CN112375179B - Bimolecular-weight-distribution binder for negative electrode and preparation method and application thereof - Google Patents
Bimolecular-weight-distribution binder for negative electrode and preparation method and application thereof Download PDFInfo
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- CN112375179B CN112375179B CN202011248434.5A CN202011248434A CN112375179B CN 112375179 B CN112375179 B CN 112375179B CN 202011248434 A CN202011248434 A CN 202011248434A CN 112375179 B CN112375179 B CN 112375179B
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- C—CHEMISTRY; METALLURGY
- C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
- C08F—MACROMOLECULAR COMPOUNDS OBTAINED BY REACTIONS ONLY INVOLVING CARBON-TO-CARBON UNSATURATED BONDS
- C08F236/00—Copolymers of compounds having one or more unsaturated aliphatic radicals, at least one having two or more carbon-to-carbon double bonds
- C08F236/02—Copolymers of compounds having one or more unsaturated aliphatic radicals, at least one having two or more carbon-to-carbon double bonds the radical having only two carbon-to-carbon double bonds
- C08F236/04—Copolymers of compounds having one or more unsaturated aliphatic radicals, at least one having two or more carbon-to-carbon double bonds the radical having only two carbon-to-carbon double bonds conjugated
- C08F236/10—Copolymers of compounds having one or more unsaturated aliphatic radicals, at least one having two or more carbon-to-carbon double bonds the radical having only two carbon-to-carbon double bonds conjugated with vinyl-aromatic monomers
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- C—CHEMISTRY; METALLURGY
- C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
- C08F—MACROMOLECULAR COMPOUNDS OBTAINED BY REACTIONS ONLY INVOLVING CARBON-TO-CARBON UNSATURATED BONDS
- C08F136/00—Homopolymers of compounds having one or more unsaturated aliphatic radicals, at least one having two or more carbon-to-carbon double bonds
- C08F136/02—Homopolymers of compounds having one or more unsaturated aliphatic radicals, at least one having two or more carbon-to-carbon double bonds the radical having only two carbon-to-carbon double bonds
- C08F136/04—Homopolymers of compounds having one or more unsaturated aliphatic radicals, at least one having two or more carbon-to-carbon double bonds the radical having only two carbon-to-carbon double bonds conjugated
- C08F136/06—Butadiene
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M10/00—Secondary cells; Manufacture thereof
- H01M10/05—Accumulators with non-aqueous electrolyte
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- H—ELECTRICITY
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- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M4/00—Electrodes
- H01M4/02—Electrodes composed of, or comprising, active material
- H01M4/62—Selection of inactive substances as ingredients for active masses, e.g. binders, fillers
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Abstract
The invention provides a bi-molecular weight distribution binder for a negative electrode, and preparation and application thereof; the binder includes a first component and a second component, wherein the first component is a butadiene-based polymer having a number average molecular weight in the range of 5 to 50 ten thousand, and the second component is a butadiene-styrene copolymer having a number average molecular weight in the range of 50 to 1000 ten thousand. The lithium ion battery prepared from the negative plate prepared from the binder for the negative electrode with the double molecular weight distribution has excellent cycle, 700 cycles more than unmodified cycle times, small expansion in the cycle process and 12% reduction in the cycle end period compared with unmodified cycle times, because the mechanical strength of added styrene polymerization is improved, the intramolecular agglomeration condition is reduced, the molecule is used as a macromolecular chain, the molecular entanglement number can be effectively ensured, and the mechanical strength in the molecular chain and among the molecular chains can be maintained.
Description
Technical Field
The invention belongs to the technical field of lithium ion batteries, and particularly relates to a bi-molecular weight distribution binder for a negative electrode, a preparation method of the bi-molecular weight distribution binder, a negative plate comprising the binder and a lithium ion battery.
Background
The lithium ion battery has the characteristics of long cycle life, no memory effect, high energy density, small environmental pollution and the like, and is widely applied to the field of digital and power automobiles in recent years, the lithium ion battery mainly comprises a positive electrode, a negative electrode, a diaphragm and electrolyte, the raw material proportion of a complete lithium ion battery must comprise active substances, conductive agents, binding agents, solvents, other additives and the like, and the binding agents have the function of binding the active substances and foil materials, the active substances and the conductive agents. Although the dosage is small, the application is not replaceable, and the lithium ion battery has important influence on the improvement of energy density, the reduction of internal resistance and the cycle life of the whole battery.
