Soap-free emulsion polymerization method, polymer emulsion and application thereof

Abstract

The invention relates to the technical field of battery materials, in particular to a soap-free emulsion polymerization method and application of prepared polymer emulsion in batteries. The soap-free emulsion polymerization method provided by the invention is not suitable for additional emulsifiers, the polymerization step is divided into two steps, the characteristic that reaction monomers simultaneously contain hydrophilic monomers and lipophilic monomers is utilized, the amphiphilic polymer emulsifier is firstly formed in the first step, and then the required polymer emulsion is formed through the second step reaction. During the period, the stability of the polymer emulsion is guaranteed by controlling the weight-average molecular weight of the polymer emulsifier obtained in the first-step reaction.

Description

Technical Field

[0001] This invention relates to the field of battery materials technology, and in particular to a method for preparing polymer emulsions used as binders in lithium batteries. Background Technology

[0002] Lithium-ion batteries are characterized by high voltage platform, high energy density, long lifespan, and zero pollution, and are now widely used in new energy vehicles, aerospace, digital devices, and home appliances. Although binders constitute a small percentage of lithium-ion batteries, they ensure the uniform dispersion and adhesion of active and conductive materials to the current collector, providing excellent connectivity between them. Furthermore, binders influence the cycle performance and rapid charge / discharge capability of lithium-ion batteries, and impart good mechanical properties and processability to the electrodes. Therefore, binders play a crucial role in lithium-ion batteries.

[0003] Commonly used battery binders include polyvinylidene fluoride (PVDF), styrene-butadiene rubber (SBR), polyacrylonitrile (PAN), polyacrylic acid (PAA), polypropylene emulsions, polytetrafluoroethylene (PTFE), and modified copolymers of these polymers. Among these binders, PAA, as an aqueous binder, significantly reduces costs due to its water-based solvent, while also enabling non-toxic and harmless production. Furthermore, the abundant carboxyl groups in the PAA backbone greatly enhance lithium-ion transport performance. Additionally, PAA exhibits low swelling in the electrolyte, ensuring electrode integrity. PAA is produced through solution polymerization, bulk polymerization, suspension polymerization, and emulsion polymerization. Emulsion polymerization, in particular, largely uses water as a solvent, avoiding expensive or toxic solvents and meeting environmentally friendly production requirements. Compared to other reactions, it also boasts a higher polymerization rate and higher molecular weight. This is due to the presence of numerous latex particles during emulsion polymerization, within which a large number of free radicals are blocked, resulting in a higher polymerization rate. On the other hand, due to electrostatic repulsion, latex particles are difficult to aggregate, resulting in a near-zero chain termination rate as free radical chains between them cannot collide. However, traditional emulsion polymerization methods, due to the use of large amounts of emulsifiers, are complex, costly, and environmentally harmful. The use of emulsifiers also affects the film-forming properties, water resistance, and adhesion of the binder. Therefore, developing emulsifier-free synthesis of polypropylene binders is of great significance. Using soap-free emulsion polymerization can reduce costs, simplify the process, and avoid the post-processing of emulsifiers and their potential environmental hazards, thus meeting current environmental protection trends.

[0004] However, polyacrylic acid emulsions obtained by existing soap-free emulsion polymerization methods have poor stability. When used as binders in lithium batteries, the resulting slurry is prone to sedimentation and demulsification under high-speed shear. Slurry sedimentation leads to uneven electrode coating, resulting in uneven distribution of adhesion on the electrode, affecting the adhesion between the active material and the current collector, causing the active material to fall off the current collector, and thus affecting battery performance and cycle performance. Summary of the Invention

[0005] A first aspect of the present invention is to provide a soap-free emulsion polymerization method, the method comprising the following steps:

[0006] Step 1: Preparation of polymeric emulsifier: The total reactive monomers are divided into two parts. Under an inert gas atmosphere, the first part of the monomers is polymerized to form a polymeric emulsifier. The first part of the monomers accounts for 10% to 30% of the total reactive monomers in molar amount. The total reactive monomers, in molar percentage, include 25% to 45% acrylic monomers, 10% to 30% acrylate monomers, and 35% to 55% nitrile monomers. The molar ratio of acrylic monomers, acrylate monomers, and nitrile monomers is the same in the first part of the monomers, the second part of the monomers, and the total reactive monomers.

