Lithium iron phosphate salt particles, method of manufacturing the same, positive electrode sheet, secondary battery, and electric device

Abstract

The present application provides a large particle low specific surface high kinetics lithium iron phosphate salt particle, the primary average particle size of the lithium iron phosphate salt particle is 500nm-3000nm, the BET specific surface area is 3m 2 / g-8m 2 / g, the carbon content of the lithium iron phosphate salt particle is Cxwt%, wherein 0.8Cx≤2.0, calculated based on the total weight of the lithium iron phosphate salt particle.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of secondary batteries, in particular to a lithium iron phosphate salt particle, a preparation method thereof, a positive electrode sheet, a secondary battery and an electric device. BACKGROUND

[0002] As a positive electrode material of lithium ion battery, lithium iron phosphate has rapidly become a global research hotspot due to its abundant resources, low price, environmental friendliness and stable voltage of two-phase reaction. The cation arrangement in lithium iron phosphate is different from that of layered ternary materials and spinel LiMn2O4. Fe 2+ is located at the 4c position of the oxygen octahedron, and Li + is located at the 4a position of the oxygen octahedron. The polyanion structure is stable, and has a high thermal decomposition temperature and good thermal stability. However, the octahedral structure FeO6 is separated by the O atom in the tetrahedral structure and is interrupted by the phosphorus oxygen tetrahedron, and a continuous FeO6 network is not formed, resulting in poor electronic conductivity of lithium iron phosphate.

[0003] The industry usually adopts means such as doping, coating and particle nanocrystallization to improve the electronic conductivity of lithium iron phosphate. At low temperature and high rate, the problems of electronic conductivity and ionic conductivity will be more prominent, and the material must be fully nanocrystallized. However, the nanocrystallized material has a high specific surface energy, which will bring a series of processing problems, such as gel, fast water absorption, difficulty in drying, low solid content of coating slurry and the like. How to improve the particle size and reduce the BET under the premise of ensuring the kinetic performance, so as to reduce the specific surface energy of the material, is a very meaningful engineering and technical progress. SUMMARY

[0004] The present application is carried out in view of the above-mentioned problems, and aims to provide a lithium iron phosphate salt particle, a preparation method thereof, a positive electrode sheet, a secondary battery and an electric device, which has a large particle size and also has excellent kinetic performance when used as a positive electrode material.

[0005] The first aspect of the present application provides a lithium iron phosphate salt particle, wherein the primary average particle size of the lithium iron phosphate salt particle is 500 nm to 3000 nm, and optionally 650 nm to 2500 nm, the BET specific surface area is 3 m 2 / g to 8 m 2 / g, and optionally 4 m 2 / g to 7 m 2 / g, the carbon content of the lithium iron phosphate salt particle is Cx wt.%, wherein 0.8≤Cx≤2.0, and 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, and optionally 3≤z≤6.

[0006] By making the primary average particle size of the lithium iron phosphate salt particles of the embodiments of the present application 500 nm to 3000 nm, the BET specific surface area 3 m 2 / g to 8 m 2 / g, the carbon content calculated based on the total weight of the lithium iron phosphate salt particles is Cx wt% (0.8≤Cx≤2.0), and further optionally, the ratio z of the BET specific surface area of the lithium iron phosphate salt particles to Cx satisfies the range of 1.5≤z≤8.5, the kinetic performance of the lithium iron phosphate salt particles with the primary average particle size in the above range can be improved; the particles can maintain a suitable micron-level size, thereby avoiding the problems of interface side reactions and processing difficulties caused by the nanometerization of the particles, and the kinetic performance caused by the particles being limited to an excessively large size is not reduced; in addition, the stirring of the slurry containing the lithium iron phosphate salt particles and the improvement of the solid content are facilitated, thereby improving the processing problems of the battery cell and further improving the volumetric energy density of the battery; in addition, the normal deintercalation of lithium ions can be avoided due to the excessively high density caused by carbon coating, thereby affecting the capacity of the battery cell.

[0007] Compared with the prior art, the lithium iron phosphate salt particles in the embodiments of the present application have a smaller specific surface area with the same content of carbon, which means that the particles contain less floating carbon, the carbon contained in the lithium iron phosphate salt particles is more uniform and dense, and each particle is coated, thereby improving the surface conductivity of the particles and improving the kinetic performance of the particles as a positive electrode material.

[0008] In any embodiment, the lithium iron phosphate salt has a molecular formula Li m Fe x P y O j Q q , wherein Q includes at least one of Al, Na, K, Mg, Cu, Mn, Cr, Zn, Pb, Ca, Co, Ni, Sr, Nb, V, Ti, B, Si, N, S, F, Cl, Br, 0.95≤m≤1.15, 0.9≤x≤1, 0.95≤y≤1, 3.5≤j≤4, 0

[0009] By making the lithium iron phosphate salt of the embodiments of the present application have the above molecular formula and being doped with one or more of the above elements, the high load phase modification of the lithium iron phosphate salt particles is realized, which helps to improve the bulk ion transport capacity of the lithium iron phosphate salt particles, and effectively solves the problem of poor kinetic performance inherent in large particles. In addition, by doping, the lithium iron phosphate salt particles can exhibit good kinetic performance when used as a positive electrode material. In the present application, the modification can specifically be doping and / or coating.

[0010] In any embodiment, the Q comprises at least one of Ti, V, Mg, Nb, and optionally Ti, and the content of Ti, V, Mg and / or Nb is 1000-10000 ppm, and optionally 2500-6000 ppm, based on the total weight of the lithium iron phosphate salt particles.

[0011] By doping one or more of the above elements in the lithium iron phosphate salt of the embodiments of the present application and allowing the content to be within the above range, the metal body phase modification of the lithium iron phosphate salt particles can be better achieved under the premise of improving the primary average particle size of the lithium iron phosphate salt particles, further improving the body phase ion transport capability of the lithium iron phosphate salt particles, and better solving the problem of poor inherent kinetic performance of large particles.

[0012] In any embodiment, the lithium iron phosphate salt particles are single crystal particles and / or polycrystal particles, and optionally, the number of single crystal particles accounts for more than 90% based on the total number of lithium iron phosphate salt particles.

[0013] In the lithium iron phosphate salt particles of the embodiments of the present application, the carbon can be in a mixed state with the lithium iron phosphate salt particles or coated on the lithium iron phosphate salt particles. Optionally, the carbon in the lithium iron phosphate salt particles is in a state of being coated on the lithium iron phosphate salt particles, so that the carbon can form a uniform and dense carbon coating layer on the surface of the lithium iron phosphate salt particles, thereby improving the surface conductivity of the particles.

[0014] In any embodiment, the capacity ratio η of the lithium iron phosphate salt particles is ≥ 88%, and η is defined as follows: a battery with the lithium iron phosphate salt particles as the positive electrode material is subjected to constant current charging and discharging twice at a rate of 0.1C in a voltage range of 2.0-3.75V, and then subjected to constant current charging and discharging once at a rate of 1C, in the charging and discharging test at a rate of 1C, the capacity value at a discharge voltage of 3.2V is recorded as C1, and the capacity value at a discharge voltage of 2.0V is recorded as C2, and η = C1 / C2, wherein the charging process includes constant voltage charging, constant voltage 3.75V, and constant voltage cutoff current 50 μA.

[0015] η represents the platform retention performance of the material, which has a strong correlation with the discharge power performance of the battery. When the value is large, the battery can still maintain good power performance when discharged to a low SOC (state of charge of the battery), i.e., the voltage drop of the battery is small when the battery is discharged at a large current at a low capacity. The capacity ratio η of the lithium iron phosphate salt particles of the embodiments of the present application is ≥ 88%, which means that the lithium iron phosphate salt particles of the present application can make the secondary battery exhibit good kinetic performance when used as the positive electrode material.

[0016] In any embodiment, the lithium iron phosphate salt particles satisfy at least one of a)-f):

[0017] a) the Dv10 of the lithium iron phosphate salt particles is ≥0.2 μm;

[0018] b) the Dv50 of the lithium iron phosphate salt particles is 0.5-5 μm;

[0019] c) the Dv90 of the lithium iron phosphate salt particles is ≤10 μm;

[0020] d) the Dv99 of the lithium iron phosphate salt particles is ≤12 μm;

[0021] e) the powder compaction density of the lithium iron phosphate salt under a pressure of 3T is ≥2.25 g / cm 3 ;

[0022] f) the powder resistivity of the lithium iron phosphate salt is less than 60 Ω·cm.

[0023] In the present application, the above-mentioned Dv10, Dv90 and Dv99 refer to the particle size corresponding to the cumulative volume percentage of 10%, 90% and 99% in the particles. The test method is the same as that of the above-mentioned Dv50, and can be determined by using the standard in the embodiments of the present application.

[0024] It should be noted that the Dv10 is a large index and thus cannot be given an upper limit, and the Dv90 is a small index and thus cannot be given a lower limit.

[0025] By making the lithium iron phosphate salt particles satisfy at least one of a)-f), the lithium iron phosphate salt particles can better achieve the above-mentioned technical effects.

[0026] In the present application, the test method of the powder resistivity is as follows: referring to the national standard GB / T33822-2017, using a powder resistivity meter (Suzhou Crystal, ST2722 type), weighing 1 g of sample (with an error of less than ±0.005 g), adding it into the charging cavity, applying a pressure of 8 MPa, respectively testing the positive resistivity and the reverse resistivity of the sample, and taking the average value of the two as the powder resistivity of the sample.

[0027] In the present application, the definition of the powder compaction density is as follows: in the process of external force compression, with the movement and deformation of the powder, larger voids are filled, the contact area between particles increases, the interatomic attractive force is generated and the mechanical bonding effect between particles is enhanced, thereby forming a compaction with a certain density and strength, and the unit is g / cm 3 .

[0028] The test method of the compacted density is as follows: referring to the national standard GB / T 24533-2009, a certain amount of powder is placed on a compaction mold, and a metal disc is placed on the upper and lower hollow parts of the mold. The powder is placed between the metal discs, and a metal cylinder is placed on the top. The mold is placed on the compaction density instrument, different pressures are set, and the thickness of the powder under different pressures can be read on the equipment. The compacted density is calculated by ρ = m / v.

[0029] According to the following powder compacted density calculation results, ρc = m / V = m / (S x H); where ρc is the powder compacted density (g / cm 3 ), m is the mass of the material (g), S is the bottom area of the mold (1.327 cm 2 ), and H is the height of the compacted sample (cm).

[0030] The second aspect of the present application provides a preparation method of 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 process is 500-760°C, and can be selected as 550-720°C; the carbon content of the material after the first sintering process is 0.01-0.79 wt%, and can be selected as 0.05-0.4 wt%; the temperature of the second sintering process is 700-800°C, and can be selected as 720-780°C; and the carbon content of the material after the second sintering process is 0.8-2.0 wt%, and can be selected as 1.0-1.6 wt%.

