Composite lithium manganese iron phosphate material, method for preparing the same, secondary battery, and electric device

Abstract

The application provides a composite lithium manganese iron phosphate material, a preparation method thereof, a secondary battery and an electric device. The composite lithium manganese iron phosphate material comprises lithium manganese iron phosphate particles and a coating layer; an upper limit voltage used by the lithium manganese iron phosphate particles is denoted as V1, and the unit is V; the coating layer is coated on at least part of the surface of the lithium manganese iron phosphate particles, and the coating layer comprises metal nanoparticles; an oxidation voltage of the metal nanoparticles is denoted as V2, and the unit is V, wherein the composite lithium manganese iron phosphate material satisfies V1V2. By coating the surface of the lithium manganese iron phosphate particles with the metal nanoparticles, the application can improve the cycle stability of the material and ensure the stable exertion of the capacity.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of batteries, in particular to a composite lithium manganese iron phosphate material, a preparation method thereof, a secondary battery and an electric device. BACKGROUND

[0002] Secondary batteries have the characteristics of high capacity and long service life, and are therefore widely used in electronic devices such as mobile phones, notebook computers, electric vehicles, electric cars, electric planes, electric ships, electric toy cars, electric toy ships, electric toy planes and electric tools.

[0003] With the increasingly wide application of batteries, the performance requirements for secondary batteries are gradually stringent. In order to improve the performance of secondary batteries, the positive active material in the secondary battery is usually optimized and improved. However, the specific capacity of the positive active material is poor when it is applied to the secondary battery, and the cycle performance of the secondary battery is poor. SUMMARY

[0004] The present application is made in view of the above problems, and aims to provide a composite lithium manganese iron phosphate material, a preparation method thereof, a secondary battery and an electric device.

[0005] The first aspect of the present application provides a composite lithium manganese iron phosphate material, the composite lithium manganese iron phosphate material comprising lithium manganese iron phosphate particles and a coating layer; the upper limit voltage of the lithium manganese iron phosphate particles is denoted as V1, and the unit is V; the coating layer is coated on at least part of the surface of the lithium manganese iron phosphate particles, and the coating layer comprises metal nanoparticles, the oxidation voltage of the metal nanoparticles is denoted as V2, and the unit is V, wherein the composite lithium manganese iron phosphate material satisfies: V1 < V2.

[0006] Thus, by coating the surface of the lithium manganese iron phosphate particles with metal nanoparticles, the present application can improve the conductivity of the coated material and reduce its electrical resistance; the performance of the metal nanoparticles is stable and is not prone to oxidation under high pressure, thereby ensuring the stability of the conductivity of the coated material during the use cycle, reducing the risk of rapid polarization increase and rapid capacity decay of the material in the later cycle, and ensuring the cycle stability and stable capacity of the material. In some embodiments, the structural formula of the lithium manganese iron phosphate particles is LiMn 1-x Fe x M y PO4, wherein 0.05≤x≤0.95; 0≤y≤1; optionally, 0≤y≤0.2; M represents a doping element, the lithium manganese iron phosphate particles comprise the doping element M, the doping element M comprises one or more elements selected from the group consisting of sulfur, nitrogen, boron, fluorine, chlorine, bromine and iodine; optionally, the doping element M comprises a sulfur element.

[0007] Therefore, the doping element M can form a bonding effect between the doping element M and the metal nanoparticles in the formation process of the metal nanoparticles, plays an anchoring role on the metal nanoparticles, and realizes the positioning growth and small size growth of the metal nanoparticles. Therefore, uniform doping of the doping element M is beneficial to the uniform distribution of the metal nanoparticles and improves the coating performance on the lithium manganese iron phosphate particles.

[0008] In some embodiments, the metal nanoparticles include one or more nanoparticles of silver, gold, platinum, palladium, rhodium, iridium, osmium, and ruthenium.

[0009] The metal nanoparticles can stably exist at the working voltage of the lithium manganese iron phosphate, and play a role in improving the conductivity of the lithium manganese iron phosphate.

[0010] In some embodiments, the mass content of the coating layer is denoted as A%, based on the total mass of the composite lithium manganese iron phosphate material, and 0.3≤A≤10; optionally, 0.3≤A≤3.

[0011] Therefore, when the mass content of the coating layer is in the above range, the thickness of the coating layer is not too thick, thereby being beneficial to the smooth migration of lithium ions, ensuring the normal progress of the cycle process, and the coating layer does not excessively occupy the space of the lithium manganese iron phosphate, ensuring the specific capacity of the composite lithium manganese iron phosphate; the thickness of the coating layer is not too thin, thereby being able to play a good coating effect on the surface of the lithium manganese iron phosphate particles and improve the overall conductivity of the coated lithium manganese iron phosphate particles.

[0012] In some embodiments, the thickness of the coating layer is denoted as H, in units of nm, and 2≤H≤100; optionally, 5≤H≤20.

[0013] Therefore, when the thickness of the coating layer is in the above range, the thickness of the coating layer is not too thick, thereby being beneficial to the smooth migration of lithium ions, ensuring the normal progress of the cycle process; the thickness of the coating layer is not too thin, thereby being able to play a good coating effect on the surface of the lithium manganese iron phosphate particles and improve the overall conductivity of the coated lithium manganese iron phosphate particles.

[0014] In some embodiments, the composite lithium manganese iron phosphate material satisfies: the average particle size of the metal nanoparticles is denoted as D1, in units of nm, and D1≤20; optionally, D1≤10.

[0015] Therefore, when the average particle size D of the metal nanoparticles is in the above range, the average particle size thereof is not too large, which can play a close coating on the lithium manganese iron phosphate particles, thereby being able to uniformly improve the overall conductivity of the lithium manganese iron phosphate particles and reduce the coating amount; the average particle size D of the metal nanoparticles is not too small, which has a certain gap that is beneficial to the migration of lithium ions and ensures the smooth progress of the cycle process.

[0016] In some embodiments, the composite lithium manganese iron phosphate material satisfies: the volume average particle size Dv50 of the lithium manganese iron phosphate particles is denoted as D2, in units of μm, 0.1≤D2≤10; optionally, 0.2≤D2≤5.

[0017] Thus, the structure of the lithium manganese iron phosphate particles of the present application is relatively stable, and the kinetic performance is relatively good, which is conducive to improving the initial coulomb efficiency of the composite lithium manganese iron phosphate particles; and is conducive to forming a uniform coating of the metal nanoparticles on the surface of the lithium manganese iron phosphate particles.

[0018] In some embodiments, the composite lithium manganese iron phosphate material satisfies: the volume average particle size Dv50 of the composite lithium manganese iron phosphate material is denoted as D, in units of μm, 0.1≤D≤10; optionally, 02≤D≤5.

[0019] Thus, when the composite lithium manganese iron phosphate material of the present application satisfies the above range, the structure is relatively stable, and the kinetic performance is relatively good, which is conducive to improving the initial coulomb efficiency of the composite lithium manganese iron phosphate material.