[0007] Step 2: Add the second monomer dropwise to the reaction system of Step 1, and the reaction yields a white stable emulsion.

[0008] Preferably, the weight-average molecular weight of the polymeric emulsifier obtained in step 1 is 8775–17867 g / mol.

[0009] Optionally, in step 1, the degree of neutralization of the acrylic monomers is 10% to 60%; more preferably, in step 1, the degree of neutralization of the acrylic monomers is 30% to 40%.

[0010] Preferably, in step 1, the initiator is persulfate, and the amount of the initiator is 0.2% to 0.4% of the total mass of the reactants.

[0011] In some possible embodiments, in step 2, during the addition of the second portion of monomers, 0.01 to 0.2% by mass of carboxymethyl cellulose salt is added.

[0012] Preferably, the carboxymethyl cellulose salt has a weight-average molecular weight of 10,700 to 50,900 g / mol.

[0013] Optionally, the acrylic monomer is selected from at least one of acrylic acid and methacrylic acid.

[0014] Optionally, the acrylate monomer is selected from one or more of butyl acrylate, butyl methacrylate, isooctyl acrylate, isooctyl methacrylate, propyl acrylate, propyl methacrylate, dodecyl acrylate, dodecyl methacrylate, tetradecyl acrylate, tetradecyl methacrylate, hexadecyl acrylate, hexadecyl methacrylate, octadecyl acrylate, and octadecyl methacrylate.

[0015] Optionally, the nitrile monomer is selected from one or more of acrylonitrile, 3-pentenonitrile, 2-pentenonitrile, 4-pentenonitrile, 2-furanacrylonitrile, 5-hexanonitrile, 6-heptenonitrile, and citronellol.

[0016] A second aspect of the present invention is to provide a polymer emulsion prepared by the above-described soap-free emulsion polymerization method.

[0017] A third aspect of the present invention is to use the above-mentioned polymer emulsion as a battery negative electrode binder.

[0018] The soap-free emulsion polymerization method provided by this invention does not use additional emulsifiers. It divides the polymerization process into two steps, cleverly utilizing the characteristic that the reactant monomers simultaneously contain both hydrophilic and lipophilic monomers. In the first step, an amphiphilic polymeric emulsifier with inherent dispersibility is formed. Then, the desired polymer emulsion is formed through the second step. During this process, the stability of the polymer emulsion is ensured by controlling the degree of neutralization of acrylic acid in the first step and the weight-average molecular weight of the polymeric emulsifier obtained in the first step.

[0019] In addition, this invention creatively explores that adding a small amount of carboxymethyl cellulose salt in the second step of the reaction can synergistically improve the viscosity and stability of the obtained polymer emulsion. Detailed Implementation

[0020] Unless otherwise stated, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. While only preferred methods and materials are described herein, any methods and materials similar or equivalent to those described herein may be used in the implementation or testing of this invention. All references to this specification are incorporated by way of citation to disclose and describe methods and / or materials associated with those references. In the event of any conflict with any incorporated reference, the content of this specification shall prevail. The terms “comprising,” “including,” “having,” “containing,” etc., as used herein are open-ended, meaning to include but not limited to. Numerical ranges in this invention should be understood to also specifically disclose each intermediate value between the upper and lower limits of the range. Any stated value or intermediate value within a stated range, as well as any other stated value or intermediate value within a stated range, and each smaller range between such values ​​is also included in this invention. The upper and lower limits of these smaller ranges may be independently included or excluded from the range. Unless otherwise specified, the experimental methods used in this invention are conventional methods. Unless otherwise specified, the materials, reagents, etc., used in this invention can be synthesized by commercially available or known methods.

