CN111900357B - Negative plate and lithium ion battery comprising same - Google Patents

Negative plate and lithium ion battery comprising same Download PDF

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Publication number
CN111900357B
CN111900357B CN202010815091.XA CN202010815091A CN111900357B CN 111900357 B CN111900357 B CN 111900357B CN 202010815091 A CN202010815091 A CN 202010815091A CN 111900357 B CN111900357 B CN 111900357B
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negative electrode
active material
material layer
carboxymethyl cellulose
electrode active
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CN111900357A (en
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陈伟平
李素丽
郭盼龙
储霖
李俊义
徐延铭
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Zhuhai Cosmx Battery Co Ltd
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Zhuhai Cosmx Battery Co Ltd
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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
    • H01M4/36—Selection of substances as active materials, active masses, active liquids
    • H01M4/362—Composites
    • H01M4/366—Composites as layered products
    • 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
    • H01M10/0525—Rocking-chair batteries, i.e. batteries with lithium insertion or intercalation in both electrodes; Lithium-ion batteries
    • 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
    • H01M4/1399—Processes of manufacture of electrodes based on electro-active 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
    • H01M4/36—Selection of substances as active materials, active masses, active liquids
    • H01M4/60—Selection of substances as active materials, active masses, active liquids of organic compounds
    • H01M4/602—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
    • 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
    • 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

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Abstract

The invention provides a negative plate and a lithium ion battery comprising the same; the negative electrode sheet comprises a negative electrode current collector, a first negative electrode active material layer and a first polymer curing layer; the first negative electrode active material layer includes a negative electrode active material and a cured product of a first carboxymethyl cellulose salt; the concentration of the cured substance of the first carboxymethyl cellulose salt is distributed in a gradient way and is gradually reduced; the first polymer cured layer includes a cured product of a first carboxymethyl cellulose salt. The introduction of the first carboxymethyl cellulose salt in the polymer curing layer greatly improves the structural strength of the negative plate in the three-dimensional direction, improves the interface bonding effect between the negative plate and the diaphragm, ensures ion conduction, reduces the expansion rate of the battery core in the cycle process, and prolongs the cycle life of the battery core.

Description

Negative plate and lithium ion battery comprising same
Technical Field
The invention belongs to the technical field of lithium ion batteries, and particularly relates to a negative plate and a lithium ion battery comprising the same.
Background
In recent years, with the increasing popularization of new energy automobiles and the rapid development of electronic devices such as notebook computers, smart phones and the like, the development and progress of lithium ion battery technology serving as an energy supply core are driven, and meanwhile, higher and higher requirements are provided for the lithium ion battery, such as higher energy density, higher charging and discharging speed, better cruising ability, looser use environment and the like.
The negative plate is one of the most main components of the lithium ion battery, and the traditional negative plate consists of a negative active material, a binder, a conductive agent, a thickening agent and a current collector; with the increasing of the energy density, the compacted density of the negative electrode is larger and larger, the content of the negative electrode active material is higher and higher, the addition amount of auxiliary materials such as a binder is reduced continuously, and meanwhile, a high-volume expanded silicon material is tried to be applied as the negative electrode active material.
However, as the amount of additives such as binders is decreased, the interfacial adhesion effect between the active material layer and the current collector of the lithium ion battery is gradually deteriorated, and the structural stability of the negative electrode sheet is gradually decreased as a silicon material having high volume expansion is present as a negative electrode active material, which further decreases the electrical properties of the lithium ion battery. Therefore, it is urgently needed to find a negative electrode sheet capable of solving and improving structural stability and improving interface bonding effect with a diaphragm and a lithium ion battery comprising the negative electrode sheet.
Disclosure of Invention
In order to overcome the defects of the prior art, the invention provides the negative plate and the lithium ion battery comprising the negative plate, wherein the negative plate has very good structural strength and stability, the lithium ion battery prepared by using the negative plate has high energy density and small cyclic expansion rate, and the service life of the lithium ion battery can be effectively prolonged.
The purpose of the invention is realized by the following technical scheme:
a negative electrode sheet comprising a negative electrode current collector, a first negative electrode active material layer, and a first polymer cured layer; the first negative electrode active material layer is arranged on the first surface of the negative electrode current collector, and the first polymer curing layer is arranged on the surface of the first negative electrode active material layer;
wherein the first anode active material layer includes an anode active material and a cured product of a first carboxymethyl cellulose salt; the cured product of the first carboxymethyl cellulose salt is distributed in the first negative electrode active material layer, and the concentration of the cured product of the first carboxymethyl cellulose salt is distributed in a gradient manner and gradually decreases along the direction of the contact surface of the first negative electrode active material layer and the first polymer cured layer facing the negative electrode current collector;
the first polymer cured layer includes a cured product of a first carboxymethyl cellulose salt.
