CN114188539A - Method for preparing negative pole piece, negative pole piece prepared by method and lithium ion battery comprising negative pole piece - Google Patents

Method for preparing negative pole piece, negative pole piece prepared by method and lithium ion battery comprising negative pole piece Download PDF

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CN114188539A
CN114188539A CN202111472345.3A CN202111472345A CN114188539A CN 114188539 A CN114188539 A CN 114188539A CN 202111472345 A CN202111472345 A CN 202111472345A CN 114188539 A CN114188539 A CN 114188539A
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active material
negative electrode
material layer
slurry
pole piece
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修忠勋
沈桃桃
乔智
王洋
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China Lithium Battery Technology Co Ltd
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China Lithium Battery Technology 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
    • H01M4/62—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
    • 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
    • H01M4/04—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
    • H01M4/13—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
    • 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
    • H01M4/624—Electric conductive fillers
    • H01M4/625—Carbon or graphite
    • 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
    • H01M4/64—Carriers or collectors
    • H01M4/66—Selection of materials
    • H01M4/661—Metal or alloys, e.g. alloy coatings
    • 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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  • Chemical Kinetics & Catalysis (AREA)
  • Electrochemistry (AREA)
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Abstract

The method for preparing the lithium ion battery negative pole piece comprises the following steps: (1) mixing a first negative electrode active material with a first binder and a first conductive agent, and adding a solvent to prepare a first negative electrode active material slurry; (2) mixing a second negative electrode active material with a second binder, a second conductive agent and a plasticizer, and adding water to prepare a second negative electrode active material slurry; (3) coating the first negative electrode active material slurry on at least one side of a negative electrode current collector to form a first active material layer; (4) and coating the second negative electrode active material slurry on the first active material layer to form a second active material layer, and preparing the lithium ion battery negative electrode piece with the active material layer in a multilayer structure. Also provided are a negative electrode plate prepared thereby and a lithium ion battery comprising the negative electrode plate. The preparation method of the invention can not only avoid the cracking of the negative pole piece, but also improve the porosity of the active material layer, reduce the direct current resistance and further improve the cycle performance of the battery.

Description

Method for preparing negative pole piece, negative pole piece prepared by method and lithium ion battery comprising negative pole piece
Technical Field
The invention relates to a lithium ion battery negative pole piece and a preparation method thereof, in particular to a lithium ion battery negative pole piece with an active material layer of a multilayer structure, a preparation method thereof and a lithium ion battery comprising the negative pole piece.
Background
In recent years, high energy density lithium ion batteries have attracted much attention, and in order to meet the requirement of high energy density, the surface density of a negative electrode plate of the lithium ion battery needs to be increased. However, in the coating and baking process of the active material layer of the negative electrode plate with high surface density, because the difference between the evaporation rates of the solvent on the surface layer of the coating and the solvent inside the coating is large and the tension of the gas-liquid interface is inconsistent, the negative electrode plate is easy to curl and crack when being dried in an oven, so that the subsequent material dropping in the winding and die cutting process is caused, the quality of the negative electrode plate is influenced, and the performance of the battery is further influenced. Meanwhile, in the rolling process of the thick pole piece, the surface coating area bears larger pressure relative to the internal coating, so that the surface porosity is relatively low, the transmission of ions to the inside of the pole piece is not facilitated, and further, the polarization degree of the lithium battery is intensified, and the quick charging and the cycle performance are adversely affected.
At present, the method for improving the processing and electrical performance of the thick electrode is generally to increase the baking times, reduce the baking temperature and increase the air outlet speed in the coating and baking process, so as to improve the cracking phenomenon, but is not beneficial to the improvement of the production efficiency, or to perform laser cutting on the thick electrode, manufacture the pole piece pore, accelerate the ion transmission, but is difficult to realize batch production.
To this end, some studies have proposed the construction of a multilayer structure in a negative electrode sheet, for example, CN 106935793 a discloses a negative electrode comprising a multilayer active material layer containing negative active materials having different compacted densities and average particle diameters to increase the porosity of the electrode surface, thereby increasing the mobility of ions to the inside of the electrode; CN 112614969A discloses multilayer negative pole piece, including first active layer and second active layer, the porosity of first active layer is greater than the porosity of second active layer, and in the charge-discharge process, higher porosity is more favorable to lithium ion transmission to promote the power performance of battery. The above documents all propose to build a multilayer structure having different porosities in the battery negative electrode to improve the ion transport performance, but fail to improve the problem of surface layer cracking. In addition, CN 112103468A selects binders with different glass transition temperatures at the surface layer and the bottom layer to construct a layer structure with different charging capabilities in the longitudinal distribution direction of the negative electrode plate. This document limits the selection range of the binder and reduces the applicability by selecting binders having different glass transition temperatures to construct a multilayer structure.