[0021] This invention provides a soap-free emulsion polymerization method, the specific steps of which are as follows:

[0022] Step 1: Preparation of polymeric emulsifier: The total reactive monomers are divided into two parts. Under an inert gas atmosphere, the first part of the monomers is polymerized for 1-3 hours to obtain the polymeric emulsifier. The first part of the monomers accounts for 10%-30% of the molar amount of the total reactive monomers. The total reactive monomers, according to molar percentage, include 25%-45% acrylic monomers, 10%-30% acrylate monomers, and 35%-55% nitrile monomers. The molar ratio of acrylic monomers, acrylate monomers, and nitrile monomers is the same in the first part of the monomers, the second part of the monomers, and the total reactive monomers.

[0023] Step 2: Add the second monomer dropwise to the reaction system of Step 1 within 2-4 hours, and react for 3-5 hours to obtain a white stable emulsion.

[0024] The above emulsion polymerization scheme does not require the addition of an emulsifier. In step 1, since both hydrophilic and lipophilic monomers are present, they can form amphiphilic molecules, thus forming a polymeric emulsifier with inherent dispersibility.

[0025] It should be understood that the reaction times in each step of the above scheme are just examples. The specific reaction times can be adjusted according to factors such as the reaction drop acceleration rate and the amount of initiator, and are not intended to limit this scheme.

[0026] In some specific embodiments, the degree of neutralization of the acrylic monomers in step 1 is 10%–60%. In some preferred embodiments, the degree of neutralization of the acrylic monomers is 30%–40%. The carboxylic acid on the acrylic acid is neutralized by the base, and the degree of neutralization is the ratio of the molar amount of neutralized acrylic acid to the total molar amount of acrylic acid. The addition of the base causes the acrylic monomers on the amphiphilic molecular chain to form acrylic salts, which makes the amphiphilic molecular chain carry a negative charge, further improving the dispersibility of the polymeric emulsifier and reducing the sedimentation of the polymeric emulsifier.

[0027] In some preferred embodiments, the weight-average molecular weight of the polymeric emulsifier obtained in step 1 is 8775–17867 g / mol. Experimental results show that controlling the molecular weight of the polymeric emulsifier within this range can effectively prevent the polymeric emulsifier from settling, thus improving the stability of the final polymer emulsion.

[0028] In some preferred embodiments, during step 2, while adding the second portion of monomers, 0.01–0.2% by mass of carboxymethyl cellulose salt is simultaneously added. Experimental results show that the addition of carboxymethyl cellulose salt effectively prevents the aggregation of latex particles and increases the stability of the emulsion.

[0029] In some preferred embodiments, the weight-average molecular weight of the carboxymethyl cellulose salt added in step 2 is 10700–50900 g / mol. Experimental results show that controlling the molecular weight of the carboxymethyl cellulose salt within this range results in a polymer emulsion with higher viscosity and better anti-settling performance.

[0030] In some specific embodiments, the initiator can be selected from persulfates, such as potassium persulfate, ammonium persulfate, and sodium persulfate; it can also be a redox initiator composed of azobisisobutyramidine hydrochloride, persulfate, and sodium sulfite; or it can be a redox initiator composed of hydrogen peroxide. When a persulfate is selected as the initiator, the preferred amount of the initiator is 0.2% to 0.4% of the total mass of the reactants.

[0031] In some specific embodiments, the inert gas used may be nitrogen, argon, or helium. The acrylic monomers are selected from at least one of acrylic acid and methacrylic acid; the acrylate monomers are selected from one or more of butyl acrylate, butyl methacrylate, isooctyl acrylate, isooctyl methacrylate, propyl acrylate, propyl methacrylate, dodecyl acrylate, dodecyl methacrylate, tetradecyl acrylate, tetradecyl methacrylate, hexadecyl acrylate, hexadecyl methacrylate, octadecyl acrylate, and octadecyl methacrylate; the nitrile monomers are selected from one or more of acrylonitrile, 3-pentenonitrile, 2-pentenonitrile, 4-pentenonitrile, 2-furanacrylonitrile, 5-hexanonitrile, 6-heptenonitrile, and citronellol.

