Secondary battery, method for manufacturing the same, and electric device

Abstract

A secondary battery includes: a positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode, and an electrolyte comprising an electrolyte salt and a solvent. The positive electrode includes a positive current collector and a positive electrode film disposed on at least one surface of the positive current collector. The positive electrode film includes first particles, the first particles including first lithium manganese iron phosphate particles, the primary average particle size s1 of the first particles being 120-600 nm, wherein the manganese leaching amount of the secondary battery is ≤50 ppm.

Description

Secondary battery, preparation method thereof and power-consuming device

[0001] Cross-references

[0002] This application claims priority to Chinese Patent Application No. 202410027059.3, filed on January 8, 2024, entitled “Secondary Battery, Preparation Method Thereof and Electrical Device”, which is incorporated herein by reference in its entirety. Technical Field

[0003] The present application relates to the technical field of secondary batteries, and in particular to a secondary battery, a preparation method thereof, and an electrical device. Background Art

[0004] As a positive electrode active material for lithium-ion batteries, manganese iron phosphate (LiFePO4) has a higher voltage platform, theoretical energy density, theoretical specific capacity, and lower cost than lithium iron phosphate. However, compared to lithium iron phosphate, LiFePO4 has poorer storage performance at high temperatures (below 60°C).

[0005] Therefore, it is necessary to provide a positive electrode active material containing lithium manganese iron phosphate, which has good high-temperature storage performance.

[0006] Summary of the Invention

[0007] The present application is made in view of the above-mentioned problems, and its purpose is to provide a secondary battery with good high-temperature storage performance, a preparation method thereof, and an electrical device.

[0008] The inventors have discovered that the above objectives can be achieved by adopting the technical solution of the present application.

[0009] A first aspect of the present application provides a secondary battery, comprising:

[0010] A positive electrode sheet, a negative electrode sheet, a separator disposed between the positive electrode sheet and the negative electrode sheet, and an electrolyte solution comprising an electrolyte salt and a solvent,

[0011] The positive electrode sheet includes a positive electrode current collector and a positive electrode film layer provided on at least one surface of the positive electrode current collector.

[0012] The positive electrode film layer includes first particles, the first particles include first manganese iron phosphate lithium salt particles, and the primary average particle size s1 of the first particles is 120-600 nm.

[0013] Among them, the manganese dissolution amount of the secondary battery is ≤50ppm.

[0014] The secondary battery of the present application has good high-temperature storage performance. In addition, the secondary battery of the present application also has good high-temperature cycle performance.

[0015] In any embodiment, the positive electrode film layer also includes second particles, the second particles include first lithium iron phosphate salt particles and / or second lithium manganese iron phosphate salt particles, the primary average particle size s2 of the second particles satisfies 500nm≤s2≤3000nm, and the Mn / Fe molar ratio of the second lithium manganese iron phosphate salt particles is ≤1.5.

[0016] When the positive electrode film layer also includes second particles, the second particles include first lithium iron phosphate salt particles and / or second lithium manganese iron phosphate salt particles, the primary average particle size s2 of the second particles satisfies 500nm≤s2≤3000nm, and the Mn / Fe molar ratio of the second lithium manganese iron phosphate salt particles is ≤1.5, the secondary battery of the present application has a lower manganese dissolution amount, better high-temperature storage performance, and better high-temperature cycle performance.

[0017] In any embodiment, based on the total area of ​​the primary particles of the positive electrode film layer, the area of ​​the first particles accounts for 60%-98%, and the area of ​​the second particles accounts for 2%-40%.

[0018] In any embodiment, the positive electrode film layer further includes first lithium iron phosphate salt particles, and the primary average particle size s2' of the first lithium iron phosphate salt particles satisfies 500 nm≤s2'≤3000 nm.

[0019] When the positive electrode film layer also includes first lithium iron phosphate salt particles, and the primary average particle size s2' of the first lithium iron phosphate salt particles satisfies 500nm≤s2'≤3000nm, the secondary battery of the present application has a lower manganese dissolution amount, better high-temperature storage performance, better high-temperature cycle performance, and a higher positive electrode slurry solid content.

[0020] In any embodiment, based on the total area of ​​the primary particles of the positive electrode film layer, the area of ​​the first manganese iron phosphate lithium salt particles accounts for 70%-98%, and the area of ​​the second lithium iron phosphate salt particles accounts for 2%-30%.

[0021] In any embodiment, the positive electrode film layer also includes third particles, the third particles include second lithium iron phosphate salt particles and / or third lithium manganese iron phosphate salt particles, the primary average particle size s3 of the third particles satisfies 30nm≤s3≤200nm, and the Mn / Fe molar ratio of the third lithium manganese iron phosphate salt particles is ≤1.5.

[0022] In any embodiment, based on the total area of ​​the primary particles of the positive electrode film layer, the area of ​​the first particles accounts for 70%-97%; the area of ​​the second particles accounts for 2%-25%; and the area of ​​the third particles accounts for 1%-10%.

[0023] In any embodiment, the positive electrode film layer further includes second lithium iron phosphate salt particles, and the primary average particle size s3 ′ of the second lithium iron phosphate salt particles satisfies 30 nm ≤ s3 ≤ 200 nm.

[0024] When the positive electrode film layer further includes second lithium iron phosphate salt particles and the primary average particle size s3' of the second lithium iron phosphate salt particles satisfies 30nm≤s3'≤200nm, the secondary battery of the present application has lower manganese dissolution, better high-temperature storage performance, and better high-temperature cycle performance.

[0025] In any embodiment, based on the total area of ​​the primary particles of the positive electrode film layer, the area of ​​the first manganese iron phosphate lithium salt particles accounts for 70%-97%; the area of ​​the first lithium iron phosphate salt particles accounts for 2-25%; and the area of ​​the second lithium iron phosphate salt particles accounts for 1-10%.

[0026] In any embodiment, the first lithium manganese iron phosphate salt particles have a molecular formula Li m1 Fe x1 Mn r1 P y1 O j1 Q1 q1 , wherein Q1 includes at least one of Al, Na, K, Mg, Cu, Cr, Zn, Pb, Ca, Co, Ni, Sr, Nb, V, Ti, B, S, Si, N, F, Cl, and Br, 0.95≤m1≤1.15, x1>0, r1>0, 0.9≤x1+r1≤1, 0.95≤y1≤1, 3.5≤j1≤4, 0≤q1≤0.1, and / or

[0027] The second lithium manganese iron phosphate salt particle has a molecular formula of Li m2 Fe x2 Mn r2 P y2 O j2 Q2 q2 , wherein Q2 includes at least one of Al, Na, K, Mg, Cu, Cr, Zn, Pb, Ca, Co, Ni, Sr, Nb, V, Ti, B, S, Si, N, F, Cl, and Br, 0.95≤m2≤1.15, x2>0, r2>0, r2 / x2≤1.5; 0.9≤x2+r2≤1, 0.95≤y2≤1, 3.5≤j2≤4, 0≤q2≤0.1, and / or

[0028] The third manganese iron phosphate lithium salt particle has the molecular formula Li m3 Fe x3 Mn r3 P y3 O j3 Q3 q3, wherein Q3 includes at least one of Al, Na, K, Mg, Cu, Cr, Zn, Pb, Ca, Co, Ni, Sr, Nb, V, Ti, B, S, Si, N, F, Cl, and Br, 0.95≤m3≤1.15, x3>0, r3>0, r3 / x3≤1.5; 0.9≤x3+r3≤1, 0.95≤y3≤1, 3.5≤j3≤4, 0≤q3≤0.1, and / or

[0029] The first lithium iron phosphate salt particles have a molecular formula of Li m4 Fe x4 P y4 O j4 Q4 q4 , wherein Q4 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, 0.95≤m4≤1.15, 0.9≤x4≤1, 0.95≤y4≤1, 3.5≤j4≤4, 0<q4≤0.1, and / or

[0030] The second lithium iron phosphate salt particles have a molecular formula of Li m5 Fe x5 P y5 O j5 Q2 q5 , wherein Q5 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, 0.95≤m5≤1.15, 0.9≤x5≤1, 0.95≤y5≤1, 3.5≤j5≤4, and 0≤q5≤0.1.

[0031] In any embodiment, in the first lithium iron phosphate salt particles, Q4 includes at least one of Ti, V, Mg, and Nb, and the content of Ti, V, Mg, and / or Nb is 1000-10000 ppm based on the total weight of the first lithium iron phosphate salt particles.

[0032] In any embodiment, the specific surface area (BET) of the first lithium iron phosphate particles is 3 m 2 / g-8m 2 / g.

[0033] When the specific surface area (BET) of the first lithium iron phosphate particles is 3 m 2 / g-8m 2 / g, the secondary battery of the present application has lower manganese dissolution amount, higher positive electrode slurry solid content, and higher positive electrode sheet compaction density.

[0034] In any embodiment, the carbon content of the first lithium iron phosphate salt particles is Cx4 wt % calculated based on the total weight of the first lithium iron phosphate salt particles, wherein 0.8≤Cx4≤2.0.

[0035] In any embodiment, based on the total weight of the first lithium iron phosphate salt particles, the carbon content of the first lithium iron phosphate salt particles is Cx4 weight %, and the ratio z4 of the specific surface area of ​​the first lithium iron phosphate salt particles to Cx4 satisfies 1.5≤z4≤8.5.

[0036] In any embodiment, the specific surface area of ​​the second manganese iron phosphate lithium salt particles is 5m 2 / g-12m 2 / g.

[0037] In any embodiment, the carbon content of the second lithium manganese iron phosphate salt particles is Cx2 wt % calculated based on the total weight of the second lithium manganese iron phosphate salt particles, wherein 0.8≤Cx2≤2.0.

[0038] In any embodiment, the primary average particle size s1' of the first lithium manganese iron phosphate salt particles is 150-210 nm, the primary average particle size s2' of the first lithium iron phosphate salt particles satisfies 870 nm ≤ s2 ≤ 3000 nm, and / or the primary average particle size s3' of the second lithium iron phosphate salt particles satisfies 60 nm ≤ s3 ≤ 200 nm.

[0039] In any embodiment, the capacity ratio of the first lithium iron phosphate salt particles is η≥88%, where η is defined as:

[0040] A battery using the first lithium iron phosphate salt particles as the positive electrode active material is charged and discharged twice at a constant current rate of 0.1C in the voltage range of 2.0V to 3.75V, and then charged and discharged once at a constant current rate of 1C. In the charge and discharge test at a rate of 1C, the capacity value at the discharge voltage of 3.2V is extracted and recorded as C1, the capacity value extracted and discharged to 2.0V is C2, and η=C1 / C2, wherein the charging process includes constant voltage charging, a constant voltage of 3.75V, and a constant voltage cutoff current of 50μA.

[0041] In any embodiment, the first lithium iron phosphate salt particles satisfy at least one of (a)-(f):

[0042] (a) Dv10 of the first lithium iron phosphate particles is ≥ 0.2 μm;

[0043] (b) the Dv50 of the first lithium iron phosphate particles is 0.5-5 μm;

[0044] (c) Dv90 of the first lithium iron phosphate particles is ≤ 10 μm;

[0045] (d) Dv99 of the first lithium iron phosphate particles is ≤ 12 μm;

[0046] (e) The powder compaction density of the first lithium iron phosphate salt under a pressure of 3 tons is ≥ 2.25 g / cm 3 ;

[0047] (f) The powder resistivity of the first lithium iron phosphate salt is less than 60Ω·cm.

[0048] In any embodiment, the aspect ratio of the second lithium iron phosphate salt particles is ≥ 1.3.

[0049] When the aspect ratio of the second lithium iron phosphate salt particles is ≥1.3, the secondary battery of the present application has better high-temperature storage performance, lower manganese dissolution, better high-temperature cycle performance, and higher gram capacity.

[0050] In any embodiment, the ratio W of the diffraction peak intensity of the (020) crystal plane to the diffraction peak intensity of the (211) crystal plane of the second lithium iron phosphate salt particles is ≥1.03.

[0051] When the ratio W of the (020) crystal plane diffraction peak intensity to the (211) crystal plane diffraction peak intensity of the second lithium iron phosphate salt particles is ≥1.03, the secondary battery of the present application has better high-temperature storage performance, lower manganese dissolution, better high-temperature cycle performance, and higher gram capacity.

[0052] In any embodiment, the first lithium iron phosphate salt particles are mainly obtained by the following preparation method:

[0053] Providing raw materials containing at least a lithium source, an iron source, a phosphorus source, optionally a carbon film-forming agent, optionally a carbon source, and optionally a modifier, and performing at least two sintering operations, wherein:

[0054] The temperature of the first sintering is 500℃-760℃, and can be optionally 550℃-720℃;

[0055] The temperature of the second sintering is 700°C-800°C, and can be optionally 720°C-780°C.

[0056] In any embodiment, the first lithium iron phosphate salt particles are mainly obtained by the following preparation method:

[0057] Providing raw materials containing at least a lithium source, an iron source, a phosphorus source, a carbon source, a carbon film-forming agent, and a modifier, and performing at least two sintering operations, wherein:

[0058] The carbon content of the material after the first sintering is 0.01 wt%-0.79 wt%, optionally 0.05 wt%-0.4 wt%;

[0059] The carbon content of the material after the second sintering is 0.8 wt%-2.0 wt%, and optionally 1.0 wt%-1.6 wt%.

[0060] In any embodiment, the preparation method of the first lithium iron phosphate salt particles comprises the following steps:

[0061] After the first sintering, a first crushing is performed, and after the second sintering, a second crushing is performed, wherein,

[0062] The Dv50 of the product after the first crushing is 300nm-1200nm, optionally 400nm-1100nm;

[0063] The Dv50 of the product after the second pulverization is 500nm-5000nm, and can be optionally 700nm-2500nm.

[0064] A second aspect of the present application provides an electric device comprising the secondary battery according to the first aspect of the present application. BRIEF DESCRIPTION OF THE DRAWINGS

[0065] FIG1 is a schematic diagram of a secondary battery according to an embodiment of the present application.

[0066] FIG. 2 is an exploded view of the secondary battery according to the embodiment of the present application shown in FIG. 1 .

[0067] FIG3 is a schematic diagram of a battery module according to an embodiment of the present application.

[0068] FIG4 is a schematic diagram of a battery pack according to an embodiment of the present application.

[0069] FIG. 5 is an exploded view of the battery pack shown in FIG. 4 according to an embodiment of the present application.

[0070] FIG6 is a schematic diagram of an electric device using a secondary battery as a power source according to an embodiment of the present application.

[0071] FIG7 is a scanning electron microscope image of first lithium iron phosphate salt particles with their primary particle sizes marked according to one embodiment of the present application.

[0072] Description of reference numerals:

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

[0074] Below, the embodiments of the secondary battery, its manufacturing method, and electrical device of the present application are described in detail with appropriate reference to the accompanying drawings. However, there may be cases where unnecessary detailed descriptions are omitted. For example, there are cases where detailed descriptions of well-known matters and repeated descriptions of actually the same structure are omitted. This is to avoid the following description from becoming unnecessarily lengthy and to facilitate the understanding of those skilled in the art. In addition, the drawings and the following description are provided for those skilled in the art to fully understand the present application and are not intended to limit the subject matter described in the claims.

[0075] " range " disclosed in the present application is limited in the form of lower limit and upper limit, and given range is limited by selecting a lower limit and an upper limit, and the selected lower limit and upper limit define the boundary of special range. The scope limited in this way can be to include end value or not include end value, and can be arbitrarily combined, that is, any lower limit can form a range with any upper limit combination. For example, if the scope of 60-120 and 80-110 is listed for specific parameters, it is understood that the scope of 60-110 and 80-120 is also expected. In addition, if the minimum range value 1 and 2 are listed, and if the maximum range value 3,4 and 5 are listed, then the following range can all be expected: 1-3, 1-4, 1-5, 2-3, 2-4 and 2-5. In this application, unless otherwise specified, the numerical range " ab " represents the abbreviation of any real number combination between a and b, wherein a and b are all real numbers. For example, a numerical range of "0-5" indicates that all real numbers between "0-5" are listed herein, and "0-5" is simply an abbreviation for these numerical combinations. Furthermore, when a parameter is expressed as an integer ≥ 2, this is equivalent to disclosing that the parameter is, for example, an integer of 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

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

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

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

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

[0080] Unless otherwise specified, the term "or" is used in this application to be inclusive. For example, the phrase "A or B" means "A, B, or both A and B." More specifically, the condition "A or B" is satisfied if any of the following conditions are met: A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or both A and B are true (or exist).

[0081] As a positive electrode active material for lithium-ion batteries, lithium manganese iron phosphate has a higher voltage platform, theoretical energy density, theoretical specific capacity, and lower cost than lithium iron phosphate. However, lithium manganese iron phosphate has poorer high-temperature (below 60°C) storage performance than lithium iron phosphate. Therefore, it is necessary to provide a positive electrode active material containing lithium manganese iron phosphate that has better high-temperature storage performance.

[0082] Based on this, this application proposes a technical solution to solve the above technical problems.

[0083] A first aspect of the present application provides a secondary battery, comprising:

[0084] A positive electrode sheet, a negative electrode sheet, a separator disposed between the positive electrode sheet and the negative electrode sheet, and an electrolyte solution comprising an electrolyte salt and a solvent,

[0085] The positive electrode sheet includes a positive electrode current collector and a positive electrode film layer provided on at least one surface of the positive electrode current collector.

[0086] The positive electrode film layer includes first particles, the first particles include first manganese iron phosphate lithium salt particles, and the primary average particle size s1 of the first particles is 120-600 nm.

[0087] Among them, the manganese dissolution amount of the secondary battery is ≤50ppm.

[0088] The ionic conductivity and electronic conductivity of lithium manganese iron phosphate salt are poor, and are greatly affected by the particle size. When the material contains lithium manganese iron phosphate salt with a larger particle size, the solid phase transport of lithium ions will be greatly restricted, and the electron transmission path during the electrochemical reaction is long, and the specific capacity and kinetic performance of the material will be significantly reduced. Studies have found that when the surface of the lithium manganese iron phosphate material is in a charged state, the surface oxidizing property is strong, and due to the Jan-Taylor effect, the surface oxygen atoms easily react with the protons of the electrolyte, resulting in manganese dissolution and electrolyte loss. Therefore, the particle size of lithium manganese iron phosphate salt should not be designed to contain too many small particles, that is, the micropowder content needs to be controlled at a low level. Experiments have found that the surface activity of small particles with a particle size of less than 120nm will increase significantly. The present application adopts lithium manganese iron phosphate salt with a lower manganese dissolution amount and optimizes the particle size at the same time, so that the secondary battery of the present application has better high temperature storage performance and better high temperature cycle performance.

[0089] In some embodiments, the primary average particle size s1 of the first particles may be 140-500 nm. In some embodiments, the primary average particle size s1 of the first particles may be 120 nm, 130 nm, 140 nm, 150 nm, 160 nm, 170 nm, 180 nm, 190 nm, 200 nm, 210 nm, 220 nm, 230 nm, 240 nm, 250 nm, 260 nm, 270 nm, 280 nm, 290 nm, 300 nm, 350 nm, 400 nm, 450 nm, 480 nm, 490 nm, 500 nm, 550 nm, 600 nm, or a range consisting of any two of the foregoing primary average particle sizes or a value within the range.

[0090] In the present application, the term "primary average particle size" refers to the average of the primary particle sizes of all particles, wherein the primary particle size refers to the longest distance connecting two points on the edge in a cross-sectional view.

[0091] The primary average particle size of the first particles can be measured by methods and equipment known in the art. For example, it can be tested by scanning electron microscopy and long diameter statistical method. Taking the first manganese iron phosphate lithium salt particles as an example, an argon ion beam is used to cut the electrode perpendicular to the large surface of the positive electrode plate to expose the cross section. The cross section is photographed using a scanning electron microscope, and the particle size of the first manganese iron phosphate lithium salt particles is statistically analyzed using the long diameter statistical method. Specifically, the total number of first manganese iron phosphate lithium salt particles with a primary particle size greater than 10nm and the sum of the primary particle sizes of the first manganese iron phosphate lithium salt particles with a primary particle size greater than 10nm can be counted in the electron microscope scanning photograph. The primary average particle size of the first manganese iron phosphate lithium salt particles = the primary particle size of the total first manganese iron phosphate lithium salt particles / the total number of first manganese iron phosphate lithium salt particles. In the primary particle size statistical process, particles with a primary particle size of 0 < ≤ 10nm are not within the statistical range.

[0092] In some embodiments, the amount of manganese leached out of the secondary battery is ≤48 ppm. In some embodiments, the amount of manganese leached out of the secondary battery is ≤45 ppm. In some embodiments, the amount of manganese leached out of the secondary battery is ≤40 ppm. In some embodiments, the amount of manganese leached out of the secondary battery is ≤38 ppm. In some embodiments, the amount of manganese leached out of the secondary battery is ≤37 ppm. In some embodiments, the amount of manganese leached out of the secondary battery is ≤35 ppm. In some embodiments, the amount of manganese leached out of the secondary battery is ≤33 ppm. In some embodiments, the amount of manganese leached out of the secondary battery is ≤31 ppm. In some embodiments, the amount of manganese leached out of the secondary battery is ≤30 ppm. In some embodiments, the amount of manganese leached out of the secondary battery is ≤28 ppm. In some embodiments, the amount of manganese leached out of the secondary battery is ≤25 ppm. In some embodiments, the amount of manganese leached out of the secondary battery is ≤24 ppm. In some embodiments, the amount of manganese leached out of the secondary battery is ≤20 ppm. In some embodiments, the amount of manganese leached out of the secondary battery is ≤18 ppm. In some embodiments, the amount of manganese leached out of the secondary battery is ≤17 ppm. In some embodiments, the manganese dissolution amount of the secondary battery is ≤16 ppm. In some embodiments, the manganese dissolution amount of the secondary battery is ≤15 ppm. In some embodiments, the manganese dissolution amount of the secondary battery is ≤14 ppm. In some embodiments, the manganese dissolution amount of the secondary battery is ≤12 ppm.

