Negative pole piece and preparation method thereof

Abstract

The invention provides a negative electrode plate and a preparation method thereof, and the preparation method specifically comprises the following steps: (1) providing first negative electrode slurry containing a first binder and second negative electrode slurry containing a second binder; (2) coating one side or two sides of a current collector with the first negative electrode slurry, and coating the first negative electrode slurry with the second negative electrode slurry; (3) drying and rolling to obtain a negative pole piece; the swelling ratio of the first binder is S1, the swelling ratio of the second binder is S2, and 1.5 < = S2 / S1 < = 4. According to the invention, the performance of the battery cell is improved by reducing the adverse effect of overlarge compaction of the surface of the pole piece on electrolyte infiltration. According to the method for preparing the negative pole piece, new equipment and production procedures do not need to be added, the method is compatible with an existing process technology, and the effect is obviously improved.

Description

Technical Field

[0001] The present invention belongs to the field of lithium-ion batteries, and in particular relates to a negative electrode sheet and a preparation method thereof. Background Art

[0002] Lithium-ion batteries (LIBs) have the advantages of high energy density, high operating voltage, long cycle life, low environmental pollution, low self-discharge, and no memory effect. They are considered to be one of the secondary batteries with the greatest development prospects. They are currently widely used in mobile phones, notebooks, cameras, portable tools, electric vehicles, energy storage, etc. With the rapid development of electric vehicles and energy storage, the demand for lithium-ion batteries is even more vigorous.

[0003] In the battery manufacturing process, the negative electrode sheet needs to be rolled before use. The thickness direction of the sheet is subjected to different forces during the rolling process. The pressure on the surface is greater than that on the bottom, resulting in a decrease in the compaction gradient from the surface to the bottom after rolling. High compaction is not conducive to the infiltration of electrolytes and the transmission of ions. The high surface compaction layer hinders the performance of the battery cell, especially in a high coating amount and high compaction design system.

[0004] To solve this problem, the industry often uses pole piece surface structure optimization technology to destroy the high compaction layer on the pole piece surface, thereby improving the pole piece dynamics and improving the performance of the battery cell. The pole piece surface etching method uses laser etching technology to etch grooves or drill blind holes on the pole piece surface to destroy the surface structure, improve electrolyte infiltration, and improve the ion transmission path. The secondary rolling method uses a secondary rolling process to make the pore gradient distribution in the thickness direction uniform, avoiding a large compaction gradient difference in the thickness direction and high surface compaction. The corona and plasma methods treat the pole piece surface through corona or plasma to remove foreign matter and some materials on the pole piece surface, which plays a role in loosening the pole piece surface structure and thereby improving electrolyte infiltration. The negative electrode embossing method embosses the pole piece before or after rolling to form some pits on the pole piece surface, which plays a certain role in retaining liquid and improving electrolyte infiltration. However, the above methods all require the addition of new equipment, production processes, and manufacturing costs. During the etching, corona and plasma treatment processes, there will be some loss of active substances, which will change the original NP ratio design of the battery cell and have a certain risk of lithium precipitation. At the same time, dust will be generated, which puts higher requirements on the dust removal capability of the equipment and process quality control. Summary of the invention

[0005] The object of the present invention is to provide a negative electrode plate and a method for preparing the same.

[0006] A first aspect of the present invention provides a method for preparing a negative electrode sheet, comprising the steps of:

[0007] (1) providing a first negative electrode slurry containing a first binder and a second negative electrode slurry containing a second binder; wherein the swelling rate of the first binder is S1, the swelling rate of the second binder is S2, and 1.5≤S2 / S1≤4 is satisfied;

[0008] (2) coating the first negative electrode slurry on one side or both sides of the current collector, and coating the second negative electrode slurry on the first negative electrode slurry;

[0009] (3) Drying and rolling to obtain the negative electrode sheet.

