Battery diaphragm, preparation method thereof and secondary battery

Abstract

The invention relates to the technical field of batteries, in particular to a battery diaphragm, a preparation method thereof and a secondary battery. The diaphragm comprises a base membrane and a coating arranged on the base membrane, the average pore size of a pore structure on the surface of the coating is 0.2-0.8 mu m, and the number of formed pores with the pore size of 0.2-0.8 mu m on the surface of the coating accounts for 60%-90% of the number of all the formed pores; a high-molecular polymer in the coating comprises a copolymer containing a first monomer unit and a second monomer unit, the first monomer unit contains cyano groups on a side chain, and a second monomer forming the second monomer unit is selected from at least one of compounds with the following structural formula; s1 represents any one of alkyl groups with 1-4 carbon atoms, S2 represents any one of hydrogen atoms, halogen atoms or alkyl groups with 1-6 carbon atoms, and S3 represents any one of hydrogen atoms or alkyl groups with 1-10 carbon atoms. According to the diaphragm, the lithium ion transmission efficiency can be effectively improved while the high cohesiveness is ensured.

Description

Technical Field

[0001] This invention relates to the field of battery technology, and more specifically, to a battery separator, a method for preparing the same, and a secondary battery. Background Technology

[0002] With the development of new energy technologies, lithium-ion batteries have been widely used due to their advantages such as high energy density, long cycle life, and environmental friendliness. However, as battery energy density and charge / discharge rate continue to increase, the requirements for core materials such as the positive and negative electrodes, separators, and electrolytes are becoming increasingly stringent. As one of the four key materials in lithium-ion batteries, the separator's core function is to isolate the positive and negative electrodes to prevent short circuits and to provide lithium-ion transport channels. Its key indicators, such as thermal stability and adhesion performance with the positive and negative electrode sheets, directly determine the battery's interface structure and internal resistance characteristics, thus significantly affecting the battery's safety and electrochemical performance.

[0003] Existing membranes mostly use polyolefins as the base membrane and polyvinylidene fluoride (PVDF) as a coating binder to modify the membrane in hopes of improving interfacial stability. However, PVDF-based bonding systems have significant drawbacks: firstly, the adhesion between PVDF and the electrode sheets is weak; secondly, fluorine-containing materials can pose serious risks to human health.

[0004] Patent application CN120261919A discloses a composite separator with a coating containing a first type of pore structure and a second type of pore structure. The second type of pore structure has a size of 30nm-150nm. In actual electrolytes, lithium ions penetrate the separator through the second type of pore structure. The smaller pore size results in smaller lithium ion transport channels, affecting lithium ion transport efficiency. Although this solution can improve the separator's adhesion performance, the lithium ion transport efficiency still needs improvement.

[0005] Therefore, there is an urgent need to develop a fluorine-free material-coated diaphragm to improve the lithium-ion transport efficiency and adhesion of the diaphragm.

[0006] In view of this, the present invention is hereby proposed. Summary of the Invention

[0007] The purpose of this invention is to provide a battery separator, its preparation method, and a secondary battery. The separator coating material of this invention is fluorine-free and can improve the adhesion between the separator and the electrode. At the same time, it can also increase the pore size of the pore structure in the coating, thereby effectively improving the lithium-ion transport efficiency.

[0008] In order to achieve the above-mentioned objectives of the present invention, the following technical solution is adopted: A battery separator includes a base film and a coating disposed on at least one surface of the base film. The coating is a porous coating, wherein the average pore size of the pore structure on the coating surface is 0.2-0.8 μm, and the number of pores with a pore size between 0.2-0.8 μm accounts for 60%-90% of the total number of pores on the coating surface. The coating comprises a polymer, the polymer comprising a copolymer containing a first monomer unit and a second monomer unit, the first monomer unit containing a cyano group on its side chain, and the second monomer forming the second monomer unit being selected from at least one compound having the following structural formula: ; Wherein, S1 represents any one of the alkyl groups having 1-4 carbon atoms, S2 represents any one of the hydrogen atoms, halogen atoms, or alkyl groups having 1-6 carbon atoms, and S3 represents any one of the hydrogen atoms or alkyl groups having 1-10 carbon atoms.

[0009] Preferably, the second monomer unit includes a long carbon chain monomer unit and a short carbon chain monomer unit, wherein the long carbon chain monomer unit is formed from long carbon chain monomers and the short carbon chain monomer unit is formed from short carbon chain monomers; in the second monomer, when the number of carbon atoms in S3 is 5-10, it is called a long carbon chain monomer, and when the number of carbon atoms in S3 is 0-4, it is called a short carbon chain monomer; the difference in the number of carbon atoms in S3 between the long carbon chain monomer and the short carbon chain monomer is ≥3.

[0010] Preferably, the long carbon chain monomer unit accounts for 40%-80% of the weight percentage of the second monomer unit; more preferably, it accounts for 40%-60%.

[0011] Preferably, the long-chain monomer includes at least one of n-pentyl acrylate, hexyl acrylate, heptyl acrylate, 2-ethylhexyl acrylate, and isononyl acrylate.

[0012] Preferably, the short-chain monomer includes at least one of butyl acrylate, propyl acrylate, ethyl acrylate, methyl acrylate, and acrylic acid.

[0013] Preferably, in the copolymer, the weight percentage of the second monomer unit is 5%-30%.