The existing lithium ion battery cathode binder in the market has two types of oiliness and water-solubility, mainly comprises systems such as polyvinyl alcohol, polyvinylidene fluoride, modified Styrene Butadiene Rubber (SBR), fluorinated rubber, polyacrylic acid and the like, the modified SBR has outstanding advantages in price and performance and is widely applied to lithium ion batteries, and the market of the water-based binder accounts for 90%. However, the existing binders are copolymers of styrene and butadiene obtained by controlling the number of free radical chains, have relatively single molecular weight, and are difficult to ensure the balance of diffusion between active substances and excellent binding performance. Meanwhile, the varieties of the domestic special binders for lithium ion batteries are few, the performances are poor, and the special binders for high-end lithium ion batteries almost completely depend on import, so that the development strength is increased, and the method has very important significance in the aspects of import substitution technology and production process.
Disclosure of Invention
Aiming at the defects of the conventional SBR (butadiene-styrene rubber) binder in the inhibition of the expansion of a negative plate in the circulation process and the influence of phenomena of reduced caking property caused by agglomeration of a binding system and the like on the long circulation life and the electrical property of a lithium ion battery, the invention aims to develop the binder for the negative electrode with double molecular weight distribution, a preparation method and application thereof, wherein the binder can ensure that a negative active substance can be quickly diffused and can also have good binding effect on the negative active substance, the circulation life of the lithium ion battery is prolonged, and the expansion of the negative plate in the circulation process is inhibited.
Specifically, the invention develops a bi-molecular weight distribution binder for a negative electrode by adopting a segmented secondary polymerization mode, wherein the binder has a bi-molecular weight distribution and comprises a first component and a second component, the first component is mainly butadiene polymer with small number average molecular weight, and the second component is mainly butadiene-styrene copolymer with large number average molecular weight. The second component is selected to effectively improve the mechanical strength of the binder, and the large-volume benzene ring can reduce molecular agglomeration, has small swelling in electrolyte and strong binding effect, and is beneficial to the cycle stability of the lithium ion battery; the first component is selected to maintain the elasticity of the binder, the small molecular weight butadiene polymer can well realize the diffusion of the binder, so that the binder can be rapidly diffused into the negative active materials, the negative active materials can be fully bonded, and the small molecular weight butadiene polymer can also ensure the wettability of the electrolyte. The invention solves the problems of low bonding strength and poor effect of inhibiting the expansion of the pole piece of the existing aqueous binder of the electrode material of the lithium ion battery, greatly prolongs the cycle life of the lithium ion battery and reduces the cycle expansion.
The purpose of the invention is realized by the following technical scheme:
a binder comprising a first component and a second component, wherein the first component is a butadiene-based polymer having a number average molecular weight in the range of 5 to 50 ten thousand, and the second component is a butadiene-styrene copolymer having a number average molecular weight in the range of 50 to 1000 ten thousand.
According to the invention, the binder has a bimodal molecular weight distribution.
According to the invention, the mass ratio of the first component to the second component is 1 (2-11), such as 1.
According to the present invention, the butadiene-based polymer includes a butadiene homopolymer.
Further, the butadiene-based polymer also includes a butadiene-styrene copolymer, that is, the first component includes a butadiene homopolymer having a number average molecular weight in the range of 5 to 50 ten thousand, and may further include a butadiene-styrene copolymer having a number average molecular weight in the range of 5 to 50 ten thousand.
According to the invention, the mass ratio of the butadiene-styrene copolymer having a number average molecular weight in the range of 5 to 50 ten thousand to the butadiene homopolymer having a number average molecular weight in the range of 5 to 50 ten thousand in the first component is (0 to 4): (10 to 6), for example, 0.
According to the present invention, the butadiene-styrene copolymer having a number average molecular weight in the range of 5 to 50 ten thousand has a higher molar percentage of butadiene repeating units than styrene repeating units. Illustratively, the mole percent content of butadiene repeat units in the butadiene-styrene copolymer having a number average molecular weight in the range of from 5 to 50 ten thousand is from 55 to 80%, such as from 55 to 60%, such as 57%; the styrene repeat units may be present in a molar percentage of 20 to 45%, such as 40 to 45%, such as 43%.
According to the present invention, the butadiene-styrene copolymer having a number average molecular weight in the range of 50 to 1000 ten thousand has a styrene repeating unit content in mole percent of 10 to 70%, for example 10 to 40%, such as 35%; the molar percentage of butadiene repeat units is 90-30%, for example 90-60%, such as 65%.
Illustratively, the butadiene-styrene copolymer has a structure as shown in formula (1),
wherein a is the number of repeating units of a styrene repeating unit, b is the number of repeating units of a 1,3 polymerized butadiene repeating unit, and c is the number of repeating units of a 1,2 polymerized butadiene repeating unit.
Preferably, the first component is a butadiene-based polymer having a number average molecular weight in the range of 10 to 40 ten thousand (e.g., in the range of 10 to 30 ten thousand), and the second component is a butadiene-styrene copolymer having a number average molecular weight in the range of 100 to 800 ten thousand (e.g., in the range of 200 to 600 ten thousand).