According to the present invention, the negative electrode sheet further includes a second negative electrode active material layer provided on a second surface of the negative electrode current collector opposite to the first surface and a second polymer cured layer provided on a surface of the second negative electrode active material layer; the composition and structure of the second anode active material layer are the same as those of the first anode active material layer, and the composition and structure of the second polymer cured layer are the same as those of the first polymer cured layer.
According to the present invention, the cured product of the first carboxymethyl cellulose salt in the first anode active material layer forms a dendritic cured structure of a gradient distribution, distributed in the first anode active material layer.
According to the present invention, the cured product of the first carboxymethyl cellulose salt in the second anode active material layer forms a dendritic cured structure in a gradient distribution, and is distributed in the second anode active material layer.
According to the present invention, in the first anode active material layer and the first polymer cured layer, the cured product of the first carboxymethyl cellulose salt accounts for 0.1 to 3 wt%, such as 0.1 wt%, 0.2 wt%, 0.5 wt%, 0.8 wt%, 1 wt%, 1.2 wt%, 1.5 wt%, 1.8 wt%, 2 wt%, 2.2 wt%, 2.5 wt%, 2.8 wt%, or 3 wt%, of the total mass of the first anode active material layer and the first polymer cured layer.
According to the present invention, in the second anode active material layer and the second polymer cured layer, the cured product of the first carboxymethyl cellulose salt accounts for 0.1 to 3 wt% of the total mass of the second anode active material layer and the second polymer cured layer, such as 0.1 wt%, 0.2 wt%, 0.5 wt%, 0.8 wt%, 1 wt%, 1.2 wt%, 1.5 wt%, 1.8 wt%, 2 wt%, 2.2 wt%, 2.5 wt%, 2.8 wt%, or 3 wt%.
According to the invention, the first carboxymethyl cellulose salt is selected from lithium carboxymethyl cellulose or sodium carboxymethyl cellulose.
According to the invention, the weight-average molecular weight M of the first carboxymethyl cellulose saltwIs 100-.
According to the present invention, the thickness of the first anode active material layer is 10 to 80 μm, such as 10 μm, 20 μm, 30 μm, 40 μm, 50 μm, 60 μm, 70 μm, or 80 μm.
According to the present invention, the thickness of the second anode active material layer is 10 to 80 μm, such as 10 μm, 20 μm, 30 μm, 40 μm, 50 μm, 60 μm, 70 μm, or 80 μm.
According to the present invention, the thickness of the first polymer cured layer is 1/1000 to 3/100, such as 1/1000, 1/500, 1/300, 1/200, 1/100, 2/100, 3/100 of the thickness of the first anode active material layer. The first polymer cured layer with the thickness in the range can avoid the blocking of the gaps between the isolating membrane and the membrane by excessive polymers.
According to the present invention, the thickness of the second polymer cured layer is 1/1000 to 3/100, such as 1/1000, 1/500, 1/300, 1/200, 1/100, 2/100, 3/100 of the thickness of the second anode active material layer. The second polymer cured layer with the thickness in the range can avoid the blocking of the gaps between the isolating membrane and the membrane by excessive polymers.
The invention also provides a lithium ion battery which comprises the negative plate.
The invention has the beneficial effects that:
the invention provides a negative plate and a lithium ion battery comprising the same; compared with the prior art, the introduction of the first carboxymethyl cellulose salt in the polymer curing layer and the negative active material layer greatly improves the structural strength of the negative plate in the three-dimensional direction, improves the interface bonding effect between the negative plate and the diaphragm, ensures ion conduction, reduces the expansion rate in the battery cell circulation process, and prolongs the cycle life of the battery cell.
Drawings
Fig. 1 is a schematic structural view of a negative electrode sheet before drying according to the present invention.
Reference numerals: reference numeral 1 denotes a negative electrode current collector, 2 denotes a slurry of the first negative electrode active material layer, and 3 denotes a slurry of the first polymer cured material layer.
Detailed Description
In one embodiment of the invention, the first carboxymethyl cellulose salt is selected from lithium carboxymethyl cellulose or sodium carboxymethyl cellulose, preferably lithium carboxymethyl cellulose.
In the invention, the first carboxymethyl cellulose salt can generate a curing reaction in the drying process to form a cured product of the first carboxymethyl cellulose salt, and the first carboxymethyl cellulose salt and the cured product thereof have better ionic conductivity and can form a lithium ion migration path.