[0031] In the preparation method of the lithium iron phosphate salt particles in the embodiments of the present application, two sintering processes are implemented. By controlling the temperature of the first sintering within the above range and making the carbon content of the intermediate after sintering within the above range, the lithium iron phosphate precursor obtained after the first sintering has a larger particle size, directly improving the powder compaction and the electrode sheet compaction density of the final product. In addition, by adding a lower content of carbon source during the first sintering, the barrier effect of the carbon layer on the growth of the lithium iron phosphate particles is greatly reduced, which is conducive to the crystallization growth of the particles at a lower temperature and the solid-phase diffusion reaction between the modifier and the lithium iron phosphate material, thereby realizing a higher concentration of metal ion modification. Compared with the traditional method of realizing particle growth at a high temperature, the above preparation method of the present application can synthesize large particles at a lower temperature, which can improve the phenomenon of cracking of the carbon layer on the particle surface at a high temperature and improve the density of the surface carbon. At the same time, during the first sintering process, the carbon source can effectively reduce the trivalent iron in the raw material, thereby improving the purity and stability of the product. By controlling the temperature of the second sintering within the above range and making the carbon content of the sintered material 0.8 wt% to 2.0 wt%, the carbon can be coated on the surface of the lithium iron phosphate salt particles, forming a lithium iron phosphate material with a uniform and dense carbon coating layer that covers each particle, thereby greatly improving the electrical conductivity of the particle surface.

[0032] In any embodiment, the lithium iron phosphate salt particles contain at least one of Ti, V, Mg, and / or Nb, and the content of the element is 1000 ppm to 10000 ppm, and optionally 2500 ppm to 6000 ppm, based on the total weight of the lithium iron phosphate salt particles.

[0033] In some embodiments, the content of the element is optionally 2500 ppm to 6000 ppm. Specifically, it can be 2500 ppm, 2600 ppm, 2700 ppm, 2800 ppm, 2900 ppm, 3000 ppm, 3100 ppm, 3200 ppm, 3300 ppm, 3400 ppm, 3500 ppm, 3600 ppm, 3700 ppm, 3800 ppm, 3900 ppm, 4000 ppm, 4100 ppm, 4200 ppm, 4300 ppm, 4400 ppm, 4500 ppm, 4600 ppm, 4700 ppm, 4800 ppm, 4900 ppm, 5000 ppm, 5100 ppm, 5200 ppm, 5300 ppm, 5400 ppm, 5500 ppm, 5600 ppm, 5700 ppm, 5800 ppm, 5900 ppm, or 6000 ppm, or a range between any two of the above values.

[0034] By doping one or more of the above elements in the lithium iron phosphate salt of the embodiments of the present application and making the content thereof within the above range, the metal body phase modification of the lithium iron phosphate salt particles can be better achieved under the premise of improving the primary average particle size of the lithium iron phosphate salt particles, further improving the body phase ion transport capacity of the lithium iron phosphate salt particles, and better solving the problem of poor kinetic performance inherent in large particles.

[0035] In any embodiment, the preparation method of the lithium iron phosphate salt particles includes performing first crushing after the first sintering and performing second crushing after the second sintering, wherein the Dv50 of the product after the first crushing is 300 nm-1200 nm, and optionally 400 nm-1100 nm; and the Dv50 of the product after the second crushing is 500 nm-5000 nm, and optionally 300 nm-2500 nm.

[0036] In the preparation method of the lithium iron phosphate salt particles of the embodiments of the present application, crushing is performed after the two sinterings. By performing first crushing after the first sintering, the Dv50 of the product is 300 nm-1200 nm, which can avoid the growth blocking of carbon materials and modification elements on the crystal, and micron-sized lithium iron phosphate precursors are obtained; by performing second crushing after the second sintering, the Dv50 of the product is 500 nm-3000 nm, which can obtain lithium iron phosphate salt particles with desired particle size, obtain a uniform and dense carbon coating layer that coats each particle, and greatly improve the surface conductivity of the particles.

[0037] In any embodiment, the preparation method of 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, the iron source, and the phosphorus source satisfies Fe:P=0.96-0.985 and Li:Fe=1.0-1.1:0.95-1.1 in terms of the atomic mole number of each element; and the carbon source and the carbon film-forming agent satisfy carbon source:carbon film-forming agent=9:1-2:8 in terms of weight ratio.

[0038] By using the raw materials containing the lithium source, the iron source, the phosphorus source, the carbon source, the modifier, and the carbon film forming agent in the above-mentioned proportions, good modification can be achieved, and the obtained lithium iron phosphate salt particle precursor is formed into a particle with a large size, so that the primary average particle diameter, the carbon content, the BET specific surface area, and the z value of the lithium iron phosphate salt particle of the first aspect of the present application can be formed well. Specifically, as described above, by setting the primary average particle diameter of the lithium iron phosphate salt particle of the embodiment of the present application to 500 nm to 3000 nm, the particle can be kept at a suitable micron level size, so that the problems of interface side reactions and processing difficulties caused by the nanometerization of the particle can be avoided (in the art, there are difficulties in processing the nanometerized particle), and the particle is not limited to a size that causes a decrease in kinetic performance. In addition, by setting the BET specific surface area of the lithium iron phosphate salt particle of the embodiment of the present application to 3 m 2 / g to 8 m 2 / g, the stirring and the solid content of the slurry containing the lithium iron phosphate salt particle can be improved, so that the processing problem of the battery cell can be improved, and the volumetric energy density of the battery can be improved. Further optionally, by setting the ratio z of the BET specific surface area of the lithium iron phosphate salt particle of the embodiment of the present application to Cx to satisfy the range of 1.5 ≤ z ≤ 8.5, the carbon contained in the lithium iron phosphate salt particle can be more uniform and dense, and each particle can be coated, so that the surface conductivity of the particle can be improved.

[0039] In any embodiment, in the preparation method of the lithium iron phosphate salt particle, 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 is optionally lithium carbonate; the iron source is an iron compound, including at least one of iron hydroxide, ferrous chloride, diiron trioxide, iron phosphate, iron pyrophosphate, ferrous oxalate, iron powder, ferric nitrate, magnetite, and hydroxyl ferric oxide, and is optionally diiron trioxide; the phosphorus source is a phosphoric acid compound, including one or more of phosphoric acid, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate, and is optionally phosphoric acid; the modifier includes at least one of titanium dioxide, vanadium pentoxide, n-butyl titanate, ammonium metavanadate, niobium ethoxide, niobium oxalate, niobium pentoxide, magnesium hydroxide, and magnesium nitrate, and is optionally titanium dioxide; the carbon source includes at least one of citric acid, glucose, sucrose, starch, fructose, and lactose, and is optionally glucose; and the carbon film forming agent includes one or a combination of several of polyethylene glycol, polyaniline, polyacrylonitrile, polyvinylpyrrolidone, and polyvinyl alcohol, and is optionally polyaniline.

[0040] By selecting the above-mentioned substances as the lithium source, iron source, phosphorus source, modifier, carbon source, and carbon film forming agent used in the preparation method of the lithium iron phosphate salt particles of the embodiments of the present application, the primary average particle size, carbon content, BET specific surface area, and z value of the lithium iron phosphate salt particles of the first aspect of the present application can be well formed. Thus, the large particle size of the lithium iron phosphate salt particles of the embodiments of the present application is more favorably achieved, and excellent kinetic performance is achieved when the lithium iron phosphate salt particles are used as a positive electrode material.

[0041] In any of the embodiments, the heating rate in the first and second sintering is each independently 2℃ / min-20℃ / min, the first sintering constant temperature time is 1h-6h; the second sintering constant temperature time is 2h-12h.

[0042] By controlling the heating rate and constant temperature time in the first and second sintering each independently within the above-mentioned range, the excessive side reactions in the sintering process caused by too fast heating can be prevented, thereby affecting the primary average particle size, carbon content, BET specific surface area, and z value of the obtained lithium iron phosphate salt particles. Thus, the large particle size of the lithium iron phosphate salt particles of the embodiments of the present application is more favorably achieved, and excellent kinetic performance is achieved when the lithium iron phosphate salt particles are used as a positive electrode material.

[0043] In any of the embodiments, after the first sintering and before the second sintering, the product after the first sintering is carbon-coated, and the amount of the carbon coating is 0.01wt%-1.99wt%, and can be 0.2wt%-1.6wt% based on the total weight of the product after the first sintering.

[0044] By carbon-coating the product after the first sintering with the above-mentioned amount of carbon before the second sintering, a uniform and dense carbon coating layer can be formed on the surface of the lithium iron phosphate salt particles during the second sintering, and each particle is coated.

[0045] In any of the embodiments, the carbon coating is performed by vapor deposition during sintering or by carbonizing a carbon source at high temperature.

[0046] In the preparation method of the lithium iron phosphate salt particles of the embodiments of the present application, the embodiment of the carbon coating is not particularly limited, and the lithium iron phosphate salt particles can be carbon-coated by vapor deposition, thereby forming a uniform and dense carbon coating layer on the surface of the lithium iron phosphate salt particles, and each particle is coated.

[0047] In any of the embodiments, after the first sintering and after the second sintering, the sintered product is crushed, and the crushing can be one or more of grinding, sanding, mechanical crushing, and airflow crushing.

[0048] In the production method of the lithium iron phosphate salt particle of the embodiment of the present application, the pulverization method is not particularly limited, and by using the specific pulverization method described above, it is possible to advantageously obtain the desired primary average particle diameter.

[0049] The third aspect of the present application provides a positive electrode tab including the lithium iron phosphate salt particle of the first aspect of the present application as a positive electrode material.

[0050] The fourth aspect of the present application provides a secondary battery including the positive electrode tab of the third aspect of the present application.

[0051] The fifth aspect of the present application provides an electric device including the secondary battery of the fourth aspect of the present application. BRIEF DESCRIPTION OF DRAWINGS

[0052] Figure 1 is a SEM and TEM image of the lithium iron phosphate salt particle of an embodiment of the present application, wherein, Figure 1 a corresponds to an image observed under SEM (scanning electron microscope) of the lithium iron phosphate salt particle of an embodiment of the present application; Figure 1 b and Figure 1 c corresponds to an image observed under TEM (transmission electron microscope) of the lithium iron phosphate salt particle of an embodiment of the present application.

[0053] Figure 2 is a SEM image of the lithium iron phosphate salt particle of an embodiment of the present application and an existing lithium iron phosphate salt particle, wherein, Figure 2 a is a SEM image of Example 10; Figure 2 b is a SEM image of an existing lithium iron phosphate salt particle.

