Positive electrode sheet, secondary battery and electric device

Abstract

Provided is a positive electrode sheet, comprising a positive electrode current collector and a positive electrode film layer, wherein the positive electrode film layer is arranged on at least one surface of the positive electrode current collector, the positive electrode film layer comprises a first positive electrode film layer and a second positive electrode film layer, and the first positive electrode film layer is located between the positive electrode current collector and the second positive electrode film layer. The first positive electrode film layer comprises a first positive electrode active material, the specific surface area of the first positive electrode active material is 12 m2 / g to 16 m2 / g, and on the basis of the total weight of the first positive electrode active material, the carbon content of the first positive electrode active material is 1.0 wt% to 1.5 wt%; and the second positive electrode film layer comprises a second positive electrode active material, the specific surface area of the second positive electrode active material is 6 m2 / g to 19 m2 / g, and on the basis of the total weight of the second positive electrode active material, the carbon content of the second positive electrode active material is 1.1 wt% to 2.1 wt%.

Description

Positive electrode sheet, secondary battery and electrical device

[0001] Cross-references

[0002] This application refers to Chinese patent application No. 202410175809.1 filed on February 7, 2024, entitled “Positive Electrode Sheet, Secondary Battery 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 positive electrode sheet, a secondary battery, and an electrical device. Background Art

[0004] Lithium-ion batteries have the advantages of high energy density, high operating voltage, long cycle life, light weight and no environmental pollution. Therefore, they have become an ideal power source for small and lightweight electronic devices such as cameras, mobile phones, laptops, digital products, etc., and are also the preferred power source for high-energy power batteries in future automobiles.

[0005] Lithium-ion batteries are primarily composed of a positive electrode, a negative electrode, a separator, and an electrolyte. In lithium-ion battery manufacturing, the positive and negative electrode slurries are first coated onto the corresponding current collectors, dried, and cold-pressed before proceeding to subsequent processes. Due to the characteristics of the positive electrode active materials (e.g., lithium iron phosphate, lithium manganese iron phosphate, etc.), the positive electrode is prone to becoming brittle after cold pressing. During the subsequent stripping and cutting processes, the active materials may crack and fall off, and the pole pieces may also break during the winding process, seriously affecting the battery's processing performance, electrochemical performance, and safety performance.

[0006] Therefore, it is necessary to provide a positive electrode plate with good plate flexibility.

[0007] Summary of the Invention

[0008] The present application is made in view of the above-mentioned problems, and its purpose is to provide a positive electrode plate, a secondary battery and an electrical device, wherein the positive electrode plate has good plate flexibility.

[0009] The inventors have discovered that the above objectives can be achieved by adopting the technical solution of the present invention.

[0010] The first aspect of the present application provides a positive electrode sheet, comprising a positive electrode current collector and a positive electrode film layer, wherein the positive electrode film layer is arranged on at least one side of the positive electrode current collector, and the positive electrode film layer comprises a first positive electrode film layer and a second positive electrode film layer, wherein the first positive electrode film layer is located between the positive electrode current collector and the second positive electrode film layer.

[0011] The first positive electrode film layer contains a first positive electrode active material, and the specific surface area of ​​the first positive electrode active material is 12m 2 / g-16m2 / g, based on the total weight of the first positive electrode active material, the carbon content of the first positive electrode active material is 1.0 wt%-1.5 wt%,

[0012] The second positive electrode film layer contains a second positive electrode active material, and the specific surface area of ​​the second positive electrode active material is 6m 2 / g-19m 2 / g, based on the total weight of the second positive electrode active material, the carbon content of the second positive electrode active material is 1.1 wt%-2.1 wt%.

[0013] The positive electrode plate of the present application has good plate flexibility.

[0014] In any embodiment, the compacted density of the first positive electrode active material is 2.4-2.7 g / cm when measured at a pressure of 3T. 3 .

[0015] When the compaction density of the first positive electrode active material is 2.4-2.7 g / cm 3 When the positive electrode sheet of the present invention is used, the battery has a higher energy density.

[0016] In any embodiment, the first positive active material comprises first lithium iron phosphate salt particles and second lithium iron phosphate salt particles, the first lithium iron phosphate salt particles have a primary average particle size of 50-300 nm, and the second lithium iron phosphate salt particles have a primary average particle size of 400-500 nm.

[0017] In any embodiment, based on the total number of the first lithium iron phosphate salt particles and the second lithium iron phosphate salt particles, the number of the first lithium iron phosphate salt particles accounts for 60%-80%, and the number of the second lithium iron phosphate salt particles accounts for 20%-40%.

[0018] In any embodiment, the second positive active material comprises third lithium iron phosphate particles, and a ratio of a charge capacity C2 of the third lithium iron phosphate particles at 60° C. to a charge capacity C1 of the third lithium iron phosphate particles at 25° C. (C2 / C1) is ≥1.02.

[0019] When C2 / C1≥1.02, the positive electrode sheet of the present application has a higher compaction density, and the battery containing the positive electrode sheet of the present application has a longer cycle life, a lower capacity attenuation degree and a higher cycle climbing degree.

[0020] In any embodiment, the carbon content of the second positive electrode active material is 1.2 wt % to 2 wt % based on the total weight of the second positive electrode active material.

[0021] When the carbon content of the second positive electrode active material is 1.2 wt % to 2 wt %, the battery comprising the positive electrode sheet of the present application has a good balance between cycle life and gram capacity.

[0022] In any embodiment, a ratio W1 / W2 of a weight W1 of the first positive electrode active material to a weight W2 of the second positive electrode active material is 1:9 to 9:1.

[0023] In any embodiment, a ratio W1 / W2 of a weight W1 of the first positive electrode active material to a weight W2 of the second positive electrode active material is 3:7 to 7:3.

[0024] When W1 / W2 is 3:7 to 7:3, the positive electrode sheet of the present application has a higher compaction density, and the battery containing the positive electrode sheet of the present application has a higher cycle climbing degree.

[0025] In any embodiment, the thickness of the first positive electrode film layer and the thickness of the second positive electrode film layer are independently 5-80 μm.

[0026] A second aspect of the present application provides a secondary battery, which includes the positive electrode sheet of the first aspect of the present application.

[0027] A third aspect of the present application provides an electrical device comprising the secondary battery according to the second aspect of the present application. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] FIG1 is a scanning electron microscope image of the first positive electrode active material in Example 1 of the present application at a magnified size of 10k.

[0029] FIG2 is a scanning electron microscope image of the second positive electrode active material in Example 1 of the present application at a magnified size of 10k.

[0030] FIG3 is a schematic diagram of a secondary battery according to an embodiment of the present application.

[0031] FIG. 4 is an exploded view of the secondary battery according to the embodiment of the present application shown in FIG. 3 .

