Manganese phosphate-based positive electrode material, preparation method and application thereof

Abstract

The application belongs to the technical field of batteries, and particularly relates to a manganese phosphate salt-based positive electrode material and a preparation method and application thereof. The application comprises the following steps: raw material components are prepared into a primary mixed material, the mass percentage of the initial carbon source in the raw material components is 1-3%; primary sintering treatment is performed; the amount of the required additional carbon source is calculated according to the carbon content in the primary sintered material and the target carbon content in the finished product, and a secondary mixed material is prepared; after secondary sintering treatment, the manganese phosphate salt-based positive electrode material is obtained by crushing; and the carbon source independently comprises a polymer carbon source and a short-chain carbon source. The application improves the uniformity of carbon layer coating and the graphitization degree by adding mixed carbon sources in steps, reduces the free carbon in the finished product, and improves the performance of the finished product, such as compaction, specific surface area, conductivity, manganese platform retention rate and the like. The mixed carbon source can realize uniform carbon layer coating and has good electronic conductivity.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of batteries, and particularly relates to a manganese phosphate salt-based positive electrode material and a preparation method and application thereof. BACKGROUND

[0002] In recent years, the positive electrode material of lithium ion batteries has made great progress. Initially, lithium cobaltate (LiCoO2) is mainly used as the material, which has a relatively high energy density. Subsequently, in order to improve safety and reduce cost, lithium iron phosphate (LiFePO4, LFP) is developed, which has better thermal stability and longer cycle life. However, LFP also has many shortcomings. First, the energy density is relatively low, which limits its use in applications with high energy demand. Second, the performance of these batteries significantly decreases in low-temperature environments, affecting their efficiency in cold regions, and the charging speed of lithium iron phosphate batteries is slower than other types of lithium ion batteries, which may affect applications that require fast charging.

[0003] Lithium manganese iron phosphate (LiFeMnPO4, LMFP) is a new type of positive electrode material for lithium ion batteries, which combines the high safety of lithium iron phosphate and the high voltage characteristics of lithium manganese phosphate. This material has attracted attention due to its environmental friendliness and relatively high energy density, but the presence of lithium manganese phosphate makes the electronic conductivity of the material worse than LFP, and the significant internal resistance in the crystal lattice leads to a very low ion diffusion coefficient. These shortcomings cause certain problems in the stability of the material during the cycle process, especially the decay of the discharge platform retention rate of the manganese segment during the cycle process, which greatly affects the high energy density advantage of the material, and is one of the urgent problems that LMFP needs to solve. SUMMARY

[0004] The purpose of the present application is to provide a manganese phosphate salt-based positive electrode material and a preparation method and application thereof, which aims to solve the problem of poor manganese platform retention rate of the existing manganese phosphate salt-based positive electrode material to a certain extent, which affects the energy density and stability of the material.

[0005] To achieve the above-mentioned application purposes, the technical solutions adopted by the present application are as follows:

[0006] In a first aspect, the present application provides a preparation method of a manganese phosphate salt-based positive electrode material, comprising the following steps:

[0007] Obtaining raw material components including a phosphorus source, a manganese source, a lithium source, other metal sources, and an initial carbon source, drying the first mixed slurry after the raw material components are mixed with a first solvent to obtain a primary mixed material; the mass percentage content of the initial carbon source in the raw material components is 1% to 3%;

[0008] The mixture is subjected to a first sintering treatment under an inert atmosphere to obtain a first sintered material;

[0009] The amount of additional carbon source required is calculated according to the carbon content in the first sintered material and the target carbon content in the finished product, and the first sintered material, the additional carbon source and a second solvent are made into a second mixed slurry and subjected to a drying treatment to obtain a second mixture; wherein the initial carbon source and the additional carbon source each independently comprise a polymer carbon source and a short-chain carbon source.

[0010] The second mixture is subjected to a second sintering treatment under an inert atmosphere, and then crushed to obtain the finished product, i.e. a manganese phosphate salt-based positive electrode material.

[0011] In some possible implementations, the mass percentage content of the initial carbon source in the raw material components is 1.5% to 2.5%.

[0012] In some possible implementations, the carbon content in the first sintered material is 0.12% to 0.36%.

[0013] In some possible implementations, the amount of additional carbon source added is 8% to 12% of the total mass of the first sintered material and the additional carbon source.

[0014] In some possible implementations, the target carbon content in the finished product is 1.6% to 1.65%.

[0015] In some possible implementations, the actual carbon content in the manganese phosphate salt-based positive electrode material is 1.4% to 1.6%.

[0016] In some possible implementations, the D50 particle size of the manganese phosphate salt-based positive electrode material is 0.9 μm to 1.5 μm.

[0017] In some possible implementations, the tap density of the manganese phosphate salt-based positive electrode material is greater than 2.19 g / cm 3 .

[0018] In some possible implementations, the specific surface area of the manganese phosphate salt-based positive electrode material is 15 m 2 / g to 18 m 2 / g.

[0019] In some possible implementations, the resistivity of the manganese phosphate salt-based positive electrode material is less than 200 Ω·cm.

[0020] In some possible implementations, the polymer carbon source comprises at least one of polyethylene glycol, polyvinyl alcohol, polyacrylic acid, carboxymethyl cellulose and starch.

[0021] In some possible implementation manners, the short-chain carbon source includes at least one of glucose, sucrose and citric acid.

[0022] In some possible implementation manners, the other metal source includes at least one of an iron source, a niobium source, a titanium source, a cobalt source, a nickel source and a vanadium source.

[0023] In some possible implementation manners, the first solvent and the second solvent each independently includes water.

[0024] In some possible implementation manners, a molar ratio of the other metal source to the manganese source is (10-X):X, where X is 2-8.

[0025] In some possible implementation manners, a ratio of a total molar amount of the manganese source and the other metal source to molar amounts of the lithium source, the phosphorus source is 1:(1-1.03):(1-1.03).

[0026] In some possible implementation manners, the first mixed slurry is prepared through a sanding process, and a particle size D50 of material in the slurry is 0.6-0.9 μm.

[0027] In some possible implementation manners, the drying condition of the first mixed slurry includes spray drying under the condition that atomization frequency is 250-350 Hz and inlet air temperature is 150-250 °C.

[0028] In some possible implementation manners, the first sintering process has a heating rate of 1-10 °C / min, a holding temperature of 400-600 °C and a holding time of 5-8 h.

[0029] In some possible implementation manners, the second mixed slurry is prepared through a sanding process, and a particle size D50 of material in the slurry is 0.6-0.9 μm.

[0030] In some possible implementation manners, the drying condition of the second mixed slurry includes spray drying under the condition that atomization frequency is 250-350 Hz and inlet air temperature is 150-250 °C.

[0031] In some possible implementation manners, the second sintering process has a heating rate of 1-10 °C / min, a holding temperature of 600-800 °C and a holding time of 6-10 h.

[0032] In a second aspect, the application provides a manganese phosphate-based positive electrode material prepared by the above method, which includes a manganese phosphate-based active core and a carbon layer coated on an outer surface of the core; the carbon layer is prepared from a polymer carbon source and a short-chain carbon source.

[0033] In some possible implementations, the manganese phosphate salt-based active core comprises lithium manganese iron phosphate.

[0034] In some possible implementations, the carbon layer in the manganese phosphate salt-based cathode material has a thickness of 1 nm to 5 nm.

[0035] In a third aspect, the application provides a cathode sheet, which comprises a current collector and a cathode active layer formed on the surface of the current collector, and the cathode active layer comprises the manganese phosphate salt-based cathode material prepared by the method described above.

[0036] In a fourth aspect, the application provides a secondary battery comprising the cathode sheet described above.

[0037] The method for preparing the manganese phosphate salt-based cathode material provided in the first aspect of the application realizes the construction of a uniform carbon coating layer by a method of adding carbon sources in steps, effectively reduces free carbon in the finished product, makes the carbon source more fully utilized, and improves the graphitization degree of the carbon coating layer. By adding the carbon source in steps, it is also found that the amount of the carbon source added before the first sintering process affects the physical and chemical properties of the finished product, such as the tap density and specific surface area, and also affects the electrochemical properties, such as the discharge specific capacity. Moreover, the mass percentage of the initial carbon source in the raw material components is controlled to be 1% to 3%, and by controlling the amount of the carbon source added before the first sintering process, the tap density, specific surface area, and electrical conductivity of the finished product can be effectively regulated, which also has a very significant effect on the manganese platform retention rate and improves the manganese platform retention rate. In addition, the mixed carbon source comprising a polymer carbon source and a short-chain carbon source can not only realize uniform carbon layer coating, but also has good electronic conductivity. The advantages and disadvantages of the two carbon sources are combined, so that the carbon source is fully utilized, and free carbon in the finished product is effectively reduced. Moreover, the use of the mixed carbon source can make the carbon coating layer more uniform and have a higher graphitization degree, which is conducive to inhibiting the dissolution of metal ions during the cycling process of the material, reducing the side reactions of the negative electrode, and improving the stability of the battery.

