CN115347192B

CN115347192B - A thick negative electrode and its preparation method and application

Info

Publication number: CN115347192B
Application number: CN202211166590.6A
Authority: CN
Inventors: 周志行; 杨倩; 金伦; 刘范芬; 苑丁丁
Original assignee: Eve Power Co Ltd
Current assignee: Eve Power Co Ltd
Priority date: 2022-09-21
Filing date: 2022-09-21
Publication date: 2024-12-31
Anticipated expiration: 2042-09-21
Also published as: CN115347192A

Abstract

The invention provides a thick negative electrode and a preparation method and application thereof, wherein the thick negative electrode comprises a current collector and a negative active material layer arranged on the surface of the current collector, the negative active material layer comprises a negative active material, a conductive agent, a dispersing agent and a combined binder, the combined binder comprises a binder A and a binder B, the binder A comprises a block copolymer, the middle of the block copolymer is a polyacrylate chain segment, both ends of the block copolymer comprise a polyacrylic acid chain segment and a polyvinyl alcohol chain segment, and the binder B comprises an emulsion type binder. The adhesive A and the adhesive B are used simultaneously, so that the negative electrode plate has good flexibility, is beneficial to processing, reduces the falling-off condition of active substances from the plate, and reduces the short-circuit risk of the electrochemical energy storage device.

Description

Negative thick electrode and preparation method and application thereof

Technical Field

The invention belongs to the technical field of lithium ion batteries, and relates to a thick negative electrode, a preparation method and application thereof.

Background

In recent years, new energy industry has rapidly developed, consumption demands of lithium ion batteries are rapidly increased, and higher requirements are put on energy density, low-temperature performance, quick charge performance, safety and cost of the lithium ion batteries in the market. The improvement of the battery performance becomes an important field of research of various material factories and battery factories. Lithium ion batteries are widely used in the fields of new energy vehicles, energy storage batteries, consumer batteries and the like. Along with popularization and development of new energy vehicles and energy storage, energy density and cost become important points of researchers, batteries prepared from pole pieces with conventional thickness are difficult to meet requirements, and the design of thick electrodes can realize that current collectors with the same area bear more electrode active materials, so that the energy density can be improved, and the cost can be reduced. The problem of electrical conductivity faced by thick electrodes affects the performance of the cell.

CN113285062A discloses a thick electrode material, a preparation method thereof and a lithium ion battery, and the thick electrode material comprises the steps of adding a conductive agent, a thickening agent, a binder and a solvent into an electrode active material with small particle size to prepare first electrode slurry, adding the conductive agent, the thickening agent, the binder and the solvent into an electrode active material with large particle size to prepare second electrode slurry, respectively conveying the first electrode slurry and the second electrode slurry to corresponding dies of a double-die-head coater through two slurry conveying systems to perform simultaneous gap coating even times, and keeping the first electrode slurry above the second electrode slurry or the second electrode slurry above the first electrode slurry during each coating, and drying and rolling to obtain the thick electrode.

CN112151743A discloses a pore-forming method of a thick electrode, a product and application thereof, wherein the pore-forming method comprises the steps of coating slurry with viscosity of 600-9000 mPas on the surface of a current collector with surface roughness Ra of more than or equal to 1 mu m, and drying to obtain the thick electrode.

The thick electrode prepared by the scheme has the problems of poor cycle performance and poor storage performance, and limits the application of the thick electrode in practice.

Disclosure of Invention

The invention aims to provide a thick negative electrode, a preparation method and application thereof, wherein the flexibility of the negative electrode is improved by compounding a dispersing agent and different binder compositions, the problems of poor wettability of electrolyte of the thick negative electrode, long lithium ion migration path and large concentration polarization are solved, the dynamic properties such as lithium precipitation, internal resistance and the like are improved, the multiplying power and low-temperature discharge property of a battery are improved, and meanwhile, the battery is ensured to have good cycle performance. In the lithium ion battery electrode, the binder provides adhesion between the electrode active material, the conductive agent, and the current collector, maintaining good contact between the active material and the conductive agent, and the current collector, and between the active material and the active material. In the charge and discharge process, the electrode structure can be effectively kept complete, a good electronic path and stable electrochemical performance are kept, and safe and effective operation of the battery is ensured.

