Positive pole piece, preparation method, lithium ion battery and electric equipment

Abstract

The invention relates to the technical field of lithium ion batteries, in particular to a positive pole piece, a preparation method, a lithium ion battery and electric equipment. The positive electrode coating comprises a first positive electrode coating and a second positive electrode coating, the first positive electrode coating is located on the surface of the current collector and comprises a first lithium iron phosphate material, the median particle size of the first lithium iron phosphate material is 1.0-1.3 mu m, and the second positive electrode coating is located on the face, away from the current collector, of the first positive electrode coating and comprises a second lithium iron phosphate material. The second positive electrode coating comprises a second lithium iron phosphate material, and the median particle size of the second lithium iron phosphate material is 0.8-2.3 [mu] m. Therefore, the first positive electrode coating comprises the first lithium iron phosphate material with relatively small particle size so as to increase the speed of lithium ion intercalation and deintercalation, and the second positive electrode coating comprises the second lithium iron phosphate material with relatively wide particle size distribution, so that the lithium ion battery has relatively high energy density, and the charging rate capability of the lithium ion battery can be improved.

Description

Technical Field

[0001] The present application relates to the technical field of lithium-ion batteries, and in particular to positive electrode sheets, preparation methods, lithium-ion batteries and electrical equipment. Background Art

[0002] As a mature battery system, lithium-ion batteries are widely used in production and life due to their high energy density. However, with the growing consumer demand, people have higher and higher demands on battery performance.

[0003] Although lithium iron phosphate materials have good cycle stability, they cannot achieve both good energy density and charge rate performance. Summary of the invention

[0004] The main purpose of this application is to provide a positive electrode plate, a preparation method, a lithium-ion battery and an electrical device, aiming to solve the above-mentioned technical problems existing in the prior art.

[0005] In order to solve the above technical problems, the technical solution of the first aspect adopted in the present application is: providing a positive electrode plate, the positive electrode plate includes a current collector and a positive electrode coating, the positive electrode coating is located on the surface of the current collector, the positive electrode coating includes a first positive electrode coating and a second positive electrode coating, the first positive electrode coating is located on the surface of the current collector, the first positive electrode coating includes a first lithium iron phosphate material, and the median particle size of the first lithium iron phosphate material is 1.0μm~1.3μm; the second positive electrode coating is located on the side of the first positive electrode coating away from the current collector, the second positive electrode coating includes a second lithium iron phosphate material, and the median particle size of the second lithium iron phosphate material is 0.8μm~2.3μm.

[0006] In one embodiment, the average particle size of the first lithium iron phosphate material is 0.5 μm to 2 μm.

[0007] In one embodiment, the average particle size of the second lithium iron phosphate material is 0.3 μm to 3 μm.

[0008] In one embodiment, the thickness of the positive electrode coating is 85 μm to 90 μm.

[0009] In one embodiment, the ratio of the thickness of the first positive electrode coating layer to the thickness of the positive electrode coating layer is (7-9):16.

[0010] In one embodiment, the compaction density of the positive electrode sheet is 2.55 g·cm -3 ~2.70g·cm -3 .

[0011] In one embodiment, the positive electrode plate further includes a conductive coating, and the conductive coating is located on the surface of the second positive electrode coating.

[0012] In one embodiment, the conductive coating comprises carbon nanotubes and conductive carbon black, and the mass ratio of the carbon nanotubes to the conductive carbon black is 1:(1-1.2).

[0013] The technical solution of the second aspect adopted in the present application is: to provide a method for preparing a positive electrode plate, the method comprising: providing a first positive electrode coating slurry and a second positive electrode coating slurry; after coating the first positive electrode coating slurry on the surface of the current collector, continuously coating the second positive electrode coating slurry on the surface of the first positive electrode coating slurry, and obtaining the positive electrode plate after drying; wherein, the first positive electrode coating slurry comprises a first lithium iron phosphate material, the median particle size of the first lithium iron phosphate material is 1.0μm to 1.3μm, and the second positive electrode coating slurry comprises a second lithium iron phosphate material, the median particle size of the second lithium iron phosphate material is 0.8μm to 2.3μm.

[0014] In one embodiment, the method for preparing the positive electrode plate further includes, after the first positive electrode coating slurry and the second positive electrode coating slurry are dried, providing a conductive coating on the surface of the second positive electrode coating, and then rolling to obtain the positive electrode plate, wherein the thickness of the conductive coating is 10 μm to 15 μm.

