Phosphate positive electrode material and preparation method and application thereof

Abstract

The invention relates to the technical field of lithium ion battery positive electrode materials, in particular to a phosphate positive electrode material and a preparation method and application thereof. The invention provides a preparation method of a phosphate positive electrode material. The preparation method comprises the following steps: preparing a phosphate precursor from a raw material at least containing a first carbon source; sintering the phosphate series precursor to obtain a phosphate series semi-finished product; and providing a second carbon source solution, introducing the phosphate series semi-finished product into the crushing equipment at a flow rate Q1, atomizing the second carbon source solution at a flow rate Q2, spraying the atomized second carbon source solution into the crushing equipment, mixing, and calcining to obtain the phosphate series positive electrode material. According to the preparation method, a conductive network is optimized through a dual carbon coating process, a precursor containing carbon is provided, the carbon forms a three-dimensional conductive skeleton in the precursor, so that a lithium ion diffusion path is shortened, the conductivity is improved, the particle surface is coated with a second carbon source through an atomization spraying method to form a compact carbon layer, and wide application is facilitated.

Description

Technical Field

[0001] This application belongs to the technical field of cathode materials for lithium-ion batteries, and particularly relates to a phosphate-based cathode material, a preparation method thereof, and an application thereof. Background Art

[0002] Due to advantages such as high safety, long cycle life, and environmental friendliness, phosphate-based cathode materials have become an important choice for cathode materials of lithium-ion batteries. However, the intrinsic conductivity of phosphate-based cathode materials is relatively low, which limits their high-rate performance. To improve their conductivity, methods such as carbon coating or carbon doping are usually adopted.

[0003] In the prior art, the influence of the addition method of the carbon source on the performance of phosphate-based cathode materials has not been systematically studied. The addition method of the carbon source will affect the carbon coating effect, thereby affecting the conventional characteristics and electrochemical performance of phosphate-based cathode materials; in addition, the type of carbon source, the addition amount, and the sintering process will also have a significant impact on the material performance.

[0004] Therefore, it is of great significance to develop a suitable carbon source addition method to improve the carbon coating effect and optimize the electrochemical performance of phosphate-based cathode materials. Summary of the Invention

[0005] The purpose of this application is to provide a phosphate-based cathode material, a preparation method thereof, and an application thereof, aiming to solve the problem that the single addition method of the carbon source in the prior art leads to poor carbon coating effect and cannot improve the electrochemical performance of phosphate-based cathode materials.

[0006] To achieve the above application purpose, the technical solution adopted in this application is as follows: In the first aspect, this application provides a preparation method of a phosphate-based cathode material, including the following steps: Prepare a phosphate-based precursor from raw materials containing at least a first carbon source; Sinter the phosphate-based precursor to obtain a phosphate-based semi-finished product; Provide a second carbon source solution, introduce the phosphate-based semi-finished product into a pulverizing device at a flow rate Q1, and at the same time atomize and spray the second carbon source solution into the pulverizing device at a flow rate Q2 for mixing, and then perform calcination to obtain a phosphate-based cathode material.

[0007] In some embodiments, the ratio of the flow rate Q1 to the flow rate Q2 is 1:0.05 - 0.5.

[0008] In some embodiments, the method for preparing the second carbon source solution includes: dissolving the second carbon source in a solvent to obtain the second carbon source solution, where the second carbon source includes at least one of sucrose, starch, citric acid, glucose, maltose, chitosan, PEG, PVA, and PS, and the solvent includes at least one of pure water, nitric acid, oxalic acid, sulfuric acid, hydrochloric acid, ethanol, and methanol.

[0009] In some embodiments, the solute mass concentration of the second carbon source solution is 1 - 50%.

[0010] In some embodiments, based on the total mass of the phosphate-based cathode material being 100%, the addition amount of the second carbon source is 0.1 - 10 wt%.

[0011] In some embodiments, the sintering includes: first performing a first-stage sintering treatment, and then performing a second-stage sintering treatment; wherein, the first-stage sintering treatment uses a heating rate of 2 - 10 °C / min to rise from room temperature to 450 - 650 °C and holds for 1 - 5 hours, and the second-stage sintering treatment uses a heating rate of 1 - 8 °C / min to rise from room temperature to 500 - 700 °C and holds for 1 - 5 hours.

[0012] In some embodiments, the calcination includes: using a heating rate of 1 - 10 °C / min to rise from room temperature to 650 - 850 °C and holding for 1 - 10 hours.

[0013] In some embodiments, in the step of preparing a phosphate-based precursor from a raw material containing at least a first carbon source, it includes: Mixing a lithium source, an iron source, a phosphorus source, a doping element compound, and a solvent to obtain a first mixture; Mixing the first carbon source and the first mixture to obtain a colloid, and obtaining a phosphate-based precursor after drying treatment.

