A high-compacted thick positive electrode sheet, a preparation method thereof and a lithium ion battery

Abstract

The present application relates to a kind of high compaction thick positive pole piece and its preparation method and lithium ion battery.The present application is designed and prepared by regulating the collocation of binder in positive paste and using double-layer coating mode, optimizes the distribution of binder in thick positive pole piece, reduces the friction between particle and particle, particle and binder, particle and current collector in pole piece, prevents the particle breakage problem caused by rolling, and improves the compaction density of thick positive pole.

Description

TECHNICAL FIELD

[0001] The present application relates to a positive electrode tab, in particular to a high-compaction-thick positive electrode tab and a preparation method thereof and a lithium ion battery. BACKGROUND

[0002] The thick electrode technology has become one of the key technologies to realize high specific energy lithium ion batteries, and has been widely designed and applied. Although the thick electrode tab has a significant advantage in improving the energy density of the lithium ion battery, there are still some process challenges in preparing a tab with excellent performance, high compaction density and low tortuosity. Among them, how to improve the compaction density of the tab and thus improve the capacity of the material is an important problem that needs to be solved urgently.

[0003] The existing technology mainly realizes the compaction of the thick electrode tab from the rolling process. The conventional process optimization includes: adopting multiple rolling to obtain a higher tab compaction; and adopting a hot pressing process to achieve a higher compaction density. Although these two methods can improve the compaction density to a certain extent, there is still a risk of particle breakage and tab fracture caused by particle overpressure, and the essential problem has not been solved. SUMMARY

[0004] Problems to be solved by the application

[0005] In view of the above, it is necessary to provide a thick positive electrode tab with high compaction density and without particle breakage and tab fracture risk, and a preparation method of the thick positive electrode tab which can improve the compaction density of the tab without overpressure.

[0006] Solution to the problem

[0007] In order to improve the compaction density and rolling effect of the thick positive electrode tab, the present application starts from the design and preparation of the thick positive electrode tab, and designs and prepares the tab by adjusting the collocation of the binder in the positive electrode slurry and adopting a double-layer coating method, thereby reducing the friction between the tab particles, the binder and the current collector, improving the compaction density of the thick positive electrode, and essentially solving the problems of particle breakage and tab fracture caused by the rolling of the thick positive electrode.

[0008] Specifically, the present application provides a high-compaction-thick positive electrode tab, which comprises a current collector, a first positive electrode coating layer on the current collector, and a second positive electrode coating layer on the first positive electrode coating layer,

[0009] The first positive electrode coating layer is formed of a first positive electrode slurry, and the first positive electrode slurry comprises a first positive electrode active main material, a first positive electrode binder, a first conductive agent, a second conductive agent and a second positive electrode binder,

[0010] The second positive electrode coating is formed from a second positive electrode slurry, the second positive electrode slurry including a second positive electrode active main material, a third positive electrode binder, a third conductive agent, a fourth conductive agent, and a fourth positive electrode binder,

[0011] The second positive electrode binder has a molecular weight greater than 1 million and a crystallinity greater than 45%,

[0012] The fourth positive electrode binder has a molecular weight less than 700,000 and a crystallinity less than 40%.

[0013] The first positive electrode binder and the third positive electrode binder each have a molecular weight greater than 1 million and a crystallinity greater than 45%,

[0014] The first positive electrode binder, the second positive electrode binder, and the third positive electrode binder each have the same or different molecular weight and crystallinity.

[0015] The first positive electrode binder, the second positive electrode binder, the third positive electrode binder, and the fourth positive electrode binder are each independently selected from one or more of polyvinylidene fluoride, polytetrafluoroethylene, an oily acrylic homopolymer.

[0016] The first conductive agent, the second conductive agent, the third conductive agent, and the fourth conductive agent are each independently selected from one or more of an oily single-walled carbon nanotube, an oily multi-walled carbon nanotube, a conductive carbon fiber, graphite, graphene, carbon black, each being the same or different;

[0017] Preferably, the first conductive agent and the third conductive agent are selected from one or more of a conductive carbon fiber, graphite, graphene, carbon black, the second conductive agent is an oily single-walled carbon nanotube, and the fourth conductive agent is an oily multi-walled carbon nanotube.

[0018] The first positive electrode active main material and the second positive electrode active main material are each independently selected from one or more of lithium iron phosphate, lithium cobaltate, lithium iron manganese phosphate, lithium manganese oxide, lithium nickel cobalt manganese oxide.

[0019] The first positive electrode active main material and the second positive electrode active main material are the same or different.

[0020] The first positive electrode slurry further includes a first positive electrode active material additive, and / or the second positive electrode slurry further includes a second positive electrode active material additive,

[0021] The first positive electrode active material additive and the second positive electrode active material additive are each independently selected from one or more of Li2S, Li2NiO2, Li5FeO4, Li2O, Li2O2;

[0022] The first positive electrode active material additive and the second positive electrode active material additive are the same or different,

[0023] Preferably, the first positive electrode active material additive and the second positive electrode active material additive are each independently selected from one or more of Li2S, Li2NiO2, Li5FeO4, Li2O, Li2O2.

[0024] According to the high-thickness high-compactness positive electrode sheet described above, the weight proportions of the materials in the first positive electrode coating are: 96.5-97.8% by mass of the first positive electrode active main material, 1.0-2.3% by mass of the first positive electrode binder, 0.2-0.8% by mass of the first conductive agent, 0.0005-0.024% by mass of the second conductive agent, and 0.00075-0.032% by mass of the second positive electrode binder.

