Lithium iron manganese phosphate positive electrode active material, preparation method thereof, positive electrode sheet, secondary battery and electric device

Abstract

The application provides a lithium iron manganese phosphate positive electrode active material and a preparation method thereof, a positive electrode sheet, a secondary battery and an electric device. The preparation method comprises the following steps: mixing, grinding and reacting an iron source, a solid base and an optional source of a doping element M at a low temperature to perform a solid phase reaction; after the grinding is completed, the obtained product is washed, impurities are removed, and the product is dried to obtain a nanoscale iron-containing oxide; the obtained nanoscale iron-containing oxide is mixed with a solvent, a lithium source, a manganese source, a phosphorus source, an optional source of a doping element N, an optional source of a doping element Q and an optional source of a doping element R according to a predetermined proportion, and then ground; after the grinding is completed, the product is obtained by spray drying and granulation to obtain a powder; and the obtained powder is sintered to obtain the lithium iron manganese phosphate positive electrode active material. The preparation method provided by the application can obtain the lithium iron manganese phosphate positive electrode active material with good electrochemical performance and high tap density.

Description

Technical Field

[0001] This application belongs to the field of battery technology, specifically relating to a lithium manganese iron phosphate positive electrode active material and its preparation method, positive electrode sheet, secondary battery and power device. Background Technology

[0002] In recent years, secondary batteries have been widely used in energy storage power systems such as hydropower, thermal power, wind power, and solar power plants, as well as in power tools, electric bicycles, electric motorcycles, electric cars, military equipment, aerospace, and many other fields. With the application and promotion of secondary batteries, their safety performance has received increasing attention. Lithium manganese iron phosphate (LFP) cathode active materials have become one of the most popular cathode active materials due to their advantages such as high capacity, good safety performance, and abundant raw material sources. However, currently prepared LFP cathode materials are insufficient to achieve both good electrochemical performance and high energy density in secondary batteries. Summary of the Invention

[0003] The purpose of this application is to provide a lithium manganese iron phosphate cathode active material and its preparation method, cathode sheet, secondary battery and power device, which can obtain a lithium manganese iron phosphate cathode active material that has both good electrochemical performance and high tap density, thereby enabling the secondary battery to have both good electrochemical performance and high energy density.

[0004] The first aspect of this application provides a method for preparing a lithium manganese iron phosphate cathode active material, comprising the following steps:

[0005] S1, Preparation of nanoscale iron-containing oxides by low-temperature solid-state reaction method:

[0006] The iron source, solid alkali, and optional dopant element M are mixed and ground to allow the components to undergo a low-temperature solid-phase reaction. After grinding, the obtained product is washed to remove impurities and dried to obtain nanoscale iron-containing oxides. The dopant element M represents a dopant element at the manganese and iron sites, and optionally includes at least one of Al, Cu, Mg, Zn, Ni, Ti, V, Zr, Co, Ga, Sn, Sb, Nb, and Ge.

[0007] S2, spray drying granulation:

[0008] The nano-sized iron oxide obtained in S1 is mixed with a solvent, a lithium source, a manganese source, a phosphorus source, a source of optional dopant element N, a source of optional dopant element Q, and a source of optional dopant element R in a predetermined ratio and then ground. After grinding, the mixture is spray-dried and granulated to obtain powder. The dopant element N represents a lithium site dopant element and optionally includes at least one of Zn, Al, Na, K, Mg, Nb, Mo, and W. The dopant element Q represents a phosphorus site dopant element and optionally includes at least one of B, S, Si, and N. The dopant element R represents an oxygen site dopant element and optionally includes at least one of S, F, Cl, and Br.

[0009] S3, sintering:

[0010] The powder obtained in S2 is sintered to obtain lithium manganese iron phosphate cathode active material.

[0011] In any embodiment of this application, in S1, the molar ratio of the iron source to the solid alkali is 1:(1-3).

[0012] In any embodiment of this application, in S1, the mixing and grinding time is 20 min to 60 min.

[0013] In any embodiment of this application, in S1, the washing and impurity removal process includes water washing and alcohol washing.

[0014] In any embodiment of this application, in S1, the drying is vacuum drying.

[0015] In any embodiment of this application, in S1, the drying temperature is below 100°C.

[0016] In any embodiment of this application, in S1, the drying time is 2h to 15h.

[0017] In any embodiment of this application, in S1, the mixing and grinding is carried out in a mortar or a ball mill jar. Optionally, the grinding speed of the mortar is 100 r / min; optionally, the ball mill rotation speed is 300 r / min to 800 r / min.

[0018] In any embodiment of this application, step S1, after mixing and grinding, further includes a settling step to allow the low-temperature solid-phase reaction to complete. Optionally, the settling time is 30 min to 120 min.

[0019] In any embodiment of this application, in step S1, a surfactant is further added and mixed and ground together with the iron source, the solid alkali, and the source of the dopant element M to adjust the morphology of the obtained nanoscale iron oxide. Optionally, the surfactant comprises polyethylene glycol; optionally, the amount of the surfactant added is less than 5% by weight, based on the total weight of the iron source, the solid alkali, and the source of the dopant element M.

[0020] In any embodiment of this application, in step S1, before mixing and grinding the iron source, the solid alkali, and the source of the dopant element M to induce a low-temperature solid-phase reaction, the method further includes: grinding the iron source, the solid alkali, and the source of the dopant element M separately to ensure the low-temperature solid-phase reaction proceeds sufficiently and to facilitate the acquisition of nanoscale iron-containing oxides with controllable morphology and size. Optionally, the grinding time for the iron source is 30 min to 90 min; optionally, the grinding time for the solid alkali is 30 min to 90 min; optionally, the grinding time for the source of the dopant element M is 30 min to 90 min.

[0021] In any embodiment of this application, in S1, the iron source is a ferric salt, optionally including at least one of ferric chloride, ferric nitrate and ferric sulfate.

[0022] In any embodiment of this application, in S1, the solid alkali includes at least one of sodium hydroxide, potassium hydroxide, sodium carbonate, and potassium carbonate, and optionally includes sodium hydroxide.

[0023] In any embodiment of this application, in S1, the source of the dopant element M includes at least one of the hydrochloride, nitrate, sulfate and acetate of the dopant element M.

[0024] In any embodiment of this application, in S2, the grinding is performed in a ball mill jar. Optionally, the ball milling speed is 300 r / min to 800 r / min; optionally, the ball milling time is 3 h to 24 h.

[0025] In any embodiment of this application, in S2, the spray drying temperature is 200°C to 250°C.

[0026] In any embodiment of this application, in S2, the solvent includes ethanol, water, or a mixture thereof.

[0027] In any embodiment of this application, in S2, the lithium source includes at least one of Li2CO3, LiOH, Li3PO4 and LiH2PO4.

[0028] In any embodiment of this application, in S2, the manganese source includes at least one of MnCO3, Mn(CH3COO)2 and MnC2O4.

[0029] In any embodiment of this application, in S2, the phosphorus source includes at least one of (NH4)2HPO4, NH4H2PO4, (NH4)3PO4, and H3PO4.

[0030] In any embodiment of this application, in S2, the source of the dopant element N includes at least one of the hydrochloride, nitrate, sulfate and acetate of the dopant element N.

[0031] In any embodiment of this application, in S2, the source of the dopant element Q includes at least one of the following: sulfate, borate, nitrate, and silicate of the dopant element Q.

[0032] In any embodiment of this application, in S2, the source of the dopant element R includes at least one of the elemental form of the dopant element R and an ammonium salt.

[0033] By selecting the sources of the aforementioned doping elements, the uniformity of their distribution can be improved, thereby enhancing the electrochemical performance of lithium manganese iron phosphate cathode active materials.

[0034] In any embodiment of this application, in step S2, a carbon source is added and mixed and ground together to obtain a carbon-coated lithium manganese iron phosphate cathode active material. Optionally, the carbon source includes at least one of organic carbon sources and inorganic carbon sources, and more preferably includes at least one of glucose, sucrose, starch, fructose, polyvinyl alcohol, polyethylene glycol, and citric acid.

[0035] In any embodiment of this application, in step S3, the sintering process includes the following steps: S31, pre-sintering the powder obtained in S2 at a low temperature of 350℃~550℃ in an air atmosphere or a protective gas atmosphere, and obtaining a pre-sintered material after the reaction is completed; S32, grinding the pre-sintered material obtained in S31 and then spray drying and granulating it to obtain a pre-sintered material powder; S33, sintering the pre-sintered material powder obtained in S32 at a high temperature of 650℃~750℃ in a protective gas atmosphere, thereby obtaining a lithium manganese iron phosphate cathode active material.

[0036] In any embodiment of this application, in S31, the low-temperature pre-sintering time is 2h to 10h.

[0037] In any embodiment of this application, in S33, the high-temperature sintering time is 2h to 10h.

[0038] In any embodiment of this application, in S32, the grinding is performed in a ball mill jar. Optionally, the ball milling speed is 300 r / min to 800 r / min; optionally, the ball milling time is 3 h to 24 h.

[0039] In any embodiment of this application, the preparation method further includes the step of: crushing the lithium manganese iron phosphate positive electrode active material obtained in S3 to the required particle size, optionally, the crushing is air jet crushing.

