Lithium iron phosphate particles and preparation method therefor, positive electrode sheet, secondary battery and electric device

Abstract

Large-particle, low-specific-surface-area and high-dynamics lithium iron phosphate particles. The primary average particle size of the lithium iron phosphate particles is 500-3000 nm, and the BET specific surface area thereof is 3 m2 / g to 8 m2 / g. By means of calculation on the basis of the total weight of the lithium iron phosphate particles, the carbon content of the lithium iron phosphate particles is Cx weight%, where 0.8≤Cx≤2.0.

Description

Lithium iron phosphate particles and preparation method thereof, positive electrode sheet, secondary battery and electrical device

[0001] Cross-references

[0002] This application refers to Chinese patent application No. 202410027073.3 filed on January 8, 2024, entitled “Lithium iron phosphate salt particles and preparation method thereof, positive electrode sheet, secondary battery and electrical device”, which is incorporated into this application in its entirety by reference. Technical Field

[0003] The present application relates to the technical field of secondary batteries, and in particular to a lithium iron phosphate salt particle and a preparation method thereof, a positive electrode sheet, a secondary battery and an electrical device. Background Art

[0004] As a cathode material for lithium-ion batteries, lithium iron phosphate has rapidly become a global research hotspot due to its abundant resources, low price, environmental friendliness, and stable voltage of two-phase reaction. The cation arrangement in lithium iron phosphate is different from that of layered ternary materials and spinel LiMn2O4, Fe 2+ At the 4c ​​position of the oxygen octahedron, Li + Located at the 4a position of the oxygen octahedron. The polyanion structure is stable, has a high thermal decomposition temperature, and has good thermal stability. However, the octahedral structure of FeO6 is separated by the O atoms in the tetrahedral structure and is interrupted by the phosphorus oxygen tetrahedron, failing to form a continuous FeO6 network, resulting in poor electronic conductivity of lithium iron phosphate.

[0005] The industry typically uses methods such as doping, coating, and particle nanosizing to improve the conductivity of lithium iron phosphate. At low temperatures and high rates, issues with electronic and ionic conductivity become more pronounced, necessitating thorough nanosizing of the material. However, nanosized materials have a high specific surface energy, which can lead to a series of processing issues, such as gelation, rapid water absorption, difficulty drying, and low solids content in the coating slurry. Increasing particle size and reducing the BET (Bet) and, therefore, the specific surface energy of the material, while maintaining kinetic performance is a highly significant engineering advancement.

[0006] Summary of the Invention

[0007] The present application is made in view of the above-mentioned problems, and its purpose is to provide a lithium iron phosphate salt particle and its preparation method, a positive electrode plate, a secondary battery and an electrical device. The lithium iron phosphate salt particle has a large particle size and also has excellent kinetic properties when used as a positive electrode material.

[0008] The first aspect of the present application provides a lithium iron phosphate salt particle, wherein the lithium iron phosphate salt particle has a primary average particle size of 500nm to 3000nm and a BET specific surface area of ​​3m 2 / g~8m 2 / g, calculated based on the total weight of the lithium iron phosphate salt particles, the carbon content of the lithium iron phosphate salt particles is Cx weight %, wherein 0.8≤Cx≤2.0.

[0009] In any embodiment, the primary average particle size of the lithium iron phosphate particles is 650 nm to 2500 nm; and / or,

[0010] The BET specific surface area of ​​lithium iron phosphate particles is 4m 2 / g~7m 2 / g; and / or,

[0011] Calculated based on the total weight of the lithium iron phosphate salt particles, the carbon content of the lithium iron phosphate salt particles is Cx weight %, where 1.0≤Cx≤1.6.

[0012] In any embodiment, the ratio z of the BET specific surface area to Cx satisfies 1.5≤z≤8.5. In any embodiment, the ratio z of the BET specific surface area to Cx satisfies 3≤z≤6.

[0013] By making the primary average particle size of the lithium iron phosphate particles of the embodiment of the present application 500nm to 3000nm and the BET specific surface area 3m 2 / g~8m 2 / g, calculated based on the total weight of the lithium iron phosphate particles, the carbon content is Cx weight % (0.8≤Cx≤2.0), and further optionally, the ratio z of the BET specific surface area of ​​the lithium iron phosphate particles to Cx satisfies the range of 1.5≤z≤8.5, which can improve the dynamic properties of the lithium iron phosphate particles with an average primary particle size within the above range; the particles are kept at a suitable micron size, thereby avoiding the interface side reactions and processing difficulties caused by nano-sizing the particles, and the particles will not be limited to an excessively large size and thus suffer a reduction in dynamic performance; in addition, it is beneficial to the stirring of the slurry containing the lithium iron phosphate particles and the increase of the solid content, thereby improving the processing problem of the battery cell, and further increasing the volume energy density of the battery; in addition, it can avoid the normal deintercalation and extraction of lithium ions due to excessively high carbon coating density, thereby affecting the performance of the battery cell capacity.

[0014] Compared with the prior art, the lithium iron phosphate salt particles in the embodiment of the present application have a smaller specific surface area when containing the same carbon content, which means that the particles contain less floating carbon, and can make the carbon contained in the lithium iron phosphate salt particles more uniform and dense and each particle is coated, thereby improving the surface conductivity of the particles and improving its kinetic performance as a positive electrode material.

[0015] In any embodiment, the lithium iron phosphate salt has the molecular formula Li m Fex P y O j Q q , wherein Q includes at least one of Al, Na, K, Mg, Cu, Mn, Cr, Zn, Pb, Ca, Co, Ni, Sr, Nb, V, Ti, B, Si, N, S, F, Cl, and Br, 0.95≤m≤1.15, 0.9≤x≤1, 0.95≤y≤1, 3.5≤j≤4, and 0<q≤0.1.

[0016] By making the lithium iron phosphate salt of the embodiment of the present application have the above molecular formula and doping it with one or more of the above elements, a high-load bulk modification of the lithium iron phosphate salt particles is achieved. This helps to improve the bulk ion transport capacity of the lithium iron phosphate salt particles and can effectively solve the problem of poor kinetic performance inherent in large particles. In addition, by performing this doping, the lithium iron phosphate salt particles can exhibit good kinetic performance when used as a positive electrode material. In this application, the modification can specifically be manifested as doping and / or coating.

[0017] In any embodiment, Q includes at least one of Ti, V, Mg, and Nb, and the content of Ti, V, Mg, and / or Nb is 1000 ppm-10000 ppm, calculated based on the total weight of the lithium iron phosphate particles.

[0018] In any embodiment, Q includes Ti. In any embodiment, the content of Ti, V, Mg and / or Nb is 2500 ppm to 6000 ppm, calculated based on the total weight of the lithium iron phosphate salt particles.

[0019] By doping the lithium iron phosphate salt of the embodiment of the present application with one or more of the above elements and making their content within the above range, it is possible to better achieve metal bulk modification of the lithium iron phosphate salt particles while improving the primary average particle size of the lithium iron phosphate salt particles, further enhance the bulk ion transport capacity of the lithium iron phosphate salt particles, and better solve the problem of poor kinetic performance inherent in large particles.

[0020] In any embodiment, the lithium iron phosphate salt particles are single crystal particles and / or polycrystalline particles. In any embodiment, based on the total number of lithium iron phosphate salt particles, the number of single crystal particles accounts for more than 90%.

[0021] In the lithium iron phosphate particles of the embodiments of the present application, the carbon may be present in a mixed state with the lithium iron phosphate particles or in a coated state on the lithium iron phosphate particles. In some embodiments, the carbon in the lithium iron phosphate particles is coated on the lithium iron phosphate particles, thereby enabling the carbon to form a uniform and dense carbon coating on the surface of the lithium iron phosphate particles, thereby improving the surface conductivity of the particles.

[0022] In any embodiment, the capacity proportion η of the lithium iron phosphate salt particles is ≥88%, and the capacity proportion η is defined as: a battery having lithium iron phosphate salt particles as a positive electrode material is charged and discharged twice at a constant current rate of 0.1C in the voltage range of 2.0V to 3.75V, and then charged and discharged once at a constant current rate of 1C. In the charge and discharge test at a rate of 1C, the capacity value at the discharge voltage of 3.2V is extracted as C1, and the capacity value at the discharge voltage to 2.0V is extracted as C2, η=C1 / C2, wherein the charging process includes constant voltage charging, a constant voltage of 3.75V, and a constant voltage cut-off current of 50μA.

[0023] η represents the platform retention performance of the material, and this value is strongly correlated with the discharge power performance of the battery. When this value is large, the battery can still maintain good power performance when discharged to a low SOC (battery state of charge), that is, the voltage drop of the battery is small when the battery is discharged at a high current at low power. The capacity ratio η of the lithium iron phosphate salt particles of the embodiment of the present application is ≥88%, indicating that the lithium iron phosphate salt particles of the present application can enable the secondary battery to exhibit good kinetic performance when used as a positive electrode material.

[0024] In any embodiment, the lithium iron phosphate salt particles satisfy at least one of a) to f):

[0025] a) Dv10 of lithium iron phosphate particles ≥ 0.2 μm;

[0026] b) the Dv50 of the lithium iron phosphate particles is 0.5-5 μm;

[0027] c) Dv90 of lithium iron phosphate particles ≤ 10 μm;

[0028] d) Dv99 of lithium iron phosphate particles ≤ 12 μm;

[0029] e) The powder compaction density of lithium iron phosphate at 3T pressure is ≥2.25g / cm 3 ;

[0030] f) The powder resistivity of the lithium iron phosphate salt is less than 60Ω·cm.

[0031] In this application, Dv10, Dv90, and Dv99 refer to the particle sizes corresponding to the 10%, 90%, and 99% cumulative particle size distribution percentages, respectively. The test method is the same as that for Dv50, and can be determined using the standards in the examples of this application.

[0032] It should be noted that Dv10 is a high index, so the upper limit cannot be given, and Dv90 is a low index, so the lower limit cannot be given.

[0033] By making the lithium iron phosphate salt particles satisfy at least one of a) to f), the lithium iron phosphate salt particles can achieve the above technical effects better.

[0034] In this application, the test method for powder resistivity is: refer to the national standard GB / T33822-2017, use a powder resistivity meter (Suzhou Jinglattice, ST2722 model), weigh 1g of sample (error within ±0.005g), add it to the feeding chamber, apply 8MPa pressure, test the forward resistivity and reverse resistivity of the sample respectively, and take the average of the two as the powder resistivity of the sample.

[0035] In this application, the definition of powder compaction density is: during the external compression process, as the powder moves and deforms, larger gaps are filled, the contact area between particles increases, the attraction between atoms is generated, and the mechanical fit between particles is enhanced, thus forming a compact with a certain density and strength. The unit is g / cm 3 .

[0036] The compaction density test method is as follows: Referring to the national standard GB / T 24533-2009, a certain amount of powder is placed on a compaction mold with a hollow center and two metal discs above and below. The powder is placed between the metal discs and a metal cylinder is placed on top. The mold is placed on a compaction density instrument and different pressures are set. The instrument can read the thickness of the powder at different pressures and calculate the compaction density using ρ = m / v.

[0037] According to the following powder compaction density calculation results, ρc=m / V=m / (S×H); where ρc is the powder compaction density (g / cm 3 ), m is the mass of the material (g), S is the bottom area of ​​the mold (1.327cm 2 ), H is the height of the sample after compaction (cm).

[0038] The second aspect of the present application provides a method for preparing lithium iron phosphate salt particles, comprising: providing raw materials containing at least a lithium source, an iron source, a phosphorus source, a carbon source, a carbon film-forming agent, and a modifier, and performing at least two sinterings, wherein the temperature of the first sintering is 500°C-760°C, and the carbon content of the material after the first sintering is 0.01 weight%-0.79 weight%; the temperature of the second sintering is 700°C-800°C, and the carbon content of the material after the second sintering is 0.8 weight%-2.0 weight%.

[0039] In any embodiment, the temperature of the first sintering is 550°C-720°C, and the carbon content of the material after the first sintering is 0.05 wt%-0.4 wt%; the temperature of the second sintering is 720°C-780°C, and the carbon content of the material after the second sintering is 1.0 wt%-1.6 wt%.

