COMPOSITE POSITIVE ELECTRODE MATERIAL, PREPARATION METHOD THEREFOR, POSITIVE ELECTRODE SHEET, SECONDARY BATTERY, AND ELECTRIC DEVICE

Abstract

A composite positive electrode material, a preparation method therefor, a positive electrode sheet, a secondary battery, and an electric device are provided. The preparation method comprises: according to a preset proportion, mixing a lithium source, a phosphorus source, an iron source, a carbon source and a carbon graphitization catalyst with a solvent to form a mixed slurry; grinding and drying the mixed slurry to obtain a mixed dry substance; and sintering the mixed dry substance to obtain a composite positive electrode material, wherein the sintering temperature is 750˚C-840˚C.

Description

Composite positive electrode material, preparation method thereof, positive electrode sheet, secondary battery and electric device Cross-reference to related applications

[0001] This application claims priority to Chinese patent application No. 202410021028.7 filed on January 5, 2024, entitled “Composite positive electrode material, preparation method thereof, positive electrode sheet, secondary battery and electrical device”, the entire contents of which are incorporated herein by reference. Technical Field

[0002] The present application relates to the field of battery technology, and in particular to a composite positive electrode material, a preparation method thereof, a positive electrode sheet, a secondary battery, and an electrical device. Background Art

[0003] Graphene is a two-dimensional material composed of carbon atoms arranged periodically in a hexagonal lattice using sp2 hybridized bonds. Theoretically, graphene is only one carbon atom thick and possesses excellent physical properties such as a large theoretical specific surface area, high electron mobility, high thermal conductivity, and a high Young's modulus. This has potential applications in a wide range of fields, including mechanics, energy storage, catalysis, electronic devices, and environmental management. However, it is important to note that the two-dimensional carbon graphene structures produced through actual synthesis are generally composed of multiple layers of carbon atoms.

[0004] Compared to traditional conductive agents in lithium-ion batteries, graphene offers superior conductivity, ultra-fast two-dimensional planar transfer properties, and a very low packing density. This means that adding a very small amount of graphene can potentially achieve the same effect as adding a large amount of traditional conductive agents. This can reduce the proportion of conductive agents, increase the proportion of electrode materials, and increase the energy density of the device.

[0005] Polyanionic cathode materials, including phosphate-based cathode materials, have poor electronic and ionic conductivity due to their inherent structural characteristics, requiring the use of highly conductive coatings and nanomaterials to improve their electrical performance. Graphene composite cathode materials are a highly effective modification method, significantly improving the electrochemical performance of the composite materials.

[0006] Since graphene preparation requires high temperatures, calcining it at high temperatures to form cathode materials can lead to significant particle growth and the formation of impurities, which can degrade the electrochemical performance of the material. Therefore, the conventional method for preparing graphene composite cathode materials is to first prepare the graphene material, then mix the graphene material with the cathode material precursor, and then sinter them. This process is complex and costly.

[0007] Summary of the Invention

[0008] The present application provides a composite positive electrode material, a preparation method thereof, a positive electrode plate, a secondary battery and an electrical device, so as to simplify the preparation process of the composite positive electrode material and reduce its cost.

[0009] The first aspect of the present application provides a method for preparing a composite positive electrode material, which comprises: mixing a lithium source, a phosphorus source, an iron source, a carbon source, and a carbon graphitization catalyst with a solvent in a predetermined proportion to form a mixed slurry; grinding and drying the mixed slurry to obtain a mixed dry matter; and sintering the mixed dry matter to obtain a composite positive electrode material, wherein the sintering temperature is 750°C-840°C.

[0010] The preparation method of the composite positive electrode material of the first embodiment of the present application adopts a one-step method to prepare a polyanionic positive electrode material while forming a carbon material containing graphene. The carbon material is coated on at least a portion of the surface of the polyanionic positive electrode material to form a coating layer, and / or is dispersed between the particles of the polyanionic positive electrode material. The graphene is connected to the coating layer or is freely distributed between the particles of the polyanionic positive electrode material, which simplifies the preparation process of the composite positive electrode material. In addition, the present application further controls the sintering temperature within the range of 750°C-840°C, so that large-size particles and small-size particles are formed in the formed composite positive electrode material, achieving a large-size particle gradation, so that the obtained composite positive electrode material has a higher powder compaction density, for example, the powder compaction density of the composite positive electrode material at 226.0738Mpa (corresponding to 3T) is ≥2.42g / cm 3 .

[0011] In any embodiment of the first aspect of the present application, the carbon graphitization catalyst includes any one or more of a metal or a metal oxide, the metal includes Fe and / or Ni, and the metal oxide includes any one or more of FeO, Fe3O4, Fe2O3, CuO, NiO, MnO, Mn3O4, V2O5, V2O3, or VO4. As catalysts, the above-mentioned metals and metal oxides are relatively stable during the sintering process and are not easily burned away.

[0012] In any embodiment of the first aspect of the present application, the average particle size of the carbon graphitization catalyst is 100 nm to 400 nm, and optionally 100 nm to 330 nm. When the nanoscale carbon graphitization catalyst within the above particle size range is mixed with other materials, a higher contact area with the carbon source can be obtained as much as possible, thereby improving the catalytic efficiency.

[0013] In any embodiment of the first aspect of the present application, the mass ratio of the carbon graphitization catalyst to the carbon source is 1:100-1:30.

[0014] In any embodiment of the first aspect of the present application, the mixed slurry further includes a lithium-site doping element raw material, a phosphorus-site doping element raw material, an iron-site doping element raw material, and an oxygen-site doping element raw material. In the mixed slurry, the total molar number of lithium in the lithium source and the doping element in the lithium-site doping element raw material is M1, the total molar number of iron in the iron source and the doping element in the iron-site doping element raw material is M2, the total molar number of phosphorus in the phosphorus source and the doping element in the phosphorus-site doping element raw material is M3, M1:M2:M3 is (1.0-1.1):(0.95-1.0):(1.0-1.1), and M2:M3 is less than 1:1, and the ratio of the mass of the carbon graphitization catalyst to the total mass of the iron source and the iron-site doping element raw material is (1:1000)-(1:100).

[0015] In any embodiment of the first aspect of the present application, M2:M3 is greater than or equal to 0.96:1 and less than 1:1.

[0016] In any embodiment of the first aspect of the present application, the polyanionic positive electrode material includes a lithium-containing phosphate, and the lithium-containing phosphate includes at least one of lithium iron phosphate and a doped and / or coated modified compound thereof.

[0017] In any embodiment of the first aspect of the present application, the carbon source includes one or more of glucose, sucrose, fructose, citric acid, starch, polyvinyl alcohol, polyethylene glycol or polyaniline.

[0018] In any embodiment of the first aspect of the present application, grinding includes ball milling and sand milling performed sequentially; the Dv50 of the insoluble particles in the mixed slurry after ball milling is 2.0μm-5.0μm, and can be optionally 2.0μm-4.0μm; the Dv50 of the insoluble particles in the mixed slurry after sand milling is 0.3μm-1.2μm, and can be optionally 0.3μm-0.4μm.

[0019] In any embodiment of the first aspect of the present application, the process of sintering the mixed dry matter includes: heating the mixed dry matter to 780°C-820°C for 2h-9h, and keeping the temperature at 780°C-820°C for 6h-20h.

[0020] In any embodiment of the first aspect of the present application, sintering is carried out in a reducing atmosphere, which includes a reducing gas and a protective gas. Optionally, the reducing gas includes any one or more of hydrogen, acetone, propylene, carbon monoxide, methanol, acetylene, methane, ethylene, and ethane; optionally, the protective gas includes any one or more of nitrogen and an inert gas.

[0021] In any embodiment of the first aspect of the present application, the volume content of the reducing gas in the reducing atmosphere is 1%-8%. The addition of the reducing atmosphere not only helps to increase the carbonization rate of the carbon source, but also significantly improves the catalytic effect of the carbon graphitization catalyst in the form of a metal oxide, further optimizes the carbon graphitization effect, and increases the graphene content in the carbon coating layer.

[0022] In any embodiment of the first aspect of the present application, after sintering, the preparation method further comprises a process of crushing the composite positive electrode material, and the Dv50 of the crushed composite positive electrode material is 0.6 μm-2.0 μm.

[0023] The second aspect of the present application provides a composite positive electrode material as described above, which comprises a polyanionic positive electrode material and a carbon material, wherein the carbon material is coated on at least a portion of the surface of the polyanionic positive electrode material and / or dispersed between the polyanionic positive electrode material particles, wherein the carbon material comprises graphene, and the powder compaction density of the composite positive electrode material at 226.0738 MPa is ≥2.42 g / cm 3 , optionally ≥2.53g / cm 3 .

