A high wettability coating separator and a method of making the same

Abstract

The application provides a high-wetting coating diaphragm and a preparation method thereof. The high-wetting coating diaphragm comprises a base film and a high-wetting coating arranged on at least one side of the base film, wherein the high-wetting coating comprises a polyvinylidene fluoride-hexafluoropropylene copolymer, nanosulfonated cellulose and fumed silica. The preparation method of the high-wetting coating diaphragm comprises the following steps: carrying out corona treatment on the surface of the base film, coating a slurry of the high-wetting coating on at least one surface of the base film, and carrying out deionized water extraction and gradient drying to obtain the high-wetting coating diaphragm. The high-wetting coating diaphragm and the preparation method thereof can effectively solve the problem of insufficient electrolyte wetting performance of the diaphragm in the prior art, which leads to the performance damage of the battery.

Description

Technical Field

[0001] This invention relates to the field of battery separator coatings, and more specifically to a highly wettable coated separator and its preparation method. Background Technology

[0002] The energy density, cycle life, and safety characteristics of lithium-ion batteries are closely related to the characteristics of the battery's internal interface. As a key internal component of the battery, the separator's core function is to physically isolate the positive and negative electrodes and provide a conduction channel for lithium ions. Its wettability to the electrolyte directly affects the battery's ionic conductivity, internal resistance, and capacity.

[0003] Currently, commercial lithium-ion battery separators are mainly polyolefin-based membranes, such as polyethylene (PE) and polypropylene (PP) membranes. These membranes are widely used due to their excellent chemical stability and mechanical properties. However, polyolefins are non-polar materials with low surface energy and poor affinity with polar electrolytes. This can easily lead to insufficient electrolyte wetting and uneven distribution, which in turn increases the battery interface impedance and causes uneven lithium deposition and lithium dendrite growth, seriously damaging the battery's cycle performance and safety performance.

[0004] To improve the electrolyte wettability of polyolefin separators, various methods have been developed in the prior art, such as hydrophilic coating, surface grafting modification, and plasma treatment. However, all of these methods have obvious drawbacks: hydrophilic coating complicates the process and increases manufacturing costs; surface grafting modification and plasma treatment can easily damage the mechanical properties and thermal stability of the separator, leading to an increase in the thermal shrinkage rate of the separator; and the improvement effect of some modification methods is not durable enough, and the wettability of the separator will rapidly decay during long-term battery use.

[0005] Therefore, developing a modification method that is simple to process, cost-controllable, does not damage the properties of the separator, and can significantly improve the electrolyte wetting rate and uniformity of the separator is of great industrial value and application significance for promoting the development of high-performance lithium-ion batteries. Summary of the Invention

[0006] In view of this, the present invention provides a highly wettable coated separator and its preparation method to solve the problem that the electrolyte wetting performance of traditional PP or PE base films is insufficient, which leads to damage to battery performance.

[0007] To achieve the above objectives, the present invention provides a highly wettable coated membrane, comprising a base membrane and a highly wettable coating disposed on at least one side of the base membrane, wherein the highly wettable coating comprises polyvinylidene fluoride-hexafluoropropylene copolymer, nano-sulfonated cellulose, and fumed silica.

[0008] In one embodiment, the base film is a PE base film or a PP base film.

[0009] In one embodiment, the thickness of the base film is 3-16 μm.

[0010] As one embodiment, the highly wettable coating satisfies at least one of the following conditions:

[0011] (1) The molecular weight of the polyvinylidene fluoride-hexafluoropropylene copolymer is 300,000 to 700,000;

[0012] (2) The sulfonated degree of the nano-sulfonated cellulose is ≥90%, the diameter is 30-70nm, and the length is ≥5μm;

[0013] (3) The thickness of the highly wettable coating is 0.5-2 μm.

[0014] As one embodiment, the highly wettable coating also includes cellulose ether derivatives.

[0015] As one embodiment, the method for preparing the slurry of the highly wettable coating includes the following steps:

[0016] S1. Polyvinylidene fluoride-hexafluoropropylene copolymer is added to a solvent, and after a first heat treatment and a first dispersion, a first mixed solution is obtained;

[0017] S2. Add nano-sulfonated cellulose to the first mixed solution, and then perform a second heat treatment and a second dispersion to obtain a second mixed solution;

[0018] S3. Add fumed silica to the second mixed solution, and after a third heat treatment and a third dispersion, obtain the slurry of the highly wettable coating;

[0019] The mass ratio of the polyvinylidene fluoride-hexafluoropropylene copolymer to the solvent is (5-7):(70-80).

[0020] The mass ratio of the nano-sulfonated cellulose to the first mixed solution is (0.6-1.3):(65-68).

[0021] The mass ratio of the fumed silica to the second mixed solution is (2.6-3.7):(60-65).

