Modified binder, and preparation method therefor and use thereof

Abstract

Provided are a modified binder, and a preparation method therefor and the use thereof. The modified binder is synthesized from polyethylene glycol, a polymerization inhibitor, an organic acid and a polymerization monomer, and has a cross-linked network structure. The modified binder has good stability in an electrolyte solution of a lithium battery, and can significantly improve the electrochemical performance and cycling performance of the lithium battery.

Description

A modified binder and its preparation method and application

[0001] This application claims priority to the Chinese patent application filed with the China Patent Office on August 22, 2023, with application number 202311056093.5. The entire contents of the above application are incorporated by reference into this application. Technical Field

[0002] The present application relates to the technical field of adhesives, and in particular to a modified adhesive and a preparation method and application thereof. Background Art

[0003] Lithium iron phosphate batteries, with their excellent cycle performance and safety, are widely used in various electronic products and electric vehicles. However, due to the olivine crystal structure of lithium iron phosphate, its actual capacity and rate performance differ significantly from theoretical values, resulting in poor electrical and lithium conductivity. In recent years, the rapid development of the electric vehicle and energy storage industries has placed higher demands on the comprehensive performance of lithium iron phosphate batteries.

[0004] As an important component of lithium iron phosphate batteries, although the amount of binder used is very small (about 1%-10% of the electrode mass), it can play a role in stabilizing the electrodes in the battery, enhancing the electronic contact between the electrode active material and the conductive agent, as well as between the electrode active material and the current collector, thereby overcoming the problems of poor electrical conductivity and poor lithium conductivity of lithium iron phosphate materials during application, and having a positive impact on the overall performance of lithium batteries. SUMMARY OF THE INVENTION

[0005] The primary binder currently used in lithium iron phosphate batteries is polyvinylidene fluoride (PVDF), an organic solvent-based binder. However, PVDF binders are difficult to process, easily swell with the electrolyte during use, and easily form lithium carbide with metallic lithium, resulting in poor electronic and ionic conductivity of the lithium iron phosphate material, thereby affecting the electrochemical performance and service life of the lithium iron phosphate battery. Therefore, there is an urgent need to provide a binder with excellent electrical conductivity and high stability in the electrolyte.

[0006] In order to improve the conductivity of the binder and its stability in the electrolyte, and thus improve the cycle performance of the lithium battery, the present application provides a modified binder and its preparation method and application.

[0007] In a first aspect, the present application provides a modified binder, which adopts the following technical solution:

[0008] A modified binder, wherein the structural formula of the modified binder is shown in Formula I:

[0009]

[0010] (I);

[0011] Wherein, the value of n1 is an integer between 3000 and 10000, the value of n2 is an integer between 3000 and 10000, the value of n3 is an integer between 3000 and 10000, and the value of n4 is an integer between 3000 and 10000;

[0012] The structural formula of the R group includes 、 、 、 Any one of .

[0013] In a second aspect, the present application provides a method for preparing a modified binder, which adopts the following technical solution:

[0014] A method for preparing a modified binder comprises the following steps:

[0015] Step 1: mixing polyethylene glycol with an organic solvent and heating to obtain a mixed solution;

[0016] Step 2: heating the mixed solution, adding a polymerization inhibitor thereto, stirring, and continuously adding an organic acid and an inorganic acid to obtain a pretreatment solution;

[0017] Step 3, heating the pretreatment solution and cooling it, adding a lithium source thereto, stopping heating after the reaction, collecting the material, and obtaining a lithium-containing crosslinking agent;

[0018] Step 4: Mix the lithium-containing crosslinking agent, dispersant and deionized water, heat them in an inert atmosphere, and continue to add initiator and polymerization monomer to obtain the modified binder.

[0019] Preferably, in the step one, the heating temperature is 50-65°C; in the step two, the mixed solution is heated to 70-75°C and stirred for 15-30 minutes; in the step three, the pretreatment solution is heated to 130-150°C, kept warm for 5-8 hours, and then cooled to 60-70°C, and the holding time after adding the lithium source is 0.8-1.2 hours; in the step four, the heating temperature is 50-60°C, the pressure is maintained at 3.4-4.0 MPa, and after the initiator and the polymerization monomer are added, the reaction is carried out until the pressure drops to 2.5-2.7 MPa to obtain the modified binder.