[0014] In some embodiments, the molar ratio of the lithium source, iron source, phosphorus source, and doping element compound is (0.9 - 1.1):(0.9 - 1.1):(0.9 - 1.1):(0 - 0.1). In some embodiments, the doping element compound includes at least one of magnesium oxide, titanium oxide, vanadium oxide, niobium oxide, zirconium oxide, and potassium oxide.

[0015] In some embodiments, the first carbon source includes at least one of sucrose, starch, citric acid, glucose, maltose, chitosan, PEG, PVA, and PS. In some embodiments, based on the total mass of the phosphate-based cathode material being 100%, the addition amount of the first carbon source is 0.5 - 50 wt%.

[0016] In a second aspect, the present application provides a phosphate-based cathode material prepared by the above-mentioned method for preparing a phosphate-based cathode material.

[0017] In some embodiments, the phosphate-based cathode material has the following characteristics: (a) The carbon content is 1% - 1.6%; (b) The resistivity is ≤20 Ω·m; (c) The specific surface area is 10 - 17 m² / g; (d) The particle size D 50 is ≤2μm.

[0018] In a third aspect, the present application provides a positive electrode sheet, which includes the above-mentioned phosphate-based cathode material or a phosphate-based cathode material prepared by the preparation method of the above-mentioned phosphate-based cathode material.

[0019] In a fourth aspect, the present application provides a secondary battery, which includes the above-mentioned positive electrode sheet.

[0020] In the preparation method of the phosphate-based cathode material provided in the first aspect of the present application, the conductive network is optimized through a double carbon coating process. A precursor prepared from a raw material containing a first carbon source is provided and sintered to obtain a phosphate-based semi-finished product, enabling carbon to form a three-dimensional conductive skeleton inside the semi-finished product to shorten the lithium ion diffusion path and improve conductivity. And the second carbon source is coated on the particle surface by atomized spraying to form a dense carbon layer. The atomized spraying method uses high-speed collision and shear force to make the second carbon source uniformly adhere to the surface of the semi-finished product particles, ensuring that the carbon layer coating is denser and more uniform, forming a "bulk-phase - interface" integrated conductive network jointly constructed by the internal and external carbon layers, which is beneficial to improving the electronic conductivity of the product and the rate performance of the product. And, through the carbon coating layer with high density and uniformity, it is more conducive to restricting the growth and aggregation of lithium iron phosphate particles, improving the dispersibility of the material, and effectively inhibiting the capacity attenuation of the material during the cycling process; this process is simple and controllable: it provides a carbon coating process suitable for large-scale production, with high practical value.

[0021] The phosphate-based cathode material provided in the second aspect of the present application includes a bulk-phase conductive carbon skeleton and a surface carbon coating layer, which not only provides a continuous electron transport channel, reduces the overall resistance, but also forms a coating layer with high uniformity and strong density, reduces interfacial side reactions, and at the same time improves the electron conduction ability between particles, having a high specific capacity, electronic conductivity and rate performance, which is beneficial to wide application.

[0022] For the positive electrode sheet provided in the third aspect of the present application, since the positive electrode sheet includes the above-mentioned phosphate-based cathode material, therefore, the obtained positive electrode sheet has the effects of high electron transport efficiency, high rate, long cycle life and high safety, providing a core material guarantee for the large-scale commercial application of lithium ion batteries.

[0023] The secondary battery provided in the fourth aspect of the present application includes the above-mentioned positive electrode sheet, so the obtained secondary battery has high cycle efficiency and high rate, which is conducive to wide application. Description of the Drawings

[0024] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the following will briefly introduce the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings in the following description are only some embodiments of the present application. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on these drawings.

[0025] Figure 1 It is the SEM analysis diagram of the phosphate-based positive electrode material provided in Embodiment 1 of the present application.

[0026] Figure 2 It is the SEM analysis diagram of the phosphate-based positive electrode material provided in Comparative Example 1 of the present application.

[0027] Figure 3 It is the SEM analysis diagram of the phosphate-based positive electrode material provided in Comparative Example 2 of the present application.

[0028] Figure 4 It is the XRD analysis diagram of the phosphate-based positive electrode materials provided in Embodiments 1-8 and Comparative Examples 1-2 of the present application. Detailed Embodiments

[0029] In order to make the technical problems to be solved, technical solutions and beneficial effects of the present application more clear and understandable, the following further details the present application in combination with embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and are not used to limit the present application.

[0030] In the present application, the term "and / or" describes the association relationship of associated objects, indicating that three relationships can exist. For example, A and / or B can represent: A exists alone, A and B exist simultaneously, and B exists alone. Wherein A and B can be singular or plural. The character " / " generally represents an "or" relationship between the front and rear associated objects.

[0031] In the present application, "at least one" means one or more, and "a plurality" means two or more. "At least one (item)" or its similar expression refers to any combination of these items, including any combination of single item (item) or plural items (items). For example, "at least one (item) of a, b, or c", or, "at least one (item) of a, b, and c" can all represent: a, b, c, a-b (that is, a and b), a-c, b-c, or a-b-c, where a, b, and c can be single or multiple respectively.

