Secondary battery, preparation method thereof and electric device

Abstract

The invention provides a secondary battery, a preparation method thereof and an electric device. The secondary battery positive pole piece comprises a positive current collector and a positive film layer arranged on at least one side of the positive current collector, the positive film layer comprises a positive material, the positive material comprises lithium iron phosphate particles, the primary average particle size of the lithium iron phosphate particles is 500-3000 nm, the BET specific surface area of the lithium iron phosphate particles is 3-8 m < 2 > / g, and by taking the total weight of the lithium iron phosphate particles as a reference, the primary average particle size of the lithium iron phosphate particles is 5-10 nm. The carbon content of the lithium iron phosphate salt particles is Cx wt%, and Cx is more than or equal to 0.8 and less than or equal to 2.0.

Description

[0001] This application is a divisional application based on the invention with application number 202410027073.3, application date January 8, 2024, applicant CATL, and invention title "Lithium iron phosphate salt particles and preparation method thereof, positive electrode sheet, secondary battery and power device". Technical Field

[0002] This application relates to the field of secondary battery technology, and in particular to a lithium iron phosphate salt particle and its preparation method, a positive electrode sheet, a secondary battery, and an electrical device. Background Technology

[0003] As a cathode material for lithium-ion batteries, lithium iron phosphate (LFP) has rapidly become a global research hotspot due to its abundant resources, low price, environmental friendliness, and stable voltage in its two-phase reaction. The cation arrangement in LFP differs from that in layered ternary materials and spinel LiMn₂O₄, with Fe... 2+ Located at the 4c ​​position of the oxygen octahedron, Li + It is located at the 4a position of the oxygen octahedron. The polyanionic structure is stable, with a high thermal decomposition temperature and good thermal stability. However, the octahedral FeO6 structure is separated by the O atoms in the tetrahedral structure and is interrupted by the phosphorus oxygen tetrahedron, thus failing to form a continuous FeO6 network, resulting in poor electronic conductivity of lithium iron phosphate.

[0004] The industry typically employs techniques such as doping, coating, and particle nanosizing to improve the conductivity of lithium iron phosphate (LFP). At low temperatures and high rates, the issues of electronic and ionic conductivity become more pronounced, necessitating significant nanosizing of the material. However, nanoscaled materials possess higher specific surface energy, leading to a series of processing challenges, such as gelation, rapid water absorption, difficulty in drying, and low solids content in coating slurries. Therefore, improving particle size and reducing BET (Body-Earth Equivalent) to lower the specific surface energy of the material while maintaining kinetic performance represents a significant engineering advancement. Summary of the Invention

[0005] This application is made in view of the aforementioned issues, and its purpose is to provide lithium iron phosphate particles and their preparation method, positive electrode sheet, secondary battery and power device, wherein the lithium iron phosphate particles have a large particle size and also have excellent kinetic performance when used as a positive electrode material.

[0006] The first aspect of this application provides lithium iron phosphate salt particles, wherein the primary average particle size of the lithium iron phosphate salt particles is 500nm~3000nm, optionally 650nm~2500nm, and the BET specific surface area is 3m². 2 / g~8m 2 / g, optional 4m 2 / g~7m2 / g, calculated based on the total weight of the lithium iron phosphate particles, the carbon content of the lithium iron phosphate particles is Cx by weight%, wherein 0.8≤Cx≤2.0, optionally 1.0≤Cx≤1.6, and further optionally, the ratio z of the BET specific surface area to Cx satisfies 1.5≤z≤8.5, optionally 3≤z≤6.

[0007] The lithium iron phosphate salt particles in the embodiments of this application have a primary average particle size of 500 nm to 3000 nm and a BET specific surface area of ​​3 m². 2 / g~8m 2 The carbon content, calculated based on the total weight of the lithium iron phosphate particles, is Cx wt% (0.8 ≤ Cx ≤ 2.0). 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. This improves the kinetic performance of lithium iron phosphate particles with a primary average particle size within the aforementioned range; it maintains suitable micron-level particle sizes, thus avoiding interfacial side reactions and processing difficulties caused by nano-sizing of particles, and preventing the reduction in kinetic performance due to excessively large particle sizes. Furthermore, it facilitates the stirring of the slurry containing the lithium iron phosphate particles and increases the solid content, thereby improving cell processing issues and increasing the volumetric energy density of the battery. Additionally, it avoids the impact of excessively high carbon coating density on the normal insertion / extraction of lithium ions, thus affecting the cell capacity.

[0008] Compared with the prior art, the lithium iron phosphate particles in the embodiments of this application have a smaller specific surface area when containing the same amount of carbon, which means that there is less floating carbon in the particles. This makes the carbon contained in the lithium iron phosphate particles more uniform and dense, and each particle is coated, thereby improving the surface conductivity of the particles and improving their kinetic performance as a positive electrode material.

[0009] In any embodiment, the lithium iron phosphate salt has the molecular formula Li m Fe x P y O j Q q 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, with 0.95≤m≤1.15, 0.9≤x≤1, 0.95≤y≤1, 3.5≤j≤4, and 0<q≤0.1.

[0010] By making the lithium iron phosphate salt of the embodiments of this application have the above-described molecular formula and simultaneously doping it with one or more of the above-described elements, a high loading volume bulk phase modification of the lithium iron phosphate salt particles is achieved. This helps to improve the bulk ion transport capability of the lithium iron phosphate salt particles and can effectively solve the problem of poor kinetic performance inherent in large particles. Furthermore, by performing this doping, the lithium iron phosphate salt particles can exhibit good kinetic performance when used as a cathode material. In this application, the modification can specifically be manifested as doping and / or coating.

[0011] In any embodiment, Q includes at least one of Ti, V, Mg, and Nb, optionally Ti, and the content of Ti, V, Mg and / or Nb is 1000ppm-10000ppm, optionally 2500ppm-6000ppm, calculated based on the total weight of the lithium iron phosphate salt particles.

[0012] By doping the lithium iron phosphate salts of the embodiments of this application with one or more of the above-mentioned elements and ensuring that their content is within the above-mentioned range, it is possible to achieve better bulk metal phase modification of the lithium iron phosphate salt particles while improving the primary average particle size, thereby further enhancing the bulk ion transport capability of the lithium iron phosphate salt particles and better solving the problem of poor kinetic performance inherent in large particles.

[0013] In any embodiment, the lithium iron phosphate particles are monocrystalline particles and / or polycrystalline particles. Optionally, based on the total number of lithium iron phosphate particles, the number of monocrystalline particles accounts for more than 90%.

[0014] In the lithium iron phosphate particles of the embodiments of this application, the carbon may exist either in a state mixed with the lithium iron phosphate particles or in a state coated on the lithium iron phosphate particles. Optionally, the carbon in the lithium iron phosphate particles may be in a state of coating on the lithium iron phosphate particles, thereby enabling the carbon to form a uniform and dense carbon coating layer on the surface of the lithium iron phosphate particles, thereby improving the surface conductivity of the particles.

[0015] In any embodiment, the capacity ratio of the lithium iron phosphate particles η ≥ 88%, where η is defined as follows: A battery with the lithium iron phosphate particles as the positive electrode material is charged and discharged twice at a constant current rate of 0.1C within a voltage range of 2.0V to 3.75V, followed by a constant current charge and discharge once at a constant current rate of 1C. In the 1C charge and discharge test, the capacity value extracted at a discharge voltage of 3.2V is recorded as C1, and the capacity value extracted at a discharge voltage of 2.0V is recorded as C2, where η = C1 / C2. The charging process includes constant voltage charging at a constant voltage of 3.75V and a constant voltage cutoff current of 50μA.

[0016] η represents the plateau retention performance of the material, and this value is strongly correlated with the battery's discharge power performance. When this value is large, the battery can still maintain good power performance when discharged to a low SOC (State of Charge), that is, the voltage drop of the battery is small when the battery discharges at a high current at a low charge level. In the embodiments of this application, the capacity ratio η of the lithium iron phosphate particles is ≥88%, indicating that the lithium iron phosphate particles of this application can enable the secondary battery to exhibit good kinetic performance when used as a positive electrode material.

[0017] In any embodiment, the lithium iron phosphate salt particles satisfy at least one of a)-f): a) The Dv10 of the lithium iron phosphate salt particles is ≥ 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 is ≤10μm; d) The Dv99 of the lithium iron phosphate salt particles is ≤12μm; e) The compacted density of the lithium iron phosphate powder under 3T pressure is ≥2.25 g / cm³. 3 ; f) The resistivity of the lithium iron phosphate powder is less than 60 Ω·cm.

[0018] In this application, Dv10, Dv90, and Dv99 refer to the particle sizes corresponding to a cumulative volumetric particle size distribution percentage of 10%, 90%, and 99%, respectively. The testing method is the same as that for Dv50, and the standard used in the embodiments of this application can be employed for determination.

[0019] It should be noted that Dv10 is a target for large-scale indicators, so no upper limit can be given; Dv90 is a target for small-scale indicators, so no lower limit can be given.

[0020] By ensuring that the lithium iron phosphate particles satisfy at least one of a)-f), the lithium iron phosphate particles can achieve the above-mentioned technical effects more effectively.

[0021] In this application, the method for testing the powder resistivity is as follows: referring to the national standard GB / T 33822-2017, a powder resistivity meter (Suzhou Jingge, ST2722 type) is used. 1g of sample (with an error within ±0.005g) is weighed and added to the feeding chamber. A pressure of 8 MPa is applied, and the forward resistivity and reverse resistivity of the sample are tested respectively. The average value of the two is taken as the powder resistivity of the sample.

[0022] In this application, the powder compaction density is defined as: during the external force compression process, as the powder moves and deforms, larger voids are filled, the contact area between particles increases, resulting in attractive forces between atoms and enhanced mechanical cohesion between particles, thereby forming a compact with a certain density and strength, with units of g / cm³. 3 .

[0023] The test method for compaction density is as follows: Referring to the national standard GB / T 24533-2009, a certain amount of powder is placed on a compaction mold. The mold has a hollow center with two metal discs at the top and bottom. 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 thickness of the powder under different pressures can be read on the instrument. The compaction density is calculated using ρ=m / v.