[0022] As one embodiment, the method for preparing the slurry of the highly wettable coating also satisfies at least one of the following conditions:

[0023] (1) The solvent includes one or more of dimethyl carbonate, ethylene carbonate, methyl ethyl carbonate, diethyl carbonate and propylene carbonate;

[0024] (2) The temperature of the first heat treatment is 50±5℃;

[0025] (3) The rotation speed of the first dispersion is 2000±500r / min, and the dispersion time is 120±10min;

[0026] (4) The temperature of the second heat treatment is 30±5℃;

[0027] (5) The rotation speed of the second dispersion is 1500±200r / min, and the dispersion time is 360±30min;

[0028] (6) The temperature of the third heat treatment is 25±5℃;

[0029] (7) The rotation speed of the third dispersion is 800±100r / min, and the time of the third dispersion is 30±5min.

[0030] To achieve the above objectives, the present invention provides a method for preparing a highly wettable coated diaphragm, comprising: subjecting a base membrane surface to corona treatment, applying a highly wettable coating slurry to at least one surface of the base membrane, and extracting with deionized water and gradient drying to obtain the highly wettable coated diaphragm.

[0031] As one embodiment, the method for preparing the highly wettable coated diaphragm also satisfies at least one of the following conditions:

[0032] (1) The parameters of the corona treatment are 7-11 W / m² / min;

[0033] (2) The coating speed is 5-10 m / min;

[0034] (3) The gradient drying is carried out in a stepped oven, and the three temperature ranges of the stepped oven along the transport direction of the base film are 45±5℃, 60±5℃ and 50±5℃ respectively.

[0035] The beneficial effects of this invention are as follows:

[0036] The high wettability coated diaphragm and its preparation method provided by this invention are simple to operate, have significant effects, and have no negative impact on the diaphragm's properties. By coating one or both sides of a PP or PE polyolefin-based membrane with an electrolyte-loving coating (high wettability coating) formed by a composite of polyvinylidene fluoride-hexafluoropropylene copolymer, nano-sulfonated cellulose (CNC), and fumed silica, combined with a process of extremely mild corona treatment of the polyolefin-based membrane, quantitative transfer roller coating, deionized water extraction, and gradient drying, the contact angle of the diaphragm to the electrolyte can be significantly reduced, the droplet penetration time can be greatly shortened, and the electrolyte concentration can be increased. Complete wetting within seconds allows for a more uniform distribution of the electrolyte within the membrane pores, resulting in an electrolyte ramp-up rate and absorption rate nearly twice that of traditional modified membranes within 1 hour. This effectively solves the problem of insufficient electrolyte wetting caused by the low surface energy and non-polar properties of traditional polyolefin-based membranes. The membrane of this invention has excellent electrolyte wettability and ensures that the electrolyte fully fills the membrane pores, significantly reducing ion migration resistance. This, in turn, reduces the overall internal resistance and interfacial impedance of the battery, which is beneficial for improving the battery's ionic conductivity and capacity utilization, and avoids problems such as uneven lithium deposition caused by uneven wetting.

[0037] In summary, compared with the prior art, the slurry of the high wettability coating used in this invention has better adhesion to PE or PP diaphragms, the diaphragm of this invention has good electrolyte wettability and uniformity, and the high wettability coating on the surface of the diaphragm of this invention improves the thermal shrinkage of the diaphragm to a certain extent. Attached Figure Description

[0038] Figure 1 This is a flowchart illustrating the preparation process of the highly wettable coated diaphragm of the present invention. Detailed Implementation

[0039] It should be noted that, unless otherwise specified, the embodiments and features described in the present invention can be combined with each other.

[0040] Unless otherwise defined, the technical or scientific terms used in this invention shall have the ordinary meaning as understood by one of ordinary skill in the art to which this invention pertains.

[0041] In this invention, "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Therefore, a feature defined with "first" and "second" may explicitly or implicitly include at least one of that feature. In the description of this application, "multiple" means at least two, such as two, three, etc., unless otherwise explicitly specified.

[0042] When a numerical range is disclosed herein, the range is considered continuous and includes the minimum and maximum values ​​of the range, as well as every value between the minimum and maximum values. Furthermore, when the range refers to integers, it includes every integer between the minimum and maximum values ​​of the range. Additionally, when multiple ranges are provided to describe a feature or characteristic, the ranges may be combined. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all subranges to which they are incorporated.

[0043] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the application.

[0044] As analyzed in the background section of this invention, traditional PP or PE base membranes have insufficient electrolyte wetting performance. To solve the above problems, this invention provides a highly wettable coated diaphragm and its preparation method.

[0045] In one embodiment of the present invention, a highly wettable coated membrane includes a base membrane and a highly wettable coating disposed on at least one side of the base membrane, wherein the highly wettable coating includes polyvinylidene fluoride-hexafluoropropylene copolymer, nano-sulfonated cellulose, and fumed silica.