In one embodiment of the present invention, the weight average molecular weight M of the first carboxymethyl cellulose saltwIs 100-. The weight average molecular weight of the first carboxymethyl cellulose salt is controlled within 100-10000, so that the coating effect of the negative active material layer caused by overlarge molecular weight can be avoided, good slurry flowability can be brought, the diffusion of the slurry forming the high polymer curing layer to the slurry forming the negative active material layer is promoted, the first carboxymethyl cellulose salt with concentration gradient distribution can be formed, the negative active material layer with a curing structure containing the first carboxymethyl cellulose salt with gradient distribution can be formed after the first carboxymethyl cellulose salt is cured, and a better structure supporting effect on the negative active material can be achieved.
In one embodiment of the present invention, the degree of substitution of the first carboxymethyl cellulose salt is 0.5 to 1.1.
In one embodiment of the invention, the negative current collector is selected from copper foil or copper foil with a functional coating.
In one embodiment of the present invention, the thickness of the negative electrode current collector is 8 to 12 μm.
In one embodiment of the present invention, the anode active material layer further includes a conductive agent, a thickener, and a binder.
In one embodiment of the present invention, the mass ratio of the negative electrode active material, the conductive agent, the thickener, and the binder in the negative electrode active material layer is (70-98.5): (0.5-10).
Preferably, the mass ratio of the negative electrode active material, the conductive agent, the thickener and the binder in the negative electrode active material layer is (76-97): (1-8).
It is also preferable that the mass ratio of the negative electrode active material, the conductive agent, the thickener, and the binder in the negative electrode active material layer is (86-97): (1-4).
In one embodiment of the present invention, the conductive agent is at least one selected from the group consisting of conductive carbon black, acetylene black, ketjen black, conductive graphite, conductive carbon fiber, carbon nanotube, metal powder, and carbon fiber.
In one embodiment of the present invention, the binder is selected from at least one of styrene-butadiene latex, polytetrafluoroethylene, and polyethylene oxide.
In one embodiment of the invention, the thickening agent is selected from a second carboxymethyl cellulose salt selected from lithium carboxymethyl cellulose or sodium carboxymethyl cellulose, preferably sodium carboxymethyl cellulose (CMC).
In one embodiment of the present invention, the negative active material is selected from at least one of artificial graphite, natural graphite, mesocarbon microbeads, lithium titanate, silicon material, or silicon carbon material.
In the invention, the carboxymethyl cellulose salt is a partially crystallized polymer, has high strength, resists electrolyte corrosion, resists aging and fatigue, and can keep stable structure for a long time. The solidified first carboxymethyl cellulose salt can be on the surface of the negative active material and forms a structural support with a certain depth with the negative active material, so that the whole negative plate structure is fixed by a frame, the expansion of the electrode in the vertical and horizontal directions in the charge-discharge process is reduced, the expansion of the plate is greatly reduced, and the effect is more obvious particularly in a silicon-doped negative electrode system with large volume expansion; meanwhile, strong hydrogen bonds and van der waals force can be formed between the first carboxymethyl cellulose salt in the polymer curing layer and the diaphragm, the interfacial adhesion effect between the negative plate and the diaphragm is enhanced, the stability of the electrode structure is maintained, and the battery cell performance is further improved. The solidified second carboxymethyl cellulose salt is uniformly dispersed in the negative electrode active material layer, and can also exert a supporting effect on the negative electrode active material.
The invention also provides a preparation method of the negative plate, which comprises the following steps:
1) preparing slurry for forming a first negative electrode active material layer and slurry for forming a first polymer cured layer, respectively;
2) and coating the slurry for forming the first negative electrode active material layer and the slurry for forming the first polymer curing layer on the first surface of the negative electrode current collector by using a double-layer coating machine, and drying to prepare the negative electrode sheet.
In one embodiment of the present invention, the method comprises the steps of:
1) preparing slurry for forming a first negative electrode active material layer and slurry for forming a first polymer cured layer, respectively;
2) coating the slurry for forming the first negative electrode active material layer and the slurry for forming the first polymer cured layer on a first surface of a negative electrode current collector by using a double-layer coater, and drying;
3) preparing a slurry for forming a second negative electrode active material layer and a slurry for forming a second polymer cured layer, respectively;
4) and coating the slurry for forming the second negative electrode active material layer and the slurry for forming the second polymer curing layer on a second surface, opposite to the first surface, of the negative electrode current collector by using a double-layer coating machine, and drying to prepare the negative electrode sheet.