[0020] Preferably, the raw materials used in the preparation process include the following raw materials in parts by weight: 2000-5000 parts of polyethylene glycol, 360-1000 parts of organic solvent, 5-15 parts of inhibitor, 350-400 parts of organic acid, 50-100 parts of inorganic acid, 20-60 parts of lithium source, 50-115 parts of dispersant, 255-495 parts of initiator, and 51700-82200 parts of polymerization monomer.

[0021] By using polyethylene glycol, an inhibitor, an organic acid and a polymer monomer, a modified binder with a cross-linked network structure can be synthesized, thereby accelerating the deintercalation and electron conduction of lithium ions on the positive electrode sheet and improving the electrochemical performance of the lithium iron phosphate battery; in addition, the modified binder has PEO side chains introduced from the polyethylene glycol raw material, which can reduce the pairing of lithium ions with hexafluorophosphate ions in the lithium battery electrolyte, thereby increasing the number of free lithium ions in the lithium iron phosphate battery; further, lithium ions are introduced into the modified binder by adding a lithium source, so that the modified binder can have a lithium replenishing effect on the positive electrode sheet, which helps to significantly improve the cycle performance of the lithium-ion battery.

[0022] Preferably, the organic solvent is selected from any one of N,N-dimethylformamide, toluene and cyclohexane.

[0023] Preferably, the polymerizable monomer includes any one of vinylidene fluoride monomer, tetrafluoroethylene monomer, hexafluoroethylene monomer, and 1,2-difluoroethylene monomer.

[0024] Preferably, the molecular weight of the polyethylene glycol is 2000-10000.

[0025] By controlling the molecular weight of polyethylene glycol, a cross-linked network structure channel suitable for the passage of lithium ions can be generated, avoiding poor lithium ion transmission performance or large accumulation of lithium ions in the negative electrode material due to the network structure channel being too narrow or too wide, which is conducive to obtaining a modified binder with excellent conductive properties, thereby improving the electrochemical performance and cycle performance of lithium batteries.

[0026] Preferably, the polymerization inhibitor includes any one of p-benzoquinone and p-hydroxyanisole.

[0027] Preferably, the organic acid includes at least one of itaconic acid, maleic acid and acrylic acid.

[0028] Preferably, the inorganic acid includes any one of concentrated sulfuric acid and concentrated nitric acid.

[0029] Preferably, the concentrated sulfuric acid refers to a sulfuric acid aqueous solution with a mass fraction greater than or equal to 70%; the concentrated nitric acid refers to a nitric acid aqueous solution with a mass fraction greater than or equal to 68%.

[0030] By using concentrated sulfuric acid, the hydroxyl groups in the polyethylene glycol can be protonated, turning it into a good ionic nucleophilic reagent, making the network cross-linked structure of the modified binder finally prepared more uniform and dense and improving the electrolyte resistance of the modified binder.

[0031] Preferably, the lithium source comprises lithium hydroxide.

[0032] Preferably, the lithium source is an aqueous lithium hydroxide solution.

[0033] Preferably, the concentration of the lithium hydroxide aqueous solution is 8wt%-12wt%.

[0034] Preferably, the dispersant includes at least one of hydroxypropyl methyl ether cellulose, polyvinyl alcohol, and cellulose acetate butyrate; and the initiator includes at least one of diisopropyl hydroperoxide, diisopropyl peroxydicarbonate, and tert-butyl peroxypivalate.

[0035] In a third aspect, the present application provides a lithium-ion battery, which adopts the following technical solution:

[0036] A lithium ion battery comprises a positive electrode sheet, a negative electrode sheet, an electrolyte, a separator and the modified binder as described above. Beneficial effects

[0037] The beneficial effects of this application are:

[0038] The modified binder synthesized in the present application has a cross-linked network structure, which can provide a continuous channel for the deintercalation and extraction of lithium ions and the conduction of electrons on the positive electrode sheet, thereby improving the conductivity of the lithium iron phosphate battery; and the polyethylene oxide (PEO) side chains generated on the cross-linked network structure can reduce the pairing of lithium ions with hexafluorophosphate ions in the lithium battery electrolyte, thereby increasing the number of free lithium ions in the lithium iron phosphate battery; further, due to the introduction of lithium ions into the modified binder, the modified binder can play a certain lithium replenishing role on the positive electrode sheet during the charge and discharge process of the lithium iron phosphate battery, and a large amount of active lithium can pass through the above-mentioned cross-linked network structure, thereby increasing the lithium ion migration number, significantly improving the battery's electrical conductivity, and the increase in the number of active lithium can also significantly increase the capacity of the lithium battery, thereby improving the electrochemical performance and cycle life of the lithium iron phosphate battery; finally, the modified binder with the above-mentioned cross-linked network structure has excellent stability in the electrolyte, which can prevent the CF bond or CH bond in the modified binder from being decomposed to form a double bond. The water, amine substances, etc. in the electrolyte have a strong destructive effect on the double bond, which will have a negative impact on the performance of the modified binder; at the same time, it can avoid the electrode being swollen by the electrolyte during use to produce chemical gel, which leads to the poor adhesion of the lithium iron phosphate active material on the current collector, thereby making the modified binder prepared in this application have excellent electrolyte resistance. In summary, the modified binder prepared in this application can significantly improve the electrochemical performance and cycle performance of the lithium iron phosphate battery after being used in the lithium iron phosphate battery.

[0039] The preparation method of the modified binder of the present application adopts the above steps to prepare the modified binder, and the final product has a smaller adsorption amount of dispersant, higher purity and lower cost; and the lithium-containing cross-linking agent prepared by the above steps has excellent polymerization performance, which can significantly improve the stability of the molecular structure of the modified binder finally prepared, thereby significantly improving the electrolyte resistance and conductivity of the binder. Modes for Carrying Out the Invention

[0040] For better understanding and implementation, the technical solution of the present application will be clearly and completely described below in conjunction with embodiments.

[0041] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art to which this application pertains. The terms used herein in the specification of this application are for the purpose of describing specific embodiments only and are not intended to limit this application.

[0042] Unless otherwise indicated, all numbers expressing quantities of ingredients, reaction conditions, and so forth used in the specification and claims are to be understood as modified by the term “about.” Accordingly, unless indicated to the contrary, the numerical parameters set forth herein are approximations that can vary depending upon the desired properties sought to be obtained.

[0043] As used herein, "and / or" means one or all of the mentioned elements.

[0044] As used herein, “including” and “comprising” encompasses the case where only the stated elements are present and also the case where there are other elements other than the stated elements.

[0045] All percentages in this application are by weight unless otherwise stated.

[0046] Unless otherwise indicated, as used in this specification, "a," "an," "an," and "the" are intended to include "at least one" or "one or more." For example, "a component" refers to one or more components, and thus more than one component is contemplated and may be employed or used in the practice of the described embodiment.

[0047] Example

[0048] Example 1

[0049] 1. Preparation of modified binder

[0050] A modified binder comprising the following raw materials in parts by weight:

[0051] 3500 parts of polyethylene glycol with a molecular weight of 5000-7000 Da, 700 parts of N,N-dimethylformamide, 10 parts of benzoquinone, 370 parts of itaconic acid, 75 parts of an 80% sulfuric acid aqueous solution, 40 parts of a 10wt% lithium hydroxide aqueous solution, 85 parts of hydroxypropyl methyl ether cellulose, 370 parts of diisopropylbenzene hydroperoxide, and 67,000 parts of vinylidene fluoride monomer;

[0052] A modified binder is prepared by the following steps:

[0053] Step 1: Mix polyethylene glycol and N,N-dimethylformamide in a light-proof environment, heat to 60° C. and stir until the polyethylene glycol is completely dissolved to obtain a mixed solution;

[0054] Step 2: heating the mixed solution to 72° C., adding benzoquinone thereto, stirring for 20 minutes, and then adding itaconic acid and the above-mentioned aqueous sulfuric acid solution, stirring for 7 minutes to obtain a pretreatment solution;

[0055] Step 3: Heat the pretreatment solution to 140° C. and keep it warm for 6 hours, then cool it to 65° C., add the lithium hydroxide aqueous solution, keep it warm for 1 hour, stop heating, collect the material, and obtain the lithium-containing crosslinking agent;

[0056] Step 4: Mix the lithium-containing cross-linking agent and hydroxypropyl methyl ether cellulose obtained in step 3 with deionized water, heat at 55° C. in a nitrogen atmosphere and maintain the pressure at 3.6 MPa, continue to add diisopropylbenzene hydroperoxide and vinylidene fluoride monomer until the pressure drops to 2.6 MPa, and obtain a modified binder with a number average molecular weight of 2130000.