[0046] Specifically, in the technical solution of this invention, polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP) has good film-forming properties, good adhesion to polyolefin-based films, and good affinity for electrolytes. It can form a continuous polymer skeleton to achieve uniform encapsulation and fixation of the other two components. Nano-sulfonated cellulose (CNC) is a strongly hydrophilic functional component. Its sulfonated polar groups significantly enhance the overall polarity of the coating, and its nanoscale aspect ratio structure can be interspersed in the PVDF-HFP polymer skeleton to form an interconnected continuous nano-network structure, providing native capillary channels for electrolyte diffusion. Fumed silica is a pore regulation and channel expansion component. Its nanoscale porous particles can fill the gaps in the network formed by PVDF-HFP and CNC, constructing a multi-level porous structure without destroying the original capillary channels. It is interconnected with the capillary channels of CNC and forms a complementary structure, significantly increasing the specific surface area of ​​the coating and the electrolyte adsorption sites. At the same time, it further broadens the electrolyte diffusion path and enhances the electrolyte adsorption and penetration capabilities. Together, these three components form a synergistic liquid absorption system. PVDF-HFP ensures the film-forming properties and structural stability of the coating, CNC provides the core hydrophilicity and basic diffusion channels, and fumed silica optimizes the pore structure and improves diffusion efficiency. The three components support and synergize with each other, resulting in a significant improvement in the wetting rate and uniformity of the diaphragm electrolyte.

[0047] In one embodiment of the present invention, the base film is a PE base film or a PP base film.

[0048] Specifically, the base film is a PE or PP polyolefin base film. PE or PP is the mainstream substrate for commercial lithium-ion battery separators, possessing excellent chemical stability, mechanical properties, and porosity, and is inexpensive, thus exhibiting good compatibility with the high wettability coating of this invention. If other base film materials (such as cellulose or non-woven fabric) are selected, there may be problems such as insufficient mechanical strength, excessive cost, or poor adhesion to the coating, resulting in situations where it cannot meet the needs of industrial production and battery applications.

[0049] In one embodiment, the thickness of the base film is 3-16 μm.

[0050] Specifically, when the thickness of the base film is less than 3 μm, the mechanical strength of the base film is insufficient, and it is easily damaged during coating and battery assembly, which may cause a short circuit in the battery. When the thickness is greater than 16 μm, the pore channels of the base film are too long, which increases the resistance to lithium ion migration, reduces the battery ion conductivity, and reduces the loading space of the battery electrode, thus affecting the battery energy density.

[0051] In one embodiment of the present invention, the highly wettable coating satisfies at least one of the following conditions:

[0052] (1) The molecular weight of the polyvinylidene fluoride-hexafluoropropylene copolymer is 300,000 to 700,000;

[0053] (2) The sulfonated degree of the nano-sulfonated cellulose is ≥90%, the diameter is 30-70nm, and the length is ≥5μm;

[0054] (3) The thickness of the highly wettable coating is 0.5-2 μm.

[0055] Specifically, when the molecular weight of the polyvinylidene fluoride-hexafluoropropylene copolymer is less than 300,000, the copolymer has poor film-forming properties, insufficient mechanical strength of the coating, and is prone to peeling off during battery cycling; when the molecular weight of the polyvinylidene fluoride-hexafluoropropylene copolymer is greater than 700,000, the viscosity of the copolymer increases significantly, making it difficult to fully dissolve in solvents, resulting in uneven dispersion of the coating slurry, and defects such as pinholes and material shortages in the coating after coating, affecting wettability and the separation performance of the separator; the molecular weight of the polyvinylidene fluoride-hexafluoropropylene copolymer between 300,000 and 700,000 balances the copolymer's solubility, film-forming properties, and coating mechanical properties.

[0056] When the degree of sulfonation of the nano-sulfonated cellulose is less than 90%, the number of polar groups on the surface of the nano-sulfonated cellulose is insufficient, the electrophilic properties decrease, and the coating wettability cannot be effectively improved; when its diameter is greater than 70 nm or its length is less than 5 μm, the nano-sulfonated cellulose cannot form a continuous nano-network structure, the capillary effect is weakened, and the electrolyte diffusion rate is reduced; when the diameter is less than 30 nm, the nano-sulfonated cellulose is prone to agglomeration, resulting in uneven slurry dispersion and uneven coating performance after coating; the parameter combination of sulfonation degree ≥90%, diameter 30-70 nm, and length ≥5 μm ensures the hydrophilicity and network structure construction ability of the nano-sulfonated cellulose.

[0057] When the thickness of the highly wettable coating is less than 0.5 μm, the coating cannot form a continuous hydrophilic network, resulting in limited improvement in wettability and insufficient support for the base film. When the thickness of the highly wettable coating is greater than 2 μm, it will prolong the transport path of lithium ions in the separator, increase the battery interface impedance, and reduce the flexibility of the separator, making it prone to coating cracking and peeling. A thickness of 0.5-2 μm ensures the improvement in wettability while avoiding a significant decrease in the ion transport performance and mechanical properties of the separator.

[0058] In one embodiment of the present invention, the highly wettable coating further includes cellulose ether derivatives.

[0059] Specifically, polyvinylidene fluoride-hexafluoropropylene copolymer can be partially replaced with cellulose ether derivatives. Cellulose ether derivatives are environmentally friendly polymer materials with lower costs and similar hydrophilicity and adhesion to PVDF-HFP. Moreover, the core structure of the coating is not changed after partial replacement. Cellulose ether derivatives can form a stable hydrophilic network with nano-sulfonated cellulose and fumed silica, which can still achieve the effect of improving the wettability of the membrane and adapt to different raw material supply and production needs.