According to the invention, the two types of polymers are uniformly dispersed in the binder.
According to the invention, the viscosity of the binder is from 1 to 60000 mPas, such as from 200 to 50000 mPas.
According to the invention, the particle size of the binder is 90-250nm, such as 100-200nm.
The invention also provides a preparation method of the adhesive, which comprises the following steps:
(1) Mixing and dispersing water, a monomer, an emulsifier and a free radical initiator to obtain an emulsion; wherein the monomers comprise butadiene and styrene;
(2) Carrying out free radical emulsion polymerization reaction, and controlling the number average molecular weight of a polymerization product to be within the range of 5-50 ten thousand;
(3) And (3) reducing the temperature of the reaction system in the step (2) to room temperature, adding a polymerization inhibitor, then increasing the reaction temperature to continue the free radical emulsion polymerization reaction, and stopping the reaction when the number average molecular weight of the polymerization product is controlled within the range of 50-1000 ten thousand, thus obtaining the binder.
According to the invention, the method further comprises the steps of:
(4) Removing residual monomers and adding a diluting component.
According to the invention, in step (1), the dispersion is carried out by high-speed shearing with an emulsifying machine, wherein the shearing speed is 300-1000rpm.
According to the present invention, in step (1), the butadiene and styrene are monomers from which the polymerization inhibitor has been removed.
According to the invention, in step (1), the mass ratio of butadiene to styrene is (30-70) to (70-30), for example 30.
According to the invention, in the step (1), the mass ratio of the monomer, the emulsifier and the free radical initiator is 100 (4-10) to (0.8-1).
According to the invention, in the step (1), the mass ratio of the monomer to the water is 100 (150-200).
According to the invention, in the step (1), the emulsifier is one or more of disproportionated potassium abietate, sodium dodecyl benzene sulfonate, OP-10, tetrahydrosodium abietate and the like.
According to the invention, in the step (1), the free radical initiator is selected from one of oxidation-reduction initiation systems consisting of an oxidizing agent, an oxidizing agent and a reducing agent; wherein the oxidant is one or more of cumene hydroperoxide, potassium sulfate peroxide, peroxyester and alkyl peroxide; the reducing agent is one or a mixture of more than two of primary fatty amine, secondary fatty amine, ethylenediamine, diethylenetriamine, triethylene tetramine and polyethylene polyamine.
In the present invention, when monomers including butadiene and styrene are mixed with water, an emulsifier and a radical initiator, the reactivity ratio of butadiene is high due to the difference in reactivity ratios between butadiene and styrene during polymerization, and thus polymerization must be performed first at the start of polymerization, and if the polymerization time is extended or the content of the initiator is increased, a part of styrene participates in the polymerization reaction after the polymerization of butadiene is completed, thereby preparing a first component including a butadiene homopolymer having a number average molecular weight in the range of 5 to 50 ten thousand and optionally a butadiene-styrene copolymer having a number average molecular weight in the range of 5 to 50 ten thousand.
According to the invention, in step (2), the temperature of the free-radical emulsion polymerization can be adjusted to the free-radical initiator system, illustratively from 30 to 60 ℃ for example 50 ℃.
According to the invention, in step (2), the free-radical emulsion polymerization is carried out for a period of time of from 1 to 4 hours, for example for 2 hours.
According to the present invention, in the step (2), the number average molecular weight of the polymerization product can be controlled by performing the radical emulsion polymerization reaction and controlling the temperature and time of the reaction, and the number average molecular weight of the polymerization product is controlled within the range of 5 to 50 ten thousand, preferably within the range of 10 to 30 ten thousand, with the standard normal distribution as the optimum uniformity requirement.
According to the invention, in step (2), the polymerization product is a butadiene homopolymer. Further, in the step (2), the polymerization product further comprises a butadiene-styrene copolymer.
According to the present invention, in the step (3), the polymerization inhibitor is at least one selected from a molecular-type polymerization inhibitor and a stable free radical-type polymerization inhibitor; wherein the molecular polymerization inhibitor is at least one selected from hydroquinone, p-benzoquinone, phenothiazine, beta-phenyl naphthylamine, p-tert-butyl catechol, methylene blue, cuprous chloride, ferric trichloride and the like; the stable free radical polymerization inhibitor is selected from 1, 1-diphenyl-2-picrylhydrazine DPPH, 2, 6-tetramethyl piperidine nitroxide radical TMP and the like.
According to the invention, in step (3), the polymerization inhibitor is added in an amount of 0.05 to 0.5wt% based on the total mass of the monomers.
