An aqueous lithium-ion battery aqueous binder and a preparation method thereof

Abstract

The application belongs to the field of new energy, and discloses a water-based binder for water-based lithium ion batteries, which is obtained by soap-free emulsion polymerization of polymerization monomers in the presence of a crosslinking agent; the polymerization monomers include 10-25 wt% of partially neutralized acrylic monomers, 25-40 wt% of nitrile monomers, 10-20 wt% of polymerizable surface-active monomers, 25-40 wt% of oil-soluble monomers, and 4-10 wt% of water-soluble monomers; the crosslinking agent accounts for 0.001-1% of the total weight of the polymerization monomers; and the neutralization degree of the partially neutralized acrylic monomers is 1-30%. The soap-free emulsion polymerization is adopted, and the use amounts of different monomers are adjusted, so that the adhesion performance and the high and low temperature performance after being used in lithium ion batteries are synergistically improved. The application also discloses a preparation method of the binder.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of new energy sources, and particularly relates to a water-based lithium ion battery water-based binder and a preparation method thereof. BACKGROUND

[0002] In recent years, lithium ion batteries (LIB) are closely related to our life, and have broad application prospects and market values in small portable electronic devices and large energy storage systems such as electric vehicles. Nowadays, lithium ion batteries are developed towards the direction of prolonging cycle stability, high energy density, high transmission rate, low cost and safety. The energy density of lithium ion batteries is affected by many factors. In addition to starting from the active material as the main component, the performance of non-active substances can also be improved to reduce the component ratio, thereby indirectly improving the energy density of the battery.

[0003] In lithium ion batteries, the binder is an important non-active material, which has a low proportion but plays an important role. It plays a binding role between the active material, the conductive agent and the current collector, stabilizes the SEI film, and makes them have overall connectivity, thereby reducing the ion transmission resistance of the electrode, and making the pole piece have good mechanical properties and processing performance to meet the needs of industrial production.

[0004] In the prior art, the scheme for the binder for batteries, especially the negative electrode binder, can be seen in the following documents:

[0005] D1: CN110982008A discloses a lithium ion battery negative water-based binder. The problem to be solved is to improve the bonding strength and dispersion performance of the binder, and improve the electrical performance of the lithium ion battery negative electrode. The lithium ion battery negative water-based binder of the application is composed of the following substances in mass percentage: 15.0-45.0% of hydrophilic monomer, 40.0-75.0% of hydrophobic monomer, 0.5-5.0% of functional monomer, 1.0-9.0% of reactive surfactant, 0.1-4.0% of initiator, 0.1-3.0% of crosslinking agent, and 0.1-0.8% of chain transfer agent.

[0006] Functional monomers are lower alkyl methacrylate, hydroxy alkyl methacrylate, polyethylene glycol vinyl, allyl ether, polypropylene glycol vinyl, allyl ether, polyethylene glycol and polypropylene glycol vinyl, allyl ether, polyethylene glycol methacrylate, polypropylene glycol methacrylate, polyethylene glycol and polypropylene glycol methacrylate, methacrylic acid C8-C30 alkyl ester, dodecyl acrylate, lauryl methacrylate, hexadecyl acrylate, octadecyl acrylate, docosyl acrylate, dodecyl vinyl ester, hexadecyl vinyl ester, octadecyl vinyl ester, docosyl vinyl ester, dodecyl vinyl ether, hexadecyl vinyl ether, octadecyl vinyl ether, docosyl vinyl ether, polyethylene glycol methacrylate dodecyl acrylate, polyethylene glycol methacrylate octadecyl acrylate.

[0007] The data of each embodiment recorded in the description are as follows:

[0008] -20℃ discharge efficiency, the discharge efficiency of example 1 is 73.1%, the discharge efficiency of example 2 is 72.4%, the discharge efficiency of example 3 is 73.8%, the discharge efficiency of example 4 is 72.6%, the discharge efficiency of example 5 is 74.6%, the discharge efficiency of example 6 is 74.4%, and the discharge efficiency of example 7 is 75.8%;

[0009] The low temperature performance of the application is 1-4 percentage points higher than that of the comparative example, and the comparative example uses CMC and SBR to combine;

[0010] D2: CN111057184A discloses a preparation method of a negative electrode sheet water-based binder, and aims to improve the production efficiency of the negative electrode sheet. The application comprises the following steps: dissolving a reactive surfactant in deionized water, adding a hydrophobic monomer, a hydrophilic monomer, a functional monomer, a crosslinking agent, and a chain transfer agent to form a pre-emulsion; dissolving the reactive surfactant in deionized water, preparing an initiator solution with deionized water, and dropping the pre-emulsion and the initiator solution into the reactive surfactant solution, cooling, incubating, pumping, and filtering to obtain a water-based binder emulsion with a solid content of 30%; neutralizing with lithium hydroxide to obtain a negative electrode sheet water-based binder.