[0010] In one or more embodiments, the swelling ratio S1 of the first binder satisfies 30%≤S1≤50%; and / or the swelling ratio S2 of the second binder satisfies 60%≤S2≤120%.

[0011] In one or more embodiments, the first binder and the second binder are each independently selected from one or more of styrene-butadiene rubber, styrene-propylene rubber and polyacrylic acid.

[0012] In one or more embodiments, the mass fraction of the first binder in the first negative electrode slurry is 1-2.5%; and / or the mass fraction of the second binder in the second negative electrode slurry is 0.5-2%.

[0013] In one or more embodiments, the current collector is a copper foil.

[0014] In one or more embodiments, the first negative electrode slurry and the second negative electrode slurry each further include active material powder, a conductive agent and other binders.

[0015] In one or more embodiments, the coating area density CW1 of the first negative electrode slurry is 1-27 mg / cm 2 The coating surface density CW2 of the second negative electrode slurry is 1-3 mg / cm 2 .

[0016] In one or more embodiments, the coating area density of the first negative electrode slurry is CW1, and the coating area density of the second negative electrode slurry is CW2, satisfying 1≤CW1 / CW2≤9.

[0017] In one or more embodiments, step (2) is coated by one or more of extrusion coating, transfer coating, spray coating, electrospinning or double-layer coating processes.

[0018] The second aspect of the present invention provides a negative electrode plate, which includes a current collector and a negative electrode material layer arranged on one side or both sides of the current collector, and the negative electrode material layer includes a first layer body arranged on the current collector and a second layer body arranged on the first layer body; wherein the first layer body contains a first binder, and the second layer body contains a second binder, the swelling rate of the first binder is S1, and the swelling rate of the second binder is S2, satisfying 1.5≤S2 / S1≤4; preferably, the first binder and the second binder are as described in the first aspect of the present invention.

[0019] In one or more embodiments, the current collector is a copper foil.

[0020] In one or more embodiments, the first layer body includes a first negative electrode slurry including the first binder, and the second layer body includes a second negative electrode slurry including the second binder.

[0021] In one or more embodiments, the coating area density CW1 of the first negative electrode slurry is 1-27 mg / cm 2 The coating surface density CW2 of the second negative electrode slurry is 1-3 mg / cm 2 .

[0022] In one or more embodiments, the coating area density of the first negative electrode slurry is CW1, and the coating area density of the second negative electrode slurry is CW2, satisfying 1≤CW1 / CW2≤9.

[0023] A third aspect of the present invention provides a battery, the battery comprising the negative electrode plate described in the second aspect of the present invention; preferably, the battery is a lithium-ion battery.

[0024] The fourth aspect of the present invention provides the use of a first binder with a swelling rate of S1 and a second binder with a swelling rate of S2 in preparing a negative electrode plate, or in reducing the battery ion diffusion impedance, or in improving the dynamic performance of the plate, wherein 1.5≤S2 / S1≤4. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 is a cross-sectional view of a first layer body and a second layer body disposed on one side of a current collector in some embodiments of the present invention. DETAILED DESCRIPTION

[0026] In order to enable those skilled in the art to understand the characteristics and effects of the present invention, the following is a general description and definition of the terms and expressions mentioned in the specification and claims. Unless otherwise specified, all technical and scientific terms used in the text are the common meanings understood by those skilled in the art for the present invention. In the event of a conflict, the definition in this specification shall prevail.

[0027] The theories or mechanisms described and disclosed herein, whether correct or incorrect, should not limit the scope of the present invention in any way, that is, the present invention can be implemented without being limited by any specific theory or mechanism.

[0028] Herein, “comprising”, “including”, “containing” and similar terms encompass the meanings of “consisting essentially of” and “consisting of”. For example, when “A comprises B and C” is disclosed herein, “A consists essentially of B and C” and “A consists of B and C” should be deemed to be disclosed herein.