Therefore, a method capable of reducing cracking of the negative electrode plate and promoting ion transmission is still needed, so that the processing performance and the electrical performance of the negative electrode plate are improved at the same time.
Disclosure of Invention
The inventor researches and discovers that a plasticizer (such as carbonates and carboxylates) is added into electrode active material slurry, and can enter molecular chains of a polymer binder in the slurry, so that the intermolecular stress of the polymer is reduced, and the long-chain motion capability is improved, so that the crystallinity of the polymer molecular chains is reduced, the glass transition temperature of the binder is reduced, the plasticity of the polymer binder is improved, the coating processability of the slurry is improved, cracking is reduced, the flexibility of a battery pole piece is improved, and meanwhile, the rapid coating process can be adapted. For the common styrene butadiene rubber SBR emulsion type binder in the active substance slurry, the coating cracking phenomenon can be improved after the plasticizer is added, but the polar plasticizer is easy to generate molecular dipole acting force with sodium alkyl sulfonate in the SBR emulsion, so that the particle size of the SBR is increased, and the phenomena of agglomeration, emulsion breaking and the like are even generated, thereby influencing the cycle performance of the battery. Therefore, the invention provides that multilayer coating is adopted in the thick electrode preparation process, a small amount of plasticizer is added in the upper layer, the cracking of the upper layer in the coating process can be avoided, and the particle size of the emulsion binder SBR in the upper layer is moderately increased under the condition of no agglomeration and emulsion breaking under the condition of adding a small amount of plasticizer, so that the porosity of the upper layer is improved, the porosity gradient of the upper layer and the lower layer is formed in the negative electrode, the ion transmission to the inside of a pole piece is facilitated, and the high energy density and quick charge requirements of the battery are met.
Specifically, the invention provides a method for preparing a negative pole piece of a lithium ion battery, which comprises the following steps:
(1) mixing a first negative electrode active material with a first binder and a first conductive agent, and adding a solvent to prepare a first negative electrode active material slurry;
(2) mixing a second negative electrode active material with a second binder, a second conductive agent and a plasticizer, and adding water to prepare a second negative electrode active material slurry;
(3) coating the first negative electrode active material slurry on at least one side of a negative electrode current collector to form a first active material layer;
(4) and coating the second negative electrode active material slurry on the first active material layer to form a second active material layer, and preparing the lithium ion battery negative electrode piece with the active material layer in a multilayer structure.
In one embodiment of the method according to the present invention, the second binder is an emulsion-type binder and is used in an amount of 2.5 to 200 wt% based on the total solid weight in the second anode active material slurry.
In another embodiment of the method according to the present invention, the emulsion type binder is an SBR-based emulsion binder having a solid content of 5 to 80 wt%.
In another embodiment of the method according to the present invention, the plasticizer is used in an amount of 0.5 to 30 wt% based on the weight of the second negative electrode active material slurry.
It is to be noted that long-chain molecules such as CMC as a binder are restricted in mobility in the negative electrode active material, resulting in poor flexibility of the negative electrode sheet. By adding the polar micromolecules as the plasticizer, the flexibility of long-chain molecules can be improved, the interfacial tension in the coating of the negative electrode slurry can be reduced, and the surface cracking phenomenon after coating can be avoided. In addition, molecular action between the plasticizer and the emulsion binder also causes the particle size of the binder to be increased, so that the plasticizer is adopted in the upper layer slurry of the double-layer coating, the processing problem of a thick electrode can be improved, and the porosity difference of an upper coating and a lower coating can be formed, thereby improving the dynamic performance of the battery. Although some electrolytes contain the above plasticizer as a solvent, and the flexibility of the negative electrode sheet can be increased by permeation, the plasticizer as a solvent of the electrolyte cannot improve the processability in the coating process, and cannot sufficiently react with binder molecules to cause agglomeration, thereby forming a difference in porosity of the coating layer.
In another embodiment of the method according to the present invention, the first negative electrode active material slurry and the second active material slurry further include an additive, and the additive may be any one of a dispersant and a thickener.
In another embodiment of the method according to the present invention, the solvent may be selected from water, NMP or any one or a combination of water and oil based solvents, depending on the type of the first binder.
It is noted that other aqueous solvents such as alcohols and non-aqueous solvents such as linear carbonates are commonly used in the art.
In another embodiment of the method according to the invention, the first active material layer and the second active material layer are dried separately after the respective coating is completed, or are dried after both layers have been coated.
In another embodiment of the method according to the present invention, a thickness ratio of the first active material layer to the second active material layer is 1:9 to 9: 1.