[0025] Preferably, the weight proportions of the materials in the first positive electrode coating are: 96.8-97.3% by mass of the first positive electrode active main material, 1.0-1.5% by mass of the first positive electrode binder, 0.3-0.6% by mass of the first conductive agent, 0.001-0.009% by mass of the second conductive agent, and 0.0015-0.012% by mass of the second positive electrode binder.

[0026] According to the high-thickness high-compactness positive electrode sheet described above, the weight proportions of the materials in the second positive electrode coating are: 97-97.8% by mass of the second positive electrode active main material, 0.8-1.3% by mass of the third positive electrode binder, 0.5-1.0% by mass of the third conductive agent, 0.00075-0.0175% by mass of the fourth conductive agent, and 0.00015-0.04% by mass of the fourth positive electrode binder.

[0027] According to the high-thickness high-compactness positive electrode sheet described above, the thickness of the thick positive electrode sheet is 160-240 μm; and the compactness density of the thick positive electrode sheet is 3.28-3.5 g / cm 3 .

[0028] The present application also provides a method for preparing a high-thickness high-compactness positive electrode sheet, the method comprising the following steps:

[0029] Step S1: determining the area density of the required positive electrode sheet;

[0030] Step S2: preparing a first positive electrode slurry and a second positive electrode slurry,

[0031] wherein the first positive electrode slurry is formed by dispersing a first positive electrode active main material, a first positive electrode binder, a first conductive agent, a second conductive agent, and a second positive electrode binder in a solvent,

[0032] the second positive electrode slurry is formed by dispersing a second positive electrode active main material, a third positive electrode binder, a third conductive agent, a fourth conductive agent, and a fourth positive electrode binder in a solvent,

[0033] the second positive electrode binder has a molecular weight greater than 1 million and a crystallinity greater than 45%,

[0034] the fourth positive electrode binder has a molecular weight less than 700 thousand and a crystallinity less than 40%;

[0035] Step S3: coating the first positive electrode slurry on the current collector according to the area density of the positive electrode tab determined in Step S1 to form a first positive electrode coating layer, coating the second positive electrode slurry on the first positive electrode coating layer to form a second positive electrode coating layer, and then drying to obtain a tab;

[0036] Step S4: rolling the tab obtained in Step S3 to obtain a high-compact-thickness positive electrode tab.

[0037] According to the preparation method described above, in Step S2, first, the second conductive agent and the second positive electrode binder are dispersed in a solvent to form a first conductive slurry, and then the first conductive slurry is mixed and dispersed with the first positive electrode active main material, the first positive electrode binder, and the first conductive agent in a solvent to obtain the first positive electrode slurry; and / or,

[0038] first, the fourth conductive agent and the fourth positive electrode binder are dispersed in a solvent to form a second conductive slurry, and then the second conductive slurry is mixed and dispersed with the second positive electrode active main material, the third positive electrode binder, and the third conductive agent in a solvent to obtain the second positive electrode slurry.

[0039] According to the preparation method described above, in Step S1, the area density of the positive electrode tab determined is 400-800 g / m 2 , preferably 480-630 g / m 2 .

[0040] According to the preparation method described above, in Step S2, the viscosity of each of the first positive electrode slurry and the second positive electrode slurry is 4000-6000 Pa·s, the solid content is 68-75 mass%, the average particle size of the slurry particles is less than 60 μm, and the scratch is less than 40 μm.

[0041] According to the preparation method described above, in Step S3, the coating area density of the first positive electrode slurry is 216-284 g / m 2, the coated face density of the second positive electrode slurry is 264-347 g / m 2 .

[0042] According to the preparation method described above, in step S4, the roll compaction density is 3.28-3.5 g / cm 3 ,

[0043] Preferably, the pressure of the rolling is 45-100T.

[0044] The present application further provides a lithium ion battery comprising the thick positive electrode sheet described above or obtained by the preparation method described above.

[0045] Effects of the application

[0046] The above technical solutions of the present application have the following beneficial effects:

[0047] (1) The present application reduces the friction between particles and particles, particles and binders, and particles and current collectors in the electrode sheet by regulating the molecular weight and crystallinity of the binder in the positive electrode slurry, thereby improving the adhesion of the positive electrode particles to the current collector; and the compaction density of the thick positive electrode sheet is improved by using a two-layer coating structure.

[0048] (2) The present application reduces the friction between the binder and the material particles by regulating the composition of the binder between different coatings in the thick positive electrode sheet, and further optimizes the distribution of the binder in the thick positive electrode sheet by using a double-layer coating preparation process, thereby improving the adhesion of the positive electrode particles to the current collector and the compaction density of the thick positive electrode sheet. BRIEF DESCRIPTION OF DRAWINGS

[0049] Figure 1 A graph showing the sheet resistance of the positive electrode sheets of Examples 1-5 and Comparative Examples 1 and 2.

[0050] Figure 2 A graph showing the peel strength of the positive electrode sheets of Examples 1-5 and Comparative Examples 1 and 2. DETAILED DESCRIPTION

[0051] Various exemplary embodiments, features, and aspects of the present application will be explained in detail below. The word "exemplary" is used herein to mean "serving as an example, instance, or illustration." Any implementation described herein as "exemplary" is not necessarily to be construed as preferred or advantageous over other implementations.

[0052] Unless otherwise stated, the units used in the present specification are international standard units, and the numerical values and numerical ranges appearing in the present application should be understood to include the systematic errors that are inevitable in industrial production.