[0054] Figure 3 is a schematic diagram exemplarily showing the particle diameter of the primary particle of the present application.

[0055] Figure 4 is a capacity performance diagram of the lithium iron phosphate salt particle of an embodiment of the present application at 0.1 C, 1 C rate.

[0056] Figure 5 is a schematic diagram of the secondary battery of an embodiment of the present application.

[0057] Figure 6 is Figure 5 is an exploded view of the secondary battery of an embodiment of the present application.

[0058] Figure 7 is a schematic diagram of the battery module of an embodiment of the present application.

[0059] Figure 8 is a schematic diagram of the battery pack of an embodiment of the present application.

[0060] Figure 9 is Figure 8 is a disassembled view of a battery pack according to an embodiment of the present application.

[0061] Figure 10 is a schematic view of a power consuming device using a secondary battery as a power source according to an embodiment of the present application.

[0062] BRIEF DESCRIPTION OF DRAWINGS

[0063] 1 battery pack; 2 upper case; 3 lower case; 4 battery module; 5 secondary battery; 51 case; 52 electrode assembly; 53 cover plate. DETAILED DESCRIPTION

[0064] Hereinafter, embodiments of a lithium iron phosphate salt particle, a method of manufacturing the same, a positive electrode sheet, a secondary battery, and a power consuming device according to the present application will be described in detail. However, there can be cases where unnecessary detailed descriptions are omitted. For example, there can be cases where detailed descriptions of matters well known in the art, repetitive descriptions of substantially identical structures are omitted. This is to avoid the following description from becoming unnecessarily lengthy and to facilitate understanding by those skilled in the art. In addition, the drawings and the following description are provided to enable those skilled in the art to fully understand the present application and are not intended to limit the subject matter recited in the claims.

[0065] The ranges disclosed herein are defined by their lower and upper limits. Ranges created by the upper and lower limits are inclusive of the endpoints. Ranges created by the upper and lower limits are also inclusive of any and all sub-ranges subsumed therein. For example, a range of "60% to 120%" or "60% to 120%" is inclusive of from 60% to 120% and all percentages subsumed therein, e.g., from 65% to 115% or from 75% to 105%, and so on. A range created by the upper and lower limits is also inclusive of the endpoints. For example, a range of "60 to 120" is inclusive of 60 and 120. A range created by the upper and lower limits is also inclusive of any and all sub-ranges subsumed therein. For example, a range of "60 to 120" is inclusive of the sub-ranges "60 to 120", "75 to 105", "60 to 105", "60 to 105", "65 to 125", "85 to 100", and so on. In the present application, unless otherwise indicated, a numerical range "a to b" means a range of any real combination of "a to b" in which "a" and "b" are both real numbers. For example, a numerical range "0 to 5" means all real numbers between "0 to 5" have been listed herein, and "0 to 5" is just a shorthand notation for these numerical combinations. In addition, when a parameter is stated to be an integer ≥ 2, it is equivalent to disclose that the parameter is, for example, an integer of 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, and so on.

[0066] Unless otherwise specified, all embodiments and optional embodiments of the present application can be combined with each other to form new technical solutions.

[0067] If not particularly specified, all the technical features of the present application and the optional technical features can be combined with each other to form new technical solutions.

[0068] If not particularly specified, all the steps of the present application can be performed in sequence or randomly, and optionally in sequence. For example, the method comprises steps (a) and (b), which means that the method can comprise steps (a) and (b) performed in sequence, or steps (b) and (a) performed in sequence. For example, the method mentioned can further comprise step (c), which means that step (c) can be added to the method in any order, for example, the method can comprise steps (a), (b) and (c), or steps (a), (c) and (b), or steps (c), (a) and (b), etc.

[0069] If not particularly specified, the "comprise" and "include" mentioned in the present application are open-ended, and can also be closed. For example, the "comprise" and "include" can mean that other components not listed can also be included or contained, or only the listed components can be included or contained.

[0070] If not particularly specified, in the present application, the term "or" is inclusive. For example, the phrase "A or B" means "A, B, or both A and B". More specifically, either of the following conditions satisfies the condition "A or B": 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 A and B are both true (or exist).

[0071] Lithium iron phosphate as a positive electrode material of a secondary battery has the problem of poor electrical conductivity. In response to this, the industry usually uses means such as doping, coating and particle nanocrystallization to improve the electrical conductivity of lithium iron phosphate.

[0072] However, the carbon coating and nanocrystallization methods commonly used in the industry introduce a loose carbon layer while reducing the average particle size, which significantly increases the specific surface area of the material, resulting in a series of processing and manufacturing problems during processing and use, increasing production manufacturing costs and reducing production yield. For example, slurry gel, electrode sheet coating cracking, powder water absorption when exposed to air, film area powder falling during slitting, small particles falling during cold pressing, etc. This series of processing problems can affect the service life of the battery.

[0073] Therefore, it is of great significance to the industrialization to prepare large-particle lithium iron phosphate positive electrode materials while improving the kinetic performance of the materials. The following will be described in detail.

[0074] [Particles of lithium iron phosphate salt]

[0075] The primary average particle size of the lithium iron phosphate salt particles of the embodiments of the present application is 500 nm to 3000 nm, optionally 650 nm to 2500 nm, and the BET specific surface area is 3 m 2 / g to 8 m 2 / g, optionally 4 m 2 / g to 7 m 2 / g, and the carbon content of the lithium iron phosphate salt particles is Cx wt.%, wherein 0.8≤Cx≤2.0, optionally 1.0≤Cx≤1.6, based on the total weight of the lithium iron phosphate salt particles.

[0076] In some embodiments, the primary average particle size of the lithium iron phosphate salt particles is optionally 500 nm, 600 nm, 650 nm, 700 nm, 790 nm, 800 nm, 870 nm, 900 nm, 920 nm, 1000 nm, 1100 nm, 1200 nm, 1250 nm, 1300 nm, 1400 nm, 1500 nm, 1600 nm, 1700 nm, 1800 nm, 1900 nm, 2000 nm, 2100 nm, 2200 nm, 2300 nm, 2400 nm, 2500 nm, 2600 nm, 2700 nm, 2800 nm, 2900 nm, or 3000 nm, or a range between any two of the aforementioned values. In some embodiments, the BET specific surface area of the lithium iron phosphate salt particles is optionally 3 m 2 / g, 4 m 2 / g, 5 m 2 / g, 6 m 2 / g, 7 m 2 / g, 7.2 m 2 / g, 7.8 m 2 / g, or 8 m 2 / g, or a range between any two of the aforementioned values. In some embodiments, the carbon content of the lithium iron phosphate salt particles is optionally 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 aforementioned values.

[0077] In this context, the term “primary average particle size” refers to the particle size value obtained by statistically analyzing the particle size of the particles in a scanning electron microscope field of view using a long diameter statistical method. In the particle size statistical process, particles with a primary average particle size less than or equal to 80 nm are not included in the statistical range. The primary average particle size of the present application is as shown in Figure 3The primary average particle diameter refers to the average particle diameter of primary particles. In the present specification, "primary particles" refer to particles having no apparent agglomeration interface in a scanning electron microscope image of the particles, but can have minute pores and point or line defects, as distinguished from powder particles that are the smallest unit having no structure such as accumulation and flocculation. The primary average particle diameter, the carbon content, and the BET specific surface area can be measured using the measurement methods described in the examples.

[0078] In the embodiments of the present application, by setting the primary average particle diameter of the lithium iron phosphate salt particles to 500 nm to 3000 nm, the BET specific surface area to 3 m 2 / g to 8 m 2 / g, and the carbon content calculated based on the total weight of the lithium iron phosphate salt particles to Cx wt% (0.8 ≤ Cx ≤ 2.0), the kinetic performance of the lithium iron phosphate salt particles having the primary average particle diameter in the above range can be improved; the particles can maintain a suitable micron-level size, thereby avoiding problems such as interface side reactions and processing difficulties that can occur when the particles are nano-sized, and kinetic performance reduction due to the particles being limited to an excessively large size; in addition, the stirring of a slurry containing the lithium iron phosphate salt particles and the improvement of the solid content can be facilitated, thereby improving the processing of the battery cell and increasing the volumetric energy density of the battery; in addition, the normal deintercalation of lithium ions can be facilitated, thereby facilitating the capacity of the battery cell.

[0079] Compared with the prior art, the lithium iron phosphate salt particles in the embodiments of the present application have a smaller specific surface area with the same carbon content, meaning that the particles contain less floating carbon, and the carbon contained in the lithium iron phosphate salt particles is more uniform and dense, and each particle is coated, thereby improving the surface conductivity of the particles and the kinetic performance of the particles as a positive electrode material.

[0080] In any of the embodiments, the ratio z of the BET specific surface area to Cx satisfies 1.5 ≤ z ≤ 8.5, and can optionally satisfy 3 ≤ z ≤ 6.

[0081] In some embodiments, the ratio z of the BET specific surface area to Cx can be 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.

[0082] 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 the present application contain less floating carbon, and the carbon coating is more uniform and dense, thereby further improving the conductivity and kinetic performance of the particles as a positive electrode material.

[0083] In any embodiment, the lithium iron phosphate salt has a molecular formula of Li m Fe x P y O j Q q wherein Q comprises 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, Br, 0.95≤m≤1.15, 0.9≤x≤1, 0.95≤y≤1, 3.5≤j≤4, 0

[0084] By making the lithium iron phosphate salt of the embodiments of the present application have the above-mentioned molecular formula while doping one or more of the above-mentioned elements, high loading amount bulk phase modification of the lithium iron phosphate salt particles is achieved, which helps to improve the bulk phase ion transport capacity of the lithium iron phosphate salt particles, and effectively solves the problem of poor inherent kinetic performance of large particles. In addition, by doping, the lithium iron phosphate salt particles exhibit good kinetic performance when used as a positive electrode material. In the present application, the modification can specifically be doping and / or coating.

[0085] In any embodiment, Q comprises at least one of Ti, V, Mg, Nb, and can optionally be Ti, the content of Ti, V, Mg and / or Nb is 1000-10000 ppm, and can optionally be 2500-6000 ppm, based on the total weight of the lithium iron phosphate salt particles. It can specifically be 2500 ppm, 2600 ppm, 2700 ppm, 2800 ppm, 2900 ppm, 3000 ppm, 3100 ppm, 3200 ppm, 3300 ppm, 3400 ppm, 3500 ppm, 3600 ppm, 3700 ppm, 3800 ppm, 3900 ppm, 4000 ppm, 4100 ppm, 4200 ppm, 4300 ppm, 4400 ppm, 4500 ppm, 4600 ppm, 4700 ppm, 4800 ppm, 4900 ppm, 5000 ppm, 5100 ppm, 5200 ppm, 5300 ppm, 5400 ppm, 5500 ppm, 5600 ppm, 5700 ppm, 5800 ppm, 5900 ppm, or 6000 ppm, or a range between any two of the above-mentioned values.

