WO2023123411A1 - 负极极片、其制备方法、含有其的二次电池及用电装置 - Google Patents

负极极片、其制备方法、含有其的二次电池及用电装置 Download PDF

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Publication number
WO2023123411A1
WO2023123411A1 PCT/CN2021/143822 CN2021143822W WO2023123411A1 WO 2023123411 A1 WO2023123411 A1 WO 2023123411A1 CN 2021143822 W CN2021143822 W CN 2021143822W WO 2023123411 A1 WO2023123411 A1 WO 2023123411A1
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Prior art keywords
negative electrode
binder
electrode sheet
sheet according
slurry
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PCT/CN2021/143822
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English (en)
French (fr)
Inventor
张铜贤
张明
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宁德时代新能源科技股份有限公司
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Priority to EP21969758.8A priority Critical patent/EP4280296A1/en
Priority to CN202180095215.4A priority patent/CN116982168A/zh
Priority to PCT/CN2021/143822 priority patent/WO2023123411A1/zh
Publication of WO2023123411A1 publication Critical patent/WO2023123411A1/zh
Priority to US18/487,613 priority patent/US20240047683A1/en

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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/62—Selection of inactive substances as ingredients for active masses, e.g. binders, fillers
    • H01M4/621—Binders
    • H01M4/622—Binders being polymers
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/13—Electrodes for accumulators with non-aqueous electrolyte, e.g. for lithium-accumulators; Processes of manufacture thereof
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M10/00—Secondary cells; Manufacture thereof
    • H01M10/05—Accumulators with non-aqueous electrolyte
    • H01M10/052—Li-accumulators
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M10/00—Secondary cells; Manufacture thereof
    • H01M10/05—Accumulators with non-aqueous electrolyte
    • H01M10/058—Construction or manufacture
    • H01M10/0585—Construction or manufacture of accumulators having only flat construction elements, i.e. flat positive electrodes, flat negative electrodes and flat separators
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/04—Processes of manufacture in general
    • H01M4/0402—Methods of deposition of the material
    • H01M4/0404—Methods of deposition of the material by coating on electrode collectors
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/04—Processes of manufacture in general
    • H01M4/0402—Methods of deposition of the material
    • H01M4/0411—Methods of deposition of the material by extrusion
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/04—Processes of manufacture in general
    • H01M4/043—Processes of manufacture in general involving compressing or compaction
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/04—Processes of manufacture in general
    • H01M4/0471—Processes of manufacture in general involving thermal treatment, e.g. firing, sintering, backing particulate active material, thermal decomposition, pyrolysis
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/13—Electrodes for accumulators with non-aqueous electrolyte, e.g. for lithium-accumulators; Processes of manufacture thereof
    • H01M4/139—Processes of manufacture
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/62—Selection of inactive substances as ingredients for active masses, e.g. binders, fillers
    • H01M4/621—Binders
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M2004/021—Physical characteristics, e.g. porosity, surface area
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M2004/026—Electrodes composed of, or comprising, active material characterised by the polarity
    • H01M2004/027—Negative electrodes
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M2220/00—Batteries for particular applications
    • H01M2220/20—Batteries in motive systems, e.g. vehicle, ship, plane
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/10—Energy storage using batteries

Definitions

  • the present application relates to the field of batteries, in particular to a negative electrode sheet, a preparation method thereof, a secondary battery containing the same, and an electrical device.
  • secondary batteries have been more and more widely used in energy storage power systems such as hydraulic, thermal, wind and solar power plants, as well as in power tools, electric bicycles, electric motorcycles, electric vehicles, military equipment, aerospace and other fields. Due to the great development of secondary batteries, higher requirements have been put forward for their safety performance.
  • the present application is made in view of the above-mentioned technical problems, and its purpose is to provide a negative electrode sheet, a preparation method thereof, a secondary battery containing the same, and an electrical device, so as to improve the safety performance of the battery.
  • the first aspect of the present application provides a negative electrode sheet, comprising: a negative electrode current collector and a negative electrode film layer located on the surface of the negative electrode current collector, the negative electrode film layer has a length consistent with the length direction of the negative electrode current collector
  • the length direction and the width direction consistent with the width direction of the negative electrode current collector, the cross-section perpendicular to the length direction of the negative electrode film layer includes a uniform thickness region in the middle of the width direction and edge regions located at both ends in the width direction.
  • the thickness decreases continuously along the direction from the uniform thickness region toward the edge region, and the width a of any one of the two edge regions and the average thickness d of the uniform thickness region satisfy the relationship: a/d ⁇ 1.5, and d ⁇ 300 ⁇ m.
  • the present application controls the thickness distribution of the negative electrode film layer, especially the thickness distribution of the edge region, so that no curling, no cracking, and good mechanical stability of the negative electrode sheet can be obtained, and due to the control, the edge of the negative electrode film
  • the width of the region is within a certain range, so that the range of the inconsistent thickness region can be reduced, the possibility of local insufficient active materials will be reduced, and local (edge) lithium precipitation will not occur due to insufficient active materials, thereby improving the secondary battery. safety.
  • a/d ⁇ 1, optionally a/d ⁇ 0.75, further optionally a/d ⁇ 0.5 As a result, the size of the edge region of the negative electrode membrane can be further reduced, and the range of the thickness inconsistent region can be reduced, thereby reducing the possibility of insufficient local active materials and preventing local (edge) lithium precipitation, thereby further improving the secondary performance. Battery safety.
  • the negative electrode film layer includes negative electrode active material, conductive agent, first binding agent and second binding agent;
  • the first binding agent includes (meth)acrylic acid, polyacrylamide , at least one of cellulose ether or its salt, starch ether or its salt;
  • the second binder includes acrylate-vinyl, styrene-butadiene rubber, nitrile rubber, acrylate, polyurethane at least one.
  • the first binder includes sodium carboxymethylcellulose (CMC-Na).
  • the weight-average molecular weight of the sodium carboxymethylcellulose is less than or equal to 400,000; it may be 200,000-300,000.
  • a slurry with a large solid component concentration that can be kept stable for a long time can be obtained during the preparation of the negative electrode film, and the processability of the prepared negative electrode sheet can be improved.
  • the degree of substitution of the sodium carboxymethylcellulose is 0.65-0.8.
  • the first binder includes sodium carboxymethyl starch (CMS-Na).
  • the dispersibility index PDI of the sodium carboxymethyl starch is 2.0-5.5, optionally 2.5-4.
  • the weight average molecular weight of the sodium carboxymethyl starch is above 400,000, optionally 400,000-600,000.
  • a slurry with a large solid component concentration that can be kept stable for a long time can be obtained during the preparation of the negative electrode film, and the processability of the prepared negative electrode sheet can be improved.
  • the second binder is styrene-butadiene rubber, and its particle size Dv50 ranges from 80nm ⁇ Dv50 ⁇ 200nm.
  • Dv50 particle size ranges from 80nm ⁇ Dv50 ⁇ 200nm.
  • the content ratio of the first binder and the second binder is in the range of 2:1-1:2, optionally 1.5:1-1:1.5.
  • the second aspect of the present application also provides the method for preparing the negative electrode sheet according to the above first aspect, comprising the following steps: coating the negative electrode slurry on the negative electrode current collector, drying, and cold pressing to obtain the negative electrode sheet.
  • the mass percentage of the solid component of the aforementioned slurry is greater than or equal to 60%, and may be 60%-75%. Therefore, in the process of coating the negative electrode slurry, it can help to obtain the specific negative electrode film layer structure of the present application, thereby efficiently improving the cycle life and safety performance of the secondary battery.
  • the above-mentioned negative electrode slurry comprises a negative electrode active material, a conductive agent, a first binder and a second binder
  • the above-mentioned preparation method includes: mixing the negative electrode active material, the conductive agent, the The step of mixing the first binder, the second binder and deionized water to obtain the above-mentioned slurry; and extrusion coating the negative electrode slurry on the surface of the above-mentioned current collector, drying, and cold pressing to obtain the above-mentioned negative electrode Pole piece steps.
  • the first binder is at least one selected from (meth)acrylic acid, polyacrylamide, cellulose ether or a salt thereof, starch ether or a salt thereof.
  • the second binder is at least one selected from acrylate-ethylene, styrene-butadiene rubber, nitrile-butadiene rubber, acrylate, and polyurethane.
  • the first binder includes sodium carboxymethylcellulose.
  • the weight-average molecular weight of the sodium carboxymethylcellulose is below 400,000, optionally 200,000-300,000.
  • the degree of substitution of the sodium carboxymethylcellulose is 0.65-0.8.
  • the first binder includes sodium starch glycolate.
  • the dispersibility index PDI of the sodium carboxymethyl starch is 2.0-5.5, optionally 2.5-4.
  • the weight average molecular weight of the sodium carboxymethyl starch is above 400,000, optionally 400,000-600,000.
  • the second binder is styrene-butadiene rubber, and its volume average particle diameter Dv50 satisfies: 80nm ⁇ Dv50 ⁇ 200nm.
  • the mass content ratio of the first binder and the second binder is in the range of 2:1-1:2, optionally 1.5:1-1:1.5.
  • the negative electrode slurry with suitable solid component content can be formulated, the stability time of the slurry can be improved, and it is helpful to obtain the negative electrode sheet with the negative electrode film thickness distribution specified in this application.
  • the third aspect of the present application provides a secondary battery, comprising the negative electrode sheet of the first aspect of the present application or the negative electrode sheet prepared according to the method of the second aspect of the present application.
  • a fourth aspect of the present application provides an electrical device including the secondary battery of the third aspect of the present application.
  • FIG. 1 is a schematic cross-sectional view of a negative electrode sheet according to an embodiment of the present application.
  • Fig. 2 is a schematic cross-sectional view of a negative electrode film layer according to an embodiment of the present application.
  • FIG. 3(A), FIG. 3(B), FIG. 3(C) and FIG. 3(D) respectively represent schematic cross-sectional views of different shapes of the edge region of the anode film layer of the present application.
  • FIG. 4 is a CCD photograph of the negative electrode sheet of an embodiment of the present application.
  • Fig. 5 is a CCD photograph of a negative electrode sheet in the prior art.
  • FIG. 6 is a flowchart of a method for preparing a negative electrode sheet according to an embodiment of the present application.
  • FIG. 7 is a schematic diagram of a secondary battery according to an embodiment of the present application.
