Diaphragm for long-circulation battery and preparation method of diaphragm

Abstract

The invention relates to the technical field of lithium ion batteries, and discloses a diaphragm for a long-circulation battery and a preparation method thereof.The diaphragm comprises a polyolefin base membrane and a composite coating, and the coating is prepared from 3-8 parts of lithium-containing MOFs powder, 14-18 parts of a nano ceramic material, 4-8 parts of a pore forming agent, 2.6-4.3 parts of an adhesive, 0.5-0.8 part of a wetting agent and 28-35 parts of deionized water. The preparation method comprises the following steps: compounding MOFs and a ceramic material through a step-by-step dispersion process, carrying out ultrasonic defoaming, coating a base membrane, and drying. Metal-MOFs are introduced into diaphragm slurry, lithium ions are inlaid in the MOFs, the MOFs and a ceramic material are fully fused, the surface of a PP or PE base membrane is coated with the mixture through roller coating, the long-circulation battery diaphragm is formed, the diaphragm has good heat resistance and liquid absorption and retention performance, the inlaid lithium ions can gradually supplement the lithium ions in a battery through special treatment, and the service life of the battery is prolonged. Therefore, the cycle index of the battery is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium-ion batteries, and particularly to a separator for long-cycle batteries and a preparation method thereof. Background Art

[0002] The separator of a lithium-ion battery is a key component of the lithium-ion battery. Its main functions include isolating the positive and negative electrodes to prevent short circuits, and at the same time allowing lithium ions to pass through, thereby realizing charge transfer during the charging and discharging process. However, the separators currently used in commercial lithium-ion batteries mainly adopt polyolefin materials such as polyethylene (PE) and polypropylene (PP). Although these materials have good mechanical strength and chemical stability, they have significant deficiencies in heat resistance and liquid absorption and retention. Polyolefin separators are prone to shrinkage at high temperatures, resulting in battery short circuits or even explosions. For example, polyethylene and polypropylene will melt at temperatures above 130 °C, causing the positive and negative electrodes to come into direct contact, thus triggering safety accidents. To solve this problem, researchers have developed various modification methods, such as coating heat-resistant ceramic materials such as alumina and boehmite on polyolefin separators. However, although these methods have improved the thermal stability of the separators to a certain extent, they cannot completely solve the essential safety problems of lithium-ion batteries. Moreover, the hydrophobicity of polyolefin materials limits the infiltration of the electrolyte, affecting the ion conduction efficiency. Insufficient infiltration of the electrolyte will lead to an increase in the internal resistance of the battery, reducing the charge and discharge performance and cycle life of the battery.

[0003] In summary, the disadvantages of lithium-ion batteries in terms of heat resistance and long cycle life limit their use in some applications with high requirements for high temperature and long life. There is an urgent need for a separator or a coated separator that can solve the above problems. Summary of the Invention

[0004] Aiming at the deficiencies of the prior art, the present invention provides a separator for long-cycle batteries and a preparation method thereof, which solve the problems of easy melting and short circuit at high temperatures of traditional polyolefin battery separators, poor electrolyte wettability leading to increased internal resistance, and short battery life caused by irreversible loss of lithium ions during the cycling process.

[0005] To achieve the above object, the present invention is realized through the following technical solutions: A separator for long-cycle batteries includes a polyolefin base film and a composite coating coated on at least one surface thereof. The composite coating is formed by curing a slurry containing the following components by mass:

[0006] 3 - 8 parts of MOFs powder containing lithium ions;

[0007] 14 - 18 parts of nano ceramic material;

[0008] 4 - 8 parts of pore former;

[0009] 2.6 - 4.3 parts of binder;

[0010] Wetting agent: 0.5 - 0.8 parts;

[0011] Deionized water: 28 - 35 parts.

[0012] Preferably, the lithium-ion-containing MOFs powder is prepared by the following steps:

[0013] a. React 12 - 18 parts by mass of lithium tetrafluoroborate with 20 - 23 parts by mass of carboxyphenyldisiloxane in a water bath at 60 - 65 °C for 15 - 30 min with a stirring speed of 50 - 80 r / min;

[0014] b. Add 2 - 5 parts by mass of benzenetricarboxylate to the product of step a, and carry out a high-pressure reaction at 150 - 180 °C for 2 - 3 h with a stirring speed of 100 - 150 r / min to obtain a suspension containing a white powder;

[0015] c. Filter the suspension containing the white powder to obtain a white solid, and grind it using a ball mill until D50 is 0.4 - 0.7 μm, D90 is 1.1 - 1.4 μm and D99 < 2 μm to obtain the lithium-ion-containing MOFS powder.

[0016] Preferably, the nano-ceramic material is selected from at least one of alumina, boehmite, magnesium hydroxide, and aluminum hydroxide, and its particle size is D50: 0.7 - 1.2 μm, D90 is 2.0 - 3.0 μm, and D99 < 4 μm.

[0017] Preferably, the pore-forming agent is an alcohol compound, the binder is a polyacrylonitrile polymer, and the wetting agent is an isomeric alcohol polyether.

[0018] Preferably, the particle size D50 of the slurry is 0.8 - 1.3 μm, D90 is 2.1 - 3.2 μm, and the viscosity is 60 - 120 mPa·s.