[0032] The above reaction is a free radical polymerization reaction, and the reaction system temperature used is also a temperature suitable for free radical polymerization, such as 60℃~70℃. The inert gas used can be nitrogen or argon.

[0033] The technical solution of the present invention will be further explained below with reference to some specific embodiments and test results.

[0034] <Preparation of Polymer Emulsifiers>

[0035] Example 1

[0036] The first batch of monomers (20% of the total molar amount of reactants) was stirred thoroughly and then neutralized with lithium hydroxide to a degree of neutralization of 10%. After neutralization, 250g of deionized water was added, nitrogen gas was introduced, and the reaction temperature was set to 65℃. Once the temperature reached 65℃, ammonium persulfate (0.2% of the total reactants by mass) was dissolved in 10g of deionized water and added dropwise to the reactor over 1 hour. After 1 hour of reaction, a portion of the emulsion was taken out to observe sedimentation, and the particle size and molecular weight of the emulsion were measured. The first batch of monomers specifically included: 10g of acrylic acid, 12.73g of acrylonitrile, and 1.3g of isooctyl acrylate (molar percentages of 35%, 55%, and 10%, respectively).

[0037] Example 2

[0038] The only difference from Example 1 is that the neutralization degree is adjusted to 15%.

[0039] Example 3

[0040] The only difference from Example 1 is that the neutralization degree is adjusted to 20%.

[0041] Example 4

[0042] The only difference from Example 1 is that the neutralization degree is adjusted to 30%.

[0043] Example 5

[0044] The only difference from Example 1 is that the neutralization degree is adjusted to 40%.

[0045] Example 6

[0046] The only difference from Example 1 is that the neutralization degree is adjusted to 60%.

[0047] Example 7

[0048] The only difference from Example 4 is that the amount of ammonium persulfate is adjusted to 0.05% of the mass of the first monomer.

[0049] Example 8

[0050] The only difference from Example 4 is that the amount of ammonium persulfate is adjusted to 0.1% of the mass of the monomer in the first part.

[0051] Example 9

[0052] The only difference from Example 4 is that the amount of ammonium persulfate is adjusted to 0.4% of the mass of the monomer in the first part.

[0053] The appearance and sedimentation properties of the polymeric emulsifiers obtained in Examples 1-6 were tested. The test method was as follows: the prepared polymeric emulsifiers were allowed to stand for 24 hours and the sedimentation phenomenon was observed. The test results are shown in Table 1.

[0054] Table 1: Results of Appearance and Sedimentation Tests for Polymer Emulsifiers

[0055] Group Neutralization Emulsion Appearance Whether it settles Example 1 10％ White granules settlement Example 2 15％ White granules settlement Example 3 20％ White granules settlement Example 4 30％ White emulsion No settlement Example 5 40％ White emulsion No settlement Example 6 60％ transparent adhesive No settlement

[0056] The molecular weight and sedimentation of the polymeric emulsifiers obtained in Examples 4 and 7-9 were tested. The test method was as follows: the sample was treated with 0.1M NaNO3 and the emulsion molecular weight was measured by pumping it into a liquid chromatograph (Waters e2695) at a flow rate of 25uL.

[0057] Table 2: Molecular weight and sedimentation test results of polymeric emulsifiers

[0058]

[0059]

[0060] As shown in Table 1, when the degree of neutralization of acrylic acid in the first monomer component is below 30%, agglomeration occurs between the polymeric emulsifiers. When the degree of neutralization is above 60%, the emulsion appears transparent. The presumed reason is that when the degree of neutralization of acrylic acid is below 30%, the polymeric emulsifiers are hydrophobic and have too low electrostatic repulsion, leading to agglomeration; when the degree of neutralization of acrylic acid is above 60%, the formed emulsifier molecular chains are hydrophilic, causing the molecular chains to expand and resulting in transparency. Therefore, the preferred degree of neutralization of acrylic acid in the preparation of polymeric emulsifiers is 30%–40%.

