A binder for a separator, a method for preparing the same, a modified separator, and a battery

Abstract

The present application relates to a kind of preparation methods of binder for diaphragm, which can obtain binder for diaphragm with good thermal stability by three-step reaction of acetal modification, grafting and crosslinking, all relevant reactions are carried out in aqueous solution and the reaction liquid is used as the final product, the process is simple and easy to implement.The present application also relates to a binder for diaphragm obtained by the above preparation method and a modified diaphragm and a battery.The ceramic-coated modified diaphragm made of the above binder for diaphragm can withstand high temperature of 150 DEG C and has high peel strength.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of battery materials, and particularly relates to a binder for a separator, a preparation method thereof, a modified separator and a battery. BACKGROUND

[0002] Although the separator does not directly participate in the electrochemical reaction of the battery, it is an indispensable part of the lithium ion battery. Its main role is to physically separate the positive and negative poles of the battery to prevent direct contact between the positive and negative poles to cause short circuit. Therefore, the separator needs to have certain thermal stability. In the industry, the thermal stability of the separator is generally improved by coating a ceramic coating on the surface of the base film. Specifically, the ceramic material is coated onto the surface of the polyolefin material separator by using an adhesive to form a ceramic coating. Such a ceramic coating can effectively improve the heat resistance, puncture resistance and other properties of the separator. The performance of the adhesive can directly determine the uniformity, firmness and long-term stability of the ceramic coating, and can also improve the comprehensive performance of the separator, especially the safety and electrochemical performance.

[0003] To improve the thermal stability of the separator, a core-shell structure adhesive specifically used for ceramic coating has been developed in the industry. The glass transition temperature of the core layer and the shell layer of the adhesive is different, which can reduce the thermal shrinkage rate of the ceramic coated separator at a higher temperature. However, the preparation of such an adhesive involves a relatively complex process and a long preparation period, which is not easy to popularize and apply. SUMMARY

[0004] The first object of the present application is to provide a preparation method of a binder for a separator for ceramic coating and having good thermal stability.

[0005] The second object of the present application is to provide a binder for a separator obtained based on the aforementioned preparation method.

[0006] The third object of the present application is to provide a modified separator obtained by using the aforementioned binder for a separator for ceramic coating.

[0007] The fourth object of the present application is to provide a battery having the aforementioned modified separator.

[0008] To achieve the above-mentioned first object, the present application provides a preparation method of a binder for a separator, which is characterized by comprising the following steps:

[0009] (1) acetal modification: 20-100 parts by weight of polyvinyl alcohol is dissolved in deionized water, and the pH of the solution is adjusted to 1-2. After adding 1-20 parts of an acetal modification agent for acetal reaction, the reaction system is cooled to room temperature and the pH is adjusted to 6-8 to obtain a first reaction solution;

[0010] (2) Grafting reaction: 20-100 parts of grafting monomer and 0.1-0.8 parts of persulfate initiator are added to the first reaction solution to perform grafting reaction to obtain a second reaction solution;

[0011] (3) Crosslinking reaction: 0.4-20 parts of crosslinking monomer are added to the second reaction solution to perform crosslinking reaction, and then the reaction system is cooled to room temperature and the pH is adjusted to 6-8 to obtain the binder for separators.

[0012] As can be seen from the above scheme, the above preparation method first introduces a six-membered ring acetal structure into the main chain by acetalization reaction of polyvinyl alcohol with an acetalization modifier, which helps to improve the cohesion, adhesion and heat resistance of the final product. Further, through graft copolymerization, the activity of the product in subsequent reactions can be improved, and the thermal stability of the final product can also be improved to some extent. Subsequently, through crosslinking reaction, the final product has a three-dimensional network molecular structure, thereby further improving the thermal stability and mechanical properties of the final product. Therefore, through the above acetal modification, grafting and crosslinking reaction steps, a binder for separators with good thermal stability can be obtained, which is suitable for application scenarios of separator ceramic coating.