[0053] In the present specification, the meaning indicated by "may" includes both the meaning of performing the process and the meaning of not performing the process.

[0054] In the present specification, the expressions "some embodiments", "other embodiments", "embodiments", and the like refer to the specific elements (for example, features, structures, properties, and / or characteristics) described in relation to the embodiments and can or can not be present in other embodiments. In addition, it should be understood that the described elements can be combined in various embodiments in any suitable manner.

[0055] In the present specification, the numerical range indicated by "numerical value A to numerical value B" refers to a range including the end point values A and B.

[0056] In the present specification, unless otherwise specified, "%" means mass percent.

[0057] First aspect

[0058] The first aspect of the present application provides a high-compaction thick positive electrode tab. The high-compaction thick positive electrode tab of the present application includes a current collector, a first positive electrode coating layer on the current collector, and a second positive electrode coating layer on the first positive electrode coating layer.

[0059] The first positive electrode coating layer is formed of a first positive electrode slurry including a first positive electrode active main material, a first positive electrode binder, a first conductive agent, a second conductive agent, and a second positive electrode binder.

[0060] The second positive electrode coating layer is formed of a second positive electrode slurry including a second positive electrode active main material, a third positive electrode binder, a third conductive agent, a fourth conductive agent, and a fourth positive electrode binder.

[0061] In the present application, the molecular weight of each of the first positive electrode binder, the second positive electrode binder, and the third positive electrode binder is greater than 1 million, and the crystallinity is greater than 45%. The molecular weight and the crystallinity of the first positive electrode binder, the second positive electrode binder, and the third positive electrode binder can each be the same or different.

[0062] Specifically, the molecular weight of the first positive electrode binder, the second positive electrode binder, and the third positive electrode binder can each independently be greater than 1 million and less than or equal to 1.5 million, for example, can be 1.01 million, 1.05 million, 1.1 million, 1.15 million, 1.2 million, 1.25 million, 1.3 million, 1.35 million, 1.4 million, 1.45 million, 1.5 million, etc. The crystallinity of the first positive electrode binder, the second positive electrode binder, and the third positive electrode binder can each independently be greater than 45% and less than or equal to 65%, for example, can be 46%, 47%, 48%, 49%, 50%, 52%, 55%, 60%, 65%, etc.

[0063] In the present application, the molecular weight of the fourth positive electrode binder is less than 0.7 million, and the crystallinity is less than 40%. For example, the molecular weight of the fourth positive electrode binder can be less than 0.7 million but greater than or equal to 0.3 million, for example, can be 0.69 million, 0.65 million, 0.6 million, 0.55 million, 0.5 million, 0.45 million, 0.4 million, 0.35 million, 0.3 million, etc. The crystallinity of the fourth positive electrode binder can be less than 40% but greater than or equal to 25%, for example, can be 39%, 38%, 37%, 36%, 35%, 34%, 33%, 32%, 31%, 30%, 29%, 28%, 27%, 26%, etc.

[0064] In the high-compact-thickness positive electrode tab of the present application, by using a binder with a larger molecular weight and a larger crystallinity in the first positive electrode coating, and a binder with a smaller molecular weight and a smaller crystallinity in the second positive electrode coating, the friction between the particles and the particles, the particles and the binder, and the particles and the current collector in the tab can be reduced, the relative slip during the rolling process can be generated, the compactness of the thick electrode tab can be improved, and the rolling effect of the tab can be ensured.

[0065] In a specific embodiment of the present application, the first positive electrode binder, the second positive electrode binder, the third positive electrode binder, and the fourth positive electrode binder can each independently be selected from one or more of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), and an oily acrylic homopolymer (oily polyacrylic acid PAA). The first positive electrode binder, the second positive electrode binder, the third positive electrode binder, and the fourth positive electrode binder can each be the same or different, and preferably they are the same binder substance.

[0066] In the present application, the first conductive agent, the second conductive agent, the third conductive agent, and the fourth conductive agent can each independently be selected from one or more of an oily single-walled carbon nanotube, an oily multi-walled carbon nanotube, a conductive carbon fiber, acetylene black, natural graphite, artificial graphite, graphene, and carbon black, and each can be the same or different.

[0067] In some preferred embodiments, the first and third conductive agents can be selected from one or more of conductive carbon fibers, acetylene black, natural graphite, artificial graphite, graphene, carbon black, the second conductive agent can be oily single-walled carbon nanotubes, and the fourth conductive agent can be oily multi-walled carbon nanotubes. By including single-walled carbon nanotubes in the first positive electrode coating layer and multi-walled carbon nanotubes in the second positive electrode coating layer, good rate performance can be ensured. In addition, by using a first conductive agent and a second conductive agent with different conductive capabilities in combination in the first positive electrode coating layer, and by using a third conductive agent and a fourth conductive agent with different conductive capabilities in combination in the second positive electrode coating layer, the conductive performance of the coating layer can be improved while ensuring good rate performance, and costs can be reduced.

[0068] In the present application, the first and second positive electrode active main materials can each independently be selected from one or more of lithium iron phosphate, lithium cobaltate, lithium iron manganese phosphate, lithium manganese oxide, lithium nickel cobalt manganese oxide. The first and second positive electrode active main materials can be the same or different.