[0032] FIG5 is a schematic diagram of a battery module according to an embodiment of the present application.

[0033] FIG6 is a schematic diagram of a battery pack according to an embodiment of the present application.

[0034] FIG. 7 is an exploded view of the battery pack shown in FIG. 6 according to an embodiment of the present application.

[0035] FIG8 is a schematic diagram of an electric device using a secondary battery as a power source according to an embodiment of the present application.

[0036] Description of reference numerals:

[0037] 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

[0038] Below, the embodiments of the positive electrode sheet, secondary battery 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.

[0039] " 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.

[0040] Unless otherwise specified, all embodiments and optional embodiments of the present application can be combined with each other to form a new technical solution.

[0041] 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.

[0042] 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.

[0043] 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.

[0044] 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).

[0045] Lithium-ion batteries are mainly composed of a positive electrode, a negative electrode, a separator and an electrolyte. In the manufacture of lithium-ion batteries, the positive and negative electrode slurries are first coated on the corresponding current collectors, dried and cold-pressed, and then the subsequent processes are carried out. Due to the characteristics of the positive electrode active materials (for example, lithium iron phosphate salts, lithium manganese iron phosphate salts, etc.), the positive electrode is prone to become brittle after cold pressing. In the subsequent stripping and cutting process, the active material may crack and fall off. The pole piece may also break during the winding process, which seriously affects the processing performance, electrochemical performance and safety performance of the battery. Therefore, it is necessary to provide a positive electrode pole piece with good pole piece flexibility.

[0046] Based on this, this application proposes a technical solution to solve the above technical problems.

[0047] The first aspect of the present application provides a positive electrode sheet, comprising a positive electrode current collector and a positive electrode film layer, wherein the positive electrode film layer is arranged on at least one side of the positive electrode current collector, and the positive electrode film layer comprises a first positive electrode film layer and a second positive electrode film layer, wherein the first positive electrode film layer is located between the positive electrode current collector and the second positive electrode film layer.

[0048] The first positive electrode film layer contains a first positive electrode active material, and the specific surface area of ​​the first positive electrode active material is 12m 2 / g-16m 2 / g, based on the total weight of the first positive electrode active material, the carbon content of the first positive electrode active material is 1.0 wt%-1.5 wt%,

[0049] The second positive electrode film layer contains a second positive electrode active material, and the specific surface area of ​​the second positive electrode active material is 6m 2 / g-19m 2 / g, based on the total weight of the second positive electrode active material, the carbon content of the second positive electrode active material is 1.1 wt%-2.1 wt%.

[0050] When the above conditions are met, the surface of the second positive electrode active material in the second positive electrode film layer has a higher carbon coating amount, which can effectively reduce the sliding resistance between the particles, and the particles between the positive electrode materials are easier to slide. Then, under the same electrode compaction conditions, the particles are easier to slide and less likely to break, thereby improving the flexibility of the electrode, so that the positive electrode electrode of the present application has better electrode flexibility.

[0051] In some embodiments, the carbon content of the first cathode active material is Cx1 wt % based on the total weight of the first cathode active material, wherein a ratio z1 of the specific surface area of ​​the first cathode active material to Cx1 satisfies 10≤z1≤13.

[0052] In some embodiments, the carbon content of the second positive electrode active material is Cx2 wt % based on the total weight of the second positive electrode active material, wherein a ratio z2 of the specific surface area of ​​the second positive electrode active material to Cx2 satisfies 6≤z2≤11.

[0053] When the above conditions are met, the degree of graphitization of the surface carbon coating is high, the interlayer slip force is small, and under the same electrode compaction conditions, the particles are easier to slide and less likely to break, so that the positive electrode of the present application has better electrode flexibility.

[0054] In some embodiments, z1 can be 10, 10.3, 11, 11.5, 12, 12.3, 12.5, 13, or a range consisting of any two of the above z1 values ​​or a value within the range.

[0055] In some embodiments, z2 can be 6, 6.5, 7, 7.5, 8, 8.5, 9, 9.5, 10, 10.5, 11, or a range consisting of any two of the above z2 values ​​or a value within the range.

[0056] In this application, the term "specific surface area" or "BET" refers to the total external surface area per unit weight of a positive electrode active material.

[0057] The BET specific surface area can be measured by methods and equipment known in the art, for example, by using Tristar 3020 in accordance with GB / T 19587-2004.

[0058] In some embodiments, the specific surface area of ​​the first positive electrode active material is 12.3 m 2 / g-15.6m 2 / g.

[0059] In some embodiments, the specific surface area BET of the first positive electrode active material may be 12 m 2 / g, 12.3m 2 / g、13m 2 / g, 13.2m 2 / g, 13.5m 2 / g、14m 2 / g, 14.4m 2 / g, 14.5m 2 / g、15m 2 / g, 15.5m 2 / g, 15.6m 2 / g、16m 2 / g, or the range consisting of the BET specific surface areas of any two of the first positive electrode active materials mentioned above or a value within the range.

[0060] If the BET of the first positive electrode active material is too high, it will increase the water absorption performance, affect the processing performance of the slurry and increase the side reaction with the electrolyte. However, if the BET of the first positive electrode active material is too low, the gram capacity will be reduced. The BET of the first positive electrode active material is controlled within 12m 2 / g-16m 2 / g, which helps to obtain better slurry processing performance and gram capacity, thereby improving the processing problems of the battery cell, and controlling the side reactions with the electrolyte within an acceptable range, thereby improving the volume energy density of the battery and the battery cell life.

[0061] In some embodiments, the specific surface area of ​​the second positive electrode active material is 6.6 m 2 / g-19m 2 / g.

[0062] In some embodiments, the specific surface area BET of the second positive electrode active material may be 6 m 2 / g, 6.5m 2 / g, 6.6m 2 / g、7m 2 / g, 7.2m 2 / g, 7.5m 2 / g、8m 2 / g、8.5m2 / g、9m 2 / g, 9.5m 2 / g, 9.9m 2 / g、10m 2 / g, 10.5m 2 / g, 10.8m 2 / g、11m 2 / g, 11.5m 2 / g、12m 2 / g, 12.5m 2 / g, 12.6m 2 / g、13m 2 / g, 13.2m 2 / g, 13.5m 2 / g、14m 2 / g, 14.4m 2 / g, 14.5m 2 / g、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, or the range consisting of the BET specific surface areas of any two of the above-mentioned second positive electrode active materials or a value within the range.