[0061] As shown in Table 2, the weight-average molecular weight of the formed polymeric emulsion decreases continuously with the increase of the amount of ammonium persulfate initiator. However, when the amount of ammonium persulfate decreases to below 0.1% of the monomer mass, the polymeric emulsion begins to precipitate. The presumed reason is that the increased free radical concentration after the initiator dosage increases leads to an increased chain termination rate, resulting in a decrease in molecular weight; when the amount of ammonium persulfate decreases to below 0.1% of the first monomer mass, the excessively large molecular weight causes the formed polymeric emulsion to become hydrophobic, leading to precipitation. Therefore, the preferred amount of ammonium persulfate is above 0.2%, and the preferred weight-average molecular weight range of the polymeric emulsion is 8775–17867 g / mol.

[0062] It should be understood that although Examples 1-9 are examples where the first monomer accounts for 20% of the total molar amount of the reactant monomers, in other specific embodiments, the ratio of the first monomer to the total molar amount of the reactant monomers can also be 10%, 15%, 25%, 30%, etc. For example, when the first monomer accounts for 10% or 30% of the total molar amount of the reactant monomers, the weight-average molecular weight of the resulting polymeric emulsifier can still be within the range of 8775-17867 g / mol by adjusting the amount of initiator.

[0063] <Polymer Emulsion Preparation>

[0064] Example 10

[0065] Based on Example 5, step 2 of the reaction was carried out. Specifically, the second portion of monomers (80% of the total molar amount of reactant monomers) was added dropwise to the reaction system of step 1 over 3 hours, and the reaction was allowed to continue for another 3 hours after the addition was complete. The second portion of monomers specifically included: 40g of acrylic acid, 50.9g of acrylonitrile, and 5g of isooctyl acrylate.

[0066] Example 11

[0067] The only difference from Example 10 is that in step 2, when the second part of the monomer is added dropwise for 1 hour (i.e., when 1 / 3 of the second part of the monomer has been added), 0.12 g of lithium carboxymethyl cellulose with a molecular weight of 10700 g / mol is dissolved in 10 g of deionized water and poured into the reaction vessel for reaction.

[0068] Examples 12-17

[0069] The only difference from Example 11 is that in step 2, the molecular weight of lithium carboxymethyl cellulose was adjusted, as shown in Table 3.

[0070] Table 3: Molecular weight of lithium carboxymethyl cellulose used in Examples 12-17

[0071]

[0072] <Performance Testing of Polymer Emulsions Obtained in Examples 10-17>

[0073] (1) Emulsion appearance, sedimentation test and viscosity test

[0074] The prepared polymer emulsion was allowed to stand for 24 hours to observe sedimentation. At the same time, the sample to be tested was prepared into an emulsion with a solid content of 10%, and after the pH was adjusted to neutral, the viscosity of the emulsion was measured on a DV2TLV viscometer with a rotation speed of 12 rpm. The test results are shown in Table 4.

[0075] Table 4: Results of appearance, sedimentation test, and viscosity test of polymer emulsions in Examples 10-17

[0076]

[0077] As shown in Table 4, the polymer emulsions prepared by adjusting the molecular weight of the polymer emulsifier to 8775-17867 g / mol did not exhibit sedimentation, indicating that the soap-free emulsion preparation method is feasible. Furthermore, the addition of lithium carboxymethyl cellulose increased the viscosity of the polymer emulsion. This is presumably because the addition of lithium carboxymethyl cellulose enhances the electrostatic repulsion between different latex particles, resulting in greater molecular chain extension and an overall increase in viscosity. Similarly, after adding lithium carboxymethyl cellulose, as in Example 10, no sedimentation occurred, indicating that the electrostatic repulsion provided by lithium carboxymethyl cellulose stabilized the latex particles and prevented agglomeration. However, when the molecular weight of lithium carboxymethyl cellulose was above 81400, slight sedimentation began to occur in the emulsion. This is presumably because the excessively long molecular chains resulted in a larger hydration radius on the surface of the latex particles, causing the lithium carboxymethyl cellulose molecular chains to entangle between different latex particles, leading to agglomeration. Therefore, in this scheme, the preferred molecular weight range of lithium carboxymethyl cellulose is between 10700-50900 g / mol.