[0029] In this article, all features such as values, quantities, contents and concentrations defined in the form of numerical ranges or percentage ranges are only for brevity and convenience. Accordingly, the description of numerical ranges or percentage ranges should be considered to have included and specifically disclosed all possible secondary ranges and individual values ​​within the range (including integers and fractions).

[0030] In this document, unless otherwise specified, percentage refers to mass percentage and ratio refers to mass ratio.

[0031] Herein, when describing embodiments or examples, it should be understood that they are not used to limit the present invention to these embodiments or examples. On the contrary, all substitutes, improvements and equivalents of the methods and materials described in the present invention can be included in the scope limited by the claims.

[0032] In this document, in order to make the description concise, not all possible combinations of various technical features in various embodiments or examples are described. Therefore, as long as there is no contradiction in the combination of these technical features, the various technical features in various embodiments or examples can be combined arbitrarily, and all possible combinations should be considered to be within the scope of this specification.

[0033] Herein, the swelling rate refers to the volume increase rate of the polymer after the solvent molecules diffuse into the polymer, and this value may be affected by the polymer preparation process. Therefore, the swelling rates of the first binder and the second binder in this article are within a range of values.

[0034] The method for measuring the swelling rate of the binder is as follows: take 1.0g of the solid mass of the binder and spread it in a 50*50mm film-forming mold, dry it in a blast oven at 60℃ for 12h to solidify it, and obtain a film. Cut a square film with a side length of 1cm. Weigh it in a drying room (25℃, humidity ≤0.5%) and record it as M1, place it in an aluminum-plastic film bag containing electrolytes of ethylene carbonate EC, ethyl methyl carbonate EMC, diethyl carbonate DEC and PC, and the volume ratio of EC:EMC:DEC:PC is 5:3:10:2, place it in a 70℃ oven for 7 days, unpack it in a drying room and take out the film, use dry filter paper to gently absorb the residual electrolyte on the surface of the film until the filter paper is no longer wetted, and then weigh it as M2. The swelling rate of the binder in the electrolyte is:

[0035] Swelling=(M2-M1) / M1*100%

[0036] The swelling ratio of the samples was calculated using the data, and the average value was taken from three samples tested in parallel.

[0037] The present invention only needs to select binders with different swelling rates, define the preparation method of the pole piece, and use the surface high swelling rate rebound system to reduce the adverse effect of excessive compaction of the pole piece surface on electrolyte infiltration, without adding new equipment and production processes, and is compatible with existing process technologies and has significantly improved effects. The surface layer of the negative pole piece of the present invention adopts a high swelling rate binder, and the swelling rate after injection is greater than that of the lower layer, so that the surface layer can absorb more electrolyte relative to the bottom layer, the liquid retention capacity is increased, and the ion diffusion capacity is higher than the bottom layer, which can balance the ion and electron transmission dynamics in the thickness direction of the pole piece.

[0038] The present invention provides a method for preparing a negative electrode sheet, comprising the steps of:

[0039] (1) providing a first negative electrode slurry containing a first binder and a second negative electrode slurry containing a second binder; wherein the swelling rate of the first binder is S1, the swelling rate of the second binder is S2, and 1.5≤S2 / S1≤4 is satisfied;

[0040] (2) coating the first negative electrode slurry on one side or both sides of the current collector, and coating the second negative electrode slurry on the first negative electrode slurry;

[0041] (3) Drying and rolling to obtain the negative electrode sheet.

[0042] The method of the present invention can reduce the ion diffusion impedance of the battery. Therefore, in some embodiments, the present invention also provides a method for reducing the impedance of the pole piece, which is preferably the ion diffusion impedance, comprising the following steps:

[0043] (1) providing a first negative electrode slurry containing a first binder and a second negative electrode slurry containing a second binder; wherein the swelling rate of the first binder is S1, the swelling rate of the second binder is S2, and 1.5≤S2 / S1≤4 is satisfied;

[0044] (2) coating the first negative electrode slurry on one side or both sides of the current collector, and coating the second negative electrode slurry on the first negative electrode slurry;

[0045] (3) Drying and rolling to obtain the negative electrode sheet.