[0013] In addition, the above reaction steps are all carried out in aqueous solution, do not involve complex reaction system control mechanism, and directly use the final reaction solution as the product, without involving purification, impurity removal and other derivative operations, so it is relatively easy to implement. The effective component type of the corresponding solution system binder product is single, the product has good stability and is resistant to storage. The ceramic coated separator made of the binder for separators prepared by the above preparation method can withstand 150℃ high temperature and has high peel strength.

[0014] Further, the weight ratio of polyvinyl alcohol to deionized water in the solution of step (1) is 1:(4-19).

[0015] As can be seen from the above scheme, since the reaction solution is used as the final product, the concentration and adhesion performance of the binder for separators are indirectly controlled by controlling the concentration of the initial substrate solution. Controlling the weight ratio of polyvinyl alcohol and deionized water in the substrate solution within the above range can ensure that the adhesion performance of the binder for separators meets the expected level.

[0016] Further, the acetalization modifier is selected from one or more of 2,3-dihydroxybenzaldehyde, 2,4-dihydroxybenzaldehyde, 2,5-dihydroxybenzaldehyde, 2,6-dihydroxybenzaldehyde, 3,4-dihydroxybenzaldehyde and 3,5-dihydroxybenzaldehyde.

[0017] As can be seen from the above scheme, the molecule of the above acetal modification agent has an aldehyde group, a benzene ring and two hydroxyl groups, can further introduce a rigid aromatic ring structure on the acetal ring structure of the main chain, and is more conducive to improving the thermal stability of the acetal modified product. The two hydroxyl groups introduced can also improve the activity of the acetal modified product in subsequent reactions.

[0018] Further, the reaction temperature of the acetal reaction in step (1) is 50-80°C, and the reaction time is 1-2h.

[0019] As can be seen from the above scheme, the above reaction temperature and reaction time in combination with the acidic condition can promote the full progress of the acetalization reaction, while also inhibiting the occurrence of aldehyde self-polymerization side reactions.

[0020] Further, the grafting monomer is selected from one or more of acrylic acid, acrylamide, acrylonitrile, methacrylic acid, methacrylamide, and methacrylonitrile.

[0021] As can be seen from the above scheme, the grafting monomer can generate corresponding free radicals under the action of the initiator and form grafting chains through addition reaction with the hydroxyl groups on the main chain of the acetal modified product, thereby improving the thermal stability of the grafting product. Meanwhile, the carboxyl group, amide group or cyano group it carries can participate in subsequent crosslinking reactions, so that the grafting reaction product has good reactivity.

[0022] Further, the crosslinking monomer is selected from one or more of N-hydroxymethyl acrylamide, N-(2-hydroxyethyl) acrylamide, and N-(2-hydroxypropyl) acrylamide.

[0023] As can be seen from the above scheme, the crosslinking monomer can generate corresponding free radicals under the action of the initiator and react with the hydroxyl groups or carboxylic acid groups on the grafting reaction product. Meanwhile, the hydroxyl group it carries can also condense with the carboxylic acid groups on the grafting reaction product to form ether bonds, thereby forming a crosslinking network and improving the thermal stability and mechanical properties of the final product.

[0024] Further, the reaction temperature of the grafting reaction in step (2) and the crosslinking reaction in step (3) is 75-80°C, and the reaction time is 1-2h.

[0025] As can be seen from the above scheme, the grafting reaction and the crosslinking reaction can use the same initiator and be carried out under relatively close reaction conditions. The above reaction temperature and reaction time range can be used as the preferred reaction conditions for both types of reactions, thereby helping to reduce the frequency of adjusting related control parameters during preparation.

[0026] To achieve the above-mentioned second object, the present application provides a binder for a diaphragm, which is prepared by the above-mentioned method for preparing a binder for a diaphragm.

[0027] As can be seen from the above scheme, the adhesive for the separator prepared by the aforementioned preparation method has good thermal stability and is suitable for application scenarios of ceramic coating of the separator.

[0028] To achieve the third object, the application provides a modified separator, comprising a base film and a coating layer, wherein the coating layer is obtained by drying the ceramic slurry coated on at least one side surface of the base film, and the ceramic slurry comprises the aforementioned adhesive for the separator.

[0029] To achieve the fourth object, the application provides a battery, comprising a positive electrode, a negative electrode and the aforementioned modified separator.