[0060] In one embodiment of the present invention, the method for preparing the slurry of the highly wettable coating includes the following steps (see [reference needed]). Figure 1 ):

[0061] S1. Polyvinylidene fluoride-hexafluoropropylene copolymer is added to a solvent, and after a first heat treatment and a first dispersion, a first mixed solution is obtained;

[0062] S2. Add nano-sulfonated cellulose to the first mixed solution, and then perform a second heat treatment and a second dispersion to obtain a second mixed solution;

[0063] S3. Add fumed silica to the second mixed solution, and after a third heat treatment and a third dispersion, obtain the slurry of the highly wettable coating;

[0064] The mass ratio of the polyvinylidene fluoride-hexafluoropropylene copolymer to the solvent is (5-7):(70-80).

[0065] The mass ratio of the nano-sulfonated cellulose to the first mixed solution is (0.6-1.3):(65-68).

[0066] The mass ratio of the fumed silica to the second mixed solution is (2.6-3.7):(60-65).

[0067] Specifically, this invention is designed based on the dispersion characteristics of each component, using PVDF-HFP, nano-sulfonated cellulose, and fumed silica added in sequence. First, PVDF-HFP is fully dissolved to form a continuous polymer matrix. Then, nano-sulfonated cellulose is added to uniformly disperse in the matrix to form a hydrophilic network. Finally, fumed silica is added to regulate the pore structure. If the order is reversed (e.g., adding fumed silica or nano-sulfonated cellulose first), it is easy to cause the nano-components to agglomerate, the polymer matrix cannot encapsulate the nanoparticles, the slurry is unevenly dispersed, and the wettability and mechanical properties of the final coating are greatly reduced.

[0068] Meanwhile, if the mass ratio of PVDF-HFP to solvent is too small, the solid content of the slurry will be too low, and a continuous coating may not be formed after coating; if the mass ratio of PVDF-HFP to solvent is too large, the viscosity of the slurry will be too high, and agglomeration will easily occur during dispersion, and defects such as sagging and accumulation will easily occur during coating.

[0069] If the mass ratio of nano-sulfonated cellulose to the first mixed solution is too small, there will be insufficient hydrophilic groups, resulting in limited improvement in coating wettability; if the mass ratio of nano-sulfonated cellulose to the first mixed solution is too large, the slurry is prone to gelation, leading to difficulty in dispersion and poor coating, and excessive nano-sulfonated cellulose may reduce the mechanical strength of the coating.

[0070] If the mass ratio of fumed silica to the second mixed solution is too small, the porous structure of the coating will be insufficient, and the electrolyte adsorption and permeation capacity will be limited. If the mass ratio of fumed silica to the second mixed solution is too high, the density of the coating will decrease, the adhesion to the base film will deteriorate, and it will easily block the original pores of the diaphragm, increasing the resistance to ion transport.

[0071] In one embodiment of the present invention, the method for preparing the slurry of the highly wettable coating further satisfies at least one of the following conditions:

[0072] (1) The solvent includes one or more of dimethyl carbonate, ethylene carbonate, methyl ethyl carbonate, diethyl carbonate and propylene carbonate;

[0073] (2) The temperature of the first heat treatment is 50±5℃;

[0074] (3) The rotation speed of the first dispersion is 2000±500r / min, and the dispersion time is 120±10min;

[0075] (4) The temperature of the second heat treatment is 30±5℃;

[0076] (5) The rotation speed of the second dispersion is 1500±200r / min, and the dispersion time is 360±30min;

[0077] (6) The temperature of the third heat treatment is 25±5℃;

[0078] (7) The rotation speed of the third dispersion is 800±100r / min, and the time of the third dispersion is 30±5min.

[0079] Specifically, dimethyl carbonate is preferred as the solvent. Dimethyl carbonate has excellent compatibility with PVDF-HFP, which can fully dissolve the copolymer. Moreover, its boiling point is moderate, and it is easy to volatilize without residue during the subsequent drying process, so as not to have a negative impact on battery performance.

[0080] The optimal dissolution temperature for PVDF-HFP is 50±5℃ during the first heat treatment. This ensures that the solvent does not evaporate too quickly, thus affecting the dispersion effect, and also minimizes the presence of undissolved particles in the slurry.

[0081] The high speed and long dispersion time of the first dispersion stage allow PVDF-HFP to fully dissolve and disperse in the solvent, forming a uniform polymer solution, which lays the foundation for subsequent coating film formation. If the speed is too low or the time is too short, the dissolution will be insufficient and undissolved substances will remain.

[0082] The optimal dispersion temperature for the second heat treatment, 30±5℃, is the optimal temperature for nano-sulfonated cellulose. This temperature will not cause the nano-sulfonated cellulose to agglomerate, nor will it reduce its dispersion activity and cause uneven dispersion.

[0083] The medium speed and long dispersion time of the second dispersion method can make the nano-sulfonated cellulose uniformly dispersed in the polymer matrix, avoid agglomeration, and form a continuous nano network. If the speed is too high, the cellulose molecular chain will be easily broken, and if the speed is too low or the time is too short, the dispersion will be uneven.