[0032] It should be understood that in various embodiments of the present application, the magnitudes of the sequence numbers of the above processes do not mean the sequence of execution. Some or all of the steps may be executed in parallel or sequentially. The execution sequence of each process should be determined according to its function and internal logic, and should not constitute any limitation to the implementation process of the embodiments of the present application.

[0033] The terms used in the embodiments of the present application are only for the purpose of describing specific embodiments, and are not intended to limit the present application. The singular forms "a" and "the" used in the embodiments of the present application and the appended claims are also intended to include the plural forms, unless the context clearly indicates otherwise.

[0034] The weight of the relevant components mentioned in the specification of the embodiments of the present application may not only refer to the specific content of each component, but also represent the proportional relationship of the weights between the components. Therefore, as long as the content of the relevant components in the specification of the embodiments of the present application is enlarged or reduced in proportion, it is within the scope disclosed in the specification of the embodiments of the present application. Specifically, the mass in the specification of the embodiments of the present application may be mass units well known in the chemical industry such as μg, mg, g, kg, etc.

[0035] The terms "first" and "second" are only used for descriptive purposes to distinguish objects such as substances from each other, and cannot be understood as indicating or implying relative importance or implicitly indicating the quantity of the indicated technical features. For example, without departing from the scope of the embodiments of the present application, the first XX may also be referred to as the second XX. Similarly, the second XX may also be referred to as the first XX. Thus, the features defined with "first" and "second" may explicitly or implicitly include one or more of such features.

[0036] The first aspect of the embodiments of the present application provides a method for preparing a phosphate-based cathode material, including the following steps: S01. Preparing a phosphate-based precursor from a raw material containing at least a first carbon source; S02. Sintering the phosphate-based precursor to obtain a phosphate-based semi-finished product; S03. Providing a second carbon source solution, introducing the phosphate-based semi-finished product into a pulverizing device at a flow rate Q1, and at the same time atomizing and spraying the second carbon source solution into the pulverizing device at a flow rate Q2 for mixing, and then performing calcination to obtain a phosphate-based cathode material.

[0037] In the preparation method of the phosphate-based cathode material provided in the first aspect of the embodiments of the present application, the conductive network is optimized through a dual carbon coating process. A precursor prepared from a raw material containing a first carbon source is provided and sintered to obtain a phosphate-based semi-finished product, enabling carbon to form a three-dimensional conductive framework inside the semi-finished product to shorten the lithium-ion diffusion path and improve conductivity. Moreover, a second carbon source is coated on the particle surface by an atomization spraying method to form a dense carbon layer. The atomization spraying method uses high-speed collision and shear force to uniformly attach the second carbon source to the surface of the semi-finished product particles, ensuring that the carbon layer coating is denser and more uniform, forming a "bulk-phase - interface" integrated conductive network jointly constructed by the internal and external carbon layers, which is beneficial to improving the electronic conductivity of the product and the rate performance of the product. Additionally, through the carbon coating layer with high density and uniformity, it is more conducive to restricting the growth and aggregation of lithium iron phosphate particles, improving the dispersibility of the material, and effectively suppressing the capacity attenuation of the material during the cycling process. This process is simple and controllable: it provides a carbon coating process suitable for large-scale production and has high practical value.

[0038] In step S01, a phosphate-based precursor is prepared from a raw material containing at least a first carbon source. A precursor containing carbon elements is provided, enabling the carbon elements to form a three-dimensional conductive framework inside the precursor to shorten the lithium-ion diffusion path and improve conductivity.

[0039] In some embodiments, in the step of preparing a phosphate-based precursor from a raw material containing at least a first carbon source, it includes: S011. Mix a lithium source, an iron source, a phosphorus source, a doping element compound, and a solvent to obtain a first mixture; S012. Mix the first carbon source and the first mixture to obtain a colloid, and after drying treatment, obtain a phosphate-based precursor.

[0040] In step S011, a lithium source, an iron source, a phosphorus source, a doping element compound, and a solvent are mixed to obtain a first mixture.

[0041] In some embodiments, the molar ratio of the lithium source, the iron source, the phosphorus source, and the doping element compound is (0.9 - 1.1):(0.9 - 1.1):(0.9 - 1.1):(0 - 0.1).

[0042] In some embodiments, the lithium source includes but is not limited to at least one of lithium carbonate, lithium hydroxide, and lithium nitrate; the iron source includes but is not limited to at least one of ferrous oxalate, ferrous sulfate, and ferrous oxide; the phosphorus source includes but is not limited to at least one of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, and phosphoric acid.

[0043] In some embodiments, the doped element compound includes at least one of magnesium oxide, titanium oxide, vanadium oxide, niobium oxide, zirconium oxide, and potassium oxide. By adding the doped element compound, a doped element is introduced into the precursor, further improving the electrochemical properties of the precursor.