[0063] If the BET of the second positive electrode active material is too high, it will increase the water absorption performance, affect the processing performance of the slurry and increase the side reaction with the electrolyte. However, if the BET of the second positive electrode active material is too low, the gram capacity will be reduced. The BET of the second positive electrode active material is controlled within 6m 2 / g-19m 2 / g, which helps to obtain better slurry processing performance and gram capacity, thereby improving the processing problems of the battery cell. At the same time, it can also regulate the initial capacity of the second active material and then regulate the degree of slow attenuation.

[0064] The carbon content of the first positive electrode active material / the second positive electrode active material can be measured by methods and equipment known in the art. For example, the carbon content can be measured by infrared absorption method after combustion in a high-frequency induction furnace according to GB / T 20123-2006 / ISO.

[0065] In some embodiments, the carbon content of the first positive electrode active material is Cx1 wt % based on the total weight of the first positive electrode active material, where 1.0≤Cx1≤1.5.

[0066] If the carbon content of the first positive electrode active material is too high, it will affect the insertion and extraction of lithium ions, and to a certain extent, affect the gram capacity of the battery. However, if the carbon content of the first positive electrode active material is too low, it will affect the conductivity of the battery, which is not conducive to its kinetic performance. Controlling the carbon content of the first positive electrode active material between 1.0 wt% and 1.5 wt% helps to obtain better kinetic performance and gram capacity. The carbon content in this weight ratio range can not affect the capacity while taking into account the conductive network between the active material particles, and can play a lubricating role between the particles, thereby achieving high kinetics and high energy density.

[0067] In some embodiments, Cx1 is 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, or a range consisting of Cx1 of any two of the first positive electrode active materials or a value within the range.

[0068] In some embodiments, the carbon contained in the first positive electrode active material is coated on the surface of its particles. In some embodiments, the carbon contained in the first positive electrode active material is embedded in its particles. In some embodiments, the carbon contained in the first positive electrode active material is partially coated on the surface of its particles and partially embedded in its particles.

[0069] In some embodiments, the carbon content of the second cathode active material is 1.1 wt % to 2.1 wt % based on the total weight of the second cathode active material. In some embodiments, the carbon content of the second cathode active material is 1.2 wt % to 2 wt % based on the total weight of the second cathode active material.

[0070] Too high a carbon content of the second positive electrode active material will affect the insertion and extraction of lithium ions, and to a certain extent, affect the gram capacity of the battery. However, too low a carbon content of the second positive electrode active material will affect the cycle life of the battery. When the carbon content of the second positive electrode active material is controlled at 1.2 wt%-2 wt%, while ensuring that the material has a slow decay effect, the reduction in initial capacity can be compensated by using it in conjunction with the first active material, thereby achieving slow decay while taking into account energy density, so that the battery containing the positive electrode sheet of the present application has a good balance between cycle life and gram capacity.

[0071] In some embodiments, Cx2 is 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, 2.1, or a range of Cx2 of any two of the second positive electrode active materials or a value within the range.

[0072] In some embodiments, the carbon contained in the second positive electrode active material is coated on the surface of its particles. In some embodiments, the carbon contained in the second positive electrode active material is embedded in its particles. In some embodiments, the carbon contained in the second positive electrode active material is partially coated on the surface of its particles and partially embedded in its particles.

[0073] In some embodiments, the compacted density of the first cathode active material is 2.4-2.7 g / cm when measured at a pressure of 3 T. 3 In some embodiments, the compacted density of the first positive electrode active material is 2.43-2.63 g / cm when measured under a pressure of 3 tons (T). 3 .

[0074] When the compaction density of the first positive electrode active material is 2.4-2.7 g / cm 3 When the volume is constant, a higher compaction density can place more active materials in a unit volume, thereby improving the energy density, so that the battery containing the positive electrode sheet of the present application has a higher energy density.

[0075] In some embodiments, the first positive active material comprises first lithium iron phosphate salt particles and second lithium iron phosphate salt particles, the first lithium iron phosphate salt particles have a primary average particle size of 50-300 nm, and the second lithium iron phosphate salt particles have a primary average particle size of 400-500 nm.

[0076] With the above-mentioned particle size combination, under the same pressure conditions, the electrode pores are smaller, thereby achieving high compaction density and improving energy density.

[0077] In some embodiments, based on the total number of the first lithium iron phosphate salt particles and the second lithium iron phosphate salt particles, the number of the first lithium iron phosphate salt particles accounts for 60%-80%, and the number of the second lithium iron phosphate salt particles accounts for 20%-40%.

[0078] The quantity distribution of the above-mentioned particle sizes can also reduce the porosity of the electrode under the same pressure conditions, thereby increasing the compaction density and energy density.

[0079] 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.

[0080] The primary average particle size of the first lithium iron phosphate salt particles / 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 the length diameter statistical method. As an example, a sample is tested using a ZEISS sigma 300 scanning electron microscope, and then tested in accordance with standard JY / T010-1996 to observe the sample morphology. The morphology of the primary particles is observed by observing particles of any 10 areas of the same size and shape in the same scanning electron microscope image (SEM) at a magnification of 10k. The 10 regions are then each subdivided into 5 positions, including the four corners and the center. The Feret diameter of any primary particle at each position under the magnification is selected, and the Feret diameter results of the 5 positions, including the four corners and the center, are averaged to obtain the particle size of the primary particles in the region. The particle sizes of the primary particles obtained in the 10 regions are then compared and counted to obtain the distribution ratio (quantity ratio) of the particle sizes of the primary particles. The particle sizes of the primary particles obtained in the 10 regions are averaged to obtain the average particle size of the primary particles (i.e., the primary average particle size). Specifically, the average value of the four sizes, namely, the size of the particle in the direction of the two adjacent sides of the rectangle circumscribed with the image and the size in the direction of the 45-degree inclination of the two adjacent sides of the rectangle circumscribed with the image, is set as the Feret diameter of the particle.

[0081] In some embodiments, the primary average particle size of the first lithium iron phosphate salt particles may be 50 nm, 100 nm, 150 nm, 200 nm, 250 nm, 300 nm, or a range consisting of any two of the foregoing primary average particle sizes or a value within the range.

[0082] In some embodiments, the primary average particle size of the second lithium iron phosphate salt particles can be 400 nm, 410 nm, 420 nm, 430 nm, 440 nm, 450 nm, 460 nm, 470 nm, 480 nm, 490 nm, 500 nm, or a range consisting of any two of the above primary average particle sizes or a value within the range.

[0083] In some embodiments, the second positive electrode active material comprises third lithium iron phosphate particles, and a ratio of a withholding capacity C2 of the third lithium iron phosphate particles at 60° C. to a withholding capacity C1 of the third lithium iron phosphate particles at 25° C. (C2 / C1) is ≥1.02.