[0093] In some embodiments, the manganese dissolution amount refers to the manganese dissolution amount after 100 cycles at 25°C.

[0094] Manganese dissolution can be measured using methods and equipment known in the art. For example, an ICP test procedure can be used: retrieve a fully charged cell after 100 cycles at 25°C, disassemble it in a glove box, remove the anode electrode in a dry room, and use a ceramic knife to gently scrape 1g of material from the center of the electrode, ensuring that the entire active material layer is scraped off to avoid different manganese concentrations at different thicknesses. Seal the bag in a ziplock bag and send it to the laboratory. Prepare an electronic scale, a heating plate, a 150ml quartz beaker, a watch glass, a funnel, a 100ml volumetric flask, pliers (heat-insulating gloves), a weighing spoon, dust-free paper, etc.; confirm whether the calibration date of the electronic scale is within the validity period. If it exceeds the validity period, it cannot be used and needs to be recalibrated; turn on the power, and the heating plate can be heated to 250℃ normally; weigh 0.2000±0.005g of powder in a beaker; acidification treatment: prepare a dilute sulfuric acid solution with a volume ratio of 1:4 (concentrated sulfuric acid: ultrapure water = 1:4); add 20ml of the prepared dilute sulfuric acid solution to the beaker; First, turn on the heating plate to preheat. After the temperature reaches 250°C, place the sample in the beaker for digestion for 30 minutes. Place a quartz cover on the mouth of the beaker to reduce acid volatilization. After the sample is digested for 30 minutes, remove it and cool it to room temperature. Transfer the digested sample to a 100ml glass volumetric flask and make up the volume. Then, follow the ICP standard test process: the test equipment temperature is 22+2°C and the humidity is <60%. Select the trace element method as the test method. Run the calibration standard: use the prepared standard solution to determine the standard curve. Select the element as Mn. Finally, the Mn dissolution amount can be obtained through testing.

[0095] In some embodiments, the positive electrode film layer also includes second particles, the second particles include first lithium iron phosphate salt particles and / or second lithium manganese iron phosphate salt particles, the primary average particle size s2 of the second particles satisfies 500nm≤s2≤3000nm, and the Mn / Fe molar ratio of the second lithium manganese iron phosphate salt particles is ≤1.5.

[0096] In a specific embodiment, as shown in the scanning electron microscope image of the first lithium iron phosphate salt particle in FIG7 , the line segment marked by the double arrow in the particle is the primary particle size defined in this application.

[0097] In some embodiments, the second particles are primary particles.

[0098] In this article, "primary particles" refer to particles that do not have obvious agglomeration interfaces in the particle scanning electron microscope image, but may have tiny pores and point or line defects, which are different from the smallest unit powder particles without structures such as stacking and flocculation.

[0099] The primary average particle size of the second particles can be measured by methods and equipment known in the art. For example, it can be tested by scanning electron microscopy and length diameter statistics. As an example, an argon ion beam is used to cut the electrode perpendicular to the large surface of the positive electrode to expose the cross section. The cross section is photographed using a scanning electron microscope, and the particle size of the second particles is statistically analyzed using the length diameter statistics method. Specifically, the total number of second particles with a primary particle size greater than 80 nm and the sum of the primary particle sizes of the second particles with a primary particle size greater than 80 nm can be counted in the electron microscope scanning photograph. The primary average particle size of the second particles = the primary particle size of the total second particles / the total number of second particles. In the process of primary particle size statistics, particles with a primary particle size of 0 < ≤ 80 nm are not within the statistical range.

[0100] By setting the primary average particle size of the second particles to 500nm-3000nm, the particles can be kept at a suitable micron-level size, thereby avoiding the problems of interfacial side reactions and processing difficulties caused by nano-sizing the particles, and preventing the particles from being limited by an excessively large size and thus reducing the kinetic performance. In addition, it is beneficial to the stirring of the slurry containing the second particles and the increase of the solid content, thereby improving the processing problems of the battery cell and further increasing the volume energy density of the battery. In addition, it can avoid the normal deintercalation and extraction of lithium ions due to the excessively high density of the carbon coating, thereby affecting the performance of the battery cell capacity.

[0101] When the positive electrode film layer also includes second particles, the second particles include first lithium iron phosphate salt particles and / or second lithium manganese iron phosphate salt particles, the primary average particle size s2 of the second particles satisfies 500nm≤s2≤3000nm, and the Mn / Fe molar ratio of the second lithium manganese iron phosphate salt particles is ≤1.5, by mixing a portion of the larger second particles into the lithium manganese iron phosphate salt material, close stacking can be achieved. At the same time, since the large particle component is lithium iron phosphate salt or lithium manganese iron phosphate salt with a low Mn / Fe molar ratio, the kinetic properties of the material are better than those of the large particle lithium manganese iron phosphate salt with a high Mn / Fe molar ratio. After the second particles crack during the cycle, since the particles do not contain manganese elements or have a low manganese content, the corresponding material's oxidizability causes the problem of electrolyte oxidation and manganese dissolution causes damage to the negative electrode SEI film. The problem will be alleviated, so that the secondary battery of the present application has a lower manganese dissolution amount, better high-temperature storage performance, and better high-temperature cycle performance.

[0102] In some embodiments, the primary average particle size of the second particles can be 250nm, 300nm, 350nm, 400nm, 440nm, 500nm, 550nm, 600nm, 650nm, 700nm, 730nm, 750nm, 780nm, 800nm, 830nm, 850nm, 870nm, 900nm, 950nm, 1000nm, 1100nm, 1200nm, 1300nm, 1400nm, 1500nm, 2000nm, 2100nm, 2200nm, 2300nm, 2400nm, 2500nm, 2600nm, 2700nm, 2800nm, 2900nm, 3000nm, 3500nm, 4000nm, or a range consisting of any two of the above primary average particle sizes or a value within the range.

[0103] In some embodiments, the primary average particle size s2 of the second particles satisfies 500nm≤s2≤3500nm. In some embodiments, the primary average particle size s2 of the second particles satisfies 500nm≤s2≤4000nm. In some embodiments, the primary average particle size s2 of the second particles satisfies 300nm≤s2≤3500nm. In some embodiments, the primary average particle size s2 of the second particles satisfies 500nm≤s2≤3000nm. In some embodiments, the primary average particle size s2 of the second particles satisfies 650nm≤s2≤2500nm. In some embodiments, the primary average particle size s2 of the second particles satisfies 650nm≤s2≤3000nm. In some embodiments, the primary average particle size s2 of the second particles satisfies 650nm≤s2≤4000nm. In some embodiments, the primary average particle size s2 of the second particles satisfies 700nm≤s2≤3000nm. In some embodiments, the primary average particle size s2 of the second particles satisfies 700 nm ≤ s2 ≤ 4000 nm. In some embodiments, the primary average particle size s2 of the second particles satisfies 800 nm ≤ s2 ≤ 3000 nm. In some embodiments, the primary average particle size s2 of the second particles satisfies 700 nm ≤ s2 ≤ 4000 nm.

[0104] In some embodiments, based on the total area of ​​the primary particles of the positive electrode film layer, the area of ​​the first particles accounts for 60%-98%, and the area of ​​the second particles accounts for 2%-40%.

[0105] In some embodiments, the area of ​​the first particles accounts for 70% to 98% of the total area of ​​the primary particles of the positive electrode film layer. In some embodiments, the area of ​​the first particles accounts for 65%, 70%, 72%, 74%, 75%, 97%, or 98%.

[0106] In some embodiments, the area proportion of the second particles is 2-30% based on the total area of ​​the primary particles of the positive electrode film layer. In some embodiments, the area proportion of the second particles is 2%, 23%, 25%, 30%, or a range consisting of any two of the foregoing area proportions or a value within the range.

[0107] The areas of the first particles and the second particles can be obtained by statistically analyzing the obtained scanning electron microscope images of the pole piece cross section using Avizo 3D software, and the corresponding area ratios can be obtained by dividing the areas of the first particles and the second particles by the total area.

[0108] In some embodiments, the second particles do not include second lithium manganese iron phosphate salt particles.

[0109] In some embodiments, the positive electrode film layer further includes first lithium iron phosphate salt particles, and the primary average particle size s2' of the first lithium iron phosphate salt particles satisfies 500 nm ≤ s2' ≤ 3000 nm.

[0110] In some embodiments, the first lithium iron phosphate salt particles are primary particles.

[0111] By setting the primary average particle size of the first lithium iron phosphate salt particles to 500nm-3000nm, the particles can be kept at a suitable micron-level size, thereby avoiding the problems of interfacial side reactions and processing difficulties caused by nano-sizing the particles, and the particles will not be limited by excessive size and thus suffer a reduction in dynamic performance; in addition, it is beneficial to the stirring of the slurry containing the first lithium iron phosphate salt particles and the increase of the solid content, thereby improving the processing problem of the battery cell and further increasing the volume energy density of the battery; in addition, it can avoid the normal deintercalation and extraction of lithium ions due to the excessive density of the carbon coating, thereby affecting the performance of the battery cell capacity.

[0112] When the positive electrode film layer also includes first lithium iron phosphate salt particles, and the primary average particle size s2' of the first lithium iron phosphate salt particles satisfies 500nm≤s2'≤3000nm, close stacking can be achieved by mixing a portion of the first lithium iron phosphate salt large particles into the manganese iron phosphate lithium salt material. At the same time, since the large particle component is lithium iron phosphate salt, the kinetic properties of the material are better than those of large particles of manganese iron phosphate lithium salt. After the large particles of lithium iron phosphate salt crack during the cycle, since the particles do not contain manganese elements, the problems of electrolyte oxidation caused by the oxidizing property of the corresponding material and damage to the negative electrode SEI film caused by manganese dissolution will be alleviated, so that the secondary battery of the present application has a lower manganese dissolution amount, better high-temperature storage performance, better high-temperature cycle performance, and a higher positive electrode slurry solid content.

[0113] In some embodiments, the primary average particle size of the first lithium iron phosphate particles can be 250nm, 300nm, 350nm, 400nm, 440nm, 500nm, 550nm, 600nm, 650nm, 700nm, 730nm, 750nm, 780nm, 800nm, 830nm, 850nm, 870nm, 900nm, 950nm, 1000nm, 1100nm, 1200nm, 1300nm, 1400nm, 1500nm, 2000nm, 2100nm, 2200nm, 2300nm, 2400nm, 2500nm, 2600nm, 2700nm, 2800nm, 2900nm, 3000nm, 3500nm, 4000nm, or a range consisting of any two of the above primary average particle sizes or a value in the range.

[0114] In some embodiments, the primary average particle size s2' of the first lithium iron phosphate salt particles satisfies 500 nm ≤ s2' ≤ 3500 nm.

[0115] When the primary average particle size s2' of the first lithium iron phosphate salt particles satisfies 500nm≤s2'≤3500nm, the secondary battery of the present application has lower manganese dissolution, better high-temperature storage performance, better high-temperature cycle performance, and higher positive electrode slurry solid content.

[0116] In some embodiments, the primary average particle size s2' of the first lithium iron phosphate salt particles satisfies 500 nm ≤ s2' ≤ 4000 nm.

[0117] When the primary average particle size s2' of the first lithium iron phosphate salt particles satisfies 500 nm ≤ s2' ≤ 4000 nm, the secondary battery of the present application has lower manganese dissolution amount, better high-temperature storage performance, and higher positive electrode slurry solid content.

[0118] In some embodiments, the primary average particle size s2' of the first lithium iron phosphate salt particles satisfies 300 nm ≤ s2' ≤ 3500 nm. In some embodiments, the primary average particle size s2' of the first lithium iron phosphate salt particles satisfies 500 nm ≤ s2' ≤ 3000 nm. In some embodiments, the primary average particle size s2' of the first lithium iron phosphate salt particles satisfies 650 nm ≤ s2' ≤ 2500 nm. In some embodiments, the primary average particle size s2' of the first lithium iron phosphate salt particles satisfies 650 nm ≤ s2' ≤ 3000 nm. In some embodiments, the primary average particle size s2' of the first lithium iron phosphate salt particles satisfies 650 nm ≤ s2' ≤ 4000 nm. In some embodiments, the primary average particle size s2' of the first lithium iron phosphate salt particles satisfies 700 nm ≤ s2' ≤ 3000 nm. In some embodiments, the primary average particle size s2' of the first lithium iron phosphate salt particles satisfies 700 nm ≤ s2' ≤ 4000 nm. In some embodiments, the primary average particle size s2' of the first lithium iron phosphate salt particles satisfies 800 nm ≤ s2' ≤ 3000 nm. In some embodiments, the primary average particle size s2' of the first lithium iron phosphate salt particles satisfies 800 nm ≤ s2' ≤ 4000 nm.

[0119] In some embodiments, the first lithium iron phosphate salt particles are single crystal particles and / or polycrystalline particles.

[0120] In this application, the term "single crystal" refers to a structurally complete crystal grown from a single nucleus. The single crystal in this application appears as a whole under a transmission electron microscope, with no grain boundaries within the single crystal.

[0121] In some embodiments, the single crystal of the present application may have tiny defects, for example, micropores inside, a small number of points and surfaces, or a small number of particles adhering to each other on the surface of a particle.

[0122] In the present application, the term "polycrystalline" refers to a crystal composed of small single crystal particles randomly oriented together, with grain boundaries existing within the polycrystalline.

[0123] In some embodiments, the proportion of single crystal particles is greater than or equal to 90% based on the total number of first lithium iron phosphate particles. Controlling the proportion of single crystal particles within the above range results in a higher proportion of single crystal particles compared to polycrystalline particles and secondary agglomerates, and the grain boundaries provide less barrier to lithium ions, which further increases the lithium ion transmission rate and improves the dynamic performance of the secondary battery.

[0124] The primary average particle size of the first lithium iron phosphate salt particles can be measured by methods and equipment known in the art. For example, it can be tested by scanning electron microscopy and long diameter statistical method. As an example, an argon ion beam is used to cut the electrode perpendicular to the large surface of the positive electrode plate to expose the cross section. The cross section is photographed by a scanning electron microscope, and the particle size of the first lithium iron phosphate salt particles is statistically analyzed by the long diameter statistical method. Specifically, the total number of first lithium iron phosphate salt particles with a primary particle size greater than 80nm and the sum of the primary particle sizes of the first lithium iron phosphate salt particles with a primary particle size greater than 80nm can be counted in the electron microscope scanning photo. The primary average particle size of the first lithium iron phosphate salt particles = the primary particle size of the total first lithium iron phosphate salt particles / the total number of first lithium iron phosphate salt particles. In the primary particle size statistical process, particles with a primary particle size of 0 < ≤ 80nm are not within the statistical range.

[0125] In some embodiments, based on the total area of ​​the primary particles of the positive electrode film layer, the area of ​​the first manganese iron phosphate lithium salt particles accounts for 70%-98%, and the area of ​​the second lithium iron phosphate salt particles accounts for 2%-30%.

[0126] In some embodiments, the area of ​​the lithium manganese iron phosphate particles accounts for 70% to 98% of the total area of ​​the primary particles of the positive electrode film layer. In some embodiments, the area of ​​the lithium manganese iron phosphate particles accounts for 65%, 70%, 72%, 74%, 75%, 97%, or 98%.

[0127] In some embodiments, the area ratio of the first lithium iron phosphate salt particles is 2-30% based on the total area of ​​the primary particles of the positive electrode film layer. In some embodiments, the area ratio of the first lithium iron phosphate salt particles is 2%, 23%, 25%, 30%, or a range consisting of any two of the above area ratios or a value within the range.

[0128] The areas of the first lithium iron phosphate salt particles and the second lithium iron phosphate salt particles can be obtained by statistically analyzing the obtained electrode cross-section scanning electron microscope images using Avizo 3D software, and the corresponding area ratios can be obtained by dividing the areas of the first lithium iron phosphate salt particles and the second lithium iron phosphate salt particles by the total area.

[0129] In some embodiments, the second particles do not include the first lithium iron phosphate salt particles.

[0130] In some embodiments, the positive electrode film layer further includes second manganese iron phosphate lithium salt particles, and the primary average particle size s2" of the second manganese iron phosphate lithium salt particles satisfies 500 nm≤s2"≤3000 nm.

[0131] In some embodiments, the second lithium manganese iron phosphate salt particles are primary particles.

[0132] By setting the primary average particle size of the second manganese iron phosphate lithium salt particles to 500nm-3000nm, the particles can be kept at a suitable micron-level size, thereby avoiding the problems of interfacial side reactions and processing difficulties caused by nano-sizing the particles, and preventing the particles from being limited by an excessively large size and thus reducing the dynamic performance; in addition, it is beneficial to the stirring of the slurry containing the second manganese iron phosphate lithium salt particles and the increase of the solid content, thereby improving the processing problem of the battery cell and further increasing the volume energy density of the battery; in addition, it can avoid the normal deintercalation and extraction of lithium ions due to the excessively high density of the carbon coating, thereby affecting the performance of the battery cell capacity.

[0133] When the positive electrode film layer also includes second manganese iron phosphate lithium salt particles, and the primary average particle size s2" of the second manganese iron phosphate lithium salt particles satisfies 500nm≤s2"≤3000nm, by mixing a portion of the second manganese iron phosphate lithium salt large particles into the manganese iron phosphate lithium salt material, tight stacking can be achieved. At the same time, since the large particle component is manganese iron phosphate lithium salt with a low Mn / Fe molar ratio, the material has better kinetic properties than the large particles of manganese iron phosphate lithium salt with a high Mn / Fe molar ratio. After the second manganese iron phosphate lithium salt large particles crack during the cycle, due to the low manganese content inside the particles, the problem of electrolyte oxidation caused by the oxidizability of the corresponding material and the problem of manganese dissolution causing damage to the negative electrode SEI film will be alleviated, so that the secondary battery of the present application has a lower manganese dissolution amount, better high-temperature storage performance, and better high-temperature cycle performance.

[0134] In some embodiments, the primary average particle size of the second manganese iron phosphate lithium salt particles can be 250nm, 300nm, 350nm, 400nm, 440nm, 500nm, 550nm, 600nm, 650nm, 700nm, 730nm, 750nm, 780nm, 800nm, 830nm, 850nm, 870nm, 900nm, 950nm, 1000nm, 1100nm, 1200nm, 1300nm, 1400nm, 1500nm, 2000nm, 2100nm, 2200nm, 2300nm, 2400nm, 2500nm, 2600nm, 2700nm, 2800nm, 2900nm, 3000nm, 3500nm, 4000nm, or a range consisting of any two of the above primary average particle sizes or a value in the range.

[0135] In some embodiments, the primary average particle size s2″ of the second manganese iron lithium phosphate salt particles satisfies 500nm≤s2″≤3500nm. In some embodiments, the primary average particle size s2″ of the second manganese iron lithium phosphate salt particles satisfies 500nm≤s2″≤4000nm. In some embodiments, the primary average particle size s2″ of the second manganese iron lithium phosphate salt particles satisfies 300nm≤s2″≤3500nm. In some embodiments, the primary average particle size s2″ of the second manganese iron lithium phosphate salt particles satisfies 500nm≤s2″≤3000nm. In some embodiments, the primary average particle size s2″ of the second manganese iron lithium phosphate salt particles satisfies 650nm≤s2″≤2500nm. In some embodiments, the primary average particle size s2″ of the second manganese iron lithium phosphate salt particles satisfies 650nm≤s2″≤3000nm. In some embodiments, the primary average particle size s2″ of the second manganese iron lithium phosphate salt particles satisfies 650nm≤s2″≤4000nm. In some embodiments, the primary average particle size s2" of the second manganese iron phosphate lithium salt particles satisfies 700nm≤s2"≤3000nm. In some embodiments, the primary average particle size s2" of the second manganese iron phosphate lithium salt particles satisfies 700nm≤s2"≤4000nm. In some embodiments, the primary average particle size s2" of the second manganese iron phosphate lithium salt particles satisfies 800nm≤s2"≤3000nm. In some embodiments, the primary average particle size s2" of the second manganese iron phosphate lithium salt particles satisfies 700nm≤s2"≤4000nm.

[0136] In some embodiments, the second lithium manganese iron phosphate salt particles are single crystal particles and / or polycrystalline particles.

[0137] In some embodiments, the proportion of single crystal particles is greater than or equal to 90% based on the total number of particles of the second manganese iron phosphate lithium salt. Controlling the proportion of single crystal particles within the above range increases the proportion of single crystal particles compared to polycrystalline particles and secondary agglomerates, and the grain boundaries provide less barrier to lithium ions, which helps further increase the lithium ion transmission rate and improve the dynamic performance of the secondary battery.