[0024] In any embodiment of the second aspect of the present application, the room temperature turbidity of the slurry formed by mixing the composite cathode material and water in a mass ratio of 1:40 is 200 FTU-400 FTU, and can be optionally 240 FTU-280 FTU.

[0025] In any embodiment of the second aspect of the present application, the mass content of the magnetic substance in the composite positive electrode material is ≤1 ppm, thereby controlling the self-discharge defect of the composite positive electrode material.

[0026] A third aspect of the present application provides a positive electrode plate, comprising a positive electrode film layer, the positive electrode film layer comprising a positive electrode active material, wherein the positive electrode active material comprises the composite positive electrode material provided in any embodiment of the second aspect.

[0027] In any embodiment of the third aspect, based on the total cross-sectional area of ​​the positive electrode active material, the cross-sectional area of ​​the composite positive electrode material with a primary particle size of 50nm-150nm accounts for 10% to 35%; the cross-sectional area of ​​the composite positive electrode material with a primary particle size of 150nm-1500nm accounts for 30% to 60%; the cross-sectional area of ​​the composite positive electrode material with a primary particle size of not less than 1500nm accounts for 10-35%.

[0028] A fourth aspect of the present application provides a secondary battery, comprising a positive electrode plate, wherein the positive electrode plate comprises the positive electrode plate provided by any embodiment of the third aspect.

[0029] A fifth aspect of the present application provides an electrical device including a secondary battery, wherein the secondary battery includes the secondary battery provided in any embodiment of the fourth aspect. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the following is a brief introduction to the drawings required for use in the embodiments of the present application. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on the drawings without creative work.

[0031] FIG1 is a transmission electron microscope (TEM) image of the composite lithium iron phosphate positive electrode material obtained in Preparation Example 1 of the present application.

[0032] FIG2 is a transmission electron microscope image of the composite lithium iron phosphate positive electrode material obtained in Comparative Preparation Example 1 of the present application.

[0033] FIG3 is a schematic diagram of a secondary battery according to an embodiment of the present application.

[0034] FIG. 4 is an exploded view of the secondary battery according to the embodiment of the present application shown in FIG. 3 .

[0035] FIG5 is a schematic diagram of a battery module according to an embodiment of the present application.

[0036] FIG6 is a schematic diagram of a battery pack according to an embodiment of the present application.

[0037] FIG. 7 is an exploded view of the battery pack shown in FIG. 6 according to an embodiment of the present application.

[0038] FIG8 is a schematic diagram of an electric device using a secondary battery as a power source according to an embodiment of the present application.

[0039] In the drawings, the drawings are not drawn to scale.

[0040] Description of reference numerals:

[0041] 1 battery pack; 2 upper box; 3 lower box; 4 battery module; 5 secondary battery cell; 51 shell; 52 electrode assembly; 53 top cover assembly. DETAILED DESCRIPTION

[0042] The following detailed description of the embodiments of the present application is provided in conjunction with the accompanying drawings and examples. The following detailed description of the embodiments and the accompanying drawings are used to illustrate the principles of the present application, but are not intended to limit the scope of the present application, that is, the present application is not limited to the described embodiments.

[0043] Below, the embodiments of the composite positive electrode material, its preparation method, positive electrode sheet, secondary battery and electric device of the present application are described in detail with appropriate reference to the accompanying drawings. However, there may be cases where unnecessary detailed descriptions are omitted. For example, there are cases where detailed descriptions of well-known matters and repeated descriptions of actually the same structure are omitted. This is to avoid the following description from becoming unnecessarily lengthy and to facilitate the understanding of those skilled in the art. In addition, the drawings and the following description are provided for those skilled in the art to fully understand the present application and are not intended to limit the subject matter described in the claims.

[0044] " range " disclosed in the present application is limited in the form of lower limit and upper limit, and given range is limited by selecting a lower limit and an upper limit, and the selected lower limit and upper limit define the boundary of special range. The scope limited in this way can be to include end value or not include end value, and can be arbitrarily combined, that is, any lower limit can form a range with any upper limit combination. For example, if the scope of 60-120 and 80-110 is listed for specific parameters, it is understood that the scope of 60-110 and 80-120 is also expected. In addition, if the minimum range value 1 and 2 are listed, and if the maximum range value 3,4 and 5 are listed, then the following range can all be expected: 1-3, 1-4, 1-5, 2-3, 2-4 and 2-5. In this application, unless otherwise specified, the numerical range " ab " represents the abbreviation of any real number combination between a and b, wherein a and b are all real numbers. For example, a numerical range of "0-5" indicates that all real numbers between "0-5" are listed herein, and "0-5" is simply an abbreviation for these numerical combinations. Furthermore, when a parameter is expressed as an integer ≥ 2, this is equivalent to disclosing that the parameter is, for example, an integer of 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0045] Unless otherwise specified, all embodiments and optional embodiments of the present application can be combined with each other to form a new technical solution.

[0046] Unless otherwise specified, all technical features and optional technical features of this application can be combined with each other to form a new technical solution.

[0047] Unless otherwise specified, all steps of the present application may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or may include steps (b) and (a) performed sequentially. For example, the method may further include step (c), indicating that step (c) may be added to the method in any order, for example, the method may include steps (a), (b) and (c), or may include steps (a), (c) and (b), or may include steps (c), (a) and (b), etc.

[0048] If there is no special explanation, the "include" and "comprising" mentioned in this application are open-ended. For example, the "include" and "comprising" may mean that other components not listed may also be included or comprised.

[0049] Unless otherwise specified, the term "or" is inclusive in this application. For example, any of the following conditions satisfies the condition "A or B": A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or both A and B are true (or exist).

[0050] [Secondary battery]

[0051] Secondary batteries, also known as rechargeable batteries or storage batteries, are batteries that can be recharged to activate the active materials after discharge and continue to be used.

[0052] Typically, a secondary battery includes a positive electrode sheet, a negative electrode sheet, a separator, and an electrolyte. During the battery's charge and discharge process, active ions (such as lithium ions or sodium ions) are embedded and released back and forth between the positive and negative electrode sheets. The separator is set between the positive and negative electrode sheets, mainly to prevent the positive and negative electrodes from short-circuiting, while allowing active ions to pass through. The electrolyte is between the positive and negative electrode sheets, mainly to conduct active ions.

[0053] As mentioned above, the current preparation of composite positive electrode materials requires the separate preparation of graphene, but the process is complex and the cost is high. To solve this problem, in the first embodiment of the present application, a method for preparing a composite positive electrode material is provided, which comprises:

[0054] A lithium source, a phosphorus source, an iron source, a carbon source, and a carbon graphitization catalyst are mixed with a solvent in a predetermined ratio to form a mixed slurry; the mixed slurry is ground and dried to obtain a mixed dry matter; the mixed dry matter is sintered to obtain a composite positive electrode material, and the sintering temperature is 750°C-840°C.

[0055] The carbon graphitization catalyst is a catalyst that catalyzes the conversion of carbon to graphene. The composite cathode material obtained by the above preparation method includes a polyanionic cathode material and a carbon material, wherein the carbon material is coated on at least a portion of the surface of the polyanionic cathode material and / or dispersed between particles of the polyanionic cathode material, and the carbon material includes graphene.

[0056] The preparation method of the composite positive electrode material of the first embodiment of the present application adopts a one-step method to prepare a polyanionic positive electrode material while forming a carbon material containing graphene. The carbon material is coated on at least a portion of the surface of the polyanionic positive electrode material to form a coating layer, and / or is dispersed between the particles of the polyanionic positive electrode material. It can be seen that the graphene is connected to the coating layer or is freely distributed between the particles of the polyanionic positive electrode material, thereby simplifying the preparation process of the composite positive electrode material. The present application further controls the sintering temperature within the range of 750℃-840℃, so that large-size particles and small-size particles are formed in the formed composite positive electrode material, and the grading of large and small particles is achieved, so that the obtained composite positive electrode material has a higher powder compaction density, thereby improving the energy density of the battery cell. For example, the powder compaction density of the composite positive electrode material at 226.0738Mpa is ≥2.42g / cm 3 .

[0057] As the sintering temperature increases, the particles of the composite positive electrode material grow, while the amount of graphene generated decreases. The preparation method of the present application controls the sintering temperature within the above range, thereby achieving the purpose of simultaneously improving the powder compaction density and the amount of graphene generated as much as possible, thereby improving the overall performance of the battery.

