CN111825804A - Copolymer latex for lithium ion secondary battery cathode, preparation method and application - Google Patents
Copolymer latex for lithium ion secondary battery cathode, preparation method and application Download PDFInfo
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- C08F257/00—Macromolecular compounds obtained by polymerising monomers on to polymers of aromatic monomers as defined in group C08F12/00
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- H01M4/02—Electrodes composed of, or comprising, active material
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Abstract
The invention discloses a copolymer latex for a lithium ion secondary battery cathode, which is prepared by copolymerizing styrene, butadiene and unsaturated carboxylic acid monomers to prepare a hard core, and copolymerizing butadiene, styrene and vinyl unsaturated monomers to prepare a soft shell, thereby preparing a latex product with a hard core and soft shell structure, and is particularly suitable for a lithium ion secondary battery cathode binder. It has been found that the copolymer latex having polymer particles with hard core and soft shell structure can improve the adhesion, thereby reducing the amount of binder and the influence of the binder on the internal resistance of the battery. The polarity and dielectric property of the polymer can be changed by the functional group introduced by copolymerization, so that the electrolyte swelling resistance of the adhesive and the influence on the internal resistance of the battery are improved.
Description
Technical Field
The present invention relates to a copolymer latex for a negative electrode of a lithium ion secondary battery. Belongs to the technical field of lithium ion secondary batteries.
Background
The negative electrode of the lithium ion secondary battery is composed of an active material graphite, a conductive additive, a binder and a current collector. The preparation method of the negative electrode generally comprises the steps of mixing and dispersing an active substance, a conductive additive and a binder into uniform slurry by taking water as a medium, coating the uniform slurry on a copper foil on a current collector, and drying and pressing the copper foil to prepare the negative electrode.
The performance of the binder is one of important indexes that determine the capacity and life of the lithium ion secondary battery. The binder is used as a non-conductive substance in an amount that affects the capacity and internal resistance of the lithium ion secondary battery, and thus the amount of the binder should be minimized while securing the adhesion. The adhesive has poor bonding performance, and the situations of powder falling and the falling of active substances and conductive additives from a current collector can occur in the manufacturing process of the negative electrode; the powder falling condition can also occur in the process of manufacturing the ultrasonic welding pole lug of the battery. In the process of charging and discharging the battery, the electrolyte swelling resistance of the adhesive is poor, and the situation that the active substance and the conductive additive fall off from the current collector can also occur, so that the service life of the battery is influenced.
Binders for negative electrodes of lithium ion secondary batteries are mainly classified into two types, one being an oily binder using an organic solvent as a dispersion medium, and the other being an aqueous binder using water as a dispersion medium. The oil-based binder is mainly a polyvinylidene fluoride-based polymer, and since the binding power to the electrode active material and the current collector is low, a larger amount of the binder is required to achieve a required binding strength, which affects the capacity of the battery. In addition, organic solvents in the oily adhesive volatilize during the manufacturing process, pollute the environment and are harmful to health, and the organic solvents are gradually replaced by aqueous adhesives at present.
The water-based adhesive mainly comprises styrene-butadiene latex, polyacrylic acid and the like. Patent application CN107868160A reports an epoxy group-containing styrene-butadiene latex, the adhesive has good cohesiveness, and does not fall off powder and fall off in the manufacturing process of a battery pole piece; is stable in electrolyte, does not swell and has high retention rate of the battery cycle capacity. Patent application CN105576284A reports a polyacrylic acid binder, which has the characteristics of small usage amount and excellent dispersibility for electrode materials such as graphite, and is excellent in battery capacity exertion and cycle performance.
Disclosure of Invention
The invention aims to overcome the defects of the prior art and provides copolymer latex for a negative electrode of a lithium ion secondary battery, a preparation method and application thereof, and the copolymer latex has excellent battery cycle performance.
In order to achieve the purpose, the invention adopts the following technical scheme:
1. a copolymer latex for a lithium ion secondary battery cathode contains latex particles with a hard core and soft shell structure, wherein the hard core is a polymer formed by copolymerizing 80-100 parts by weight of styrene, 0-15 parts by weight of butadiene and 0-5 parts by weight of unsaturated carboxylic acid monomers, the glass transition temperature of the hard core is above 60 ℃, the soft shell is a polymer formed by copolymerizing 30-80 parts by weight of butadiene, 20-70 parts by weight of styrene and 0-30 parts by weight of vinyl unsaturated monomers, and the hard core and the soft shell respectively account for 5-20% and 80-95% of the weight of the latex particles.
Preferably, the unsaturated carboxylic acid is selected from one or more of acrylic acid, methacrylic acid, maleic acid, fumaric acid and itaconic acid.