[0138] The primary average particle size of the second manganese iron phosphate lithium salt particles can be measured by methods and equipment known in the art. For example, it can be tested by scanning electron microscopy and long diameter statistical method. As an example, an argon ion beam is used to cut the electrode perpendicular to the large surface of the positive electrode plate to expose the cross section. The cross section is photographed using a scanning electron microscope, and the particle size of the second manganese iron phosphate lithium salt particles is statistically analyzed using the long diameter statistical method. Specifically, the total number of second manganese iron phosphate lithium salt particles with a primary particle size greater than 80nm and the sum of the primary particle sizes of the second manganese iron phosphate lithium salt particles with a primary particle size greater than 80nm can be counted in the electron microscope scanning photo. The primary average particle size of the second manganese iron phosphate lithium salt particles = the primary particle size of the total second manganese iron phosphate lithium salt particles / the total number of second manganese iron phosphate lithium salt particles. In the primary particle size statistical process, particles with a primary particle size of 0 < ≤ 80nm are not within the statistical range.

[0139] In some embodiments, based on the total area of ​​the primary particles of the positive electrode film layer, the area of ​​the first manganese iron phosphate lithium salt particles accounts for 70%-98%, and the area of ​​the second manganese iron phosphate lithium salt particles accounts for 2%-30%.

[0140] In some embodiments, the area of ​​the lithium manganese iron phosphate particles accounts for 70% to 98% of the total area of ​​the primary particles of the positive electrode film layer. In some embodiments, the area of ​​the lithium manganese iron phosphate particles accounts for 65%, 70%, 72%, 74%, 75%, 97%, or 98%.

[0141] In some embodiments, the second lithium manganese iron phosphate particles account for 2-30% of the total area of ​​the primary particles of the positive electrode film layer. In some embodiments, the second lithium manganese iron phosphate particles account for 2%, 23%, 25%, 30%, or a range consisting of any two of the foregoing area percentages or a value within the range.

[0142] The areas of the first manganese iron phosphate lithium salt particles and the second manganese iron phosphate lithium salt particles can be obtained by statistically analyzing the obtained electrode cross-section scanning electron microscope images using Avizo 3D software, and the corresponding area ratio can be obtained by dividing the respective areas by the total area.

[0143] In some embodiments, the positive electrode film layer also includes third particles, the third particles include second lithium iron phosphate salt particles and / or third lithium manganese iron phosphate salt particles, the primary average particle size s3 of the third particles satisfies 30nm≤s3≤200nm, and the Mn / Fe molar ratio of the third lithium manganese iron phosphate salt particles is ≤1.5.

[0144] By adding a small amount of ultrafine particles to the positive electrode film layer, the space utilization rate can be further improved. The ultrafine particles can be adjusted to the gap between the medium-sized first manganese iron phosphate lithium salt particles and the ultra-large first manganese iron phosphate lithium salt particles / second manganese iron phosphate lithium salt particles, further improving the volume energy density, that is, further improving the compaction density of the electrode. The ratio of ultrafine lithium iron phosphate or manganese iron phosphate lithium salt with a low Mn / Fe molar ratio to ultrafine manganese iron phosphate lithium salt with a high Mn / Fe molar ratio will have some loss on the voltage platform, but the gram capacity and surface stability have been greatly improved, especially the surface stability. The ultrafine manganese iron phosphate lithium salt material with a high Mn / Fe molar ratio is difficult to form a good coating. In the charging state, the surface has a strong oxidizing property. At the same time, due to the Jan-Taylor effect, the small particles have a high specific surface energy, and the problem of manganese dissolution is prominent, which worsens the battery cycle life. The secondary battery of the present application controls the first manganese iron phosphate lithium salt particles to have a relatively moderate particle size, contains less ultrafine powder and ultra-large particles, and the material exhibits good cycle life and gram capacity. Furthermore, the secondary battery of the present application achieves a good compaction density by adding large-particle lithium iron phosphate (LiFePO4) with a low Mn / Fe molar ratio and small-particle lithium iron phosphate (LiFePO4) with a low Mn / Fe molar ratio to the positive electrode film layer. For the positive electrode active material, an idealized dense packing model indicates that to achieve a good compaction density, the material must contain some large particles and ultrafine powder to form a dense packing, thereby achieving a high positive electrode sheet compaction density while achieving good high-temperature cycle performance.

[0145] In some embodiments, the positive electrode film layer further includes second lithium iron phosphate salt particles, and the primary average particle size s3' of the second lithium iron phosphate salt particles satisfies 30 nm ≤ s3' ≤ 100 nm.

[0146] In some embodiments, the positive electrode film layer also includes second lithium iron phosphate salt particles, and the primary average particle size of the second lithium iron phosphate salt particles can be 30nm, 40nm, 50nm, 60nm, 70nm, 80nm, 90nm, 100nm, 110nm, 120nm, 130nm, 140nm, 150nm, 160nm, 170nm, 180nm, 190nm, 200nm, or a range consisting of any two of the above primary average particle sizes or a value within the range.

[0147] The primary average particle size of the third particles can be measured by methods and equipment known in the art. For example, it can be tested by scanning electron microscopy and long diameter statistical method. As an example, an argon ion beam is used to cut the electrode perpendicular to the large surface of the positive electrode to expose the cross section. The cross section is photographed by a scanning electron microscope, and the particle size of the third particles is statistically analyzed by the long diameter statistical method. Specifically, the total number of third particles with a primary particle size greater than 10nm and the sum of the primary particle sizes of the third particles with a primary particle size greater than 10nm can be counted in the electron microscope scanning photo. The primary average particle size of the third particles = the primary particle size of the total third particles / the total number of third particles. In the process of primary particle size statistics, particles with a primary particle size of 0 < ≤ 10nm are not within the statistical range.

[0148] In some embodiments, based on the total area of ​​the primary particles of the positive electrode film layer, the area of ​​the first particles accounts for 70%-97%; the area of ​​the second particles accounts for 2%-25%; and the area of ​​the third particles accounts for 1%-10%.

[0149] In some embodiments, the area of ​​the first particles accounts for 70% to 97% of the total area of ​​the primary particles in the positive electrode film. In some embodiments, the area of ​​the first particles accounts for 65%, 70%, 72%, 74%, 75%, 97%, or 98% of the total area of ​​the primary particles in the positive electrode film. In some embodiments, the area of ​​the first particles accounts for 65% to 74% of the total area of ​​the primary particles in the positive electrode film.

[0150] In some embodiments, the area ratio of the second particles is 2-25% based on the total area of ​​the primary particles of the positive electrode film layer. In some embodiments, the area ratio of the second particles is 2%, 23%, 25%, 30%, or a range consisting of any two of the above area ratios or a value within the range based on the total area of ​​the primary particles of the positive electrode film layer. In some embodiments, the area ratio of the second particles is 23-25% based on the total area of ​​the primary particles of the positive electrode film layer.

[0151] In some embodiments, the area ratio of the third particles is 1-10% based on the total area of ​​the primary particles of the positive electrode film layer. In some embodiments, the area ratio of the third particles is 1%, 3%, 5%, 10%, or a range consisting of any two of the above area ratios or a value within the range based on the total area of ​​the primary particles of the positive electrode film layer.

[0152] The areas of the first particle, the second particle, and the third particle can be obtained by statistically analyzing the obtained scanning electron microscope images of the pole piece cross section using Avizo 3D software, and the corresponding area ratio can be obtained by dividing the area of ​​each particle by the total area.

[0153] In some embodiments, the third particles do not include third lithium manganese iron phosphate particles.

[0154] In some embodiments, the positive electrode film layer further includes second lithium iron phosphate salt particles, and the primary average particle size s3' of the second lithium iron phosphate salt particles satisfies 30 nm ≤ s3' ≤ 200 nm.

[0155] By adding a small amount of ultrafine particles to the positive electrode film layer, the space utilization rate can be further improved. The ultrafine particles can be adjusted to the gap between the medium manganese iron phosphate lithium salt particles and the ultra-large manganese iron phosphate lithium salt particles, further improving the volume energy density, that is, further improving the compaction density of the electrode. The ratio of ultrafine lithium iron phosphate to ultrafine manganese iron phosphate lithium salt will have some loss on the voltage platform, but the gram capacity and surface stability have been greatly improved, especially the surface stability. Ultrafine manganese iron phosphate lithium salt materials with a high Mn / Fe molar ratio are difficult to form a good coating. In the charged state, the surface has a strong oxidizing property. At the same time, due to the Jan-Taylor effect, the small particles have a high specific surface energy, and the problem of manganese dissolution is prominent, which worsens the battery cycle life. The secondary battery of the present application controls the manganese iron phosphate lithium salt particles to have a relatively moderate particle size, contains less ultrafine powder and ultra-large particles, and the material exhibits a good cycle life and gram capacity. In addition, the secondary battery of the present application ensures a good compaction density by adding large particles of lithium iron phosphate and small particles of lithium iron phosphate materials to the positive electrode film layer. For the positive electrode active material, deduction from an idealized close-packing model shows that to achieve a good compaction density, the material must contain some large particles and ultrafine powder to form a dense packing, thereby achieving good high-temperature cycling performance while also having a high positive electrode sheet compaction density. When the positive electrode film layer also includes second lithium iron phosphate salt particles, and the primary average particle size s3' of the second lithium iron phosphate salt particles satisfies 30nm≤s3'≤200nm, the secondary battery of the present application has low manganese dissolution, good high-temperature storage performance, and good high-temperature cycling performance.

[0156] In some embodiments, the positive electrode film layer further includes second lithium iron phosphate salt particles, and the primary average particle size s3' of the second lithium iron phosphate salt particles satisfies 30 nm ≤ s3' ≤ 100 nm.

[0157] In some embodiments, the positive electrode film layer also includes second lithium iron phosphate salt particles, and the primary average particle size of the second lithium iron phosphate salt particles can be 30nm, 40nm, 50nm, 60nm, 70nm, 80nm, 90nm, 100nm, 110nm, 120nm, 130nm, 140nm, 150nm, 160nm, 170nm, 180nm, 190nm, 200nm, or a range consisting of any two of the above primary average particle sizes or a value within the range.

[0158] The primary average particle size of the second lithium iron phosphate salt particles can be measured by methods and equipment known in the art. For example, it can be tested by scanning electron microscopy and long diameter statistical method. As an example, an argon ion beam is used to cut the electrode perpendicular to the large surface of the positive electrode plate to expose the cross section. The cross section is photographed using a scanning electron microscope, and the particle size of the second lithium iron phosphate salt particles is statistically analyzed using the long diameter statistical method. Specifically, the total number of second lithium iron phosphate salt particles with a primary particle size greater than 10nm and the sum of the primary particle sizes of the second lithium iron phosphate salt particles with a primary particle size greater than 10nm can be counted in the electron microscope scanning photograph. The primary average particle size of the second lithium iron phosphate salt particles = the primary particle size of the total second lithium iron phosphate salt particles / the total number of second lithium iron phosphate salt particles. In the primary particle size statistical process, particles with a primary particle size of 0 < ≤ 10nm are not within the statistical range.

[0159] In some embodiments, based on the total area of ​​the primary particles of the positive electrode film layer, the area of ​​the first manganese iron phosphate lithium salt particles accounts for 70%-97%; the area of ​​the first lithium iron phosphate salt particles accounts for 2-25%; and the area of ​​the second lithium iron phosphate salt particles accounts for 1-10%.

[0160] In some embodiments, the area of ​​the first lithium manganese iron phosphate salt particles accounts for 70% to 97% of the total area of ​​the primary particles of the positive electrode film layer. In some embodiments, the area of ​​the first lithium manganese iron phosphate salt particles accounts for 65%, 70%, 72%, 74%, 75%, 97%, or 98% of the total area of ​​the primary particles of the positive electrode film layer. In some embodiments, the area of ​​the first lithium manganese iron phosphate salt particles accounts for 65% to 74% of the total area of ​​the primary particles of the positive electrode film layer.

[0161] In some embodiments, the area ratio of the first lithium iron phosphate salt particles is 2-25% based on the total area of ​​the primary particles of the positive electrode film layer. In some embodiments, the area ratio of the first lithium iron phosphate salt particles is 2%, 23%, 25%, 30%, or a range consisting of any two of the above area ratios or a value within the range based on the total area of ​​the primary particles of the positive electrode film layer. In some embodiments, the area ratio of the first lithium iron phosphate salt particles is 23-25% based on the total area of ​​the primary particles of the positive electrode film layer.

[0162] In some embodiments, the area ratio of the second lithium iron phosphate salt particles is 1-10% based on the total area of ​​the primary particles of the positive electrode film layer. In some embodiments, the area ratio of the second lithium iron phosphate salt particles is 1%, 3%, 5%, 10%, or a range consisting of any two of the above area ratios or a value within the range based on the total area of ​​the primary particles of the positive electrode film layer.

[0163] The areas of the first manganese iron phosphate lithium salt particles, the first lithium iron phosphate salt particles, and the second lithium iron phosphate salt particles can be obtained by statistically analyzing the obtained electrode cross-section scanning electron microscope images using Avizo 3D software, and the corresponding area ratio can be obtained by dividing the respective areas by the total area.

[0164] In some embodiments, the third particles do not include the second lithium iron phosphate salt particles.

[0165] In some embodiments, the positive electrode film layer further includes third manganese iron phosphate lithium salt particles, and the primary average particle size s3" of the third manganese iron phosphate lithium salt particles satisfies 30 nm ≤ s3" ≤ 200 nm.

[0166] By adding a small amount of ultrafine particles to the positive electrode film layer, the space utilization rate can be further improved. The ultrafine particles can be adjusted to the gap between the medium-sized first manganese iron lithium salt particles and the ultra-large first manganese iron lithium salt particles / second manganese iron lithium salt particles, further improving the volume energy density, that is, further improving the compaction density of the electrode. The ratio of ultrafine manganese iron lithium salt with a low Mn / Fe molar ratio to ultrafine manganese iron lithium salt with a high Mn / Fe molar ratio will have some loss on the voltage platform, but the gram capacity and surface stability have been greatly improved, especially the surface stability. The ultrafine manganese iron lithium salt material with a high Mn / Fe molar ratio is difficult to form a good coating. In the charging state, the surface has a strong oxidizing property. At the same time, due to the Jan-Taylor effect, the small particles have a high specific surface energy, and the problem of manganese dissolution is prominent, which worsens the battery cycle life. The secondary battery of the present application controls the first manganese iron lithium salt particles to have a relatively moderate particle size, contains less ultrafine powder and ultra-large particles, and the material exhibits good cycle life and gram capacity. Furthermore, the secondary battery of the present application achieves a good compaction density by adding large-particle lithium iron phosphate (LiFePO4) with a low Mn / Fe molar ratio and small-particle lithium manganese iron phosphate (LiFePO4) with a low Mn / Fe molar ratio to the positive electrode film layer. For the positive electrode active material, an idealized close-packing model indicates that to achieve a good compaction density, the material must contain some large particles and ultrafine powder to form a dense packing, thereby achieving a high positive electrode sheet compaction density while achieving good high-temperature cycling performance.

[0167] In some embodiments, the positive electrode film layer further includes third manganese iron phosphate lithium salt particles, and the primary average particle size s3" of the third manganese iron phosphate lithium salt particles satisfies 30 nm ≤ s3" ≤ 100 nm.

[0168] In some embodiments, the positive electrode film layer further includes third manganese iron phosphate lithium salt particles, and the primary average particle size of the third manganese iron phosphate lithium salt particles can be 30nm, 40nm, 50nm, 60nm, 70nm, 80nm, 90nm, 100nm, 110nm, 120nm, 130nm, 140nm, 150nm, 160nm, 170nm, 180nm, 190nm, 200nm, or a range consisting of any two of the above primary average particle sizes or a value within the range.

[0169] The primary average particle size of the third manganese iron phosphate lithium salt particles can be measured by methods and equipment known in the art. For example, it can be tested by scanning electron microscopy and long diameter statistical method. As an example, an argon ion beam is used to cut the electrode perpendicular to the large surface of the positive electrode plate to expose the cross section. The cross section is photographed using a scanning electron microscope, and the particle size of the third manganese iron phosphate lithium salt particles is statistically analyzed using the long diameter statistical method. Specifically, the total number of third manganese iron phosphate lithium salt particles with a primary particle size greater than 10nm and the sum of the primary particle sizes of the third manganese iron phosphate lithium salt particles with a primary particle size greater than 10nm can be counted in the electron microscope scanning photo. The primary average particle size of the third manganese iron phosphate lithium salt particles = the primary particle size of the total third manganese iron phosphate lithium salt particles / the total number of third manganese iron phosphate lithium salt particles. In the primary particle size statistical process, particles with a primary particle size of 0 < ≤ 10nm are not within the statistical range.

[0170] In some embodiments, based on the total area of ​​the primary particles of the positive electrode film layer, the area of ​​the first manganese iron phosphate lithium salt particles accounts for 70%-97%; the area of ​​the second manganese iron phosphate lithium salt particles accounts for 2-25%; and the area of ​​the third manganese iron phosphate lithium salt particles accounts for 1-10%.

[0171] In some embodiments, the area of ​​the first lithium manganese iron phosphate salt particles accounts for 70% to 97% of the total area of ​​the primary particles of the positive electrode film layer. In some embodiments, the area of ​​the first lithium manganese iron phosphate salt particles accounts for 65%, 70%, 72%, 74%, 75%, 97%, or 98% of the total area of ​​the primary particles of the positive electrode film layer. In some embodiments, the area of ​​the first lithium manganese iron phosphate salt particles accounts for 65% to 74% of the total area of ​​the primary particles of the positive electrode film layer.

[0172] In some embodiments, the area ratio of the second lithium manganese iron phosphate salt particles is 2-25% based on the total area of ​​the primary particles of the positive electrode film layer. In some embodiments, the area ratio of the second lithium manganese iron phosphate salt particles is 2%, 23%, 25%, 30%, or a range consisting of any two of the above area ratios or a value within the range based on the total area of ​​the primary particles of the positive electrode film layer. In some embodiments, the area ratio of the second lithium manganese iron phosphate salt particles is 23-25% based on the total area of ​​the primary particles of the positive electrode film layer.

[0173] In some embodiments, the area ratio of the third lithium manganese iron phosphate particles is 1-10% based on the total area of ​​the primary particles of the positive electrode film layer. In some embodiments, the area ratio of the third lithium manganese iron phosphate particles is 1%, 3%, 5%, 10%, or a range consisting of any two of the above area ratios or a value within the range based on the total area of ​​the primary particles of the positive electrode film layer.

[0174] The areas of the first manganese iron phosphate lithium salt particles, the second manganese iron phosphate lithium salt particles, and the third manganese iron phosphate lithium salt particles can be obtained by statistically analyzing the obtained electrode cross-section scanning electron microscope images using Avizo 3D software, and the corresponding area ratio can be obtained by dividing the respective areas by the total area.

[0175] In some embodiments, the first lithium manganese iron phosphate salt particles have the molecular formula Li m1 Fe x1 Mn r1 P y1 O j1 Q1 q1 , wherein Q1 includes at least one of Al, Na, K, Mg, Cu, Cr, Zn, Pb, Ca, Co, Ni, Sr, Nb, V, Ti, B, S, Si, N, F, Cl, and Br, 0.95≤m1≤1.15, x1>0, r1>0, 0.9≤x1+r1≤1, 0.95≤y1≤1, 3.5≤j1≤4, 0≤q1≤0.1, and / or

[0176] The second lithium manganese iron phosphate salt particle has a molecular formula of Li m2 Fe x2 Mn r2 P y2 O j2 Q2 q2, wherein Q2 includes at least one of Al, Na, K, Mg, Cu, Cr, Zn, Pb, Ca, Co, Ni, Sr, Nb, V, Ti, B, S, Si, N, F, Cl, and Br, 0.95≤m2≤1.15, x2>0, r2>0, r2 / x2≤1.5; 0.9≤x2+r2≤1, 0.95≤y2≤1, 3.5≤j2≤4, 0≤q2≤0.1, and / or

[0177] The third manganese iron phosphate lithium salt particle has the molecular formula Li m3 Fe x3 Mn r3 P y3 O j3 Q3 q3 , wherein Q3 includes at least one of Al, Na, K, Mg, Cu, Cr, Zn, Pb, Ca, Co, Ni, Sr, Nb, V, Ti, B, S, Si, N, F, Cl, and Br, 0.95≤m3≤1.15, x3>0, r3>0, r3 / x3≤1.5; 0.9≤x3+r3≤1, 0.95≤y3≤1, 3.5≤j3≤4, 0≤q3≤0.1, and / or

[0178] The first lithium iron phosphate salt particles have a molecular formula of Li m4 Fe x4 P y4 O j4 Q4 q4 , wherein Q4 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, 0.95≤m4≤1.15, 0.9≤x4≤1, 0.95≤y4≤1, 3.5≤j4≤4, 0<q4≤0.1, and / or

[0179] The second lithium iron phosphate salt particles have a molecular formula of Li m5 Fe x5 P y5 O j5 Q2 q5 , wherein Q5 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, 0.95≤m5≤1.15, 0.9≤x5≤1, 0.95≤y5≤1, 3.5≤j5≤4, and 0≤q5≤0.1.