  • FIG. 8 is an exploded view of the secondary battery according to one embodiment of the present application shown in FIG. 7 .
  • Fig. 9 is a schematic diagram of an electrical device according to an embodiment of the present application.
  • Negative electrode sheet Negative electrode film layer
  • 12 Negative electrode current collector
  • 111 Uniform thickness area
  • 112 Edge area
  • 5 Secondary battery
  • 51 Housing
  • 52 Electrode assembly
  • 53 Top cover assembly .
  • ranges disclosed herein are defined in terms of lower and upper limits, and a given range is defined by selecting a lower limit and an upper limit that define the boundaries of the particular range. Ranges defined in this manner may be inclusive or exclusive and may be combined arbitrarily, ie any lower limit may be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a particular parameter, it is understood that ranges of 60-110 and 80-120 are contemplated. Additionally, 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 are all expected: 1-3, 1-4, 1-5, 2- 3, 2-4 and 2-5.
  • the numerical range "a-b” represents an abbreviated representation of any combination of real numbers between a and b, where a and b are both real numbers.
  • the numerical range "0-5" indicates that all real numbers between "0-5" have been listed in this article, and "0-5" is only an abbreviated representation of the combination of these values.
  • a certain parameter is an integer ⁇ 2
  • the method includes steps (a) and (b), which means that the method may include steps (a) and (b) performed in sequence, and may also include steps (b) and (a) performed in sequence.
  • steps (c) means that step (c) may be added to the method in any order, for example, the method may include steps (a), (b) and (c) , may also include steps (a), (c) and (b), may also include steps (c), (a) and (b) and so on.
  • the “comprising” and “comprising” mentioned in this application mean open or closed.
  • the “comprising” and “comprising” may mean that other components not listed may be included or included, or only listed components may be included or included.
  • the term "or” is inclusive unless otherwise stated.
  • the phrase "A or B” means “A, B, or both A and B.” More specifically, the condition "A or B” is satisfied by either of the following: 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).
  • a secondary battery is generally composed of a positive electrode, a negative electrode, an electrolyte, and the like.
  • the negative electrode is formed by mixing negative electrode active materials, binders and additives to form a slurry, uniformly coating on one or both sides of the current collector, and drying, rolling and other treatments.
  • corner gaskets, etc. are used at the extrusion coating port in the extrusion coating device to control the flow rate of the slurry at the edge when the slurry is extruded, thereby thinning the coating.
  • the thickness of the edge of the layer can avoid the occurrence of the edge being too thick.
  • the control of the thickness by this "thinning" technology is not good, which will lead to a wide range of thinning areas on the edge of the pole piece (as shown in Fig. 5), thereby easily leading to the local active material shortage in the edge part of the negative pole sheet during the operation of the secondary battery, thereby easily producing edge lithium precipitation, affecting the use safety of the secondary battery.
  • the inventors of the present application have found through special research that by using a specific binder in the negative electrode film layer preparation process to obtain a slurry with specific properties, it can be coated by simple extrusion (without additionally adding a guide for thinning). Corner gaskets, etc.) can solve the problem of "thick edge phenomenon" existing in the previous extrusion coating method, and can obtain a negative electrode diaphragm with a small range of edge area with thinned thickness, so that there will be no edge area due to excessive
  • the negative electrode sheet obtained by the present invention has no curling, no cracking, and no edge lithium precipitation, and the use of such a negative electrode sheet can provide a battery with low internal resistance, long cycle life and excellent safety performance. secondary battery.
  • a negative electrode sheet 1 comprising: a negative electrode collector 12 and a negative electrode film layer 11 located on the surface of the negative electrode collector 12 .
  • the negative electrode film layer 11 may be formed on one side or both sides of the negative electrode current collector 12 .
  • the negative electrode film layer 11 has a length direction consistent with the length direction of the negative electrode current collector 12 and a width direction consistent with the width direction of the negative electrode current collector 12 (FIG. 1 is a schematic cross-sectional view perpendicular to the length direction of the negative electrode current collector 12).
  • the negative electrode film layer 11 includes a uniform thickness region 111 located in the middle of the width direction and edge regions 112 located at both ends of the width direction. In either of the two edge regions, the thickness is along the direction from The region of uniform thickness decreases continuously in the direction of the edge region.
  • the width a of any one of the two edge regions and the average thickness d of the uniform thickness region satisfy the relationship: a/d ⁇ 1.5, and d ⁇ 300 ⁇ m.
  • the negative electrode sheet produced by the "shaving" extrusion coating process of the chamfer gasket also has the problem of insufficient local active materials.
  • the negative electrode sheet 1 of this embodiment does not curl or crack, and does not produce edge lithium deposition. By using such a negative electrode sheet, a secondary battery with a longer cycle life and excellent safety performance can be provided.
  • the uniform thickness region 111 refers to a region where the height from the upper surface of the negative electrode film layer to the lower surface thereof is substantially the same. According to the manufacturing process, the thickness of the uniform thickness region 111 may have a certain range of deviation, that is, the upper surface of the uniform thickness region 111 may have a certain degree of roughness. In some embodiments, the coefficient of variation CVd of the thickness of the uniform thickness region 111 is ⁇ 5%.
  • the edge region 112 is located on both ends of the negative electrode film layer, and its thickness continuously decreases until it is zero along the direction from the uniform thickness region toward the end.
  • the trapezoid can be an isosceles trapezoid or a non-isosceles trapezoid .
  • the cross-sectional shape of any one of the two edge regions 112 of the negative electrode film layer 11 is such that the thickness decreases continuously along the direction from the uniform thickness region toward the edge region, and the width a and thickness of the edge region 112 are uniform
  • the average thickness d of the region 111 satisfies the relational expression: a/d ⁇ 1.5.
  • the cross-sectional shape of the edge region 112 can also be a shape whose sides are curved (that is, the sides are curved surfaces) as shown in FIGS. It is a shape in which the sides are composed of two or more oblique lines with different slopes (the sides are composed of two or more slopes with different slopes) (as shown in Figure 3(C)). In fact, as long as it does not affect the general trend of the thickness continuously decreasing along the direction from the uniform thickness region to the edge region, it may also include the occurrence of operational errors in the direction from the uniform thickness region to the edge region. Occasional cases of increased thickness (as shown in Figure 3(D)).
  • the distinction and measurement of the uniform thickness region 111 and the edge region 112 can be carried out by observing the section of the negative electrode sheet perpendicular to its length direction with an optical microscope (CCD) or an electron microscope (SEM, STEM, etc.) Measurement.
  • FIG. 4 shows a partial CCD photograph of the negative electrode membrane of an example of this embodiment.
  • the middle part where the thickness is substantially uniform is regarded as the uniform thickness region, and the region where the thickness continuously decreases to zero in the direction from the uniform thickness region toward the end is regarded as the edge region.
  • the projected length of the edge region on the bottom edge of the pole piece is taken as the width a of the edge region.
  • a and the average thickness d of the uniform thickness region satisfy the relational expression: a/d ⁇ 1.5.
  • the range of the edge area where the thickness is thinned in the negative electrode film is very small, that is, the area where the thickness is inconsistent is very small, and the end of the edge area with a particularly small thickness is not far from the area with a uniform thickness. Therefore, during the working process of the secondary battery, it is easier for electrons or lithium ions to migrate to regions with uniform thickness, thereby reducing the possibility of local shortage of active materials, reducing local lithium deposition, and improving the safety of the secondary battery.
  • the thickness of the uniform thickness region is controlled to be d ⁇ 300 ⁇ m, optionally 50 ⁇ m ⁇ d ⁇ 300 ⁇ m.
  • a/d ⁇ 1, optionally a/d ⁇ 0.75, further optionally a/d ⁇ 0.5 can be further reduced, and the range of the thickness inconsistency region can be reduced, thereby reducing the possibility of insufficient local active materials and preventing local (edge) precipitation of lithium, thereby further improving the secondary Battery safety.
  • a can be 0, which directly eliminates the edge region, so that there is no possibility of local shortage of active materials, and further improves the safety of the secondary battery.
  • the negative electrode film layer includes a negative electrode active material, a conductive agent, a first binder, and a second binder.
  • the first binder includes at least one of (meth)acrylic acid, polyacrylamide, cellulose ether or its salt, starch ether or its salt.
  • the first binder is preferably sodium carboxymethyl cellulose (CMC-Na) or sodium carboxymethyl starch (CMS-Na).
  • the first binder when the first binder is sodium carboxymethyl cellulose, its weight average molecular weight (Mw) is controlled below 400,000; for example, it can be 200,000-400,000, 200,000-300,000.
  • the weight-average molecular weight of sodium carboxymethyl cellulose commonly used in the prior art is usually greater than 400,000 (generally more than 500,000), and the inventor finds that if the weight-average molecular weight of sodium carboxymethyl cellulose is reduced, it can be added to the negative electrode diaphragm.
  • 400,000 generally more than 500,000
  • the first binder when the first binder is sodium carboxymethyl cellulose, its degree of substitution is 0.65-0.8. Thereby, the cohesive force of the slurry can be further improved, the cohesive force of the negative electrode film layer can be improved, coating cracking can be reduced, and the processability of the negative electrode sheet can be improved.
  • sodium carboxymethyl starch when sodium carboxymethyl starch is selected as the first binder, its dispersibility index PDI is controlled at 2.0-5.5, and can be optionally 2.5-4.
  • PDI dispersibility index
  • the water retention of the slurry can be improved, so that a slurry with a large solid component concentration that can be kept stable for a long time can be obtained in the preparation process of the negative electrode diaphragm, and the stability of the slurry can be improved and the negative electrode sheet can be improved. machinability.
  • sodium carboxymethyl starch when sodium carboxymethyl starch is selected as the first binder, its weight average molecular weight (Mw) is controlled above 400,000, and may be 400,000-600,000. In this way, the coating cracking of the negative electrode film layer can be reduced.
  • the first binder when the first binder is sodium carboxymethyl starch, its degree of substitution (degree of substitution) is 0.7-0.9.
  • degree of substitution degree of substitution
  • the cohesive force of the slurry can be further improved, the cohesive force of the negative electrode film layer can be improved, coating cracking can be reduced, and the processability of the negative electrode sheet can be improved.
  • the second adhesive includes at least one of acrylate-vinyl, styrene-butadiene rubber, nitrile rubber, acrylate, and polyurethane.