[0058] The carbon graphitization catalyst used in the present application can be selected from conventional carbon graphitization catalysts. In some embodiments, the carbon graphitization catalyst includes any one or more of metals or metal oxides, the metal includes Fe and / or Ni, and the metal oxide includes any one or more of FeO, Fe3O4, Fe2O3, CuO, NiO, MnO, Mn3O4, V2O5, V2O3 or VO4. As catalysts, the above-mentioned metals and metal oxides are relatively stable during the sintering process and are not easily burned away. If metal residues appear during the metal-catalyzed carbon graphitization process, the battery self-discharge K value will be affected; preferably, the residues of the above-mentioned metal oxides have no effect on the battery self-discharge K value.

[0059] In some embodiments, the carbon source dosage can be set based on the carbonization rate of different carbon sources, with the carbon formed after carbonization accounting for 0.6%-2.5% of the weight content of the composite positive electrode material as a reference. The carbonization rates of the above-mentioned different carbon sources can be obtained through experience in this field and will not be repeated in this application.

[0060] In some embodiments, the carbon source comprises one or more of glucose, sucrose, fructose, citric acid, starch, polyvinyl alcohol, polyethylene glycol, or polyaniline, wherein glucose, sucrose, fructose, and citric acid are small molecule carbon sources, and starch, polyvinyl alcohol, polyethylene glycol, and polyaniline are high molecule carbon sources.

[0061] Since the type of raw materials used in the one-step method of the present application is large, the proportion of carbon source in the whole slurry is relatively small, and the proportion of carbon graphitization catalyst as a catalyst in the mixed slurry is even less. In order to further improve the catalytic effect of carbon graphitization catalyst on carbon source, in some embodiments, the average particle size of carbon graphitization catalyst is 100nm-400nm, which can be optionally 100nm-330nm, such as 100nm-200nm, 200nm-330nm, 300nm-400nm. When the nano-scale carbon graphitization catalyst within the above particle size range is mixed with other materials, a higher contact area with the carbon source can be obtained as much as possible, thereby improving the catalytic efficiency. The above average particle size is calculated based on the longest diameter of the primary particles.

[0062] In some embodiments, in order to maximize the conversion rate of graphene and better improve the conductivity of the polyanionic cathode material, in some embodiments, the mass ratio of the carbon graphitization catalyst to the carbon source is 1:100-1:30, such as 1:100, 5:312, 10:312, and can be optionally 1:63-1:30.

[0063] In some embodiments, the mixed slurry further comprises a lithium doping element raw material, a phosphorus doping element raw material, an iron doping element raw material, and an oxygen doping element raw material. The amounts or ratios of the lithium source, phosphorus source, iron source, lithium doping element raw material, iron doping element raw material, phosphorus doping element raw material, phosphorus doping element raw material, and oxygen doping element raw material used in the present application can be based on the amounts or ratios used in the synthesis of conventional phosphate-based positive electrode materials, or based on the composition of the phosphate-based positive electrode material to be prepared. Generally, in order to improve performance such as cycle performance, specific capacity, or rate, a relatively large amount of iron source and iron doping element raw material is selected. In some embodiments, in the mixed slurry, the total molar number of lithium in the lithium source and the doping element in the lithium-site doping element raw material is M1, the total molar number of iron in the iron source and the doping element in the iron-site doping element raw material is M2, the total molar number of phosphorus in the phosphorus source and the doping element in the phosphorus-site doping element raw material is M3, M1:M2:M3 is (1.0-1.1): (0.95-1.0) (1.0-1.1), and M2:M3 is less than 1:1. When a metal oxide is used as a carbon graphitization catalyst, the metal oxide can not only be used as a catalyst, but the metal elements therein can be used as additional doping elements to enter the skeleton structure of the phosphate-based positive electrode material during sintering. Therefore, the amount of iron in the iron source and the doping element in the iron-site doping element raw material can be appropriately reduced. In the above molar ratio, M2 is less than M3, and the smaller M2 is, the larger the particle size of the composite positive electrode material obtained under the same calcination conditions and the greater the compaction density.

[0064] Of course, if the mixed slurry does not include the corresponding lithium doping element raw materials, phosphorus doping element raw materials, iron doping element raw materials, and oxygen doping element raw materials, the above M1, M2, and M3 also do not include the molar number of the corresponding doping elements.

[0065] However, if the ratio of M2:M3 is too small, the specific capacity of the composite cathode material will be reduced. In some embodiments, M2:M3 is greater than or equal to 0.96:1 and less than 1:1, thereby improving both the compaction density and the specific capacity as much as possible.

[0066] When a metal oxide is selected as a carbon graphitization catalyst, although it can be used as a doping element, its main function is still to catalyze the conversion of carbon to graphene. In some embodiments, the ratio of the mass of the carbon graphitization catalyst to the total mass of the iron source and the iron-doping element raw material is (1:1000)-(1:100), preferably (1:300)-(1:100), so that the effects of the catalyst, iron source and iron-doping element are fully exerted, and excessive residue of the carbon graphitization catalyst will not be caused. In addition, the amount of graphitization catalyst used within the above-mentioned ratio range also makes the graphene content produced at an appropriate level, avoiding the negative effects easily caused by too high a graphene content, such as excessive viscosity of the positive electrode slurry and difficulty in construction.

[0067] The phosphorus source, iron source, optional lithium-doping element raw material, optional iron-doping element raw material, phosphorus-doping element raw material, optional phosphorus-doping element raw material, phosphorus-doping element raw material, and optional oxygen-doping element raw material used in this application can be selected from salt substances such as sulfates, nitrates, carbonates, hydrochlorides, phosphates, and oxalates of each element.

[0068] In some embodiments, the polyanionic cathode material comprises a lithium-containing phosphate, wherein the lithium-containing phosphate comprises at least one of lithium iron phosphate and a doped and / or coated modified compound thereof. In some embodiments, the prepared lithium-containing phosphate comprises a molecular formula of Li m A a Fe x D d P y E e O z G gA comprises at least one element selected from the group consisting of Al, Na, K, and Mg; D comprises at least one element selected from the group consisting of Cu, Mn, Cr, Zn, Pb, Ca, Co, Ni, Sr, Nb, Ti, and V; E comprises at least one element selected from the group consisting of B, S, Si, and N; and G comprises at least one element selected from the group consisting of S, F, Cl, and Br. m is selected from the range of 0.5 to 1.15 (e.g., 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, or 1.1); a is selected from the range of 0 to 0.1 (e.g., 0, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09 or 0.1); x is selected from the range of 0.5 to 1 (such as 0.5, 0.6, 0.7, 0.8, 0.9 or 1); d is selected from the range of 0 to 0.5 (such as 0, 0.1, 0.2, 0.3, 0.4 or 0.5); y is selected from the range of 0.5 to 1 (such as 0.5, 0.6, 0.7, 0.8, 0.9 or 1); e is selected from the range of 0 to 0.5 (such as 0, 0.1, 0.2, 0.3, 0.4 or 0.5); z is selected from the range of 3.5 to 4 (such as 3.5, 3.6, 3.7, 3.8, 3.9 or 4.0); g is selected from the range of 0 to 0.5 (such as 0, 0.1, 0.2, 0.3, 0.4 or 0.5). For example, the element A is a lithium doping element, the element D is an iron doping element, the element E is a phosphorus doping element, and the element G is an oxygen doping element. The carbon material described in the above embodiments is coated on the surface of the lithium-containing phosphate and / or dispersed between the particles of the lithium-containing phosphate.

[0069] In some embodiments, the iron source includes at least one of ferric phosphate and ferrous oxalate; the lithium source includes at least one of lithium dihydrogen phosphate, lithium monohydrogen phosphate, lithium nitrate, lithium carbonate, lithium phosphate, lithium hydroxide or lithium acetate; and the phosphorus source includes at least one of ammonium dihydrogen phosphate, ammonium phosphate, lithium phosphate, phosphoric acid and ferric phosphate.

[0070] The amount of solvent in the mixed slurry affects the grinding and mixing effect and the drying efficiency. When the solvent is more, it is beneficial to improve the uniformity of the material; when the solvent is less, it is beneficial to improve the drying efficiency. The above-mentioned solvent can be selected from commonly used volatile solvents, such as water, ethanol, etc. In some embodiments, the solvent is water. In some embodiments, the mass ratio of the total mass of the lithium source, phosphorus source, iron source, optional lithium-doping element raw material, optional iron-doping element raw material, phosphorus-doping element raw material, optional phosphorus-doping element raw material, phosphorus-doping element raw material, and optional oxygen-doping element raw material to the solvent is (1-3): (1-7).