[0040] The second aspect of this application provides a lithium manganese iron phosphate cathode active material obtained by the preparation method of the first aspect of this application, which has the chemical formula Li a N b Fe x Mn y M 1-x-y P1- m Q m O 4-n R n The doping element M represents a doping element at the manganese and iron sites, optionally including at least one of Al, Cu, Mg, Zn, Ni, Ti, V, Zr, Co, Ga, Sn, Sb, Nb, and Ge; the doping element N represents a doping element at the lithium site, optionally including at least one of Zn, Al, Na, K, Mg, Nb, Mo, and W; the doping element Q represents a doping element at the phosphorus site, optionally including at least one of B, S, Si, and N; and the doping element R represents a doping element at the oxygen site, optionally including at least one of S, F, Cl, and Br. The values ​​are: b = 0.9 to 1.1, x = 0.001 to 0.999, y = 0.499 to 0.80, 1-xy = 0.001 to 0.1, m = 0.001 to 0.1, n = 0.001 to 0.1, and the lithium manganese iron phosphate positive electrode active material is electrically neutral, with a tap density of 1.4 g / cm³. 3 The above is an optional value of 1.47 g / cm³. 3 ~1.82g / cm 3 This can improve the volumetric energy density of secondary batteries.

[0041] In any embodiment of this application, the volume average particle size Dv50 of the lithium manganese iron phosphate cathode active material is 300 nm to 500 nm. This improves the electrochemical performance of the secondary battery.

[0042] In any embodiment of this application, the lithium manganese iron phosphate cathode active material has a spherical or near-spherical single crystal morphology. This can improve the electrochemical performance and volumetric energy density of the secondary battery.

[0043] A third aspect of this application provides a positive electrode sheet, including a positive current collector and a positive electrode film layer disposed on at least one surface of the positive current collector, wherein the positive electrode film layer includes a lithium manganese iron phosphate positive electrode active material obtained by the preparation method of the first aspect of this application or a lithium manganese iron phosphate positive electrode active material of the second aspect of this application.

[0044] In any embodiment of this application, the content of the lithium manganese iron phosphate positive electrode active material in the positive electrode film layer is 50% by weight or more, optionally 90% by weight to 99% by weight, based on the total weight of the positive electrode film layer.

[0045] The fourth aspect of this application provides a secondary battery, including a lithium manganese iron phosphate positive electrode active material obtained by the preparation method of the first aspect of this application, or a lithium manganese iron phosphate positive electrode active material of the second aspect of this application, or a positive electrode sheet of the third aspect of this application.

[0046] The fifth aspect of this application provides an electrical device, including the secondary battery of the fourth aspect of this application.

[0047] Beneficial effects

[0048] The preparation method of lithium manganese iron phosphate cathode active material provided in this application is simple, energy-efficient, and low-cost, and can obtain nanoscale lithium manganese iron phosphate cathode active material with both good electrochemical performance and high tap density, for example, a tap density of 1.4 g / cm³. 3 The initial specific capacity is above 138 mAh / g, which is far higher than that of nanoscale lithium manganese iron phosphate cathode active materials obtained by traditional high-temperature solid-state sintering methods. Therefore, when the lithium manganese iron phosphate cathode active material obtained by the preparation method of this application is applied to a secondary battery, a secondary battery that balances good electrochemical performance and high volumetric energy density can be obtained. The power device of this application includes the secondary battery of this application, and therefore has at least the same advantages as the secondary battery. Attached Figure Description

[0049] To more clearly illustrate the technical solutions of the embodiments of this application, the accompanying drawings used in the embodiments of this application will be briefly described below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on the drawings without creative effort.

[0050] Figure 1 This is a schematic diagram of one embodiment of the secondary battery of this application.

[0051] Figure 2 yes Figure 1 An exploded view of the implementation method of the secondary battery.

[0052] Figure 3 This is a schematic diagram of one embodiment of the battery module of this application.

[0053] Figure 4 This is a schematic diagram of one embodiment of the battery pack of this application.

[0054] Figure 5 yes Figure 4 An exploded view of an embodiment of the battery pack shown.

[0055] Figure 6 This is a schematic diagram of one embodiment of an electrical device that uses a secondary battery as a power source, as described in this application.

[0056] Figure 7 This is a scanning electron microscope image of the nanoscale iron-containing oxide prepared in Example 1.

[0057] Figure 8 This is a scanning electron microscope image of the nanoscale lithium manganese iron phosphate positive electrode active material prepared in Example 1.

[0058] The accompanying drawings are not necessarily drawn to scale. The reference numerals are as follows: 1 Battery pack, 2 Upper casing, 3 Lower casing, 4 Battery module, 5 Secondary battery, 51 Housing, 52 Electrode assembly, 53 Cover plate. Detailed Implementation

[0059] The following detailed description, with appropriate reference to the accompanying drawings, discloses embodiments of the lithium manganese iron phosphate positive electrode active material, its preparation method, positive electrode sheet, secondary battery, and power application device of this application. However, unnecessary detailed descriptions may be omitted. For example, detailed descriptions of well-known matters and repetitive descriptions of practically identical structures may be omitted. This is to avoid unnecessarily lengthy descriptions and to facilitate understanding by those skilled in the art. Furthermore, the accompanying drawings and the following description are provided for the purpose of enabling those skilled in the art to fully understand this application and are not intended to limit the subject matter of the claims.

[0060] The "range" disclosed in this application is defined by a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of a particular range. Ranges defined in this way can include or exclude endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a specific parameter, it is expected that ranges of 60-110 and 80-120 are also included. Furthermore, if minimum range values ​​of 1 and 2 are listed, and if maximum range values ​​of 3, 4, and 5 are listed, then the following ranges are all expected: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In this application, unless otherwise stated, the numerical range "ab" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0-5" have been listed in this article; "0-5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is stated as an integer ≥2, it is equivalent to disclosing that the parameter is, for example, an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0061] Unless otherwise specified, all embodiments and optional embodiments of this application may be combined with each other to form new technical solutions, and such technical solutions should be considered to be included in the disclosure of this application.

[0062] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions, and such technical solutions shall be deemed to be included in the disclosure of this application.

[0063] Unless otherwise specified, all steps in this application may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps S1 and S2, indicating that the method may include steps S1 and S2 performed sequentially, or it may include steps S2 and S1 performed sequentially. For example, the method may also include step S3, indicating that step S3 may be added to the method in any order. For example, the method may include steps S1, S2, and S3, or it may include steps S1, S3, and S2, or it may include steps S3, S1, and S2, etc.

[0064] Unless otherwise specified, the terms "comprising" and "including" as used in this application can be open-ended or closed-ended. For example, "comprising" and "including" can mean that other components not listed may also be included, or that only the listed components may be included.

[0065] Unless otherwise specified, the term "or" is inclusive in this application. For example, the phrase "A or B" means "A, B, or both A and B". More specifically, the condition "A or B" is satisfied by any of the following conditions: A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or both A and B are true (or exist).

[0066] In this application, the terms "multiple" or "various" refer to two or more kinds.

[0067] Currently, the main method for preparing lithium manganese iron phosphate cathode active materials is high-temperature solid-state sintering, which involves mixing various raw materials to obtain a precursor, followed by high-temperature sintering to obtain the lithium manganese iron phosphate cathode active material. However, the morphology and size of lithium manganese iron phosphate cathode active materials prepared by existing technologies are difficult to control, and they are mostly micro- and nano-sized particles, which affects their electrochemical performance.

[0068] Nano-sized lithium manganese iron phosphate cathode active materials obtained through processes such as high-speed ball milling can reduce polarization, shorten lithium-ion transport distance, and improve electrochemical performance. However, nano-sizing significantly increases the specific surface area of ​​the lithium manganese iron phosphate cathode active material, which in turn significantly reduces its tap density, resulting in a lower volumetric energy density of the secondary battery, which cannot meet the requirements of high-energy-density secondary batteries.

[0069] Therefore, lithium manganese iron phosphate cathode active materials obtained by the traditional high-temperature solid-state sintering method are difficult to achieve both good electrochemical performance and high tap density.

[0070] The inventors of this application made a surprising discovery during their research: by preparing nanoscale iron-containing oxides using a low-temperature solid-state reaction method beforehand, and then using a spray drying granulation process and a sintering process, they were able to obtain nanoscale lithium manganese iron phosphate cathode active materials that combine good electrochemical performance and high tap density.

[0071] Preparation method

[0072] Specifically, the first aspect of this application provides a method for preparing lithium manganese iron phosphate cathode active material, including the following steps S1, S2 and S3.

[0073] S1, Preparation of nanoscale iron-containing oxides by low-temperature solid-state reaction method.

[0074] The iron source, solid alkali, and optional dopant element M are mixed and ground to induce a low-temperature solid-phase reaction in each component. After grinding, the obtained product is washed to remove impurities and dried to obtain nanoscale iron-containing oxides. The dopant element M represents a dopant element at manganese and iron sites.

[0075] S2, spray drying granulation.

[0076] The nano-sized iron oxide obtained in S1 is mixed with solvent, lithium source, manganese source, phosphorus source, optional dopant element N source, optional dopant element Q source and optional dopant element R source in a predetermined ratio and then ground. After grinding, the mixture is spray-dried and granulated to obtain powder. In this mixture, dopant element N represents lithium site dopant, dopant element Q represents phosphorus site dopant, and dopant element R represents oxygen site dopant.

[0077] S3, sintering.

[0078] The powder obtained in S2 is sintered to obtain lithium manganese iron phosphate cathode active material.