[0015] In one embodiment, the conductive coating includes carbon nanotubes and conductive carbon black, and the mass ratio of the carbon nanotubes to the conductive carbon black is 1:(1-1.2).

[0016] A third aspect of the present application provides a lithium-ion battery, comprising the positive electrode sheet of the first aspect and / or a method for preparing the positive electrode sheet of the second aspect.

[0017] A fourth aspect of the present application provides an electrical device, comprising the lithium-ion battery of the third aspect.

[0018] In the technical solution of the present application, the positive electrode coating includes a first positive electrode coating and a second positive electrode coating. The first positive electrode coating is located on the surface of the current collector, and the second positive electrode coating is located on the surface of the first positive electrode coating. The first positive electrode coating includes a first lithium iron phosphate material with a smaller particle size, which utilizes the faster electron transfer speed close to the current collector side and combines the faster lithium ion transmission speed of the smaller particle size to increase the speed of lithium ion insertion and deinsertion, thereby improving the charge rate performance. The second positive electrode coating includes a second lithium iron phosphate material with a wider particle size distribution to increase the tap density of the lithium ion battery, thereby improving the charge rate performance of the lithium ion battery while also having a higher energy density. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 It is a schematic structural diagram of a positive electrode sheet according to an embodiment of the present application;

[0020] Figure 2 It is a comparison diagram of the charging curves of Example 1 and Comparative Example 1.

[0021] Reference numerals: positive electrode sheet 10 ; current collector 100 ; first positive electrode coating 200 ; second positive electrode coating 300 ; conductive coating 400 . DETAILED DESCRIPTION

[0022] Below, the battery cells, batteries and electrical equipment 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 structures 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.

[0023] The "range" disclosed in the present application is defined in the form of a lower limit and an upper limit, 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 range defined in this way can be inclusive or exclusive of the end values, and can be arbitrarily combined, that is, any lower limit can be combined with any upper limit to form a range. For example, if a range of 60 to 120 and 80 to 110 is listed for a particular parameter, it is understood that a range of 60 to 110 and 80 to 120 is also expected. In addition, if the minimum range values ​​1 and 2 are listed, and if the maximum range values ​​3, 4 and 5 are listed, the following ranges can all be expected: 1-3, 1 to 4, 1 to 5, 2-3, 2 to 4 and 2 to 5. In the present application, unless otherwise specified, the numerical range "a to b" represents an abbreviation of any real number combination between a and b, where a and b are both real numbers. For example, the numerical range "0-5" means that all real numbers between "0-5" are listed in this document, and "0-5" is just an abbreviation of these numerical combinations. In addition, when a parameter is expressed as an integer ≥ 2, it 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.

[0024] Unless otherwise specified, all embodiments and optional embodiments of the present application can be combined with each other to form a new technical solution.

[0025] 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.

[0026] Unless otherwise specified, all steps of the present application may be performed sequentially or randomly, preferably sequentially. For example, a method includes steps (a) and (b), which means that the method may include steps (a) and (b) performed sequentially, or may include steps (b) and (a) performed sequentially. For example, a method may also include step (c), which means 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.

[0027] If there is no special explanation, the "include" and "comprising" mentioned in this application represent open-ended or closed-ended expressions. For example, "include" and "comprising" may represent that other components not listed may also be included or only listed components may be included or only listed components may be included.

[0028] If not specifically stated, in this application, the term "or" is inclusive. For example, the phrase "A or B" means "A, B, or both A and B". More specifically, any 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 both A and B are true (or exist).

[0029] Lithium iron phosphate material has a stable olivine structure. During the charge and discharge process, its structure changes little and is not prone to collapse and damage, thus ensuring the long-term stability of the battery. However, since the theoretical capacity of lithium iron phosphate material itself is not high (about 170mAh / g), and the electronic conductivity of lithium iron phosphate material itself is poor, it is easy to produce polarization, increase internal resistance, and the greater the charging current, the greater the capacity loss. Therefore, the energy density and charging rate performance of lithium iron phosphate are poor.

[0030] Based on the above problems, the first aspect of the present application provides a positive electrode sheet. Figure 1 , Figure 1 A schematic structural diagram of an embodiment of the positive electrode plate provided in the present application.

