and cross-sectional SEM images of the target product synthesized in this comparative example, respectively. As can be seen from the figures, the degree of primary particle compatibility of the target product synthesized in this comparative example is improved compared with that of Comparative Example 1, but it is still poor. The particles are severely dispersed, and the secondary particles composed of them are very loose and porous with a high porosity.

[0178] Comparative Example 3

[0179] This example provides a positive electrode active material, including spherical secondary particles formed by the stacking of primary particles. The primary particles are sodium iron phosphate pyrophosphate and carbon material coating at least part of the surface of the sodium iron phosphate pyrophosphate. The average particle size of the primary particles is 305.8 nm. The average porosity of the secondary particles is 20.11%, and the average pore size of the secondary particles is 154.2 nm.

[0180] Its preparation method includes the following steps:

[0181] 1) 3 mol of phosphorus iron precursor (chemical formula Fe3(HPO4)3PO4·2.57H2O; exhibiting an amorphous structure, crystal structure as shown in Figure 1, crystallinity approximately 0%; specific surface area 23 m²) was selected. 2 / g; tap density is 1.1g / cm³ 3 2 mol sodium carbonate and 2 mol glucose were added to 12 L of water and mixed evenly to obtain a first mixed slurry with a solid content of about 10%.

[0182] 2) Place the first mixed slurry into a sand mill, add sand milling media, and sand mill for 3 hours. The particle size (Dmax) of the solid particles in the second mixed slurry after sand milling is about 550 nm.

[0183] 3) The second mixed slurry is spray-dried at an inlet air temperature of 170°C to obtain a solid product;

[0184] 4) The solid product obtained in step 3) is placed in a nitrogen atmosphere and heated to 550°C at a heating rate of 10°C / min, and held at that temperature for 10 hours to obtain the final product positive electrode active material powder.

[0185] Figures 15 and 16 are high-magnification SEM images and cross-sectional SEM images of the target product synthesized in this comparative example, respectively. As can be seen from the figures, the primary particle compatibility of the target product synthesized in this comparative example is also very low, and the particles are severely dispersed. The secondary particles composed of these particles are very loose and porous as a whole, with a high porosity.

[0186] Experimental Example 1

[0187] This example provides a series of positive electrode sheets, each including the positive electrode active material of the above embodiments or comparative examples.

[0188] Its preparation method includes the following steps:

[0189] The positive electrode active material is mixed with carbon nanotubes and PVDF at a mass ratio of 100:5:4, and N-methylpyrrolidone solvent is added until the solid content is 50%. The mixture is mechanically stirred for 4 hours to prepare a conductive slurry. The conductive slurry is coated onto an aluminum foil current collector and dried at 100°C to obtain the positive electrode sheet.

[0190] Test Example 1

[0191] Electrode compaction density test: The positive electrode sheets prepared in Experiment 1 using the various embodiments and comparative examples were compressed to different compaction densities using an electrode roll press. The rolled electrode sheets were folded in half 3 times, and the state of the electrode sheets after folding was observed. The results are recorded in Table 2, and the ultimate compaction density of each positive electrode sheet is recorded in Table 1.

[0192] Figure 7 shows the positive electrode sheet prepared using the positive electrode active material of Example 1 at 2.35 g / cm³. 3 The image shows a folded image at a compacted density of 2.35 g / cm³. 3 Under the compacted density of 1.95 g / cm³, the bent positive electrode sheet did not show obvious breakage and was in good condition; Figure 12 shows the positive electrode sheet prepared using the positive electrode active material of Comparative Example 1 at 1.95 g / cm³. 3 The image shows a folded image at a compacted density of 1.95 g / cm³. 3 Under the compaction density, the bent positive electrode sheet breaks directly, indicating that the positive electrode sheet made of the positive electrode active material of Comparative Example 1 cannot withstand such a high compaction density. This is mainly because the porosity of the positive electrode active material of Comparative Example 1 is too high, and it needs to be compressed with higher pressure, which will cause the electrode sheet to be damaged.