[0084] The optimal dispersion temperature for fumed silica is 25±5℃ for the third heat treatment. The low temperature can prevent the solvent from evaporating rapidly and at the same time prevent the hydrophilic network formed in the previous step from being destroyed, ensuring that the fumed silica is uniformly dispersed in the coating.

[0085] The low speed and short dispersion time of the third dispersion method can ensure uniform dispersion of fumed silica without damaging the already formed PVDF-HFP and CNC network structure.

[0086] To achieve the above objectives, the present invention provides a method for preparing a highly wettable coated diaphragm, comprising: subjecting a base membrane surface to corona treatment, applying a highly wettable coating slurry to at least one surface of the base membrane, and extracting with deionized water and gradient drying to obtain the highly wettable coated diaphragm.

[0087] Specifically, deionized water extraction can remove residual solvents and insufficiently dispersed small molecule impurities from the coating, preventing residual solvents from reacting with the electrolyte in the battery. At the same time, it optimizes the pore structure of the coating and improves electrolyte wettability. If extraction is not performed, residual impurities will lead to increased battery interfacial impedance and affect battery performance.

[0088] Gradient drying using a stepped oven replaces direct high-temperature drying, avoiding coating cracking and blistering caused by rapid solvent evaporation, while also preventing thermal shrinkage of the base film due to sudden heating, thus ensuring the structural integrity and performance uniformity of the diaphragm.

[0089] In one embodiment of the present invention, the method for preparing the highly wettable coated diaphragm further satisfies at least one of the following conditions:

[0090] (1) The parameters of the corona treatment are 7-11 W / m² / min;

[0091] (2) The coating speed is 5-10 m / min;

[0092] (3) The gradient drying is carried out in a stepped oven, and the three temperature ranges of the stepped oven along the transport direction of the base film are 45±5℃, 60±5℃ and 50±5℃ respectively.

[0093] Specifically, the corona treatment of the present invention differs from conventional strong corona treatment. It only moderately increases the polar groups on the surface of the base film by using a parameter of 7-11 W / m² / min, thereby improving the adhesion between the base film and the highly wettable coating and reducing the interfacial impedance. If corona treatment is not performed, the surface of the base film is non-polar, resulting in poor adhesion to the coating and easy coating detachment. If the corona treatment is too strong, it will damage the bulk structure and mechanical properties of the base film, leading to an increase in the thermal shrinkage rate of the base film.

[0094] Regarding the gradient drying process, the temperature of the first zone is 45±5℃, which can initially dry the deionized water in the coating and slowly evaporate the moisture to avoid rapid moisture loss that could cause pinholes and blistering in the coating. The temperature of the second zone is 60±5℃, which can remove residual organic solvents in the coating at high temperature to ensure that there are no solvent residues. The temperature of the third zone is 50±5℃, which can perform low-temperature shaping of the coating to improve the bonding strength between the coating and the base film, while preventing the diaphragm from warping due to sudden cooling after high temperature, thus ensuring the flatness and structural stability of the diaphragm. Changing the order of the temperature zones or deviating from the temperature range can lead to problems such as coating cracking, solvent residue, and thermal shrinkage of the base film.

[0095] In summary, the high wettability coated separator and its preparation method provided by the present invention can effectively solve the problem of insufficient electrolyte wetting performance of separators in the prior art, which leads to damage to battery performance.

[0096] The present invention will be further described in detail below with reference to specific embodiments, which should not be construed as limiting the scope of protection claimed by the present invention. Furthermore, in the following embodiments, unless otherwise specified, the reagents and equipment used are commercially available. If specific processing conditions and methods are not explicitly described in the later embodiments, conditions and methods known in the art can be used for processing.

[0097] The technical solution of the present invention will be further described below with reference to specific embodiments.

[0098] Note: This invention takes single-sided coating of 9μm PE base film as an example. This invention is applicable to single-sided / double-sided coating using 3-20μm PP or PE base film. This solution does not have any particular limitation on the dispersant, which can be any dispersant known in the prior art, as long as it can achieve the purpose of this application.

[0099] Example 1

[0100] Slurry for preparing highly wettable coatings:

[0101] S1. Take 5.2 parts by mass of polyvinylidene fluoride-hexafluoropropylene copolymer and add it to 72.7 parts by mass of dimethyl carbonate. Mix with a high-speed dispersant for 123 min at a dispersion speed of 2016 r / min. Maintain the mixing temperature at 49.8℃ during the mixing process to obtain the first mixed solution.

[0102] S2. Take 0.64 parts by weight of CNC powder and add it to 66 parts by weight of the first mixed solution. Mix it for 368 minutes using a high-speed disperser at a speed of 1530 r / min and maintain the mixing temperature at 32℃ to obtain the second solution.

[0103] S3. Take 2.75 parts by weight of fumed silica powder and add it to 63.8 parts by weight of the second solution. Mix with a high-speed dispersant for 28 minutes at a dispersing speed of 806 r / min. Maintain the mixing temperature at 23.4℃ during the mixing process to obtain a slurry with a high wettability coating.