[0031] The positive electrode plate 10 includes: a current collector and a positive electrode coating, the positive electrode coating is located on the surface of the current collector 100, the positive electrode coating includes a first positive electrode coating 200 and a second positive electrode coating 300, the first positive electrode coating 200 is located on the surface of the current collector 100, the first positive electrode coating 200 includes a first lithium iron phosphate material, and the median particle size of the first lithium iron phosphate material is 1.0μm~1.3μm; the second positive electrode coating 300 is located on the side of the first positive electrode coating 200 away from the current collector 100, the second positive electrode coating 300 includes a second lithium iron phosphate material, and the median particle size of the second lithium iron phosphate material is 0.8μm~2.3μm.

[0032] In the technical solution of the embodiment of the present application, the positive electrode coating includes a first positive electrode coating 200 and a second positive electrode coating 300. The first positive electrode coating 200 is located on the surface of the current collector 100, and the second positive electrode coating 300 is located on the surface of the first positive electrode coating 200. The first positive electrode coating 200 includes a first lithium iron phosphate material with a smaller particle size, and utilizes the characteristics of a faster electron transfer speed on the side close to the current collector 100, and combines the faster lithium ion transmission speed of the smaller particle size to increase the speed of lithium ion insertion and deinsertion, thereby improving the charging rate performance. The second positive electrode coating 300 includes a second lithium iron phosphate material with a wider particle size distribution, which can reduce the gap between the second lithium iron phosphate materials in the second positive electrode coating 300 to increase the tap density of the lithium ion battery, thereby improving the charging rate performance of the lithium ion battery while also having a higher energy density.

[0033] The median particle size of the first lithium iron phosphate material may be 1 μm, 1.1 μm, 1.2 μm, 1.3 μm, etc., or a range consisting of any two of the above values, for example, 1 μm to 1.2 μm, 1.2 μm to 1.3 μm, etc. The median particle size of the second lithium iron phosphate material may be 0.8 μm, 1.2 μm, 1.7 μm, 2.0 μm, 2.3 μm, etc., or a range consisting of any two of the above values, for example, 0.8 μm to 1.7 μm, 1.7 μm to 2.3 μm.

[0034] In one embodiment, the average particle size of the first lithium iron phosphate material is 0.5 μm to 2 μm.

[0035] The first lithium iron phosphate material has a smaller average particle size, which can shorten the electron transfer path and reduce the transmission distance of lithium ions, thereby improving the charging rate performance of the positive electrode plate 10.

[0036] The average particle size of the first lithium iron phosphate material may be 0.5 μm, 0.9 μm, 1.3 μm, 1.6 μm, 2 μm, etc., or a range consisting of any two of the above values, for example, 0.5 μm to 1.3 μm, 1.3 μm to 2 μm, etc.

[0037] In one embodiment, the average particle size of the second lithium iron phosphate material is 0.3 μm to 3 μm.

[0038] The average particle size of the second lithium iron phosphate material is larger than that of the first lithium iron phosphate material, and the range of the average particle size is wider, which can increase the compaction density of the positive electrode sheet 10 and further increase the energy density of the positive electrode sheet 10 .

[0039] The particle size of the second lithium iron phosphate material may be 0.3 μm, 1 μm, 1.6 μm, 2.4 μm, 3 μm, etc., or a range consisting of any two of the above values, for example, 0.3 μm to 1.6 μm, 1.6 μm to 3 μm, etc.

[0040] In one embodiment, the thickness of the positive electrode coating is 85 μm to 90 μm.

[0041] When the thickness of the positive electrode coating is within the above range, the positive electrode plate 10 can have both lower internal resistance and higher energy density.

[0042] Among them, the thickness of the positive electrode coating can be 85μm, 86μm, 87μm, 88μm, 89μm, 90μm, etc., or a range consisting of any two of the above values, for example, 85μm-87μm, 87μm-88μm, 88μm-90μm, etc.

[0043] In one embodiment, the ratio of the thickness of the first positive electrode coating 200 to the thickness of the positive electrode coating is (7-9):16.

[0044] The ratio of the thickness of the first positive electrode coating 200 to the thickness of the positive electrode coating is within the above range, so that the thickness of the first positive electrode coating 200 and the thickness of the second positive electrode coating 300 of the positive electrode plate 10 are more appropriate, thereby making the charge rate performance and energy density of the positive electrode plate 10 better.

[0045] The ratio of the thickness of the first positive electrode coating 200 to the thickness of the positive electrode coating may be 7:16, 8:16, 9:16, etc., or a range consisting of any two of the above values, for example, (7-8):16, (8-9):16, etc.

[0046] In one embodiment, the compaction density of the positive electrode sheet 10 is 2.55 g·cm -3 ~2.70g·cm -3 .