  • the preferred second binder is styrene-butadiene rubber.
  • the particle size Dv50 of the styrene-butadiene rubber is preferably in the range of 80nm ⁇ Dv50 ⁇ 200nm.
  • the content ratio of the first binder and the second binder is in the range of 2:1-1:2, optionally 1.5:1-1:1.5.
  • the negative electrode active material can be a negative electrode active material known in the art for secondary batteries.
  • the negative electrode active material may include at least one of the following materials: artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based material, tin-based material, lithium titanate, and the like.
  • the silicon-based material may be selected from at least one of elemental silicon, silicon-oxygen compounds, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys.
  • the tin-based material may be selected from at least one of simple tin, tin oxide compounds and tin alloys.
  • the present application is not limited to these materials, and other conventional materials that can be used as negative electrode active materials of batteries can also be used. These negative electrode active materials may be used alone or in combination of two or more.
  • the conductive agent may be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.
  • the negative electrode film layer 11 may optionally include other binders.
  • Other binders can be selected from at least one of polyvinyl alcohol (PVA), sodium alginate (SA), carboxymethyl chitosan (CMCS) and the like.
  • the negative electrode current collector 12 in the negative electrode sheet 1 can be a metal foil or a composite current collector.
  • copper foil can be used as the metal foil.
  • the composite current collector may include a base layer of polymer material and a metal layer formed on at least one surface of the base material of polymer material.
  • Composite current collectors can be formed by metal materials (copper, copper alloys, nickel, nickel alloys, titanium, titanium alloys, silver and silver alloys, etc.) on polymer material substrates (such as polypropylene (PP), polyethylene terephthalic acid It is formed on substrates such as ethylene glycol ester (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).
  • the negative electrode sheet 1 is formed by making the negative electrode material constituting the negative electrode film layer into negative electrode slurry and coating it on one or both sides of the negative electrode current collector.
  • FIG. 6 shows a method for preparing a negative electrode sheet according to an embodiment.
  • the negative electrode active material, the conductive agent, and the first binder are mixed to obtain a first mixture (step S1).
  • a double dispersing disk double planetary mixer with self-rotating kneading and stirring function can be used for 15-30 minutes under the conditions of autorotation speed 25-100rpm and dispersion speed 600-1000rpm.
  • step S2 add the second binding agent and deionized water to the first mixture, and use a kneader (for example, a dual-dispersion disc double planetary mixer with a rotation kneading and stirring function to rotate 25-100rpm and disperse at a speed of 0-200rpm) Kneading for 60-120 min (step S2) to obtain the slurry for the negative electrode film layer.
  • a kneader for example, a dual-dispersion disc double planetary mixer with a rotation kneading and stirring function to rotate 25-100rpm and disperse at a speed of 0-200rpm
  • the mass percentage of solid components of the obtained slurry is greater than or equal to 60%, optionally 60%-75%, and standing in the air at 25°C can maintain the uniformity of the slurry for more than 4 days without water Analyze the phenomenon of agglomeration with solids, will not block the extrusion slit, and will not settle, delaminate, etc.
  • Using such a slurry can easily obtain the negative electrode membrane in which the thickness distribution of the negative electrode film layer in the above embodiment is properly controlled by extrusion coating, thereby improving the yield of the negative electrode sheet.
  • Extrusion coating can be extruded through a 50-500um width slit at a pump speed of 50-300ml/min by using, for example, a twin-screw extrusion coater.
  • step S4 put the negative electrode current collector coated with the slurry for the negative electrode film layer in a drying oven adjusted to 120°C for 2min-120min to dry (step S4), so as to obtain the negative electrode sheet of this embodiment.
  • the selection of the negative electrode active material, conductive agent, first binder, and second binder used in the preparation method of the negative electrode sheet is as described in the above-mentioned embodiment of the negative electrode sheet.
  • a secondary battery is provided.
  • a secondary battery typically includes a positive pole piece, a negative pole piece, an electrolyte, and a separator.
  • active ions are intercalated and extracted back and forth between the positive electrode and the negative electrode.
  • the electrolyte plays the role of conducting ions between the positive pole piece and the negative pole piece.
  • the separator is arranged between the positive pole piece and the negative pole piece, which mainly plays a role in preventing the short circuit of the positive and negative poles, and at the same time allows ions to pass through.
  • the negative electrode sheet in the secondary battery uses the negative electrode sheet described in the above embodiment.
  • the positive electrode sheet includes a positive electrode collector and a positive electrode film layer arranged on at least one surface of the positive electrode collector, and the positive electrode film layer includes a positive electrode active material.
  • the positive electrode current collector can be a metal foil or a composite current collector.
  • aluminum foil can be used as the metal foil.
  • the composite current collector may include a polymer material base and a metal layer formed on at least one surface of the polymer material base.
  • the composite current collector can be formed by forming metal materials (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer material substrate (such as polypropylene (PP), polyethylene terephthalic acid It is formed on substrates such as ethylene glycol ester (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).
  • PP polypropylene
  • PET polyethylene glycol ester
  • PBT polybutylene terephthalate
  • PS polystyrene
  • PE polyethylene
  • the positive electrode active material may be a positive electrode active material known in the art for batteries.
  • the positive active material may include at least one of the following materials: olivine-structured lithium-containing phosphate, lithium transition metal oxide, and their respective modified compounds.
  • the present application is not limited to these materials, and other conventional materials that can be used as positive electrode active materials of batteries can also be used. These positive electrode active materials may be used alone or in combination of two or more.
  • lithium transition metal oxides may include, but are not limited to, lithium cobalt oxides (such as LiCoO 2 ), lithium nickel oxides (such as LiNiO 2 ), lithium manganese oxides (such as LiMnO 2 , LiMn 2 O 4 ), lithium Nickel cobalt oxide, lithium manganese cobalt oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide (such as LiNi 1/3 Co 1/3 Mn 1/3 O 2 (also referred to as NCM 333 ), LiNi 0.5 Co 0.2 Mn 0.3 O 2 (also abbreviated as NCM 523 ), LiNi 0.5 Co 0.25 Mn 0.25 O 2 (also abbreviated as NCM 211 ), LiNi 0.6 Co 0.2 Mn 0.2 O 2 (also abbreviated as NCM 622 ), LiNi At least one of 0.8 Co 0.1 Mn 0.1 O 2 (also referred to as NCM 811 ), lithium nickel cobalt aluminum oxide (such as LiNi
  • the olivine structure contains Examples of lithium phosphates may include, but are not limited to, lithium iron phosphate (such as LiFePO 4 (also may be abbreviated as LFP)), composite materials of lithium iron phosphate and carbon, lithium manganese phosphate (such as LiMnPO 4 ), lithium manganese phosphate and carbon At least one of a composite material, lithium manganese iron phosphate, and a composite material of lithium manganese iron phosphate and carbon.
  • lithium iron phosphate such as LiFePO 4 (also may be abbreviated as LFP)
  • composite materials of lithium iron phosphate and carbon such as LiMnPO 4
  • LiMnPO 4 lithium manganese phosphate and carbon
  • the positive electrode film layer may further optionally include a binder.
  • the binder may include polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene At least one of ethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer and fluorine-containing acrylate resin.
  • the positive electrode film layer may also optionally include a conductive agent.
  • the conductive agent may include at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.
  • the positive electrode sheet can be prepared in the following manner: the above-mentioned components used to prepare the positive electrode sheet, such as positive electrode active material, conductive agent, binder and any other components, are dispersed in a solvent (such as N -methylpyrrolidone) to form a positive electrode slurry; the positive electrode slurry is coated on the positive electrode current collector, and after drying, cold pressing and other processes, the positive electrode sheet can be obtained.
  • a solvent such as N -methylpyrrolidone
  • the electrolyte plays the role of conducting ions between the positive pole piece and the negative pole piece.
  • the present application has no specific limitation on the type of electrolyte, which can be selected according to requirements.
  • electrolytes can be liquid, gel or all solid.
  • the electrolyte is an electrolytic solution.
  • the electrolyte solution includes an electrolyte salt and a solvent.
  • the electrolyte salt may be selected from lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bisfluorosulfonyl imide, lithium bistrifluoromethanesulfonyl imide, trifluoromethane At least one of lithium sulfonate, lithium difluorophosphate, lithium difluorooxalate borate, lithium difluorooxalate borate, lithium difluorodifluorooxalatephosphate and lithium tetrafluorooxalatephosphate.
  • the solvent may be selected from ethylene carbonate, propylene carbonate, ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, Butylene carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate At least one of ester, 1,4-butyrolactone, sulfolane, dimethyl sulfone, methyl ethyl sulfone and diethyl sulfone.
  • the electrolyte may optionally include additives.
  • additives may include negative electrode film-forming additives, positive electrode film-forming additives, and additives that can improve certain performances of the battery, such as additives that improve battery overcharge performance, additives that improve high-temperature or low-temperature performance of batteries, and the like.
  • a separator is further included in the secondary battery.
  • the present application has no particular limitation on the type of the isolation membrane, and any known porous structure isolation membrane with good chemical stability and mechanical stability can be selected.
  • the material of the isolation film can be selected from at least one of glass fiber, non-woven fabric, polyethylene, polypropylene and polyvinylidene fluoride.
  • the separator can be a single-layer film or a multi-layer composite film, without any particular limitation. When the separator is a multilayer composite film, the materials of each layer may be the same or different, and there is no particular limitation.
  • the positive pole piece, the negative pole piece and the separator can be made into an electrode assembly through a winding process or a lamination process.
  • the secondary battery may include an outer package.
  • the outer package can be used to package the above-mentioned electrode assembly and electrolyte.
  • the outer packaging of the secondary battery may be a hard case, such as a hard plastic case, aluminum case, steel case, and the like.
  • the outer packaging of the secondary battery may also be a soft bag, such as a bag-type soft bag.
  • the material of the soft case may be plastic, and examples of the plastic include polypropylene, polybutylene terephthalate, polybutylene succinate, and the like.
  • FIG. 7 shows a square-shaped secondary battery 5 as an example.
  • the outer package may include a housing 51 and a cover 53 .
  • the housing 51 may include a bottom plate and a side plate connected to the bottom plate, and the bottom plate and the side plates enclose to form an accommodating cavity.
  • the housing 51 has an opening communicating with the accommodating cavity, and the cover plate 53 can cover the opening to close the accommodating cavity.
