A PMMA coated diaphragm and its preparation method and application

Abstract

The present application discloses a PMMA coated diaphragm and its preparation method and application, which relates to the technical field of secondary batteries. It comprises a base film, which is a porous film; a coating, which is arranged on at least one side of the base film, and comprises small-particle PMMA and large-particle PMMA, wherein the large-particle PMMA is embedded between the small-particle PMMA, and the large-particle PMMA is formed by demulsification and agglomeration of small-particle PMMA. It solves the problems of low liquid storage and poor air permeability of the diaphragm coated with small-particle PMMA in the prior art, and powder shedding of the large-particle PMMA coating.

Description

Technical Field

[0001] The present application relates to the technical field of secondary batteries, and in particular to a PMMA-coated diaphragm and a preparation method and application thereof. Background Art

[0002] As an important component of the battery, the performance of the diaphragm can determine the overall performance of the battery. When the diaphragm and the pole piece are not bonded, the battery becomes soft and has no hardness, which makes it difficult to install the whole device. For this reason, it is often necessary to coat an organic bonding layer on the surface of the diaphragm. When the diaphragm and the pole piece are hot pressed, the diaphragm has a certain degree of adhesion, thereby giving the battery a certain degree of hardness. Most of the existing technologies coat a PVDF layer on the outer surface of the diaphragm. PVDF can play an adhesive role when the diaphragm and the pole piece are hot pressed, and the diaphragm and the pole piece are tightly bonded together. For example, the Chinese invention patent with announcement number CN105552277B uses PVDF microspheres to coat a 0.1-0.5μm PVDF bonding layer on one or both sides of the diaphragm, which improves the bonding between the diaphragm and the pole piece, thereby increasing the hardness of the battery.

[0003] With the EU's introduction of the fluorine ban policy in 2025, non-fluorine adhesives have attracted much attention. The future development trend of lithium battery separators is to use non-fluorine adhesives to replace PVDF, such as PMMA coating. PMMA is an amorphous organic adhesive, generally existing in primary particle size particles, ranging in size from 0.5 to 15μm. Most of the coating of PMMA is mainly roller coating. When small-particle PMMA is coated, the adhesion is higher than that of PVDF coating, and the particle powder shedding performance is good. However, when the gram weight of small-particle PMMA coating is low, the adhesion with the pole piece will be seriously weakened, which will reduce the overall hardness of the battery; when the gram weight of small-particle PMMA coating is high, although the high adhesion performance is guaranteed, the high gram weight coating increases the coating coverage, aggravates the deterioration of the air permeability of the coated separator, thereby increasing the internal resistance and reducing the cycle performance. At the same time, the performance of storing electrolyte is reduced by small-particle PMMA coating. In this regard, the existing technology introduces the coating of large-particle PMMA. Compared with the coating of small-particle PMMA, the coating of large-particle PMMA has a lower overall coverage and improved air permeability at the same coating weight. At the same time, the performance of storing electrolyte is also increased. However, the biggest disadvantage of the large-particle PMMA coating is that the coated diaphragm is prone to powdering. This is because the contact between the large-particle PMMA and the coated base film is a tangent contact. Compared with the small-particle PMMA, the proportion of the contact area with the base film is seriously reduced. Therefore, the coated diaphragm suffers from severe powdering, which often leads to defective soft batteries. Summary of the invention

[0004] The purpose of the present application is to provide a PMMA coated diaphragm and a preparation method and application thereof, so as to solve the problems in the prior art of low liquid storage and poor air permeability of diaphragms coated with small-particle PMMA and powder shedding of large-particle PMMA.

[0005] To achieve the above-mentioned purpose, the embodiment of the present application adopts the following technical scheme: a PMMA-coated diaphragm, comprising: a base film, which is a porous film; a coating, which is arranged on at least one side surface of the base film, and the coating comprises small-particle PMMA and large-particle PMMA, the large-particle PMMA is embedded between the small-particle PMMA, and the large-particle PMMA is formed by demulsifying and agglomerating small-particle PMMA.