[0047] The compaction density of the positive electrode sheet 10 is within the above range, so that the positive electrode sheet 10 can have more positive electrode active materials per unit volume to provide more active lithium, thereby improving the energy density of the positive electrode sheet 10 .

[0048] The compaction density of the positive electrode sheet 10 can be 2.55 g·cm -3 , 2.6 g·cm -3 , 2.65g·cm -3 , 2.7 g·cm -3 etc., or a range consisting of any two of the above values, for example, 2.55 g·cm -3 ~2.65g·cm -3 , 2.65g·cm-3 ~2.7g·cm -3 wait.

[0049] In one embodiment, the positive electrode plate 10 further includes a conductive coating 400 , and the conductive coating 400 is located on the surface of the second positive electrode coating 300 .

[0050] The conductive coating 400 located on the surface of the second positive electrode coating 300 can increase the electron transfer rate of the second positive electrode coating, thereby improving the charge rate performance of the positive electrode plate 10 .

[0051] In one embodiment, the conductive coating 400 includes carbon nanotubes and conductive carbon black, and the mass ratio of the carbon nanotubes to the conductive carbon black is 1:(1-1.2).

[0052] The conductive material in the conductive coating 400 includes carbon nanotubes and conductive carbon black. The conductive carbon black has a high specific surface area and good dispersibility, and can form nodes of a conductive network in the matrix. The carbon nanotubes have a high aspect ratio and excellent conductivity, and can serve as a bridge of the conductive path to connect each conductive carbon black particle, thereby forming a more efficient conductive network, improving the electronic conduction rate of the second positive electrode coating 300, reducing the internal resistance of the positive electrode plate 10, and thus improving the charge rate performance of the positive electrode plate 10.

[0053] The second aspect of the present application provides a method for preparing a positive electrode plate 10, the method comprising: providing a first positive electrode coating slurry and a second positive electrode coating slurry; after coating the first positive electrode coating slurry on the surface of the current collector 100, continuously coating the second positive electrode coating slurry on the surface of the first positive electrode coating slurry, and obtaining the positive electrode plate 10 after drying; wherein the first positive electrode coating slurry comprises a first lithium iron phosphate material, the median particle size of the first lithium iron phosphate material is 1.0 μm to 1.3 μm, and the second positive electrode coating slurry comprises a second lithium iron phosphate material, the median particle size of the second lithium iron phosphate material is 0.8 μm to 2.3 μm.

[0054] In the technical solution of the embodiment of the present application, the preparation method of the positive electrode plate 10 is to first apply the first positive electrode coating slurry to the surface of the current collector, and then continuously apply the second positive electrode coating slurry to the surface of the first positive electrode coating slurry. That is to say, after the first positive electrode coating slurry is applied to the surface of the current collector 100, there is no need to dry the first positive electrode coating slurry, and the second positive electrode coating slurry is directly applied to the surface of the first positive electrode coating slurry. In this way, the first positive electrode coating 200 and the second positive electrode coating 300 can be made closer, reducing the risk of the second positive electrode coating 300 falling off the surface of the first positive electrode coating 200. The first positive electrode coating slurry includes a first lithium iron phosphate material, and the median particle size of the first lithium iron phosphate material can shorten the transfer path of electrons within the above range, and reduce the transmission distance of lithium ions, thereby improving the charging rate performance of the positive electrode plate 10. The second positive electrode coating slurry includes a second lithium iron phosphate material. The particle size of the second lithium iron phosphate material is larger than that of the first lithium iron phosphate, and the range of the particle size is wider, which can increase the compaction density of the positive electrode plate 10 and further increase the energy density of the positive electrode plate 10.

[0055] In one embodiment, the preparation method of the positive electrode plate 10 further includes, after the first positive electrode coating slurry and the second positive electrode coating slurry are dried, setting a conductive coating 400 on the surface of the second positive electrode coating 300, and then rolling to obtain the positive electrode plate 10, wherein the thickness of the conductive coating 400 is 10 μm to 15 μm.

[0056] In some embodiments, the conductive coating 400 may be disposed on the surface of the second positive electrode coating 300 by vapor deposition; or the conductive coating 400 may be disposed on the surface of the second positive electrode coating 300 by spraying or the like.