[0011] -20℃ discharge efficiency, the discharge efficiency of example 1 is 73.1%, the discharge efficiency of example 2 is 72.4%, the discharge efficiency of example 3 is 73.8%, the discharge efficiency of example 4 is 72.6%, the discharge efficiency of example 5 is 74.6%, the discharge efficiency of example 6 is 74.4%, and the discharge efficiency of example 7 is 75.8%;

[0012] In addition, in D2 and D3, the influence of the adhesive on the high-temperature performance is not studied, and it is found through the research of the project that the composition of the monomer and the high-temperature performance have a strong correlation.

[0013] Therefore, the core of the project is how to improve the high-temperature and low-temperature performance of the water-based lithium ion battery adhesive synchronously. SUMMARY

[0014] In view of the shortcomings of the prior art, the purpose of the present application is to provide a water-based lithium ion battery adhesive, which uses soap-free emulsion polymerization to realize the synergistic improvement of the adhesive performance and the high and low temperature performance after being used in lithium ion batteries by adjusting the amount of different monomers.

[0015] The secondary purpose of the present application is to provide a preparation method of the adhesive.

[0016] To achieve the purpose of the present application, the following technical scheme is adopted: a water-based lithium ion battery adhesive is obtained by soap-free emulsion polymerization of polymerization monomers in the presence of a crosslinking agent; the polymerization monomers include 10-25wt% of partially neutralized acrylic monomers, 25-40wt% of nitrile monomers, 10-20wt% of polymerizable surface active monomers, 25-40wt% of oil-soluble monomers, and 4-10% of water-soluble monomers.

[0017] The crosslinking agent accounts for 0.001-1% of the total weight of the polymerization monomers.

[0018] The neutralization degree of the partially neutralized acrylic monomers is 1-30%.

[0019] In the above-mentioned water-based lithium ion battery adhesive, the polymerization monomers include 15-25wt% of partially neutralized acrylic monomers, 25-30wt% of nitrile monomers, 13-18wt% of polymerizable surface active monomers, 30-40wt% of oil-soluble monomers, and 5-9% of water-soluble monomers.

[0020] The crosslinking agent accounts for 0.01-0.5% of the total weight of the polymerization monomers.

[0021] In the above-mentioned water-based lithium ion battery adhesive, the acrylic monomers are acrylic acid or methacrylic acid.

[0022] The nitrile monomers are one or more of acrylonitrile, alpha-methyl acrylonitrile, alpha-ethyl acrylonitrile, alpha-isopropyl acrylonitrile, and alpha-n-hexyl acrylonitrile.

[0023] The polymerizable surface-active monomer is one or more of N-vinylpyrrolidone, sodium p-styrenesulfonate, 2-acrylamido-2-methylpropanesulfonic acid, allyl polyether, dimethyldiallylammonium chloride, sodium methylallylsulfonate, sodium allylsulfonate, N,N-bisallyldodecylamine, octadecyl polyethylene glycol acrylate, octadecyl polyethylene glycol methacrylate, methoxy polyethylene glycol methacrylate;

[0024] The oil-soluble monomer is one or more of methyl (meth) acrylate, methyl acrylate, ethyl (meth) acrylate, ethyl acrylate, propyl (meth) acrylate, propyl acrylate, lauryl (meth) acrylate, lauryl acrylate, stearyl (meth) acrylate, stearyl acrylate, C13-C16 (meth) acrylate, C13-C16 acrylate, butyl (meth) acrylate, butyl acrylate, isooctyl (meth) acrylate, isooctyl acrylate, isobornyl methacrylate, styrene, vinyl acetate, alkoxyphenol acrylate, glycidyl (meth) acrylate;

[0025] The water-soluble monomer is one or more of acrylamide, hydroxyethyl acrylate, hydroxyethyl methacrylate.

[0026] In the above aqueous lithium ion battery aqueous binder, the oil-soluble monomer is composed of a first oil-soluble monomer and a second oil-soluble monomer; the first oil-soluble monomer is at least one of butyl acrylate, butyl (meth) acrylate, propyl (meth) acrylate, propyl acrylate, methyl (meth) acrylate, methyl acrylate, ethyl (meth) acrylate, ethyl acrylate, and the second oil-soluble monomer is at least one of isooctyl acrylate, isooctyl (meth) acrylate.

[0027] The water-soluble monomer is hydroxyethyl acrylate or hydroxyethyl methacrylate.

[0028] In the above aqueous lithium ion battery aqueous binder, the mass ratio of the first oil-soluble monomer and the second oil-soluble monomer is 1-5:1-5.