Further, in the method of the present invention, the current collector may use, for example, a copper foil; the first and second anode active materials may each be independently selected from graphite materials; the first and second conductive agents may each be independently selected from various conventional lithium ion battery conductive agents, such as carbon black, conductive graphite, carbon fibers, carbon nanotubes, graphene, and the like; the first binder can be selected from CMC, polyacrylic acid, polytetrafluoroethylene, polyvinyl alcohol, etc., and also can be emulsion type binder, such as SBR. The coating of the first active material layer and the second active material layer may be performed by, for example, a doctor blade type, roll transfer type, slit extrusion type. Drying can be carried out in an oven at a temperature of, for example, 50-140 ℃ after coating is complete.
The invention also provides a lithium ion battery negative electrode plate prepared by the method, which comprises a negative electrode current collector, a first active material layer coated on the negative electrode current collector and a second active material layer coated on the first active material layer, wherein the porosity of the second active material layer is greater than that of the first active material layer.
The invention further provides a lithium ion battery which comprises the negative pole piece with the multilayer structure.
As mentioned above, the invention adopts a multilayer coating method, and the upper active substance slurry adopts SBR emulsion type binder and is added with plasticizer, thereby improving the surface cracking problem when the high surface density pole piece is coated. Meanwhile, the surface sizing agent is only selected and added with a small amount of plasticizer, so that the particle size of SBR can be increased, the porosity of the surface layer can be improved, the pore gradient can be formed between the upper coating and the lower coating, ions can be transmitted to the inside of the pole piece, the polarization phenomenon of a thick electrode can be improved, the dynamics of the battery can be improved, the requirements of high energy density and quick charge can be met, and the degradation of the cycle performance caused by the large-scale emulsion breaking of the SBR can be avoided by only adding a small amount of plasticizer on the surface.
Drawings
Fig. 1 is a photograph of an anode prepared according to example 1;
fig. 2 is a photograph of an anode prepared according to example 2;
fig. 3 is a photograph of an anode prepared according to example 3;
fig. 4 is a photograph of an anode prepared according to example 4;
fig. 5 is a photograph of an anode prepared according to example 5;
fig. 6 is a photograph of an anode prepared according to comparative example 1;
fig. 7 is a photograph of an anode prepared according to comparative example 2; and
fig. 8 is a photograph of the anode prepared according to comparative example 3.
Detailed Description
The present application is described in further detail below with reference to the figures and examples. The features and advantages of the present application will become more apparent from the description.
In addition, the technical features described below in the different embodiments of the present application may be combined with each other as long as they do not conflict with each other.
In the following examples and comparative examples, the porosity of the anode active material layer was measured by the following method: and (3) placing the pole piece with a certain mass in hexadecane, soaking for 1h, taking out, and weighing the pole piece to change the mass before and after the pole piece is sucked on filter paper.
For the prepared battery, a direct current internal resistance (DCR) test was performed at 25 ℃ using a direct current internal resistance tester, and the test procedure was as follows: charging to 4.35V at constant current at 1C, charging to 0.05C at constant voltage, standing for 10min, and discharging to 50% SOC at constant current at 1C; and performing cycle performance test at 45 ℃ by using a cycle characteristic tester, wherein the test procedure comprises the following steps: charging to 4.35V at constant current at 1C, charging to 0.05C at constant voltage, standing for 10min, and discharging to 2.75V at constant current at 1C.
The sources and performance parameters of the raw materials used in the following examples and comparative examples are as follows:
the negative electrode graphite 1 was purchased from new materials science and technology ltd of beite rui (jiangsu);
the negative electrode graphite 2 was purchased from Ningbo fir, Inc.;
conductive carbon black (SP) was purchased from tianjin yi borry chemical ltd;
sodium carboxymethylcellulose (CMC) was purchased from cheng wei yi scientific ltd;
styrene butadiene rubber emulsion binder (SBR) was purchased from sikawa skirle technologies ltd;
the plasticizer Propylene Carbonate (PC) was purchased from shanghai kossi polymers ltd;
the ternary positive electrode active material (NCM) is purchased from Xiamenxianzhan tungsten new energy materials Co.Ltd;
the positive electrode conductive agent (CNT) is purchased from Shanghai Zi-reagent factory;
positive binder (PVDF) was purchased from suzhou, logistical plastication, inc;
the negative current collector copper foil is purchased from Guangzhou Nano new material technology Co., Ltd;
the positive current collector carbon-coated aluminum foil is purchased from Guangzhou Nano new material technology GmbH;
the ceramic diaphragm is purchased from new materials of Yunnan Enjie GmbH.