Preferably, the ethylenically unsaturated monomer is selected from one or more of methyl acrylate, ethyl acrylate, butyl acrylate, octyl acrylate, methyl methacrylate, acrylonitrile, acrylic acid, methacrylic acid, maleic acid, fumaric acid, itaconic acid, hydroxyethyl acrylate, hydroxypropyl acrylate.
Preferably, the hard core is a polymer copolymerized by 80-100 parts by weight of styrene, 10-15 parts by weight of butadiene and 3-5 parts by weight of unsaturated carboxylic acid monomer, and the soft shell is a polymer copolymerized by 30-80 parts by weight of butadiene, 20-70 parts by weight of styrene and 20-30 parts by weight of vinyl unsaturated monomer.
Preferably, the soft shell is a polymer prepared by copolymerizing 30-80 parts of butadiene, 20-70 parts of styrene, 20-30 parts of vinyl unsaturated monomer and 0.5-0.8 part of 1-allyl-3-butylimidazolium tetrafluoroborate modified nitrogen, sulfur and fluorine co-doped carbon nanotube.
More preferably, the specific method for modifying the 1-allyl-3-butylimidazolium tetrafluoroborate by weight parts is as follows: mixing and stirring 1 part of 1-allyl-3-butylimidazole tetrafluoroborate and 5-7 parts of isopropanol uniformly, adding 3-4 parts of nitrogen, sulfur and fluorine co-doped carbon nanotubes, stirring and reacting at 70-80 ℃ for 2-3 hours under the atmosphere of nitrogen, naturally cooling to room temperature (25 ℃), centrifuging to obtain precipitate, washing, and drying.
Still preferably, the preparation method of the nitrogen, sulfur and fluorine co-doped carbon nanotube comprises the following steps: adding 1 part of carbon nano tube into 30-40 parts of dimethyl sulfoxide, uniformly dispersing by ultrasonic waves, adding 0.008-0.01 part of 2-methylpyridine and 1.8-2 parts of N-butyl-N-methylpiperidine bis (trifluoromethanesulfonyl) imide salt, and uniformly dispersing by ultrasonic waves to obtain a suspension; and then transferring the suspension into a hydrothermal reaction kettle, reacting for 6-8 hours at the temperature of 60-70 ℃, filtering, washing and drying to obtain the catalyst.
2. The preparation method of the copolymer latex for the negative electrode of the lithium ion secondary battery comprises the following specific steps:
(1) fully stirring and mixing styrene, butadiene and unsaturated carboxylic acid monomers, an emulsifier and deionized water according to the formula ratio to obtain a core layer monomer pre-emulsion;
(2) fully stirring and mixing butadiene, styrene and vinyl unsaturated monomers with a formula ratio, an emulsifier and deionized water to obtain a shell layer monomer pre-emulsion;
(3) mixing 30-50% of the total mass of the nuclear layer monomer pre-emulsion obtained in the step (1) with a first part of pH regulator and a first part of initiator, stirring and heating to 70-75 ℃, and stirring for 4-6 hours under heat preservation to obtain nuclear seed microemulsion;
(4) heating the nuclear seed microemulsion prepared in the step (3) to 75-80 ℃, then respectively and uniformly dripping the rest nuclear layer monomer pre-emulsion and the second part of initiator into the nuclear layer monomer pre-emulsion and the second part of initiator simultaneously, and continuing to keep the temperature at 75-80 ℃ for 30-40 minutes after dripping to prepare nuclear polymer emulsion;
(5) heating the nuclear polymer emulsion prepared in the step (4) to 80-85 ℃, adding a second part of pH regulator, then respectively and uniformly dropwise adding the shell layer monomer pre-emulsion obtained in the step (2) and a third part of initiator into the nuclear polymer emulsion, preserving heat for 30-40 minutes at 80-85 ℃ after dropwise adding, and then cooling to room temperature to obtain the polymer emulsion.
Preferably, in the steps (1) and (2), the dosages of the emulsifier and the deionized water are respectively 0.02-0.04 times and 1.5-2 times of the total weight of the core layer monomer or the shell layer monomer; the emulsifier is sodium dodecyl sulfate and nonylphenol polyoxyethylene ether according to a mass ratio of 1: 0.2 to 0.3, respectively.
Preferably, the pH regulator is selected from any one of sodium carbonate, sodium bicarbonate or disodium hydrogen phosphate; the initiator is selected from potassium persulfate or ammonium persulfate; the initiator is prepared into an aqueous solution with the mass concentration of 20-30% and is added dropwise; the dosage of the first part of pH regulator is 0.2-0.4% of the total weight of the core layer monomer, the total amount of the first part of initiator and the second part of initiator is 0.4-0.7% of the total weight of the core layer monomer, and the weight ratio of the first part of initiator to the second part of initiator is 1: 1.1 to 1.3; the dosage of the pH regulator in the second part is 0.2-0.4% of the total weight of the shell monomer, and the dosage of the initiator in the third part is 0.4-0.7% of the total weight of the shell monomer.