[0057] 2. Preparation of positive electrode

[0058] A positive electrode slurry was prepared as follows: lithium iron phosphate positive electrode active material, acetylene black as a conductive agent, and the modified binder described above were added to a vacuum mixer in a mass ratio of 97.9:0.9:1.2 and mixed. NMP as a solvent was then added to the mixed slurry, and the mixed slurry was stirred in a vacuum mixer until it was uniform, thereby obtaining the positive electrode slurry of this embodiment. The positive electrode slurry was evenly coated on both surfaces of the positive electrode current collector aluminum foil, dried at room temperature, and then transferred to an oven for further drying. After drying in the oven, a semi-finished positive electrode sheet was obtained, which was then cold pressed and cut into pieces to obtain the positive electrode sheets to be assembled.

[0059] 3. Preparation of negative electrode sheet

[0060] The negative electrode slurry was prepared as follows: artificial graphite, conductive agent acetylene black, thickener CMC, and binder SBR were added to a vacuum mixer at a mass ratio of 96.4:1:1.2:1.4 and mixed. Deionized water was then added to the mixture, and the mixture was stirred in a vacuum mixer until it became uniform, thereby obtaining the negative electrode slurry of this embodiment.

[0061] The above-mentioned negative electrode slurry is evenly coated on both surfaces of the negative electrode current collector copper foil, dried at room temperature and then transferred to an oven for further drying. After drying in the oven, a negative electrode sheet semi-finished product is obtained, and then the negative electrode sheet semi-finished product is cold pressed and cut to obtain the negative electrode sheet to be assembled.

[0062] 4. Assembly of lithium batteries

[0063] A commercially available polyethylene film is used as the lithium-ion battery separator, and a commercially available electrolyte suitable for batteries with a 4.2V (charge limit) voltage system is used. The positive and negative electrode sheets, along with the separator, are wound together to form a bare cell. The bare cell undergoes packaging, liquid injection, static aging, formation, and capacity grading to produce the finished battery.

[0064] Example 2

[0065] 1. Preparation of modified binder

[0066] A modified binder comprising the following raw materials in parts by weight:

[0067] 2000 parts of polyethylene glycol with a molecular weight of 2000-3000 Da, 1000 parts of toluene, 5 parts of p-hydroxyanisole, 400 parts of maleic acid, 100 parts of a 70% by weight aqueous sulfuric acid solution, 20 parts of a 12% by weight aqueous lithium hydroxide solution, 115 parts of polyvinyl alcohol, 255 parts of diisopropyl peroxydicarbonate, and 82,200 parts of tetrafluoroethylene monomer;

[0068] A modified binder is prepared by the following steps:

[0069] Step 1: Mix polyethylene glycol and toluene in a light-proof environment, heat to 50° C., and stir until the polyethylene glycol is completely dissolved to obtain a mixed solution;

[0070] Step 2: heating the mixed solution to 75° C., adding p-hydroxyanisole thereto, stirring for 15 minutes, and then adding maleic acid and the above-mentioned sulfuric acid aqueous solution, stirring for 10 minutes to obtain a pretreatment solution;

[0071] Step 3: Heat the pretreatment solution to 130° C. and keep it warm for 8 hours, then cool it to 60° C., add the lithium hydroxide aqueous solution, keep it warm for 1.2 hours, then stop heating and collect the material to obtain a lithium-containing crosslinking agent;

[0072] Step 4: Mix the lithium-containing cross-linking agent and polyvinyl alcohol in step 3 with deionized water, heat at 50°C in a nitrogen atmosphere and maintain the pressure at 4.0 MPa, continue to add diisopropyl peroxydicarbonate and tetrafluoroethylene monomer until the pressure drops to 2.5 MPa, and obtain a modified binder with a number average molecular weight of 2300000.

[0073] 2. Preparation of positive electrode

[0074] A positive electrode slurry was prepared as follows: lithium iron phosphate positive electrode active material, acetylene black as a conductive agent, and the modified binder described above were added to a vacuum mixer in a mass ratio of 97.9:0.9:1.2 and mixed. NMP as a solvent was then added to the mixed slurry, and the mixed slurry was stirred in a vacuum mixer until it was uniform, thereby obtaining the positive electrode slurry of this embodiment. The positive electrode slurry was evenly coated on both surfaces of the positive electrode current collector aluminum foil, dried at room temperature, and then transferred to an oven for further drying. After drying in the oven, a semi-finished positive electrode sheet was obtained, which was then cold pressed and cut into pieces to obtain the positive electrode sheets to be assembled.