[0038] The manganese phosphate salt-based cathode material provided in the second aspect of the application is prepared by the method described above, and comprises a manganese phosphate salt-based active core and a carbon layer coated on the outer surface of the core. The carbon layer is prepared from a polymer carbon source and a short-chain carbon source. The carbon layer uniformly and stably coats the surface of the manganese phosphate salt-based active core, which not only improves the structural stability of the manganese phosphate salt-based cathode material, but also improves the electrical conductivity of the manganese phosphate salt-based cathode material, improves the cycling stability of the manganese phosphate salt-based cathode material, improves the stability of the interface between the cathode material and the electrolyte, and slows down the stress caused by volume change. These are all very beneficial to the manganese platform retention rate of the manganese phosphate salt-based cathode material during the cycling process, which can improve the manganese platform retention rate and improve the electrochemical performance of the manganese phosphate salt-based cathode material.

[0039] In the positive electrode sheet of the third aspect of the present application, the manganese phosphate salt-based positive electrode material is used in the positive electrode active layer, the manganese phosphate salt-based positive electrode material comprises a manganese phosphate salt-based active core and a carbon layer covering the outer surface, and the conductivity, structural stability, capacity and other characteristics of the manganese phosphate salt-based positive electrode material are improved. Therefore, the stability, energy density, rate capability, cycle performance and other electrochemical properties of the positive electrode sheet are improved.

[0040] The secondary battery provided in the fourth aspect of the present application comprises the positive electrode sheet with excellent electrochemical properties such as good stability, high energy density, good rate capability and good cycle stability, and thus the energy density and cycle stability and other electrochemical properties of the secondary battery are improved. BRIEF DESCRIPTION OF DRAWINGS

[0041] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the drawings needed in the embodiments or prior art description will be briefly introduced as follows. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative labor.

[0042] Figure 1 is a flowchart of the preparation method of the manganese phosphate salt-based positive electrode material provided in the embodiments of the present application;

[0043] Figure 2 is an XRD graph of the manganese iron lithium phosphate positive electrode material prepared in Examples 1-3 and Comparative Examples 1-3 provided in the embodiments of the present application;

[0044] Figure 3 is an SEM graph of the manganese iron lithium phosphate positive electrode material prepared in Examples 1-3 and Comparative Examples 1-3 provided in the embodiments of the present application;

[0045] Figure 4 is a TEM image of the manganese iron lithium phosphate positive electrode material prepared in Example 1 of the embodiments of the present application;

[0046] Figure 5 is a charge-discharge curve graph of the manganese iron lithium phosphate positive electrode material prepared in Examples 1-3 and Comparative Examples 1-3 provided in the embodiments of the present application under 0.1C and 1C conditions;

[0047] Figure 6 is the discharge specific capacity of the manganese iron lithium phosphate positive electrode material prepared in Examples 1-3 and Comparative Examples 1-3 provided in the embodiments of the present application when cycled under 1C conditions;

[0048] Figure 7 is the discharge manganese platform retention rate of the manganese iron lithium phosphate positive electrode material prepared in Examples 1-3 and Comparative Examples 1-3 provided in the embodiments of the present application when cycled under 1C conditions;

[0049] Figure 8 is the discharge specific capacity of the lithium iron manganese phosphate positive electrode material prepared in Examples 1-3 and Comparative Examples 1-3 of the present application under 1C condition at 40°C after cycling;

[0050] Figure 9 is the discharge manganese platform retention rate of the lithium iron manganese phosphate positive electrode material prepared in Examples 1-3 and Comparative Examples 1-3 of the present application under 1C condition at 40°C after cycling;

[0051] Figure 10 is the charge-discharge curve of the lithium iron manganese phosphate positive electrode material prepared in Examples 1-3 of the present application under 0.1C, 0.5C, 1C, 2C, 3C conditions;

[0052] Figure 11 is the manganese section discharge platform proportion of the lithium iron manganese phosphate positive electrode material prepared in Examples 1-3 of the present application under 0.1C, 0.5C, 1C, 2C, 3C conditions. DETAILED DESCRIPTION

[0053] In order to make the technical problems to be solved by the present application, the technical solutions and beneficial effects clearer and more apparent, the present application will be further described in detail below with reference to the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and not intended to limit the present application.

[0054] In the present application, the term "and / or" describes the association relationship of the associated objects, which means that there can be three kinds of relationships, for example, A and / or B can mean that A exists alone, A and B exist together, and B exists alone. Wherein A and B can be singular or plural. The character " / " generally represents an "or" relationship between the associated objects before and after it.

[0055] In the present application, "at least one" means one or more, and "multiple" means two or more. "At least one of the following" or similar expressions means any combination of these items, including any combination of single item or multiple items. For example, "at least one of a, b or c", or "at least one of a, b and c", can mean a, b, c, a-b (i.e. a and b), a-c, b-c, or a-b-c, wherein a, b and c can be single or multiple.

[0056] It should be understood that in various embodiments of the present application, the size of the sequence number of the above-mentioned processes does not mean the order of execution, and part or all of the steps can be executed in parallel or in sequence, and the execution order of each process should be determined according to its function and inherent logic, and should not constitute any limitation on the implementation process of the embodiments of the present application.

[0057] The terminology used in the embodiments of the present application is for the purpose of describing particular embodiments only and is not intended to be limiting of the present application. As used in the description of the embodiments of the present application and the appended claims, the singular forms "a," "an," and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise.

[0058] The weight of the related components mentioned in the embodiments of the present application can refer to the specific content of each component, but also can represent the proportional relationship between the weights of each component. Therefore, as long as the content of the related components in the embodiments of the present application is proportionally enlarged or reduced, it is within the scope disclosed in the embodiments of the present application. Specifically, the mass in the embodiments of the present application can be μg, mg, g, kg and other mass units commonly known in the chemical field.

[0059] The terms "first", "second", etc. are only used for the purpose of description and are used to distinguish objects such as substances from each other, and cannot be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. For example, without departing from the scope of the embodiments of the present application, the first XX can also be referred to as the second XX, and similarly, the second XX can also be referred to as the first XX. Therefore, the features limited by "first" and "second" can explicitly or implicitly include one or more of the features.

[0060] The first aspect of the embodiments of the present application provides a preparation method of a manganese phosphate salt-based positive electrode material, as shown in the accompanying drawings, comprising the following steps: Figure 1 The first aspect of the embodiments of the present application provides a preparation method of a manganese phosphate salt-based positive electrode material, as shown in the accompanying drawings, comprising the following steps:

[0061] S10. Obtain raw material components including a phosphorus source, a manganese source, a lithium source, other metal sources and an initial carbon source, dry the first mixed slurry prepared by mixing the raw material components with a first solvent to obtain a primary mixed material; the mass percentage content of the initial carbon source in the raw material components is 1% to 3%;

[0062] S20. Perform a primary sintering treatment on the mixed material under an inert atmosphere to obtain a primary sintered material;

[0063] S30. Calculate the amount of additional carbon source required according to the carbon content in the primary sintered material and the target carbon content in the finished product, dry the second mixed slurry prepared by mixing the primary sintered material, the additional carbon source and a second solvent to obtain a secondary mixed material; wherein the initial carbon source and the additional carbon source each independently include a polymer carbon source and a short-chain carbon source;

[0064] S40. After performing a secondary sintering treatment on the secondary mixed material under an inert atmosphere, the finished product, i.e. the manganese phosphate salt-based positive electrode material, is obtained by crushing.

[0065] The preparation method of the manganese phosphate salt-based positive electrode material provided by the first aspect of the embodiments of the present application has at least the following beneficial effects:

[0066] Firstly, the carbon source is added in the first sintering process and the second sintering process, respectively, so that the construction of the uniform carbon coating layer is realized by a step-by-step carbon source adding method, the free carbon in the finished product is effectively reduced, the carbon source is more fully utilized, and the graphitization degree of the carbon coating layer is improved. This can inhibit the dissolution of Mn, Fe and other metal ions in the manganese phosphate salt-based positive electrode material in the cycle process, and is conducive to improving the manganese platform retention rate of the manganese phosphate salt-based positive electrode material in the cycle process. Moreover, by step-by-step adding of the carbon source, it is found that the amount of the carbon source added before the first sintering process has an effect on the physical and chemical properties such as the tap density and specific surface area of the finished product, and an effect on the electrochemical properties such as the discharge specific capacity.