[0020] The second aspect of the present application provides a preparation method of a composite lithium manganese iron phosphate material, the method comprising: providing lithium manganese iron phosphate particles; providing a conductive precursor to the lithium manganese iron phosphate particles, and heat treating the conductive precursor to reduce the conductive precursor to form a coating layer coating the lithium manganese iron phosphate particles, the coating layer comprising metal nanoparticles, wherein the upper limit voltage of the lithium manganese iron phosphate particles is denoted as V1, in units of V; the coating layer is coated on at least part of the surface of the lithium manganese iron phosphate particles, the coating layer comprises metal nanoparticles, the oxidation voltage of the metal nanoparticles is denoted as V2, in units of V, and the composite lithium manganese iron phosphate material satisfies: V1<V2.

[0021] In some embodiments, the step of providing the lithium manganese iron phosphate particles comprises: doping a doping element M into the lithium manganese iron phosphate particles, the doping element M comprising one or more of sulfur, nitrogen, boron, fluorine, chlorine, bromine, and iodine; optionally, the doping element M comprises a sulfur element.

[0022] In some embodiments, the temperature of the heat treatment is 400°C to 1000°C; and / or the time of the heat treatment is 2h to 6h.

[0023] Thus, by adjusting the conditions of the heat treatment, the present application can control the size of the metal nanoparticles formed; the temperature of the heat treatment is not too high, which is conducive to the moderate growth of the metal nanoparticles, the particles of which are not too large, and the doping element can easily play an anchoring effect, so that the metal nanoparticles are uniformly distributed on the surface of the lithium manganese iron phosphate particles, and the particle size of the particles formed is relatively moderate, which is conducive to forming a good coating effect on the lithium manganese iron phosphate particles.

[0024] In some embodiments, the conductive precursor includes one or more of nitrate, chloride, bromide, iodide, sulfate, phosphate, acetate, and acetylacetone; and / or the conductive precursor includes one or more of silver, gold, platinum, palladium, rhodium, iridium, osmium, and ruthenium ions.

[0025] In some embodiments, the molar content of the conductive precursor is b%, based on the total molar ratio of the conductive precursor and the lithium manganese iron phosphate particles, and is 0.25≤b≤14.5%; alternatively, it is 0.40≤b≤4.60%.

[0026] Therefore, this application regulates the quality of the coating layer formed on the surface of lithium manganese iron phosphate particles by controlling the molar content of the conductive precursor. Since lithium manganese iron phosphate is the main material in the composite lithium manganese iron phosphate, the coating layer has little impact on the overall specific capacity of the material, thus ensuring the overall specific capacity of the material. Moreover, the coating layer can uniformly coat the lithium manganese iron phosphate, improve the overall conductivity of the composite lithium manganese iron phosphate, and facilitate the capacity utilization of lithium manganese iron phosphate.

[0027] A third aspect of this application provides a secondary battery including a positive electrode, said positive electrode comprising a composite lithium manganese iron phosphate material as described in any embodiment of the first aspect of this application or a composite lithium manganese iron phosphate material obtained by the method described in any embodiment of the second aspect of this application.

[0028] The fourth aspect of this application also provides an electrical device, including a secondary battery as described in the third aspect of this application. Attached Figure Description

[0029] To more clearly illustrate the technical solutions of the embodiments of this application, the drawings used in the embodiments of this application will be briefly introduced 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.

[0030] Figure 1 This is a schematic diagram of one embodiment of the secondary battery of this application.

[0031] Figure 2 yes Figure 1 An exploded view of the implementation method of the secondary battery.

[0032] Figure 3 This is a schematic diagram of one embodiment of the battery module of this application.

[0033] Figure 4 This is a schematic diagram of one embodiment of the battery pack of this application.

[0034] Figure 5 yes Figure 4 An exploded view of an embodiment of the battery pack shown.

[0035] 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.

[0036] Figure 7 These are the cycle curves of Example 1 and Comparative Example 1.

[0037] The accompanying drawings may not be drawn to scale.

[0038] The annotations in the attached figures are explained as follows:

[0039] 1. Battery pack; 2. Upper casing; 3. Lower casing; 4. Battery module;

[0040] 5. Secondary battery; 51. Housing; 52. Electrode assembly;

[0041] 53. Cover plate;

[0042] 6. Electrical appliances. Detailed Implementation

[0043] The following detailed description discloses the composite lithium manganese iron phosphate material, its preparation method, secondary battery, and embodiment of the power supply 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 to enable those skilled in the art to fully understand this application and are not intended to limit the subject matter of the claims.

[0044] 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.

[0045] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions. Unless otherwise specified, all technical features and optional technical features of this application can be combined to form new technical solutions.

[0046] Unless otherwise specified, all steps of this application may be performed sequentially or randomly, preferably sequentially. For example, if the method includes steps (a) and (b), it means that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, if the method may also include step (c), it means that step (c) may be added to the method in any order. For example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0047] 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.

[0048] 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).

[0049] In this application, the terms "multiple" or "various" refer to two or more kinds.

[0050] Lithium manganese iron phosphate (LMP) possesses an olivine structure and its theoretical capacity is comparable to that of lithium iron phosphate (LFP). Due to the presence of manganese (Mn), its electrode potential is relatively high, giving it the potential for high energy density. However, LMP crystals also exhibit a hexagonal close-packed structure, with lithium and iron atoms occupying the 4a and 4c sites of the octahedrons, respectively, and phosphorus atoms occupying the 4c ​​site of the tetrahedron. The FeO6 (MnO6) octahedra and PO4 tetrahedra are cross-connected, resulting in good stability. Even if all lithium ions are released during charging, structural collapse is unlikely. Furthermore, the phosphorus atoms in the material form PO4 tetrahedra through strong covalent bonds with PO, making it difficult for oxygen atoms to escape from the structure, thus contributing to the material's high stability. However, because the material is connected by PO4 tetrahedra, it lacks a continuous FeO6 (MnO6) octahedral network, leading to poor electrical conductivity, low electronic conductivity, and a low lithium-ion diffusion coefficient. This makes the synthesis of reversibly charge-discharge LMP difficult, and its poor conductivity limits its development.

[0051] The inventors discovered that, in order to improve the conductivity of lithium manganese iron phosphate, a conductive carbon layer is usually coated on the surface of the lithium manganese iron phosphate particles. However, after in-depth research, the inventors found that, since lithium manganese iron phosphate is suitable for high voltage systems, such as >4V, the oxidation of the carbon layer is enhanced under high voltage systems. This may lead to the generation of gas by the oxidation of the carbon layer. After long-term use, the conductivity of the material decreases, the impedance increases, and the cycle performance deteriorates.