In one embodiment of the present invention, the method comprises the steps of:
1) preparing slurry for forming a first negative electrode active material layer, slurry for forming a first polymer cured layer, slurry for forming a second negative electrode active material layer and slurry for forming a second polymer cured layer, respectively;
2) and coating the slurry for forming the first negative electrode active material layer and the slurry for forming the first polymer curing layer on a first surface of a negative electrode current collector by using a double-layer coating machine, coating the slurry for forming the second negative electrode active material layer and the slurry for forming the second polymer curing layer on a second surface, opposite to the first surface, of the negative electrode current collector, and drying to prepare the negative electrode sheet.
In step 1), the solid content of the slurry for forming the first anode active material layer is 40 wt% to 45 wt%.
In the step 1), the viscosity of the slurry for forming the first cured polymer layer is 100-1000mPa · s.
In the step 2), the drying temperature is 45-100 ℃, and the drying time is 10-600 s.
In step 2), as shown in fig. 1, when the slurry for forming the polymer cured layer is coated on the surface of the slurry for forming the negative electrode active material layer, two slurry layers are formed on the surface of the negative electrode current collector, and after the coating is completed, the slurry for forming the polymer cured layer on the upper layer permeates into the slurry for forming the negative electrode active material layer on the lower layer, so as to form a first carboxymethyl cellulose salt with a concentration gradient distribution; in the drying process, along with the continuous volatilization of moisture, the first carboxymethyl cellulose salt is adsorbed on the surface of the negative electrode active material and is solidified to form a gradient dendritic solidified structure, and the concentration of a solidified substance of the first carboxymethyl cellulose salt is distributed in a gradient manner and gradually reduced along the direction of the contact surface of the first negative electrode active material layer and the first polymer solidified layer facing the negative electrode current collector. Forming a second carboxymethyl cellulose salt with uniformly distributed concentration in the slurry for forming the negative electrode active material layer of the lower layer; in the drying process, as the moisture is continuously volatilized, the second carboxymethyl cellulose salt is adsorbed on the surface of the negative active material and is solidified, and a solidified structure with uniformly distributed concentration is formed.
In step 3) and step 1), the solid content of the slurry for forming the second anode active material layer is 40 wt% to 45 wt%.
In the step 3) and the step 1), the viscosity of the slurry for forming the second cured polymer layer is 100-1000mPa · s.
In the step 4), the drying temperature is 45-100 ℃, and the drying time is 10-600 s.
In step 4), as shown in fig. 1, when the slurry for forming the polymer cured layer is coated on the surface of the slurry for forming the negative electrode active material layer, two slurry layers are formed on the surface of the negative electrode current collector, and after the coating is completed, the slurry for forming the polymer cured layer on the upper layer permeates into the slurry for forming the negative electrode active material layer on the lower layer, so as to form a first carboxymethyl cellulose salt with a concentration gradient distribution; in the drying process, along with the continuous volatilization of moisture, the first carboxymethyl cellulose salt is adsorbed on the surface of the negative electrode active material and is solidified to form a gradient dendritic solidified structure, and the concentration of a solidified substance of the first carboxymethyl cellulose salt is distributed in a gradient manner and gradually reduced along the direction of the contact surface of the first negative electrode active material layer and the first polymer solidified layer facing the negative electrode current collector. Forming a second carboxymethyl cellulose salt with uniformly distributed concentration in the slurry for forming the negative electrode active material layer of the lower layer; in the drying process, as the moisture is continuously volatilized, the second carboxymethyl cellulose salt is adsorbed on the surface of the negative active material and is solidified, and a solidified structure with uniformly distributed concentration is formed.
The present invention will be described in further detail with reference to specific examples. It is to be understood that the following examples are only illustrative and explanatory of the present invention and should not be construed as limiting the scope of the present invention. All the technologies realized based on the above-mentioned contents of the present invention are covered in the protection scope of the present invention.
The experimental methods used in the following examples are all conventional methods unless otherwise specified; reagents, materials and the like used in the following examples are commercially available unless otherwise specified.
In the description of the present invention, it should be noted that the terms "first", "second", etc. are used for descriptive purposes only and do not indicate or imply relative importance.
Example 1
Lithium carboxymethyl cellulose (Mw ≈ 500) is dispersed in water to prepare a slurry 1 with a concentration of 20.0% and a viscosity of 489 mPas;
preparing a slurry 2 by using water as a solvent according to the mass ratio of a negative electrode active substance (artificial graphite), a conductive agent (super-p), a thickening agent (CMC) and a binder (SBR) of 96:1.0:1.5:1.5, coating the slurry 2 and the slurry 1 on two sides of a copper foil with the thickness of 8 mu m by a double-die head coating machine, and drying and rolling to obtain a negative electrode sheet;
the two side surfaces of the current collector of the negative plate respectively comprise a negative active material layer and a polymer curing layer, and the solidified substance of the lithium carboxymethyl cellulose accounts for 2 wt% of the total mass of the negative active material layer and the polymer curing layer; the thickness of the polymer cured layer is 1/1000-3/100 of the thickness of the negative electrode active material layer.