[0067] Secondly, the embodiment of the present application finds that the amount of the carbon content in the first sintering process has a very significant effect on the performance of the finished product, so the mass percentage of the initial carbon source in the raw material components is controlled to be 1% to 3% before the first sintering process, and the amount of the carbon source required for the second sintering process is calculated according to the carbon content in the first sintered material and the target carbon content in the finished product. By controlling the amount of the carbon source added before the first sintering process, the performance of the finished product such as the tap density, specific surface area and electrical conductivity can be effectively regulated, and this also has a very significant effect on the manganese platform retention rate. If too much carbon source is added before the first sintering process, then the amount of the carbon source added after the first sintering process is too small, which can easily lead to the formation of a large number of extremely small particles, resulting in serious agglomeration, and at this time the carbon supplement effect of the finished product is not good, which is not conducive to improving the performance of the finished product. If too little carbon source is added before the first sintering process, then the metal components such as the manganese source may not be fully reduced in the sintering process, which is not conducive to the formation of the manganese phosphate salt-based positive electrode material crystal nucleus, and thus there will be more impurities in the finished product prepared, so an appropriate amount of carbon source should be added before the first sintering process.

[0068] Thirdly, a mixed carbon source including a polymer carbon source and a short-chain carbon source is used, which can realize uniform carbon layer coating and has good electronic conductivity. The short-chain carbon source is a small molecule carbon source, which can improve the specific surface area of the material, increase the reaction sites and electronic conductivity, but may result in uneven coating and cannot effectively inhibit the dissolution of transition metals. The polymer carbon source is a high molecular polymer, which is conducive to the formation of a thin film-like carbon coating layer when used as a carbon source, can reduce the specific surface area of the material and improve the tap density of the material, but the electronic conductivity of the material will generally decrease. Therefore, the use of the mixed carbon source combines the advantages and disadvantages of the two carbon sources, so that the carbon source is fully utilized and the free carbon in the finished product is effectively reduced, and the use of the mixed carbon source can make the carbon coating layer thickness more uniform and the graphitization degree higher, which is conducive to inhibiting the dissolution of metal ions in the cycle process, reducing the side reactions of the negative electrode, and improving the stability of the battery.

[0069] Therefore, the preparation method of the manganese phosphate salt positive electrode material in the embodiments of the present application effectively improves the manganese platform retention rate of the prepared manganese phosphate salt positive electrode material in the normal temperature and high temperature cycle processes, greatly improves the specific discharge capacity of the material, and the rate performance is also very excellent, and the material has excellent double-charging and double-discharging capacity.

[0070] In the step S10, after the phosphorus source, the manganese source, the lithium source, the other metal source and the initial carbon source are dissolved and dispersed in the first solvent, a sand milling process is performed to mix the materials to prepare a first mixed slurry.

[0071] In some possible implementations, the first solvent includes water, which has good dissolving and dispersing performance for the components of the raw materials, and can avoid the influence of solvent residues on the subsequent sintering process and the performance of the finished product.

[0072] In some possible implementations, the initial carbon source includes a polymer carbon source and a short-chain carbon source. In this case, the mixed carbon source combines the advantages and disadvantages of the two carbon sources, so that the carbon source is fully utilized, the free carbon in the finished product is effectively reduced, the thickness of the carbon coating layer is more uniform, the graphitization degree is higher, which is beneficial to reducing the side reactions of the negative electrode and improving the electrochemical performance of the material.

[0073] In some possible implementations, the polymer carbon source includes at least one of polyethylene glycol, polyvinyl alcohol, polyacrylic acid, carboxymethyl cellulose and starch. When these high molecular polymers are used as carbon sources, they are beneficial to the formation of a thin film-shaped carbon coating layer, which can reduce the specific surface area of the material and improve the compaction density of the material.

[0074] In some possible implementations, the short-chain carbon source includes at least one of glucose, sucrose and citric acid. These small molecule carbon sources can improve the specific surface area of the material, increase the reaction sites and electronic conductivity.

[0075] In some embodiments, the mass ratio of the polymer carbon source and the short-chain carbon source can be 1:1, 1:2, 2:1, 1:3, 3:1, or any point value or interval value between any two point values.

[0076] In some possible implementations, the manganese source includes one or more of manganese chloride tetrahydrate, manganese sulfate, manganese nitrate, manganese acetate tetrahydrate and manganese dioxide.

[0077] In some possible implementations, the lithium source includes one or more of lithium dihydrogen phosphate, lithium hydroxide, lithium carbonate and lithium acetate.

[0078] In some possible implementations, the phosphorus source includes one of ammonium dihydrogen phosphate and lithium dihydrogen phosphate.

[0079] In some possible implementation manners, the other metal source includes at least one of an iron source, a niobium source, a titanium source, a cobalt source, a nickel source, and a vanadium source; and the other metal source can further provide the electrochemical performance of the manganese phosphate salt-based positive electrode material through doping.

[0080] In some possible implementation manners, the iron source includes one or more of iron sulfate, anhydrous ferric chloride, ferric chloride hexahydrate, ferric nitrate nonahydrate, iron oxide, and triiron tetroxide.

[0081] The raw material components used in the above embodiments have high purity, and the above raw material components have good solubility, which is beneficial to the preparation of the manganese phosphate salt-based positive electrode active material.

[0082] In some possible implementation manners, the molar ratio of the other metal source to the manganese source is (10-X):X, where X is 2-8, and specific values can be 8:2, 7:3, 6:4, 5:5, 4:6, 3:7, 2:8, or any interval value between any two of the above values.

[0083] In some possible implementation manners, the total molar amount of the manganese source and the other metal source to the molar amount of the lithium source and the phosphorus source is 1:(1-1.03):(1-1.03). In this case, the appropriate increase of the lithium source helps to refine the grain size and relieve the sintering and fusion between the grains, and helps to improve the rate discharge performance of the manganese phosphate salt-based positive electrode material. The appropriate increase of the phosphorus source relative to the manganese source and other metal sources promotes the growth of primary particles and reduces particle agglomeration.

[0084] In some possible implementation manners, the manganese phosphate salt-based positive electrode material is lithium iron manganese phosphate, and the iron source, the manganese source, the phosphorus source, the lithium source, and the initial carbon source are dispersed in deionized water, and then sand milling is performed to uniformly mix the materials to obtain a first mixed slurry. The iron source and the manganese source are added in a manner that the molar ratio of Fe to Mn is 4:6; and the lithium source and the phosphorus source are added in a manner that Li:P:(Fe+Mn) = 1.03:1.03:1.0.

[0085] In some possible implementation manners, the mass percentage of the initial carbon source in the raw material component is 1.5% to 2.5%; specifically, it can be 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2.0%, 2.1%, 2.2%, 2.3%, 2.4%, 2.5%, or any interval value between any two point values. In this case, the carbon content added in the first sintering and the second sintering processes is more conducive to improving the coating effect of the carbon material on the active material, improving the particle size, specific surface area, morphology, and other characteristics of the manganese phosphate salt-based positive electrode material, and thus better improving the physical and chemical properties such as the tap density and active specific surface area of the manganese phosphate salt-based positive electrode material, and improving the electrochemical properties such as the capacity and cycle stability of the manganese phosphate salt-based positive electrode material.

[0086] In some possible implementation manners, the first mixed slurry is prepared through a sanding process, and the particle size D50 of the material in the slurry is 0.6 μm to 0.9 μm; specifically, it can be 0.6 μm, 0.7 μm, 0.8 μm, 0.9 μm, or any interval value between any two point values. In this case, the sanding method is used for mixing the material, and the sanding process ensures the uniformity of the material mixing and also can control the particle size distribution of the material. Moreover, the particle size D50 of the material in the slurry is 0.6 μm to 0.9 μm, which can ensure sufficient mixing of the raw materials and also can make the smallest particle debris as few as possible, because the more the sanding times, the greater the difference between the particle sizes of large and small particles. The qualified particle size in the sanding process can make the particle size of the finished product more concentrated and uniform.

[0087] In some possible implementation manners, the conditions for drying the first mixed slurry include: spray drying under the conditions that the atomization frequency is 250 Hz to 350 Hz and the inlet air temperature is 150°C to 250°C. In this case, the material is dried through spray drying and the morphology of the finished product particles is controlled to a certain extent; the spray drying process can control the morphology of the material, so that the sphericity is higher and the electrical performance is more favorable. For example, the atomization frequency of the spray drying can be 250 Hz, 260 Hz, 280 Hz, 300 Hz, 320 Hz, 350 Hz, or any interval value between any two point values; the inlet air temperature can be 150°C, 160°C, 180°C, 200°C, 220°C, 250°C, or any interval value between any two point values.