  • the positive pole piece, the negative pole piece and the separator can be formed into an electrode assembly 52 through a winding process or a lamination process.
  • the electrode assembly 52 is packaged in the accommodating cavity. Electrolyte is infiltrated in the electrode assembly 52 .
  • the number of electrode assemblies 52 contained in the secondary battery 5 can be one or more, and those skilled in the art can select according to specific actual needs.
  • the present application also provides an electric device, the electric device includes the secondary battery provided in the present application.
  • the secondary battery can be used as a power source of the electric device, and can also be used as an energy storage unit of the electric device.
  • the electric devices may include mobile devices (such as mobile phones, notebook computers, etc.), electric vehicles (such as pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, etc.) , electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc., but not limited thereto.
  • a secondary battery can be selected according to its use requirements, or it can be assembled into a battery module or a battery pack for use.
  • FIG. 9 is an example of an electrical device.
  • the electric device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle.
  • a battery pack or a battery module assembled from the secondary battery may be used.
  • a device may be a cell phone, tablet, laptop, or the like.
  • the device is generally required to be light and thin, and a secondary battery can be used as a power source.
  • the negative electrode slurry, negative electrode sheet and secondary battery of Examples 1-23 and Comparative Examples 1-4 were prepared according to the following method.
  • the artificial graphite as negative active material, the carbon black Super P as conductive agent, the first binding agent of the kind shown in table 1 and the second binding agent are mixed by the mass ratio of 96.5: 0.5: 1.5: 1.5, and An appropriate amount of deionized water was added to adjust the mass percentage of solid components of the negative electrode slurry to the values shown in Table 1.
  • EC ethylene carbonate
  • EMC ethylmethyl carbonate
  • DEC diethyl carbonate
  • the slurry prepared in the above (1) is extruded through a slit with a width of 100 ⁇ m using an extrusion coater with a pressure sensor, and the control flow is 100 ml/min, and the pressure value measured by the pressure sensor is read, and It is described in Table 2 below.
  • Severe cracking the crack length > 2cm or the average number of cracks > 10 in a 2m pole piece.
  • d represents the average thickness of the uniform thickness region
  • the uniform thickness region refers to the region where there is no obvious change in the thickness of the middle part in the width direction of the negative electrode film under the observation of an optical microscope (CCD) or an electron microscope.
  • CCD optical microscope
  • d 10 are randomly measured 10 times within any 100 ⁇ m width of the negative electrode film layer, and the variation coefficient CV d of the thickness is ⁇ 5%, It is considered that the thickness varies uniformly within the width of 100 ⁇ m.
  • a represents the length of the projection in the width direction of the edge regions located at both ends in the width direction.
  • the edge region refers to the observation of the optical microscope (CCD), from the position where the negative electrode film layer appears on the current collector to the end of the uniform thickness region. The area where the edge begins to show a continuous decrease in thickness.
  • the weight-average molecular weight of sodium carboxymethyl starch is in the scope of 400000-600000 , the stability and ease of extrusion of the slurry as well as the processability and safety of the prepared negative electrode sheet can be taken into account.
  • Examples 17-20 show that other water-retaining materials such as sodium carboxymethylcellulose (CMC-Na) can also be used to prepare the negative pole sheet of the present invention as the first binding agent, wherein sodium carboxymethylcellulose
  • CMC-Na sodium carboxymethylcellulose
  • the weight average molecular weight is less than or equal to 400000, especially in the range of 200000-300000, the stability and extrudability of the slurry and the processability and safety of the prepared negative electrode sheet can be taken into account.
  • the present application is not limited to the above-mentioned embodiments.