Example 2
Dispersing lithium carboxymethyl cellulose (Mw ≈ 1000) in water to obtain slurry 1 with concentration of 10.0% and viscosity of 372mPa · s;
preparing a slurry 2 by using water as a solvent according to the mass ratio of 96:1.0:1.5:1.5 of a negative electrode active substance (artificial graphite), a conductive agent (super-p), a thickening agent (CMC) and a binder (SBR) and coating the slurry 2 and the slurry 1 on two sides of a copper foil with the thickness of 8 mu m by a double-die head coating machine, and drying and rolling to obtain a negative electrode sheet;
the two side surfaces of the current collector of the negative plate respectively comprise a negative active material layer and a polymer curing layer, and the solidified substance of the lithium carboxymethyl cellulose accounts for 2 wt% of the total mass of the negative active material layer and the polymer curing layer; the thickness of the polymer cured layer is 1/1000-3/100 of the thickness of the negative electrode active material layer.
Example 3
Lithium carboxymethyl cellulose (Mw ≈ 2000) is dispersed in water to prepare 5.0% slurry 1 with viscosity of 519mPa · s;
preparing a slurry 2 by using water as a solvent according to the mass ratio of 96:1.0:1.5:1.5 of a negative electrode active substance (artificial graphite), a conductive agent (super-p), a thickening agent (CMC) and a binder (SBR) and coating the slurry 2 and the slurry 1 on two sides of a copper foil with the thickness of 8 mu m by a double-die head coating machine, and drying and rolling to obtain a negative electrode sheet;
the two side surfaces of the current collector of the negative plate respectively comprise a negative active material layer and a polymer curing layer, and the solidified substance of the lithium carboxymethyl cellulose accounts for 2 wt% of the total mass of the negative active material layer and the polymer curing layer; the thickness of the polymer cured layer is 1/1000-3/100 of the thickness of the negative electrode active material layer.
Example 4
Dispersing lithium carboxymethyl cellulose (Mw ≈ 5000) in water to obtain slurry 1 with concentration of 4.0% and viscosity of 481 mPas;
preparing a slurry 2 by using water as a solvent according to the mass ratio of 96:1.0:1.5:1.5 of a negative electrode active substance (artificial graphite), a conductive agent (super-p), a thickening agent (CMC) and a binder (SBR) and coating the slurry 2 and the slurry 1 on two sides of a copper foil with the thickness of 8 mu m by a double-die head coating machine, and drying and rolling to obtain a negative electrode sheet;
the two side surfaces of the current collector of the negative plate respectively comprise a negative active material layer and a polymer curing layer, and the solidified substance of the lithium carboxymethyl cellulose accounts for 2 wt% of the total mass of the negative active material layer and the polymer curing layer; the thickness of the polymer cured layer is 1/1000-3/100 of the thickness of the negative electrode active material layer.
Example 5
Dispersing lithium carboxymethyl cellulose (Mw is approximately equal to 10000) in water to prepare slurry 1 with the concentration of 2.0 percent and the viscosity of 850 mPas;
preparing a slurry 2 by using water as a solvent according to the mass ratio of 96:1.0:1.5:1.5 of a negative electrode active substance (artificial graphite), a conductive agent (super-p), a thickening agent (CMC) and a binder (SBR) and coating the slurry 2 and the slurry 1 on two sides of a copper foil with the thickness of 8 mu m by a double-die head coating machine, and drying and rolling to obtain a negative electrode sheet;
the two side surfaces of the current collector of the negative plate respectively comprise a negative active material layer and a polymer curing layer, and the solidified substance of the lithium carboxymethyl cellulose accounts for 2 wt% of the total mass of the negative active material layer and the polymer curing layer; the thickness of the polymer cured layer is 1/1000-3/100 of the thickness of the negative electrode active material layer.