[0040] In the preparation method of lithium iron phosphate salt particles of the embodiment of the present application, two sintering processes are implemented. Among them, by controlling the temperature of the first sintering within the above range and making the carbon content of the intermediate after sintering within the above range, the lithium iron phosphate precursor obtained after the first sintering can have a larger particle size, which directly improves the powder compaction of the final product and the electrode compaction density. In addition, by adding a lower content of carbon source during the first sintering, the barrier effect of the carbon layer on the growth process of lithium iron phosphate particles is greatly reduced, which is conducive to the crystallization growth of particles at a lower temperature, and is also conducive to the solid-phase diffusion reaction between the modifier and the lithium iron phosphate material, thereby achieving a higher concentration of metal ion modification. Compared with the traditional method of using high temperature to achieve particle growth, the above-mentioned preparation method of the present application can first synthesize large particles at a lower temperature, which can improve the phenomenon of cracking of the carbon layer on the surface of the particles at high temperature and improve the density of the surface carbon; at the same time, during the first sintering process, the carbon source can effectively reduce the trivalent iron in the raw material, thereby improving the purity and stability of the product; by controlling the temperature of the second sintering within the above-mentioned range and making the carbon content of the sintered material 0.8 wt%-2.0 wt%, carbon can be coated on the surface of the lithium iron phosphate salt particles to form a lithium iron phosphate material with a uniform and dense carbon coating layer on which each particle is coated, thereby greatly improving the surface conductivity of the particles.

[0041] In any embodiment, the lithium iron phosphate salt particles contain at least one of the elements Ti, V, Mg, and / or Nb, and the content of the element is 1000ppm-10000ppm, calculated based on the total weight of the lithium iron phosphate salt particles. In some embodiments, the lithium iron phosphate salt particles contain at least one of the elements Ti, V, Mg, and / or Nb, and the content of the element is 2500ppm-6000ppm, calculated based on the total weight of the lithium iron phosphate salt particles.

[0042] By doping the lithium iron phosphate salt of the embodiment of the present application with one or more of the above elements and making their content within the above range, it is possible to better achieve metal bulk modification of the lithium iron phosphate salt particles while improving the primary average particle size of the lithium iron phosphate salt particles, further enhance the bulk ion transport capacity of the lithium iron phosphate salt particles, and better solve the problem of poor kinetic performance inherent in large particles.

[0043] In any embodiment, in the preparation method of lithium iron phosphate salt particles, a first crushing is performed after the first sintering, and a second crushing is performed after the second sintering, wherein the Dv50 of the product after the first crushing is 300nm-1200nm; the Dv50 of the product after the second crushing is 500nm-5000nm.

[0044] In any embodiment, the Dv50 of the product after the first pulverization is 400 nm-1100 nm; the Dv50 of the product after the second pulverization is 300 nm-2500 nm.

[0045] In the preparation method of lithium iron phosphate salt particles according to the embodiments of the present application, two post-sintering crushing steps are implemented. By performing the first crushing after the first sintering, the product has a Dv50 of 300nm-1200nm, which can avoid the growth barrier of the carbon material and the modifying elements on the crystals, and obtain a micron-sized lithium iron phosphate precursor. By performing the second crushing after the second sintering, the product has a Dv50 of 500nm-3000nm, and lithium iron phosphate salt particles of the desired particle size can be obtained. A uniform and dense carbon coating is obtained, and each particle is coated, significantly improving the surface conductivity of the particles.

[0046] In any embodiment, the preparation method of lithium iron phosphate salt particles includes: providing raw materials containing at least a lithium source, an iron source, a phosphorus source, a carbon source, a carbon film-forming agent, and a modifier, wherein the mixing ratio of the lithium source, the iron source, and the phosphorus source is calculated based on the atomic molar number of each element, satisfying Fe:P=0.96-0.985, Li:Fe=1.0-1.1:0.95-1.1; the carbon source and the carbon film-forming agent are calculated based on the weight ratio satisfying carbon source:carbon film-forming agent=9:1-2:8.

[0047] By using raw materials containing a lithium source, an iron source, a phosphorus source, a carbon source, a modifier, and a carbon film-forming agent in the above-mentioned proportions, it is possible to achieve good modification, and to form the obtained lithium iron phosphate salt particle precursor into particles of larger size, thereby being able to well form the primary average particle size, carbon content, BET specific surface area, and z value of the lithium iron phosphate salt particles of the first aspect of this application. Specifically, as described above, by making the primary average particle size of the lithium iron phosphate salt particles of the embodiment of the present application 500nm to 3000nm, the particles can be kept at a suitable micron-level size, thereby avoiding the problems of interfacial side reactions and processing difficulties caused by nano-sizing the particles (in this field, it is difficult to process nano-sized particles), and the particles will not be limited to a large size and thus reduce the kinetic performance. In addition, by making the BET specific surface area of ​​the lithium iron phosphate salt particles of the embodiment of the present application 3m 2 / g~8m 2 / g, which can facilitate the stirring of the slurry containing the lithium iron phosphate salt particles and increase the solid content, thereby improving the processing of the battery cell and further increasing the volumetric energy density of the battery. Further optionally, by ensuring that the ratio z of the BET specific surface area to Cx of the lithium iron phosphate salt particles in the embodiment of the present application satisfies the range of 1.5≤z≤8.5, the carbon contained in the lithium iron phosphate salt particles can be made more uniform and dense, and each particle can be coated, thereby improving the surface conductivity of the particles.

[0048] In any embodiment, in the preparation method of lithium iron phosphate salt particles, the lithium source is a lithium compound, including one or more of lithium dihydrogen phosphate, lithium oxalate, lithium carbonate, lithium oxide, lithium hydroxide and lithium acetate; the iron source is an iron compound, including at least one of ferric hydroxide, ferrous chloride, ferric oxide, ferric phosphate, ferrous pyrophosphate, ferrous oxalate, iron powder, ferric nitrate, ferric oxide and ferric oxyhydroxide; the phosphorus source is a phosphate compound, including one or more of phosphoric acid, ammonium dihydrogen phosphate and diammonium hydrogen phosphate; the modifier includes at least one of titanium dioxide, vanadium pentoxide, n-butyl titanate, ammonium metavanadate, niobium ethanol, niobium oxalate, niobium pentoxide, magnesium hydroxide and magnesium nitrate; the carbon source includes at least one of citric acid, glucose, sucrose, starch, fructose and lactose; the carbon film-forming agent includes one or more of polyethylene glycol, polyaniline, polyacrylonitrile, polyvinyl pyrrolidone and polyvinyl alcohol.

[0049] In any embodiment, in the preparation method of lithium iron phosphate salt particles, the lithium source is a lithium compound, including lithium carbonate; the iron source is an iron compound, including ferric oxide; the phosphorus source is a phosphate compound, including phosphoric acid; the modifier includes titanium dioxide; the carbon source includes glucose; and the carbon film-forming agent includes polyaniline.

[0050] By selecting the above substances as the lithium source, iron source, phosphorus source, modifier, carbon source, and carbon film-forming agent respectively used in the method for preparing the lithium iron phosphate salt particles according to the embodiment of the present application, the primary average particle size, carbon content, BET specific surface area, and z-value of the lithium iron phosphate salt particles according to the first aspect of the present application can be well formed. This can better achieve a large particle size of the lithium iron phosphate salt particles according to the embodiment of the present application, while also achieving excellent kinetic performance when used as a positive electrode material.

[0051] In any embodiment, the heating rates in the first sintering and the second sintering are independently 2°C / min-20°C / min, the first sintering constant temperature time is 1h-6h, and the second sintering constant temperature time is 2h-12h.

[0052] By independently controlling the heating rate and holding time during the first and second sintering processes within the aforementioned ranges, it is possible to prevent excessive side reactions during the sintering process caused by excessively rapid heating, thereby affecting the primary average particle size, carbon content, BET specific surface area, and z-value of the resulting lithium iron phosphate salt particles. This allows for a better realization of the large particle size of the lithium iron phosphate salt particles of the embodiments of the present application, while also achieving excellent kinetic performance when used as a positive electrode material.

[0053] In any embodiment, in the second sintering, the product after the first sintering is carbon-coated, and the amount of carbon coating is 0.01 wt%-1.99 wt% based on the total weight of the product after the first sintering. In any embodiment, the amount of carbon coating is 0.2 wt%-1.6 wt% based on the total weight of the product after the first sintering.

[0054] By coating the product of the first sintering with carbon within the above-mentioned content range after the first sintering and before the second sintering, a uniform and dense carbon coating layer can be formed on the surface of the lithium iron phosphate particles during the second sintering process, and each particle is coated.

[0055] In any embodiment, the carbon coating is performed by vapor deposition during the sintering process, or by carbonizing the carbon source at a high temperature.

[0056] In the preparation method of lithium iron phosphate salt particles of the embodiment of the present application, the implementation method of carbon coating is not particularly limited. The lithium iron phosphate salt particles are carbon-coated by vapor deposition, thereby forming a uniform and dense carbon coating layer on the surface of the lithium iron phosphate salt particles and coating each particle.

[0057] In any embodiment, the pulverization includes one or more of grinding, sand milling, mechanical crushing, and air flow crushing.

[0058] In the method for preparing lithium iron phosphate salt particles according to the embodiment of the present application, the pulverization method is not particularly limited. By adopting the above-mentioned specific pulverization method, it is advantageous to obtain a desired primary average particle size.

[0059] The third aspect of the present application provides a positive electrode plate, which comprises the lithium iron phosphate salt particles of the first aspect of the present application as a positive electrode material.

[0060] The fourth aspect of the present application provides a secondary battery comprising the positive electrode sheet according to the third aspect of the present application.

[0061] A fifth aspect of the present application provides an electrical device comprising the secondary battery according to the fourth aspect of the present application. BRIEF DESCRIPTION OF THE DRAWINGS

[0062] Figure 1 is the SEM and TEM images of the lithium iron phosphate salt particles according to an embodiment of the present application, wherein Figure 1a corresponds to the image of the lithium iron phosphate salt particles according to an embodiment of the present application observed under SEM (scanning electron microscope); Figure 1b and Figure 1c correspond to the images of the lithium iron phosphate salt particles according to an embodiment of the present application observed under TEM (transmission electron microscope).

[0063] FIG2 is an SEM image of an embodiment of the present application and existing lithium iron phosphate salt particles, wherein FIG2a is an SEM image of Example 10; FIG2b is an SEM image of existing lithium iron phosphate salt particles.

[0064] FIG. 3 is a schematic diagram exemplarily showing the particle size of primary particles of the present application.

[0065] FIG4 is a diagram showing the capacity performance of lithium iron phosphate particles according to an embodiment of the present application at rates of 0.1C and 1C.

[0066] FIG5 is a schematic diagram of a secondary battery according to an embodiment of the present application.

[0067] FIG. 6 is an exploded view of the secondary battery according to the embodiment of the present application shown in FIG. 5 .

[0068] FIG7 is a schematic diagram of a battery module according to an embodiment of the present application.

[0069] FIG8 is a schematic diagram of a battery pack according to an embodiment of the present application.

[0070] FIG. 9 is an exploded view of the battery pack shown in FIG. 8 according to an embodiment of the present application.

[0071] FIG. 10 is a schematic diagram of an electric device using a secondary battery as a power source according to an embodiment of the present application.

[0072] Description of reference numerals:

[0073] 1 battery pack; 2 upper box; 3 lower box; 4 battery module; 5 secondary battery; 51 shell; 52 electrode assembly; 53 cover plate. DETAILED DESCRIPTION

[0074] The following describes in detail the embodiments of the lithium iron phosphate salt particles and their preparation methods, positive electrode sheets, secondary batteries, and electrical devices of the present application. However, there may be cases where unnecessary detailed descriptions are omitted. For example, there may be cases where detailed descriptions of well-known matters and repeated descriptions of actually the same structures are omitted. This is to avoid the following description from becoming unnecessarily lengthy and to facilitate the understanding of those skilled in the art. In addition, the drawings and the following description are provided for those skilled in the art to fully understand the present application and are not intended to limit the subject matter described in the claims.