  • the above-mentioned embodiments are merely examples, and within the scope of the technical solutions of the present application, embodiments that have substantially the same configuration as the technical idea and exert the same effects are included in the technical scope of the present application.
  • various modifications conceivable by those skilled in the art are added to the embodiments, and other forms constructed by combining some components in the embodiments are also included in the scope of the present application. .

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Abstract

本申请提供了一种负极极片及其制备方法，本申请的负极极片包含：负极集流体和位于负极集流体表面的负极膜层，负极膜层的长度方向以及宽度方向与集流体一致，负极膜层的垂直于长度方向的截面包括位于宽度方向上中间部位的厚度均匀区域以及位于宽度方向上两端的边缘区域，在任一边缘区域，厚度沿着从厚度均匀区域朝向该边缘区域的方向连续减小，任一边缘区域的宽度a与厚度均匀区域的平均厚度d满足关系式：a/d≤1.5，且d≤300μm。由此，可以得到不卷边、不开裂，机械稳定性好的负极极片，其用于二次电池中不会出现局部活性材料不足的现象，不会产生边缘析锂，从而可以促进负极活性物质材料最大限度地发挥高容量。

Description

负极极片、其制备方法、含有其的二次电池及用电装置 技术领域
本申请涉及电池领域，尤其涉及一种负极极片及其制备方法、含有其的二次电池以及用电装置。
背景技术
近年来，二次电池越来越广泛应用于水力、火力、风力和太阳能电站等储能电源系统，以及电动工具、电动自行车、电动摩托车、电动汽车、军事装备、航空航天等多个领域。由于二次电池取得了极大的发展，因此对其安全性能也提出了更高的要求。
因此，如何使电池具有较好的安全性能仍是亟待解决的问题。
发明内容
本申请是鉴于上述技术问题而进行的，其目的在于提供一种负极极片及其制备方法、含有其的二次电池以及用电装置，以改善电池的安全性能。
为了达到上述目的，本申请的第一方面提供了一种负极极片，包含：负极集流体和位于负极集流体表面的负极膜层，所述负极膜层具有与负极集流体的长度方向一致的长度方向以及与负极集流体的宽度方向一致的宽度方向，负极膜层的垂直于长度方向的截面包括位于宽度方向上中间部位的厚度均匀区域以及位于宽度方向上两端的边缘区域，在两端的边缘区域的任一者，厚度沿着从厚度均匀区域朝向该边缘区域的方向连续减小，两个边缘区域中的任一者的宽度a与厚度均匀区域的平均厚度d满足关系式：a/d≤1.5，且d≤300μm。
由此，本申请通过控制负极膜层的厚度分布，尤其是边缘区域的厚度分布，从而可以得到不卷边、不开裂，机械稳定性好的负极极片， 并且由于控制使负极膜片的边缘区域的宽度在一定范围内，从而可以减小厚度不一致区域的范围，减少出现局部活性材料不足的可能性，不会出现因活性材料不足导致的局部(边缘)析锂，从而提高二次电池的安全性。
在任意实施方式中，a/d≤1，可选为a/d≤0.75，进一步可选为a/d≤0.5。由此，可以进一步减小负极膜片的边缘区域的大小，减小厚度不一致区域的范围，从而可以减少出现局部活性材料不足的可能性，不会局部(边缘)析锂，从而进一步提高二次电池的安全性。
在任意实施方式中，负极膜层包含负极活性材料、导电剂、第一粘结剂和第二粘结剂；可选地，所述第一粘结剂包括(甲基)丙烯酸、聚丙烯酰胺、纤维素醚或其盐、淀粉醚或其盐中的至少一种；可选地，所述第二粘结剂包括丙烯酸酯-乙烯、丁苯橡胶、丁腈橡胶、丙烯酸酯、聚氨酯中的至少一种。由此，可以得到制备过程中通过简单的挤压涂布就容易得到上述适当地控制负极膜层的厚度分布的负极膜片的浆料，并使该浆料能够长时间保持稳定。
在任意实施方式中，所述第一粘结剂包括羧甲基纤维素钠(CMC-Na)。
在任意实施方式中，所述羧甲基纤维素钠的重均分子量小于等于400000；可选为200000-300000。由此，可以在负极膜片的制备过程中得到可长时间保持稳定的具有较大固体组分浓度的浆料，并且可改善所制备的负极极片的可加工性。
在任意实施方式中，所述羧甲基纤维素钠的取代度为0.65-0.8。
在任意实施方式中，所述第一粘结剂包括羧甲基淀粉钠(CMS-Na)。
在任意实施方式中，所述羧甲基淀粉钠的分散性指数PDI为2.0-5.5，可选为2.5-4。由此，可以在负极膜片的制备过程中得到可长时间保持稳定的具有较大固体组分浓度的浆料，并且可改善所制备的负极极片的可加工性。
在任意实施方式中，所述羧甲基淀粉钠的重均分子量为400000以上，可选为400000-600000。由此，可以在负极膜片的制备过程中得到可长时间保持稳定的具有较大固体组分浓度的浆料，并且可改善所制 备的负极极片的可加工性。
在任意实施方式中，所述第二粘结剂为丁苯橡胶，其粒径Dv50的范围为80nm≤Dv50≤200nm。通过具备这样的小粒径胶粒，可充分发挥粘结效果，提高浆料的内聚力，进一步由于胶粒粒径小，还可增大其覆盖石墨端面的可能性，因而可提高极片的直流阻抗。
在任意实施方式中，所述第一粘结剂和所述第二粘结剂的含量比例范围为2∶1-1∶2，可选为1.5∶1-1∶1.5。由此，可以在负极膜片的制备过程中得到可长时间保持稳定的具有较大固体组分浓度的浆料，并且可改善所制备的负极极片的可加工性。
本申请的第二方面还提供上述第一方面的负极极片的制备方法，包括如下步骤：将负极浆料涂布于所述负极集流体，经干燥，冷压获得所述负极极片。
在任意实施方式中，所负极述浆料的固体组分质量百分含量大于等于60％，可选为60％-75％。由此，在负极浆料涂布的过程中可有助于得到本申请特定的负极膜层结构，从而高效地提高二次电池的循环寿命以及安全性能。
在任意的实施方式中，上述负极浆料包含负极活性材料、导电剂、第一粘结剂和第二粘结剂，上述制备方法包括：将所述负极活性材料、所述导电剂、所述第一粘结剂、所述第二粘结剂和去离子水混合获得上述浆料的步骤；以及将所述负极浆料挤压涂布于上述集流体表面，经干燥，冷压获得上述负极极片的步骤。
在任意的实施方式中，所述第一粘结剂为选自(甲基)丙烯酸、聚丙烯酰胺、纤维素醚或其盐、淀粉醚或其盐中的至少一种。
在任意的实施方式中，所述第二粘结剂为选自丙烯酸酯-乙烯、丁苯橡胶、丁腈橡胶、丙烯酸酯、聚氨酯中的至少一种。
在任意的实施方式中，所述第一粘结剂包括羧甲基纤维素钠。
在任意的实施方式中，所述羧甲基纤维素钠的重均分子量为400000以下，可选为200000-300000。
在任意的实施方式中，所述羧甲基纤维素钠的取代度为0.65-0.8。
在任意的实施方式中，所述第一粘结剂包括羧甲基淀粉钠。
在任意的实施方式中，所述羧甲基淀粉钠的分散性指数PDI为2.0-5.5，可选为2.5-4。
在任意的实施方式中，所述羧甲基淀粉钠的重均分子量为400000以上，可选为400000-600000。
在任意的实施方式中，所述第二粘结剂为丁苯橡胶，其体积平均粒径Dv50满足：80nm≤Dv50≤200nm。
在任意的实施方式中，所述第一粘结剂和所述第二粘结剂的质量含量比例范围为2∶1-1∶2，可选为1.5∶1-1∶1.5。
由此，可调配出具有合适固体组分含量的负极浆料，提高浆料的稳定时间，并且有助于得到本申请规定的负极膜层厚度分布的负极极片。
本申请的第三方面提供一种二次电池，包括本申请第一方面的负极极片或者包括根据本申请第二方面的方法制备的负极极片。
本申请的第四方面提供一种用电装置，包括本申请的第三方面的二次电池。
附图说明
图1是本申请一个实施方式的负极极片截面示意图。
图2是本申请一个实施方式的负极膜层截面示意图。
图3(A)、图3(B)、图3(C)和图3(D)分别表示本申请负极膜层的边缘区域的不同形状的截面示意图。
图4所示的本申请一个实施方式的负极极片的CCD照片。
图5是现有技术的负极极片的CCD照片。
图6是本申请一个实施方式的负极极片的制备方法的流程图。
图7是本申请一个实施方式的二次电池的示意图。
图8是图7所示的本申请一个实施方式的二次电池的分解图。
图9是本申请一个实施方式的用电装置的示意图。
附图标记说明：
1：负极极片；11：负极膜层；12：负极集流体；111：厚度均匀区域；112：边缘区域；5：二次电池；51：壳体；52：电极组件；53： 顶盖组件。
具体实施方式
以下，适当地参照附图详细说明具体公开了本申请的负极极片及其制造方法、二次电池和用电装置的实施方式。但是会有省略不必要的详细说明的情况。例如，有省略对已众所周知的事项的详细说明、实际相同结构的重复说明的情况。这是为了避免以下的说明不必要地变得冗长，便于本领域技术人员的理解。此外，附图及以下说明是为了本领域技术人员充分理解本申请而提供的，并不旨在限定权利要求书所记载的主题。
本申请所公开的“范围”以下限和上限的形式来限定，给定范围是通过选定一个下限和一个上限进行限定的，选定的下限和上限限定了特别范围的边界。这种方式进行限定的范围可以是包括端值或不包括端值的，并且可以进行任意地组合，即任何下限可以与任何上限组合形成一个范围。例如，如果针对特定参数列出了60-120和80-110的范围，理解为60-110和80-120的范围也是预料到的。此外，如果列出的最小范围值1和2，和如果列出了最大范围值3，4和5，则下面的范围可全部预料到：1-3、1-4、1-5、2-3、2-4和2-5。在本申请中，除非有其他说明，数值范围“a-b”表示a到b之间的任意实数组合的缩略表示，其中a和b都是实数。例如数值范围“0-5”表示本文中已经全部列出了“0-5”之间的全部实数，“0-5”只是这些数值组合的缩略表示。另外，当表述某个参数为≥2的整数，则相当于公开了该参数为例如整数2、3、4、5、6、7、8、9、10、11、12等。
如果没有特别的说明，本申请的所有实施方式以及可选实施方式可以相互组合形成新的技术方案。
如果没有特别的说明，本申请的所有技术特征以及可选技术特征可以相互组合形成新的技术方案。
如果没有特别的说明，本申请的所有步骤可以顺序进行，也可以随机进行，优选是顺序进行的。例如，所述方法包括步骤(a)和(b)，表示所述方法可包括顺序进行的步骤(a)和(b)，也可以包括顺序进行的步 骤(b)和(a)。例如，所述提到所述方法还可包括步骤(c)，表示步骤(c)可以任意顺序加入到所述方法，例如，所述方法可以包括步骤(a)、(b)和(c)，也可包括步骤(a)、(c)和(b)，也可以包括步骤(c)、(a)和(b)等。