Example 6
Dispersing lithium carboxymethyl cellulose (Mw ≈ 5000) in water to obtain slurry 1 with concentration of 4.0% and viscosity of 481 mPas;
preparing a slurry 2 by using water as a solvent according to the mass ratio of 96:1.0:1.5:1.5 of a negative electrode active substance (artificial graphite), a conductive agent (super-p), a thickening agent (CMC) and a binder (SBR) and coating the slurry 2 and the slurry 1 on two sides of a copper foil with the thickness of 8 mu m by a double-die head coating machine, and drying and rolling to obtain a negative electrode sheet;
the two side surfaces of the current collector of the negative plate respectively comprise a negative active material layer and a polymer curing layer, and the solidified substance of the lithium carboxymethyl cellulose accounts for 0.1 wt% of the total mass of the negative active material layer and the polymer curing layer; the thickness of the polymer cured layer is 1/1000-3/100 of the thickness of the negative electrode active material layer.
Example 7
Dispersing lithium carboxymethyl cellulose (Mw ≈ 5000) in water to obtain slurry 1 with concentration of 4.0% and viscosity of 481 mPas;
preparing a slurry 2 by using water as a solvent according to the mass ratio of 96:1.0:1.5:1.5 of a negative electrode active substance (artificial graphite), a conductive agent (super-p), a thickening agent (CMC) and a binder (SBR) and coating the slurry 2 and the slurry 1 on two sides of a copper foil with the thickness of 8 mu m by a double-die head coating machine, and drying and rolling to obtain a negative electrode sheet;
the two side surfaces of the current collector of the negative plate respectively comprise a negative active material layer and a polymer curing layer, and the solidified substance of the lithium carboxymethyl cellulose accounts for 0.2 wt% of the total mass of the negative active material layer and the polymer curing layer; the thickness of the polymer cured layer is 1/1000-3/100 of the thickness of the negative electrode active material layer.
Example 8
Dispersing lithium carboxymethyl cellulose (Mw ≈ 5000) in water to obtain slurry 1 with concentration of 4.0% and viscosity of 481 mPas;
preparing a slurry 2 by using water as a solvent according to the mass ratio of 96:1.0:1.5:1.5 of a negative electrode active substance (artificial graphite), a conductive agent (super-p), a thickening agent (CMC) and a binder (SBR) and coating the slurry 2 and the slurry 1 on two sides of a copper foil with the thickness of 8 mu m by a double-die head coating machine, and drying and rolling to obtain a negative electrode sheet;
the two side surfaces of the current collector of the negative plate respectively comprise a negative active material layer and a polymer curing layer, and the solidified substance of the lithium carboxymethyl cellulose accounts for 5 wt% of the total mass of the negative active material layer and the polymer curing layer; the thickness of the polymer cured layer is 1/1000-3/100 of the thickness of the negative electrode active material layer.
Example 9
Dispersing lithium carboxymethyl cellulose (Mw ≈ 200000) in water to obtain 1.0% slurry 1 with viscosity of 14500 mPas;
preparing a slurry 2 by using water as a solvent according to the mass ratio of 96:1.0:1.5:1.5 of a negative electrode active substance (artificial graphite), a conductive agent (super-p), a thickening agent (CMC) and a binder (SBR) and coating the slurry 2 and the slurry 1 on two sides of a copper foil with the thickness of 8 mu m by a double-die head coating machine, and drying and rolling to obtain a negative electrode sheet;
the negative electrode sheet comprises a negative electrode active material layer and a polymer curing layer on the two side surfaces of a current collector, wherein the carboxymethyl cellulose lithium has a relatively large molecular weight and cannot be blended with slurry for forming the negative electrode active material layer, and a solidified substance of the carboxymethyl cellulose lithium is mainly concentrated in the polymer curing layer; the content of the solidified material of lithium carboxymethyl cellulose was 2 wt% based on the total mass of the negative electrode active material layer and the polymer solidified layer; the thickness of the polymer cured layer is 1/1000-3/100 of the thickness of the negative electrode active material layer.
Comparative example 1
Adding a negative electrode active substance (artificial graphite), a conductive agent (super-p), a thickening agent (CMC) and a binder (SBR) into water according to the mass ratio of 96:1.0:1.5:1.5, uniformly mixing, coating on two sides of a copper foil with the thickness of 8 mu m, drying and rolling to obtain the negative electrode sheet.
Table 1 parameters of negative electrode sheets prepared in examples and comparative examples
Figure BDA0002632389160000111
Test example 1
Preparing a lithium ion battery:
preparing a positive plate: adding a positive electrode active substance (lithium cobaltate), a conductive agent (super-p) and a binder (PVDF) into N-methyl pyrrolidone according to the mass ratio of 97:2:1, uniformly mixing, coating on a positive electrode current collector (aluminum foil), drying at 90 ℃, rolling by using a roller press, cutting into pieces, cutting, drying in vacuum, and welding tabs to obtain the positive electrode sheet.