Example 1
< preparation of negative electrode >
(1) Preparation of first negative active material slurry
1.1 preparing a raw material of a first negative electrode active material slurry according to a weight ratio of graphite, SP, CMC, SBR, 97%, 0.8%, 1%, 1.2%;
1.2 dry mixing of powder: dry-mixing the graphite and the SP prepared in the above way in a stirring kettle;
1.3, glue solution preparation: dissolving CMC prepared in the above manner in deionized water, and dispersing for 120min at the rotating speed of 1800rmp to obtain CMC glue solution with the solid content of 1.4 wt%;
1.4 mixing the slurry: stirring and dispersing the dry powder obtained from 1.2, the 50% CMC glue solution obtained from 1.3 and a proper amount of deionized water for 60min, then adding the rest 50% CMC glue solution and a proper amount of deionized water to enable the solid content to be 50 wt%, continuing stirring for 60min, then adding the SBR emulsion prepared above (the solid content is 50 wt%) and stirring for 60min, and adding a proper amount of deionized water to adjust the viscosity of the slurry to 3000-5000 mPas.
(2) Preparation of second negative active material slurry
A second negative active material slurry was prepared in the same manner as the preparation of the first negative active material slurry described above, except that in the slurry-combining step of 1.4, the plasticizer PC was added in an amount of 1 wt% based on the weight of the slurry used, before the SBR emulsion was added.
(3) Preparation of the negative electrode
Sequentially coating the first negative active material slurry and the second negative active material slurry on a negative current collector copper foil, drying and rolling to obtain a double-sided coating density of 250g/m2Negative electrode sheet (mass without base material). Through the determination, the method has the advantages that,the porosity of the negative electrode sheet is shown in table 1 below, and the coated state of the negative electrode sheet is shown in fig. 1, and it is apparent that there is no crack.
< preparation of Positive electrode >
Carrying out positive electrode slurry mixing according to the proportion of 97.5 percent to 1.5 percent to 1 percent of ternary positive electrode active material (NCM), conductive agent (CNT) and binder (PVDF) to obtain positive electrode active material slurry with the solid content of 70 weight percent; and uniformly coating the positive active substance slurry on a carbon-coated aluminum foil, drying and rolling to obtain the positive pole piece.
< preparation of Battery >
Sequentially laminating the positive pole piece, the ceramic diaphragm and the negative pole piece, and placing the laminated positive pole piece, the ceramic diaphragm and the negative pole piece in a shell; then, the electrolyte was injected, formed, and fixed in volume to obtain a battery a 1. The obtained battery a1 was subjected to a DCR test at 25 ℃ and a cycle test at 45 ℃ in an incubator, and the results are shown in table 1.
Example 2
< preparation of negative electrode >
(1) Preparation of first negative active material slurry
1.1 preparing a raw material of a first negative electrode active material slurry in a weight ratio of graphite SP, CMC, PAA 97%, 0.8%, 1%, 1.2%;
1.2 dry mixing of powder: dry-mixing the graphite and the SP prepared in the above way in a stirring kettle;
1.3, glue solution preparation: dissolving CMC prepared in the above manner in deionized water, and dispersing for 120min at the rotating speed of 1800rmp to obtain CMC glue solution with the solid content of 1.4 wt%;
1.4 mixing the slurry: stirring and dispersing the dry powder obtained from 1.2, the 50% CMC glue solution obtained from 1.3 and a proper amount of deionized water for 60min, then adding the rest 50% CMC glue solution, PAA and a proper amount of deionized water to enable the solid content to be 50 wt%, continuously stirring for 60min, adding a proper amount of deionized water, and adjusting the viscosity of the slurry to 3000-5000mPa & s.
(2) Preparation of second negative active material slurry
2.1 preparing a raw material of a first negative electrode active material slurry according to a solid weight ratio of graphite, SP, CMC, SBR, 97%, 0.8%, 1%, and 1.2%;
2.2 dry mixing of powder: dry-mixing the graphite and the SP prepared in the above way in a stirring kettle;
2.3, glue solution preparation: dissolving CMC prepared in the above manner in deionized water, and dispersing for 120min at the rotating speed of 1800rmp to obtain CMC glue solution with the solid content of 1.4 wt%;
2.4 mixing the slurry: stirring and dispersing the dry powder obtained from 1.2, 50% of CMC glue solution obtained from 1.3 and a proper amount of deionized water for 60min, then adding the rest 50% of CMC glue solution, plasticizer PC with the total solid mass of 2 wt% and a proper amount of deionized water to enable the solid content to be 50 wt%, continuously stirring for 60min, then adding the prepared SBR emulsion with the solid content of 50% and stirring for 60min, adding a proper amount of deionized water, and adjusting the viscosity of the slurry to 3000-5000 mPas.
