Positive electrode sheet, secondary battery, and electric device

Abstract

The application provides a positive electrode tab, a secondary battery, and an electric device. In particular, the disclosure provides a positive electrode tab, comprising a positive electrode current collector and a positive electrode film layer, the positive electrode film layer being arranged 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, the first positive electrode film layer being located between the positive electrode current collector and the second positive electrode film layer, wherein 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 m 2 / g-16 m 2 / g, the carbon content of the first positive electrode active material is 1.0 wt%-1.5 wt% based on the total weight of the first positive electrode active material, 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 m 2 / g-19 m 2 / g, and the carbon content of the second positive electrode active material is 1.1 wt%-2.1 wt% based on the total weight of the second positive electrode active material.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of secondary batteries, in particular to a positive electrode sheet, a secondary battery and an electric device. BACKGROUND

[0002] Lithium ion batteries have the advantages of high energy density, high working voltage, long cycle life, light weight, no environmental pollution, etc., and thus become the ideal power source for the miniaturization and light weight of electronic devices such as video cameras, mobile phones, notebook computers and digital products, and are also the preferred power source for future high-energy power batteries for automobiles.

[0003] A lithium ion battery mainly consists of a positive electrode, a negative electrode, a separator and an electrolyte. In the production of a lithium ion battery, the positive and negative electrode slurries are first coated on the corresponding current collectors, dried, cold-pressed, and then subjected to subsequent processes. Due to the characteristics of the positive electrode active material (for example, lithium iron phosphate salt, lithium manganese iron phosphate salt, etc.), the positive electrode is prone to brittle sheet after cold pressing, and active material cracking and falling off may occur during the subsequent slitting and cutting process, and the positive electrode sheet may also be broken during winding, which seriously affects the processing performance, electrochemical performance and safety performance of the battery.

[0004] Therefore, it is necessary to provide a positive electrode sheet with good sheet flexibility. SUMMARY

[0005] The present application is made in view of the above-mentioned problems, and aims to provide a positive electrode sheet, a secondary battery and an electric device, which has good sheet flexibility.

[0006] The present application has found that the above-mentioned problems can be solved by using the technical solutions of the present application.

[0007] The first aspect of the present application provides a positive electrode sheet, comprising a positive electrode current collector and a positive electrode film layer, the positive electrode film layer being arranged 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, the first positive electrode film layer being located between the positive electrode current collector and the second positive electrode film layer, wherein

[0008] The first positive electrode film layer comprises a first positive electrode active material, the specific surface area of the first positive electrode active material being 12 m 2 / g-16 m 2 / g, the carbon content of the first positive electrode active material being 1.0 wt% - 1.5 wt% based on the total weight of the first positive electrode active material,

[0009] The second positive electrode film layer comprises a second positive electrode active material, the specific surface area of the second positive electrode active material being 6 m 2 / g-19 m 2The carbon content of the second positive electrode active material is 1.1-2.1 wt% based on the total weight of the second positive electrode active material.

[0010] The positive electrode tab of the present application has good tab flexibility.

[0011] In any embodiment, the compaction density of the first positive electrode active material is 2.4-2.7 g / cm 3 .

[0012] When the compaction density of the first positive electrode active material is 2.4-2.7 g / cm 3 , the battery comprising the positive electrode tab of the present application has high energy density.

[0013] In any embodiment, the first positive electrode active material comprises first lithium iron phosphate salt particles and second lithium iron phosphate salt particles, the primary average particle size of the first lithium iron phosphate salt particles is 50-300 nm, and the primary average particle size of the second lithium iron phosphate salt particles is 400-500 nm.

[0014] In any embodiment, the number ratio of the first lithium iron phosphate salt particles to the total number of the first lithium iron phosphate salt particles and the second lithium iron phosphate salt particles is 60%-80%, and the number ratio of the second lithium iron phosphate salt particles is 20%-40%.

[0015] In any embodiment, the second positive electrode active material comprises third lithium iron phosphate salt particles, and the ratio C2 / C1 of the discharge capacity C2 of the third lithium iron phosphate salt particles at 60°C to the discharge capacity C1 of the third lithium iron phosphate salt particles at 25°C is ≥1.02.

[0016] When C2 / C1≥1.02, the positive electrode tab of the present application has high compaction density, and the battery comprising the positive electrode tab of the present application has long cycle life, low capacity decay degree, and high cycle climbing degree.

[0017] In any embodiment, the carbon content of the second positive electrode active material is 1.2-2 wt% based on the total weight of the second positive electrode active material.

[0018] When the carbon content of the second positive electrode active material is 1.2-2 wt%, the battery comprising the positive electrode tab of the present application has a good balance of cycle life and specific capacity.

[0019] In any embodiment, 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 to 9:1.

[0020] In any embodiment, 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 3:7 to 7:3.

[0021] When W1 / W2 is 3:7 to 7:3, the positive electrode tab of the present application has a higher compaction density, and the battery comprising the positive electrode tab of the present application has a higher cycle climbing degree.

[0022] In any embodiment, the thickness of the first positive electrode film layer and the thickness of the second positive electrode film layer are independently from each other 5-80 μm.

[0023] The second aspect of the present application provides a secondary battery comprising the positive electrode tab of the first aspect of the present application.

[0024] The third aspect of the present application provides an electric device comprising the secondary battery of the second aspect of the present application. BRIEF DESCRIPTION OF DRAWINGS

[0025] Figure 1 is a scanning electron microscope image of the first positive electrode active material in Example 1 of the present application at a magnification of 10k.

[0026] Figure 2 is a scanning electron microscope image of the second positive electrode active material in Example 1 of the present application at a magnification of 10k.

[0027] Figure 3 is a schematic diagram of a secondary battery of an embodiment of the present application.

[0028] Figure 4 is Figure 3 is an exploded view of the secondary battery of an embodiment of the present application.

[0029] Figure 5 is a schematic diagram of a battery module of an embodiment of the present application.

[0030] Figure 6 is a schematic diagram of a battery pack of an embodiment of the present application.

[0031] Figure 7 is Figure 6 is an exploded view of the battery pack of an embodiment of the present application.

[0032] Figure 8 is a schematic diagram of an electric device using the secondary battery of an embodiment of the present application as a power source.

[0033] REFERENCE SIGNS:

[0034] 1 battery pack; 2 upper case; 3 lower case; 4 battery module; 5 secondary battery; 51 housing; 52 electrode assembly; 53 top cap assembly DETAILED DESCRIPTION

[0035] Hereinafter, specific embodiments of the positive electrode sheet, secondary battery, and electrical device of the present application are concretely explained with appropriate reference to the accompanying drawings. However, there are cases where unnecessary detailed explanations are omitted. For example, there are cases where detailed explanations of matters known to those skilled in the art, repeated explanations of substantially identical structures are omitted. This is to avoid the following explanation from becoming unnecessarily lengthy and to facilitate understanding by those skilled in the art. Furthermore, the accompanying drawings and the following explanation are provided so that those skilled in the art can fully understand the present application, and are not intended to limit the subject matter recited in the claims.