[0024] Based on the following powder compaction density calculation results, ρc = m / V = m / (S×H); where ρc is the powder compaction density (g / cm³). 3 m is the mass of the material weighed (g), and S is the bottom area of ​​the mold (1.327 cm²). 2 H is the height of the compacted sample (cm).

[0025] The second aspect of this 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 sintering processes, wherein the temperature of the first sintering is 500℃-760℃, optionally 550℃-720℃, and the carbon content of the material after the first sintering is 0.01 wt%-0.79 wt%, optionally 0.05 wt%-0.4 wt%; the temperature of the second sintering is 700℃-800℃, optionally 720℃-780℃, and the carbon content of the material after the second sintering is 0.8 wt%-2.0 wt%, optionally 1.0 wt%-1.6 wt%.

[0026] In the method for preparing lithium iron phosphate salt particles according to the embodiments of this application, two sintering processes are performed. By controlling the temperature of the first sintering within the aforementioned range and ensuring that the carbon content of the intermediate after sintering is within the aforementioned range, the lithium iron phosphate precursor obtained after the first sintering can have a larger particle size, directly improving the powder compaction and electrode compaction density of the final product. Furthermore, by adding a low amount of carbon source during the first sintering, the obstructive effect of the carbon layer on the growth process of lithium iron phosphate particles is greatly reduced, which is beneficial for particle crystallization growth at a lower temperature. Simultaneously, it facilitates the solid-phase diffusion reaction between the modifier and the lithium iron phosphate material, thereby achieving a higher concentration of metal ion modification. Compared to traditional methods that use high temperatures to achieve particle growth, the preparation method described in this application can first synthesize large particles at a lower temperature, which can improve the phenomenon of cracking of the carbon layer on the particle surface at high temperatures 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 materials, improving the purity and stability of the product. By controlling the temperature of the second sintering within the above range and making the carbon content of the sintered material 0.8%-2.0% by weight, 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 each particle, which greatly improves the conductivity of the particle surface.

[0027] In any embodiment, the lithium iron phosphate particles contain at least one of the elements Ti, V, Mg, and / or Nb, and the content of the element is 1000ppm-10000ppm, optionally 2500ppm-6000ppm, based on the total weight of the lithium iron phosphate particles.

[0028] In some embodiments, the content of the element may be selected as 2500ppm-6000ppm. Specifically, the values ​​can be 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 any range between any two of the above values.

[0029] By doping the lithium iron phosphate salts of the embodiments of this application with one or more of the above-mentioned elements and ensuring that their content is within the above-mentioned range, it is possible to achieve better bulk metal phase modification of the lithium iron phosphate salt particles while improving the primary average particle size, thereby further enhancing the bulk ion transport capability of the lithium iron phosphate salt particles and better solving the problem of poor kinetic performance inherent in large particles.

[0030] In any embodiment, the method for preparing lithium iron phosphate particles involves a first pulverization after the first sintering and a second pulverization after the second sintering. The Dv50 of the product after the first pulverization is 300nm-1200nm, optionally 400nm-1100nm; the Dv50 of the product after the second pulverization is 500nm-5000nm, optionally 300nm-2500nm.

[0031] In the method for preparing lithium iron phosphate particles according to the embodiments of this application, pulverization is performed after two sintering processes. By performing a first pulverization after the first sintering, the Dv50 of the product is 300nm-1200nm, which avoids the growth barrier of carbon materials and modifying elements on crystals, and obtains micron-sized lithium iron phosphate precursors. By performing a second pulverization after the second sintering, the Dv50 of the product is 500nm-3000nm, which can obtain lithium iron phosphate particles with the desired particle size, and obtain a uniform and dense carbon coating layer in which each particle is coated, which greatly improves the surface conductivity of the particles.

[0032] In any embodiment, the method for preparing the 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, iron source, and phosphorus source, based on the atomic molar number of each element, satisfies Fe∶P=0.96-0.985 and Li∶Fe=1.0-1.1∶0.95-1.1; and the carbon source and carbon film-forming agent, based on a weight ratio, satisfy carbon source∶carbon film-forming agent=9∶1-2∶8.

[0033] By using raw materials containing lithium source, iron source, phosphorus source, carbon source, modifier, and carbon film-forming agent in the above-mentioned proportions, good modification can be achieved, and the resulting lithium iron phosphate particle precursor can be formed into larger particles. This allows for better shaping of the primary average particle size, carbon content, BET specific surface area, and z-value of the lithium iron phosphate particles according to the first aspect of this application. Specifically, as described above, by setting the primary average particle size of the lithium iron phosphate particles in the embodiments of this application to 500 nm to 3000 nm, the particles can maintain a suitable micron-level size, thereby avoiding interfacial side reactions and processing difficulties that arise when nano-sizing the particles (processing nano-sized particles is difficult in this art), and preventing the reduction in kinetic performance due to excessively large particle size. Furthermore, by setting the BET specific surface area of ​​the lithium iron phosphate particles in the embodiments of this application to 3 m², 2 / g~8m 2 The / g ratio facilitates the stirring of the slurry containing the lithium iron phosphate particles and increases the solid content, thereby improving the cell processing and increasing the volumetric energy density of the battery. Further, optionally, by ensuring that the BET specific surface area to Cx ratio z of the lithium iron phosphate particles in the embodiments of this application satisfies the range of 1.5 ≤ z ≤ 8.5, the carbon contained in the lithium iron phosphate particles can be made more uniform and dense, and each particle can be coated, thereby improving the surface conductivity of the particles.

[0034] In any embodiment, in the method for preparing the 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, and lithium carbonate may be selected; 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, iron(II,III) oxide, and ferric hydroxide, and iron(II,III) oxide may be selected; the phosphorus source is a phosphoric acid compound, including phosphoric acid, lithium dihydrogen phosphate, and lithium oxalate. The modifier comprises one or more of ammonium and diammonium hydrogen phosphate, optionally phosphoric acid; the modifier comprises at least one of titanium dioxide, vanadium pentoxide, tetrabutyl titanate, ammonium metavanadate, niobium ethoxide, niobium oxalate, niobium pentoxide, magnesium hydroxide, and magnesium nitrate, optionally titanium dioxide; the carbon source comprises at least one of citric acid, glucose, sucrose, starch, fructose, and lactose, optionally glucose; the carbon film-forming agent comprises one or more combinations of polyethylene glycol, polyaniline, polyacrylonitrile, polyvinylpyrrolidone, and polyvinyl alcohol, optionally polyaniline.

[0035] By selecting the aforementioned substances as the lithium source, iron source, phosphorus source, modifier, carbon source, and carbon film-forming agent used in the preparation method of lithium iron phosphate particles according to the embodiments of this application, the primary average particle size, carbon content, BET specific surface area, and z-value of the lithium iron phosphate particles of the first aspect of this application can be well obtained. This further achieves a larger particle size for the lithium iron phosphate particles of the embodiments of this application, while also achieving excellent kinetic performance when used as a cathode material.

[0036] In any embodiment, the heating rate in the first sintering and the second sintering are each independently 2℃ / min-20℃ / min, the isothermal time in the first sintering is 1h-6h, and the isothermal time in the second sintering is 2h-12h.

[0037] By independently controlling the heating rate and isothermal time in the first and second sintering processes within the aforementioned ranges, excessive side reactions during sintering due to rapid heating can be prevented, thus avoiding negative impacts on the primary average particle size, carbon content, BET specific surface area, and z-value of the obtained lithium iron phosphate particles. This results in a better achievement of the large particle size of the lithium iron phosphate particles described in this application, while also achieving excellent kinetic performance when used as a cathode material.

[0038] In any embodiment, 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%-1.99% by weight, or optionally 0.2%-1.6% by weight, based on the total weight of the product after the first sintering.

[0039] By coating the product after 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.

[0040] In any embodiment, the carbon coating is performed by vapor deposition during sintering or by carbonizing the carbon source at high temperature.

[0041] In the method for preparing lithium iron phosphate particles according to the embodiments of the present invention, the implementation of the carbon coating is not particularly limited. It can be selected to use a chemical vapor deposition method to carbon coat the lithium iron phosphate particles, thereby forming a uniform and dense carbon coating layer on the surface of the lithium iron phosphate particles, in which each particle is coated.

[0042] In any embodiment, after the first sintering and after the second sintering, the sintered product is pulverized, and the pulverization can be selected from one or more of grinding, sand milling, mechanical crushing, and air jet crushing.

[0043] In the method for preparing lithium iron phosphate salt particles according to the embodiments of the present invention, the pulverization method is not particularly limited. By adopting the specific pulverization method described above, it is beneficial to obtain the desired primary average particle size.

[0044] A third aspect of this application provides a positive electrode sheet comprising lithium iron phosphate particles as a positive electrode material according to the first aspect of this application.

[0045] A fourth aspect of this application provides a secondary battery that includes the positive electrode sheet of the third aspect of this application.

[0046] The fifth aspect of this application provides an electrical device that includes the secondary battery of the fourth aspect of this application. Attached Figure Description

[0047] Figure 1 These are SEM and TEM images of lithium iron phosphate salt particles according to an embodiment of this application, wherein, Figure 1 a corresponds to an image of lithium iron phosphate salt particles observed under SEM (scanning electron microscope) according to an embodiment of this application; Figure 1 b and Figure 1 c corresponds to an image of lithium iron phosphate salt particles observed under TEM (transmission electron microscopy) according to an embodiment of this application.

[0048] Figure 2 These are SEM images of an embodiment of this application and existing lithium iron phosphate particles, wherein, Figure 2 a is the SEM image of Example 10; Figure 2 b is an SEM image of existing lithium iron phosphate particles.

[0049] Figure 3 This is an exemplary schematic diagram showing the particle size of a primary particle according to this application.

[0050] Figure 4 This is a capacity performance diagram of lithium iron phosphate salt particles according to an embodiment of this application at 0.1C and 1C rates.

[0051] Figure 5 This is a schematic diagram of a secondary battery according to one embodiment of this application.

[0052] Figure 6 yes Figure 5 An exploded view of a secondary battery according to one embodiment of this application is shown.