[0069] In addition, the first positive electrode slurry of the present application can also optionally include a first positive electrode active material additive, and the second positive electrode slurry can also optionally include a second positive electrode active material additive. In the present application, the positive electrode active material additive is mainly a lithium supplement agent. Specifically, the first and second positive electrode active material additives can each independently be selected from one or more of lithium sulfide (Li2S), lithium nickelate (Li2NiO2), lithium ferrite (Li5FeO4, LFO), lithium oxide (Li2O), lithium peroxide (Li2O2). The first and second positive electrode active material additives can be the same or different. By adding the above additives, the loss of lithium in the positive electrode material can be compensated. The amount of the first and second positive electrode active material additives added is each selected from 0.5-2.5 mass%, for example, can be 0.5 mass%, 0.8 mass%, 1.0 mass%, 1.2 mass%, 1.5 mass%, 1.8 mass%, 2.0 mass%, 2.2 mass%, 2.5 mass%, etc.

[0070] In the present application, the proportions of the materials used in the first and second positive electrode coating layers are regulated. Specifically, the weight proportions of the materials used in the first positive electrode coating layer can be: 96.5-97.8 mass% of the first positive electrode active main material, 1.0-2.3 mass% of the first positive electrode binder, 0.2-0.8 mass% of the first conductive agent, 0.0005-0.024 mass% of the second conductive agent, 0.00075-0.032 mass% of the second positive electrode binder.

[0071] In some preferred embodiments, the weight proportions of the materials used in the first positive electrode coating layer can be: 96.8-97.3 mass% of the first positive electrode active main material, 1.0-1.5 mass% of the first positive electrode binder, 0.3-0.6 mass% of the first conductive agent, 0.001-0.009 mass% of the second conductive agent, 0.0015-0.012 mass% of the second positive electrode binder.

[0072] In the present application, the weight proportions of the materials used in the second positive electrode coating layer can be: 97-97.8 mass% of the second positive electrode active main material, 0.8-1.3 mass% of the third positive electrode binder, 0.5-1.0 mass% of the third conductive agent, 0.00075-0.0175 mass% of the fourth conductive agent, 0.00015-0.04 mass% of the fourth positive electrode binder.

[0073] In the present application, the thickness of the first positive electrode coating layer can be 70-110 μm, the thickness of the second positive electrode coating layer can be 90-130 μm, and the thickness of the high-pressing-thick positive electrode sheet can be 160-240 μm. For example, the thickness of the first positive electrode coating layer can be 70 μm, 80 μm, 90 μm, 100 μm, 110 μm, etc., the thickness of the second positive electrode coating layer can be 90 μm, 100 μm, 110 μm, 120 μm, 130 μm, etc., and the thickness of the high-pressing-thick positive electrode sheet can be 160 μm, 170 μm, 180 μm, 190 μm, 200 μm, 210 μm, 220 μm, 230 μm, 240 μm, etc. The high-pressing-thick positive electrode sheet of the present application can have a pressing density of 3.28-3.5 g / cm 3 .

[0074] The current collector used in the positive electrode sheet of the present application is not particularly limited, and a current collector conventionally used in the art can be used, for example, can be an aluminum foil, etc.

[0075] Second aspect

[0076] The second aspect of the present application provides a method for preparing the high-pressing-thick positive electrode sheet of the first aspect described above.

[0077] The method for preparing the high-pressing-thick positive electrode sheet of the present application comprises the following steps:

[0078] Step S1: determining the area density of the required positive electrode sheet;

[0079] Step S2: preparing a first positive electrode slurry and a second positive electrode slurry,

[0080] wherein the first positive electrode slurry is formed by dispersing a first positive electrode active main material, a first positive electrode binder, a first conductive agent, a second conductive agent, and a second positive electrode binder in a solvent,

[0081] The second positive electrode slurry is formed by dispersing a second positive electrode active main material, a third positive electrode binder, a third conductive agent, a fourth conductive agent, and a fourth positive electrode binder in a solvent,

[0082] The second positive electrode binder has a molecular weight greater than 1 million and a crystallinity greater than 45%,

[0083] The fourth positive electrode binder has a molecular weight less than 700 thousand and a crystallinity less than 40%;

[0084] Step S3: The first positive electrode slurry is coated on the current collector according to the area density of the positive electrode tab determined in step S1 to form a first positive electrode coating layer, the second positive electrode slurry is coated on the first positive electrode coating layer to form a second positive electrode coating layer, and then drying is performed to obtain a tab;

[0085] Step S4: The tab obtained in step S3 is rolled to obtain a high-compactness thick positive electrode tab.

[0086] The following describes each step in detail.

[0087] Step S1

[0088] In step S1, the area density parameters of the positive electrode tab required for output are calculated through cell design. In some specific embodiments, the area density of the positive electrode tab can range from 400 to 800 g / m 2 , preferably from 480 to 630 g / m 2 . For example, the area density of the positive electrode tab can be 420 g / m 2 , 450 g / m 2 , 500 g / m 2 , 520 g / m 2 , 550 g / m 2 , 580 g / m 2 , 600 g / m 2 , 620 g / m 2 , etc.

[0089] Step S2

[0090] In step S2, the first positive electrode slurry and the second positive electrode slurry are prepared.

[0091] The types, proportions, molecular weights, crystallinities, etc. of the materials used in the first positive electrode slurry and the second positive electrode slurry in step S2 are the same as those in the first aspect described above, and will not be described again here. As described above in the first aspect, the second positive electrode binder has a molecular weight greater than 1 million and a crystallinity greater than 45%, and the fourth positive electrode binder has a molecular weight less than 700 thousand and a crystallinity less than 40%.