[0036] The "ranges" disclosed in the present application are defined in the form of lower and upper limits, and a given range is defined by selecting a lower limit and an upper limit, and the selected lower limit and upper limit define the boundaries of the particular range. The ranges defined in this way can be inclusive or exclusive of the end values, and can be arbitrarily combined, i.e., any lower limit can be combined with any upper limit to form a range. For example, if the ranges 60-120 and 80-110 are listed for a particular parameter, it is understood that the ranges 60-110 and 80-120 are also contemplated. Furthermore, if the minimum range values 1 and 2 are listed, and if the maximum range values 3, 4, and 5 are listed, then the following ranges are all contemplated: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In the present application, unless otherwise specified, a numerical range "a-b" represents a shorthand manner of describing all of the real combinations of a to b, where a and b are both real numbers. For example, the numerical range "0-5" indicates that all of the real numbers between "0-5" have been listed herein, and "0-5" is merely a shorthand manner of describing these numerical combinations. In addition, when it is stated that a parameter is an integer ≥ 2, it is equivalent to disclose that the parameter is, for example, the integer 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0037] If not specifically stated, all embodiments and optional embodiments of the present application can be combined with each other to form new technical solutions.

[0038] If not specifically stated, all technical features and optional technical features of the present application can be combined with each other to form new technical solutions.

[0039] If not specifically stated, all steps of the present application can be carried out in sequence or randomly, preferably in sequence. For example, the method comprises steps (a) and (b) means that the method can comprise steps (a) and (b) in sequence, or steps (b) and (a) in sequence. For example, the method can further comprise step (c) means that step (c) can be added to the method in any order, for example, the method can comprise steps (a), (b) and (c), or steps (a), (c) and (b), or steps (c), (a) and (b), etc.

[0040] If not specifically stated, the present application refers to "including" and "comprising" as open-ended, and can also be closed. For example, "including" and "comprising" can mean that other components not listed can also be included or contained, or only the listed components can be included or contained.

[0041] If not specifically stated, in the present application, the term "or" is inclusive. For example, the phrase "A or B" means "A, B, or both A and B". More specifically, any one of the following conditions satisfies the condition "A or B": A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or A and B are both true (or exist).

[0042] A lithium ion battery mainly consists of a positive electrode, a negative electrode, a separator and an electrolyte. In the manufacture of a lithium ion battery, the positive and negative electrode slurries are coated on the corresponding current collectors, dried, cold-pressed, and then subjected to subsequent processes. Due to the characteristics of the positive active material (e.g., lithium iron phosphate salt, lithium manganese iron phosphate salt, etc.), the positive electrode is prone to brittle problems after cold-pressing, and active material cracking and falling off may occur during subsequent slitting and cutting processes. In the winding process, the positive electrode may also be broken, which seriously affects the processing performance, electrochemical performance and safety performance of the battery. Therefore, it is necessary to provide a positive electrode sheet with good flexibility.

[0043] Based on this, the present application provides a technical solution to solve the above technical problems.

[0044] The first aspect of the present application provides a positive electrode sheet, comprising a positive electrode current collector and a positive electrode film layer, the positive electrode film layer being arranged 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, the first positive electrode film layer being located between the positive electrode current collector and the second positive electrode film layer, wherein

[0045] 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, and a carbon content of the first positive electrode active material is 1.0 wt% - 1.5 wt% based on a total weight of the first positive electrode active material,

[0046] The second positive electrode film layer comprises a second positive electrode active material, and a specific surface area of the second positive electrode active material is 6 m 2 / g - 19 m 2 / g, and a carbon content of the second positive electrode active material is 1.1 wt% - 2.1 wt% based on a total weight of the second positive electrode active material.

[0047] When the above conditions are met, the second positive electrode active material surface in the second positive electrode film layer has a high carbon coating amount, which can effectively reduce the sliding resistance between particles, and the particles between the positive electrode materials are more easily to slide, thereby under the same positive plate compaction condition, the particles are more easily to slide and are not prone to breakage, which improves the flexibility of the positive plate, and the positive plate of the present application has good flexibility.

[0048] In some embodiments, the carbon content of the first positive electrode active material is Cx1 wt% based on a total weight of the first positive electrode active material, and a ratio z1 of the specific surface area of the first positive electrode active material to the Cx1 satisfies 10 ≤ z1 ≤ 13.

[0049] In some embodiments, the carbon content of the second positive electrode active material is Cx2 wt% based on a total weight of the second positive electrode active material, and a ratio z2 of the specific surface area of the second positive electrode active material to the Cx2 satisfies 6 ≤ z2 ≤ 11.

[0050] When the above conditions are met, the graphitization degree of the surface carbon-coated graphite is high, and the interlayer sliding force is small, and under the same positive plate compaction condition, the particles are more easily to slide and are not prone to breakage, which makes the positive plate of the present application have good flexibility.

[0051] In some embodiments, z1 can be 10, 10.3, 11, 11.5, 12, 12.3, 12.5, 13, or a range or a value in the range composed of any two of the above z1 values.

[0052] 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 or a value in the range composed of any two of the above z2 values.

[0053] In the present application, the term "specific surface area" or "BET" refers to the total external surface area per unit weight of the positive electrode active material.

[0054] The specific surface area BET can be measured by methods and equipment known in the art. For example, it can be measured using Tristar 3020 according to GB / T 19587-2004.

[0055] In some embodiments, the specific surface area of the first positive electrode active material is 12.3 m 2 / g - 15.6 m 2 / g.

[0056] In some embodiments, the specific surface area BET of the first positive electrode active material can be 12 m 2 / g, 12.3 m 2 / g, 13 m 2 / g, 13.2 m 2 / g, 13.5 m 2 / g, 14 m 2 / g, 14.4 m 2 / g, 14.5 m 2 / g, 15 m 2 / g, 15.5 m 2 / g, 15.6 m 2 / g, 16 m 2 / g, or a range or a value in the range consisting of the specific surface area BET of any two of the first positive electrode active materials described above.

[0057] A BET of the first positive electrode active material that is too high will increase the water absorption, affect the processing performance of the slurry and increase the side reactions with the electrolyte. However, a BET of the first positive electrode active material that is too low will decrease the gravimetric capacity. Controlling the BET of the first positive electrode active material within 12 m 2 / g - 16 m 2 / g will help to obtain a better processing performance of the slurry and gravimetric capacity, thus improving the processing problems of the battery cell and controlling the side reactions with the electrolyte within an acceptable range, thereby increasing the volumetric energy density of the battery and the life of the battery cell.

[0058] In some embodiments, the specific surface area of the second positive electrode active material is 6.6 m 2 / g - 19 m 2 / g.

[0059] In some embodiments, the specific surface area BET of the second positive electrode active material can be 6 m 2 / g, 6.5 m 2 / g, 6.6 m 2 / g, 7 m 2 / g, 7.2 m 2 / g, 7.5 m 2 / g, 8 m 2 / g, 8.5 m2 / g, 9 m 2 / g, 9.5 m 2 / g, 9.9 m 2 / g, 10 m 2 / g, 10.5 m 2 / g, 10.8 m 2 / g, 11 m 2 / g, 11.5 m 2 / g, 12 m 2 / g, 12.5 m 2 / g, 12.6 m 2 / g, 13 m 2 / g, 13.2 m 2 / g, 13.5 m 2 / g, 14 m 2 / g, 14.4 m 2 / g, 14.5 m 2 / g, 15 m 2 / g, 15.5 m 2 / g, 16 m 2 / g, 16.5 m 2 / g, 17 m 2 / g, 17.5 m 2 / g, 18 m 2 / g, 18.5 m 2 / g, 19 m 2 / g, or a range or a value in the range of specific surface area BET of any two of the above second positive electrode active materials.