[0086] In some embodiments, the lithium iron phosphate salt has a molecular formula of Li m A a Fe x D d Py E e O z G g , the A comprises at least one element of Al, Na, K or Mg; the D comprises at least one element of Cu, Mn, Cr, Zn, Pb, Ca, Co, Ni, Sr, Nb, Ti or V; the E comprises at least one element of B, S, Si or N; the G comprises at least one element of 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; the g is selected from the range of 0 to 0.1.

[0087] By doping one or more of the above elements in the lithium iron phosphate salt of the embodiments of the present application and making the content thereof within the above range, the metal body phase modification of the lithium iron phosphate salt particles can be better achieved under the premise of improving the primary average particle size of the lithium iron phosphate salt particles, further improving the body phase ion transport capacity of the lithium iron phosphate salt particles, and better solving the problem of poor kinetic performance inherent in large particles.

[0088] In any embodiment, the lithium iron phosphate salt particles are single crystal particles and / or polycrystal particles, and optionally, the number ratio of the single crystal particles is more than 90% based on the total number of the lithium iron phosphate salt particles. Single crystal refers to a structure complete crystal grown from one crystal nucleus, and there is no grain boundary in the single crystal; polycrystal is a crystal combined randomly by a large number of small single crystal particles, and there is a grain boundary in the polycrystal. Different from perfect single crystal, the large single crystal in the present application can have small defects, such as internal micropores, a small amount of point and surface defects, or a small amount of particles adhering to each other on the surface of a particle. However, the single crystal referred to herein is an integral whole under the view of TEM photo.

[0089] In the embodiments of the present application, the number ratio of the single crystal particles is controlled within the above range based on the total number of the lithium iron phosphate salt particles. Compared with polycrystal and secondary agglomerates, the single crystal particle ratio is high, the grain boundary barrier to lithium ions is smaller, the transmission rate of lithium ions is faster, and the kinetic performance is better.

[0090] In the lithium iron phosphate salt particles of the embodiments of the present application, the carbon can be in a state of mixing with the lithium iron phosphate salt particles or in a state of coating the lithium iron phosphate salt particles. Optionally, the carbon in the lithium iron phosphate salt particles is in a state of coating the lithium iron phosphate salt particles, so that the carbon can form a uniform and dense carbon coating layer on the surface of the lithium iron phosphate salt particles, thereby improving the surface conductivity of the particles.

[0091] In any of the embodiments, the capacity ratio η of the lithium iron phosphate salt particles is ≥ 88%, where η is defined as follows: a battery having the lithium iron phosphate salt particles as the positive electrode material is subjected to constant current charge and discharge twice in a voltage range of 2.0 V to 3.75 V at a rate of 0.1 C, and then subjected to constant current charge and discharge once at a rate of 1 C, in the charge and discharge test at a rate of 1 C, the capacity value at a discharge voltage of 3.2 V is denoted as C1, and the capacity value at a discharge voltage of 2.0 V is denoted as C2, and η = C1 / C2, wherein the charging process includes constant voltage charging, constant voltage 3.75 V, and constant voltage cutoff current 50 μA.

[0092] The η value can represent the kinetic performance of the lithium iron phosphate particles, which can be adjusted by adjusting the primary average particle size of the particles, the carbon content, the ratio of the carbon source and the film forming agent, the modifier and its content. The η represents the platform holding performance of the material, which has a strong correlation with the discharge power performance of the battery. When the value is larger, the battery can still maintain good power performance when discharged at low SOC (state of charge of the battery), i.e., the voltage drop of the battery is smaller when the battery is discharged at a large current at low capacity. The capacity ratio η of the lithium iron phosphate salt particles of the embodiments of the present application is ≥ 88%, which means that the lithium iron phosphate salt particles of the present application can make the secondary battery exhibit good kinetic performance when used as the positive electrode material.

[0093] In any of the embodiments, the lithium iron phosphate salt particles satisfy at least one of a)-f):

[0094] a) the Dv10 of the lithium iron phosphate salt particles is ≥ 0.2 μm;

[0095] b) the Dv50 of the lithium iron phosphate salt particles is 0.5 μm-5 μm;

[0096] c) the Dv90 of the lithium iron phosphate salt particles is ≤ 10 μm;

[0097] d) the Dv99 of the lithium iron phosphate salt particles is ≤ 12 μm;

[0098] e) the powder compaction density of the lithium iron phosphate salt under a pressure of 3T is ≥ 2.25 g / cm 3 ;

[0099] f) the powder resistivity of the lithium iron phosphate salt is less than 60 Ω·cm.

[0100] In the present application, the above-mentioned Dv10, Dv90 and Dv99 refer to the particle size corresponding to the cumulative volume percentage of 10%, 90% and 99% of the particles, respectively. The test method is the same as that of Dv50, which can be determined by using the standard in the embodiments of the present application.

[0101] It should be noted that the Dv10 is a large target, so it cannot give an upper limit, and the Dv90 is a small target, so it cannot give a lower limit.

[0102] By making the lithium iron phosphate salt particles satisfy at least one of a)-f), the lithium iron phosphate salt particles can better achieve the above-mentioned technical effects.

[0103] In the present application, the test method of the above-mentioned Dv10, Dv90 and Dv99 is the same as that of Dv50, which can be determined by using the standard in the embodiments of the present application.

[0104] In the present application, the test method of the powder resistivity is as follows: referring to the national standard GB / T33822-2017, using a powder resistivity instrument (Suzhou Crystal, ST2722 type), weighing 1 g of sample (error within ±0.005 g), adding it into the charging cavity, applying a pressure of 8 MPa, testing the forward resistivity and reverse resistivity of the sample respectively, and taking the average value of the two as the powder resistivity of the sample.

[0105] In the present application, the powder compaction density is defined as follows: during the compression process of external force, with the movement and deformation of the powder, the larger voids are filled, the contact area between particles increases, the interatomic attractive force is generated and the mechanical bonding effect between particles is enhanced, thereby forming a compaction with a certain density and strength, and the unit is g / cm 3 .

[0106] The test method of the 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 diameter of the mold is known), and a metal disc is placed on the middle hollow of the mold. The powder is placed between the metal discs, and a metal cylinder is placed on the top. The mold is placed on the compaction density instrument, different pressures are set, and the thickness of the powder under different pressures can be read on the equipment. The compaction density is calculated by ρ = m / v.

[0107] According to the following powder compaction density calculation result, ρc = m / V = m / (S*H); in the formula, ρc is the powder compaction density (g / cm 3 ), m is the mass of the material (g), S is the bottom area of the mold (1.327 cm 2H is the height of the sample after compaction (cm).

[0108] Method for preparing lithium iron phosphate salt particles

[0109] The method for preparing lithium iron phosphate salt particles of the embodiments of the present application can prepare the lithium iron phosphate salt particles of the present application. The method for preparing lithium iron phosphate salt particles of the embodiments of the present application comprises: providing raw materials containing at least a lithium source, an iron source, a phosphorus source, a carbon source, a carbon film forming agent, and a modifier, and performing at least two times of sintering, wherein the temperature of the first sintering is 500-760°C, which can be optionally 550-720°C, the carbon content of the material after the first sintering is 0.01-0.79% by weight, which can be optionally 0.05-0.4% by weight; the temperature of the second sintering is 700-800°C, which can be optionally 720-780°C, the carbon content of the material after the second sintering is 0.8-2.0% by weight, which can be optionally 1.0-1.6% by weight.

[0110] In some embodiments, the temperature of the first sintering can be optionally 500°C, 510°C, 520°C, 530°C, 540°C, 550°C, 560°C, 570°C, 580°C, 590°C, 600°C, 610°C, 620°C, 630°C, 640°C, 650°C, 655°C, 660°C, 670°C, 680°C, 690°C, 700°C, 710°C, 720°C, 730°C, 740°C, 750°C, 760°C, or 760°C, or a range between any two of the above values. The temperature of the second sintering can be optionally 700°C, 710°C, 720°C, 730°C, 740°C, 750°C, 755°C, 760°C, 770°C, 780°C, 790°C, or 800°C, or a range between any two of the above values. The carbon content of the material after the first sintering can be optionally 0.01% by weight, 0.05% by weight, 0.10% by weight, 0.15% by weight, 0.2% by weight, 0.25% by weight, 0.3% by weight, 0.35% by weight, 0.4% by weight, 0.45% by weight, 0.5% by weight, 0.55% by weight, 0.6% by weight, 0.65% by weight, 0.7% by weight, 0.75% by weight, 0.79% by weight, and the carbon content of the material after the second sintering is 0.8-2.0% by weight, which can be optionally 0.8% by weight, 0.9% by weight, 1% by weight, 1.1% by weight, 1.2% by weight, 1.3% by weight, 1.4% by weight, 1.5% by weight, 1.6% by weight, 1.7% by weight, 1.8% by weight, 1.9% by weight, or 2.0% by weight, or a range between any two of the above values.

[0111] In the preparation method of the lithium iron phosphate salt particles in the embodiments of the present application, two sintering processes are implemented. By controlling the temperature of the first sintering within the above range and making the carbon content of the intermediate after sintering within the above range, the lithium iron phosphate precursor obtained after the first sintering has a larger particle size, directly improving the powder compaction and the electrode sheet compaction density of the final product. In addition, by adding a lower content of carbon source during the first sintering, the barrier effect of the carbon layer on the growth of the lithium iron phosphate particles is greatly reduced, which is conducive to the crystallization growth of the particles at a lower temperature and the solid-phase diffusion reaction between the modifier and the lithium iron phosphate material, thereby realizing a higher concentration of metal ion modification. Compared with the traditional method of realizing particle growth at a high temperature, the above preparation method of the present application can synthesize large particles at a lower temperature, which can improve the phenomenon of cracking of the carbon layer on the particle surface at a high temperature and improve the density of the surface carbon. At the same time, during the first sintering process, the carbon source can effectively reduce the trivalent iron in the raw material, thereby improving the purity and stability of the product. By controlling the temperature of the second sintering within the above range and making the carbon content of the sintered material 0.8 wt% to 2.0 wt%, the carbon can be coated on the surface of the lithium iron phosphate salt particles, forming a lithium iron phosphate material with a uniform and dense carbon coating layer that covers each particle, thereby greatly improving the electrical conductivity of the particle surface.

[0112] In any embodiment, the lithium iron phosphate salt particles contain at least one of Ti, V, Mg, and / or Nb, and the content of the element is 1000 ppm to 10000 ppm, and optionally 2500 ppm to 6000 ppm, based on the total weight of the lithium iron phosphate salt particles.