(3) Preparation of the negative electrode
And sequentially coating the first negative active material slurry and the second negative active material slurry on the negative current collector copper foil, and drying and rolling to obtain the negative pole piece with the double-sided coating density of 250g/m2 (without the mass of the base material). The porosity of the negative electrode sheet was measured and shown in table 1 below, and the coated state of the negative electrode sheet was shown in fig. 2, and it was apparent that there was no crack.
< preparation of Positive electrode >
Carrying out positive electrode slurry mixing according to the proportion of 97.5 percent to 1.5 percent to 1 percent of ternary positive electrode active material (NCM), conductive agent (CNT) and binder (PVDF) to obtain positive electrode active material slurry with the solid content of 70 weight percent; and uniformly coating the positive active substance slurry on a carbon-coated aluminum foil, drying and rolling to obtain the positive pole piece.
< preparation of Battery >
Sequentially laminating the positive pole piece, the ceramic diaphragm and the negative pole piece, and placing the laminated positive pole piece, the ceramic diaphragm and the negative pole piece in a shell; then, the electrolyte was injected, formed, and fixed in volume to obtain a battery a 2. The obtained battery a2 was subjected to a DCR test at 25 ℃ and a cycle test at 45 ℃ in an incubator, and the results are shown in table 1.
Example 3
< preparation of negative electrode >
(1) Preparation of first negative active material slurry
1.1 preparing a raw material of a first negative electrode active material slurry according to a weight ratio of graphite, SP, CMC, SBR, 97%, 0.8%, 1%, 1.2%;
1.2 dry mixing of powder: dry-mixing the graphite and the SP prepared in the above way in a stirring kettle;
1.3, glue solution preparation: dissolving CMC prepared in the above manner in deionized water, and dispersing for 120min at the rotating speed of 1800rmp to obtain CMC glue solution with the solid content of 1.4 wt%;
1.4 mixing the slurry: stirring and dispersing the dry powder obtained from 1.2, the 50% CMC glue solution obtained from 1.3 and a proper amount of deionized water for 60min, then adding the rest 50% CMC glue solution and a proper amount of deionized water to enable the solid content to be 50 wt%, continuously stirring for 60min, then adding the prepared SBR emulsion with the solid content of 50% and stirring for 60min, adding a proper amount of deionized water, and adjusting the viscosity of the slurry to 3000-5000mPa & s.
(2) Preparation of second negative active material slurry
A second negative electrode active material slurry was prepared in the same manner as the preparation of the first negative electrode active material slurry described above, except that NMP, a plasticizer, which was 2 wt% of the total solid weight in the second negative electrode active material slurry, was added before the SBR emulsion was added in the slurry combining step of 1.4.
(3) Preparation of the negative electrode
And sequentially coating the first negative active material slurry and the second negative active material slurry on the negative current collector copper foil, and drying and rolling to obtain the negative pole piece with the double-sided coating density of 250g/m2 (without the mass of the base material). The porosity of the negative electrode sheet was measured and shown in table 1 below, and the coated state of the negative electrode sheet was as shown in fig. 3, and it was apparent that there was no crack.
< preparation of Positive electrode >
Carrying out positive electrode slurry mixing according to the proportion of 97.5 percent to 1.5 percent to 1 percent of ternary positive electrode active material (NCM), conductive agent (CNT) and binder (PVDF) to obtain positive electrode active material slurry with the solid content of 70 weight percent; and uniformly coating the positive active substance slurry on a carbon-coated aluminum foil, drying and rolling to obtain the positive pole piece.
< preparation of Battery >
Sequentially laminating the positive pole piece, the ceramic diaphragm and the negative pole piece, and placing the laminated positive pole piece, the ceramic diaphragm and the negative pole piece in a shell; then, the electrolyte was injected, formed, and fixed in volume to obtain a battery a 3. The obtained battery a3 was subjected to a DCR test at 25 ℃ and a cycle test at 45 ℃ in an incubator, and the results are shown in table 1.
Example 4
< preparation of negative electrode >
(1) Preparation of first negative active material slurry
1.1 preparing a raw material of a first negative electrode active material slurry according to a weight ratio of graphite, SP, CMC, SBR, 97%, 0.8%, 1%, 1.2%;
1.2 dry mixing of powder: dry-mixing the graphite and the SP prepared in the above way in a stirring kettle;
1.3, glue solution preparation: dissolving CMC prepared in the above manner in deionized water, and dispersing for 120min at the rotating speed of 1800rmp to obtain CMC glue solution with the solid content of 1.4 wt%;
1.4 mixing the slurry: stirring and dispersing the dry powder obtained from 1.2, the 50% CMC glue solution obtained from 1.3 and a proper amount of deionized water for 60min, then adding the rest 50% CMC glue solution and a proper amount of deionized water to enable the solid content to be 50 wt%, continuously stirring for 60min, then adding the prepared SBR emulsion with the solid content of 50% and stirring for 60min, adding a proper amount of deionized water, and adjusting the viscosity of the slurry to 3000-5000mPa & s.