[0060] A BET of the second positive electrode active material that is too high will increase the water absorption, affect the processing performance of the slurry and increase the side reaction with the electrolyte. However, a BET of the second positive electrode active material that is too low will reduce the gram capacity. Controlling the BET of the second positive electrode active material in the range of 6 m 2 / g-19 m 2 / g will help to obtain better processing performance of the slurry and gram capacity, thereby improving the processing problems of the battery cell, and also can regulate the initial capacity of the second active material to play high and low, and further regulate the degree of slow attenuation.

[0061] The carbon content of the first positive electrode active material / second positive electrode active material can be measured by methods and devices known in the art. For example, the test can be performed by using the high-frequency induction furnace combustion followed by infrared absorption method according to GB / T 20123-2006 / ISO.

[0062] In some embodiments, the carbon content of the first positive electrode active material is Cx1wt%, based on the total weight of the first positive electrode active material, wherein 1.0≤Cx1≤1.5.

[0063] The carbon content of the first positive electrode active material that is too high will affect the intercalation and deintercalation of lithium ions, to some extent, affecting the gram capacity of the battery. However, the carbon content of the first positive electrode active material that is too low will affect the conductivity of the battery, thereby being not conducive to the kinetic performance thereof. Controlling the carbon content of the first positive electrode active material to be between 1.0 wt% and 1.5 wt% helps to obtain better kinetic performance and gram capacity. The carbon content in this weight percentage range can take into account the conductive network between active material particles while not affecting the capacity performance, and can play a lubricating role between particles, thereby realizing high kinetics and high energy density.

[0064] In some embodiments, Cx1 is 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, or a range or a value in the range composed of the Cx1 of any two of the above first positive electrode active materials.

[0065] In some embodiments, the carbon contained in the first positive electrode active material is coated on the surface of the particles thereof. In some embodiments, the carbon contained in the first positive electrode active material is embedded in the particles thereof. In some embodiments, the carbon contained in the first positive electrode active material is partially coated on the surface of the particles thereof and partially embedded in the particles thereof.

[0066] In some embodiments, the carbon content of the second positive electrode active material is 1.1 wt% to 2.1 wt% based on the total weight of the second positive electrode active material. In some embodiments, 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.

[0067] The carbon content of the second positive electrode active material that is too high will affect the intercalation and deintercalation of lithium ions, to some extent, affecting the gram capacity of the battery. However, the carbon content of the second positive electrode active material that is too low will affect the cycle life of the battery. Controlling the carbon content of the second positive electrode active material to be between 1.2 wt% and 2 wt% can ensure that the material has a slow decay effect, while the reduction in initial capacity performance can be compensated for by the use of the first active material, thereby realizing slow decay while taking into account the 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.

[0068] 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 or a value in the range composed of the Cx2 of any two of the above second positive electrode active materials.

[0069] In some embodiments, the carbon contained in the second positive electrode active material is coated on the surface of the particles thereof. In some embodiments, the carbon contained in the second positive electrode active material is embedded in the particles thereof. In some embodiments, the carbon contained in the second positive electrode active material is partially coated on the surface of the particles thereof and partially embedded in the particles thereof.

[0070] In some embodiments, the first positive electrode active material has a compaction density of 2.4-2.7 g / cm 3 In some embodiments, the first positive electrode active material has a compaction density of 2.43-2.63 g / cm 3 .

[0071] When the first positive electrode active material has a compaction density of 2.4-2.7 g / cm 3 , a higher compaction density can place more active material in a unit volume under the condition of a fixed volume, thereby improving the energy density, so that the battery containing the positive electrode sheet of the present application has a higher energy density.

[0072] In some embodiments, the first positive electrode active material contains 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.

[0073] The above particle size combination has fewer pores in the electrode sheet under the same pressure condition, thereby achieving high compaction density and improving energy density.

[0074] In some embodiments, the first lithium iron phosphate salt particles account for 60%-80% of the total number of the first lithium iron phosphate salt particles and the second lithium iron phosphate salt particles, and the second lithium iron phosphate salt particles account for 20%-40% of the total number of the first lithium iron phosphate salt particles and the second lithium iron phosphate salt particles.

[0075] The above particle size distribution of the particles can also reduce the pores in the electrode sheet under the same pressure condition, thereby improving the compaction density and the energy density.

[0076] In the present application, the term "primary average particle size" refers to the average value of the primary particle sizes of all particles, wherein the primary particle size refers to the longest distance between two points connected by an edge in a cross-sectional view.

[0077] The primary average particle size of the first lithium iron phosphate salt particles / second lithium iron phosphate salt particles can be measured by methods and devices known in the art. For example, the test can be performed by scanning electron microscopy and long diameter statistics. As an example, the sample is tested by a ZEISS sigma 300 scanning electron microscope, and then tested in accordance with the standard JY / T010-1996, and the sample morphology is observed. The morphology of the primary particles is observed, and the particles in any 10 regions of the same scanning electron microscope (SEM) image under 10k magnification are observed. Then the 10 regions are each subdivided into four corners and a center, and the feret particle diameter of any one primary particle at each position under the magnification is selected, and the feret particle diameter results at the four corners and the center are averaged to obtain the particle diameter of the primary particles in the region. Then the particle diameters of the primary particles obtained from the 10 regions are compared and counted to obtain the distribution ratio (number ratio) of the particle diameters of the primary particles. The particle diameters of the primary particles obtained from the 10 regions are averaged to obtain the average particle diameter of the primary particles (i.e., the primary average particle size). Specifically, the average of the four dimensions of the particle, i.e., the size of the particle in the direction of the adjacent two sides of the rectangle circumscribed by the image, and the size in the direction of the 45-degree inclination of the adjacent two sides of the rectangle circumscribed by the image, is set as the feret particle diameter of the particle.

[0078] In some embodiments, the primary average particle size of the first lithium iron phosphate salt particles can be 50 nm, 100 nm, 150 nm, 200 nm, 250 nm, 300 nm, or a range or a value in the range composed of any two of the above primary average particle sizes.

[0079] 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 or a value in the range composed of any two of the above primary average particle sizes.

[0080] In some embodiments, the second positive electrode active material comprises third lithium iron phosphate salt particles, and the ratio C2 / C1 of the discharge capacity C2 of the third lithium iron phosphate salt particles at 60°C to the discharge capacity C1 of the third lithium iron phosphate salt particles at 25°C is ≥1.02.

[0081] When C2 / C1≥1.02, the capacity of the active material cannot be fully exerted at the initial stage of battery cycling, and the capacity gradually exerts with the activation of the active material during the continuous cycling of the battery, thereby prolonging the capacity retention rate at the initial stage of battery cycling. Therefore, when C2 / C1≥1.02, the positive electrode sheet of the application has a higher compaction density, and the battery comprising the positive electrode sheet of the application has a longer cycle life, a lower capacity decay degree, and a higher cycle climbing degree.