According to the invention, in step (3), the temperature of the free-radical emulsion polymerization can be adjusted to the free-radical initiator system, illustratively from 30 to 60 ℃ for example 50 ℃. The time of the free radical emulsion polymerization reaction is 2-8h. Increasing the reaction temperature may facilitate further reaction of the remaining monomers.
According to the present invention, in the step (3), the number average molecular weight of the polymerization product can be controlled by performing the radical emulsion polymerization reaction while controlling the temperature and time of the reaction, and the number average molecular weight of the polymerization product is controlled within a range of 50 to 1000 ten thousand, and it is preferable that the number average molecular weight distribution peak of the polymerization product is controlled to about 500 ten thousand, and the standard normal distribution is used as the optimum uniformity requirement.
According to the present invention, in the step (3), the polymerization product is a butadiene-styrene copolymer.
The invention also provides the adhesive prepared by the method.
The invention also provides a negative plate which comprises the binder.
According to the present invention, the negative electrode sheet includes a negative electrode current collector, and a negative electrode active material layer provided on at least one side of the negative electrode current collector, the negative electrode active material layer including a negative electrode active material and the above-described binder.
According to the invention, the negative plate is prepared by coating slurry on one side or two sides of a negative current collector, and the slurry comprises a negative active material and the binder.
Illustratively, the anode active material layer includes 0.5 to 5wt% of the above-described binder, preferably 0.8 to 3wt% of the above-described binder, and further preferably 0.8 to 2.5wt% of the above-described binder, the base being the total mass of the anode active material layer.
According to the invention, the negative active material is selected from one or a combination of more of artificial graphite, natural graphite, hard carbon, mesocarbon microbeads and the like.
According to the invention, the negative current collector is a single-optical-surface copper foil, a double-optical-surface copper foil, a carbon-coated copper foil or a porous copper foil.
According to the invention, the negative plate further comprises a conductive agent, wherein the conductive agent is selected from at least one of carbon black, graphite, acetylene black, graphene and carbon nanotubes.
According to the invention, the negative plate further comprises a dispersant, wherein the dispersant is selected from sodium carboxymethyl cellulose or lithium carboxymethyl cellulose.
The invention also provides a lithium ion battery which comprises the negative plate.
According to the present invention, the binder accounts for 0.5 to 5wt% of the total mass of the anode active material layer, preferably the binder accounts for 0.8 to 3wt% of the total mass of the anode active material layer, and further preferably the binder accounts for 0.8 to 2.5wt% of the total mass of the anode active material layer.
According to the invention, the lithium ion battery further comprises a positive plate, a diaphragm and electrolyte.
The invention has the beneficial effects that:
the invention provides a bi-molecular weight distribution binder for a negative electrode, a preparation method and application thereof; it has the following advantages:
1. the method for preparing the bimolecular-weight-distribution binder for the negative electrode is innovative, the product yield is high, and the current situation that the current SBR modification method is single is further improved;
2. the binder for the negative electrode with the double molecular weight distribution is well dispersed, and small molecules can be quickly, uniformly and fully diffused in a negative active material, so that the membrane resistance of a pole piece is smaller;
3. the lithium ion battery prepared by the negative plate prepared by the bi-molecular weight distribution negative electrode binder has excellent cycle and less expansion in the cycle process, because the butadiene-styrene copolymer with high molecular weight is added, the mechanical strength of the binder can be improved, the intramolecular agglomeration condition is reduced, and the molecule is used as a macromolecular chain, so that the molecular entanglement number can be effectively ensured, and the mechanical strength in the molecular chain and among the molecular chains can be maintained;
4. the binder for the negative electrode with the bimolecular distribution is small in usage amount, and the modified binder is only added by 0.8wt% (the content of the binder in the conventional one-time SBR is 1.2 wt%), so that the energy density of the lithium ion battery can be greatly improved.
As described above, the binder for a negative electrode having a bimodal distribution can combine good adhesion, workability, curing strength, and rapid diffusion performance in the negative electrode active material.
Detailed Description
The preparation method of the present invention will be described in further detail with reference to specific examples. It is to be understood that the following examples are only illustrative and explanatory of the present invention and should not be construed as limiting the scope of the present invention. All the technologies realized based on the above-mentioned contents of the present invention are covered in the protection scope of the present invention.
The experimental methods used in the following examples are all conventional methods unless otherwise specified; reagents, materials and the like used in the following examples are commercially available unless otherwise specified.