[0075] 3. Preparation of negative electrode sheet

[0076] The negative electrode slurry was prepared as follows: artificial graphite, conductive agent acetylene black, thickener CMC, and binder SBR were added to a vacuum mixer at a mass ratio of 96.4:1:1.2:1.4 and mixed. Deionized water was then added to the mixture, and the mixture was stirred in a vacuum mixer until it became uniform, thereby obtaining the negative electrode slurry of this embodiment.

[0077] The above-mentioned negative electrode slurry is evenly coated on both surfaces of the negative electrode current collector copper foil, dried at room temperature and then transferred to an oven for further drying. After drying in the oven, a negative electrode sheet semi-finished product is obtained, and then the negative electrode sheet semi-finished product is cold pressed and cut to obtain the negative electrode sheet to be assembled.

[0078] 4. Assembly of lithium batteries

[0079] A commercially available polyethylene film is used as the lithium-ion battery separator, and a commercially available electrolyte suitable for batteries with a 4.2V (charge limit) voltage system is used. The positive and negative electrode sheets, along with the separator, are wound together to form a bare cell. The bare cell undergoes packaging, liquid injection, static aging, formation, and capacity grading to produce the finished battery.

[0080] Example 3

[0081] 1. Preparation of modified binder

[0082] A modified binder comprising the following raw materials in parts by weight:

[0083] 5000 parts of polyethylene glycol with a molecular weight of 9000-10000 Da, 360 parts of cyclohexane, 15 parts of benzoquinone, 350 parts of acrylic acid, 50 parts of a 90% by weight aqueous sulfuric acid solution, 60 parts of an 8% by weight aqueous lithium hydroxide solution, 50 parts of cellulose acetate butyrate, 495 parts of tert-butyl peroxypivalate, and 51700 parts of vinylidene fluoride monomer;

[0084] A modified binder is prepared by the following steps:

[0085] Step 1: Mix polyethylene glycol and cyclohexane in a light-proof environment, heat to 65° C. and stir until the polyethylene glycol is completely dissolved to obtain a mixed solution;

[0086] Step 2: heating the mixed solution to 70° C., adding benzoquinone thereto, stirring for 30 minutes, and then adding acrylic acid and the above-mentioned sulfuric acid aqueous solution, stirring for 5 minutes to obtain a pretreatment solution;

[0087] Step 3: Heat the pretreatment solution to 150° C. and keep it warm for 5 hours, then cool it to 70° C., add the lithium hydroxide aqueous solution, keep it warm for 0.8 hours, then stop heating and collect the material to obtain a lithium-containing crosslinking agent;

[0088] Step 4: Mix the lithium-containing cross-linking agent and cellulose acetate butyrate in step 3 with deionized water, heat at 60°C in a nitrogen atmosphere and maintain the pressure at 3.4 MPa, continue to add tert-butyl peroxypivalate and 1,2-difluoroethylene monomer until the pressure drops to 2.7 MPa, and obtain a modified binder with a number average molecular weight of 2,000,000.

[0089] 2. Preparation of positive electrode

[0090] A positive electrode slurry was prepared as follows: lithium iron phosphate positive electrode active material, acetylene black as a conductive agent, and the modified binder described above were added to a vacuum mixer in a mass ratio of 97.9:0.9:1.2 and mixed. NMP as a solvent was then added to the mixed slurry, and the mixed slurry was stirred in a vacuum mixer until it was uniform, thereby obtaining the positive electrode slurry of this embodiment. The positive electrode slurry was evenly coated on both surfaces of the positive electrode current collector aluminum foil, dried at room temperature, and then transferred to an oven for further drying. After drying in the oven, a semi-finished positive electrode sheet was obtained, which was then cold pressed and cut into pieces to obtain the positive electrode sheets to be assembled.

[0091] 3. Preparation of negative electrode sheet

[0092] The negative electrode slurry was prepared as follows: artificial graphite, conductive agent acetylene black, thickener CMC, and binder SBR were added to a vacuum mixer at a mass ratio of 96.4:1:1.2:1.4 and mixed. Deionized water was then added to the mixture, and the mixture was stirred in a vacuum mixer until it became uniform, thereby obtaining the negative electrode slurry of this embodiment.

[0093] The above-mentioned negative electrode slurry is evenly coated on both surfaces of the negative electrode current collector copper foil, dried at room temperature and then transferred to an oven for further drying. After drying in the oven, a negative electrode sheet semi-finished product is obtained, and then the negative electrode sheet semi-finished product is cold pressed and cut to obtain the negative electrode sheet to be assembled.