[0079] This application uses nanoscale iron-containing oxides as raw materials in the preparation of lithium manganese iron phosphate cathode active materials. Unlike traditional high-temperature solid-state reaction methods for preparing iron-containing oxides, this application uses a low-temperature solid-state reaction method to prepare nanoscale iron-containing oxides. This method eliminates the need for high-temperature sintering; instead, it requires mixing and grinding the iron source, solid alkali, and optional dopant element M at a lower temperature to allow the raw material components to undergo a chemical reaction in a short time, resulting in a one-step synthesis of the iron-containing oxide. Furthermore, the morphology and size of the iron-containing oxides obtained through this low-temperature solid-state reaction method are controllable, with particle sizes at the nanometer level. The obtained iron-containing oxides also exhibit high purity, low impurities, and a 100% reaction conversion rate.

[0080] In preparing lithium manganese iron phosphate cathode active material, this application uses nano-sized iron oxide as raw material. Compared with using ferrous oxalate or iron phosphate as raw material, the preparation method of this application can effectively control the morphology and size of the obtained lithium manganese iron phosphate cathode active material. Its particle morphology is more regular, the particle size is at the nanoscale, and the particle size distribution is more uniform, thus exhibiting good electrochemical performance. At the same time, the obtained lithium manganese iron phosphate cathode active material also has high tap density, thereby improving the volumetric energy density of the secondary battery. In addition, the obtained lithium manganese iron phosphate cathode active material also has the advantages of high purity and high batch stability.

[0081] Furthermore, traditional high-temperature solid-state sintering methods often employ high-speed ball milling to refine the morphology and size of the raw materials (e.g., iron, manganese, phosphorus, lithium, etc.) used in the preparation of lithium manganese iron phosphate cathode active materials, aiming to achieve a uniform distribution of elements (e.g., lithium, manganese, iron, etc.). However, the results obtained are unsatisfactory, thus affecting the electrochemical performance of the lithium manganese iron phosphate cathode active material and the secondary battery. Additionally, high-speed ball milling increases energy consumption and production costs. This application uses nanoscale iron oxides as raw materials in the preparation of lithium manganese iron phosphate cathode active materials, thereby obtaining lithium manganese iron phosphate cathode active materials with a uniform molecular-level mixture of lithium, manganese, iron, and various doping elements. This avoids the problems of large raw material particles, uneven element distribution, and high energy consumption associated with traditional high-temperature solid-state sintering methods.

[0082] This application achieves precise control over the ratio of iron to manganese in the preparation of lithium manganese iron phosphate cathode active materials. This results in lithium manganese iron phosphate cathode active materials with fewer crystal defects, such as fewer cation antisite defects (e.g., Li / Mn antisite defects), with a Li / Mn antisite defect ratio of <0.2%. This is beneficial for lithium-ion transport and enabling high-rate charging. Crystal defects refer to locations where the internal structure of a crystal is disrupted. Li / Mn antisite defects refer to the defects in the Li / Mn cation lattice within the lithium manganese iron phosphate cation. + and Mn 2+ The positions of Li and Mn are interchanged. The Li / Mn antisite defect ratio refers to the ratio of Li to Mn in the active material of lithium manganese iron phosphate cathode. 2+ Interchangeable Li + Zhan Li + Percentage of the total amount. Since the Li+ transport channel is a one-dimensional channel, Mn 2+ In Li + It is difficult to migrate in the transmission channel, therefore, the Mn of the inversion defect is difficult to migrate. 2+ It will hinder Li + The transmission of Li / Mn antisite defects can be determined according to JIS K 0131-1996 by X-ray diffraction (XRD). For example, the XRD results of the lithium manganese iron phosphate cathode active material test sample can be compared with the PDF (Powder Diffraction File) card of the standard crystal to obtain the Li / Mn antisite defect ratio.

[0083] Therefore, the preparation method of this application is simple, energy-efficient, and low-cost, and can obtain nanoscale lithium manganese iron phosphate cathode active materials that combine good electrochemical performance and high tap density, for example, a tap density of 1.4 g / cm³. 3The initial specific capacity is above 138 mAh / g, which is much higher than that of nanoscale lithium manganese iron phosphate cathode active materials obtained by traditional high-temperature solid-state sintering methods. Therefore, when the lithium manganese iron phosphate cathode active material obtained by the preparation method of this application is applied to secondary batteries, a secondary battery that balances good electrochemical performance and high volumetric energy density can be obtained.

[0084] In some embodiments, in S1, the iron source is a ferric salt, optionally including at least one of ferric chloride, ferric nitrate, and ferric sulfate, and the source of the dopant element M includes at least one of the hydrochloride, nitrate, sulfate, and acetate of the dopant element M. Therefore, in this application, the nanoscale iron-containing oxide can be ferric oxide (Fe2O3), or it can be a solid solution of ferric oxide (Fe2O3) and an oxide of the dopant element M.

[0085] In some embodiments, in S1, the solid alkali includes at least one of sodium hydroxide, potassium hydroxide, sodium carbonate, and potassium carbonate, and optionally includes sodium hydroxide.

[0086] In some embodiments, in S1, the molar ratio of the iron source to the solid alkali is 1:(1-3).

[0087] During the mixing and grinding process of an iron source, a solid alkali, and an optional dopant element M, an iron hydroxide, either doped or undoped with dopant element M, is first formed. Then, under the action of the solid alkali, the doped or undoped iron hydroxide rapidly transforms into nanoscale iron oxides, such as ferric oxide, or a solid solution of ferric oxide and an oxide of dopant element M. In the low-temperature solid-phase reaction, the melting point (or decomposition temperature) of the doped or undoped iron hydroxide is relatively low, thus rapidly generating nanoscale iron oxides under the action of the solid alkali. During the mixing and grinding process, the reactants first change from a solid state to a slurry state, and then from a slurry state to a powder state. The grinding process is particle-free, and the obtained iron oxide particles are small, with a size in the nanometer range, for example, between 100 nm and 800 nm.

[0088] The dopant element M represents a dopant element at both the manganese and iron sites. In some embodiments, the dopant element M optionally includes at least one selected from Al, Cu, Mg, Zn, Ni, Ti, V, Zr, Co, Ga, Sn, Sb, Nb, and Ge. More preferably, the dopant element M includes at least one selected from Al, Cu, Mg, Zn, and Ni.

[0089] In some embodiments, in S1, the mixed grinding can be performed by a suitable grinding method known in the art, for example, the mixed grinding can be performed in a mortar or a ball mill jar. Optionally, the grinding speed in the mortar is 100 r / min. Optionally, the ball mill rotation speed is 300 r / min to 800 r / min.

[0090] In some embodiments, in S1, the mixing and grinding time is 20 min to 60 min.

[0091] In some embodiments, S1 includes a settling step after mixing and grinding to allow the low-temperature solid-phase reaction to complete. Optionally, the settling time is 30 min to 120 min.

[0092] In some embodiments, in S1, the sum of the mixing and grinding time and the settling time is greater than or equal to 90 min, thereby ensuring that the low-temperature solid-phase reaction is complete and that the obtained nanoscale iron oxide has higher purity.

[0093] In some embodiments, in S1, before mixing and grinding the iron source, the solid alkali, and the source of the dopant element M to induce a low-temperature solid-phase reaction, the method further includes: grinding the iron source, the solid alkali, and the source of the dopant element M separately to ensure a sufficient low-temperature solid-phase reaction and to facilitate the acquisition of nanoscale iron-containing oxides with controllable morphology and size. Optionally, the grinding time for the iron source is 30 min to 90 min. Optionally, the grinding time for the solid alkali is 30 min to 90 min. Optionally, the grinding time for the source of the dopant element M is 30 min to 90 min.

[0094] In some embodiments, in S1, a surfactant is further added and mixed and milled together with the iron source, the solid alkali, and the source of the dopant element M to adjust the morphology of the obtained nanoscale iron oxide. Optionally, the surfactant includes polyethylene glycol, for example, at least one of polyethylene glycol 200, polyethylene glycol 300, polyethylene glycol 400, and polyethylene glycol 600. Optionally, the amount of surfactant added is less than 5% by weight, based on the total weight of the iron source, the solid alkali, and the source of the dopant element M.

[0095] In some embodiments, in S1, the washing and impurity removal process includes water washing and alcohol washing, optionally using ethanol as a solvent for alcohol washing. Optionally, the number of water washings is 2 to 4 times, and the number of alcohol washings is 2 to 4 times.

[0096] In some embodiments, in S1, the drying is vacuum drying.

[0097] In some embodiments, in S1, the drying temperature is below 100°C.

[0098] In some embodiments, in S1, the drying time is 2h to 15h.

[0099] In some embodiments, in S2, the lithium source may be a lithium-containing compound known in the art that can be used to prepare lithium manganese iron phosphate cathode active materials. For example, the lithium source includes at least one of Li2CO3, LiOH, Li3PO4 and LiH2PO4.

[0100] In some embodiments, in S2, the manganese source may be a manganese-containing compound known in the art that can be used to prepare lithium manganese iron phosphate cathode active materials. For example, the manganese source includes at least one of MnCO3, Mn(CH3COO)2 and MnC2O4.

[0101] In some embodiments, in S2, the phosphorus source may be a phosphorus-containing compound known in the art that can be used to prepare lithium manganese iron phosphate cathode active materials. For example, the phosphorus source includes at least one of (NH4)2HPO4, NH4H2PO4, (NH4)3PO4 and H3PO4.

[0102] The dopant element N represents a lithium site dopant element. In some embodiments, the dopant element N may optionally include at least one of Zn, Al, Na, K, Mg, Nb, Mo, and W. In some embodiments, in S2, the source of the dopant element N may optionally include at least one of the hydrochloride, nitrate, sulfate, and acetate salts of the dopant element N.