Example 1
Mixing 65 parts by mass of butadiene (the same below), 35 parts by mass of styrene, 200 parts by mass of water, 5 parts by mass of disproportionated rosin potassium emulsifier and 0.8 part by mass of potassium persulfate, emulsifying for 20min under the condition of 500rpm of rotation speed by using an emulsifying machine to complete dispersion, then raising the temperature to 50 ℃ to perform a free radical emulsion polymerization reaction, reducing the temperature of a reaction system to room temperature after 2 hours of reaction, adding 0.25 part by mass of hydroquinone, uniformly stirring, raising the reaction temperature to 50 ℃ again to promote further reaction of the remaining monomers, and completely terminating the reaction after 4 hours, removing the remaining monomers to obtain a binder, wherein the first component in the binder comprises a butadiene homopolymer with a molecular weight distribution peak of 10 ten thousand, and the second component comprises a butadiene-styrene copolymer with a molecular weight distribution peak of 500 ten thousand (wherein the molar percentage content of butadiene is 57%, the molar percentage content of styrene is 43%), and the mass ratio of the first component to the second component is 1.
Dispersing 93wt% of positive active material NCM111, 2wt% of binder PVDF, 4wt% of conductive carbon black and 1wt% of carbon nano tube in N-methyl pyrrolidone, stirring to obtain uniform positive slurry, uniformly coating the positive slurry on two sides of the carbon-coated aluminum foil, baking at 100-130 ℃ for 4h, rolling, and compacting to 2.5-3.0 g/cm 3 Obtaining a positive plate;
dispersing 95wt% of negative active substance artificial graphite, 1.8wt% of the prepared binder, 2wt% of conductive carbon black and 1.2wt% of sodium carboxymethylcellulose in solvent water, stirring to obtain uniform negative slurry, uniformly coating the negative slurry on two sides of the carbon-coated copper foil, baking at 70-100 ℃ for 4h, rolling by using a roller press, wherein the compaction density is 1.25-1.35 g/cm 3 Obtaining a negative pole piece;
and packaging the positive plate, the negative plate and the diaphragm into a battery cell, then injecting electrolyte, and performing the procedures of formation, hot pressing, secondary sealing and the like to obtain the lithium ion battery.
Example 2
The other operations are the same as example 1, except that the binder preparation method is different:
mixing 50 parts of butadiene (by mass, the same below), 50 parts of styrene, 200 parts of water, 5 parts of disproportionated rosin potassium emulsifier and 0.8 part of potassium persulfate, emulsifying for 20min under the condition of 500rpm by using an emulsifying machine to complete dispersion, then raising the temperature to 50 ℃ to perform free radical emulsion polymerization reaction, reducing the temperature of a reaction system to room temperature after 3h of reaction, adding 0.25 part of hydroquinone, uniformly stirring, raising the reaction temperature to 50 ℃ again to promote further reaction of the residual monomers, completely terminating the reaction after 7h, and removing the residual monomers to obtain a binder, wherein the first component in the binder comprises a butadiene homopolymer with a molecular weight distribution peak of 10 ten thousand, and the second component comprises a butadiene-styrene copolymer with a molecular weight distribution peak of 900 ten thousand (wherein the molar percentage content of butadiene is 30% and the molar percentage content of styrene is 70%), and the mass ratio of the first component to the second component is 1.
Example 3
The other operations are the same as example 1, except that the binder preparation method is different:
mixing 70 parts by mass of butadiene (the same below), 30 parts by mass of styrene, 150 parts by mass of water, 5 parts by mass of disproportionated rosin potassium emulsifier and 0.9 part by mass of potassium persulfate, emulsifying for 20min at the rotation speed of 500rpm by using an emulsifying machine to complete dispersion, and then raising the temperature to 50 ℃ to perform free radical emulsion polymerization, wherein the monomer conversion rate is about 20% after 2 hours of reaction; reducing the temperature of a reaction system to room temperature, adding 0.25 part of hydroquinone, uniformly stirring, raising the reaction temperature to 50 ℃ again to promote further reaction of the residual monomers, completely terminating the reaction after 4 hours, and removing the residual monomers to obtain the binder, wherein a first component in the binder comprises a butadiene homopolymer with a molecular weight distribution peak of 25 ten thousand and a butadiene-styrene copolymer with a molecular weight distribution peak of 5 ten thousand, the mass ratio of the butadiene homopolymer to the butadiene-styrene copolymer is 10, a second component comprises a butadiene-styrene copolymer with a molecular weight distribution peak of 800 ten thousand (wherein the molar percentage of the butadiene is 45%, the molar percentage of the styrene is 55%), and the mass ratio of the first component to the second component is 1.