[0075] The "ranges" disclosed herein are defined in terms of lower and upper limits, where a given range is defined by selecting a lower limit and an upper limit, and the selected lower and upper limits define the boundaries of the particular range. Ranges defined in this manner can be inclusive or exclusive of the end values ​​and can be combined arbitrarily, i.e., any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60 to 120 and 80 to 110 are listed for a particular parameter, it is understood that ranges of 60 to 110 and 80 to 120 are also contemplated. Furthermore, if minimum range values ​​of 1 and 2 are listed, and if maximum range values ​​of 3, 4, and 5 are listed, the following ranges are all contemplated: 1 to 3, 1 to 4, 1 to 5, 2 to 3, 2 to 4, and 2 to 5. In this application, unless otherwise indicated, the numerical range "a to b" is a shorthand representation of any combination of real numbers between a and b, where a and b are both real numbers. For example, a numerical range of "0-5" indicates that all real numbers between "0-5" are listed herein, and "0-5" is simply an abbreviation for these numerical combinations. Furthermore, when a parameter is expressed as an integer ≥ 2, this is equivalent to disclosing that the parameter is, for example, an integer of 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0076] Unless otherwise specified, all embodiments and optional embodiments of the present application can be combined with each other to form a new technical solution.

[0077] Unless otherwise specified, all technical features and optional technical features of this application can be combined with each other to form a new technical solution.

[0078] Unless otherwise specified, all steps of the present application may be performed sequentially, randomly, or optionally sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or may include steps (b) and (a) performed sequentially. For example, the method may further include step (c), indicating that step (c) may be added to the method in any order, for example, the method may include steps (a), (b), and (c), or may include steps (a), (c), and (b), or may include steps (c), (a), and (b), etc.

[0079] Unless otherwise specified, the terms "include" and "comprising" used in this application may be open-ended or closed-ended. For example, "include" and "comprising" may mean that other components not listed may also be included or that only the listed components are included.

[0080] Unless otherwise specified, the term "or" is used in this application to be inclusive. For example, the phrase "A or B" means "A, B, or both A and B." More specifically, the condition "A or B" is satisfied if any of the following conditions are met: 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).

[0081] Lithium iron phosphate, the cathode material for secondary batteries, suffers from poor conductivity. To address this, the industry typically uses methods such as doping, coating, and particle nano-sizing to improve its conductivity.

[0082] The carbon coating and nano-sizing methods currently used in the industry reduce the primary average particle size while introducing a loose carbon layer, significantly increasing the material's specific surface area. This can lead to a series of processing and manufacturing issues during processing and use, increasing production costs and reducing production yields. These issues include slurry gelation, cracking during electrode coating, water absorption by powder exposed to air, powder loss from the film during electrode slitting, and small particle shedding during cold pressing. These processing issues can affect battery life.

[0083] Therefore, preparing large-particle lithium iron phosphate cathode materials and improving their kinetic properties are of great significance for industrialization. This is explained in detail below.

[0084] [Lithium iron phosphate particles]

[0085] In an embodiment of the present application, the primary average particle size of the lithium iron phosphate salt particles is 500 nm to 3000 nm. In some embodiments, the primary average particle size of the lithium iron phosphate salt particles is 650 nm to 2500 nm.

[0086] In some embodiments, the BET specific surface area of ​​the lithium iron phosphate particles is 3 m 2 / g~8m 2 In some embodiments, the BET specific surface area of ​​the lithium iron phosphate particles is 4 m 2 / g~7m 2 / g.

[0087] In some embodiments, the carbon content of the lithium iron phosphate salt particles is Cx weight percent, calculated based on the total weight of the lithium iron phosphate salt particles, where 0.8 ≤ Cx ≤ 2.0. In some embodiments, the carbon content of the lithium iron phosphate salt particles is Cx weight percent, calculated based on the total weight of the lithium iron phosphate salt particles, where 1.0 ≤ Cx ≤ 1.6.

[0088] In some embodiments, the primary average particle size of the lithium iron phosphate particles is 500 nm, 600 nm, 650 nm, 700 nm, 790 nm, 800 nm, 870 nm, 900 nm, 920 nm, 1000 nm, 1100 nm, 1200 nm, 1250 nm, 1300 nm, 1400 nm, 1500 nm, 1600 nm, 1700 nm, 1800 nm, 1900 nm, 2000 nm, 2100 nm, 2200 nm, 2300 nm, 2400 nm, 2500 nm, 2600 nm, 2700 nm, 2800 nm, 2900 nm or 3000 nm, or a range between any two of the above values.

[0089] In some embodiments, the BET specific surface area of ​​the lithium iron phosphate particles is 3 m 2 / g、4m 2 / g、5m 2 / g、6m 2 / g、7m 2 / g, 7.2m 2 / g、7.8m 2 / g or 8m 2 / g, or the range between any two of the above values.

[0090] In some embodiments, the carbon content of the lithium iron phosphate particles is 0.8 weight %, 0.9 weight %, 0.94 weight %, 1.0 weight %, 1.1 weight %, 1.2 weight %, 1.3 weight %, 1.4 weight %, 1.5 weight %, 1.6 weight %, 1.7 weight %, 1.8 weight %, 1.9 weight % or 2.0 weight %, or a range between any two of the above values.

[0091] In this article, the term "primary average particle size" refers to the particle size value obtained by statistically analyzing the particle size of the particles using the long diameter statistical method under the field of view of a scanning electron microscope. Among them, in the particle size statistical process, particles with a primary average particle size less than or equal to 80nm are not included in the statistical range. The primary average particle size of this application is shown in Figure 3. The primary average particle size refers to the average particle size of the primary particles. In this article, "primary particles" refer to particles that do not have obvious agglomeration interfaces in the particle scanning electron microscope image, but may have tiny pores and point or line defects, which are different from powder particles that are the smallest units of structures such as stacking and flocculation. The method for determining the primary average particle size, carbon content and BET specific surface area can be carried out using the determination method described in the examples.

[0092] In the embodiment of the present application, the primary average particle size of the lithium iron phosphate particles is 500 nm to 3000 nm, and the BET specific surface area is 3 m 2 / g~8m 2 / g, the carbon content calculated based on the total weight of the lithium iron phosphate particles is Cx weight % (0.8≤Cx≤2.0), which can improve the dynamic properties of lithium iron phosphate particles with an average primary particle size within the above range; the particles are kept in a suitable micron-level size, thereby avoiding the interface side reactions and processing difficulties caused by nano-sizing the particles, and the particles will not be limited to an excessively large size and thus suffer a reduction in dynamic performance; in addition, it is beneficial to the stirring of the slurry containing the lithium iron phosphate particles and the increase of the solid content, thereby improving the processing problem of the battery cell and further increasing the volume energy density of the battery; in addition, it can avoid the normal deintercalation and extraction of lithium ions due to the excessively high density of the carbon coating, thereby affecting the performance of the battery cell capacity.

[0093] Compared with the prior art, the lithium iron phosphate salt particles in the embodiment of the present application have a smaller specific surface area when containing the same carbon content, which means that the particles contain less floating carbon, and can make the carbon contained in the lithium iron phosphate salt particles more uniform and dense and each particle is coated, thereby improving the surface conductivity of the particles and improving its kinetic performance as a positive electrode material.

[0094] In some embodiments, the ratio z of the BET specific surface area to Cx satisfies 1.5≤z≤8.5. In some embodiments, the ratio z of the BET specific surface area to Cx satisfies 3≤z≤6.

[0095] In some embodiments, the ratio z of the BET specific surface area to Cx is 1.5, 2.0, 2.5, 3.0, 3.9, 3.5, 3.75, 4.0, 4.17, 4.5, 5.0, 5.5, 5.71, 5.83, 6.0, 6.5, 7.0, 7.5, 8.0 or 8.5, or a range between any two of the above values.

[0096] By further limiting the ratio z to satisfy the above range, it can be intuitively reflected that the lithium iron phosphate salt particles in the embodiment of the present application contain less floating carbon and the carbon coating is more uniform and dense, thereby further improving its conductivity and kinetic performance as a positive electrode material.

[0097] In some embodiments, the lithium iron phosphate salt has the molecular formula Li m Fe x P y O j Q q , wherein Q includes at least one of Al, Na, K, Mg, Cu, Mn, Cr, Zn, Pb, Ca, Co, Ni, Sr, Nb, V, Ti, B, Si, N, S, F, Cl, and Br, 0.95≤m≤1.15, 0.9≤x≤1, 0.95≤y≤1, 3.5≤j≤4, and 0<q≤0.1.

[0098] By making the lithium iron phosphate salt of the embodiment of the present application have the above molecular formula and doping it with one or more of the above elements, a high-load bulk modification of the lithium iron phosphate salt particles is achieved. This helps to improve the bulk ion transport capacity of the lithium iron phosphate salt particles and can effectively solve the problem of poor kinetic performance inherent in large particles. In addition, by performing this doping, the lithium iron phosphate salt particles can exhibit good kinetic performance when used as a positive electrode material. In this application, the modification can specifically be manifested as doping and / or coating.

[0099] In some embodiments, Q comprises at least one of Ti, V, Mg, and Nb. In some embodiments, Q is Ti.

[0100] In some embodiments, the content of Ti, V, Mg and / or Nb is 1000ppm-10000ppm, calculated based on the total weight of the lithium iron phosphate salt particles. In some embodiments, the content of Ti, V, Mg and / or Nb is 2500ppm-6000ppm, for example, 2500ppm, 2600ppm, 2700ppm, 2800ppm, 2900ppm, 3000ppm, 3100ppm, 3200ppm, 3300ppm, 3400ppm, 3500ppm, 3600ppm, 3700ppm, 3800ppm, 3900ppm, 4000ppm, 4100ppm, 4200ppm, 4300ppm, 4400ppm, 4500ppm, 4600ppm, 4700ppm, 4800ppm, 4900ppm, 5000ppm, 5100ppm, 5200ppm, 5300ppm, 5400ppm, 5500ppm, 5600ppm, 5700ppm, 5800ppm, 5900ppm, 6000ppm, 6100ppm, 6200ppm, 6300ppm, 6400ppm, 6500ppm, 6600ppm, 6700ppm, 6800ppm, 6900ppm, 7000ppm, 7100ppm, 7100ppm, 7200ppm, 7300ppm, 7400ppm, 7500ppm, 7600ppm, 7700ppm, 7800ppm, 7900ppm, 8000ppm, 8100ppm, 8100ppm, 8100ppm, 8100ppm 400ppm, 4500ppm, 4600ppm, 4700ppm, 4800ppm, 4900ppm, 5000ppm, 5100ppm, 5200ppm, 5300ppm, 5400ppm, 5500ppm, 5600ppm, 5700ppm, 5800ppm, 5900ppm or 6000ppm, or a range between any two of the above values.

[0101] In some embodiments, the lithium iron phosphate salt has the molecular formula Li m A a Fe x D d P y E e O z G g , A includes at least one element selected from Al, Na, K or Mg; D includes at least one element selected from Cu, Mn, Cr, Zn, Pb, Ca, Co, Ni, Sr, Nb, Ti or V; E includes at least one element selected from B, S, Si or N; G includes at least one element selected from S, F, Cl or Br; m is selected from the range of 0.95 to 1.15; a is selected from the range of 0 to 0.1; x is selected from the range of 0.95 to 1; d is selected from the range of 0 to 0.1; y is selected from the range of 0.95 to 1; e is selected from the range of 0 to 0.1; z is selected from the range of 3.5 to 4; g is selected from the range of 0 to 0.1.

[0102] By doping the lithium iron phosphate salt of the embodiment of the present application with one or more of the above elements and making their content within the above range, it is possible to better achieve metal bulk modification of the lithium iron phosphate salt particles while improving the primary average particle size of the lithium iron phosphate salt particles, further enhance the bulk ion transport capacity of the lithium iron phosphate salt particles, and better solve the problem of poor kinetic performance inherent in large particles.

[0103] In some embodiments, the lithium iron phosphate salt particles are single crystal particles and / or polycrystalline particles. In some embodiments, based on the total number of lithium iron phosphate salt particles, the number of single crystal particles accounts for more than 90%. A single crystal refers to a structurally complete crystal grown from a single crystal nucleus, and there are no grain boundaries in the single crystal; a polycrystal is a crystal formed by a large number of small single crystal particles randomly oriented and combined, and there are grain boundaries inside the polycrystal. Different from a perfect single crystal, the large single crystal mentioned in this article may have tiny defects, such as micropores inside, a small number of points and surfaces, or a small number of particles sticking to each other on the surface of a particle. However, the single crystal referred to in this article is a whole in the field of view of the TEM photo.