[0104] Preparation of highly wettable coated diaphragms:

[0105] A1. Substrate corona treatment: The PE substrate film is subjected to a very slight corona treatment with a corona treatment parameter of 7.6 W / m² / min.

[0106] A2. Slurry Coating: The high wettability coating slurry from S3 is coated onto one side of the PE base film using a micro-metering anilox roller at a speed of 5.8 m / min. After coating, deionized water is used for extraction. After extraction, the film is dried in a stepped oven with three temperature zones: 43.6℃, ​​60.5℃, and 51.6℃. After drying, a PE base film with a high wettability coating thickness of 1.2 μm is obtained, which is the high wettability coated diaphragm.

[0107] The resulting highly wettable coated diaphragm is then wound up normally for storage.

[0108] Example 2

[0109] Slurry for preparing highly wettable coatings:

[0110] S1. Take 5.9 parts by weight of polyvinylidene fluoride-hexafluoropropylene copolymer and add it to 75.4 parts by weight of dimethyl carbonate. Mix with a high-speed dispersant for 118 min at a dispersion speed of 1997 r / min. Maintain the mixing temperature at 52.3℃ during the mixing process to obtain the first mixed solution.

[0111] S2. Take 0.69 parts by weight of CNC powder and add it to 67.2 parts by weight of the first mixed solution. Mix it for 372 minutes using a high-speed disperser at a speed of 1510 r / min and maintain the mixing temperature at 30.3℃ to obtain the second solution.

[0112] S3. Take 2.94 parts by weight of fumed silica powder and add it to 61.3 parts by weight of the second solution. Mix with a high-speed dispersant for 30 minutes at a dispersion speed of 824 r / min. Maintain the mixing temperature at 24.3℃ during the mixing process to obtain a slurry with a high wettability coating.

[0113] A1. Substrate corona treatment: The PE substrate film is subjected to a very slight corona treatment with a corona treatment parameter of 7.9 W / m² / min.

[0114] A2. Slurry Coating: The high wettability coating slurry from S3 is coated onto one side of the PE base film using a micro-metering anilox roller at a speed of 6.9 m / min. After coating, deionized water is used for extraction. After extraction, the film is dried in a stepped oven with three temperature zones: 45.8℃, 59.4℃, and 49.5℃. After drying, a PE base film with a high wettability coating thickness of 1.4 μm is obtained, which is the high wettability coated diaphragm.

[0115] The resulting highly wettable coated diaphragm is then wound up normally for storage.

[0116] Example 3

[0117] Slurry for preparing highly wettable coatings:

[0118] S1. Take 6.4 parts by mass of polyvinylidene fluoride-hexafluoropropylene copolymer and add it to 77.4 parts by mass of dimethyl carbonate. Mix with a high-speed dispersant for 124 min at a dispersion speed of 2042 r / min. Maintain the mixing temperature at 50.1℃ during the mixing process to obtain the first mixed solution.

[0119] S2. Take 0.73 parts by weight of CNC powder and add it to 66.7 parts by weight of the first mixed solution. Mix it for 357 minutes using a high-speed disperser at a speed of 1620 r / min and maintain the mixing temperature at 32.4℃ to obtain the second solution.

[0120] S3. Take 3.4 parts by mass of fumed silica powder and add it to 64.1 parts by mass of the second solution. Mix with a high-speed dispersant for 31 min at a dispersing speed of 843 r / min. Maintain the mixing temperature at 25.6℃ during the mixing process to obtain a slurry with a high wettability coating.

[0121] A1. Substrate corona treatment: The PE substrate film is subjected to very slight corona treatment with a corona treatment parameter of 8.4 W / m² / min.

[0122] A2. Slurry Coating: The high wettability coating slurry from S3 is coated onto one side of the PE base film using a micro-metering anilox roller at a speed of 7.4 m / min. After coating, deionized water is used for extraction. After extraction, the film is dried in a stepped oven with three temperature zones: 46.2℃, 61.3℃, and 48.7℃. After drying, a PE base film with a high wettability coating thickness of 1.1 μm is obtained, which is the high wettability coated diaphragm.

[0123] The resulting highly wettable coated diaphragm is then wound up normally for storage.

[0124] Comparative Example 1

[0125] Slurry for preparing ceramic coatings:

[0126] Take 35 parts by weight of alumina powder, add 35 parts by weight of ultrapure water and 0.9 parts by weight of dispersant, stir and disperse for 60 min, add 2.2 parts by weight of pore-forming agent under ultrasonic mixing conditions, continue stirring and dispersing for 30 min, add 3.3 parts by weight of binder and 0.1 parts by weight of wetting agent, continue ultrasonic mixing for 15 min to obtain ceramic coating slurry; wherein, ultrasonic mixing is performed in a planetary mixer under vacuum conditions, the planetary mixer's rotation speed is 2000 r / min, the revolution speed is 40 r / min, and the ultrasonic frequency is 6 kHz;

[0127] The ceramic coating slurry was applied to one side of the PE base film using a micro metering anilox roller at a speed of 7.1 m / min. After coating, the film was dried in a stepped oven with three temperature zones: 45.1℃, 60.3℃, and 49.8℃. After drying, a PE base film with a ceramic coating thickness of 2.1 μm was obtained.