[0084] When C2 / C1 ≥ 1.02, the capacity of the active material cannot be fully utilized in the early stages of the battery cycle. During the continuous cycling of the battery, as the active material is activated, the capacity is gradually utilized, thereby extending the capacity retention rate in the early stages of the battery cycle. Therefore, when C2 / C1 ≥ 1.02, the positive electrode sheet of the present application has a higher compaction density, and the battery containing the positive electrode sheet of the present application has a longer cycle life, lower capacity decay, and higher cycle ramp rate.

[0085] The buckle capacitance of the third lithium iron phosphate salt particle is measured as follows:

[0086] When measuring the button capacity of the third lithium iron phosphate salt particles, first prepare a button battery: mix the third lithium iron phosphate salt particles, the adhesive polyvinylidene fluoride (PVDF), and the conductive agent acetylene black in a weight ratio of 95:5:5, add an appropriate amount of N-methylpyrrolidone (NMP) solvent, and stir and mix thoroughly to form a uniform positive electrode slurry; apply this slurry on an aluminum foil with a thickness of 1μm for the positive electrode current collector, then dry and cold press it, and then punch it into small discs with a diameter of 14mm as the positive electrode. Use a lithium sheet as the negative electrode, use a 12μm thick polypropylene isolation film and a common lithium iron phosphate electrolyte to assemble it into a button battery. Then,

[0087] For the button cell capacity (C2) of the third lithium iron phosphate salt particles at 60°C: At 60°C, charge and discharge at a rate of 0.1C and record the corresponding gram capacity. The test conditions are as follows: At 60°C, let the button cell rest for 3 hours, charge at a constant current of 0.1C to the upper cutoff voltage (3.75V), then charge at a constant voltage of 50μA, let it rest for 5 minutes, and record the charge capacity of the button cell. The charge gram capacity (C2) of the third lithium iron phosphate salt particles at 60°C = the charge capacity of the button cell at 60°C / the weight of the third lithium iron phosphate salt particles in the button cell.

[0088] For the button capacity C2 (i.e., gram capacity) of the third lithium iron phosphate salt particles at 25°C: At 25°C, charge and discharge at a rate of 0.1C and record the corresponding gram capacity. The test conditions are as follows: At 25°C, the button battery was allowed to stand for 3 hours, charged at a constant current of 0.1C to the upper cutoff voltage (3.75V), then charged at a constant voltage to 50μA, allowed to stand for 5 minutes, and the charge capacity of the button battery was recorded. The charge gram capacity C1 of the third lithium iron phosphate salt particles at 25°C = the charge capacity of the button battery at 25°C / the weight of the third lithium iron phosphate salt particles in the button battery.

[0089] In some embodiments, C2 / C1 can be 1, 1.01, 1.02, 1.03, 1.04, 1.05, 1.06, 1.07, 1.08, 1.09, 1.1, or a range consisting of any two of the above C2 / C1 or a value within the range.

[0090] In some embodiments, the primary average particle size of the third lithium iron phosphate particles may be 200 nm, 250 nm, 300 nm, 350 nm, 400 nm, 450 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.

[0091] The method for measuring the primary average particle size of the third lithium iron phosphate salt particles may be the same as the method for measuring the primary average particle size of the first lithium iron phosphate salt particles.

[0092] In some embodiments, the Dv50 of the third lithium iron phosphate salt particles can be 0.1μm, 0.2μm, 0.3μm, 0.4μm, 0.5μm, 0.6μm, 0.7μm, 0.8μm, 0.9μm, 1μm, 1.1μm, 1.2μm, 1.3μm, 1.4μm, 1.5μm, or a range consisting of any two of the above Dv50 or a value within the range.

[0093] Dv50 can be measured by methods and equipment known in the art. For example, it can be measured with a laser particle size analyzer (Malvern Master Size 3000) with reference to GB / T19077-2016 / ISO13320:2009. The specific test process is as follows: take an appropriate amount of the sample to be tested, and ensure that the sample concentration is 8% to 12% shading. Add 20mL of deionized water and ultrasonicate for 5 minutes at an ultrasonic frequency of 53KHz and an ultrasonic power of 120W to ensure that the sample is completely dispersed. Then, the sample is measured according to the GB / T19077-2016 / ISO13320:2009 standard.

[0094] In some embodiments, a ratio W1 / W2 of the weight W1 of the first positive electrode active material to the weight W2 of the second positive electrode active material is 1:9 to 9:1.

[0095] In some embodiments, a ratio W1 / W2 of a weight W1 of the first positive electrode active material to a weight W2 of the second positive electrode active material is in a range of 3:7 to 7:3.

[0096] When W1 / W2 is 3:7 to 7:3, the first active material layer ensures the energy density of the battery, while the second active material layer provides a slow decay effect while optimizing the flexibility of the battery electrode. Therefore, when W1 / W2 is 3:7 to 7:3, the positive electrode electrode of the present application has a higher compaction density, and the battery containing the positive electrode electrode of the present application has a higher cycle climbing degree.

[0097] In some embodiments, the thickness of the first positive electrode film layer and the thickness of the second positive electrode film layer are independently 5-80 μm. In some embodiments, the thickness of the first positive electrode film layer and the thickness of the second positive electrode film layer are independently 8-72 μm. In some embodiments, the thickness of the first positive electrode film layer and the thickness of the second positive electrode film layer are independently 24-56 μm. In some embodiments, the thickness of the first positive electrode film layer and the thickness of the second positive electrode film layer are independently 30-50 μm. In some embodiments, the thickness of the first positive electrode film layer and the thickness of the second positive electrode film layer are independently 5μm, 8μm, 10μm, 15μm, 16μm, 20μm, 24μm, 25μm, 30μm, 32μm, 35μm, 40μm, 45μm, 48μm, 50μm, 55μm, 56μm, 60μm, 64μm, 65μm, 70μm, 72μm, 75μm, 80μm, or a range consisting of any two of the above thicknesses or a value in the range.

[0098] A second aspect of the present application provides a secondary battery, which includes the positive electrode sheet of the first aspect of the present application.

[0099] A third aspect of the present application provides an electrical device comprising the secondary battery according to the second aspect of the present application.

[0100] In addition, the secondary battery and the electric device of the present application will be described below with reference to the drawings as appropriate.

[0101] In one embodiment of the present application, a secondary battery is provided.

[0102] 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.

[0103] [Positive electrode]

[0104] 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, and the positive electrode film layer includes a positive electrode active material.

[0105] 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.

[0106] 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.).

[0107] 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.

[0108] 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.

[0109] 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.

[0110] [Negative electrode]

[0111] 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.

[0112] 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.

[0113] 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.).