(2) Preparation of second negative active material slurry
A second negative electrode active material slurry was prepared in the same manner as the preparation of the first negative electrode active material slurry described above, except that in the slurry combining step of 1.4, the plasticizer PC was added in an amount of 20 wt% based on the total solid weight in the second negative electrode active material slurry, before the SBR emulsion was added.
(3) Preparation of the negative electrode
And sequentially coating the first negative active material slurry and the second negative active material slurry on the negative current collector copper foil, and drying and rolling to obtain the negative pole piece with the double-sided coating density of 250g/m2 (without the mass of the base material). The porosity of the negative electrode sheet was measured and shown in table 1 below, and the coated state of the negative electrode sheet was as shown in fig. 4, and it was apparent that there was no crack.
< preparation of Positive electrode >
Carrying out positive electrode slurry mixing according to the proportion of 97.5 percent to 1.5 percent to 1 percent of ternary positive electrode active material (NCM), conductive agent (CNT) and binder (PVDF) to obtain positive electrode active material slurry with the solid content of 70 weight percent; and uniformly coating the positive active substance slurry on a carbon-coated aluminum foil, drying and rolling to obtain the positive pole piece.
< preparation of Battery >
Sequentially laminating the positive pole piece, the ceramic diaphragm and the negative pole piece, and placing the laminated positive pole piece, the ceramic diaphragm and the negative pole piece in a shell; then, the electrolyte was injected, formed, and fixed in volume to obtain a battery a 4. The obtained battery a4 was subjected to a DCR test at 25 ℃ and a cycle test at 45 ℃ in an incubator, and the results are shown in table 1.
Example 5
< preparation of negative electrode >
(1) Preparation of first negative active material slurry
1.1 preparing a raw material of a first negative electrode active material slurry according to a weight ratio of graphite, SP, CMC, SBR, 97%, 0.8%, 1%, 1.2%;
1.2 dry mixing of powder: dry-mixing the graphite and the SP prepared in the above way in a stirring kettle;
1.3, glue solution preparation: dissolving CMC prepared in the above manner in deionized water, and dispersing for 120min at the rotating speed of 1800rmp to obtain CMC glue solution with the solid content of 1.4 wt%;
1.4 mixing the slurry: stirring and dispersing the dry powder obtained from 1.2, the 50% CMC glue solution obtained from 1.3 and a proper amount of deionized water for 60min, then adding the rest 50% CMC glue solution and a proper amount of deionized water to enable the solid content to be 50 wt%, continuously stirring for 60min, then adding the prepared SBR emulsion with the solid content of 50% and stirring for 60min, adding a proper amount of deionized water, and adjusting the viscosity of the slurry to 3000-5000mPa & s.
(2) Preparation of second negative active material slurry
A second negative electrode active material slurry was prepared in the same manner as the preparation of the first negative electrode active material slurry described above, except that in the slurry-combining step of 1.4, after the SBR emulsion was added, NMP, which is a plasticizer in an amount of 2 wt% based on the total solid weight in the second negative electrode active material slurry, was added.
(3) Preparation of the negative electrode
And sequentially coating the first negative active material slurry and the second negative active material slurry on the negative current collector copper foil, and drying and rolling to obtain the negative pole piece with the double-sided coating density of 250g/m2 (without the mass of the base material). The porosity of the negative electrode sheet was measured and shown in table 1 below, and the coated state of the negative electrode sheet was as shown in fig. 5, and it was apparent that there was no crack.
< preparation of Positive electrode >
Carrying out positive electrode slurry mixing according to the proportion of 97.5 percent to 1.5 percent to 1 percent of ternary positive electrode active material (NCM), conductive agent (CNT) and binder (PVDF) to obtain positive electrode active material slurry with the solid content of 70 weight percent; and uniformly coating the positive active substance slurry on a carbon-coated aluminum foil, drying and rolling to obtain the positive pole piece.
< preparation of Battery >
Sequentially laminating the positive pole piece, the ceramic diaphragm and the negative pole piece, and placing the laminated positive pole piece, the ceramic diaphragm and the negative pole piece in a shell; then, the electrolyte was injected, formed, and fixed in volume to obtain a battery a 5. The obtained battery a5 was subjected to a DCR test at 25 ℃ and a cycle test at 45 ℃ in an incubator, and the results are shown in table 1.