[0104] In an embodiment of the present application, the proportion of single crystal particles is controlled within the above range based on the total number of lithium iron phosphate particles. Compared with polycrystalline and secondary agglomerates, the proportion of single crystal particles is high, the barrier of grain boundaries to lithium ions is smaller, the transmission rate of lithium ions is faster, and the kinetic performance is better.

[0105] In the lithium iron phosphate particles of the embodiments of the present application, the carbon may be present in a state mixed with the lithium iron phosphate particles or coated on the lithium iron phosphate particles. Alternatively, the carbon in the lithium iron phosphate particles may be coated on the lithium iron phosphate particles, thereby enabling the carbon to form a uniform and dense carbon coating on the surface of the lithium iron phosphate particles, thereby improving the surface conductivity of the particles.

[0106] In some embodiments, the capacity proportion of lithium iron phosphate salt particles η ≥ 88%, and η is defined as: a battery having lithium iron phosphate salt particles as a positive electrode material is charged and discharged twice at a constant current rate of 0.1C in the voltage range of 2.0V to 3.75V, and then charged and discharged once at a constant current rate of 1C. In the charge and discharge test at the rate of 1C, the capacity value at the discharge voltage of 3.2V is extracted and recorded as C1, and the capacity value at the discharge voltage to 2.0V is extracted as C2, η = C1 / C2, wherein the charging process includes constant voltage charging, a constant voltage of 3.75V, and a constant voltage cut-off current of 50μA.

[0107] The η value can characterize the kinetic properties of the lithium iron phosphate particles, and can be adjusted by adjusting the primary average particle size of the particles, carbon content, the ratio of carbon source and film-forming agent, modifier and its content. η represents the platform retention performance of the material, and this value is strongly correlated with the discharge power performance of the battery. When this value is large, the battery can still maintain good power performance when discharged to a low SOC (battery state of charge), that is, the voltage drop of the battery is small when the battery is discharged at a high current at low power. The capacity proportion η of the lithium iron phosphate salt particles of the embodiment of the present application is ≥88%, indicating that the lithium iron phosphate salt particles of the present application can enable the secondary battery to exhibit good kinetic properties when used as a positive electrode material.

[0108] In some embodiments, the lithium iron phosphate salt particles satisfy at least one of a) to f):

[0109] a) Dv10 of lithium iron phosphate particles ≥ 0.2 μm;

[0110] b) the Dv50 of the lithium iron phosphate particles is 0.5 μm-5 μm;

[0111] c) Dv90 of lithium iron phosphate particles ≤ 10 μm;

[0112] d) Dv99 of lithium iron phosphate particles ≤ 12 μm;

[0113] e) The powder compaction density of lithium iron phosphate at 3T pressure is ≥2.25g / cm 3 ;

[0114] f) The powder resistivity of the lithium iron phosphate salt is less than 60Ω·cm.

[0115] In this application, Dv10, Dv90, and Dv99 refer to the particle sizes corresponding to the 10%, 90%, and 99% cumulative particle size distribution percentages, respectively. The test method is the same as that for Dv50, and can be determined using the standards in the examples of this application.

[0116] It should be noted that the Dv10 is a large index, so the upper limit cannot be given, and the Dv90 is a small index, so the lower limit cannot be given.

[0117] By making the lithium iron phosphate salt particles satisfy at least one of a) to f), the lithium iron phosphate salt particles can achieve the above technical effects better.

[0118] In the present application, the test methods for the above-mentioned Dv10, Dv90, and Dv99 are the same as those for the above-mentioned Dv50, and can be measured using the standards in the examples of the present application.

[0119] In this application, the test method for powder resistivity is as follows: referring to the national standard GB / T 33822-2017, a powder resistivity meter (Suzhou Jingge, ST2722 model) is used, 1 gram (g) of sample is weighed (the error is within ±0.005g), added to the feeding chamber, and a pressure of 8 MPa is applied. The forward resistivity and reverse resistivity of the sample are tested respectively, and the average value of the two is taken as the powder resistivity of the sample.

[0120] In this application, the definition of powder compaction density is: during the external compression process, as the powder moves and deforms, larger gaps are filled, the contact area between particles increases, the attraction between atoms is generated, and the mechanical fit between particles is enhanced, thus forming a compact with a certain density and strength. The unit is g / cm 3 .

[0121] In this application, the compaction density test method is as follows: Referring to the national standard GB / T 24533-2009, a certain amount of powder is placed in a special compaction mold (of known diameter). The mold has a hollow center with two metal discs above and below. The powder is placed between the metal discs and a metal cylinder is placed on top. The mold is placed on a compaction density instrument and different pressures are set. The instrument can read the powder thickness at different pressures and calculate the compaction density using ρ = m / v.

[0122] According to the following powder compaction density calculation results, ρc=m / V=m / (S*H); where ρc is the powder compaction density (g / cm 3 ), m is the mass of the material (g), S is the bottom area of ​​the mold (1.327cm 2 ), H is the height of the sample after compaction (cm).

[0123] [Preparation method of lithium iron phosphate particles]

[0124] The method for preparing lithium iron phosphate salt particles according to an embodiment of the present application can prepare the lithium iron phosphate salt particles according to the present application. The method for preparing lithium iron phosphate salt particles according to an embodiment of the present application comprises: providing raw materials containing at least a lithium source, an iron source, a phosphorus source, a carbon source, a carbon film-forming agent, and a modifier, and performing at least two sintering operations, wherein the temperature of the first sintering operation is 500°C to 760°C, and the carbon content of the material after the first sintering operation is 0.01 wt% to 0.79 wt%; and the temperature of the second sintering operation is 700°C to 800°C, and the carbon content of the material after the second sintering operation is 0.8 wt% to 2.0 wt%.

[0125] In some embodiments, the preparation method of lithium iron phosphate salt particles includes: providing raw materials containing at least a lithium source, an iron source, a phosphorus source, a carbon source, a carbon film-forming agent, and a modifier, and performing at least two sinterings, wherein the temperature of the first sintering is 550°C-720°C, and the carbon content of the material after the first sintering is 0.05 wt%-0.4 wt%; the temperature of the second sintering is 720°C-780°C, and the carbon content of the material after the second sintering is 1.0 wt%-1.6 wt%.

[0126] In some embodiments, the temperature of the first sintering is 500°C, 510°C, 520°C, 530°C, 540°C, 550°C, 560°C, 570°C, 580°C, 590°C, 600°C, 610°C, 620°C, 630°C, 640°C, 650°C, 655°C, 660°C, 670°C, 680°C, 690°C, 700°C, 710°C, 720°C, 730°C, 740°C, 750°C, 760°C or 760°C, or a range between any two of the above values.

[0127] In some embodiments, the temperature of the second sintering is 700°C, 710°C, 720°C, 730°C, 740°C, 750°C, 755°C, 760°C, 770°C, 780°C, 790°C or 800°C, or a range between any two of the above values.

[0128] In some embodiments, the carbon content of the material after the first sintering is 0.01 weight %, 0.05 weight %, 0.10 weight %, 0.15 weight %, 0.2 weight %, 0.25 weight %, 0.3 weight %, 0.35 weight %, 0.4 weight %, 0.45 weight %, 0.5 weight %, 0.55 weight %, 0.6 weight %, 0.65 weight %, 0.7 weight %, 0.75 weight %, 0.79 weight %, or a range between any two of the above values.

[0129] In some embodiments, the carbon content of the material after the second sintering is 0.8 wt% to 2.0 wt%, 0.8 wt%, 0.9 wt%, 1 wt%, 1.1 wt%, 1.2 wt%, 1.3 wt%, 1.4 wt%, 1.5 wt%, 1.6 wt%, 1.7 wt%, 1.8 wt%, 1.9 wt% or 2.0 wt%, or a range between any two of the above values.

[0130] In the preparation method of lithium iron phosphate salt particles of the embodiment of the present application, two sintering processes are implemented. Among them, by controlling the temperature of the first sintering within the above range and making the carbon content of the intermediate after sintering within the above range, the lithium iron phosphate precursor obtained after the first sintering can have a larger particle size, which directly improves the powder compaction of the final product and the electrode compaction density. In addition, by adding a lower content of carbon source during the first sintering, the barrier effect of the carbon layer on the growth process of lithium iron phosphate particles is greatly reduced, which is conducive to the crystallization growth of particles at a lower temperature, and is also conducive to the solid-phase diffusion reaction between the modifier and the lithium iron phosphate material, thereby achieving a higher concentration of metal ion modification. Compared with the traditional method of using high temperature to achieve particle growth, the above-mentioned preparation method of the present application can first synthesize large particles at a lower temperature, which can improve the phenomenon of cracking of the carbon layer on the surface of the particles at high temperature and improve the density of the surface carbon; at the same time, during the first sintering process, the carbon source can effectively reduce the trivalent iron in the raw material, thereby improving the purity and stability of the product; by controlling the temperature of the second sintering within the above-mentioned range and making the carbon content of the sintered material 0.8 wt%-2.0 wt%, carbon can be coated on the surface of the lithium iron phosphate salt particles to form a lithium iron phosphate material with a uniform and dense carbon coating layer on which each particle is coated, thereby greatly improving the surface conductivity of the particles.

[0131] In some embodiments, the lithium iron phosphate salt particles contain at least one of Ti, V, Mg, and / or Nb elements, and the content of the element is 1000 ppm-10000 ppm based on the total weight of the lithium iron phosphate salt particles.

[0132] In some embodiments, the element is present in an amount of 2500 ppm to 6000 ppm. Specifically it is 2500ppm, 2600ppm, 2700ppm, 2800ppm, 2900ppm, 3000ppm, 3100ppm, 3200ppm, 3300ppm, 3400ppm, 3500ppm, 3600ppm, 3700ppm, 3800ppm, 3900ppm, 4000ppm, 4100ppm, 4200ppm, 4300ppm, 4400ppm, 4500ppm, 4600ppm, 4700ppm, 4800ppm, 4900ppm, 5000ppm, 5100ppm, 5200ppm, 5300ppm, 5400ppm, 5500ppm, 5600ppm, 5700ppm, 5800ppm, 5900ppm or 6000ppm, or the range between any two of the above values.

[0133] By doping the lithium iron phosphate salt of the embodiment of the present application with one or more of the above elements and making their content within the above range, it is possible to better achieve metal bulk modification of the lithium iron phosphate salt particles while improving the primary average particle size of the lithium iron phosphate salt particles, further enhance the bulk ion transport capacity of the lithium iron phosphate salt particles, and better solve the problem of poor kinetic performance inherent in large particles.

[0134] In some embodiments, in the preparation method of lithium iron phosphate salt particles, a first crushing is performed after the first sintering, and a second crushing is performed after the second sintering, wherein the Dv50 of the product after the first crushing is 300nm-1200nm; the Dv50 of the product after the second crushing is 500nm-5000nm.

[0135] In some embodiments, in the preparation method of lithium iron phosphate salt particles, a first crushing is performed after the first sintering, and a second crushing is performed after the second sintering, wherein the Dv50 of the product after the first crushing is 400nm-1100nm; the Dv50 of the product after the second crushing is 300nm-2500nm.

[0136] In some embodiments, the Dv50 of the product after the first pulverization is 300 nm, 350 nm, 400 nm, 500 nm, 550 nm, 600 nm, 700 nm, 800 nm, 900 nm, 1000 nm, 1100 nm, or 1200 nm, or a range between any two of the above values.

[0137] In some embodiments, the Dv50 of the product after the second crushing is 500 nm, 600 nm, 700 nm, 800 nm, 900 nm, 1000 nm, 1100 nm, 1200 nm, 1300 nm, 1400 nm, 1500 nm, 1600 nm, 1700 nm, 1800 nm, 1900 nm, 2000 nm, 2300 nm, 2500 nm, 2700 nm, 3000 nm, 3300 nm, 3500 nm, 3700 nm, 4000 nm, 4300 nm, 4500 nm, 4700 nm, 5000 nm, or a range between any two of the above values.