[0193] Experimental Example 2

[0194] This example provides a series of batteries, including the positive electrode, negative electrode, separator, and electrolyte of Example 1, the preparation of which includes the following steps:

[0195] The negative electrode active material (hard carbon material), negative electrode conductive agent (conductive carbon black (SP)), and negative electrode binder (of which, sodium carboxymethyl cellulose (CMC-Na) 1 part by weight and styrene-butadiene rubber (SBR) 2.5 parts by weight) are mixed in a weight ratio of 95.3:1.2:3.5. The mixture is prepared by a wet process (water is added to the above mixture and stirred under the action of a vacuum mixer until the mixture becomes a uniform and fluid negative electrode slurry). The slurry is coated on both sides of the negative electrode current collector (aluminum foil), and then dried (temperature: 85℃, time: 5h), rolled and die-cut to obtain the negative electrode sheet.

[0196] Preparation of electrolyte

[0197] In an argon-filled glove box (moisture content <10ppm, oxygen content <1ppm), dimethyl carbonate (DMC) and diethyl carbonate (DEC) are mixed uniformly at a volume ratio of 1:1 to obtain a mixed solution. Sodium perchlorate is added to the mixed solution to make the concentration of sodium perchlorate 1mol / L, and the mixture is stirred evenly to obtain a non-aqueous electrolyte.

[0198] Preparation of sodium-ion batteries

[0199] The positive electrode sheet from step (1), the separators from each embodiment and comparative example, and the negative electrode sheet from step (2) are wound together to obtain bare batteries without electrolyte injection. The bare batteries are placed in outer packaging foil, and the electrolyte from step (3) is injected into the dried bare batteries. After vacuum sealing, standing, formation, shaping, sorting and other processes, the desired sodium-ion batteries are obtained.

[0200] Test Example 2

[0201] 2) Battery Capacity Test: The batteries prepared in Example 2 were tested on a LAND CT 2001C secondary battery performance testing device at 298±1K. Each example and comparative battery was subjected to charge-discharge cycle tests at a current density of 0.1C (1C=129mA / g). The steps were as follows: rest for 10 min; constant current charging to 4V cutoff, then constant current discharging to 2V cutoff. The battery capacity during charging and discharging was recorded, and the results are shown in Table 1.

[0202] Figure 8 shows the first charge-discharge capacity curve of the battery prepared using the positive electrode active material of Example 1.

[0203] Table 1

[0204] As can be seen from Table 1, the internal porosity of the secondary particles of the positive electrode active material prepared in the examples is all less than 15%, while the internal porosity of the secondary particles of the positive electrode active material in the comparative examples is all higher. This high internal porosity will directly have an adverse effect on the compaction density of the electrode sheet, thereby reducing the energy density of the battery cell.

[0205] Table 2

[0206] As shown in Table 2, the porosity of the secondary particles of the positive electrode active material has a significant impact on the ultimate compaction density of the positive electrode sheet. The two exhibit a very obvious negative correlation: the lower the porosity of the secondary particles of the positive electrode active material, the higher the ultimate compaction density of the positive electrode sheet. However, excessive porosity can directly lead to complete breakage of the electrode sheet when it is rolled to a higher compaction density, or even prevent it from being rolled to a higher compaction density.

[0207] Furthermore, from the comparison of Example 1, Example 8 and Comparative Example 3, we can find that the primary particle size of the positive electrode active material is mainly related to the milling time and the milling particle size. The longer the milling time, the smaller the milling particle size, the smaller the primary particle size of the finished material, and the lower the porosity inside the particles. This indicates that the smaller the primary particles, the more beneficial it is to improve the compaction density of the positive electrode sheet.