[0046] The method of the present invention can improve the dynamic performance of the pole piece. Therefore, in some embodiments, the present invention also provides a method for improving the dynamic performance of the pole piece, comprising the following steps:

[0047] (1) providing a first negative electrode slurry containing a first binder and a second negative electrode slurry containing a second binder; wherein the swelling rate of the first binder is S1, the swelling rate of the second binder is S2, and 1.5≤S2 / S1≤4 is satisfied;

[0048] (2) coating the first negative electrode slurry on one side or both sides of the current collector, and coating the second negative electrode slurry on the first negative electrode slurry;

[0049] (3) Drying and rolling to obtain the negative electrode sheet.

[0050] Herein, preferably, the swelling rate S1 of the first binder satisfies 30%≤S1≤50%, such as 35%, 40%, 45% or in a range between any two values, preferably 38%≤S1≤42%. Preferably, the swelling rate S2 of the second binder satisfies 60%≤S2≤120%, such as 65%, 70%, 75%, 80%, 85%, 90%, 95%, 100%, 110% or in a range between any two values, preferably 88%≤S2≤92%.

[0051] Herein, the ratio of S2 to S1 satisfies 1.5≤S2 / S1≤4, preferably 1.8≤S2 / S1≤3.5, 2≤S2 / S1≤3 or 2.2≤S2 / S1≤2.5.

[0052] Herein, the first binder and the second binder may be independently selected from one or more of styrene-butadiene rubber, styrene-propylene rubber and polyacrylic acid, as long as 1.5≤S2 / S1≤4 is satisfied.

[0053] Herein, the swelling ratio of styrene-butadiene rubber can be 40-120%, such as 40%, 50%, 60%, 70%, 80%, 90%, 100%, 110% or between any two numerical ranges. Herein, the swelling ratio of styrene-propylene rubber can be 80-150%, such as 90%, 100%, 110%, 120%, 130%, 140% or between any two numerical ranges. Herein, the swelling ratio of polyacrylic acid can be 20-50%, such as 30%, 35%, 40% or between any two numerical ranges.

[0054] Herein, the mass fraction of the first binder in the first negative electrode slurry may be 1-2.5%, such as 1.2%, 1.4%, 1.6%, 1.8%, 2%, 2.2%, 2.4% or in a range between any two values, preferably 1.7-1.9%. The mass fraction of the second binder in the second negative electrode slurry is 0.5-2%, such as 0.8%, 1%, 1.2%, 1.4%, 1.6%, 1.8% or in a range between any two values, preferably 1.3-1.7%.

[0055] Herein, the first negative electrode slurry and the second negative electrode slurry may also include conventional components in the negative electrode slurry in the art. In some embodiments, the first negative electrode slurry and the second negative electrode slurry also include active material powder, conductive agent and other binders. The active material powder can be an active material powder for negative electrodes well known in the art, including but not limited to one or more of graphite, hard carbon, mesophase microspheres, silicon oxygen materials and silicon carbon materials, preferably graphite. The conductive agent may be conductive carbon black. Other binders may be binders commonly used in negative electrode materials, such as sodium carboxymethyl cellulose.

[0056] Herein, the mass fraction of the active material powder in the first negative electrode slurry or the second negative electrode slurry may be 93-97%, such as 94%, 95%, 96% or in the range between any two numerical values, preferably 95.5-96.5%. The mass fraction of the conductive carbon black in the first negative electrode slurry or the second negative electrode slurry may be 1-1.5%, such as 1.1%, 1.2%, 1.3%, 1.4% or in the range between any two numerical values, preferably 1.1-1.3%. The mass fraction of other binders in the first negative electrode slurry or the second negative electrode slurry may be 0.8-1.2%, such as 0.9%, 1%, 1.1% or in the range between any two numerical values.