[0019] Preferably, the thickness of the composite coating is 2 - 4 μm, the polyolefin-based film is made of polyethylene or polypropylene, and the thickness of the base film is 3 - 20 μm.

[0020] The present invention also provides a method for preparing the separator for the long-cycle battery, including the following steps:

[0021] S1. Mix 3 - 8 parts by mass of the lithium-ion-containing MOFS powder with 28 - 35 parts by mass of deionized water, and disperse it at a self-rotation speed of 1000 - 1500 r / min for 10 - 20 min;

[0022] S2. Add 14 - 18 parts by mass of the nano-ceramic material to the product of step S1, and disperse it at a self-rotation speed of 2000 - 2500 r / min for 60 - 90 min;

[0023] S3. Add 4 - 8 parts by mass of pore former, 2.6 - 4.3 parts by mass of binder, and 0.5 - 0.8 parts by mass of wetting agent to the product in step S2, disperse at a self - rotation speed of 400 - 800 r / min for 30 - 60 min to obtain a slurry.

[0024] S4. Coat the slurry on the surface of the polyolefin - based film and dry at 60 - 85 °C for 2 - 4 min to form a coating.

[0025] Preferably, in steps S1, S2 and S3, a double - planetary mixer is used for stirring, and the revolution speed is maintained at 35 - 45 r / min.

[0026] Preferably, in step S4, the coating is carried out by quantitative transfer coating method, and the coating linear speed is 40 - 60 m / min.

[0027] Preferably, before coating in step S4, the slurry is treated by ultrasonic wave at 5 - 8 kHz and degassed under vacuum of ≤ - 0.8 MPa for 10 - 20 min.

[0028] The present invention provides a separator for long - cycle batteries and its preparation method. It has the following beneficial effects:

[0029] 1. Through the structural design of the MOFs - ceramic composite coating, the present invention significantly improves the heat resistance of the separator while maintaining the mechanical properties of the polyolefin - based film. Compared with the prior art of simply coating inorganic ceramic particles, this composite structure forms a thermally stable network through the chemical bonding between the MOFs skeleton and ceramic particles, effectively inhibiting the melting and shrinkage of the base film at high temperatures and solving the core problem of high - temperature short - circuit failure of traditional separators.

[0030] 2. Based on the synergistic mechanism of MOFs multi - level pores and pore formers, the present invention realizes the efficient infiltration and long - term storage of electrolytes. Aiming at the problem of uneven electrolyte distribution caused by the hydrophobicity of traditional ceramic coatings, this technology enables the electrolyte to quickly penetrate and uniformly stay through the synergistic effect of MOFs micropore adsorption and macropore diversion of pore formers, fundamentally overcoming the defect of increased battery internal resistance caused by insufficient infiltration.

[0031] 3. Through the design of the MOFs lithium - ion slow - release system, the present invention dynamically maintains the lithium - ion concentration in the electrolyte during the cycling process. Compared with the prior art of directly adding lithium salts to accelerate side reactions, this scheme utilizes the selective storage and controllable release characteristics of MOFs pores for lithium ions, avoiding both the decomposition loss of free lithium salts and continuously supplementing the loss of active lithium in the electrode, breaking through the technical bottleneck of limited cycle life of traditional batteries. Description of the Drawings

[0032] Figure 1 It is a flow chart of the preparation method of the present invention;

[0033] Figure 2 This is the electron micrograph of Example 1 of the present invention (magnification: 5.00KX;

[0034] Figure 3 This is the electron micrograph of Comparative Example 1 of the present invention (magnification: 5.00KX). Detailed implementation manners

[0035] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0036] Please refer to the attached Figure 1 , an embodiment of the present invention provides a separator for a long-cycle battery, including a polyolefin-based film and a composite coating coated on at least one surface thereof. The composite coating is formed by curing a slurry containing the following components by mass:

[0037] 3-8 parts of MOFs powder containing lithium ions;

[0038] The three-dimensional coordination structure of MOFs fixes lithium ions (Li + ) through chemical bonds. Its microporous channels (1-2 nm) physically limit the release rate of lithium ions, avoiding the decomposition of the electrolyte caused by one-time dissolution, and realizing continuous lithium replenishment during cycling. The high specific surface area of MOFs (>800 m 2 / g) adsorbs the electrolyte through capillary action, improves the liquid retention amount, extends the battery cycle life (lithium ion dynamic balance), and improves the electrolyte retention rate at high temperatures (reducing the risk of dry-out).

[0039] 14-18 parts of nano-ceramic materials;

[0040] Nano-ceramic particles (melting point > 2000 °C) form a rigid framework in the coating, inhibiting the high-temperature shrinkage of the polyolefin-based film (thermal shrinkage rate at 150 °C < 5%). The hydroxyl groups (-OH) on the ceramic surface form weak coordination with Li + in the electrolyte, reducing the activation energy of ion migration, preventing high-temperature short circuit, and improving the ionic conductivity (internal resistance reduced by more than 30%).

[0041] 4-8 parts of pore-forming agent;

[0042] The pore-forming agent (such as ethanol) volatilizes during drying to form large-sized connected pores (100-500 nm), forming a gradient pore structure with the micropores of MOFs and the mesopores of ceramics (10-50 nm). The large pores serve as fast penetration channels for the electrolyte, shortening the wetting time.