[0092] In the present application, the first positive electrode slurry can be formed by dispersing the first positive electrode active main material, the first positive electrode binder, the first conductive agent, the second conductive agent, and the second positive electrode binder in a solvent.

[0093] In some preferred embodiments, the second conductive agent and the second positive electrode binder can be first dispersed in a solvent to form a first conductive slurry, and then the first conductive slurry can be mixed and dispersed in a solvent with the first positive electrode active main material, the first positive electrode binder, and the first conductive agent to obtain the first positive electrode slurry.

[0094] In the first conductive slurry of the present application, the content of the second conductive agent can be about 1-3% by mass of the total amount of the first conductive slurry, and the content of the second positive electrode binder can be about 1.5-4% by mass of the total amount of the first conductive slurry, with the balance being a solvent. The viscosity of the first conductive slurry and the second conductive slurry can each be less than 3000 Pa·s, for example, 2900 Pa·s, 2700 Pa·s, 2500 Pa·s, 2200 Pa·s, 2000 Pa·s, 1800 Pa·s, 1500 Pa·s, etc.

[0095] In the present application, the second positive electrode slurry can be formed by dispersing the second positive electrode active main material, the third positive electrode binder, the third conductive agent, the fourth conductive agent, and the fourth positive electrode binder in a solvent.

[0096] In some preferred embodiments, the fourth conductive agent and the fourth positive electrode binder can be first dispersed in a solvent to form a second conductive slurry, and then the second conductive slurry can be mixed and dispersed in a solvent with the second positive electrode active main material, the third positive electrode binder, and the third conductive agent to obtain the second positive electrode slurry.

[0097] In the second conductive slurry of the present application, the content of the fourth conductive agent can be about 1.5-3.5% by mass of the total amount of the second conductive slurry, and the content of the fourth positive electrode binder can be about 0.3-8% by mass of the total amount of the second conductive slurry, with the balance being a solvent.

[0098] By first preparing the first conductive slurry and / or the second conductive slurry, the second conductive agent and / or the fourth conductive agent can be better dispersed in the positive electrode slurry.

[0099] In step S2, the solvent for the first positive electrode slurry, the second positive electrode slurry, the first conductive slurry, and the second conductive slurry is not particularly limited and can be a commonly used solvent in the art, for example, N-methyl pyrrolidone (NMP), butylene glycol methyl ether, dimethyl sulfoxide, etc. The solvent for the first positive electrode slurry, the second positive electrode slurry, the first conductive slurry, and the second conductive slurry can be the same or different, and is preferably the same.

[0100] In a specific embodiment, the first positive electrode active main material, the first positive electrode binder, the first conductive agent, the second conductive agent, and the second positive electrode binder (or the second conductive agent and the second positive electrode binder are replaced by the first conductive paste), and the optional first positive electrode active material additive are mixed in a certain proportion, and then a certain amount of solvent is added for dispersion, so as to prepare the first positive electrode coating; the second positive electrode active main material, the third positive electrode binder, the third conductive agent, the fourth conductive agent, and the fourth positive electrode binder (or the fourth conductive agent and the fourth positive electrode binder are replaced by the second conductive paste), and the optional second positive electrode active material additive are mixed in a certain proportion, and then a certain amount of solvent is added for dispersion, so as to prepare the second positive electrode paste.

[0101] In the present application, the viscosity of the first positive electrode paste and the second positive electrode paste can be 4000-6000 Pa·s, the solid content can be 68-75 mass%, the average particle size of the paste particles is less than 60 μm, and the scratch is less than 40 μm.

[0102] Specifically, the viscosity of the first positive electrode paste and the second positive electrode paste can be 4200 Pa·s, 4500 Pa·s, 4800 Pa·s, 5000 Pa·s, 5200 Pa·s, 5500 Pa·s, 5800 Pa·s, etc. The solid content of the first positive electrode paste and the second positive electrode paste can be 69 mass%, 70 mass%, 71 mass%, 72 mass%, 73 mass%, 74 mass%, etc. The average particle size of the paste particles can be 59 μm, 55 μm, 50 μm, 45 μm, 40 μm, 35 μm, 30 μm, 25 μm, 20 μm, 15 μm, 10 μm, 5 μm, 1 μm, etc. The scratch of the first positive electrode paste and the second positive electrode paste can be 39 μm, 35 μm, 30 μm, 25 μm, 20 μm, 15 μm, 10 μm, etc.

[0103] Step S3

[0104] In the present application, the coating of the paste is carried out in a two-layer coating manner, so as to further optimize the distribution of the binder in the thick electrode plate, not only improve the adhesion of the positive electrode particles and the current collector, but also control the composition of the binder between different coating layers through the paste, and further reduce the friction between the positive electrode material particles and the binder to improve the compaction density of the thick electrode plate.

[0105] Specifically, according to the area density determined in step S1, the first positive electrode paste is coated on one surface of the current collector to form a first positive electrode coating layer; the second positive electrode paste is coated on the surface of the first positive electrode coating layer to form a second positive electrode coating layer, and then dried, so as to obtain a positive electrode plate.

[0106] In the present application, the coating surface density of the first positive electrode slurry can be 216-284 g / m 2 , for example, can be 220 g / m 2 , 230 g / m 2 , 240 g / m 2 , 250 g / m 2 , 260 g / m 2 , 270 g / m 2 , 275 g / m 2 , 280 g / m 2 , etc. The coating surface density of the second positive electrode slurry can be 264-347 g / m 2 , for example, can be 270 g / m 2 , 280 g / m 2 , 290 g / m 2 , 300 g / m 2 , 310 g / m 2 , 320 g / m 2 , 330 g / m 2 , 340 g / m 2 , 345 g / m 2 , etc. The sum of the coating surface density of the first positive electrode slurry and the coating surface density of the second positive electrode slurry is the actual surface density of the positive electrode sheet.