[0082] The measurement method of the discharge capacity of the third lithium iron phosphate salt particles is as follows:

[0083] When measuring the discharge capacity of the third lithium iron phosphate salt particles, first, a discharge battery is prepared: the third lithium iron phosphate salt particles, the binder polyvinylidene fluoride (PVDF), and the conductive agent acetylene black are mixed in a weight ratio of 95:5:5, an appropriate amount of N-methyl pyrrolidone (NMP) solvent is added, and the mixture is fully stirred and mixed to form a uniform positive electrode slurry; the slurry is coated on an aluminum foil with a positive electrode current collector thickness of 1 μm, followed by drying and cold pressing, and then punched into small round pieces with a diameter of 14 mm as the positive electrode. Lithium sheet is used as the negative electrode, 12 μm thick polypropylene separator film and lithium iron phosphate electrolyte are used, and the discharge battery is assembled. Then,

[0084] For the discharge capacity C2 of the third lithium iron phosphate salt particles at 60°C: under the condition of 60°C, charge and discharge at a rate of 0.1C and record the corresponding gram capacity. The test conditions are as follows: under the condition of 60°C, the discharge battery is placed for 3 hours, charged to the upper limit cutoff voltage (3.75V) at a constant current of 0.1C, then charged to 50μA at a constant voltage, and placed for 5 minutes, and the charge capacity of the discharge battery is recorded. The charge gram capacity C2 of the third lithium iron phosphate salt particles at 60°C = the charge capacity of the discharge battery at 60°C / the weight of the third lithium iron phosphate salt particles in the discharge battery.

[0085] For the discharge capacity C2 of the third lithium iron phosphate salt particles at 25°C (i.e., gram capacity): under the condition of 25°C, charge and discharge at a rate of 0.1C and record the corresponding gram capacity. The test conditions are as follows: under the condition of 25°C, the discharge battery is placed for 3 hours, charged to the upper limit cutoff voltage (3.75V) at a constant current of 0.1C, then charged to 50μA at a constant voltage, and placed for 5 minutes, and the charge capacity of the discharge battery is recorded. The charge gram capacity C1 of the third lithium iron phosphate salt particles at 25°C = the charge capacity of the discharge battery at 25°C / the weight of the third lithium iron phosphate salt particles in the discharge battery.

[0086] 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 or a value in the range between any two of the aforementioned C2 / C1.

[0087] In some embodiments, the primary average particle size of the third lithium iron phosphate salt particles can be 200 nm, 250 nm, 300 nm, 350 nm, 400 nm, 450 nm, 500 nm, 550 nm, 600 nm, or a range or a value in the range between any two of the aforementioned primary average particle sizes.

[0088] The method for measuring the primary average particle size of the third lithium iron phosphate salt particles can be the same as the method for measuring the primary average particle size of the first lithium iron phosphate salt particles.

[0089] In some embodiments, the Dv50 of the third lithium iron phosphate salt particles can be 0.1 pm, 0.2 pm, 0.3 pm, 0.4 pm, 0.5 pm, 0.6 pm, 0.7 pm, 0.8 pm, 0.9 pm, 1 pm, 1.1 pm, 1.2 pm, 1.3 pm, 1.4 pm, 1.5 pm, or a range or a value in the range between any two of the aforementioned Dv50.

[0090] The Dv50 can be measured by methods and devices known in the art. For example, GB / T 19077-2016 / ISO 13320:2009 can be referred to, and a laser particle size analyzer (Malvern Master Size 3000) can be used for the determination. The specific test procedure is as follows: an appropriate amount of the sample to be measured is taken, and the sample concentration is ensured to be 8% to 12% obscuration, 20 mL of deionized water is added, and the sample is ultrasonically dispersed for 5 min at an ultrasonic frequency of 53 KHz and an ultrasonic power of 120 W to ensure that the sample is completely dispersed, and then the sample is measured according to the GB / T 19077-2016 / ISO 13320:2009 standard.

[0091] In some embodiments, 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 to 9:1.

[0092] In some embodiments, 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 3:7 to 7:3.

[0093] When W1 / W2 is 3:7 to 7:3, the first active material layer guarantees the energy density of the battery, and the second active material layer optimizes the flexibility of the battery pole piece while providing a slow attenuation effect. Therefore, when W1 / W2 is 3:7 to 7:3, the positive pole piece of the application has a higher compaction density, and the battery containing the positive pole piece of the application has a higher cycle climbing degree.

[0094] 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 5 μm, 8 μm, 10 μm, 15 μm, 16 μm, 20 μm, 24 μm, 25 μm, 30 μm, 32 μm, 35 μm, 40 μm, 45 μm, 48 μm, 50 μm, 55 μm, 56 μm, 60 μm, 64 μm, 65 μm, 70 μm, 72 μm, 75 μm, 80 μm, or a range or a value in the range consisting of any two of the above thicknesses.

[0095] The second aspect of the application provides a secondary battery comprising the positive pole piece of the first aspect of the application.

[0096] The third aspect of the application provides an electric device comprising the secondary battery of the second aspect of the application.

[0097] In addition, the secondary battery and the electric device of the application are described below with appropriate reference to the accompanying drawings.

[0098] In one embodiment of the application, a secondary battery is provided.

[0099] Generally, a secondary battery comprises a positive pole piece, a negative pole piece, an electrolyte, and a separator. During the charging and discharging process of the battery, active ions are embedded and extracted between the positive pole piece and the negative pole piece. The electrolyte plays a role in conducting ions between the positive pole piece and the negative pole piece. The separator is arranged between the positive pole piece and the negative pole piece, mainly to prevent the short circuit of the positive and negative poles, while allowing ions to pass through.

[0100] [Positive pole piece]

[0101] The positive pole piece comprises a positive current collector and a positive film layer arranged on at least one surface of the positive current collector, wherein the positive film layer comprises a positive active material.

[0102] As an example, the positive electrode current collector has two surfaces opposite in the thickness direction thereof, and the positive electrode film layer is provided on either one or both of the two surfaces of the positive electrode current collector.

[0103] In some embodiments, the positive electrode current collector can employ a metal foil or a composite current collector. For example, as the metal foil, an aluminum foil can be employed. The composite current collector can include a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer. The composite current collector can be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver, silver alloy, etc.) on a polymer material base material (e.g., a base material of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0104] In some embodiments, the positive electrode film layer can further optionally include a binder. As an example, the binder can include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), a vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, a vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, a tetrafluoroethylene-hexafluoropropylene copolymer, and a fluorine-containing acrylic ester resin.

[0105] In some embodiments, the positive electrode film layer can further optionally include a conductive agent. As an example, the conductive agent can include at least one of super conductive carbon, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0106] In some embodiments, the positive electrode tab can be prepared by dispersing the above-described components for preparing the positive electrode tab, such as the positive electrode active material, the conductive agent, the binder, and any other components, in a solvent (e.g., N-methylpyrrolidone) to form a positive electrode slurry, coating the positive electrode slurry on the positive electrode current collector, and subjecting the same to a drying, cold-pressing, or the like process to obtain the positive electrode tab.

[0107] [Negative electrode tab]

[0108] The negative electrode tab includes a negative electrode current collector and a negative electrode film layer provided on at least one surface of the negative electrode current collector, the negative electrode film layer including a negative electrode active material.

[0109] As an example, the negative electrode current collector has two surfaces opposite in the thickness direction thereof, and the negative electrode film layer is provided on either one or both of the two surfaces of the negative electrode current collector.