[0052] In view of this, the inventors have improved the coating structure of lithium manganese iron phosphate by coating the surface of the lithium manganese iron phosphate particles with a coating layer containing metal nanoparticles. This coating layer improves the conductivity of lithium manganese iron phosphate, which is beneficial for its capacity utilization. Furthermore, the coating layer is stable and improves the overall structural stability of lithium manganese iron phosphate, thus improving the cycle performance of the secondary batteries in which it is applied. The following is a detailed description of this application.

[0053] Composite lithium manganese iron phosphate material

[0054] Firstly, this application proposes a composite lithium manganese iron phosphate material.

[0055] The described lithium manganese iron phosphate composite material includes lithium manganese iron phosphate particles and a coating layer; the upper limit voltage of the lithium manganese iron phosphate particles is denoted as V1, with the unit of V; the coating layer coats at least part of the surface of the lithium manganese iron phosphate particles, and the coating layer includes metal nanoparticles, and the oxidation voltage of the metal nanoparticles is denoted as V2, with the unit of V. Among them, the lithium manganese iron phosphate composite material satisfies: V1 < V2.

[0056] The upper limit voltage V1 of lithium manganese iron phosphate refers to the upper limit charging voltage when lithium manganese iron phosphate is used as a positive electrode active material in a secondary battery. At this voltage, the stability of the material can be basically ensured for lithium manganese iron phosphate. For example, the upper limit voltage V1 of lithium manganese iron phosphate can be set to 3.7V to 4.5V. V1 can be set by the manufacturer.

[0057] The oxidation voltage V2 of the metal nanoparticles refers to the voltage at which the metal nanoparticles lose electrons and become metal ions. V2 can be tested by linear sweep voltammetry (LSV). The metal to be measured is used as the working electrode, and the lithium sheet is used as the reference electrode. The voltage is scanned from 0V to 5V or a higher voltage at a scanning speed of 0.15mV / s to 50mV / s. The voltage when the oxidation current significantly increases is V2. The scanning speed can be selected as 1mV / s to 5mV / s.

[0058] In this application, setting V1 < V2 can ensure that the metal nanoparticles are basically not oxidized during the long-term cyclic charge and discharge process of the secondary battery, and guarantee the structural stability of the lithium manganese iron phosphate composite material.

[0059] The lithium manganese iron phosphate particles have the structural formula LiMn 1-x Fe x M y PO4, where 0.05 ≤ x ≤ 0.95; 0 ≤ y ≤ 1; optionally, 0 ≤ y ≤ 0.2.

[0060] M represents a doping element, that is, the lithium manganese iron phosphate can be a material modified by a doping element or a material without a doping element. The doping element can be a cation doping element or an anion doping element. The lithium manganese iron phosphate can be charged and discharged at a high voltage of about 4.45V. Due to the above potential relationship satisfied by the lithium manganese iron phosphate composite material, the metal nanoparticles can stably exist under the working voltage of the lithium manganese iron phosphate and are not easily oxidized, so that they can play a long-term stable coating role on the lithium manganese iron phosphate particles, improve the structural stability of the coated lithium manganese iron phosphate, and improve the cycle performance of the lithium manganese iron phosphate composite material.

[0061] Lithium manganese iron phosphate (LMP) particles typically have a small particle size, while the particles in the coating layers of related technologies are relatively large, making it difficult for the coating layer to form a uniform coating on the surface of LMP particles, thus failing to achieve a good coating effect. The coating layer of this application includes metal nanoparticles. The metal particles in the coating layer have a small particle size, at the nanometer scale, which can be dispersed on the surface of the LMP particles to form a uniform coating, resulting in better overall coating performance. Furthermore, due to the small particle size of the metal nanoparticles, the contact between adjacent metal nanoparticles is closer, and the metal nanoparticles can fill the gaps between adjacent layers within the coating layer. This results in better conductivity of the coating layer composed of metal nanoparticles, which can reduce conductivity resistance to a certain extent and improve the conductivity of the coated LMP particles. Improved conductivity is beneficial for maximizing the capacity of the LMP particles.

[0062] This application improves the conductivity and reduces the conductivity resistance of lithium manganese iron phosphate particles by coating them with metal nanoparticles. Furthermore, the metal nanoparticles are stable and do not easily oxidize under high voltage, thus ensuring the conductivity stability of the coated material during its service life. This reduces the risk of rapid polarization increase and rapid capacity decay in the later stages of cycling, ensuring the stable cycling stability and capacity of the material.

[0063] To further improve the coating effect of the coating layer and enhance the conductivity of the composite lithium manganese iron phosphate material, in some embodiments, the lithium manganese iron phosphate particles include a dopant element M, which includes one or more elements selected from sulfur, nitrogen, boron, fluorine, chlorine, bromine, iodine, etc.; optionally, the dopant element M includes sulfur.

[0064] The doping element M is incorporated in the form of anions, which can occupy oxygen vacancies in lithium manganese iron phosphate particles during charging and discharging, thereby stabilizing the structure of lithium manganese iron phosphate. Furthermore, the doping element M can increase the electron density of lithium manganese iron phosphate particles, improve the conductivity and lithium capacity of lithium manganese iron phosphate, improve the overall conductivity of composite lithium manganese iron phosphate materials, and facilitate capacity utilization and improve cycle performance.

[0065] During the formation of metal nanoparticles, the dopant element M can form bonds with the metal nanoparticles, acting as an anchor to achieve targeted and small-size growth. Therefore, uniform doping of the dopant element M is beneficial for the uniform distribution of metal nanoparticles and improves the coating performance of lithium manganese iron phosphate particles.

[0066] In some embodiments, the metal nanoparticles include one or more nanoparticles selected from silver, gold, platinum, palladium, rhodium, iridium, osmium, ruthenium, mercury, and thallium. These metal nanoparticles are stable under the operating voltage of lithium manganese iron phosphate, thus improving the conductivity of the lithium manganese iron phosphate. Optionally, the metal nanoparticles include one or more nanoparticles selected from silver, gold, platinum, palladium, rhodium, iridium, osmium, and ruthenium. These metals have lower toxicity and are more suitable for battery systems.

[0067] In some embodiments, the mass content of the coating layer is denoted as A% based on the total mass of the composite lithium manganese iron phosphate material, and 0.3 ≤ A ≤ 10%; alternatively, 0.3 ≤ A ≤ 3%.

[0068] In this application, the mass content of the coating layer is a term known in the art and can be determined using methods or equipment known in the art. Specifically, a certain weight of composite lithium manganese iron phosphate material is weighed, digested with aqua regia, and then the solution is diluted. The weight of the coating metal in the diluted solution is tested by ICP. The mass percentage of the coating layer is calculated based on the tested metal weight and the weight of the composite lithium manganese iron phosphate material used.