[0094] 4. Assembly of lithium batteries

[0095] A commercially available polyethylene film is used as the lithium-ion battery separator, and a commercially available electrolyte suitable for batteries with a 4.2V (charge limit) voltage system is used. The positive and negative electrode sheets, along with the separator, are wound together to form a bare cell. The bare cell undergoes packaging, liquid injection, static aging, formation, and capacity grading to produce the finished battery.

[0096] Example 4

[0097] 1. Preparation of modified binder

[0098] A modified binder comprising the following raw materials in parts by weight:

[0099] 4000 parts of polyethylene glycol with a molecular weight of 3000-5000 Da, 800 parts of cyclohexane, 7 parts of benzoquinone, 390 parts of acrylic acid, 60 parts of an 80% by weight aqueous sulfuric acid solution, 60 parts of a 9% by weight aqueous lithium hydroxide solution, 90 parts of cellulose acetate butyrate, 300 parts of tert-butyl peroxypivalate, and 70,000 parts of hexafluoroethylene monomer;

[0100] A modified binder is prepared by the following steps:

[0101] Step 1: Mix polyethylene glycol and cyclohexane in a light-proof environment, heat to 55° C. and stir until the polyethylene glycol is completely dissolved to obtain a mixed solution;

[0102] Step 2: heating the mixed solution to 70° C., adding benzoquinone thereto, stirring for 25 minutes, and then adding acrylic acid and the above-mentioned aqueous sulfuric acid solution, stirring for 7 minutes to obtain a pretreatment solution;

[0103] Step 3: Heat the pretreatment solution to 135° C. and keep it warm for 7 hours, then cool it to 60° C., add the lithium hydroxide aqueous solution, keep it warm for 0.8 hours, then stop heating and collect the material to obtain a lithium-containing crosslinking agent;

[0104] Step 4: Mix the lithium-containing cross-linking agent and cellulose acetate butyrate in step 3 with deionized water, heat at 65°C in a nitrogen atmosphere and maintain the pressure at 3.8 MPa, continue to add tert-butyl peroxypivalate and hexafluoroethylene monomer until the pressure drops to 2.6 MPa, and obtain a modified binder with a number average molecular weight of 2,500,000.

[0105] 2. Preparation of positive electrode

[0106] A positive electrode slurry was prepared as follows: lithium iron phosphate positive electrode active material, acetylene black as a conductive agent, and the modified binder described above were added to a vacuum mixer in a mass ratio of 97.9:0.9:1.2 and mixed. NMP as a solvent was then added to the mixed slurry, and the mixed slurry was stirred in a vacuum mixer until it was uniform, thereby obtaining the positive electrode slurry of this embodiment. The positive electrode slurry was evenly coated on both surfaces of the positive electrode current collector aluminum foil, dried at room temperature, and then transferred to an oven for further drying. After drying in the oven, a semi-finished positive electrode sheet was obtained, which was then cold pressed and cut into pieces to obtain the positive electrode sheets to be assembled.

[0107] 3. Preparation of negative electrode sheet

[0108] The negative electrode slurry was prepared as follows: artificial graphite, conductive agent acetylene black, thickener CMC, and binder SBR were added to a vacuum mixer at a mass ratio of 96.4:1:1.2:1.4 and mixed. Deionized water was then added to the mixture, and the mixture was stirred in a vacuum mixer until it became uniform, thereby obtaining the negative electrode slurry of this embodiment.

[0109] The above-mentioned negative electrode slurry is evenly coated on both surfaces of the negative electrode current collector copper foil, dried at room temperature and then transferred to an oven for further drying. After drying in the oven, a negative electrode sheet semi-finished product is obtained, and then the negative electrode sheet semi-finished product is cold pressed and cut to obtain the negative electrode sheet to be assembled.

[0110] 4. Assembly of lithium batteries

[0111] A commercially available polyethylene film is used as the lithium-ion battery separator, and a commercially available electrolyte suitable for batteries with a 4.2V (charge limit) voltage system is used. The positive and negative electrode sheets, along with the separator, are wound together to form a bare cell. The bare cell undergoes packaging, liquid injection, static aging, formation, and capacity grading to produce the finished battery.

[0112] Example 5

[0113] A modified binder, which differs from Example 1 in that the molecular weight of the polyethylene glycol is 11000-12000 Da; the rest of the components remain the same as Example 1.