[0029] In the above aqueous lithium ion battery aqueous binder, the degree of neutralization of the partially neutralized acrylic monomer is 5-30%.

[0030] In the above aqueous lithium ion battery aqueous binder, the crosslinking agent is one or more of divinylbenzene, polyethylene glycol (200) bisacrylate, polyethylene glycol (400) bisacrylate, polyethylene glycol (600) bisacrylate, polyethylene glycol (200) bismethacrylate, polyethylene glycol (400) bismethacrylate, polyethylene glycol (600) bismethacrylate / N,N-methylene bisacrylamide, pentaerythritol triacrylate, pentaerythritol tetraacrylate, trimethylolpropane triacrylate, trimethylolpropane tri(3-aziridinyl propionate), polyurethane acrylate, dipentaerythritol diacrylate, dipropylene glycol diacrylate, ethoxylated (30) bisphenol A diacrylate.

[0031] Meanwhile, the application also discloses a preparation method of the above binder, comprising the following steps:

[0032] Step 1: preparing an aqueous solution of partially neutralized acrylic monomers;

[0033] Step 2: adding the remaining monomers into the aqueous solution of step 1, and adding an initiator to initiate a free radical polymerization reaction in the presence of inert gas to obtain the product.

[0034] In the above preparation method of the binder, the neutralizing agent used in step 1 is one or more of sodium hydroxide, potassium hydroxide, lithium hydroxide, ammonia, magnesium hydroxide, triethanolamine, sodium carbonate, sodium bicarbonate, 2-amino-2-methyl-1-propanol;

[0035] The initiator is one or more of ammonium sulfate, potassium persulfate, sodium persulfate, azobisdimethylaminoformamide hydrochloride, a redox initiator composed of persulfate and sodium sulfite, and a redox initiator composed of hydrogen peroxide;

[0036] The reaction temperature of step 2 is the initiation temperature of the initiator;

[0037] The reaction time of step 2 is 2-24 h.

[0038] In the above preparation method of the binder, the product obtained in step 2 is an emulsion, which can be obtained by filtering, neutralizing to a neutral system, and removing low molecular weight substances to obtain the aqueous lithium ion battery aqueous binder.

[0039] Compared with the prior art, the application has the following beneficial effects:

[0040] (1) The aqueous negative electrode binder has excellent thickening and suspending properties, and the obtained slurry has excellent stability and anti-settling properties;

[0041] (2) The negative electrode binder of the present application has good water dispersibility and operability, and its use only needs to be directly added to the negative electrode slurry formula, without the need for additional pretreatment or post-neutralization, and without the need to add CMC, simplifying the homogenizing process and being simple to operate, and the water dispersion time is 10-15 min, and the dispersion is relatively fast.

[0042] (3) The present application is based on monomers, preferably, the polymer synthesized by the soap-free emulsion polymerization method has excellent high-temperature cycle performance, and has better performance than the SBR+CMC system;

[0043] (4) The low-temperature electrochemical performance of the material is basically equivalent to or slightly better than that of SBR+CMC, and the internal resistance and the first discharge efficiency are better than those of SBR+CMC.

[0044] The specific reasons for the above beneficial effects are that in the technical scheme of the present application, by effectively controlling the selection and amount of various monomers and using the soap-free emulsion polymerization method for synthesis, the thickening, suspension performance and the most core advantage of the present application, i.e., the improvement and optimization of high-temperature cycle performance, are comprehensively realized.

[0045] Specifically, in the technical scheme of the present application, the amount of each monomer is sorted from more to less, and the order is nitrile and oil-soluble monomer, acrylic monomer, polymerizable surface active monomer, and water-soluble monomer.

[0046] Based on this, on the one hand, acrylonitrile is conducive to the construction of lithium ion channels and the improvement of adhesion, so that the appropriate amount of acrylonitrile can significantly improve the stability of the battery material; the polymerizable surface active monomer is conducive to ensuring the stability of the material polymerization and application process, and its ionic group is also conducive to optimizing the electrical performance, and sufficient reactive surface activity is an important guarantee for the performance of the product to meet the standards, so it is necessary to appropriately increase the reactive surface activity under the premise of ensuring the stability of the system; the oil-soluble monomer and the water-soluble monomer are soft monomers, and excessive use of soft monomers will deteriorate the electrolyte swelling performance of the material, and thus the long-term cycle performance of the battery will be deteriorated.