[0103] The dopant element Q represents a phosphorus site dopant element. In some embodiments, the dopant element Q may optionally include at least one of B, S, Si, and N. In some embodiments, in S2, the source of the dopant element Q may optionally include at least one of a sulfate, borate, nitrate, and silicate of the dopant element Q.

[0104] The dopant element R represents the dopant element at the oxygen site. In some embodiments, the dopant element R may optionally include at least one of S, F, Cl, and Br. In some embodiments, in S2, the source of the dopant element R may optionally include at least one of the elemental form of the dopant element R and an ammonium salt.

[0105] By selecting the sources of the aforementioned doping elements, the uniformity of their distribution can be improved, thereby enhancing the electrochemical performance of lithium manganese iron phosphate cathode active materials.

[0106] In some embodiments, in S2, the solvent includes ethanol, water, or a mixture thereof.

[0107] In some embodiments, in S2, the grinding can be performed by a suitable grinding method known in the art, for example, the grinding can be performed in a ball mill jar. Optionally, the ball milling speed is 300 r / min to 800 r / min. Optionally, the ball milling time is 3 h to 24 h.

[0108] In some embodiments, in S2, the spray drying temperature can be a temperature conventional for spray drying in the art. Optionally, the spray drying temperature is 200°C to 250°C.

[0109] In some embodiments, in step S2, a carbon source is also added and mixed and ground together to obtain a carbon-coated lithium manganese iron phosphate cathode active material. Optionally, the carbon source includes at least one of an organic carbon source and an inorganic carbon source, and more preferably includes at least one of glucose, sucrose, starch, fructose, polyvinyl alcohol, polyethylene glycol, and citric acid.

[0110] In some embodiments, in S3, the sintering process is carried out in a muffle furnace.

[0111] In some embodiments, in S3, the sintering process can be a stepwise sintering process. Optionally, the stepwise sintering process includes the following steps: S31, pre-sintering the powder obtained in S2 at a low temperature of 350℃~550℃ in an air atmosphere or a protective gas atmosphere, and obtaining a pre-sintered material after the reaction is completed; S32, grinding the pre-sintered material obtained in S31 and then spray drying and granulating it to obtain a pre-sintered material powder; S33, sintering the pre-sintered material powder obtained in S32 at a high temperature of 650℃~750℃ in a protective gas atmosphere, thereby obtaining the lithium manganese iron phosphate cathode active material.

[0112] In some embodiments, in S31, the low-temperature pre-sintering time is 2h to 10h.

[0113] In some embodiments, in S33, the high-temperature sintering time is 2h to 10h.

[0114] In some embodiments, in S31, the protective gas atmosphere may be nitrogen, an inert gas, or a combination thereof.

[0115] In some embodiments, in S33, the protective gas atmosphere may be nitrogen, an inert gas, or a combination thereof.

[0116] In some embodiments, in S32, the grinding can be performed by a suitable grinding method known in the art, for example, the grinding can be performed in a ball mill jar. Optionally, the ball milling speed is 300 r / min to 800 r / min. Optionally, the ball milling time is 3 h to 24 h.

[0117] In some embodiments, in S32, the spray drying temperature can be a temperature conventional for spray drying in the art. Optionally, the spray drying temperature is 200°C to 250°C.

[0118] In some embodiments, the preparation method further includes the step of crushing the lithium manganese iron phosphate cathode active material obtained in S3 to the desired particle size. Optionally, the crushing is air jet crushing.

[0119] In some embodiments, the preparation method includes the following steps: mixing and grinding an iron source, a solid alkali, and a source of optional dopant element M for 20 min to 60 min, then allowing the mixture to stand for 30 min to 120 min to allow the components to undergo a low-temperature solid-phase reaction; washing and drying the obtained product to obtain nanoscale iron-containing oxide, wherein the dopant element M represents a dopant element at manganese and iron sites; mixing the obtained nanoscale iron-containing oxide with a solvent, a lithium source, a manganese source, a phosphorus source, a carbon source, a source of optional dopant element N, a source of optional dopant element Q, and a source of optional dopant element R in a predetermined ratio, and then grinding the mixture. After grinding, the material is spray-dried and granulated to obtain powder, wherein the doping element N represents the lithium site doping element, the doping element Q represents the phosphorus site doping element, and the doping element R represents the oxygen site doping element. The obtained powder is then pre-sintered at a low temperature of 350℃~550℃ in air or a protective gas atmosphere for 2h~10h to obtain a pre-sintered material. The obtained pre-sintered material is then ground and spray-dried to obtain a pre-sintered material powder. The obtained pre-sintered material powder is then sintered at a high temperature of 650℃~750℃ in a protective gas atmosphere for 2h~10h to obtain the lithium manganese iron phosphate cathode active material.

[0120] In the preparation method of this application, the amount of each of the doping elements M, N, Q, and R sources added depends on the target doping amount; the amounts of iron, lithium, manganese, and phosphorus sources added conform to the stoichiometry of the lithium manganese iron phosphate cathode active material. In some embodiments, the amount of lithium source added can be slightly excessive, for example, it can be 100% to 110% of the theoretical mass of the lithium source, where the theoretical mass of the lithium source refers to the mass of the lithium source calculated based on the stoichiometry of the lithium manganese iron phosphate cathode active material.

[0121] In the preparation method of this application, unless otherwise specified, all raw materials can be purchased directly, and each raw material may or may not contain water of crystallization. Taking ferric chloride as an example, in the preparation method of this application, either anhydrous ferric chloride or ferric chloride hexahydrate FeCl3·6H2O can be used.

[0122] Lithium manganese iron phosphate cathode active material

[0123] The second aspect of this application provides a lithium manganese iron phosphate cathode active material, which is obtained by the preparation method of the first aspect of this application.

[0124] The lithium manganese iron phosphate cathode active material has the chemical formula Li a N b Fe x Mn y M 1-x-y P 1-m Q m O 4-n R n The doping element M represents a doping element at the manganese and iron sites, and optionally includes at least one of Al, Cu, Mg, Zn, Ni, Ti, V, Zr, Co, Ga, Sn, Sb, Nb, and Ge. The doping element N represents a doping element at the lithium site, and optionally includes at least one of Zn, Al, Na, K, Mg, Nb, Mo, and W. The doping element Q represents a doping element at the phosphorus site, and optionally includes at least one of B, S, Si, and N. The doping element R represents a doping element at the oxygen site, and optionally includes at least one of S, F, Cl, and Br. a is 0.9 to 1.1, b is 0 to 0.1, x is 0.001 to 0.999, y is 0.001 to 0.999, 1-xy is 0 to 0.1, m is 0 to 0.1, and n is 0 to 0.1. The lithium manganese iron phosphate cathode active material is electrically neutral.

[0125] Optionally, x is from 0.199 to 0.50 and y is from 0.499 to 0.80, thereby enabling the lithium manganese iron phosphate cathode active material to have better electrochemical performance.

[0126] Optionally, x∶y is 5∶5 to 2∶8, thereby enabling the lithium manganese iron phosphate cathode active material to have better electrochemical performance.

[0127] Optionally, 1-xy is 0.001 to 0.1.

[0128] Optionally, b can be between 0.001 and 0.1.

[0129] Optionally, m can be 0.001 to 0.1.

[0130] Optionally, n can be between 0.001 and 0.1.

[0131] Therefore, the lithium manganese iron phosphate cathode active material has higher structural stability and better electrochemical performance.

[0132] The tap density of the lithium manganese iron phosphate cathode active material obtained by the above preparation method of this application is 1.4 g / cm³. 3 The above is an optional value of 1.47 g / cm³.3 ~1.82g / cm 3 This can improve the volumetric energy density of secondary batteries.

[0133] The volume average particle size (Dv50) of the lithium manganese iron phosphate cathode active material obtained by the above preparation method of this application is 300 nm to 500 nm. This can improve the electrochemical performance of secondary batteries.

[0134] The lithium manganese iron phosphate cathode active material obtained by the above preparation method of this application has a spherical or near-spherical single crystal morphology. This can improve the electrochemical performance and volumetric energy density of the secondary battery.

[0135] Positive electrode sheet

[0136] A third aspect of this application provides a positive electrode sheet, comprising a positive current collector and a positive electrode film layer disposed on at least one surface of the positive current collector, wherein the positive electrode film layer comprises a lithium manganese iron phosphate positive electrode active material obtained by the preparation method of the first aspect of this application or a lithium manganese iron phosphate positive electrode active material of the second aspect of this application. The positive current collector has two surfaces opposite to each other in its thickness direction, and the positive electrode film layer is disposed on either or both of the two opposite surfaces of the positive current collector.

[0137] In some embodiments, the content of the positive electrode active material in the positive electrode film layer is 50% by weight or more, optionally 90% by weight to 99% by weight, based on the total weight of the positive electrode film layer.

[0138] The positive electrode film layer does not exclude components other than the lithium manganese iron phosphate positive electrode active material obtained by the preparation method of the first aspect of this application or the lithium manganese iron phosphate positive electrode active material of the second aspect of this application. For example, the positive electrode film layer may also include other positive electrode active materials. Optionally, the other positive electrode active materials may include at least one of lithium transition metal oxides and their modified compounds. As an example, the other positive electrode active materials may include at least one of lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium nickel cobalt oxide, lithium manganese cobalt oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, and their respective modified compounds.

[0139] In some embodiments, the positive electrode film may optionally include a positive electrode conductive agent. This application does not impose any particular limitation on the type of positive electrode conductive agent. As an example, the positive electrode conductive agent includes at least one of superconducting carbon, conductive graphite, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0140] In some embodiments, the positive electrode film layer may optionally include a positive electrode binder. This application does not impose any particular limitation on the type of positive electrode binder. As an example, the positive electrode binder may include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), PVDF-tetrafluoroethylene-propylene terpolymer, PVDF-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resins.