[0138] As used herein, the term "Dv50" refers to the particle size at which the volume cumulative particle size distribution percentage in the particles reaches 50%. Dv50 can be measured using the measurement method described in the Examples.

[0139] In the preparation method of lithium iron phosphate salt particles according to the embodiments of the present application, two post-sintering crushing steps are implemented. By performing the first crushing after the first sintering, the product has a Dv50 of 300nm-1200nm, which can avoid the growth barrier of the carbon material and the modifying elements on the crystals, and obtain a micron-sized lithium iron phosphate precursor. By performing the second crushing after the second sintering, the product has a Dv50 of 500nm-3000nm, and lithium iron phosphate salt particles of the desired particle size can be obtained. A uniform and dense carbon coating is obtained, and each particle is coated, significantly improving the surface conductivity of the particles.

[0140] In some embodiments, the preparation method of lithium iron phosphate salt particles includes: providing raw materials containing at least a lithium source, an iron source, a phosphorus source, a carbon source, a carbon film-forming agent, and a modifier, wherein the mixing ratio of the lithium source, the iron source, and the phosphorus source is calculated based on the atomic molar number of each element, satisfying Fe:P=0.96-0.985:1, Li:Fe=1.0-1.1:0.95-1.1; the carbon source and the carbon film-forming agent are calculated based on the weight ratio satisfying carbon source:carbon film-forming agent=9:1-2:8.

[0141] In some embodiments, the mixing ratio of the iron source and the phosphorus source, calculated on the basis of the atomic moles of each element, satisfies Fe:P=0.96:1, Fe:P=0.965:1, Fe:P=0.97:1, Fe:P=0.975:1, Fe:P=0.98:1 or Fe:P=0.985:1.

[0142] In some embodiments, the mixing ratio of the lithium source to the iron source, calculated on the basis of the atomic moles of each element, satisfies Li:Fe=1.0:1.1, Li:Fe=0.99:1.1, Li:Fe=0.98:1.1, Li:Fe=0.97:1.1, Li:Fe=0.96:1.1 or Li:Fe=0.95:1.1.

[0143] In some embodiments, the weight ratio of the carbon source to the carbon film-forming agent may be 9:1, 8:2, 7:3, 6:4, 5:5, 4:6, 3:7 or 2:8.

[0144] By using raw materials containing a lithium source, an iron source, a phosphorus source, a carbon source, a modifier, and a carbon film-forming agent in the above-mentioned proportions, it is possible to achieve good modification, and to form the obtained lithium iron phosphate salt particle precursor into particles of larger size, thereby being able to well form the primary average particle size, carbon content, BET specific surface area, and z value of the lithium iron phosphate salt particles of the first aspect of this application. Specifically, as described above, by making the primary average particle size of the lithium iron phosphate salt particles of the embodiment of the present application 500nm to 3000nm, the particles can be kept at a suitable micron-level size, thereby avoiding the problems of interfacial side reactions and processing difficulties caused by nano-sizing the particles (in this field, it is difficult to process nano-sized particles), and the particles will not be limited to a reduced dynamic performance due to excessive size. In addition, by making the BET specific surface area of ​​the lithium iron phosphate salt particles of the embodiment of the present application 3m 2 / g~8m 2 / g, which can facilitate the stirring of the slurry containing the lithium iron phosphate salt particles and increase the solid content, thereby improving the processing of the battery cell and further increasing the volumetric energy density of the battery. Further optionally, by ensuring that the ratio z of the BET specific surface area to Cx of the lithium iron phosphate salt particles in the embodiment of the present application satisfies the range of 1.5≤z≤8.5, the carbon contained in the lithium iron phosphate salt particles can be made more uniform and dense, and each particle can be coated, thereby improving the surface conductivity of the particles.

[0145] In some embodiments, in the method for preparing lithium iron phosphate particles, the lithium source is a lithium compound, including one or more of lithium dihydrogen phosphate, lithium oxalate, lithium carbonate, lithium oxide, lithium hydroxide, and lithium acetate. In some embodiments, the lithium source is lithium carbonate.

[0146] In some embodiments, in the method for preparing lithium iron phosphate particles, the iron source is an iron compound, including at least one of ferric hydroxide, ferrous chloride, ferric oxide, ferric phosphate, ferric pyrophosphate, ferrous oxalate, iron powder, ferric nitrate, ferrosoferric oxide, and ferric oxyhydroxide. In some embodiments, the iron source is ferric oxide.

[0147] In some embodiments, in the method for preparing lithium iron phosphate salt particles, the phosphorus source is a phosphoric acid compound, including one or more of phosphoric acid, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate. In some embodiments, the phosphorus source is phosphoric acid.

[0148] In some embodiments, in the method for preparing lithium iron phosphate particles, the modifier includes at least one of titanium dioxide, vanadium pentoxide, n-butyl titanate, ammonium metavanadate, niobium ethoxide, niobium oxalate, niobium pentoxide, magnesium hydroxide, and magnesium nitrate. In some embodiments, the modifier is titanium dioxide.

[0149] In some embodiments, in the method for preparing lithium iron phosphate particles, the carbon source includes at least one of citric acid, glucose, sucrose, starch, fructose, and lactose. In some embodiments, the carbon source is glucose.

[0150] In some embodiments, in the method for preparing lithium iron phosphate particles, the carbon film-forming agent includes one or a combination of polyethylene glycol, polyaniline, polyacrylonitrile, polyvinyl pyrrolidone, and polyvinyl alcohol. In some embodiments, the carbon film-forming agent is polyaniline.

[0151] By selecting the above substances as the lithium source, iron source, phosphorus source, modifier, carbon source, and carbon film-forming agent respectively used in the method for preparing the lithium iron phosphate salt particles according to the embodiment of the present application, the primary average particle size, carbon content, BET specific surface area, and z-value of the lithium iron phosphate salt particles according to the first aspect of the present application can be well formed. This can better achieve a large particle size of the lithium iron phosphate salt particles according to the embodiment of the present application, while also achieving excellent kinetic performance when used as a positive electrode material.

[0152] In some embodiments, the heating rates in the first sintering and the second sintering are independently 2° C. / min-20° C. / min, the constant temperature time of the first sintering is 1 h-6 h, and the constant temperature time of the second sintering is 2 h-12 h.

[0153] In some embodiments, the heating rates in the first sintering and the second sintering are each independently 2°C / min, 5°C / min, 7°C / min, 10°C / min, 13°C / min, 15°C / min, 17°C / min, or 20°C / min.

[0154] In some embodiments, the constant temperature sintering time of the first sintering is 1-6 hours. In some embodiments, the constant temperature sintering time of the first sintering is 1 hour, 1.5 hours, 2 hours, 2.5 hours, 3 hours, 3.5 hours, 4 hours, 4.5 hours, 5 hours, 5.5 hours or 6 hours.

[0155] In some embodiments, the constant temperature sintering time of the second sintering is 2-12 hours. In some embodiments, the constant temperature sintering time of the second sintering is 1 hour, 1.5 hours, 2 hours, 2.5 hours, 3 hours, 3.5 hours, 4 hours, 4.5 hours, 5 hours, 5.5 hours, 6 hours, 6.5 hours, 7 hours, 7.5 hours, 8 hours, 8.5 hours, 9 hours, 9.5 hours, 10 hours, 10.5 hours, 11 hours, 11.5 hours or 12 hours.

[0156] By independently controlling the heating rate and holding time during the first and second sintering processes within the aforementioned ranges, it is possible to prevent excessive side reactions during the sintering process caused by excessively rapid heating, thereby affecting the primary average particle size, carbon content, BET specific surface area, and z-value of the resulting lithium iron phosphate salt particles. This allows for a better realization of the large particle size of the lithium iron phosphate salt particles of the embodiments of the present application, while also achieving excellent kinetic performance when used as a positive electrode material.

[0157] In some embodiments, after the first sintering and before the second sintering, the product after the first sintering is carbon-coated, and the amount of carbon coating is 0.01 wt%-1.99 wt% based on the total weight of the product after the first sintering. In some embodiments, based on the total weight of the product after the first sintering, the amount of carbon coating is 0.2 wt%-1.6 wt%, specifically 0.2 wt%, 0.3 wt%, 0.4 wt%, 0.5 wt%, 0.6 wt%, 0.7 wt%, 0.8 wt%, 0.9 wt%, 1.0 wt%, 1.1 wt%, 1.2 wt%, 1.3 wt%, 1.4 wt%, 1.5 wt% or 1.6 wt%, or a range between any two of the above values.

[0158] By coating the product of the first sintering with carbon within the above-mentioned content range after the first sintering and before the second sintering, a uniform and dense carbon coating layer can be formed on the surface of the lithium iron phosphate particles during the second sintering process, and each particle is coated.

[0159] In some embodiments, carbon coating is performed by vapor deposition during sintering, or by carbonizing a carbon source at high temperature.

[0160] In the preparation method of the lithium iron phosphate salt particles of the embodiment of the present application, the embodiment of carbon coating is not particularly limited. In some embodiments, the embodiment of carbon coating is to carbon-coat the lithium iron phosphate salt particles by using a vapor deposition method, thereby forming a uniform and dense carbon coating layer on the surface of the lithium iron phosphate salt particles and coating each particle.

[0161] In some embodiments, after the first sintering and the second sintering, the sintered product is pulverized, and the pulverization includes one or more of grinding, sand milling, mechanical crushing, and air flow crushing.

[0162] In the method for preparing lithium iron phosphate salt particles according to the embodiment of the present application, the pulverization method is not particularly limited. By adopting the above-mentioned specific pulverization method, it is advantageous to obtain a desired primary average particle size.

[0163] [Positive electrode]

[0164] The positive electrode sheet includes a positive electrode current collector and a positive electrode film layer disposed on at least one surface of the positive electrode current collector. The positive electrode film layer includes the lithium iron phosphate salt particles of the first aspect of the present application as a positive electrode material. The lithium iron phosphate salt particles of the present application have a large particle size when used in a secondary battery and also have excellent kinetic performance when used as a positive electrode material.

[0165] In some embodiments, the positive electrode current collector may be a metal foil or a composite current collector. For example, the battery aluminum foil of the present application is used as the metal foil. The composite current collector may include a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer. The composite current collector may be formed by forming the battery aluminum foil of the present application on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0166] In some embodiments, the positive electrode material may be a positive electrode material for a battery that is well known in the art. As an example, the positive electrode material may include at least one of the following materials: a lithium-containing phosphate with an olivine structure, a lithium transition metal oxide, and their respective modified compounds. However, the present application is not limited to these materials, and other traditional materials that can be used as positive electrode materials for batteries may also be used. These positive electrode materials may be used alone or in combination of two or more. Examples of lithium transition metal oxides may include, but are not limited to, lithium cobalt oxide (such as LiCoO2), lithium nickel oxide (such as LiNiO2), lithium manganese oxide (such as LiMnO2, LiMn2O4), lithium nickel cobalt oxide, lithium manganese cobalt oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide (such as LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (also referred to as NCM 333 ), LiNi 0.5 Co 0.2 Mn 0.3 O2 (also referred to as NCM 523 ), LiNi 0.5 Co 0.25 Mn 0.25 O2 (also referred to as NCM 211 ), LiNi 0.6 Co 0.2 Mn 0.2 O2 (also referred to as NCM 622 ), LiNi 0.8 Co 0.1 Mn 0.1 O2 (also referred to as NCM 811 ), lithium nickel cobalt aluminum oxide (such as LiNi 0.85 Co0.15 Al 0.05 O2) and its modified compounds. Examples of olivine-structured lithium-containing phosphates may include, but are not limited to, at least one of lithium iron phosphate (such as LiFePO4 (also referred to as LFP)), a composite material of lithium iron phosphate and carbon, lithium manganese phosphate (such as LiMnPO4), a composite material of lithium manganese phosphate and carbon, lithium iron manganese phosphate, and a composite material of lithium iron manganese phosphate and carbon.

[0167] In some embodiments, the positive electrode film layer may further optionally include a binder. As an example, the binder may include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), a vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, a vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, a tetrafluoroethylene-hexafluoropropylene copolymer, and a fluorine-containing acrylate resin.