[0057] In step (2), the current collector may be a copper foil. Step (2) may be applied by one or more of extrusion coating, transfer coating, spraying, electrospinning or a double-layer coating process. In some embodiments, a double-layer coating process is used for coating. The coating surface density CW1 of the first negative electrode slurry may be 1-27 mg / cm2 , for example 3mg / cm 2 , 5mg / cm 2 , 7mg / cm 2 , 9mg / cm 2 、11mg / cm 2 、13mg / cm 2 、15mg / cm 2 、17mg / cm 2 、19mg / cm 2 , 22mg / cm 2 , 25mg / cm 2 Or between any two values, preferably 4-6 mg / cm 2 The coating surface density CW2 of the second negative electrode slurry can be 1-3 mg / cm 2 , for example 1.4 mg / cm 2 、1.8mg / cm 2 , 2.2mg / cm 2 , 2.6mg / cm 2 Or between any two values, preferably 1.5-2.5 mg / cm 2 . Preferably, CW1 and CW2 satisfy 1≤CW1 / CW2≤9, for example, CW1 / CW2 is 5:5, 6:4, 7:3, 8:2 or within the range of any two ratios, preferably 1.5≤CW1 / CW2≤7, 2≤CW1 / CW2≤5 or 2.1≤CW1 / CW2≤3. The inventors found through research that the amount of the second binder with a higher swelling rate cannot be too high, otherwise the thickness of the electrode sheet will increase too much after injection, which is not conducive to the overall capacity design of the battery cell. The use of a second binder with a higher swelling rate is to increase the liquid absorption and liquid retention capacity of the surface layer after injection, to increase the lithium ion transmission rate, so that the high electron transmission rate of the surface layer (coated with the second negative electrode slurry) and the bottom layer (coated with the first negative electrode slurry) of the electrode sheet is balanced, therefore, the coating surface density of the first negative electrode slurry and the second negative electrode slurry requires a suitable ratio. In general, the coating surface density of the surface layer is lower and the coating surface density of the bottom layer is higher.

[0058] The present invention also provides a negative electrode plate, which includes a current collector and a negative electrode material layer arranged on one side or both sides of the current collector. The negative electrode material layer includes a first layer body arranged on the current collector and a second layer body arranged on the first layer body. The first layer body is made of a first negative electrode slurry. The second layer body is made of a second negative electrode slurry. The first layer body contains a first binder, and the second layer body contains a second binder. The swelling rate of the first binder is S1, and the swelling rate of the second binder is S2, satisfying 1.5≤S2 / S1≤4. The second negative electrode slurry adopts a high swelling rate binder system relative to the first negative electrode slurry. The definitions of S1 and S2 are as described in any embodiment of this invention. The pore and thickness gradient difference between the second layer body and the first layer body is reduced, which can reduce the adverse effect of the large surface compaction of the negative electrode material layer caused by rolling on the electrolyte infiltration, and improve the performance of the battery cell. In some embodiments, the negative electrode plate is a negative electrode plate prepared by the method of the present invention.

[0059] Herein, the first layer body includes a first negative electrode slurry containing a first binder. The second layer body includes a second negative electrode slurry containing a second binder. In some embodiments, the first negative electrode slurry, the second negative electrode slurry and the current collector are as described in any embodiment herein.

[0060] The present invention also provides a battery, which includes the negative electrode sheet of the present invention. The battery can be a lithium ion battery or a sodium ion battery. In some embodiments, the battery is a lithium ion battery.

[0061] When the negative electrode sheet prepared by the method of the present invention is used in a battery, the ion diffusion impedance can be reduced and the dynamic performance of the sheet can be improved. Therefore, the present invention also provides the use of a first binder with a swelling rate of S1 and a second binder with a swelling rate of S2 in the preparation of a negative electrode sheet, or in reducing the ion diffusion impedance of a battery, or in improving the dynamic performance of a sheet, wherein 1.5≤S2 / S1≤4. In some embodiments, the first binder and the second binder are as described in any embodiment of the present invention.