[0141] In some embodiments, the positive current collector may be a metal foil or a composite current collector. An example of a metal foil is aluminum foil. The composite current collector may include a polymer substrate and a metal layer formed on at least one surface of the polymer substrate. An example of the metal material may include at least one of aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy. An example of the polymer substrate may include at least one of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), and polyethylene (PE).

[0142] The positive electrode film is typically formed by coating a positive electrode slurry onto a positive electrode current collector, followed by drying and cold pressing. The positive electrode slurry is typically formed by dispersing the positive electrode active material, optional conductive agent, optional binder, and any other components in a solvent and stirring until homogeneous. The solvent may be N-methylpyrrolidone (NMP), but is not limited to this.

[0143] Secondary batteries

[0144] A fourth aspect of this application provides a secondary battery. A secondary battery, also known as a rechargeable battery or accumulator, is a battery that can be recharged after discharge to activate its active materials and continue to be used. Typically, a secondary battery includes an electrode assembly and an electrolyte. The electrode assembly includes a positive electrode, a negative electrode, and a separator. The separator is disposed between the positive and negative electrodes, primarily serving to prevent short circuits between the positive and negative electrodes while allowing active ions to pass through. The electrolyte, located between the positive and negative electrodes, conducts active ions.

[0145] [Positive electrode plate]

[0146] The positive electrode used in the secondary battery of this application is the positive electrode described in any embodiment of the third aspect of the embodiments of this application.

[0147] [Negative electrode plate]

[0148] In some embodiments, the negative electrode sheet includes a negative current collector and a negative electrode film layer disposed on at least one surface of the negative current collector and comprising a negative electrode active material. For example, the negative current collector has two surfaces opposite each other in its thickness direction, and the negative electrode film layer is disposed on either or both of the two opposite surfaces of the negative current collector.

[0149] The negative electrode active material may be any negative electrode active material known in the art for use in secondary batteries. As an example, the negative electrode active material includes, but is not limited to, at least one of natural graphite, artificial graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, and lithium titanate. The silicon-based material may include at least one of elemental silicon, silicon oxide, silicon-carbon composite, silicon-nitrogen composite, and silicon alloy. The tin-based material may include at least one of elemental tin, tin oxide, and tin alloy. This application is not limited to these materials, and other conventionally known materials that can be used as negative electrode active materials for secondary batteries may also be used. These negative electrode active materials may be used alone or in combination of two or more.

[0150] In some embodiments, the negative electrode film layer may optionally include a negative electrode conductive agent. This application does not impose any particular limitation on the type of negative electrode conductive agent. As an example, the negative electrode conductive agent may include at least one of superconducting carbon, conductive graphite, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0151] In some embodiments, the negative electrode film layer may optionally include a negative electrode binder. This application does not impose any particular limitation on the type of negative electrode binder. As an example, the negative electrode binder may include at least one of styrene-butadiene rubber (SBR), water-soluble unsaturated resin SR-1B, waterborne acrylic resins (e.g., polyacrylic acid PAA, polymethacrylic acid PMAA, sodium polyacrylate PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), and carboxymethyl chitosan (CMCS).

[0152] In some embodiments, the negative electrode film may optionally include other additives. As an example, other additives may include thickeners, such as sodium carboxymethyl cellulose (CMC), PTC thermistor materials, etc.

[0153] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. As an example of a metal foil, copper foil may be used. The composite current collector may include a polymer base material and a metal material layer formed on at least one surface of the polymer base material. As an example, the metal material may include at least one of copper, copper alloys, nickel, nickel alloys, titanium, titanium alloys, silver, and silver alloys. As an example, the polymer base material may include at least one of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), and polyethylene (PE).

[0154] The negative electrode film layer is typically formed by coating a negative electrode slurry onto a negative electrode current collector, followed by drying and cold pressing. The negative electrode slurry is typically formed by dispersing a negative electrode active material, optional conductive agent, optional binder, and other optional additives in a solvent and stirring until homogeneous. The solvent can be N-methylpyrrolidone (NMP) or deionized water, but is not limited to these.

[0155] The negative electrode sheet does not exclude other additional functional layers besides the negative electrode film layer. For example, in some embodiments, the negative electrode sheet of this application further includes a conductive undercoat layer (e.g., composed of a conductive agent and an adhesive) sandwiched between the negative electrode current collector and the negative electrode film layer and disposed on the surface of the negative electrode current collector. In other embodiments, the negative electrode sheet of this application further includes a protective layer covering the surface of the negative electrode film layer.

[0156] [Electrolytes]

[0157] This application does not impose specific limitations on the type of electrolyte, which can be selected according to actual needs. For example, the electrolyte can be selected from at least one of solid electrolytes and liquid electrolytes (i.e., electrolyte solutions).

[0158] In some embodiments, the electrolyte is an electrolyte solution comprising an electrolyte salt and a solvent.

[0159] The type of electrolyte salt is not specifically limited and can be selected according to actual needs. In some embodiments, as an example, the electrolyte salt may include at least one of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium perchlorate (LiClO4), lithium hexafluoroarsenate (LiAsF6), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium trifluoromethanesulfonate (LiTFS), lithium difluorooxalate borate (LiDFOB), lithium dioxalate borate (LiBOB), lithium difluorophosphate (LiPO2F2), lithium difluorodioxalate phosphate (LiDFOP), and lithium tetrafluorooxalate phosphate (LiTFOP).

[0160] The type of solvent is not specifically limited and can be selected according to actual needs. In some embodiments, as an example, the solvent may include at least one of ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), butyl carbonate (BC), fluoroethylene carbonate (FEC), methyl formate (MF), methyl acetate (MA), ethyl acetate (EA), propyl acetate (PA), methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), methyl butyrate (MB), ethyl butyrate (EB), 1,4-butyrolactone (GBL), sulfolane (SF), dimethyl sulfone (MSM), methyl ethyl sulfone (EMS), and diethyl sulfone (ESE).

[0161] In some embodiments, the electrolyte may optionally include additives. For example, the additives may include negative electrode film-forming additives, positive electrode film-forming additives, and additives that can improve certain battery performance, such as additives that improve battery overcharge performance, additives that improve battery high-temperature performance, and additives that improve battery low-temperature power performance.

[0162] [Isolation membrane]

[0163] Secondary batteries using electrolytes, as well as some secondary batteries using solid electrolytes, also include a separator. The separator is disposed between the positive and negative electrodes, primarily serving to prevent short circuits between the positive and negative electrodes while allowing active ions to pass through. This application does not impose any particular limitation on the type of separator; any known porous separator with good chemical and mechanical stability can be selected.

[0164] In some embodiments, the material of the separator may include at least one selected from glass fiber, nonwoven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator may be a single-layer film or a multi-layer composite film. When the separator is a multi-layer composite film, the materials of each layer may be the same or different.

[0165] In some embodiments, the positive electrode, the separator, and the negative electrode can be fabricated into an electrode assembly using a winding process and / or a stacking process.

[0166] In some embodiments, the secondary battery may include an outer packaging. This outer packaging may be used to encapsulate the electrode assembly and electrolyte described above.

[0167] In some embodiments, the outer packaging of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. The outer packaging of the secondary battery can also be a soft pack, such as a pouch. The material of the soft pack can be plastic, such as at least one of polypropylene (PP), polybutylene terephthalate (PBT), and polybutylene succinate (PBS).

[0168] This application does not impose any particular limitation on the shape of the secondary battery; it can be cylindrical, square, or any other arbitrary shape. Figure 1 This is an example of a square-structured secondary battery 5.

[0169] In some embodiments, such as Figure 2 As shown, the outer packaging may include a housing 51 and a cover 53. The housing 51 may include a base plate and side plates connected to the base plate, the base plate and side plates enclosing a receiving cavity. The housing 51 has an opening communicating with the receiving cavity, and the cover 53 is used to cover the opening to close the receiving cavity. The positive electrode, negative electrode, and separator may be formed into an electrode assembly 52 by a winding process and / or a stacking process. The electrode assembly 52 is encapsulated in the receiving cavity. Electrolyte is immersed in the electrode assembly 52. ​​The secondary battery 5 may contain one or more electrode assemblies 52, which can be adjusted according to actual needs.

[0170] The method for preparing the secondary battery described in this application is well known. In some embodiments, a positive electrode, a separator, a negative electrode, and an electrolyte can be assembled to form a secondary battery. As an example, the positive electrode, separator, and negative electrode can be formed into an electrode assembly through a winding or stacking process. The electrode assembly is then placed in an outer packaging, dried, and injected with an electrolyte. After vacuum sealing, settling, formation, and shaping, a secondary battery is obtained.

[0171] In some embodiments of this application, the secondary battery according to this application can be assembled into a battery module. The number of secondary batteries contained in the battery module can be multiple, and the specific number can be adjusted according to the application and capacity of the battery module.

[0172] Figure 3 This is a schematic diagram of battery module 4 as an example. Figure 3 As shown, in battery module 4, multiple secondary batteries 5 can be arranged sequentially along the length of battery module 4. Of course, they can also be arranged in any other manner. Furthermore, these multiple secondary batteries 5 can be fixed in place using fasteners.

[0173] Optionally, the battery module 4 may also include a housing with a receiving space in which a plurality of secondary batteries 5 are received.