Preferably, 1-allyl-3-butylimidazolium tetrafluoroborate modified nitrogen, sulfur and fluorine co-doped carbon nanotubes are further added in the step (2).
3. The copolymer latex for the negative electrode of the lithium ion secondary battery is used as a binder of negative electrode slurry of the lithium ion secondary battery.
4. The negative electrode slurry of the lithium ion battery prepared by the copolymer latex is prepared by mixing a negative electrode active substance, a conductive agent, a dispersing agent, deionized water and the copolymer latex.
Preferably, the mass ratio of the negative electrode active material, the conductive agent, the dispersant, and the copolymer latex is 100: 1: 1: 0.5 to 2; the solid content (mass content) of the negative electrode slurry is 40-60%.
Preferably, the negative active material is selected from any one or more of a silicon-carbon composite material, natural graphite or artificial graphite; the conductive agent is selected from any one of Ketjen black, acetylene black, superconducting carbon black, carbon nanotubes, carbon fibers or graphene; the dispersing agent is carboxymethyl cellulose or salt thereof.
5. The preparation method of the lithium ion battery cathode slurry comprises the steps of mixing and stirring the cathode active material, the conductive agent, the dispersing agent and the deionized water to obtain a premix, adding the copolymer latex into the premix, and uniformly stirring to obtain the lithium ion battery cathode slurry.
6. The lithium ion battery cathode material is prepared by uniformly coating the cathode slurry on the surface of a conductive matrix, and drying to remove a solvent.
The invention has the beneficial effects that:
the invention uses styrene, butadiene and unsaturated carboxylic acid monomer to copolymerize and make into hard core, uses butadiene, styrene, vinyl unsaturated monomer to copolymerize and make into soft shell, and then prepare and get a latex product with hard core soft shell structure, especially suitable for the lithium ion secondary battery negative pole binder. It has been found that the copolymer latex having polymer particles with hard core and soft shell structure can improve the adhesion, thereby reducing the amount of binder and the influence of the binder on the internal resistance of the battery. The polarity and dielectric property of the polymer can be changed by the functional group introduced by copolymerization, so that the electrolyte swelling resistance of the adhesive and the influence on the internal resistance of the battery are improved.
The copolymer latex of the core-shell polymer particles of the present invention is produced using a seeded emulsion polymerization process. Firstly, a seed latex of a copolymer forming core is produced by a known emulsion polymerization method, and the average particle diameter of the seed latex forming core is 50-90 nm.
The shell polymer is formed by polymerization in the presence of a seed latex constituting the core. The particle size of the obtained copolymer latex is between 100 and 200 nm. In order to be able to form core-shell polymer particles, the aqueous phase cannot contain emulsifier micelles, otherwise the monomers used to form the shell polymer would enter the emulsifier micelles to form new particles.
The invention also introduces 1-allyl-3-butylimidazolium tetrafluoroborate modified nitrogen, sulfur and fluorine co-doped carbon nanotubes into the soft shell monomer, the carbon nanotubes have excellent electrical properties, the applicant firstly utilizes N-butyl-N-methylpiperidine bis (trifluoromethanesulfonyl) imide salt to dope nitrogen, sulfur and fluorine into the carbon nanotubes and then utilizes 1-allyl-3-butylimidazolium tetrafluoroborate to modify, on one hand, the compatibility and dispersibility of the carbon nanotubes in a system are thoroughly solved through double bond reaction, on the other hand, active sites are increased through doping, fluorine is introduced, and hydrogen bond action is formed with water, so that the stability of latex is improved, and the adhesive property of the latex and the cycle performance of the obtained battery are greatly improved through the two actions.
Detailed Description
The present invention will be further illustrated by the following examples, which are intended to be merely illustrative and not limitative.
Examples 1 to 7
The specific formula of the copolymer latex for the negative electrode of the lithium ion secondary battery is shown in Table 1.