[0128] The obtained PE base film with ceramic coating is wound up normally for storage.

[0129] Comparative Example 2

[0130] Slurry for preparing ceramic coatings:

[0131] Take 35 parts by weight of alumina powder, add 35 parts by weight of ultrapure water, 0.9 parts by weight of dispersant and 0.76 parts by weight of CNC powder, and disperse by stirring for 60 minutes. Under ultrasonic mixing conditions, add 2.2 parts by weight of pore-forming agent and continue stirring and dispersing for 30 minutes. Add 3.3 parts by weight of binder and 0.1 parts by weight of wetting agent, and continue ultrasonic mixing for 15 minutes to obtain a ceramic coating slurry. The ultrasonic mixing is performed in a planetary mixer under vacuum conditions. The planetary mixer has a rotation speed of 2000 r / min, a revolution speed of 40 r / min, and an ultrasonic frequency of 6 kHz.

[0132] The ceramic coating slurry was applied to one side of the PE base film using a micro-metering anilox roller at a speed of 6.5 m / min. After coating, the film was dried in a stepped oven with three temperature zones: 45.1℃, 60.3℃, and 49.8℃. After drying, a PE base film with a ceramic coating thickness of 1.9 μm was obtained.

[0133] The obtained PE base film with ceramic coating is wound up normally for storage.

[0134] Comparative Example 3

[0135] The difference from Comparative Example 1 is that, based on Comparative Example 1, the PE base film was subjected to a very slight corona treatment with a corona treatment parameter of 8.6 W / m² / min; at the same time, the coating speed was adjusted to 6.7 m / min; finally, a PE base film with a ceramic coating thickness of 2.0 μm was obtained.

[0136] The obtained PE base film with ceramic coating is wound up normally for storage.

[0137] Comparative Example 4

[0138] The difference from Comparative Example 2 is that, based on Comparative Example 2, the PE base film was subjected to a very slight corona treatment with a corona treatment parameter of 7.4 W / m² / min; at the same time, the coating speed was adjusted to 7.3 m / min; and finally, a PE base film with a ceramic coating thickness of 1.8 μm was obtained.

[0139] The obtained PE base film with ceramic coating is wound up normally for storage.

[0140] Implementation effect

[0141] The highly wettable coated membranes obtained in Examples 1-3 and Comparative Examples 1-4 were tested, including:

[0142] Appearance test: Visually inspect the surface of the battery separator to confirm its color and integrity, and determine whether there are any obvious defects, contamination, discoloration or unevenness.

[0143] Thickness test: Use a precision thickness gauge (caliper) to measure multiple points on each sample, and take the average value to obtain the thickness of the battery separator;

[0144] Areal density test: Use a balance to measure the mass of the sample, then measure the area of ​​the sample, and calculate the areal density according to the formula: areal density = sample mass / sample area;

[0145] Breakdown voltage test: Lay the diaphragm flat on the test conductive plate of the withstand voltage insulation analyzer and test 50 breakdown points (the distance between two adjacent breakdown points is 5cm). Take the average value of the 50 breakdown points to obtain the breakdown voltage.

[0146] Air permeability test: The air permeability of each sample was tested using an air permeability tester, and the time it took for 100 ml of air to pass through was recorded;

[0147] Shrinkage test: The battery separator sample was heated in an environment of 150°C for 1 hour, and then the shrinkage of the sample in both the MD and TD directions was measured.

[0148] Contact angle test: The contact angle of the electrolyte on the surface of the battery separator is measured using a contact angle tester;

[0149] Peel strength test: The peel strength of the diaphragm sample was tested using a universal testing machine according to GB / T2792-2014. The peel speed was 50 mm / min, the sample width was 10 mm, and the data acquisition frequency was 100 Hz.

[0150] Electrolyte climbing height in 1 hour: Cut the sample into strips of 10cm×2cm and immerse them vertically into a container filled with electrolyte to a depth of 1cm. Mark the height of the electrolyte rising in the diaphragm with a marker and record the total climbing height after 1 hour.

[0151] Liquid absorption rate: Cut the sample into 100mm×100mm pieces, weigh the dried sample using an electronic balance, then immerse it completely in the electrolyte and let it stand for 30 minutes. After removing it, hang it for 10 seconds to remove excess electrolyte from the surface, and weigh the sample after liquid absorption. Calculation formula: Liquid absorption rate = (mass after liquid absorption - mass before liquid absorption) / mass before liquid absorption × 100%.

[0152] The test results are shown in Table 1.