[0043] 2.6 - 4.3 parts of adhesive;

[0044] The molecular chains of the adhesive are bonded to the surfaces of MOFs, ceramic particles and the base film simultaneously through van der Waals forces and hydrogen bonds, forming a "ceramic - MOFs - base film" trinity structure, which improves the elastic deformation ability of the molecular chains to offset the coating stress changes during cycling.

[0045] 0.5 - 0.8 parts of wetting agent;

[0046] The wetting agent is oriented on the surface of the slurry, reducing the liquid - solid interfacial tension (surface tension < 30 mN / m), promoting the spreading of the slurry on the base film. The wetting agent wraps the MOFs / ceramic particles and inhibits agglomeration through steric hindrance effect.

[0047] 28 - 35 parts of deionized water.

[0048] The MOFs powder containing lithium ions is prepared through the following steps:

[0049] a. React 12 - 18 parts by mass of lithium tetrafluoroborate with 20 - 23 parts by mass of carboxyphenyldisiloxane in a water bath at 60 - 65 °C for 15 - 30 min with a stirring speed of 50 - 80 r / min;

[0050] b. Add 2 - 5 parts by mass of benzenetricarboxylate to the product of step a, and carry out a high - pressure reaction at 150 - 180 °C for 2 - 3 h with a stirring speed of 100 - 150 r / min to obtain a suspension containing white powder;

[0051] c. Filter the suspension containing white powder to obtain a white solid, and grind it using a ball mill until D50 is 0.4 - 0.7 μm, D90 is 1.1 - 1.4 μm and D99 < 2 μm to obtain the MOFS powder containing lithium ions.

[0052] Specifically, lithium tetrafluoroborate is used as the lithium source, and the BF4 - anion coordinates with the silicon - oxygen bond in carboxyphenyldisiloxane to form a stable MOFs skeleton and embed Li + (LiBF4 → Li + + BF4 - )

[0053] Carboxyphenyldisiloxane: The organic ligand constructs a three - dimensional porous structure (pore size 1 - 2 nm) through Si - O - Si bonds. The carboxyl functional groups cross - link with benzenetricarboxylate to enhance the thermal stability of MOFs (decomposition temperature > 300 °C).

[0054] High temperature and high pressure conditions promote the reaction of benzene tricarboxylate with the hydrolysis product (LiOH) of LiBF4 to form stable metal-organic coordination bonds (the bond energy of Li-O-C is about 400 kJ / mol).

[0055] The nano-ceramic material is selected from at least one of alumina, boehmite, magnesium hydroxide, and aluminum hydroxide, with a particle size of D50: 0.7 - 1.2 μm, D90 of 2.0 - 3.0 μm, and D99 < 4 μm.

[0056] The nano-ceramic material selected in the present invention realizes the synergistic improvement of multiple properties through specific particle size design. Taking alumina as an example, its D50 particle size is controlled within the range of 0.7 - 1.2 μm, which can form a dense packing structure in the coating. At high temperatures (150 - 200 °C), the physical anchoring effect between particles inhibits the melting shrinkage of the polyolefin-based film. The high melting point characteristic of ceramic particles (>2000 °C) directly blocks the heat conduction path, reducing the thermal shrinkage rate of the separator from >50% of traditional materials to <5%. At the same time, the design with D90 controlled within 2.0 - 3.0 μm avoids the blockage of the microporous structure of MOFs by over-sized particles. The hydroxyl groups on the ceramic surface enhance the wettability with ester solvents (such as ethylene carbonate) in the electrolyte through hydrogen bonding, and the contact angle is significantly reduced from over 80° of the unmodified one to below 30°, and the electrolyte penetration time is shortened to within 3 seconds.

[0057] Furthermore, the particle size upper limit of D99 < 4 μm strictly limits the existence of abnormally large particles in the coating, ensuring the uniformity of the slurry flow during the coating process and avoiding the coating thickness fluctuation (the fluctuation range < ±0.3 μm) caused by local particle aggregation. When boehmite (AlOOH) is selected, its flaky structure can fill the voids between spherical alumina particles to form an interlocking network, improving the mechanical strength of the coating (the peel strength > 1.5 N / cm). The water molecules released by the decomposition reaction of magnesium hydroxide and aluminum hydroxide at high temperatures (such as Mg(OH)2 → MgO + H2O↑) can dilute the combustible gas generated by the decomposition of the electrolyte, realizing the function of flame retardancy and explosion suppression.

[0058] The combined application of different ceramic materials can produce a synergistic effect. For example, when alumina and boehmite are compounded, the rigid skeleton of alumina and the lamellar structure of boehmite jointly construct multi-scale pores. The micropores (1-2 nm) of MOFs are responsible for storing the electrolyte and regulating the diffusion of lithium ions, the ceramic mesopores (10-50 nm) serve as fast ion transport channels, and the macropores (100-500 nm) formed by the pore former accelerate the initial infiltration of the electrolyte. The synergy of the three increases the ionic conductivity by more than 40% compared with a single ceramic coating. This multi-level pore structure fundamentally solves the problem of increased battery internal resistance caused by poor wettability of traditional diaphragms. At the same time, through the combination of the thermal stability of the ceramic-MOFs composite coating and the lithium ion slow-release function, it breaks through the technical limitation that single material modification cannot balance high-temperature safety and long cycle life.