[0014] Preferably, the first monomer forming the first monomer unit includes at least one of acrylonitrile, methacrylonitrile, cyanoacrylate, fumaric acid, and p-cyano-styrene.

[0015] Preferably, the weight-average molecular weight of the polymer is 100,000 to 2,000,000.

[0016] Preferably, the glass transition temperature (Tg) of the polymer is 20-140°C.

[0017] Preferably, the swelling degree of the polymer in the lithium electrolyte is 10%-200%, more preferably 75%-95%.

[0018] Preferably, the coating further contains inorganic particles; wherein the inorganic particles include at least one of alumina, boehmite, barium sulfate, barium titanate, magnesium hydroxide, silicon dioxide, and aluminum nitride, and / or the D50 particle size of the inorganic particles is 0.1-2.0 μm, and / or the weight ratio of the inorganic particles to the polymer is 10-70:30-90.

[0019] Preferably, the thickness of the coating on one side is 0.1-3 μm.

[0020] Preferably, the base film comprises a polyethylene base film and / or a polypropylene base film.

[0021] Preferably, the coating has a Gurly value of 10s / 100cc to 100s / 100cc.

[0022] Preferably, the Gury value of the coating increases by 1%-20% after immersion in lithium electrolyte.

[0023] The method for preparing the battery separator according to any one of the foregoing embodiments includes the following steps: A slurry containing a polymer, a solvent, and a pore-forming agent is prepared, and the slurry is coated on at least one side surface of a base film. After coagulation bath treatment, the film is dried to obtain the battery separator. The pore-forming agent includes at least one of water, methanol, ethanol, tripropylene glycol, polyethylene glycol, and polyvinylpyrrolidone.

[0024] Preferably, the slurry further contains inorganic particles; wherein the inorganic particles include at least one of alumina, boehmite, barium sulfate, barium titanate, magnesium hydroxide, silicon dioxide, and aluminum nitride, and / or the D50 particle size of the inorganic particles is 0.1-2.0 μm, and / or the weight ratio of the inorganic particles to the polymer is 10-70:30-90.

[0025] Preferably, the amount of pore-forming agent added is 0.5%-20% of the mass of the slurry.

[0026] Preferably, the solvent is a good solvent for polymers, and the good solvent includes at least one of NMP, DMAC, DMF, and DMSO.

[0027] Preferably, the coagulation bath includes a primary coagulation bath, a secondary coagulation bath, a tertiary coagulation bath, a quaternary coagulation bath, and a quinary coagulation bath. The primary, secondary, and tertiary coagulation baths are all mixed solutions of a good solvent for the polymer and water, with the concentration of the good solvent decreasing sequentially. The quaternary and quinary coagulation baths are both deionized water.

[0028] A secondary battery, comprising the battery separator described in any of the foregoing embodiments.

[0029] Compared with the prior art, the beneficial effects of the present invention are as follows: (1) The average pore size of the pore structure on the surface of the diaphragm coating of the present invention is 0.2-0.8μm, and the number of pores with a pore size between 0.2-0.8μm accounts for 60%-90% of the total number of pores. The diaphragm has a large lithium-ion transport channel, which can effectively improve the lithium-ion transport efficiency, while ensuring the coating coverage and the contact area between the coating and the electrode sheet, thereby ensuring the adhesion performance.

[0030] (2) In the diaphragm coating of the present invention, a fluorine-free polymer is used as the binder component, which is clean and environmentally friendly. The polymer used includes a copolymer containing a first monomer unit and a second monomer unit. The abundant cyano groups of the first monomer unit form an intermolecular interactive network through hydrogen bonding and dipole interaction, which has strong oxidation resistance. The introduction of the second monomer unit disrupts the regularity of its molecular chain, so that the polymer has better dispersibility and solubility in the solvent. At the same time, the second monomer unit provides adhesive ester groups, which can improve the adhesion between the diaphragm and the electrode sheet and achieve a wider bonding temperature and pressure range. Meanwhile, by controlling the difference in the number of carbon atoms in the long carbon chain monomer and the short carbon chain monomer S3 in the second monomer, the hydrophobicity of the polymer is improved, which is conducive to the formation of a larger pore structure in the polymer coating during solution phase transformation, thereby improving the lithium ion transport efficiency.

[0031] (3) The process flow of the present invention is short, the operation is simple, and it is easy to industrialize. By adding a pore-forming agent to the slurry, the polymer solution is made to produce liquid-liquid phase separation in advance. After being immersed in the coagulation bath, the pore structure formed is enlarged, the lithium ion transport channel of the membrane is enlarged, and the lithium ion transport efficiency is improved. Attached Figure Description

[0032] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0033] Figure 1 This is a surface morphology diagram of the diaphragm coating provided in Embodiment 1 of the present invention. Detailed Implementation

[0034] The technical solution of the present invention will be clearly and completely described below with reference to the accompanying drawings and specific embodiments. However, those skilled in the art will understand that the embodiments described below are some embodiments of the present invention, but not all embodiments, and are only used to illustrate the present invention, and should not be regarded as limiting the scope of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall be followed. Where the manufacturers of reagents or instruments are not specified, they are all conventional products that can be purchased commercially.