[0062] The present invention has the following beneficial effects:

[0063] (1) The present invention utilizes the expansion of the surface high swelling rate binder system to reduce the surface compaction of the electrode, improve the electrolyte infiltration, and improve the performance of the battery cell.

[0064] (2) The high swelling rate system has stronger liquid absorption and liquid retention capabilities and a faster ion transfer rate. The high swelling rate system on the surface of the electrode can achieve a fast ion path on the surface of the electrode and cooperate with the fast electron path in the bottom layer to balance the ion and electron transfer in the thickness direction of the electrode, thereby making lithium embedded and evenly distributed inside the negative electrode, thereby improving the performance of the battery cell.

[0065] (3) In the method for preparing the negative electrode plate of the present invention, no new equipment or production process is required, and the method is compatible with the existing process technology and has significantly improved effects.

[0066] The present invention will be further described below in conjunction with specific examples. It should be understood that these examples are only used to illustrate the present invention and are not intended to limit the scope of the present invention. The experimental methods in the following examples without specifying specific conditions are usually based on conventional conditions or the conditions recommended by the manufacturer. Unless otherwise stated, percentages and parts are calculated by weight.

[0067] The method of the double-layer coating process is as follows: a conventional double-layer coating die is used for coating, that is, the upper and lower layers have their own feeding systems and are coated on the electrode at the same time. The double-layer coating technology is a relatively mature process in the industry. The upper and lower layers are fed separately, and the first layer and the second layer are coated simultaneously in the same coating die.

[0068] The measurement method of the coating surface density of the first layer and the second layer is as follows: ① Stop feeding at the upper lip, and coat, dry, and punch out small discs at the lower lip to weigh and test the coating surface density CW1 of the lower layer (first layer); ② Feed, dry, and punch out small discs at the upper and lower lips at the same time to weigh and test the overall coating surface density CW; ③ CW2=CW-CW1 to obtain the coating surface density of the upper layer (second layer).

[0069] The chemical abbreviations and Chinese names of the reagents used in the examples correspond to the following:

[0070] LiFePO4: lithium iron phosphate;

[0071] SP: conductive carbon black;

[0072] CMC: sodium carboxymethyl cellulose;

[0073] SBR: styrene butadiene rubber;

[0074] PVDF: polyvinylidene fluoride;

[0075] PAA: polyacrylic acid.

[0076] Example 1

[0077] Preparation of positive electrode sheet: LiFePO4, SP and PVDF were uniformly dispersed in NMP at a mass ratio of 96.3%:1.5%:1.5% to form positive electrode slurry. The current collector was aluminum foil. The positive electrode slurry was coated on both sides of the aluminum foil. The surface density of the single-side coating was 16.36 mg / cm 2 , and then dried and rolled to make the positive electrode sheet.

[0078] Preparation of negative electrode sheet: artificial graphite, SP, CMC and styrene-butadiene rubber (purchased from Shanghai Daoying Industrial Co., Ltd., GD1332L, swelling rate 40%) were uniformly dispersed in deionized water at a mass ratio of 96%: 1.2%: 1%: 1.8% to form negative electrode slurry 1; artificial graphite, SP, CMC and styrene-propylene rubber (purchased from Shanghai Daoying Industrial Co., Ltd., RA-181, swelling rate 90%) were uniformly dispersed in deionized water at a mass ratio of 96.3%: 1.2%: 1%: 1.5% to form negative electrode slurry 2; slurry 1 and slurry 2 were simultaneously coated on the current collector copper foil by a double-layer coating process, and the coating surface density CW1 / CW2 was controlled to be 7 / 3, and the lower layer ( Figure 1 The coating area density CW1 of the first layer body shown in FIG. 1 is 5.24 mg / cm 2 ,upper layer( Figure 1 The coating area density CW2 of the second layer body shown in FIG. 1 is 2.25 mg / cm 2 , negative electrode slurry 1 and negative electrode slurry 2 are coated on both sides of the current collector, so both sides of the current collector contain the first layer and the second layer. After coating and drying, the coating is rolled to make a negative electrode sheet.