[0059] The pore former is an alcohol compound, the binder is a polyacrylonitrile polymer, and the wetting agent is an isomeric alcohol polyether.

[0060] As the core component for pore construction, alcohol compounds (such as ethanol and isopropanol) in the pore former selectively volatilize during the drying stage of the slurry, forming a multi-level pore network that penetrates the coating. Its low boiling point characteristics (the boiling point of ethanol is 78 °C) ensure complete escape during the drying process at 60-85 °C, leaving a connected macropore structure of 100-500 nm, forming a gradient infiltration path with the MOFs micropores (1-2 nm) and the ceramic mesopores (10-50 nm). This design reduces the electrolyte contact angle from 50° of traditional diaphragms to less than 30°, shortens the infiltration time to within 3 seconds, and at the same time reduces the pore tortuosity from 2.5 to 1.3, significantly improving the lithium ion migration rate (ionic conductivity > 1.2 mS / cm). Compared with inorganic pore formers (such as ammonium bicarbonate), the complete volatilization characteristics of alcohols avoid the damage to the chemical stability of the battery caused by high-temperature decomposition residues.

[0061] The polyacrylonitrile molecular chain forms strong hydrogen bond interactions (bond energy is about 20 kJ / mol) with the hydroxyl groups (-OH) on the surface of ceramic particles through cyano groups (-CN). At the same time, its linear structure contracts during the drying process to generate cohesive force, tightly anchoring the MOFs and ceramic particles on the surface of the base film. This bonding system increases the coating peel strength to more than 1.5 N / cm (about 0.8 N / cm for traditional PVDF binders), and there is no swelling or peeling phenomenon after soaking in the electrolyte for 240 h. Compared with water-soluble binders such as carboxymethyl cellulose (CMC), the hydrophobic characteristics (contact angle > 90°) of polyacrylonitrile are more compatible with the electrolyte, avoiding the problem of pore blockage caused by binder swelling during cycling.

[0062] The isomeric alcohol polyether molecules adsorb on the surface of MOFs / ceramic particles through the hydrophobic end, and the hydrophilic end extends into the aqueous phase to form a steric hindrance layer (with a thickness of about 5 - 10 nm), effectively inhibiting particle agglomeration (the slurry particle size distribution D90 < 3.2 μm). Its dynamic surface tension (< 35 mN / m) enables the slurry to spread evenly on the surface of the base film (the thickness fluctuation < ±0.2 μm), while reducing the shear stress required during the coating process (50% lower than that without adding a wetting agent). Compared with traditional fluorocarbon wetting agents, isomeric alcohol polyethers do not contain fluorine elements, avoiding side reactions of electrolyte decomposition caused by the release of fluorides at high temperatures (such as the generation of HF), and ensuring the long-term chemical stability of the battery system.

[0063] The D50 of the slurry particle size is 0.8 - 1.3 μm, D90 is 2.1 - 3.2 μm, and the viscosity is 60 - 120 mPa·s.

[0064] The thickness of the composite coating is 2 - 4 μm, the polyolefin base film is made of polyethylene or polypropylene, and the thickness of the base film is 3 - 20 μm.

[0065] A preparation method for a separator for a long-cycle battery includes the following steps:

[0066] S1. Mix 3 - 8 parts by mass of MOFS powder containing lithium ions with 28 - 35 parts by mass of deionized water, and disperse at a self-rotation speed of 1000 - 1500 r / min for 10 - 20 min;

[0067] The shear force generated by the self-rotation speed (> 10 4 s -1 ) fully opens the MOFs particle aggregates (the initial particle size D50 ≈ 0.5 μm) to form a uniform suspension, avoiding uneven local lithium ion concentration during subsequent coating. Deionized water as a solvent avoids the pollution of the electrolyte by the residue of organic solvents (such as NMP), and at the same time, the hydrogen bond interaction between the hydroxyl groups (-OH) on the surface of MOFs and water molecules enhances the dispersion stability.

[0068] S2. Add 14 - 18 parts by mass of nano-ceramic materials to the product of step S1, and disperse at a self-rotation speed of 2000 - 2500 r / min for 60 - 90 min;

[0069] Through high-speed self-rotation at 2000 - 2500 r / min (the shear rate > 5 × 10 4 s -1), nanoceramic particles (such as alumina) and MOFs form an interpenetrating structure of "ceramic-anchored MOFs", where the ceramic particles are embedded in the periphery of the MOFs pores, which not only inhibits the collapse of the MOFs pore channels but also enhances the interfacial bonding force through the chemical bonding between the hydroxyl groups (-OH) on the ceramic surface and the carboxyl groups (-COOH) of the MOFs. The nanoceramic D50 (0.7 - 1.2 μm) and MOFs D50 (0.4 - 0.7 μm) form a graded packing, optimizing the porosity to 40 - 50%, and shortening the ion transport path.

