Battery separator preparation method, battery separator, secondary battery, battery module, battery pack and electrical device.

Abstract

This application relates to a method for preparing a battery separator, a battery separator, a secondary battery, a battery module, a battery pack, and an electrical device. The battery separator includes a polyimide film, which is prepared by imidization of a polyamic acid-amine salt with a cross-linked structure. This results in a narrower pore size distribution in the polyimide film and better mechanical properties, thereby benefiting the performance of the secondary battery, battery module, battery pack, and electrical device.

Description

Technical Field

[0001] This application relates to the field of batteries, specifically to a method for preparing a battery separator, a battery separator, a secondary battery, a battery module, a battery pack, and an electrical device. Background Technology

[0002] A battery separator is a porous insulating film and an important component of a battery. It separates the positive and negative electrodes, preventing short circuits between them inside the battery. The separator contains nanoscale pores that allow lithium ions and other ions to pass freely during charging and discharging, providing a pathway for the rapid transport of lithium ions between the positive and negative electrodes.

[0003] In cases of overcharging or improper use, the battery may overheat internally or externally, causing the internal temperature to exceed 160°C. Existing secondary battery separators are mainly polyolefin separators, which are prone to shrinkage or melting at high temperatures, leading to short circuits caused by contact between the positive and negative electrodes inside the battery, and even causing battery combustion or explosion accidents.

[0004] Therefore, developing a separator material that possesses both excellent high-temperature resistance and mechanical properties is a technical challenge that urgently needs to be solved in the current battery field. Summary of the Invention

[0005] In view of the problems existing in the background art, this application provides a method for preparing a battery separator, a battery separator prepared by the method, a secondary battery, a battery module, a battery pack, and an electrical device.

[0006] In a first aspect, this application provides a method for preparing a battery separator, the battery separator comprising a polyimide porous membrane as shown in formula (II), wherein the polyimide porous membrane is prepared by an imidization reaction of a polyamic acid-amine salt with a cross-linked structure as shown in formula (I).

[0007]

[0008] Wherein, A and B are independently selected from one of the substituted or unsubstituted aromatic, alicyclic, heterocyclic or aliphatic groups;

[0009] The M1—Y—M2 has a bis-tertiary amine structure;

[0010] n is selected from 10 2 ~10 5 .

[0011] Compared to existing technologies, this application has at least the following advantages: This application provides a method for preparing a polyimide battery separator, which uses a polyamic acid-amine salt with a cross-linked structure to obtain the separator through an imidization reaction. Compared to using a linear polyamic acid-amine salt, the polyimide film prepared by this invention using a polyamic acid-amine salt with a cross-linked structure has a narrower pore size distribution in the film. In addition to basic electronic insulation and ion conduction properties, the film also possesses excellent mechanical properties and good high-temperature resistance.

[0012] Optionally, the two tertiary amine bonds in M1—Y—M2 are respectively bonded to the two -COOH groups in formula (I).

[0013] Optionally, the bistertiary amine structure can be -NR1R2—Z—NR3R4, wherein R1, R2, R3, and R4 are the same or different, and R1, R2, R3, and R4 are independently selected from one of C1-C40 alkyl groups, C3-C20 alicyclic groups, C3-C30 heterocyclic groups, or C5-C30 aromatic groups; Z is selected from one of C1-C40 alkyl groups, C3-C20 alicyclic groups, C3-C30 heterocyclic groups, and C5-C30 aromatic groups.

[0014] Optionally, M1—Y—M2 includes at least one of the following structural formulas:

[0015]

[0016] Among them, R5, R6, R7, R8, R9, R 10 R 11 R 12 R 13 Each group is independently selected from one of the following: C1-C40 alkyl groups, C3-C20 alicyclic groups, C3-C30 heterocyclic groups, and C5-C30 aromatic groups.

[0017] Optionally, A and B are independently selected from one of the following: C6-C30 aromatic groups, C3-C20 alicyclic groups, C3-C30 heterocyclic groups, and C1-C30 aliphatic groups.