The preparation method is characterized by comprising the following steps:
(1) fully stirring and mixing styrene, butadiene and unsaturated carboxylic acid monomers, an emulsifier and deionized water according to the formula ratio to obtain a core layer monomer pre-emulsion;
(2) fully stirring and mixing butadiene, styrene and vinyl unsaturated monomers with a formula ratio, an emulsifier and deionized water to obtain a shell layer monomer pre-emulsion;
(3) mixing 40% of the total mass of the nuclear layer monomer pre-emulsion obtained in the step (1) with a first part of pH regulator and a first part of initiator, stirring and heating to 72 ℃, and keeping the temperature and stirring for 5 hours to obtain nuclear seed microemulsion;
(4) heating the nuclear seed microemulsion prepared in the step (3) to 78 ℃, then respectively and uniformly dripping the rest nuclear layer monomer pre-emulsion and the second part of initiator into the nuclear layer monomer pre-emulsion and the second part of initiator simultaneously, and continuing to keep the temperature at 78 ℃ for 35 minutes after dripping to prepare nuclear polymer emulsion;
(5) heating the nuclear polymer emulsion prepared in the step (4) to 82 ℃, adding a second part of pH regulator, then respectively and uniformly dropwise adding the shell layer monomer pre-emulsion obtained in the step (2) and a third part of initiator simultaneously, preserving heat for 35 minutes at 82 ℃ after dropwise adding, and then cooling to room temperature to obtain the polymer.
In the steps (1) and (2), the dosages of the emulsifier and the deionized water are respectively 0.03 time and 1.8 time of the total weight of the core layer monomer or the shell layer monomer; the emulsifier is sodium dodecyl sulfate and nonylphenol polyoxyethylene ether according to a mass ratio of 1: 0.25 and mixing.
The pH regulator is sodium carbonate, sodium bicarbonate or disodium hydrogen phosphate; the initiator is potassium persulfate or ammonium persulfate; the initiator is prepared into an aqueous solution with the mass concentration of 25 percent, and the aqueous solution is dropwise added with the feed; the dosage of the first part of pH regulator is 0.3 percent of the total weight of the core layer monomer, the total amount of the first part of initiator and the second part of initiator is 0.5 percent of the total weight of the core layer monomer, and the weight ratio of the first part of initiator to the second part of initiator is 1: 1.2; the dosage of the pH regulator of the second part is 0.3 percent of the total weight of the shell layer monomers, and the dosage of the initiator of the third part is 0.6 percent of the total weight of the shell layer monomers.
The average particle size of the latex particles was determined according to the test requirements of a malvern (Nano-ZS90) laser particle sizer.
TABLE 1 formulation compositions of examples 1-7
Example 8
The copolymer latex for the negative electrode of the lithium ion secondary battery contains latex particles with a hard core and soft shell structure, wherein the average particle size is 100nm, the hard core is a polymer formed by copolymerizing 80 parts of styrene, 15 parts of butadiene and 1 part of unsaturated carboxylic acid monomer, the glass transition temperature of the hard core is above 60 ℃, the soft shell is a polymer formed by copolymerizing 80 parts of butadiene, 20 parts of styrene, 30 parts of vinyl unsaturated monomer and 0.5 part of nitrogen-sulfur-fluorine co-doped carbon nano tube, and the hard core and the soft shell respectively account for 20% and 80% of the weight of the latex particles.
The unsaturated carboxylic acid is acrylic acid.
The vinyl unsaturated monomer is methyl acrylate.
The hard core is a polymer formed by copolymerizing 100 parts of styrene, 10 parts of butadiene and 5 parts of unsaturated carboxylic acid monomers, and the soft shell is a polymer formed by copolymerizing 30 parts of butadiene, 70 parts of styrene and 20 parts of vinyl unsaturated monomers.
The preparation method of the nitrogen, sulfur and fluorine co-doped carbon nano tube comprises the following steps of: adding 1 part of carbon nano tube into 40 parts of dimethyl sulfoxide, uniformly dispersing by ultrasonic waves, adding 0.008 part of 2-methylpyridine and 2 parts of N-butyl-N-methylpiperidine bis (trifluoromethanesulfonyl) imide salt, and uniformly dispersing by ultrasonic waves to obtain a suspension; and then transferring the suspension into a hydrothermal reaction kettle, reacting for 8 hours at 60 ℃, filtering, washing and drying to obtain the catalyst.
The preparation method of the copolymer latex for the negative electrode of the lithium ion secondary battery comprises the following specific steps:
(1) fully stirring and mixing styrene, butadiene and unsaturated carboxylic acid monomers, an emulsifier and deionized water according to the formula ratio to obtain a core layer monomer pre-emulsion;
(2) fully stirring and mixing butadiene, styrene, vinyl unsaturated monomer, nitrogen-sulfur-fluorine co-doped carbon nano tube, emulsifier and deionized water according to the formula ratio to obtain a shell monomer pre-emulsion;
(3) mixing 30% of the total mass of the nuclear layer monomer pre-emulsion obtained in the step (1) with a first part of pH regulator and a first part of initiator, stirring and heating to 75 ℃, and keeping the temperature and stirring for 4 hours to obtain nuclear seed microemulsion;
(4) heating the nuclear seed microemulsion prepared in the step (3) to 80 ℃, then respectively and uniformly dripping the rest nuclear layer monomer pre-emulsion and the second part of initiator into the nuclear layer monomer pre-emulsion and the second part of initiator simultaneously, and continuing to keep the temperature at 75 ℃ for 40 minutes after dripping to prepare nuclear polymer emulsion;
(5) heating the nuclear polymer emulsion prepared in the step (4) to 80 ℃, adding a second part of pH regulator, then respectively and uniformly dropwise adding the shell layer monomer pre-emulsion obtained in the step (2) and a third part of initiator simultaneously, preserving heat for 30 minutes at 85 ℃ after dropwise adding, and then cooling to room temperature to obtain the polymer.