[0070] S3. Add 4 - 8 parts by mass of pore former, 2.6 - 4.3 parts by mass of binder, and 0.5 - 0.8 parts by mass of wetting agent to the product of step S2, and disperse at a self-rotation speed of 400 - 800 r / min for 30 - 60 min to obtain a slurry;

[0071] The low speed of 400 - 800 r / min avoids damaging the MOFs-ceramic composite. At the same time, the wetting agent (isomeric alcohol polyether) reduces the interfacial tension (<30 mN / m) by adsorbing on the particle surface, enabling the pore former (ethanol) to be evenly distributed in the slurry. After drying, interconnected macropores of 100 - 500 nm are formed. The polyacrylonitrile molecular chains contract during drying, and the cyano groups (-CN) form hydrogen bonds (bond energy about 20 kJ / mol) with the particle surface, forming a three-dimensional cross-linked network after curing to inhibit the cyclic shedding of the coating.

[0072] S4. Coating the slurry on the surface of the polyolefin-based film and drying at 60 - 85 °C for 2 - 4 min to form a coating.

[0073] Controlling the wet coating thickness (5 - 8 μm) through slit extrusion, and curing to 2 - 4 μm after drying, precisely matching the thermal expansion coefficient of the base film (≈1×10 -4 / °C) to avoid curling and cracking. The stepwise temperature increase from 60 - 85 °C allows the wetting agent to slowly migrate to the surface of the coating to form a hydrophilic layer (contact angle <30°). At the same time, the pore former (ethanol) preferentially volatilizes to construct open pore channels, and LiBF4 in the MOFs pore channels remains stable and does not decompose at <100 °C.

[0074] In steps S1, S2, and S3, a double planetary mixer is used for stirring, and the revolution speed is maintained at 35 - 45 r / min.

[0075] In step S4, the coating is carried out by quantitative transfer coating method, and the coating line speed is 40 - 60 m / min.

[0076] Before coating in step S4, the slurry is treated by ultrasonic wave at 5 - 8 kHz combined with vacuum degassing at ≤ - 0.8 MPa for 10 - 20 min.

[0077] The instantaneous collapse of ultrasonic cavitation bubbles generates local high pressure (>100 MPa), breaking the hidden bubbles (size > 50 μm) in the slurry, and the negative pressure of ≤ -0.8 MPa extracts the μm-sized bubbles (< 10 μm), avoiding the uneven current density caused by local holes (pore diameter > 1 μm) in the coating

[0078] Example 1:

[0079] Please refer to the appendix Figure 2 :

[0080] The first step: Preparation of MOFS powder containing lithium ions

[0081] Take 13 parts by mass of lithium tetrafluoroborate powder, add it to 20 parts by mass of carboxyphenyl disiloxane, heat it in a water bath and stir. The water bath heating temperature is 60 °C, the stirring speed is 55 r / min, and the reaction time is 16 min to obtain solution A

[0082] Take 2.5 parts by mass of benzene tricarboxylate, add it to solution A, mix evenly to obtain mixed solution B. Put mixed solution B into a high-pressure reactor for synthesis reaction. The reaction time is 2 h, the reaction temperature is 158 °C, and the stirring parameter is 130 r / min. After the reaction, a suspension containing white powder is obtained. Filter the suspension to obtain a white solid, and use a ball mill to grind the white solid to obtain MOFS powder containing lithium ions, denoted as powder C

[0083] The second step: Preparation of the coating slurry for the lithium-ion separator

[0084] Take 3 parts by mass of powder C, add it to 28 parts by mass of deionized water, and use a double planetary mixer for high-speed dispersion. The dispersion time is 10 min, the revolution speed is 35 r / min, and the rotation speed is 1000 r / min to obtain mixed solution D

[0085] Add 14 parts by mass of alumina to mixed solution D, and use a double planetary mixer for high-speed dispersion. The dispersion time is 60 min, the revolution speed is 35 r / min, and the rotation speed is 2000 r / min to obtain mixed solution F

[0086] Add 4.3 parts by mass of pore former, 2.6 parts by mass of binder and 0.558 parts by mass of wetting agent to mixed solution F, and use a double planetary mixer for stirring. The dispersion time is 35 min, the revolution speed is 37 r / min, and the rotation speed is 400 r / min to obtain the coating slurry for the lithium battery separator

[0087] After the above steps are completed, ultrasonic treatment and vacuum pumping are performed on the slurry to eliminate the hidden bubbles in the slurry and produce the finished slurry. Among them, the ultrasonic frequency is 6 kHz, the vacuum degree is ≤ -0.8 Mpa, and the ultrasonic treatment and vacuum pumping time is 15 min.

[0088] In the above technical solution, the particle size of powder C is D50: 0.448 μm, D90 is 1.138 μm, and D99 is 1.785 μm.

[0089] The particle size D50 of the finished slurry is 0.843 μm, D90 is 2.879 μm, and the viscosity is 78 mPa·s.

[0090] The above-mentioned finished slurry is coated on the PE diaphragm by a single-sided or double-sided quantitative transfer coating method.

[0091] In the above technical solution, the drying time of the ceramic layer after coating is 2 min, and the temperature is 70 °C.

[0092] In the above technical solution, the linear speed of the coater is 40 m / min.

[0093] In the above technical solution, the thickness of the coating formed by coating is 2.3 μm.

[0094] Example 2:

[0095] First step: Preparation of MOFS powder containing lithium ions

[0096] Take 15 parts by mass of lithium tetrafluoroborate powder, add it to 23 parts by mass of carboxyphenyl disiloxane, heat it in a water bath and stir. The water bath heating temperature is 65 °C, the stirring speed is 55 r / min, and the reaction time is 20 min to obtain solution A.