[0047] On the other hand, although excessive use of oil-soluble monomers will deteriorate the swelling performance of the material, within a reasonable use range, controlling the amount to be more than that of acrylic monomers can achieve good high-temperature cycle effect guarantee; because the appropriate use of oil-soluble monomers can improve the dispersion performance of the present binder for conductive materials and the retention of the adhesion performance under high-temperature conditions, the damage to the high-temperature cycle performance of the battery caused by the reduction of adhesion under high-temperature conditions can be minimized; although excessive use of oil-soluble monomers will cause swelling problems, under appropriate conditions, the high-temperature swelling effect is not significant, and the hydrophobic bonding effect formed with the water-soluble monomer can inhibit the damage to the adhesion of the binder under high temperature, achieving the result of improving the high-temperature cycle performance.

[0048] Meanwhile, the amount of acrylonitrile should be controlled to be greater than that of acrylic monomers. Within a reasonable range, cyano (acrylonitrile) and carboxyl (acrylic acid) have a stronger ability to capture lithium ions and are more conducive to the construction of lithium ion transport channels. However, excessive use of acrylic monomers will reduce the resistance to electrolyte swelling. Therefore, controlling the amount of acrylonitrile to acrylic monomers is an important step in ensuring high-temperature cycling performance.

[0049] Finally, although the water-soluble monomer is used in small quantities in the formulation of this invention, it should be used as much as possible within an appropriate range, and it is an indispensable component. It can synergize with the oil-soluble monomer because the type of oil-soluble monomer has a significant impact on viscosity. Generally speaking, a small amount of oil-soluble monomer will lead to a decrease in viscosity; however, as the amount of oil-soluble monomer increases, a certain amount of oil-soluble monomer and hydrophilic monomer, especially water-soluble monomer, will form hydrophilic-hydrophobic association to a certain extent, thereby playing a certain role in thickening and viscosity enhancement. The two have a synergistic effect, and there is no strict positive correlation. This synergy is particularly significant at high temperatures. Attached Figure Description

[0050] Appendix Figure 1 Image of a sample of the adhesive;

[0051] Appendix Figure 2 Thermal stability test of the negative electrode binder;

[0052] Appendix Figure 3 Images show the application effect of the binder in the negative electrode homogenization. Detailed Implementation

[0053] The technical solution of the present invention will be further illustrated below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely illustrative of the present invention and should not be construed as limiting the invention in any way.

[0054] Example 1

[0055] This embodiment provides an aqueous lithium battery adhesive, the preparation method of which includes the following steps:

[0056] Dissolve 48 g of acrylic acid in 100 g of water, under the circulation of frozen water, add lithium hydroxide drop by drop for neutralization, the neutralization degree is 5%; then add 60 g of acrylonitrile (AN), 40 g of butyl acrylate (BA), 40 g of isooctyl acrylate (EHA), 16 g of sodium p-styrenesulfonate, 16 g of hydroxyethyl acrylate (HEA), 16 g of 2-acrylamido-2-methylpropanesulfonic acid (AMPS), 0.24 g of N,N methylene bisacrylamide, then add a certain amount of deionized water and stir quickly to make it emulsify completely, and blow nitrogen for about 1 h to remove oxygen; after blowing nitrogen for a certain time, start to heat to 60℃, add 1.0 g of initiator potassium persulfate to initiate polymerization, the reaction time is 12 h, after the reaction is completed, the product is a stable milky white emulsion; remove the gel block by filtration, and the emulsion obtained after filtration is neutralized to pH 6.5-8.0, and low molecular weight is removed to obtain a high viscosity water-based lithium battery binder, and the residual monomer and oligomer are removed by distillation under reduced pressure.

[0057] Example 2

[0058] The same as example 1, except that the neutralization degree is adjusted to 10%.

[0059] Example 3

[0060] The same as example 1, except that the neutralization degree is adjusted to 50%.

[0061] Example 4

[0062] The same as example 1, except that the neutralization degree is adjusted to 90%.

[0063] Example 5

[0064] The same as example 1, except that the water-soluble monomer is composed of 8 g of hydroxyethyl acrylate and 8 g of acrylamide, and the neutralization degree of acrylic acid is 20%.

[0065] Example 6

[0066] The same as example 1, except that the hydroxyethyl acrylate is replaced by acrylamide, and the neutralization degree of acrylic acid is 30%.

[0067] Example 7

[0068] The same as example 1, except that the sodium p-styrenesulfonate is changed to polyethylene glycol octadecyl methacrylate.

[0069] Example 8

[0070] The same as example 1, except that the oil-soluble monomer is composed of ethyl acrylate and isooctyl acrylate in a ratio of 1:1, and the total amount remains unchanged.

[0071] Example 9

[0072] The same as example 1, except that the oil-soluble monomer consists of 80 g of butyl acrylate.

[0073] Example 10

[0074] The same as example 1, except that the oil-soluble monomer consists of 80 g of butyl acrylate.

[0075] Examples 11-20

[0076] The process steps of examples 11-20 are the same as example 1, except as noted in Table 1 below.