[0079] The positive electrode sheet, negative electrode sheet and separator are made into dry battery cells, which are then assembled, injected with liquid and aged to make symmetrical batteries and single-chip batteries.

[0080] Comparative Example 1

[0081] Preparation of positive electrode sheet: The same preparation method as in Example 1 is adopted.

[0082] Preparation of negative electrode sheet: Artificial graphite, SP, CMC and styrene-butadiene rubber (GD1332L purchased from Shanghai Daoying Industrial Co., Ltd., swelling rate 40%) were uniformly dispersed in deionized water at a mass ratio of 96%:1.2%:1%:1.8% to form a negative electrode slurry, which was coated on both sides of the current collector copper foil. The single-layer coating surface density was 7.49 mg / cm 2 , dried, and rolled to make the negative electrode sheet.

[0083] The positive electrode sheet, negative electrode sheet and separator are made into dry battery cells, which are then assembled, injected with liquid and aged to make symmetrical batteries and single-chip batteries.

[0084] Comparative Example 2

[0085] The preparation method of Example 1 was repeated, except that the binder in negative electrode slurry 1 was styrene acrylic rubber (swelling rate 90%), the binder in negative electrode slurry 2 was styrene butadiene rubber (swelling rate 40%), and S2 / S1<1.5.

[0086] Comparative Example 3

[0087] The preparation method of Example 1 was repeated, except that the binder in the negative electrode slurry 1 was polyacrylic acid (purchased from Shenzhen Yanyi New Materials Co., Ltd., BAP-S66, swelling rate 30%), the binder in the negative electrode slurry 2 was styrene acrylic rubber (purchased from Shanghai Daoying Industrial Co., Ltd., S401, swelling rate 130%), and S2 / S1>4.

[0088] Performance Testing

[0089] Symmetrical battery test: The pole pieces were assembled into symmetrical batteries, the separator was a 9μm PE separator, the electrolyte was a LiPF6 solution, the solvent was a mixture of dimethyl carbonate (DMC), ethyl methyl carbonate (EMC) and ethylene carbonate (EC), the volume ratio of DMC, EMC and EC was 20:55:25, and the LiPF6 concentration was 1.2mol / L. The EIS test characterized the ion diffusion impedance (Rct), the surface pole piece high swelling binder system, the increased liquid absorption and liquid retention capacity, the improved pole piece dynamics, and the reduced Rct, indicating that the surface high swelling binder has a significant improvement effect.

[0090] Monolithic battery test: The obtained pole piece and lithium metal are assembled into a monolithic battery, the separator is a 9μm PE separator, the electrolyte is a LiPF6 solution, the solvent is a mixture of dimethyl carbonate (DMC), ethyl methyl carbonate (EMC) and ethylene carbonate (EC), the volume ratio of DMC, EMC and EC is 20:55:25, and the LiPF6 concentration is 1.2mol / L. Based on the 0.5C rate capacity, 1C and 1.5C rate tests are added. The higher the ratio of capacity at high rate to 0.5C capacity, the better the dynamic performance of the pole piece. According to the electrode structure design of the present invention, the main reasons for the improvement of the electrode dynamics performance are: ① The surface high swelling rate binder system expands after absorbing the electrolyte, breaking the influence of the high surface compaction caused by the cold pressing process of the electrode on the dynamics, and at the same time increasing the surface porosity of the electrode to a certain extent, improving the surface lithium ion transmission dynamics; ② The surface high swelling rate binder system, the large swelling rate indicates that it has strong liquid absorption and liquid retention capabilities, so that the surface electrolyte is in a relatively abundant state, the ion transmission capacity is increased, and the surface dynamics are further improved.