[0018] Optionally, the substituents in A and B are independently selected from one of C1-C20 alkyl, C1-C20 fluoroalkyl, carboxyl, nitro, C1-C10 alkoxy, acetyl, and hydroxyl.

[0019] Optionally, in the method for preparing the battery separator provided in this application, the battery separator includes an aromatic polyimide film as shown in formula (IV), wherein the aromatic polyimide film as shown in formula (IV) is prepared by an imidization reaction of an aromatic polyamic acid-amine salt with a cross-linked structure as shown in formula (III).

[0020]

[0021] in, Selected from:

[0022]

[0023] One of them;

[0024] R′ is selected from:

[0025]

[0026] One of them;

[0027] R″ is selected from:

[0028]

[0029] One of them;

[0030] -Ar2- is selected from:

[0031] One of them;

[0032] X and X′ are each independently selected from one of the following groups: hydrogen, alkyl, cycloalkyl, aryl, fluoroalkyl, hydroxyl, alkoxy, phenoxy, cyano, nitro, amino, acetamino, ester, acyl, halogen, and carboxyl.

[0033] M1—Y—M2 has a bistertiary amine structure; n is selected from 10 2 ~10 5 .

[0034] The polyimide battery separator prepared by imidization reaction using the above-mentioned aromatic polyamic acid-amine salt with cross-linked structure has high high temperature resistance and excellent mechanical strength, which can significantly improve the safety performance of the battery.

[0035] Optionally, X and X′ are each independently selected from one of -H, -Br, -Cl, -F, -NO2, -CN, -OH, -CH3, -CH2CH3, -CH2CH2CH3, isopropyl, isobutyl, tert-butyl, cyclopentyl, cyclohexyl, phenyl, and naphthyl.

[0036] Optionally, Ar1 is selected from one of benzene, biphenyl, pyridine, naphthalene, binaphthalene, diphenyl ether, diphenyl sulfone, benzophenone, diphenylpropane, diphenylhexafluoropropane, diphenylsilane, phenyl benzoate, and phenylbenzamide; Ar2 is selected from one of benzene, biphenyl, pyridine, naphthalene, binaphthalene, diphenyl ether, diphenyl sulfone, benzophenone, diphenylpropane, diphenylhexafluoropropane, diphenylsilane, phenyl benzoate, and phenylbenzamide.

[0037] Optionally, in the battery separator preparation method provided in this application, the preparation method of the porous membrane with cross-linked polyamic acid-amine salt includes the following steps:

[0038] Step (1): Polymerize dianhydride and diamine in a polar solvent to prepare a polyamic acid solution;

[0039] Step (2): Add a tertiary amine to the polyamic acid solution to obtain a solution containing polyamic acid-amine salt, and coat the above solution onto the surface of the carrier plate to form a coating containing polyamic acid-amine salt;

[0040] Step (3): Dry the coating or immerse the coating in a poor solvent to cure the coating and obtain a polyamic acid-amine salt film with a cross-linked structure.

[0041] Optionally, in step (1), the polar solvent is selected from at least one of N,N′-dimethylformamide, N,N′-dimethylacetamide, N-methylpyrrolidone, dimethyl sulfoxide, and a mixture of tetrahydrofuran / methanol.

[0042] Optionally, in step (2), the bis-tertiary amine comprises at least one of the following structural formulas:

[0043]

[0044] Among them, R5, R6, R7, R8, R9, R 10 R 11 R 12 R 13 Each group is independently selected from one of the following: C1-C40 alkyl groups, C3-C20 alicyclic groups, C3-C30 heterocyclic groups, and C5-C30 aromatic groups.