[0113] In some embodiments, the content of the element is optionally 2500 ppm to 6000 ppm. Specifically, it can be 2500 ppm, 2600 ppm, 2700 ppm, 2800 ppm, 2900 ppm, 3000 ppm, 3100 ppm, 3200 ppm, 3300 ppm, 3400 ppm, 3500 ppm, 3600 ppm, 3700 ppm, 3800 ppm, 3900 ppm, 4000 ppm, 4100 ppm, 4200 ppm, 4300 ppm, 4400 ppm, 4500 ppm, 4600 ppm, 4700 ppm, 4800 ppm, 4900 ppm, 5000 ppm, 5100 ppm, 5200 ppm, 5300 ppm, 5400 ppm, 5500 ppm, 5600 ppm, 5700 ppm, 5800 ppm, 5900 ppm, or 6000 ppm, or a range between any two of the above values.

[0114] By doping one or more of the above elements in the lithium iron phosphate salt of the embodiments of the present application and making the content thereof within the above range, the metal body phase modification of the lithium iron phosphate salt particles can be better achieved under the premise of improving the primary average particle size of the lithium iron phosphate salt particles, further improving the body phase ion transport capacity of the lithium iron phosphate salt particles, and better solving the problem of poor kinetic performance inherent in large particles.

[0115] In any embodiment, the method for preparing the lithium iron phosphate salt particles comprises a first crushing after the first sintering and a second crushing after the second sintering, wherein the Dv50 of the product after the first crushing is 300-1200 nm, optionally 400-1100 nm; and the Dv50 of the product after the second crushing is 500-5000 nm, optionally 300-2500 nm.

[0116] In some embodiments, the Dv50 of the product after the first crushing is 300 nm, 350 nm, 400 nm, 500 nm, 550 nm, 600 nm, 700 nm, 800 nm, 900 nm, 1000 nm, 1100 nm, or 1200 nm, or a range between any two of the above values. In some embodiments, the Dv50 of the product after the second crushing is 500 nm, 600 nm, 700 nm, 800 nm, 900 nm, 1000 nm, 1100 nm, 1200 nm, 1300 nm, 1400 nm, 1500 nm, 1600 nm, 1700 nm, 1800 nm, 1900 nm, 2000 nm, 2300 nm, 2500 nm, 2700 nm, 3000 nm, 3300 nm, 3500 nm, 3700 nm, 4000 nm, 4300 nm, 4500 nm, 4700 nm, 5000 nm, or a range between any two of the above values.

[0117] In this context, the term "Dv50" refers to the particle size corresponding to the cumulative volume particle size distribution percentage of 50% in the particles. The determination of the Dv50 can be performed using the determination method described in the examples.

[0118] In the preparation method of the lithium iron phosphate salt particles in the embodiments of the present application, crushing is implemented after the two sintering. By performing the first crushing after the first sintering, the Dv50 of the product is 300 nm-1200 nm, which can avoid the growth blocking of the carbon material and the modified element to the crystal, and micron-level lithium iron phosphate precursor is obtained; by performing the second crushing after the second sintering, the Dv50 of the product is 500 nm-3000 nm, which can obtain lithium iron phosphate salt particles with desired particle size, and a uniform and dense carbon coating layer is obtained, which greatly improves the surface conductivity of the particles.

[0119] In any embodiment, the preparation method of the lithium iron phosphate salt particles comprises: providing raw materials containing at least a lithium source, an iron source, a phosphorus source, a carbon source, a carbon film forming agent, and a modifier, wherein the mixing ratio of the lithium source, the iron source, and the phosphorus source satisfies Fe:P=0.96-0.985:1 and Li:Fe=1.0-1.1:0.95-1.1 in terms of atomic moles of each element; and the carbon source and the carbon film forming agent satisfy carbon source:carbon film forming agent=9:1-2:8 in terms of weight ratio.

[0120] In some embodiments, the mixing ratio of the iron source and the phosphorus source satisfies Fe:P=0.96:1, Fe:P=0.965:1, Fe:P=0.97:1, Fe:P=0.975:1, Fe:P=0.98:1, or Fe:P=0.985:1 in terms of atomic moles of each element.

[0121] In some embodiments, the mixing ratio of the lithium source and the iron source satisfies Li:Fe=1.0:1.1, Li:Fe=0.99:1.1, Li:Fe=0.98:1.1, Li:Fe=0.97:1.1, Li:Fe=0.96:1.1, or Li:Fe=0.95:1.1 in terms of atomic moles of each element.

[0122] In some embodiments, the weight ratio of the carbon source and the 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.

[0123] By using the raw materials containing the lithium source, the iron source, the phosphorus source, the carbon source, the modifier, and the carbon film forming agent in the above-mentioned proportions, good modification can be achieved, and the obtained lithium iron phosphate salt particle precursor is formed into a particle with a large size, so that the primary average particle diameter, the carbon content, the BET specific surface area, and the z value of the lithium iron phosphate salt particle of the first aspect of the present application can be formed well. Specifically, as described above, by setting the primary average particle diameter of the lithium iron phosphate salt particle of the embodiment of the present application to 500 nm to 3000 nm, the particle can be kept at a suitable micron level size, so that the problems of interface side reactions and processing difficulties caused by the nanometerization of the particle can be avoided (in the art, there are difficulties in processing the nanometerized particle), and the particle is not limited to a size that causes a decrease in kinetic performance. In addition, by setting the BET specific surface area of the lithium iron phosphate salt particle of the embodiment of the present application to 3 m 2 / g to 8 m 2 / g, the stirring and the solid content of the slurry containing the lithium iron phosphate salt particle can be improved, so that the processing problem of the battery cell can be improved, and the volumetric energy density of the battery can be improved. Further optionally, by setting the ratio z of the BET specific surface area of the lithium iron phosphate salt particle of the embodiment of the present application to Cx to satisfy the range of 1.5 ≤ z ≤ 8.5, the carbon contained in the lithium iron phosphate salt particle can be more uniform and dense, and each particle can be coated, so that the surface conductivity of the particle can be improved.

[0124] In any embodiment, in the preparation method of the lithium iron phosphate salt particle, 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 is optionally lithium carbonate; the iron source is an iron compound, including at least one of iron hydroxide, ferrous chloride, diiron trioxide, iron phosphate, iron pyrophosphate, ferrous oxalate, iron powder, ferric nitrate, magnetite, and hydroxyl ferric oxide, and is optionally diiron trioxide; the phosphorus source is a phosphoric acid compound, including one or more of phosphoric acid, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate, and is optionally phosphoric acid; the modifier includes at least one of titanium dioxide, vanadium pentoxide, n-butyl titanate, ammonium metavanadate, niobium ethoxide, niobium oxalate, niobium pentoxide, magnesium hydroxide, and magnesium nitrate, and is optionally titanium dioxide; the carbon source includes at least one of citric acid, glucose, sucrose, starch, fructose, and lactose, and is optionally glucose; and the carbon film forming agent includes one or a combination of several of polyethylene glycol, polyaniline, polyacrylonitrile, polyvinylpyrrolidone, and polyvinyl alcohol, and is optionally polyaniline.

[0125] By selecting the above-mentioned substances as the lithium source, the iron source, the phosphorus source, the modifier, the carbon source, and the carbon film former used in the preparation method of the lithium iron phosphate salt particles of the embodiments of the present application, the primary average particle size, the carbon content, the BET specific surface area, and the z value of the lithium iron phosphate salt particles of the first aspect of the present application can be well formed. Thus, the large particle size of the lithium iron phosphate salt particles of the embodiments of the present application is more favorably achieved, and at the same time, excellent kinetic performance is achieved when the lithium iron phosphate salt particles are used as a positive electrode material.

[0126] In any embodiment, the heating rate in the first and second sintering is each independently 2°C / min-20°C / min, the first sintering constant temperature time is 1h-6h; the second sintering constant temperature time is 2h-12h.

[0127] In some embodiments, the heating rate in the first and second sintering is 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.

[0128] In some embodiments, the constant temperature sintering time of the first sintering is 1-6h. In some embodiments, the constant temperature sintering time of the first sintering can be selected to be 1h, 1.5h, 2h, 2.5h, 3h, 3.5h, 4h, 4.5h, 5h, 5.5h, or 6h.

[0129] In some embodiments, the constant temperature sintering time of the second sintering is 2-12h. In some embodiments, the constant temperature sintering time of the second sintering can be selected to be 1h, 1.5h, 2h, 2.5h, 3h, 3.5h, 4h, 4.5h, 5h, 5.5h, 6h, 6.5h, 7h, 7.5h, 8h, 8.5h, 9h, 9.5h, 10h, 10.5h, 11h, 11.5h, or 12h.

[0130] By controlling the heating rate and the constant temperature time in the first and second sintering to be each independently within the above-mentioned range, the excessive side reactions in the sintering process caused by too fast heating can be prevented, thereby affecting the primary average particle size, the carbon content, the BET specific surface area, and the z value of the obtained lithium iron phosphate salt particles. Thus, the large particle size of the lithium iron phosphate salt particles of the embodiments of the present application is more favorably achieved, and at the same time, excellent kinetic performance is achieved when the lithium iron phosphate salt particles are used as a positive electrode material.

[0131] 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 the carbon-coating is 0.01-1.99 wt%, optionally 0.2-1.6 wt%, and specifically 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, or 1.6 wt%, or a range between any two of the above values, based on the total weight of the product after the first sintering.

[0132] By carbon-coating the product after the first sintering with the above amount of carbon, a uniform and dense carbon-coating layer can be formed on the surface of the lithium iron phosphate salt particles during the second sintering.

[0133] In any embodiment, the carbon-coating is performed by vapor deposition during sintering, or by carbonization of a carbon source at high temperature.

[0134] In the method for preparing the lithium iron phosphate salt particles of the embodiments of the present application, the embodiment of the carbon-coating is not particularly limited, and the lithium iron phosphate salt particles can be carbon-coated by vapor deposition, so that a uniform and dense carbon-coating layer can be formed on the surface of the lithium iron phosphate salt particles.

[0135] In any embodiment, after the first sintering and the second sintering, the sintered product is crushed, and the crushing can be one or more of grinding, sanding, mechanical crushing, and airflow crushing.

[0136] In the method for preparing the lithium iron phosphate salt particles of the embodiments of the present application, the crushing method is not particularly limited, and by using the above specific crushing method, the desired average primary particle size can be obtained.

[0137] [Positive electrode sheet]

[0138] The positive electrode sheet comprises a positive electrode current collector and a positive electrode film layer arranged on at least one surface of the positive electrode current collector, and the positive electrode film layer comprises the lithium iron phosphate salt particles of the first aspect of the present application as a positive electrode material. The lithium iron phosphate salt particles of the present application have a large particle size when applied to a secondary battery, and also have excellent kinetic performance when used as a positive electrode material.