[0078] (2) Emulsion mechanical stability test

[0079] The prepared emulsion was stirred at 2000 rpm to observe whether the emulsion broke down. Under the conditions of absorbance of 1.42 and refractive index of 1, a laser particle size analyzer was used to measure the particle size before and after mechanical stirring by taking the average of three measurements. The viscosity before and after stirring was measured by a DV2TLV viscometer. The results of the mechanical stability test of the emulsion are shown in Table 5.

[0080] Table 5: Results of Emulsion Mechanical Stability Test

[0081]

[0082] As shown in Table 5, the emulsion prepared using the soap-free emulsion polymerization process provided in this embodiment of the invention did not exhibit demulsification under high mechanical stirring conditions. However, without the use of lithium carboxymethyl cellulose, the particle size and viscosity of the emulsion changed significantly before and after mechanical stirring. After stabilization with lithium carboxymethyl cellulose, the particle size and viscosity of the emulsion only changed slightly before and after stirring, and no demulsification occurred. The presumed reason is that when lithium carboxymethyl cellulose is used, there is a strong electrostatic repulsion between the latex particles, resulting in strong mechanical stirring stability of the emulsion.

[0083] (3) Freeze-thaw and high-temperature stability test

[0084] Equal masses of the prepared emulsions were placed at 3℃ and 50℃ for 30 days, respectively. After being placed at room temperature for 2 hours, the emulsions were observed to see if they had broken down. Simultaneously, under conditions of absorbance of 1.42 and refractive index of 1, a laser particle size analyzer was used to measure the particle size before and after freeze-thaw and high-temperature treatments, taking the average of three measurements. The viscosity before and after stirring was measured using a DV2TLV viscometer, and the results are shown in Table 6.

[0085] Table 6: Results of Freeze-Thaw and High-Temperature Stability Tests

[0086]

[0087] As can be seen from Table 6, the particle size and viscosity of the emulsion did not change significantly after treatment at 3℃ and 50℃, indicating that the polymer emulsion obtained by the soap-free emulsion polymerization process provided in this embodiment of the invention still has good stability at both lower and higher temperatures.

[0088] (4) Ion stability test

[0089] Using Ca at a concentration of 1 mol / L 2+ The emulsion was treated, and the emulsion demulsification was observed. Simultaneously, the precipitate was filtered and weighed. Meanwhile, a laser particle size analyzer was used under conditions of absorbance 1.42 and refractive index 1, and the average of three measurements was taken to measure the Ca content. 2+ Particle size before and after treatment. Ca was measured using a DV2TLV viscometer. 2+ The viscosity before and after treatment is shown in Table 7.

[0090] Table 7: Results of Ion Stability Test

[0091]

[0092] As can be seen from the results in Table 7, in Example 11, 13g of Ca at a concentration of 1mol / L was added. 2+ No demulsification occurred afterward compared to Example 10, and 15g of 1mol / L Ca was added. 2+The amount of precipitate that appeared was lower than in Example 10. The presumed reason is that, with the increase of Ca... 2+ With increasing dosage, the emulsion particle size increases, and Ca... 2+ The addition of lithium carboxymethyl cellulose disrupts the charge balance within the emulsion, weakening the electrostatic repulsion between latex particles and leading to agglomeration, which increases the particle size. The addition of lithium carboxymethyl cellulose enhances the strength between charges and improves the emulsion's ability to resist positively charged ions.

[0093] Examples 18-21

[0094] The only difference between Examples 18-21 and Example 12 is that the ratio of the reactants was adjusted, as shown in Table 8.