[0088] In the step S20, the following steps are performed:

[0089] In some possible implementations, the temperature rising rate of the first sintering process is 1-10 ℃ / min, the holding temperature is 400-600 ℃, and the holding time is 5-8 h. In this case, the manganese phosphate salt-based positive electrode material is preliminarily formed in the first sintering process, and the size of the particles and the discharge of volatile components can be well balanced. If the temperature is high or the sintering time is long, the crystal grows faster, and enough carbon is not added at the beginning to coat and limit the growth of the particles. Therefore, the particle size is large after the first sintering, which makes the average primary particle size (statistically obtained from the SEM image) of the finished product large and the difference between the large and small particles larger, which is not conducive to the electrochemical performance. On the contrary, if the temperature is low or the sintering time is short, the crystal nucleus is preliminarily generated, although the particles will not grow excessively, but the volatile components are not completely removed, the amount of carbon source supplemented during sanding will be reduced, and more volatile substances will continue to be generated during the second sintering, which is extremely unfavorable for compaction.

[0090] For example, the temperature rising rate of the first sintering process can be 1 ℃ / min, 2 ℃ / min, 3 ℃ / min, 4 ℃ / min, 5 ℃ / min, 6 ℃ / min, 7 ℃ / min, 8 ℃ / min, 9 ℃ / min, 10 ℃ / min, or any interval value between any two point values of the typical but non-limiting values; the holding temperature can be 400 ℃, 450 ℃, 500 ℃, 550 ℃, 600 ℃, or any interval value between any two point values of the typical but non-limiting values; and the holding time can be 5 h, 6 h, 7 h, 8 h, or any interval value between any two point values of the typical but non-limiting values.

[0091] In some possible implementations, the inert atmosphere in the first sintering process includes at least one of nitrogen, argon, and helium.

[0092] In some possible implementations, the carbon content in the first sintered material is 0.12%-0.36%. In this case, the carbon content in the first sintered material is conducive to regulating the performance of the finished product, such as compaction, specific surface area, and electrical conductivity, and also has a very significant impact on the manganese platform retention rate. For example, the carbon content in the first sintered material can be 0.12%, 0.15%, 0.18%, 0.20%, 0.23%, 0.25%, 0.28%, 0.30%, 0.33%, 0.36%, or any interval value between any two point values of the typical but non-limiting values.

[0093] In the step S30, the carbon content in the primary sintered material is detected, and then the amount of the carbon source to be added is calculated according to the carbon content in the primary sintered material and the target carbon content in the finished product. The finished product refers to the target product, i.e., the manganese phosphate salt-based positive electrode material. The amount of the carbon source added is positively correlated with the carbon content in the finished product. After the carbon content in the primary sintered material is detected, the amount of the carbon source to be added can be calculated according to the target carbon content in the finished product. Based on this, the amount of the carbon source to be added in the secondary sintering process can be obtained.

[0094] In some embodiments, the amount of the carbon source to be added can be calculated according to the formula: added carbon source = mass of the primary sintered material * (target carbon content in the finished product - carbon content in the primary sintered material) * K, wherein K is an adjustment coefficient for adjusting the original data to eliminate the influence of different dimensions, data ranges or system errors on the calculation results. Through the adjustment coefficient, the data from different sources or under different conditions can be made comparable, and the accuracy and reliability of the analysis can be improved. For example, K = 2-10.

[0095] In some possible implementations, the target carbon content in the finished product is 1.6%-1.65%, and can be 1.6%, 1.61%, 1.62%, 1.63%, 1.64%, 1.65% or any interval value between any two point values. In this case, the carbon content in the finished product can form a complete and uniform coating layer, improve the particle size, specific surface area, morphology and other characteristics of the manganese phosphate salt-based positive electrode material, thereby improving the compaction density, active specific surface area, cycle stability and other performances of the manganese phosphate salt-based positive electrode material, and also being conducive to ensuring the capacity of the manganese phosphate salt-based positive electrode material.

[0096] In some possible implementations, the amount of the carbon source to be added is calculated according to the carbon content in the primary sintered material and the target carbon content in the finished product, and the amount of the carbon source to be added is 8%-12% of the total mass of the primary sintered material and the added carbon source. In this case, the amount of the carbon source to be added is conducive to forming a complete carbon coating layer, improving the morphology of the manganese phosphate salt-based positive electrode material, and controlling the actual carbon content in the manganese phosphate salt-based positive electrode material to about 1.6%, thereby improving the electrochemical performance of the manganese phosphate salt-based positive electrode material. For example, the amount of the carbon source to be added is 8%, 9%, 10%, 11%, 12% or any interval value between any two point values of the total mass of the primary sintered material and the added carbon source.

[0097] In some possible implementations, the second solvent includes water, which has good dissolving and dispersing performance for the primary sintered material and the added carbon source.

[0098] In some possible implementations, the additional carbon source includes a polymer carbon source and a short-chain carbon source. In some possible implementations, the polymer carbon source includes at least one of polyethylene glycol, polyvinyl alcohol, polyacrylic acid, carboxymethyl cellulose, and starch. In some possible implementations, the short-chain carbon source includes at least one of glucose, sucrose, and citric acid. In this case, the mixed carbon source adopted combines the advantages and disadvantages of the two carbon sources, so that the carbon source is fully utilized, the free carbon in the product is effectively reduced, the carbon coating layer is more uniform in thickness, the graphitization degree is higher, and the side reactions of the negative electrode are reduced, and the electrochemical performance of the material is improved.

[0099] In some possible implementations, the second mixed slurry is prepared by sanding process, and the particle size D50 of the material in the slurry is 0.6 μm to 0.9 μm. Specifically, it can be 0.6 μm, 0.7 μm, 0.8 μm, 0.9 μm, or any interval value between any two point values. In this case, the sanding method is used to mix the material, and the sanding process ensures the uniformity of the material mixing and also can control the particle size distribution of the material. Moreover, the particle size D50 of the material in the slurry is 0.6 μm to 0.9 μm, which can ensure sufficient mixing of the raw materials and also can minimize the amount of small particle debris, because the more the sanding frequency, the greater the difference between the particle sizes of large and small particles. The qualified particle size in the sanding range can make the particle size of the finished product more concentrated and uniform.

[0100] In some possible implementations, the drying conditions of the second mixed slurry include: spray drying under the conditions that the atomization frequency is 250 Hz to 350 Hz and the inlet air temperature is 150°C to 250°C. In this case, the material is dried by spray drying and the morphology of the finished product particles is controlled to a certain extent; the spray drying process can control the morphology of the material to make it more spherical, which is more conducive to the performance of the electrical performance. For example, the atomization frequency of the spray drying can be 250 Hz, 260 Hz, 280 Hz, 300 Hz, 320 Hz, 350 Hz, or any interval value between any two point values. The inlet air temperature can be 150°C, 160°C, 180°C, 200°C, 220°C, 250°C, or any interval value between any two point values.

[0101] In the above step S40:

[0102] In some possible implementations, the heating rate of the secondary sintering process is 1°C / min to 10°C / min, the holding temperature is 600°C to 800°C, and the time length is 6 h to 10 h. In this case, the lithium manganese iron phosphate crystal can be more complete, the crystallinity is higher, and the graphitization degree of the carbon coating layer is improved.

[0103] Exemplarily, the heating rate of the secondary sintering process can be 1℃ / min, 2℃ / min, 3℃ / min, 4℃ / min, 5℃ / min, 6℃ / min, 7℃ / min, 8℃ / min, 9℃ / min, 10℃ / min, or any interval value between any two point values of the above typical but non-limiting point values; the holding temperature can be 600℃, 650℃, 700℃, 750℃, 800℃, or any interval value between any two point values of the above typical but non-limiting point values; and the holding time can be 6h, 7h, 8h, 9h, 10h, or any interval value between any two point values of the above typical but non-limiting point values.

[0104] In some possible implementations, the inert atmosphere in the secondary sintering process includes at least one of nitrogen, argon, and helium.

[0105] In some possible implementations, the sintered product is crushed by an air flow mill. In some embodiments, the frequency of the classification wheel of the air flow mill is between 40 Hz and 60 Hz.