[0168] In some embodiments, the positive electrode film layer may further include a conductive agent. For example, the conductive agent may include at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0169] In some embodiments, the positive electrode sheet can be prepared by the following method: the components used to prepare the positive electrode sheet, such as the positive electrode material, the conductive agent, the binder and any other components are dispersed in a solvent (such as N-methylpyrrolidone) to form a positive electrode slurry; the positive electrode slurry is coated on the positive electrode collector, and after drying, cold pressing and other processes, the positive electrode sheet can be obtained.

[0170] [Negative electrode]

[0171] The negative electrode sheet includes a negative electrode current collector and a negative electrode film layer provided on at least one surface of the negative electrode current collector, wherein the negative electrode film layer includes a negative electrode active material.

[0172] As an example, the negative electrode current collector has two surfaces opposite to each other in its thickness direction, and the negative electrode film layer is disposed on either or both of the two opposite surfaces of the negative electrode current collector.

[0173] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, copper foil may be used as the metal foil. The composite current collector may include a polymer base layer and a metal layer formed on at least one surface of the polymer base material. The composite current collector may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy, etc.) on a polymer base material (such as a base material of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0174] In some embodiments, the negative electrode active material may be a negative electrode active material for a battery that is well known in the art. As an example, the negative electrode active material may include at least one of the following materials: artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, and lithium titanate. The silicon-based material may be selected from at least one of elemental silicon, silicon oxides, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. The tin-based material may be selected from at least one of elemental tin, tin oxides, and tin alloys. However, the present application is not limited to these materials, and other traditional materials that can be used as negative electrode active materials for batteries may also be used. These negative electrode active materials may be used alone or in combination of two or more.

[0175] In some embodiments, the negative electrode film layer may further 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).

[0176] In some embodiments, the negative electrode film layer may further include a conductive agent, which may be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0177] In some embodiments, the negative electrode film layer may optionally include other additives, such as a thickener (eg, sodium carboxymethyl cellulose (CMC-Na)).

[0178] In some embodiments, the negative electrode sheet can be prepared by the following method: the components used to prepare the negative electrode sheet, such as the negative electrode active material, the conductive agent, the binder and any other components are dispersed in a solvent (such as deionized water) to form a negative electrode slurry; the negative electrode slurry is coated on the negative electrode collector, and after drying, cold pressing and other processes, the negative electrode sheet can be obtained.

[0179] [Electrolytes]

[0180] The electrolyte conducts ions between the positive and negative electrodes. This application does not specify the type of electrolyte, and the electrolyte can be selected based on the needs. For example, the electrolyte can be liquid, gel, or solid.

[0181] In some embodiments, the electrolyte is an electrolyte solution, which includes an electrolyte salt and a solvent.

[0182] In some embodiments, the electrolyte salt may be selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalatoborate, lithium dioxalatoborate, lithium difluorodioxalatophosphate, and lithium tetrafluorooxalatophosphate.

[0183] In some embodiments, the solvent can be selected from at least one of ethylene carbonate, propylene carbonate, ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methylpropyl carbonate, ethylpropyl carbonate, butylene carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, cyclopentane, dimethyl sulfone, methyl ethyl sulfone and diethyl sulfone.

[0184] In some embodiments, the electrolyte may optionally include additives. For example, the additives may include negative electrode film-forming additives, positive electrode film-forming additives, and additives that can improve certain battery properties, such as additives that improve battery overcharge performance, and additives that improve battery high or low temperature performance.

[0185] [Isolation film]

[0186] In some embodiments, the secondary battery further includes a separator. The present application has no particular limitation on the type of separator, and any known porous separator with good chemical and mechanical stability can be selected.

[0187] In some embodiments, the material of the separator can be selected from at least one of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single-layer film or a multi-layer composite film, without particular limitation. When the separator is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

[0188] [Secondary battery]

[0189] In one embodiment of the present application, a secondary battery is provided, which includes a positive electrode plate.

[0190] Typically, a secondary battery consists of a positive electrode, a negative electrode, an electrolyte, and a separator. During the battery's charge and discharge processes, active ions are inserted and removed between the positive and negative electrodes. The electrolyte conducts ions between the positive and negative electrodes. The separator, located between the positive and negative electrodes, primarily prevents short circuits between the positive and negative electrodes while allowing ions to pass through.

[0191] In some embodiments, the positive electrode sheet, the negative electrode sheet, and the separator can be formed into an electrode assembly through a winding process or a lamination process.

[0192] In some embodiments, the secondary battery may include an outer packaging that can be used to encapsulate the electrode assembly and the electrolyte.

[0193] In some embodiments, the outer packaging of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. Alternatively, the outer packaging of the secondary battery can be a soft shell, such as a pouch-type soft shell. The soft shell can be made of plastic, such as polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0194] The present application has no particular limitation on the shape of the secondary battery, which may be cylindrical, square, or any other shape. For example, FIG5 shows a secondary battery 5 having a square structure as an example.

[0195] In some embodiments, referring to Figure 6, the outer packaging may include a shell 51 and a cover plate 53. The shell 51 may include a bottom plate and a side plate connected to the bottom plate, and the bottom plate and the side plate enclose a receiving cavity. The shell 51 has an opening connected to the receiving cavity, and the cover plate 53 can be covered on the opening to close the receiving cavity. The positive electrode sheet, the negative electrode sheet and the isolation membrane can be formed into an electrode assembly 52 through a winding process or a lamination process. The electrode assembly 52 is encapsulated in the receiving cavity. The electrolyte is infiltrated in the electrode assembly 52. ​​The number of electrode assemblies 52 contained in the secondary battery 5 can be one or more, and those skilled in the art can select according to specific actual needs.

[0196] In some embodiments, secondary batteries can be assembled into a battery module. The number of secondary batteries contained in the battery module can be one or more. The specific number can be selected by those skilled in the art according to the application and capacity of the battery module.

[0197] Figure 7 shows an example battery module 4. Referring to Figure 7 , within the battery module 4, multiple secondary batteries 5 may be arranged sequentially along the length of the battery module 4. Of course, any other arrangement is also possible. Furthermore, the multiple secondary batteries 5 may be secured together using fasteners.

[0198] Optionally, the battery module 4 may further include a housing having a receiving space, and the plurality of secondary batteries 5 are received in the receiving space.

[0199] In some embodiments, the battery modules described above may also be assembled into a battery pack. The battery pack may contain one or more battery modules, and the specific number may be selected by those skilled in the art based on the application and capacity of the battery pack.

[0200] Figures 8 and 9 illustrate an example battery pack 1. Referring to Figures 8 and 9 , the battery pack 1 may include a battery box and multiple battery modules 6 disposed within the battery box. The battery box comprises an upper case 2 and a lower case 3. The upper case 2 can be positioned over the lower case 3 to form an enclosed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the battery box.

[0201] [Electrical devices]

[0202] In addition, the present application also provides an electrical device, which includes at least one of the secondary batteries, battery modules, or battery packs provided in the present application. The secondary batteries, battery modules, or battery packs can be used as power sources for the electrical devices, and can also be used as energy storage units for the electrical devices. Electrical devices may include mobile devices (such as mobile phones, laptops, etc.), electric vehicles (such as pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc., but are not limited thereto.

[0203] As an electrical device, a secondary battery, a battery module or a battery pack can be selected according to its usage requirements.

[0204] Figure 10 shows an example of an electric device. This device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. To meet the high power and high energy density requirements of the secondary battery, a battery pack or battery module can be used.

[0205] Another example device may be a mobile phone, a tablet computer, a notebook computer, etc. Such a device is generally required to be lightweight and thin, and may use a secondary battery as a power source.

[0206] Example

[0207] Below, the embodiment of the present application is described. The embodiment described below is exemplary and is only used to explain the present application, and is not to be construed as limiting the present application. Where specific techniques or conditions are not specified in the embodiments, the techniques or conditions described in the literature in this area or the product specifications are used. Reagents or instruments used that do not specify the manufacturer are conventional products that can be obtained commercially.

[0208] Example 1

[0209] 1. Preparation of lithium iron phosphate particles

[0210] Lithium carbonate, ferric oxide, phosphoric acid, glucose, titanium dioxide (based on the total weight of the lithium iron phosphate particles, the amount of titanium dioxide added is sufficient to ensure a titanium content of 5000 ppm in the prepared lithium iron phosphate particles), and polyaniline are weighed separately. The weight ratios of Li, Fe, and P satisfy the following: Fe:P = 0.968:1, Li:Fe = 1:0.98, and the weight ratio of glucose to polyaniline satisfies the following: glucose:polyaniline = 1:2. The amount of glucose added is sufficient to ensure that the carbon content of the lithium iron phosphate precursor after the first sintering is completed is 0.15% by weight. Water is added to the above substances to obtain a mixture slurry.

[0211] The mixture was mixed in a ball mill and ground in a sand mill to a slurry with a solids content of 38% and a Dv50 of 0.40 μm. The mixture was then spray-dried (the negative pressure of the high-speed spray dryer was -650 to -200 Pa, the inlet temperature was 300°C to 360°C, and the outlet temperature was 100°C to 140°C). The dried reactants were placed in a sintering furnace for the first sintering process. The heating rate was controlled at 5°C / min, the holding temperature was controlled at 650°C, and the holding time was 4 hours. After cooling, the material was pulverized using a mechanical mill to obtain a powder.

[0212] Glucose as a carbon source and polyaniline as a carbon film-forming agent were added to the resulting powder, which was then mixed with water to produce a material with a solid content of 40%. The amounts of glucose and polyaniline added were such that the carbon content of the product after the second sintering was 1.2% (based on the total weight of the lithium iron phosphate particles), and the weight ratio of glucose to polyaniline was 1:2. The material was processed using a ball mill and a sand mill to produce a slurry with a Dv50 value of insoluble matter of 550 nm. The slurry was then spray-dried (using a high-speed spray dryer with a negative pressure of -650 to -200 Pa, an inlet temperature of 300°C to 360°C, and an outlet temperature of 100°C to 140°C). The dried reactants were then placed in a sintering furnace for a second low-temperature sintering (heating rate controlled at 5°C / min, sintering temperature of 750°C, and sintering time of 4 hours). After the material is cooled, it is crushed for the second time to an average particle size of 870 nm. After demagnetization, lithium iron phosphate salt particles are obtained, with a carbon content of 1.2% and a Ti content of 5000 ppm in the lithium iron phosphate salt particles.

[0213] The obtained lithium iron phosphate salt particles were observed under SEM (scanning electron microscope), and the results are shown in FIG1a ; the obtained lithium iron phosphate salt particles were observed under TEM (transmission electron microscope), and the results are shown in FIG1b and FIG1c .

[0214] 2. Preparation of positive electrode sheet

[0215] 2.0 wt% polyvinylidene fluoride binder was fully dissolved in N-methylpyrrolidone (NMP), and then 1.0 wt% Super P, 0.5 wt% carbon nanotubes, and 96.5 wt% of the above-mentioned positive electrode materials were added and stirred to obtain a positive electrode slurry. The slurry was evenly coated on the surface of the current collector aluminum foil and then transferred to a vacuum drying oven to completely dry. The dried electrode sheet was roll-pressed and punched to obtain the positive electrode sheet.

[0216] 3. Preparation of negative electrode sheet

[0217] The active material artificial graphite, the conductive agent carbon black, the binder styrene-butadiene rubber (SBR), and the thickener sodium carboxymethyl cellulose (CMC Na) are dissolved in the solvent deionized water in a weight ratio of 96.7:1.3:0.8:1.2, and mixed evenly to prepare a negative electrode slurry; the negative electrode slurry is evenly coated on the negative electrode current collector copper foil once or multiple times, and the negative electrode sheet is obtained after drying, cold pressing, and slitting.

[0218] 4. Preparation of electrolyte

[0219] In an argon atmosphere glove box (H2O < 0.1ppm, O2 < 0.1ppm), organic solvents ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed uniformly in a volume ratio of 3:7, and LiPF6 lithium salt was dissolved in the organic solvent to prepare a solution with a weight content of 12.5% ​​to obtain an electrolyte.

[0220] 5. Isolation film

[0221] Polypropylene film is used as the isolation film.