[0044] In some embodiments, the raw materials further include a manganese source, and the manganese source includes at least one of manganese dioxide, manganese sesquioxide, manganese tetraoxide, manganese sulfate, manganese nitrate, manganese chloride, manganese dihydrogen phosphate, manganese oxalate, manganese acetate, manganese citrate, manganese glycinate, and manganese gluconate.

[0045] In some embodiments, the molar ratio of the lithium source, the iron source + manganese source, the phosphorus source, and the doped element compound is (0.9 - 1.1):(0.9 - 1.1):(0.9 - 1.1):(0 - 0.1).

[0046] In step S012, the first carbon source and the first mixture are mixed to obtain a colloid, and after drying treatment, a phosphate-based precursor is obtained.

[0047] In some embodiments, the first carbon source includes at least one of sucrose, starch, citric acid, glucose, maltose, chitosan, PEG, PVA, and PS.

[0048] In some embodiments, based on the total mass of the phosphate-based cathode material being 100%, the addition amount of the first carbon source is 0.5 - 50 wt%. If the addition amount of the first carbon source is too small, carbon cannot form a conductive network inside the precursor, which is not conducive to improving the electronic conductivity; if the addition amount of the first carbon source is too large, it will affect the properties of the precursor.

[0049] In some specific embodiments, based on the total mass of the phosphate-based cathode material being 100%, the addition amount of the first carbon source includes but is not limited to typical but non-limiting values such as 0.5 wt%, 1 wt%, 5 wt%, 10 wt%, 15 wt%, 20 wt%, 25 wt%, 30 wt%, 35 wt%, 40 wt%, 45 wt%, 50 wt%, etc.

[0050] Further, the first carbon source and the first mixture are mixed to obtain a colloid, and after drying treatment, a phosphate-based precursor is obtained.

[0051] In step S02, the phosphate-based precursor is sintered to obtain a phosphate-based semi-finished product.

[0052] In some embodiments, the sintering includes: providing an inert atmosphere, first performing a first-stage sintering treatment, and then performing a second-stage sintering treatment; Among them, in the first-stage sintering treatment, the temperature is raised from room temperature to 450 - 650 °C at a heating rate of 2 - 10 °C / min and held for 1 - 5 hours. In the second-stage sintering treatment, the temperature is raised from room temperature to 500 - 700 °C at a heating rate of 1 - 8 °C / min and held for 1 - 5 hours. The two-stage sintering treatment is mainly used to control the growth of crystal nuclei.

[0053] In step S03, a second carbon source solution is provided. The phosphate-based precursor is introduced into the pulverizing device at a flow rate Q1, and at the same time, the second carbon source solution is atomized and sprayed into the pulverizing device at a flow rate Q2 for mixing, and then sintered and calcined to obtain the phosphate-based cathode material. By providing the second carbon source solution and atomizing and spraying it to uniformly coat the surface of the particles during the pulverizing process, a continuous and dense nano-carbon layer is formed, which can effectively inhibit the electrode-electrolyte side reaction and reduce the interfacial impedance. Moreover, with the atomizing and spraying method used, the second carbon source solution is atomized and sprayed synchronously during the gas flow pulverizing process, and the carbon source solution is uniformly attached to the surface of the newly formed particles by high-speed collision and shear force, avoiding the hard agglomeration problem caused by secondary drying in the traditional step-by-step process.

[0054] In some embodiments, the preparation method of the second carbon source solution includes: dissolving the second carbon source in a solvent to obtain the second carbon source solution, where the second carbon source includes at least one of sucrose, starch, citric acid, glucose, maltose, chitosan, PEG, PVA, and PS, and the solvent includes at least one of pure water, nitric acid, oxalic acid, sulfuric acid, hydrochloric acid, ethanol, and methanol.

[0055] In some embodiments, the solute mass concentration of the second carbon source solution is 1 - 50%.

[0056] In some embodiments, based on the total mass of the phosphate-based cathode material being 100%, the addition amount of the second carbon source is 0.1 - 10 wt%. If the addition amount of the second carbon source is too much, it will affect the growth of the phosphate cathode material, thus affecting the material performance. If the addition amount of the second carbon source is too little, it will affect the coating effect, thus affecting the material performance.

[0057] In some specific embodiments, based on the total mass of the phosphate-based cathode material being 100%, the addition amount of the second carbon source includes but is not limited to typical but non-limiting values such as 0.1 wt%, 0.5 wt%, 1 wt%, 1.5 wt%, 2 wt%, 2.5 wt%, 3 wt%, 3.5 wt%, 4 wt%, 4.5 wt%, 5 wt%, 5.5 wt%, 6 wt%, 6.5 wt%, 7 wt%, 7.5 wt%, 8 wt%, 8.5 wt%, 9 wt%, 9.5 wt%, 10 wt%, etc.

[0058] In some embodiments, the comminution device includes any one of a jet mill, a mechanical mill, a butterfly nest mill, a sand mill, and a ball mill.