[0077] Table 1 Formulation Table

[0078]

[0079] Comparative Example 1

[0080] The weight ratio of CMC and SBR is: 1.5:2.0.

[0081] Comparative Examples 2-5

[0082] The same as example 1, except as noted in Table 2 below.

[0083] Table 2 Formulation Table

[0084]

[0085] Performance Testing

[0086] Test Item 1: Emulsion Viscosity Test

[0087] The sample to be tested is prepared into an emulsion with a solid content of 15%, and a viscosity test is performed. The viscosity test method is as follows: a digital rotary viscometer is used for testing, the rotation speed is 20 rpm, and the viscosity data is obtained by testing.

[0088] The test results are shown in Table 3 below:

[0089] Table 3 Viscosity Test Results (unit: mPa·s)

[0090]

[0091]

[0092] Test Result Analysis:

[0093] The following conclusions can be drawn from the experimental results:

[0094] 1. As can be seen from Examples 1-6, when the neutralization degree of acrylic acid is less than 30%, the emulsion can remain in a stable state.

[0095] When the neutralization degree is higher than 50%, the ionization of acrylic acid leads to too strong hydration ability of the emulsion capsule, and the emulsion capsule is easy to stick. At the same time, a large amount of acrylonitrile is used in the present case, and the hydration ability of acrylonitrile is also strong, and the two superimpose, so that the preferred suitable range of the present case is relatively limited.

[0096] 2. As can be seen from Examples 1, 11-15, within a reasonable range, the more the acrylic acid, the greater the viscosity, and the more the oil-soluble monomer, the greater the viscosity may be. The reason for the difference between the viscosity of Example 15 and the viscosity of Example 12 is that the oil-soluble monomer is too little;

[0097] The reason for the positive correlation between the content of acrylic acid and the size of the viscosity is that the neutralized acrylic acid has the effect of alkali thickening, so the higher the content of acrylic acid, the greater the viscosity of the glue solution. However, from the swelling point of view, if the amount of acrylic acid exceeds the normal range, it will cause the problem of adhesive swelling, so it is important to use acrylic acid within a reasonable range.

[0098] The reason for the possible positive correlation between the content of the oil-soluble monomer and the size of the viscosity is that the type of oil-soluble monomer has a great influence on the viscosity. Generally, a small amount of oil-soluble monomer will cause the viscosity to decrease. However, with the increase of oil-soluble monomer, a certain amount of oil-soluble monomer and hydrophilic monomer, especially water-soluble monomer, will form a hydrophobic bonding effect to a certain extent, thus playing a certain thickening effect. The two belong to synergistic effect, and there is no strict positive correlation.

[0099] 3. As can be seen from Examples 1 and 16, when the amount of water-soluble monomer is small, it has little effect on the viscosity. It can be seen that within a reasonable range, the reduction of water-soluble monomer has little effect on the viscosity performance.

[0100] The main role of the water-soluble monomer of the present application is electrochemical performance, which will be analyzed later.

[0101] 4. As can be seen from Examples 1 and 17, when the amount of water-soluble monomer is large, it has little effect on the viscosity. Compared with Example 1, the viscosity is slightly reduced, mainly due to the reduction of the amount of acrylic acid. It can be seen that within a reasonable range, the increase of water-soluble monomer has little effect on the viscosity performance. However, in high temperature state, it cooperates with oil-soluble monomer to have a relatively obvious effect on inhibiting the decrease of the adhesion performance in high temperature state.

[0102] 5. From Example 2, Example 5 and Example 6, it can be seen that pre-neutralization and post-reaction neutralization have no obvious trend on the viscosity of the product, but too much neutralization degree has a significant effect on the stability of the reaction system;

[0103] 6. From Example 1, Examples 7-10, it can be seen that in the art, the adjustment of the reaction-type emulsifier and the oil-soluble monomer has a slight difference in the effect on the viscosity according to different raw materials.

[0104] 7. From Comparative Examples 2-5 and Example 1, it can be seen that the adjustment of the amount of monomer has no significant effect on the viscosity of the product.

[0105] Test item 2

[0106] Characterization / test of the application performance of the negative electrode sheet

[0107] Battery electrode sheet preparation test: the binder is mixed with the conductive main material, water, etc. in a certain proportion, homogenized to a certain viscosity and fineness, then filtered, coated, and prepared into a soft package battery for performance evaluation.

[0108] Thermal stability test: the sample is dried for 3h and then subjected to TG test. Test conditions: 10℃ / min, 35-800℃ N2 atmosphere, gas flow 20mL / min, 10℃ / min, 800-900℃ air atmosphere, gas flow 20mL / min; the thermal stability curve of the product is obtained and analyzed.