[0106] In some possible implementations, the actual carbon content in the manganese phosphate salt-based positive electrode material is 1.4% to 1.6%. In this case, the actual carbon content in the manganese phosphate salt-based positive electrode material can form a complete and uniform coating layer, improve the particle size, specific surface area, morphology, and other characteristics of the manganese phosphate salt-based positive electrode material, thereby improving the tap density, active specific surface area, cycle stability, and other performances of the manganese phosphate salt-based positive electrode material, and also be conducive to ensuring the capacity of the manganese phosphate salt-based positive electrode material. Exemplarily, the actual carbon content in the manganese phosphate salt-based positive electrode material can be 1.4%, 1.5%, 1.6%, or any interval value between any two point values of the above typical but non-limiting point values.

[0107] In some possible implementations, the D50 particle size of the manganese phosphate salt-based positive electrode material is 0.9 μm to 1.5 μm. In this case, the particle size of the manganese phosphate salt-based positive electrode material is small, the active specific surface area is high, and the ion insertion and extraction are facilitated, thereby improving the rate performance of the manganese phosphate salt-based positive electrode material. Exemplarily, the D50 particle size of the manganese phosphate salt-based positive electrode material can be 0.9 μm, 0.933 μm, 1 μm, 1.13 μm, 1.2 μm, 1.3 μm, 1.35 μm, 1.4 μm, 1.5 μm, or any interval value between any two point values of the above typical but non-limiting point values.

[0108] In some possible implementations, the tap density of the manganese phosphate salt-based positive electrode material is greater than 2.19 g / cm 3; in this case, the compaction density of the manganese phosphate salt-based positive electrode material is high, which is conducive to improving the capacity of the manganese phosphate salt-based positive electrode material and providing the energy density of the positive electrode sheet. For example, the compaction density of the manganese phosphate salt-based positive electrode material can be 2.199 g / cm 3 , 2.319 g / cm 3 , 2.269 g / cm 3 , 2.49 g / cm 3 , 2.59 g / cm 3 , 2.69 g / cm 3 , 2.79 g / cm 3 , 2.89 g / cm 3 , 2.99 g / cm 3 , 3.19 g / cm 3 , 3.39 g / cm 3 , 3.59 g / cm 3 , or an interval value between any two point values.

[0109] In some possible implementation manners, the specific surface area of the manganese phosphate salt-based positive electrode material is 15 m 2 / g to 18 m 2 / g; in this case, the manganese phosphate salt-based positive electrode material has a higher specific surface area, which improves the working efficiency of the material and also makes the manganese phosphate salt-based positive electrode material have better electrode sheet processing performance. For example, the specific surface area of the manganese phosphate salt-based positive electrode material can be 15 m 2 / g, 16 m 2 / g, 17 m 2 / g, 18 m 2 / g, or an interval value between any two point values.

[0110] In some possible implementation manners, the resistivity of the manganese phosphate salt-based positive electrode material is lower than 200 Ω·cm. In this case, the manganese phosphate salt-based positive electrode material has low resistivity and high conductivity, which is conducive to ion and electron transmission and improves the charge and discharge rate performance of the manganese phosphate salt-based positive electrode material. For example, the resistivity of the manganese phosphate salt-based positive electrode material can be 197.2 Ω·cm, 180 Ω·cm, 170 Ω·cm, 160 Ω·cm, 150 Ω·cm, 120 Ω·cm, 100 Ω·cm, 80 Ω·cm, 60 Ω·cm, 51.1 Ω·cm, 50 Ω·cm, 40 Ω·cm, or an interval value between any two point values.

[0111] In a second aspect, the embodiments of the present application provide a manganese phosphate salt-based positive electrode material prepared by the above method, which comprises a manganese phosphate salt-based active core and a carbon layer coated on the outer surface of the core; the carbon layer is prepared from a polymer carbon source and a short-chain carbon source.

[0112] The manganese phosphate-based cathode material of this application is prepared by the above method, comprising a manganese phosphate-based active core and a carbon layer coated on the outer surface of the core; the carbon layer is prepared by a polymer carbon source and a short-chain carbon source. The uniform and stable coating of the carbon layer on the surface of the manganese phosphate-based active core particles not only improves the structural stability of the manganese phosphate-based cathode material, but also enhances its conductivity, cycle stability, and the stability of the interface between the cathode material and the electrolyte, mitigating stress caused by volume changes. These factors are extremely beneficial to the manganese plateau retention rate during cycling, improving the manganese plateau retention rate during both room temperature and high temperature cycling, significantly increasing the material's discharge specific capacity, and exhibiting excellent rate performance with outstanding charge-discharge capability, thus significantly improving the electrochemical performance of the manganese phosphate-based cathode material.

[0113] In some possible implementations, the manganese phosphate active core includes lithium manganese iron phosphate, which combines the high safety of lithium iron phosphate with the high voltage characteristics of lithium manganese phosphate.

[0114] In some possible implementations, the carbon layer thickness in the manganese phosphate-based cathode material is 1 nm to 5 nm. This is beneficial for improving the structure and cycle stability of the manganese phosphate-based cathode material, as well as its electrochemical performance, such as conductivity. For example, the carbon layer thickness in the manganese phosphate-based cathode material can be any typical but non-limiting value, such as 1 nm, 2 nm, 3 nm, 4 nm, or 5 nm, or a range between any two values.

[0115] Thirdly, embodiments of this application provide a positive electrode sheet, including a current collector and a positive electrode active layer formed on the surface of the current collector, wherein the positive electrode active layer contains the aforementioned manganese phosphate-based positive electrode material, or a manganese phosphate-based positive electrode material prepared by the aforementioned method.

[0116] In the positive electrode of this application embodiment, the aforementioned manganese phosphate-based positive electrode material is used in the positive electrode active layer. This manganese phosphate-based positive electrode material includes a manganese phosphate-based active core and a carbon layer covering the outer surface, which improves the conductivity, structural stability, and capacity of the manganese phosphate-based positive electrode material. Therefore, the stability, energy density, rate performance, cycle performance, and other electrochemical performance of the positive electrode are improved.

[0117] In some possible implementations, the preparation of the positive electrode active layer includes the following steps: mixing the above-mentioned manganese phosphate-based positive electrode material, conductive agent and binder to form an electrode slurry, coating the electrode slurry onto the current collector, and then preparing the positive electrode sheet through steps such as drying, rolling and die cutting.

[0118] In some possible implementation manners, the mass percentage of the manganese phosphate salt-based positive electrode material in the positive electrode active layer of the positive electrode sheet is 90% to 95%. Specifically, the mass percentage of the manganese phosphate salt-based positive electrode material in the positive electrode active material layer can be 90%, 91%, 92%, 93%, 94%, 95%, or the like.

[0119] In some possible implementation manners, the current collector of the positive electrode sheet includes but is not limited to any one of a copper foil and an aluminum foil.

[0120] In some possible implementation manners, the content of the binder in the positive electrode sheet active material layer is 2 wt% to 5 wt%. In specific embodiments, the content of the binder can be 2 wt%, 3 wt%, 4 wt%, 5 wt%, or the like typical but non-limiting content.

[0121] In some possible implementation manners, the binder includes one or more of polyvinylidene chloride, soluble polytetrafluoroethylene, butadiene rubber, hydroxypropyl methyl cellulose, methyl cellulose, carboxymethyl cellulose, polyvinyl alcohol, acrylonitrile copolymer, sodium alginate, chitosan, and a chitosan derivative.

[0122] In some possible implementation manners, the content of the conductive agent in the positive electrode sheet active material layer is 1 wt% to 5 wt%. In specific embodiments, the content of the conductive agent can be 3 wt%, 4 wt%, 5 wt%, or the like typical but non-limiting content.

[0123] In some possible implementation manners, the conductive agent includes one or more of graphite, carbon black, acetylene black, graphene, carbon fiber, C 60 , and a carbon nanotube.

[0124] In a fourth aspect, the embodiments of the present application provide a secondary battery, which contains the above positive electrode sheet.

[0125] The secondary battery provided by the embodiments of the present application has good stability, high energy density, good rate performance, and good cycle stability, and thus the energy density and the electrochemical performance such as cycle stability of the secondary battery are improved.

[0126] The negative electrode sheet, the electrolyte, and the separator in the secondary battery of the embodiments of the present application are not specifically limited and can be applied to any battery system.

[0127] In some possible implementation manners, the negative electrode active material of the secondary battery includes but is not limited to a carbon material such as graphite, soft carbon (such as coke), hard carbon, or a nitride, a tin-based oxide, a tin-based oxide, a tin alloy, and a nano negative electrode material. The current collector includes but is not limited to any one of a copper foil and an aluminum foil.

[0128] In some possible embodiments, the step of manufacturing the negative electrode sheet comprises: mixing the negative electrode active material, the conductive agent such as conductive carbon black, the binder such as carboxymethyl cellulose and butadiene rubber, and the solvent such as water in a mass ratio of (80-99):(1-5):(2-10):100 to form a negative electrode mixed slurry, vacuum degassing, discharging, coating on a coating machine, rolling, slitting, and die cutting to obtain the negative electrode sheet.