[0006] In the above technical scheme, the embodiment of the present application is coated with a mixture of large and small particle sizes of PMMA in a certain proportion, which solves the problem of low liquid storage and high air permeability of small particle size PMMA coating and the problem of powder loss of large particle size PMMA coating, improves the hardness of the battery and the liquid storage performance of the battery, thereby improving the battery cycle performance. Under the coating of the same gram weight of PMMA, due to the high proportion of small particle size PMMA in contact with the base film, it is not easy to lose powder, and the large particle size PMMA is embedded between the small particle size PMMA, and the contact point of the large particle size PMMA enables the large particle size PMMA to be firmly fixed on the surface of the coated diaphragm, and it is not easy to lose powder. In addition, the large particle size PMMA is obtained by demulsification and agglomeration of small particle size PMMA, thereby ensuring the consistency of other properties of large and small particle size PMMA except particle size, thereby reducing some defects at the battery end caused by the difference in the main material, and the large particle size PMMA is formed by demulsification and agglomeration of small particle size PMMA. Compared with the large particle size PMMA of the primary particle size, the proportion of the synthetic monomer coated inside it is greatly reduced, thereby reducing the gas production at the battery end and improving the cycle performance of the battery.

[0007] Further, according to an embodiment of the present application, the number of connected particles of the small-size PMMA is less than or equal to 6, the large-size PMMA particles are not connected to each other, and the number of connected particles of the small-size PMMA and a single large-size PMMA particle is 3 to 10.

[0008] Further, according to an embodiment of the present application, the mass ratio of the small-particle PMMA to the large-particle PMMA is 2:8~4:6.

[0009] Further, according to an embodiment of the present application, the Tg of the small-particle PMMA is 55~70°C.

[0010] Further, according to an embodiment of the present application, the coating also includes an adhesive and a wetting agent.

[0011] Further, according to an embodiment of the present application, the adhesive is one or more of polyacrylate, polyacrylamide, polystyrene, and polyvinyl alcohol.

[0012] Further, according to an embodiment of the present application, the wetting agent is one or more of ethylene oxide, nonylphenol polyoxyethylene ether, and polyoxyethylene polyoxypropylene block copolymer.

[0013] Further, according to an embodiment of the present application, the porous film is a PP film, a PE film, or a PP / PE composite film.

[0014] In order to achieve the above object, the present application also discloses a method for preparing a PMMA-coated diaphragm, comprising the following steps:

[0015] Preparation of large-particle PMMA: Take a small-particle PMMA emulsion of primary particle size and place it in a blender, add sodium chloride solution, stir at high speed to break the emulsion, and after the emulsion is broken, filter it with a 200-mesh filter and take the filtrate for standby use;

[0016] Prepare the coating mixture: mix the filtrate and the small-size PMMA emulsion of the primary particle size, add the adhesive and the wetting agent, and stir evenly;

[0017] Coating: Use micro-gravure coating technology to coat the coating mixture on the surface of the diaphragm, and dry it to obtain a coated diaphragm.

[0018] In order to achieve the above-mentioned purpose, the embodiments of the present application also disclose an application of a PMMA-coated diaphragm in a lithium battery.

[0019] Compared with the prior art, the present application has the following beneficial effects: the present application solves the problem of low liquid storage and high air permeability of small-particle PMMA coating and the problem of powder shedding of large-particle PMMA coating by coating with mixed large-particle PMMA in a certain proportion, improves the hardness of the battery and the liquid storage performance of the battery, thereby improving the battery cycle performance. When the same weight of PMMA is coated, the small-particle PMMA has a higher contact ratio with the base film and is not easy to shed powder. The large-particle PMMA is embedded between the small-particle PMMA. The contact points of the large-particle PMMA and the small-particle PMMA enable the large-particle PMMA to be firmly fixed on the surface of the coated diaphragm and is not easy to shed powder. In addition, large-particle PMMA is obtained by demulsifying and agglomerating small-particle PMMA, thereby ensuring the consistency of other properties of large and small-particle PMMA except particle size, thereby reducing some defects at the battery end caused by differences in the main materials. Large-particle PMMA is formed by demulsifying and agglomerating small-particle PMMA. Compared with large-particle PMMA of primary particle size, the proportion of synthetic monomers coated inside it is greatly reduced, thereby reducing the gas production at the battery end and improving the battery's cycle performance. DETAILED DESCRIPTION