Example 4
The other operations are the same as example 1, except that the preparation method of the binder is different:
mixing 35 parts of butadiene (by mass, the same below), 65 parts of styrene, 200 parts of water, 5 parts of disproportionated rosin potassium emulsifier and 0.8 part of potassium persulfate, emulsifying for 20min under the condition of 500rpm by using an emulsifying machine to complete dispersion, then raising the temperature to 50 ℃ to perform a free radical emulsion polymerization reaction, reducing the temperature of a reaction system to room temperature after 4 hours of reaction, adding 0.15 part of hydroquinone, uniformly stirring, raising the reaction temperature to 50 ℃ again to promote further reaction of the remaining monomers, completely terminating the reaction after 4 hours, and removing the remaining monomers to obtain a binder, wherein a first component in the binder comprises a butadiene homopolymer with a molecular weight distribution peak of 35 ten thousand and a butadiene-styrene copolymer with a molecular weight distribution peak of 8 thousand, wherein the mass ratio of the butadiene homopolymer to the butadiene-styrene copolymer is 7.
Example 5
The other operations are the same as example 1, except that the addition amount of each component in the negative electrode sheet is as follows:
95.5wt% of artificial graphite, which is a negative electrode active material, 0.8wt% of the binder prepared above, 2.5wt% of conductive carbon black, and 1.2wt% of sodium carboxymethylcellulose were dispersed in solvent water.
Comparative example 1
The other operations are the same as example 1, except that the binder preparation method is different:
mixing 65 parts (by mass, the same below) of butadiene, 35 parts of styrene, 200 parts of water, 5 parts of disproportionated rosin potassium emulsifier and 0.8 part of potassium persulfate, emulsifying for 20min by using an emulsifying machine under the condition of 500rpm of rotation speed to complete dispersion, then raising the temperature to 50 ℃ to perform free radical emulsion polymerization reaction, completely terminating the reaction after 5h of reaction, and removing residual monomers to obtain a binder, wherein the binder is a butadiene-styrene copolymer, the molar ratio of butadiene repeating units to styrene repeating units is 26, and the molecular weight distribution peak is 700 ten thousand.
Comparative example 2
The other operations are the same as example 1, except that the binder preparation method is different:
mixing 50 parts of butadiene (mass part, the same below), 50 parts of styrene, 200 parts of water, 5 parts of disproportionated rosin potassium emulsifier and 0.8 part of potassium persulfate, emulsifying for 20min by using an emulsifying machine under the condition of 500rpm to complete dispersion, then raising the temperature to 50 ℃ to perform free radical emulsion polymerization reaction, completely terminating the reaction after 5h of reaction, and removing residual monomers to obtain the binder, wherein the binder is a butadiene-styrene copolymer, the molar ratio of butadiene repeating units to styrene repeating units is 2, and the molecular weight distribution peak is 900 ten thousand.
Comparative example 3
The other operations are the same as example 1, except that the binder preparation method is different:
mixing 70 parts of butadiene (by mass, the same below), 30 parts of styrene, 150 parts of water, 5 parts of disproportionated rosin potassium emulsifier and 0.8 part of potassium persulfate, emulsifying for 20min by using an emulsifying machine at the rotation speed of 500rpm to complete dispersion, then raising the temperature to 50 ℃ to perform free radical emulsion polymerization reaction, completely terminating the reaction after 5h of reaction, and removing residual monomers to obtain the adhesive, wherein the adhesive is a butadiene-styrene copolymer, the molar ratio of a butadiene repeating unit to a styrene repeating unit is 14, and the molecular weight distribution peak is 500 ten thousand.
Comparative example 4
The other operations are the same as example 1, except that the binder preparation method is different:
mixing 35 parts of butadiene (mass part, the same below), 65 parts of styrene, 200 parts of water, 5 parts of disproportionated rosin potassium emulsifier and 0.8 part of potassium persulfate, emulsifying for 20min by using an emulsifying machine under the condition of 500rpm to complete dispersion, then raising the temperature to 50 ℃ to perform free radical emulsion polymerization reaction, completely terminating the reaction after 5h of reaction, and removing residual monomers to obtain a binder, wherein the binder is a butadiene-styrene copolymer, the molar ratio of butadiene repeating units to styrene repeating units is 14, and the molecular weight distribution peak is 1000 ten thousand.
Comparative example 5
The other operations are the same as the comparative example 1, and only the difference is that the addition amount of each component in the negative plate is as follows:
95wt% of artificial graphite, which is a negative active material, 1.2wt% of the binder prepared in comparative example 1, 2wt% of conductive carbon black, and 1.8wt% of sodium carboxymethylcellulose were dispersed in solvent water.