Comparative example 1
The first negative electrode active material slurry was prepared by mixing negative electrode graphite SP, CMC SBR 97%, 0.8%, 1%, and 1.2%, and the second negative electrode active material slurry was prepared by mixing negative electrode graphite 2, SP, CMC SBR 97%, 0.8%, 1%, and 1.2%. The solid contents of the two cathode slurries are both 50 wt%, and the two slurries are mixed according to the following steps:
(1) dry mixing of powder: adding graphite and SP into a 5L stirring kettle according to a formula, and stirring and dry-mixing;
(2) preparing CMC glue solution: dissolving CMC in deionized water according to a proportion, and dispersing for 120min at the rotating speed of 1800rmp to obtain CMC glue solution with the solid content of 1.4 wt%;
(3) mixing the slurry: taking the dry powder in the step (1), the 50% CMC glue solution in the step (2) and a solvent (deionized water) according to a formula, and stirring and dispersing for 60 min; adding the rest 50% of CMC glue solution and solvent, and stirring for 60 min; then adding SBR emulsion and stirring for 60min, finally adding a proper amount of deionized water to adjust the viscosity to 3000-5000 mPa.s.
Sequentially coating the first negative electrode active material slurry and the second negative electrode active material slurry on the copper foil of the negative electrode current collector, and measuring the porosity of the coating, wherein the double-sided coating density is 250g/m2(without the mass of the base material), and rolling after coating to obtain the negative pole piece. The porosity of the negative electrode sheet was measured and shown in table 1 below, and the negative electrode sheet was coated in the state shown in fig. 6, and cracks were clearly observed.
A positive electrode and a battery were produced by the same method and procedure as in example 1, to obtain battery C1. The obtained battery C1 was subjected to a DCR test at 25 ℃ and a cycle test at 45 ℃ in an incubator, and the results are shown in table 1.
Comparative example 2
Mixing the negative electrode graphite SP, CMC, SBR 97%, 0.8%, 1% and 1.2% to prepare negative electrode active substance slurry, wherein the solid content of the slurry is 50 wt%, and mixing the slurry according to the following steps:
(1) dry mixing of powder: adding graphite and SP into a 5L stirring kettle according to a formula, and stirring and dry-mixing;
(2) preparing CMC glue solution: dissolving CMC in deionized water according to a proportion, and dispersing for 120min at the rotating speed of 1800rmp to obtain CMC glue solution with the solid content of 1.4 wt%;
(3) mixing the slurry: taking the dry powder in the step (1), the 50% CMC glue solution in the step (2) and a solvent (deionized water) according to a formula, and stirring and dispersing for 60 min; adding the rest 50% of CMC glue solution and solvent, and stirring for 60 min; then adding SBR emulsion and stirring for 60min, finally adding a proper amount of deionized water to adjust the viscosity to 3000-5000 mPa.s.
Coating the obtained slurry on the copper foil of the negative current collector, and measuring the porosity of the coating, wherein the double-sided coating density is 250g/m2(without the mass of the base material), and rolling after coating to obtain the negative pole piece. Through the determination, the method has the advantages that,the porosity of the negative electrode sheet is shown in table 1 below, and the negative electrode sheet was coated in a state shown in fig. 7, and cracks were evident.
A positive electrode and a battery were produced by the same method and procedure as in example 1, to obtain battery C2. The obtained battery C2 was subjected to a DCR test at 25 ℃ and a cycle test at 45 ℃ in an incubator, and the results are shown in table 1.
Comparative example 3
Mixing the graphite of the negative electrode, SP, CMC, SBR, 97%, 0.8%, 1% and 1.2%, adding plasticizer PC accounting for 1% of the total solid mass in the slurry of the negative electrode active material, mixing to prepare slurry of the negative electrode active material, wherein the solid content is 50 wt%, and mixing the slurry according to the following steps:
(1) dry mixing of powder: adding graphite and SP into a 5L stirring kettle according to a formula, and stirring and dry-mixing;
(2) preparing CMC glue solution: dissolving CMC in deionized water according to a proportion, and dispersing for 120min at the rotating speed of 1800rmp to obtain CMC glue solution with the solid content of 1.4 wt%;
(3) mixing the slurry: taking the dry powder in the step (1), the 50% CMC glue solution in the step (2) and a solvent (deionized water) according to a formula, and stirring and dispersing for 60 min; adding the rest 50% of CMC glue solution and solvent, and stirring for 60 min; continuously adding the plasticizer PC, and stirring for 60 min; and finally adding SBR emulsion, stirring for 60min, and adding a proper amount of deionized water to adjust the viscosity to 3000-5000mPa & s.
Coating the obtained slurry on the copper foil of the negative current collector, and measuring the porosity of the coating, wherein the double-sided coating density is 250g/m2(without the mass of the base material), and rolling after coating to obtain the negative pole piece. The porosity of the negative electrode sheet was measured and shown in table 1 below, and the coated state of the negative electrode sheet was as shown in fig. 8, and it was apparent that there was no crack.