[0139] In some embodiments, the positive current collector can employ a metal foil or a composite current collector. For example, as a metal foil, the battery aluminum foil of the present application is employed. The composite current collector can include a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer. The composite current collector can be formed by forming the battery aluminum foil of the present application on a polymer material base material such as a base material of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), or the like.

[0140] In some embodiments, the positive electrode material can employ a positive electrode material for a battery as known in the art. As an example, the positive electrode material can include at least one of a lithium-containing phosphate of an olivine structure, a lithium transition metal oxide, and a modified compound of each thereof. However, the present application is not limited to these materials, and other conventional materials that can be used as a positive electrode material for a battery can also be used. These positive electrode materials can be used alone only one or two or more can be used in combination. Among them, examples of the lithium transition metal oxide can include, but are not limited to, lithium cobalt oxide (e.g., LiCoO2), lithium nickel oxide (e.g., LiNiO2), lithium manganese oxide (e.g., LiMnO2, LiMn2O4), lithium nickel cobalt oxide, lithium manganese cobalt oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide (e.g., LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (which can also be referred to simply as NCM 333 ), LiNi 0.5 Co 0.2 Mn 0.3 O2 (which can also be referred to simply as NCM 523 ), LiNi 0.5 Co 0.25 Mn 0.25 O2 (which can also be referred to simply as NCM 211 ), LiNi 0.6 Co 0.2 Mn 0.2 O2 (which can also be referred to simply as NCM 622 ), LiNi 0.8 Co 0.1 Mn 0.1 O2 (which can also be referred to simply as NCM 811 ), lithium nickel cobalt aluminum oxide (e.g., LiNi 0.85 Co 0.15 Al 0.05O2) and modified compounds thereof, etc. Examples of the lithium-containing phosphate of olivine structure can include, but are not limited to, at least one of lithium iron phosphate (such as LiFePO4 (which can also be referred to simply as LFP)), a composite of lithium iron phosphate and carbon, lithium manganese phosphate (such as LiMnPO4), a composite of lithium manganese phosphate and carbon, lithium manganese iron phosphate, and a composite of lithium manganese iron phosphate and carbon.

[0141] In some embodiments, the positive electrode film layer further optionally includes a binder. As an example, the binder can include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), a vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, a vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, a tetrafluoroethylene-hexafluoropropylene copolymer, and a fluorine-containing acrylate resin.

[0142] In some embodiments, the positive electrode film layer further optionally includes a conductive agent. As an example, the conductive agent can include at least one of super P, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0143] In some embodiments, the positive electrode tab can be prepared by dispersing the components for preparing the positive electrode tab, such as the positive electrode material, the conductive agent, the binder, and any other components, in a solvent (such as N-methylpyrrolidone) to form a positive electrode slurry, coating the positive electrode slurry on a positive electrode current collector, and subjecting the same to processes such as drying, cold pressing, etc. to obtain the positive electrode tab.

[0144] [Negative electrode tab]

[0145] The negative electrode tab includes a negative electrode current collector and a negative electrode film layer disposed on at least one surface of the negative electrode current collector, the negative electrode film layer including a negative electrode active material.

[0146] As an example, the negative electrode current collector has two surfaces opposite in the thickness direction thereof, and the negative electrode film layer is disposed on either one or both of the two opposite surfaces of the negative electrode current collector.

[0147] In some embodiments, the negative electrode current collector can employ a metal foil or a composite current collector. For example, as the metal foil, a copper foil can be employed. The composite current collector can include a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer. The composite current collector can be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, silver alloy, etc.) on a polymer material base layer (such as a base layer of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0148] In some embodiments, the negative active material can employ a negative active material for a battery known in the art. As an example, the negative active material can include at least one of artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based material, tin-based material, and lithium titanate, etc. The silicon-based material can be selected from at least one of elemental silicon, silicon oxide compound, silicon-carbon composite, silicon-nitrogen composite, and silicon alloy. The tin-based material can be selected from at least one of elemental tin, tin oxide compound, and tin alloy. However, the present application is not limited to these materials, and other conventional materials that can be used as a negative active material for a battery can also be used. These negative active materials can be used alone or in combination of two or more.

[0149] In some embodiments, the negative film layer can further optionally include a binder. The binder can 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).

[0150] In some embodiments, the negative film layer can further optionally include a conductive agent. The conductive agent can be selected from at least one of super conductive carbon, acetylene black, carbon black, ketjen black, carbon dot, carbon nanotube, graphene, and carbon nanofiber.

[0151] In some embodiments, the negative film layer can further optionally include other auxiliary agents, such as thickening agents (e.g., sodium carboxymethyl cellulose (CMC-Na)) and the like.

[0152] In some embodiments, the negative electrode sheet can be prepared by dispersing the components for preparing the negative electrode sheet, such as the negative active material, the conductive agent, the binder, and any other components, in a solvent (e.g., deionized water) to form a negative electrode slurry, coating the negative electrode slurry on a negative current collector, and then performing processes such as drying, cold pressing, and the like to obtain the negative electrode sheet.

[0153] [Electrolyte]

[0154] The electrolyte functions to conduct ions between the positive electrode sheet and the negative electrode sheet. The type of the electrolyte is not particularly limited in the present application, and can be selected as needed. For example, the electrolyte can be in a liquid state, a gel state, or a full solid state.

[0155] In some embodiments, the electrolyte employs an electrolyte solution. The electrolyte solution includes an electrolyte salt and a solvent.

[0156] In some embodiments, the electrolyte salt can be selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bisfluorosulfonylimide, lithium bis-trifluoromethanesulfonylimide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorobisoxalate borate, lithium bisoxalate borate, lithium difluorobisoxalate phosphate, and lithium tetrafluorobisoxalate phosphate.

[0157] In some embodiments, the solvent can 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, butylene carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, cyclobutane sulfone, dimethyl sulfone, methyl ethyl sulfone, and diethyl sulfone.

[0158] In some embodiments, the electrolyte solution can further optionally include an additive. For example, the additive can include a negative electrode film-forming additive, a positive electrode film-forming additive, and can further include an additive capable of improving certain performance of the battery, such as an additive for improving overcharge performance of the battery, an additive for improving high-temperature or low-temperature performance of the battery, etc.

[0159] [Separator]

[0160] In some embodiments, the secondary battery further includes a separator. The type of the separator is not particularly limited in the present application, and any known porous structure separator having good chemical stability and mechanical stability can be used.

[0161] In some embodiments, the material of the separator can be selected from at least one of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single layer film or a multi-layer composite film, and is not particularly limited. When the separator is a multi-layer composite film, the materials of the respective layers can be the same or different, and are not particularly limited.

[0162] [Secondary battery]

[0163] In one embodiment of the present application, a secondary battery is provided, which includes the positive electrode sheet.

[0164] Generally, a secondary battery includes a positive electrode sheet, a negative electrode sheet, an electrolyte, and a separator. During charging and discharging of the battery, active ions are repeatedly intercalated and deintercalated between the positive electrode sheet and the negative electrode sheet. The electrolyte functions to conduct ions between the positive electrode sheet and the negative electrode sheet. The separator is disposed between the positive electrode sheet and the negative electrode sheet, and mainly functions to prevent short circuiting of the positive and negative electrodes, while allowing ions to pass through.

[0165] In some embodiments, the positive electrode sheet, the negative electrode sheet, and the separator can be made into an electrode assembly through a roll-pressing process or a stacking process.

[0166] In some embodiments, the secondary battery can include an outer package. The outer package can be used to enclose the electrode assembly and the electrolyte.

[0167] In some embodiments, the outer package of the secondary battery can be a hard case, such as a hard plastic case, an aluminum case, a steel case, etc. The outer package of the secondary battery can also be a soft package, such as a pouch-type soft package. The material of the soft package can be plastic, and as plastic, polypropylene, polybutylene terephthalate, polybutylene succinate, etc. can be listed.

[0168] The present application does not have a particular limitation on the shape of the secondary battery, which can be cylindrical, square, or any other shape. For example, Figure 5 is a square structure of the secondary battery 5 as an example.

[0169] In some embodiments, referring to Figure 6 , the outer package can include a case 51 and a cover plate 53. The case 51 can include a bottom plate and a side plate connected to the bottom plate, and the bottom plate and the side plate enclose a receiving cavity. The case 51 has an opening communicating with the receiving cavity, and the cover plate 53 can be provided on the opening to close the receiving cavity. The positive electrode sheet, the negative electrode sheet, and the separator can form an electrode assembly 52 through a winding process or a stacking process. The electrode assembly 52 is packaged in the receiving cavity. The electrolyte is impregnated in the electrode assembly 52. The number of electrode assemblies 52 contained in the secondary battery 5 can be one or more, which can be selected by those skilled in the art according to the specific actual needs.

[0170] In some embodiments, the secondary battery can be assembled into a battery module, and the number of secondary batteries contained in the battery module can be one or more, and the specific number can be selected by those skilled in the art according to the application and capacity of the battery module.

[0171] Figure 7 is a battery module 4 as an example. Referring to Figure 7 , in the battery module 4, a plurality of secondary batteries 5 can be arranged in sequence along the length direction of the battery module 4. Of course, it can also be arranged in any other way. Further, the plurality of secondary batteries 5 can be fixed by fasteners.

[0172] Optionally, the battery module 4 can also include a housing having a receiving space, and the plurality of secondary batteries 5 are received in the receiving space.

[0173] In some embodiments, the above-mentioned battery module can also be assembled into a battery pack, and the number of battery modules contained in the battery pack can be one or more, and the specific number can be selected by those skilled in the art according to the application and capacity of the battery pack.

[0174] Figure 8 andFigure 9 is a battery pack 1 as an example. Referring to Figure 8 and Figure 9 In the battery pack 1, a battery case and a plurality of battery modules 6 disposed in the battery case can be included. The battery case includes an upper case 2 and a lower case 3, and the upper case 2 can be disposed on the lower case 3 and form an enclosed space for accommodating the battery modules 4. The plurality of battery modules 4 can be arranged in the battery case in any manner.

[0175] [Power consuming device]

[0176] In addition, the present application also provides a power consuming device, which includes at least one of the secondary battery, the battery module, or the battery pack provided by the present application. The secondary battery, the battery module, or the battery pack can be used as a power supply of the power consuming device, or can be used as an energy storage unit of the power consuming device. The power consuming device can include a mobile device (such as a mobile phone, a notebook computer, etc.), an electric vehicle (such as a pure electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, an electric bicycle, an electric scooter, an electric golf cart, an electric truck, etc.), an electric train, a ship and a satellite, an energy storage system, etc., but is not limited thereto.

[0177] As the power consuming device, the secondary battery, the battery module, or the battery pack can be selected according to the use requirement thereof.