[0035] A first aspect of the present invention provides a battery separator, comprising a base film and a coating disposed on at least one surface of the base film. The coating is a porous coating, wherein the average pore size of the pore structure on the coating surface is 0.2-0.8 μm, for example, it can be any single value or a range of any two values ​​selected from 0.2 μm, 0.3 μm, 0.44 μm, 0.56 μm, 0.66 μm, and 0.8 μm; the number of pores with a pore size between 0.2-0.8 μm on the coating surface accounts for 60%-90% of the total number of pores, for example, it can be any single value or a range of any two values ​​selected from 60%, 65%, 70%, 75%, 80%, 85%, and 90%. The coating comprises a polymer, which includes a copolymer containing a first monomer unit and a second monomer unit, wherein the first monomer unit contains a cyano group on its side chain, and the second monomer forming the second monomer unit is selected from at least one compound having the following structural formula: ; Wherein, S1 represents any one of the alkyl groups having 1-4 carbon atoms, S2 represents any one of the hydrogen atoms, halogen atoms, or alkyl groups having 1-6 carbon atoms, and S3 represents any one of the hydrogen atoms or alkyl groups having 1-10 carbon atoms.

[0036] The membrane coating of this invention has a large pore structure with an average pore size of 0.2-0.8 μm. The number of pores with a diameter between 0.2-0.8 μm accounts for 60%-90% of all pores on the coating surface. This results in a large lithium-ion transport channel, high pore concentration, and minimal pore size variation, effectively improving lithium-ion transport efficiency while ensuring coating coverage and maintaining the contact area between the coating and the electrode, thus guaranteeing adhesion performance. If the average pore size is too small, the coating may swell and clog pores when immersed in the electrolyte, leading to poor lithium-ion transport efficiency. If the average pore size is too large, the pore concentration will be low, and the pore size variation will be large, affecting lithium-ion conduction. Alternatively, the coating coverage may be small, reducing the contact area between the coating and the positive and negative electrodes, weakening adhesion. If the proportion of pores with a diameter between 0.2-0.8 μm is too low, it indicates a large pore size variation, which affects lithium-ion conduction.

[0037] The polymer in the coating of this invention comprises a copolymer containing a first monomer unit and a second monomer unit. The abundant cyano groups in the first monomer unit form an intermolecular interactive network through hydrogen bonding and dipole interactions, exhibiting strong oxidation resistance. The introduction of the second monomer unit disrupts the regularity of its molecular chain, giving the polymer better dispersibility and solubility in solvents. At the same time, the second monomer unit provides adhesive ester groups, which can significantly improve the adhesion between the polymer and the positive and negative electrodes containing active materials, achieving a wider range of bonding temperature and pressure.

[0038] In some specific embodiments of the present invention, the second monomer unit includes a long-chain monomer unit and a short-chain monomer unit. The long-chain monomer unit is formed from long-chain monomers, and the short-chain monomer unit is formed from short-chain monomers. In the second monomer, when the number of carbon atoms in S3 is 5-10, it is called a long-chain monomer; when the number of carbon atoms in S3 is 0-4, it is called a short-chain monomer. When the number of carbon atoms in S3 is 0, S3 is hydrogen atoms. The difference in the number of carbon atoms in S3 between the long-chain monomer and the short-chain monomer is ≥3. For example, the difference can be any value from 3, 4, 5, 6, 7, 8, 9, or a range of any two values. When the difference in the number of carbon atoms in S3 between the long-chain monomer and the short-chain monomer is ≥3, the polarity difference between the long-chain monomer and the short-chain monomer is relatively large, which can increase the hydrophobicity of the polymer. This is beneficial for the polymer coating to form a larger pore structure during solution phase transition, increasing the lithium-ion transport channels of the membrane and thus improving the lithium-ion transport efficiency.

[0039] In some specific embodiments of the present invention, the weight percentage of long carbon chain monomer units in the second monomer unit is 40%-80% by weight, for example, it can be any one value or a range of any two values ​​from 40%, 48%, 55%, 62%, 70%, 80%; the balance is short carbon chain monomers; in some preferred embodiments, the weight percentage of long carbon chain monomer units in the second monomer unit is 40%-60%, and the balance is short carbon chain monomers.

[0040] When the number of carbon atoms in the S3 group is greater than 10, it will increase the difficulty of copolymerizing the second monomer with the first monomer. At the same time, the long carbon chain monomer unit has poor solubility, which reduces the solubility of the polymer. Therefore, the number of carbon atoms in the S3 group in the long carbon chain monomer unit should be controlled to be ≤10.

[0041] When the proportion of long-chain monomer units in the second monomer unit is too high (e.g., >80%), the ability of the polymer to absorb electrolyte increases, resulting in increased swelling and a larger increase in the Gury value after immersion in the diaphragm electrolyte; at the same time, it reduces the solubility of the polymer. When the proportion of long carbon chain monomer units in the second monomer unit is too low and the proportion of short carbon chain monomer units is too high, the adhesion between the short carbon chain monomer units and the positive and negative electrode plates becomes worse because the short carbon chain monomer units have higher polarity and stronger rigidity than the long carbon chain monomer units.