A positive electrode and a battery were produced by the same method and procedure as in example 1, to obtain battery C3. The obtained battery C3 was subjected to a DCR test at 25 ℃ and a cycle test at 45 ℃ in an incubator, and the results are shown in table 1.
Table 1 porosity and electrical property test results of the batteries prepared in examples and comparative examples
Figure BDA0003392975230000131
As can be seen from the photographs of fig. 1 to 8 and the results of table 1, the addition of the plasticizer to the slurry helps to improve the coating state of the negative electrode sheet and avoid cracking; compared with the results of porosity when no plasticizer was added in comparative examples 1 and 2, porosity was significantly increased when a single-layer negative active material slurry with 1 wt% plasticizer added was coated in comparative example 3, indicating that the plasticizer causes an increase in the particle size of SBR and increases porosity, while porosity was also increased when a multi-layer coating was applied in example 1 and only the plasticizer was added in the upper layer, indicating that the plasticizer applied only to the surface layer increases porosity of the upper layer and forms a porosity gradient. Comparing the DCR data of example 1 and comparative example 2 and comparative example 3, respectively, it can be seen that the DCR of the battery is reduced after 1 wt% PC is added to the negative active material slurry regardless of whether the multi-layer coating is performed, whereas the multi-layer coating of comparative example 1 and comparative example 3 is more effective in reducing the DCR than the single-layer coating, which indicates that the formation of the multi-layer structure and the formation of the pore gradient between the upper layer and the lower layer by adding the plasticizer only at the upper layer contribute to the improvement of the dynamic performance of the battery. Comparing the cycle performance data at 45 ℃, it can be seen that the addition of the plasticizer reduces the capacity retention at 45 ℃ cycle, but compared to comparative example 3, example 1 has no significant effect on the cycle performance because the plasticizer coating amount is relatively small compared to a single layer because the plasticizer is added only to the second negative electrode active material slurry.
In conclusion, according to the preparation method of the negative pole piece, the multi-layer coating is adopted, and a small amount of plasticizer is added on the upper layer, so that the surface cracking of the coating in the coating process is avoided, the grain size of the SBR on the upper layer is increased, the porosity gradient of the upper layer and the lower layer is formed, the ion transmission to the inside of the pole piece is facilitated, and the high energy density and quick charge requirements of the battery are met.
The present application has been described above in connection with preferred embodiments, which are, however, merely exemplary and illustrative. On the basis of the above, the present application can be subjected to various substitutions and modifications, and the present application is in the protection scope of the present application.

Claims (10)

1. A method for preparing a negative pole piece of a lithium ion battery comprises the following steps:
(1) mixing a first negative electrode active material with a first binder and a first conductive agent, and adding a solvent to prepare a first negative electrode active material slurry;
(2) mixing a second negative electrode active material with a second binder, a second conductive agent and a plasticizer, and adding water to prepare a second negative electrode active material slurry;
(3) coating the first negative electrode active material slurry on at least one side of a negative electrode current collector to form a first active material layer;
(4) and coating the second negative electrode active material slurry on the first active material layer to form a second active material layer, and preparing the lithium ion battery negative electrode piece with the active material layer in a multilayer structure.
2. The method according to claim 1, wherein the second binder is an emulsion-type binder and is used in an amount of 2.5 to 200 wt% based on the total solid weight in the second negative electrode active material slurry.
3. The method according to claim 2, wherein the emulsion-type binder is an SBR-based emulsion binder having a solid content of 5 to 80 wt%.
4. The method according to claim 1, wherein the plasticizer is used in an amount of 0.5 to 30 wt% based on the weight of the second negative electrode active material slurry.
5. The method according to claim 1, wherein the first negative electrode active material slurry and the second active material slurry further contain an additive, which may be any one of a dispersant or a thickener.
6. The method according to claim 1, wherein the solvent is selected from water, NMP or any one or a combination of water and oil based solvents according to the kind of the first binder.
7. The method according to claim 1, wherein the first active material layer and the second active material layer are dried separately after the respective coating is completed, or dried after both layers are coated.
8. The method according to claim 1, wherein a thickness ratio of the first active material layer to the second active material layer is 1:9 to 9: 1.
9. A lithium ion battery negative electrode sheet prepared according to the method of any preceding claim, comprising a negative electrode current collector, a first active material layer coated over the negative electrode current collector, and a second active material layer coated over the first active material layer, wherein the porosity of the second active material layer is greater than the porosity of the first active material layer.
10. A lithium ion battery comprising the negative electrode tab of claim 9.
CN202111472345.3A 2021-12-06 2021-12-06 Method for preparing negative pole piece, negative pole piece prepared by method and lithium ion battery comprising negative pole piece Pending CN114188539A (en)

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