[0071] On the one hand, the above-mentioned grinding can refine the insoluble particles in the mixed slurry, and on the other hand, promote the mixing of insolubles. In some embodiments, the above-mentioned grinding includes ball milling and sand milling performed sequentially; the Dv50 of the insoluble particles in the mixed slurry after ball milling is 2.0μm-5.0μm, and can be optionally 2.0μm-4.0μm; the Dv50 of the insoluble particles in the mixed slurry after sand milling is 0.3μm-1.2μm, and can be optionally 0.3μm-0.4μm. The particle size of the insoluble particles is adjusted by controlling the time and speed of ball milling and sand milling. In some embodiments, the ball milling temperature is adjusted to 25℃-35℃, the ball milling speed is 800rpm-2000rpm, and the ball milling time is 0.5h-4h to obtain insoluble particles in the above-mentioned Dv50 range. In some embodiments, the sand milling temperature is adjusted to 25° C.-35° C., the sand milling speed is adjusted to 5000 rpm-10000 rpm, and the sand milling time is adjusted to 0.5 h-4 h to obtain insoluble particles within the above Dv50 range.

[0072] The Dv50 was measured using a laser particle size analyzer (Malvern 3000).

[0073] In some embodiments, the drying process in the above preparation method can employ commonly used drying methods in the art, such as spray drying and heat drying. In some embodiments, spray drying is selected. The inlet and outlet air temperatures of spray drying affect the dry state of the mixed dry matter, such as particle size and moisture content. In some embodiments, the inlet air temperature during spray drying is controlled to be 200°C to 300°C, and the outlet air temperature is controlled to be 90°C to 120°C. Under these conditions, spray drying can produce mixed dry matter particles with good uniformity and a moisture content of less than 2%.

[0074] The process of sintering the mixed dry matter can be based on a conventional sintering process. In some embodiments, the sintering process includes heating the mixed dry matter to 780°C-820°C for 2-9 hours and maintaining the temperature at 780°C-820°C for 6-20 hours, for example, heating to 780°C, 790°C, 800°C, 810°C, or 820°C. By controlling the sintering conditions, a balance is achieved between the powder compaction density and graphene content of the resulting composite positive electrode material, thereby optimizing the discharge capacity and energy density of the battery.

[0075] In some embodiments, sintering is performed in a reducing atmosphere, which includes any one or more of hydrogen, acetone, propylene, carbon monoxide, methanol, acetylene, methane, ethylene, and ethane. In some embodiments, the reducing atmosphere further includes a protective gas, which is nitrogen and / or an inert gas.

[0076] The addition of a reducing atmosphere not only helps to increase the carbonization rate of the carbon source, but also has a significant improvement effect on the catalytic effect of the carbon graphitization catalyst in the form of metal oxides, further optimizing the carbon graphitization effect and allowing the composite positive electrode material to contain more graphene.

[0077] In some embodiments, the reducing gas content in the reducing atmosphere is 1% to 8% by volume. It should be noted that this volume content is calculated based on the volumes of the reducing gas and protective gas introduced. A reducing gas content within this range not only improves the carbonization rate but also facilitates the conversion of metal elements from the various metal-containing raw materials used to prepare the positive electrode material into the positive electrode material.

[0078] The composite positive electrode material obtained after sintering is agglomerated. In order to facilitate the subsequent preparation of positive electrode sheets, in some embodiments, after sintering, the preparation method also includes a process of crushing the composite positive electrode material. In order to further improve the compaction density of the composite positive electrode material, in some embodiments, the Dv50 of the crushed composite positive electrode material is adjusted to 0.6μm-2.0μm.

[0079] The second embodiment of the present application provides a composite positive electrode material, which includes a polyanionic positive electrode material and a carbon material, wherein the carbon material is coated on at least a portion of the surface of the polyanionic positive electrode material and / or dispersed between the polyanionic positive electrode material particles, and the carbon material includes graphene. The powder compaction density of the composite positive electrode material at 226.0738 MPa is ≥2.42 g / cm 3 , optionally ≥2.53g / cm 3 .

[0080] The composite cathode material can be prepared using the aforementioned preparation method. The presence of graphene in the composite cathode material improves its conductivity. Furthermore, the composite cathode material has a high compactness, thereby enabling the battery to have a higher energy density.

[0081] The content of graphene not only affects the conductivity of the composite positive electrode material, but also affects the compaction density of the composite positive electrode material and the turbidity of the positive electrode slurry. When the graphene content increases, on the one hand, due to the increase in graphene conductivity, the discharge capacity of the battery cell can be improved. On the other hand, due to the layered characteristics of graphene, the surface lubricity of the composite positive electrode material will be further improved, which is conducive to close contact between particles, thereby increasing the compaction density. Since the adhesion of graphene between particles increases with the increase of graphene content, the turbidity of the positive electrode slurry containing the composite positive electrode material increases. If the graphene content in the composite positive electrode material is too high, the turbidity of the composite positive electrode material will be too high, which will make it difficult to coat the positive electrode slurry, affecting the construction efficiency and the morphology of the positive electrode film layer. Moreover, if the graphene content in the composite positive electrode material is too high, the effective proportion of active materials will be reduced, which will lead to reduced electrode compaction and reduced discharge capacity. In some embodiments, the graphene content is measured by the turbidity of a slurry formed by mixing the composite positive electrode material and water at a mass ratio of 1:40. In order to improve the three properties of conductivity, compaction density, and coating properties of the positive electrode slurry as much as possible, the room temperature turbidity of the slurry formed by mixing the composite positive electrode material and water at a mass ratio of 1:40 is 200FTU-400FTU, and optionally 240FTU-280FTU. The room temperature in this application is 15°C-35°C as commonly understood in the art.

[0082] The turbidity was measured using a HACH 2100Q turbidity meter according to the instructions.

[0083] In some embodiments, the mass content of the magnetic substance in the composite positive electrode material is ≤1 ppm, thereby controlling the self-discharge of the composite positive electrode material and extending the life of the battery having the composite positive electrode material.

[0084] [Positive electrode]

[0085] A positive electrode sheet includes a positive electrode film layer, the positive electrode film layer includes a positive electrode active material, and the positive electrode active material includes any one of the composite positive electrode materials provided in the second embodiment.

[0086] Since the composite positive electrode material of the present application contains graphene, the conductivity and compaction density of the composite positive electrode material are improved, so the compaction density and coulombic efficiency of the positive electrode sheet having the composite positive electrode material are also better.

[0087] As an example, the positive electrode current collector has two surfaces opposite to each other in its thickness direction, and the positive electrode film layer is disposed on either or both of the two opposite surfaces of the positive electrode current collector.

[0088] In some embodiments of the present application, based on the total cross-sectional area of ​​the positive electrode active material, the cross-sectional area of ​​the composite positive electrode material with a primary particle size of 50nm-150nm accounts for 10% to 35%; the cross-sectional area of ​​the composite positive electrode material with a primary particle size of 150nm-1500nm accounts for 30% to 60%; the cross-sectional area of ​​the composite positive electrode material with a primary particle size of not less than 1500nm accounts for 10-35%. The composite positive electrode material in the positive electrode sheet contains small-sized particles, medium-sized particles and large-sized particles, and the particle sizes between the particles are graded, which can further improve the compaction density of the positive electrode sheet. The above-mentioned composite positive electrode materials of different particle sizes can be adjusted by using the different particle sizes of the raw material particles during preparation, or composite positive electrode materials with different particle size ranges can be mixed to obtain them.

[0089] The above cross-sectional area percentages were determined using the following method: the positive electrode sheet was cut perpendicularly to the positive electrode sheet using an argon ion beam, the cross-section was exposed, and the cross-section was photographed using a scanning electron microscope. The longest diameter of the lithium iron phosphate particles was statistically analyzed using the length-diameter method. In the cross-sectional image, the primary particle size refers to the longest distance between two points along the edge.

[0090] Since particles with a primary particle size of ≤50nm are prone to adhesion, there are large errors in statistics and it is difficult to clearly identify them individually. Therefore, during the particle size statistics process, particles with a primary particle size of ≤50nm are not included in the statistical range.

[0091] The quantity content of the composite positive electrode material with a primary particle size of 50 nm-150 nm is calculated by dividing the quantity of the composite positive electrode material with a primary particle size of 50 nm-150 nm in the cross section by the total quantity of the positive electrode material.

[0092] The quantity content of the composite positive electrode material with a primary particle size of 150 nm to 1500 nm is calculated by dividing the quantity of the composite positive electrode material with a primary particle size of 150 nm to 1500 nm in the cross section by the total quantity of the positive electrode material.