[0180] In some embodiments, the first lithium manganese iron phosphate salt particles have the molecular formula Li m1 Fe x1 Mnr1 P y1 O j1 Q1 q1 , wherein m1 can be 0.95, 0.98, 1.00, 1.03, 1.05, 1.08, 1.10, 1.13, or 1.15, x1+r1 can be 0.9 or 1.0, y1 can be 0.95, 0.98, or 1.00, j1 can be 3.5, 3.6, 3.7, 3.8, 3.9, or 4, and q1 can be 0, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, or 0.1. In some embodiments, r1 / x1 ≥ 1.5.

[0181] In some embodiments, the second lithium manganese iron phosphate salt particles have the molecular formula Li m2 Fe x2 Mn r2 P y2 O j2 Q2 q2 , wherein m2 can be 0.95, 0.98, 1.00, 1.03, 1.05, 1.08, 1.10, 1.13, or 1.15, x2+r2 can be 0.9 or 1.0, y2 can be 0.95, 0.98, or 1.00, j2 can be 3.5, 3.6, 3.7, 3.8, 3.9, or 4, and q2 can be 0, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, or 0.1. In some embodiments, r2 / x2 ≤ 1.5. In some embodiments, 0 ≤ r2 / x2 ≤ 1.5. In some embodiments, 0.43 ≤ r2 / x2 ≤ 1.5.

[0182] When the Mn / Fe molar ratio of the second manganese iron phosphate lithium salt particles is ≤1.5 (ie, r2 / x2≤1.5), the secondary battery of the present application has lower manganese dissolution, better high-temperature storage performance, better high-temperature cycle performance, and higher gram capacity.

[0183] In some embodiments, the third lithium manganese iron phosphate salt particles have the molecular formula Li m3 Fe x3 Mn r3 P y3 O j3 Q3 q3, wherein m3 can be 0.95, 0.98, 1.00, 1.03, 1.05, 1.08, 1.10, 1.13, or 1.15, x3+r3 can be 0.9 or 1.0, y3 can be 0.95, 0.98, or 1.00, j3 can be 3.5, 3.6, 3.7, 3.8, 3.9, or 4, and q3 can be 0, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, or 0.1. In some embodiments, r3 / x3 ≤ 1.5. In some embodiments, 0 ≤ r3 / x3 ≤ 1.5. In some embodiments, 0.43 ≤ r3 / x3 ≤ 1.5.

[0184] When the Mn / Fe molar ratio of the third manganese iron phosphate lithium salt particles is ≤1.5 (i.e., r3 / x3≤1.5), the secondary battery of the present application has a lower manganese dissolution amount, better high-temperature storage performance, better high-temperature cycle performance, higher gram capacity, higher positive electrode slurry solid content, and higher positive electrode sheet compaction density.

[0185] In some embodiments, the first lithium iron phosphate salt particles have the molecular formula Li m4 Fe x4 P y4 O j4 Q4 q4 , m4 can be 0.95, 0.98, 1.00, 1.03, 1.05, 1.08, 1.10, 1.13, 1.15, x4 can be 0.9, 1.0, y4 can be 0.95, 0.98, 1.00, 1.03, 1.05, 1.08, 1.10, 1.13, 1.15, j4 can be 3.5, 3.6, 3.7, 3.8, 3.9, 4, and q4 can be 0, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.1.

[0186] In some embodiments, the second lithium iron phosphate salt particles have the molecular formula Li m5 Fe x5 P y5 O j5 Q2 q5, m5 can be 0.95, 0.98, 1.00, 1.03, 1.05, 1.08, 1.10, 1.13, 1.15, x5 can be 0.9, 1.0, y5 can be 0.95, 0.98, 1.00, 1.03, 1.05, 1.08, 1.10, 1.13, 1.15, j5 can be 3.5, 3.6, 3.7, 3.8, 3.9, 4, and q5 can be 0, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.1.

[0187] Modifying the first and / or second lithium iron phosphate salt particles with elements Q4 and / or Q5 helps improve the ion transport capacity of the positive electrode active material. These elements can create vacancies in the particle lattice or change the interatomic bond lengths, facilitating the movement of lithium ions within the lattice, thereby effectively improving the conductivity of the particles themselves and enhancing the kinetic properties of the positive electrode active material. In this application, the modification can specifically be manifested as doping and / or coating.

[0188] In some embodiments, in the first lithium iron phosphate salt particles, Q4 includes at least one of Ti, V, Mg, and Nb, and the content of Ti, V, Mg, and / or Nb is 1000-10000 ppm based on the total weight of the first lithium iron phosphate salt particles.

[0189] In some embodiments, the content of Q4 in the first lithium iron phosphate salt particles is 1000 ppm, 2000 ppm, 2500 ppm, 3000 ppm, 3500 ppm, 4000 ppm, 4500 ppm, 5000 ppm, 5500 ppm, 6000 ppm, 6500 ppm, 7000 ppm, 7500 ppm, 8000 ppm, 8500 ppm, 9000 ppm, 9500 ppm, 10000 ppm, or a range consisting of any two of the above Q4 contents or a value within the range.

[0190] The content of Q4 in the lithium iron phosphate particles, for example, the content of Ti, can be measured by methods and equipment known in the art. For example, the test can be performed with reference to GB / T 33822-2017.

[0191] The content of modifying elements in existing lithium iron phosphate particles is generally low or no modifying elements are added. Increasing the content of Q4 elements in the first lithium iron phosphate particles helps to further improve the bulk ion transport capacity of the first lithium iron phosphate particles and improve its kinetic performance. However, as the content of Q4 elements further increases, the bulk ion transport capacity may not continue to increase, and it may also occupy the position of lithium ions, affecting the performance of gram capacity. The range of Q4 elements in this application helps to further obtain better kinetic performance and gram capacity.

[0192] In some embodiments, in the first lithium iron phosphate salt particles, Q4 includes Ti, and the content of Ti is 1000-10000 ppm, optionally 2500-6000 ppm, calculated based on the total weight of the first lithium iron phosphate salt particles.

[0193] In some embodiments, in the first lithium iron phosphate salt particles, Q4 includes V, and the content of V is 1000-10000 ppm, optionally 2500-6000 ppm, calculated based on the total weight of the first lithium iron phosphate salt particles.

[0194] In some embodiments, in the first lithium iron phosphate salt particles, Q4 includes Nb, and the content of Nb is 1000-10000 ppm, optionally 2500-6000 ppm, calculated based on the total weight of the first lithium iron phosphate salt particles.

[0195] Calculated based on the total weight of the first lithium iron phosphate salt particles, when the Ti content in the first lithium iron phosphate salt particles is 2500-6000ppm, by further appropriately increasing the Ti element content in the first lithium iron phosphate salt particles, it helps to further improve the bulk ion transport capacity of the first lithium iron phosphate salt particles, so that the secondary battery of the present application has better high-temperature storage performance and higher gram capacity.

[0196] In some embodiments, the specific surface area of ​​the first lithium iron phosphate salt particles is 3 m 2 / g-8m 2 / g.

[0197] In some embodiments, the specific surface area BET of the first lithium iron phosphate particles can be 3 m 2 / g, 3.5m 2 / g、4m 2 / g, 4.5m 2 / g, 4.6m 2 / g、5m 2 / g, 5.5m 2 / g、6m 2 / g, 6.5m 2 / g、7m 2 / g, 7.5m 2 / g、8m 2 / g、8.5m 2 / g、9m 2 / g, 9.5m 2 / g、10m 2 / g, 10.5m 2 / g、11m 2 / g, 11.5m 2 / g、12m 2 / g, or a range consisting of any two of the above BET values ​​or a value within the range.

[0198] In this application, the term "specific surface area" or "BET" refers to the total area per unit mass of particles. In this application, the BET of lithium iron phosphate particles is related to factors such as the primary average particle size of the lithium iron phosphate particles, the carbon content, the density of the carbon coating, the degree of fit between the carbon and the particles, and the looseness and porosity of the particles.

[0199] The BET specific surface area of ​​particles can be measured using methods and equipment known in the art. For example, the test can be performed using the gas adsorption method in accordance with GB / T 19587-2017. As an example, lithium iron phosphate particles are placed as a sample in a sample tube. The sample tube is immersed in liquid nitrogen at -196°C. The amount of nitrogen adsorbed on the solid surface at different pressures is measured at a relative pressure of 0.05-0.30. The monolayer adsorption of the sample is calculated based on the BET multilayer adsorption theory and its formula, thereby obtaining the specific surface area of ​​the sample.

[0200] If the BET of the first lithium iron phosphate salt particles is too high, the particles will increase their water absorption performance and affect the processing performance of the mixed positive electrode slurry. However, if the BET of the first lithium iron phosphate salt particles is too low, its gram capacity will be reduced. The BET of the first lithium iron phosphate salt particles is controlled within 3m 2 / g-8m 2 / g, which is conducive to further balancing the processing performance of the positive electrode slurry and the improvement of the solid content, thereby improving the processing problems of the battery cell and further increasing the volume energy density of the battery.

[0201] When the specific surface area of ​​the first lithium iron phosphate salt particles is 3m 2 / g-8m 2 / g, by introducing large particles with a smaller BET, the specific surface energy of the positive electrode active material can be reduced, the spontaneous agglomeration and polymerization of the material after the dispersion process can be slowed down, the viscosity of the slurry can be reduced, and a higher solid content can be achieved. At the same time, the reduction in the specific surface area of ​​the positive electrode active material and the corresponding reduction in the active reaction area can reduce the side reactions of the electrode under high voltage. The first lithium iron phosphate salt particles have a low specific surface area, which can also reduce the overall water absorption of the material. The reduction in moisture reduces the side reactions of the battery during the charge and discharge process, so that the secondary battery of the present application has a lower manganese dissolution amount, a higher positive electrode slurry solid content, and a higher positive electrode sheet compaction density.

[0202] In some embodiments, the specific surface area of ​​the first lithium iron phosphate salt particles is 3 m 2 / g-6m 2 / g.

[0203] When the specific surface area (BET) of the first lithium iron phosphate particles is 3 m 2 / g-6m 2 / g, the secondary battery of the present application has good high-temperature storage performance, good high-temperature cycle performance, and high positive electrode sheet compaction density.

[0204] In some embodiments, the carbon content of the first lithium iron phosphate salt particles is Cx4 wt %, calculated based on the total weight of the first lithium iron phosphate salt particles, wherein 0.8≤Cx4≤2.0.

[0205] When 0.8≤Cx4≤2.0, by controlling the carbon content of large-particle lithium iron phosphate salt within a certain range, the conductivity and coating integrity of the large-particle lithium iron phosphate salt material can be adjusted. The higher the carbon content, the better the kinetic performance, the better the conductivity of the particles, the improved coating integrity, and the reduction of side reactions on the surface of the material. However, too high a carbon content will lead to a high specific surface area of ​​the material, deteriorating the cycle performance. High carbon coating will lead to partial carbon redundancy in the form of floating carbon or ineffective coating, deteriorating the processing performance of the material, especially having a greater impact on the solid content of the material pulping. The carbon content range of this application helps to further obtain better kinetic performance and processing performance.

[0206] In some embodiments, the carbon content of the first lithium iron phosphate salt particles is 0.8 weight%, 0.9 weight%, 1.0 weight%, 1.1 weight%, 1.15 weight%, 1.2 weight%, 1.3 weight%, 1.4 weight%, 1.5 weight%, 1.6 weight%, 1.7 weight%, 1.8 weight%, 1.9 weight%, 2.0 weight%, or a range consisting of any two of the above carbon contents or a value within the range.

[0207] In some embodiments, the carbon content of the first lithium iron phosphate salt particles is Cx4 wt %, calculated based on the total weight of the first lithium iron phosphate salt particles, wherein 0.8≤Cx4≤1.2.

[0208] Calculated based on the total weight of the first lithium iron phosphate salt particles, the carbon content of the first lithium iron phosphate salt particles is Cx4 weight %. When 0.8≤Cx4≤1.2, the secondary battery of the present application has better high-temperature storage performance, lower manganese dissolution, higher positive electrode slurry solid content, better high-temperature cycle performance, and higher positive electrode sheet compaction density.

[0209] In some embodiments, the carbon contained in the first lithium iron phosphate salt particles is coated on the surface of the particles. In some embodiments, the carbon contained in the first lithium iron phosphate salt particles is embedded in the particles. In some embodiments, the carbon contained in the first lithium iron phosphate salt particles is partially coated on the surface of the particles and partially embedded in the particles.

[0210] In some embodiments, the ratio z4 of the specific surface area of ​​the first lithium iron phosphate salt particles to Cx4 satisfies 1.5≤z4≤8.5. In some embodiments, the ratio z4 of the specific surface area of ​​the first lithium iron phosphate salt particles to Cx4 satisfies 3≤z4≤6.

[0211] In some embodiments, z4 can be 1.5, 1.8, 2, 2.3, 2.5, 2.8, 3, 3.2, 3.5, 3.75, 3.8, 4, 4.2, 4.3, 4.5, 4.58, 4.8, 4.98, 5, 5.3, 5.42, 5.5, 5.71, 5.8, 5.83, 6, 6.3, 6.5, 6.7, 6.8, 7, 7.3, 7.5, 7.8, 7.9, 8, 8.3, 8.5, 8.6, or a range consisting of any two of the above z4 values ​​or a value within the range.

[0212] The ratio z4 of the specific surface area BET of the first lithium iron phosphate salt particles to Cx4 can characterize the uniform density of the carbon contained in the first lithium iron phosphate salt particles. When the primary average particle size and carbon content of the first lithium iron phosphate salt particles remain unchanged, the lower the ratio z4, the higher the carbon coating utilization rate in the particles, the less floating carbon, and the more uniform and dense the carbon contained in the first lithium iron phosphate salt particles. Improving the uniform density of the contained carbon is conducive to further improving the kinetic performance and gram capacity of the first lithium iron phosphate salt particles, but too high a carbon density may affect the insertion and extraction of lithium ions, and to a certain extent, affect the kinetic performance and gram capacity of the secondary battery. The ratio z4 range of the present application is conducive to the carbon contained in the first lithium iron phosphate salt particles having a suitable uniform density, thereby helping to improve the conductivity of the particle surface, and helping to further improve the gram capacity and kinetic performance of the secondary battery.

[0213] In some embodiments, a ratio z4 of the specific surface area of ​​the first lithium iron phosphate salt particles to Cx4 satisfies 3.8≤z4≤5.

[0214] When the ratio z4 of the specific surface area of ​​the first lithium iron phosphate salt particles to Cx4 satisfies 3.8≤z4≤5, the secondary battery of the present application has better high-temperature storage performance, lower manganese dissolution, higher positive electrode slurry solid content, better high-temperature cycle performance, and higher positive electrode sheet compaction density.

[0215] In some embodiments, the primary average particle size s1 of the first particles is 120-600 nm, the primary average particle size s2 of the second particles satisfies 650 nm ≤ s2 ≤ 3000 nm, and / or the primary average particle size s3 of the third particles satisfies 30 nm ≤ s3 ≤ 100 nm.

[0216] In some embodiments, the primary average particle size s1' of the first lithium manganese iron phosphate salt particles is 120-600 nm, the primary average particle size s2' of the first lithium iron phosphate salt particles satisfies 650 nm ≤ s2' ≤ 3000 nm, and / or the primary average particle size s3' of the second lithium iron phosphate salt particles satisfies 30 nm ≤ s3' ≤ 100 nm.

[0217] In some embodiments, the primary average particle size s1' of the first lithium manganese iron phosphate salt particles is 150-210 nm, the primary average particle size s2' of the first lithium iron phosphate salt particles satisfies 650 nm ≤ s2' ≤ 3000 nm, and / or the primary average particle size s3' of the second lithium iron phosphate salt particles satisfies 30 nm ≤ s3' ≤ 100 nm.

[0218] In some embodiments, the primary average particle size s1' of the first lithium manganese iron phosphate salt particles is 150-210 nm, the primary average particle size s2' of the first lithium iron phosphate salt particles satisfies 870 nm ≤ s2' ≤ 3000 nm, and / or the primary average particle size s3' of the second lithium iron phosphate salt particles satisfies 60 nm ≤ s3' ≤ 200 nm.

[0219] In some embodiments, the primary average particle size s1' of the first manganese iron phosphate lithium salt particles is 120-600 nm, the primary average particle size s2" of the second manganese iron phosphate lithium salt particles satisfies 650 nm ≤ s2" ≤ 3000 nm, and / or the primary average particle size s3" of the third manganese iron phosphate lithium salt particles satisfies 30 nm ≤ s3" ≤ 100 nm.

[0220] In some embodiments, the primary average particle size s1' of the first manganese iron phosphate lithium salt particles is 150-210 nm, the primary average particle size s2" of the second manganese iron phosphate lithium salt particles satisfies 650 nm ≤ s2" ≤ 3000 nm, and / or the primary average particle size s3" of the third manganese iron phosphate lithium salt particles satisfies 30 nm ≤ s3" ≤ 100 nm.

[0221] In some embodiments, the capacity ratio of the first lithium iron phosphate salt particles is η≥88%, where η is defined as:

[0222] A battery using the first lithium iron phosphate salt particles as the positive electrode active material is charged and discharged twice at a constant current rate of 0.1C in the voltage range of 2.0V to 3.75V, and then charged and discharged once at a constant current rate of 1C. In the charge and discharge test at a rate of 1C, the capacity value at the discharge voltage of 3.2V is extracted and recorded as C1, and the capacity value at the discharge voltage to 2.0V is extracted as C2, η=C1 / C2, wherein the charging process includes constant voltage charging, a constant voltage of 3.75V, and a constant voltage cut-off current of 50μA.

[0223] In some embodiments, n can be 88%, 88.1%, 89%, 90%, 90.1%, 91%, 92%, 92.2%, 93%, 94%, 94.1%, 94.5%, 95%, 95.1%, or a range consisting of any two of the above n values ​​or a value within the range.

[0224] The η value of the first lithium iron phosphate salt particles can be measured by methods and equipment known in the art. As an example, a button cell is first prepared. The specific button cell preparation process is as follows: 2.0000g of the first lithium iron phosphate salt particles are mixed with 0.1111g of conductive carbon black and 0.1111g of polyvinylidene fluoride, and then added to 2.5g of the organic solvent N-methylpyrrolidone. After thorough mixing, a slurry is formed. The slurry is coated on aluminum foil with a coating thickness of 140 microns, dried under vacuum at 120°C for 2 hours, and punched into discs with a diameter of 13mm using a punch. The tablets are pressed using a tablet press at 10Mpa and kept in vacuum at 120°C for 12 hours to obtain a positive electrode sheet. The positive electrode sheet is weighed, and the loading amount of the lithium iron phosphate salt particles is 11-12mg. Button cells were assembled in an argon-protected glove box, with a metallic lithium sheet as the negative electrode, an electrolyte consisting of a 1:1 volume ratio of EC (ethylene carbonate) and DMC (1,2-dimethyl carbonate) mixed solvent, LiPF6 electrolyte, and a Celgard 2400 microporous polyethylene membrane as the separator.

[0225] The prepared button cell was tested for electrical performance on a blue battery tester. Specifically, the button cell was charged and discharged twice at a constant current rate of 0.1C within a voltage range of 2.0V to 3.75V, followed by a single charge and discharge at a constant current rate of 1C. During the 1C charge and discharge test, the capacity value at a discharge voltage of 3.2V was recorded as C1, and the capacity value at a discharge voltage of 2.0V was recorded as C2, with η = C1 / C2. The charging process included constant voltage charging at a constant voltage of 3.75V and a constant voltage cutoff current of 50μA.

[0226] The capacity ratio η of the first lithium iron phosphate salt particles can reflect its dynamic performance and platform retention performance, and can be adjusted by adjusting the primary average particle size, carbon content, carbon source and carbon film-forming agent ratio, modifier and its content of the first lithium iron phosphate salt particles. The η value of the first lithium iron phosphate salt particles of the present application is ≥88%, which has good dynamic performance. At the same time, when the η value is high, the secondary battery can still maintain good power performance when discharged to a low state of charge (SOC), that is, the battery with a high η value has a smaller voltage drop value when discharged at low power and high current.

[0227] In some embodiments, the aspect ratio of the second lithium iron phosphate salt particles is ≥ 1.1. In some embodiments, the aspect ratio of the second lithium iron phosphate salt particles is ≥ 1.3.

[0228] When the aspect ratio of the second lithium iron phosphate salt particles is ≥1.3, the material has good crystallinity, so that the secondary battery of the present application has good high-temperature storage performance, low manganese dissolution, good high-temperature cycle performance, and high gram capacity.

[0229] In this application, the term "aspect ratio" refers to the ratio of the diameter of a primary particle in the long axis direction to the diameter in the short axis direction. Assuming the diameter in the long axis direction is a and the diameter in the short axis direction is b, the aspect ratio = a / b; this parameter is often used to describe the morphology of particles and can be used to measure their sphericity.

[0230] The aspect ratio of the second lithium iron phosphate salt particles can be measured using methods and equipment known in the art. For example, the following method can be used to test: a cold-pressed electrode is cut perpendicularly to the electrode surface using an Ar particle beam to expose the end face, and an image is obtained using a scanning electron microscope. The scanning electron microscope image of the electrode section is then analyzed using imaging software to measure the major diameter a and minor diameter b of the particles, and their ratio is obtained, which is the aspect ratio of the particles.