In the steps (1) and (2), the dosages of the emulsifier and the deionized water are respectively 0.04 time and 1.5 times of the total weight of the core layer monomer or the shell layer monomer; the emulsifier is sodium dodecyl sulfate and nonylphenol polyoxyethylene ether according to a mass ratio of 1: 0.3 and mixing.
The pH regulator is sodium carbonate; the initiator is ammonium persulfate; the initiator is prepared into an aqueous solution with the mass concentration of 20 percent, and is added dropwise; the dosage of the first part of pH regulator is 0.4 percent of the total weight of the core layer monomer, the total amount of the first part of initiator and the second part of initiator is 0.4 percent of the total weight of the core layer monomer, and the weight ratio of the first part of initiator to the second part of initiator is 1: 1.3; the dosage of the pH regulator of the second part is 0.2 percent of the total weight of the shell layer monomers, and the dosage of the initiator of the third part is 0.7 percent of the total weight of the shell layer monomers.
The average particle size of the latex particles was determined according to the test requirements of a malvern (Nano-ZS90) laser particle sizer.
Example 9
A copolymer latex for a negative electrode of a lithium ion secondary battery, which comprises latex particles having a hard core-soft shell structure, wherein the average particle diameter of the latex particles is 100nm, the hard core is a polymer obtained by copolymerizing 100 parts of styrene, 5 parts of butadiene and 5 parts of unsaturated carboxylic acid monomers, the glass transition temperature of the hard core is 60 ℃ or higher, the soft shell is a polymer obtained by copolymerizing 30 parts of butadiene, 70 parts of styrene, 20 parts of vinyl unsaturated monomers and 0.8 part of 1-allyl-3-butylimidazolium tetrafluoroborate-modified carbon nanotubes, and the hard core and the soft shell respectively account for 5% and 95% of the weight of the latex particles.
The unsaturated carboxylic acid is methacrylic acid and maleic acid, and the mass ratio of the methacrylic acid to the maleic acid is 1: 1.
the vinyl unsaturated monomer is octyl acrylate and itaconic acid, and the mass ratio of the octyl acrylate to the itaconic acid is 1: 1.
the hard core is a polymer formed by copolymerizing 80 parts of styrene, 15 parts of butadiene and 3 parts of unsaturated carboxylic acid monomers, and the soft shell is a polymer formed by copolymerizing 80 parts of butadiene, 20 parts of styrene and 30 parts of vinyl unsaturated monomers.
The specific method for modifying the 1-allyl-3-butylimidazolium tetrafluoroborate comprises the following steps in parts by weight: mixing 1 part of 1-allyl-3-butylimidazole tetrafluoroborate and 5 parts of isopropanol, uniformly stirring, adding 4 parts of carbon nanotubes, stirring and reacting at 70 ℃ for 3 hours under the nitrogen atmosphere, naturally cooling to room temperature (25 ℃), centrifuging to obtain a precipitate, washing, and drying.
The preparation method of the copolymer latex for the negative electrode of the lithium ion secondary battery comprises the following specific steps:
(1) fully stirring and mixing styrene, butadiene and unsaturated carboxylic acid monomers, an emulsifier and deionized water according to the formula ratio to obtain a core layer monomer pre-emulsion;
(2) fully stirring and mixing butadiene, styrene, vinyl unsaturated monomers, 1-allyl-3-butylimidazolium tetrafluoroborate-modified carbon nanotubes, an emulsifier and deionized water according to the formula ratio to obtain a shell monomer pre-emulsion;
(3) mixing 50% of the total mass of the nuclear layer monomer pre-emulsion obtained in the step (1) with a first part of pH regulator and a first part of initiator, stirring and heating to 70 ℃, and keeping the temperature and stirring for 6 hours to obtain nuclear seed microemulsion;
(4) heating the nuclear seed microemulsion prepared in the step (3) to 75 ℃, then respectively and uniformly dripping the rest nuclear layer monomer pre-emulsion and the second part of initiator into the nuclear layer monomer pre-emulsion and the second part of initiator simultaneously, and continuing to keep the temperature at 80 ℃ for 30 minutes after dripping to prepare nuclear polymer emulsion;
(5) heating the nuclear polymer emulsion prepared in the step (4) to 85 ℃, adding a second part of pH regulator, then respectively and uniformly dropwise adding the shell layer monomer pre-emulsion obtained in the step (2) and a third part of initiator simultaneously, preserving heat for 40 minutes at 80 ℃ after dropwise adding, and then cooling to room temperature to obtain the polymer.