[0045] Optionally, the tertiary amine may be selected from one or more of N,N,N',N'-tetramethylhexanediamine, N,N,N',N'-tetraethylbutanediamine, N,N,N',N'-tetraethylethylenediamine, 4,4-bipyridine, 2,2-bipyridine, 4-diethylaminopyridine, N,N'-dimethylpiperazine, N,N'-diethylpiperazine, 1,4-diazabicyclo[2.2.2]octane, N,N'-dimethyl-N,N'-dicyclohexylhexanediamine, N-butylimidazole, N-ethylimidazole, and N,N'-dimethyl-N,N'-dibenzylpropanediamine.

[0046] Optionally, in step (2), the molar amount of the bis-tertiary amine is 10% to 100% of the molar amount of the carboxylic acid groups in the polyamic acid solution, and optionally 30% to 80%.

[0047] Optionally, step (2) may also include adding a surfactant and / or a nucleating agent to the polyamic acid solution.

[0048] Optionally, in the battery separator preparation method provided in this application, in step (3), the temperature for drying the coating is 50 to 100 degrees Celsius, and the drying time is 0.1 to 5 hours.

[0049] Optionally, in the battery separator preparation method provided in this application, in step (3), the unsuitable solvent is selected from organic solvents with a boiling point below 200°C or water; optionally, the unsuitable solvent is selected from at least one of water, methanol, ethanol, n-propanol, isopropanol, n-butanol, ethylene glycol, ethylene glycol monomethyl ether, dimethoxyethyl ether, acetone, methyl ethyl ketone, tetrahydrofuran, dioxane and mixtures thereof.

[0050] Optionally, in the battery separator preparation method provided in this application, the imidization method includes thermal imidization or chemical imidization.

[0051] Optionally, in the battery separator preparation method provided in this application, the polyimide is selected from one of aliphatic polyimide, aromatic polyimide, aliphatic copolyimide, aromatic copolyimide, aliphatic polyimide block, aliphatic polyimide graft copolymer, aromatic polyimide block, and aromatic polyimide graft copolymer.

[0052] Secondly, this application provides a battery separator prepared according to the method of the first aspect of this application.

[0053] Thirdly, this application provides a secondary battery, which includes a battery separator prepared according to the method of the first aspect of this application.

[0054] Fourthly, this application provides a battery module, which includes the secondary battery described in the third aspect of this application.

[0055] Fifthly, this application provides a battery pack, which includes a secondary battery according to the third aspect of this application or a battery module according to the fourth aspect of this application.

[0056] In a sixth aspect, this application provides an electrical device comprising a secondary battery according to the third aspect of this application, a battery module according to the fourth aspect of this application, or a battery pack according to the fifth aspect of this application; wherein the secondary battery, battery module, or battery pack serves as the power source or energy storage unit for the electrical device. Attached Figure Description

[0057] Figure 1 This is a scanning electron microscope image of the polyimide battery separator PMDA-ODA in Example 1 of this application;

[0058] Figure 2 This is a scanning electron microscope image of the polyimide battery separator BPDA-PPD in Example 2 of this application;

[0059] Figure 3 This is a scanning electron microscope image of the polyimide battery separator TB-BTDA in Example 3 of this application;

[0060] Figure 4 This is a scanning electron microscope image of the polyimide battery separator TFMB-ODPA in Example 4 of this application;

[0061] Figure 5 This is a scanning electron microscope image of the polyimide battery separator MDA-6FDA in Example 5 of this application;

[0062] Figure 6 This is a scanning electron microscope image of the polyimide battery separator DDS-BPADA in Example 6 of this application;

[0063] Figure 7 This is a scanning electron microscope image of the polyimide battery separator PMDA-ODA in Comparative Example 1 of this application;

[0064] Figure 8 This is a scanning electron microscope image of the polyimide battery separator PMDA-ODA in Comparative Example 2 of this application;

[0065] Figure 9 This is a perspective view of a secondary battery according to a specific embodiment of this application;

[0066] Figure 10 yes Figure 9 An exploded view of the secondary battery shown.

[0067] Figure 11 This is a perspective view of a battery module according to a specific embodiment of this application;

[0068] Figure 12This is a perspective view of a battery pack according to a specific embodiment of this application;

[0069] Figure 13 yes Figure 12 An exploded view of the battery pack shown.