[0069] When the mass content of the coating layer is within the above-mentioned range, the thickness of the coating layer will not be too thick, thus facilitating the smooth migration of lithium ions and ensuring the normal operation of the cycling process. Furthermore, the coating layer will not excessively encroach on the space of the lithium manganese iron phosphate, ensuring the specific capacity of the composite lithium manganese iron phosphate. Conversely, the thickness of the coating layer will not be too thin, thus providing good coating of the surface of the lithium manganese iron phosphate particles and improving the overall conductivity of the coated lithium manganese iron phosphate particles. For example, the mass content A% of the coating layer can be 0.3%, 0.5%, 0.8%, 1.0%, 1.5%, 2.0%, 2.5%, 3.0%, 3.5%, 4.0%, 5.0%, 6.0%, 7.0%, 8.0%, 9.0%, or 10%; or a range consisting of any two of the above values.

[0070] In some embodiments, the thickness of the coating layer is denoted as H, in nm, where 2 ≤ H ≤ 100; alternatively, 5 ≤ H ≤ 20.

[0071] In this application, the thickness of the coating layer is a term known in the art and can be measured using methods or equipment known in the art. Specifically, a certain amount of composite lithium manganese iron phosphate material can be taken as a sample and subjected to high-resolution transmission electron microscopy (HRTEM) analysis to obtain HRTEM images. Then, the thickness at multiple (e.g., more than 30) different locations on the HRTEM images is measured, and the average value is taken as the average thickness of the coating layer. From the HRTEM images, it can be seen that there are obvious grain boundaries between the coating layer and the lithium manganese iron phosphate particles.

[0072] When the thickness of the coating layer is within the above range, the coating layer will not be too thick, which is conducive to the smooth migration of lithium ions and ensures the normal operation of the cycling process; the coating layer will not be too thin, which can play a good coating role on the surface of lithium manganese iron phosphate particles and improve the overall conductivity of the coated lithium manganese iron phosphate particles.

[0073] For example, the thickness H nm of the coating layer can be 2 nm, 3 nm, 5 nm, 10 nm, 20 nm, 30 nm, 50 nm, 60 nm, 80 nm or 100 nm; or a range of any two of the above values.

[0074] In some embodiments, the average particle size D of the metal nanoparticles is denoted as D1, in nm, and D1 ≤ 20; alternatively, D1 ≤ 10.

[0075] In this application, the average particle size D of the metal nanoparticles has a meaning known in the art and can be measured using methods or equipment known in the art; the metal coating is granular, and a certain amount of composite lithium manganese iron phosphate material can be taken as a sample, and high-resolution transmission electron microscopy analysis can be performed to obtain HRTEM images. Then, the particle size of multiple (e.g., more than 30) metal nanoparticles at different positions can be measured on the HRTEM images, and the average value can be taken as the average particle size D of the metal nanoparticles.

[0076] When the average particle size D of the metal nanoparticles is within the aforementioned range, its average particle size will not be too large, allowing for a tight coating of the lithium manganese iron phosphate particles. This uniformly improves the overall conductivity of the lithium manganese iron phosphate particles and reduces the coating weight. Conversely, the average particle size D of the metal nanoparticles will not be too small, as it contains sufficient porosity to facilitate lithium ion migration and ensure smooth cycling. For example, the average particle size of the metal nanoparticles can be 2 nm, 3 nm, 5 nm, 6 nm, 8 nm, 10 nm, 12 nm, 15 nm, 18 nm, or 20 nm; or any combination of two of the aforementioned values.

[0077] In some embodiments, the volume average particle size Dv50 of the lithium manganese iron phosphate particles is denoted as D2, in μm, and 0.1≤D2≤10; optionally, 0.2≤D2≤5.

[0078] In this application, the volume average particle size Dv50 of the material has a well-known meaning in the art, representing the particle size corresponding to a cumulative volume distribution percentage of 50%, which can be tested using instruments and methods known in the art. For example, it can be conveniently tested using a laser particle size analyzer, such as the Mastersizer 2000E laser particle size analyzer from Malvern Instruments Ltd., UK, in accordance with GB / T19077-2016 Particle Size Distribution Laser Diffraction Method.

[0079] When lithium manganese iron phosphate particles meet the above range, the first and second particles in the lithium manganese iron phosphate particles may coexist, or only one of them may exist. The structure of lithium manganese iron phosphate particles is relatively stable and the kinetic performance is relatively good, which is conducive to improving the first coulombic efficiency of composite lithium manganese iron phosphate particles; and it is also conducive to the formation of uniform coating of metal nanoparticles on the surface of lithium manganese iron phosphate particles.

[0080] In some embodiments, the volume average particle size Dv50 of the composite lithium manganese iron phosphate material is denoted as D, with units of μm, and 0.1≤D≤10; optionally, 0.2≤D≤5.

[0081] The thickness of the coating layer differs from the particle size of lithium manganese iron phosphate by orders of magnitude. The coating layer thickness contributes negligibly to the particle size of the composite lithium manganese iron phosphate. Therefore, the particle size difference between lithium manganese iron phosphate particles and composite lithium manganese iron phosphate particles is not significant at the micrometer level. When the composite lithium manganese iron phosphate material meets the above-mentioned range, its structure is relatively stable, and its kinetic performance is relatively good, which is beneficial to improving the first coulombic efficiency of the composite lithium manganese iron phosphate material.

[0082] Method for preparing composite lithium iron phosphate material

[0083] Secondly, this application proposes a method for preparing a composite lithium manganese iron phosphate material, which can prepare the composite lithium manganese iron phosphate material according to any embodiment of the first aspect of this application.

[0084] The method includes:

[0085] Step S100: Provide lithium iron phosphate granules;

[0086] Step S200: A conductive precursor is provided to the lithium manganese iron phosphate particles, and the conductive precursor is heat-treated to reduce the conductive precursor to form a coating layer covering the lithium manganese iron phosphate particles, the coating layer comprising metal nanoparticles.

[0087] The surface of lithium manganese iron phosphate particles is coated with metal nanoparticles, which can improve the conductivity of the coated material and reduce its conductivity resistance. Moreover, the metal nanoparticles are stable and not easily oxidized under high pressure, thus ensuring the conductivity of the coated material during its service life, reducing the risk of rapid polarization and rapid capacity decay in the later stages of cycling, and ensuring the cycling stability and stable capacity of the material.

[0088] Particularly, when the composite lithium iron phosphate material meets the following requirements: the upper limit voltage of the lithium iron manganese phosphate particles is denoted as V1, with the unit of V; the coating layer covers at least part of the surface of the lithium iron manganese phosphate particles, and the coating layer includes metal nanoparticles, the oxidation voltage of the metal nanoparticles is denoted as V2, with the unit of V, and the composite lithium iron manganese phosphate material satisfies: V1 < V2, which can improve the conductivity and cycling performance of the composite lithium iron phosphate material.

[0089] In some embodiments, in step S100, the lithium iron manganese phosphate particles can be prepared by various methods. For example, solid-phase method, co-precipitation method, sol-gel method, hydrothermal / solvent thermal method, etc. Next, the solid-phase method will be taken as an example for illustration. The lithium source, manganese source, iron source, phosphorus source, etc. are added to a solvent according to a set molar ratio, such as 1:(1 - m):m:1, and mixed into a slurry. The slurry is ball-milled and then dried; then it is sintered at a high temperature under a protective gas to obtain the lithium iron phosphate material. Among them, 0.05 ≤ m ≤ 0.95, preferably 0.2 ≤ m ≤ 0.5.