[0109] The characterization / test results are as follows:

[0110] Figure 2 is the thermal performance test of Example 1, which shows that it has good thermal stability. Figure 2 It can be seen that it has obvious decomposition at 394℃, which also shows that it has good thermal stability.

[0111] Figure 3 is the negative electrode application using a self-made binder, which has excellent flexibility and processing performance, and the addition amount is 2.0-3.0%, and no additional plasticizer is needed. The prepared electrode sheet has no powder dropping, cracking and other adverse phenomena during coating and baking.

[0112] The related formula and results of the battery electrode sheet preparation test and the thermal stability test are as follows in Table 4.

[0113] Table 4: Slurry formula and stability test results

[0114] Color paste Experiment 1 Experiment 2 Experiment 3 Experiment 4 Experiment 5 Experiment 6 Use of binder Example 11 Comparative Example 1 Comparative Example 2 Comparative Example 3 Comparative Example 4 Comparative Example 5 Binder amount 3％ 2％ 3％ 3％ 3％ 3％ CMC 0％ 1.5％ 0％ 0％ 0％ 0％ Conductive carbon 1.5％ 1.5％ 1.5％ 1.5％ 1.5％ 1.5％ Graphite 95.5％ 95.5％ 95.5％ 95.5％ 95.5％ 95.5％ Paste viscosity 4500 mPa-s 2200 mPa-s 6500 mPa-s 4800 mPa-s 5300 mPa-s 8000 mPa-s Paste stability 24 h stable 24 h stable 24 h stable 24 h stable 24 h stable 24 h stable

[0115] The negative electrode sheet of the battery is prepared using the binder, and the electrochemical performance test is carried out. The negative electrode sheet of the battery is prepared using the binder, and the electrochemical performance test is carried out.

[0116] The manufacturing method of the negative electrode sheet is as follows: a mixture of negative active material graphite or graphite and other active materials in different mass ratios, conductive agent acetylene black, and binder are mixed in a certain mass ratio, a solvent deionized water is added, and stirring is performed under the action of a vacuum stirrer until the system is uniform to obtain a negative electrode slurry. The negative electrode slurry is uniformly coated on both surfaces of the negative electrode current collector copper foil, dried at room temperature, then transferred to an oven for continuous drying, and then cold-pressed and cut to obtain the negative electrode sheet;

[0117] The manufacturing method of the lithium ion battery is as follows: the positive electrode sheet, the separator, and the negative electrode sheet are sequentially stacked with the separator between the positive and negative electrode sheets to play a separating role, and then wound to obtain a bare cell; the bare cell is placed in an outer packaging shell, dried, and then electrolyte is injected in a glove box, and the lithium ion battery is obtained after processes such as vacuum packaging, standing, formation, and shaping;

[0118] The test method of the first discharge efficiency is as follows: the formed battery is placed in a 45℃ oven, the battery pole is connected to a charging device, then charged at 0.3C constant current to 4.6V constant voltage until the current is less than 0.05C, then discharged at 0.3C to 2.8V, the charging process capacity C1 and the discharging process capacity C2 are extracted, and the battery first efficiency IC = C2 / C1*100% is calculated;

[0119] The test method of the internal resistance is as follows: first step: place the lithium ion battery at room temperature;

[0120] Second step: continuously discharge the lithium ion battery at a first preset discharge current I1 within a first preset discharge time interval, and measure the first discharge cutoff voltage U1 of the lithium ion battery in real time after the discharge is completed;

[0121] Third step: continue to discharge the lithium ion battery at a second preset discharge current I2 within a second preset discharge time interval, and measure the second discharge cutoff voltage U2 of the lithium ion battery in real time after the discharge is completed;

[0122] Fourth step: the resistance meter calculates the direct current internal resistance of the lithium ion battery in real time according to the preset direct current internal resistance calculation formula;

[0123] The test method of the high-temperature cycle performance is as follows: the batteries of the above examples and comparative examples are tested for high-temperature cycle performance using an electrochemical workstation (temperature 45℃, 1C constant current and constant voltage charging, cutoff current 0.05C, 30min standing after charging is completed, 1C constant current discharging, 30min standing after discharging is completed; cycle test is performed according to the above steps until the capacity attenuation is 80% of the initial capacity;

[0124] The test method for normal temperature cycle performance is as follows: the batteries of the above examples and the comparative examples are subjected to high temperature cycle test by using an electrochemical workstation (temperature is 25 DEG C, 1C constant current constant voltage charging, cut-off current is 0.05C, charging is stopped for 30 min, 1C constant current discharging, discharging is stopped for 30 min; cycle test is carried out according to the above steps until the capacity attenuation is 80% of the initial capacity;

[0125] The electrochemical performance test results can be seen from Table 5.