[0042] In this invention, the second monomer unit contains both long-chain and short-chain monomer units. By controlling the number of carbon atoms in the S3 group of the long-chain monomer unit to be 5-10 and by reasonably controlling the weight ratio of the long-chain monomer unit in the second monomer unit, the aforementioned defects can be overcome, improving the solubility of the polymer in the solvent and its adhesion to the positive and negative electrodes. By controlling the difference in the number of carbon atoms in the S3 group between the long-chain and short-chain monomers to be ≥3, the polarity difference between the long-chain and short-chain monomers is relatively large, which increases the hydrophobicity of the polymer. The pore-forming agent is generally highly polar, while the polymer is hydrophobic and relatively weakly polar. The large polarity difference between the pore-forming agent and the polymer makes it easy for the pore-forming agent and the polymer in the slurry to undergo liquid-liquid phase separation, which is beneficial for the formation of a larger pore structure in the polymer coating during solution phase transformation, thereby improving lithium-ion transport efficiency.

[0043] In some specific embodiments of the present invention, the long carbon chain monomer includes at least one of n-pentyl acrylate, hexyl acrylate, heptyl acrylate, 2-ethylhexyl acrylate, and isononyl acrylate.

[0044] In some specific embodiments of the present invention, the short carbon chain monomer includes at least one of butyl acrylate, propyl acrylate, ethyl acrylate, methyl acrylate, and acrylic acid.

[0045] In some specific embodiments of the present invention, the weight percentage of the second monomer unit in the copolymer is 5%-30%, for example, it can be any value or a range of any two values ​​from 5%, 10%, 15%, 20%, 25%, 30%. When the weight percentage of the second monomer unit is less than 5%, the molecular chain formed by the first monomer unit in the polymer has high regularity and high polarity, resulting in reduced solubility in the solvent. After film formation, the adhesion between the polymer and the non-polar polyolefin base film is reduced, thereby reducing the adhesion between the separator and the electrode. When the weight percentage of the second monomer unit is greater than 30%, the proportion of the first monomer unit decreases, resulting in increased polymer swelling. After the separator is soaked in electrolyte, it is easy to clog the pores, and the gas permeability of the separator increases.

[0046] In some specific embodiments of the present invention, the first monomer forming the first monomer unit includes at least one of acrylonitrile, methacrylonitrile, cyanoacrylate, fumaric acid, and p-acrylonitrile styrene.

[0047] In some specific embodiments of the present invention, the weight-average molecular weight of the polymer is 100,000 to 2,000,000. For example, it can be any one value or a range of any two values ​​from 100,000, 200,000, 400,000, 600,000, 800,000, 1,000,000, 1,500,000, and 2,000,000.

[0048] In some specific embodiments of the present invention, the glass transition temperature Tg of the polymer is 20-140°C, for example, it can be any one value or a range of any two values ​​among 20°C, 40°C, 60°C, 80°C, 100°C, 120°C, and 140°C.

[0049] In some embodiments of the present invention, the copolymer in the polymer is mainly obtained by polymerization of a first monomer (such as acrylonitrile) and a second monomer (such as hexyl acrylate, ethyl acrylate and acrylic acid); the polymerization reaction is carried out in water under the initiator.

[0050] In some embodiments of the present invention, the initiators used in the polymerization reaction include, but are not limited to, azobisisobutyronitrile (AIBN), potassium persulfate, ammonium persulfate, benzoyl peroxide, and di-tert-butyl peroxide, and the amount of initiator used is 0.2%-0.4% of the total amount of monomers used.

[0051] In some embodiments of the present invention, the polymerization reaction system also contains a chain transfer agent. As an example, the chain transfer agent includes, but is not limited to, thiol chain transfer agents, such as n-dodecyl mercaptan. The amount of chain transfer agent used is 0.05%-0.3% of the total amount of monomers used. The specific amount can be adjusted according to the target molecular weight of the copolymer.

[0052] In some specific embodiments of the present invention, the swelling degree of the polymer in the lithium electrolyte is 10%-200%, for example, it can be any one value or a range of any two values ​​from 10%, 30%, 50%, 76%, 92%, 103%, 150%, and 200%; preferably 75%-95%. Excessive swelling will affect ion conduction, thereby affecting the electrochemical performance of the battery; insufficient swelling will lead to reduced adhesion between the separator and the electrode. Controlling the swelling degree within the above range is beneficial for obtaining better adhesion and electrochemical performance.

[0053] In some specific embodiments of the present invention, the content of the polymer (including copolymers containing a first monomer unit and a second monomer unit) in all polymers within the coating is ≥80% by weight. For example, it can be any one value or a range of any two values ​​among 80%, 85%, 90%, 95%, and 100%.

[0054] In some specific embodiments of the present invention, the coating further contains inorganic particles; wherein the inorganic particles include at least one selected from alumina, boehmite, barium sulfate, barium titanate, magnesium hydroxide, silicon dioxide, and aluminum nitride; and / or, the D50 particle size of the inorganic particles is 0.1-2.0 μm, for example, it can be any single value or a range of any two values ​​selected from 0.1 μm, 0.5 μm, 1.0 μm, 1.5 μm, and 2.0 μm; and / or, the weight ratio of the inorganic particles to the polymer is 10-70:30-90, for example, it can be any single value or a range of any two values ​​selected from 10:90, 20:80, 30:70, 40:60, 50:50, 60:40, and 70:30. Introducing inorganic particles into the separator helps to improve the thermal stability of the separator and ensure the safety of the battery.