[0107] In the present application, the temperature and time for drying after coating the first positive electrode slurry and the second positive electrode slurry are not particularly limited, for example, the temperature can be 60-120℃, and the time can be 20-40 min.

[0108] Step S4

[0109] In step S4, the sheet obtained in step S3 is rolled to obtain a high-compaction thick positive electrode sheet. The rolling compaction density in step S4 can be 3.28-3.5 g / cm 3 . The thickness of the finally obtained high-compaction thick positive electrode sheet can be 160-240 μm, and the rolling pressure can be 45-100 T.

[0110] Third aspect

[0111] A third aspect of the present application provides a lithium ion battery comprising the thick positive electrode sheet described above or obtained by the preparation method described above.

[0112] Examples

[0113] The embodiments of the present application will be described in detail by way of examples as follows, but those skilled in the art will understand that the following examples are only for illustration of the present application and should not be regarded as limiting the scope of the present application. The specific conditions not noted in the examples are carried out according to the conventional conditions or the conditions recommended by the manufacturer. The reagents or instruments used are not noted by the manufacturer, which are all conventional products that can be obtained by purchase.

[0114] Preparation of the slurry employed in Examples 1-5

[0115] Preparation of the first conductive slurry: the oily single-walled carbon nanotube CNTs as the second conductive agent and the PVDF as the second positive electrode binder are dry powder mixed at a weight ratio of 1:6, then the solvent NMP is added for high-speed dispersion, and the preparation of the conductive slurry is completed when the viscosity is 2600 mPa·s and the solid content is 1.2 mass%.

[0116] Preparation of the first positive electrode slurry: the first positive electrode active material NCM811, the first positive electrode binder PVDF, the first conductive agent acetylene black, the first conductive slurry, and the first positive electrode active material additive lithium ferrite are dry powder mixed at a ratio of 97 mass%, 1.2 mass%, 0.3 mass%, 0.6 mass%, and 1.35 mass%, respectively, the solvent NMP is added for further mixing to prepare the slurry, and the viscosity of the slurry is 5500 mPa·s.

[0117] After the preparation of the slurry, a 200-mesh sieve is selected for sieving the slurry, and the viscosity of the sieved slurry is tested by a viscometer using a 3# rotor at 12 rpm.

[0118] In addition, the particle and scratch tests are carried out as follows: the slurry is dropped into the deepest part of the doctor blade fineness meter groove, and the doctor blade is vertically contacted to the flat plate within 1-2 s to complete the scraping of the slurry from the maximum scale to the minimum scale, and within 5 s after scraping, the uniform particle exposure is observed along the incident angle of 20-30°, and if there are 5-10 particles in a 3-mm strip of the slurry, it is recorded as the maximum fineness of the slurry. After the parameters of the slurry are qualified, it is stored slowly under vacuum.

[0119] Preparation of the second conductive slurry: the oily multi-walled carbon nanotube as the fourth conductive agent and the PVDF as the fourth positive electrode binder are dry powder mixed at a weight ratio of 2:3, then the solvent NMP is added for high-speed dispersion, and the preparation of the conductive slurry is completed when the viscosity is 2300 mPa·s and the solid content is 4.6 mass%.

[0120] Preparation of the second positive electrode slurry: the second positive electrode active material NCM622, the third positive electrode binder PVDF, the third conductive agent conductive carbon black, the second conductive slurry, and the second positive electrode active material additive lithium ferrite were dry powder mixed in a ratio of 97.3 mass%, 1.0 mass%, 0.6 mass%, 0.1 mass%, and 1.0 mass%, respectively, and a solvent NMP was added for further mixing to prepare a slurry, and the viscosity of the slurry was 5000 mPa·s. The viscosity, particle, and scratch tests were the same as those of the first positive electrode slurry described above, and the slurry parameters were qualified before vacuum slow storage.

[0121] The particle and scratch test results of the first slurry and the second slurry prepared in Comparative Example 1, Comparative Example 2, and Examples 1-5 are shown in Table 1.

[0122] Table 1

[0123]

[0124]

[0125] The two slurries were coated in a double-layer coating manner, wherein the first coating area was the first positive electrode slurry, and the first positive electrode slurry coating area density was 246 g / m 2 ; the second coating area was coated on the first coating area, and the slurry was the second positive electrode slurry, and the second positive electrode slurry coating area density was 284 g / m 2 .

[0126] Examples 1-5 were all prepared in a double-layer coating manner, wherein the component formula of the first positive electrode slurry and the second positive electrode slurry remained unchanged, the molecular weight and crystallinity of the first positive electrode binder PVDF in the first positive electrode slurry and the second positive electrode binder PVDF in the first conductive slurry are listed in Table 2, and the molecular weight and crystallinity of the third positive electrode binder PVDF in the second positive electrode slurry and the fourth positive electrode binder PVDF in the second conductive slurry are listed in Table 3.

[0127] Comparative Example 1 was single-layer coated with the first positive electrode slurry described above, and the coating area density was 530 g / m 2 .