[0093] The cross-sectional area percentage of the composite positive electrode material having a primary particle size of not less than 1500 nm is calculated by dividing the cross-sectional area of ​​the composite positive electrode material having a primary particle size of not less than 1500 nm in the cross section by the area of ​​the total positive electrode material.

[0094] The area of ​​the active material was obtained by statistical analysis of the scanning electron microscopy images of the positive electrode cross-section obtained using Avizo 3D software.

[0095] In some embodiments, the positive electrode current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil. The composite current collector may include a polymer material base and a metal layer formed on at least one surface of the polymer material base. The composite current collector may be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0096] In some embodiments, the positive electrode film layer may further optionally include a binder. As an example, the binder may include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), a vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, a vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, a tetrafluoroethylene-hexafluoropropylene copolymer, and a fluorine-containing acrylate resin.

[0097] In some embodiments, the positive electrode film layer may further include a conductive agent. For example, the conductive agent may include at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0098] In some embodiments, the positive electrode sheet can be prepared by the following method: the components for preparing the positive electrode sheet, such as the positive electrode active material, the conductive agent, the binder and any other components, are dispersed in a solvent (such as N-methylpyrrolidone) to form a positive electrode slurry; the positive electrode slurry is coated on the positive electrode current collector, and after drying, cold pressing and other processes, the positive electrode sheet can be obtained.

[0099] [Negative electrode]

[0100] The negative electrode sheet includes a negative electrode current collector and a negative electrode film layer provided on at least one surface of the negative electrode current collector, wherein the negative electrode film layer includes a negative electrode active material.

[0101] As an example, the negative electrode current collector has two surfaces opposite to each other in its thickness direction, and the negative electrode film layer is disposed on either or both of the two opposite surfaces of the negative electrode current collector.

[0102] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, copper foil may be used as the metal foil. The composite current collector may include a polymer base layer and a metal layer formed on at least one surface of the polymer base material. The composite current collector may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy, etc.) on a polymer base material (such as a base material of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0103] In some embodiments, the negative electrode active material may be a negative electrode active material for a battery that is well known in the art. As an example, the negative electrode active material may include at least one of the following materials: artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, and lithium titanate. The silicon-based material may be selected from at least one of elemental silicon, silicon oxides, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. The tin-based material may be selected from at least one of elemental tin, tin oxides, and tin alloys. However, the present application is not limited to these materials, and other traditional materials that can be used as negative electrode active materials for batteries may also be used. These negative electrode active materials may be used alone or in combination of two or more.

[0104] In some embodiments, the negative electrode film layer may further include a binder. For example, the binder may be selected from at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

[0105] In some embodiments, the negative electrode film layer may further include a conductive agent. For example, the conductive agent may be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0106] In some embodiments, the negative electrode film layer may optionally include other additives, such as a thickener (eg, sodium carboxymethyl cellulose (CMC-Na)).

[0107] In some embodiments, the negative electrode sheet can be prepared by the following method: the components for preparing the negative electrode sheet, such as the negative electrode active material, the conductive agent, the binder and any other components, are dispersed in a solvent (such as deionized water) to form a negative electrode slurry; the negative electrode slurry is coated on the negative electrode current collector, and after drying, cold pressing and other processes, the negative electrode sheet can be obtained.

[0108] [Electrolytes]

[0109] The electrolyte conducts ions between the positive and negative electrodes. This application does not specify the type of electrolyte, and the electrolyte can be selected based on the needs. For example, the electrolyte can be liquid, gel, or solid.

[0110] In some embodiments, the electrolyte is liquid and includes an electrolyte salt and a solvent.

[0111] In some embodiments, the electrolyte salt may be selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalatoborate, lithium dioxalatoborate, lithium difluorodioxalatophosphate, and lithium tetrafluorooxalatophosphate.

[0112] In some embodiments, the solvent can be selected from at least one of ethylene carbonate, propylene carbonate, ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methylpropyl carbonate, ethylpropyl carbonate, butylene carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, cyclopentane, dimethyl sulfone, methyl ethyl sulfone and diethyl sulfone.

[0113] In some embodiments, the electrolyte may optionally include additives. For example, the additives may include negative electrode film-forming additives, positive electrode film-forming additives, and additives that can improve certain battery properties, such as additives that improve battery overcharge performance, and additives that improve battery high or low temperature performance.

[0114] [Isolation film]

[0115] In some embodiments, the secondary battery further includes a separator. The present application has no particular limitation on the type of separator, and any known porous separator with good chemical and mechanical stability can be selected.

[0116] In some embodiments, the material of the separator can be selected from at least one of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single-layer film or a multi-layer composite film, without particular limitation. When the separator is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

[0117] In some embodiments, the positive electrode sheet, the negative electrode sheet, and the separator can be formed into an electrode assembly through a winding process or a lamination process.

[0118] In some embodiments, the secondary battery includes a secondary battery cell, or includes a battery module and a battery pack.

[0119] In some embodiments, the secondary battery may include an outer packaging that can be used to encapsulate the electrode assembly and the electrolyte.

[0120] In some embodiments, the outer packaging of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. Alternatively, the outer packaging of the secondary battery can be a soft shell, such as a pouch-type soft shell. The soft shell can be made of plastic, such as polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0121] The present application has no particular limitation on the shape of the secondary battery cell, which can be cylindrical, square, or any other shape. For example, FIG3 shows a secondary battery cell 5 with a square structure as an example.

[0122] In some embodiments, referring to Figure 4, the outer packaging may include a shell 51 and a top cover assembly 53. Among them, the shell 51 may include a bottom plate and a side plate connected to the bottom plate, and the bottom plate and the side plate enclose a receiving cavity. The shell 51 has an opening connected to the receiving cavity, and the top cover assembly 53 can be covered on the opening to close the receiving cavity. The positive electrode sheet, the negative electrode sheet and the isolation membrane can form an electrode assembly 52 through a winding process or a lamination process. The electrode assembly 52 is encapsulated in the receiving cavity. The electrolyte is infiltrated in the electrode assembly 52. ​​The number of electrode assemblies 52 contained in the secondary battery cell 5 can be one or more, and those skilled in the art can select according to specific actual needs.

[0123] In some embodiments, secondary battery cells may be assembled into a battery module. The battery module may contain one or more secondary battery cells. The specific number may be selected by those skilled in the art based on the application and capacity of the battery module.

[0124] Figure 5 shows an example battery module 4. Referring to Figure 5 , in the battery module 4, multiple secondary battery cells 5 may be arranged sequentially along the length of the battery module 4. Of course, they may also be arranged in any other manner. Furthermore, the multiple secondary battery cells 5 may be secured together using fasteners.

[0125] Optionally, the battery module 4 may further include a housing having an accommodation space, and the plurality of secondary battery cells 5 are accommodated in the accommodation space.

[0126] In some embodiments, the battery modules described above may also be assembled into a battery pack. The battery pack may contain one or more battery modules, and the specific number may be selected by those skilled in the art based on the application and capacity of the battery pack.

[0127] Figures 6 and 7 illustrate an example battery pack 1. Referring to Figures 6 and 7 , the battery pack 1 may include a battery box and multiple battery modules 4 disposed within the battery box. The battery box comprises an upper case 2 and a lower case 3. The upper case 2 can be positioned over the lower case 3 to form an enclosed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the battery box.

[0128] In addition, the present application also provides an electrical device, which includes the secondary battery provided in the present application. The secondary battery can be used as a power source for the electrical device, or as an energy storage unit for the electrical device. The electrical device may include mobile devices (such as mobile phones, laptops, etc.), electric vehicles (such as pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc., but is not limited thereto.

[0129] As the electrical device, a secondary battery cell, a battery module or a battery pack can be selected according to its usage requirements.

[0130] Figure 8 shows an example of an electric device. This device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. To meet the high power and high energy density requirements of the secondary battery, a battery pack or battery module can be used.

[0131] [Example]

[0132] Below, the embodiment of the present application is described. The embodiment described below is exemplary and is only used to explain the present application, and is not to be construed as limiting the present application. Where specific techniques or conditions are not specified in the embodiments, the techniques or conditions described in the literature in this area or the product specifications are used. Reagents or instruments used that do not specify the manufacturer are conventional products that can be obtained commercially.