[0178] Figure 10 is a power consuming device as an example. The power consuming device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. In order to meet the requirement of high power and high energy density of the secondary battery for the power consuming device, the battery pack or the battery module can be used.

[0179] The device as another example can be a mobile phone, a tablet computer, a notebook computer, etc. The device usually requires thinning, and the secondary battery can be used as a power supply.

[0180] Embodiment

[0181] Hereinafter, the embodiments of the present application will be described. The embodiments described below are exemplary and are only used to explain the present application, and cannot be understood as a limitation of the present application. If the specific technology or condition is not indicated in the embodiments, the technology or condition described in the literature in the art or according to the product instruction is used. If the reagent or instrument is not indicated by the manufacturer, it is a conventional product that can be obtained by market purchase.

[0182] Embodiment 1

[0183] 1. Preparation of lithium iron phosphate salt particles

[0184] 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 and Li:Fe = 1:0.98. 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.

[0185] 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.

[0186] 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. 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 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.

[0187] 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.

[0188] 2. Preparation of the positive electrode sheet

[0189] 2.0 wt% polyvinylidene fluoride binder was dissolved in N-methyl pyrrolidone (NMP) and 1.0 wt% Super P, 0.5 wt% carbon nanotubes were added and mixed with 96.5 wt% of the above positive electrode material to obtain a positive electrode slurry. The slurry was uniformly coated on the surface of the current collector aluminum foil and then transferred to a vacuum drying oven for complete drying. The dried electrode sheet was rolled and punched to obtain a positive electrode sheet.

[0190] 3. Preparation of a negative electrode sheet

[0191] An active material artificial graphite, a conductive agent carbon black, a binder styrene butadiene rubber (SBR), and a thickening agent sodium carboxymethyl cellulose (CMC Na) were dissolved in a solvent deionized water at a weight ratio of 96.7:1.3:0.8:1.2 to prepare a negative electrode slurry. The negative electrode slurry was uniformly coated on a negative electrode current collector copper foil one or more times, and then dried, cold-pressed, and cut to obtain a negative electrode sheet.

[0192] 4. Preparation of an electrolyte

[0193] In an argon atmosphere glove box (H2O < 0.1 ppm, O2 < 0.1 ppm), organic solvents ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed at a volume ratio of 3:7, and LiPF6 lithium salt was dissolved in the organic solvents to prepare a solution with a weight content of 12.5% to obtain an electrolyte.

[0194] 5. Separation film

[0195] A polypropylene film was used as the separation film.

[0196] 6. Preparation of a lithium ion battery

[0197] The above positive electrode sheet, separation film, and negative electrode sheet were stacked in order with the separation film between the positive and negative electrode sheets to play a separation role, and then wound to obtain a bare cell. The bare cell was welded with tabs, and then placed in an aluminum shell and baked at 80°C to remove water. Then, the electrolyte was injected and sealed to obtain a non-charged battery. The non-charged battery was subjected to the processes of standing, hot and cold pressing, formation, shaping, and capacity testing in sequence to obtain the lithium ion battery of Example 1.

[0198] The lithium ion battery obtained in Example 1 was subjected to the performance tests described below, and the results are shown in Table 1.

[0199] Example 2

[0200] The Dv50 particle size of the particles crushed after the first sintering was controlled to be 350 nm, thereby obtaining lithium iron phosphate particles with an average particle size of 500 nm. Otherwise, the same processes as in Example 1 were performed.

[0201] Example 3

[0202] The Dv50 particle diameter of the particles crushed after the first sintering was controlled to be 1200 nm, whereby lithium iron phosphate particles having a primary average particle diameter of 3000 nm were obtained, and otherwise, the same as in Example 1 was performed.

[0203] Example 4

[0204] The temperature of the first sintering and the second sintering was changed to 620°C and 740°C, respectively, and the Ti content of the product was controlled to be 2500 ppm, and otherwise, the same as in Example 1 was performed.

[0205] Example 5

[0206] The temperature of the first sintering and the second sintering was changed to 655°C and 755°C, respectively, and the Ti content was controlled to be 6000 ppm, and otherwise, the same as in Example 1 was performed.

[0207] Example 6

[0208] The modifying element was changed from Ti to V, and otherwise, the same as in Example 1 was performed.

[0209] Example 7

[0210] The modifying element was changed from Ti to Nb, and otherwise, the same as in Example 1 was performed.

[0211] Example 8

[0212] The primary average particle diameter of the lithium iron phosphate particles was controlled to be 920 nm, the BET specific surface area was controlled to be 3.0 m 2 / g, and the carbon content was controlled to be 0.8 wt%, and otherwise, the same as in Example 1 was performed.

[0213] Example 9

[0214] The primary average particle diameter of the lithium iron phosphate particles was controlled to be 790 nm, the BET specific surface area was controlled to be 8.0 m 2 / g, and the carbon content was controlled to be 1.4 wt%, and otherwise, the same as in Example 1 was performed.

[0215] Example 10

[0216] The carbon content after the first sintering was controlled to be 0.05 wt%, and the primary average particle diameter of the product was controlled to be 1250 nm, the BET specific surface area was controlled to be 6.0 m 2 / g, and the carbon content was controlled to be 1.2 wt%, and otherwise, the same as in Example 1 was performed.

[0217] Example 11

[0218] The primary average particle diameter of the lithium iron phosphate salt particles was regulated to 520 nm, the BET specific surface area was regulated to 6.8 m 2 / g, the carbon content after the first sintering was regulated to 0.4% by weight, and otherwise the same as in Example 1.

[0219] Example 12

[0220] The BET specific surface area of the product was regulated to 7.2 m 2 / g, and the carbon content was 1.6% by weight, and otherwise the same as in Example 1.

[0221] Example 13

[0222] The BET specific surface area of the product was regulated to 7.8 m 2 / g, and the carbon content was 2% by weight, and otherwise the same as in Example 1.

[0223] Example 14

[0224] The temperature of the first sintering was regulated to 620°C, the temperature of the second sintering was regulated to 730°C, the weight ratio of glucose and polyaniline satisfied glucose: polyaniline = 1:4, the BET specific surface area of the lithium iron phosphate salt particles was 3.0 m 2 / g, and the carbon content was 2% by weight, and otherwise the same as in Example 1.

[0225] Example 15

[0226] The BET specific surface area of the lithium iron phosphate salt particles was regulated to 8.0 m 2 / g, and the carbon content was 0.94% by weight, and otherwise the same as in Example 1.

[0227] Example 16

[0228] The primary average particle diameter of the lithium iron phosphate salt particles was regulated to 650 nm, the Dv50 after the first sintering was regulated to 450 nm, and otherwise the same as in Example 1.

[0229] Example 17

[0230] The primary average particle diameter of the lithium iron phosphate salt particles was regulated to 2500 nm, the BET specific surface area was regulated to 5.5 m 2 / g, and otherwise the same as in Example 1.

[0231] Comparative Example 1

[0232] The BET specific surface area of the lithium iron phosphate salt particles was regulated to 2.4 m 2 / g, and the carbon content was 0.7% by weight, and otherwise the same as in Example 1.

[0233] Comparative Example 2

[0234] The carbon content was regulated to 1.7 wt%, and the BET specific surface area of the product was regulated to 12 m 2 / g, and otherwise the same as in Example 1.

[0235] Comparative Example 3

[0236] The temperature of the second sintering was regulated to 830°C, and the BET specific surface area of the product was regulated to 9.5 m 2 / g, and otherwise the same as in Example 1.

[0237] Comparative Example 4

[0238] The primary average particle diameter of the product was regulated to 5000 nm, and the BET specific surface area was regulated to 4 m 2 / g, and otherwise the same as in Example 1.

[0239] Comparative Example 5

[0240] The primary sintering was used in the production process, and the BET specific surface area of the product was regulated to 12 m 2 / g, and otherwise the same as in Example 1.

[0241] Comparative Example 6

[0242] The temperature of the first sintering was regulated to 760°C, the primary average particle diameter of the product was regulated to 3200 nm, and the BET specific surface area was regulated to 4.8 m 2 / g, and otherwise the same as in Example 1.

[0243] Comparative Example 7

[0244] The carbon content of the product after the first sintering was regulated to 1.1 wt%, and the primary average particle diameter of the product was regulated to 300 nm, and the BET specific surface area was regulated to 13 m 2 / g, and otherwise the same as in Example 1.

[0245] Comparative Example 8

[0246] The carbon content of the product was regulated to 0.5% without using a carbon film former in the production process, the temperature of the first sintering was regulated to 600°C, the temperature of the second sintering was regulated to 720°C, and otherwise the same as in Example 1.

[0247] I. Measurement of the relevant parameters of the lithium iron phosphate salt particles

[0248] 1. Primary average particle diameter

[0249] The argon ion beam is used to cut the pole piece vertically to the large surface of the pole piece to expose the cross section, and the cross section is photographed by a scanning electron microscope. The longest diameter of the lithium iron phosphate salt particles is statistically analyzed by a length-diameter statistical method. Among them, "one-time average particle size" refers to the average value of the particle size of all particles, which is equal to the total particle size value divided by the total number of particles. Among them, the one-time particle size in the cross-sectional view refers to the longest distance between the two points connected by the edge. Specifically, the total number of lithium iron phosphate salt particles with a one-time particle size greater than 80 nm and the one-time particle size of lithium iron phosphate salt particles with a one-time particle size greater than 80 nm can be counted in the electron microscope scanning photograph. The one-time average particle size of lithium iron phosphate salt particles = one-time particle size of total lithium iron phosphate salt particles / total number of lithium iron phosphate salt particles. Among them, the particles with a one-time average particle size less than or equal to 80 nm are not included in the statistical range during the above particle size statistical process.

[0250] 2、BET specific surface area

[0251] The specific surface area is tested by gas adsorption method according to the test standard GB / T 19587-2017, and the specific process is as follows: lithium iron phosphate particles are taken as samples, the sample tube is immersed in-196℃ liquid nitrogen, the adsorption amount of nitrogen on the solid surface under different pressures is measured under 0.05-0.30 relative pressure, and the monolayer adsorption amount of the sample is obtained based on the BET multilayer adsorption theory and its formula, so as to calculate the specific surface area of the material.

[0252] 3、Dv50

[0253] According to GB / T 19077.1-2016, the Dv50 value of lithium iron phosphate salt particles is measured by using a laser particle size analyzer (Malvern Master Size 3000). In addition, the Dv10, Dv90 and Dv99 of the present application are also measured.

[0254] 4、Carbon content

[0255] After the lithium iron phosphate salt particles are burned by a high-frequency induction furnace, the carbon content is tested by infrared absorption method, and the specific testing process is based on the standard GB / T 20123-2006 / ISO 15350:2000.