[0097] Take 3.6 parts by mass of benzene tricarboxylate, add it to solution A, mix evenly to obtain mixed solution B. Put mixed solution B into a high-pressure reactor for synthesis reaction. The reaction time is 2.5 h, the reaction temperature is 165 °C, and the stirring parameter is 130 r / min. After the reaction, a suspension containing white powder is obtained. Filter the suspension to obtain a white solid, and use a ball mill to grind the white solid to obtain MOFS powder containing lithium ions, denoted as powder C.

[0098] Second step: Preparation of lithium-ion diaphragm coating slurry

[0099] Take 3.6 parts by mass of powder C, add it to 30 parts by mass of deionized water, and use a double planetary mixer for high-speed dispersion. The dispersion time is 10 min, the revolution speed is 40 r / min, and the rotation speed is 1000 r / min to obtain mixed solution D.

[0100] Add 16 parts by mass of alumina to the mixed solution D, and use a double planetary mixer for high-speed dispersion. The dispersion time is 60 min, the revolution speed is 40 r / min, and the rotation speed is 2500 r / min to obtain the mixed solution F.

[0101] Add 4.8 parts by mass of pore former, 3.2 parts by mass of binder and 0.657 parts by mass of wetting agent to the mixed solution F, and use a double planetary mixer for stirring. The dispersion time is 35 min, the revolution speed is 40 r / min, and the rotation speed is 500 r / min to obtain the lithium battery separator coating slurry.

[0102] After the above steps are completed, perform ultrasonic treatment and vacuum pumping on the slurry to eliminate the hidden bubbles in the slurry to make the finished slurry. Among them, the ultrasonic frequency is 6 kHz, the vacuum degree is ≤ -0.8 Mpa, and the ultrasonic treatment and vacuum pumping time is 15 min.

[0103] In the above technical solution, the particle size of the powder C is D50: 0.453 μm, D90 is 1.142 μm, and D99 is 1.737 μm.

[0104] The particle size D50 of the finished slurry is 0.857 μm, D90 is 2.749 μm, and the viscosity is 84 mPa·s.

[0105] The above-mentioned finished slurry is coated on the PE separator unidirectionally or bidirectionally by a quantitative transfer coating method.

[0106] In the above technical solution, the drying time of the ceramic layer after coating is 3 min, and the temperature is 70 °C.

[0107] In the above technical solution, the linear speed of the coater is 40 m / min.

[0108] In the above technical solution, the thickness of the coating formed by coating is 2.5 μm

[0109] Example 3:

[0110] First step: Preparation of MOFS powder containing lithium ions

[0111] Take 17 parts by mass of lithium tetrafluoroborate powder, add it to 22 parts by mass of carboxyphenyldisiloxane, heat it by water bath and stir. The water bath heating temperature is 70 °C, the stirring speed is 60 r / min, and the reaction time is 20 min to obtain solution A.

[0112] Take 4.2 parts by mass of benzene tricarboxylate and add it to Solution A. Mix evenly to obtain Mixed Solution B. Put Mixed Solution B into a high-pressure reactor for synthesis reaction. The reaction time is 3 h, the reaction temperature is 175 °C, and the stirring parameter is 150 r / min. After the reaction, a suspension containing white powder is obtained. Filter the suspension to obtain a white solid. Use a ball mill to grind the white solid to obtain MOFS powder containing lithium ions, denoted as Powder C.

[0113] Second step: Prepare the lithium-ion separator coating slurry

[0114] Take 4.2 parts by mass of Powder C and add it to 32 parts by mass of deionized water. Use a double planetary mixer for high-speed dispersion. The dispersion time is 10 min, the revolution speed is 40 r / min, and the rotation speed is 1000 r / min to obtain Mixed Solution D.

[0115] Add 18 parts by mass of alumina to Mixed Solution D. Use a double planetary mixer for high-speed dispersion. The dispersion time is 80 min, the revolution speed is 40 r / min, and the rotation speed is 2500 r / min to obtain Mixed Solution F.

[0116] Add 5.2 parts by mass of pore former, 3.4 parts by mass of binder, and 0.75 part by mass of wetting agent to Mixed Solution F. Use a double planetary mixer for stirring. The dispersion time is 50 min, the revolution speed is 40 r / min, and the rotation speed is 700 r / min to obtain the lithium battery separator coating slurry.

[0117] After the above steps are completed, perform ultrasonic treatment and vacuum pumping on the slurry to eliminate the hidden bubbles in the slurry and make the finished slurry. Among them, the ultrasonic frequency is 6 kHz, the vacuum degree is ≤ -0.8 Mpa, and the ultrasonic treatment and vacuum pumping time is 15 min.

[0118] In the above technical solution, the particle size of Powder C is D50: 0.442 μm, D90 is 1.084 μm, and D99 is 1.713 μm.

[0119] The particle size D50 of the finished slurry is 0.843 μm, D90 is 2.672 μm, and the viscosity is 79 mPa·s.

[0120] The above-mentioned finished slurry is coated on the PE separator by a quantitative transfer coating method on one side or both sides.

[0121] In the above technical solution, the drying time of the ceramic layer after coating is 3 min, and the temperature is 70 °C.

[0122] In the above technical solution, the linear speed of the coater is 45 m / min.