[0133] Preparation Example 1

[0134] A mixed carbon source formed by lithium carbonate, iron phosphate, ammonium dihydrogen phosphate, glucose and polyethylene glycol in a mass ratio of 90:35, a carbon graphitization catalyst Fe2O3 and water are mixed to form a mixed slurry, wherein the molar ratio of lithium element in lithium carbonate, iron element in iron phosphate, and phosphorus element in iron phosphate and ammonium dihydrogen phosphate is 1.01:1:1.03, the mass ratio of the total mass of lithium carbonate, iron phosphate and ammonium dihydrogen phosphate to water is 2:3, the mass of Fe2O3 is 0.5% of the mass of iron phosphate, the average particle size of Fe2O3 is in the range of 200nm-330nm, and the mass ratio of Fe2O3 to the mixed carbon source is 5:312.

[0135] The mixed slurry is sequentially ball milled, sand milled and spray dried to obtain a mixed dry matter, wherein the ball milling temperature is 25-35°C, the ball milling time is 2 hours, and the ball milling speed is 1000 rpm; the Dv50 of the particles of the insoluble matter in the mixed slurry after ball milling is 4 μm; the sand milling temperature is 25-35°C, the sand milling time is 2 hours, and the sand milling speed is 8000 rpm, and the Dv50 of the particles of the insoluble matter in the mixed slurry after sand milling is 0.4 μm; and the inlet air temperature during the spray drying process is 250°C and the outlet air temperature is 100°C.

[0136] The mixed dry matter was sintered and crushed to obtain a composite lithium iron phosphate cathode material. The sintering atmosphere included nitrogen and acetylene, and the volume ratio of acetylene to nitrogen was controlled to be 2:98 by adjusting the nitrogen and acetylene flow rates during the sintering process. The sintering process was heated to 780°C and then held at that temperature for a heating time t1 and a holding time t2, respectively. t1 was 6 hours, and t2 was 13 hours. The resulting composite lithium iron phosphate cathode material included lithium iron phosphate and a carbon coating coated on at least a portion of the surface of the lithium iron phosphate. A TEM image of the composite lithium iron phosphate cathode material (Telescope F200SG2, Thermo Scientific) in Figure 1 shows that the carbon coating is adhered to graphene. The Dv50 value of the crushed composite lithium iron phosphate cathode material was 1.2 μm.

[0137] Preparation Example 2

[0138] On the basis of Preparation Example 1, Fe2O3 with an average particle size in the range of 100nm-200nm was used to replace the Fe2O3 in Preparation Example 1, and the rest was the same as Preparation Example 1 to obtain the composite lithium iron phosphate positive electrode material of Preparation Example 2.

[0139] Preparation Example 3

[0140] On the basis of Preparation Example 1, Fe2O3 with an average particle size in the range of 300nm-400nm was used to replace the Fe2O3 in Preparation Example 1, and the rest was the same as Preparation Example 1 to obtain the composite lithium iron phosphate positive electrode material of Preparation Example 3.

[0141] Preparation Example 4

[0142] On the basis of Preparation Example 1, Fe2O3 with an average particle size of less than 100 nm was used to replace the Fe2O3 in Preparation Example 1, and the rest was the same as Preparation Example 1 to obtain the composite lithium iron phosphate positive electrode material of Preparation Example 4.

[0143] Preparation Example 5

[0144] On the basis of Preparation Example 1, Fe2O3 with an average particle size in the range of 420nm-550nm was used to replace the Fe2O3 in Preparation Example 1, and the rest was the same as Preparation Example 1 to obtain the composite lithium iron phosphate positive electrode material of Preparation Example 5.

[0145] Preparation Example 6

[0146] On the basis of Preparation Example 1, the amount of Fe2O3 was adjusted so that the mass ratio of Fe2O3 to the mixed carbon source was 1:100, and the mass ratio of Fe2O3 to iron phosphate was 0.312:100; the rest was the same as Preparation Example 1, and the composite lithium iron phosphate positive electrode material of Preparation Example 6 was obtained.

[0147] Preparation Example 7

[0148] Based on Preparation Example 1, the amount of Fe2O3 was adjusted so that the mass ratio of Fe2O3 to the mixed carbon source was 10:312, and the mass ratio of Fe2O3 to iron phosphate was 1:100; the rest was the same as Preparation Example 1, and the composite lithium iron phosphate positive electrode material of Preparation Example 7 was obtained.

[0149] Preparation Example 8

[0150] On the basis of Preparation Example 1, the amount of Fe2O3 was adjusted so that the mass ratio of Fe2O3 to the mixed carbon source was 1:312, and the mass ratio of Fe2O3 to iron phosphate was 1:1000; the rest was the same as Preparation Example 1, and the composite lithium iron phosphate positive electrode material of Preparation Example 8 was obtained.

[0151] Preparation Example 9

[0152] On the basis of Preparation Example 1, the amount of Fe2O3 was adjusted so that the mass ratio of Fe2O3 to the mixed carbon source was 0.5:312, and the mass ratio of Fe2O3 to iron phosphate was 1:2000; the rest was the same as Preparation Example 1, and the composite lithium iron phosphate positive electrode material of Preparation Example 9 was obtained.

[0153] Preparation Example 10

[0154] Based on Preparation Example 1, the amount of Fe2O3 was adjusted so that the mass ratio of Fe2O3 to the mixed carbon source was 20:312, and the mass ratio of Fe2O3 to iron phosphate was 1:50; the rest was the same as Preparation Example 1, and the composite lithium iron phosphate positive electrode material of Preparation Example 10 was obtained.

[0155] Preparation Example 11

[0156] On the basis of Preparation Example 1, the ball milling time and ball milling speed of the mixed slurry, and the time and speed of sand milling were adjusted so that the Dv50 of the insoluble particles in the mixed slurry after ball milling was 5 μm, and the Dv50 of the insoluble particles in the mixed slurry after sand milling was 1.2 μm. The rest was the same as in Preparation Example 1, and the composite lithium iron phosphate positive electrode material of Preparation Example 11 was obtained.

[0157] Preparation Example 12

[0158] On the basis of Preparation Example 1, the ball milling time and ball milling speed of the mixed slurry, and the time and speed of sand milling were adjusted so that the Dv50 of the insoluble particles in the mixed slurry after ball milling was 2 μm, and the Dv50 of the insoluble particles in the mixed slurry after sand milling was 0.3 μm. The rest was the same as in Preparation Example 1, and the composite lithium iron phosphate positive electrode material of Preparation Example 12 was obtained.

[0159] Preparation Example 13

[0160] On the basis of Preparation Example 1, carbon monoxide was used instead of acetylene during sintering, and the rest was the same as Preparation Example 1, to obtain the composite lithium iron phosphate positive electrode material of Preparation Example 13.

[0161] Preparation Example 14

[0162] On the basis of Preparation Example 1, the amount of acetylene and nitrogen introduced during sintering was adjusted so that the volume ratio of acetylene to nitrogen was 8:92, and the rest was the same as Preparation Example 1 to obtain the composite lithium iron phosphate positive electrode material of Preparation Example 14.

[0163] Preparation Example 15

[0164] On the basis of Preparation Example 1, acetylene was not used during sintering, and the rest was the same as Preparation Example 1, to obtain the composite lithium iron phosphate positive electrode material of Preparation Example 15.

[0165] Preparation Example 16

[0166] On the basis of Preparation Example 1, Fe3O4 was used instead of Fe2O3 as the carbon graphitization catalyst, and the rest was the same as Preparation Example 1 to obtain the composite lithium iron phosphate positive electrode material of Preparation Example 16.

[0167] Preparation Example 17

[0168] On the basis of Preparation Example 1, FeO was used instead of Fe2O3 as the carbon graphitization catalyst, and the rest was the same as Preparation Example 1 to obtain the composite lithium iron phosphate positive electrode material of Preparation Example 17.

[0169] Preparation Example 18

[0170] On the basis of Preparation Example 1, Fe powder was used instead of Fe2O3 as the carbon graphitization catalyst, and the rest was the same as Preparation Example 1 to obtain the composite lithium iron phosphate positive electrode material of Preparation Example 18.

[0171] Preparation Example 19

[0172] On the basis of Preparation Example 1, Ni powder was used to replace Fe2O3 as the carbon graphitization catalyst, and the rest was the same as Preparation Example 1 to obtain the composite lithium iron phosphate positive electrode material of Preparation Example 19.

[0173] Preparation Example 20

[0174] On the basis of Preparation Example 1, NiO was used instead of Fe2O3 as the carbon graphitization catalyst, and the rest was the same as Preparation Example 1 to obtain the composite lithium iron phosphate positive electrode material of Preparation Example 20.

[0175] Preparation Example 21

[0176] On the basis of Preparation Example 1, Mn3O4 was used instead of Fe2O3 as the carbon graphitization catalyst, and the rest was the same as Preparation Example 1 to obtain the composite lithium iron phosphate positive electrode material of Preparation Example 21.