[0055] In some specific embodiments of the present invention, the thickness of the coating on one side of the diaphragm is 0.1-3 μm. For example, it can be any one value or a range of any two values ​​from 0.1 μm, 0.5 μm, 1.0 μm, 1.5 μm, 1.9 μm, 2.4 μm, and 3.0 μm.

[0056] In some specific embodiments of the present invention, the base film used includes polyethylene base film and / or polypropylene base film.

[0057] In some specific embodiments of the present invention, the Gurly value of the coating is 10s / 100cc-100s / 100cc, for example, it can be any one value or a range of any two values ​​among 10s / 100cc, 35s / 100cc, 43s / 100cc, 60s / 100cc, 80s / 100cc, and 100s / 100cc.

[0058] In some specific embodiments of the present invention, the growth rate of the Gury value of the coating after immersion in lithium electrolyte is 1%-20%, for example, it can be any one value or a range of any two values ​​among 1%, 6.6%, 9.6%, 13.4%, 15%, and 20%; the growth rate of the Gury value refers to the growth rate of the Gury value after the diaphragm is immersed in lithium salt electrolyte at 85°C for 24 hours.

[0059] A second aspect of the present invention provides a method for preparing a battery separator according to any one of the foregoing embodiments, comprising the following steps: A slurry containing a polymer, solvent and pore-forming agent is prepared, the slurry is coated on at least one side of the base film, and dried after coagulation bath treatment to obtain a battery separator. The pore-forming agent includes at least one of water, methanol, ethanol, tripropylene glycol, polyethylene glycol, and polyvinylpyrrolidone.

[0060] The preparation process of this invention is simple. By adding a pore-forming agent to the slurry, the polymer solution undergoes liquid-liquid phase separation in advance. After immersion in the coagulation bath, the pore structure formed is enlarged, and the lithium-ion transport channels of the membrane are enlarged, which is beneficial to improving the lithium-ion transport efficiency.

[0061] In some specific embodiments of the present invention, the slurry further contains inorganic particles; wherein, the inorganic particles include at least one of alumina, boehmite, barium sulfate, barium titanate, magnesium hydroxide, silicon dioxide, and aluminum nitride, and / or, the D50 particle size of the inorganic particles is 0.1-2.0 μm (including but not limited to any one value or a range of any two values ​​among 0.1 μm, 0.5 μm, 1.0 μm, 1.5 μm, and 2.0 μm), and / or, the weight ratio of the inorganic particles to the weight of the polymer is 10-70:30-90 (including but not limited to any one value or a range of any two values ​​among 10:90, 20:80, 30:70, 40:60, 50:50, 60:40, and 70:30).

[0062] In some specific embodiments of the present invention, the solvent used is a good solvent for polymers, including at least one of NMP, DMAC, DMF, and DMSO.

[0063] In some specific embodiments of the present invention, the method for preparing the battery separator specifically includes the following steps: dissolving a polymer in a first solvent, adding inorganic particles, then adding a mixture of a second solvent and a pore-forming agent, mixing evenly to form a slurry, coating the slurry onto at least one side surface of a base membrane, and drying after coagulation bath treatment to obtain the battery separator. Both the first solvent and the second solvent are good solvents for the polymer (e.g., at least one of NMP, DMAC, DMF, and DMSO), and the first solvent and the second solvent can be the same or different.

[0064] In some specific embodiments of the present invention, the amount of pore-forming agent added is 0.5%-20% of the slurry mass. For example, it can be any one value or a range of any two values ​​from 0.5%, 1%, 4%, 8%, 10%, 15%, 20%.

[0065] In some specific embodiments of the present invention, the coagulation bath includes a primary coagulation bath, a secondary coagulation bath, a tertiary coagulation bath, a quaternary coagulation bath, and a quinary coagulation bath; wherein, the primary coagulation bath, the secondary coagulation bath, and the tertiary coagulation bath are all mixed solutions of a good solvent for polymers and water, and the concentration of the good solvent decreases sequentially; the quaternary coagulation bath and the quinary coagulation bath are both deionized water.

[0066] A third aspect of the present invention provides a secondary battery comprising the battery separator described in any of the foregoing embodiments.

[0067] The following detailed description of some embodiments of the present invention is provided in conjunction with specific application examples. Unless otherwise specified, all raw materials used in the embodiments can be obtained commercially available.

[0068] Example 1 (1) Add 135g of hexyl acrylate, 141g of ethyl acrylate, 965g of acrylonitrile and 4g of acrylic acid to a reactor containing 2kg of deionized water, heat to 65℃, stir evenly and then gradually add 3g of azobisisobutyronitrile and 1g of n-dodecyl mercaptan. React for 10h to generate a polymer emulsion. After spray drying the emulsion, obtain polymer powder. (2) Place 510g NMP into a beaker, add 48g of polymer powder, stir and dissolve at 25℃ and 1500rpm for 2h to form a polymer solution; (3) Stir 150g NMP and 30g deionized water at 800rpm for 10min to form a pore-forming agent solution; (4) Take 12g of alumina powder with D50 of 0.6μm, add it to the polymer solution, stir at 1500rpm for 3h, then add the pore-forming agent solution and stir at 1200rpm for 30min to form a mixed slurry; (5) The above mixed slurry is coated on one side of a polyethylene film with a thickness of 7 μm, and then passed through a coagulation bath of aqueous solution with concentrations of 50% NMP, 30% NMP, and 10% NMP and two deionized water coagulation baths in sequence. After that, it is dried in an oven at 65°C to form a porous coating and obtain a battery separator. The thickness of the coating is shown in Table 2.