[0123] In the above technical solution, the thickness of the coating formed by coating is 2.0 μm

[0124] In the above embodiment, the thickness range of the base film used is 3 - 20 μm, and a 9-μm base film is taken as an example.

[0125] Comparative Example 1:

[0126] Please refer to the atta Figure 3 :

[0127] An aqueous ceramic coating slurry is obtained through the following steps:

[0128] Take 18 parts by mass of alumina and add it to 31 parts by mass of deionized water. Use a double planetary mixer for high-speed dispersion. The dispersion time is 30 min, the revolution speed is 40 r / min, and the rotation speed is 2500 r / min. Then add 4.7 parts by mass of pore-forming agent, 3.2 parts by mass of binder, and 0.65 parts by mass of wetting agent. Use a double planetary mixer for stirring. The dispersion time is 30 min, the revolution speed is 40 r / min, and the rotation speed is 800 r / min to obtain the lithium battery separator coating slurry.

[0129] After the above steps are completed, the slurry is subjected to ultrasonic treatment and vacuum pumping to eliminate the hidden bubbles in the slurry to make the finished product slurry. Among them, the ultrasonic frequency is 7 kHz, the vacuum degree is ≤ -0.8 Mpa, and the ultrasonic treatment and vacuum pumping time is 15 min.

[0130] In the above technical solution, the particle size D50 of the finished product slurry is 0.986 μm, D90 is 1.636 μm, and the viscosity is 113.1 mPa·s.

[0131] In the above embodiment, the drying time of the ceramic layer after coating is 3 min, and the temperature is 75 °C.

[0132] In the above embodiment, the linear speed of the coater is 40 m / min.

[0133] In the above embodiment, the thickness of the coating formed by coating is 2.8 μm.

[0134] In the above embodiment, the thickness of the base film used is 9 μm.

[0135] Experimental description:

[0136] Experimental materials:

[0137] Base film: Polyethylene (PE) separator (thickness 9 μm, porosity 40%)

[0138] Raw materials:

[0139] Examples 1 - 3: Lithium tetrafluoroborate (LiBF4, purity ≥ 99.9%), carboxyphenyldisiloxane, benzene tricarboxylate, nano-aluminum oxide (D50: 0.7 - 1.2 μm), isopropanol (pore former), polyacrylonitrile binder (solid content 30%), isomeric alcohol polyether wetting agent

[0140] Comparative Example 1: Nano-aluminum oxide (same as in the examples), isopropanol, polyacrylonitrile binder, isomeric alcohol polyether

[0141] Equipment:

[0142] Double planetary mixer (revolution speed 35 - 45 r / min, rotation speed 400 - 2500 r / min adjustable)

[0143] High-pressure reactor (pressure resistance 5 MPa, temperature range 50 - 200 °C)

[0144] Ultrasonic cleaner (frequency 5 - 8 kHz)

[0145] Quantitative transfer coater (linear speed 40 - 60 m / min, coating accuracy ±0.2 μm)

[0146] Ball mill (zirconia balls, particle size 0.5 - 1 mm)

[0147] Experimental procedures:

[0148] Preparation processes for Examples 1 - 3:

[0149] Preparation of MOF powder:

[0150] Mix lithium tetrafluoroborate and carboxyphenyldisiloxane by mass (e.g., in Example 1: 13 parts of LiBF4 + 20 parts of carboxyphenyldisiloxane), stir in a water bath at 60 - 70 °C (55 - 60 r / min) for 15 - 30 min to obtain solution A.

[0151] Add benzene tricarboxylate (e.g., 2.5 parts in Example 1), transfer the mixture to a high-pressure reactor after mixing, and react at 150 - 180 °C and 100 - 150 r / min for 2 - 3 h to form a white suspension.

[0152] Filter the suspension, ball mill the white solid to D50: 0.44 - 0.45 μm (D99 < 2 μm) to obtain lithium-containing MOF powder C.

[0153] Preparation of slurry:

[0154] Primary dispersion: Disperse powder C (3 - 4.2 parts) and deionized water (28 - 32 parts) at a rotation speed of 1000 r / min for 10 min.

[0155] Secondary dispersion: Add alumina (14 - 18 parts), and disperse at a self-rotation speed of 2000 - 2500 r / min for 60 - 80 min.

[0156] Tertiary dispersion: Add pore former (4.3 - 5.2 parts), binder (2.6 - 3.4 parts), wetting agent (0.558 - 0.75 parts), and disperse at a self-rotation speed of 400 - 700 r / min for 35 - 50 min.

[0157] Defoaming treatment: Perform defoaming for 15 min with 6 kHz ultrasonic wave combined with a vacuum of ≤ - 0.8 MPa to obtain a slurry (viscosity 78 - 84 mPa·s, D50: 0.84 - 0.86 μm).

[0158] Separator coating:

[0159] The slurry is coated on the PE base film at a linear speed of 40 - 45 m / min, dried at 70°C for 2 - 3 min to form a 2.0 - 2.5 μm coating.

[0160] Preparation process of Comparative Example 1:

[0161] Omit the MOFs synthesis step, directly mix alumina (18 parts), pore former (4.7 parts), binder (3.2 parts), wetting agent (0.65 parts) and deionized water (31 parts), and the remaining steps are the same as those in the example.