[0208] Furthermore, from the comparison of Examples 1-5 and Comparative Example 2, we can see that the parameters of the iron-phosphorus precursor affect its compatibility activity. The iron-phosphorus precursor that meets the conditions of this application can effectively help the primary particles to fuse, reduce the porosity inside the secondary particles, and thus improve the ultimate compaction density of the positive electrode sheet.

[0209] Furthermore, as can be seen from Examples 1, 6, and 7, changing the elemental ratio of the precursor and the types of sodium and phosphorus sources does not have a significant impact on the porosity inside the particles.

[0210] Furthermore, as can be seen from Examples 1 and Comparative Examples 1-3, the use of a phosphorus-iron precursor with high phase fusion reaction activity and the particle size of the solid particles in the second mixed slurry after thorough sand milling are mutually reinforcing and synergistic. They must be simultaneously satisfied in order to minimize the porosity inside the particles and increase the ultimate compaction density of the electrode.

[0211] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of this application.

Claims

1. A positive electrode active material, characterized in that, It includes secondary particles formed by the stacking of primary particles, the primary particles comprising iron-based phosphate and carbon material located on at least a portion of the surface of the iron-based phosphate; the internal average porosity of the secondary particles is less than or equal to 15%.

2. The positive electrode active material according to claim 1, characterized in that, The average internal porosity of the secondary particles is greater than or equal to 7%.

3. The positive electrode active material according to claim 1 or 2, characterized in that, The average particle size of the primary particles is less than or equal to 120 nm.

4. The positive electrode active material according to claim 3, characterized in that, The average particle size of the primary particles is 75nm-100nm.

5. The positive electrode active material according to any one of claims 1-4, characterized in that, The average pore size inside the secondary particles is less than or equal to 100 nm; and / or, the secondary particles are spherical or near-spherical.

6. The positive electrode active material according to any one of claims 1-5, characterized in that, The iron-based phosphates include sodium iron-based phosphates.

7. The positive electrode active material according to claim 6, characterized in that, The iron-based sodium phosphate salt includes at least one of sodium ferric pyrophosphate, sodium ferric phosphate, and sodium ferric pyrophosphate.

8. A method for preparing a positive electrode active material as described in any one of claims 1-7, characterized in that, Includes the following steps: The positive electrode active material is obtained by grinding, spray drying, and sintering a mixed slurry comprising a phosphorus iron precursor, a metal ion source, a carbon source, and a solvent; wherein the mixed slurry comprises solid particles, the particle size of which after grinding is less than or equal to 300 nm, and the phosphorus iron precursor satisfies at least one of the following conditions: 1) The crystallinity of the phosphorus-iron precursor is less than or equal to 30%; 2) The water of crystallization content of the phosphorus-iron precursor is greater than or equal to 10%; 3) The specific area of ​​the phosphorus-iron precursor is greater than or equal to 15 m². 2 / g; 4) The tap density of the phosphorus-iron precursor is greater than or equal to 1 g / cm³. 3 .

9. The preparation method according to claim 8, characterized in that, The mixed slurry also contains a phosphorus source; and / or, In the mixed slurry, the molar ratio of phosphorus, metal ions, and iron is 4:(3.8-4.2):(2.8-3.2).

10. The preparation method according to claim 8, characterized in that, The particle size of the solid particles after grinding is 50nm-300nm.

11. The preparation method according to claim 8, characterized in that, The sintering process includes the following steps: Heat to 450℃-600℃ at a heating rate of 5-20℃ / min, and hold for 5-30 hours.

12. A positive electrode plate, characterized in that, Includes the positive electrode active material as described in any one of claims 1-7.

13. The positive electrode sheet according to claim 12, characterized in that, The compaction density of the positive electrode sheet is greater than 2.05 g / cm³. 3 .

14. A battery, characterized in that, It includes a negative electrode and a positive electrode as described in claim 12 or 13.

15. A battery pack, characterized in that, Includes the battery as described in claim 14.

16. An electrical appliance, characterized in that, Includes the battery of claim 14 or the battery pack of claim 15.

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