[0030] The preparation method of the application can obtain the adhesive for the separator with good thermal stability through three steps of acetal modification, grafting and crosslinking, the related reactions are all carried out in aqueous solution and the reaction liquid is used as the final product, the process is simple and easy to implement. The ceramic-coated modified separator made therefrom can withstand a high temperature of 150℃ and has a high peeling strength. DETAILED DESCRIPTION

[0031] The application provides a preparation method of an adhesive for a separator, comprising the following steps:

[0032] (1) acetal modification: 20-100 parts by weight of polyvinyl alcohol is dissolved in deionized water, the pH of the solution is adjusted to 1-2, 1-20 parts by weight of an acetal modification agent is added for acetal reaction, then the reaction system is cooled to room temperature and the pH is adjusted to 6-8 to obtain a first reaction liquid;

[0033] (2) grafting reaction: 20-100 parts by weight of a grafting monomer and 0.1-0.8 parts by weight of a persulfate initiator are added to the first reaction liquid for grafting reaction to obtain a second reaction liquid;

[0034] (3) crosslinking reaction: 0.4-20 parts by weight of a crosslinking monomer is added to the second reaction liquid for crosslinking reaction, then the reaction system is cooled to room temperature and the pH is adjusted to 6-8 to obtain the adhesive for the separator.

[0035] Preferably, the weight ratio of polyvinyl alcohol to deionized water in step (1) is 1: (4-19).

[0036] Preferably, the acetal modification agent is selected from one or more of 2,3-dihydroxybenzaldehyde, 2,4-dihydroxybenzaldehyde, 2,5-dihydroxybenzaldehyde, 2,6-dihydroxybenzaldehyde, 3,4-dihydroxybenzaldehyde and 3,5-dihydroxybenzaldehyde.

[0037] Preferably, the reaction temperature of the acetal reaction in step (1) is 50-80℃, and the reaction time is 1-2h.

[0038] Preferably, the grafting monomer is selected from one or more of acrylic acid, acrylamide, acrylonitrile, methacrylic acid, methacrylamide, methacrylonitrile.

[0039] Preferably, the crosslinking monomer is selected from one or more of N-methylol acrylamide, N-(2-hydroxyethyl) acrylamide, N-(2-hydroxypropyl) acrylamide.

[0040] Preferably, the reaction temperature of the grafting reaction in step (2) and the crosslinking reaction in step (3) is 75-80°C, and the reaction time is 1-2h.

[0041] The present application also provides a binder for a separator, which is prepared by the above-mentioned method for preparing a binder for a separator.

[0042] The present application also provides a modified separator, comprising a base film and a coating layer, which is obtained by drying a ceramic slurry coated on at least one side surface of the base film, the ceramic slurry comprising the above-mentioned binder for a separator.

[0043] The present application finally provides a battery, comprising a positive electrode, a negative electrode and the above-mentioned modified separator.

[0044] The technical solutions in the embodiments of the present application will be clearly and completely described below. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by a person of ordinary skill in the art without creative work fall within the protection scope of the present application.

[0045] Binder for a separator and method for preparing the same

[0046] The binder for a separator of Example 1 is prepared according to the following steps:

[0047] (1) Acetal modification: 30g of polyvinyl alcohol and 270g of deionized water are put into a reaction kettle, and stirred at 95±2°C water bath until the polyvinyl alcohol is completely dissolved, and then cooled to room temperature. The pH of the above solution is adjusted to 1-2 using an acid regulator A, and then heated to 50°C. 10g of 3,4-dihydroxybenzaldehyde is put into the reaction kettle, and acetal reaction is carried out for 2h. Then the reaction system is cooled to room temperature, and the pH is adjusted to 6-8 using an acidity regulator B to obtain a first reaction solution.

[0048] (2) Grafting reaction: 10g of methacrylic acid, 10g of methacrylamide, 10g of methacrylonitrile and 0.1g of ammonium persulfate initiator are added to the first reaction solution, and then stirred and dissolved. The temperature is raised to 75°C, and grafting reaction is carried out for 1h to obtain a second reaction solution.