[0095] Table 8: Reaction monomer ratios in Examples 18-21

[0096]

[0097]

[0098] Electrolyte swelling tests were conducted on the polymer emulsions prepared in Examples 10-11 and Examples 18-20. Test method: 30g of a neutral emulsion with 6% solids content was placed in an 8×10cm aluminum-plastic film, dried at 70℃, and then cut into three films of different shapes. These films were then immersed in the same mass of electrolyte at 70℃ for 24 hours. The mass of the films before and after immersion was recorded to obtain the electrolyte swelling. After drying the films, the electrolyte swelling was calculated. The test results are shown in Table 9.

[0099] Table 9: Electrolyte swelling and dissolution test results

[0100]

[0101] The results in Table 9 show that adding a small amount of lithium carboxymethyl cellulose has no effect on the swelling and dissolution of the electrolyte in the film. However, after reducing the amount of isooctyl acrylate, both the swelling and dissolution of the electrolyte in the film decreased. The reason for this is that isooctyl acrylate has similar compatibility with the electrolyte, which leads to the partial dissolution of the molecular chain in the electrolyte.

[0102] <Electrical performance and slurry stability testing>

[0103] The polymer emulsions prepared in Examples 10-11 and 18-21 were used as binders to fabricate electrode materials. Comparative Example 1 was set as a control group, using sodium carboxymethyl cellulose (CMC) as a binder. The following performance tests were conducted:

[0104] (1) Electrode slurry sedimentation test: The active material (graphite): binder: conductive agent (Super P): SBR (styrene-butadiene rubber) was added to the mixing tank in a mass ratio of 95.9:1.5:0.6:2 and stirred evenly to obtain a slurry. The slurry was allowed to stand for 96 hours and the sedimentation phenomenon was observed. The results are shown in Table 10.

[0105] (2) Electrode preparation: The slurry prepared in (1) is evenly coated onto copper foil with a scraper, dried in an oven at 70°C for 1-2 hours, and then placed in a vacuum drying oven for another 1 hour.

[0106] (3) Peel strength test: According to GB / T 2792-2014 "Test method for peel strength of adhesive tape", copper foil electrode sheet is used as the substrate, 180° peel method is adopted, peel speed is 10mm / min, 5 samples per group, and the average value is taken.

[0107] The peel strength of the adhesive is calculated by the following formula:

[0108] σ180°=F / B

[0109] Where σ180° is the 180° peel strength (N / m), F is the peel force (N), and B is the sample width (m).

[0110] (4) Cyclic performance: The above electrodes were used to prepare lithium-ion batteries. Under the condition of charge-discharge rate of 1C, the batteries were activated with a small current. The average value of the first three discharge capacities was taken as the initial capacity, and the discharge capacities of 100, 300 and 500 cycles were taken as the capacity after cycling. The test results are shown in Table 11.

[0111] Table 10: Test results of battery slurry sedimentation, viscosity and adhesion.

[0112]

[0113] The results in Table 10 show that, comparing Examples 10 and 11, the addition of lithium carboxymethyl cellulose increased the slurry viscosity, indicating that lithium carboxymethyl cellulose had a thickening effect to some extent, consistent with the emulsion viscosity test results. However, the addition of lithium carboxymethyl cellulose had no effect on the adhesive strength. The slurries in Examples 18 and 21 experienced sedimentation, presumably due to a lack of ester monomers, resulting in poor dispersion between them and the active and conductive materials. No sedimentation occurred in Examples 10, 11, 19, and 20 after the addition of isooctyl acrylate, indicating that the addition of the ester increased the dispersibility between the adhesive and the active material. The adhesive strength test results show that the higher the acrylonitrile content, the greater the adhesive strength, presumably because acrylonitrile contains the strongly polar cyano group, which can chelate with the current collector to improve adhesive strength. By comparing CMC and SBR, the polymer emulsion provided in the embodiments of this invention has a significant advantage in adhesive strength when used as an adhesive.