[0114] In some embodiments, the negative electrode active material may be a negative electrode active material for a battery that is well known in the art. As an example, the negative electrode active material may include at least one of the following materials: artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, and lithium titanate. The silicon-based material may be selected from at least one of elemental silicon, silicon oxides, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. The tin-based material may be selected from at least one of elemental tin, tin oxides, and tin alloys. However, the present application is not limited to these materials, and other traditional materials that can be used as negative electrode active materials for batteries may also be used. These negative electrode active materials may be used alone or in combination of two or more.

[0115] 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).

[0116] 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.

[0117] In some embodiments, the negative electrode film layer may further include other additives, such as a thickener (eg, sodium carboxymethyl cellulose (CMC-Na)).

[0118] 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.

[0119] [Electrolytes]

[0120] 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.

[0121] In some embodiments, the electrolyte is an electrolyte solution, which includes an electrolyte salt and a solvent.

[0122] In some embodiments, the electrolyte salt may be selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bisfluorosulfonyl imide, lithium bistrifluoromethanesulfonyl imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalatoborate, lithium bisoxalatoborate, lithium difluorodioxalatophosphate, and lithium tetrafluorooxalatophosphate.

[0123] 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.

[0124] In some embodiments, the electrolyte may optionally include additives. For example, the additives may include negative electrode film-forming additives, positive electrode film-forming additives, and additives that can improve certain battery properties, such as additives that improve battery overcharge performance, and additives that improve battery high or low temperature performance.

[0125] [Isolation film]

[0126] 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.

[0127] 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.

[0128] 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.

[0129] In some embodiments, the secondary battery may include an outer packaging that can be used to encapsulate the electrode assembly and the electrolyte.

[0130] 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.

[0131] 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, FIG3 shows a secondary battery 5 having a square structure as an example.

[0132] In some embodiments, referring to Figure 4, 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.

[0133] 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.

[0134] Figure 5 shows an example battery module 4. Referring to Figure 5 , 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.

[0135] 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.

[0136] 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.

[0137] Figures 6 and 7 illustrate an example battery pack 1. Referring to Figures 6 and 7 , 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 positioned over the lower case 3 to form an enclosed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the battery box.

[0138] 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 are not limited to these.

[0139] As the electrical device, a secondary battery, a battery module or a battery pack can be selected according to its usage requirements.

[0140] Figure 8 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.

[0141] 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.

[0142] Example

[0143] 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.

[0144] 1. Preparation method

[0145] Example 1

[0146] 1) Preparation of positive electrode active materials and positive electrode slurry

[0147] The preparation method of the first positive electrode active material is as follows: ferrous oxalate and lithium dihydrogen phosphate (weight ratio 1:2) are used as raw materials, polyethylene glycol is used as a carbon source, the weight of the carbon source accounts for 17% of the total weight of the raw materials (ferrous oxalate + lithium dihydrogen phosphate + polyethylene glycol), 2000ppm TiO2 (based on the total weight of the raw materials) is added as an additive, and the particle size Dv50 is ground to 0.4μm by a sand mill, and then spray-dried, and the outlet temperature is controlled at 150°C to obtain a spherical dried material. The spherical dried material is sintered under a nitrogen atmosphere, with a controlled potting amount of 5kg, a nitrogen flow rate of 300L / min, a heating rate of 3°C / min, a sintering temperature of 780°C, and a constant temperature period of 12h to obtain a sintered material. The sintered material was pulverized using a jet mill at a classifier frequency of 50 Hz to obtain a first positive electrode active material. The primary particle size of each particle was measured. Particles with a primary particle size of ≤350 nm were counted and the average particle size was calculated to obtain the number and average particle size of the first lithium iron phosphate salt particles. Particles with a primary particle size of >350 nm were counted and the average particle size was calculated to obtain the number and average particle size of the second lithium iron phosphate salt particles. Based on the total number of the first lithium iron phosphate salt particles and the second lithium iron phosphate salt particles, the primary average particle size of the first lithium iron phosphate salt particles was 150 nm, accounting for 70% of the total number, and the primary average particle size of the second lithium iron phosphate salt particles was 450 nm, accounting for 30% of the total number.

[0148] The first positive electrode active material, binder polyvinylidene fluoride (PVDF), and conductive agent acetylene black were mixed in a weight ratio of 97.3:2:0.7, and an appropriate amount of N-methylpyrrolidone (NMP) solvent was added and stirred thoroughly to form a uniform first positive electrode slurry with a viscosity of 6000 mPa·s.

[0149] The second positive electrode active material is prepared as follows: iron phosphate and lithium carbonate are used as raw materials, and glucose and polyethylene glycol are added as carbon sources in a weight ratio of 7:3, with the weight of the carbon source accounting for 17% of the total weight of the raw materials. The iron phosphate: lithium carbonate: carbon source are mixed in a weight ratio of 3.85:1:1 and wet-grinded with water. The resulting slurry has a Dv50 of 300 nm. The slurry is spray-dried and then sintered in a roller hearth furnace at 760°C for 24 hours, with nitrogen introduced during the sintering process and a heating rate of 3°C / min. The material is then naturally cooled to a temperature of <80°C before being discharged to obtain a calcined material. The calcined material is subjected to air flow crushing, screening, demagnetization, and vacuum packaging. The classifier frequency of the air flow crushing is controlled at 80 Hz and the induced draft fan frequency is controlled at 40 Hz to obtain the second positive electrode active material, wherein the third lithium iron phosphate salt particles have a primary average particle size of 400 nm and a Dv50 of 0.7 μm.

[0150] The second positive electrode active material, binder polyvinylidene fluoride (PVDF), and conductive agent acetylene black were mixed in a weight ratio of 97.3:2:0.7, and an appropriate amount of N-methylpyrrolidone (NMP) solvent was added and stirred thoroughly to form a uniform second positive electrode slurry with a viscosity of 6000 mPa·s.

[0151] 2) Preparation of positive electrode sheet

[0152] The first positive electrode slurry was coated on a 15 μm thick aluminum foil with a coating weight of 160 mg / 1540.25 cm 2 , and then dried to form the first positive electrode film layer. The specific surface area of ​​the first positive electrode active material is 13.2m 2 / g, the carbon content of the first positive electrode active material is Cx1 weight %, wherein Cx1 is 1.2, and the ratio of the specific surface area of ​​the first positive electrode active material to Cx1 is z1=12.

[0153] The second positive electrode slurry was coated on the first positive electrode film layer with a coating weight of 160 mg / 1540.25 cm 2 , and then dried to form the second positive electrode film layer. The specific surface area of ​​the second positive electrode active material is 10.8m 2 / g, the carbon content of the second positive electrode active material is Cx2 weight %, wherein Cx2 is 1.8, and the ratio of the specific surface area of ​​the second positive electrode active material to Cx2 is z2=6.

[0154] The aluminum foil, the first positive electrode film layer and the second positive electrode film layer together form the positive electrode sheet.