[0090] The lithium source can include one or more of lithium hydroxide, lithium carbonate, lithium nitrate, lithium dihydrogen phosphate, lithium hydrogen phosphate, and lithium oxalate.

[0091] The manganese source can include manganese carbonate and / or manganese oxalate.

[0092] The iron source can include one or more of ferrous carbonate, ferrous acetate, ferrous sulfate, iron nitrate, iron phosphate, iron acrylate, and ferrous oxalate.

[0093] The phosphorus source can include one or more of ammonium phosphate, ammonium dihydrogen phosphate, ammonium hydrogen phosphate, and iron phosphate.

[0094] The solvent can be deionized water and / or ethanol.

[0095] The protective gas can include one or more of nitrogen, argon, and helium.

[0096] Furthermore, the doping element M is doped into the lithium iron manganese phosphate particles. The doping element M includes one element or multiple elements such as sulfur, nitrogen, boron, fluorine, chlorine, bromine, iodine, etc.; optionally, the doping element M includes sulfur element. Specifically, a raw material containing the doping element M can be continuously added to the slurry, then ball-milled, dried, and then sintered at a high temperature under a protective gas to obtain the lithium iron manganese phosphate material doped with the doping element.

[0097] The sulfur source can include one or several of thiourea, carbon disulfide, hydrogen sulfide, sulfur powder, and sodium dodecylbenzenesulfonate. The nitrogen source can include one or more of ammonium chloride, ammonium iodide, ammonium formate, ammonium acetate, hexamethylenetetramine, and glucosamine.

[0098] The boron source may include one or more of boron dioxide, boron trioxide, boric acid, tetraphenylboronic acid, boron carbide, and tributyl borate.

[0099] The molar ratio of sulfur to lithium is (0.01-0.05):1.

[0100] Lithium manganese iron phosphate particles can also be prepared by dry mixing, and the specific preparation process can adopt the existing processes in this field.

[0101] In some embodiments, in step S200, the conductive precursor may include one or more of nitrates, chlorides, bromides, iodides, sulfates, phosphates, acetates, and acetylacetonates. That is, the anion of the conductive precursor is one or more of nitrate, chloride, bromide, iodide, sulfate, phosphate, acetate, and acetylacetonates.

[0102] The cations of the conductive precursor may include one or more of the following ions: silver ion, gold ion, platinum ion, palladium ion, rhodium ion, iridium ion, osmium ion, and ruthenium ion.

[0103] For example, the conductive precursor is a reasonable combination of any of the above anions and any of the above cations. For example, the conductive precursor can be an inorganic precursor or an organic precursor, such as silver nitrate, silver chloride, gold nitrate, silver acetylacetonate, etc.

[0104] The conductive precursor facilitates reduction under a reducing atmosphere, and nitrate ions can decompose and are not easily retained in the composite lithium manganese iron phosphate material; chloride ions, bromide ions, etc. can react with hydrogen ions to form hydrogen chloride or hydrogen bromide gas and volatilize out of the system, and are not easily retained in the composite lithium manganese iron phosphate material, thus having little impact on the composite lithium manganese iron phosphate material.

[0105] In some embodiments, in step S200, based on the total molar ratio of the conductive precursor and lithium manganese iron phosphate particles, the molar content of the conductive precursor is b%, 0.25≤b≤14.5; optionally, 0.40≤b≤4.60. Taking silver ions as an example of the cation of the conductive precursor, 0.45≤b≤13.7; optionally, 0.70≤b≤4.20.

[0106] By controlling the molar content of the conductive precursor, the quality of the coating layer formed by the conductive precursor on the surface of lithium manganese iron phosphate particles can be controlled. In the composite lithium manganese iron phosphate, lithium manganese iron phosphate is the main material, and the coating layer has little impact on the overall specific capacity of the material, which can ensure the overall specific capacity of the material. Moreover, the coating layer can uniformly coat the lithium manganese iron phosphate, improve the overall conductivity of the composite lithium manganese iron phosphate, and is conducive to the capacity utilization of lithium manganese iron phosphate.

[0107] In some embodiments, in step S200, the temperature of the heat treatment can be from 400°C to 1000°C; and / or the heat treatment time can be from 2 hours to 6 hours.

[0108] By controlling the heat treatment conditions, the size of the metal nanoparticles can be controlled. A moderately high heat treatment temperature promotes appropriate growth of the nanoparticles, preventing them from becoming excessively large and facilitating the anchoring effect of the dopant elements. This results in a uniform distribution of metal nanoparticles on the surface of lithium manganese iron phosphate particles, with a relatively moderate particle size that effectively coats the particles. Conversely, a moderately low heat treatment temperature ensures that the metal ions in the conductive precursor are fully reduced to elemental metals.

[0109] Correspondingly, controlling the heat treatment time can also achieve similar effects. Specifically, sufficient heat treatment time can fully reduce the metal ions in the conductive precursor to elemental metals and ensure that the elemental metals do not grow excessively. The nanoparticles are relatively small in size and can uniformly coat the surface of lithium manganese iron phosphate particles.

[0110] In some embodiments, in step S200, heat treatment can be performed under a reducing atmosphere, allowing the conductive precursor to be sufficiently reduced to generate metal nanoparticles. For example, the reducing atmosphere can be a mixture of argon and hydrogen, with hydrogen comprising a relatively small proportion, such as 5%.

[0111] Secondary battery

[0112] Thirdly, this application proposes a secondary battery.

[0113] A secondary battery consists of a positive electrode, a negative electrode, an electrolyte, and a separator. During charging and discharging, active ions move back and forth between the positive and negative electrodes, inserting and releasing. The electrolyte acts as a conductor of ions between the positive and negative electrodes. The separator, positioned between the positive and negative electrodes, primarily prevents short circuits while allowing ions to pass through.

[0114] [Positive electrode plate]

[0115] In some embodiments, the positive electrode includes a positive current collector and a positive electrode film layer disposed on at least one surface of the positive current collector. For example, the positive current collector has two surfaces opposite 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.

[0116] In some embodiments, the positive electrode film layer includes a positive electrode active material, which may include a composite lithium iron manganese oxide material according to any embodiment of the first aspect of this application or a composite lithium iron manganese oxide material prepared by the method described in any embodiment of the second aspect of this application.

[0117] In some embodiments, the positive electrode film layer 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 one or more combinations selected from superconducting carbon, conductive graphite, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers. In some embodiments, the mass percentage of the positive electrode conductive agent is less than 5% based on the total mass of the positive electrode film layer.