[0091] Table 1

[0092] plan Rct / mΩ 1C / 0.5C 1.5C / 0.5C Example 1 18.1 92.1％ 90.3％ Comparative Example 1 24.8 88.2％ 85.7％ Comparative Example 2 22.3 89.3％ 86.5％ Comparative Example 3 21.9 90.3％ 87.6％

Claims

1. A method for preparing a negative electrode sheet, characterized in that: Includes steps: (1) providing a first negative electrode slurry containing a first binder and a second negative electrode slurry containing a second binder; wherein the swelling rate of the first binder is S1, the swelling rate of the second binder is S2, and 1.5≤S2 / S1≤4 is satisfied; (2) coating the first negative electrode slurry on one side or both sides of the current collector, and coating the second negative electrode slurry on the first negative electrode slurry; (3) Drying and rolling to obtain the negative electrode sheet.

2. The method according to claim 1, characterized in that The swelling ratio S1 of the first binder satisfies 30%≤S1≤50%; and / or the swelling ratio S2 of the second binder satisfies 60%≤S2≤120%.

3. The method according to claim 1, characterized in that The first binder and the second binder are each independently selected from one or more of styrene-butadiene rubber, styrene-propylene rubber and polyacrylic acid.

4. The method according to claim 1, characterized in that The mass fraction of the first binder in the first negative electrode slurry is 1-2.5%; and / or the mass fraction of the second binder in the second negative electrode slurry is 0.5-2%.

5. The method according to claim 1, characterized in that: The current collector is copper foil; and / or, The first negative electrode slurry and the second negative electrode slurry each further include active material powder, a conductive agent and other binders.

6. The method according to claim 1, characterized in that: The coating surface density CW1 of the first negative electrode slurry is 1-27 mg / cm 2 The coating surface density CW2 of the second negative electrode slurry is 1-3 mg / cm 2 ; and / or The coating surface density of the first negative electrode slurry is CW1, and the coating surface density of the second negative electrode slurry is CW2, satisfying 1≤CW1 / CW2≤9.

7. The method according to claim 1, characterized in that Step (2) is coated by one or more of extrusion coating, transfer coating, spraying, electrospinning or double-layer coating processes.

8. A negative electrode plate, comprising a current collector and a negative electrode material layer disposed on one side or both sides of the current collector, characterized in that: The negative electrode material layer includes a first layer body arranged on the current collector and a second layer body arranged on the first layer body; wherein the first layer body contains a first binder, the second layer body contains a second binder, the swelling rate of the first binder is S1, the swelling rate of the second binder is S2, and 1.5≤S2 / S1≤4 is satisfied.

9. The negative electrode sheet according to claim 8, characterized in that: The current collector is copper foil; and / or The first layer body includes a first negative electrode slurry containing the first binder, and the second layer body includes a second negative electrode slurry containing the second binder.

10. The negative electrode sheet according to claim 9, characterized in that: The coating surface density CW1 of the first negative electrode slurry is 1-27 mg / cm 2 The coating surface density CW2 of the second negative electrode slurry is 1-3 mg / cm 2 ; and / or The coating area density of the first negative electrode slurry is CW1, and the coating area density of the second negative electrode slurry is CW2, satisfying 1≤CW1 / CW2≤9.

11. The negative electrode sheet according to claim 9, characterized in that: It is prepared by the method described in any one of claims 1 to 7.

12. A battery, characterized in that: The battery comprises the negative electrode sheet according to any one of claims 8 to 11.

13. Use of a first binder with a swelling rate of S1 and a second binder with a swelling rate of S2 in preparing a negative electrode sheet, or in reducing battery ion diffusion impedance, or in improving the dynamic performance of a sheet, wherein: 1.5≤S2 / S1≤4.

14. The use according to claim 13, characterized in that The first adhesive and the second adhesive are as described in any one of claims 2 to 4.

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