[0059] In some embodiments, the phosphate-based precursor is introduced into the comminution device at a flow rate Q1, and at the same time, the second carbon source solution is atomized and sprayed into the comminution device at a flow rate Q2 for mixing. The ratio of the flow rate Q1 to the flow rate Q2 is 1:0.05 - 0.5. The flow rate Q1 is the flow rate of the phosphate-based precursor, and the flow rate Q2 is the flow rate of the second carbon source solution. The Q1 / Q2 ratio directly affects the contact time and coverage density between the carbon source solution and the semi-finished product particles, resulting in insufficient carbon coating and affecting the material properties. If the ratio is too low (Q2 is too large), it may cause an overly thick carbon layer or local agglomeration. By adjusting the flow rate ratio, the size of the atomized droplets of the carbon source solution and the adsorption ability of the particle surface are adjusted, so that the thickness of the carbon layer in the finished material is controllable, forming a continuous and dense conductive network; and ensuring uniform distribution of the carbon material, improving the conductivity. Therefore, by precisely controlling the Q1 / Q2 ratio, the synergy of carbon coating uniformity, conductive network optimization, particle dispersion improvement, and process economy can be achieved, ultimately endowing the phosphate cathode material with high specific capacity, long cycle life, and excellent rate performance.

[0060] In some specific embodiments, the ratio of the flow rate Q1 to the flow rate Q2 includes but is not limited to typical but non-limiting values such as 1:0.05, 1:0.1, 1:0.15, 1:0.2, 1:0.25, 1:0.3, 1:0.35, 1:0.4, 1:0.45, 1:0.5, etc.

[0061] Further, calcination is carried out to obtain the phosphate-based cathode material.

[0062] In some embodiments, the calcination includes: heating from room temperature to 650 - 850 °C at a heating rate of 1 - 10 °C / min and holding for 1 - 10 hours. After sintering, calcination treatment is carried out to achieve carbon network reconstruction. Since after primary sintering, the carbon coating may be uneven or carbonization may be incomplete, resulting in a discontinuous conductive network, secondary carbon supplementation can be used to supplement the carbon lost in the primary sintering, and secondary sintering can promote the bridging between carbon particles to form a more uniform coating layer, experiment with carbon network reconstruction, and increase the electron conduction path.

[0063] The second aspect of the embodiments of the present application provides a phosphate-based cathode material, which includes a phosphate core material and a carbon coating layer provided on the surface of the phosphate core material, wherein the carbon coating layer is prepared from a first carbon source and a second carbon source respectively.

[0064] The phosphate-based cathode material provided in the second aspect of the embodiments of the present application includes a bulk conductive carbon skeleton and a surface carbon coating layer, which not only provides a continuous electron transport channel, reduces the overall resistance, but also forms a coating layer with high uniformity and strong compactness, reduces interfacial side reactions, and at the same time improves the electron conduction ability between particles, having a high specific capacity, electron conductivity and rate performance, which is conducive to wide application.

[0065] In some embodiments, the carbon content of the phosphate-based cathode material is 1% - 1.6%.

[0066] In some embodiments, the resistivity of the phosphate-based cathode material is ≤20 Ω·m.

[0067] In some embodiments, the specific surface area of the phosphate-based cathode material is 10 - 17 m² / g.

[0068] In some embodiments, the particle size D of the phosphate-based cathode material 50 is ≤2 μm.

[0069] The third aspect of the embodiments of the present application provides a positive electrode sheet, which includes the above-mentioned phosphate-based cathode material or a phosphate-based cathode material prepared by the preparation method of the above-mentioned phosphate-based cathode material.

[0070] For the positive electrode sheet provided in the third aspect of the embodiments of the present application, since the positive electrode sheet includes the above-mentioned phosphate-based cathode material, therefore, the obtained positive electrode sheet has the effects of high electron transport efficiency, high rate, long cycle and high safety, providing a core material guarantee for the large-scale commercial application of lithium-ion batteries.

[0071] The fourth aspect of the embodiments of the present application provides a secondary battery, which includes the above-mentioned positive electrode sheet.

[0072] For the secondary battery provided in the fourth aspect of the embodiments of the present application, since the provided secondary battery includes the above-mentioned positive electrode sheet, therefore, the obtained secondary battery has a high cycle efficiency and high rate, which is conducive to wide application.

[0073] The following is illustrated with specific embodiments.