[0118] 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 one or more combinations selected from polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), PVDF-tetrafluoroethylene-propylene terpolymer, PVDF-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resins. In some embodiments, the mass percentage of the positive electrode binder is less than 5% based on the total mass of the positive electrode film layer.

[0119] In some embodiments, the positive current collector may be a metal foil or a composite current collector. Examples of metal foils include aluminum foil or aluminum alloy foil. The composite current collector may include a polymer substrate and a metal layer formed on at least one surface of the polymer substrate. Examples of the metal material include one or more combinations selected from aluminum, aluminum alloys, nickel, nickel alloys, titanium, titanium alloys, silver, and silver alloys. The polymer substrate may include one or more combinations selected from polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), and polyethylene (PE).

[0120] 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 usually 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 can be N-methylpyrrolidone (NMP), but is not limited to it.

[0121] [Negative electrode plate]

[0122] 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, the negative electrode film layer including a negative electrode active material.

[0123] As an example, the negative electrode current collector has two surfaces opposite each other in its own thickness direction, and the negative electrode film layer is disposed on either or both of the two opposite surfaces of the negative electrode current collector.

[0124] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, copper foil may be used as the metal foil. The composite current collector may include a polymer substrate and a metal layer formed on at least one surface of the polymer substrate. The composite current collector may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0125] In some embodiments, the negative electrode active material may be a negative electrode active material known in the art for use in batteries. As an example, the negative electrode active material may include at least one of the following materials: artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, and lithium titanate, etc. The silicon-based material may be selected from at least one of elemental silicon, silicon oxide compounds, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. The tin-based material may be selected from at least one of elemental tin, tin oxide compounds, and tin alloys. However, this application is not limited to these materials, and other conventional materials that can be used as negative electrode active materials for batteries may also be used. These negative electrode active materials may be used alone or in combination of two or more.

[0126] In some embodiments, the negative electrode film layer may optionally include a binder. The binder may be selected from at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

[0127] In some embodiments, the negative electrode film may optionally include a conductive agent. The conductive agent may be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0128] In some embodiments, the negative electrode film may optionally include other additives, such as thickeners (e.g., sodium carboxymethyl cellulose (CMC-Na)).

[0129] In some embodiments, the negative electrode sheet can be prepared by dispersing the components used to prepare the negative electrode sheet, such as the negative electrode active material, conductive agent, binder and any other components, in a solvent (e.g., deionized water) to form a negative electrode slurry; coating the negative electrode slurry onto the negative electrode current collector, and then obtaining the negative electrode sheet after drying, cold pressing and other processes.

[0130] [Electrolytes]

[0131] The electrolyte acts as a conductor of ions between the positive and negative electrodes. This application does not impose specific restrictions on the type of electrolyte; it can be selected according to requirements. For example, the electrolyte can be liquid, gel, or entirely solid.

[0132] In some embodiments, the electrolyte is an electrolyte solution. The electrolyte solution includes an electrolyte salt and a solvent.

[0133] In some embodiments, the electrolyte is an electrolyte solution. The electrolyte solution includes an electrolyte salt and a solvent.

[0134] As an example, lithium salts may include one or more combinations selected from 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).

[0135] As an example, the organic solvent may include one or more combinations selected from 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 ester carbonate (BC), 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).

[0136] In some embodiments, the electrolyte may optionally include additives. For example, additives may include negative electrode film-forming additives, positive electrode film-forming additives, and may also include additives that can improve certain battery performance, such as additives that improve battery overcharge performance, additives that improve battery high-temperature or low-temperature performance, etc.

[0137] [Isolation membrane]

[0138] In some embodiments, the secondary battery also includes a separator. 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.

[0139] In some embodiments, the material of the separator can be selected from at least one of glass fiber, nonwoven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single-layer film or a multi-layer composite film, without particular limitation. When the separator is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

[0140] In some implementations, the positive electrode, negative electrode, and separator can be fabricated into an electrode assembly using a winding or stacking process.

[0141] 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.

[0142] In some embodiments, such as Figure 1 and 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 forming 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. Positive electrode sheets, negative electrode sheets, and a separator may be formed into an electrode assembly 52 through a winding process 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 requirements.

[0143] 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.

[0144] 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.

[0145] 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.

[0146] Optionally, the battery module 4 may also include a housing with a receiving space in which a plurality of secondary batteries 5 are received.

[0147] 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.

[0148] 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.

[0149] Electric device

[0150] Fourthly, this application provides an electrical device, which includes at least one of the secondary battery, battery module, and battery pack described in this application. The secondary battery, battery module, and battery pack can be used as the power source for the electrical device or as the energy storage unit of the electrical device. The electrical device can 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.

[0151] Electrical devices can be equipped with secondary batteries, battery modules, or battery packs depending on their usage requirements.

[0152] Figure 6This is a schematic diagram of an example electrical device. The electrical device 6 is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. To meet the high power and high energy density requirements of this electrical device, a battery pack 1 or a battery module can be used.

[0153] 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.

[0154] Example

[0155] The following describes the embodiments of this application. The embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they shall be performed in accordance with the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments whose manufacturers are not specified are all conventional products that can be obtained commercially.

[0156] Example 1

[0157] 1. Preparation of positive electrode sheet

[0158] 1 mol of lithium source (e.g., lithium hydroxide), 0.6 mol of manganese source (e.g., manganese carbonate), 0.4 mol of iron source (e.g., ferrous carbonate), and 1 mol of phosphorus source (e.g., ammonium phosphate) were successively added to 35 mol of deionized water and stirred until homogeneous to obtain a mixed slurry. 0.03 mol of sulfur source (e.g., sulfur powder) was slowly added to the mixed slurry. After ball milling at 600 rpm for 6 hours, the mixture was vacuum dried at 80°C for 12 hours and calcined at 800°C for 12 hours under a nitrogen atmosphere. The resulting solid product was then ground and pulverized to obtain sulfur-doped lithium manganese iron phosphate particles.

[0159] Lithium manganese iron phosphate particles were used as the positive electrode active material. The positive electrode active material and silver nitrate were mixed at a molar ratio of 1:0.027 and treated at 600℃ for 4 hours under a reducing atmosphere (Ar / H2) to obtain silver-coated lithium manganese iron phosphate material, i.e., composite lithium manganese iron phosphate material.

[0160] The composite lithium manganese iron phosphate material, conductive carbon black, and binder polyvinylidene fluoride (PVDF) were thoroughly mixed in an appropriate amount of solvent NMP at a weight ratio of 97.5:1.4:1.1 to form a uniform positive electrode slurry. The positive electrode slurry was uniformly coated on the surface of the positive electrode current collector aluminum foil (12 μm thick), and after drying and cold pressing, the positive electrode sheet was obtained.

[0161] 2. Preparation of negative electrode sheet

[0162] A copper foil with a thickness of 8μm was used as the negative electrode current collector.