[0053] Figure 7 This is a schematic diagram of a battery module according to one embodiment of this application.

[0054] Figure 8 This is a schematic diagram of a battery pack according to one embodiment of this application.

[0055] Figure 9 yes Figure 8 An exploded view of a battery pack according to one embodiment of this application is shown.

[0056] Figure 10 This is a schematic diagram of an electrical device that uses a secondary battery as a power source according to one embodiment of this application.

[0057] Explanation of reference numerals in the attached figures: 1 Battery pack; 2 Upper housing; 3 Lower housing; 4 Battery module; 5 Secondary battery; 51 Casing; 52 Electrode assembly; 53 Cover plate. Detailed Implementation

[0058] The following describes in detail the embodiments of the lithium iron phosphate salt particles, their preparation method, positive electrode sheet, secondary battery, and power device of this application. However, unnecessary detailed descriptions may be omitted. For example, detailed descriptions of well-known matters and repetitive descriptions of practically identical structures may be omitted. This is to avoid making the following description unnecessarily lengthy and to facilitate understanding by those skilled in the art. Furthermore, the accompanying drawings and the following description are provided to enable those skilled in the art to fully understand this application and are not intended to limit the subject matter of the claims.

[0059] The "range" disclosed in this application is defined by a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of a particular range. Ranges defined in this way can include or exclude endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60~120 and 80~110 are listed for a specific parameter, it is also expected that ranges of 60~110 and 80~120 are also included. Furthermore, if minimum range values ​​of 1 and 2 are listed, and if maximum range values ​​of 3, 4, and 5 are listed, then the following ranges are all expected: 1~3, 1~4, 1~5, 2~3, 2~4, and 2~5. In this application, unless otherwise stated, the numerical range "a~b" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0~5" indicates that all real numbers between "0~5" have been listed in this article; "0~5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is stated as an integer ≥2, it is equivalent to disclosing that the parameter is, for example, an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0060] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0061] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions.

[0062] Unless otherwise specified, all steps in this application may be performed sequentially or randomly, optionally sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, the method may also 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 it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0063] Unless otherwise specified, the terms "comprising" and "including" as used in this application can be open-ended or closed-ended. For example, "comprising" and "including" can mean that other components not listed may also be included, or that only the listed components may be included.

[0064] Unless otherwise specified, the term "or" is inclusive in this application. For example, the phrase "A or B" means "A, B, or both A and B". More specifically, the condition "A or B" is satisfied by any of the following conditions: A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or both A and B are true (or exist).

[0065] Lithium iron phosphate (LFP) is used as the cathode material in secondary batteries, but it suffers from poor conductivity. To address this, the industry typically employs techniques such as doping, coating, and particle nanostructuring to improve the conductivity of LFP.

[0066] Currently, the carbon coating and nano-sizing techniques commonly used in the industry, while reducing the average primary particle size, introduce a loose carbon layer, which significantly increases the specific surface area of ​​the material. This leads to a series of processing and manufacturing problems during the material's processing and use, increasing production costs and reducing yield. Examples include slurry gelation, electrode coating cracking, powder absorbing moisture when exposed to air, powder loss from the film area during electrode slitting, and small particle shedding during cold pressing. All of these processing problems can potentially affect battery life.

[0067] Therefore, the preparation of large-particle lithium iron phosphate cathode materials, while simultaneously improving their kinetic properties, is of great significance for industrialization. This will be explained in detail below.

[0068] [Lithium iron phosphate granules] The lithium iron phosphate salt particles in the embodiments of this application have a primary average particle size of 500nm~3000nm, optionally 650nm~2500nm, and a BET specific surface area of ​​3m². 2 / g~8m 2 / g, optional 4m 2 / g~7m 2 / g, calculated based on 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, or optionally 1.0≤Cx≤1.6.

[0069] In some embodiments, the primary average particle size of the lithium iron phosphate particles can be selected as 500nm, 600nm, 650nm, 700nm, 790nm, 800nm, 870nm, 900nm, 920nm, 1000nm, 1100nm, 1200nm, 1250nm, 1300nm, 1400nm, 1500nm, 1600nm, 1700nm, 1800nm, 1900nm, 2000nm, 2100nm, 2200nm, 2300nm, 2400nm, 2500nm, 2600nm, 2700nm, 2800nm, 2900nm, or 3000nm, or a range between any two of the above values. In some embodiments, the BET specific surface area of ​​the lithium iron phosphate particles can be selected as 3m². 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 a range between any two of the above values. In some embodiments, the carbon content of the lithium iron phosphate particles may be selected as 0.8 wt%, 0.9 wt%, 0.94 wt%, 1.0 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.

[0070] In this document, the term "first-order average particle size" refers to the particle size value obtained by statistical analysis of particle size using the major axis statistics method under a scanning electron microscope field of view. During the particle size statistics process, particles with a first-order average particle size less than or equal to 80 nm are not included in the statistical range. The first-order average particle size of this application is as follows: Figure 3As shown. The primary average particle size refers to the average particle size of the primary particles. In this document, "primary particle" refers to a particle that does not have obvious agglomeration interfaces in the particle scanning electron microscope image, but may have tiny pores and point or line defects, distinguishing it from powder particles that are the smallest units without aggregation or flocculation structures. The determination methods for the primary average particle size, carbon content, and BET specific surface area can be performed using the determination methods described in the examples.

[0071] In the embodiments of this application, the primary average particle size of lithium iron phosphate salt particles is 500nm~3000nm and the BET specific surface area is 3m². 2 / g~8m 2 The carbon content, calculated based on the total weight of the lithium iron phosphate particles, is Cx wt% (0.8 ≤ Cx ≤ 2.0). This improves the kinetic performance of lithium iron phosphate particles with a primary average particle size within the above range; it maintains suitable micron-level particle size, thereby avoiding interfacial side reactions and processing difficulties caused by nano-sizing of particles, and preventing the reduction in kinetic performance due to excessively large particle size; furthermore, it facilitates the stirring of the slurry containing the lithium iron phosphate particles and increases the solid content, thereby improving cell processing issues and increasing the volumetric energy density of the battery; additionally, it avoids the impact of excessively high carbon coating density on the normal insertion and extraction of lithium ions, thus affecting the cell capacity.

[0072] Compared with the prior art, the lithium iron phosphate particles in the embodiments of this application have a smaller specific surface area when containing the same amount of carbon, which means that there is less floating carbon in the particles. This makes the carbon contained in the lithium iron phosphate particles more uniform and dense, and each particle is coated, thereby improving the surface conductivity of the particles and improving their kinetic performance as a positive electrode material.

[0073] In any implementation, the ratio z of the BET specific surface area to Cx satisfies 1.5≤z≤8.5, or optionally satisfies 3≤z≤6.

[0074] In some embodiments, the ratio z of the BET specific surface area to Cx can be selected as 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.

[0075] 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 embodiments of this 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.

[0076] In any embodiment, the lithium iron phosphate salt has the molecular formula Li m Fe x P y O j Q q 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, with 0.95≤m≤1.15, 0.9≤x≤1, 0.95≤y≤1, 3.5≤j≤4, and 0<q≤0.1.

[0077] By making the lithium iron phosphate salt of the embodiments of this application have the above-described molecular formula and simultaneously doping it with one or more of the above-described elements, a high loading volume bulk phase modification of the lithium iron phosphate salt particles is achieved. This helps to improve the bulk ion transport capability of the lithium iron phosphate salt particles and can effectively solve the problem of poor kinetic performance inherent in large particles. Furthermore, by performing this doping, the lithium iron phosphate salt particles can exhibit good kinetic performance when used as a cathode material. In this application, the modification can specifically be manifested as doping and / or coating.

[0078] In any embodiment, Q includes at least one of Ti, V, Mg, and Nb, optionally Ti, and the content of Ti, V, Mg and / or Nb is 1000ppm-10000ppm, optionally 2500ppm-6000ppm, calculated based on the total weight of the lithium iron phosphate salt particles. Specifically, the values ​​can be 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 any range between any two of the above values.

[0079] In some embodiments, the lithium iron phosphate salt has the molecular formula Li m A a Fe x D d Py E e O z G g The A includes at least one element selected from Al, Na, K, or Mg; the D includes at least one element selected from Cu, Mn, Cr, Zn, Pb, Ca, Co, Ni, Sr, Nb, Ti, or V; the E includes at least one element selected from B, S, Si, or N; the G includes at least one element selected from S, F, Cl, or Br; the m is selected from the range of 0.95 to 1.15; the a is selected from the range of 0 to 0.1; the x is selected from the range of 0.95 to 1; the d is selected from the range of 0 to 0.1; the y is selected from the range of 0.95 to 1; the e is selected from the range of 0 to 0.1; the z is selected from the range of 3.5 to 4; and the g is selected from the range of 0 to 0.1.

[0080] By doping the lithium iron phosphate salts of the embodiments of this application with one or more of the above-mentioned elements and ensuring that their content is within the above-mentioned range, it is possible to achieve better bulk metal phase modification of the lithium iron phosphate salt particles while improving the primary average particle size, thereby further enhancing the bulk ion transport capability of the lithium iron phosphate salt particles and better solving the problem of poor kinetic performance inherent in large particles.

[0081] In any embodiment, the lithium iron phosphate particles are single-crystal particles and / or polycrystalline particles. Optionally, based on the total number of lithium iron phosphate 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, without grain boundaries; a polycrystalline crystal is a crystal formed by the random orientation of a large number of small single-crystal particles, with grain boundaries present inside. Unlike a perfect single crystal, the large single crystals mentioned herein may have minor defects, such as internal micropores, a small number of point and surface defects, or a small number of particles adhering to each other on the surface of a single particle. However, the single crystal referred to herein appears as a single entity in a TEM image.

[0082] In the embodiments of this application, based on the total number of lithium iron phosphate salt particles, the proportion of single crystal particles is controlled within the above-mentioned range. Compared with polycrystalline and secondary agglomerates, a higher proportion of single crystal particles results in less obstruction of lithium ions by grain boundaries, faster lithium ion transport rate, and better kinetic performance.