[0128] Comparative Example 2 was coated in a double-layer coating manner, wherein the first coating area was the second positive electrode slurry described above, and the coating area density was 246 g / m 2 ; the second coating area was coated on the first coating area, and the second coating area was the first positive electrode slurry described above, and the coating area density was 284 g / m 2 .

[0129] Table 2: The molecular weight and crystallinity of the first positive electrode binder and the second positive electrode binder PVDF in the first conductive slurry in the first positive electrode slurry are as follows:

[0130]

[0131]

[0132] Table 3: The molecular weight and crystallinity of PVDF in the third positive electrode binder in the second positive electrode slurry and the fourth positive electrode binder in the second conductive slurry are as follows:

[0133]

[0134] Examples 1-5 and Comparative Examples 1 and 2 were prepared according to the above requirements, including a first positive electrode slurry and a second positive electrode slurry. Electrode coating was performed according to step S4. After drying the first positive electrode slurry at 90°C, the second positive electrode slurry was coated, and then dried at 90°C for 30 minutes to obtain the electrode. The electrode sheets from the examples and comparative examples were rolled under a rolling pressure of 60T to obtain good electrode sheets. All electrode sheets were prepared according to a ratio of 3.45 g / cm³. 3 The electrode sheet is designed and compacted using roller pressing. The electrode sheet thickness and rolling condition are shown in Table 4.

[0135] Table 4

[0136]

[0137]

[0138] By calculating the electrode state and actual compaction density of different embodiments and comparative examples, it can be seen that by controlling the molecular weight and crystallinity of the binder in the slurry and adopting a double-layer coating method, the compaction density of the electrode is effectively improved. This further demonstrates that the thick positive electrode design of the present invention has a significant impact on the compaction of the thick positive electrode. It also shows that different binder combinations do indeed reduce the friction between particles, particles and binder, and particles and current collector in the electrode, resulting in relative slippage during the rolling process, improving the compaction of the thick positive electrode, and ensuring the rolling effect of the electrode.

[0139] In addition, the film resistance and peel strength of the electrode sheets in the embodiments and comparative examples were also tested, and the results are as follows: Figure 1 and Figure 2 As shown.

[0140] according to Figure 1 The diaphragm resistance shown is Figure 2The data of the peel strength shown indicate that the thick positive electrode plate designed in the application has higher compaction density, and the reduction of the compaction density reduces the film resistance of the electrode plate, because when the compaction density is increased, the contact between the electrode plate particles is more close, the electron passage between the electrode plate particles and the conductive agent is reduced, and the film resistance of the electrode plate is reduced; meanwhile, when the compaction density of the electrode plate is increased, the collocation of the binder in the application improves the distribution of the binder, and finally improves the peel strength of the electrode plate.

[0141] The skilled in the art can easily understand the technical solutions provided by the application according to the above description and guidance, and the skilled in the art will further make relevant expansion and improvement on the above embodiments. Therefore, the application is not limited to the above specific embodiments, and the skilled in the art should understand that any obvious improvement, replacement or modification made on the basis of the application all belongs to the protection scope of the application. In addition, although some specific and restrictive terms are used in the specification, these terms are only used to further illustrate the technical solutions of the application, and do not constitute any limitation on the application.

Claims

1. A high-voltage, thick positive electrode sheet, characterized in that, The high-pressure, thick positive electrode sheet includes a current collector, a first positive electrode coating on the current collector, and a second positive electrode coating on the first positive electrode coating. The first positive electrode coating is formed from a first positive electrode slurry, which includes a first positive electrode active material, a first positive electrode binder, a first conductive agent, a second conductive agent, and a second positive electrode binder. The second positive electrode coating is formed from a second positive electrode slurry, which includes a second positive electrode active material, a third positive electrode binder, a third conductive agent, a fourth conductive agent, and a fourth positive electrode binder. The second positive electrode binder has a molecular weight greater than 1 million and a crystallinity greater than 45%. The fourth positive electrode binder has a molecular weight of less than 700,000 and a crystallinity of less than 40%. The first positive electrode binder and the third positive electrode binder each have a molecular weight greater than 1 million and a crystallinity greater than 45%.

2. The high-pressure solid-thickness positive electrode sheet according to claim 1, wherein the molecular weight and crystallinity of the first positive electrode binder, the second positive electrode binder and the third positive electrode binder are the same or different.

3. The high-pressure solid-thickness positive electrode sheet according to claim 2, wherein the first positive electrode binder, the second positive electrode binder, the third positive electrode binder and the fourth positive electrode binder are each independently selected from one or more of polyvinylidene fluoride, polytetrafluoroethylene, and oily acrylic homopolymer.

4. The high-pressure solid-thickness positive electrode sheet according to any one of claims 1-3, wherein the first conductive agent, the second conductive agent, the third conductive agent and the fourth conductive agent are each independently selected from one or more of oily single-walled carbon nanotubes, oily multi-walled carbon nanotubes, conductive carbon fibers, graphite, graphene and carbon black, and they may be the same or different.

5. The high-pressure solid-thickness positive electrode sheet according to any one of claims 1-3, wherein the first conductive agent and the third conductive agent are selected from one or more of conductive carbon fiber, graphite, graphene, and carbon black, the second conductive agent is an oily single-walled carbon nanotube, and the fourth conductive agent is an oily multi-walled carbon nanotube.

6. The high-voltage solid-thickness positive electrode sheet according to any one of claims 1-3, wherein the first positive electrode active material and the second positive electrode active material are each independently selected from one or more of lithium iron phosphate, lithium cobalt oxide, lithium manganese iron phosphate, lithium manganese oxide, and lithium nickel cobalt manganese oxide; The first positive electrode active material may be the same as or different from the second positive electrode active material.