[0057] Since the positive electrode sheet 10 has both the first positive electrode coating 200 and the second positive electrode coating 300, the second positive electrode coating 300 is far away from the current collector 100. Therefore, the electron transfer rate of the second positive electrode coating 300 is slower than that of the first positive electrode coating 200. After the first positive electrode coating 200 and the second positive electrode coating 300 are dried, a conductive coating 400 is arranged on the surface of the second positive electrode coating 300. Since the thickness of the conductive coating 400 is relatively thin, the mass proportion of the conductive coating 400 can be greatly reduced, thereby making the active material proportion of the positive electrode sheet 10 larger, and making the positive electrode sheet 10 have a larger capacity. While the conductive coating 400 is located on the surface of the second positive electrode coating 300, the conductive material in the conductive coating 400 will also penetrate into the gap inside the second positive electrode coating 300 to form a conductive network on the surface and the internal gap of the second positive electrode coating 300, which can increase the electron transfer speed of the second positive electrode coating 300, thereby improving the charging rate performance of the positive electrode sheet 10.

[0058] The thickness of the conductive coating 400 may be 10 μm, 12 μm, 13 μm, 15 μm, etc., or a range consisting of any two of the above values, for example, 10 μm to 13 μm, 13 μm to 15 μm, etc.

[0059] In one embodiment, the conductive coating 400 includes carbon nanotubes and conductive carbon black, and the mass ratio of the carbon nanotubes to the conductive carbon black is 1:(1-1.2).

[0060] The conductive material in the conductive coating 400 includes carbon nanotubes and conductive carbon black. The conductive carbon black has a high specific surface area and good dispersibility, and can form nodes of a conductive network in the matrix. The carbon nanotubes have a high aspect ratio and excellent conductivity, and can serve as a bridge of the conductive path to connect each conductive carbon black particle, thereby forming a more efficient conductive network, improving the electronic conduction rate of the second positive electrode coating 300, reducing the internal resistance of the positive electrode plate 10, and thus improving the charge rate performance of the positive electrode plate 10.

[0061] The mass ratio of carbon nanotubes to conductive carbon black may be 1:1, 1:1.1, 1:1.2, etc., or a range consisting of any two of the above values, for example, 1:(1-1.1), 1:(1.1-1.2), etc.

[0062] The third aspect of the present application provides a lithium ion battery, including the positive electrode sheet 10 of the first aspect or / and the method for preparing the positive electrode sheet 10 of the second aspect. The lithium ion battery in the embodiment of the present application has at least the same advantages as the positive electrode sheet 10 of the first aspect, or / and the same advantages as the lithium ion battery prepared by the method for preparing the positive electrode sheet 10 of at least the second aspect.

[0063] A fourth aspect of the present application provides an electrical device, comprising the lithium-ion battery of the third aspect. The electrical device in the embodiment of the present application at least has the same advantages as the lithium-ion battery of the third aspect.

[0064] The lithium-ion battery disclosed in the embodiments of the present application can be used in electrical equipment using lithium-ion batteries as power sources or various energy storage systems using batteries as energy storage elements. Electrical equipment can be, but is not limited to, mobile phones, tablets, laptops, electric toys, electric tools, battery cars, electric cars, ships, spacecraft, etc. Among them, electric toys can include fixed or mobile electric toys, for example, game consoles, electric car toys, electric ship toys, and electric airplane toys, etc., and spacecraft can include airplanes, rockets, space shuttles, and spacecrafts, etc.

[0065] In order to make the technical problems, technical solutions and beneficial effects solved by the embodiments of the present application clearer, the following will be further described in detail with reference to the embodiments and the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all of the embodiments. The following description of at least one exemplary embodiment is actually only illustrative and is by no means intended to limit the present application and its applications. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in the field without creative work are within the scope of protection of the present application.

[0066] Example 1

[0067]

Preparation of positive electrode

[0068] Preparation of the first positive electrode coating slurry: The first lithium iron phosphate material, 1-methyl-2-pyrrolidone and acetylene black are mixed in a mass ratio of 8:1:1 in 5% polyvinylidene fluoride, and the solid content is controlled at about 50%.

[0069] Preparation of the second positive electrode coating slurry: The second lithium iron phosphate material, 1-methyl-2-pyrrolidone and acetylene black are mixed in a mass ratio of 8:1:1 in 5% polyvinylidene fluoride, and the solid content is controlled at about 50%.