[0162] Experimental results:

[0163] Table 1 Test results of separator performance

[0164]

[0165]

[0166] Table 2 Battery cycle performance test

[0167] Project Example 1 Example 2 Example 3 Comparative Example 1 Remaining battery capacity after 2000 charge and discharge cycles 92 93 92 76 Remaining battery capacity after 3000 charge and discharge cycles 87 88 86 57

[0168] Summary of the experiment: As can be seen from Table 1, based on the above test data, when there are no obvious differences in other performances, the heat resistance, liquid absorption capacity and liquid retention capacity of the separator of the present invention are all superior to those of the ordinary alumina coating. This is because the MOFs and the ceramic material are fully integrated, improving the rigidity of the coating and enhancing the heat resistance of the coated separator. Moreover, due to the unique porous structure and high specific surface area of MOFs, the porosity is derived from the coordination connection between metal ions and organic ligands, forming regular pores and channels. This property can store the electrolyte and enable lithium ions to shuttle quickly between the positive and negative electrodes, thus increasing the cycle performance of the battery. From Table 2, it can be obtained that the battery made of the separator of the present invention still maintains a battery capacity of more than 90% after 2000 charge and discharge cycles, while the ordinary separator only retains 76% of the battery capacity. This is because in the present invention, the lithium ions embedded during the preparation of MOFs will also be gradually released by the soaking of the electrolyte during use, always keeping the lithium ion concentration in the battery at a relatively high level, so as to maintain a relatively high level of the battery capacity all the time.

[0169] Although the embodiments of the present invention have been shown and described, those of ordinary skill in the art can understand that various changes, modifications, substitutions and variations can be made to these embodiments without departing from the principle and spirit of the present invention. The scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A long cycle battery separator, characterized in that: The invention comprises a polyolefin-based film and a composite coating coated on at least one surface thereof, wherein the composite coating is formed by curing a slurry comprising the following components by weight: 3-8 parts of MOFs powder containing lithium ions; 14-18 parts of nano ceramic material; 4-8 parts of pore-forming agent; Adhesive 2.6-4.3 parts; Wetting agent 0.5-0.8 parts; 28-35 parts of deionized water.

2. A long cycle battery separator according to claim 1, characterized in that: The lithium ion-containing MOFs powder is prepared by the following steps: a. Stir 12-18 parts by mass of lithium tetrafluoroborate and 20-23 parts by mass of carboxyphenyl disiloxane in a water bath at 60-65°C for 15-30min at a stirring speed of 50-80r / min; b. Add 2-5 parts by mass of benzene trimesate to the product of step a, react at 150-180 ° C under high pressure for 2-3h, stirring speed 100-150r / min, to obtain a suspension containing a white powder; c. The suspension containing the white powder was filtered to obtain a white solid, which was ground using a ball mill until D50 was 0.4-0.7 μm, D90 was 1.1-1.4 μm and D99 < 2 μm to obtain MOFS powder containing lithium ions.

3. A long cycle battery separator according to claim 1, characterized in that: The nano ceramic material is selected from at least one of alumina, boehmite, magnesium hydroxide and aluminum hydroxide, and has a particle size of D50: 0.7-1.2 μm, D90 of 2.0-3.0 μm, and D99 < 4 μm.

4. A long cycle battery separator according to claim 1, characterized in that: The pore-forming agent is an alcohol compound, the adhesive is a polyacrylonitrile polymer, and the wetting agent is an isomeric alcohol polyether.

5. A long cycle battery separator according to claim 1, characterized in that: The particle size D50 of the slurry is 0.8-1.3 μm, D90 is 2.1-3.2 μm, and the viscosity is 60-120 mPa·s.

6. A long cycle battery separator according to claim 1, characterized in that: The composite coating has a thickness of 2-4 μm, and the polyolefin base film is made of polyethylene or polypropylene, and the base film has a thickness of 3-20 μm.

7. A method for preparing a long cycle battery separator according to any one of claims 1 to 6, characterized in that: The following steps are involved: S1. Mix 3-8 parts by mass of MOFS powder containing lithium ions with 28-35 parts by mass of deionized water, and disperse at a rotation speed of 1000-1500 r / min for 10-20 min; S2, adding 14-18 parts by mass of nano-ceramic material to the product of step S1, and dispersing at a rotation speed of 2000-2500 r / min for 60-90 min; S3, adding 4-8 parts by mass of a pore former, 2.6-4.3 parts by mass of a binder, and 0.5-0.8 parts by mass of a wetting agent to the product of step S2, and dispersing at a rotation speed of 400-800 r / min for 30-60 min to obtain a slurry; S4. Apply the slurry on the surface of the polyolefin base film and dry it at 60-85° C. for 2-4 minutes to form a coating.

8. The method for preparing a long cycle battery separator according to claim 7, characterized in that: The stirring in steps S1, S2 and S3 all uses a double planetary mixer, and the revolution speed is maintained at 35-45r / min.

9. The method for preparing a long cycle battery separator according to claim 7, characterized in that: In the step S4, the coating is carried out by quantitative transfer coating, and the coating line speed is 40-60 m / min.

10. The method for preparing a long cycle battery separator according to claim 7, characterized in that: In the step S4, the slurry is subjected to 5-8kHz ultrasonic treatment combined with ≤-0.8MPa vacuum degassing for 10-20min before coating.

Images