In the steps (1) and (2), the dosages of the emulsifier and the deionized water are respectively 0.02 time and 2 times of the total weight of the core layer monomer or the shell layer monomer; the emulsifier is sodium dodecyl sulfate and nonylphenol polyoxyethylene ether according to a mass ratio of 1: 0.2 and mixing.
The pH regulator is sodium bicarbonate; the initiator is potassium persulfate; the initiator is prepared into an aqueous solution with the mass concentration of 30 percent, and is added dropwise; the dosage of the first part of pH regulator is 0.2 percent of the total weight of the core layer monomer, the total amount of the first part of initiator and the second part of initiator is 0.7 percent of the total weight of the core layer monomer, and the weight ratio of the first part of initiator to the second part of initiator is 1: 1.1; the dosage of the pH regulator of the second part is 0.4 percent of the total weight of the shell layer monomers, and the dosage of the initiator of the third part is 0.4 percent of the total weight of the shell layer monomers.
The average particle size of the latex particles was determined according to the test requirements of a malvern (Nano-ZS90) laser particle sizer.
Example 10
The copolymer latex for the negative electrode of the lithium ion secondary battery contains latex particles with a hard core and soft shell structure, wherein the average particle size is 100nm, the hard core is a polymer formed by copolymerizing 90 parts of styrene, 10 parts of butadiene and 3 parts of unsaturated carboxylic acid monomers in parts by weight, the glass transition temperature of the hard core is more than 60 ℃, the soft shell is a polymer formed by copolymerizing 50 parts of butadiene, 45 parts of styrene, 25 parts of vinyl unsaturated monomers and 0.6 part of 1-allyl-3-butylimidazolium tetrafluoroborate modified nitrogen, sulfur and fluorine co-doped carbon nano tubes, and the hard core and the soft shell respectively account for 10% and 90% of the weight of the latex particles.
The unsaturated carboxylic acid is maleic acid, fumaric acid and itaconic acid, and the mass ratio of the maleic acid to the fumaric acid to the itaconic acid is 1: 1: 1.
the vinyl unsaturated monomer is octyl acrylate, maleic acid and hydroxypropyl acrylate, and the mass ratio of the octyl acrylate, the maleic acid and the hydroxypropyl acrylate is 1: 1: 1.
the hard core is a polymer formed by copolymerizing 90 parts of styrene, 12 parts of butadiene and 4 parts of unsaturated carboxylic acid monomers, and the soft shell is a polymer formed by copolymerizing 55 parts of butadiene, 48 parts of styrene and 25 parts of vinyl unsaturated monomers.
The specific method for modifying the 1-allyl-3-butylimidazolium tetrafluoroborate comprises the following steps in parts by weight: mixing 1 part of 1-allyl-3-butylimidazole tetrafluoroborate and 6 parts of isopropanol, uniformly stirring, adding 3.5 parts of nitrogen, sulfur and fluorine co-doped carbon nanotube, stirring and reacting at 75 ℃ for 2.5 hours under the atmosphere of nitrogen, naturally cooling to room temperature (25 ℃), centrifuging to obtain precipitate, washing and drying.
The preparation method of the nitrogen, sulfur and fluorine co-doped carbon nano tube comprises the following steps of: adding 1 part of carbon nano tube into 35 parts of dimethyl sulfoxide, uniformly dispersing by ultrasonic waves, adding 0.009 part of 2-methylpyridine and 1.9 parts of N-butyl-N-methylpiperidine bis (trifluoromethanesulfonyl) imide salt, and uniformly dispersing by ultrasonic waves to obtain a suspension; and then transferring the suspension into a hydrothermal reaction kettle, reacting for 7 hours at 65 ℃, filtering, washing and drying to obtain the catalyst.