[0020] In order to make the purpose and technical solution of the present invention clearly and completely described, and the advantages more clearly understood, the embodiments of the present invention are further described in detail below. It should be understood that the specific embodiments described herein are part of the embodiments of the present invention, rather than all of the embodiments, and are only used to explain the embodiments of the present invention, and are not used to limit the embodiments of the present invention. All other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0021] In the description of the present invention, it should be noted that the terms "center", "middle", "upper", "lower", "left", "right", "inner", "outer", "top", "bottom", "side", "vertical", "horizontal" and the like indicating directions or positional relationships are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the devices or elements referred to must have a specific direction, be constructed and operated in a specific direction, and therefore cannot be understood as limiting the present invention. In addition, the terms "one", "first", "second", "third", "fourth", "fifth", "sixth" are only used for descriptive purposes and cannot be understood as indicating or implying relative importance.

[0022] In the description of the present invention, it should be noted that, unless otherwise clearly specified and limited, the terms "installed", "connected", and "connected" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or it can be indirectly connected through an intermediate medium, or it can be the internal communication of two components. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0023] For the purpose of simplicity and illustration, the principles of the embodiments are mainly described by reference to examples. In the following description, many specific details are proposed to provide a thorough understanding of the embodiments. However, it is obvious that for those of ordinary skill in the art, these embodiments may not be limited to these specific details in practice. In some instances, known methods and structures are not described in detail to avoid making these embodiments unnecessarily difficult to understand. In addition, all embodiments can be used in combination with each other.

[0024] The present application discloses a PMMA coated diaphragm, comprising a base film and a coating arranged on at least one side of the base film, wherein the base film is a porous film of a PP film, a PE film, or a PP / PE composite film, and the coating comprises small-particle PMMA, large-particle PMMA, an adhesive, and a wetting agent. In this regard, by coating with mixed large-particle PMMA, the problems of low liquid storage and high air permeability of small-particle PMMA coating and powder shedding of large-particle PMMA coating are solved, and the hardness and liquid storage performance of the battery are improved, thereby improving the battery cycle performance.

[0025] Furthermore, in this embodiment, the large-particle PMMA is embedded between the small-particle PMMA. When coated with the same gram weight of PMMA, since the small-particle PMMA has a higher contact ratio with the base film, it is not easy to lose powder. The large-particle PMMA is embedded between the small-particle PMMA. The contact points of the large and small-particle PMMA enable the large-particle PMMA to be firmly fixed on the surface of the coated diaphragm, and it is not easy to lose powder. Specifically, the number of connected particles of small-particle PMMA is less than or equal to 6, and the large-particle PMMA is not connected to each other. The number of connected particles of small-particle PMMA and a single large-particle PMMA is 3 to 10. The coverage rate of small-particle PMMA is 5 to 16.7%, and the number of particles per unit area is 6.35*10 10 ~5.90*10 11 Particles / m 2 The coverage of large-size PMMA is 2.5~5.0%, and the number of particles per unit area is 8.84*10 8 ~3.98*10 9 Particles / m 2 .

[0026] In addition, the large-particle PMMA used in this application is obtained by demulsifying and agglomerating small-particle PMMA, thereby ensuring the consistency of other properties of large-particle PMMA except particle size, thereby reducing some defects at the battery end caused by differences in the main materials. Large-particle PMMA is formed by demulsifying and agglomerating small-particle PMMA. Compared with large-particle PMMA of primary particle size, the proportion of synthetic monomers coated inside it is greatly reduced, thereby reducing the gas production at the battery end and improving the cycle performance of the battery.