[0049] (3) Cross-linking reaction: 1 g of N-hydroxymethyl acrylamide, 1 g of N-(2-hydroxyethyl) acrylamide and 1 g of N-(2-hydroxypropyl) acrylamide were added to the second reaction solution, and after the cross-linking reaction was continued at 75°C for 1 h, the reaction system was cooled to room temperature and the pH was adjusted to 6-8 with the acidity regulator B to obtain the binder for separators.

[0050] It should be noted that the same commercially available polyvinyl alcohol (PVA) raw material was used as the reaction substrate in this embodiment and the subsequent related embodiments and comparative examples, and specifically, PVA-2488 or PVA-2499 type polyvinyl alcohol can be used, both of which have a degree of polymerization of 2400-2500, the former has an average molecular weight of 117000-124000, and the latter has an average molecular weight of 118000-124000.

[0051] The acidity regulator A can be a solution of any one or more of hydrochloric acid, sulfuric acid, acetic acid, formic acid and phosphoric acid, and the acidity regulator B can be a solution of any one or more of sodium hydroxide, potassium hydroxide, sodium bicarbonate, sodium acetate, sodium pyrophosphate, sodium carbonate and ammonia.

[0052] The preparation idea of the binder for separators in Example 2 is substantially the same as that in Example 1, except that in step (1), the addition amount of polyvinyl alcohol is 20 g, the addition amount of deionized water is 280 g, 2.5 g of 2,3-dihydroxybenzaldehyde and 2.5 g of 2,6-dihydroxybenzaldehyde are used as the acetal modification agent, the acetal reaction is carried out at 65°C, and the reaction time is 2 h; in step (2), 20 g of acrylic acid, 20 g of acrylamide and 20 g of acrylonitrile are added as grafting monomers, the addition amount of ammonium persulfate initiator is 0.4 g, the grafting reaction temperature is 80°C, and the reaction time is 1 h; in step (3), 4 g of N-hydroxymethyl acrylamide and 4 g of N-(2-hydroxyethyl) acrylamide are added as cross-linking monomers, the cross-linking reaction temperature is 80°C, and the reaction time is 1 h.

[0053] The preparation idea of the binder for separators in Example 3 is substantially the same as that in Example 1, except that in step (1), the addition amount of polyvinyl alcohol is 50 g, the addition amount of deionized water is 200 g, 8 g of 2,4-dihydroxybenzaldehyde and 8 g of 3,5-dihydroxybenzaldehyde are used as the acetal modification agent, the acetal reaction is carried out at 70°C, and the reaction time is 1 h; in step (2), 50 g of acrylamide and 50 g of acrylonitrile are added as grafting monomers, the addition amount of ammonium persulfate initiator is 0.8 g, the grafting reaction temperature is 75°C, and the reaction time is 2 h; in step (3), 7 g of N-hydroxymethyl acrylamide and 7 g of N-(2-hydroxypropyl) acrylamide are added as cross-linking monomers, the cross-linking reaction temperature is 75°C, and the reaction time is 2 h.

[0054] The preparation idea of the separator binder of Example 4 is substantially the same as that of Example 1, except that: in step (1), the addition amount of polyvinyl alcohol is 70 g, the addition amount of deionized water is 1330 g, 1 g of 2,5-dihydroxybenzaldehyde is used as the acetal modification agent, the acetal reaction is carried out at 80℃, and the reaction time is 2 h; in step (2), 10 g of acrylamide and 10 g of acrylonitrile are added as grafting monomers, the addition amount of ammonium persulfate initiator is 0.6 g, the grafting reaction temperature is 80℃, and the reaction time is 1 h; in step (3), 0.4 g of N-(2-hydroxyethyl) acrylamide and 0.4 g of N-(2-hydroxypropyl) acrylamide are added as crosslinking monomers, the crosslinking reaction temperature is 80℃, and the reaction time is 1 h.

[0055] The preparation idea of the separator binder of Example 5 is substantially the same as that of Example 1, except that: in step (1), the addition amount of polyvinyl alcohol is 100 g, the addition amount of deionized water is 1200 g, 7 g of 3,4-dihydroxybenzaldehyde is used as the acetal modification agent, the acetal reaction is carried out at 55℃, and the reaction time is 1.5 h; in step (2), 45 g of methacrylic acid is added as the grafting monomer, the addition amount of ammonium persulfate initiator is 0.2 g, the grafting reaction temperature is 75℃, and the reaction time is 1.5 h; in step (3), 0.4 g of N-hydroxymethyl acrylamide is added as the crosslinking monomer, the crosslinking reaction temperature is 75℃, and the reaction time is 1.5 h.