[0155] 3) Preparation of negative electrode sheet

[0156] The negative electrode active material, graphite, the thickener, sodium carboxymethyl cellulose, the binder, styrene-butadiene rubber, and the conductive agent, acetylene black, were mixed in a weight ratio of 97:1:1:1. Deionized water was added and the mixture was stirred in a vacuum mixer to produce a negative electrode slurry. The slurry was evenly coated onto 8μm-thick copper foil. The negative electrode sheet was then dried, cold-pressed, and slit.

[0157] 4) Preparation of electrolyte

[0158] A mixture of ethylene carbonate, ethyl methyl carbonate, and diethyl carbonate was prepared in a volume ratio of 20:20:60. A fully dried lithium salt was dissolved in the mixture, and 10 wt% of fluoroethylene carbonate was added as an additive. The electrolyte solution was obtained. The lithium salt concentration was 1 mol / L. The entire process was performed in an argon atmosphere glove box with a water content of <10 ppm.

[0159] 5) Isolation film

[0160] A polyethylene film with a thickness of 12 μm was used as the separator.

[0161] 6) Preparation of batteries

[0162] The positive electrode sheet, separator, and negative electrode sheet are stacked in order, with the separator positioned between the cathode and anode to provide isolation. The cells are then wound to form a bare cell. The bare cell is then placed in an outer package, injected with the prepared electrolyte, and packaged, injected, formed, and vented to produce a lithium-ion battery.

[0163] Example 2

[0164] The difference between Example 2 and Example 1 is that the specific surface area of ​​the first positive electrode active material is 14.4, so the ratio of the specific surface area of ​​the first positive electrode active material to Cx1 is z1=12.

[0165] Example 3

[0166] The difference between Example 3 and Example 1 is that the specific surface area of ​​the first positive electrode active material is 15.6, so the ratio of the specific surface area of ​​the first positive electrode active material to Cx1 is z1=13.

[0167] Example 4

[0168] The difference between Example 4 and Example 1 is that the ratio of the withheld capacity C2 of the third lithium iron phosphate salt particles at 60°C to the withheld capacity C1 of the third lithium iron phosphate salt particles at 25°C is C2 / C1=1.04, the specific surface area of ​​the second positive electrode active material is 14.4, and thus the ratio of the specific surface area of ​​the second positive electrode active material to Cx2 is z2=8.

[0169] Example 5

[0170] The difference between Example 5 and Example 1 is that: C2 / C1=1.025, the specific surface area of ​​the second positive electrode active material is 18m 2 / g, the specific surface area of ​​the second positive electrode active material=18, so the ratio of the specific surface area of ​​the second positive electrode active material to Cx2 is z2=10.

[0171] Example 6

[0172] The difference between Example 6 and Example 1 is that: C2 / C1=1.02.

[0173] Example 7

[0174] The difference between Example 7 and Example 1 is that the specific surface area of ​​the second positive electrode active material is 7.2 m 2 / g, the carbon content is Cx2 weight%, where Cx2 is 1.2.

[0175] Example 8

[0176] The difference between Example 8 and Example 1 is that the specific surface area of ​​the second positive electrode active material is 12m 2 / g, the carbon content is Cx2 weight%, where Cx2 is 2.

[0177] Example 9

[0178] The difference between Example 9 and Example 1 is that the specific surface area of ​​the second positive electrode active material is 6.6 m 2 / g, the carbon content is Cx2 weight%, where Cx2 is 1.1.

[0179] Example 10

[0180] The difference between Example 10 and Example 1 is that the specific surface area of ​​the second positive electrode active material is 12.6 m 2 / g, the carbon content is Cx2 weight%, where Cx2 is 2.1.

[0181] Example 11

[0182] The difference between Example 11 and Example 1 is that the primary average particle size of the first lithium iron phosphate salt particles is 300 nm, and the number accounts for 60% (based on the total number of the first lithium iron phosphate salt particles and the second lithium iron phosphate salt particles), and the primary average particle size of the second lithium iron phosphate salt particles is 400 nm, and the number accounts for 40% (based on the total number of the first lithium iron phosphate salt particles and the second lithium iron phosphate salt particles); in addition, the ratio W1 / W2 of the weight W1 of the first positive electrode active material to the weight W2 of the second positive electrode active material is 1:9 (the total weight of the first positive electrode active material and the second positive electrode active material is the same as in Example 1).

[0183] Example 12

[0184] The difference between Example 12 and Example 1 is that the primary average particle size of the first lithium iron phosphate salt particles is 50 nm, and the number accounts for 80% (based on the total number of the first lithium iron phosphate salt particles and the second lithium iron phosphate salt particles), and the primary average particle size of the second lithium iron phosphate salt particles is 500 nm, and the number accounts for 20% (based on the total number of the first lithium iron phosphate salt particles and the second lithium iron phosphate salt particles); in addition, W1 / W2 is 9:1 (the total weight of the first positive electrode active material and the second positive electrode active material is the same as in Example 1).

[0185] Example 13

[0186] The difference between Example 13 and Example 1 is that W1 / W2 is 3:7 (the total weight of the first positive electrode active material and the second positive electrode active material is the same as that in Example 1).

[0187] Example 14

[0188] The difference between Example 14 and Example 1 is that W1 / W2 is 7:3 (the total weight of the first positive electrode active material and the second positive electrode active material is the same as that in Example 1).

[0189] Example 15

[0190] The difference between Example 15 and Example 1 is that W1 / W2 is 1:9 (the total weight of the first positive electrode active material and the second positive electrode active material is the same as that in Example 1).

[0191] Example 16

[0192] The difference between Example 16 and Example 1 is that W1 / W2 is 9:1 (the total weight of the first positive electrode active material and the second positive electrode active material is the same as that in Example 1).

[0193] Example 17

[0194] The difference between Example 17 and Example 1 is that: C2 / C1=1.01; the specific surface area of ​​the second positive electrode active material is 19m 2 / g, so z2 is 10.5.

[0195] Example 18

[0196] The difference between Example 18 and Example 1 is that the specific surface area of ​​the first positive electrode active material is 12.3 m 2 / g, the carbon content is Cx1 weight %, where Cx1 is 1, and thus z1 is 12.3.

[0197] Example 19

[0198] The difference between Example 19 and Example 1 is that the specific surface area of ​​the first positive electrode active material is 15.5 m 2 / g, the carbon content is Cx1 weight %, where Cx1 is 1.5 and thus z1 is 10.3.

[0199] Comparative Example 1

[0200] The difference between Comparative Example 1 and Example 1 is that the second positive electrode film layer is not included, and the aluminum foil and the first positive electrode film layer are combined to form the positive electrode sheet.