[0070] Figure 14 This is a schematic diagram of an electrical device according to a specific embodiment of this application.

[0071] The reference numerals in the attached figures are explained as follows:

[0072] 1 Battery Pack

[0073] 2 upper box

[0074] 3 lower box

[0075] 4 Battery Module

[0076] 5. Secondary batteries

[0077] 51. Housing

[0078] 52 Electrode Assembly

[0079] 53 Top Cover Assembly Detailed Implementation

[0080] The present application will be further described below with reference to specific embodiments. It should be understood that these specific embodiments are for illustrative purposes only and are not intended to limit the scope of the present application.

[0081] Methods for preparing battery separators

[0082] The first aspect of this application provides a method for preparing a battery separator, the battery separator comprising a polyimide porous membrane as shown in formula (II), wherein the polyimide porous membrane is prepared by an imidization reaction of a polyamic acid-amine salt with a cross-linked structure as shown in formula (I).

[0083]

[0084] Wherein, A and B are independently selected from one of the substituted or unsubstituted aromatic, alicyclic, heterocyclic, and aliphatic groups; M1—Y—M2 is a bis-tertiary amine structure; n is selected from 10 2 ~10 5 .

[0085] Compared to polyolefin separators, polyimide possesses excellent thermal stability, chemical stability, and high mechanical properties, with a long-term operating temperature reaching up to 300°C, making it the best-performing thin-film insulating material currently available. However, the publicly disclosed fabrication techniques for porous polyimide membranes share a common characteristic: after the formation of micropores, the mechanical properties of the polyimide film significantly degrade, making it difficult to meet the practical application requirements of battery separators.

[0086] Among the publicly available methods for forming porous polyimide films, the main approaches include the template method, the phase inversion method, and the electrospinning method. The template method uses an inorganic porogen incompatible with polyimide as a template. After mixing the template with polyamic acid, imidization is performed to obtain a polyimide composite film containing the porogen. The porogen is then removed using a template etchant to prepare the porous polyimide film. Commonly used porogens in the template method include silica and sodium carbonate, but this method is difficult to completely remove the porogen, resulting in uneven micropore distribution and poor continuity. In particular, residual porogens severely degrade the mechanical properties of the membrane. The preparation of porous polyimide films can also employ the well-known precipitation-phase inversion technique. The basic process of precipitation-phase inversion involves processing a polyamic acid solution into the desired film shape, drying it to form a semi-dry film, and then immersing it in a poor solvent to achieve phase separation. In precipitation-reverse phase inversion (DRPI) technology, micropores typically only form on the film surface. If through-holes form on both sides of the film, the film loses its mechanical properties and becomes difficult to form a self-supporting membrane. Furthermore, the pore size of films produced by DRPI is usually larger than 500 nm, making them unsuitable for use as lithium-ion battery separators. Electrospinning technology can produce polyimide nonwoven fabrics, which can fabricate polyimide films with high porosity. However, these films typically have low strength and insufficient mechanical properties, making them unable to withstand the external forces exerted on lithium ions during assembly and charge-discharge cycles.

[0087] The battery separator prepared according to the embodiments of this application is a self-supporting, high-temperature resistant, and high-strength polyimide porous membrane. Micropores are uniformly distributed on the surface and inside of the porous membrane. These micropores are formed during the imidization process of the polyamic acid-amine salt membrane layer, resulting in the separation of the polyimide from the free amine and residual solvent. This complex phase separation molding process induced by the chemical reaction exhibits a unique stepwise phase separation behavior, which differs from the traditional physical phase separation process based on supersaturation. Therefore, this unique stepwise phase separation behavior results in a narrower and more continuous pore size distribution, which is one reason why the polyimide porous membrane prepared by the method of this application has greater mechanical strength. Secondly, this application uses a polyamic acid-amine salt with a cross-linked structure as a raw material, and through the imidization process, forms a microporous polyimide porous membrane. Compared with using a linear polyamic acid-amine salt as a raw material, the prepared polyimide porous membrane has significant advantages in mechanical properties.