[0110] In some embodiments, the negative current collector can employ a metal foil or a composite current collector. For example, as a metal foil, a copper foil can be employed. The composite current collector can include a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer. The composite current collector can be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, silver alloy, etc.) on a polymer material base layer (e.g., a base layer of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0111] In some embodiments, the negative active material can employ a negative active material for a battery known in the art. As an example, the negative active material can include at least one of artificial graphite, natural graphite, soft carbon, hard carbon, a silicon-based material, a tin-based material, and lithium titanate, etc. The silicon-based material can be selected from at least one of elemental silicon, a silicon oxide compound, a silicon-carbon composite, a silicon-nitrogen composite, and a silicon alloy. The tin-based material can be selected from at least one of elemental tin, a tin oxide compound, and a tin alloy. However, the present application is not limited to these materials, and other conventional materials that can be used as a negative active material for a battery can also be used. These negative active materials can be used alone or in combination of two or more.

[0112] In some embodiments, the negative film layer can further optionally include a binder. The binder can be selected from at least one of styrene butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

[0113] In some embodiments, the negative film layer can further optionally include a conductive agent. The conductive agent can be selected from at least one of super conductive carbon, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0114] In some embodiments, the negative film layer can further optionally include other auxiliary agents, such as a thickening agent (e.g., sodium carboxymethyl cellulose (CMC-Na)) and the like.

[0115] In some embodiments, the negative electrode sheet can be prepared by dispersing the above-described components for preparing the negative electrode sheet, such as the negative active material, the conductive agent, the binder, and any other components, in a solvent (e.g., deionized water) to form a negative electrode slurry; coating the negative electrode slurry on a negative current collector; and drying, cold-pressing, and the like to obtain the negative electrode sheet.

[0116] [Electrolyte]

[0117] The electrolyte functions to conduct ions between the positive electrode sheet and the negative electrode sheet. The type of electrolyte is not particularly limited in the present application and can be selected as desired. For example, the electrolyte can be liquid, gel, or all-solid.

[0118] In some embodiments, the electrolyte employs an electrolyte solution. The electrolyte solution includes an electrolyte salt and a solvent.

[0119] In some embodiments, the electrolyte salt can be selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bisfluorosulfonylimide, lithium bis-trifluoromethanesulfonylimide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluoro oxalate borate, lithium di-oxalate borate, lithium difluoro di-oxalate phosphate, and lithium tetrafluoro oxalate phosphate.

[0120] In some embodiments, the solvent can be selected from at least one of ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butylene carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, cyclobutane sulfone, dimethyl sulfone, methyl ethyl sulfone, and diethyl sulfone.

[0121] In some embodiments, the electrolyte solution can optionally further include an additive. For example, the additive can include a negative electrode film-forming additive, a positive electrode film-forming additive, and can further include an additive capable of improving certain properties of the battery, such as an additive for improving overcharge performance of the battery, an additive for improving high-temperature or low-temperature performance of the battery, etc.

[0122] [Separator]

[0123] In some embodiments, the secondary battery further includes a separator. The type of separator is not particularly limited in the present application and any known porous structure separator having good chemical stability and mechanical stability can be used.

[0124] In some embodiments, the material of the separator can be selected from at least one of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single layer film or a multi-layer composite film, and is not particularly limited. When the separator is a multi-layer composite film, the materials of the layers can be the same or different, and are not particularly limited.

[0125] In some embodiments, the positive electrode sheet, the negative electrode sheet, and the separator can be used to make an electrode assembly through a winding process or a stacking process.

[0126] In some embodiments, the secondary battery can include an outer package. The outer package can be used to package the electrode assembly and the electrolyte described above.

[0127] In some embodiments, the outer package of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, or the like. The outer package of the secondary battery can also be a soft package, such as a pouch soft package. The material of the soft package can be plastic, and as plastic, polypropylene, polybutylene terephthalate, polybutylene succinate, or the like can be listed.

[0128] The shape of the secondary battery is not particularly limited in the present application, and it can be cylindrical, square, or any other shape. For example, Figure 3 is a square structure of a secondary battery 5 as an example.

[0129] In some embodiments, referring to Figure 4 , the outer package can include a shell 51 and a cover plate 53. The shell 51 can include a bottom plate and a side plate connected to the bottom plate, and the bottom plate and the side plate enclose a receiving cavity. The shell 51 has an opening communicating with the receiving cavity, and the cover plate 53 can be provided on the opening to close the receiving cavity. The positive electrode sheet, the negative electrode sheet, and the separator can form an electrode assembly 52 through a winding process or a stacking process. The electrode assembly 52 is packaged in the receiving cavity. The electrolyte is impregnated in the electrode assembly 52. The number of electrode assemblies 52 contained in the secondary battery 5 can be one or more, and the person skilled in the art can select according to the specific actual demand.

[0130] In some embodiments, the secondary battery can be assembled into a battery module, and the number of secondary batteries contained in the battery module can be one or more, and the specific number can be selected by the person skilled in the art according to the application and capacity of the battery module.

[0131] Figure 5 is a battery module 4 as an example. Referring to Figure 5 , in the battery module 4, a plurality of secondary batteries 5 can be arranged in sequence along the length direction of the battery module 4. Of course, it can also be arranged in any other way. Further, the plurality of secondary batteries 5 can be fixed by fasteners.

[0132] Optionally, the battery module 4 can also include a housing having a receiving space, and the plurality of secondary batteries 5 are received in the receiving space.

[0133] In some embodiments, the above-mentioned battery module can also be assembled into a battery pack, and the number of battery modules contained in the battery pack can be one or more, and the specific number can be selected by the person skilled in the art according to the application and capacity of the battery pack.

[0134] Figure 6 and Figure 7 is a battery pack 1 as an example. Referring to Figure 6 and Figure 7In the battery pack 1, a battery case and a plurality of battery modules 4 disposed in the battery case can be included. The battery case includes an upper case 2 and a lower case 3, and the upper case 2 is capable of being disposed on the lower case 3 and forms an enclosed space for accommodating the battery modules 4. The plurality of battery modules 4 can be arranged in the battery case in any manner.

[0135] In addition, the application also provides a power consuming device, which includes at least one of the secondary battery, the battery module, or the battery pack provided by the application. The secondary battery, the battery module, or the battery pack can be used as a power supply of the power consuming device, and can also be used as an energy storage unit of the power consuming device. The power consuming device can include a mobile device (such as a mobile phone, a notebook computer, etc.), an electric vehicle (such as a pure electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, an electric bicycle, an electric scooter, an electric golf cart, an electric truck, etc.), an electric train, a ship and a satellite, an energy storage system, etc., but is not limited thereto.

[0136] As the power consuming device, the secondary battery, the battery module, or the battery pack can be selected according to the use requirement thereof.

[0137] Figure 8 The power consuming device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. In order to meet the requirement of the power consuming device for high power and high energy density of the secondary battery, the battery pack or the battery module can be used.

[0138] The power consuming device as another example can be a mobile phone, a tablet computer, a notebook computer, etc. The device usually requires thinning, and the secondary battery can be used as a power supply.

[0139] Example

[0140] Hereinafter, the embodiments of the application are described. The embodiments described below are exemplary and are only used to explain the application, and cannot be understood as a limitation of the application. If the specific technology or condition is not indicated in the embodiments, the technology or condition described in the literature in the art or according to the product instruction is used. If the reagent or instrument is not indicated by the manufacturer, it is a conventional product that can be obtained by market purchase.