The preparation method of the copolymer latex for the negative electrode of the lithium ion secondary battery comprises the following specific steps:
(1) fully stirring and mixing styrene, butadiene and unsaturated carboxylic acid monomers, an emulsifier and deionized water according to the formula ratio to obtain a core layer monomer pre-emulsion;
(2) fully stirring and mixing butadiene, styrene and vinyl unsaturated monomers with a formula ratio, an emulsifier and deionized water to obtain a shell layer monomer pre-emulsion;
(3) mixing 40% of the total mass of the nuclear layer monomer pre-emulsion obtained in the step (1) with a first part of pH regulator and a first part of initiator, stirring and heating to 72 ℃, and keeping the temperature and stirring for 5 hours to obtain nuclear seed microemulsion;
(4) heating the nuclear seed microemulsion prepared in the step (3) to 78 ℃, then respectively and uniformly dripping the rest nuclear layer monomer pre-emulsion and the second part of initiator into the nuclear layer monomer pre-emulsion and the second part of initiator simultaneously, and continuing to keep the temperature at 78 ℃ for 35 minutes after dripping to prepare nuclear polymer emulsion;
(5) heating the nuclear polymer emulsion prepared in the step (4) to 83 ℃, adding a second part of pH regulator, then respectively and uniformly dropwise adding the shell layer monomer pre-emulsion obtained in the step (2) and a third part of initiator simultaneously, preserving heat for 35 minutes at 82 ℃ after dropwise adding, and then cooling to room temperature to obtain the polymer.
In the steps (1) and (2), the dosages of the emulsifier and the deionized water are respectively 0.03 time and 1.8 time of the total weight of the core layer monomer or the shell layer monomer; the emulsifier is sodium dodecyl sulfate and nonylphenol polyoxyethylene ether according to a mass ratio of 1: 0.25 and mixing.
The pH regulator is disodium hydrogen phosphate; the initiator is ammonium persulfate; the initiator is prepared into an aqueous solution with the mass concentration of 25 percent, and the aqueous solution is dropwise added with the feed; the dosage of the first part of pH regulator is 0.3 percent of the total weight of the core layer monomer, the total amount of the first part of initiator and the second part of initiator is 0.6 percent of the total weight of the core layer monomer, and the weight ratio of the first part of initiator to the second part of initiator is 1: 1.2; the dosage of the pH regulator of the second part is 0.3 percent of the total weight of the shell layer monomers, and the dosage of the initiator of the third part is 0.6 percent of the total weight of the shell layer monomers.
The average particle size of the latex particles was determined according to the test requirements of a malvern (Nano-ZS90) laser particle sizer.
Comparative example
A copolymer latex is copolymerized by 130 parts of styrene, 40 parts of butadiene, 20 parts of itaconic acid and 10 parts of vinyl unsaturated monomers (5 parts of methyl methacrylate, 2 parts of acrylonitrile, 1 part of methacrylic acid and 2 parts of itaconic acid) in parts by weight, and the specific preparation method is as follows: fully stirring and mixing the monomers with the emulsifier and the deionized water according to the formula ratio, adding the pH regulator and the initiator, stirring and reacting for 6 hours at 75 ℃, and naturally cooling to room temperature to obtain the product. Wherein the dosage of the emulsifier and the deionized water is respectively 0.03 time and 1.8 time of the total weight of the core layer monomer or the shell layer monomer; the emulsifier is sodium dodecyl sulfate and nonylphenol polyoxyethylene ether according to a mass ratio of 1: 0.25, mixing; the pH regulator is sodium bicarbonate; the initiator is ammonium persulfate; the initiator is prepared into an aqueous solution with the mass concentration of 20-30% and is added dropwise.
Test examples
The copolymer latexes of examples 1-10 and comparative examples of the invention were used to prepare negative electrode slurries with natural graphite as the negative electrode active material. The negative electrode slurry comprises the following components in percentage by weight: natural graphite/conductive agent/sodium carboxymethylcellulose/copolymer latex 100/1/1/1 (weight ratio). The solid content of the anode slurry was 51%.
And coating the prepared negative electrode slurry on a current collector copper foil, and drying and rolling to obtain a negative electrode plate. In the process of manufacturing the negative pole piece, the phenomena of powder falling and peeling-off do not occur. The peel strength of the negative electrode sheet was measured in accordance with the JISK6854-2 standard. And testing the peel strength of the negative pole piece.
The negative pole piece is matched with the lithium cobaltate positive pole to assemble a square lithium ion battery for application test. The charge/discharge end voltage of the test cell was 4.0V. The viscosity, peel strength and cycle performance of the battery were measured, and the results are shown in table 2.
TABLE 2 comparison of Properties
As can be seen from Table 2, the latexes obtained in examples 1 to 10 have high viscosity, high peel strength and good cycle performance, and the capacity retention rate of the battery is more than 96% (standard is more than or equal to 94%) after 50 cycles at 0.5C, the capacity retention rate of the battery is more than 94.5% (standard is more than or equal to 90%) after 100 cycles, and the capacity retention rate of the battery is more than 90.0% (standard is more than or equal to 80%) after 300 cycles, which meet the standards. In embodiments 8 to 10, carbon nanotubes are introduced into the shell layer, and thus, indexes of the battery are significantly better, in embodiment 8, the nitrogen, sulfur and fluorine co-doped carbon nanotubes are not modified, in embodiment 9, the carbon nanotubes are not doped, and indexes are slightly inferior to those of embodiment 10.