[0070] After the first positive electrode coating slurry is applied on one side of the aluminum foil, the wet film thickness of the first positive electrode coating is 40μm, and the second positive electrode coating slurry is continuously applied on the first positive electrode coating slurry coated on the surface of the aluminum foil, and the total wet film thickness of the first positive electrode coating and the second positive electrode coating is 80μm. After drying, a conductive coating is set on the surface of the second positive electrode coating by vapor deposition, and the positive electrode sheet is obtained after baking, rolling and cutting. Among them, the thickness of the conductive coating is 10μm, and the conductive coating is a mixture of carbon nanotubes and conductive carbon black, and the mass ratio of carbon nanotubes to conductive carbon black is 1:1.1. The thickness of the positive electrode coating is 87μm.

[0071] The median particle size of the first lithium iron phosphate is 1.1 μm, and the average particle size is 1.2 μm. The median particle size of the second lithium iron phosphate is 1.5 μm, and the average particle size is 2.1 μm.

[0072]

Preparation of negative electrode sheet

[0073] The negative electrode active materials graphite, acetylene black, styrene-butadiene rubber (SBR), and carboxymethyl cellulose (CMC) are uniformly dispersed in deionized water at a mass ratio of 96:2:1:1, and the solid content is controlled at about 50%. The mixed black slurry is coated on both sides of the copper foil, and then baked, rolled, and cut to obtain the negative electrode sheet, wherein the thickness of the negative electrode active layer is 22 μm.

[0074]

Preparation of electrolyte

[0075] The following components were weighed in percentage by mass: 12.5% ​​of lithium hexafluorophosphate, 0.5% of lithium difluorooxalatoborate, 30% of ethylene carbonate, 43% of propylene carbonate, 10% of ethyl methyl carbonate, 2% of fluoroethylene carbonate, 1,3-propane sultone, 0.5% of tris(trimethylsilyl)phosphite, and 0.5% of 3-trimethylsilyl-2-oxazolidinone, and mixed evenly to obtain an electrolyte.

[0076]

Diaphragm

[0077] Polyethylene (PE) is used as a base film and a nano-alumina coating is coated on the base film as an isolation membrane, wherein the thickness of the polyethylene base film is 7 μm, the porosity is 34%, the average particle size / volume average particle size of the nano-alumina in the alumina coating is 0.5 μm, and the thickness of the alumina coating is 2 μm.

[0078]

Battery preparation

[0079] The positive electrode sheets, separators and negative electrode sheets are made into a battery electrode group with 10 positive electrode sheets and 11 negative electrode sheets through a lamination process. The battery electrode group is hot pressed and the tabs are welded, and then placed in an aluminum-plastic film; and the electrolyte is injected. The battery after injection is packaged, stored, formed, aged, and divided into different capacities to obtain a lithium-ion battery.

[0080] Example 2

[0081] The difference from Example 1 is that in the preparation of the positive electrode sheet, the median particle size of the first lithium iron phosphate is 1 μm and the average particle size is 0.5 μm. The rest is the same as Example 1 and will not be described again.

[0082] Example 3

[0083] The difference from Example 1 is that in the preparation of the positive electrode sheet, the median particle size of the first lithium iron phosphate is 1.2 μm and the average particle size is 2 μm. The rest is the same as Example 1 and will not be repeated here.

[0084] Example 4

[0085] The difference from Example 1 is that in the preparation of the positive electrode sheet, the median particle size of the second lithium iron phosphate is 0.8 μm and the average particle size is 0.3 μm. The rest is the same as Example 1 and will not be repeated here.

[0086] Example 5

[0087] The difference from Example 1 is that in the preparation of the positive electrode sheet, the median particle size of the second lithium iron phosphate is 2.3 μm and the average particle size is 3 μm. The rest is the same as Example 1 and will not be repeated here.

[0088] Example 6

[0089] The difference from Example 1 is that in the preparation of the positive electrode sheet, the thickness of the positive electrode sheet is 85 μm, and the ratio of the thickness of the first positive electrode coating to the thickness of the positive electrode coating is 7:16. The rest is the same as Example 1 and will not be repeated here.

[0090] Example 7

[0091] The difference from Example 1 is that in the preparation of the positive electrode sheet, the thickness of the positive electrode sheet is 90 μm, and the ratio of the thickness of the first positive electrode coating to the thickness of the positive electrode coating is 9:16. The rest is the same as Example 1 and will not be repeated here.

[0092] Example 8

[0093] The difference from Example 1 is that in the preparation of the positive electrode sheet, the mass ratio of carbon nanotubes to conductive carbon black is 1:1. The rest is the same as Example 1 and will not be described again.

[0094] Example 9

[0095] The difference from Example 1 is that in the preparation of the positive electrode sheet, the mass ratio of carbon nanotubes to conductive carbon black is 1:1.2. The rest is the same as Example 1 and will not be described again.