Comparative example 6
The other operations are the same as example 1, except that the molecular weight distribution of the binder is different:
mixing 50 parts by mass of butadiene (the same below), 50 parts by mass of styrene, 200 parts by mass of water, 5 parts by mass of disproportionated rosin potassium emulsifier and 0.8 part by mass of potassium persulfate, emulsifying for 20min under the condition of 500rpm of rotation speed by using an emulsifying machine to complete dispersion, then raising the temperature to 50 ℃ to perform free radical emulsion polymerization reaction, reducing the temperature of a reaction system to room temperature after 0.3h of reaction, adding 0.15 part of hydroquinone, uniformly stirring, raising the reaction temperature to 50 ℃ again to promote further reaction of residual monomers, and completely terminating the reaction after 2h, removing the residual monomers to obtain a binder, wherein the first component in the binder comprises a butadiene homopolymer with a molecular weight distribution peak of 1 ten thousand, and the second component comprises a butadiene-styrene copolymer with a molecular weight distribution peak of 800 ten thousand (wherein the molar percentage content of butadiene is 35%, the molar percentage content of styrene is 65%), and the mass ratio of the first component to the second component is 1.
Comparative example 7
The other operations are the same as example 1, except that the molecular weight distribution of the binder is different:
mixing 50 parts by mass of butadiene (the same below), 50 parts by mass of styrene, 200 parts by mass of water, 5 parts by mass of disproportionated rosin potassium emulsifier and 0.8 part by mass of potassium persulfate, emulsifying for 20min under the condition of 500rpm of rotation speed by using an emulsifying machine to complete dispersion, then raising the temperature to 50 ℃ to perform free radical emulsion polymerization reaction, reducing the temperature of a reaction system to room temperature after 1h of reaction, adding 0.05 part of hydroquinone, uniformly stirring, raising the reaction temperature to 50 ℃ again to promote further reaction of residual monomers, completely terminating the reaction after 7h, and removing the residual monomers to obtain a binder, wherein the first component in the binder comprises a butadiene homopolymer with a molecular weight distribution peak of 6 ten thousand, and the second component comprises a butadiene-styrene copolymer with a molecular weight distribution peak of 2000 ten thousand (wherein the molar percentage content of butadiene is 60% and the molar percentage content of styrene is 40%), and the mass ratio of the first component to the second component is 1.
Comparative example 8
The other operations were the same as example 1 except that the binder was different, and the binder of this comparative example was a commercially available SBR-based binder having a composition of a butadiene-styrene copolymer and in which the molar percentage of butadiene was 52%, the molar percentage of styrene was 48%, and the molecular weight distribution peak was 400 ten thousand, which was purchased from japanese tumbler (ZEON).
The lithium ion batteries of the above examples and comparative examples were subjected to performance tests, the test procedures being as follows:
(1) Testing of resistance properties of membranes
The testing method comprises the steps of adopting a four-probe method testing principle to test the diaphragm resistance by using a two-probe resistance tester, cutting a pole piece into a square size of 4cm multiplied by 8cm, then placing the pole piece below the two probes, connecting the two probes with a resistance meter through two poles, rotating a handle of a testing device, extruding the pole piece by stable pressure on the probes, controlling the pressure through a pressure gauge, reading resistance data of the resistance meter after the pressure reaches a certain pressure, wherein the data is a relative value of the resistance of the pole piece.
(2) Capacity retention at 45 ℃ 3C/3C
Thickness D of full-electricity cell before test 0 Placing the battery in a (45 + -3) deg.C environment, standing for 3 hr, after the battery temperature reaches 45 deg.C, charging the battery to 4.3V at constant voltage according to constant current 3C (3C: 3 times of rated capacity as current, the same below), charging to cut-off current 0.05C at constant voltage, standing for 5min, discharging to 3V at 3C, and recording initial capacity Q 0 When the number of cycles reaches the required number (1000 cycles) and the capacity fading rate is less than 80%, the previous discharge capacity is taken as the capacity Q of the battery 2 Calculating capacity retention rate (%), taking out the battery full, standing for 3 hours at normal temperature, and testing full thickness D 2 The thickness change rate (%) was calculated, and the results are shown in Table 1. The calculation formula used therein is as follows:
retention ratio of cycle capacity = Q 2 /Q 0 X 100%. Thickness expansion rate = ((D) 2 -D 0 )/D 0 )×100％。
(3) Analysis of powder falling of battery under circulation
The batteries in any SOC state (SOC: state of charge, 100% SOC: full charge, 50% SOC: half charge) were disassembled at 45 ℃ cycle in the above examples and comparative examples, and the presence or absence of the dusting phenomenon at the interface of the negative electrode sheet and the separator facing the negative electrode was observed.
Table 1 results of performance test of lithium ion batteries of examples and comparative examples
As can be seen from the above examples and comparative examples, the sheet resistances of the binders having bimolecular weight distributions in the present invention were all lower than those of the comparative examples under the same conditions, indicating that the low molecular weight distribution polymer of the first component diffused more uniformly between the conductive agent and the negative active material, ensuring good contact at the interface between the conductive agent and the negative active material, reducing the internal resistance, and thus the sheet resistance of the resulting electrode sheet was smaller.