[0083] In the lithium iron phosphate particles of the embodiments of this application, the carbon may exist either in a state mixed with the lithium iron phosphate particles or in a state coated on the lithium iron phosphate particles. Optionally, the carbon in the lithium iron phosphate particles may be in a state of coating on the lithium iron phosphate particles, thereby enabling the carbon to form a uniform and dense carbon coating layer on the surface of the lithium iron phosphate particles, thereby improving the surface conductivity of the particles.

[0084] In any embodiment, the capacity ratio of the lithium iron phosphate particles η ≥ 88%, where η is defined as follows: A battery with the lithium iron phosphate particles as the positive electrode material is charged and discharged twice at a constant current rate of 0.1C within a voltage range of 2.0V to 3.75V, followed by a constant current charge and discharge once at a constant current rate of 1C. In the 1C charge and discharge test, the capacity value extracted at a discharge voltage of 3.2V is recorded as C1, and the capacity value extracted at a discharge voltage of 2.0V is recorded as C2, where η = C1 / C2. The charging process includes constant voltage charging at a constant voltage of 3.75V and a constant voltage cutoff current of 50μA.

[0085] The η value characterizes the kinetic performance of lithium iron phosphate particles and can be adjusted by modifying the primary average particle size, carbon content, ratio of carbon source and film-forming agent, and modifier and its content. η represents the plateau retention performance of the material, and this value is strongly correlated with the battery's discharge power performance. When this value is large, the battery can still maintain good power performance when discharged to a low SOC (state of charge), meaning that the voltage drop is small when the battery discharges at a high current at low charge levels. In the embodiments of this application, the capacity ratio η of the lithium iron phosphate salt particles is ≥88%, indicating that the lithium iron phosphate salt particles of this application can enable the secondary battery to exhibit good kinetic performance when used as a positive electrode material.

[0086] In any embodiment, the lithium iron phosphate salt particles satisfy at least one of a)-f): a) The Dv10 of the lithium iron phosphate salt particles is ≥ 0.2 μm; b) The Dv50 of the lithium iron phosphate salt particles is 0.5μm-5μm; c) The Dv90 of the lithium iron phosphate salt particles is ≤10μm; d) The Dv99 of the lithium iron phosphate salt particles is ≤12μm; e) The compacted density of the lithium iron phosphate powder under 3T pressure is ≥2.25 g / cm³. 3 ; f) The resistivity of the lithium iron phosphate powder is less than 60 Ω·cm.

[0087] In this application, Dv10, Dv90, and Dv99 refer to the particle sizes corresponding to a cumulative volumetric particle size distribution percentage of 10%, 90%, and 99%, respectively. The testing method is the same as that for Dv50, and the standard used in the embodiments of this application can be employed for determination.

[0088] It should be noted that Dv10 is a target for large-scale indicators, so no upper limit can be given; Dv90 is a target for small-scale indicators, so no lower limit can be given.

[0089] By ensuring that the lithium iron phosphate particles satisfy at least one of a)-f), the lithium iron phosphate particles can achieve the above-mentioned technical effects more effectively.

[0090] In this application, the test methods for Dv10, Dv90, and Dv99 are the same as those for Dv50, and the standards in the embodiments of this application can be used for measurement.

[0091] In this application, the method for testing the powder resistivity is as follows: referring to the national standard GB / T 33822-2017, a powder resistivity meter (Suzhou Jingge, ST2722 type) is used. 1g of sample (with an error within ±0.005g) is weighed and added to the feeding chamber. A pressure of 8 MPa is applied, and the forward resistivity and reverse resistivity of the sample are tested respectively. The average value of the two is taken as the powder resistivity of the sample.

[0092] In this application, the powder compaction density is defined as: during the external force compression process, as the powder moves and deforms, larger voids are filled, the contact area between particles increases, resulting in attractive forces between atoms and enhanced mechanical cohesion between particles, thereby forming a compact with a certain density and strength, with units of g / cm³. 3 .

[0093] The test method for compaction density is as follows: Referring to the national standard GB / T 24533-2009, a certain amount of powder is placed on a special compaction mold (the mold diameter is known). The mold has a hollow center with two metal discs, one at the top and one at the bottom. 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 thickness of the powder under different pressures can be read on the instrument. The compaction density is calculated using ρ=m / v.

[0094] Based on the following powder compaction density calculation results, ρc = m / V = m / (S) H); where ρc is the compacted density of the powder (g / cm³). 3 m is the mass of the material weighed (g), and S is the bottom area of ​​the mold (1.327 cm²). 2 H is the height of the compacted sample (cm).

[0095] [Preparation method of lithium iron phosphate salt particles] The method for preparing lithium iron phosphate particles according to the embodiments of this application can prepare the lithium iron phosphate particles described in this application. The method for preparing lithium iron phosphate particles according to the embodiments of this application 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 sintering processes, wherein the temperature of the first sintering is 500℃-760℃, optionally 550℃-720℃, and the carbon content of the material after the first sintering is 0.01 wt%-0.79 wt%, optionally 0.05 wt%-0.4% by weight; the temperature of the second sintering is 700℃-800℃, optionally 720℃-780℃, and the carbon content of the material after the second sintering is 0.8 wt%-2.0 wt%, optionally 1.0 wt%-1.6 wt%.

[0096] In some embodiments, the temperature of the first sintering can be selected from 500℃, 510℃, 520℃, 530℃, 540℃, 550℃, 560℃, 570℃, 580℃, 590℃, 600℃, 610℃, 620℃, 630℃, 640℃, 650℃, 655℃, 660℃, 670℃, 680℃, 690℃, 700℃, 710℃, 720℃, 730℃, 740℃, 750℃, 760℃, or 760℃, or a range between any two of the above values. The temperature of the second sintering can be selected from 700℃, 710℃, 720℃, 730℃, 740℃, 750℃, 755℃, 760℃, 770℃, 780℃, 790℃, or 800℃, or a range between any two of the above values. After the first sintering, the carbon content of the material can be selected as 0.01 wt%, 0.05 wt%, 0.10 wt%, 0.15 wt%, 0.2 wt%, 0.25 wt%, 0.3 wt%, 0.35 wt%, 0.4 wt%, 0.45 wt%, 0.5 wt%, 0.55 wt%, 0.6 wt%, 0.65 wt%, 0.7 wt%, 0.75 wt%, or 0.79 wt%. After the second sintering, the carbon content of the material is 0.8 wt% to 2.0 wt%, and can be selected as 0.8 wt%, 0.9 wt%, 1 wt%, 1.1%, 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 any range between the above two values.

[0097] In the method for preparing lithium iron phosphate salt particles according to the embodiments of this application, two sintering processes are performed. By controlling the temperature of the first sintering within the aforementioned range and ensuring that the carbon content of the intermediate after sintering is within the aforementioned range, the lithium iron phosphate precursor obtained after the first sintering can have a larger particle size, directly improving the powder compaction and electrode compaction density of the final product. Furthermore, by adding a low amount of carbon source during the first sintering, the obstructive effect of the carbon layer on the growth process of lithium iron phosphate particles is greatly reduced, which is beneficial for particle crystallization growth at a lower temperature. Simultaneously, it facilitates the solid-phase diffusion reaction between the modifier and the lithium iron phosphate material, thereby achieving a higher concentration of metal ion modification. Compared to traditional methods that use high temperatures to achieve particle growth, the preparation method described in this application can first synthesize large particles at a lower temperature, which can improve the phenomenon of cracking of the carbon layer on the particle surface at high temperatures 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 materials, improving the purity and stability of the product. By controlling the temperature of the second sintering within the above range and making the carbon content of the sintered material 0.8%-2.0% by weight, 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 each particle, which greatly improves the conductivity of the particle surface.

[0098] In any embodiment, the lithium iron phosphate particles contain at least one of the elements Ti, V, Mg, and / or Nb, and the content of the element is 1000ppm-10000ppm, optionally 2500ppm-6000ppm, based on the total weight of the lithium iron phosphate particles.

[0099] In some embodiments, the content of the element may be selected as 2500ppm-6000ppm. Specifically, the values ​​can be 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 any range between any two of the above values.

[0100] By doping the lithium iron phosphate salts of the embodiments of this application with one or more of the above-mentioned elements and ensuring that their content is within the above-mentioned range, it is possible to achieve better bulk metal phase modification of the lithium iron phosphate salt particles while improving the primary average particle size, thereby further enhancing the bulk ion transport capability of the lithium iron phosphate salt particles and better solving the problem of poor kinetic performance inherent in large particles.

[0101] In any embodiment, the method for preparing lithium iron phosphate particles involves a first pulverization after the first sintering and a second pulverization after the second sintering. The Dv50 of the product after the first pulverization is 300nm-1200nm, optionally 400nm-1100nm; the Dv50 of the product after the second pulverization is 500nm-5000nm, optionally 300nm-2500nm.

[0102] In some embodiments, the Dv50 of the product after the first pulverization is 300nm, 350nm, 400nm, 500nm, 550nm, 600nm, 700nm, 800nm, 900nm, 1000nm, 1100nm, or 1200nm, or a range between any two of the above values. In some embodiments, the Dv50 of the product after the second pulverization is 500nm, 600nm, 700nm, 800nm, 900nm, 1000nm, 1100nm, 1200nm, 1300nm, 1400nm, 1500nm, 1600nm, 1700nm, 1800nm, 1900nm, 2000nm, 2300nm, 2500nm, 2700nm, 3000nm, 3300nm, 3500nm, 3700nm, 4000nm, 4300nm, 4500nm, 4700nm, 5000nm, or a range between any two of the above values.

[0103] In this document, the term "Dv50" refers to the particle size at which the cumulative volumetric size distribution percentage in the particle reaches 50%. The Dv50 can be determined using the determination method described in the examples.

[0104] In the method for preparing lithium iron phosphate particles according to the embodiments of this application, pulverization is performed after two sintering processes. By performing a first pulverization after the first sintering, the Dv50 of the product is 300nm-1200nm, which avoids the growth barrier of carbon materials and modifying elements on crystals, and obtains micron-sized lithium iron phosphate precursors. By performing a second pulverization after the second sintering, the Dv50 of the product is 500nm-3000nm, which can obtain lithium iron phosphate particles with the desired particle size, and obtain a uniform and dense carbon coating layer in which each particle is coated, which greatly improves the surface conductivity of the particles.