[0027] Based on this, the above-mentioned PMMA coated diaphragm is prepared by the following method:

[0028] Preparation of large-particle PMMA: Place a small-particle PMMA emulsion of primary particle size in a blender, add sodium chloride solution, stir at high speed to break the emulsion, filter with a 200-mesh diaphragm after the emulsion is broken, and take the filtrate for standby use;

[0029] Prepare the coating mixture: mix the filtrate and the small-size PMMA emulsion of the primary particle size, add the adhesive and the wetting agent, and stir evenly;

[0030] Coating: Use micro-gravure coating technology to coat the coating mixture on the surface of the diaphragm, and dry it to obtain a coated diaphragm.

[0031] Among them, the particle size of the small-particle PMMA used is 0.6μm~1.0μm, the particle size of the large-particle PMMA is 4μm~6μm, the small-particle PMMA: large-particle PMMA=2:8~4:6, and the Tg (glass transition temperature) of PMMA is 55~70℃. The adhesive used is one or more of polyacrylate, polyacrylamide, polystyrene, and polyvinyl alcohol, accounting for 2%~6% of the total PMMA mass ratio. The wetting agent used is one or more of ethylene oxide, nonylphenol polyoxyethylene ether, and polyoxyethylene polyoxypropylene block copolymer, accounting for 0.1%~1% of the total PMMA mass ratio. The thickness of the base film used is 5μm~16μm. The gram weight of the coating is 0.2g / m 2 .

[0032] The technical scheme and technical effects of the present application are further described below by listing embodiments and comparative examples, but the present application is not limited to these embodiments.

[0033] [Example 1]

[0034] Step 1: Place a small-particle PMMA emulsion with a Tg (glass transition temperature) of 62°C, a solid content of 20%, and a primary particle size of 0.65 μm in a blender, add an appropriate amount of 0.5 Mol / L sodium chloride solution, control the stirring speed and time, and obtain a large-particle PMMA emulsion with a solid content of 20% and a secondary particle size of 5.5 μm. Filter with a 200-mesh filter and take the filtrate for standby use;

[0035] Step 2: Mix 250 parts of the small-sized PMMA of the primary particle size in step 1 and 750 parts of the large-sized PMMA filtrate of the secondary particle size obtained in step 1, add 10 parts of polyacrylate, 1 part of ethylene oxide and 3000 parts of deionized water, stir evenly to obtain a mixed solution;

[0036] Step 3: Use micro-gravure coating technology to coat the mixed solution on the surface of the diaphragm and dry it to obtain a surface density of 0.2g / m 2 of coated diaphragms.

[0037] [Example 2]

[0038] In step 1 of Example 1, small-particle PMMA with a Tg of 65°C, a solid content of 20%, and a primary particle size of 0.80 μm was taken, and the stirring speed and time were adjusted to obtain a large-particle PMMA emulsion with a solid content of 20% and a secondary particle size of 4.8 μm; 260 parts of small-particle PMMA and 740 parts of large-particle PMMA were mixed, and the rest was the same as in Example 1.

[0039] [Example 3]

[0040] In step 1 of Example 1, small-particle PMMA with a Tg of 58°C, a solid content of 20%, and a primary particle size of 0.94 μm was taken, and the stirring speed and time were adjusted to obtain a large-particle PMMA emulsion with a solid content of 20% and a secondary particle size of 5.0 μm; 350 parts of small-particle PMMA and 650 parts of large-particle PMMA were mixed, and the rest was the same as in Example 1.

[0041] [Example 4]

[0042] In step 1 of Example 1, small-particle PMMA with a Tg of 60°C, a solid content of 20%, and a primary particle size of 0.85 μm was taken, and the stirring speed and time were adjusted to obtain a large-particle PMMA emulsion with a solid content of 20% and a secondary particle size of 4.5 μm; 300 parts of small-particle PMMA and 700 parts of large-particle PMMA were mixed, and the rest was the same as in Example 1.