[0074] Example 1 Phosphate-based cathode material and its preparation method The preparation method includes the following steps: (1) Dissolve and mix ferric nitrate, ammonium dihydrogen phosphate, and lithium carbonate with a molar ratio of 0.97:1:1.02, then add a doping element compound and stir, and then mix with a primary carbon source to obtain a mixed colloid. The mixed colloid is dried, crushed, sintered at 530 °C for 2 h at a heating rate of 5.5 °C / min in a nitrogen atmosphere, and then sintered at 630 °C for 2.5 h at a heating rate of 2 °C / min to obtain a semi-finished product; (2) Dissolve the secondary carbon source in a solvent; the obtained semi-finished product passes through a crushing device at a flow rate of 100 kg / h. At the same time, the solution dissolving the secondary carbon source is sprayed into the crushing device at a flow rate of 10 kg / h. After the two are mixed during the crushing process, they are calcined at 780 °C for 6 h at a heating rate of 4.5 °C / min in a nitrogen atmosphere to obtain a finished lithium iron phosphate cathode material; The primary carbon source is glucose, and the addition amount of the primary carbon source is 10 wt% of the total mass of the phosphate-based cathode material; The doping element compound is titanium oxide, and the addition amount of titanium oxide is 0.4 wt% of the theoretical mass of the phosphate-based cathode material; The secondary carbon source is glucose, and the addition amount is 1 wt% of the mass of the prepared phosphate-based cathode material. The solvent for dissolving the secondary carbon source is water, and the addition amount is 9 wt% of the mass of the prepared phosphate-based cathode material.

[0075] Example 2 This example provides a method for preparing a lithium iron phosphate cathode material. The difference between the preparation method and Example 1 is only that, except for replacing the addition amount of the secondary carbon source of 1 wt% in step (2) with 2 wt% and the addition amount of the solvent of 9 wt% with 8 wt%, the rest are the same as in Example 1.

[0076] Example 3 This example provides a method for preparing a lithium iron phosphate cathode material. The difference between the preparation method and Example 1 is only that, except for replacing the addition amount of the secondary carbon source of 1 wt% in step (2) with 3 wt% and the addition amount of the solvent of 9 wt% with 7 wt%, the rest are the same as in Example 1.

[0077] Example 4 This example provides a method for preparing a lithium iron phosphate cathode material. The difference between the preparation method and Example 1 is only that, except for changing the secondary carbon source glucose to PEG in step (2), the rest are the same as in Example 1.

[0078] Example 5 This example provides a method for preparing a lithium iron phosphate cathode material. The difference between the preparation method and Example 2 is only that, except for changing the secondary carbon source glucose to PEG in step (2), the rest are the same as in Example 2.

[0079] Example 6 This example provides a preparation method of a lithium iron phosphate cathode material. The difference between the preparation method and that of Example 3 is only that in step (2), the secondary carbon source glucose is changed to PEG, and the rest is the same as in Example 3.

[0080] Example 7 This example provides a preparation method of a lithium iron phosphate cathode material. The difference between the preparation method and that of Example 1 is only that in step (2), the flow rate of the secondary carbon source solution is changed from 10 kg / h to 5 kg / h. The secondary carbon source is glucose, the addition amount is 1 wt% of the finished product quality, the solvent for dissolving the secondary carbon source is water, and the addition amount is 4 wt% of the finished product quality.

[0081] Example 8 This example provides a preparation method of a lithium iron phosphate cathode material. The difference between the preparation method and that of Example 1 is only that in step (2), the flow rate of the secondary carbon source solution is changed from 10 kg / h to 20 kg / h. The secondary carbon source is glucose, the addition amount is 1 wt% of the finished product quality, the solvent for dissolving the secondary carbon source is water, and the addition amount is 19 wt% of the finished product quality.

[0082] Comparative Example 1 A preparation method of a lithium iron phosphate cathode material is provided, including the following steps: (1) Ferric nitrate, ammonium dihydrogen phosphate, and lithium carbonate with a molar ratio of 0.97:1:1.02 are dissolved and mixed, then a doping element compound is added and stirred, and then mixed with a primary carbon source to obtain a mixed colloid. The colloid is dried, crushed, sintered at 530 °C for 2 h at a heating rate of 5.5 °C / min in a nitrogen atmosphere, and then sintered at 630 °C for 2.5 h at a heating rate of 2 °C / min to obtain a semi-finished product; (2) After mixing the secondary carbon source with the semi-finished product obtained in step (1) during the crushing process, sinter at 780 °C for 6 h at a heating rate of 4.5 °C / min in a nitrogen atmosphere to obtain the finished lithium iron phosphate cathode material; The primary carbon source is glucose, the doping element compound is titanium oxide, and the addition amount of titanium oxide is 0.4 wt% of the theoretical mass of the phosphate-based cathode material; The secondary carbon source is glucose, and the addition amount is 1 wt% of the mass of the semi-finished product in step (1).

[0083] Comparative Example 2 This example provides a preparation method of a lithium iron phosphate cathode material. The difference between the preparation method and that of Comparative Example 1 is only that step (2) is changed to: after crushing the semi-finished product, calcine at 780 °C for 6 h at a heating rate of 4.5 °C / min in a nitrogen atmosphere to obtain the finished lithium iron phosphate cathode material.

[0084] Performance test (1)Analyze the scanning electron microscope analysis and X-ray diffractometer analysis of the finished materials of each example and comparative example.

[0085] (2)Analyze the carbon content, resistivity, specific surface area, particle size D of the finished materials of each example and comparative example 50 .