[0056] The preparation idea of the separator binder of Example 6 is substantially the same as that of Example 1, except that: in step (1), the addition amount of polyvinyl alcohol is 45 g, the addition amount of deionized water is 765 g, 20 g of 2,6-dihydroxybenzaldehyde is used as the acetal modification agent, the acetal reaction is carried out at 60℃, and the reaction time is 1 h; in step (2), 35 g of acrylonitrile and 35 g of methacrylamide are added as the grafting monomers, the addition amount of ammonium persulfate initiator is 0.7 g, the grafting reaction temperature is 80℃, and the reaction time is 1.5 h; in step (3), 20 g of N-(2-hydroxyethyl) acrylamide is added as the crosslinking monomer, the crosslinking reaction temperature is 80℃, and the reaction time is 1.5 h.

[0057] The separator binders prepared in Examples 1-6 are all light yellow clear transparent solutions, and no precipitates or gels are generated after long-term storage at room temperature.

[0058] Modified separator examples and performance tests

[0059] The separator binders prepared in Examples 1-6 are all light yellow clear transparent solutions, and no precipitates or gels are generated after long-term storage at room temperature.

[0060] a. 120 g of alumina powder as ceramic material and 1.2 g of sodium carboxymethyl cellulose as dispersant were poured into 151.8 g of deionized water, and stirred at 500 rpm for 30 min to obtain an intermediate suspension;

[0061] b. 27 g of the binder for the separator as prepared in Example 1 was added to the intermediate suspension, and stirring was continued at 500 rpm for 30 min to obtain a ceramic slurry.

[0062] The ceramic slurries prepared from the binders for the separator of Examples 1 to 6 were uniformly coated on one side of a polyolefin lithium battery separator having a thickness of 7 µm as a base film using a wire bar coater of 15 µm gauge, and dried to form a coating layer having a thickness of 1 to 2 µm, thereby obtaining modified separators corresponding to Examples 1 to 6.

[0063] Comparative Example 1 used a commercially available ordinary separator adhesive, the composition of which was an acrylate emulsion. Comparative Example 2 used a commercially available high-temperature-resistant separator adhesive, the main component of which was an epoxy emulsion. Comparative Example 3 used a binder for the separator prepared by substantially the same method as in Example 1, except that the acetal modification step was not involved. The ceramic slurries were prepared by referring to the aforementioned procedure, and the modified separators were prepared.

[0064] The modified separators corresponding to Examples 1 to 6 and Comparative Examples 1 to 3 were subjected to the following performance tests:

[0065] 1. Peeling strength test: The modified separators corresponding to the above examples and comparative examples were cut into 15 mm x 120 mm strips, and the side of the modified separator without the coating layer was tightly adhered to the plane of a glass slide. The side with the coating layer was tightly adhered to a 3M tape. One end of the sample was fixed, and the other end was pulled by the 3M tape. The force required to pull the tape at a constant speed of 300 mm / min was recorded. The average value of multiple samples was taken as the peeling strength test value.

[0066] 2. Heat shrinkage test: The modified separator was cut into 5 cm x 10 cm, and was treated in a 150℃ electric hot air drying oven for 30 min. The length of the modified separator in the transverse and longitudinal directions before and after baking was measured, and the heat shrinkage rate was calculated. The average value of three samples was taken as the test value of the transverse and longitudinal heat shrinkage rates.

[0067] Table 1: Sample peeling strength and heat shrinkage test table

[0068]

[0069] As can be seen from Table 1, the ceramic-coated modified separator prepared using the binder for separators of Examples 1-6 has high peeling strength, and the transverse and longitudinal shrinkage rates at 150°C are both less than 3%, indicating that it also has good thermal stability and can withstand high temperatures of 150°C.