[0231] In some embodiments, the ratio of the diffraction peak intensity of the (020) crystal plane to the diffraction peak intensity of the (211) crystal plane of the second lithium iron phosphate salt particles, W, is ≥ 0.98. In some embodiments, the ratio of the diffraction peak intensity of the (020) crystal plane to the diffraction peak intensity of the (211) crystal plane of the second lithium iron phosphate salt particles, W, is ≥ 1.03. In some embodiments, the ratio of the diffraction peak intensity of the (020) crystal plane to the diffraction peak intensity of the (211) crystal plane of the second lithium iron phosphate salt particles, W, is ≥ 1.08.

[0232] When the ratio W of the diffraction peak intensity of the (020) crystal plane to the diffraction peak intensity of the (211) crystal plane of the second lithium iron phosphate salt particles is ≥1.03, the material has good crystallinity; in addition, a higher W value indicates that the (020) crystal plane is more exposed, which is more conducive to the deintercalation of lithium ions, thereby making the secondary battery of the present application have better high-temperature storage performance, lower manganese dissolution, better high-temperature cycle performance, and higher gram capacity.

[0233] The ratio W of the diffraction peak intensity of the (020) crystal plane to the diffraction peak intensity of the (211) crystal plane can be measured by methods and equipment known in the art. For example, it can be tested by the following method: the diffraction peak intensity ratio of the second lithium iron phosphate salt particles is tested by X-ray, and the second lithium iron phosphate salt particles are placed on the test platform of the X-ray diffractometer (model Shimadzu XRD-7000) using a copper target X-ray diffractometer, and the starting angle of the scan is 10°, the ending angle is 90°, and the step length is 0.013. Then, the test is started to obtain the diffraction pattern of the second lithium iron phosphate salt particles in the diffraction angle range of 10° to 90°, and the ratio W of the diffraction peak intensity of the (020) crystal plane to the diffraction peak intensity of the (211) crystal plane is determined based on the diffraction pattern.

[0234] In some embodiments, the specific surface area of ​​the second lithium iron phosphate salt particles is 11 m 2 / g-14m 2 In some embodiments, the specific surface area of ​​the second lithium iron phosphate particles is 11 m 2 / g-12m 2 In some embodiments, the specific surface area of ​​the second lithium iron phosphate particles is 12 m 2 / g-14m 2 / g.

[0235] In some embodiments, the carbon content of the second lithium iron phosphate salt particles is Cx5% by weight, calculated based on the total weight of the second lithium iron phosphate salt particles, and the ratio z5 of the specific surface area of ​​the second lithium iron phosphate salt particles to Cx5 satisfies 8≤z5≤11. In some embodiments, 8.1≤z5≤10.4. In some embodiments, the carbon content of the second lithium iron phosphate salt particles is Cx5% by weight, calculated based on the total weight of the second lithium iron phosphate salt particles, and the ratio z5 of the specific surface area of ​​the second lithium iron phosphate salt particles to Cx5 is 8.1, 8.9, 10.4, or a range consisting of any two of the above z5s or a value within that range.

[0236] In some embodiments, the carbon contained in the second lithium iron phosphate salt particles is coated on the particle surface. In some embodiments, the carbon contained in the second lithium iron phosphate salt particles is embedded in the particles. In some embodiments, the carbon contained in the second lithium iron phosphate salt particles is partially coated on the particle surface and partially embedded in the particles.

[0237] In some embodiments, the specific surface area of ​​the first manganese iron phosphate lithium salt particles is 8-20 m 2 In some embodiments, the specific surface area of ​​the first manganese iron phosphate lithium salt particles is 10-20m 2 / g.

[0238] In some embodiments, the specific surface area of ​​the first manganese iron phosphate lithium salt particles is 8m 2 / g、9m 2 / g、10m 2 / g, 10.5m 2 / g、11m 2 / g、12m 2 / g、13m 2 / g、14m 2 / g、15m 2 / g、16m 2 / g、17m 2 / g、18m 2 / g、19m 2 / g, 20m 2 / g, or a range consisting of any two of the above specific surface areas or a value within the range.

[0239] In some embodiments, the specific surface area of ​​the first manganese iron phosphate lithium salt particles is 10-18m 2 In some embodiments, the specific surface area of ​​the first manganese iron phosphate lithium salt particles is 10-17m 2 / g.

[0240] When the specific surface area of ​​the first manganese iron phosphate lithium salt particles is 10m 2 / g-17m2 / g, the secondary battery of the embodiment of the present application has good high-temperature storage performance, low manganese dissolution, good high-temperature cycle performance, and high positive electrode sheet compaction density.

[0241] In some embodiments, the carbon content of the first manganese iron phosphate lithium salt particles is Cx1 wt %, calculated based on the total weight of the first manganese iron phosphate lithium salt particles, wherein 1.2≤Cx1≤1.8.

[0242] In some embodiments, the carbon content of the lithium manganese iron phosphate particles is Cx1 weight %, calculated based on the total weight of the first lithium manganese iron phosphate particles, wherein Cx1 is 1.2, 1.3, 1.4, 1.5, 1.6, 1.65, 1.7, 1.8, or a range consisting of any two of the above Cx1 or a value within the range.

[0243] In some embodiments, the carbon contained in the first lithium manganese iron phosphate salt particles is coated on the particle surface. In some embodiments, the carbon contained in the first lithium manganese iron phosphate salt particles is embedded in the particles. In some embodiments, the carbon contained in the first lithium manganese iron phosphate salt particles is partially coated on the particle surface and partially embedded in the particles.

[0244] In some embodiments, the ratio z1 of the specific surface area of ​​the first manganese iron phosphate lithium salt particles to Cx1 satisfies 7≤z1≤13. In some embodiments, the ratio z of the specific surface area of ​​the first manganese iron phosphate lithium salt particles to Cx1 satisfies 7.1≤z1≤12.5. In some embodiments, the ratio z1 of the specific surface area of ​​the first manganese iron phosphate lithium salt particles to Cx is 7, 7.1, 7.3, 7.5, 7.6, 7.9, 8.3, 8.6, 8.8, 9.1, 9.3, 9.4, 12.5, 13, or a range consisting of any two of the above z1 or a value within the range.

[0245] In some embodiments, the specific surface area of ​​the second manganese iron phosphate lithium salt particles is 5m 2 / g-12m 2 / g.

[0246] In some embodiments, the specific surface area BET of the second manganese iron phosphate lithium salt particles can be 5m 2 / g, 5.5m 2 / g、6m 2 / g, 6.5m 2 / g、7m 2 / g, 7.5m 2 / g、8m 2 / g、8.5m 2 / g、9m 2 / g, 9.5m 2 / g、10m 2 / g, 10.5m 2 / g、11m 2 / g, 11.5m 2 / g、12m 2 / g, or a range consisting of any two of the above BET values ​​or a value within the range.

[0247] In some embodiments, the carbon content of the second lithium manganese iron phosphate salt particles is Cx2 wt %, calculated based on the total weight of the second lithium manganese iron phosphate salt particles, wherein 0.8≤Cx2≤2.0.

[0248] In some embodiments, the carbon content of the second manganese iron phosphate lithium salt particles is 0.8 weight%, 0.9 weight%, 1.0 weight%, 1.1 weight%, 1.15 weight%, 1.2 weight%, 1.3 weight%, 1.4 weight%, 1.5 weight%, 1.6 weight%, 1.7 weight%, 1.8 weight%, 1.9 weight%, 2.0 weight%, or a range consisting of any two of the above carbon contents or a value within the range.

[0249] In some embodiments, the specific surface area of ​​the third manganese iron phosphate lithium salt particles is 15m 2 / g-30m 2 / g.

[0250] In some embodiments, the specific surface area BET of the third manganese iron phosphate lithium salt particles can be 15m 2 / g, 15.5m 2 / g、16m 2 / g, 16.5m 2 / g、17m 2 / g, 17.5m 2 / g、18m 2 / g, 18.5m 2 / g、19m 2 / g, 19.5m 2 / g, 20m 2 / g, 20.5m 2 / g, 21m 2 / g, 21.5m 2 / g、22m 2 / g, 22.5m 2 / g, 23m 2 / g, 23.5m 2 / g、24m 2 / g, 24.5m 2 / g, 25m 2 / g, 25.5m 2 / g、26m 2 / g, 26.5m 2 / g、27m 2 / g, 27.5m 2 / g、28m 2 / g, 28.5m 2 / g、29m 2 / g, 29.5m 2 / g、30m 2 / g, or a range consisting of any two of the above BET values ​​or a value within the range.

[0251] In some embodiments, the carbon content of the third lithium manganese iron phosphate salt particles is Cx3 wt % calculated based on the total weight of the third lithium manganese iron phosphate salt particles, wherein 0.8≤Cx3≤2.0.

[0252] In some embodiments, the carbon content of the third manganese iron phosphate lithium salt particles is 0.8 weight%, 0.9 weight%, 1.0 weight%, 1.1 weight%, 1.15 weight%, 1.2 weight%, 1.3 weight%, 1.4 weight%, 1.5 weight%, 1.6 weight%, 1.7 weight%, 1.8 weight%, 1.9 weight%, 2.0 weight%, or a range consisting of any two of the above carbon contents or a value within the range.

[0253] In some embodiments, the first lithium iron phosphate salt particles satisfy at least one of (a)-(f):

[0254] (a) Dv10 of the first lithium iron phosphate particles is ≥ 0.2 μm;

[0255] (b) the Dv50 of the first lithium iron phosphate particles is 0.5-5 μm;

[0256] (c) Dv90 of the first lithium iron phosphate particles is ≤ 10 μm;

[0257] (d) Dv99 of the first lithium iron phosphate particles is ≤ 12 μm;

[0258] (e) The powder compaction density of the first lithium iron phosphate salt under a pressure of 3 tons is ≥ 2.25 g / cm 3 ;

[0259] (f) The powder resistivity of the first lithium iron phosphate salt is less than 60Ω·cm.

[0260] In this application, the term "Dv10" refers to the particle size corresponding to when the volume cumulative particle size distribution percentage in the particles reaches 10%.

[0261] In this application, the term "Dv90" refers to the particle size corresponding to when the volume cumulative particle size distribution percentage in the particles reaches 90%.

[0262] In this application, the term "Dv99" refers to the particle size corresponding to when the volume cumulative particle size distribution percentage in the particles reaches 99%.

[0263] In this application, the term "powder compaction density" refers to the density of a compact with a certain density and strength formed during the external compression process. As the powder moves and deforms, larger gaps are filled, the contact area between particles increases, the attraction between atoms is generated, and the mechanical fit between particles is enhanced. The unit is g / cm 3 .

[0264] In some embodiments, the first lithium iron phosphate salt particles have a Dv10 < Dv50.

[0265] In some embodiments, the first lithium iron phosphate salt particles have a Dv90>Dv50.

[0266] In some embodiments, the Dv50 of the first lithium iron phosphate particles can be 500nm, 600nm, 700nm, 800nm, 900nm, 1000nm, 1100nm, 1200nm, 1300nm, 1400nm, 1500nm, 1700nm, 1900nm, 2000nm, 2300nm, 2500nm, 2700nm, 2900nm, 3000nm, 3200nm, 3400nm, 3600nm, 3800nm, 4000nm, 4200nm, 4400nm, 4600nm, 4800nm, 5000nm, or a range composed of the Dv50 of any two of the above second pulverization products or a value within this range.

[0267] The Dv10, Dv50, Dv90, and Dv99 of the first lithium iron phosphate salt particles can be measured by methods and equipment known in the art. For example, they can be measured using a laser particle size analyzer (Malvern Master Size 3000) with reference to GB / T19077.1-2016.

[0268] Under different pressures, the powder compaction density of the first lithium iron phosphate salt particles can be measured by methods and equipment known in the art. For example, it can be measured using a compaction density instrument with reference to GB / T 24533-2009. Specifically, a certain amount of lithium iron phosphate salt particles are placed on a special compaction mold (the mold diameter is known), and the mold is hollow in the middle with a metal disc on the top and bottom. The lithium iron phosphate salt particles are placed between the metal discs, a metal cylinder is placed on the top, and the mold is placed on a compaction density instrument. Different pressures are set (for example, 3T). The thickness of the lithium iron phosphate salt particles under different pressures can be read on the device. The powder compaction density of the lithium iron phosphate salt particles is ρ=m / v, where v=(S×H), m is the mass of the lithium iron phosphate salt particles, S is the bottom area of ​​the mold, and H is the thickness of the lithium iron phosphate salt particles after compaction.

[0269] The powder resistivity of the first lithium iron phosphate salt particles can be measured using methods and equipment known in the art. For example, the powder resistivity meter (Suzhou Jingge, ST2722 model) can be used for measurement with reference to GB / T 33822-2017. Specifically, a certain amount of lithium iron phosphate salt particles (e.g., 1 g) is weighed and added to the feeding chamber of the powder resistivity meter. A pressure of 8 MPa is applied, and the forward resistivity and reverse resistivity of the lithium iron phosphate salt particles are measured respectively. The average of the two values ​​is taken as the powder resistivity of the lithium iron phosphate salt particles.

[0270] By making the first lithium iron phosphate salt particles satisfy at least one of (a) to (f), the first lithium iron phosphate salt particles can better achieve the technical effects of the present application.

[0271] In some embodiments, the first lithium iron phosphate salt particles are mainly obtained by the following preparation method:

[0272] Providing raw materials containing at least a lithium source, an iron source, a phosphorus source, optionally a carbon film-forming agent, optionally a carbon source, and optionally a modifier, and performing at least two sintering operations, wherein:

[0273] The temperature of the first sintering is 500℃-760℃, and can be optionally 550℃-720℃;

[0274] The temperature of the second sintering is 700°C-800°C, and can be optionally 720°C-780°C.

[0275] In some embodiments, the temperature of the first sintering may be 500°C, 530°C, 550°C, 580°C, 600°C, 630°C, 650°C, 680°C, 700°C, 710°C, 720°C, 730°C, 740°C, 750°C, 760°C, or a range consisting of any two of the above first sintering temperatures or a value within this range; the temperature of the second sintering may be 700°C, 710°C, 720°C, 730°C, 740°C, 750°C, 760°C, 770°C, 780°C, 790°C, 800°C, or a range consisting of any two of the above second sintering temperatures or a value within this range.

[0276] In some embodiments, the heating rates of the first sintering and the second sintering are each independently 2° C. / min to 20° C. / min.

[0277] In some embodiments, the heating rates in the first sintering and the second sintering are each independently 2°C / min, 5°C / min, 7°C / min, 10°C / min, 13°C / min, 15°C / min, 17°C / min, or 20°C / min.

[0278] In some embodiments, the constant temperature sintering time of the first sintering is 1-6 hours. In some embodiments, the constant temperature sintering time of the first sintering can be 1 hour, 1.5 hours, 2 hours, 2.5 hours, 3 hours, 3.5 hours, 4 hours, 4.5 hours, 5 hours, 5.5 hours or 6 hours.

[0279] In some embodiments, the constant temperature sintering time of the second sintering is 2-12 hours. In some embodiments, the constant temperature sintering time of the second sintering can be 1 hour, 1.5 hours, 2 hours, 2.5 hours, 3 hours, 3.5 hours, 4 hours, 4.5 hours, 5 hours, 5.5 hours, 6 hours, 6.5 hours, 7 hours, 7.5 hours, 8 hours, 8.5 hours, 9 hours, 9.5 hours, 10 hours, 10.5 hours, 11 hours, 11.5 hours or 12 hours.

[0280] Compared to the traditional preparation method that uses high temperature to achieve particle growth, the first lithium iron phosphate salt particles of the embodiment of the present application are sintered twice. Controlling the temperature of the two sinterings is conducive to preparing the first lithium iron phosphate salt particles having the primary average particle size and specific surface area of ​​the present application. Furthermore, controlling the heating rate, sintering temperature and constant temperature sintering time of the first sintering and / or the second sintering helps to reduce side reactions, thereby better preparing the first lithium iron phosphate salt particles of the present application. Furthermore, in the conventional high-temperature sintering process, the carbon coating layer on the surface of the particles is prone to cracking, reducing the integrity of the carbon coating. The present application performs large particle synthesis at low temperature, which is conducive to improving the consistency and uniformity of the surface carbon coating.

[0281] In some embodiments, the first lithium iron phosphate salt particles are mainly obtained by the following preparation method:

[0282] Providing raw materials containing at least a lithium source, an iron source, a phosphorus source, a carbon source, a carbon film-forming agent, and a modifier, and performing at least two sintering operations, wherein:

[0283] The carbon content of the material after the first sintering is 0.01 wt%-0.79 wt%, optionally 0.05 wt%-0.4 wt%;

[0284] The carbon content of the material after the second sintering is 0.8 wt%-2.0 wt%, and optionally 1.0 wt%-1.6 wt%.

[0285] In some embodiments, the carbon content of the material after the first sintering may be 0.01 weight %, 0.05 weight %, 0.1 weight %, 0.15 weight %, 0.20 weight %, 0.25 weight %, 0.30 weight %, 0.35 weight %, 0.40 weight %, 0.45 weight %, 0.50 weight %, 0.55 weight %, 0.60 weight %, 0.65 weight %, 0.70 weight %, 0.75 weight %, 0.79 weight %, or a range consisting of any two of the above carbon contents or a value in this range; the carbon content of the material after the second sintering may be 0.8 weight %, 0.9 weight %, 1.0 weight %, 1.1 weight %, 1.2 weight %, 1.3 weight %, 1.4 weight %, 1.5 weight %, 1.6 weight %, 1.7 weight %, 1.8 weight %, 1.9 weight %, 2.0 weight %, or a range consisting of any two of the above carbon contents or a value in this range.

[0286] In the preparation method of the embodiment of the present application, adding a carbon source before the first sintering can effectively reduce the trivalent iron in the raw material, thereby improving the purity and stability of the product. Furthermore, by controlling the temperature of the first sintering and the carbon content of the intermediate after the first sintering within the above range, it is helpful to increase the primary particle size of the first lithium iron phosphate salt particle precursor obtained after the first sintering. Specifically, during the first sintering process, a lower carbon content is conducive to reducing the barrier effect of the carbon layer on the growth process of the first lithium iron phosphate particles, which is conducive to the crystallization growth of the first lithium iron phosphate salt particle precursor at a lower temperature. At the same time, it is also conducive to the solid-phase diffusion reaction between the modifier that may be added and the first lithium iron phosphate salt material, thereby facilitating the realization of a higher concentration of metal ion modification. By controlling the temperature of the second sintering and the carbon content of the sintered material within the above range, it is helpful to better coat the carbon on the surface of the first lithium iron phosphate salt particles to form a uniform and dense carbon coating layer, which is conducive to improving the surface conductivity of the first lithium iron phosphate particles, and improving its kinetic properties and gram capacity.

[0287] In some embodiments, raw materials containing at least a lithium source, an iron source, a phosphorus source, a carbon source, a modifier, and a carbon film-forming agent are provided and sintered at least twice.

[0288] In some embodiments, the mixing ratio of the lithium source, the iron source, and the phosphorus source satisfies, based on the atomic moles of each element, Fe:P=(0.96-0.985):1, and Li:Fe=(1.0-0.95):1.1.

[0289] In some embodiments, the mixing ratio of the iron source and the phosphorus source, calculated on the basis of the atomic moles of each element, satisfies Fe:P=0.96:1, Fe:P=0.965:1, Fe:P=0.97:1, Fe:P=0.975:1, Fe:P=0.98:1 or Fe:P=0.985:1.

[0290] In some embodiments, the mixing ratio of the lithium source and the iron source, calculated on the basis of the atomic moles of each element, satisfies Li:Fe=1.0:1.1, Li:Fe=0.99:1.1, Li:Fe=0.98:1.1, Li:Fe=0.97:1.1, Li:Fe=0.96:1.1 or Li:Fe=0.95:1.1.

[0291] In some embodiments, the weight ratio of the carbon source to the carbon film-forming agent is (9-0.25) : 1. In some embodiments, the weight ratio of the carbon source to the carbon film-forming agent may be 9:1, 8:1, 7:1, 6:1, 5:1, 4:1, 3:1, 2:1, 1:1, 0.8:1, 0.5:1 or 0.25:1.

[0292] In some embodiments, the lithium source is a lithium-containing compound. In some embodiments, the lithium source includes at least one of lithium dihydrogen phosphate, lithium oxalate, lithium carbonate, lithium oxide, lithium hydroxide, and lithium acetate. In some embodiments, the lithium source includes lithium carbonate.

[0293] In some embodiments, the iron source is an iron-containing compound. In some embodiments, the iron source includes at least one of ferric hydroxide, ferrous chloride, ferric oxide, ferric phosphate, ferric pyrophosphate, ferrous oxalate, iron powder, ferric nitrate, ferric oxide, and ferric oxyhydroxide. In some embodiments, the iron source includes ferric oxide.

[0294] In some embodiments, the phosphorus source is a phosphoric acid compound. In some embodiments, the phosphorus source includes at least one of phosphoric acid, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate. In some embodiments, the phosphorus source includes phosphoric acid.

[0295] In some embodiments, the carbon source comprises at least one of citric acid, glucose, sucrose, starch, fructose, and lactose. In some embodiments, the carbon source comprises glucose.

[0296] In some embodiments, the carbon film-forming agent comprises at least one of polyethylene glycol, polyaniline, polyacrylonitrile, polyvinyl pyrrolidone, and polyvinyl alcohol. In some embodiments, the carbon film-forming agent comprises polyaniline.

[0297] In some embodiments, the modifier comprises at least one of titanium dioxide, vanadium pentoxide, n-butyl titanate, ammonium metavanadate, niobium ethoxide, niobium oxalate, niobium pentoxide, magnesium hydroxide, and magnesium nitrate. In some embodiments, the modifier comprises titanium dioxide.