[0129] In some possible embodiments, the separator can block electrons and allow ions to pass. For example, the separator can include, but is not limited to, at least one material selected from the group consisting of polypropylene fiber, polyacrylonitrile fiber, polyvinyl formal fiber, poly(ethylene terephthalate), polyethylene terephthalate, polyamide fiber, and poly(p-phenyleneterephthalamide).

[0130] In some possible embodiments, the electrolyte can include at least one soluble metal salt. In some specific embodiments, the metal salt can include at least one of LiClO4, LiBF4, LiPF6, LiAsF6, LiCF3SO3, LiTDI, Li[(CF3SO2)2N], Li[(FSO2)2N], Li[(C m F 2m+1 SO2)(C n F 2n+1 SO2)N], wherein m and n are natural numbers. These electrolyte salts can ensure high ionic conductivity of the electrolyte, and do not cause harmful side reactions with the electrode material, the electrolyte, the separator, and the like, and have good chemical stability.

[0131] In some possible embodiments, the secondary battery can include at least one of a battery cell, a battery module, and a battery pack.

[0132] In some possible embodiments, the battery cell type can include a lithium ion battery, and a new battery such as a lithium air battery and a lithium metal battery.

[0133] In some possible embodiments, the battery cell of the present application can be assembled into a battery module, and the number of battery cells included in the battery module can be multiple, and the specific number can be adjusted according to the application and capacity of the battery module. Further, the battery module can further include a housing having an accommodation space, and the plurality of battery cells are accommodated in the accommodation space.

[0134] In some possible embodiments, the battery cell and / or the battery module can be further assembled into a battery pack, and the number of battery cells or battery modules included in the battery pack can be adjusted according to the application and capacity of the battery pack.

[0135] For the above-mentioned implementation details and operations of the present application to be clearly understood by those skilled in the art, and the significant performance of the manganese phosphate salt positive electrode material and its preparation method and application of the embodiments of the present application, the following will be illustrated by multiple examples.

[0136] Example 1

[0137] A lithium manganese iron phosphate, the preparation comprising steps of:

[0138] 1. 276.18 g (1.83 mol) of iron phosphate and 238.81 g (2.75 mol) of manganese dioxide are added to 3 kg of deionized water, and after being fully stirred with a tool for 3-5 min, 169.13 g (2.29 mol) of lithium carbonate is added, and then the tool is fully stirred for 3-5 min, and then 10 g of glucose and 10 g of polyvinyl alcohol are added, and the tool is fully mixed and uniform, and finally 316.33 g (2.75 mol) of ammonium dihydrogen phosphate is slowly added, and bubbles can be seen after being added, which is because the reaction between ammonium dihydrogen phosphate and lithium carbonate generates CO2 gas, the designed LMFP has Mn:Fe=6:4, the solid content is about 30%, the mass of the carbon source added is 2% of the total mass of the feed, and the mass ratio of the two carbon sources is 1:1. After the reaction is completed and the material is fully mixed, sand milling is started, the feeding speed is controlled by adjusting the peristaltic pump, and the sand milling particle size D50 is controlled to be between 0.6-0.9 μm. The sand-milled material is spray dried, the feeding speed is controlled by adjusting the diaphragm pump, the atomization frequency is adjusted to 280 Hz, the inlet air temperature is 200°C, and the material is dried.

[0139] 2. The dried material is placed in a large boat, and sintering is carried out in a tube furnace under the protection of a nitrogen atmosphere, the temperature is raised from room temperature to 500°C at a rate of 5°C / min, and sintering is carried out for 6 h, and after the temperature is lowered to room temperature, the primary sintered sample is taken out.

[0140] 3. The carbon content of the primary sintered sample is tested, the amount of carbon source to be added is calculated according to the carbon content of 1.6% of the finished product, glucose and polyvinyl alcohol are still added in a mass ratio of 1:1, and 10% of the mass of the feed is added as carbon source, i.e. 50 g of glucose and 50 g of polyvinyl alcohol. The mixed primary sintered sample and the added carbon source are continuously subjected to sand milling, and the sand milling particle size D50 is controlled to be between 0.6 μm and 0.9 μm. The sand-milled material is spray dried, the feeding speed is controlled by adjusting the diaphragm pump, the atomization frequency is adjusted to 280 Hz, the inlet air temperature is 200°C, and the material is dried.

[0141] 4. Put the dried material into a large boat, and sinter in a tube furnace under the protection of nitrogen atmosphere. The temperature is raised from room temperature to 750℃ at a rate of 5℃ / min, and sinter for 8h. After the temperature drops to room temperature, the sample is taken out, and the second sintered sample is obtained. The second sintered sample is pulverized by airflow mill, and the classification wheel frequency is controlled at 40Hz. The pulverized sample is the finished product of lithium manganese iron phosphate.

[0142] Example 2

[0143] A lithium manganese iron phosphate, which is different from example 1 in that the mass of the carbon source added in step 1 is 1% of the total mass of the feed, i.e. 5g of glucose and 5g of polyvinyl alcohol are added. Correspondingly, in step 3, 11% of the mass of the feed is added as a carbon source, i.e. 55g of glucose and 55g of polyvinyl alcohol.

[0144] Example 3

[0145] A lithium manganese iron phosphate, which is different from example 1 in that the mass of the carbon source added in step 1 is 3% of the total mass of the feed, i.e. 15g of glucose and 15g of polyvinyl alcohol are added. Correspondingly, in step 3, 9% of the mass of the feed is added as a carbon source, i.e. 45g of glucose and 45g of polyvinyl alcohol.

[0146] Comparative Example 1

[0147] A lithium manganese iron phosphate, the preparation of which comprises the steps of:

[0148] 1. Add 276.18g (1.83mol) of iron phosphate and 238.81g (2.75mol) of manganese dioxide to 3kg of deionized water, and mix well with a tool for 3-5min. Then add 169.13g (2.29mol) of lithium carbonate, and mix well with a tool for 3-5min. Then add a certain amount of glucose and a certain amount of polyvinyl alcohol, and mix well with a tool. Finally, slowly add 316.33g (2.75mol) of ammonium dihydrogen phosphate. It can be seen that bubbles are generated after the addition, because the reaction between ammonium dihydrogen phosphate and lithium carbonate generates CO2 gas. The design of the LMFP is Mn:Fe=6:4, and the solid content is about 30%. The mass of the carbon source required is calculated according to the carbon content of 1.6% in the finished product, and the mass ratio of glucose to polyvinyl alcohol is 1:1. After the reaction is completed and the material is well mixed, sand milling is started. The feeding speed is controlled by adjusting the peristaltic pump, and the particle size D50 of the sand milling is controlled between 0.6μm and 0.9μm. The sand-milled material is spray dried. The feeding speed is controlled by adjusting the diaphragm pump, and the atomization frequency is adjusted to 280Hz. The inlet air temperature is 200℃, and the material is dried until it is completely dried.

[0149] 2, the dried material is placed in a large boat, sintered in a tube furnace under the protection of nitrogen atmosphere, the temperature is raised to 500℃ from room temperature at a rate of 5℃ / min, sintered for 6h, and then taken out after the temperature drops to room temperature to obtain the first sintered sample.

[0150] 3, the first sintered material is placed in a large boat after simple crushing by a high mixer, sintered in a tube furnace under the protection of nitrogen atmosphere, the temperature is raised to 750℃ from room temperature at a rate of 5℃ / min, sintered for 8h, and then taken out after the temperature drops to room temperature to obtain the second sintered sample.

[0151] 4, the second sintered sample is crushed by an air jet mill, and the frequency of the classification wheel is controlled at 40Hz, and the crushed sample is the finished lithium manganese iron phosphate product.

[0152] Comparative Example 2

[0153] A lithium manganese iron phosphate, which is different from Example 1 in that only glucose is added as the carbon source in step 1, i.e. the addition amount of glucose is 20g, and the remaining steps remain the same as Example 1.

[0154] Comparative Example 3

[0155] A lithium manganese iron phosphate, which is different from Example 1 in that only polyvinyl alcohol is added as the carbon source in step 1, i.e. the addition amount of polyvinyl alcohol is 20g, and the remaining steps remain the same as Example 1.