[0114] Example 6

[0115] A modified binder is provided, which differs from Example 1 in that an aqueous nitric acid solution with a mass fraction of 68% is used instead of the aqueous sulfuric acid solution in Example 1; the rest of the solution remains the same as in Example 1.

[0116] Example 7

[0117] A modified binder, which is different from Example 1 in that the weight portion of the lithium source is 10 parts; the rest of the parts are consistent with Example 1.

[0118] Example 8

[0119] A modified binder, which is different from Example 1 in that the weight portion of the lithium source is 80 parts; the rest of the parts are consistent with Example 1.

[0120] Comparative Example 1

[0121] A modified binder, which differs from Example 1 in that polyethylene glycol is not used; the rest of the components are the same as those in Example 1.

[0122] Comparative Example 2

[0123] A modified binder, which differs from Example 1 in that no lithium source is used; the rest of the components are the same as Example 1.

[0124] Detection method

[0125] 1. Electrolyte resistance performance test

[0126] The modified adhesives prepared in the above embodiments and comparative examples were subjected to an electrolyte tolerance test. The specific steps were as follows: 70 g of the adhesive diluted to 20 wt% by the products of each embodiment and comparative example were poured into a mold and transferred to an oven. The adhesive was baked at 60 ° C for 8 h, 90 ° C for 4 h, 120 ° C for 2 h, and 140 ° C for 15 min to obtain an adhesive film; the adhesive film was then punched into a dumbbell shape, its weight was weighed using a micrometer balance, its thickness was measured using a micrometer, and the relevant data were recorded as the first adhesive film. The film was prepared for standby use; the first film was placed in an aluminum-plastic bag, and an electrolyte 40 times the weight of the film was injected and sealed; the sealed aluminum-plastic bag was moved into a 40°C oven and baked for 15 days; the film in the aluminum-plastic bag was removed, and the electrolyte on the surface of the film was absorbed with absorbent paper, and its weight was weighed with a micrometer and its thickness was measured with a micrometer, and the relevant data were recorded as a second film for standby use; the weight change rate and thickness change rate were calculated respectively, and the calculation results were recorded in Table 1; among them, the smaller the weight change rate and thickness change rate, the better the electrolyte resistance.

[0127] 2. Conductive performance test

[0128] The modified adhesives prepared in the above examples and comparative examples were tested for their electrical conductivity. The specific steps are as follows: (1) 70 g of the adhesive diluted to 20 wt % from the products of each example and comparative example were poured into a mold and transferred to an oven. The adhesive was baked at 60°C for 8 h, 90°C for 4 h, 120°C for 2 h, and 140°C for 15 min to obtain an adhesive film; (2) 5 cm × 5 cm adhesive films were cut and the conductivity of the films was tested using a conductivity tester. The conductivity of the films was recorded in Table 1.

[0129] 3. Cycle performance test

[0130] The lithium batteries prepared in the above embodiments and comparative examples were subjected to a cycle performance test, and the specific steps are as follows: 1C / 1C cycle test: At 25°C, the prepared battery was charged at a 1C rate and discharged at a 1C rate, and a full charge and discharge cycle test was performed for 1000 cycles. The capacity retention rate is recorded in Table 1.

[0131] Table 1

[0132]

[0133] In combination with Examples 1-4, Comparative Examples 1-2 and Table 1, it can be seen that by adding polyethylene glycol and introducing a lithium source, not only can a cross-linked network molecular structure with channels that are conducive to the insertion and extraction of lithium ions be formed on the surface of the positive electrode of the lithium battery, but the content of active lithium in the lithium battery can also be significantly increased. Due to the presence of the above-mentioned cross-linked network structure, the conductivity of the lithium battery can be further improved, as well as the capacity and cycle performance of the lithium battery can be improved; and the binder having the cross-linked network structure has the lowest weight change rate and thickness change rate, which means that the modified binder prepared in the present application has excellent electrolyte resistance, which improves its structural stability in the electrolyte during the cycle of the lithium battery.

[0134] Combining Example 1, Example 5 and Table 1, it can be seen that when the molecular weight of polyethylene glycol is too high, the weight change rate and thickness change rate of the binder increase slightly, and the cycle performance decreases slightly. This is because the change in the molecular weight of polyethylene glycol causes the ions in the cross-linked network structure of the modified binder to pass through the pores unevenly, enabling the modified binder to have excellent conductive properties.