[0105] In any embodiment, the method for preparing the 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, iron source, and phosphorus source, based on the atomic molar number of each element, satisfies Fe∶P=0.96-0.985∶1 and Li∶Fe=1.0-1.1∶0.95-1.1; and the carbon source and carbon film-forming agent, based on a weight ratio, satisfy carbon source∶carbon film-forming agent=9∶1-2∶8.

[0106] In some implementations, the mixing ratio of iron source to phosphorus source, based on the atomic molar number of each element, satisfies the following conditions: 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.

[0107] In some embodiments, the mixing ratio of lithium source to iron source, based on the atomic molar number of each element, satisfies the following: 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.

[0108] In some embodiments, the weight ratio of carbon source to carbon film-forming agent can be selected as 9:1, 8:2, 7:3, 6:4, 5:5, 4:6, 3:7 or 2:8.

[0109] By using raw materials containing lithium source, iron source, phosphorus source, carbon source, modifier, and carbon film-forming agent in the above-mentioned proportions, good modification can be achieved, and the resulting lithium iron phosphate particle precursor can be formed into larger particles. This allows for better shaping of the primary average particle size, carbon content, BET specific surface area, and z-value of the lithium iron phosphate particles according to the first aspect of this application. Specifically, as described above, by setting the primary average particle size of the lithium iron phosphate particles in the embodiments of this application to 500 nm to 3000 nm, the particles can maintain a suitable micron-level size, thereby avoiding interfacial side reactions and processing difficulties (processing nano-sized particles is difficult in this art), and preventing the reduction in kinetic performance due to excessively large particle size. Furthermore, by setting the BET specific surface area of ​​the lithium iron phosphate particles in the embodiments of this application to 3 m², 2 / g~8m 2 The / g ratio facilitates the stirring of the slurry containing the lithium iron phosphate particles and increases the solid content, thereby improving the cell processing and increasing the volumetric energy density of the battery. Further, optionally, by ensuring that the BET specific surface area to Cx ratio z of the lithium iron phosphate particles in the embodiments of this application satisfies the range of 1.5 ≤ z ≤ 8.5, the carbon contained in the lithium iron phosphate particles can be made more uniform and dense, and each particle can be coated, thereby improving the surface conductivity of the particles.

[0110] In any embodiment, in the method for preparing the 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, and lithium carbonate may be selected; 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, iron(II,III) oxide, and ferric hydroxide, and iron(II,III) oxide may be selected; the phosphorus source is a phosphoric acid compound, including phosphoric acid, lithium dihydrogen phosphate, and lithium oxalate. The modifier comprises one or more of ammonium and diammonium hydrogen phosphate, optionally phosphoric acid; the modifier comprises at least one of titanium dioxide, vanadium pentoxide, tetrabutyl titanate, ammonium metavanadate, niobium ethoxide, niobium oxalate, niobium pentoxide, magnesium hydroxide, and magnesium nitrate, optionally titanium dioxide; the carbon source comprises at least one of citric acid, glucose, sucrose, starch, fructose, and lactose, optionally glucose; the carbon film-forming agent comprises one or more combinations of polyethylene glycol, polyaniline, polyacrylonitrile, polyvinylpyrrolidone, and polyvinyl alcohol, optionally polyaniline.

[0111] By selecting the aforementioned substances as the lithium source, iron source, phosphorus source, modifier, carbon source, and carbon film-forming agent used in the preparation method of lithium iron phosphate particles according to the embodiments of this application, the primary average particle size, carbon content, BET specific surface area, and z-value of the lithium iron phosphate particles of the first aspect of this application can be well obtained. This further achieves a larger particle size for the lithium iron phosphate particles of the embodiments of this application, while also achieving excellent kinetic performance when used as a cathode material.

[0112] In any embodiment, the heating rate in the first sintering and the second sintering are each independently 2℃ / min-20℃ / min, the isothermal time in the first sintering is 1h-6h, and the isothermal time in the second sintering is 2h-12h.

[0113] 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.

[0114] In some embodiments, the isothermal sintering time for the first sintering is 1-6 hours. In some embodiments, the isothermal sintering time for the first sintering can be selected as 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.

[0115] In some embodiments, the isothermal sintering time for the second sintering is 2-12 hours. In some embodiments, the isothermal sintering time for the second sintering can be selected as 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.

[0116] By independently controlling the heating rate and isothermal time in the first and second sintering processes within the aforementioned ranges, excessive side reactions during sintering due to rapid heating can be prevented, thus avoiding negative impacts on the primary average particle size, carbon content, BET specific surface area, and z-value of the obtained lithium iron phosphate particles. This results in a better achievement of the large particle size of the lithium iron phosphate particles described in this application, while also achieving excellent kinetic performance when used as a cathode material.

[0117] In any embodiment, after the first sintering and before the second sintering, the product after the first sintering is carbon-coated. Based on the total weight of the product after the first sintering, the amount of carbon coating is 0.01 wt% to 1.99 wt%, optionally 0.2 wt% to 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 any range between the above two values.

[0118] By coating the product after 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.

[0119] In any embodiment, the carbon coating is performed by vapor deposition during sintering or by carbonizing the carbon source at high temperature.

[0120] In the method for preparing lithium iron phosphate particles according to the embodiments of the present invention, the implementation of the carbon coating is not particularly limited. It can be selected to use a chemical vapor deposition method to carbon coat the lithium iron phosphate particles, thereby forming a uniform and dense carbon coating layer on the surface of the lithium iron phosphate particles, in which each particle is coated.

[0121] In any embodiment, after the first sintering and after the second sintering, the sintered product is pulverized, and the pulverization can be selected from one or more of grinding, sand milling, mechanical crushing, and air jet crushing.

[0122] In the method for preparing lithium iron phosphate salt particles according to the embodiments of the present invention, the pulverization method is not particularly limited. By adopting the specific pulverization method described above, it is beneficial to obtain the desired primary average particle size.

[0123] [Positive electrode plate] The positive electrode includes a positive current collector and a positive electrode film layer disposed on at least one surface of the positive current collector. The positive electrode film layer includes lithium iron phosphate particles as the positive electrode material according to the first aspect of this application. The lithium iron phosphate particles of this application have a large particle size when used in secondary batteries, and also exhibit excellent kinetic performance when used as a positive electrode material.

[0124] 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 this application may be used as the metal foil. The composite current collector may include a polymer material substrate and a metal layer formed on at least one surface of the polymer material substrate. The composite current collector may be formed by forming the battery aluminum foil of this application on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0125] In some embodiments, the cathode material may be a cathode material known in the art for use in batteries. As an example, the cathode material may include at least one of the following materials: lithium phosphates with an olivine structure, lithium transition metal oxides, and their respective modified compounds. However, this application is not limited to these materials, and other conventional materials that can be used as battery cathode materials may also be used. These cathode materials may be used alone or in combination of two or more. Examples of lithium transition metal oxides include, but are not limited to, lithium cobalt oxides (such as LiCoO2), lithium nickel oxides (such as LiNiO2), lithium manganese oxides (such as LiMnO2, LiMn2O4), lithium nickel cobalt oxides, lithium manganese cobalt oxides, lithium nickel manganese oxides, and lithium nickel cobalt manganese oxides (such as LiNi). 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (also known as NCM) 333 LiNi 0.5 Co 0.2 Mn 0.3 O2 (also known as NCM) 523 LiNi 0.5 Co 0.25 Mn 0.25 O2 (also known as NCM) 211 LiNi 0.6 Co 0.2 Mn 0.2 O2 (also known as NCM) 622 LiNi 0.8 Co 0.1 Mn 0.1 O2 (also known as NCM) 811 ), lithium nickel cobalt aluminum oxide (such as LiNi) 0.85 Co 0.15 Al 0.05At least one of O2 and its modified compounds. Examples of lithium phosphates with an olivine structure include, but are not limited to, lithium iron phosphate (such as LiFePO4 (also referred to as LFP)), lithium iron phosphate and carbon composites, lithium manganese phosphate (such as LiMnPO4), lithium manganese phosphate and carbon composites, lithium manganese iron phosphate, and lithium manganese iron phosphate and carbon composites.

[0126] In some embodiments, the positive electrode film layer may optionally include a binder. As an example, the binder may include at least one selected from polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), PVDF-tetrafluoroethylene-propylene terpolymer, PVDF-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin.

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

[0128] In some embodiments, the positive electrode sheet can be prepared by dispersing the components used to prepare the positive electrode sheet, such as positive electrode material, conductive agent, binder and any other components, in a solvent (e.g. N-methylpyrrolidone) to form a positive electrode slurry; coating the positive electrode slurry onto a positive electrode current collector, and then obtaining the positive electrode sheet after drying, cold pressing and other processes.

[0129] [Negative electrode plate] The negative electrode sheet includes a negative current collector and a negative electrode film layer disposed on at least one surface of the negative current collector, the negative electrode film layer including a negative electrode active material.

[0130] As an example, the negative electrode current collector has two surfaces opposite each other in its own thickness direction, and the negative electrode film layer is disposed on either or both of the two opposite surfaces of the negative electrode current collector.

[0131] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, copper foil may be used as the metal foil. The composite current collector may include a polymer substrate and a metal layer formed on at least one surface of the polymer substrate. The composite current collector may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0132] In some embodiments, the negative electrode active material may be a negative electrode active material known in the art for use in batteries. As an example, the negative electrode active material may include at least one of the following materials: artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, and lithium titanate, etc. The silicon-based material may be selected from at least one of elemental silicon, silicon oxide compounds, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. The tin-based material may be selected from at least one of elemental tin, tin oxide compounds, and tin alloys. However, this application is not limited to these materials, and other conventional materials that can be used as negative electrode active materials for batteries may also be used. These negative electrode active materials may be used alone or in combination of two or more.

[0133] In some embodiments, the negative electrode film layer may optionally include an adhesive. The adhesive 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).