[0153]

[0154] Table 1 Comparison of diaphragm properties

[0155] Based on the comparative analysis of the data in Table 1, the results of Examples 1-3 and Comparative Examples 1-4 show that, as can be seen from Table 1, the electrolyte wettability of the diaphragm of the present invention is significantly better than that of the comparative examples, while keeping other properties unchanged. The contact angle of the diaphragm of the present invention is only about 50% of that of the comparative examples, the electrolyte creep rate in 1 hour is almost twice that of the comparative examples, and the liquid absorption rate is also almost twice that of the comparative examples. Furthermore, the heat shrinkage performance of the diaphragm of the present invention at 150°C is also better than that of the comparative examples. In addition, the diaphragm of the present invention uses corona treatment on the surface of the base film, which can increase the adhesion between the coating slurry and the diaphragm, reduce the contact interface resistance, and the network structure formed by polyvinylidene fluoride-hexafluoropropylene copolymer, CNC (nano-sulfonated cellulose), and fumed silica is stably attached to the surface of the base film through physical adsorption and chemical action, allowing the electrolyte to diffuse rapidly through capillary effect, thereby achieving the purpose of the invention.

[0156] In summary, the high wettability coated separator and its preparation method provided by this invention, by coating a polyolefin-based membrane with an electrolyte-friendly organic solution using quantitative transfer roller coating technology, can greatly improve the wettability of the base membrane. The separator treated by this invention exhibits a significantly reduced contact angle with the electrolyte, a significantly shortened droplet penetration time, and a reduced interfacial impedance. The excellent wettability ensures that the electrolyte is fully filled within the separator pores, significantly reducing ion migration resistance and thus lowering the overall internal resistance of the battery. In other words, the high wettability coated separator and its preparation method provided by this invention can effectively solve the problem of insufficient electrolyte wettability of separators in the prior art, which leads to impaired battery performance.

[0157] The above description is merely a preferred embodiment of this disclosure and is not intended to limit this disclosure. Various modifications and variations can be made to this disclosure by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this disclosure should be included within the scope of protection of this disclosure.

Claims

1. A highly wettable coated diaphragm, characterized in that, It includes a base film and a highly wettable coating disposed on at least one side of the base film, the highly wettable coating comprising polyvinylidene fluoride-hexafluoropropylene copolymer, nano-sulfonated cellulose and fumed silica.

2. The diaphragm according to claim 1, characterized in that, The base film is a PE base film or a PP base film.

3. The diaphragm according to claim 1, characterized in that, The thickness of the base film is 3-16 μm.

4. The diaphragm according to claim 1, characterized in that, The highly wettable coating satisfies at least one of the following conditions: (1) The molecular weight of the polyvinylidene fluoride-hexafluoropropylene copolymer is 300,000 to 700,000; (2) The sulfonated degree of the nano-sulfonated cellulose is ≥90%, the diameter is 30-70nm, and the length is ≥5μm; (3) The thickness of the highly wettable coating is 0.5-2 μm.

5. The diaphragm according to claim 1, characterized in that, The highly wettable coating also includes cellulose ether derivatives.

6. The diaphragm according to claim 1, characterized in that, The method for preparing the highly wettable coating slurry includes the following steps: S1. Polyvinylidene fluoride-hexafluoropropylene copolymer is added to a solvent, and after a first heat treatment and a first dispersion, a first mixed solution is obtained; S2. Add nano-sulfonated cellulose to the first mixed solution, and then perform a second heat treatment and a second dispersion to obtain a second mixed solution; S3. Add fumed silica to the second mixed solution, and after a third heat treatment and a third dispersion, obtain the slurry of the highly wettable coating; The mass ratio of the polyvinylidene fluoride-hexafluoropropylene copolymer to the solvent is (5-7):(70-80). The mass ratio of the nano-sulfonated cellulose to the first mixed solution is (0.6-1.3):(65-68). The mass ratio of the fumed silica to the second mixed solution is (2.6-3.7):(60-65).

7. The diaphragm according to claim 6, characterized in that, The method for preparing the slurry of the highly wettable coating also satisfies at least one of the following conditions: (1) The solvent includes one or more of dimethyl carbonate, ethylene carbonate, methyl ethyl carbonate, diethyl carbonate and propylene carbonate; (2) The temperature of the first heat treatment is 50±5℃; (3) The rotation speed of the first dispersion is 2000±500r / min, and the dispersion time is 120±10min; (4) The temperature of the second heat treatment is 30±5℃; (5) The rotation speed of the second dispersion is 1500±200r / min, and the dispersion time is 360±30min; (6) The temperature of the third heat treatment is 25±5℃; (7) The rotation speed of the third dispersion is 800±100r / min, and the time of the third dispersion is 30±5min.

8. A method for preparing a highly wettable coated diaphragm as described in any one of claims 1-4, characterized in that, include: The base membrane surface is subjected to corona treatment, and a slurry of a highly wettable coating is applied to at least one surface of the base membrane. After deionized water extraction and gradient drying, the highly wettable coated membrane is obtained.

9. The preparation method according to claim 8, characterized in that, The slurry for the highly wettable coating is the slurry for the highly wettable coating prepared according to claim 6 or 7.

10. The preparation method according to claim 8, characterized in that, The method for preparing the highly wettable coated diaphragm also satisfies at least one of the following conditions: (1) The parameters of the corona treatment are 7-11 W / m² / min; (2) The coating speed is 5-10 m / min; (3) The gradient drying is carried out in a stepped oven, and the three temperature ranges of the stepped oven along the transport direction of the base film are 45±5℃, 60±5℃ and 50±5℃ respectively.

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