[0156] Further, in order to verify the progressiveness of the embodiments of the present application, the above examples and comparative examples are respectively subjected to the following performance tests:

[0157] 1, carbon content test of the first sintered sample: HCS-140 high-frequency infrared carbon and sulfur analyzer is used;

[0158] 2, carbon content test of the finished lithium manganese iron phosphate product: HCS-140 high-frequency infrared carbon and sulfur analyzer is used;

[0159] 3, particle size D50 test of the finished lithium manganese iron phosphate product: Malvern 3000 laser particle size analyzer is used;

[0160] 4, compactness density test of the finished lithium manganese iron phosphate product: electronic pressure testing machine UTM7305 is used;

[0161] 5, specific surface area test of the finished lithium manganese iron phosphate product: JW-TQ-D4 degassing station of JW-DX dynamic adsorption specific surface instrument of Jingmiao Gaobo is used;

[0162] 6, resistivity test of the finished lithium manganese iron phosphate product: ST2742B type automatic powder resistivity tester is used;

[0163] The above test results are shown in Table 1 as follows:

[0164] Table 1

[0165]

[0166]

[0167] From the above Table 1 test results, it can be seen that the carbon content of the primary sintering sample has a significant effect on the conventional performance of the finished product. With the increase of the amount of carbon source added before primary sintering, the compaction of the material continuously decreases, the specific surface area first decreases and then increases, the resistance continuously increases, and the D50 first decreases and then increases. Therefore, the amount of carbon source added before primary sintering can be used to adjust the various indicators of the finished product to adapt to various occasions. It can be seen that when the carbon source content added before primary sintering increases from 1% to 3% (i.e. Example 2 and Example 3), the compaction decreases by 5.19%, the resistivity increases by more than 5 times, and the D50 increases by 19.46%. This shows that the amount of carbon added before primary sintering has a relatively obvious effect on the conventional indicators of the finished product.

[0168] Among them, the resistivity of the finished product of Example 3 reaches 325.8 Ω·cm mainly because: the total amount of carbon added in Step 1 and Step 3 of the application should be consistent, and the amount of carbon source added in Step 3 has a greater effect on the resistivity, and the more the amount added, the lower the resistivity. In Example 3, the carbon source added in Step 1 is 3%, and the carbon source added in Step 3 is only 9%. The decrease in the amount of carbon source added in Step 3 leads to an increase in the resistivity of the finished product of Example 3.

[0169] At the same time, it is found that the resistivity of Comparative Example 1 is relatively large, and the compaction is the smallest among all samples, which shows that directly adding all carbon sources before primary sintering is not conducive to the conventional indicators of the finished product. Comparative Example 2 uses glucose as a single carbon source sample. It can be seen that the compaction is not high, and the specific surface area is large, which is not conducive to the processability of the material. Comparative Example 3 uses polyvinyl alcohol as a single carbon source sample. It can be seen that the compaction is good, but the specific surface is low. This is because the high molecular polymer coats a dense carbon layer on the surface of the particles during sintering, resulting in large finished product particles and low porosity, which is not conducive to the transmission of lithium ions.

[0170] 7. The X-ray diffraction patterns of the lithium manganese iron phosphate prepared in Examples 1-3 are shown in the following figure: Figure 2 As can be seen, the lithium manganese iron phosphate prepared in Examples 1-3 all match well with the standard card of lithium manganese phosphate with orthorhombic olivine structure (PDF #77-0178), proving the successful preparation of lithium manganese iron phosphate with good crystallinity, and proving that the method strategy is achievable.

[0171] 8. The scanning electron microscope images of the lithium manganese iron phosphate prepared in Examples 1-3 and Comparative Examples 1-3 are shown in the following figure: Figure 3As shown, the lithium manganese iron phosphate prepared in the examples all exhibit near-spherical or elliptical morphologies. Comparison reveals that the sample in Example 1 shows very little particle agglomeration, exhibits the best particle uniformity, and has the highest sphericity. The material particles in Example 2 are larger, resulting in a longer lithium-ion transport path, which to some extent hinders electrochemical performance. The particle agglomeration in Example 3 is relatively severe, which to some extent impedes the contact between the material and the electrolyte, also negatively impacting electrochemical performance.

[0172] However, the lithium manganese iron phosphate prepared in Comparative Example 1 exhibited severe agglomeration. This was due to the addition of a large amount of carbon source before the first sintering, resulting in numerous extremely small particles and severe agglomeration. In Comparative Example 2, only glucose, a short-chain carbon source, was added, resulting in an uneven carbon coating layer with significant differences in particle size, making the material prone to side reactions with the electrolyte. In Comparative Example 3, only polyvinyl alcohol, a high-molecular-weight polymer carbon source, was added, resulting in an overly dense carbon coating layer that reduced the specific surface area of ​​the material, thereby reducing the number of active sites. Simultaneously, the electronic conductivity of the material decreased significantly, leading to severe polarization during electrochemical reactions in the battery, which is highly detrimental to its electrical performance.

[0173] 9. Appendix Figure 4 The transmission electron microscope image of Example 1 clearly shows that it has a good carbon coating layer, which is the main reason for its high manganese plateau retention rate.

[0174] 10. To investigate the electrochemical performance of lithium manganese iron phosphate prepared in the examples and comparative examples, the LAND electrochemical testing system was used to test the electrochemical performance of the materials, specifically including the following steps:

[0175] ① First, the prepared lithium manganese iron phosphate cathode material is passed through a 200-mesh sieve, and then placed in a petri dish to dry for more than 2 hours at a drying temperature of 130℃ to fully remove moisture;

[0176] ② The dried lithium manganese iron phosphate cathode material is mixed with conductive carbon black, polyvinylidene fluoride (PVDF), dispersant and N-methylpyrrolidone (NMP) in a 100mL ball mill jar. Zirconia beads are selected for the ball milling, and the beads of different sizes are mixed in a 1:1 ratio. The mixture is stirred in a ball mill at a rate of 450r / min for 3h to obtain the cathode slurry for lithium-ion batteries with an active material content of 91%.

[0177] ③ The positive electrode slurry is evenly coated onto the aluminum foil with a carbon layer by a coating machine, then dried at 120°C for 10 minutes, and then rolled under 12MPa pressure. The appropriate positive electrode sheet is then obtained by a sheet-making machine of a certain specification. The active material content is weighed and calculated to calculate the subsequent charge and discharge specific capacity.

[0178] (4) The prepared positive electrode sheet is fixed on the positive electrode shell with glue to avoid the occurrence of electrode sheet deviation phenomenon when the positive electrode shell is finally placed. After fixing, dry at 130°C for more than two hours to completely remove the dispersing agent NMP. At the same time, the negative electrode shell, spring and steel sheet are ultrasonically washed and dried for use;

[0179] (5) Start assembling the button cell in the glove box, make sure that the moisture content and oxygen content are below 0.5 ppm. First, place the negative electrode shell at the bottom, then place the spring, steel sheet and lithium sheet negative electrode in sequence, then add 80 μL of LiPF6 electrolyte dissolved in a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC), then place the Celgard 2500 separator, then add 80 μL of electrolyte, and then place the positive electrode shell with the positive electrode sheet. Finally, place the preliminarily assembled battery in the tablet press and set the pressure to 650 Kg, the speed switching to 4.0 s, the pressure holding to 1.5 s, the battery placed in the middle, the negative electrode on top, and the battery assembled after wiping off the excess electrolyte.

[0180] (6) Use the LAND electrochemical test system to test the electrochemical performance of the button cell, with the discharge cutoff voltage set to 2.0 V, the charge cutoff voltage set to 4.3 V, and the nominal specific capacity set to 150 mAh g -1 Test the charge-discharge curves of the lithium manganese iron phosphate positive electrode active material at 0.1 C and 1 C rates, the cycle curves at different temperatures at 1 C, and the rate curves at 0.1 C, 0.5 C, 1 C, 2 C and 3 C at room temperature.

[0181] The test results of the charge-discharge performance at 0.1 C and 1 C at room temperature are shown in Table 2:

[0182] Table 2

[0183]

[0184] From the charge-discharge performance of each lithium manganese iron phosphate product at 0.1 C and 1 C at room temperature in Table 2 above, it can be seen that Example 1 with 2% carbon content before primary sintering has the best button cell performance. When the carbon content before primary sintering is increased from 1% to 3%, the button cell performance first increases and then decreases, but the performance of 3% addition is better than that of 1%. At the same time, it can also be observed that adding too much carbon source before primary sintering (Comparative Example 4), adding less carbon source before primary sintering (Comparative Example 5), adding all carbon sources before primary sintering (Comparative Example 1) or adding a single carbon source before primary sintering (Comparative Examples 2 and 3) are not conducive to the performance of the material.