In order to achieve the aim of the invention, the invention adopts the following technical scheme:

In a first aspect, the invention provides a thick negative electrode, which comprises a current collector and a negative active material layer arranged on the surface of the current collector, wherein the negative active material layer comprises a negative active material, a conductive agent, a dispersing agent and a combined binder, the combined binder comprises a binder A and a binder B, the binder A comprises a block copolymer, a polyacrylate chain segment is arranged in the middle of the block copolymer, both ends of the block copolymer comprise a polyacrylic acid chain segment and a polyvinyl alcohol chain segment, and the binder B comprises an emulsion binder.

In the thick electrode of the negative electrode, the binder A is a block copolymer binder, wherein the block copolymer is provided with a polyacrylate chain segment in the middle, and two ends of the block copolymer are respectively provided with a polyacrylic acid (PAA) chain segment and a polyvinyl alcohol (PVA) chain segment. The polyacrylate chain segment has certain swelling capacity to electrolyte, forms an ion-conducting channel, and is favorable for improving the wettability of the electrolyte in the thick electrode. The carboxyl groups in the polyacrylic acid (PAA) chain segments and the hydroxyl groups in the CMC form a three-dimensional network structure, and the electrode expansion is inhibited. Wherein the cross-linking is also generated between the different binder a chains by a condensation reaction between the hydroxyl groups of one chain and the carboxyl groups of the other chain, which further enhances the stability of the three-dimensional network structure.

Preferably, the monomer unit of the binder B includes any one or a combination of at least two of an aromatic vinyl monomer unit, an aromatic conjugated diene monomer unit, an alkenyl unsaturated carboxylic acid monomer unit, an unsaturated carboxylic acid alkyl ester monomer unit, or an acrylonitrile monomer unit.

Preferably, the binder B comprises styrene-butadiene latex.

In the thick negative electrode, the binder B is emulsion type, and the negative electrode active material, the negative electrode active material and the current collector are bonded in a point-to-point mode. The binder B has better flexibility, the binder A is a solution type multipolymer, the molecular structure of the binder A has rich functional groups, and a reticular structure is formed in the electrode in a surface-to-surface mode, so that on one hand, the formed reticular structure is beneficial to improving the binding force between anode active substances and between the anode active substances and a current collector and improving the stripping force of a pole piece of a thick electrode, on the other hand, the existence of the reticular structure also reserves channels for embedding and stripping lithium ions, but the binder A has high brittleness and softness active balance, and the pole piece active substances are easy to fall off during pole piece processing and charging and discharging. Therefore, the synergistic effect exerted by the adhesive B and the adhesive A further improves the stripping force of the negative electrode plate prepared by the slurry and improves the flexibility of the plate. In addition, the single use of the binder a forms an unstable carboxylate with the electrolyte to remain in the SEI film, forming an SEI film that cannot exist stably, deteriorating charge and discharge efficiency and cycle performance of the electrochemical device.

The thick electrode of the negative electrode simultaneously comprises a binder A and a binder B, and the electrode plate independently comprising the binder A is hard and brittle and has excellent dynamic performance. The binder A and the binder B are used simultaneously, so that the negative electrode plate has good flexibility, is beneficial to processing, reduces the falling-off condition of active substances from the plate, and reduces the short-circuit risk of the electrochemical energy storage device.

Preferably, the dispersing agent comprises any one or a combination of at least two of sodium carboxymethyl cellulose, sodium carboxypropyl cellulose or sodium alginate.

Preferably, the negative electrode active material includes graphite.

Preferably, the conductive agent includes any one or a combination of at least two of conductive carbon black, graphene, carbon nanotubes, or conductive fibers.

Preferably, the current collector comprises copper foil.

Preferably, the mass fraction of the negative electrode active material is 91.8 to 98.9%, for example, 91.8%, 92%, 94%, 95% or 98.9%, based on 100% of the mass of the negative electrode active material layer.

Preferably, the mass fraction of the dispersant is 0.1-2.2%, for example, 0.1%, 0.5%, 1%, 1.5% or 2.2%.

Preferably, the mass fraction of the binder A is 0.5-2.1%, for example, 0.5%, 0.8%, 1%, 1.5% or 2.1%.

Preferably, the mass fraction of the binder B is 0.1-1.7%, for example, 0.1%, 0.5%, 1%, 1.2% or 1.7%.

Preferably, the mass fraction of the conductive agent is 0.4-5%, for example, 0.4%, 1%, 2%, 3%, 4% or 5%.