[0298] By using the raw materials in the above ratio, it is advantageous to form the first lithium iron phosphate salt particles of the present application.

[0299] In some embodiments, the method for preparing the first lithium iron phosphate salt particles comprises the following steps:

[0300] After the first sintering, a first crushing is performed, and after the second sintering, a second crushing is performed, wherein,

[0301] The Dv50 of the product after the first crushing is 300nm-1200nm, optionally 400nm-1100nm;

[0302] The Dv50 of the product after the second pulverization is 500nm-5000nm, and can be optionally 700nm-2500nm.

[0303] In this application, the term "Dv50" refers to the particle size corresponding to when the volume cumulative particle size distribution percentage in the particles reaches 50%.

[0304] In some embodiments, the Dv50 of the product after the first crushing can be 300nm, 400nm, 500nm, 600nm, 700nm, 800nm, 900nm, 1000nm, 1100nm, 1200nm, or a range consisting of the Dv50 of any two of the above first crushing products or a value within the range.

[0305] In some embodiments, the Dv50 of the product after the second crushing can be 500nm, 600nm, 700nm, 800nm, 900nm, 1000nm, 1100nm, 1200nm, 1300nm, 1400nm, 1500nm, 1700nm, 1900nm, 2000nm, 2300nm, 2500nm, 2700nm, 2900nm, 3000nm, 3200nm, 3400nm, 3600nm, 3800nm, 4000nm, 4200nm, 4400nm, 4600nm, 4800nm, 5000nm, or a range consisting of the Dv50 of any two of the above second crushing products, or a value within the range.

[0306] In some embodiments, the pulverization includes one or more of mechanical crushing, grinding, sand milling, and air flow crushing.

[0307] The Dv50 of the particles can be measured using methods and equipment commonly used in the art. As an example, it can be measured using a laser particle size analyzer (Malvern Master Size 3000) with reference to GB / T19077.1-2016.

[0308] Controlling the Dv50 of the product after the first pulverization within the above range helps reduce the growth barrier effect of the added carbon source and any modified elements on the crystals of the first lithium iron phosphate salt particle precursor, thereby facilitating the preparation of micron-sized lithium iron phosphate salt particle precursors. Controlling the Dv50 value of the product after the second pulverization within the above range helps to obtain the first lithium iron phosphate salt particles having the primary average particle size of the present application.

[0309] A second aspect of the present application provides an electric device comprising the secondary battery according to the first aspect of the present application.

[0310] In addition, the secondary battery and the electric device of the present application will be described below with reference to the drawings as appropriate.

[0311] In one embodiment of the present application, a secondary battery is provided.

[0312] Typically, a secondary battery consists of a positive electrode, a negative electrode, an electrolyte, and a separator. During the battery's charge and discharge processes, active ions are inserted and removed between the positive and negative electrodes. The electrolyte conducts ions between the positive and negative electrodes. The separator, located between the positive and negative electrodes, primarily prevents short circuits between the positive and negative electrodes while allowing ions to pass through.

[0313] [Positive electrode]

[0314] The positive electrode sheet includes a positive electrode current collector and a positive electrode film layer arranged on at least one surface of the positive electrode current collector.

[0315] As an example, the positive electrode current collector has two surfaces opposite to each other in its thickness direction, and the positive electrode film layer is disposed on either or both of the two opposite surfaces of the positive electrode current collector.

[0316] In some embodiments, the positive electrode current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil. The composite current collector may include a polymer material base and a metal layer formed on at least one surface of the polymer material base. The composite current collector may be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0317] In some embodiments, the positive electrode film layer may further optionally include a binder. As an example, the binder may include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), a vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, a vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, a tetrafluoroethylene-hexafluoropropylene copolymer, and a fluorine-containing acrylate resin.

[0318] In some embodiments, the positive electrode film layer may further include a conductive agent. For example, the conductive agent may include at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0319] In some embodiments, the positive electrode sheet can be prepared by the following method: the components for preparing the positive electrode sheet, such as the positive electrode active material, the conductive agent, the binder and any other components, are dispersed in a solvent (such as N-methylpyrrolidone) to form a positive electrode slurry; the positive electrode slurry is coated on the positive electrode current collector, and after drying, cold pressing and other processes, the positive electrode sheet can be obtained.

[0320] [Negative electrode]

[0321] The negative electrode sheet includes a negative electrode current collector and a negative electrode film layer provided on at least one surface of the negative electrode current collector, wherein the negative electrode film layer includes a negative electrode active material.

[0322] As an example, the negative electrode current collector has two surfaces opposite to each other in its thickness direction, and the negative electrode film layer is disposed on either or both of the two opposite surfaces of the negative electrode current collector.

[0323] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, copper foil may be used as the metal foil. The composite current collector may include a polymer base layer and a metal layer formed on at least one surface of the polymer base material. The composite current collector may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy, etc.) on a polymer base material (such as a base material of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0324] In some embodiments, the negative electrode active material may be a negative electrode active material for a battery that is well known in the art. As an example, the negative electrode active material may include at least one of the following materials: artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, lithium titanate, etc. The silicon-based material may be selected from at least one of elemental silicon, silicon oxides, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. The tin-based material may be selected from at least one of elemental tin, tin oxides, and tin alloys. However, the present application is not limited to these materials, and other traditional materials that can be used as negative electrode active materials for batteries may also be used. These negative electrode active materials may be used alone or in combination of two or more.

[0325] In some embodiments, the negative electrode film layer may further include a binder. The binder may be selected from at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

[0326] In some embodiments, the negative electrode film layer may further include a conductive agent, which may be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0327] In some embodiments, the negative electrode film layer may optionally include other additives, such as a thickener (eg, sodium carboxymethyl cellulose (CMC-Na)).

[0328] In some embodiments, the negative electrode sheet can be prepared by the following method: the components for preparing the negative electrode sheet, such as the negative electrode active material, the conductive agent, the binder and any other components, are dispersed in a solvent (such as deionized water) to form a negative electrode slurry; the negative electrode slurry is coated on the negative electrode current collector, and after drying, cold pressing and other processes, the negative electrode sheet can be obtained.

[0329] [Electrolytes]

[0330] The electrolyte conducts ions between the positive and negative electrodes. This application does not specify the type of electrolyte, and the electrolyte can be selected based on the needs. For example, the electrolyte can be liquid, gel, or solid.

[0331] In some embodiments, the electrolyte is an electrolyte solution comprising an electrolyte salt and a solvent.

[0332] In some embodiments, the electrolyte salt may be selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalatoborate, lithium dioxalatoborate, lithium difluorodioxalatophosphate, and lithium tetrafluorooxalatophosphate.

[0333] In some embodiments, the solvent can be selected from at least one of ethylene carbonate, propylene carbonate, ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methylpropyl carbonate, ethylpropyl carbonate, butylene carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, cyclopentane, dimethyl sulfone, methyl ethyl sulfone and diethyl sulfone.

[0334] In some embodiments, the electrolyte may further include additives. For example, the additives may include negative electrode film-forming additives, positive electrode film-forming additives, and additives that can improve certain battery properties, such as additives that improve battery overcharge performance, and additives that improve battery high or low temperature performance.

[0335] [Isolation film]

[0336] In some embodiments, the secondary battery further includes a separator. The present application has no particular limitation on the type of separator, and any known porous separator with good chemical and mechanical stability can be selected.

[0337] In some embodiments, the material of the separator can be selected from at least one of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single-layer film or a multi-layer composite film, without particular limitation. When the separator is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

[0338] In some embodiments, the positive electrode sheet, the negative electrode sheet, and the separator can be formed into an electrode assembly through a winding process or a lamination process.

[0339] In some embodiments, the secondary battery may include an outer packaging that can be used to encapsulate the electrode assembly and the electrolyte.

[0340] In some embodiments, the outer packaging of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. Alternatively, the outer packaging of the secondary battery can be a soft shell, such as a pouch-type soft shell. The soft shell can be made of plastic, such as polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0341] The present application has no particular limitation on the shape of the secondary battery, which may be cylindrical, square, or any other shape. For example, FIG1 shows a secondary battery 5 with a square structure as an example.

[0342] In some embodiments, referring to Figure 2, the outer packaging may include a shell 51 and a cover plate 53. The shell 51 may include a bottom plate and a side plate connected to the bottom plate, and the bottom plate and the side plate enclose a receiving cavity. The shell 51 has an opening connected to the receiving cavity, and the cover plate 53 can be covered on the opening to close the receiving cavity. The positive electrode sheet, the negative electrode sheet and the isolation membrane can be formed into an electrode assembly 52 through a winding process or a lamination process. The electrode assembly 52 is encapsulated in the receiving cavity. The electrolyte is infiltrated in the electrode assembly 52. ​​The number of electrode assemblies 52 contained in the secondary battery 5 can be one or more, and those skilled in the art can select according to specific actual needs.

[0343] In some embodiments, secondary batteries can be assembled into a battery module. The number of secondary batteries contained in the battery module can be one or more. The specific number can be selected by those skilled in the art according to the application and capacity of the battery module.

[0344] Figure 3 shows an example battery module 4. Referring to Figure 3 , within the battery module 4, multiple secondary batteries 5 may be arranged sequentially along the length of the battery module 4. Of course, any other arrangement is also possible. Furthermore, the multiple secondary batteries 5 may be secured together using fasteners.

[0345] Optionally, the battery module 4 may further include a housing having a receiving space, and the plurality of secondary batteries 5 are received in the receiving space.

[0346] In some embodiments, the battery modules described above may also be assembled into a battery pack. The battery pack may contain one or more battery modules, and the specific number may be selected by those skilled in the art based on the application and capacity of the battery pack.

[0347] Figures 4 and 5 illustrate an example battery pack 1. Referring to Figures 4 and 5 , the battery pack 1 may include a battery box and multiple battery modules 4 disposed within the battery box. The battery box comprises an upper case 2 and a lower case 3. The upper case 2 can be placed over the lower case 3 to form an enclosed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the battery box.

[0348] In addition, the present application also provides an electric device, which includes at least one of the secondary battery, battery module, or battery pack provided in the present application. The secondary battery, battery module, or battery pack can be used as a power source for the electric device, and can also be used as an energy storage unit for the electric device. The electric device may include mobile devices (such as mobile phones, laptops, etc.), electric vehicles (such as pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc., but is not limited thereto.

[0349] As the electrical device, a secondary battery, a battery module or a battery pack can be selected according to its usage requirements.

[0350] Figure 6 shows an example of an electric device. This device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. To meet the high power and high energy density requirements of the secondary battery, a battery pack or battery module can be used.

[0351] Another example device may be a mobile phone, a tablet computer, a notebook computer, etc. Such a device is generally required to be lightweight and thin, and may use a secondary battery as a power source.

[0352] Example

[0353] Below, the embodiment of the present application is described. The embodiment described below is exemplary and is only used to explain the present application, and is not to be construed as limiting the present application. Where specific techniques or conditions are not specified in the embodiments, the techniques or conditions described in the literature in this area or the product specifications are used. Reagents or instruments used that do not specify the manufacturer are conventional products that can be obtained commercially.

[0354] Example 1

[0355] (1) Preparation of positive electrode slurry:

[0356] Preparation of the first lithium iron phosphate particles:

[0357] Lithium carbonate, ferric oxide, phosphoric acid, glucose, titanium dioxide (based on the total weight of the lithium iron phosphate particles, the amount of titanium dioxide added is such that the titanium content in the prepared lithium iron phosphate particles is 5000ppm), and polyaniline are weighed separately, wherein the weight ratios of the elements Li, Fe, and P satisfy the following: Fe:P=0.968:1, Li:Fe=1:0.98, and the weight ratio of glucose to polyaniline satisfies the following: glucose:polyaniline=1:2. The amount of glucose added is such that after the first sintering is completed, the carbon content accounts for 0.15% of the weight of the lithium iron phosphate precursor. Water is added to the above substances to obtain a mixture slurry.

[0358] The mixture was mixed in a ball mill and ground in a sand mill to a slurry with a solids content of 38% and a Dv50 of 0.40 μm. The mixture was then spray-dried (the negative pressure of the high-speed spray dryer was -650 to -200 Pa, the inlet temperature was 300°C to 360°C, and the outlet temperature was 100°C to 140°C). The dried reactants were placed in a sintering furnace for the first sintering process. The heating rate was controlled at 5°C / min, the holding temperature was controlled at 650°C, and the holding time was 4 hours. After cooling, the material was pulverized using a mechanical mill to obtain a powder.

[0359] Glucose as a carbon source and polyaniline as a carbon film-forming agent were added to the resulting powder, which was then mixed with water to produce a material with a solids content of 40%. The amounts of glucose and polyaniline added were such that the carbon content of the product after the second sintering was 1.2% (based on the total weight of the lithium iron phosphate particles), and the weight ratio of glucose to polyaniline was 1:2. A slurry was obtained using a ball mill and a sand mill, with a Dv50 value 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°C to 360°C, and an outlet temperature of 100°C to 140°C). The dried reactants were then placed in a sintering furnace for a second low-temperature sintering (heating rate controlled at 5°C / min, sintering temperature of 750°C, and sintering time of 4 hours). After the material is cooled, it is crushed for the second time to an average particle size of 870 nm. After demagnetization, lithium iron phosphate salt particles are obtained, with a carbon content of 1.2% and a Ti content of 5000 ppm in the lithium iron phosphate salt particles.

[0360] The first lithium iron phosphate salt particles (relevant parameters are shown in Table 1) and the first lithium manganese iron phosphate salt particles (purchased from Guangdong Bangpu Recycling Technology Co., Ltd., item number CPP-001-001, relevant parameters are shown in Table 2) are blended according to the area ratio listed in Table 4 to obtain a positive electrode active material. The above-mentioned blended positive electrode active material, conductive agent conductive carbon black, binder polyvinylidene fluoride and dispersant PVP are mixed in a weight percentage of 93:4.0:2.5:0.5 and N-methylpyrrolidone is added. The mixture is fully mixed, stirred, and dispersed to form a positive electrode slurry.

[0361] (2) Preparation of positive electrode sheet:

[0362] The viscosity of the mixed and stirred slurry was adjusted to 8000-20000mPa.s until the slurry was not stratified, and the slurry was coated with 420mg / 1540mm by double-sided double-control coating equipment. 2 It is coated on the surface of the substrate Al foil, and then dried, cold pressed, cut and sliced ​​to finally obtain the positive electrode sheet.

[0363] (3) Preparation of negative electrode sheet:

[0364] Artificial graphite, conductive carbon black, binder styrene-butadiene rubber (SBR) and thickener sodium carboxymethyl cellulose (CMC) were mixed uniformly according to the weight percentage of 95:1.0:2.0:2.0 and added with deionized water. After stirring and dispersion, the negative electrode slurry was obtained. The negative electrode slurry was prepared at 211 mg / 1540 mm 2 It is coated on the Cu foil substrate, and then dried, cold pressed, cut and sliced ​​to obtain the negative electrode sheet;

[0365] (4) Preparation of batteries:

[0366] The positive electrode sheet, isolation film, and negative electrode sheet are stacked in order. The isolation film must be able to isolate the anode and cathode. The bare battery cell is obtained by winding, and the bare battery cell is placed in the outer packaging. The electrolyte is injected and the lithium-ion battery is finally obtained after the packaging, formation, exhaust and other processes.

[0367] Example 2

[0368] The main difference between Example 2 and Example 1 is that the average particle size of the first manganese iron phosphate lithium salt particles is 210nm, and the specific surface area is 12m 2 / g, and a carbon content of 1.4% by weight. The first manganese iron phosphate lithium salt particles were purchased from Guangdong Brunp Recycling Technology Co., Ltd. with the item number CPP-001-002 (Guangdong Brunp Recycling Technology Co., Ltd.).

[0369] Example 3

[0370] The main difference between Example 3 and Example 2 is that the average particle size of the first manganese iron phosphate lithium salt particles is 120nm, and the specific surface area is 15m 2 / g, and a carbon content of 1.65% by weight. The first lithium manganese iron phosphate particles were purchased from Guangdong Brunp Recycling Technology Co., Ltd. with the item number SH-LMFP-1.

[0371] Example 4

[0372] The main difference between Example 4 and Example 2 is that the average particle size of the first manganese iron phosphate lithium salt particles is 600nm and the specific surface area is 10m 2 / g, the first manganese iron phosphate lithium salt particles were purchased from Guangdong Brunp Recycling Technology Co., Ltd. with the product number SH-LMFP-2.

[0373] Example 5

[0374] The main difference between Example 5 and Example 2 is that the average particle size of the first lithium iron phosphate salt particles is 250 nm and the specific surface area is 9.5 m 2 / g.

[0375] Example 6

[0376] The main difference between Example 6 and Example 2 is that the average particle size of the first lithium iron phosphate salt particles is 300 nm and the specific surface area is 7.5 m 2 / g.

[0377] Example 7

[0378] The main difference between Example 7 and Example 2 is that the average particle size of the first lithium iron phosphate salt particles is 4000 nm and the specific surface area is 4.5 m 2 / g.

[0379] Example 8

[0380] The main difference between Example 8 and Example 2 is that the average particle size of the first manganese iron phosphate lithium salt particles is 480nm and the specific surface area is 11m 2 / g, the first manganese iron phosphate lithium salt particles were purchased from Guangdong Brunp Recycling Technology Co., Ltd. with the product number CPP-001-003.

[0381] Example 9

[0382] The main difference between Example 9 and Example 2 is that the average particle size of the first lithium iron phosphate salt particles is 3000nm and the specific surface area is 5m 2 / g.

[0383] Example 10

[0384] The main difference between Example 10 and Example 2 is that the average particle size of the first lithium iron phosphate salt particles is 3500 nm and the specific surface area is 4.6 m 2 / g.

[0385] Example 11

[0386] The main difference between Example 11 and Example 2 is that the average particle size of the first lithium iron phosphate salt particles is 500 nm and the specific surface area is 7 m 2 / g.

[0387] Example 12

[0388] The main difference between Example 12 and Example 2 is that the specific surface area of ​​the first manganese iron phosphate lithium salt particles is 17m 2 / g, and a carbon content of 1.8 wt%. The first manganese iron phosphate lithium salt particles were purchased from Guangdong Brunp Recycling Technology Co., Ltd. with the item number CPP-001-005.

[0389] Example 13

[0390] The main difference between Example 13 and Example 2 is that the specific surface area of ​​the first manganese iron phosphate lithium salt particles is 10m 2 / g, with a carbon content of 1.2% by weight. The first manganese iron phosphate lithium salt particles were purchased from Guangdong Brunp Recycling Technology Co., Ltd. with the item number CPP-001-009.

[0391] Example 14

[0392] The main difference between Example 14 and Example 2 is that the specific surface area of ​​the first lithium iron phosphate salt particles is 5.5m 2 / g, and the carbon content is 1.15 wt%.

[0393] Example 15

[0394] The main difference between Example 15 and Example 2 is that the specific surface area of ​​the first lithium iron phosphate salt particles is 5m 2 / g, and the carbon content is 1.2% by weight.

[0395] Example 16

[0396] Compared with Example 2, Example 16 mainly differs in that the titanium content of the first lithium iron phosphate salt particles is 2500 ppm.

[0397] Example 17

[0398] Compared with Example 2, Example 17 mainly differs in that the titanium content of the first lithium iron phosphate salt particles is 6000 ppm.

[0399] Example 18

[0400] The main difference between Example 18 and Example 2 is that vanadium pentoxide is used instead of titanium dioxide, and the specific surface area of ​​the first lithium iron phosphate particles is 8m 2 / g.

[0401] Example 19

[0402] Compared with Example 2, Example 19 mainly differs in that niobium pentoxide is used instead of titanium dioxide.

[0403] Example 20

[0404] The main difference between Example 20 and Example 2 is that the specific surface area of ​​the first lithium iron phosphate salt particles is 3m 2 / g, and the carbon content is 0.8 wt%.

[0405] Example 21

[0406] The main difference between Example 21 and Example 2 is that the specific surface area of ​​the first lithium iron phosphate salt particles is 12m 2 / g, and the carbon content is 1.4 wt%.

[0407] Example 22

[0408] The main difference between Example 22 and Example 2 is that the primary average particle size of the first manganese iron phosphate lithium salt particles is 500nm and the specific surface area is 10.5m 2 / g, the first manganese iron phosphate lithium salt particles were purchased from Guangdong Brunp Recycling Technology Co., Ltd. with the item number CPP-001-013.

[0409] Example 23

[0410] The main difference between Example 23 and Example 2 is that the average particle size of the first manganese iron phosphate lithium salt particles is 140nm, and the specific surface area is 20m 2 / g, the first manganese iron phosphate lithium salt particles were purchased from Guangdong Brunp Recycling Technology Co., Ltd. with the item number CPP-001-026.

[0411] Example 24

[0412] Compared with Example 2, Example 24 mainly differs in that the area of ​​the first manganese iron phosphate lithium salt particles accounts for 70%, and the area of ​​the first lithium iron phosphate salt particles accounts for 30%.

[0413] Example 25

[0414] Compared with Example 2, Example 25 mainly differs in that the area of ​​the first manganese iron phosphate lithium salt particles accounts for 98%, and the area of ​​the first lithium iron phosphate salt particles accounts for 2%.