[0069] Example 2 The preparation method of Example 2 is the same as that of Example 1, except that the amount of raw materials used in step (1) is different; Step (1) of this embodiment includes: adding 155g of hexyl acrylate, 121g of ethyl acrylate, 965g of acrylonitrile and 4g of acrylic acid into a reaction vessel containing 2kg of deionized water, heating to 65°C, stirring evenly and then gradually adding 3g of azobisisobutyronitrile and 1g of n-dodecyl mercaptan, reacting for 10h to generate a polymer emulsion, and spray drying the emulsion to obtain polymer powder; The remaining preparation conditions were the same as in Example 1.

[0070] Example 3 The preparation method of Example 3 is the same as that of Example 1, except that the amount of raw materials used in step (1) is different; Step (1) of this embodiment includes: adding 175g of hexyl acrylate, 101g of ethyl acrylate, 965g of acrylonitrile and 4g of acrylic acid into a reaction vessel containing 2kg of deionized water, heating to 65°C, stirring evenly and then gradually adding 3g of azobisisobutyronitrile and 1g of n-dodecyl mercaptan, reacting for 10h to generate a polymer emulsion, and spray drying the emulsion to obtain polymer powder; The remaining preparation conditions were the same as in Example 1.

[0071] Example 4 The preparation method of Example 4 is the same as that in Example 1, except that in step (1), ethyl acrylate is replaced with propyl acrylate; The remaining preparation conditions were the same as in Example 1.

[0072] Example 5 The preparation method of Example 5 is the same as that in Example 1, except that: in step (1), hexyl acrylate is replaced with an equal amount of heptyl acrylate; in step (3), deionized water is replaced with an equal amount of tripropylene glycol. The remaining preparation conditions were the same as in Example 1.

[0073] Example 6 The preparation method of Example 6 is the same as that in Example 1, except that in step (3), 177g of NMP and 8g of deionized water are stirred at 800rpm for 10min to form a pore-forming agent solution. The remaining preparation conditions were the same as in Example 1.

[0074] Comparative Example 1 The preparation method of Comparative Example 1 is the same as that in Example 1, except that step (1) is not performed and the polymer powder in step (2) is replaced with polyvinylidene fluoride. The remaining preparation conditions were the same as in Example 1.

[0075] Comparative Example 2 The preparation method of Comparative Example 2 is the same as that in Example 1, except that in step (1), ethyl acrylate is replaced with butyl acrylate; The remaining preparation conditions were the same as in Example 1.

[0076] Comparative Example 3 The preparation method of Comparative Example 3 is the same as that in Example 1, except that in step (3), deionized water is replaced with NMP; The remaining preparation conditions were the same as in Example 1.

[0077] Comparative Example 4 The preparation method of Comparative Example 4 is the same as that in Example 1, except that: step (2) is changed to: put 690g NMP into a beaker, add 24g polymer powder, stir and dissolve at 25℃ and 1500rpm for 2h, then add 24g polyimide and continue stirring for 1h to form a polymer solution; step (3) is deleted, and in step (4), alumina powder is directly added to the polymer solution and stirred to form a mixed slurry; The remaining preparation conditions were the same as in Example 1.

[0078] Comparative Example 5 The preparation method of Comparative Example 5 is the same as that in Example 1, except that the amount of raw materials used in step (1) is different; In this comparative example, step (1) includes: adding 215g of hexyl acrylate, 229g of ethyl acrylate, 797g of acrylonitrile, and 4g of acrylic acid to a reaction vessel containing 2kg of deionized water, heating to 65℃, stirring evenly, and then gradually adding 3g of azobisisobutyronitrile and 1g of n-dodecyl mercaptan. The reaction is carried out for 10h to generate a polymer emulsion. After spray drying, the emulsion is used to obtain polymer powder. The remaining preparation conditions were the same as in Example 1.

[0079] Comparative Example 6 The preparation method of Comparative Example (6) is the same as that in Example (1), except that: in step (1), 135g of hexyl acrylate and 141g of ethyl acrylate are replaced with 276g of butyl acrylate; in step (3), deionized water is replaced with NMP. The remaining preparation conditions were the same as in Example 1.

[0080] Test methods 1. Swelling degree test of polymers Take 1g of polymer M1, dissolve it in NMP, pour it into a petri dish, and dry it at 60℃ to obtain a polymer film. Place the film in LiPF6 electrolyte (EC:PC:DEC:EP=3:1:3:3, 1 Mol / L LiPF6) at 60℃ for 1 day, filter it, and weigh the polymer M2g after soaking. The swelling degree of the polymer is (M2- M1) / M1×100%.

[0081] 2. Gurly value test of the coating Polyolefin-based membrane and diaphragm samples, each 200mm × 200mm in size, were tested using a Wang Yan-style gas permeability meter. Under a set pressure, gas was passed through the sample, and the time it took for a certain volume of gas to pass through was recorded, expressed as s / 100cc. Six points were tested, and the average value was taken. The gas permeability values ​​of the porous membrane and diaphragm were G1 and G2, respectively, and the Gurly value of the coating was (G2-G1) s / 100cc.