[0086] (3)Prepare the finished materials obtained from each example and comparative example into positive electrode sheets, assemble them into secondary batteries, and then conduct 0.1C charge specific capacity measurement, 0.1C discharge specific capacity measurement, efficiency, 1C charge specific capacity measurement, and 1C discharge specific capacity measurement.

[0087] Result analysis (1)Analyze the scanning electron microscope analysis and X-ray diffractometer analysis of the finished materials of each example and comparative example. The scanning electron microscope analysis diagrams of Example 1, Comparative Example 1, and Comparative Example 2 are respectively as Figure 1 , Figure 2 and Figure 3 shown. It can be seen that by changing the secondary carbon source replenishment method, the size of the primary particles of the material can be effectively controlled; compared with Comparative Example 1 and Comparative Example 2, in Example 1, the secondary carbon source is replenished by spraying, and the primary particles are significantly reduced; simultaneously, the particle dispersion of the material is better, and the agglomerates are reduced; it is beneficial to the electrochemical performance of the material.

[0088] Figure 4 The XRD images of Examples 1-8 and Comparative Examples 1-2 are shown. It can be seen from the XRD images that adjusting the secondary carbon source addition method, addition amount, and flow ratio will not produce impurity phases.

[0089] (2)Analyze the carbon content, resistivity, specific surface area, particle size D of the finished materials of each example and comparative example 50 . As shown in Table 1, by comparing Examples 1 to 3, it can be seen that as the addition amount of the secondary carbon source increases, the overall carbon content of the finished material also increases, the resistivity decreases, the specific surface area increases, and the particle size D 50 decreases.

[0090] The resistivity range of the finished materials obtained in the examples is between 6.3 Ω·m and 18.31 Ω·m. The resistivity of the finished material obtained in Comparative Example 1 is 20.84 Ω·m, and the resistivity of the finished material obtained in Comparative Example 2 is as high as 38.65 Ω·m, which is much higher than that of the finished materials obtained in the examples. Resistivity is closely related to the electrical conductivity of the material. A lower resistivity is beneficial to improving the charge and discharge efficiency of the material. It can be found that when the carbon content is higher, the resistivity is relatively lower, indicating that the increase in carbon content may help improve the electrical conductivity of the material.

[0091] The specific surface area of the finished material obtained in the examples is between 12.08 m² / g and 16.83 m² / g. The specific surface area of the finished material obtained in Comparative Example 1 is 10.06 m² / g, and the specific surface area of the finished material obtained in Comparative Example 2 is 7.06 m² / g, which is lower than that of the examples. A larger specific surface area is beneficial to the full contact between the material and the electrolyte, thereby improving the performance of the battery. As the carbon content increases, the specific surface area tends to increase, which may make the battery have better reaction activity during charge and discharge.

[0092] The particle size D of the finished material obtained in the examples 50 is between 0.91 μm and 1.26 μm, and the distribution is relatively uniform. The particle size D of the finished material obtained in Comparative Example 1 50 is 1.4 μm, and the particle size D of the finished material obtained in Comparative Example 2 50 is 2.07 μm, which is relatively larger than that of the finished material obtained in the examples. A smaller particle size helps to increase the specific surface area of the material and improve the charge and discharge performance of the battery.

[0093] In summary, it can be seen that different ways of adding the secondary carbon source will change the carbon coating effect, thereby affecting the conductivity of the material. By adding the secondary carbon source in the form of spraying, compared with the conventional solid addition method, the carbon content of the finished product increases, D 50 decreases, and the rate performance is improved.

[0094] (3) Prepare the finished materials obtained in each example and comparative example into positive electrode sheets, assemble them into secondary batteries, and then conduct 0.1 C charge specific capacity measurement, 0.1 C discharge specific capacity measurement, initial efficiency, 1 C charge specific capacity measurement, and 1 C discharge specific capacity measurement. As shown in Table 1, at 25°C and 0.1 C, the charge specific capacity and discharge specific capacity of the examples and comparative examples are both above 150 mAh / g, and the values are relatively close. However, at 25°C and 1 C, the discharge specific capacity of the examples is significantly higher than that of the comparative examples. This shows that at a higher charge and discharge rate, the materials in the examples can better maintain their performance and have better rate performance. This may be the result of the combined action of factors such as appropriate carbon content, lower resistivity, larger specific surface area, and smaller particle size in the examples.

[0095] The initial efficiency of the secondary battery including the finished material obtained in the examples is between 99.18% and 100.40%. The initial efficiency of the secondary battery including the finished material obtained in Comparative Example 1 is 98.10%, and the initial efficiency of the secondary battery including the finished material obtained in Comparative Example 2 is 96.07%. The initial efficiency of the secondary battery including the finished material obtained in the examples is significantly higher than that of the secondary battery including the finished material obtained in the comparative examples.

[0096] It can be seen that with the increase of the addition amount of the secondary carbon source, the carbon content of the material product increases, the conductivity of the material is improved, the resistivity decreases, which is beneficial to the rate performance of the material; with the increase of the addition amount of the secondary carbon source, the coated particles are more uniform, the growth of the particles is restricted, the primary particles become smaller, the secondary particles are synchronously reduced, and the specific surface area increases accordingly.