[0185] In addition, the charge-discharge curves of the lithium iron manganese phosphate prepared in Examples 1-3 and Comparative Examples 1-3 under 0.1C and 1C conditions are shown in FIG. 1, and it can be seen that the lithium iron manganese phosphate prepared in Example 1 has the best performance of the coin half-cell under 0.1C and 1C conditions. Figure 5

[0186] FIG. 1 shows the charge-discharge curves of the lithium iron manganese phosphate prepared in Examples 1-3 and Comparative Examples 1-3 under 0.1C and 1C conditions. Figure 6 Figure 7 FIG. 2 shows the discharge specific capacity of the lithium iron manganese phosphate prepared in Examples 1-3 and Comparative Examples 1-3 when cycled under 1C conditions. It can be found that the discharge specific capacity of each material tends to be stable after a certain number of cycles, but the manganese platform retention rate thereof shows a continuous decay trend. Among them, the sample in Example 1 can still maintain a manganese platform discharge retention rate of 52.25% after 100 cycles, while the manganese platform retention rates of Examples 2-3, Comparative Examples 1-3 after cycling are 14.49%, 43.74%, 22.21%, 26.46%, and 23.47%, respectively. It can be seen that the sample prepared in Example 1 has a very obvious advantage.

[0187] FIG. 3 shows the discharge specific capacity of the lithium iron manganese phosphate prepared in Examples 1-3 and Comparative Examples 1-3 when cycled under 1C conditions at 40°C. Figure 8 Figure 9 FIG. 4 shows the discharge manganese platform retention rate of the lithium iron manganese phosphate prepared in Examples 1-3 and Comparative Examples 1-3 when cycled under 1C conditions at 40°C. It can be found that the discharge specific capacity of each material tends to be stable after a certain number of cycles, but the manganese platform retention rate thereof shows a continuous decay trend. Among them, the sample in Example 1 can still maintain a manganese platform discharge retention rate of 57.58% after 100 cycles, while the manganese platform retention rates of Examples 2-3, Comparative Examples 1-3 after cycling are 20.01%, 50.42%, 35.45%, 42.46%, and 33.21%, respectively. It can be seen that the sample prepared in Example 1 has a very obvious advantage.

[0188] The discharge specific energy is obtained by multiplying the discharge specific capacity by the discharge voltage. It can also be seen that the lithium iron manganese phosphate prepared in Example 1 has excellent performance, and the discharge specific capacity thereof hardly decays after cycling. It is also found that the sample prepared in Example 1 also hardly has a capacity climbing phenomenon, which indicates that the polarization degree of the material is small.

[0189] FIG. 5 shows the discharge specific energy of the lithium iron manganese phosphate prepared in Examples 1-3 and Comparative Examples 1-3 when cycled under 1C conditions at 40°C. Figure 10 ​​​The rate curve of examples 1-3 at 25℃ under 0.1C, 0.5C, 1C, 2C, 3C conditions; it can be seen that the capacity decay and manganese segment discharge platform ratio decay of example 1 at high rate are significantly smaller than those of example 2, and example 3 performs better at high temperature than at normal temperature, and the manganese segment discharge platform ratio decay is close to that of example 1.

[0190] The Figure 11 The manganese segment discharge platform retention rate change diagram of examples 1-3 at 25℃ under 0.1C, 0.5C, 1C, 2C, 3C conditions. It can be seen that the manganese segment discharge platform ratio of example 1 after increasing the rate is 62.56%, 62.59, 61.21%, 57%, and 52.77% respectively; the manganese segment discharge platform ratio of example 2 after increasing the rate is 64%, 57.4%, 47.77%, 36.03%, and 27.13% respectively; the manganese segment discharge platform ratio of example 3 after increasing the rate is 63.1%, 63.14%, 60.47%, 54.42%, and 48.92% respectively. It can be seen that the finished product prepared by example 1 has obvious advantages when charging and discharging at high rate.

[0191] The above only describes the preferred embodiments of the present application and is 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 manganese phosphate-based cathode material, characterized in that, Includes the following steps: A raw material composition including a phosphorus source, a manganese source, a lithium source, other metal sources and an initial carbon source is obtained. The raw material composition is then mixed with a first solvent to form a first mixed slurry, which is then dried to obtain a primary mixed material. The initial carbon source in the raw material components has a mass percentage content of 1% to 3%; the other metal sources include at least one of the following: iron source, niobium source, titanium source, cobalt source, nickel source, and vanadium source. The mixture is subjected to a single sintering process under an inert atmosphere to obtain a single-sintered material; the holding temperature of the single sintering process is 400℃~600℃. The required amount of additional carbon source is calculated based on the carbon content in the primary sintering material and the target carbon content in the finished product. The primary sintering material, the additional carbon source, and the second solvent are then mixed into a second slurry and dried to obtain a secondary mixture. The initial carbon source and the additional carbon source each independently include a polymer carbon source and a short-chain carbon source. The polymer carbon source includes at least one of polyethylene glycol, polyvinyl alcohol, polyacrylic acid, carboxymethyl cellulose, and starch. The short-chain carbon source includes at least one of glucose, sucrose, and citric acid. Under an inert atmosphere, the secondary mixture is subjected to a secondary sintering treatment, and then pulverized to obtain the finished product, namely, manganese phosphate-based cathode material; the holding temperature of the secondary sintering treatment is 600℃~800℃.

2. The method for preparing the manganese phosphate-based cathode material as described in claim 1, characterized in that, The initial carbon source in the raw material components has a mass percentage content of 1.5% to 2.5%. And / or, the carbon content in the primary sintering material is 0.12%~0.36%; And / or, the amount of the added carbon source is 8% to 12% of the total mass of the primary sintering material and the added carbon source; And / or, the target carbon content in the finished product is 1.6% to 1.65%.

3. The method for preparing the manganese phosphate-based cathode material as described in claim 2, characterized in that, The actual carbon content in the manganese phosphate-based cathode material is 1.4% to 1.6%. And / or, the D50 particle size of the manganese phosphate-based cathode material is 0.9 μm to 1.5 μm; And / or, the compaction density of the manganese phosphate-based cathode material is greater than 2.19 g / cm³. 3 ; And / or, the specific surface area of ​​the manganese phosphate-based cathode material is 15 m². 2 / g~18m 2 / g; And / or, the resistivity of the manganese phosphate-based cathode material is less than 200 Ω∙cm.

4. The method for preparing the manganese phosphate-based cathode material according to any one of claims 1 to 3, characterized in that, The first solvent and the second solvent each independently comprise water; And / or, the molar ratio of the other metal source to the manganese source is (10~X):X, where X takes the value of 2~8; And / or, the ratio of the total molar amount of the manganese source and the other metal sources to the molar amount of the lithium source and the phosphorus source is 1:(1~1.03):(1~1.03).

5. The method for preparing the manganese phosphate-based cathode material as described in claim 4, characterized in that, The first mixed slurry was prepared by a grinding process, and the particle size D50 of the material in the slurry was 0.6μm~0.9μm; And / or, the conditions for drying the first mixed slurry include: spray drying under the conditions of atomization frequency conversion of 250 Hz to 350 Hz and inlet air temperature of 150℃ to 250℃; And / or, the heating rate of the first sintering process is 1℃ / min to 10℃ / min, the holding temperature is 400℃ to 600℃, and the duration is 5h to 8h.

6. The method for preparing the manganese phosphate-based cathode material according to any one of claims 1 to 3 or 5, characterized in that, The second mixed slurry was prepared by a grinding process, and the particle size D50 of the material in the slurry was 0.6μm~0.9μm; And / or, the conditions for drying the second mixed slurry include: spray drying under the conditions of atomization frequency conversion of 250 Hz to 350 Hz and inlet air temperature of 150℃ to 250℃; And / or, the heating rate of the secondary sintering treatment is 1℃ / min to 10℃ / min, the holding temperature is 600℃ to 800℃, and the duration is 6h to 10h.

7. A manganese phosphate-based cathode material prepared by the method according to any one of claims 1 to 6, characterized in that, It includes a manganese phosphate-based active core and a carbon layer coating the outer surface of the core; the carbon layer is made from a polymer carbon source and a short-chain carbon source.

8. The manganese phosphate-based cathode material as described in claim 7, characterized in that, The manganese phosphate-based active core includes lithium iron manganese phosphate; And / or, the carbon layer thickness in the manganese phosphate-based cathode material is 1 nm to 5 nm.

9. A positive electrode plate, characterized in that, It includes a current collector and a positive electrode active layer formed on the surface of the current collector, wherein the positive electrode active layer contains a manganese phosphate-based positive electrode material as described in any one of claims 1 to 6, or a manganese phosphate-based positive electrode material prepared by the method described in any one of claims 7 to 8.

10. A secondary battery, characterized in that, The secondary battery includes the positive electrode as described in claim 9.

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