CN115347192A

CN115347192A - Thick cathode electrode and preparation method and application thereof

Info

Publication number: CN115347192A
Application number: CN202211166590.6A
Authority: CN
Inventors: 周志行; 杨倩; 金伦; 刘范芬; 苑丁丁
Original assignee: Hubei Eve Power Co Ltd
Current assignee: Hubei Eve Power Co Ltd
Priority date: 2022-09-21
Filing date: 2022-09-21
Publication date: 2022-11-15

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, the two 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 binder A and the binder B are used simultaneously, so that the negative pole piece has good flexibility, processing is facilitated, the situation that active substances fall off from the pole piece is reduced, and the short-circuit risk of the electrochemical energy storage device is reduced.

Description

Thick cathode 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 cathode electrode and a preparation method and application thereof.

Background

In recent years, new energy industries are rapidly developed, the consumption demand of lithium ion batteries is rapidly increased, and the market puts higher requirements on the energy density, the low-temperature performance, the quick charge performance, the safety and the cost of the lithium ion batteries. The improvement of battery performance is a key area for research of various material manufacturers and battery manufacturers. Lithium ion batteries are now widely used in the fields of new energy vehicles, energy storage batteries, consumer batteries, and the like. With the popularization and development of new energy vehicles and energy storage, energy density and cost become the key points of attention of researchers; the battery prepared by the pole piece with the conventional thickness is difficult to meet the requirement, and the design of the thick electrode can realize that the current collector with the same area bears more electrode active materials, so that the energy density can be improved, and the cost can be reduced. The problem of conductivity faced by thick electrodes affects the performance of the battery.

CN113285062A discloses a thick electrode material, a preparation method thereof and a lithium ion battery, which comprises the following steps: adding a conductive agent, a thickening agent, a binder and a solvent into the electrode active material with small particle size to prepare first electrode slurry; adding a conductive agent, a thickening agent, a binder and a solvent into the electrode active material with large particle size to prepare second electrode slurry; and respectively conveying the first electrode slurry and the second electrode slurry to corresponding die heads of a double-die-head coating machine through two sets of slurry conveying systems to simultaneously coat the first electrode slurry and the second electrode slurry in a clearance mode for an even number of times, keeping the first electrode slurry above the second electrode slurry or keeping 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 a use thereof, wherein the pore-forming method comprises the steps of coating slurry with the viscosity of 6000 m-9000 mPa & s on the surface of a current collector with the surface roughness Ra being 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 the application of the thick electrode in practice is limited.

Disclosure of Invention

The invention aims to provide a thick negative electrode and a preparation method and application thereof, and the invention improves the flexibility of the negative electrode by compounding and using a dispersing agent and different types of binder compositions, solves the problems of poor wettability of electrolyte of the thick electrode, long lithium ion migration path and large concentration polarization, improves the dynamic performance such as lithium precipitation, internal resistance and the like, improves the multiplying power and low-temperature discharge performance of the battery, and simultaneously ensures that the battery has good cycle performance. In an electrode for a lithium ion battery, a binder provides adhesion between an electrode active material, a conductive agent, and a current collector, and maintains good contact between an active material and the conductive agent, and between the active material and the active material. In the charging and discharging process, the integrity of the electrode structure can be effectively kept, a good electronic path and stable electrochemical performance are kept, and the safe and effective work of the battery is ensured.

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

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

Preferably, the binder B includes styrene-butadiene latex.

In the thick negative electrode, the binder B is in an emulsion type and binds a negative active substance, a negative active substance and a current collector in a point-point mode. The binder B has good 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 power between negative active substances and between the negative active substances and a current collector and improving the pole piece stripping force of a thick electrode; on the other hand, the existence of the network structure also reserves the channels for the lithium ions to be inserted and extracted, but the binding agent A has large brittleness and poor flexibility, and the pole piece active substance is easy to fall off during the pole piece processing and charging and discharging. Therefore, the adhesive B and the adhesive A play a synergistic effect, the stripping force of the negative pole piece prepared by the slurry is further improved, and the flexibility of the pole piece is improved. In addition, the binder a used alone may form unstable carboxylate with an electrolyte to remain in an SEI film, form an SEI film that cannot exist stably, and deteriorate the charge and discharge efficiency and cycle performance of an electrochemical device.

The thick negative electrode simultaneously contains the binder A and the binder B, and the electrode plate independently containing 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 pole piece has good flexibility, processing is facilitated, the situation that active substances fall off from the pole piece is reduced, and the short-circuit risk of the electrochemical energy storage device is reduced.

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

Preferably, the negative active material includes graphite.

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

Preferably, the current collector comprises a copper foil.

Preferably, the negative electrode active material has a mass fraction of 91.8 to 98.9% based on 100% by mass of the negative electrode active material layer, for example: 91.8%, 92%, 94%, 95%, 98.9%, etc.