[0177] Preparation Example 22

[0178] On the basis of Preparation Example 1, V2O5 was used instead of Fe2O3 as the carbon graphitization catalyst, and the rest was the same as Preparation Example 1 to obtain the composite lithium iron phosphate positive electrode material of Preparation Example 22.

[0179] Preparation Example 23

[0180] On the basis of Preparation Example 1, ferrocene was used instead of Fe2O3 as the carbon graphitization catalyst, and the rest was the same as Preparation Example 1 to obtain the composite lithium iron phosphate positive electrode material of Preparation Example 23.

[0181] Preparation Example 24

[0182] On the basis of Preparation Example 1, the amount of ammonium dihydrogen phosphate was reduced so that the molar ratio of lithium in lithium carbonate, iron in iron phosphate, and phosphorus in iron phosphate and ammonium dihydrogen phosphate was 1.01:1:1. The rest was the same as in Preparation Example 1 to obtain the composite lithium iron phosphate positive electrode material of Preparation Example 24.

[0183] Preparation Example 25

[0184] The sintering temperature of Preparation Example 1 was adjusted to 750° C., the heating time t1 was 2 h, the holding time t2 was 20 h, and the rest was the same as Preparation Example 1.

[0185] Preparation Example 26

[0186] The sintering temperature of Preparation Example 1 was adjusted to 840° C., the heating time t1 was 9 h, the holding time t2 was 6 h, and the rest was the same as Preparation Example 1.

[0187] Preparation Example 27

[0188] The sintering temperature of Preparation Example 1 was adjusted to 750° C., the heating time t1 was 6 h, the holding time t2 was 13 h, and the rest was the same as Preparation Example 1.

[0189] Preparation Example 28

[0190] The sintering temperature of Preparation Example 1 was adjusted to 820° C., the heating time t1 was 6 h, the holding time t2 was 13 h, and the rest was the same as Preparation Example 1.

[0191] Comparative Preparation Example 1

[0192] The difference from Preparation Example 1 is that Fe2O3 is not used as a catalyst. The TEM image of the obtained composite lithium iron phosphate positive electrode material is shown in Figure 2, in which no graphene is observed.

[0193] Comparative Preparation Example 2

[0194] The sintering temperature of Preparation Example 1 was adjusted to 720° C., the heating time t1 was 6 h, the holding time t2 was 13 h, and the rest was the same as Preparation Example 1.

[0195] Comparative Preparation Example 3

[0196] The sintering temperature of Preparation Example 1 was adjusted to 880° C., the heating time t1 was 6 h, the holding time t2 was 13 h, and the rest was the same as Preparation Example 1.

[0197] Turbidity was tested using a HACH 2100Q turbidity meter. The test method is as follows:

[0198] Sample preparation: At room temperature, peel a 50mL centrifuge tube and weigh 1±0.01g of sample; weigh four replicates for each sample group; peel an electronic balance and weigh 40g±0.1g of ultrapure water and powder; place the centrifuge tube on a longitudinal mixer and stir at 80r / min for 2h; centrifuge: place the sample evenly in the centrifuge; set the centrifuge speed to "1500r / min" for 10min.

[0199] Calibration: Press the "Standard Curve" button in the lower left corner. Use a lens cleaning cloth to clean the surface of the standard solution bottle. After removing any water stains and fingerprints, add 0 FTU (pure water), 20 FTU, 100 FTU, and 800 FTU standard solutions that have been shaken evenly to remove bubbles. Press the "Read" button to perform alignment calibration. After calibration, press the "Done" button to save the results.

[0200] Test: At room temperature, steadily remove the centrifuge tube and rubber sleeve from the centrifuge and place them on the table. Hold a 3mL rubber-tipped dropper at the 1.5mL mark and insert it vertically into the center of the centrifuge tube to draw 2mL of solution at a time. Place the sample bottle on an electronic balance and remove the tares. Weigh 0.5±0.005g of solution and add 10±0.1g of ultrapure water. Use a lens cleaning cloth to wipe off water stains and fingerprints on the surface of the sample bottle, then place it in the measuring port and close the lid. Click to confirm the test and record the data.

[0201] Method for determining the mass content of magnetic materials in composite lithium iron phosphate positive electrode materials:

[0202] 1) Weigh 1 kg of sample and place it in a plastic bucket. Add 6 L of deionized water. Use plastic tubing to cover a φ24 mm x 240 mm magnetic bar (magnetic induction strength of 6000 GS). Then, heat-seal the tub with heat-sealing clips. Place the magnetic bar in the tub and seal it. Set the roller mixing drum to 60 rpm and the stirring time to 15 minutes. Place the sealed tub on the equipment and stir.

[0203] 2) Prepare another clean bucket and add 5 ± 0.2 L of deionized water to the bucket. Flush the magnetic material on the plastic tube into the solvent, reseal the magnetic rod, and repeat the above steps twice to ensure the accuracy of the magnetic material extraction;

[0204] 3) Prepare a clean 1L beaker and rinse all the magnetic material on the plastic tube into the beaker. Use a magnetic block to hold the bottom of the beaker and rinse 2-3 times to remove the non-magnetic material.

[0205] 4) Use a graduated cylinder to measure 10 mL of deionized water and add it to a 100 mL beaker. Then, measure 10 mL of 36%-38% hydrochloric acid and slowly add it to the beaker. Pour the prepared hydrochloric acid solution into the beaker, seal it, and sonicate it in an ultrasonicator for 2 minutes.

[0206] 5) After the ultrasonic treatment is complete, remove the beaker and use a magnetic block on the bottom of the beaker to absorb and gather the magnetic material. Pour the acid solution in the beaker into a waste liquid bucket. Rinse the magnetic material in the beaker three times and add an appropriate amount of deionized water for filtration.

[0207] 6) Use a filter with a pore size of 0.45 μm. After filtration, place the filter paper with magnetic particles on the surface onto a cleanliness microscope slide and dry it in an oven at 60°C for (10 ± 2) minutes. After drying, weigh the filter and calculate the magnetic content. A higher magnetic content in the positive electrode material indicates more severe self-discharge and a shorter battery life.

[0208] Powder compaction density measurement: A certain amount of powder is placed in a compaction die of known diameter (for example, the Sansi Zongheng UTM7305). Powder is placed between two metal sheets, one above and one below the die. A pressure of 226.0738 MPa (3 T) is applied while measuring the corresponding powder thickness. The compaction density is calculated using the formula ρ = m / v. Specific procedures can be performed in accordance with the standard: GB / T24533-2009.

[0209] Electrode analysis: The positive electrode sheet was cut perpendicularly to the sheet using an argon ion beam, exposing the cross-section. The cross-section was photographed using a scanning electron microscope, and the longest diameter of the lithium iron phosphate particles was statistically analyzed using the length-diameter statistical method. The area of ​​the active material was determined by statistically analyzing the obtained scanning electron micrographs of the positive electrode sheet cross-section using Avizo 3D software. The cross-sectional area percentage (A) of the composite positive electrode material with a primary particle size of 50nm-150nm was recorded; the cross-sectional area percentage (B) of the composite positive electrode material with a primary particle size of 150nm-1500nm was recorded; and the cross-sectional area percentage (C) of the composite positive electrode material with a primary particle size of not less than 1500nm was recorded. The primary particle size in the cross-sectional image refers to the longest distance between two points along the edge.

[0210] Gram Capacity Test:

[0211] The battery preparation and testing process is as follows: 2.0000g of the positive electrode active material, conductive carbon black, and PVDF from the corresponding preparation example were mixed in a mass ratio of 0.9:0.05:0.05, and then the organic solvent NMP (N-methylpyrrolidone) was added. After thorough mixing, the mixture was coated onto aluminum foil to form a 140-micron-thick film, dried under vacuum at 120°C for 2 hours, punched into 13-mm-diameter discs using a punch, and pressed using a tablet press at 10 MPa. The tablets were then kept at 120°C under vacuum for 12 hours, and the positive electrode sheet was weighed. Button cells were assembled in an argon-protected glove box, using a metallic lithium sheet as the negative electrode, an electrolyte consisting of a 1:1 volume ratio of EC (ethylene carbonate) and DMC (1,2-dimethyl carbonate), the electrolyte being LiPF6, and a Celgard 2400 microporous polyethylene membrane as the separator. The assembled batteries were tested for electrical performance on a blue-light tester. The specific capacity was measured over a voltage range of 2.0V to 3.75V using a constant current of 0.1C for two weeks, followed by a constant current of 1C for two weeks. The charging process was performed with a constant voltage of 3.75V and a cut-off current of 50μA.