[0174] In some embodiments, the battery modules described above can also be assembled into a battery pack, and the number of battery modules contained in the battery pack can be adjusted according to the application and capacity of the battery pack.

[0175] Figure 4 and Figure 5 This is a schematic diagram of battery pack 1 as an example. Figure 4 and Figure 5 As shown, the battery pack 1 may include a battery box and multiple battery modules 4 disposed within the battery box. The battery box includes an upper body 2 and a lower body 3. The upper body 2 covers the lower body 3, forming a closed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the battery box.

[0176] Electrical appliances

[0177] A fifth aspect of this application provides an electrical device, which includes at least one of the secondary battery, battery module, or battery pack described in this application. The secondary battery, battery module, or battery pack can be used as a power source for the electrical device or as an energy storage unit for the electrical device. The electrical device may be, but is not limited to, mobile devices (e.g., mobile phones, laptops, etc.), electric vehicles (e.g., pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc.

[0178] The electrical device can be equipped with a secondary battery, battery module, or battery pack according to its usage requirements.

[0179] Figure 6 This is a schematic diagram of an example electrical device. The device could be a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. To meet the device's requirements for high power and high energy density, a battery pack or battery module can be used.

[0180] Another example of an electrical device could be a mobile phone, tablet, or laptop. These devices typically require a slim and lightweight design and can use rechargeable batteries as their power source.

[0181] Example

[0182] The following embodiments describe the disclosure of this application in more detail. These embodiments are merely illustrative, as various modifications and variations will be apparent to those skilled in the art within the scope of the disclosure of this application. Unless otherwise stated, all parts, percentages, and ratios reported in the following embodiments are based on weight, and all reagents used in the embodiments are commercially available or synthesized by conventional methods and can be used directly without further processing, and the instruments used in the embodiments are commercially available.

[0183] Example 1

[0184] FeCl3·6H2O (38 mmol), MgCl2·6H2O (2 mmol), and NaOH (60 mmol) were added to a mortar and ground for 60 min to refine the particles. Then, NaOH was added to FeCl3·6H2O and MgCl2·6H2O and the mixture was ground for another 30 min to obtain a powdered precursor. The precursor was then allowed to stand for 60 min to ensure complete reaction. The obtained powder was washed with water and ethanol three times each, and then dried in a vacuum drying oven at 80 °C for 12 h to obtain nano-sized iron oxide.

[0185] The obtained nano-sized iron oxide (20 mmol) was placed in a ball mill jar with Li2CO3 (50 mmol), MnC2O4 (60 mmol), NH4H2PO4 (100 mmol) and sucrose (15 mmol), and a small amount of ethanol and water were added as solvents. The mixture was ball milled at 500 r / min for 8 h to obtain a slurry. The obtained slurry was then transferred to a spray drying equipment for spray drying and granulation to obtain powder. The drying temperature was set at 210 °C.

[0186] The obtained powder was sintered in a muffle furnace at 400℃ in air atmosphere for 5 hours, and then naturally cooled to room temperature to obtain a pre-sintered material. The pre-sintered material was then placed in a ball mill jar with a small amount of ethanol and water as solvents, and ball-milled at 800 r / min for 4 hours to obtain a slurry. The slurry was transferred to a spray dryer for spray drying and granulation to obtain pre-sintered material powder, with the drying temperature set at 210℃. The pre-sintered material powder was then sintered in a muffle furnace at 700℃ in nitrogen atmosphere for 10 hours, and then naturally cooled to room temperature. The powder was then subjected to air jet milling to obtain the final product, namely, nano-scale lithium manganese iron phosphate cathode active material LiFe. 0.38 Mn 0.60 Mg 0.02 PO4. The content of each element can be detected by inductively coupled plasma atomic emission spectrometry (ICP).

[0187] Example 2

[0188] Fe(NO3)3·9H2O (39.5 mmol), CoSO4·7H2O (0.5 mmol), and NaOH (120 mmol) were added to a mortar and ground for 60 min to refine the particles. Then, NaOH was added to Fe(NO3)3·9H2O and CoSO4·7H2O and the mixture was ground for another 30 min to obtain a powdered precursor. The precursor was then allowed to stand for 120 min to ensure complete reaction. The obtained powder was washed with water and ethanol three times each, and then dried in a vacuum drying oven at 80 °C for 12 h to obtain nano-sized iron oxide.

[0189] The obtained nano-sized iron oxide (20 mmol) was placed in a ball mill jar with LiOH·H2O (105 mmol), MnCO3 (60 mmol), (NH4)2HPO4 (100 mmol), and glucose (15 mmol). A small amount of ethanol and water were added as solvents, and the mixture was ball-milled at 600 r / min for 8 h to obtain a slurry. The obtained slurry was then transferred to a spray drying equipment for spray drying and granulation to obtain powder. The drying temperature was set to 210 °C.

[0190] The obtained powder was sintered in a muffle furnace at 450℃ under a nitrogen atmosphere for 10 hours, and then naturally cooled to room temperature to obtain a pre-sintered material. The pre-sintered material was then placed in a ball mill jar with a small amount of ethanol and water as solvents, and ball-milled at 800 r / min for 12 hours to obtain a slurry. The slurry was transferred to a spray dryer for spray drying and granulation to obtain pre-sintered material powder, with the drying temperature set at 210℃. The pre-sintered material powder was then sintered in a muffle furnace at 650℃ under a nitrogen atmosphere for 6 hours, and then naturally cooled to room temperature. The powder was then subjected to air jet milling to obtain the final product, namely the lithium manganese iron phosphate cathode active material LiFe. 0.395 Mn 0.60 Co 0.005 PO4. The content of each element can be detected by inductively coupled plasma atomic emission spectrometry (ICP).

[0191] Example 3

[0192] FeCl3·6H2O (39 mmol), CoCl2·6H2O (1 mmol), and NaOH (80 mmol) were added to a mortar and ground for 60 min to refine the particles. Then, NaOH was added to FeCl3·6H2O and CoCl2·6H2O and the mixture was ground for another 30 min to obtain a powdered precursor. The precursor was then allowed to stand for 120 min to ensure complete reaction. The obtained powder was washed with water and ethanol three times each, and then dried in a vacuum drying oven at 80 °C for 12 h to obtain nano-sized iron oxide.

[0193] The obtained nano-sized iron oxide (20 mmol) was placed in a ball mill jar with LiOH·H2O (105 mmol), MnCO3 (60 mmol), (NH4)2HPO4 (100 mmol), and glucose (15 mmol). A small amount of ethanol and water were added as solvents, and the mixture was ball-milled at 600 r / min for 8 h to obtain a slurry. The obtained slurry was then transferred to a spray drying equipment for spray drying and granulation to obtain powder. The drying temperature was set to 210 °C.

[0194] The obtained powder was sintered in a muffle furnace at 450℃ under a nitrogen atmosphere for 10 hours, and then naturally cooled to room temperature to obtain a pre-sintered material. The pre-sintered material was then placed in a ball mill jar with a small amount of ethanol and water as solvents and ball-milled at 800 r / min for 12 hours to obtain a slurry. The slurry was transferred to a spray dryer for spray drying and granulation to obtain pre-sintered material powder, with the drying temperature set at 210℃. The pre-sintered material powder was then sintered in a muffle furnace at 650℃ under a nitrogen atmosphere for 6 hours, and then naturally cooled to room temperature. The powder was then subjected to air jet milling to obtain the final product, namely, nano-scale lithium manganese iron phosphate cathode active material LiFe. 0.39 Mn 0.60 Co 0.01 PO4. The content of each element can be detected by inductively coupled plasma atomic emission spectrometry (ICP).

[0195] Example 4

[0196] FeCl3·6H2O (39.9 mmol), ZnCl2 (0.1 mmol), and NaOH (70 mmol) were added to a mortar and ground separately for 60 min to refine the particles. Then, NaOH was added to FeCl3·6H2O and ZnCl2 and the mixture was ground for another 30 min to obtain a powdered precursor. The precursor was then allowed to stand for 60 min to ensure complete reaction. The obtained powder was washed with water and ethanol three times each, and then dried in a vacuum drying oven at 80 °C for 12 h to obtain nano-sized iron oxide.

[0197] The obtained nano-sized iron oxide (20 mmol) was placed in a ball mill jar with Li2CO3 (51 mmol), MnC2O4 (60 mmol), NH4H2PO4 (100 mmol) and sucrose (15 mmol), and a small amount of ethanol and water were added as solvents. The mixture was ball milled at 600 r / min for 6 h to obtain a slurry. The obtained slurry was then transferred to a spray drying equipment for spray drying and granulation to obtain powder. The drying temperature was set at 210 °C.

[0198] The obtained powder was sintered in a muffle furnace at 400℃ in air atmosphere for 5 hours, and then naturally cooled to room temperature to obtain a pre-sintered material. The pre-sintered material was then placed in a ball mill jar with a small amount of ethanol and water as solvents, and ball-milled at 500 r / min for 4 hours to obtain a slurry. The slurry was transferred to a spray dryer for spray drying to obtain pre-sintered material powder, with the drying temperature set at 210℃. The pre-sintered material powder was then sintered in a muffle furnace at 700℃ in nitrogen atmosphere for 10 hours, and then naturally cooled to room temperature. The powder was then subjected to air jet milling to obtain the final product, namely, nano-scale lithium manganese iron phosphate cathode active material LiFe. 0.399 Mn 0.60 Zn 0.001 PO4. The content of each element can be detected by inductively coupled plasma atomic emission spectrometry (ICP).