In a second aspect, the present invention provides a method for preparing the thick negative electrode according to the first aspect, the method comprising the steps of:

(1) Mixing a dispersing agent and a solvent to obtain a dispersing agent solution, mixing a negative electrode active material and a conductive agent to obtain a mixture, mixing the mixture with the dispersing agent solution, and dispersing for one time to obtain a mixed solution;

(2) Mixing part of the binder A with the mixed solution obtained in the step (1) for secondary dispersion, adding a solvent and the rest of the binder A for tertiary dispersion, adding the binder B, and stirring to obtain negative electrode slurry;

(3) And coating the negative electrode slurry on the surface of a current collector, drying, rolling and cutting to obtain the negative electrode thick electrode.

The preparation method of the invention is characterized in that a dispersing agent, a binder A and a binder B are added. The dispersing agent is added preferentially to uniformly wrap the surface of the anode active material, so that the anode active material is dispersed on one hand, and a stable SEI film is formed on the other hand, and the problem that the capacity decay is faster in the charge-discharge cycle process due to the creation of electrochemical energy storage when a single binder is used as the binder and the dispersing agent is avoided.

Preferably, the solid content of the dispersant solution in the step (1) is 1-2%, for example, 1%, 1.2%, 1.5%, 1.8% or 2%.

Preferably, the substitution degree of the dispersant is 0.5-1, for example, 0.5, 0.62, 0.71, 0.85 or 1.

Preferably, the part of the binder A in the step (2) accounts for 20-35% of the total mass of the binder A, for example, 20%, 22%, 25%, 30% or 35% and the like.

Preferably, the thickness of the current collector in the step (3) is 5-10 μm, for example, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm or 10 μm.

In a third aspect, the present invention provides a lithium ion battery comprising a negative thick electrode according to the first aspect.

Compared with the prior art, the invention has the following beneficial effects:

(1) The invention improves the flexibility of the thick electrode of the negative electrode by compounding the dispersing agent and the binder compositions of different types, solves the problems of poor wettability of electrolyte of the thick electrode, long migration path of lithium ions and large concentration polarization, improves dynamic properties such as lithium precipitation, internal resistance and the like, improves the multiplying power and low-temperature discharge property of the battery, and ensures that the battery has good cycle performance. In the lithium ion battery electrode, the binder provides adhesion between the electrode active material, the conductive agent, and the current collector, maintaining good contact between the active material and the conductive agent, and the current collector, and between the active material and the active material. In the charge and discharge process, the electrode structure can be effectively kept complete, a good electronic path and stable electrochemical performance are kept, and safe and effective operation of the battery is ensured.

(2) The capacity retention rate of the battery prepared by the thick electrode can reach more than 90.1% after being cycled at 1C/1C for 3000 circles under 25 ℃, the capacity retention rate can reach more than 89.42% after being stored at 60 ℃ for 30 days, the capacity recovery rate can reach more than 93.24% after being stored at 60 ℃ for 30 days, the 1C discharge rate can reach more than 93.10% under 25 ℃, and the 0.2C low-temperature discharge rate can reach more than 92.74% under 0 ℃.

Drawings

Fig. 1 is a schematic view of the binder of example 1 in a thick negative electrode.

FIG. 2 is a graph showing the cycle performance of the electrodes prepared in example 1 and comparative example 1.

Detailed Description

The technical scheme of the invention is further described by the following specific embodiments. It will be apparent to those skilled in the art that the examples are merely to aid in understanding the invention and are not to be construed as a specific limitation thereof.

Example 1

The embodiment provides a thick negative electrode, and the preparation method of the thick negative electrode is as follows:

(1) Mixing sodium carboxymethylcellulose with water to obtain a dispersant solution with a solid content of 1.5%, wherein the substitution degree of the dispersant is 0.75, mixing graphite and conductive carbon black to obtain a mixture, mixing the mixture with the dispersant solution, and dispersing for the first time to obtain a mixed solution;

(2) Mixing 32% of binder A with the mixed solution obtained in the step (1) for secondary dispersion, adding a solvent and the rest 68% of binder A for tertiary dispersion, adding binder styrene-butadiene latex, and stirring to obtain negative electrode slurry;

(3) Coating the negative electrode slurry on the surface of a copper foil with the thickness of 8.2 mu m, drying, rolling and cutting to obtain the negative electrode thick electrode;

The mass ratio of graphite, conductive carbon black, sodium carboxymethylcellulose, binder A and binder B in the thick negative electrode is 97.2:0.5:0.4:1.4:0.5;

a schematic of the binder in a thick negative electrode is shown in fig. 1.