[0134] In some embodiments, the negative electrode film may optionally include a conductive agent. The conductive agent may be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0135] In some embodiments, the negative electrode film may optionally include other additives, such as thickeners (e.g., sodium carboxymethyl cellulose (CMC-Na)).

[0136] In some embodiments, the negative electrode sheet can be prepared by dispersing the components used to prepare the negative electrode sheet, such as negative electrode active material, conductive agent, binder and any other components, in a solvent (e.g., deionized water) to form a negative electrode slurry; coating the negative electrode slurry onto a negative electrode current collector, and then obtaining the negative electrode sheet after drying, cold pressing and other processes.

[0137] [Electrolytes] The electrolyte acts as a conductor of ions between the positive and negative electrodes. This application does not impose specific restrictions on the type of electrolyte; it can be selected according to requirements. For example, the electrolyte can be liquid, gel, or entirely solid.

[0138] In some embodiments, the electrolyte is an electrolyte solution. The electrolyte solution includes an electrolyte salt and a solvent.

[0139] 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 difluorooxalate borate, lithium dioxalate borate, lithium difluorodioxalate phosphate, and lithium tetrafluorooxalate phosphate.

[0140] In some embodiments, the solvent may be selected from at least one of ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butyl carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, sulfolane, dimethyl sulfone, methyl ethyl sulfone, and diethyl sulfone.

[0141] In some embodiments, the electrolyte may optionally include additives. For example, additives may include negative electrode film-forming additives, positive electrode film-forming additives, and may also include additives that can improve certain battery performance, such as additives that improve battery overcharge performance, additives that improve battery high-temperature or low-temperature performance, etc.

[0142] [Isolation membrane] In some embodiments, the secondary battery also includes a separator. This application does not impose any particular limitation on the type of separator; any known porous separator with good chemical and mechanical stability can be selected.

[0143] In some embodiments, the material of the separator can be selected from at least one of glass fiber, nonwoven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single-layer film or a multi-layer composite film, without particular limitation. When the separator is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

[0144] [Rechargeable Battery] In one embodiment of this application, a secondary battery is provided, which includes the positive electrode plate.

[0145] Typically, a secondary battery consists of a positive electrode, a negative electrode, an electrolyte, and a separator. During charging and discharging, active ions move back and forth between the positive and negative electrodes, inserting and releasing. The electrolyte acts as a conductor between the positive and negative electrodes. The separator, positioned between the positive and negative electrodes, primarily prevents short circuits while allowing ions to pass through.

[0146] In some implementations, the positive electrode, negative electrode, and separator can be fabricated into an electrode assembly using a winding or stacking process.

[0147] In some embodiments, the secondary battery may include an outer packaging. This outer packaging may be used to encapsulate the electrode assembly and the electrolyte.

[0148] In some embodiments, the outer packaging of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. The outer packaging of the secondary battery can also be a soft pack, such as a pouch. The material of the soft pack can be plastic; examples of plastics include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0149] This application does not impose any particular limitation on the shape of the secondary battery; it can be cylindrical, square, or any other arbitrary shape. For example, Figure 5 This is an example of a square-structured secondary battery 5.

[0150] In some implementations, refer to Figure 6 The outer packaging may include a housing 51 and a cover 53. The housing 51 may include a base plate and side plates connected to the base plate, the base plate and side plates forming a receiving cavity. The housing 51 has an opening communicating with the receiving cavity, and the cover 53 can be placed over the opening to close the receiving cavity. The positive electrode, negative electrode, and separator may be formed into an electrode assembly 52 by a winding process or a stacking process. The electrode assembly 52 is encapsulated within the receiving cavity. Electrolyte is immersed in the electrode assembly 52. ​​The secondary battery 5 may contain one or more electrode assemblies 52, which can be selected by those skilled in the art according to specific practical needs.

[0151] In some implementations, the secondary batteries can be assembled into a battery module, and the number of secondary batteries contained in the battery module can be one or more, the specific number of which can be selected by those skilled in the art according to the application and capacity of the battery module.

[0152] Figure 7 This is battery module 4, used as an example. (See reference...) Figure 7 In battery module 4, multiple secondary batteries 5 can be arranged sequentially along the length of battery module 4. Of course, they can also be arranged in any other manner. Furthermore, these multiple secondary batteries 5 can be fixed in place using fasteners.

[0153] Optionally, the battery module 4 may also include a housing with a receiving space in which a plurality of secondary batteries 5 are received.

[0154] In some embodiments, the battery modules described above can also be assembled into a battery pack, and the number of battery modules contained in the battery pack can be one or more, the specific number of which can be selected by those skilled in the art according to the application and capacity of the battery pack.

[0155] Figure 8 and Figure 9 This is battery pack 1 as an example. (See reference...) Figure 8 and Figure 9 The battery pack 1 may include a battery box and multiple battery modules 6 disposed within the battery box. The battery box includes an upper body 2 and a lower body 3, with the upper body 2 covering the lower body 3 to form a closed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the battery box.

[0156] [Electrical appliances] In addition, this application also provides an electrical device, which includes at least one of the secondary battery, battery module, or battery pack provided in this application. The secondary battery, battery module, or battery pack can be used as a power source for the electrical device, or as an energy storage unit for the electrical device. The electrical device may include, but is not limited to, mobile devices (e.g., mobile phones, laptops, etc.), electric vehicles (e.g., pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc.

[0157] As an electrical device, a secondary battery, battery module, or battery pack can be selected according to its usage requirements.

[0158] Figure 10 This is an example of an electrical device. The device could be 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 for this device, a battery pack or battery module can be used.

[0159] Another example device could be a mobile phone, tablet, or laptop. These devices typically require a slim and lightweight design and can use a rechargeable battery as their power source.

[0160] Example The following describes embodiments of this application. The embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments used, unless otherwise specified, are all conventional products that can be obtained commercially.

[0161] Example 1 1. Preparation of lithium iron phosphate salt particles Lithium carbonate, ferric oxide, phosphoric acid, glucose, titanium dioxide (based on the total weight of lithium iron phosphate particles, with the amount of titanium dioxide added ensuring a titanium content of 5000 ppm in the prepared lithium iron phosphate particles), and polyaniline were weighed separately. The weight ratios of Li, Fe, and P elements were: Fe∶P = 0.968∶1, Li∶Fe = 1∶0.98, and the weight ratio of glucose to polyaniline was: glucose∶polyaniline = 1∶2. The amount of glucose added was such that after the first sintering, the carbon content accounted for 0.15% of the weight of the lithium iron phosphate precursor. Water was added to the above substances to obtain a slurry mixture.

[0162] The mixture was homogenized using a ball mill and then ground using a sand mill to obtain a slurry with a solid content of 38% and a Dv50 of 0.40 μm. 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℃ to 360℃, and an outlet temperature of 100℃ to 140℃). The dried reactants were then loaded into a sintering furnace for the first sintering, with a heating rate controlled at 5℃ / min, a holding temperature of 650℃, and a holding time of 4 hours. After cooling, the material was mechanically ground to obtain powder.

[0163] Glucose as a carbon source and polyaniline as a carbon film-forming agent were added to the obtained powder, and then mixed with water to obtain 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 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 obtain a slurry with a Dv50 of 550 nm for the insoluble matter. Spray drying was then performed (high-speed spray dryer with a negative pressure of -650 to -200 Pa, an inlet temperature of 300℃ to 360℃, and an outlet temperature of 100℃ to 140℃). The dried reactants were then loaded into a sintering furnace for a second low-temperature sintering (heating rate controlled at 5℃ / min, sintering temperature at 750℃, and sintering time at 4 h). After the material cools, it is crushed a second time to an average particle size of 870 nm. After demagnetization, lithium iron phosphate particles are obtained with a carbon content of 1.2% and a Ti element content of 5000 ppm.

[0164] The obtained lithium iron phosphate particles were observed under SEM (scanning electron microscope), and the results are as follows. Figure 1 of Figure 1 As shown in figure a; the obtained lithium iron phosphate salt particles were observed under TEM (transmission electron microscopy), and the results are as follows. Figure 1 b and Figure 1 As shown in c.

[0165] 2. Preparation of the positive electrode sheet 2.0 wt% polyvinylidene fluoride binder was fully dissolved in N-methylpyrrolidone (NMP), then 1.0 wt% Super P, 0.5 wt% carbon nanotubes, and 96.5 wt% of the above-mentioned cathode material were added and stirred until homogeneous to obtain a cathode slurry. The slurry was uniformly coated onto the surface of the current collector aluminum foil and then transferred to a vacuum drying oven for complete drying. The dried electrode was then rolled and punched to obtain the cathode electrode sheet.

[0166] 3. Preparation of negative electrode sheet The active material artificial graphite, conductive agent carbon black, binder styrene-butadiene rubber (SBR), and thickener sodium carboxymethyl cellulose (CMC Na) are dissolved in deionized water at 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 uniformly coated onto the negative electrode current collector copper foil once or multiple times, and then dried, cold-pressed, and slit to obtain the negative electrode sheet.

[0167] 4. Preparation of electrolyte In an argon atmosphere glove box (H2O < 0.1 ppm, O2 < 0.1 ppm), ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed uniformly at a volume ratio of 3:7. LiPF6 lithium salt was dissolved in the organic solvent to prepare a solution with a weight content of 12.5%, thus obtaining the electrolyte.

[0168] 5. Separating membrane Polypropylene film is used as the separator.

[0169] 6. Preparation of lithium-ion batteries The positive electrode, separator, and negative electrode are stacked in sequence, with the separator positioned between the positive and negative electrodes to provide isolation. The resulting bare cell is then wound, tabs are welded onto it, and the cell is placed in an aluminum casing. It is then baked at 80°C to remove moisture, followed by the injection of electrolyte and sealing to obtain a non-charged battery. This non-charged battery then undergoes a series of processes including settling, hot and cold pressing, formation, shaping, and capacity testing to obtain the lithium-ion battery of Example 1.

[0170] The lithium-ion battery obtained in Example 1 was subjected to performance tests described later, and the results are shown in 1.

[0171] Example 2 By controlling the Dv50 particle size of the pulverized particles after the first sintering to be 350 nm, lithium iron phosphate particles with an average primary particle size of 500 nm were obtained. Otherwise, the process was carried out in the same manner as in Example 1.