[0096] Example 10

[0097] The difference from Example 1 is that in the preparation of the positive electrode sheet, after the first positive electrode coating and the second positive electrode coating are applied to the aluminum foil, the positive electrode sheet is obtained by directly baking, rolling and cutting, and no conductive coating is provided. The rest is the same as Example 1 and will not be repeated here.

[0098] Comparative Example 1

[0099] The difference from Example 1 is that in the preparation of the positive electrode sheet, the first positive electrode coating slurry is not prepared, and the second positive electrode coating slurry is directly coated on the surface of the aluminum foil, and the wet film thickness of the coating is 80 μm. The rest is the same as Example 1 and will not be repeated here.

[0100] Comparative Example 2

[0101] The difference from Example 1 is that in the preparation of the positive electrode sheet, no second positive electrode coating slurry is prepared, and the first positive electrode coating slurry is directly coated on the surface of the aluminum foil, and the wet film thickness of the coating is 80 μm. The rest is the same as Example 1 and will not be repeated here.

[0102] The relevant parameter testing methods in the above embodiments and comparative examples are as follows:

[0103] (1) Energy density test:

[0104] After the battery is temperature balanced in an environment of 25±2℃, it is charged to 3.65V at a constant current and constant voltage of 0.33C, with a cut-off current of 0.05C. After standing for 30min, it is discharged to 2.5V at a constant current of 0.33C and stood for 30min. The charge and discharge are repeated three times, and the average power of the three discharges is taken as the energy of the battery cell. The energy is divided by the mass of the battery cell to obtain the energy density of the battery cell.

[0105] (2) Charging voltage test: After the battery is temperature balanced at 25±2°C, charge it to 3.65V at a constant current and voltage of 0.33C, with a cut-off current of 0.05C. Obtain the charging voltage-capacity curve and read the voltage platform (charging potential).

[0106] (3) Compacted density test: Take a certain mass of powder and place it in a special compaction container. Place the container on the compaction density instrument and set different pressures to apply different pressures to the powder to compact it. After unloading the pressure, read the thickness of the powder under different pressures on the instrument display and calculate the compaction density of the powder by ρ = m / V.

[0107] (4) Particle size test (median particle size and average particle size): Take an appropriate amount of powder sample and place it in a special container, add 20-25 mL of deionized water to meet the shading degree of the sample concentration of 8% to 12%, and ultrasonically disperse for 10 minutes to make the powder sample evenly dispersed. Then, according to the national standard "GB / T19077-2016", use Malvern 2000 laser particle size analyzer to measure the median particle size and average particle size of the particles.

[0108] Table 1 Preparation process and performance parameters of positive electrode sheets of Examples 1 to 10 and Comparative Examples 1 to 2

[0109]

[0110] In summary, if Figure 2 As shown, Figure 2 1 is a comparison chart of the charging rate performance of Example 1 and Comparative Example 1. Figure 2 It can be clearly seen that the charging curve of Example 1 at a charging current of 0.33C, the charging voltage platform curve of Example 1 is about 3.32V, while the charging voltage platform curve of Comparative Example 1 at the same charging current of 0.33C is about 3.41V, indicating that at the same current, the polarization voltage of Example 1 is smaller than the polarization voltage of Comparative Example 1, thereby indicating that the charging rate performance of Example 1 is better than that of Comparative Example 1.

[0111] As shown in Table 1, Table 1 is the preparation process and performance parameters of the positive electrode sheets of Examples 1 to 10 and Comparative Examples 1 to 2. Compared with Comparative Examples 1 to 2, Examples 1 to 10 are provided with a first positive electrode coating and a second positive electrode coating on the surface of the aluminum foil in sequence. Comparative Examples 1 to 2 are to directly apply the first positive electrode coating or the second positive electrode coating to the aluminum foil alone. Among them, Comparative Example 1 directly applies the second positive electrode coating to the aluminum foil alone, so that the positive electrode sheet has a good energy density, but the charging potential is high and the charging rate performance is poor. Comparative Example 2 directly applies the first positive electrode coating to the aluminum foil alone, so that the positive electrode sheet has a lower charging potential and better charging rate performance, but the energy density is low. Examples 1 to 10 successively apply the first positive electrode coating and the second positive electrode coating, and then combine the characteristics of the first positive electrode coating with a lower charging potential and the second positive electrode coating with a higher energy density, so that the positive electrode sheet has a lower charging potential, better charging rate performance, and a higher energy density. By regulating the particle size of the first lithium iron phosphate material in the first positive electrode coating, the particle size of the second lithium iron phosphate material in the second positive electrode coating, and the thickness of the first positive electrode coating and the positive electrode coating, the energy density and charging potential of the positive electrode plate are optimized, so that the energy density and charging rate performance of Example 1 are better. In addition, it can be seen from Examples 1 and 10 that the charging potential of the positive electrode plate can be effectively reduced by providing a conductive coating on the surface of the second positive electrode coating by vapor deposition, thereby making the positive electrode plate have better charging rate performance. Finally, the energy density of the positive electrode plate in Example 1 reaches 183Wh / Kg at a charging rate of 0.33C, and the charging potential is only 3.32V.