[0114] Table 11: Battery Cycle Performance Test Results

[0115]

[0116] As shown in Table 11, a comparison between Example 10 and Example 11 reveals that the addition of lithium carboxymethyl cellulose in step 2 of the polymer emulsion preparation process has no impact on the battery's cycle performance. Examples 10, 19, and 20 demonstrate that the amount of isooctyl acrylate added should be controlled within a certain range; excessively high levels may lead to a decrease in battery cycle performance. However, the polymer emulsion binder provided in these embodiments of the invention exhibits a significant advantage in cycle performance compared to CMC.

[0117] It should be understood that the above experimental phenomena were not expected by the inventors, and the reason for the speculation is a reasonable conjecture made by the inventors based on the experimental results.

[0118] Although the embodiments of the present invention have been disclosed above, they are not limited to the applications listed in the specification and embodiments. They can be applied to various fields suitable for the present invention. For those skilled in the art, other modifications can be easily made. Therefore, without departing from the general concept defined by the claims and their equivalents, the present invention is not limited to the specific details.

Claims

1. A soap-free emulsion polymerization method, characterized in that, Includes the following steps: Step 1: Preparation of polymeric emulsifier: The total reactive monomers are divided into two parts. Under an inert gas atmosphere, the first part of the monomers is polymerized to form a polymeric emulsifier. The first part of the monomers accounts for 10% to 30% of the total molar amount of the total reactant monomers; The total reactive monomers, by molar percentage, comprise 25%–45% acrylic monomers, 10%–30% acrylate monomers, and 35%–55% nitrile monomers; the molar ratios of acrylic monomers, acrylate monomers, and nitrile monomers are the same in the first part of the monomers, the second part of the monomers, and the total reactive monomers. Step 2: Add the second monomer dropwise to the reaction system of Step 1, and the reaction yields a white stable emulsion.

2. The soap-free emulsion polymerization method according to claim 1, characterized in that, In step 1, the degree of neutralization of the acrylic monomers is 10% to 60%.

3. The soap-free emulsion polymerization method according to claim 2, characterized in that, In step 1, the degree of neutralization of the acrylic monomers is 30% to 40%.

4. The soap-free emulsion polymerization method according to claim 1, characterized in that, The weight-average molecular weight of the polymeric emulsifier obtained in step 1 is 8775–17867 g / mol.

5. The soap-free emulsion polymerization method according to claim 1, characterized in that, In step 1, the initiator is persulfate, and the amount of the initiator is 0.2% to 0.4% of the total mass of the reactants.

6. The soap-free emulsion polymerization method according to any one of claims 1 to 5, characterized in that: In step 2, during the dropwise addition of the second batch of monomers, carboxymethyl cellulose salt is added at a mass of 0.01 to 0.2% of the total reactant monomers.

7. The soap-free emulsion polymerization method according to claim 6, characterized in that, The weight-average molecular weight of the carboxymethyl cellulose salt is 10700–50900 g / mol.

8. The soap-free emulsion polymerization method according to any one of claims 1 to 5, characterized in that: The acrylic monomer is selected from at least one of acrylic acid and methacrylic acid; The acrylate monomers are selected from one or more of butyl acrylate, butyl methacrylate, isooctyl acrylate, isooctyl methacrylate, propyl acrylate, propyl methacrylate, dodecyl acrylate, dodecyl methacrylate, tetradecyl acrylate, tetradecyl methacrylate, hexadecyl acrylate, hexadecyl methacrylate, octadecyl acrylate, and octadecyl methacrylate. The nitrile monomers are selected from one or more of acrylonitrile, 3-pentenonitrile, 2-pentenonitrile, 4-pentenonitrile, 2-furanacrylonitrile, 5-hexanonitrile, 6-heptenonitrile, and citronellol.

9. A polymer emulsion, characterized in that, It is prepared by the soap-free emulsion polymerization method according to any one of claims 1 to 8.

10. An application of a polymer emulsion, characterized in that, The polymer emulsion of claim 9 is used as a binder for the negative electrode of a lithium battery.

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