7. The high-pressure solid-thickness positive electrode sheet according to any one of claims 1-3, wherein the first positive electrode slurry further comprises a first positive electrode active material additive, and / or, the second positive electrode slurry further comprises a second positive electrode active material additive. The first positive electrode active material additive and the second positive electrode active material additive are each independently selected from one or more of Li2S, Li2NiO2, Li5FeO4, Li2O, and Li2O2; The first positive electrode active material additive may be the same as or different from the second positive electrode active material additive.

8. The high-pressure solid-thickness positive electrode sheet according to claim 7, wherein the amount of the first positive electrode active material additive and the second positive electrode active material additive added is selected from 0.5-2.5 by mass.

9. The high-pressure solid-thickness positive electrode sheet according to any one of claims 1-3, wherein the weight ratio of each material in the first positive electrode coating is: 96.5-97.8% by mass of the first positive electrode active material, 1.0-2.3% by mass of the first positive electrode binder, 0.2-0.8% by mass of the first conductive agent, 0.0005-0.024% by mass of the second conductive agent, and 0.00075-0.032% by mass of the second positive electrode binder.

10. The high-pressure solid-thickness positive electrode sheet according to any one of claims 1-3, wherein the weight ratio of each material in the first positive electrode coating is: 96.8-97.3% by mass of the first positive electrode active material, 1.0-1.5% by mass of the first positive electrode binder, 0.3-0.6% by mass of the first conductive agent, 0.001-0.009% by mass of the second conductive agent, and 0.0015-0.012% by mass of the second positive electrode binder.

11. The high-pressure solid-thickness positive electrode sheet according to any one of claims 1-3, wherein the weight ratio of each material in the second positive electrode coating is: 97-97.8% by mass of the second positive electrode active material, 0.8-1.3% by mass of the third positive electrode binder, 0.5-1.0% by mass of the third conductive agent, 0.00075-0.0175% by mass of the fourth conductive agent, and 0.00015-0.04% by mass of the fourth positive electrode binder.

12. The high-compacted thick positive electrode sheet according to any one of claims 1-3, wherein the thickness of the thick positive electrode sheet is 160-240 μm; and the compaction density of the thick positive electrode sheet is 3.28-3.5 g / cm³. 3 .

13. A method for preparing a high-pressure, thick positive electrode sheet, characterized in that, The preparation method includes the following steps: Step S1: Determine the required areal density of the positive electrode sheet; Step S2: Prepare the first positive electrode slurry and the second positive electrode slurry. The first positive electrode slurry is formed by dispersing the first positive electrode active material, the first positive electrode binder, the first conductive agent, the second conductive agent, and the second positive electrode binder in a solvent. The second positive electrode slurry is formed by dispersing the second positive electrode active material, the third positive electrode binder, the third conductive agent, the fourth conductive agent, and the fourth positive electrode binder in a solvent. The second positive electrode binder has a molecular weight greater than 1 million and a crystallinity greater than 45%. The fourth positive electrode binder has a molecular weight of less than 700,000 and a crystallinity of less than 40%. The first positive electrode binder and the third positive electrode binder each have a molecular weight greater than 1 million and a crystallinity greater than 45%. Step S3: Based on the areal density of the positive electrode sheet determined in step S1, the first positive electrode slurry is coated onto the current collector to form a first positive electrode coating. The second positive electrode slurry is then coated onto the first positive electrode coating to form a second positive electrode coating. Finally, the coating is dried to obtain the electrode sheet. Step S4: Roll the electrode sheet obtained in step S3 to obtain a high-pressure, thick positive electrode sheet.

14. The preparation method according to claim 13, wherein in step S2, the second conductive agent and the second positive electrode binder are first dispersed in a solvent to form a first conductive slurry, and then the first conductive slurry is mixed with the first positive electrode active material, the first positive electrode binder, and the first conductive agent and dispersed in a solvent to obtain the first positive electrode slurry; and / or, First, the fourth conductive agent and the fourth positive electrode binder are dispersed in a solvent to form a second conductive slurry. Then, the second conductive slurry is mixed with the second positive electrode active material, the third positive electrode binder and the third conductive agent and dispersed in a solvent to obtain the second positive electrode slurry.

15. The preparation method according to claim 13 or 14, wherein the areal density of the positive electrode sheet determined in step S1 is 400-800 g / m². 2 .

16. The preparation method according to claim 13 or 14, wherein the areal density of the positive electrode sheet determined in step S1 is 480-630 g / m². 2 .

17. The preparation method according to claim 13 or 14, wherein in step S2, the viscosity of the first positive electrode slurry and the second positive electrode slurry are 4000-6000 Pa•s, the solid content is 68-75% by mass%, the average particle size of the slurry particles is less than 60 μm, and the scratches are less than 40 μm.

18. The preparation method according to claim 13 or 14, wherein in step S3, the coating surface density of the first positive electrode slurry is 216-284 g / m³. 2 The surface density of the second positive electrode slurry is 264-347 g / m³. 2 .

19. The preparation method according to claim 13 or 14, wherein in step S4, the compaction density by roller pressing is 3.28-3.5 g / cm³. 3 ; The pressure of the roller is 45-100T.

20. A lithium-ion battery, characterized in that, It includes the thick positive electrode sheet as described in any one of claims 1-12 or the thick positive electrode sheet obtained by the preparation method described in any one of claims 13-19.

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