[0112] The above are only implementation methods of the present application, and are not intended to limit the patent scope of the present application. Any equivalent structure or equivalent process transformation made using the contents of the present application specification and drawings, or directly or indirectly used in other related technical fields, are also included in the patent protection scope of the present application.

Claims

1. A positive electrode sheet, characterized in that: The positive electrode sheet comprises: current collector; A positive electrode coating, located on the surface of the current collector, the positive electrode coating comprising a first positive electrode coating and a second positive electrode coating, the first positive electrode coating being located on the surface of the current collector, the first positive electrode coating comprising a first lithium iron phosphate material, and the median particle size of the first lithium iron phosphate material being 1.0 μm to 1.3 μm; The second positive electrode coating is located on a side of the first positive electrode coating away from the current collector, and the second positive electrode coating comprises a second lithium iron phosphate material, wherein the median particle size of the second lithium iron phosphate material is 0.8 μm to 2.3 μm.

2. The positive electrode sheet according to claim 1, characterized in that: The average particle size of the first lithium iron phosphate material is 0.5 μm to 2 μm.

3. The positive electrode sheet according to claim 1, characterized in that: The average particle size of the second lithium iron phosphate material is 0.3 μm to 3 μm.

4. The positive electrode sheet according to claim 1, characterized in that: The thickness of the positive electrode coating is 85 μm to 90 μm.

5. The positive electrode sheet according to claim 4, characterized in that: The ratio of the thickness of the first positive electrode coating to the thickness of the positive electrode coating is (7-9):

16.

6. The positive electrode sheet according to claim 1, characterized in that: The compaction density of the positive electrode sheet is 2.55 g·cm -3 ~2.70g·cm -3 .

7. The positive electrode sheet according to any one of claims 1 to 6, characterized in that: The positive electrode plate also includes a conductive coating, and the conductive coating is located on the surface of the second positive electrode coating.

8. The positive electrode sheet according to claim 7, characterized in that: The conductive coating comprises carbon nanotubes and conductive carbon black, and the mass ratio of the carbon nanotubes to the conductive carbon black is 1:(1-1.2).

9. A method for preparing a positive electrode sheet according to any one of claims 1 to 8, characterized in that: include: providing a first positive electrode coating slurry and a second positive electrode coating slurry; After coating the first positive electrode coating slurry on the surface of the current collector, continuously coating the second positive electrode coating slurry on the surface of the first positive electrode coating slurry, and obtaining a positive electrode sheet after drying; The first positive electrode coating slurry includes a first lithium iron phosphate material, the median particle size of the first lithium iron phosphate material is 1.0 μm to 1.3 μm, and the second positive electrode coating slurry includes a second lithium iron phosphate material, the median particle size of the second lithium iron phosphate material is 0.8 μm to 2.3 μm.

10. The preparation method according to claim 9, characterized in that: The method for preparing the positive electrode plate also includes, after the first positive electrode coating slurry and the second positive electrode coating slurry are dried, providing a conductive coating on the surface of the second positive electrode coating, and then rolling to obtain the positive electrode plate, wherein the thickness of the conductive coating is 10 μm to 15 μm.

11. The preparation method according to claim 10, characterized in that: The conductive coating comprises carbon nanotubes and conductive carbon black, and the mass ratio of the carbon nanotubes to the conductive carbon black is 1:(1-1.2).

12. A lithium ion battery, characterized in that: The lithium-ion battery comprises the positive electrode sheet as claimed in any one of claims 1 to 8, or / and the method for preparing the positive electrode sheet as claimed in any one of claims 9 to 11.

13. An electrical equipment, characterized in that: The electrical equipment comprises the lithium-ion battery provided in claim 12.

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