Preparing an electrolyte: uniformly mixing Ethylene Carbonate (EC), Propylene Carbonate (PC) and Ethyl Methyl Carbonate (EMC) according to the volume ratio of 1:1:1, adding lithium hexafluorophosphate to prepare 1mol/L electrolyte, and adding Vinylene Carbonate (VC) with the mass of 2% of the electrolyte and fluoroethylene carbonate with the mass of 5% of the electrolyte as additives to prepare the final electrolyte.
Winding the positive plate, the negative plate and the diaphragm (12 mu m polyethylene porous bare film) obtained by the preparation into a bare cell according to a conventional mode, placing the bare cell in a pit-punched aluminum-plastic film after hot pressing, and performing vacuum drying for 24 hours after pre-packaging; and testing that the moisture of the positive plate, the negative plate and the diaphragm is below 200ppm, injecting electrolyte, and then carrying out vacuum packaging to obtain the lithium ion battery.
And (3) testing the cycle retention rate and the cycle expansion rate of the prepared lithium ion battery:
at 25 ℃, the cell thickness D in the initial half-cell state was recorded1Charging to 4.2V at constant current and constant voltage of 1C, standing for 10min, discharging to 3.0V at constant current of 1C, standing for 10min, and recording initial discharge capacity C1(ii) a The capacity retention rate of different cycles of charge-discharge cycle is tested by the cycle of the discharge working step, and the last discharge capacity is marked as C2Capacity retention rate of C2/C1(ii) a And testing the thickness D of the battery in a full-charge state at the end of the cycle2Thickness expansion ratio of (D)2-D1)/D1. The test results are shown in tables 2 and 3.
From the data in table 2, it can be seen that:
comparing the data of the comparative example 1 and the data of the examples 1 to 5, it can be seen that the cycle retention rate of the lithium ion battery with the negative electrode sheet with the cured substance of the first carboxymethyl cellulose salt introduced is obviously improved, because the cured substance of the first carboxymethyl cellulose salt provides a good support effect for the structural stability of the negative electrode sheet at the later cycle stage, the problem of negative electrode sheet dusting is reduced, and meanwhile, the carboxymethyl cellulose lithium in the high-molecular cured layer has good affinity with the diaphragm, the interface bonding effect with the diaphragm is improved, and the occurrence of side reactions is reduced.
Table 2 test results of cycle retention rate of lithium ion battery prepared in table 2
Number of cycles 200T 500T 800T 1000T
Comparative example 1 96.00％ 91.10％ 80.10％ 75.40％
Example 9 93.30％ 87.50％ 77.50％ 70.30％
Example 1 96.00％ 91.30％ 82.00％ 78.90％
Example 2 96.30％ 91.50％ 82.90％ 79.80％
Example 3 97.10％ 92.20％ 85.80％ 83.10％
Example 4 97.20％ 92.50％ 86.30％ 84.90％
Example 5 96.60％ 91.00％ 82.10％ 78.70％
Example 6 96.20％ 91.50％ 81.80％ 76.70％
Practice ofExample 7 96.60％ 91.80％ 82.20％ 77.40％
Example 8 95.30％ 88.10％ 79.80％ 73.70％
Secondly, as can be seen from comparison of data in comparative example 1 and example 9, when the molecular weight of the selected lithium carboxymethyl cellulose is too large, the cycle performance is deteriorated, because the lithium carboxymethyl cellulose with large molecular weight is not easy to permeate into the negative active material layer during coating, the three-dimensional structure curing effect is reduced, and meanwhile, a large amount of non-conductive lithium carboxymethyl cellulose forms a thick polymer curing layer to block the gaps of the diaphragm and the infiltration of electrolyte, thereby hindering the electron and ion conductivity and causing the performance deterioration.
③ from the data comparison of examples 1-5, it can be seen that the selection of the first salts of carboxymethyl cellulose with different molecular weights has an influence on the performance, and the performance is best when the weight average molecular weight is about 5000.
From the data in table 3, it can be seen that:
it can be seen from the results of the cycle expansion tests of comparative example 1 and examples 1 to 5 that the cycle expansion rates of the lithium ion batteries incorporating the negative electrode sheet including the cured product of the first carboxymethyl cellulose salt are all reduced because the first carboxymethyl cellulose salt permeates from the surface to the inside of the negative electrode active material layer, the formed gradient cured negative electrode active material layer helps the negative electrode sheet, the rebound of the negative electrode sheet during the cycle process is reduced, the interface adhesion is firmer, and the expansion rate increase caused by the interface deterioration is reduced.