[0222] 6. Preparation of lithium-ion batteries

[0223] The positive electrode sheet, separator, and negative electrode sheet are stacked in order, with the separator positioned between the positive and negative electrode sheets to provide isolation. The cells are then wound to obtain a bare cell. The tabs are welded to the bare cell and placed in an aluminum shell. The cells are then baked at 80°C to remove moisture. The electrolyte is then injected and sealed to obtain an uncharged battery. The uncharged battery then undergoes a series of steps, including resting, hot and cold pressing, formation, shaping, and capacity testing, to obtain the lithium-ion battery of Example 1.

[0224] The lithium-ion battery obtained in Example 1 was subjected to the performance test described below, and the results are shown in Figure 1.

[0225] Example 2

[0226] The same procedure as in Example 1 was followed except that the Dv50 particle size of the particles pulverized after the primary sintering was controlled to be 350 nm, thereby obtaining lithium iron phosphate particles having a primary average particle size of 500 nm.

[0227] Example 3

[0228] The same procedure as in Example 1 was followed except that the Dv50 particle size of the particles pulverized after the primary sintering was controlled to be 1200 nm, thereby obtaining lithium iron phosphate particles having a primary average particle size of 3000 nm.

[0229] Example 4

[0230] The same procedures as in Example 1 were followed except that the temperatures of the first sintering and the second sintering were changed to 620° C. and 740° C., respectively, and the Ti content of the product was controlled to 2500 ppm.

[0231] Example 5

[0232] The same procedures as in Example 1 were carried out except that the temperatures of the first sintering and the second sintering were changed to 655° C. and 755° C., respectively, and the Ti content was adjusted to 6000 ppm.

[0233] Example 6

[0234] The same procedure as in Example 1 was carried out except that the modifying element was replaced by V instead of Ti.

[0235] Example 7

[0236] The same procedure as in Example 1 was carried out except that the modifying element was replaced by Nb from Ti.

[0237] Example 8

[0238] The average particle size of lithium iron phosphate particles is 920nm and the BET specific surface area is 3.0m 2 / g, and the carbon content was 0.8 wt %, the same procedure as in Example 1 was carried out.

[0239] Example 9

[0240] The average particle size of lithium iron phosphate particles is regulated to 790nm and the BET specific surface area is 8.0m 2 / g, and the carbon content was 1.4 wt %, the same procedures as in Example 1 were carried out.

[0241] Example 10

[0242] The carbon content after the first sintering was adjusted to 0.05 wt%, and the primary average particle size of the product was adjusted to 1250 nm and the BET specific surface area was adjusted to 6.0 m 2 / g, and the carbon content was 1.2 wt %, the same procedure as in Example 1 was carried out.

[0243] Example 11

[0244] The average particle size of lithium iron phosphate particles is 520nm and the BET specific surface area is 6.8m 2 / g, and the carbon content after the first sintering was adjusted to 0.4 wt % in the same manner as in Example 1.

[0245] Example 12

[0246] The BET specific surface area of ​​the regulated product is 7.2 m 2 / g, and the carbon content was 1.6 wt %, the same procedure as in Example 1 was carried out.

[0247] Example 13

[0248] The BET specific surface area of ​​the regulated product is 7.8 m 2 / g, and the carbon content was 2 wt %, the same procedure as in Example 1 was carried out.

[0249] Example 14

[0250] The temperature of the first sintering was controlled at 620°C, and the temperature of the second sintering was controlled at 730°C. The weight ratio of glucose to polyaniline satisfied glucose:polyaniline = 1:4. The BET specific surface area of ​​the lithium iron phosphate particles was 3.0 m 2 / g, and the carbon content was 2 wt %, the same procedure as in Example 1 was carried out.

[0251] Example 15

[0252] The BET specific surface area of ​​lithium iron phosphate particles is regulated to 8.0 m 2 / g, and the carbon content was 0.94 wt %, the same procedure as in Example 1 was carried out.

[0253] Example 16

[0254] The same procedure as in Example 1 was followed except that the primary average particle size of the lithium iron phosphate particles was adjusted to 650 nm and the Dv50 after the first sintering was adjusted to 450 nm.

[0255] Example 17

[0256] The average particle size of lithium iron phosphate particles is regulated to 2500nm and the BET specific surface area is 5.5m 2 / g, the same procedure as in Example 1 was carried out except for this.

[0257] Comparative Example 1

[0258] The BET specific surface area of ​​lithium iron phosphate particles is regulated to 2.4 m 2 / g, and the carbon content was 0.7 wt %, the same procedure as in Example 1 was carried out.

[0259] Comparative Example 2

[0260] The carbon content was adjusted to 1.7% by weight, so that the BET specific surface area of ​​the product was 12 m 2 / g, the same procedure as in Example 1 was carried out except for this.

[0261] Comparative Example 3

[0262] The temperature of the second sintering was adjusted to 830 °C, and the BET specific surface area of ​​the product was adjusted to 9.5 m 2 / g, the same procedure as in Example 1 was carried out except for this.

[0263] Comparative Example 4

[0264] The average particle size of the product was 5000 nm and the BET specific surface area was 4 m 2 / g, the same procedure as in Example 1 was carried out except for this.

[0265] Comparative Example 5

[0266] During the preparation process, one-time sintering was used and the BET specific surface area of ​​the product was controlled to be 12 m 2 / g, the same procedure as in Example 1 was carried out except for this.

[0267] Comparative Example 6

[0268] The temperature of the first sintering was controlled at 760°C, the average particle size of the product was controlled to be 3200 nm, and the BET specific surface area was controlled to be 4.8 m 2 / g, the same procedure as in Example 1 was carried out except for this.

[0269] Comparative Example 7

[0270] The carbon content of the product after the first sintering was controlled to be 1.1 wt%, and the average particle size of the product was regulated to be 300 nm and the BET specific surface area was regulated to be 13 m 2 / g, the same procedure as in Example 1 was carried out except for this.

[0271] Comparative Example 8

[0272] No carbon film-forming agent was used in the preparation process. The temperature of the first sintering was controlled at 600°C, the temperature of the second sintering was controlled at 720°C, and the carbon content of the product was controlled at 0.5%. Except for this, the process was carried out in the same manner as in Example 1.

[0273] 1. Determination of relevant parameters of lithium iron phosphate particles

[0274] 1. Primary average particle size

[0275] The electrode is cut open perpendicularly to the large surface of the electrode using an argon ion beam to expose the cross section, which is photographed using a scanning electron microscope. The longest diameter of the lithium iron phosphate particles is statistically analyzed using the length-diameter statistical method. The "primary average particle size" refers to the average value of the primary particle size of all particles, which is numerically equal to the total particle size value divided by the total number of particles. The primary particle size in the cross-sectional view refers to the longest distance between two points along the edge. Specifically, the total number of lithium iron phosphate particles with a primary particle size greater than 80 nm and the sum of the primary particle sizes of lithium iron phosphate particles with a primary particle size greater than 80 nm can be counted in the electron microscope scanning photograph. The primary average particle size of the lithium iron phosphate particles = the primary particle size of the total lithium iron phosphate particles / the total number of lithium iron phosphate particles. In the above-mentioned particle size statistical process, particles with a primary average particle size less than or equal to 80 nm are not included in the statistical range.

[0276] 2. BET specific surface area

[0277] The specific surface area was tested by gas adsorption method according to the GB / T19587-2017 test standard, as follows: lithium iron phosphate granular salt was taken as a sample, the sample tube was immersed in liquid nitrogen at -196°C, and the adsorption amount of nitrogen on the solid surface at different pressures was measured at a relative pressure of 0.05-0.30. The single-molecule adsorption amount of the sample was obtained based on the BET multilayer adsorption theory and its formula, thereby calculating the specific surface area of ​​the material.

[0278] 3. Dv50

[0279] The Dv50 value of the lithium iron phosphate particles was measured using a laser particle size analyzer (Malvern Master Size 3000) with reference to GB / T 19077.1-2016. Furthermore, the Dv10, Dv90, and Dv99 values ​​of the present application were also measured in the same manner.

[0280] 4. Carbon content

[0281] The lithium iron phosphate particles are burned in a high-frequency induction furnace and then the carbon content is tested using the infrared absorption method. The specific testing process is based on the standard GB / T 20123-2006 / ISO 15350:2000.

[0282] 5. Morphology test

[0283] Example 10 and the existing lithium iron phosphate positive electrode material were tested using a ZEISS sigma 300 scanning electron microscope and then tested according to the standard JY / T010-1996. The sample morphology was observed. The observation results are shown in Figures 2a and 2b.

[0284] 2. Determination of battery performance

[0285] 1. Battery capacity in grams

[0286] A 2.0000g sample was mixed with 0.1111g of conductive carbon black and 0.1111g of PVDF (at a mass ratio of 0.9:0.05:0.05). 2.5g of the organic solvent NMP (N-methylpyrrolidone) was then added and thoroughly mixed. The mixture was then coated onto aluminum foil to form a 140-micron-thick film, dried under vacuum at 120°C for 2 hours, punched into 13mm discs using a hole punch, and pressed using a tablet press at 10 MPa. The discs were then insulated at 120°C for 12 hours. The positive electrode sheet was weighed, and the active material loading was 11-12mg. Coin-type cells were assembled in an argon-protected glove box, using a lithium metal sheet as the negative electrode. The electrolyte consisted of a 1:1 (volume) mixture of EC (ethylene carbonate) and DMC (1,2-dimethyl carbonate), with LiPF6 as the electrolyte and a Celgard 2400 microporous polyethylene membrane as the separator. The assembled cells were tested for electrical performance on a blue-light tester. The specific capacity is measured by charging / discharging twice at a constant current of 0.1C, followed by two cycles at a constant current of 1C, within the voltage range of 2.0V to 3.75V. The charging process is followed by a constant voltage step at 3.75V and a cutoff current of 50μA. The gram capacity is the discharge capacity at the first 0.1C cycle.

[0287] 2. Battery capacity ratio when discharged from 1C to 3.2V (η value)

[0288] The battery preparation and testing process is as follows: 2.0000g of sample was mixed with 0.1111g of conductive carbon black and 0.1111g of PVDF (at a mass ratio of 0.9:0.05:0.05), followed by the addition of 2.5g of the organic solvent NMP (N-methylpyrrolidone). After thorough mixing, the mixture was coated onto aluminum foil to form a 140-micron-thick film. The film was then dried under vacuum at 120°C for 2 hours. The film was punched into 13mm-diameter discs using a hole punch. The discs were pressed using a tablet press at 10 MPa and kept at 120°C for 12 hours. The positive electrode was weighed, and the active material loading was 11-12mg. Coin-type cells were assembled in an argon-protected glove box with a lithium metal sheet as the negative electrode, an electrolyte consisting of a 1:1 (volume) mixture of EC (ethylene carbonate) and DMC (1,2-dimethyl carbonate), the electrolyte being LiPF6, and a Celgard 2400 microporous polyethylene membrane as the separator. The assembled batteries were tested for electrical performance on a blue battery tester. Within the 2.0V to 3.75V voltage range, the specific capacity was measured by charging / discharging at a constant current of 0.1C for two weeks, followed by a constant current of 1C for two weeks. The charging process was performed with a constant voltage of 3.75V and a cutoff current of 50μA.

[0289] The capacity value extracted when the discharge voltage is 3.2V is recorded as C1, and the capacity value extracted when the discharge voltage is reduced to 2.0V is recorded as C2, where η=C1 / C2. The charging process includes constant voltage charging, a constant voltage of 3.75V, and a constant voltage cutoff current of 50μA.

[0290] 3. Slurry solid content

[0291] Prepare an electronic balance (accuracy 0.0001), an oven, and a glass drying tray. Take 8-10g of the positive electrode slurry sample and spread it evenly on the sample tray. Record the mass of the slurry before drying as A. Close the oven door and heat. As heating continues, the temperature in the oven continues to rise, reaching 130°C for 5 hours. After drying, cool the sample in the oven and remove it. Record the mass of the dried slurry. Repeat the drying process several times until the sample reaches a constant weight, recording the mass after drying as B. Slurry solids content = (A / B) × 100%.

[0292] 4. Magnetic material content

[0293] 1) Weigh 1 kg of sample and place it in a plastic bucket. Add 6 L of deionized water. Use plastic tubing to cover a 24 mm diameter, 240 mm length magnetic bar (magnetic field strength 6000 gauss). Heat-seal the tub with heat-sealing clips. Place the magnetic bar in the tub, seal it, and stir at 60 rpm for 15 minutes.