Example 2

(1) Mixing sodium carboxymethylcellulose with water to obtain a dispersant solution with a solid content of 1.5%, wherein the substitution degree of the dispersant is 0.71, mixing graphite and conductive carbon black to obtain a mixture, mixing the mixture with the dispersant solution, and dispersing for one time to obtain a mixed solution;

(2) Mixing 30% of binder A with the mixed solution obtained in the step (1) for secondary dispersion, adding a solvent and the rest 70% of binder A for tertiary dispersion, adding binder styrene-butadiene latex, and stirring to obtain negative electrode slurry;

(3) Coating the negative electrode slurry on the surface of a copper foil with the thickness of 8 mu m, drying, rolling and cutting to obtain the negative electrode thick electrode;

The mass ratio of graphite, conductive carbon black, sodium carboxymethylcellulose, binder A and binder B in the thick electrode of the negative electrode is 96.7:0.6:0.5:1.8:0.4.

Example 3

This example differs from example 1 only in that the mass ratio of the binder a in the anode active material layer is 0.3%, and other conditions and parameters are exactly the same as in example 1.

Example 4

This example differs from example 1 only in that the mass ratio of the binder a in the anode active material layer is 2.4%, and other conditions and parameters are exactly the same as in example 1.

Example 5

This example differs from example 1 only in that the mass ratio of the binder B in the anode active material layer is 0.05%, and other conditions and parameters are exactly the same as in example 1.

Example 6

The present example differs from example 1 only in that the mass ratio of the binder B in the anode active material layer is 2%, and other conditions and parameters are exactly the same as in example 1.

Comparative example 1

This comparative example differs from example 1 only in that no binder a will be added, other conditions are exactly the same as in example 1.

Comparative example 2

This comparative example differs from example 1 only in that no binder B will be added, and other conditions and parameters are exactly the same as example 1.

Comparative example 3

This comparative example differs from example 1 only in that binder a is replaced by equal mass of acrylate, polyacrylic acid and polyacryl alcohol, the other conditions and parameters being exactly the same as example 1.

Performance test:

Adding a certain amount of adhesive polyvinylidene fluoride powder into N-methyl pyrrolidone solvent, stirring and mixing uniformly to prepare the glue solution with the solid content of 8%. Adding the positive active materials lithium iron phosphate, carbon nano tube, conductive carbon black and polyvinylidene fluoride into a stirrer according to the mass ratio of 97.5 percent to 0.4 percent to 0.5 percent to 1.6 percent, fully stirring uniformly to obtain positive slurry, coating the prepared slurry on a current collector, coating the current collector with carbon aluminum foil with the thickness of 16 mu m, drying in an oven, rolling and cutting to form a positive plate for preparing a battery, taking the thick negative electrode obtained in examples 1-6 and comparative examples 1-3 as a negative electrode, adopting a Polyethylene (PE) porous polymeric film as a separation film, and preparing a soft-package lithium ion battery by adopting LiPF ₆/(DEC+EMC+EC) of 1.00mol/L as an electrolyte, wherein the performance test is carried out on the lithium ion battery, and the test results are shown in table 1:

TABLE 1

As can be seen from Table 1, according to the negative electrode of the present invention, the capacity retention rate of the battery prepared from examples 1-2 at 25 ℃ for 1C/1C cycle 3000 cycles can be up to 90.1%, the capacity retention rate of the battery stored at 60 ℃ for 30 days can be up to 89.42%, the capacity recovery rate of the battery stored at 60 ℃ for 30 days can be up to 93.24%, the discharge rate of the battery at 25 ℃ can be up to 93.10%, and the low-temperature discharge rate of the battery at 0 ℃ at 0.2C can be up to 92.74%.

As can be seen from comparison of examples 1 and examples 3-4, in the active material layer of the thick electrode for the negative electrode, the mass ratio of the binder A influences the performance of the thick electrode, the mass fraction of the binder A is controlled to be 0.5-2.1%, the prepared thick electrode for the negative electrode has good performance, if the mass ratio of the binder A is too small, the performance improvement of a battery cell is not obvious, and if the mass ratio of the binder A is too large, the negative electrode sheet becomes brittle and hard, the active material is seriously shed in the processing process, and the performance of the battery cell is deteriorated.

As can be seen from comparison of examples 1 and examples 5 to 6, in the active material layer of the thick electrode for a negative electrode according to the present invention, the mass ratio of the binder B affects the performance, the mass fraction of the binder B is controlled to be 0.1 to 1.7%, the performance of the thick electrode for a negative electrode is better, if the mass ratio of the binder B is too small, the synergistic effect with the binder a cannot be fully exerted, the cell performance is poor, and if the mass ratio of the binder B is too large, the cell impedance becomes large, and the low-temperature discharge performance of the cell becomes poor.