如果没有特别的说明，本申请所提到的“包括”和“包含”表示开放式，也可以是封闭式。例如，所述“包括”和“包含”可以表示还可以包括或包含没有列出的其他组分，也可以仅包括或包含列出的组分。
如果没有特别的说明，在本申请中，术语“或”是包括性的。举例来说，短语“A或B”表示“A，B，或A和B两者”。更具体地，以下任一条件均满足条件“A或B”：A为真(或存在)并且B为假(或不存在)；A为假(或不存在)而B为真(或存在)；或A和B都为真(或存在)。
二次电池一般由正极、负极、电解液等组成。其中，负极是通过将负极活性物质、粘合剂和添加剂等混合制成浆料，均匀涂布于集流体单侧或两侧，并经干燥、滚压等处理而成。
但是，在涂布浆料的过程中，由于浆料的表面张力及浆料的流变特性等，会在边缘聚集更厚的活性物质层，即，产生所谓的“厚边现象”。这样在干燥过程中较厚的边缘区域与中间区域之间会产生应力差异，造成极片边缘开裂的问题。但是，现有缓解极片“厚边现象”的方法基本上都对极片厚度的控制不精，容易造成所制备的电池在工作中出现局部活性材料不足、边缘局部析锂等问题，影响电池的使用安全。
现有技术中，为了解决这样的问题，会在挤压涂布装置中在挤压涂布口使用导角垫片等来控制挤出浆料时位于边缘的浆料的流量，从而削薄涂层边缘的厚度，避免边缘过厚情况的产生。但是由于所使用的负极涂层浆料的粘度、流动性等的限制，通过这种“削薄”技术对厚度的控制并不佳，会导致极片边缘厚度减薄区域范围很宽(如图5)，从而容易导致在二次电池的工作中负极极片的边缘部分局部活性材料不足，从而容易产生边缘析锂，影响二次电池的使用安全。
本申请发明人通过专门研究发现，通过在负极膜层制备过程中使 用特定的粘结剂得到具有特定性状的浆料，从而可通过简单的挤压涂布(不需另行加入削薄用的导角垫片等)就可以解决之前挤压涂布方式中存在的“厚边现象”的问题，并且可以得到厚度减薄的边缘区域范围很小的负极膜片，因而不会产生因边缘区域过大所导致的局部活性材料不足、边缘析锂的问题。通过本发明得到的负极极片不卷边、不开裂，并且不会产生边缘析锂，通过使用这样的负极极片可提供具有较低的内阻、较长的循环寿命以及优异的安全性能的二次电池。
[负极极片]
参照图1，本申请的一个实施方式中，提出了一种负极极片1，包含：负极集流体12和位于负极集流体12表面的负极膜层11。负极膜层11可以形成于负极集流体12的单面，也可以形成于双面。负极膜层11具有与负极集流体12的长度方向一致的长度方向以及与负极集流体12的宽度方向一致的宽度方向(图1为垂直于负极集流体12的长度方向的截面示意图)。从垂直于长度方向的截面观察，负极膜层11包括位于宽度方向上中间部位的厚度均匀区域111以及位于宽度方向上两端的边缘区域112，在两个边缘区域的任一者，厚度沿着从厚度均匀区域朝向该边缘区域的方向连续减小。两个边缘区域中的任一者的宽度a与厚度均匀区域的平均厚度d满足关系式：a/d≤1.5，且d≤300μm。
这样，不仅消除了以往的一般挤压涂布工艺制造的负极极片中存在的“厚边现象”的问题，而且边缘区域的范围本身也控制得比较小，因而不会像现有的通过附加导角垫片的“削薄”挤压涂布工艺所制造的负极极片一样存在局部活性材料不足的问题。本实施方式这样的负极极片1不卷边、不开裂，不产生边缘析锂，通过使用这样的负极极片可提供具有较长的循环寿命以及优异的安全性能的二次电池。
如图2所示，厚度均匀区域111是指负极膜层的上表面距离其下表面的高度基本一致的区域。根据制备工艺，厚度均匀区域111的厚度可以有一定范围的偏差，即，厚度均匀区域111的上表面可以具有一定程度的粗糙度。在一些实施方式中，厚度均匀区域111的厚度的变异系数CVd≤5％。边缘区域112位于负极膜层的两端上，其厚度沿 着从所述厚度均匀区域朝向该端部的方向连续减小直至为0。
作为负极膜层11的典型截面形状，可例举如以靠近负极集流体的一边为下底边的梯形形状(如图2所示)，该梯形可以是等腰梯形，也可以是非等腰梯形。另外，负极膜层11的两个边缘区域112中的任一者的截面形状只要是厚度沿着从厚度均匀区域朝向该边缘区域的方向连续减小，且该边缘区域112的宽度a与厚度均匀区域111的平均厚度d满足关系式：a/d≤1.5即可。边缘区域112的截面形状除了可以是如图2所示的三角形外，还可以是图3(A)、3(B)所示的侧边为曲线(即，侧面为曲面)的形状，也可以是侧边由两条或多条斜率不同的斜线构成(侧面由两个或多个坡度不同的斜面构成)的形状(如图3(C)所示)。事实上，只要不影响厚度沿着从厚度均匀区域朝向该边缘区域的方向连续减小的总体趋势，也可以包括在沿着从厚度均匀区域朝向该边缘区域的方向上出现因操作误差等导致的偶然的厚度增加的情况(如图3(D)所示)。
在本实施方式中，厚度均匀区域111和边缘区域112的区分和测量可通过对负极极片的垂直于其长度方向的截面进行光学显微镜(CCD)、电子显微镜(SEM、STEM等)观察而进行测量。图4中示出了本实施方式的一例的负极膜片的局部CCD照片。在CCD照片中，将厚度基本保持均匀的中间部分作为厚度均匀区域，并将厚度在从厚度均匀区域朝向该端部的方向连续地减小直至为0的区域作为边缘区域。将边缘区域在位于极片的下底边上的投影长度作为边缘区域的宽度a，本实施方式中，a与厚度均匀区域的平均厚度d满足关系式：a/d≤1.5。这样负极膜片中厚度减薄的边缘区域范围很小，即，厚度不一致的区域很小，并且边缘区域的厚度特别小的端部距离厚度均匀区域的距离不远。因此，在二次电池工作过程中，电子或锂离子向厚度均匀区域的迁移更为容易，因此减少了出现局部活性材料不足的可能性，减少局部析锂，提高了二次电池的安全性。另外，在本实施方式中，从经济性以及电池的循环寿命等观点考虑，控制厚度均匀区域的厚度为d≤300μm，可选为50μm≤d≤300μm。
在一些实施方式中，a/d≤1，可选为a/d≤0.75，进一步可选为a/d ≤0.5。由此，可以进一步减小负极膜层的边缘区域的大小，减小厚度不一致区域的范围，从而可以减少出现局部活性材料不足的可能性，不会局部(边缘)析锂，从而进一步提高二次电池的安全性。在理想的状态下，a可以为0，这样直接消除了边缘区域，从而完全不会出现局部活性材料不足的可能性，进一步提高了二次电池的安全性。
在一些实施方式中，负极膜层包含负极活性材料、导电剂、第一粘结剂和第二粘结剂。
第一粘结剂包括(甲基)丙烯酸、聚丙烯酰胺、纤维素醚或其盐、淀粉醚或其盐中的至少一种。其中，从提高负极膜层的粘结力，减少涂布开裂的观点出发，优选第一粘结剂为羧甲基纤维素钠(CMC-Na)或者羧甲基淀粉钠(CMS-Na)。
在一些实施方式中，当第一粘结剂选用羧甲基纤维素钠情况下，其重均分子量(Mw)控制在400000以下；例如可以为200000-400000，200000-300000。现有技术中常用的羧甲基纤维素钠的重均分子量通常大于400000(一般在500000以上)，发明人发现，如果将羧甲基纤维素钠的重均分子量降低，可以在负极膜片的制备过程中得到可长时间保持稳定的具有较大固体组分浓度的浆料，并且可改善所制备的负极极片的可加工性。
在一些实施方式中，当第一粘结剂选用羧甲基纤维素钠情况下，其取代度(degree of substitution)为0.65-0.8。由此，可以进一步提高浆料的内聚力，提高负极膜层的粘结力，减少涂布开裂，并提高负极极片的可加工性。
在一些实施方式中，当第一粘结剂选用羧甲基淀粉钠的情况下，其分散性指数PDI控制在2.0-5.5，可选为2.5-4。由此，可以提高浆料的保水性，从而可以在负极膜片的制备过程中得到可长时间保持稳定的具有较大固体组分浓度的浆料，提高浆料的稳定性并改善负极极片的可加工性。
在一些实施方式中，当第一粘结剂选用羧甲基淀粉钠的情况下，其重均分子量(Mw)控制在400000以上，可选为400000-600000。由此可以减少负极膜层涂布开裂的情况。
在一些实施方式中，当第一粘结剂选用羧甲基淀粉钠的情况下，其取代度(degree of substitution)为0.7-0.9。由此可以进一步从提高浆料的内聚力，提高负极膜层的粘结力，减少涂布开裂，并提高负极极片的可加工性。
在一些实施方式中，第二粘结剂包括丙烯酸酯-乙烯、丁苯橡胶、丁腈橡胶、丙烯酸酯、聚氨酯中的至少一种。由此，可以得到制备过程中通过简单的挤压涂布就容易得到上述适当地控制负极膜层的厚度分布的负极膜片的浆料，并使该浆料能够长时间保持稳定。
在一些实施方式中，从充分发挥第一粘结剂和第二粘结剂的协同作用，提高浆料的粘结性，减少涂布开裂等的观点出发，优选第二粘结剂为丁苯橡胶。
在一些实施方式中，从充分发挥粘结效果，提高浆料内聚力，减少挤压涂布时所需的的压力的观点出发，优选丁苯橡胶的粒径Dv50的范围为80nm≤Dv50≤200nm。
在一些实施方式中，第一粘结剂和第二粘结剂的含量比例范围为2∶1-1∶2，可选为1.5∶1-1∶1.5。由此，可以提高浆料的保水性，可得到能够长时间保持稳定的具有较大固体组分浓度的浆料，并且可改善所制备的负极极片的可加工性。
在一些实施方式中，负极活性材料可采用本领域公知的用于二次电池的负极活性材料。作为示例，负极活性材料可包括以下材料中的至少一种：人造石墨、天然石墨、软炭、硬炭、硅基材料、锡基材料和钛酸锂等。所述硅基材料可选自单质硅、硅氧化合物、硅碳复合物、硅氮复合物以及硅合金中的至少一种。所述锡基材料可选自单质锡、锡氧化合物以及锡合金中的至少一种。但本申请并不限定于这些材料，还可以使用其他可被用作电池负极活性材料的传统材料。这些负极活性材料可以仅单独使用一种，也可以将两种以上组合使用。
在一些实施方式中，导电剂可选自超导碳、乙炔黑、炭黑、科琴黑、碳点、碳纳米管、石墨烯及碳纳米纤维中的至少一种。
在一些实施方式中，负极膜层11还可选地包括其它粘结剂。其它粘结剂可选自聚乙烯醇(PVA)、海藻酸钠(SA)、及羧甲基壳聚糖 (CMCS)等中的至少一种。
负极极片1中的负极集流体12可采用金属箔片或复合集流体。例如，作为金属箔片，可以采用铜箔。复合集流体可包括高分子材料基层和形成于高分子材料基材至少一个表面上的金属层。复合集流体可通过将金属材料(铜、铜合金、镍、镍合金、钛、钛合金、银及银合金等)形成在高分子材料基材(如聚丙烯(PP)、聚对苯二甲酸乙二醇酯(PET)、聚对苯二甲酸丁二醇酯(PBT)、聚苯乙烯(PS)、聚乙烯(PE)等的基材)上而形成。
[负极极片的制备方法]
以下，对本实施方式的负极极片1的制备方法进行说明。
负极极片1通过将构成负极膜层的负极材料做成负极浆料，涂布于负极集流体的单面或双面而成。
图6中示出一个实施方式的负极极片的制备方法。首先，将负极活性材料、导电剂、第一粘结剂混合，得到第一混合物(步骤S1)。混合过程中可使用带自转捏合搅拌功能的双分散盘双行星搅拌机，在自转速度25-100rpm、分散转速600-1000rpm的条件下进行15-30min。