[0082] 3. Gury value growth rate test of the coating Take the diaphragm, test the Gurly value S1, and then place it in an appropriate amount of electrolyte (EC:PC:DEC:EP=3:1:3:3, 1Mol / L LiPF6) at 85℃ for 24h. Dry the diaphragm at 60℃ and test the Gurly value S2 of the diaphragm. The growth rate of the Gurly value of the coating is (S2-S1) / S1×100%.

[0083] 4. Coating surface pore size test Ten 10K magnification images of the diaphragm coating surface morphology were obtained using a scanning electron microscope (SEM). Using the Nano Measurer software, the sizes of all pores on the coating surface in the 20K magnification images were measured as d1, d2, ..., dn. The number of pores with a diameter between 0.2 μm and 0.8 μm (a) and the total number of pores (n) in the images were recorded. The average pore size D was calculated as D = (d1 + d2 + ... + dn) / n (μm). The ratio of the number of pores with a diameter between 0.2 μm and 0.8 μm to the total number of pores was calculated, i.e., the pore density (a / n × 100%).

[0084] 5. Adhesion between the diaphragm and the electrode The separator and negative electrode sheet were cut into 2cm strips. The coated side of the separator was then bonded to the negative electrode sheet using a hot press. The adhesion between the separator and the negative electrode sheet was then tested at 180° using a tensile testing machine. The unit of adhesion strength is N / m. The hot pressing conditions were set to 95℃, 1.5MPa, and 3min. The tensile testing machine speed was set to 60mm / min. The negative electrode sheet composition was: 96% artificial graphite, 1.5% CMC, 1.5% SBR, and 1% SP, with an areal density of 135g / m³. 2 Compacted density -1.56 g / cm³ 3 .

[0085] 6. Ionic conductivity performance Inside an argon-filled glove box, a 1616 button cell was fabricated using a separator. An appropriate amount of electrolyte (EC:PC:DEC:EP = 3:1:3:3, 1 Mol / L LiPF6) was added. Using an electrochemical workstation, the AC impedance was measured, yielding σ = L / (Rb × A), where σ is the ionic conductivity (mS / cm); L is the separator thickness (cm); Rb is the intrinsic resistance of the separator (Ω); and A is the effective area (cm²). 2 ).

[0086] The test results are shown in Tables 1 and 2. In Table 1, the proportion of long carbon chain monomer units refers to the weight proportion of long carbon chain monomer units in the second monomer unit, and the proportion of the second monomer unit refers to the weight proportion of the second monomer unit in the copolymer.

[0087] Table 1

[0088] Table 2

[0089] As shown in Tables 1 and 2, in Examples 1-3, the proportion of long-chain monomer units increased, the growth rate of polymer swelling and Gury value of the membrane coating increased, and the ionic conductivity of the membrane decreased. The coating surface had a high pore concentration and relatively high ionic conductivity.

[0090] In Example 4, based on Example 1, the difference in the number of carbon atoms in S3 of the long-chain monomer and the short-chain monomer decreased, the hydrophobicity of the polymer weakened, the average pore size of the coating surface decreased, and the ionic conductivity of the membrane decreased. In Example 5, based on Example 1, the difference in the number of carbon atoms in S3 of the long-chain monomer and the short-chain monomer increased, the hydrophobicity of the polymer was enhanced, the average pore size of the coating surface increased, and the ionic conductivity of the membrane increased.

[0091] In Example 6, the amount of pore-forming agent was reduced, the average pore size of the coating surface was reduced, and the ionic conductivity of the membrane was reduced.

[0092] Comparative Example 1 uses polyvinylidene fluoride as the polymer, and the membrane has relatively low adhesion to the negative electrode.

[0093] Compared with the Example 2, the difference in the number of carbon atoms in S3 between the long-chain monomer and the short-chain monomer decreased, the hydrophobicity of the polymer was weakened, the average pore size of the coating surface decreased, the pore concentration rate of the coating decreased, the growth rate of the Gury value of the diaphragm coating increased, and the ionic conductivity of the diaphragm decreased.

[0094] In Comparative Example 3, no pore-forming agent was used, resulting in a decrease in the average pore size of the coating surface, a decrease in the pore concentration rate of the coating, an increase in the growth rate of the Gury value of the diaphragm coating, and a decrease in the ionic conductivity of the diaphragm.

[0095] In Comparative Example 4, the proportion of polymer in the coating decreased, the adhesion of the diaphragm to the negative electrode decreased, the average pore size of the coating surface decreased, the pore concentration rate of the coating decreased, the growth rate of the Gury value of the diaphragm coating increased, and the ionic conductivity of the diaphragm decreased.

[0096] In Comparative Example 5, the proportion of the first monomer decreased, the growth rate of the polymer swelling degree and the Gury value of the membrane coating increased, the membrane was prone to pore blockage when immersed in electrolyte, and the membrane ionic conductivity decreased.

[0097] In Comparative Example 6, the removal of long-chain monomer units enhances the polarity of the polymer, weakens its hydrophobicity, reduces the average pore size of the coating surface, lowers the pore concentration rate of the coating, increases the growth rate of the Gury value of the diaphragm coating, and reduces the ionic conductivity of the diaphragm.

[0098] Depend on Figure 1 It can be seen that the membrane coating prepared in the embodiments of the present invention is a porous coating, and most of the pores on the surface have a pore size between 0.2-0.8 μm, which has a large lithium ion transport channel, a high pore concentration rate, and a small difference in pore size, which is beneficial to lithium ion conduction.