[0212] Count the first 0.1C charging capacity in grams C1 and the first 1C discharging capacity in grams C2.

[0213] The test results are recorded in Table 1.

[0214] Table 1

[0215] According to the comparison between each preparation example and comparative preparation example 1, it can be seen that when a graphitization catalyst is used in the process of preparing the composite lithium iron phosphate positive electrode material, the turbidity of the slurry is significantly increased, indicating that graphene is generated therein, and the discharge gram capacity of the corresponding button battery is increased.

[0216] A comparison of Preparation Examples 1 to 5 shows that the particle size of the graphitization catalyst used affects the turbidity of the slurry, that is, the graphene content. When the particle size increases to 200 nm, the turbidity of the slurry increases with the increase in the particle size of the graphitization catalyst, indicating an increase in the graphene content in the resulting material and an increase in the catalytic efficiency of the graphitization catalyst. However, when the particle size of the graphitization catalyst continues to increase to 400 nm, the turbidity of the slurry does not increase, indicating that its catalytic efficiency has not increased significantly. As the particle size of the graphitization catalyst continues to increase, the turbidity of the slurry actually decreases, indicating that the catalytic effect of the graphitization catalyst is affected by the excessive particle size.

[0217] According to the comparison of Preparation Example 1 and Preparation Examples 6 to 10, it can be seen that as the amount of graphitization catalyst increases, the turbidity of the slurry increases, indicating that the graphene content in the obtained material increases; however, when the graphene content is too much, the actual proportion of lithium iron phosphate active material in the composite material decreases, resulting in electrode compaction and reduced capacity.

[0218] A comparison of Preparation Example 1 and Preparation Examples 16 to 23 shows that the catalytic efficiency of various graphitization catalysts varies. Furthermore, when metals are used as graphitization catalysts in Preparation Examples 18 and 19, significant metal residues remain in the positive electrode material, as evidenced by the magnetic material content shown in Table 1. The presence of magnetic material affects battery self-discharge, and thus battery life. Furthermore, when ferrocene is used as the graphitization catalyst in Preparation Example 23, the corresponding slurry has a lower turbidity and a lower graphene content, resulting in less graphene production, due to the ease of burning off during calcination.

[0219] According to the comparative examples of Preparation Examples 1, 27, and 28 and Comparative Preparation Examples 2 and 3, it can be seen that within the range of 750°C to 820°C, the proportion of medium-sized particles in the composite positive electrode material is relatively large, which makes the powder compaction of the material more suitable, and the turbidity of the slurry is relatively large, that is, the graphene content is relatively high, so the battery's discharge capacity is higher. The sintering temperature of Comparative Preparation Example 2 is below 750°C. The lower the sintering temperature, the greater the proportion of small-sized particles in the composite positive electrode material, the smaller the powder compaction, and thus the lower the energy density of the battery cell. The sintering temperature of the Comparative Preparation Example is above 840°C. The higher the sintering temperature, the lower the turbidity of the composite positive electrode material slurry, which leads to a smaller amount of graphene generated and a lower discharge capacity of the corresponding button battery.

[0220] Although the present application has been described with reference to preferred embodiments, various modifications may be made thereto and components may be substituted with equivalents without departing from the scope of the present application. In particular, the various technical features described in the various embodiments may be combined in any manner as long as there are no structural conflicts. The present application is not limited to the specific embodiments disclosed herein, but encompasses all technical solutions within the scope of the claims.

Claims

1. A method for preparing a composite cathode material, wherein, The preparation method includes: Mixing a lithium source, a phosphorus source, an iron source, a carbon source, and a carbon graphitization catalyst in a predetermined ratio with a solvent to form a mixed slurry; Grinding and drying the mixed slurry to obtain a mixed dry matter; Sintering the mixed dry matter to obtain a composite cathode material, and the sintering temperature is 750°C - 840°C.

2. The preparation method according to claim 1, wherein, The carbon graphitization catalyst includes any one or more of metals or metal oxides. The metals include Fe and / or Ni, and the metal oxides include any one or more of FeO, Fe3O4, Fe2O3, CuO, NiO, MnO, Mn3O4, V2O5, V2O3, or VO4.

3. The preparation method according to claim 1 or 2, wherein The average particle size of the carbon graphitization catalyst is 100nm - 400nm.

4. The preparation method according to any one of claims 1 to 3, wherein, The mass ratio of the carbon graphitization catalyst to the carbon source is 1:100 - 1:

30.

5. The preparation method according to any one of claims 1 to 4, wherein The mixed slurry further includes a lithium-site doping element raw material, a phosphorus-site doping element raw material, an iron-site doping element raw material, and an oxygen-site doping element raw material. In the mixed slurry, the total number of moles of lithium element in the lithium source and the doping element in the lithium-site doping element raw material is M1, the total number of moles of iron element in the iron source and the doping element in the iron-site doping element raw material is M2, and the total number of moles of phosphorus element in the phosphorus source and the doping element in the phosphorus-site doping element raw material is M3. M1:M2:M3 is (1.0 - 1.1):(0.95 - 1.0):(1.0 - 1.1), and M2:M3 is less than 1:

1. The mass ratio of the carbon graphitization catalyst to the total mass of the iron source and the iron-site doping element raw material is (1:1000) - (1:100).

6. The preparation method according to claim 5, wherein, The M2:M3 is greater than or equal to 0.96:1 and less than 1:

1.

7. The preparation method according to any one of claims 1 to 6, wherein, The polyanion-type cathode material includes a lithium-containing phosphate, and the lithium-containing phosphate includes at least one of lithium iron phosphate and its doped and / or coated modified compounds.

8. The preparation method according to any one of claims 1 to 7, wherein, The carbon source includes one or more of glucose, sucrose, fructose, citric acid, starch, polyvinyl alcohol, polyethylene glycol, or polyaniline.

9. The preparation method according to any one of claims 1 to 8, wherein The grinding includes ball milling and sand milling in sequence; the Dv50 of the insoluble particles in the mixed slurry after ball milling is 2.0μm - 5.0μm; the Dv50 of the insoluble particles in the mixed slurry after sand milling is 0.3μm - 1.2μm.

10. The preparation method according to any one of claims 1 to 9, wherein The process of sintering the mixed dry matter includes: Heating the mixed dry matter to 780°C - 820°C, and the heating time is 2h - 9h, and keeping it at 780°C - 820°C for 6h - 20h.

11. The preparation method according to any one of claims 1 to 10, wherein, The sintering is carried out in a reducing atmosphere, and the reducing atmosphere includes a reducing gas and a protective gas.

12. The preparation method according to claim 11, wherein, The reducing gas includes any one or more of hydrogen, acetone, propylene, carbon monoxide, methanol, acetylene, methane, ethylene, or ethane.

13. A composite cathode material, the composite cathode material comprising a polyanion-type cathode material and a carbon material, the carbon material being coated on at least a part of the surface of the polyanion-type cathode material and / or dispersed between the polyanion-type cathode material particles, the carbon material comprising graphene, and the powder compaction density of the composite cathode material being ≥ 2.42 g / cm at 226.0738 Mpa 3 .

14. The composite cathode material according to claim 13, wherein, The powder tap density of the composite cathode material under 226.0738 Mpa ≥ 2.53 g / cm 3 .

15. The composite cathode material according to claim 13 or 14, wherein The room temperature turbidity of the slurry formed by mixing the composite cathode material and water at a mass ratio of 1:40 is 200FTU - 400FTU, and / or the mass content of the magnetic substance in the composite cathode material ≤ 1ppm.

16. A positive electrode sheet, comprising a positive electrode film layer, and the positive electrode film layer comprises a positive electrode active material, wherein, The positive electrode active material includes the composite positive electrode material described in any one of claims 13 to 15.

17. The positive electrode sheet according to claim 16, wherein, Based on the total cross-sectional area of the positive electrode active material, the cross-sectional area ratio of the composite positive electrode material with a primary particle size of 50 nm - 150 nm is 10% - 35%; the cross-sectional area ratio of the composite positive electrode material with a primary particle size of 150 nm - 1500 nm is 30% - 60%; the cross-sectional area ratio of the composite positive electrode material with a primary particle size of not less than 1500 nm is 10 - 35%.

18. A secondary battery, comprising a positive electrode tab, wherein, The positive electrode sheet includes the positive electrode sheet described in claim 16 or 17.

19. An electric device includes a secondary battery, wherein, The secondary battery includes the secondary battery described in claim 18.

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