[0097] Table 1

[0098] In summary, a phosphate-based cathode material is provided to supplement the carbon source by spraying, which can effectively improve the carbon coating effect and optimize the electrochemical performance of the lithium iron phosphate cathode material. The specific beneficial effects include: 1. Improve conductivity: By optimizing the carbon coating effect, the conductivity of the material is significantly improved, and its high rate performance is improved.

[0099] 2. Enhance dispersibility: Provide a better carbon coating atmosphere, restrict the growth and aggregation of lithium iron phosphate particles, and enhance the dispersibility of the material; 3. Improve cycle stability: Through the uniform carbon coating layer, the capacity decay of the material during the cycle is effectively inhibited.

[0100] 4. The process is simple and controllable: A carbon coating process suitable for large-scale production is provided, which has high practical value.

[0101] The above are only the preferred embodiments of the present application and are not intended to limit the present application. Any modifications, equivalent replacements, and improvements made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A method for preparing a phosphate-based positive electrode material, characterized in that: The steps include: Prepare a phosphate precursor from a raw material comprising at least a first carbon source; Sintering the phosphate precursor to obtain a phosphate semi-finished product; A second carbon source solution is provided, the phosphate-based semi-finished product is passed into a pulverizing device at a flow rate of Q1, and the second carbon source solution is sprayed into the pulverizing device at a flow rate of Q2 for mixing, and then calcined to obtain a phosphate-based positive electrode material.

2. The method for preparing a phosphate-based positive electrode material according to claim 1, characterized in that: The ratio of the flow rate Q1 to the flow rate Q2 is 1:(0.05~0.5).

3. The method for preparing a phosphate-based positive electrode material according to claim 1, characterized in that: The preparation method of the second carbon source solution comprises: dissolving the second carbon source in a solvent to obtain the second carbon source solution, wherein each of the second carbon sources is selected from at least one of sucrose, starch, citric acid, glucose, maltose, chitosan, PEG, PVA, and PS, and the solvent comprises at least one of pure water, nitric acid, oxalic acid, sulfuric acid, hydrochloric acid, ethanol, and methanol; and / or, The solute mass concentration of the second carbon source solution is 1-50%; and / or, Based on the total mass of the phosphate-based positive electrode material being 100%, the added amount of the second carbon source is 0.1-10 wt%.

4. The method for preparing a phosphate-based positive electrode material according to claim 1, characterized in that: The sintering comprises: firstly performing a first stage sintering treatment, and then performing a second stage sintering treatment; wherein, the first stage sintering treatment adopts a heating rate of 2-10°C / min from room temperature to 450-650°C, and keeps the temperature for 1-5 hours, and the second stage sintering treatment adopts a heating rate of 1-8°C / min from room temperature to 500-700°C, and keeps the temperature for 1-5 hours; and / or, The calcination comprises: heating the temperature from room temperature to 650-850° C. at a heating rate of 1-10° C. / min, and keeping the temperature for 1-10 hours.

5. The method for preparing a phosphate-based positive electrode material according to claim 1, characterized in that: The step of preparing a phosphate precursor from a raw material containing at least a first carbon source comprises: Mixing a lithium source, an iron source, a phosphorus source, a doping element compound and a solvent to obtain a first mixture; The first carbon source and the first mixture are mixed to obtain a colloid, which is then dried to obtain a phosphate-based precursor.

6. The method for preparing a phosphate-based positive electrode material according to claim 5, characterized in that: The molar ratio of the lithium source, the iron source, the phosphorus source and the doping element compound is (0.9-1.1): (0.9-1.1): (0.9-1.1): (0-0.1); and / or, The doping element compound includes at least one of magnesium oxide, titanium oxide, vanadium oxide, niobium oxide, zirconium oxide and potassium oxide; and / or, The first carbon source comprises at least one of sucrose, starch, citric acid, glucose, maltose, chitosan, PEG, PVA, PS, and / or, Taking the total mass of the phosphate-based positive electrode material as 100%, the added amount of the first carbon source is 0.5-50 wt%.

7. A phosphate-based positive electrode material, characterized in that: The positive electrode material is prepared by the method for preparing the phosphate-based positive electrode material according to any one of claims 1 to 6.

8. The phosphate-based positive electrode material according to claim 7, characterized in that: The phosphate-based positive electrode material has the following characteristics: (a) Carbon content is 1.0%~1.6%; (b) Resistivity ≤ 20Ω·m; (c) Specific surface area of ​​10-17 m² / g; (d) Particle size D 50 ≤2 μm.

9. A positive electrode sheet, characterized in that: The positive electrode plate comprises the phosphate-based positive electrode material described in any one of claims 7 to 8.

10. A secondary battery, characterized in that: Including the positive electrode sheet as described in claim 9.

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