Preferably, the mass fraction of the dispersant is 0.1 to 2.2%, for example: 0.1%, 0.5%, 1%, 1.5%, 2.2%, etc.

Preferably, the mass fraction of the binder a is 0.5 to 2.1%, for example: 0.5%, 0.8%, 1%, 1.5%, 2.1%, etc.

Preferably, the mass fraction of the binder B is 0.1 to 1.7%, for example: 0.1%, 0.5%, 1%, 1.2%, 1.7%, etc.

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

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

(1) Mixing a dispersant and a solvent to obtain a dispersant solution, mixing a negative active material and a conductive agent to obtain a mixture, mixing the mixture and the dispersant solution, and performing primary dispersion 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, and carrying out drying, rolling and cutting treatment to obtain the thick negative electrode.

The preparation method of the invention adds the dispersant, the binder A and the binder B. The dispersant is preferentially added to uniformly wrap the surface of the negative active material, so that the dispersant is used for dispersing the negative active material on one hand and is helpful for forming a stable SEI film on the other hand; the problem that the capacity attenuation is fast in the charge-discharge cycle process due to electrochemical energy storage when a single binder is used as the binder and the dispersing agent is solved.

Preferably, the dispersant solution of step (1) has a solid content of 1 to 2%, for example: 1%, 1.2%, 1.5%, 1.8%, 2%, etc.

Preferably, the degree of substitution of the dispersant is from 0.5 to 1, for example: 0.5, 0.62, 0.71, 0.85, 1, etc.

Preferably, the part of the binder A in the step (2) accounts for 20 to 35 percent of the total mass of the binder A, such as: 20%, 22%, 25%, 30%, 35%, etc.

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

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

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

(1) According to the invention, by compounding and using the dispersing agent and different types of binder compositions, the flexibility of the thick negative electrode is improved, the problems of poor wettability, long lithium ion migration path and large concentration polarization of the electrolyte of the thick electrode are solved, the dynamic properties such as lithium precipitation, internal resistance and the like are improved, the multiplying power and low-temperature discharge performance of the battery are improved, and 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, and maintains 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 charging and discharging process, the integrity of the electrode structure can be effectively maintained, a good electronic path and stable electrochemical performance are maintained, and the safe and effective work of the battery is ensured.

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

Drawings

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

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

Detailed Description

The technical solution of the present invention is further described below by way of specific embodiments. It should be understood by those skilled in the art that the examples are only for the understanding of the present invention and should not be construed as the specific limitation of the present invention.

Example 1

The embodiment provides a thick negative electrode, and a preparation method of the thick negative electrode comprises the following steps:

(1) Mixing sodium carboxymethylcellulose and water to obtain a dispersant solution with the 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 and the dispersant solution, and performing primary dispersion 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 remaining 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, and drying, rolling and cutting to obtain a 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;

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

Example 2

(1) Mixing sodium carboxymethylcellulose and water to obtain a dispersant solution with the 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 and the dispersant solution, and performing primary dispersion 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 remaining 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 a negative electrode thick electrode;

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

Example 3

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

Example 4

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

Example 5

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

Example 6

This example is different 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 those in example 1.

Comparative example 1

This comparative example differs from example 1 only in that no binder a was added, and the other conditions and parameters were exactly the same as those of example 1.

Comparative example 2

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

Comparative example 3

The comparative example differs from example 1 only in that the adhesive A was replaced by an equal mass of acrylate, polyacrylic acid and polyallyl alcohol, and the other conditions and parameters were exactly the same as those of example 1.

And (3) performance testing:

adding a certain amount of binder polyvinylidene fluoride powder into an N-methyl pyrrolidone solvent, and uniformly stirring and mixing to prepare a glue solution with solid content of 8%. Adding the positive active material lithium iron phosphate, the carbon nano tube, the conductive carbon black and the 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 and uniformly stirring to obtain positive slurry, coating the prepared slurry on a current collector, coating the current collector with carbon and aluminum foil with the thickness of 16 mu m, and then drying by an oven, rolling and cutting into pieces to form a positive plate for preparing the battery, wherein the negative thick electrodes obtained in the examples 1-6 and the comparative examples 1-3 are used as negative electrodes, a Polyethylene (PE) porous polymeric film is used as an isolating film, and 1.00mol/L LiPF (lithium fluoride) is used as a negative electrode ₆ (DEC + EMC + EC) is used as electrolyte to prepare a soft package lithium ion battery, the lithium ion battery is subjected to performance test, and the test result is shown in table 1:

TABLE 1

As can be seen from table 1, in example 1-2, the capacity retention rate of 3000 cycles at 1C/1C at 25 ℃ of the battery prepared from the thick negative electrode of the present invention can reach 90.1% or more, the capacity retention rate of 89.42% or more after 30-day storage at 60 ℃ can reach 93.24% or more after 30-day storage at 60 ℃, the 1C discharge rate at 25 ℃ can reach 93.10% or more, and the 0.2C low-temperature discharge rate at 0 ℃ can reach 92.74% or more.