[0088] In some embodiments of this application, the two tertiary amine bonds in M1—Y—M2 are respectively bonded to the two -COOH groups in formula (I).

[0089] In some embodiments of this application, the bistertiary amine structure can be -NR1R2—Z—NR3R4, wherein R1, R2, R3, and R4 are the same or different, and R1, R2 and R3, R4 are independently selected from one of C1-C40 alkyl groups, C3-C20 alicyclic groups, C3-C30 heterocyclic groups, or C5-C30 aromatic groups; Z is selected from one of C1-C40 alkyl groups, C3-C20 alicyclic groups, C3-C30 heterocyclic groups, and C5-C30 aromatic groups.

[0090] In some embodiments of this application, M1-Y-M2 includes at least one of the following structural formulas:

[0091]

[0092] Among them, R5, R6, R7, R8, R9, R 10 R 11 R 12 R 13 Each group is independently selected from one of the following: C1-C40 alkyl groups, C3-C20 alicyclic groups, C3-C30 heterocyclic groups, and C5-C30 aromatic groups.

[0093] In some embodiments of this application, A and B are independently selected from one of the following: C6-C30 aromatic groups, C3-C20 alicyclic groups, C3-C30 heterocyclic groups, and C1-C30 aliphatic groups.

[0094] In some embodiments of this application, the substituents in A and B are independently selected from one of C1-C20 alkyl, C1-C20 fluoroalkyl, carboxyl, nitro, C1-C10 alkoxy, acetyl, and hydroxyl.

[0095] In some embodiments of this application, the battery separator includes an aromatic polyimide film of formula (IV), wherein the aromatic polyimide film of formula (IV) is prepared by an imidization reaction using an aromatic polyamic acid-amine salt with a crosslinked structure as shown in formula (III).

[0096]

[0097] in,

[0098] Selected from:

[0099]

[0100]

[0101] One of them;

[0102] R′ is selected from:

[0103]

[0104] One of them;

[0105] R″ is selected from:

[0106]

[0107] One of them;

[0108] -Ar2- is selected from:

[0109] One of them;

[0110] X and X′ are each independently selected from one of the following groups: hydrogen, alkyl, cycloalkyl, aryl, fluoroalkyl, hydroxyl, alkoxy, phenoxy, cyano, nitro, amino, acetamino, ester, acyl, halogen, and carboxyl.

[0111] M1—Y—M2 has a bis-tertiary amine structure;

[0112] n is selected from 10 2 ~10 5 .

[0113] In the above embodiments, the aromatic polyimide battery separator prepared using an aromatic polyamic acid-amine salt film with a cross-linked structure has significantly improved high-temperature resistance and higher mechanical strength.

[0114] In some embodiments of this application, X and X′ are each independently preferred from one of -H, -Br, -Cl, -F, -NO2, -CN, -H, -CH3, -CH2CH3, -CH2CH2CH3, isopropyl, isobutyl, tert-butyl, cyclopentyl, cyclohexyl, phenyl, and naphthyl.

[0115] In some embodiments of this application, Ar1 is selected from benzene, biphenyl, pyridine, naphthalene, binaphthalene, diphenyl ether, diphenyl sulfone, benzophenone, diphenylpropane, diphenylhexafluoropropane, diphenylsilane, phenyl benzoate, and phenylbenzamide; Ar2 is selected from benzene, biphenyl, pyridine, naphthalene, binaphthalene, diphenyl ether, diphenyl sulfone, benzophenone, diphenylpropane, diphenylhexafluoropropane, diphenylsilane, phenyl benzoate, and phenylbenzamide.