[0043] [Example 5]

[0044] In step 1 of Example 1, small-particle PMMA with a Tg of 68°C, a solid content of 20%, and a primary particle size of 0.75 μm was taken, and the stirring speed and time were adjusted to obtain a large-particle PMMA emulsion with a solid content of 20% and a secondary particle size of 5.2 μm; 380 parts of small-particle PMMA and 620 parts of large-particle PMMA were mixed, and the rest was the same as in Example 1.

[0045] [Example 6]

[0046] In step 1 of Example 1, small-particle PMMA with a Tg of 65°C, a solid content of 20%, and a primary particle size of 0.96 μm was taken, and the stirring speed and time were adjusted to obtain a large-particle PMMA emulsion with a solid content of 20% and a secondary particle size of 4.2 μm; 250 parts of small-particle PMMA and 750 parts of large-particle PMMA were mixed, and the rest was the same as in Example 1.

[0047] [Example 7]

[0048] In step 1 of Example 1, small-particle PMMA with a Tg of 64°C, a solid content of 20%, and a primary particle size of 0.64 μm was taken, and the stirring speed and time were adjusted to obtain a large-particle PMMA emulsion with a solid content of 20% and a secondary particle size of 5.6 μm; 350 parts of small-particle PMMA and 650 parts of large-particle PMMA were mixed, and the rest was the same as in Example 1.

[0049] [Comparative Example 1]

[0050] In step 1 of Example 1, take small-particle PMMA with a Tg of 59°C, a solid content of 20%, and a primary particle size of 0.66 μm, adjust the stirring speed and time, and obtain a large-particle PMMA emulsion with a solid content of 20% and a secondary particle size of 5.0 μm; take 500 parts of small-particle PMMA and 500 parts of large-particle PMMA and mix them, and the rest is the same as Example 1.

[0051] [Comparative Example 2]

[0052] In step 1 of Example 1, small-particle PMMA with a Tg of 61°C, a solid content of 20%, and a primary particle size of 0.82 μm was taken, and the stirring speed and time were adjusted to obtain a large-particle PMMA emulsion with a solid content of 20% and a secondary particle size of 4.7 μm; 120 parts of small-particle PMMA and 880 parts of large-particle PMMA were mixed, and the rest was the same as in Example 1.

[0053] [Comparative Example 3]

[0054] In step 1 of Example 1, small-particle PMMA with a Tg of 63°C, a solid content of 20%, and a primary particle size of 1.50 μm was taken, and the stirring speed and time were adjusted to obtain a large-particle PMMA emulsion with a solid content of 20% and a secondary particle size of 4.5 μm; 320 parts of small-particle PMMA and 680 parts of large-particle PMMA were mixed, and the rest was the same as in Example 1.

[0055] [Comparative Example 4]

[0056] In step 1 of Example 1, small-particle PMMA with a Tg of 64°C, a solid content of 20%, and a primary particle size of 0.84 μm was taken, and the stirring speed and time were adjusted to obtain a large-particle PMMA emulsion with a solid content of 20% and a secondary particle size of 7.5 μm; 350 parts of small-particle PMMA and 650 parts of large-particle PMMA were mixed, and the rest was the same as in Example 1.

[0057] [Comparative Example 5]

[0058] In step 1 of Example 1, small-particle PMMA with a Tg of 60°C, a solid content of 20%, and a primary particle size of 0.50 μm was taken, and the stirring speed and time were adjusted to obtain a large-particle PMMA emulsion with a solid content of 20% and a secondary particle size of 3.8 μm; 390 parts of small-particle PMMA and 610 parts of large-particle PMMA were mixed, and the rest was the same as in Example 1.