[0126] Table 5 electrochemical performance test results

[0127]

[0128] Result analysis:

[0129] 1. From the first charge-discharge efficiency, thickness and voltage, all samples show relatively similar performance;

[0130] 2. The internal resistance of the polymer type binder is lower than that of SBR+CMC, and the possible reason is that: this type of binder has rich polar groups, and the cyan group and the carboxyl group have stronger capture ability for lithium ions, which is more conducive to the construction of lithium ion transmission channel; at the same time, this type of binder has excellent dispersion performance and adhesion to the conductive main material, good dispersion performance represents that the material is more uniform, and the uniformity of the material and the resistance value are positively correlated; it represents that the adhesion between the material and the electrode sheet is better, which can avoid the separation of material particles at the micro level caused by electrochemical reaction, solvent and other factors, so as to reduce the resistance, so the resistance has obvious advantage compared with SBR+CMC.

[0131] 3. From the normal temperature cycle performance and high temperature cycle performance, the performance of example 1 of the application is better than SBR+CMC, and SBR+CMC is generally better than the comparative example.

[0132] It is shown that too little acrylonitrile (comparative example 2), too little reactive surface activity (comparative example 3), too much oil-soluble monomer (comparative example 4) and too much water-soluble monomer (comparative example 5) are not conducive to the normal temperature performance and high temperature cycle performance.

[0133] The possible reason is that:

[0134] On the one hand, acrylonitrile is conducive to the construction of lithium ion channel and the improvement of adhesion, so that the appropriate amount of acrylonitrile is used to significantly improve the stability of the battery material; the reactive surface activity is conducive to ensuring the stability of the material polymerization and application process, and the ionic group thereof is also conducive to the optimization of the electrical performance, so it is necessary to appropriately increase the reactive surface activity under the premise of ensuring the stability of the system; the oil-soluble monomer and the water-soluble monomer are soft monomers, and excessive use of soft monomers will deteriorate the electrolyte swelling resistance of the material, and thus the long-term cycle performance of the battery will be deteriorated;

[0135] In another aspect, although excessive use of oil-soluble monomers can deteriorate the swelling performance of the material, within a reasonable use range, controlling the amount to be more than that of the acrylic monomers can achieve better high-temperature cycle effect guarantee; because of the appropriate use of oil-soluble monomers, the dispersion performance of the binder to the conductive material and the retention of the bonding performance under high temperature can be improved, so that the damage to the high-temperature cycle performance of the battery caused by the reduction of the bonding force under high-temperature conditions can be weakened as much as possible; although excessive use of oil-soluble monomers can cause swelling problems, under appropriate conditions, the high-temperature swelling effect is not significant, and the hydrophobic bonding effect formed with water-soluble monomers can inhibit the damage to the high-temperature bonding force of the binder, achieving the result of improving the high-temperature cycle performance.

[0136] At the same time, the amount of acrylonitrile should also be controlled to be more than that of the acrylic monomers, and within a reasonable use range, the cyan group (acrylonitrile) and the carboxyl group (acrylic acid) have stronger capture ability for lithium ions, which is more conducive to the construction of lithium ion transmission channels, but excessive use of acrylic monomers can deteriorate the electrolyte swelling resistance, therefore, controlling the amount of acrylonitrile to be more than that of the acrylic monomers is an important link to guarantee the high-temperature cycle performance.

[0137] Referring to Test Item 2, the negative electrode sheets were prepared with the binders of Example 5, Example 7, Example 11, Example 12, Example 17, and Example 18, and the batteries were assembled and tested according to Test Item 2, and the test results are shown in Table 6 below.

[0138] Table 6: Test results of electrochemical performance

[0139]

[0140] Result analysis:

[0141] 1. According to the results of Example 1, Example 5, Example 8, Example 11, Example 12, Example 17, and Example 18, the performance trend is Example 8 > Example 17 > Example 1 > Example 12 > Example 5 > Example 11 > Example 18;

[0142] The performance of Example 8 is the best, and combined with Example 17 and Comparative Example 2, it is shown that within a suitable use range, the amount of water-soluble monomers has a more significant effect on the electrochemical performance, and the selection of the type of oil-soluble monomers also has a more significant effect on the electrochemical performance. This also provides directional guidance for the subsequent performance optimization of the binder.

[0143] 2. The performance of Example 7 is not as good as that of Example 1, and the performance of Example 18 is not as good as that of Example 1, which shows that sodium p-styrenesulfonate is a better reactive surface active agent, and the preferred amount of crosslinking agent is about 0.2-0.5wt%

[0144] 3. The performance of Example 11 and Example 12 is slightly worse than Example 1, which shows that viscosity is not a decisive factor for high temperature performance.