[0099] Although the present invention has been illustrated and described with specific embodiments, it should be understood that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; those skilled in the art should understand that modifications can be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein, without departing from the spirit and scope of the present invention; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention; therefore, this means that all such substitutions and modifications that fall within the scope of the present invention are included in the appended claims.

Claims

1. A battery separator, characterized in that, The coating includes a base film and a coating disposed on at least one surface of the base film. The coating is a porous coating, wherein the average pore size of the pore structure on the coating surface is 0.2-0.8 μm, and the number of pores with a pore size between 0.2-0.8 μm accounts for 60%-90% of the total number of pores on the coating surface. The coating comprises a polymer, the polymer comprising a copolymer containing a first monomer unit and a second monomer unit, the first monomer unit containing a cyano group on its side chain, and the second monomer forming the second monomer unit being selected from at least one compound having the following structural formula: ； Wherein, S1 represents any one of the alkyl groups having 1-4 carbon atoms, S2 represents any one of the hydrogen atoms, halogen atoms, or alkyl groups having 1-6 carbon atoms, and S3 represents any one of the hydrogen atoms or alkyl groups having 1-10 carbon atoms.

2. The battery separator according to claim 1, characterized in that, The second monomer unit includes long-chain monomer units and short-chain monomer units. The long-chain monomer unit is formed from long-chain monomers, and the short-chain monomer unit is formed from short-chain monomers. In the second monomer, when the number of carbon atoms in S3 is 5-10, it is called a long-chain monomer, and when the number of carbon atoms in S3 is 0-4, it is called a short-chain monomer. The difference in the number of carbon atoms in S3 between the long-chain monomer and the short-chain monomer is ≥3.

3. The battery separator according to claim 2, characterized in that, The long carbon chain monomer unit accounts for 40%-80% of the weight of the second monomer unit; preferably 40%-60%.

4. The battery separator according to claim 2 or 3, characterized in that, It meets at least one of the following characteristics: (1) The long carbon chain monomer includes at least one of n-pentyl acrylate, hexyl acrylate, heptyl acrylate, 2-ethylhexyl acrylate, and isononyl acrylate; (2) The short carbon chain monomers include at least one of butyl acrylate, propyl acrylate, ethyl acrylate, methyl acrylate, and acrylic acid.

5. The battery separator according to claim 1, characterized in that, It meets at least one of the following characteristics: (1) In the copolymer, the weight percentage of the second monomer unit is 5%-30%; (2) The first monomer forming the first monomer unit includes at least one of acrylonitrile, methacrylonitrile, cyanoacrylate, fumaric acid, and p-acrylonitrile; (3) The weight-average molecular weight of the polymer is 100,000 to 2,000,000; (4) The glass transition temperature (Tg) of the polymer is 20-140℃; (5) The swelling degree of the polymer in the lithium electrolyte is 10%-200%, preferably 75%-95%.

6. The battery separator according to claim 1, characterized in that, The coating also contains inorganic particles; wherein the inorganic particles include at least one of alumina, boehmite, barium sulfate, barium titanate, magnesium hydroxide, silicon dioxide, and aluminum nitride, and / or the D50 particle size of the inorganic particles is 0.1-2.0 μm, and / or the weight ratio of the inorganic particles to the weight of the polymer is 10-70:30-90.

7. The battery separator according to claim 1, characterized in that, It meets at least one of the following characteristics: (1) The thickness of the coating on one side is 0.1-3 μm; (2) The base film includes a polyethylene base film and / or a polypropylene base film; (3) The Gurly value of the coating is 10s / 100cc-100s / 100cc; (4) The Gury value of the coating increases by 1%-20% after being immersed in lithium electrolyte.

8. The method for preparing the battery separator according to any one of claims 1-7, characterized in that, Includes the following steps: A slurry containing a polymer, a solvent, and a pore-forming agent is prepared, and the slurry is coated on at least one side surface of a base film. After coagulation bath treatment, the film is dried to obtain the battery separator. The pore-forming agent includes at least one of water, methanol, ethanol, tripropylene glycol, polyethylene glycol, and polyvinylpyrrolidone.

9. The method for preparing the battery separator according to claim 8, characterized in that, It meets at least one of the following characteristics: (1) The slurry further contains inorganic particles; wherein the inorganic particles include at least one of alumina, boehmite, barium sulfate, barium titanate, magnesium hydroxide, silicon dioxide, and aluminum nitride, and / or the D50 particle size of the inorganic particles is 0.1-2.0 μm, and / or the weight ratio of the inorganic particles to the weight of the polymer is 10-70:30-90; (2) The amount of the pore-forming agent added is 0.5%-20% of the mass of the slurry; (3) The coagulation bath includes a primary coagulation bath, a secondary coagulation bath, a tertiary coagulation bath, a quaternary coagulation bath and a quinary coagulation bath. The primary coagulation bath, the secondary coagulation bath and the tertiary coagulation bath are all mixed solutions of the good solvent of the polymer and water, and the concentration of the good solvent decreases in sequence. The quaternary coagulation bath and the quinary coagulation bath are both deionized water.

10. A secondary battery, characterized in that, Includes the battery separator as described in any one of claims 1-7.

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