[0256] 5、Morphology test

[0257] Example 10 and the existing lithium iron phosphate positive electrode material are tested by ZEISS sigma 300 scanning electron microscope, and then tested according to the standard JY / T010-1996, and the sample morphology is observed. The observation results are shown in Figure 2 a、b.

[0258] II. Determination of battery performance

[0259] 1、Battery gram capacity

[0260] 2.0000g sample was mixed with 0.1111g conductive carbon black, 0.1111g PVDF (mass ratio 0.9:0.05:0.05), 2.5g organic solvent NMP (N-methyl pyrrolidone) was added, and after being mixed evenly, a film with a thickness of 140 microns was coated on an aluminum foil, vacuum dried at 120°C for 2h, punched into a 13mm diameter circle using a puncher, and pressed using a tablet press at 10Mpa, vacuum incubated at 120°C for 12h, and the weight of the positive electrode sheet was measured, with the active material loading being 11-12mg. A button cell was assembled in an argon-protected glove box, with a lithium metal sheet as the negative electrode, a mixed solvent of EC (ethylene carbonate) and DMC (1,2-dimethyl carbonate) in a volume ratio of 1:1 as the electrolyte, LiPF6 as the electrolyte, and a Celgard 2400 microporous polyethylene membrane as the separator. The assembled battery was tested for electrical performance on a blue light tester. The specific capacity was tested by constant current charging / discharging at 0.1C for two times and then at 1C for two times in the voltage range of 2.0V-3.75V. There was a constant voltage process during the charging process, with a constant voltage of 3.75V and a constant voltage cutoff current of 50μA. The gram capacity was the discharge capacity under the first 0.1C cycle.

[0261] 2. The battery 1C discharge capacity ratio (η value) at 3.2V

[0262] The preparation and testing process of the battery was as follows: 2.0000g sample was mixed with 0.1111g conductive carbon black, 0.1111g PVDF (mass ratio 0.9:0.05:0.05), 2.5g organic solvent NMP (N-methyl pyrrolidone) was added, and after being mixed evenly, a film with a thickness of 140 microns was coated on an aluminum foil, vacuum dried at 120°C for 2h, punched into a 13mm diameter circle using a puncher, and pressed using a tablet press at 10Mpa, vacuum incubated at 120°C for 12h, and the weight of the positive electrode sheet was measured, with the active material loading being 11-12mg. A button cell was assembled in an argon-protected glove box, with a lithium metal sheet as the negative electrode, a mixed solvent of EC (ethylene carbonate) and DMC (1,2-dimethyl carbonate) in a volume ratio of 1:1 as the electrolyte, LiPF6 as the electrolyte, and a Celgard 2400 microporous polyethylene membrane as the separator. The assembled battery was tested for electrical performance on a blue light tester. The specific capacity was tested by constant current charging / discharging at 0.1C for two weeks and then at 1C for two weeks in the voltage range of 2.0V-3.75V. There was a constant voltage process during the charging process, with a constant voltage of 3.75V and a constant voltage cutoff current of 50μA.

[0263] The capacity value at the extraction discharge voltage of 3.2 V is denoted as C1, the capacity value at the extraction discharge voltage of 2.0 V is denoted as C2, and η = C1 / C2, wherein the charging process includes constant voltage charging, constant voltage of 3.75 V, and constant voltage cutoff current of 50 μA.

[0264] 3. Solid content of the slurry

[0265] Prepare an electronic balance (accuracy 0.0001), an oven, and a glass drying tray. Take 8-10 g of the positive electrode slurry sample and evenly spread it on a sample tray. Record the mass of the slurry before drying as A. Close the oven door and start heating. As the heating proceeds, the temperature in the oven is continuously increased to 130 °C for 5 h. After drying, take out the sample from the oven and record the mass of the slurry after drying. Repeat the drying process several times, and record the mass of the sample after the mass is constant as B. The solid content of the slurry = (A / B) x 100%.

[0266] 4. Magnetic substance content

[0267] 1) Take 1 kg of the sample and put it into a plastic bucket. Add 6 L of deionized water. Use a plastic tube to cover a magnetic bar (magnetic field strength 6000 Gauss) with a diameter of 24 mm and a length of 240 mm. Then use a heat-sealing clamp to heat seal the magnetic bar in the plastic bucket. After sealing, stir the mixture at a speed of 60 rpm for 15 min.

[0268] 2) Prepare another clean bucket. Add 5±0.2 L of deionized water into the bucket. Flush the magnetic substance on the plastic tube into the solvent. Re-seal the magnetic bar. Repeat the above step 2 times to ensure the accuracy of the amount of the magnetic substance extracted.

[0269] 3) Prepare a clean 1 L beaker. Flush all the magnetic substance on the plastic tube into the beaker. Use a magnetic block to absorb the bottom of the beaker and rinse it 1-2 times.

[0270] 4) Use a measuring cylinder to take 10 mL of deionized water and add it into a 100 mL beaker. Then take 10 mL of 36%-38% hydrochloric acid and slowly add it into the beaker. Pour the prepared hydrochloric acid solution into the beaker. After sealing, put the beaker into an ultrasonic instrument and ultrasonicate for 2 min.

[0271] 5) After ultrasonication, take out the beaker and use a magnetic block outside the bottom of the beaker to absorb and gather the magnetic substance. Pour the acid solution in the beaker into a waste liquid bucket. Rinse the magnetic substance in the beaker 3 times and inject an appropriate amount of deionized water for filtration.

[0272] 6) Use a filter membrane with a pore size of 0.45 microns. After filtration, place the filter paper with the magnetic particles on the surface of a clean microscope slide and put it into an oven for drying at 60 °C for (10±2) min. After drying, weigh the filter membrane to calculate the content of the magnetic substance.

[0273] The higher the magnetic substance content, the more serious the self-discharge phenomenon of the battery, and the shorter the service life of the battery.

[0274] 5. Measurement of kinetic performance

[0275] The kinetic performance of the battery in the present application is specifically manifested as the measurement of the direct current internal resistance (power performance).

[0276] 25℃ power performance test:

[0277] Capacity calibration: after the lithium ion batteries prepared from each example and the comparative example were kept at 25℃ for 2h, they were charged at 0.33C to 3.65V, and then charged at 3.65V to 0.05C, after the charging was completed, the measured batteries were kept at 25℃ for 2h, and then discharged at 0.33C to 2.5V, and the normal temperature discharge capacity was recorded as C0;

[0278] Adjustment of SOC (state of charge of the battery): after the lithium ion battery with calibrated capacity was kept at 25℃ for 2h, it was discharged at 1 / 3C0 discharge rate for 144min to adjust the lithium ion battery capacity to 10% SOC;

[0279] Power test: after the lithium ion battery with 10% SOC was kept at 25℃ for 2h, it was discharged at 3C0 discharge rate under pulse current I for 30s, and the voltage before 3C0 discharge was recorded as V1, and the voltage at the end of 30s discharge was recorded as V2; the value of direct current internal resistance (V1-V2) / I was calculated, and the power performance of the battery could be represented by this data.

[0280] III. Parameters and performances in examples and comparative examples

[0281] The batteries of each example and the comparative example were prepared according to the above method, and each performance parameter was measured, and the specific values are shown in Table 1.

[0282] Table 1

[0283]

[0284] According to the content of Table 1, it can be seen that the primary average particle size, BET specific surface area and carbon content of the lithium iron phosphate salt particles obtained in Examples 1-17 of the present application all meet the limitations of the present application, and the results are that the η value is greater than or equal to 88%, indicating that the discharge power performance of the battery is strong, and when the battery is discharged to a low SOC (state of charge of the battery), it can still maintain good power performance, that is, when the battery is discharged at a large current at a low power, the voltage drop of the battery is small. In addition, the kinetic performance value of the lithium iron phosphate salt particles in the examples is low, indicating that the mechanical performance of the battery is excellent. The gravimetric capacity of the lithium iron phosphate salt particles is high, the solid content of the slurry is high, and the content of magnetic substances is low, indicating that the energy density and service life characteristics of the battery are both excellent.

[0285] 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.

[0286] 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 lithium iron phosphate salt granule, characterized in that, The lithium iron phosphate salt particles have a primary average particle size of 500nm-3000nm and a BET specific surface area of ​​3m². 2 / g-8m 2 / g, 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 0.8 ≤ Cx ≤ 2.

0.

2. The lithium iron phosphate salt particles according to claim 1, characterized in that, The primary average particle size of the lithium iron phosphate salt particles is 650 nm-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 lithium iron phosphate salt particles 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 lithium iron phosphate salt particles according to claim 3, characterized in that, The Q includes at least one of Ti, V, Mg, and Nb. Based on the total weight of the lithium iron phosphate salt particles, the content of Ti, V, Mg, and / or Nb is 1000ppm-10000ppm.

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

6. The lithium iron phosphate salt particles 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-3.75V, followed by one charge and discharge cycle 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 lithium iron phosphate salt particles according to any one of claims 1 to 6, characterized in that, The lithium iron phosphate salt particles satisfy at least one of a)-f): a) The Dv10 of the lithium iron phosphate salt particles 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 3 tons of pressure 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 lithium iron phosphate salt particles, characterized in that, include: Provide raw materials containing at least lithium source, iron source, phosphorus source, carbon source, carbon film-forming agent, and modifier, and perform at least two sintering processes, wherein after the first sintering, the raw materials are sequentially crushed, carbon source is added, and then sintered a second time. 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 lithium iron phosphate salt particles 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 lithium iron phosphate salt particles according to claim 8 or 9, characterized in that, After the second sintering, a second pulverization is performed, wherein... 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 lithium iron phosphate salt particles according to claim 10, characterized in that, The pulverization is one or more of grinding, sand milling, mechanical crushing, and air jet crushing.

12. The method for preparing lithium iron phosphate salt particles according to any one of claims 8 to 11, 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.

13. The method for preparing lithium iron phosphate salt particles according to any one of claims 8 to 12, 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 more of polyethylene glycol, polyaniline, polyacrylonitrile, polyvinylpyrrolidone, and polyvinyl alcohol.

14. The method for preparing lithium iron phosphate salt particles according to any one of claims 8 to 13, 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.

15. The method for preparing lithium iron phosphate salt particles according to any one of claims 8 to 14, 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%-1.99% by weight based on the total weight of the product after the first sintering.

16. The method for preparing lithium iron phosphate salt particles according to claim 15, 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.

17. A positive electrode plate, characterized in that, The lithium iron phosphate particles comprising any one of claims 1 to 7 or prepared by any one of claims 8 to 16 are used as positive electrode materials.

18. A secondary battery, characterized in that, It includes the positive electrode sheet as described in claim 17.

19. An electrical appliance, characterized in that, It includes the secondary battery as described in claim 18.

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