② from the comparison of the swelling ratio data of comparative example 1 and example 9, it is understood that the use of lithium carboxymethylcellulose having a high molecular weight not only fails to improve the cycle swelling ratio but also deteriorates because the polymer cured layer which has not penetrated into the inside brings about more resistance effect and lithium is more easily precipitated during the cycle, resulting in deterioration of the swelling ratio.
Comparing the expansion rate data of the embodiment 1-5 shows that the expansion rate of the selected carboxymethyl cellulose lithium is the lowest, wherein the weight average molecular weight of the selected carboxymethyl cellulose lithium is about 5000.
Table 3 results of cycle expansion test of the prepared lithium ion battery
Number of cycles 200T 500T 800T 1000T
Comparative example 1 7.80％ 9.10％ 10.20％ 15.30％
Example 9 8.10％ 9.70％ 11.40％ 21.80％
Example 1 7.10％ 8.80％ 9.60％ 12.30％
Example 2 6.90％ 8.40％ 9.40％ 10.50％
Example 3 6.10％ 7.90％ 8.70％ 9.50％
Example 4 4.40％ 5.10％ 7.20％ 7.60％
Example 5 6.80％ 8.50％ 9.70％ 11.00％
Example 6 7.60％ 9.00％ 10.00％ 14.10％
Example 7 7.50％ 8.90％ 9.90％ 12.60％
Example 8 7.56％ 9.06％ 9.77％ 13.81％
The embodiments of the present invention have been described above. However, the present invention is not limited to the above embodiment. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims (12)

1. A negative electrode sheet, wherein the negative electrode sheet comprises a negative electrode current collector, a first negative electrode active material layer, and a first polymer cured layer; the first negative electrode active material layer is arranged on the first surface of the negative electrode current collector, and the first polymer curing layer is arranged on the surface of the first negative electrode active material layer;
wherein the first anode active material layer includes an anode active material and a cured product of a first carboxymethyl cellulose salt; the cured product of the first carboxymethyl cellulose salt is distributed in the first negative electrode active material layer, and the concentration of the cured product of the first carboxymethyl cellulose salt is distributed in a gradient manner and gradually decreases along the direction of the contact surface of the first negative electrode active material layer and the first polymer cured layer facing the negative electrode current collector;
the first polymer cured layer comprises a cured product of a first carboxymethyl cellulose salt;
the cured product of the first carboxymethyl cellulose salt in the first negative electrode active material layer forms a dendritic cured structure in a gradient distribution, and is distributed in the first negative electrode active material layer.
2. The negative electrode sheet according to claim 1, wherein the negative electrode sheet further comprises a second negative electrode active material layer provided on a second surface of the negative electrode current collector opposite to the first surface, and a second polymer cured layer provided on a surface of the second negative electrode active material layer; the composition and structure of the second anode active material layer are the same as those of the first anode active material layer, and the composition and structure of the second polymer cured layer are the same as those of the first polymer cured layer.
3. The negative electrode sheet according to claim 2, wherein the cured product of the first carboxymethyl cellulose salt in the second negative electrode active material layer forms a dendritic cured structure of a gradient distribution, distributed in the second negative electrode active material layer.
4. The negative electrode sheet according to claim 1, wherein the cured product of the first carboxymethyl cellulose salt accounts for 0.1 to 3 wt% of the total mass of the first negative electrode active material layer and the first polymer cured layer.
5. The negative electrode sheet according to claim 2, wherein the cured product of the first carboxymethyl cellulose salt accounts for 0.1 to 3 wt% of the total mass of the second negative electrode active material layer and the second polymer cured layer.
6. The negative electrode sheet of claim 1, wherein the first carboxymethyl cellulose salt is selected from lithium carboxymethyl cellulose or sodium carboxymethyl cellulose.
7. The negative electrode sheet of claim 1, wherein the first carboxymethyl cellulose salt has a weight average molecular weight MwIs 100-.
8. The negative electrode sheet according to claim 1, wherein the thickness of the first negative electrode active material layer is 10 to 80 μm.
9. The negative electrode sheet according to claim 2, wherein the thickness of the second negative electrode active material layer is 10 to 80 μm.
10. The negative-electrode sheet according to any one of claims 1 to 9, wherein the thickness of the first polymer cured layer is 1/1000 to 3/100 of the thickness of the first negative-electrode active material layer.
11. The negative electrode sheet according to claim 2, wherein the thickness of the second polymer cured layer is 1/1000 to 3/100 of the thickness of the second negative electrode active material layer.
12. A lithium ion battery comprising the negative electrode sheet of any one of claims 1 to 11.
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