[0163] The negative electrode active material graphite, the binder styrene-butadiene rubber (SBR), the thickener sodium carboxymethyl cellulose (CMC-Na), and the conductive agent carbon black (Super P) are mixed thoroughly in an appropriate amount of deionized water at a weight ratio of 96.2:1.8:1.2:0.8 to form a uniform negative electrode slurry. The negative electrode slurry is then uniformly coated onto the surface of the negative electrode current collector copper foil. After drying and cold pressing, the negative electrode sheet is obtained.

[0164] 3. Separating membrane

[0165] Porous polyethylene (PE) membrane is used as the separator.

[0166] 4. Preparation of electrolyte

[0167] In an environment with a water content of less than 10 ppm, non-aqueous organic solvents ethylene carbonate (EC) and diethyl carbonate (DMC) are mixed at a volume ratio of 1:1 to obtain an electrolyte solvent. Then, lithium salt is mixed with the mixed solvent to prepare an electrolyte with a lithium salt concentration of 1 mol / L.

[0168] 5. Preparation of secondary batteries

[0169] The positive electrode, separator, and negative electrode are stacked in sequence, with the separator acting as a separator between the positive and negative electrodes. The electrode assembly is then wound up. The electrode assembly is placed in an outer packaging shell, dried, and then injected with electrolyte. After vacuum sealing, settling, formation, and shaping, a lithium-ion battery is obtained.

[0170] Example 2

[0171] Examples 2-1 to 2-3 prepared secondary batteries in a similar manner to Example 1, except that the types of conductive precursors were adjusted in Examples 2-1 to 2-3.

[0172] Example 3

[0173] Examples 3-1 to 3-8 prepared secondary batteries in a similar manner to Example 1, except that the molar amount of the conductive precursor silver nitrate was adjusted in Examples 3-1 to 3-8.

[0174] Example 4

[0175] Examples 4-1 to 4-5 were prepared using a method similar to that of Example 1, except that the heat treatment conditions were adjusted in Examples 4-1 to 4-5.

[0176] Example 5

[0177] Example 5 prepared a secondary battery using a method similar to Example 1. The difference from Example 1 is that the composite lithium manganese iron phosphate particles in Example 5 were not doped with sulfur. The preparation process is as follows:

[0178] 1 mol of lithium source (e.g., lithium hydroxide), 0.6 mol of manganese source (e.g., manganese carbonate), 0.4 mol of iron source (e.g., ferrous carbonate), and 1 mol of phosphorus source (e.g., ammonium phosphate) were successively added to 35 mol of deionized water and stirred until homogeneous to obtain a mixed slurry. The slurry was ball-milled at 600 rpm for 6 hours, then vacuum-dried at 80°C for 12 hours, and calcined at 800°C for 12 hours under a nitrogen atmosphere. The resulting solid product was then ground and pulverized to obtain lithium manganese iron phosphate particles.

[0179] After mixing lithium manganese iron phosphate particles and silver nitrate at a molar ratio of 1:0.027, the mixture was treated at 600℃ for 4 hours under a reducing atmosphere (Ar / H2) to obtain silver-coated lithium manganese iron phosphate material, i.e., composite lithium manganese iron phosphate material.

[0180] Comparative Example 1

[0181] Comparative Example 1 prepared a secondary battery using a method similar to that of Example 1. The difference from Example 1 is that the composite lithium manganese iron phosphate particles of Comparative Example 1 were not coated.

[0182] Comparative Example 2

[0183] Comparative Example 2 prepared a secondary battery using a similar method to Example 1. The difference from Example 1 is that the composite lithium manganese iron phosphate particles in Comparative Example 2 had a carbon coating layer. The specific preparation process is as follows:

[0184] 1 mol of lithium source (e.g., lithium hydroxide), 0.6 mol of manganese source (e.g., manganese carbonate), 0.4 mol of iron source (e.g., ferrous carbonate), and 1 mol of phosphorus source (e.g., ammonium phosphate) were successively added to 35 mol of deionized water and stirred until homogeneous to obtain a mixed slurry. The slurry was ball-milled at 600 rpm for 6 hours, then vacuum-dried at 80°C for 12 hours, and calcined at 800°C for 12 hours under a nitrogen atmosphere. The resulting solid product was then ground and pulverized to obtain lithium manganese iron phosphate particles.

[0185] Lithium manganese iron phosphate particles and glucose were mixed at a molar ratio of 1:01 and then treated at 600℃ for 4 hours under a reducing atmosphere (Ar / H2) to obtain carbon-coated lithium manganese iron phosphate material, i.e., composite lithium manganese iron phosphate material.

[0186] The relevant parameters of the embodiments and comparative examples are shown in Table 1.

[0187] Table 1

[0188]

[0189] As shown in Table 1, the lithium manganese iron phosphate particles in Comparative Example 1 lacked a coating layer, resulting in structural instability during long-term charge-discharge cycles. Compared to Comparative Example 1, Comparative Example 2 incorporated a carbon-containing coating layer on the surface of the lithium manganese iron phosphate particles, providing excellent protection and improving the cycle performance of the secondary battery. However, the carbon may be oxidized under high voltage conditions, leading to a decline in the system's cycle performance. Figure 7 The figure shows the cycling curves of Comparative Example 1 and Example 1; the embodiments of this application have relatively stable performance due to the coating layer containing metal nanoparticles, which can significantly improve the cycling performance of the system; and can ensure the utilization of the specific capacity of lithium manganese iron phosphate.

[0190] Compared to Comparative Example 1, Examples 3-1 to 3-8 adjusted the mass content of the coating layer, which can improve the cycle performance of the secondary battery. Among them, although the mass content of the coating layer in Examples 3-8 is lower and the silver content is less, it still shows an effect of improving cycle performance compared to Comparative Example 1.

[0191] Comparative Example 2, with sufficient carbon coating, improves cycle performance. However, the oxidation of carbon weakens its protective ability against lithium manganese iron phosphate particles, which deteriorates cycle performance to some extent. Compared to Comparative Example 2, some examples in Examples 3-1 to 3-8, when the mass content of the coating layer is similar to that of carbon coating in Comparative Example 2, significantly improve the coating effect and enhance cycle performance.

[0192] Compared to Example 5, Example 1 incorporates sulfur into lithium manganese iron phosphate particles, which can anchor the metal nanoparticles, improve the uniformity of metal nanoparticle coating, and enhance the cycle performance of the system.

[0193] Test section:

[0194] 1. Specific capacity of positive electrode active material

[0195] The above-prepared positive electrode active material, conductive carbon black, and binder polyvinylidene fluoride (PVDF) were mixed evenly with solvent N-methylpyrrolidone (NMP) at a mass ratio of 91.6:1.8:6.6 to form a slurry. The prepared slurry was coated onto copper foil and dried in an oven for later use. Then, a lithium metal sheet was used as the counter electrode, a polyethylene (PE) film was used as the separator, a few drops of the same electrolyte as the above-mentioned secondary battery were added, and the CR2430 coin cell was assembled in an argon-protected glove box.