[0199] Example 5

[0200] FeCl3·6H2O (40 mmol) and NaOH (60 mmol) were added to a mortar and ground separately for 60 min to refine the particles. Then, NaOH was added to FeCl3·6H2O and the mixture was ground for another 30 min to obtain a powdered precursor. The precursor was then allowed to stand for 60 min to ensure complete reaction. The obtained powder was washed with water and ethanol three times each, and then dried in a vacuum drying oven at 80 °C for 12 h to obtain nano-sized ferric oxide.

[0201] The obtained nano-sized ferric oxide (20 mmol) was placed in a ball mill jar with Li2CO3 (50 mmol), MnC2O4 (60 mmol), NH4H2PO4 (100 mmol) and sucrose (15 mmol), and a small amount of ethanol and water were added as solvents. The mixture was ball milled at 500 r / min for 8 h to obtain a slurry. The obtained slurry was then transferred to a spray drying equipment for spray drying and granulation to obtain powder. The drying temperature was set at 210℃.

[0202] The obtained powder was sintered in a muffle furnace at 400℃ in air atmosphere for 5 hours, and then naturally cooled to room temperature to obtain a pre-sintered material. The pre-sintered material was then placed in a ball mill jar with a small amount of ethanol and water as solvents, and ball-milled at 800 r / min for 4 hours to obtain a slurry. The slurry was transferred to a spray dryer for spray drying and granulation to obtain pre-sintered material powder, with the drying temperature set at 210℃. The pre-sintered material powder was then sintered in a muffle furnace at 700℃ in nitrogen atmosphere for 10 hours, and then naturally cooled to room temperature. The powder was then subjected to air jet milling to obtain the final product, namely, nano-scale lithium manganese iron phosphate cathode active material LiFe. 0.40 Mn 0.60PO4. The content of each element can be detected by inductively coupled plasma atomic emission spectrometry (ICP).

[0203] Comparative Example 1

[0204] FeC2O4 (40 mmol), Li2CO3 (50 mmol), MnC2O4 (60 mmol), NH4H2PO4 (100 mmol), and sucrose (15 mmol) were placed in a ball mill jar, and a small amount of ethanol and water were added as solvents. The mixture was ball milled at 500 r / min for 8 h to obtain a slurry. The obtained slurry was then transferred to a spray drying equipment for spray drying and granulation to obtain powder. The drying temperature was set to 210 °C.

[0205] The obtained powder was sintered in a muffle furnace at 400℃ in air atmosphere for 5 hours, and then naturally cooled to room temperature to obtain a pre-sintered material. The pre-sintered material was then placed in a ball mill jar with a small amount of ethanol and water as solvents, and ball-milled at 800 r / min for 4 hours to obtain a slurry. The slurry was transferred to a spray dryer for spray drying and granulation to obtain pre-sintered material powder, with the drying temperature set at 210℃. The pre-sintered material powder was then sintered in a muffle furnace at 700℃ in nitrogen atmosphere for 10 hours, and then naturally cooled to room temperature. The powder was then subjected to air jet milling to obtain the final product, namely, lithium manganese iron phosphate cathode active material LiFe. 0.40 Mn 0.60 PO4. The content of each element can be detected by inductively coupled plasma atomic emission spectrometry (ICP).

[0206] Comparative Example 2

[0207] AR-grade micron-sized Fe2O3 particles (20 mmol, Dv50 approximately 10 μm, purchased from Sinopharm Chemical Reagent Co., Ltd.) were placed in a ball mill jar with Li2CO3 (50 mmol), MnC2O4 (60 mmol), NH4H2PO4 (100 mmol), and sucrose (15 mmol). A small amount of ethanol and water were added as solvents, and the mixture was ball-milled at 500 r / min for 8 h to obtain a slurry. The obtained slurry was then transferred to a spray drying device for spray drying and granulation to obtain powder. The drying temperature was set at 210 °C.

[0208] The obtained powder was sintered in a muffle furnace at 400℃ in air atmosphere for 5 hours, and then naturally cooled to room temperature to obtain a pre-sintered material. The pre-sintered material was then placed in a ball mill jar with a small amount of ethanol and water as solvents, and ball-milled at 800 r / min for 4 hours to obtain a slurry. The slurry was transferred to a spray dryer for spray drying and granulation to obtain pre-sintered material powder, with the drying temperature set at 210℃. The pre-sintered material powder was then sintered in a muffle furnace at 700℃ in nitrogen atmosphere for 10 hours, and then naturally cooled to room temperature. The powder was then subjected to air jet milling to obtain the final product, namely, lithium manganese iron phosphate cathode active material LiFe. 0.40 Mn 0.60 PO4. The content of each element can be detected by inductively coupled plasma atomic emission spectrometry (ICP).

[0209] Comparative Example 3

[0210] The micron-sized Fe2O3 particles used in Comparative Example 2 were crushed to the nanoscale size by high-speed ball milling, and then placed in a ball mill jar with Li2CO3 (50 mmol), MnC2O4 (60 mmol), MgCl2·6H2O (2 mmol), NH4H2PO4 (100 mmol), and sucrose (15 mmol). A small amount of ethanol and water were added as solvents, and the mixture was ball-milled at 500 r / min for 8 h to obtain a slurry. The obtained slurry was then transferred to a spray drying device for spray drying and granulation to obtain powder, and the drying temperature was set to 210 °C.

[0211] The obtained powder was sintered in a muffle furnace at 400℃ in air atmosphere for 5 hours, and then naturally cooled to room temperature to obtain a pre-sintered material. The pre-sintered material was then placed in a ball mill jar with a small amount of ethanol and water as solvents, and ball-milled at 800 r / min for 4 hours to obtain a slurry. The slurry was transferred to a spray dryer for spray drying and granulation to obtain pre-sintered material powder, with the drying temperature set at 210℃. The pre-sintered material powder was then sintered in a muffle furnace at 700℃ in nitrogen atmosphere for 10 hours, and then naturally cooled to room temperature. The powder was then subjected to air jet milling to obtain the final product, namely, nano-scale lithium manganese iron phosphate cathode active material LiFe. 0.38 Mn 0.60 Mg 0.02 PO4. The content of each element can be detected by inductively coupled plasma atomic emission spectrometry (ICP).

[0212] Test section

[0213] (1) Volume average particle size Dv50 test

[0214] The volume average particle size (Dv50) refers to the particle size corresponding to a cumulative volume distribution percentage of 50% for the material. This application uses a Malvem Master Size 3000 laser particle size analyzer to test the volume average particle size (Dv50) of the prepared lithium manganese iron phosphate cathode active material. The testing standard can be found in GB / T 19077-2016.

[0215] (2) Tap density test

[0216] The tap density of the prepared lithium manganese iron phosphate cathode active material was tested using a BT-301 powder tap density tester from Dandong Better Instruments Co., Ltd. The testing standard is GB / T 5162-2006.

[0217] (3) Initial capacity test of button cell

[0218] The prepared lithium manganese iron phosphate positive electrode active material, binder polyvinylidene fluoride (PVDF), and conductive agent acetylene black were added to N-methylpyrrolidone (NMP) at a weight ratio of 90:5:5, and stirred in a drying chamber to form a slurry. The slurry was then coated onto aluminum foil, dried, and cold-pressed to form a positive electrode sheet. The coating amount was 10 mg / cm². 2 The compacted density is 2.0 g / cm³. 3 .

[0219] A coin cell was assembled in a coin cell box using a lithium sheet as the negative electrode and a 1 mol / L LiPF6 solution in a 1:1:1 volume ratio of ethylene carbonate (EC), diethyl carbonate (DEC), and dimethyl carbonate (DMC) as the electrolyte.

[0220] Under a constant temperature environment of 25℃, the coin cell is charged to 4.3V at a constant current of 0.1C, and then charged at a constant voltage of 4.3V until the current is less than or equal to 0.05mA. After standing for 5 minutes, it is discharged to 2.0V at a constant current of 0.1C. The discharge capacity obtained at this time is the initial specific capacity of the coin cell.

[0221] Table 1

[0222]

[0223] Figure 7 This is a scanning electron microscope (SEM) image of the nanoscale iron-containing oxide prepared in Example 1. Figure 8 This is a scanning electron microscope (SEM) image of the nanoscale lithium manganese iron phosphate cathode active material prepared in Example 1. From... Figure 7 It can be seen that this application prepared iron-containing oxides with regular morphology and nanoscale particle size via a low-temperature solid-state reaction method. From Figure 8It can be seen that, in preparing lithium manganese iron phosphate cathode active materials, this application uses nano-sized iron oxides as raw materials, which can produce lithium manganese iron phosphate cathode active materials with regular morphology and particle size at the nanometer level. The test results of Examples 1-5 also show that the preparation method of this application can also obtain nano-sized lithium manganese iron phosphate cathode active materials that balance high initial specific capacity and high tap density.

[0224] Comparative Examples 1 and 2 used ferrous oxalate and micron-sized ferric oxide as raw materials to prepare lithium manganese iron phosphate cathode active materials by the traditional high-temperature solid-state sintering method. However, the obtained lithium manganese iron phosphate cathode active materials could not achieve both high initial specific capacity and high tap density.

[0225] Comparative Example 3 uses nano-sized ferric oxide obtained from high-speed ball milling in Comparative Example 2 as raw material to prepare lithium manganese iron phosphate cathode active material. However, the tap density of the obtained lithium manganese iron phosphate cathode active material is low. At the same time, since the doping element Mg is introduced in the spray drying granulation and sintering process, it is difficult to achieve a uniform distribution of doping elements such as Mg, iron, manganese and lithium. Therefore, compared with Example 1, the initial specific capacity of the lithium manganese iron phosphate cathode active material prepared in Comparative Example 3 is significantly reduced.