[0145] In summary, to achieve effective guarantee of high temperature performance, at least the following technical means should be adopted:

[0146] 1. The amount of acrylonitrile and oil-soluble monomers should be greater than that of acrylic acid;

[0147] 2. The amount of water-soluble monomers should be as much as possible within a suitable range, but cannot be excessive;

[0148] 3. A sufficient amount of reactive surface activity is an important guarantee for the product performance to meet the standards.

[0149] The applicant states that the process of the present application is illustrated by the above examples, but the present application is not limited to the above process steps, nor does it mean that the present application must rely on the above process steps to be implemented. It should be understood by those skilled in the art that any improvement on the present application, equivalent replacement of the raw materials selected by the present application, addition of auxiliary ingredients, selection of specific methods, etc. all fall within the protection scope and disclosure scope of the present application.

Claims

1. An aqueous binder for lithium-ion batteries, characterized in that, The polymer is obtained by soap-free emulsion polymerization of polymeric monomers in the presence of a crosslinking agent; the polymeric monomers include 10-25 wt% partially neutralized acrylic monomers, 25-40 wt% nitrile monomers, 10-20 wt% polymerizable surfactant monomers, 25-40 wt% oil-soluble monomers, and 4-10% water-soluble monomers. The crosslinking agent accounts for 0.001% to 1% of the total weight of the polymer monomers; The degree of neutralization of partially neutralized acrylic monomers ranges from 5% to 30%. The polymerizable surfactant monomer is one or more of sodium p-styrene sulfonate, 2-acrylamido-2-methylpropanesulfonic acid, sodium methylallyl sulfonate, and sodium allyl sulfonate. The oil-soluble monomer is composed of a first oil-soluble monomer and a second oil-soluble monomer; the first oil-soluble monomer is at least one of butyl acrylate, butyl methacrylate, propyl methacrylate, propyl acrylate, methyl methacrylate, methyl acrylate, ethyl methacrylate, and ethyl acrylate; and the second oil-soluble monomer is at least one of isooctyl acrylate and isooctyl methacrylate. The water-soluble monomer is hydroxyethyl acrylate or hydroxyethyl methacrylate.

2. The aqueous binder for lithium-ion batteries according to claim 1, characterized in that, The polymerizable monomers include 15-25 wt% partially neutralized acrylic monomers, 25-30 wt% nitrile monomers, 13-18 wt% polymerizable surfactant monomers, 30-40 wt% oil-soluble monomers, and 5-9% water-soluble monomers. The crosslinking agent accounts for 0.01 to 0.5% of the total weight of the polymer monomers.

3. The aqueous binder for aqueous lithium-ion batteries according to claim 1 or 2, characterized in that, The acrylic monomer is acrylic acid or methacrylic acid; The nitrile monomer is one or more of acrylonitrile, α-methylacrylonitrile, α-ethylacrylonitrile, α-isopropylacrylonitrile, and α-n-hexylacrylonitrile.

4. The aqueous binder for lithium-ion batteries according to claim 3, characterized in that, The mass ratio of the first oil-soluble monomer to the second oil-soluble monomer is 1-5:1-5.

5. The aqueous binder for lithium-ion batteries according to claim 1, characterized in that, The crosslinking agent is one or more of the following: divinylbenzene, N,N-methylenebisacrylamide, pentaerythritol triacrylate, pentaerythritol tetraacrylate, trimethylolpropane triacrylate, trimethylolpropane tri(3-acrylidinepropionate), polyurethane acrylate, dipropylene glycol diacrylate, and dipropylene glycol diacrylate.

6. A method for preparing an adhesive as described in any one of claims 1-5, characterized in that, Includes the following steps: Step 1: Prepare a partially neutralized aqueous solution of acrylic monomers; Step 2: Add the remaining monomer to the aqueous solution from Step 1, and in the presence of an inert gas, add an initiator to initiate a free radical polymerization reaction to obtain the product.

7. The method for preparing the adhesive according to claim 6, characterized in that, The neutralizing agent used in step 1 is one or more of the following: sodium hydroxide, potassium hydroxide, lithium hydroxide, ammonia, magnesium hydroxide, triethanolamine, sodium carbonate, sodium bicarbonate, and 2-amino-2-methyl-1-propanol. The initiator is one or more of the following: potassium persulfate, sodium persulfate, azobisisobutyramidine hydrochloride, redox initiators composed of persulfate and sodium sulfite, and redox initiators composed of hydrogen peroxide; The reaction temperature in step 2 is the initiation temperature of the initiator; The reaction time for step 2 is 2–24 hours.

8. The method for preparing the adhesive according to claim 6, characterized in that, The product obtained in step 2 is an emulsion. The emulsion is filtered, neutralized to neutrality, and then subjected to de-alcoholization to obtain an aqueous binder for water-based lithium-ion batteries.

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