[0141] I. Preparation method

[0142] Example 1

[0143] 1) Preparation of positive active material and positive electrode slurry

[0144] 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), 2000 ppm TiO2 (based on the total weight of the raw materials) is added as an additive, the particle size Dv50 is ground to 0.4 μm by a sand mill, then spray drying is performed, the outlet temperature is controlled at 150°C, and spherical dried material is obtained. The spherical dried material is sintered in a nitrogen atmosphere, the pot loading is controlled at 5 kg, the nitrogen flow is 300 L / min, the heating rate is 3°C / min, and the sintering temperature is 780°C, the constant temperature section is 12 h, and the sintered material is obtained. The sintered material is subjected to air jet crushing using an air jet crusher under the condition that the frequency of the classifier is 50 Hz to obtain the first positive electrode active material. The primary particle size of each particle in the first positive electrode active material is measured. For particles with a primary particle size ≤ 350 nm, counting is performed and the average particle size is calculated to obtain the number and average particle size of the first lithium iron phosphate salt particles. For particles with a primary particle size > 350 nm, counting is performed and the average particle size is 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 is 150 nm, and the number ratio is 70%. The primary average particle size of the second lithium iron phosphate salt particles is 450 nm, and the number ratio is 30%.

[0145] The first positive electrode active material, the adhesive polyvinylidene fluoride (PVDF), and the conductive agent acetylene black are mixed in a weight ratio of 97.3:2:0.7, an appropriate amount of N-methyl pyrrolidone (NMP) solvent is added, and the mixture is stirred thoroughly to form a uniform first positive electrode slurry with a viscosity of 6000 mPa·s.

[0146] The preparation method of the second positive electrode active material is as follows: iron phosphate and lithium carbonate are used as raw materials, and glucose and polyethylene glycol in a weight ratio of 7:3 are added as a carbon source, and the weight of the carbon source accounts for 17% of the total weight of the raw materials. The above iron phosphate: lithium carbonate: carbon source are mixed in a weight ratio of 3.85:1:1, then wet grinding is performed with water as the solvent, the Dv50 of the slurry after grinding is 300 nm, the obtained slurry is subjected to spray drying, and then sintering is performed in a roller hearth furnace, the sintering temperature is 760°C for 24 h, nitrogen is introduced during the sintering process, and the heating rate is 3°C / min. Then the material is discharged after natural cooling to a material temperature < 80°C to obtain the calcined material. The calcined material is subjected to air jet crushing, screening, and magnetic removal, then vacuum packaging is performed, the classifier frequency of the air jet crushing is controlled at 80 Hz, and the frequency of the induced draft fan is controlled at 40 Hz to obtain the second positive electrode active material, wherein the primary average particle size of the third lithium iron phosphate salt particles is 400 nm, and the Dv50 is 0.7 μm.

[0147] The second positive electrode active material, the adhesive polyvinylidene fluoride (PVDF), and the conductive agent acetylene black are mixed in a weight ratio of 97.3:2:0.7, an appropriate amount of N-methyl pyrrolidone (NMP) solvent is added, and the mixture is stirred thoroughly to form a uniform second positive electrode slurry with a viscosity of 6000 mPa·s.

[0148] 2) Preparation of the positive electrode sheet

[0149] The first positive electrode slurry is coated on a 15 μm thick aluminum foil with a coating weight of 160 mg / 1540.25 cm 2 , and then dried to form a first positive electrode film layer. The specific surface area of the first positive electrode active material is 13.2 m 2 / g, the carbon content of the first positive electrode active material is Cx1 wt.%, where Cx1 is 1.2, and the ratio of the specific surface area of the first positive electrode active material to Cx1 is z1 = 12.

[0150] The second positive electrode slurry is coated on the first positive electrode film layer with a coating weight of 160 mg / 1540.25 cm 2 , and then dried to form a second positive electrode film layer. The specific surface area of the second positive electrode active material is 10.8 m 2 / g, the carbon content of the second positive electrode active material is Cx2 wt.%, where Cx2 is 1.8, and the ratio of the specific surface area of the second positive electrode active material to Cx2 is z2 = 6.

[0151] The aluminum foil, the first positive electrode film layer, and the second positive electrode film layer together form the positive electrode sheet.

[0152] 3) Preparation of the negative electrode sheet

[0153] The negative electrode active material graphite, the thickening agent sodium carboxymethyl cellulose, the adhesive styrene-butadiene rubber, and the conductive agent acetylene black are mixed in a weight ratio of 97:1:1:1, and deionized water is added to obtain a negative electrode slurry under the action of a vacuum stirrer. The negative electrode slurry is uniformly coated on a copper foil with a thickness of 8 μm. Then, drying, cold pressing, and slitting are performed to obtain the negative electrode sheet.

[0154] 4) Preparation of the electrolyte

[0155] Vinyl carbonate, methyl ethyl carbonate, and diethyl carbonate are configured into a mixed solution in a volume ratio of 20:20:60, and then a fully dried lithium salt is dissolved in the mixed solution. Then, 10 wt.% of fluoroethylene carbonate additive is added and mixed uniformly to obtain the electrolyte. The concentration of the lithium salt is 1 mol / L. The entire operation process is performed in an argon atmosphere glove box with a water content of <10 ppm.

[0156] 5) Separation film

[0157] A polyethylene film having a thickness of 12 μm is used as the separator.

[0158] 6) Preparation of the battery

[0159] The positive electrode sheet, the separator, and the negative electrode sheet are stacked in order with the separator between the positive and negative electrodes to function as a separator, and are wound to obtain a bare battery cell. The bare battery cell is placed in an outer package, and electrolyte solution is injected and sealed, and the processes of injection, formation, and degassing are performed to obtain a lithium ion battery.

[0160] Example 2

[0161] Example 2 differs from Example 1 in that the specific surface area of the first positive electrode active material = 14.4, and thus the ratio z1 of the specific surface area of the first positive electrode active material to Cx1 = 12.

[0162] Example 3

[0163] Example 3 differs from Example 1 in that the specific surface area of the first positive electrode active material = 15.6, and thus the ratio z1 of the specific surface area of the first positive electrode active material to Cx1 = 13.

[0164] Example 4

[0165] Example 4 differs from Example 1 in that the ratio C2 / C1 of the discharge capacity C2 at 60°C to the discharge capacity C1 at 25°C of the third lithium iron phosphate salt particles = 1.04, the specific surface area of the second positive electrode active material = 14.4, and thus the ratio z2 of the specific surface area of the second positive electrode active material to Cx2 = 8.

[0166] Example 5

[0167] Example 5 differs from Example 1 in that C2 / C1 = 1.025, the specific surface area of the second positive electrode active material = 18 m 2 / g, the specific surface area of the second positive electrode active material = 18, and thus the ratio z2 of the specific surface area of the second positive electrode active material to Cx2 = 10.

[0168] Example 6

[0169] Example 6 differs from Example 1 in that C2 / C1 = 1.02.

[0170] Example 7

[0171] Example 7 differs from Example 1 in that the specific surface area of the second positive electrode active material = 7.2 m 2 / g, and the carbon content = Cx2 wt%, where Cx2 = 1.2.