[0172] Example 3 By controlling the Dv50 particle size of the pulverized particles after the first sintering to be 1200 nm, lithium iron phosphate particles with an average primary particle size of 3000 nm were obtained. Otherwise, the process was carried out in the same manner as in Example 1.

[0173] Example 4 The temperatures of the first and second sintering were changed to 620°C and 740°C, respectively, and the Ti content of the product was controlled to be 2500 ppm. Otherwise, the process was the same as in Example 1.

[0174] Example 5 The temperatures of the first and second sintering were changed to 655°C and 755°C, respectively, and the Ti content was adjusted to 6000 ppm. Otherwise, the process was the same as in Example 1.

[0175] Example 6 The modification element was replaced with V, and otherwise the process was the same as in Example 1.

[0176] Example 7 The modification element was replaced with Nb, and otherwise the process was the same as in Example 1.

[0177] Example 8 The primary average particle size of lithium iron phosphate salt particles was adjusted to 920 nm, and the BET specific surface area was adjusted to 3.0 m². 2 / g, and a carbon content of 0.8% by weight, otherwise, it was carried out in the same manner as in Example 1.

[0178] Example 9 The primary average particle size of lithium iron phosphate salt particles was adjusted to 790 nm, and the BET specific surface area was adjusted to 8.0 m². 2 / g, and a carbon content of 1.4% by weight, otherwise, it was carried out in the same manner as in Example 1.

[0179] Example 10 The carbon content after the first sintering was adjusted to 0.05% by weight, 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 μm. 2 / g, and a carbon content of 1.2% by weight, otherwise, it was carried out in the same manner as in Example 1.

[0180] Example 11 The primary average particle size of lithium iron phosphate salt particles was adjusted to 520 nm, and the BET specific surface area was adjusted to 6.8 m². 2 / g, the carbon content after the first sintering was adjusted to 0.4% by weight, otherwise, it was carried out in the same manner as in Example 1.

[0181] Example 12 The BET specific surface area of ​​the regulated product is 7.2 m². 2 / g, and a carbon content of 1.6% by weight, otherwise, it was carried out in the same manner as in Example 1.

[0182] Example 13 The BET specific surface area of ​​the regulated product is 7.8 m². 2 / g, and a carbon content of 2% by weight, otherwise, it was carried out in the same manner as in Example 1.

[0183] Example 14 The first sintering temperature was set at 620℃, and the second sintering temperature was set at 730℃. The weight ratio of glucose to polyaniline was 1:4. The BET specific surface area of ​​the lithium iron phosphate particles was 3.0 m². 2 / g, and a carbon content of 2% by weight, otherwise, it was carried out in the same manner as in Example 1.

[0184] Example 15 The BET specific surface area of ​​lithium iron phosphate salt particles was adjusted to 8.0 m². 2 / g, and a carbon content of 0.94% by weight, otherwise, it was carried out in the same manner as in Example 1.

[0185] Example 16 The average particle size of the lithium iron phosphate salt particles was adjusted to 650 nm and the Dv50 after the first sintering was 450 nm. Otherwise, the process was the same as in Example 1.

[0186] Example 17 The primary average particle size of lithium iron phosphate salt particles was adjusted to 2500 nm, and the BET specific surface area was adjusted to 5.5 m². 2 / g, except that, the procedure is the same as in Example 1.

[0187] Comparative Example 1 The BET specific surface area of ​​lithium iron phosphate salt particles was adjusted to 2.4 m². 2 / g, and a carbon content of 0.7% by weight, otherwise, it was carried out in the same manner as in Example 1.

[0188] Comparative Example 2 The carbon content was adjusted to 1.7% by weight, resulting in a BET specific surface area of ​​12 m² for the product. 2 / g, except that, the procedure is the same as in Example 1.

[0189] Comparative Example 3 The temperature of the second sintering was adjusted to 830℃, and the BET specific surface area of ​​the product was adjusted to 9.5 m². 2 / g, except that, the procedure is the same as in Example 1.

[0190] Comparative Example 4 The primary average particle size of the controlled product was 5000 nm, and the BET specific surface area was 4 m². 2 / g, except that, the procedure is the same as in Example 1.

[0191] Comparative Example 5 The preparation process employs a single sintering step, and the BET specific surface area of ​​the product is controlled to be 12 m². 2 / g, except that, the procedure is the same as in Example 1.

[0192] Comparative Example 6 The temperature of the first sintering was controlled at 760℃, resulting in an average primary particle size of 3200 nm and a BET specific surface area of ​​4.8 m². 2 / g, except that, the procedure is the same as in Example 1.

[0193] Comparative Example 7 The carbon content of the product after the first sintering was controlled at 1.1% by weight, and the primary average particle size of the product was adjusted to 300 nm, and the BET specific surface area was adjusted to 13 m². 2 / g, except that, the procedure is the same as in Example 1.

[0194] Comparative Example 8 No carbon film-forming agent was used in the preparation process. The temperature of the first sintering was controlled at 600°C and the temperature of the second sintering was controlled at 720°C. The carbon content of the product was controlled to be 0.5%. Otherwise, the process was the same as in Example 1.

[0195] I. Determination of relevant parameters of lithium iron phosphate salt particles 1. Primary average particle size The electrode was cut open perpendicular to its large surface using an argon ion beam, exposing the cross-section. Scanning electron microscopy (SEM) images were taken of the cross-section, and the longest diameter of the lithium iron phosphate (LFP) particles was statistically analyzed using a length-diameter statistical method. The "first-order average particle size" refers to the average of the first-order particle sizes of all particles, numerically equal to the total particle size divided by the total number of particles. In the cross-sectional image, the first-order particle size is defined as the longest distance connecting two points along the edge. Specifically, the total number of LFP particles with a first-order particle size greater than 80 nm and the sum of the first-order particle sizes of these particles were counted in the SEM images. The first-order average particle size of LFP particles = total first-order particle size of LFP particles / total number of LFP particles. Particles with a first-order average particle size less than or equal to 80 nm were not included in the statistical analysis.

[0196] 2. BET specific surface area The specific surface area was tested using the gas adsorption method, according to the GB / T19587-2017 testing standard. The specific steps were as follows: Lithium iron phosphate granular salt was used as the sample, and the sample tube was immersed in liquid nitrogen at -196℃. The amount of nitrogen adsorbed on the solid surface under different pressures of 0.05-0.30 was measured. Based on the BET multilayer adsorption theory and its formula, the monolayer adsorption amount of the sample was obtained, and the specific surface area of ​​the material was calculated.

[0197] 3. Dv50 Referring to GB / T19077.1-2016, the Dv50 value of lithium iron phosphate particles was determined using a laser particle size analyzer (Malvern Master Size 3000). Furthermore, the Dv10, Dv90, and Dv99 values ​​described in this application were also determined in the same manner.

[0198] 4. Carbon content The carbon content of lithium iron phosphate salt particles was tested by infrared absorption method after combustion in a high-frequency induction furnace. The specific testing procedure was in accordance with standard GB / T 20123-2006 / ISO 15350:2000.

[0199] 5. Morphological testing Example 10 and existing lithium iron phosphate cathode materials were tested using a ZEISS Sigma 300 scanning electron microscope, and then tested according to standard JY / T010-1996. The morphology of the samples was observed, and the observation results are shown in [reference missing]. Figure 2 a, b.

[0200] II. Battery Performance Measurement 1. Battery capacity 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), and then 2.5g of organic solvent NMP (N-methylpyrrolidone) was added. After thorough mixing, the mixture was coated onto aluminum foil to form a 140-micron thick film. The film was then vacuum-dried at 120℃ for 2 hours, punched into 13mm discs, and pressed into sheets at 10MPa using a tablet press. The sheets were then vacuum-treated at 120℃ for 12 hours. The weight of the positive electrode sheet was measured, revealing an active material loading of 11-12mg. A coin cell was assembled in an argon-protected glove box, using a lithium metal sheet as the negative electrode, a 1:1 volume ratio of EC (ethylene carbonate) and DMC (1,2-dimethyl carbonate) mixed solvent, LiPF6 as the electrolyte, and a Celgard 2400 microporous polyethylene membrane as the separator. The assembled battery was then tested for electrical performance using a blue electrode tester. Within a voltage range of 2.0V to 3.75V, the specific capacity was tested by charging / discharging twice at a constant current of 0.1C, followed by charging / discharging twice at a constant current of 1C. A constant voltage period of 3.75V was maintained during the charging process, with a constant voltage cutoff current of 50μA. The specific capacity is the discharge capacity under the first 0.1C cycle.

[0201] 2. Battery capacity percentage when discharged at 1C to 3.2V (η value) The battery preparation and testing process is as follows: 2.0000g of sample is mixed with 0.1111g of conductive carbon black and 0.1111g of PVDF (at a mass ratio of 0.9:0.05:0.05), and then 2.5g of organic solvent NMP (N-methylpyrrolidone) is added. After thorough mixing, the mixture is coated onto aluminum foil to form a film with a thickness of 140 micrometers. The film is then vacuum-dried at 120℃ for 2 hours, punched into discs with a diameter of 13mm, and pressed into discs at 10MPa using a tablet press. The discs are then vacuum-insulated at 120℃ for 12 hours. The weight of the positive electrode disc is measured, and the loading of active material is 11-12mg. A coin cell is assembled in an argon-protected glove box, using lithium metal as the negative electrode, a 1:1 volume ratio of EC (ethylene carbonate) and DMC (1,2-dimethyl carbonate) mixed solvent, LiPF6 as the electrolyte, and a Celgard 2400 microporous polyethylene membrane as the separator. The assembled battery was tested for electrical performance using a Blue Battery Tester. Within a voltage range of 2.0V to 3.75V, it was charged / discharged at a constant current of 0.1C for two weeks, followed by a constant current charge / discharge of 1C for two weeks to test specific capacity. A constant voltage period of 3.75V was maintained during the charging process, with a constant voltage cutoff current of 50μA.