Meanwhile, from the view of cycle data, the cycle number and the expansion rate of examples 1 to 5 are lower than those of comparative examples 1 to 7, because the polymer with double molecular weight distribution in the examples exerts a synergistic effect, the low molecular weight polymer binder mainly containing butadiene diffuses into the active material, so that the internal bonding uniformity is ensured, the high molecular weight polymer binder mainly containing styrene improves the mechanical strength between the active materials through internal crosslinking, and the higher cohesive force ensures that the electrolyte cannot swell the binder, so that the stability of the interface is ensured. Therefore, the number of cycles and the expansion ratio in examples 1 to 5 were superior to those in comparative examples 1 to 7. Specifically, it is seen from example 5 and comparative example 5 that the binder of the present invention still circulates better than the conventional SBR binder with a small amount of binder, and the energy density is improved by 7 points. From the above comparative examples 6 and 7, it can be seen that the cycle number is very small when the peak of the molecular weight distribution of the first component is 1 ten thousand and the peak of the molecular weight distribution of the second component is 2000 ten thousand, and the difference between the molecular weights of the two components is too large, so that the viscosity is not well controlled when preparing the slurry, the slurry dispersibility is poor, the coating of the pole piece is not uniform, the resistance of the polarized large membrane of the pole piece is increased, and the cycle number is small, and the expansion is large.
The embodiments of the present invention have been described above. However, the present invention is not limited to the above embodiment. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.
Claims (8)
1. A binder comprising a first component and a second component, wherein the first component is a butadiene-based polymer having a number average molecular weight in the range of 5 to 50 ten thousand, and the second component is a butadiene-styrene copolymer having a number average molecular weight in the range of 50 to 1000 ten thousand;
the mass ratio of the first component to the second component is 1 (2-11);
the butadiene-based polymer includes a butadiene homopolymer; or, the butadiene-based polymer includes a butadiene homopolymer and a butadiene-styrene copolymer;
in the first component, the mass ratio of a butadiene-styrene copolymer with the number average molecular weight ranging from 5 ten thousand to 50 ten thousand to a butadiene homopolymer with the number average molecular weight ranging from 5 ten thousand to 50 ten thousand is (0-4) to (10-6);
the butadiene-styrene copolymer with the number average molecular weight ranging from 5 ten thousand to 50 ten thousand has the butadiene repeating unit content of 55 to 80 percent in mol percentage; the mol percentage content of the styrene repeating unit is 20-45%;
the mol percentage content of the styrene repeating unit in the butadiene-styrene copolymer with the number average molecular weight of 50-1000 ten thousand is 10-70%; the molar percentage content of the butadiene repeating unit is 90-30%.
2. A method of preparing the binder of claim 1, the method comprising the steps of:
(1) Mixing and dispersing water, a monomer, an emulsifier and a free radical initiator to obtain an emulsion; wherein the monomers comprise butadiene and styrene;
(2) Carrying out free radical emulsion polymerization reaction, and controlling the number average molecular weight of a polymerization product to be within the range of 5-50 ten thousand;
(3) And (3) reducing the temperature of the reaction system in the step (2) to room temperature, adding a polymerization inhibitor, then increasing the reaction temperature to continue the free radical emulsion polymerization reaction, and stopping the reaction when the number average molecular weight of the polymerization product is controlled within the range of 50-1000 ten thousand, thus obtaining the binder.
3. The method according to claim 2, wherein in the step (1), the mass ratio of the butadiene to the styrene is (50-70): (50-30); and/or the presence of a gas in the gas,
the mass ratio of the monomer, the emulsifier and the free radical initiator is 100 (4-10) to 0.8-1; and/or the presence of a gas in the gas,
the mass ratio of the monomer to the water is 100 (150-200).
4. The process of claim 2, wherein in step (2), the polymerization product is a butadiene homopolymer; and/or the presence of a gas in the gas,
the polymerization product also comprises butadiene-styrene copolymer.
5. The process according to claim 2, wherein in the step (3), the polymerization inhibitor is added in an amount of 0.05 to 0.5wt% based on the total mass of the monomers.
6. The method of claim 2, wherein the polymerization product is a butadiene-styrene copolymer.
7. A negative electrode sheet comprising the binder of claim 1;
the negative electrode sheet comprises a negative electrode current collector and an active material layer arranged on at least one side of the negative electrode current collector, wherein the active material layer comprises a negative electrode active material and the binder.
8. A lithium ion battery comprising the negative electrode sheet of claim 7, wherein the binder accounts for 0.5 to 5wt% of the total mass of the negative electrode active material layer.
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