[0201] Comparative Example 2

[0202] The difference between Comparative Example 2 and Example 2 is that the second positive electrode film layer is not included, and the aluminum foil and the first positive electrode film layer are combined to form the positive electrode sheet.

[0203] Comparative Example 3

[0204] The difference between Comparative Example 3 and Example 3 is that the second positive electrode film layer is not included, and the aluminum foil and the first positive electrode film layer are combined to form the positive electrode sheet.

[0205] 2. Battery performance test

[0206] 1) Primary average particle size

[0207] The samples were tested using a ZEISS sigma 300 scanning electron microscope and then tested according to the standard JY / T010-1996 to observe the sample morphology.

[0208] The morphology of the primary particles is observed, and particles of the same size and shape in any 10 areas of the same scanning electron microscope (SEM) at a magnification of 10k are observed. The 10 areas are then divided into 5 positions, including the four corners and the center. The Feret particle size of any primary particle at each position under this magnification is selected, and the Feret particle size results of the 5 positions, including the four corners and the center, are averaged to obtain the particle size of the primary particles in this area. The particle sizes of the primary particles obtained in the 10 areas are then compared and counted to obtain the distribution ratio (quantity ratio) of the particle size of the primary particles. The particle sizes of the primary particles obtained in the 10 areas are averaged to obtain the average particle size of the primary particles (i.e., the primary average particle size). Specifically, the average value of the four sizes, namely, the size of the particle in the direction of the two adjacent sides of the rectangle circumscribed with the image and the size in the direction of the 45-degree inclination of the two adjacent sides of the rectangle circumscribed with the image, is set as the Feret particle size of the particle.

[0209] 2) Dv50

[0210] The laser particle size analyzer Malvern 3000 (MasterSizer 3000) was used for measurement according to the standard procedure: GB / T19077-2016 / ISO13320:2009.

[0211] The specific test process is as follows: take an appropriate amount of the sample to be tested, and ensure that the sample concentration is 8% to 12% shading. Add 20mL of deionized water and ultrasonicate for 5 minutes at a frequency of 53KHz and a power of 120W to ensure that the sample is completely dispersed. Then, measure the sample according to the GB / T19077-2016 / ISO13320:2009 standard.

[0212] 3) Specific surface area

[0213] The specific surface area was determined according to the standard GB / T 19587-2004 using a Tristar 3020 instrument.

[0214] 4) Carbon content

[0215] The carbon content is determined according to the standard GB / T 20123-2006 / ISO using the infrared absorption method after combustion in a high-frequency induction furnace.

[0216] 5) Compacted density:

[0217] Powder compaction density

[0218] A certain amount of powder is placed in a compaction die of known diameter, with a metal sheet placed above and below the die. The powder is placed in the middle. Pressure (3T) is applied while measuring the corresponding powder thickness to determine the powder volume. The compaction density, ρ, is calculated using the formula ρ = m / v. Specific procedures can be followed in accordance with GB / T24533-2009.

[0219] Pole compaction density

[0220] Cut the electrode into 1000mm long sheets; roll the positive electrode sheet under a certain pressure. Due to the ductility of aluminum foil, the length of the membrane is 1006mm, and the punching is 1540.25mm. 2 By measuring the weight and thickness of the small disc, the compacted density can be calculated.

[0221] 6) 60℃ / 25℃ withholding capacity

[0222] The third lithium iron phosphate salt particles, the binder polyvinylidene fluoride (PVDF), and the conductive agent acetylene black were mixed in a weight ratio of 95:5:5. An appropriate amount of N-methylpyrrolidone (NMP) solvent was added and thoroughly stirred to form a uniform positive electrode slurry. This slurry was coated onto a 1μm thick aluminum foil used as the positive electrode current collector, then dried and cold-pressed. It was then punched into small discs with a diameter of 14mm to serve as the positive electrode. A button cell was assembled using a lithium sheet as the negative electrode, a 12μm thick polypropylene separator, and a common lithium iron phosphate electrolyte. Charge and discharge cycles were performed at a rate of 0.1C at 60°C / 25°C, and the corresponding gram capacity was recorded.

[0223] The test conditions are as follows: at 25°C, the button battery was allowed to stand for 3 hours, charged at a constant current of 0.1C to the upper cutoff voltage (3.75V), then charged at a constant voltage to 50μA, allowed to stand for 5 minutes, and the charging capacity of the button battery was recorded. The charging gram capacity of the third lithium iron phosphate salt particles at 25°C C1 = the charging capacity of the button battery at 25°C / the weight of the third lithium iron phosphate salt particles in the button battery. At 60°C, the button battery was allowed to stand for 3 hours, charged at a constant current of 0.1C to the upper cutoff voltage (3.75V), then charged at a constant voltage to 50μA, allowed to stand for 5 minutes, and the charging capacity of the button battery was recorded. The charging gram capacity of the third lithium iron phosphate salt particles at 60°C C2 = the charging capacity of the button battery at 60°C / the weight of the third lithium iron phosphate salt particles in the button battery.

[0224] 7) Cycle life test

[0225] 1. Let the battery sit at 20°C for 120 minutes

[0226] 2. 1C discharge to 2.5V

[0227] 3. Let it stand at 25℃ for 30 minutes

[0228] 4. 1C constant current charging to 3.65V, constant voltage charging, cut-off current 0.05C

[0229] 5. Let stand at 25℃ for 5 minutes

[0230] 6. 1C discharge to 2.5V

[0231] 7. Let stand at 25℃ for 5 minutes

[0232] 8. The above is one charge and discharge cycle of the battery. Repeat until the battery capacity decays to 80% of the initial value. Record the number of cycles.

[0233] 8) Capacity attenuation

[0234] Capacity retention rate at the 1000th cycle = (discharge capacity at the 1000th cycle / discharge capacity at the initial cycle) × 100%.

[0235] 9) Cycle climbing degree

[0236] Record the number of cycles during the first 3000 cycles where the capacity retention rate is continuously above 100%. If the number of cycles continuously above 100% is ≥10, it is determined that a cycle ramp has occurred during the cycling process, and the maximum capacity retention rate during the cycling process when the capacity retention rate is above 100% is recorded. The cycle ramp is the maximum capacity retention rate minus 100%. If the number of cycles continuously above 100% is <10, it is determined that no cycle ramp has occurred during the cycling process and is recorded as "-".

[0237] 10) Pole flexibility

[0238] At room temperature and with humidity below 10%, cut the electrode into a shape 5-10cm long and 2-4cm wide. Fold it in half, roll it, and then use a 2kg roller to roll it at a constant speed of 2-3m / min. Observe whether the fold of the electrode is light-transmissive and qualitatively judge the electrode's flexibility. The n-fold light transmittance test indicates that in the test of electrode pieces produced from the same batch, the electrode piece must be folded back and forth and rolled at the same fold n times before light is transmissive. The larger the n value, the better the electrode's flexibility.