[0135] In combination with Example 1, Example 6 and Table 1, it can be seen that when concentrated nitric acid is used in the process of synthesizing the modified binder, the weight change rate and thickness change rate of the modified binder are slightly increased relative to Example 1, and the cycle performance is slightly decreased, but the overall performance remains at a high level; this is because concentrated nitric acid can also increase the degree of protonation of hydroxyl groups in polyethylene glycol, which is conducive to the formation of a uniform and stable cross-linked network structure, improve the conductivity of the modified binder, and have a positive effect on the electrolyte resistance of the modified binder.

[0136] In combination with Example 1, Examples 7-8 and Table 1, it can be seen that by introducing a certain concentration of lithium ions into the binder, the amount of active lithium in the lithium battery can be increased while avoiding the rapid generation of lithium dendrites at the negative electrode of the lithium battery due to the increase in the amount of active lithium; when the amount of lithium source introduced is too small, it is not conducive to improving the cycle performance of the lithium battery, and when the amount of lithium source introduced is too much, it is easy to cause unbalanced transfer of lithium ions between the positive and negative electrodes of the lithium battery, significantly reducing the overall performance of the lithium battery.

Claims

1. A modified binder, wherein the structural formula of the modified binder is as shown in Formula I: （Ⅰ）； in, The value of n1 is an integer between 3000 and 10000, the value of n2 is an integer between 3000 and 10000, the value of n3 is an integer between 3000 and 10000, and the value of n4 is an integer between 3000 and 10000; The structural formula of the R group includes 、 、 、 Any one of .

2. A method for preparing a modified binder, comprising the following steps: Step 1, mixing polyethylene glycol with an organic solvent, and heating to obtain a mixed solution; Step 2, heating the mixed solution, adding a polymerization inhibitor thereto, stirring, and continuing to add an organic acid and an inorganic acid to obtain a pretreatment solution; Step 3, heating the pretreatment solution and cooling it, adding a lithium source thereto, stopping heating after the reaction, collecting the material, and obtaining a lithium-containing crosslinking agent; Step 4: Mix the lithium-containing crosslinking agent, dispersant and deionized water, heat them in an inert atmosphere, and continue to add initiator and polymerization monomer to obtain the modified binder.

3. The method for preparing the modified binder according to claim 2, wherein: In the step 1, the heating temperature is 50-65°C; In the step 2, the mixed solution is heated to 70-75°C and stirred for 15-30 minutes; in the step 3, the pretreatment solution is heated to 130-150°C, kept warm for 5-8 hours, and then cooled to 60-70°C, and the holding time after adding the lithium source is 0.8-1.2 hours; in the step 4, the heating temperature is 50-60°C, the pressure is maintained at 3.4-4.0MPa, and after the initiator and the polymerization monomer are added, the reaction is carried out until the pressure drops to 2.5-2.7MPa to obtain the modified binder.

4. A method for preparing a modified binder according to claim 2, wherein: The raw materials used in the preparation process are calculated by weight and include the following raw materials in parts by weight: 2000-5000 parts of polyethylene glycol, 360-1000 parts of organic solvent, 5-15 parts of inhibitor, 350-400 parts of organic acid, 50-100 parts of inorganic acid, 20-60 parts of lithium source, 50-115 parts of dispersant, 255-495 parts of initiator, and 51700-82200 parts of polymerization monomer.

5. A method for preparing a modified binder according to claim 2, wherein: The molecular weight of the polyethylene glycol is 2000-10000 Da.

6. A method for preparing a modified binder according to claim 2, wherein: The organic acid includes at least one of itaconic acid, maleic acid and acrylic acid.

7. A method for preparing a modified binder according to claim 2, wherein: The inorganic acid includes any one of concentrated sulfuric acid and concentrated nitric acid.

8. A method for preparing a modified binder according to claim 2, wherein: The lithium source includes lithium hydroxide.

9. A method for preparing a modified binder according to claim 2, wherein: The dispersant includes at least one of hydroxypropyl methyl ether cellulose, polyvinyl alcohol, and cellulose acetate butyrate; the initiator includes at least one of diisopropyl hydroperoxide, diisopropyl peroxydicarbonate, and tert-butyl peroxypivalate.

10. A lithium ion battery comprising a positive electrode sheet, a negative electrode sheet, an electrolyte, a separator and the modified binder according to claim 1.

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