[0202] The capacity value extracted at a discharge voltage of 3.2V is denoted as C1, and the capacity value extracted at a discharge voltage of 2.0V is denoted as C2. η = C1 / C2. The charging process includes constant voltage charging, with a constant voltage of 3.75V and a constant voltage cutoff current of 50μA.

[0203] 3. Solid content of slurry Prepare an electronic balance (accuracy 0.0001), an oven, and a glass drying tray. Take 8-10g of the positive electrode slurry sample and evenly spread it on the sample tray. Record the slurry mass before drying as A. Close the oven door and heat. As heating progresses, the oven temperature continuously increases, reaching 130℃ for 5 hours. After drying, remove the sample from the oven and record the mass of the dried slurry. Repeat the drying process multiple times until the sample reaches a constant weight, recording the mass after drying as B. The slurry solid content = (A / B) × 100%.

[0204] 4. Magnetic substance content 1) Weigh 1kg of sample and put it into a plastic bucket. Add 6L of deionized water. Use a plastic tube to cover a magnetic rod with a diameter of 24mm and a length of 240mm (magnetic field strength of 6000 Gauss). Then use a heat-sealing clamp to heat-seal the magnetic rod into the plastic bucket and seal it together. Stir at a speed of 60 rpm for 15 minutes. 2) Prepare another clean bucket, add 5±0.2L of deionized water to the bucket, rinse 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. 3) Prepare a clean 1L beaker, rinse all the magnetic material on the plastic tube into the beaker, use a magnetic block to hold the bottom of the beaker, and rinse 1-2 times. 4) Measure 10 mL of deionized water using a graduated cylinder 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 place it in an ultrasonic instrument for sonication for 2 minutes. 5) After the ultrasound is completed, remove the beaker and use a magnetic block to attract and gather the magnetic material on the bottom of the beaker. Pour the acid in the beaker into the waste liquid bucket. Rinse the magnetic material in the beaker 3 times and add an appropriate amount of deionized water for filtration.

[0205] 6) Using a filter membrane with a pore size of 0.45 micrometers, after filtration, place the filter paper with magnetic particles on its surface on a slide of a cleanliness microscope and put it in an oven to dry at 60℃ for (10±2) min. After drying, weigh the filter membrane and calculate the content of magnetic material.

[0206] The higher the content of magnetic materials, the more severe the self-discharge phenomenon of the cell and the shorter the battery life.

[0207] 5. Determination of dynamic performance In this application, the dynamic performance of the battery is specifically reflected in the measurement of DC internal resistance (power performance).

[0208] 25℃ Power Performance Test: Capacity calibration: The lithium-ion batteries prepared in each example 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 3.65V to 0.05C. After charging, the batteries were left to stand 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. Adjusting SOC (State of Charge): After keeping the calibrated lithium-ion battery at 25°C for 2 hours, discharge it at 1 / 3C0 discharge rate for 144 minutes to adjust the lithium-ion battery capacity to 10% SOC. Power test: After the lithium-ion battery with 10% SOC is left to stand at 25℃ for 2 hours, it is discharged for 30 seconds at a discharge rate of 3C0 under pulse current I. The voltage before 3C0 discharge is recorded as V1 and the voltage at the end of 30 seconds of discharge is recorded as V2. The value of DC internal resistance (V1-V2) / I is calculated. This data can characterize the power performance of the battery.

[0209] III. Parameters and Performance in Examples and Comparative Examples Batteries for each embodiment and comparative example were prepared according to the above method, and various performance parameters were measured, as shown in Table 1.

[0210] Table 1

[0211] As shown in Table 1, the primary average particle size, BET specific surface area, and carbon content of the lithium iron phosphate particles obtained in Examples 1-17 of this application all meet the requirements of this application. As a result, their η values ​​are all greater than or equal to 88%, indicating strong battery discharge power performance. Even when the battery is discharged to a low SOC (State of Charge), it can still maintain good power performance, meaning that the voltage drop is small when the battery is discharged at a high current at low charge levels. Furthermore, the low kinetic performance values ​​of the lithium iron phosphate particles in the examples indicate excellent battery mechanical properties. The high specific capacity of the lithium iron phosphate particles, the high solids content of the slurry, and the low magnetic material content indicate excellent energy density and lifespan characteristics of the battery.

[0212] Furthermore, as shown in Table 1, in Comparative Examples 1-8, regardless of whether the parameter characteristics of the lithium iron phosphate particles themselves were changed or the parameter characteristics in their preparation process were changed, the resulting lithium iron phosphate particles did not meet the requirements of this application. The size characteristics, kinetic performance, specific capacity, solid content of the slurry, or magnetic material content of the resulting lithium iron phosphate particles were significantly reduced, which obviously could not meet the requirements of this application.

[0213] It should be noted that this application is not limited to the described embodiments. The described embodiments are merely examples, and any embodiments with the same structure and effect as the technical concept within the scope of this application are included in the technical scope of this application. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, are also included in the scope of this application without departing from the spirit of this application.

Claims

1. A secondary battery, characterized in that, The device includes a positive electrode sheet, which comprises a positive current collector and a positive electrode film layer disposed on at least one side of the positive current collector. The positive electrode film layer comprises a positive electrode material, which comprises lithium iron phosphate particles. The primary average particle size of the lithium iron phosphate particles is 500 nm-3000 nm, and the BET specific surface area is 3 m². 2 / g-8m 2 / g, calculated based on the total weight of the lithium iron phosphate particles, the carbon content of the lithium iron phosphate particles is Cx by weight%, where 0.8≤Cx≤2.

0.

2. The secondary battery according to claim 1, characterized in that, The primary average particle size of the lithium iron phosphate salt particles is 650 nm-2500 nm; and / or, The BET specific surface area of ​​the lithium iron phosphate particles is 4 m². 2 / g-7m 2 / g; and / or, Based on the total weight of the lithium iron phosphate particles, the carbon content of the lithium iron phosphate particles is calculated to be Cx% by weight, where 1.0 ≤ Cx ≤ 1.

6.

3. The secondary battery according to claim 1 or 2, characterized in that, The lithium iron phosphate particles have the molecular formula Li m Fe x P y O j Q q 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, and Br, with 0.95≤m≤1.15, 0.9≤x≤1, 0.95≤y≤1, 3.5≤j≤4, and 0<q≤0.

1.

4. The secondary battery 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 1000ppm-10000ppm.

5. The secondary battery according to any one of claims 1 to 4, characterized in that, The lithium iron phosphate salt particles are monocrystalline particles and / or polycrystalline particles.

6. The secondary battery according to any one of claims 1 to 5, characterized in that, The capacity ratio of the lithium iron phosphate particles η ≥ 88%, where η is defined as follows: A battery containing the lithium iron phosphate particles as the positive electrode material is charged and discharged twice at a constant current rate of 0.1C within a voltage range of 2.0V to 3.75V, followed by a constant current charge and discharge once at a constant current rate of 1C. In the 1C charge and discharge test, the capacity value extracted at a discharge voltage of 3.2V is recorded as C1, and the capacity value extracted at a discharge voltage of 2.0V is recorded as C2. η = C1 / C2. The charging process includes constant voltage charging at a constant voltage of 3.75V and a constant voltage cutoff current of 50μA.

7. The secondary battery 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 is ≥ 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 is ≤10μm; d) The Dv99 of the lithium iron phosphate salt particles is ≤12μm; e) The compacted density of the lithium iron phosphate powder under a pressure of 3 tons is ≥2.25 g / cm³. 3 ; f) The resistivity of the lithium iron phosphate powder is less than 60 Ω·cm.

8. A method for preparing a secondary battery, characterized in that, This includes the preparation of lithium iron phosphate salt particles. The preparation of lithium iron phosphate particles specifically 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 sintering processes, wherein... The first sintering temperature is 500℃-760℃, and the carbon content of the material after the first sintering is 0.01%-0.79% by weight. The second sintering temperature is 700℃-800℃, and the carbon content of the material after the second sintering is 0.8%-2.0% by weight.

9. The method for preparing a secondary battery according to claim 8, characterized in that, The lithium iron phosphate particles contain at least one of the elements Ti, V, Mg, and / or Nb, and the content of the element is 1000ppm-10000ppm based on the total weight of the lithium iron phosphate particles.

10. The method for preparing a secondary battery according to claim 8 or 9, characterized in that, After the first sintering, a first pulverization is performed; after the second sintering, a second pulverization is performed. The Dv50 of the product after the first pulverization is 300nm-1200nm; The Dv50 of the product after the second pulverization is 500nm-5000nm.

11. The method for preparing a secondary battery according to any one of claims 8 to 10, characterized in that, include: The raw materials provided contain at least a lithium source, an iron source, a phosphorus source, a carbon source, a carbon film-forming agent, and a modifier. The mixing ratio of the lithium source, iron source, and phosphorus source, based on the atomic molar number of each element, satisfies Fe∶P=0.96-0.985 and Li∶Fe=1.0-1.1∶0.95-1.

1. The carbon source and 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 a secondary battery 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 ferric hydroxide, ferrous chloride, ferric oxide, ferric phosphate, ferric pyrophosphate, ferrous oxalate, iron powder, ferric nitrate, iron(II) oxide, and ferric hydroxide. 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 method for preparing a secondary battery according to any one of claims 8 to 12, characterized in that, The heating rates in the first and second sintering processes are each independently 2℃ / min-20℃ / min; the holding time for the first sintering is 1h-6h; and the holding time for the second sintering is 2h-12h.

14. The method for preparing a secondary battery according to claims 8 to 13, characterized in that, In the second sintering, the product after the first sintering is carbon-coated, and the amount of carbon coating is 0.01% to 1.99% by weight based on the total weight of the product after the first sintering.

15. The method for preparing a secondary battery according to claim 14, characterized in that, The carbon coating is achieved through vapor deposition during sintering or by carbonizing the carbon source and coating it with carbon at high temperatures.

16. The method for preparing a secondary battery according to any one of claims 10 to 15, characterized in that, The pulverization can be selected from one or more of grinding, sand milling, mechanical crushing, and air jet crushing.

17. An electrical appliance, characterized in that, The battery cell comprising any one of claims 1-7 and / or the secondary battery prepared by the preparation method according to any one of claims 8-16.

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