[0239] 3. Analysis of test results of various embodiments and comparative examples

[0240] Batteries of various embodiments and comparative examples were prepared according to the above methods, and various performance parameters were measured. The positive electrode film parameters are shown in Table 1, and the performance test results are shown in Table 2.

[0241] Table 2: Performance test results

[0242] According to the above results, the positive electrode sheets in Examples 1-19 all include a positive electrode current collector and a positive electrode film layer, the positive electrode film layer is arranged on at least one side of the positive electrode current collector, the positive electrode film layer includes a first positive electrode film layer and a second positive electrode film layer, the first positive electrode film layer is located between the positive electrode current collector and the second positive electrode film layer, wherein the first positive electrode film layer contains a first positive electrode active material, and the specific surface area of ​​the first positive electrode active material is 12m 2 / g-16m 2 / g, based on the total weight of the first positive electrode active material, the carbon content of the first positive electrode active material is 1.0 wt%-1.5 wt%, the second positive electrode film layer comprises a second positive electrode active material, and the specific surface area of ​​the second positive electrode active material is 6 m 2 / g-19m 2 / g, based on the total weight of the second positive electrode active material, the carbon content of the second positive electrode active material is 1.1 wt%-2.1 wt%. From the comparison between Examples 1-19 and Comparative Examples 1-3, it can be seen that the positive electrode sheets of the embodiments of the present application have good sheet flexibility.

[0243] Figure 1 illustrates the microscopic morphology of the first positive electrode active material in Example 1 of the present application. As can be seen from the figure, the first positive electrode active material in Example 1 of the present application comprises a uniform mixture of large and small particles, with the small particles densely filling the pores of the large particles, resulting in a high compaction density of the first positive electrode film layer.

[0244] Figure 2 illustrates the microscopic morphology of the second cathode active material in Example 1 of the present application. As can be seen from the figure, the second cathode active material in Example 1 of the present application has a relatively uniform particle distribution, consisting primarily of medium-sized particles with no large or small particles. The absence of large particles allows for balanced dynamics, while the absence of small particles reduces side reactions, facilitating a longer cycle life for the battery cell.

[0245] From the comparison between Examples 1, 4-6 and Example 17, it can be seen that when the second positive electrode active material contains third lithium iron phosphate salt particles, and the ratio of the withheld capacity C2 of the third lithium iron phosphate salt particles at 60°C to the withheld capacity C1 of the third lithium iron phosphate salt particles at 25°C is C2 / C1≥1.02, the positive electrode plate of the embodiment of the present application has a higher compaction density, and the battery containing the positive electrode plate of the embodiment of the present application has a longer cycle life, a lower degree of capacity attenuation and a higher degree of cycle climbing.

[0246] From the comparison of Examples 1, 7-8 and Examples 9-10, it can be seen that, based on the total weight of the second positive electrode film layer, when the carbon content of the second positive electrode active material is 1.2 wt%-2 wt%, the battery comprising the positive electrode plate of the embodiment of the present application has a good balance between cycle life and gram capacity. When the carbon content of the second positive electrode active material is less than 1.2% (Example 9), the battery comprising the positive electrode plate of the embodiment of the present application has a shorter cycle life. When the carbon content of the second positive electrode active material is greater than 2% (Example 10), the battery comprising the positive electrode plate of the embodiment of the present application has a lower gram capacity.

[0247] From the comparison of Examples 1, 13-14 and 15-16, it can be seen that when the ratio W1 / W2 of the weight of the first positive electrode active material W1 to the weight of the second positive electrode active material W2 is 3:7 to 7:3, the positive electrode sheet of the embodiment of the present application has a higher compaction density, and the battery containing the positive electrode sheet of the embodiment of the present application has a higher cycle climbing degree. When W1 / W2 is less than 3:7 (Example 15), the positive electrode sheet of the embodiment of the present application has a lower compaction density. When W1 / W2 is greater than 7:3 (Example 16), the battery containing the positive electrode sheet of the embodiment of the present application has a lower cycle climbing degree.

[0248] 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 positive electrode sheet comprising a positive electrode current collector and a positive electrode film layer, wherein the positive electrode film layer is disposed on at least one side of the positive electrode current collector, the positive electrode film layer comprising a first positive electrode film layer and a second positive electrode film layer, wherein the first positive electrode film layer is located between the positive electrode current collector and the second positive electrode film layer, The first positive electrode film layer comprises a first positive electrode active material, and the specific surface area of the first positive electrode active material is 12m 2 / g-16m 2 / g, based on the total weight of the first positive electrode active material, the carbon content of the first positive electrode active material is 1.0 wt %-1.5 wt %, The second positive electrode film layer comprises a second positive electrode active material, and the specific surface area of the second positive electrode active material is 6m 2 / g-19m 2 / g, based on the total weight of the second positive electrode active material, the carbon content of the second positive electrode active material is 1.1 wt%-2.1 wt%.

2. The positive electrode sheet according to claim 1, wherein the compaction density of the first positive electrode active material is 2.4-2.7 g / cm when measured under a pressure of 3 T. 3 .

3. The positive electrode plate according to claim 1 or 2, wherein the first positive electrode active material comprises first lithium iron phosphate salt particles and second lithium iron phosphate salt particles, the first lithium iron phosphate salt particles have an average primary particle size of 50-300 nm, and the second lithium iron phosphate salt particles have an average primary particle size of 400-500 nm.

4. The positive electrode plate according to any one of claims 1 to 3, wherein based on the total number of the first lithium iron phosphate salt particles and the second lithium iron phosphate salt particles, the number of the first lithium iron phosphate salt particles accounts for 60%-80%, and the number of the second lithium iron phosphate salt particles accounts for 20%-40%. 5 . The positive electrode sheet according to claim 1 , wherein the carbon content of the second positive electrode active material is 1.2 wt % to 2 wt % based on the total weight of the second positive electrode active material. 6 . The positive electrode sheet according to claim 1 , wherein a ratio W1 / W2 of a weight W1 of the first positive electrode active material to a weight W2 of the second positive electrode active material is 1:9 to 9:

1. 7 . The positive electrode sheet according to claim 1 , wherein a ratio W1 / W2 of a weight W1 of the first positive electrode active material to a weight W2 of the second positive electrode active material is 3:7 to 7:

3. 8 . The positive electrode sheet according to claim 1 , wherein the thickness of the first positive electrode film layer and the thickness of the second positive electrode film layer are independently 5-80 μm. 9 . A secondary battery comprising the positive electrode sheet according to claim 1 . 10 . An electric device comprising the secondary battery according to claim 9 .

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