[0226] It should be noted that this application is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same structure and effect as the technical concept within the scope of this application are included in the technical scope of this application. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, without departing from the spirit of this application, are also included in the scope of this application.

Claims

1. A method for preparing a lithium manganese iron phosphate cathode active material, comprising the following steps: S1, Preparation of nanoscale iron-containing oxides by low-temperature solid-state reaction method: Iron source, solid alkali, and dopant element M are mixed and ground to induce a low-temperature solid-phase reaction. After grinding, the resulting product is washed to remove impurities and dried to obtain nano-sized iron-containing oxides. The doping element M represents the doping element at the manganese and iron sites; S2, spray drying granulation: The nano-sized iron oxide obtained in S1 is mixed with solvent, lithium source, manganese source, phosphorus source, source of doping element N, source of doping element Q and source of doping element R in a predetermined ratio and then ground. After grinding, the powder is obtained by spray drying. In this case, the doping element N represents the lithium site doping element, the doping element Q represents the phosphorus site doping element and the doping element R represents the oxygen site doping element. S3, sintering: S31, the powder obtained in S2 is pre-sintered at low temperature at 400℃~550℃ in air or protective gas atmosphere, and the pre-sintered material is obtained after the reaction is completed; S32, the pre-sintered material obtained in S31 is ground and then spray-dried and granulated to obtain pre-sintered material powder; S33, the pre-sintered material powder obtained in S32 is sintered at high temperature of 700℃~750℃ in a protective gas atmosphere to obtain lithium manganese iron phosphate cathode active material.

2. The preparation method according to claim 1, wherein, The doping element M includes at least one of Al, Cu, Mg, Zn, Ni, Ti, V, Zr, Co, Ga, Sn, Sb, Nb, and Ge.

3. The preparation method according to claim 1, wherein, The doping element N includes at least one of Zn, Al, Na, K, Mg, Nb, Mo, and W; the doping element Q includes at least one of B, S, Si, and N; and the doping element R includes at least one of S, F, Cl, and Br.

4. The preparation method according to claim 1, wherein, In S1, The molar ratio of the iron source to the solid alkali is 1:(1-3); and / or, The mixing and grinding time is 20 min to 60 min; and / or, The washing and impurity removal process includes water washing and alcohol washing; and / or, The drying is vacuum drying; and / or, The drying temperature is below 100°C; and / or, The drying time is 2 hours to 15 hours.

5. The preparation method according to claim 1, wherein, In S1, the mixing and grinding is carried out in a mortar or ball mill jar.

6. The preparation method according to claim 5, wherein, The grinding speed of the mortar is 100 r / min.

7. The preparation method according to claim 5, wherein, The ball milling speed is 300 r / min to 800 r / min.

8. The preparation method according to claim 1, wherein, In S1, after the mixing and grinding are completed, a settling step is also included.

9. The preparation method according to claim 8, wherein, The settling time is 30 min to 120 min.

10. The preparation method according to claim 1, wherein, In S1, a surfactant is also added and mixed and ground together with the iron source, the solid alkali, and the source of the dopant element M.

11. The preparation method according to claim 10, wherein, The surfactant includes polyethylene glycol.

12. The preparation method according to claim 10, wherein, The amount of surfactant added is less than 5% by weight, based on the total weight of the iron source, the solid alkali, and the source of the dopant element M.

13. The preparation method according to claim 1, wherein, In S1, before mixing and grinding the iron source, the solid alkali, and the source of the doping element M to induce a low-temperature solid-phase reaction in each component, the step further includes grinding the iron source, the solid alkali, and the source of the doping element M separately.

14. The preparation method according to claim 13, wherein, The grinding time for the iron source is 30 min to 90 min.

15. The preparation method according to claim 13, wherein, The grinding time for the solid alkali is 30 min to 90 min.

16. The preparation method according to claim 13, wherein, The source grinding time for the dopant element M is 30 min to 90 min.

17. The preparation method according to claim 1, wherein, In S1, The iron source is a ferric salt; and / or, The solid alkali includes at least one selected from sodium hydroxide, potassium hydroxide, sodium carbonate, and potassium carbonate; and / or, The source of the dopant element M includes at least one of the hydrochloride, nitrate, sulfate and acetate salts of the dopant element M.

18. The preparation method according to claim 1, wherein, The iron source includes at least one of ferric chloride, ferric nitrate and ferric sulfate; The solid alkali includes sodium hydroxide; In S2, The grinding is carried out in a ball mill jar.

19. The preparation method according to claim 18, wherein, The ball mill rotation speed is 300 r / min to 800 r / min; and / or, The spray drying temperature is 200℃~250℃.

20. The preparation method according to claim 19, wherein, The ball milling time is 3h to 24h.

21. The preparation method according to claim 1, wherein, In S2, The solvent includes ethanol, water, or a mixture thereof; and / or, The lithium source includes at least one of Li₂CO₃, LiOH, Li₃PO₄, and LiH₂PO₄; and / or, The manganese source includes at least one of MnCO3, Mn(CH3COO)2, and MnC2O4; and / or, The phosphorus source includes at least one of (NH4)2HPO4, NH4H2PO4, (NH4)3PO4, and H3PO4; and / or, The source of the dopant element N includes at least one of the following: hydrochloride, nitrate, sulfate, and acetate of the dopant element N; and / or, The source of the dopant element Q includes at least one of the following: sulfate, borate, nitrate, and silicate of the dopant element Q; and / or, The source of the dopant element R includes at least one of the elemental form of dopant element R and an ammonium salt.

22. The preparation method according to claim 1, wherein, In S2, a carbon source is also added for mixing and grinding.

23. The preparation method according to claim 22, wherein, The carbon source includes at least one of organic carbon sources and inorganic carbon sources.

24. The preparation method according to claim 22, wherein, The carbon source includes at least one of glucose, sucrose, starch, fructose, polyvinyl alcohol, polyethylene glycol, and citric acid.

25. The preparation method according to claim 1, wherein, In S31, the low-temperature pre-sintering time is 2h to 10h; and / or, In S33, the high-temperature sintering time is 2h to 10h.

26. The preparation method according to claim 1, wherein, In S32, the grinding is carried out in a ball mill jar.

27. The preparation method according to claim 26, wherein, The ball milling speed is 300 r / min to 800 r / min.

28. The preparation method according to claim 26, wherein, The ball milling time is 3h to 24h.

29. The preparation method according to claim 1, further comprising the step of: The lithium iron phosphate cathode active material obtained in S3 is crushed to the required particle size, and the crushing is airflow crushing.

30. A lithium manganese iron phosphate cathode active material obtained by the preparation method of claim 1, having the chemical formula Li a N b Fe x Mn y M 1-x-y P 1-m Q m O 4-n R n The doping element M represents the manganese and iron sites, the doping element N represents the lithium site, the doping element Q represents the phosphorus site, and the doping element R represents the oxygen site. a is 0.9 to 1.1, b is 0 to 0.1, x is 0.001 to 0.999, y is 0.001 to 0.999, 1-xy is 0 to 0.1, m is 0 to 0.1, and n is 0 to 0.

1. Furthermore, the lithium manganese iron phosphate cathode active material is electrically neutral. The tap density of the lithium manganese iron phosphate cathode active material is 1.4 g / cm³. 3 above.

31. The lithium manganese iron phosphate cathode active material according to claim 30, wherein, The doping element M includes at least one of Al, Cu, Mg, Zn, Ni, Ti, V, Zr, Co, Ga, Sn, Sb, Nb and Ge; The doping element N includes at least one of Zn, Al, Na, K, Mg, Nb, Mo, and W; The doping element Q includes at least one of B, S, Si, and N; The doping element R includes at least one of S, F, Cl and Br; The tap density of the lithium manganese iron phosphate cathode active material is 1.47 g / cm³. 3 ~1.82g / cm 3 .

32. The lithium manganese iron phosphate cathode active material according to claim 30, wherein, b is 0.001 to 0.1; x is 0.199 to 0.50; y is 0.499 to 0.80; 1-xy is 0.001 to 0.1; m is 0.001 to 0.1; n is 0.001 to 0.

1.

33. The lithium iron manganese phosphate cathode active material according to claim 30, wherein, The volume average particle size Dv50 of the lithium manganese iron phosphate cathode active material is 300 nm to 500 nm; and / or, The lithium manganese iron phosphate cathode active material has a spherical or near-spherical single crystal morphology.

34. A positive electrode sheet, comprising a positive current collector and a positive electrode film layer disposed on at least one surface of the positive current collector, wherein, The positive electrode film layer comprises lithium manganese iron phosphate positive electrode active material obtained by the preparation method of any one of claims 1-29 or lithium manganese iron phosphate positive electrode active material of any one of claims 30-33.

35. The positive electrode sheet according to claim 34, wherein, The content of the lithium manganese iron phosphate positive electrode active material in the positive electrode film layer is more than 50% by weight, based on the total weight of the positive electrode film layer.

36. The positive electrode sheet according to claim 34, wherein, The content of the lithium manganese iron phosphate cathode active material in the cathode film layer is 90% to 99% by weight.

37. A secondary battery comprising a lithium manganese iron phosphate positive electrode active material obtained by any one of claims 1-29, or a lithium manganese iron phosphate positive electrode active material obtained by any one of claims 30-33, or a positive electrode sheet as described in claim 34.

38. An electrical device comprising the secondary battery of claim 37.

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