The comparison of the cycle performance of the electrode sheets of example 1 and comparative example 1 is shown in fig. 2, and the comparison of example 1 and comparative examples 1-2 shows that the electrode sheet of the invention contains both binder a and soft binder B, and the electrode sheet containing binder a alone is hard and brittle and has excellent dynamic performance. The adhesive A and the adhesive B are used simultaneously, so that the negative electrode plate has good flexibility, is beneficial to processing, reduces the falling-off condition of active substances from the plate, and reduces the short-circuit risk of the electrochemical energy storage device.

As can be seen from comparison of example 1 and comparative example 3, in the negative electrode according to the present invention, the binder a is a block copolymer binder, the block copolymer has a polyacrylate segment in the middle, and a polyacrylic acid (PAA) segment and a polyvinyl alcohol (PVA) segment at both ends, respectively. The polyacrylate chain segment has certain swelling capacity to electrolyte, forms an ion-conducting channel, and is favorable for improving the wettability of the electrolyte in the thick electrode. The carboxyl groups in the polyacrylic acid (PAA) chain segments and the hydroxyl groups in the CMC form a three-dimensional network structure, and the electrode expansion is inhibited. Wherein the cross-linking is also generated between the different binder a chains by a condensation reaction between the hydroxyl groups of one chain and the carboxyl groups of the other chain, which further enhances the stability of the three-dimensional network structure.

The applicant declares that the above is only a specific embodiment of the present invention, but the scope of the present invention is not limited thereto, and it should be apparent to those skilled in the art that any changes or substitutions that are easily conceivable within the technical scope of the present invention disclosed by the present invention fall within the scope of the present invention and the disclosure.

Claims

1. The thick negative electrode is characterized by comprising a current collector and a negative active material layer arranged on the surface of the current collector, wherein the negative active material layer comprises a negative active material, a conductive agent, a dispersing agent and a combined binder, the combined binder comprises a binder A and a binder B, the binder A is a block copolymer, the middle of the block copolymer is a polyacrylate chain segment, both ends of the block copolymer comprise a polyacrylic acid chain segment and a polyvinyl alcohol chain segment, and the binder B comprises an emulsion binder;

the adhesive B is styrene-butadiene latex;

The mass fraction of the binder A is 0.5-2.1% and the mass fraction of the binder B is 0.1-1.7% based on 100% of the mass of the negative electrode active material layer.

2. The negative thick electrode of claim 1, wherein the monomer unit of binder B comprises any one or a combination of at least two of an aromatic vinyl monomer unit, an aromatic conjugated diene monomer unit, an alkenyl unsaturated carboxylic acid monomer unit, an unsaturated carboxylic acid alkyl ester monomer unit, or an acrylonitrile monomer unit.

3. The negative thick electrode of claim 1, wherein the dispersant comprises any one or a combination of at least two of sodium carboxymethyl cellulose, sodium carboxypropyl cellulose, or sodium alginate.

4. The negative thick electrode of claim 1, wherein the negative active material comprises graphite.

5. The negative thick electrode of claim 1, wherein the conductive agent comprises any one or a combination of at least two of conductive carbon black, graphene, carbon nanotubes, or conductive fibers.

6. The negative thick electrode of claim 1, wherein the current collector comprises copper foil.

7. The thick anode electrode according to claim 1, wherein the mass fraction of the anode active material is 91.8 to 98.9% based on 100% of the mass of the anode active material layer.

8. The negative electrode according to claim 1, wherein the mass fraction of the dispersant is 0.1 to 2.2% based on 100% of the mass of the negative electrode active material layer.

9. The negative electrode according to claim 1, wherein the mass fraction of the conductive agent is 0.4 to 5% based on 100% of the mass of the negative electrode active material layer.

10. A method for producing the negative electrode thick electrode according to any one of claims 1 to 9, comprising the steps of:

11. The method of claim 10, wherein the dispersant solution of step (1) has a solids content of 1 to 2%.

12. The method of claim 10, wherein the dispersant has a degree of substitution of 0.5 to 1.

13. The method of claim 10, wherein the portion of the binder a in step (2) is 20-35% of the total mass of the binder a.

14. The method of claim 10, wherein the thickness of the current collector in step (3) is 5 to 10 μm.

15. A lithium ion battery comprising a negative thick electrode according to any one of claims 1-9.