[0172] Example 8

[0173] Example 8 differs from Example 1 in that the specific surface area of the second positive electrode active material is 12 m2 / g, the carbon content is Cx2% by weight, where Cx2 is 2. 2 / g, the carbon content is Cx2% by weight, where Cx2 is 2.

[0174] Example 9

[0175] Example 9 differs from Example 1 in that the specific surface area of the second positive electrode active material is 6.6 m2 / g, the carbon content is Cx2% by weight, where Cx2 is 1.1. 2 / g, the carbon content is Cx2% by weight, where Cx2 is 2.

[0176] Example 10

[0177] Example 10 differs from Example 1 in that the specific surface area of the second positive electrode active material is 12.6 m2 / g, the carbon content is Cx2% by weight, where Cx2 is 2.1. 2 / g, the carbon content is Cx2% by weight, where Cx2 is 2.

[0178] Example 11

[0179] Example 11 differs from Example 1 in that the primary average particle diameter of the first lithium-iron-phosphate salt particles is 300 nm, the number fraction is 60% (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 diameter of the second lithium-iron-phosphate salt particles is 400 nm, the number fraction is 40% (based on the total number of the first lithium-iron-phosphate salt particles and the second lithium-iron-phosphate salt particles); furthermore, 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).

[0180] Example 12

[0181] Example 12 differs from Example 1 in that the primary average particle diameter of the first lithium-iron-phosphate salt particles is 50 nm, the number fraction is 80% (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 diameter of the second lithium-iron-phosphate salt particles is 500 nm, the number fraction is 20% (based on the total number of the first lithium-iron-phosphate salt particles and the second lithium-iron-phosphate salt particles); furthermore, 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).

[0182] Example 13

[0183] Example 13 differs from Example 1 in 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 in Example 1).

[0184] Example 14

[0185] Example 14 differs from Example 1 in 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 in Example 1).

[0186] Example 15

[0187] Example 15 differs from Example 1 in 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 in Example 1).

[0188] Example 16

[0189] Example 16 differs from Example 1 in 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 in Example 1).

[0190] Example 17

[0191] Example 17 differs from Example 1 in that C2 / C1 = 1.01; the specific surface area of the second positive electrode active material is 19 m2 / g, so that z2 is 10.5. 2

[0192] Example 18

[0193] Example 18 differs from Example 1 in that the specific surface area of the first positive electrode active material is 12.3 m2 / g, the carbon content is Cx1 wt.%, where Cx1 is 1, so that z1 is 12.3. 2

[0194] Example 19

[0195] Example 19 differs from Example 1 in that the specific surface area of the first positive electrode active material is 15.5 m2 / g, the carbon content is Cx1 wt.%, where Cx1 is 1.5, so that z1 is 10.3. 2

[0196] Comparative Example 1

[0197] Comparative Example 1 differs from Example 1 in that the second positive electrode film layer is not included, the aluminum foil and the first positive electrode film layer are combined to be the positive electrode tab.

[0198] Comparative Example 2

[0199] Comparative Example 2 differs from Example 2 in that the second positive electrode film layer is not included, the aluminum foil and the first positive electrode film layer are combined to be the positive electrode tab.

[0200] Comparative Example 3

[0201] Comparative Example 3 differs from Example 3 in that the second positive electrode film layer is not included, the aluminum foil and the first positive electrode film layer are combined to be the positive electrode tab. ​​​

[0202] II. Battery performance test

[0203] 1) Primary average particle size

[0204] The sample was tested by ZEISS sigma 300 scanning electron microscope, and then tested according to the standard JY / T010-1996, and the sample morphology was observed.

[0205] The morphology of the primary particles was observed, and the particles in any 10 regions with the same size and shape under the same scanning electron microscope (SEM) at 10k magnification were observed. Then the 10 regions were subdivided into four corners and centers, a total of 5 positions, and the feret particle size of any one primary particle at each position under the magnification was selected, and the feret particle size results of the four corners and centers were averaged to obtain the particle size of the primary particles in the region. Then the particle sizes of the primary particles obtained from the 10 regions were compared and counted to obtain the distribution ratio (number ratio) of the particle size of the primary particles; the particle sizes of the primary particles obtained from the 10 regions were averaged to obtain the average particle size of the primary particles (i.e., the primary average particle size). Specifically, the average of the four dimensions of the particle, i.e., the size of the particle in the direction of the adjacent two sides of the rectangle circumscribed by the image, and the size in the direction of the 45-degree inclination of the adjacent two sides of the rectangle circumscribed by the image, was set as the feret particle size of the particle.

[0206] 2) Dv50

[0207] A laser particle size analyzer MasterSizer3000 was used to determine the Dv50 according to the standard process: GB / T19077-2016 / ISO13320:2009.

[0208] The specific test process is as follows: take an appropriate amount of sample to be tested, and the sample concentration can be guaranteed to be 8%~12% optical density, add 20mL deionized water, and simultaneously ultrasonic for 5min, ultrasonic frequency 53KHz, ultrasonic power 120W, to ensure that the sample is completely dispersed, and then determine the sample according to the standard GB / T19077-2016 / ISO13320:2009.

[0209] 3) Specific surface area

[0210] The specific surface area was determined according to the standard GB / T 19587-2004, and the test instrument was Tristar3020.

[0211] 4) Carbon content

[0212] The carbon content was determined according to the standard GB / T 20123-2006 / ISO, and the high-frequency induction furnace combustion followed by infrared absorption method was used for testing.

[0213] 5) Compacted density:

[0214] Powder compaction density

[0215] A certain amount of powder is compacted onto a special mold of known diameter. The mold has a metal plate at the top and a metal plate at the bottom, with the powder placed in the middle. Pressure (3T) is applied while simultaneously measuring the corresponding powder thickness to obtain the powder volume. The compaction density ρ is then calculated using the formula ρ = m / v. Specific procedures can be performed according to standard GB / T24533-2009.

[0216] The compaction density of the electrode

[0217] The electrode sheet is cut into 1000mm long films; the positive electrode sheet is rolled under certain pressure. Due to the ductility of aluminum foil, the film length is 1006mm, and it is punched to 1540.25mm. 2 The compaction density can be calculated by measuring the weight and thickness of the small circular disc.

[0218] 6) 60℃ / 25℃ cladding capacity

[0219] Lithium iron phosphate (LiFePO4) salt particles, polyvinylidene fluoride (PVDF) binder, and acetylene black conductive agent were mixed in a weight ratio of 95:5:5. An appropriate amount of N-methylpyrrolidone (NMP) solvent was added, and the mixture was thoroughly stirred to form a uniform positive electrode slurry. This slurry was coated onto an aluminum foil with a positive electrode current collector thickness of 1 μm, then dried, cold-pressed, and finally punched into small circular pieces with a diameter of 14 mm as the positive electrode. Using lithium foil as the negative electrode, a 12 μm thick polypropylene separator, and a commonly used lithium iron phosphate electrolyte, a coin cell was assembled. The cells were charged and discharged at a rate of 0.1C at 60℃ / 25℃, and the corresponding specific capacity was recorded.