[0294] 2) Prepare another clean bucket and add 5 ± 0.2 L of deionized water to the bucket. Flush the magnetic material on the plastic tube into the solvent, reseal the magnetic rod, and repeat the above steps twice to ensure the accuracy of the magnetic material extraction;

[0295] 3) Prepare a clean 1L beaker, rinse all the magnetic material on the plastic tube into the beaker, and use a magnetic block to attract the bottom of the beaker and rinse 1-2 times;

[0296] 4) Use a graduated cylinder to measure 10 mL of deionized water and add it to a 100 mL beaker. Then, measure 10 mL of 36%-38% hydrochloric acid and slowly add it to the beaker. Pour the prepared hydrochloric acid solution into the beaker, seal it, and sonicate it in an ultrasonicator for 2 minutes.

[0297] 5) After the ultrasonic treatment is complete, remove the beaker and use a magnetic block on the bottom of the beaker to absorb and gather the magnetic material. Pour the acid solution in the beaker into a waste liquid bucket. Rinse the magnetic material in the beaker three times and add an appropriate amount of deionized water for filtration.

[0298] 6) Using a 0.45 μm pore size filter, place the filter paper with the magnetic particles on the surface of the cleanliness microscope slide and dry it in an oven at 60°C for 10 ± 2 minutes. After drying, weigh the filter and calculate the magnetic content.

[0299] The higher the magnetic material content, the more serious the self-discharge of the battery cell and the shorter the battery life.

[0300] 5. Determination of kinetic properties

[0301] In this application, the dynamic performance of the battery is specifically reflected in the measurement of the DC internal resistance (power performance).

[0302] 25℃ power performance test:

[0303] Capacity calibration: The lithium-ion batteries prepared in each embodiment and comparative example were kept at 25°C for 2 hours, then charged at a constant current of 0.33C to 3.65V, and then charged at a constant voltage of 0.05C at 3.65V. After charging, the tested batteries were left at rest at 25°C for 2 hours, and then discharged at a DC current of 0.33C to 2.5V. The discharge capacity at room temperature was recorded as C0.

[0304] Adjust the SOC (battery state of charge): After keeping the calibrated capacity lithium-ion battery at 25°C for 2 hours, discharge it at a 1 / 3C0 discharge rate for 144 minutes to adjust the lithium-ion battery capacity to 10% SOC;

[0305] Power test: After a 10% SOC lithium-ion battery is left at 25°C for 2 hours, it is discharged at a discharge rate of 3C0 for 30 seconds under a pulse current I. The voltage before 3C0 discharge is recorded as V1, and the voltage at the end of the 30-second discharge is recorded as V2. The DC internal resistance (V1-V2) / I is calculated, and this data can be used to characterize the battery's power performance.

[0306] 3. Parameters and Performance in Examples and Comparative Examples

[0307] Batteries of various examples and comparative examples were prepared according to the above methods, and various performance parameters were measured, as shown in Table 1.

[0308] Table 1

[0309] According to the contents of Table 1, the primary average particle size, BET specific surface area and carbon content of the lithium iron phosphate salt particles obtained in Examples 1-17 of the present application all meet the requirements of the present application. As a result, their η values ​​are all greater than or equal to 88%, indicating that the battery has strong discharge power performance. When the battery is discharged to a low SOC (battery state of charge), it can still maintain good power performance, that is, the voltage drop of the battery is small when the battery is discharged at a high current at a low power level. In addition, the dynamic performance values ​​of the lithium iron phosphate salt particles in the embodiment are low, indicating that the mechanical properties of the battery are excellent. The lithium iron phosphate salt particles have a high gram capacity, a high solid content of the slurry and a low content of magnetic material, indicating that the energy density and life characteristics of the battery are excellent.

[0310] In addition, according to Table 1, in Comparative Examples 1-8, whether by changing the parameter characteristics of the lithium iron phosphate salt particles themselves or changing the parameter characteristics in the preparation process, the resulting lithium iron phosphate salt particles do not meet the limitations of this application. The resulting lithium iron phosphate salt particles have at least one of the size characteristics, dynamic properties, gram capacity, solid content of the slurry or magnetic material content significantly reduced, which obviously cannot meet the requirements of this application.

[0311] It should be noted that the present application is not limited to the above-described embodiments. The above-described embodiments are merely examples, and any embodiments having substantially the same structure and effect as the technical concept within the scope of the present application are all included within the technical scope of the present application. In addition, without departing from the scope of the present application, any other modifications that can be imagined by those skilled in the art to the embodiments, or any other methods constructed by combining some of the constituent elements in the embodiments are also included within the scope of the present application.

Claims

1. A lithium iron phosphate salt particle, characterized in that, The primary average particle size of the lithium iron phosphate salt particles is 500 nm to 3000 nm, and the BET specific surface area is 3 m 2 / g to 8 m 2 / g, Based on the total weight of the lithium iron phosphate salt particles, the carbon content of the lithium iron phosphate salt particles is Cx wt%, where 0.8 ≤ Cx ≤ 2.

0.

2. The lithium iron phosphate salt particles according to claim 1, characterized in that, The primary average particle size of the lithium iron phosphate salt particles is 650 nm to 2500 nm; and / or, The BET specific surface area of the lithium iron phosphate salt particles is 4 m 2 / g to 7 m 2 / g; and / or, Based on the total weight of the lithium iron phosphate salt particles, the carbon content of the lithium iron phosphate salt particles is Cx wt%, where 1.0 ≤ Cx ≤ 1.

6.

3. The lithium iron phosphate salt particles according to claim 1 or 2, characterized in that, The lithium iron phosphate salt particles have the molecular formula Li m Fe x P y O j Q q , where Q includes at least one of Al, Na, K, Mg, Cu, Mn, Cr, Zn, Pb, Ca, Co, Ni, Sr, Nb, V, Ti, B, S, Si, N, F, Cl, Br, 0.95 ≤ m ≤ 1.15, 0.9 ≤ x ≤ 1, 0.95 ≤ y ≤ 1, 3.5 ≤ j ≤ 4, 0 < q ≤ 0.

1.

4. The lithium iron phosphate salt particles according to claim 3, characterized in that, The Q includes at least one of Ti, V, Mg, and Nb, Based on the total weight of the lithium iron phosphate salt particles, the content of Ti, V, Mg, and / or Nb is 1000 ppm - 10000 ppm.

5. The lithium iron phosphate salt particles according to any one of claims 1 to 4, characterized in that, The lithium iron phosphate salt particles are single crystal particles and / or polycrystal particles.

6. The lithium iron phosphate salt particles according to any one of claims 1 to 5, characterized in that, The capacity ratio η of the lithium iron phosphate salt particles ≥ 88%, where η is defined as follows: A battery containing the lithium iron phosphate salt particles as the cathode material is subjected to constant current charge and discharge twice at a rate of 0.1C within a voltage range of 2.0V to 3.75V, and then subjected to constant current charge and discharge once at a rate of 1C. During the charge and discharge test at a rate of 1C, the capacity value at a discharge voltage of 3.2V is extracted and denoted as C1, and the capacity value at a discharge voltage of 2.0V is C2, η = C1 / C2, where the charging process includes constant voltage charging, constant voltage of 3.75V, and a constant voltage cut-off current of 50 μA.

7. The lithium iron phosphate salt particles according to any one of claims 1 to 6, characterized in that, The lithium iron phosphate salt particles satisfy at least one of a)-f): a) The Dv10 of the lithium iron phosphate salt particles ≥ 0.2 μm; b) The Dv50 of the lithium iron phosphate salt particles is 0.5 - 5 μm; c) The Dv90 of the lithium iron phosphate salt particles ≤ 10 μm; d) The Dv99 of the lithium iron phosphate salt particles ≤ 12 μm; e) The powder tap density of the lithium iron phosphate salt under a pressure of 3 tons ≥ 2.25 g / cm 3 ; f) The powder resistivity of the lithium iron phosphate is less than 60 Ω·cm.

8. A method for preparing lithium iron phosphate particles, characterized in that, Including: Providing raw materials containing at least a lithium source, an iron source, a phosphorus source, a carbon source, a carbon film-forming agent, and a modifier, and performing at least two sinterings, where, The temperature of the first sintering is 500°C - 760°C, and the carbon content of the material after the first sintering is 0.01 wt% - 0.79 wt%; The temperature of the second sintering is 700°C - 800°C, and the carbon content of the material after the second sintering is 0.8 wt% - 2.0 wt%.

9. The method for preparing the lithium iron phosphate salt particles according to claim 8, wherein The lithium iron phosphate salt particles contain at least one of the elements Ti, V, Mg, and / or Nb. Based on the total weight of the lithium iron phosphate salt particles, the content of the element is 1000 ppm - 10000 ppm.

10. The method for preparing lithium iron phosphate particles according to claim 8 or 9, characterized in that, Performing a first pulverization after the first sintering and performing a second pulverization after the second sintering, where, The Dv50 of the product after the first pulverization is 300 nm - 1200 nm; The Dv50 of the product after the second pulverization is 500 nm - 5000 nm.

11. The preparation method of the lithium iron phosphate salt particles according to any one of claims 8 to 10, characterized in that, Including: Provided are raw materials containing at least a lithium source, an iron source, a phosphorus source, a carbon source, a carbon film-forming agent, and a modifier. Among them, the mixing ratio of the lithium source, the iron source, and the phosphorus source, based on the atomic molar number of each element, satisfies Fe∶P = 0.96 - 0.985, Li∶Fe = 1.0 - 1.1∶0.95 - 1.1; the carbon source and the carbon film-forming agent, based on the weight ratio, satisfy carbon source∶carbon film-forming agent = 9∶1 - 2∶8.

12. The method for preparing lithium iron phosphate salt particles according to any one of claims 8 to 11, characterized in that, The lithium source is a lithium compound, including one or more of lithium dihydrogen phosphate, lithium oxalate, lithium carbonate, lithium oxide, lithium hydroxide, and lithium acetate; The iron source is an iron compound, including at least one of iron hydroxide, ferrous chloride, iron(III) oxide, iron phosphate, iron pyrophosphate, ferrous oxalate, iron powder, iron nitrate, iron(II,III) oxide, and iron oxyhydroxide; The phosphorus source is a phosphoric acid compound, including one or more of phosphoric acid, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate; The modifier includes at least one of titanium dioxide, vanadium pentoxide, tetrabutyl titanate, niobium pentoxide, niobium oxalate, niobium ethoxide, magnesium hydroxide, magnesium nitrate, and ammonium metavanadate; The carbon source includes at least one of citric acid, glucose, sucrose, starch, fructose, and lactose; The carbon film-forming agent includes one or a combination of polyethylene glycol, polyaniline, polyacrylonitrile, polyvinylpyrrolidone, and polyvinyl alcohol.

13. The preparation method of the lithium iron phosphate salt particles according to any one of claims 8 to 12, characterized in that, The heating rate during the first sintering and the second sintering is independently 2°C / min - 20°C / min; the holding time at constant temperature during the first sintering is 1 h - 6 h; the holding time at constant temperature during the second sintering is 2 h - 12 h.

14. The method for preparing lithium iron phosphate particles according to claims 8 to 13, characterized in that, During the second sintering, carbon coating is performed on the product after the first sintering. Based on the total weight of the product after the first sintering, the amount of carbon coating is 0.01 wt% - 1.99 wt%.

15. The method for preparing lithium iron phosphate particles according to claim 14, wherein The carbon coating is carried out by chemical vapor deposition during the sintering process or by carbonizing the carbon source to wrap carbon at high temperature.

16. The method for preparing lithium iron phosphate particles according to any one of claims 10 to 15, characterized in that, The pulverization includes one or more of grinding, sanding, mechanical crushing, and jet milling.

17. A positive electrode plate, characterized in that, Using the lithium iron phosphate salt particles described in any one of claims 1 to 7 or the lithium iron phosphate salt particles prepared by the preparation method of the lithium iron phosphate salt particles described in any one of claims 8 to 16 as the cathode material.

18. A secondary battery, characterized in that, Including the cathode electrode sheet described in claim 17.

19. An electrical device, characterized in that, Including the secondary battery described in claim 18.

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