然后，向第一混合物中加入第二粘结剂、去离子水，使用捏合机(例如带自转捏合搅拌功能的双分散盘双行星搅拌机以自转25-100rpm、分散转速0-200rpm的条件)进行捏合60-120min(步骤S2)，得到负极膜层用浆料。所得到的浆料的固体组分质量百分含量大于等于60％，可选为60％-75％，并且在25℃下空气中静置可以维持浆料的均匀性4天以上而不发生水分析出和固体团聚现象，不会堵塞挤压狭缝，并且不会沉降、分层等。使用这样的浆料，可以容易地通过挤压涂布得到上述实施方式的适当控制负极膜层的厚度分布的负极膜片，从而提高负极极片的成品率。
将上述浆料通过挤压涂布而涂布于负极集流体的单面或者双面(步骤S3)。挤压涂布可通过使用例如双螺杆挤压涂布机在50-300ml/min的泵速下于50-500um宽度狭缝挤出。
然后再将涂布有负极膜层用浆料的负极集流体置于调节至120℃的干燥箱内进行干燥2min-120min(步骤S4)，从而得到本实施方式 的负极极片。
关于负极极片的制备方法中所使用的负极活性材料、导电剂、第一粘结剂、第二粘结剂的选择如上述负极极片的实施方式中所描述。
另外，以下适当参照附图对本申请的二次电池和用电装置进行说明。
本申请的一个实施方式中，提供一种二次电池。
通常情况下，二次电池包括正极极片、负极极片、电解质和隔离膜。在电池充放电过程中，活性离子在正极极片和负极极片之间往返嵌入和脱出。电解质在正极极片和负极极片之间起到传导离子的作用。隔离膜设置在正极极片和负极极片之间，主要起到防止正负极短路的作用，同时可以使离子通过。
本实施方式中，二次电池中的负极极片使用上述实施方式中所描述的负极极片。
[正极极片]
正极极片包括正极集流体以及设置在正极集流体至少一个表面的正极膜层，所述正极膜层包括正极活性材料。
在一些实施方式中，所述正极集流体可采用金属箔片或复合集流体。例如，作为金属箔片，可采用铝箔。复合集流体可包括高分子材料基层和形成于高分子材料基层至少一个表面上的金属层。复合集流体可通过将金属材料(铝、铝合金、镍、镍合金、钛、钛合金、银及银合金等)形成在高分子材料基材(如聚丙烯(PP)、聚对苯二甲酸乙二醇酯(PET)、聚对苯二甲酸丁二醇酯(PBT)、聚苯乙烯(PS)、聚乙烯(PE)等的基材)上而形成。
在一些实施方式中，正极活性材料可采用本领域公知的用于电池的正极活性材料。作为示例，正极活性材料可包括以下材料中的至少一种：橄榄石结构的含锂磷酸盐、锂过渡金属氧化物及其各自的改性化合物。但本申请并不限定于这些材料，还可以使用其他可被用作电池正极活性材料的传统材料。这些正极活性材料可以仅单独使用一种，也可以将两种以上组合使用。其中，锂过渡金属氧化物的示例可包括但不限于锂钴氧化物(如LiCoO 2)、锂镍氧化物(如LiNiO 2)、锂锰氧 化物(如LiMnO 2、LiMn 2O 4)、锂镍钴氧化物、锂锰钴氧化物、锂镍锰氧化物、锂镍钴锰氧化物(如LiNi 1/3Co 1/3Mn 1/3O 2(也可以简称为NCM 333)、LiNi 0.5Co 0.2Mn 0.3O 2(也可以简称为NCM 523)、LiNi 0.5Co 0.25Mn 0.25O 2(也可以简称为NCM 211)、LiNi 0.6Co 0.2Mn 0.2O 2(也可以简称为NCM 622)、LiNi 0.8Co 0.1Mn 0.1O 2(也可以简称为NCM 811)、锂镍钴铝氧化物(如LiNi 0.85Co 0.15Al 0.05O 2)及其改性化合物等中的至少一种。橄榄石结构的含锂磷酸盐的示例可包括但不限于磷酸铁锂(如LiFePO 4(也可以简称为LFP))、磷酸铁锂与碳的复合材料、磷酸锰锂(如LiMnPO 4)、磷酸锰锂与碳的复合材料、磷酸锰铁锂、磷酸锰铁锂与碳的复合材料中的至少一种。
在一些实施方式中，正极膜层还可选地包括粘结剂。作为示例，所述粘结剂可以包括聚偏氟乙烯(PVDF)、聚四氟乙烯(PTFE)、偏氟乙烯-四氟乙烯-丙烯三元共聚物、偏氟乙烯-六氟丙烯-四氟乙烯三元共聚物、四氟乙烯-六氟丙烯共聚物及含氟丙烯酸酯树脂中的至少一种。
在一些实施方式中，正极膜层还可选地包括导电剂。作为示例，所述导电剂可以包括超导碳、乙炔黑、炭黑、科琴黑、碳点、碳纳米管、石墨烯及碳纳米纤维中的至少一种。
在一些实施方式中，可以通过以下方式制备正极极片：将上述用于制备正极极片的组分，例如正极活性材料、导电剂、粘结剂和任意其他的组分分散于溶剂(例如N-甲基吡咯烷酮)中，形成正极浆料；将正极浆料涂覆在正极集流体上，经烘干、冷压等工序后，即可得到正极极片。
[电解质]
电解质在正极极片和负极极片之间起到传导离子的作用。本申请对电解质的种类没有具体的限制，可根据需求进行选择。例如，电解质可以是液态的、凝胶态的或全固态的。
在一些实施方式中，所述电解质采用电解液。所述电解液包括电解质盐和溶剂。
在一些实施方式中，电解质盐可选自六氟磷酸锂、四氟硼酸锂、高氯酸锂、六氟砷酸锂、双氟磺酰亚胺锂、双三氟甲磺酰亚胺锂、三 氟甲磺酸锂、二氟磷酸锂、二氟草酸硼酸锂、二草酸硼酸锂、二氟二草酸磷酸锂及四氟草酸磷酸锂中的至少一种。
在一些实施方式中，溶剂可选自碳酸亚乙酯、碳酸亚丙酯、碳酸甲乙酯、碳酸二乙酯、碳酸二甲酯、碳酸二丙酯、碳酸甲丙酯、碳酸乙丙酯、碳酸亚丁酯、氟代碳酸亚乙酯、甲酸甲酯、乙酸甲酯、乙酸乙酯、乙酸丙酯、丙酸甲酯、丙酸乙酯、丙酸丙酯、丁酸甲酯、丁酸乙酯、1，4-丁内酯、环丁砜、二甲砜、甲乙砜及二乙砜中的至少一种。
在一些实施方式中，所述电解液还可选地包括添加剂。例如添加剂可以包括负极成膜添加剂、正极成膜添加剂，还可以包括能够改善电池某些性能的添加剂，例如改善电池过充性能的添加剂、改善电池高温或低温性能的添加剂等。
[隔离膜]
在一些实施方式中，二次电池中还包括隔离膜。本申请对隔离膜的种类没有特别的限制，可以选用任意公知的具有良好的化学稳定性和机械稳定性的多孔结构隔离膜。
在一些实施方式中，隔离膜的材质可选自玻璃纤维、无纺布、聚乙烯、聚丙烯及聚偏二氟乙烯中的至少一种。隔离膜可以是单层薄膜，也可以是多层复合薄膜，没有特别限制。在隔离膜为多层复合薄膜时，各层的材料可以相同或不同，没有特别限制。
在一些实施方式中，正极极片、负极极片和隔离膜可通过卷绕工艺或叠片工艺制成电极组件。
在一些实施方式中，二次电池可包括外包装。该外包装可用于封装上述电极组件及电解质。
在一些实施方式中，二次电池的外包装可以是硬壳，例如硬塑料壳、铝壳、钢壳等。二次电池的外包装也可以是软包，例如袋式软包。软包的材质可以是塑料，作为塑料，可列举出聚丙烯、聚对苯二甲酸丁二醇酯以及聚丁二酸丁二醇酯等。
本申请对二次电池的形状没有特别的限制，其可以是圆柱形、方形或其他任意的形状。例如，图7是作为一个示例的方形结构的二次电池5。
在一些实施方式中，参照图8，外包装可包括壳体51和盖板53。其中，壳体51可包括底板和连接于底板上的侧板，底板和侧板围合形成容纳腔。壳体51具有与容纳腔连通的开口，盖板53能够盖设于所述开口，以封闭所述容纳腔。正极极片、负极极片和隔离膜可经卷绕工艺或叠片工艺形成电极组件52。电极组件52封装于所述容纳腔内。电解液浸润于电极组件52中。二次电池5所含电极组件52的数量可以为一个或多个，本领域技术人员可根据具体实际需求进行选择。
另外，本申请还提供一种用电装置，所述用电装置包括本申请提供的二次电池。所述二次电池可以用作所述用电装置的电源，也可以用作所述用电装置的能量存储单元。所述用电装置可以包括移动设备(例如手机、笔记本电脑等)、电动车辆(例如纯电动车、混合动力电动车、插电式混合动力电动车、电动自行车、电动踏板车、电动高尔夫球车、电动卡车等)、电气列车、船舶及卫星、储能系统等，但不限于此。
作为所述用电装置，可以根据其使用需求来选择二次电池，或者将其组装为电池模块或电池包来进行使用。
图9是作为一个示例的用电装置。该用电装置为纯电动车、混合动力电动车、或插电式混合动力电动车等。为了满足该用电装置对二次电池的高功率和高能量密度的需求，可以采用由二次电池组装而成的电池包或电池模块。
作为另一个示例的装置可以是手机、平板电脑、笔记本电脑等。该装置通常要求轻薄化，可以采用二次电池作为电源。
实施例
以下，说明本申请的实施例。下面描述的实施例是示例性的，仅用于解释本申请，而不能理解为对本申请的限制。实施例中未注明具体技术或条件的，按照本领域内的文献所描述的技术或条件或者按照产品说明书进行。所用试剂或仪器未注明生产厂商者，均为可以通过市购获得的常规产品。
实施例1-23、比较例1-4
按照下述方法制备实施例1-23、比较例1-4的负极浆料、负极极 片以及二次电池。
(1)负极浆料的制备
将作为负极活性材料的人造石墨、作为导电剂的炭黑Super P、表1所示种类的第一粘结剂和第二粘结剂按96.5∶0.5∶1.5∶1.5的质量比进行混合，并加入适量的去离子水调节负极浆料的固体组分质量百分含量至表1所示的值。
(2)负极极片的制备
将上述(1)中得到的负极浆料涂覆于负极集流体铜箔的表面上，之后经过烘干、冷压、分切，得到负极极片，其中，控制挤压涂布机的泵速不同以使负极膜层冷压后的平均厚度d成为表1所示的厚度。
【表1】
Figure PCTCN2021143822-appb-000001
Figure PCTCN2021143822-appb-000002
(3)二次电池的制备
分别准备下述正极极片、隔离膜和电解液。
正极极片
将作为正极活性材料LFP、作为导电剂的炭黑Super P、作为粘结剂的PVDF按质量比96∶2∶2在适量的NMP中充分搅拌混合，使其形成均匀的正极浆料；以保证上述负极极片和正极极片的活性物质容量比CB＝1.1的条件将正极浆料涂覆于正极集流体铝箔的表面上。经干燥、冷压得到正极极片，正极极片的压实密度为2.4g/cm 3。
隔离膜
使用PP/PE复合隔离膜。
电解液
将碳酸亚乙酯(EC)、碳酸甲乙酯(EMC)、碳酸二乙酯(DEC)按体积比1∶1∶1混合，然后将LiPF6均匀溶解在上述溶液中并调节LiPF6的浓度为1mol/L，得到电解液。
将正极极片、隔离膜以及负极极片按顺序堆叠并卷绕，得到电极组件；将电极组件放入外包装中，加入上述制备的电解液，经封装、静置、化成、老化等工序后，得到二次电池。
测试方法
(a)负极浆料相关性能测试：
1、浆料稳定保存时间的测试
取上述(1)中制备的浆料在室温条件下静置，使用1ml注射器(针头直径330μm)从尾部装入浆料，并从针头部挤出浆料，每天定时进行测试，将直至挤出时浆料出现阻塞针孔而无法完全挤出1ml浆料时的天数(天)作为浆料的稳定保存时间，并将其记载于表2中。
2、浆料的100μm狭缝挤出压力
使用附带压力传感器的挤压涂布机将上述(1)中制备的浆料通过 宽度为100μm的狭缝挤出，并控制流量为100ml/min，读取压力传感器测出的压力数值，并将其记载于下述表2中。