[0220] The test conditions are as follows: At 25℃, the coin cell battery was left to stand for 3 hours, then charged at a constant current of 0.1C to the upper limit cutoff voltage (3.75V), and then charged at a constant voltage to 50μA. After standing for 5 minutes, the charging capacity of the coin cell battery was recorded. The specific charging capacity of the lithium iron phosphate (LFP) particles at 25℃, C1, = the charging capacity of the coin cell battery at 25℃ / the weight of the LFP particles in the coin cell battery. At 60℃, the coin cell battery was left to stand for 3 hours, then charged at a constant current of 0.1C to the upper limit cutoff voltage (3.75V), and then charged at a constant voltage to 50μA. After standing for 5 minutes, the charging capacity of the coin cell battery was recorded. The specific charging capacity of the LFP particles at 60℃, C2, = the charging capacity of the coin cell battery at 60℃ / the weight of the LFP particles in the coin cell battery.

[0221] 7) Cyclic life test

[0222] 1. The cell is rested at 20℃ for 120 min

[0223] 2. 1C discharge to 2.5V

[0224] 3. Rest at 25℃ for 30 min

[0225] 4. 1C constant current charge to 3.65V, constant voltage charge, cut-off current 0.05C

[0226] 5. Rest at 25℃ for 5 min

[0227] 6. 1C discharge to 2.5V

[0228] 7. Rest at 25℃ for 5 min

[0229] 8. The above is one charge-discharge cycle of the battery, which is repeated until the battery capacity

[0230] decays to 80% of the initial value, and the cycle number is recorded.

[0231] 8) Capacity decay degree

[0232] The capacity retention rate at the 1000th cycle = (discharge capacity at the 1000th cycle / initial discharge capacity of the cycle) x 100%.

[0233] 9) Cycle ramping degree

[0234] Record the number of cycles with a capacity retention rate continuously higher than 100% in the first 3000 cycles. If the number of cycles continuously higher than 100% is ≥10, it is determined that the cycle process has a cycle ramping degree, and the maximum capacity retention rate when the cycle process has a capacity retention rate higher than 100% is recorded, and the cycle ramping degree is the maximum capacity retention rate value minus 100%. If the number of cycles continuously higher than 100% is <10, it is determined that the cycle process does not have a cycle ramping degree, and is recorded as "-".

[0235] 10) Pole piece flexibility

[0236] In an environment with normal temperature and air humidity below 10%, the pole piece is cut into a shape with a length of 5-10 cm and a width of 2-4 cm, and is folded and rolled, and a 2Kg roller is used to press at a uniform speed of 2-3 m / min, and the light transmission at the fold of the pole piece is observed, and the flexibility degree of the pole piece is qualitatively judged. n-fold light transmission means that in the test of the pole pieces prepared in the same batch, the pole piece is folded back and forth at the same fold n times before it transmits light, and the larger the n value, the better the flexibility of the pole piece.

[0237] III. Analysis of test results of each example and comparative example

[0238] The batteries of each example and comparative example were prepared according to the above method, and each performance parameter was measured, the positive electrode film layer parameters are shown in Table 1, and the performance test results are shown in Table 2.

[0239]

[0240] Table 2: Performance test results

[0241]

[0242] According to the above results, the positive electrode tab in each of Examples 1-19 includes 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, the specific surface area of the first positive electrode active material is 12 m 2 / g-16 m 2 / g, the carbon content of the first positive electrode active material is 1.0 wt%-1.5 wt% based on the total weight of the first positive electrode active material, the second positive electrode film layer contains a second positive electrode active material, the specific surface area of the second positive electrode active material is 6 m 2 / g-19 m 2 / g, the carbon content of the second positive electrode active material is 1.1 wt%-2.1 wt% based on the total weight of the second positive electrode active material. As can be seen from the comparison of Examples 1-19 and Comparative Examples 1-3, the positive electrode tab of the present application has good tab flexibility.

[0243] Figure 1 shows the microstructure of the first positive electrode active material in Example 1 of the present application. Figure 1 Figure 1 shows the microstructure 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 contains uniformly mixed large particles and small particles, the small particles densely fill the pores of the large particles, so that the first positive electrode film layer has a high compaction density.

[0244] Figure 2 shows the microstructure of the second positive electrode active material in Example 1 of the present application. Figure 2 Figure 2 shows the microstructure of the second positive electrode active material in Example 1 of the present application. As can be seen from the figure, the second positive electrode active material in Example 1 of the present application is uniformly distributed, mainly medium particles, no large and small particles, no large particles can take into account the kinetics, no small particles can reduce side reactions, which is conducive to achieving longer cycle life of the battery.

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

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

[0247] From the comparison of Example 1, 13-14 and Example 15-16, it can be seen that when 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 3:7 to 7:3, the positive electrode plate of the present application has a higher compaction density, and at the same time the battery comprising the positive electrode plate of the present application has a higher cycle climbing degree. When W1 / W2 <3:7 (Example 15), the positive electrode plate of the present application has a lower compaction density. When W1 / W2 >7:3 (Example 16), the battery comprising the positive electrode plate 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 only examples, and embodiments having substantially the same configuration and playing the same role and effect as the technical idea within the scope of the technical solution of the present application are all included in the technical scope of the present application. In addition, within the scope of the main idea of the present application, various modifications that can be thought of by those skilled in the art are applied to the embodiments, and other ways are constructed by combining part of the constituent elements in the embodiments, which are also included in the scope of the present application.

Claims

1. A positive electrode sheet, comprising a positive current collector and a positive electrode film layer, the positive electrode film layer being disposed on at least one side of the positive current collector, the positive electrode film layer comprising a first positive electrode film layer and a second positive electrode film layer, the first positive electrode film layer being located between the positive current collector and the second positive electrode film layer, wherein... The first positive electrode film layer contains a first positive electrode active material, the specific surface area of ​​which is 12 m². 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 contains a second positive electrode active material, the specific surface area of ​​which is 6m². 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%-2.1% by weight; The carbon content of the second positive electrode active material is greater than that of the first positive electrode active material; The first positive electrode active material comprises first lithium iron phosphate particles and second lithium iron phosphate particles. The first lithium iron phosphate particles have a primary average particle size of 50-300 nm, and the second lithium iron phosphate particles have a primary average particle size of 400-500 nm.

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 / cm³ when measured under 3T pressure. 3 .

3. The positive electrode sheet according to claim 1 or 2, wherein, based on the total number of the first lithium iron phosphate particles and the second lithium iron phosphate particles, the number of the first lithium iron phosphate particles accounts for 60%-80%, and the number of the second lithium iron phosphate particles accounts for 20%-40%.

4. The positive electrode sheet according to claim 1 or 2, wherein the carbon content of the second positive electrode active material is 1.2% to 2% by weight based on the total weight of the second positive electrode active material.

5. The positive electrode sheet according to claim 1 or 2, wherein the weight ratio W1 of the first positive electrode active material to the weight W2 of the second positive electrode active material is W1 / W2, which is 1:9 to 9:

1.

6. The positive electrode sheet according to claim 1 or 2, wherein the weight ratio W1 of the first positive electrode active material to the weight W2 of the second positive electrode active material is W1 / W2, which is 3:7 to 7:

3.

7. The positive electrode sheet according to claim 1 or 2, wherein the thickness of the first positive electrode film and the thickness of the second positive electrode film are independently 5-80 μm.

8. A secondary battery comprising a positive electrode sheet according to any one of claims 1-7.

9. An electrical device comprising the secondary battery of claim 8.

Images