The comparative example adopts direct polymerization, does not form a core-shell structure, and has obviously poor performances.
Although the present invention has been described with reference to the specific embodiments, it is not intended to limit the scope of the present invention, and various modifications and variations can be made by those skilled in the art without inventive changes based on the technical solution of the present invention.
Claims (8)
1. The copolymer latex for the negative electrode of the lithium ion secondary battery is characterized in that latex particles contained in the copolymer latex have a hard core and soft shell structure, wherein the hard core is a polymer formed by copolymerizing 80-100 parts by weight of styrene, 0-15 parts by weight of butadiene and 0-5 parts by weight of unsaturated carboxylic acid monomers, the glass transition temperature of the hard core is above 60 ℃, the soft shell is a polymer formed by copolymerizing 30-80 parts by weight of butadiene, 20-70 parts by weight of styrene and 0-30 parts by weight of vinyl unsaturated monomers, and the hard core and the soft shell respectively account for 5-20% and 80-95% of the weight of the latex particles.
2. The copolymer latex for the negative electrode of the lithium ion secondary battery according to claim 1, wherein the unsaturated carboxylic acid is one or more selected from acrylic acid, methacrylic acid, maleic acid, fumaric acid, and itaconic acid.
3. The copolymer latex for negative electrodes of lithium ion secondary batteries according to claim 1, wherein the ethylenically unsaturated monomer is selected from one or more of methyl acrylate, ethyl acrylate, butyl acrylate, octyl acrylate, methyl methacrylate, acrylonitrile, acrylic acid, methacrylic acid, maleic acid, fumaric acid, itaconic acid, hydroxyethyl acrylate, and hydroxypropyl acrylate.
4. The method for preparing the copolymer latex for the negative electrode of the lithium ion secondary battery according to any one of claims 1 to 3, which is characterized by comprising the following steps:
(1) fully stirring and mixing styrene, butadiene and unsaturated carboxylic acid monomers, an emulsifier and deionized water according to the formula ratio to obtain a core layer monomer pre-emulsion;
(2) fully stirring and mixing butadiene, styrene and vinyl unsaturated monomers with a formula ratio, an emulsifier and deionized water to obtain a shell layer monomer pre-emulsion;
(3) mixing 30-50% of the total mass of the nuclear layer monomer pre-emulsion obtained in the step (1) with a first part of pH regulator and a first part of initiator, stirring and heating to 70-75 ℃, and stirring for 4-6 hours under heat preservation to obtain nuclear seed microemulsion;
(4) heating the nuclear seed microemulsion prepared in the step (3) to 75-80 ℃, then respectively and uniformly dripping the rest nuclear layer monomer pre-emulsion and the second part of initiator into the nuclear layer monomer pre-emulsion and the second part of initiator simultaneously, and continuing to keep the temperature at 75-80 ℃ for 30-40 minutes after dripping to prepare nuclear polymer emulsion;
(5) heating the nuclear polymer emulsion prepared in the step (4) to 80-85 ℃, adding a second part of pH regulator, then respectively and uniformly dropwise adding the shell layer monomer pre-emulsion obtained in the step (2) and a third part of initiator into the nuclear polymer emulsion, preserving heat for 30-40 minutes at 80-85 ℃ after dropwise adding, and then cooling to room temperature to obtain the polymer emulsion.
5. Use of the copolymer latex for a negative electrode of a lithium ion secondary battery according to any one of claims 1 to 3 as a binder for a slurry of a negative electrode of a lithium ion secondary battery.
6. A lithium ion battery negative electrode slurry prepared by using the copolymer latex of any one of claims 1 to 3, which is prepared by mixing a negative electrode active material, a conductive agent, a dispersant, deionized water and the copolymer latex of any one of claims 1 to 3.
7. The preparation method of the lithium ion battery negative electrode slurry, according to claim 6, is characterized in that the negative electrode active material, the conductive agent, the dispersing agent and the deionized water are mixed and stirred to obtain a premix, and then the copolymer latex is added into the premix and stirred uniformly to obtain the lithium ion battery negative electrode slurry.
8. A lithium ion battery cathode material is characterized in that the cathode slurry of claim 6 is uniformly coated on the surface of a conductive substrate, and the conductive substrate is dried to remove a solvent.
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| CN112341961A (en) * | 2020-10-28 | 2021-02-09 | 欣旺达电动汽车电池有限公司 | Adhesive, diaphragm and preparation method thereof |
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