(b)负极极片相关性能测试：
1、负极极片的卷绕鼓边情况
将在上述(2)的负极极片的制备过程中，将进行了单层涂布且干燥压实之前的极片使用直径为10cm的卷筒，以极片的宽度方向为轴向进行收紧卷绕，卷绕100m极片。测量卷筒轴向中心处的周长l 1与负极膜层边缘1mm处周长l 2，以鼓边度y＝(l 2-l 1)/l 1计算鼓边度值，并以下述标准判定鼓边程度，并将结果示于下述表2中。
评价基准：
不鼓边：y≤0.5％；
轻微鼓边：0.5＜y≤1％；
严重鼓边：y＞1％。
2、负极膜层的涂布开裂情况
在上述(2)的负极极片的制备过程中，在将浆料涂布于负极集流体并于120℃烘道中干燥后，取4段2m干燥后极片，观察极片外观，以下述标准判定表面开裂程度，并将结果示于下述表2中。
评价基准：
不开裂：无明显肉眼可见裂纹；
轻微开裂：肉眼可见裂纹，所有裂纹长度≤2cm且2m极片平均裂纹数目≤10；
严重开裂：出现裂纹长度＞2cm或2m极片平均裂纹数目＞10。
3、负极膜层截面形状
将上述(2)中得到的干燥冷压后负极极片中裁剪出包括一个完整的垂直于集流体长度方向的横截面的样品，对该完整的垂直于集流体长度方向的横截面进行金相研磨或等离子抛光，并将其背面用水晶胶固定于样品台后，置于光学显微镜(CCD)或电子显微镜下观察并量出负极极片中负极膜层的a，计算a/d值，并将其示于表2中。其中，d表示厚度均匀区域的平均厚度，该厚度均匀区域是指在光学显微镜(CCD)或电子显微镜的观察下，在负极膜层的宽度方向上中间部位 的厚度没有出现明显变化的区域。“厚度没有出现明显变化”定义为：在负极膜层的任一100μm宽度内任意量取10次厚度数值d 1、d 2、...d 10，其厚度的变异系数CV d≤5％，视为在该100μm宽度内厚度均匀变化。a表示位于宽度方向上两端的边缘区域在宽度方向上的投影的长度，该边缘区域是指光学显微镜(CCD)的观察下，从集流体上开始出现负极膜层的位置至在厚度均匀区域端缘开始出现厚度连续降低趋势的位置为止的区域。
4、负极极片的充放电后边缘析锂情况
在25℃下，对每个实施例和对比例制备10个二次电池，分别以1C满充、以1C满放重复10次后，再将锂离子二次电池以3C满充，然后拆解出负极极片，观察各电池中负极极片边缘析锂情况，统计出现边缘析锂的二次电池的个数，并以边缘析锂的二次电池的个数/10作为边缘析锂概率示于表2中。
Figure PCTCN2021143822-appb-000003
由实施例1-23与比较例1和3、4相比可知，负极膜层的a/d与二次电池充放电后负极析锂的产生情况密切相关，在控制所制备的负极膜层厚度d≤300μm的情况下，a/d≤1.5时，大大降低了二次电池边缘析锂概率，从而可以提高二次电池的安全性。进一步可知，在a/d≤1时，可进一步降低二次电池边缘析锂概率至1/10以下，在a/d≤0.75时，甚至未发现二次电池出现边缘析锂情况。
由除厚度不同其它实验参数均相同的实施例9-14与比较例2的结果比较可知，在控制d≤300um时，可明显改善负极极片的边缘鼓边和涂布开裂情况，提高极片可加工性能，并且还能够明显改善二次电池充放电后负极析锂的产生情况，从而可以提高二次电池的安全性。
由实施例4-8的结果比较可知，通过将浆料固体组分含量调节在60％-75％范围内，可获得a/d较小的负极极片，尤其是当固体组分含量调节为60-70％范围时，可进一步降低挤压涂布的唇口压力，更易于挤出加工，从而可以降低制备过程中的设备磨损。
由实施例1-4和实施例10的结果比较可知，在使用羧甲基淀粉钠(CMS-Na)作为第一粘结剂时，通过采用2.0≤PDI≤5.5的羧甲基淀粉钠，可以获得高稳定性浆料，并且可降低挤压涂布的唇口压力至0.75Mpa以下，即，可以得到易于挤出加工的浆料，从而可以降低制备过程中的设备磨损。
进一步由同样使用羧甲基淀粉钠(CMS-Na)作为第一粘结剂的实施例10、15-16的结果比较可知，在羧甲基淀粉钠的重均分子量在400000-600000的范围内时，均可兼顾浆料的稳定性和易于挤出性以及所制备的负极极片的加工性和安全性。
实施例17-20的结果表明，也可使用其它保水性材料如羧甲基纤维素钠(CMC-Na)作为第一粘结剂来制备本发明负极极片，其中在羧甲基纤维素钠的重均分子量在小于等于400000，尤其是在200000-300000的范围内时，可兼顾浆料的稳定性和易于挤出性以及所制备的负极极片的加工性和安全性。
另外，实施例1-20与实施例21-23的结果比较表明也可使用其 它种类的第二粘结剂来制备本发明负极极片。
此外，还进一步考察了实施例10-11，14、17、21以及对比例1-3中得到的负极极片所制备的二次电池的容量保持率，按照下述方法测定了这些各例的二次电池在45℃条件下循环500次后的容量保持率。
在45℃下，将二次电池以1C倍率恒流充电至充电截止电压3.65V，之后恒压充电至电流≤0.05C，静置10min，再以0.5C倍率恒流放电至放电截止电压2.5V，静置10min，此为第一个充放电循环，放电容量记为C 0。按照此方法对电池进行500次(cls)充放电循环测试，第500次的放电容量为C 1。以C 1/C 0作为45℃下500次循环容量保持率进行计算，并将结果示于下述表3中。
【表3】
Figure PCTCN2021143822-appb-000004
由表3的结果可知，在负极膜层的a/d的值大于1.5的情况下，还会导致容量保持率降低。另外，从比较例2的结果可知，在负极膜层过厚的情况下，可能由于较厚极片较低的动力学，也会进一步恶化二次电池的循环容量保持率。
需要说明的是，本申请不限定于上述实施方式。上述实施方式仅为示例，在本申请的技术方案范围内具有与技术思想实质相同的构成、发挥相同作用效果的实施方式均包含在本申请的技术范围内。此外，在不脱离本申请主旨的范围内，对实施方式施加本领域技术人员能够想到的各种变形、将实施方式中的一部分构成要素加以组 合而构筑的其它方式也包含在本申请的范围内。

Claims (20)

  1. 一种负极极片，包括负极集流体和位于所述负极集流体表面的负极膜层，
    所述负极膜层具有与所述负极集流体的长度方向一致的长度方向以及与所述负极集流体的宽度方向一致的宽度方向，
    所述负极膜层的垂直于长度方向的截面包括位于所述宽度方向上中间部位的厚度均匀区域以及位于所述宽度方向上两端的边缘区域，
    在所述边缘区域的任一者，厚度沿着从所述厚度均匀区域朝向该边缘区域的方向连续减小，
    所述边缘区域中的任一者的宽度a与所述厚度均匀区域的平均厚度d满足关系式：a/d≤1.5，
    且d≤300μm。
  2. 如权利要求1所述的负极极片，其中，a/d≤1；可选地，a/d≤0.75。
  3. 如权利要求1或2所述的负极极片，其中，所述负极膜层包含负极活性材料、导电剂、第一粘结剂和第二粘结剂；
    可选地，所述第一粘结剂包括(甲基)丙烯酸、聚丙烯酰胺、纤维素醚或其盐、淀粉醚或其盐中的至少一种；
    可选地，所述第二粘结剂包括丙烯酸酯-乙烯、丁苯橡胶、丁腈橡胶、丙烯酸酯、聚氨酯中的至少一种。
  4. 根据权利要求3所述的负极极片，其中，所述第一粘结剂包括羧甲基纤维素钠，所述羧甲基纤维素钠的重均分子量为400000以下，可选为200000-300000。
  5. 根据权利要求3或4所述的负极极片，其中，所述第一粘结剂 包括羧甲基纤维素钠，所述羧甲基纤维素钠的取代度为0.65-0.8。
  6. 根据权利要求3-5中任一项所述的负极极片，其中，所述第一粘结剂包括羧甲基淀粉钠，所述羧甲基淀粉钠的分散性指数PDI为2.0-5.5，可选为2.5-4。
  7. 根据权利要求3-6中任一项所述的负极极片，其中，所述第一粘结剂包括羧甲基淀粉钠，所述羧甲基淀粉钠的重均分子量为400000以上，可选为400000-600000。
  8. 根据权利要求3-7中任一项所述的负极极片，其中，所述第二粘结剂为丁苯橡胶，其体积平均粒径Dv50满足：80nm≤Dv50≤200nm。
  9. 根据权利要求3-8中任一项所述的负极极片，其中，所述第一粘结剂和所述第二粘结剂的质量含量比例范围为2∶1-1∶2，可选为1.5∶1-1∶1.5。
  10. 一种负极极片的制备方法，所述负极极片的制备方法包括如下步骤：
    将负极浆料涂布于所述负极集流体并经干燥、冷压从而获得所述负极极片，所述负极极片是权利要求1或2所述的负极极片。
  11. 根据权利要求10所述的负极极片的制备方法，其中，所述负极浆料的固体组分质量百分含量大于等于60％，可选为60％-75％。
  12. 根据权利要求10或11所述的负极极片的制备方法，其中，
    所述负极浆料包含负极活性材料、导电剂、第一粘结剂和第二粘结剂，
    所述制备方法包括：将所述负极活性材料、所述导电剂、所述第一粘结剂、所述第二粘结剂和去离子水混合获得所述浆料的步骤；以 及将所述浆料挤压涂布于所述集流体表面，经干燥、冷压以获得所述负极极片的步骤，
    可选地，所述第一粘结剂为选自(甲基)丙烯酸、聚丙烯酰胺、纤维素醚或其盐、淀粉醚或其盐中的至少一种；
    可选地，所述第二粘结剂为选自丙烯酸酯-乙烯、丁苯橡胶、丁腈橡胶、丙烯酸酯、聚氨酯中的至少一种。
  13. 根据权利要求12所述的负极极片的制备方法，其中，所述第一粘结剂包括羧甲基纤维素钠，所述羧甲基纤维素钠的重均分子量为400000以下，可选为200000-300000。
  14. 根据权利要求12或13所述的负极极片的制备方法，其中，所述第一粘结剂包括羧甲基纤维素钠，所述羧甲基纤维素钠的取代度为0.65-0.8。
  15. 根据权利要求12-14中任一项所述的负极极片的制备方法，其中，所述第一粘结剂包括羧甲基淀粉钠，所述羧甲基淀粉钠的分散性指数PDI为2.0-5.5，可选为2.5-4。
  16. 根据权利要求12-15中任一项所述的负极极片的制备方法，其中，所述第一粘结剂包括羧甲基淀粉钠，所述羧甲基淀粉钠的重均分子量为400000以上，可选为400000-600000。
  17. 根据权利要求12-16中任一项所述的负极极片的制备方法，其中，所述第二粘结剂为丁苯橡胶，其体积平均粒径Dv50满足：80nm≤Dv50≤200nm。
  18. 根据权利要求12-17中任一项所述的负极极片的制备方法，其中，所述第一粘结剂和所述第二粘结剂的质量含量比例范围为2∶1-1∶2，可选为1.5∶1-1∶1.5。
  19. 一种二次电池，其中，具备权利要求1-9中任一项所述的负极极片或通过权利要求10-18中任一项所述的负极极片的制备方法制造的负极极片。
  20. 一种用电装置，其中，包括权利要求19所述的二次电池。
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