[0059] [Comparative Example 6]

[0060] Step 1: Mix 350 parts of small-particle PMMA with a Tg of 63°C, a solid content of 20%, and a primary particle size of 0.68 μm and 750 parts of large-particle PMMA with a Tg of 63°C, a solid content of 20%, and a primary particle size of 5.2 μm, add 10 parts of polyacrylate, 1 part of ethylene oxide and 3000 parts of deionized water, stir evenly, and obtain a mixed solution;

[0061] Step 2: Use micro-gravure coating technology to coat the mixed liquid on the surface of the diaphragm, dry it, and obtain a coated diaphragm with a surface density of 0.2g / m2.

[0062] The microstructure of the coated diaphragm prepared in the above embodiment is observed as follows: The coated diaphragm is taken and the microscopic particle distribution of the coated diaphragm at 1000x is photographed using a scanning electron microscope. The coverage = π*d 2 *n / 4, n=m / ρ (d is the particle size of the corresponding PMMA, n is 1m 2 The number of PMMA particles with corresponding diameters on the diaphragm, m is 1m 2 The PMMA mass corresponding to the diaphragm is g). The observation results are summarized in Table 1.

[0063] Table 1

[0064]

[0065] As shown in Table 1, in Examples 1 to 7, under appropriate ratios and particle sizes, the obtained coated diaphragms were tested under microscopic conditions, and the microstructural properties were all within the requirements of this patent. In Comparative Example 1, the proportion of small-particle PMMA is too high, the maximum number of connected particles and the number of particles connected to a single large-particle PMMA are both increased, and the overall coverage is also increased. In Comparative Example 2, the proportion of large-particle PMMA is too high, and there are too few small-particle PMMA particles, resulting in a decrease in the minimum number of connected particles between small-particle PMMA and a single large-particle PMMA. In Comparative Example 3, the particle size of small-particle PMMA increases, and the overall number of particles decreases, resulting in a decrease in the minimum number of connected particles between small-particle PMMA and a single large-particle PMMA. In Comparative Example 4, the particle size of large-particle PMMA increases, and the overall number of particles decreases, resulting in an increase in the maximum number of connected particles between small-particle PMMA and a single large-particle PMMA. In comparative example 5, the particle sizes of both small-size PMMA and large-size PMMA are reduced, resulting in an increase in the overall number of particles, and the increase in small-size PMMA particles is greater. Therefore, the maximum number of connected small-size PMMA particles and the number of particles connected to a single large-size PMMA particle are increased, and the overall coverage rate is also increased.

[0066] The performance of the prepared coated diaphragm was tested below, mainly testing the air permeability growth rate, the powder loss of the coated diaphragm, the adhesion to the electrode, the expansion coefficient, the ion conductivity performance and the lithium precipitation. The test method is as follows:

[0067] Permeability growth rate: Permeability refers to the time required for 100 ml of gas to pass through a fixed area diaphragm. Permeability growth rate = (coated diaphragm permeability - base membrane permeability) / base membrane permeability * 100%;

[0068] Powder loss of coated diaphragm: Use dye friction color fastness tester RT-300S for testing, take the coated diaphragm and cut it into 30cm*2cm long strip samples, weigh it, and fix the long strip sample with the coating film facing up. Put lens paper on the friction piece and place it on the diaphragm. Start the power supply, rub the friction piece back and forth on the diaphragm 5 times, take out the diaphragm after friction, shake it gently three times, and measure the mass of the diaphragm again. The difference between the two weights divided by the total weight of the strip diaphragm coating is the powder loss rate of the coated diaphragm;

[0069] Adhesion to the electrode: Adhesion between the coated diaphragm and the positive electrode at 3MPa, 3min, 80℃. The positive electrode is composed of 960 parts of lithium iron phosphate + 12 parts of conductive carbon black + 60 parts of PVDF with a melting point of 140℃.