[0070] As can be further seen from Table 1, the transverse and longitudinal thermal shrinkage rates of the modified separator prepared using the acrylate emulsion of Comparative Example 1 are significantly higher than those of Examples 1-6 and both exceed 3%, and the peeling strength is also significantly lower than the test data of Examples 1-6, indicating that the modified separator prepared using the binder for separators provided by the present application has better thermal stability at 150°C than the modified separator prepared using the existing acrylate binder. The longitudinal and transverse thermal shrinkage rates of the modified separator prepared using the epoxy resin emulsion of Comparative Example 2 also exceed 3%, and there is a significant gap with the test data of Examples 1-6, and the peeling strength is also lower than the test data of Examples 1-6, indicating that the thermal stability of the modified separator prepared using the binder for separators provided by the present application at 150°C is also better than that of the modified separator prepared using the existing high-temperature-resistant epoxy resin binder.

[0071] In addition, the transverse and longitudinal thermal shrinkage rates of Comparative Example 3 both exceed 3% compared with Examples 1-6, and there is also a significant gap in the peeling strength, indicating that acetal modification of the reaction substrate polyvinyl alcohol can significantly improve the adhesion and heat resistance of the final product.

[0072] Each of the embodiments in the specification is described in a progressive manner, and each embodiment focuses on the differences from other embodiments. The same or similar parts between the embodiments can be referred to each other.

[0073] The above description of the disclosed embodiments enables a person skilled in the art to implement or use the present application. Various modifications to these embodiments will be apparent to those skilled in the art, and the general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present application. Therefore, the present application will not be limited to these embodiments shown herein, but will conform to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for preparing an adhesive for diaphragms, characterized in that, Includes the following steps: (1) Acetal modification: By weight, 20-100 parts of polyvinyl alcohol are dissolved in deionized water and the pH of the solution is adjusted to 1-2. 1-20 parts of acetalization modifier are added to carry out the acetal reaction. The reaction system is then cooled to room temperature and the pH is adjusted to 6-8 to obtain the first reaction solution. The acetalization modifier is selected from one or more of 2,3-dihydroxybenzaldehyde, 2,4-dihydroxybenzaldehyde, 2,5-dihydroxybenzaldehyde, 2,6-dihydroxybenzaldehyde, 3,4-dihydroxybenzaldehyde, and 3,5-dihydroxybenzaldehyde. (2) Grafting reaction: 20-100 parts of grafting monomer and 0.1-0.8 parts of persulfate initiator are added to the first reaction solution to carry out the grafting reaction and obtain the second reaction solution. The grafting monomer is selected from one or more of acrylic acid, acrylamide, acrylonitrile, methacrylic acid, methacrylamide, and methacrylonitrile. (3) Crosslinking reaction: Add 0.4 to 20 parts of crosslinking monomer to the second reaction solution to carry out the crosslinking reaction, cool the reaction system to room temperature and adjust the pH to 6 to 8 to obtain the membrane adhesive. The crosslinking monomer is selected from one or more of N-hydroxymethylacrylamide, N-(2-hydroxyethyl)acrylamide and N-(2-hydroxypropyl)acrylamide.

2. The method for preparing the diaphragm adhesive as described in claim 1, characterized in that: The weight ratio of polyvinyl alcohol to deionized water in the solution of step (1) is 1:(4~19).

3. The method for preparing the diaphragm adhesive as described in claim 1, characterized in that: The reaction temperature of the acetal reaction in step (1) is 50~80℃ and the reaction time is 1~2h.

4. The method for preparing the diaphragm adhesive as described in claim 1, characterized in that: The reaction temperature for the grafting reaction in step (2) and the crosslinking reaction in step (3) are both 75~80℃, and the reaction time is both 1~2h.

5. An adhesive for diaphragms, characterized in that: It is prepared by the method for preparing the diaphragm adhesive according to any one of claims 1 to 4.

6. A modified diaphragm, comprising a base membrane and a coating layer, characterized in that: The coating layer is obtained by coating a ceramic slurry onto at least one side of the base membrane and drying it, the ceramic slurry comprising the membrane adhesive of claim 5.

7. A battery, characterized in that: It includes a positive electrode, a negative electrode, and the modified separator as described in claim 6.

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