Compared with the embodiment 1 and the embodiment 3-4, in the active material layer of the negative electrode thick electrode, the mass ratio of the binder A influences the performance of the negative electrode thick electrode, the mass fraction of the binder A is controlled to be 0.5-2.1%, the prepared negative electrode thick electrode has good performance, if the mass ratio of the binder A is too small, the performance of the battery cell is not obviously improved, if the mass ratio of the binder A is too large, the negative electrode plate becomes brittle and hard, and the active material falls off seriously in the processing process, so that the performance of the battery cell becomes poor.

Compared with the examples 5 to 6, the active material layer of the thick negative electrode of the present invention has the mass ratio of the binder B that affects the performance thereof, and the mass fraction of the binder B is controlled to be 0.1 to 1.7%, so that the thick negative electrode has good performance, and if the mass ratio of the binder B is too small, the synergistic effect with the binder a cannot be sufficiently exerted, the cell performance is poor, and if the mass ratio of the binder B is too large, the cell impedance is increased, and the low-temperature discharge performance of the cell is deteriorated.

The comparison graph of the cycle performance of the pole pieces in example 1 and comparative example 1 is shown in fig. 2, and can be obtained by comparing example 1 and comparative examples 1-2, the negative pole piece of the invention simultaneously comprises the binder A and the soft binder B, and the pole piece of the invention separately comprises the binder A, which is hard and brittle, and has excellent dynamic performance. The binder A and the binder B are used simultaneously, so that the negative pole piece has good flexibility, processing is facilitated, the situation that active substances fall off from the pole piece is reduced, and the short-circuit risk of the electrochemical energy storage device is reduced.

Comparing example 1 with comparative example 3, the binder a in the thick negative electrode of the present invention is a block copolymer binder, the middle of the block copolymer is a polyacrylate segment, and the two ends are a polyacrylic acid (PAA) segment and a polyvinyl alcohol (PVA) segment, respectively. The polyacrylate chain segment has certain swelling capacity on the electrolyte, and an ion-conducting channel is formed, so that the wettability of the electrolyte in the thick electrode is improved. Carboxyl in polyacrylic acid (PAA) chain segment and hydroxyl in CMC form a three-dimensional network structure to inhibit electrode expansion. Wherein, the cross-linking between different binder A chains is also generated through condensation reaction, the condensation reaction is generated between hydroxyl of one chain and carboxyl of the other chain, and the condensation reaction further enhances the stability of a three-dimensional network structure.

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

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 comprises a block copolymer, a polyacrylate chain segment is arranged in the middle of the block copolymer, polyacrylic acid chain segments and polyvinyl alcohol chain segments are arranged at two ends of the block copolymer, and the binder B comprises an emulsion binder.

2. The negative thick electrode of claim 1, wherein the monomer unit of the 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 ethylenically unsaturated carboxylic acid monomer unit, an unsaturated carboxylic acid alkyl ester monomer unit, or a vinylcyanide monomer unit;

preferably, the binder B includes styrene-butadiene latex.

3. The thick negative electrode of claim 1 or 2, wherein the dispersant comprises any one of sodium carboxymethylcellulose, sodium carboxymethylcellulose or sodium alginate, or a combination of at least two thereof.

4. The negative thick electrode of any of claims 1-3, wherein the negative active material comprises graphite;

preferably, the conductive agent comprises any one of conductive carbon black, graphene, carbon nanotubes or conductive fibers or a combination of at least two of the same;

preferably, the current collector includes a copper foil.

5. The negative thick electrode according to any one of claims 1 to 4, wherein the negative active material has a mass fraction of 91.8 to 98.9% based on 100% by mass of the negative active material layer;

preferably, the mass fraction of the dispersant is 0.1-2.2%;

preferably, the mass fraction of the binder A is 0.5-2.1%;

preferably, the mass fraction of the binder B is 0.1-1.7%;

preferably, the mass fraction of the conductive agent is 0.4 to 5%.

6. A method for preparing a thick negative electrode according to any one of claims 1 to 5, comprising the steps of:

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

7. The method of claim 6, wherein the dispersant solution of step (1) has a solid content of 1 to 2%;

preferably, the degree of substitution of the dispersant is 0.5 to 1.

8. The method according to claim 6 or 7, wherein the portion of the binder A in the step (2) is 20 to 35% by mass of the total mass of the binder A.

9. The manufacturing method according to any one of claims 6 to 8, wherein the thickness of the current collector in the step (3) is 5 to 10 μm.

10. A lithium ion battery comprising a thick negative electrode according to any one of claims 1 to 5.