[0070] Expansion coefficient: After the diaphragm and the pole piece are hot-pressed and wound and the electrolyte is injected, the battery thickness expansion coefficient is tested at a high temperature of 65°C and sealed for 20 days. The expansion coefficient = thickness after 20 days of sealing swelling / thickness when sealed;

[0071] Ionic conductivity performance: In an argon glove box, the diaphragm was made into a 2016 button battery, and an appropriate amount of electrolyte (EC:PC:DEC = =1:4:3, 2Mol / L LiPF 6 ), using the AC impedance test in the electrochemical workstation, we can get, σ=L / (Rb*A), where σ is the ionic conductivity (S cm -1 ); L is the thickness of the diaphragm (cm); Rb is the intrinsic resistance of the diaphragm (Ω); A is the effective area (cm 2 );

[0072] Lithium deposition: The battery is cycled 500 times at 0.5C charge and discharge, and then the battery is disassembled to observe the lithium deposition of the battery.

[0073] The test results are summarized in Table 2.

[0074] Table 2

[0075]

[0076] As shown in Table 2, in Examples 1 to 7, at appropriate ratios and particle sizes, the overall air permeability growth rate of the obtained coated diaphragm is relatively small; in contrast, in Comparative Examples 1 and 5, the proportion of small-particle PMMA is high or the particle size of small-particle PMMA is small, resulting in a large coverage rate of small-particle PMMA and aggravated pore blockage of the base membrane, so the overall air permeability growth rate is relatively large.

[0077] The powder loss of the coated diaphragm is related to the large particle size PMMA. The small particle size PMMA and large particle size PMMA in Examples 1 to 7 have good overall powder loss under appropriate particle size and ratio. In Comparative Example 1, the small particle size PMMA accounts for a large proportion, and the large particle size PMMA particle size is within the appropriate range, so the overall powder loss is good; in Comparative Example 2, the large particle size PMMA accounts for a large proportion, and the small particle size PMMA provides few bonding sites, so the powder loss is serious; in Comparative Example 3, the particle size of the small particle size PMMA exceeds the appropriate range, and its own powder loss worsens, so the overall powder loss is serious; in Comparative Example 4, the particle size of the large particle size PMMA exceeds the appropriate range. Although the small particle size PMMA can provide enough bonding sites, the particle size is too large, the overall surface roughness is too large, and the powder loss is aggravated; in Comparative Example 5, the particle size of the small / large particle size PMMA is lower than the appropriate range, and the overall roughness is reduced, so the powder loss is good; Comparative Example 6 is the same as the embodiment.

[0078] In Examples 1 to 7, within the appropriate range of particle size and ratio, the adhesion between the coated diaphragm and the pole piece is relatively good; in Comparative Example 1, the proportion of large-particle PMMA is small, resulting in a decrease in the overall number of large-particle PMMA particles, and a smaller number of bonding sites with the large-particle PMMA pole piece, so the adhesion is reduced; in Comparative Examples 2 to 4, the overall coated diaphragm has serious powder shedding, resulting in reduced adhesion between the diaphragm and the pole piece; in Comparative Examples 5 / 6, the overall proportion of large-particle PMMA is within an appropriate range, and the overall powder shedding of the coated diaphragm is good, so the adhesion with the pole piece is better.

[0079] In Examples 1 to 7, within a reasonable ratio and particle size range, the battery cells made by coating the diaphragm have sufficient adhesion and swelling space, so the battery expansion coefficient is 1.000; in Comparative Examples 1 to 4, the adhesion between the diaphragm and the electrode is reduced, which easily leads to battery cell expansion; in Comparative Example 5, the particle size of large-particle PMMA is smaller than that of Examples 1 to 7, and after hot pressing with the electrode, the swelling space that can be provided is limited, so the battery expansion coefficient is also large; in Comparative Example 6, large-particle PMMA is used. During the synthesis process, many synthetic monomers are coated inside the particles. Large-particle PMMA absorbs electrolyte at the battery end to swell, and the released synthetic monomers are prone to generate gas, so the overall expansion coefficient of the battery is large.