[0415] Example 26

[0416] Compared with Example 2, Example 26 mainly differs in that the titanium content of the first lithium iron phosphate salt particles is 1000 ppm.

[0417] Example 27

[0418] Compared with Example 2, Example 27 mainly differs in that the titanium content of the first lithium iron phosphate salt particles is 10,000 ppm.

[0419] Example 28

[0420] (1) Preparation of positive electrode slurry:

[0421] The source / preparation method of the first manganese iron phosphate lithium salt particles and the first iron phosphate lithium salt particles are exactly the same as those in Example 2.

[0422] The second lithium iron phosphate salt particles were purchased from Xiamen Tungsten New Energy Technology Co., Ltd. with the product number CPF-014-001. The relevant parameters are shown in Table 3.

[0423] The first manganese iron phosphate lithium salt particles, the first lithium iron phosphate salt particles, and the second lithium iron phosphate salt particles are blended according to the area ratio listed in Table 1 to obtain a positive electrode active material, and the above-mentioned blended positive electrode active material, conductive agent conductive carbon black, binder polyvinylidene fluoride and dispersant PVP are mixed according to the weight percentage of 93:4.0:2.5:0.5 and N-methylpyrrolidone is added, and the mixture is fully mixed, stirred, and dispersed to form a positive electrode slurry.

[0424] (2) Preparation of positive electrode sheet:

[0425] Exactly the same as Example 2.

[0426] (3) Preparation of negative electrode sheet:

[0427] Exactly the same as Example 2.

[0428] (4) Preparation of batteries:

[0429] Exactly the same as Example 2.

[0430] Example 29

[0431] Compared with Example 28, Example 29 mainly differs in that the area of ​​the first manganese iron phosphate lithium salt particles accounts for 70%, the area of ​​the first lithium iron phosphate salt particles accounts for 25%, and the area of ​​the second lithium iron phosphate salt particles accounts for 5%.

[0432] Example 30

[0433] Compared with Example 28, Example 30 mainly differs in that the area of ​​the first manganese iron phosphate lithium salt particles accounts for 65%, the area of ​​the first lithium iron phosphate salt particles accounts for 25%, and the area of ​​the second lithium iron phosphate salt particles accounts for 10%.

[0434] Example 31

[0435] Compared with Example 28, Example 31 mainly differs in that the area of ​​the first manganese iron phosphate lithium salt particles accounts for 74%, the area of ​​the first lithium iron phosphate salt particles accounts for 25%, and the area of ​​the second lithium iron phosphate salt particles accounts for 1%.

[0436] Example 32

[0437] The main difference between Example 32 and Example 28 is that the average particle size of the second lithium iron phosphate salt particles is 30 nm and the specific surface area is 14 m 2 / g, the second lithium iron phosphate particles were purchased from Xiamen Tungsten New Energy Technology Co., Ltd. with the item number XW-LFP-1.

[0438] Example 33

[0439] The main difference between Example 33 and Example 28 is that the primary average particle size of the second lithium iron phosphate salt particles is 200 nm and the specific surface area is 11 m 2 / g, the second lithium iron phosphate particles were purchased from Xiamen Tungsten New Energy Technology Co., Ltd. with the item number XW-LFP-2.

[0440] Example 34

[0441] Compared with Example 28, Example 34 mainly differs in that the aspect ratio of the second lithium iron phosphate salt particles is 1.3, the ratio of the (020) crystal plane diffraction peak intensity to the (211) crystal plane diffraction peak intensity W = 1.03, and the second lithium iron phosphate salt particles were purchased from Xiamen Tungsten New Energy Technology Co., Ltd. with the product number XW-LFP-3.

[0442] Example 35

[0443] Compared with Example 28, Example 35 mainly differs in that: the aspect ratio of the second lithium iron phosphate salt particles is 1.3, the ratio of the (020) crystal plane diffraction peak intensity to the (211) crystal plane diffraction peak intensity of the second lithium iron phosphate salt particles is W = 1.08, and the second lithium iron phosphate salt particles were purchased from Xiamen Tungsten New Energy Technology Co., Ltd. with the product number XW-LFP-4.

[0444] Example 36

[0445] Compared with Example 28, the main difference between Example 36 is that the second lithium iron phosphate particles (purchased from Guangdong Brunp Recycling Technology Co., Ltd., item number CPP-010-01; relevant parameters are shown in Table 2) are used instead of the first lithium iron phosphate particles, and the third lithium iron phosphate particles (purchased from Guangdong Brunp Recycling Technology Co., Ltd., item number CPP-100-05; relevant parameters are shown in Table 3) are used instead of the second lithium iron phosphate particles; the area of ​​the first lithium iron phosphate particles accounts for 75%, the area of ​​the second lithium iron phosphate particles accounts for 23%, and the area of ​​the third lithium iron phosphate particles accounts for 2%.

[0446] Example 37

[0447] Compared with Example 36, Example 37 mainly differs in that the Mn / Fe molar ratio of the second manganese iron phosphate lithium salt particles is 0.67 (purchased from Guangdong Brunp Recycling Technology Co., Ltd., product number CPP-010-03; relevant parameters are shown in Table 2).

[0448] Example 38

[0449] Compared with Example 36, the main difference between Example 38 is that the Mn / Fe molar ratio of the second manganese iron phosphate lithium salt particles is 0.43 (purchased from Guangdong Brunp Recycling Technology Co., Ltd., item number CPP-010-07; relevant parameters are shown in Table 2), and the Mn / Fe molar ratio of the third manganese iron phosphate lithium salt particles is 1.5 (purchased from Guangdong Brunp Recycling Technology Co., Ltd., item number CPP-100-06; relevant parameters are shown in Table 3).

[0450] Example 39

[0451] Compared with Example 38, Example 39 mainly differs in that the Mn / Fe molar ratio of the third manganese iron phosphate lithium salt particles is 0.67 (purchased from Guangdong Brunp Recycling Technology Co., Ltd., product number CPP-100-067; relevant parameters are shown in Table 3).

[0452] Example 40

[0453] Compared with Example 2, Example 40 mainly differs in that second lithium iron phosphate particles (purchased from Guangdong Brunp Recycling Technology Co., Ltd., product number CPP-010-01; relevant parameters are shown in Table 2) are used instead of the first lithium iron phosphate particles.

[0454] Example 41

[0455] Compared with Example 40, Example 41 mainly differs in that the Mn / Fe molar ratio of the second manganese iron phosphate lithium salt particles is 0.43 (purchased from Guangdong Brunp Recycling Technology Co., Ltd., product number CPP-010-043; relevant parameters are shown in Table 2).

[0456] Comparative Example 1

[0457] Compared with Example 2, Comparative Example 1 mainly differs in that the first lithium iron phosphate salt particles are not included.

[0458] Comparative Example 2

[0459] Compared with Example 28, Comparative Example 2 mainly differs in that the first lithium iron phosphate salt particles are not included.

[0460] Comparative Example 3

[0461] Compared with Example 40, the main difference between Comparative Example 3 is that the Mn / Fe molar ratio of the second manganese iron phosphate lithium salt particles is 4 (purchased from Guangdong Brunp Recycling Technology Co., Ltd., product number CPP-010-40; relevant parameters are shown in Table 2).

[0462] Comparative Example 4

[0463] Compared with Example 40, the main difference between Comparative Example 4 is that third manganese iron phosphate lithium salt particles with a Mn / Fe molar ratio of 4 (purchased from Guangdong Brunp Recycling Technology Co., Ltd., product number CPP-100-40; relevant parameters are shown in Table 3) are used instead of the second manganese iron phosphate lithium salt particles.

[0464] 2. Battery performance test

[0465] 1) Primary average particle size

[0466] First manganese iron phosphate lithium salt particles:

[0467] The electrode is cut open perpendicularly to the large surface of the electrode using an argon ion beam to expose the cross section, which is photographed using a scanning electron microscope. The longest diameter of the lithium manganese iron phosphate particles is statistically analyzed using the length diameter statistical method. The "primary average particle size" refers to the average value of the primary particle size of all particles, which is numerically equal to the total particle size value divided by the total number of particles. The primary particle size in the cross-section diagram refers to the longest distance between two points along the edge. Specifically, the total number of lithium manganese iron phosphate particles with a primary particle size greater than 10 nm and the sum of the primary particle sizes of lithium manganese iron phosphate particles with a primary particle size greater than 10 nm can be counted in the electron microscope scanning photograph. The primary average particle size of the lithium manganese iron phosphate particles = the primary particle size of the total lithium manganese iron phosphate particles / the total number of lithium manganese iron phosphate particles. In the above particle size statistical process, particles with a primary average particle size less than or equal to 10 nm are not included in the statistical range.

[0468] First lithium iron phosphate particles:

[0469] The electrode is cut open perpendicularly to the large surface of the electrode using an argon ion beam to expose the cross section, which is photographed using a scanning electron microscope. The longest diameter of the first lithium iron phosphate salt particles is statistically analyzed using the length diameter statistical method. The "primary average particle size" refers to the average value of the primary particle size of all particles, which is numerically equal to the total particle size value divided by the total number of particles. The primary particle size in the cross-sectional view refers to the longest distance between two points along the edge. Specifically, the total number of first lithium iron phosphate salt particles with a primary particle size greater than 80 nm and the sum of the primary particle sizes of the first lithium iron phosphate salt particles with a primary particle size greater than 80 nm can be counted in the electron microscope scanning photograph. The primary average particle size of the first lithium iron phosphate salt particles = the primary particle size of the total first lithium iron phosphate salt particles / the total number of first lithium iron phosphate salt particles. In the above 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.

[0470] Second manganese iron phosphate lithium salt particles:

[0471] The electrode is cut open perpendicularly to the large surface of the electrode using an argon ion beam to expose the cross section, which is photographed using a scanning electron microscope. The longest diameter of the second manganese iron phosphate lithium salt particles is statistically analyzed using the length diameter statistical method. The "primary average particle size" refers to the average value of the primary particle size of all particles, which is numerically equal to the total particle size value divided by the total number of particles. The primary particle size in the cross-sectional view refers to the longest distance between two points along the edge. Specifically, the total number of second manganese iron phosphate lithium salt particles with a primary particle size greater than 80nm and the sum of the primary particle sizes of the second manganese iron phosphate lithium salt particles with a primary particle size greater than 80nm can be counted in the electron microscope scanning photograph. The primary average particle size of the second manganese iron phosphate lithium salt particles = the primary particle size of the total second manganese iron phosphate lithium salt particles / the total number of second manganese iron phosphate lithium salt particles. In the above particle size statistical process, particles with a primary average particle size less than or equal to 80nm are not included in the statistical range.

[0472] Second lithium iron phosphate particles:

[0473] The electrode is cut open perpendicularly to the large surface of the electrode using an argon ion beam to expose the cross section, which is photographed using a scanning electron microscope. The longest diameter of the second lithium iron phosphate salt particles is statistically analyzed using the length diameter statistical method. The "primary average particle size" refers to the average value of the primary particle size of all particles, which is numerically equal to the total particle size value divided by the total number of particles. The primary particle size in the cross-section diagram refers to the longest distance connecting two points along the edge. Specifically, the total number of second lithium iron phosphate salt particles with a primary particle size greater than 10 nm and the sum of the primary particle sizes of the second lithium iron phosphate salt particles with a primary particle size greater than 10 nm can be counted in the electron microscope scanning photograph. The primary average particle size of the second lithium iron phosphate salt particles = the primary particle size of the total second lithium iron phosphate salt particles / the total number of second lithium iron phosphate salt particles. In the above particle size statistical process, particles with a primary average particle size less than or equal to 10 nm are not included in the statistical range.

[0474] Third manganese iron phosphate lithium salt particles:

[0475] The electrode is cut open perpendicularly to the large surface of the electrode using an argon ion beam to expose the cross section, which is photographed using a scanning electron microscope. The longest diameter of the third manganese iron phosphate lithium salt particles is statistically analyzed using the length diameter statistical method. The "primary average particle size" refers to the average of the primary particle sizes of all particles, which is numerically equal to the total particle size value divided by the total number of particles. In the cross-sectional view, the primary particle size refers to the longest distance between two points along the edge. Specifically, the total number of third manganese iron phosphate lithium salt particles with a primary particle size greater than 10 nm and the sum of the primary particle sizes of the third manganese iron phosphate lithium salt particles with a primary particle size greater than 10 nm can be counted in the electron microscope scanning photograph. The primary average particle size of the third manganese iron phosphate lithium salt particles = the primary particle size of the total third manganese iron phosphate lithium salt particles / the total number of third manganese iron phosphate lithium salt particles. In the above particle size statistical process, particles with a primary average particle size less than or equal to 10 nm are not included in the statistical range.

[0476] 2) Dv50

[0477] With reference to GB / T19077.1-2016, the Dv50 value of the particles was measured using a laser particle size analyzer (Malvern Master Size 3000). In addition, the Dv10, Dv90, and Dv99 values ​​of the present application were also measured in the same manner.

[0478] 3) Specific surface area

[0479] The specific surface area was tested by gas adsorption method according to the GB / T19587-2017 test standard, as follows: lithium iron phosphate salt particles / lithium manganese iron phosphate salt particles were taken as samples, the sample tube was immersed in liquid nitrogen at -196°C, and the adsorption amount of nitrogen on the solid surface at different pressures was measured at a relative pressure of 0.05-0.30. The monolayer adsorption amount of the sample was obtained based on the BET multilayer adsorption theory and its formula, thereby calculating the specific surface area of ​​the material.

[0480] 4) Carbon content

[0481] The lithium iron phosphate particles / lithium manganese iron phosphate particles are burned in a high-frequency induction furnace and then the carbon content is tested using an infrared absorption method. The specific testing process is based on the standard GB / T 20123-2006 / ISO 15350:2000.

[0482] 5) Content of Q element (e.g., Ti element) in the first lithium iron phosphate particles

[0483] The content of Q element in the first lithium iron phosphate salt particles is tested with reference to GB / T 33822-2017.

[0484] 6) Area ratio

[0485] The areas of the first manganese iron phosphate lithium salt particles, the first lithium iron phosphate particles, the second manganese iron phosphate lithium salt particles, the second lithium iron phosphate particles, and the third manganese iron phosphate lithium salt particles were obtained by statistically analyzing the obtained electrode cross-section scanning electron microscope images using Avizo 3D software, and the corresponding area ratio was obtained by dividing the area of ​​each by the total area.

[0486] 7) The ratio of the diffraction peak intensity of the (020) crystal plane to the diffraction peak intensity of the (211) crystal plane W

[0487] The diffraction peak intensity ratio of the second lithium iron phosphate salt particles was tested by X-ray. The second lithium iron phosphate salt particles were placed on the test platform of the X-ray diffractometer (model Shimadzu XRD-7000) using a copper target X-ray diffractometer. The starting angle of the scan was 10°, the ending angle was 90°, the step length was 0.013, and then the test was started to obtain the diffraction pattern of the second lithium iron phosphate salt particles in the diffraction angle range of 10° to 90°. The ratio W of the diffraction peak intensity of the (020) crystal plane to the diffraction peak intensity of the (211) crystal plane was determined based on the diffraction pattern.

[0488] 8) Aspect ratio

[0489] The aspect ratio is the ratio of the diameter of the major axis of a primary particle to the diameter of the minor axis. Assuming the diameter in the major axis is a and the diameter in the minor axis is b, the aspect ratio = a / b. This parameter is often used to describe the morphology of particles and can be used to measure their sphericity.

[0490] Testing method: Take a cold-pressed electrode and cut it open perpendicular to its broad surface using an Ar particle beam, exposing the end face. Then, use a scanning electron microscope to obtain an image. Then, use imaging software to analyze the SEM image of the electrode section, measure the major diameter (a) and minor diameter (b) of the particles, and obtain their ratio, which is the particle aspect ratio.

[0491] 9) The capacity ratio η of the first lithium iron phosphate particles in the embodiment and the comparative example when discharged to 3.2V and discharged to 2.0V respectively

[0492] First, a button battery is prepared. The specific button battery preparation process is as follows: 2.0000g of the first lithium iron phosphate salt particles are mixed with 0.1111g of conductive carbon black and 0.1111g of polyvinylidene fluoride, and then added to 2.5g of the organic solvent N-methylpyrrolidone. After thorough mixing, a slurry is formed. The slurry is coated on aluminum foil with a coating thickness of 140 microns, vacuum-dried at 120°C for 2h, and punched into a disc with a diameter of 13mm using a punch. Use a tablet press to press the tablet at 10Mpa and vacuum-keep it at 120°C for 12h to obtain a positive electrode sheet. The weight of the positive electrode sheet is weighed, where the loading amount of the first lithium iron phosphate salt particles is 11-12mg. Button cells were assembled in an argon-protected glove box, with a metallic lithium sheet as the negative electrode, an electrolyte consisting of a 1:1 volume ratio of EC (ethylene carbonate) and DMC (1,2-dimethyl carbonate) mixed solvent, LiPF6 electrolyte, and a Celgard 2400 microporous polyethylene membrane as the separator.

[0493] The prepared button cell was tested for electrical performance on a blue battery tester. Specifically, the button cell was charged and discharged twice at a constant current rate of 0.1C within a voltage range of 2.0V to 3.75V, followed by a single charge and discharge at a constant current rate of 1C. During the 1C charge and discharge test, the capacity value at a discharge voltage of 3.2V was recorded as C1, and the capacity value at a discharge voltage of 2.0V was recorded as C2, with η = C1 / C2. The charging process included constant voltage charging at a constant voltage of 3.75V and a constant voltage cutoff current of 50μA.

[0494] 10) Manganese dissolution

[0495] Retrieve the fully charged cell after 100 cycles at 25°C and disassemble it in the glove box. Remove the anode electrode in the dry room and use a ceramic knife to gently scrape 1g of material from the center of the electrode, ensuring that the entire active material layer is scraped off to avoid different manganese content at different thicknesses. Seal the bag and send it to the laboratory. Prepare an electronic scale, heating plate, 150ml quartz beaker, watch glass, funnel, 100ml volumetric flask, pliers (heat-insulating gloves), weighing spoon, dust-free paper, etc.; confirm whether the electronic scale calibration date is within the validity period. It cannot be used after the validity period has expired and needs to be recalibrated; turn on the power and the heating plate can heat up to 250℃ normally; weigh 0.2000±0.005g of powder into a beaker; acidification treatment: prepare a dilute sulfuric acid solution with a volume ratio of 1:4 (concentrated sulfuric acid: ultrapure water = 1:4); add 20ml of the prepared dilute sulfuric acid solution into the beaker; turn on the heating plate for preheating in advance, and after the temperature reaches 250℃, place the sample for digestion for 30 minutes, and place a quartz cover on the beaker to reduce the volatilization of the acid; after the sample is digested for 30in, take it out and cool it to room temperature; transfer the digested sample to a 100ml glass volumetric flask and make up the volume; then proceed according to the ICP standard test process: the test equipment temperature is 22+2℃, and the humidity is <60%; select the trace element method as the test method; run the calibration standard: use the prepared standard solution to determine the standard curve; select the element as Mn; finally, the Mn dissolution amount can be obtained after testing.

[0496] 11) Solid content of positive electrode slurry

[0497] Prepare an electronic balance (accuracy 0.0001), an oven, and a glass drying tray. Take 8-10g of the positive electrode slurry sample and spread it evenly on the sample tray. Record the mass of the slurry before drying as A. Close the oven door and heat. As heating continues, the temperature in the oven continues to rise, reaching 130°C for 5 hours. After drying, cool the sample in the oven and remove it. Record the mass of the dried slurry. Repeat the drying process several times until the sample reaches a constant weight, recording the mass after drying as B. Slurry solids content = (A / B) × 100%.

[0498] 12) Pole compaction density

[0499] The "pole compaction density" referred to in this application refers to the "ultimate compaction density" of the pole piece, and the test method is as follows:

[0500] The double-sided coated electrode is cold-pressed by a roller press to test the elongation of the electrode after cold pressing, and the flexibility of the electrode after cold pressing is also evaluated.

[0501] By increasing the pressure of the roller press, pole pieces with different compaction densities will be obtained. As the pressure increases, the compaction density of the pole piece increases, the elongation of the pole piece increases, and the flexibility of the pole piece decreases. A pole piece with too high an elongation can easily cause the pole piece to warp, while a pole piece with too low flexibility can easily lead to brittle fracture. Therefore, the smaller of the compaction density corresponding to the pole piece elongation of 6‰ or the number of times the pole piece is folded three times is defined as the limit compaction density.

[0502] The compacted density is calculated by dividing the weight of the single-sided positive electrode film layer by the volume of the positive electrode film layer.

[0503] 13) Gram capacity

[0504] The specific test process is briefly described as follows: (1) Place the battery in a 40°C oven and let it sit for 2 hours until the battery temperature remains at 40°C; (2) Discharge the battery at a constant current of 1 / 3C to 2.0V; (3) Pause for 5 minutes; (4) Charge the battery at a constant current of 1 / 3C to 4.1V, then charge at a constant voltage of 4.1V until the cutoff current reaches 0.05C; (5) Pause for 5 minutes; (6) Discharge the battery at a constant current of 1 / 3C to 2.0V. This step is the actual battery cell capacity test. Combined with the mass of the positive electrode active material, the gram capacity of the battery can be calculated: gram capacity = capacity / mass of the positive electrode active material.