[0196] After the obtained coin cells were allowed to stand for 12 hours, they were discharged at 25°C with a constant current of 0.05C to 0.005V, allowed to stand for 10 minutes, and then discharged again with a constant current of 50μA to 0.005V. After standing for 10 minutes, they were discharged again with a constant current of 10μA to 0.005V. Then, they were charged at a constant current of 0.1C to 2V, and the charging capacity was recorded. The ratio of the charging capacity to the mass of the negative electrode active material is the initial specific capacity of the negative electrode active material.

[0197] 2. Cycle performance of secondary batteries

[0198] At 25°C, the prepared secondary battery was charged at a constant current of 0.33C to the charging cutoff voltage of 4.4V, then charged at a constant voltage to a current of 0.05C, allowed to stand for 5 minutes, and then discharged at a constant current of 0.33C to the discharge cutoff voltage of 2.8V. Its initial capacity was recorded as C0. Then, charging was performed according to the strategy shown in Table 2, followed by discharging at 0.33C. The discharge capacity Cn for each cycle was recorded until the cycle capacity retention rate (i.e., Cn / C0×100%) reached 80%, and the number of cycles was recorded. A higher number of cycles indicates better cycle performance of the secondary battery.

[0199] Table 2

[0200] State of charge SOC of secondary battery Charge rate (C) 0～10％ 0.33 10％～20％ 5.2 20％～30％ 4.5 30％～40％ 4.2 40％～50％ 3.3 50％～60％ 2.6 60％～70％ 2.0 70％～80％ 1.5 80％～100％ 0.33

[0201] Although this application has been described with reference to preferred embodiments, various modifications can be made thereto and components can be replaced with equivalents without departing from the scope of this application. In particular, the technical features mentioned in the various embodiments can be combined in any manner, provided there is no structural conflict. This application is not limited to the specific embodiments disclosed herein, but includes all technical solutions falling within the scope of the claims.

Claims

1. A composite lithium manganese iron phosphate material, comprising: The upper limit voltage for lithium iron phosphate granules is denoted as V1, and the unit is V. A coating layer is applied to at least a portion of the surface of the lithium manganese iron phosphate particles. The coating layer comprises metal nanoparticles, and the oxidation voltage of the metal nanoparticles is denoted as V², in volts (V). Wherein, the composite lithium manganese iron phosphate material satisfies: V1 < V2, The lithium manganese iron phosphate particles include a dopant element M, which includes one or more elements selected from sulfur, nitrogen, boron, fluorine, chlorine, bromine and iodine.

2. The composite lithium manganese iron phosphate material according to claim 1, wherein, The lithium manganese iron phosphate particles have the structural formula LiMn. 1-x Fe x M y PO4, where 0.05≤x≤0.95; 0<y≤1.

3. The composite lithium manganese iron phosphate material according to claim 2, wherein, 0≤y≤0.2。 4. The composite lithium manganese iron phosphate material according to claim 1, wherein, The doping element M includes sulfur.

5. The composite lithium manganese iron phosphate material according to any one of claims 1 to 4, wherein, The metal nanoparticles include one or more nanoparticles selected from silver, gold, platinum, palladium, rhodium, iridium, osmium, and ruthenium.

6. The composite lithium manganese iron phosphate material according to any one of claims 1 to 5, wherein, Based on the total mass of the composite lithium manganese iron phosphate material, the mass content of the coating layer is denoted as A%, and 0.3 ≤ A ≤ 10%.

7. The composite lithium manganese iron phosphate material according to claim 6, wherein, 0.3≤A≤3。 8. The composite lithium manganese iron phosphate material according to any one of claims 1 to 7, wherein, The thickness of the coating layer is denoted as H, with units of nm, and 2 ≤ H ≤ 100.

9. The composite lithium manganese iron phosphate material according to claim 8, wherein, 5≤H≤20。 10. The composite lithium manganese iron phosphate material according to any one of claims 1 to 9, wherein, The composite lithium manganese iron phosphate material satisfies at least one of conditions (1) to (3): (1) The average particle size of the metal nanoparticles is denoted as D1, with the unit being nm, and D1≤20; (2) The volume average particle size Dv50 of the lithium manganese iron phosphate particles is denoted as D2, with the unit being μm, and 0.1≤D2≤10; (3) The volume average particle size Dv50 of the composite lithium manganese iron phosphate material is denoted as D, and the unit is μm, 0.1≤D≤10.

11. The composite lithium manganese iron phosphate material according to claim 10, wherein, D1≤10。 12. The composite lithium manganese iron phosphate material according to claim 10, wherein, 0.2≤D2≤5。 13. The composite lithium manganese iron phosphate material according to claim 10, wherein, 0.2≤D≤5。 14. A method for preparing a composite lithium manganese iron phosphate material as described in any one of claims 1 to 13, comprising: Lithium iron phosphate manganese phosphate granules are available; A conductive precursor is provided to the lithium manganese iron phosphate particles, and the conductive precursor is heat-treated to reduce the conductive precursor to form a coating layer covering the lithium manganese iron phosphate particles, the coating layer comprising metal nanoparticles. in, The upper limit voltage for the use of the lithium manganese iron phosphate particles is denoted as V1, and the unit is V; A coating layer is applied to at least a portion of the surface of the lithium manganese iron phosphate particles. The coating layer comprises metal nanoparticles, and the oxidation voltage of the metal nanoparticles is denoted as V², in units of volts (V). The composite lithium manganese iron phosphate material satisfies: V1 < V2. The step of providing lithium manganese iron phosphate particles includes: The doping element M is incorporated into the lithium manganese iron phosphate particles. The doping element M includes one or more elements selected from sulfur, nitrogen, boron, fluorine, chlorine, bromine and iodine.

15. The method according to claim 14, wherein, The doping element M includes sulfur.

16. The method according to claim 14 or 15, wherein, The heat treatment temperature is from 400°C to 1000°C; and / or The heat treatment time is 2 hours to 6 hours.

17. The method according to any one of claims 14 to 16, wherein, The conductive precursor includes one or more of nitrate, chloride, bromide, iodide, sulfate, phosphate, acetate, and acetylacetonate; and / or The conductive precursor includes one or more of the following: silver ions, gold ions, platinum ions, palladium ions, rhodium ions, iridium ions, osmium ions, and ruthenium ions.

18. The method according to any one of claims 14 to 17, wherein, Based on the total molar ratio of the conductive precursor and the lithium manganese iron phosphate particles, the molar content of the conductive precursor is b%, and 0.25≤b≤14.5%.

19. The method of claim 18, wherein, 0.40≤b≤4.60。 20. A secondary battery comprising a positive electrode, said positive electrode comprising a composite lithium manganese iron phosphate material as described in any one of claims 1 to 13 or a composite lithium manganese iron phosphate material obtained by the method as described in any one of claims 14 to 19.

21. An electrical device comprising the secondary battery as described in claim 20.

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