[0080] In Examples 1 to 7, the air permeability and adhesion to the electrode are good within a reasonable ratio and particle size range, so the overall ionic conductivity of the coated diaphragm is good; in Comparative Examples 1 to 5, the coated diaphragm is either too permeable or the overall adhesion to the electrode is insufficient, so the overall ionic conductivity is slightly poor; in Comparative Example 6, large-particle PMMA with a primary particle size is used, which has a greater effect on the penetration of lithium ions, so the overall ionic conductivity of the coated diaphragm is poor.

[0081] In Examples 1 to 7, the overall performance of the diaphragm is good within a reasonable ratio and particle size range, and the large-particle PMMA is formed by demulsification and agglomeration of primary-particle PMMA, so the lithium deposition of the battery is good; in Comparative Example 6, the large-particle PMMA adopts a primary-particle large-particle PMMA, and because it contains more synthetic monomers inside, it is easy to produce gas, which seriously affects the shuttling of lithium ions, so the lithium deposition of the battery is serious; the coated diaphragms in other comparative examples are either too air permeable or insufficiently bonded to the pole piece, so the overall lithium deposition of the battery is also serious.

[0082] Although the above describes the illustrative specific implementation methods of the present application so that technicians in this technical field can understand the present application, the present application is not limited to the scope of the specific implementation methods. For ordinary technicians in this technical field, as long as various changes are within the spirit and scope of the present application defined and determined by the attached claims, all application creations using the concept of the present application are protected.

Claims

1. A PMMA coated diaphragm, characterized in that: include: A basement membrane, wherein the basement membrane is a porous film; A coating, wherein the coating is disposed on at least one side of the base film, the coating comprises small-diameter PMMA and large-diameter PMMA, the large-diameter PMMA is embedded between the small-diameter PMMA, and the large-diameter PMMA is formed by demulsification and agglomeration of small-diameter PMMA; The particle size of the small-size PMMA used is 0.6 μm~1.0 μm, the particle size of the large-size PMMA obtained is 4 μm~6 μm, and the mass ratio of the small-size PMMA to the large-size PMMA is 2:8~4:6; The number of connected particles of the small-size PMMA is less than or equal to 6, the large-size PMMA particles are not connected to each other, and the number of connected particles of the small-size PMMA and a single large-size PMMA particle is 3 to 10; The small-particle PMMA is a small-particle PMMA with a primary particle size.

2. A PMMA coated diaphragm according to claim 1, characterized in that: The Tg of the small-particle PMMA is 55-70°C.

3. A PMMA coated diaphragm according to claim 1, characterized in that: The coating also includes a binder and a wetting agent.

4. A PMMA coated diaphragm according to claim 3, characterized in that: The adhesive is one or more of polyacrylate, polyacrylamide, polystyrene and polyvinyl alcohol.

5. A PMMA coated diaphragm according to claim 3, characterized in that: The wetting agent is one or more of ethylene oxide, nonylphenol polyoxyethylene ether, and polyoxyethylene polyoxypropylene block copolymer.

6. A PMMA coated diaphragm according to claim 1, characterized in that: The porous film is a PP film, a PE film, or a PP / PE composite film.

7. A method for preparing a PMMA coated diaphragm as claimed in claim 1, characterized in that: The following steps are involved: Preparation of large-particle PMMA: Take a small-particle PMMA emulsion of primary particle size and place it in a blender, add sodium chloride solution, stir at high speed to break the emulsion, and after the emulsion is broken, filter it with a 200-mesh filter and take the filtrate for standby use; Prepare the coating mixture: mix the filtrate and the small-particle PMMA emulsion of the primary particle size, add the adhesive and the wetting agent, and stir evenly; Coating: Use micro-gravure coating technology to coat the coating mixture on the surface of the diaphragm, and dry it to obtain a coated diaphragm.

8. Use of the PMMA coated diaphragm according to any one of claims 1 to 6 or the PMMA coated diaphragm prepared by the preparation method of the PMMA coated diaphragm according to claim 7 in a lithium battery.

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