[0505] 14) High temperature storage performance

[0506] The high-temperature storage performance was characterized by storing the battery at 60°C in a fully charged state (charged to 4.1V) and detecting the time it takes for the battery capacity to decay to 80% of the initial value.

[0507] 1. After 10 minutes of storage, discharge the battery at a constant current of 0.33Cn (Cn is the capacity of the battery at 40°C and 1 / 3C) to 2V.

[0508] 2. Set aside for 10 minutes, charge at a constant current of 0.33Cn to 4.1V, and then charge at a constant voltage with a cut-off current of 0.02Cn.

[0509] 3. Set aside for 2 hours, store at 60°C for 30 days, then take out and test the reversible capacity of the battery cell.

[0510] The specific test process is briefly described as follows: (1) Place the battery in a 40°C oven for 2 hours, until the battery temperature remains at 40°C; (2) Discharge at a constant current of 1 / 3C to 2.0V; (3) Pause for 5 minutes; (4) Charge at a constant current of 1 / 3C to 4.1V, then charge at a constant voltage of 4.1V to a cutoff current of 0.05C; (5) Pause for 5 minutes; (6) Discharge at a constant current of 1 / 3C to 2.0V. This step is for actual cell capacity testing.

[0511] 4. Test regularly until the reversible capacity of the cell decays to 80% of its initial capacity.

[0512] 15) High temperature cycle performance

[0513] The high temperature cycle performance is characterized by using 1C / 1C cycling at 60°C and detecting the number of cycles when the battery capacity decays to 80% of the initial value.

[0514] The specific process is briefly described as follows: (1) Place the battery in a 45°C oven and let it sit for 2 hours until the battery temperature remains at 25°C; (2) Charge the battery at a constant current of 1C to 3.65V, and continue charging at a constant voltage until the charging current is less than 0.05C; (3) Pause for 5 minutes; (4) Discharge the battery at a constant current of 1C to 2.5V; (5) Pause for 5 minutes. Steps (2) to (6) constitute one charge and discharge cycle of the battery. Repeat steps (2) to (6) until the battery capacity decays to 80% of its initial value.

[0515] 16) Mn / Fe molar ratio

[0516] The test method for the Mn molar fraction can be performed by methods and equipment known in the art, for example: referring to the nano-lithium iron phosphate chemical analysis method in the national standard GB T33822-2017, the molar content of the Mn element and the Fe element are tested, and the Mn molar fraction of the material can be obtained by calculation.

[0517] 3. Analysis of test results of various embodiments and comparative examples

[0518] Batteries of various embodiments and comparative examples were prepared according to the above method, and various performance parameters were measured. The parameters of the positive electrode active material are shown in Tables 1 to 3, the area ratio is shown in Table 4, and the performance test results are shown in Table 5.

[0519] Table 1: Related parameters of the first particle

[0520] Table 2: Related parameters of the second particle

[0521] Table 3: Related parameters of the third particle

[0522] Table 4: Area ratio of the first particle, the second particle, and the third particle

[0523] Table 5: Performance test results

[0524] According to the above results, the secondary batteries in Examples 1-41 all include:

[0525] A positive electrode sheet, a negative electrode sheet, a separator disposed between the positive electrode sheet and the negative electrode sheet, and an electrolyte solution comprising an electrolyte salt and a solvent,

[0526] The positive electrode sheet includes a positive electrode current collector and a positive electrode film layer provided on at least one surface of the positive electrode current collector.

[0527] The positive electrode film layer includes first particles, the first particles include first manganese iron phosphate lithium salt particles, and the primary average particle size s1 of the first particles is 120-600 nm.

[0528] Among them, the manganese dissolution amount of the secondary battery is ≤50ppm.

[0529] From the comparison between Examples 1-41 and Comparative Examples 1-4, it can be seen that the secondary batteries of the present invention have good high-temperature storage performance. In addition, the secondary batteries of the present invention also have good high-temperature cycle performance.

[0530] From the comparison between Example 1-41 and Comparative Example 1-3, it can be seen that when the positive electrode film layer also includes second particles, the second particles include first lithium iron phosphate salt particles and / or second lithium manganese iron phosphate salt particles, the primary average particle size s2 of the second particles satisfies 500nm≤s2≤3000nm, and the Mn / Fe molar ratio of the second lithium manganese iron phosphate salt particles is ≤1.5, the secondary battery of the embodiment of the present application has a lower manganese dissolution amount, better high-temperature storage performance, and better high-temperature cycle performance.

[0531] From the comparison of Examples 36-41 with Comparative Example 3, it can be seen that when the Mn / Fe molar ratio of the second manganese iron phosphate lithium salt particles is ≤1.5, the secondary battery of the embodiment of the present application has a lower manganese dissolution amount, better high-temperature storage performance, better high-temperature cycle performance, and higher gram capacity.

[0532] From the comparison of Example 1-41 with Comparative Examples 1-2 and 4, it can be seen that when the positive electrode film layer also includes second particles, the second particles include first lithium iron phosphate salt particles and / or second lithium manganese iron phosphate salt particles, the primary average particle size s2 of the second particles satisfies 500nm≤s2≤3000nm, and the Mn / Fe molar ratio of the second lithium manganese iron phosphate salt particles is ≤1.5, the secondary battery of the embodiment of the present application has a lower manganese dissolution amount, better high-temperature storage performance, and better high-temperature cycle performance.

[0533] From the comparison of Examples 36-39 with Comparative Example 4, it can be seen that when the Mn / Fe molar ratio of the third manganese iron phosphate lithium salt particles is ≤1.5, the secondary battery of the embodiment of the present application has a lower manganese dissolution amount, better high-temperature storage performance, better high-temperature cycle performance, higher gram capacity, higher positive electrode slurry solid content, and higher positive electrode sheet compaction density.

[0534] From the comparison between Examples 2, 9, and 11 and Examples 5-6, it can be seen that when the positive electrode film layer also includes first lithium iron phosphate salt particles and the primary average particle size s1 of the first lithium iron phosphate salt particles satisfies 500nm≤s1≤3000nm, the secondary battery of the embodiment of the present application has a lower manganese dissolution amount, better high-temperature storage performance, better high-temperature cycle performance, and a higher positive electrode slurry solid content.

[0535] From the comparison between Examples 2, 9, 10-11 and Examples 5-6, it can be seen that when the positive electrode film layer also includes first lithium iron phosphate salt particles and the primary average particle size s1 of the first lithium iron phosphate salt particles satisfies 500nm≤s1≤3500nm, the secondary battery of the embodiment of the present application has a lower manganese dissolution amount, better high-temperature storage performance, better high-temperature cycle performance, and a higher positive electrode slurry solid content.

[0536] From the comparison between Examples 2, 7, 9, 10-11 and Examples 5-6, it can be seen that when the positive electrode film layer also includes first lithium iron phosphate salt particles and the primary average particle size s1 of the first lithium iron phosphate salt particles satisfies 500nm≤s1≤4000nm, the secondary battery of the embodiment of the present application has a lower manganese dissolution amount, better high-temperature storage performance, and a higher positive electrode slurry solid content.

[0537] From the comparison between Example 2 and Examples 28-35, it can be seen that when the positive electrode film layer also includes second lithium iron phosphate salt particles and the primary average particle size s2 of the second lithium iron phosphate salt particles satisfies 30nm≤s2≤200nm, the secondary battery of the embodiment of the present application has a lower manganese dissolution amount, better high-temperature storage performance, and better high-temperature cycle performance.

[0538] From the comparison between Examples 2, 16-17 and Examples 26-27, it can be seen that, calculated based on the total weight of the first lithium iron phosphate salt particles, when the Ti content in the first lithium iron phosphate salt particles is 2500-6000 ppm, the secondary battery of the embodiment of the present application has better high-temperature storage performance and higher gram capacity.

[0539] From the comparison between Examples 2, 14-15, 18, 20 and Example 21, it can be seen that when the specific surface area of ​​the first lithium iron phosphate salt particles is 3m 2 / g-8m 2 / g, the secondary battery of the embodiment of the present application has a lower manganese dissolution amount, a higher positive electrode slurry solid content, and a higher positive electrode sheet compaction density.

[0540] From the comparison between Examples 2, 10-11, 16 and Example 17, it can be seen that when the specific surface area of ​​the first lithium iron phosphate salt particles is 3m 2 / g-6m 2 / g, the secondary battery of the embodiment of the present application has good high-temperature storage performance, good high-temperature cycle performance, and high positive electrode sheet compaction density.

[0541] From the comparison between Examples 2, 14-15, and 20 and Example 21, it can be seen that, calculated based on the total weight of the first lithium iron phosphate salt particles, the carbon content of the first lithium iron phosphate salt particles is Cx1 weight %, and when 0.8≤Cx1≤1.2, the secondary battery of the embodiment of the present application has better high-temperature storage performance, lower manganese dissolution amount, higher positive electrode slurry solid content, better high-temperature cycle performance, and higher positive electrode sheet compaction density.

[0542] From the comparison between Examples 2, 14-15, and 20 and Example 21, it can be seen that when the ratio z1 of the specific surface area of ​​the first lithium iron phosphate salt particles to Cx1 satisfies 3.8≤z1≤5, the secondary battery of the embodiment of the present application has better high-temperature storage performance, lower manganese dissolution, higher positive electrode slurry solid content, better high-temperature cycle performance, and higher positive electrode sheet compaction density.

[0543] From the comparison of Examples 34-35 and Example 28, it can be seen that when the aspect ratio of the second lithium iron phosphate salt particles is ≥1.3, the secondary battery of the embodiment of the present application has better high-temperature storage performance, lower manganese dissolution, better high-temperature cycle performance, and higher gram capacity.

[0544] From the comparison of Examples 34-35 and Example 28, it can be seen that when the ratio W of the (020) crystal plane diffraction peak intensity to the (211) crystal plane diffraction peak intensity of the second lithium iron phosphate salt particles is ≥1.03, the secondary battery of the embodiment of the present application has better high-temperature storage performance, lower manganese dissolution, better high-temperature cycle performance, and higher gram capacity.

[0545] From the comparison between Examples 2, 12-13 and Example 23, it can be seen that when the specific surface area of ​​the lithium manganese iron phosphate particles is 10m 2 / g-17m 2 / g, the secondary battery of the embodiment of the present application has good high-temperature storage performance, low manganese dissolution, good high-temperature cycle performance, and high positive electrode sheet compaction density.

[0546] It should be noted that the present application is not limited to the above-mentioned embodiments. The above-mentioned embodiments are merely examples, and any embodiments having substantially the same structure and effect as the technical concept within the scope of the present application are all included in the technical scope of the present application. In addition, without departing from the scope of the present application, any other embodiments that can be conceived by those skilled in the art and that combine some of the constituent elements in the embodiments are also included in the scope of the present application.

Claims

1. A secondary battery, comprising: a positive electrode plate, a negative electrode plate, a separator disposed between the positive electrode plate and the negative electrode plate, and an electrolytic solution comprising an electrolyte salt and a solvent, wherein the positive electrode plate comprises a positive electrode current collector and a positive electrode film layer disposed on at least one surface of the positive electrode current collector, the positive electrode film layer comprises first particles, the first particles comprise first lithium iron manganese phosphate salt particles, and the primary average particle size s1 of the first particles is 120 - 600 nm, wherein the manganese dissolution amount of the secondary battery is ≤ 50 ppm.

2. The secondary battery according to claim 1, wherein the positive electrode film layer further comprises second particles, the second particles comprise first lithium iron phosphate salt particles and / or second lithium iron manganese phosphate salt particles, the primary average particle size s2 of the second particles satisfies 500 nm ≤ s2 ≤ 3000 nm, and the Mn / Fe molar ratio of the second lithium iron manganese phosphate salt particles is ≤ 1.

5.

3. The secondary battery according to claim 2, wherein based on the total area of the primary particles of the positive electrode film layer, the area proportion of the first particles is 60% - 98%, and the area proportion of the second particles is 2% - 40%.

4. The secondary battery according to any one of claims 1 - 3, wherein the positive electrode film layer further comprises first lithium iron phosphate salt particles, and the primary average particle size s2' of the first lithium iron phosphate salt particles satisfies 500 nm ≤ s2' ≤ 3000 nm.

5. The secondary battery according to any one of claims 2 - 4, wherein based on the total area of the primary particles of the positive electrode film layer, the area proportion of the first lithium iron manganese phosphate salt particles is 70% - 98%, and the area proportion of the first lithium iron phosphate salt particles is 2% - 30%.

6. The secondary battery according to any one of claims 2 - 5, wherein the positive electrode film layer further comprises third particles, the third particles comprise second lithium iron phosphate salt particles and / or third lithium iron manganese phosphate salt particles, the primary average particle size s3 of the third particles satisfies 30 nm ≤ s3 ≤ 200 nm, and the Mn / Fe molar ratio of the third lithium iron manganese phosphate salt particles is ≤ 1.

5.

7. The secondary battery according to claim 6, wherein based on the total area of the primary particles of the positive electrode film layer, the area proportion of the first particles is 70% - 97%; the area proportion of the second particles is 2% - 25%; the area proportion of the third particles is 1% - 10%.

8. The secondary battery according to any one of claims 2 - 7, wherein the positive electrode film layer further comprises second lithium iron phosphate salt particles, and the primary average particle size s3' of the second lithium iron phosphate salt particles satisfies 30 nm ≤ s3' ≤ 200 nm.

9. The secondary battery according to any one of claims 6 - 8, wherein based on the total area of the primary particles of the positive electrode film layer, the area proportion of the first lithium iron manganese phosphate salt particles is 70% - 97%; the area proportion of the first lithium iron phosphate salt particles is 2 - 25%; the area proportion of the second lithium iron phosphate salt particles is 1 - 10%.

10. The secondary battery according to any one of claims 1-9, wherein the first lithium iron manganese phosphate salt particles have the molecular formula Li m1 Fe x1 Mn r1 P y1 O j1 Q1 q1 , where Q1 comprises at least one of Al, Na, K, Mg, Cu, Cr, Zn, Pb, Ca, Co, Ni, Sr, Nb, V, Ti, B, S, Si, N, F, Cl, Br, 0.95 ≤ m1 ≤ 1.15, x1 > 0, r1 > 0, 0.9 ≤ x1 + r1 ≤ 1, 0.95 ≤ y1 ≤ 1, 3.5 ≤ j1 ≤ 4, 0 ≤ q1 ≤ 0.1, and / or The second lithium iron manganese phosphate particle has the molecular formula Li m2 Fe x2 Mn r2 P y2 O j2 Q2 q2 , where Q2 includes at least one of Al, Na, K, Mg, Cu, Cr, Zn, Pb, Ca, Co, Ni, Sr, Nb, V, Ti, B, S, Si, N, F, Cl, Br, and 0.95 ≤ m2 ≤ 1.15, x2 > 0, r2 > 0, r2 / x2 ≤ 1.5; 0.9 ≤ x2 + r2 ≤ 1, 0.95 ≤ y2 ≤ 1, 3.5 ≤ j2 ≤ 4, 0 ≤ q2 ≤ 0.1, and / or The third lithium iron manganese phosphate particle has the molecular formula Li m3 Fe x3 Mn r3 P y3 O j3 Q3 q3 , where Q3 includes at least one of Al, Na, K, Mg, Cu, Cr, Zn, Pb, Ca, Co, Ni, Sr, Nb, V, Ti, B, S, Si, N, F, Cl, Br, 0.95 ≤ m3 ≤ 1.15, x3 > 0, r3 > 0, r3 / x3 ≤ 1.5; 0.9 ≤ x3 + r3 ≤ 1, 0.95 ≤ y3 ≤ 1, 3.5 ≤ j3 ≤ 4, 0 ≤ q3 ≤ 0.1, and / or The first lithium iron phosphate salt particle has the molecular formula Li m4 Fe x4 P y4 O j4 Q4 q4 , where Q4 includes at least one of Al, Na, K, Mg, Cu, Mn, Cr, Zn, Pb, Ca, Co, Ni, Sr, Nb, V, Ti, B, S, Si, N, F, Cl, Br, 0.95 ≤ m4 ≤ 1.15, 0.9 ≤ x4 ≤ 1, 0.95 ≤ y4 ≤ 1, 3.5 ≤ j4 ≤ 4, 0 < q4 ≤ 0.1, and / or The second lithium iron phosphate particle has a molecular formula Li m5 Fe x5 P y5 O j5 Q2 q5 , where Q5 includes at least one of Al, Na, K, Mg, Cu, Mn, Cr, Zn, Pb, Ca, Co, Ni, Sr, Nb, V, Ti, B, S, Si, N, F, Cl, Br, 0.95 ≤ m5 ≤ 1.15, 0.9 ≤ x5 ≤ 1, 0.95 ≤ y5 ≤ 1, 3.5 ≤ j5 ≤ 4, 0 ≤ q5 ≤ 0.

1.

11. The secondary battery according to claim 10, wherein in the first lithium iron phosphate salt particles, the Q4 includes at least one of Ti, V, Mg, and Nb, and based on the total weight of the first lithium iron phosphate salt particles, the content of Ti, V, Mg, and / or Nb is 1000 - 10000 ppm.

12. The secondary battery according to any one of claims 2-11, wherein the specific surface area of the first lithium iron phosphate salt particles is 3 m 2 / g - 8 m 2 / g.

13. The secondary battery according to any one of claims 2-12, wherein, based on the total weight of the first lithium iron phosphate particles, the carbon content of the first lithium iron phosphate particles is Cx4% by weight, wherein, 0.8 ≤ Cx4 ≤ 2.

0.

14. The secondary battery according to any one of claims 2-13, wherein the specific surface area of the second lithium iron manganese phosphate salt particles is 5 m 2 / g - 12 m 2 / g.

15. The secondary battery according to any one of claims 2-14, wherein, based on the total weight of the second lithium iron manganese phosphate particles, the carbon content of the second lithium iron manganese phosphate particles is Cx2% by weight, wherein, 0.8 ≤ Cx2 ≤ 2.

0.

16. The secondary battery according to any one of claims 1 - 15, wherein the primary average particle size s1' of the first lithium manganese iron phosphate salt particles is 150 - 210 nm, the primary average particle size s2' of the first lithium iron phosphate salt particles satisfies 870 nm ≤ s2' ≤ 3000 nm, and / or the primary average particle size s3' of the second lithium iron phosphate salt particles satisfies 60 nm ≤ s3' ≤ 200 nm.

17. The secondary battery according to any one of claims 2 - 16, wherein the capacity ratio η of the first lithium iron phosphate salt particles ≥ 88%, and the η is defined as: The battery with the first lithium iron phosphate salt particles as the positive electrode active material is subjected to constant current charge and discharge twice at a rate of 0.1C in the voltage range of 2.0V to 3.75V, and then subjected to constant current charge and discharge once at a rate of 1C. In the charge and discharge test at the rate of 1C, the capacity value with a discharge voltage of 3.2V is extracted and denoted as C1, and the capacity value when discharged to 2.0V is C2, and η = C1 / C2, where, The charging process includes constant voltage charging, with a constant voltage of 3.75 V and a constant voltage cut-off current of 50 μA.

18. The secondary battery according to any one of claims 2 - 17, wherein the first lithium iron phosphate salt particles satisfy at least one of (a) - (f): (a) The Dv10 of the first lithium iron phosphate salt particles ≥ 0.2 μm; (b) The Dv50 of the first lithium iron phosphate salt particles is 0.5 - 5 μm; (c) The Dv90 of the first lithium iron phosphate salt particles ≤ 10 μm; (d) The Dv99 of the first lithium iron phosphate salt particles ≤ 12 μm; (e) The powder tap density of the first lithium iron phosphate salt under a pressure of 3 tons is ≥ 2.25 g / cm 3 ; (f) The powder resistivity of the first lithium iron phosphate is less than 60 Ω·cm.

19. The secondary battery according to any one of claims 6 - 18, wherein the aspect ratio of the second lithium iron phosphate salt particles ≥ 1.

3.

20. The secondary battery according to any one of claims 6 - 19, wherein the ratio W of the diffraction peak intensity of the (020) crystal plane to the diffraction peak intensity of the (211) crystal plane of the second lithium iron phosphate salt particles ≥ 1.

03.

21. The secondary battery according to any one of claims 2 - 20, wherein the first lithium iron phosphate salt particles are mainly obtained by the following preparation method: Providing raw materials containing at least a lithium source, an iron source, a phosphorus source, a carbon film-forming agent, a carbon source, and a modifier, and performing at least two sinterings, wherein, The temperature of the first sintering is 500°C - 760°C; The temperature of the second sintering is 700°C - 800°C.

22. The secondary battery according to claim 21, wherein the first lithium iron phosphate salt particles are mainly obtained by the following preparation method: Providing raw materials containing at least a lithium source, an iron source, a phosphorus source, a carbon source, a carbon film-forming agent, and a modifier, and performing at least two sinterings, wherein, The carbon content of the material after the first sintering is 0.01 wt% - 0.79 wt%; The carbon content of the material after the second sintering is 0.8 wt% - 2.0 wt%.

23. The secondary battery according to claim 21 or 22, wherein the method for preparing the first lithium iron phosphate salt particles comprises the following steps: Performing first pulverization after the first sintering and performing second pulverization after the second sintering, wherein, The Dv50 of the product after the first pulverization is 300 nm - 1200 nm; The Dv50 of the product after the second pulverization is 500 nm - 5000 nm.

24. An electrical device comprising the secondary battery according to any one of claims 1 - 23.

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