[0116] In some embodiments of this application, the method for preparing a polyamic acid-amine salt film with a crosslinked structure includes the following steps:

[0117] Step (1): Polymerize dianhydride and diamine in a polar solvent to prepare a polyamic acid solution;

[0118] Step (2): Add a tertiary amine to the polyamic acid solution to obtain a solution containing polyamic acid-amine salt, and coat the above solution onto the surface of the carrier plate to form a coating containing polyamic acid-amine salt;

[0119] Step (3): Dry the above coating or immerse the above coating in a poor solvent to cure the coating and obtain a polyamic acid-amine salt film with a cross-linked structure.

[0120] In some embodiments of this application, a polyamic acid-amine salt film with a crosslinked structure as shown in formula (I) is prepared using the following method:

[0121] Step (1): Polymerize the dianhydride shown in formula (V) and the diamine shown in formula (VI) in a polar solvent to prepare a polyamic acid solution;

[0122] Step (2): Add a bis-tertiary amine to the polyamic acid solution. The bis-tertiary amine may have a structure of -NR1R2—Z—NR3R4, where R1, R2, R3, and R4 may be the same or different. R1, R2, R3, and R4 are independently selected from one of C1-C40 alkyl groups, C3-C20 alicyclic groups, C3-C30 heterocyclic groups, or C5-C30 aromatic groups. Z is selected from one of C1-C40 alkyl groups, C3-C20 alicyclic groups, C3-C30 heterocyclic groups, and C5-C30 aromatic groups. A solution containing polyamic acid-amine salt is obtained, and the solution is coated onto the surface of a carrier plate to form a coating containing polyamic acid-amine salt.

[0123] Step (3): Dry the above coating and / or immerse the above coating in a poor solvent to cure the coating into a polyamic acid-amine salt film with a cross-linked structure as shown in Formula (I).

[0124]

[0125] in,

[0126] A and B are independently selected from one of the substituted or unsubstituted aromatic, alicyclic, heterocyclic, and aliphatic groups; M1—Y—M2 is a bis-tertiary amine structure, and n is selected from 10 2 ~10 5 .

[0127] In some embodiments of this application, the substituents in A and B are independently selected from one of alkyl, fluoroalkyl, carboxyl, nitro, alkoxy, acetyl, and hydroxyl groups. In some embodiments of this application, the alkyl group can be a C1-C6 alkyl group. In some embodiments of this application, the fluoroalkyl group can be a C1-C6 fluoroalkyl group. The amount of fluorine atom substitution for carbon atom in the fluoroalkyl group can be partial or complete. In some embodiments of this application, the alkoxy group can be a C1-C6 alkoxy group.

[0128] In some embodiments of this application, in step (1), A in the dianhydride represented by formula (V) is selected from benzene, biphenyl, pyridine, naphthalene, binaphthalene, diphenyl ether, diphenyl sulfone, benzophenone, diphenylpropane, diphenylhexafluoropropane, diphenylsilane, phenyl benzoate or phenylbenzamide.

[0129] In some embodiments of this application, in step (1), B in the diamine represented by formula (VI) is selected from benzene, biphenyl, pyridine, naphthalene, binaphthalene, diphenyl ether, diphenyl sulfone, benzophenone, diphenylpropane, diphenylhexafluoropropane, diphenylsilane, phenyl benzoate, and phenylbenzamide.

[0130] In some embodiments of this application, in step (1), the preparation of a polyamic acid solution by polymerizing the dianhydride represented by formula (V) and the diamine represented by formula (VI) in a polar solvent can be achieved by conventional methods in the art. For example, polyamic acid can be prepared by reacting an aromatic dianhydride and an aromatic diamine in an aprotic dipole solvent. Aprotic dipole solvents include, but are not limited to, N,N′-dimethylformamide (DMF), N,N′-dimethylacetamide (DMAc), N-methylpyrrolidone, dimethyl sulfoxide (DMSO), tetrahydrofuran / methanol mixtures (THF / MeOH), etc. Typically, the diamine monomer is first dissolved in the solvent, and then the dianhydride is added in one batch or in small batches. The reaction is carried out under an inert atmosphere such as N2 or Ar. After obtaining a very viscous polyamic acid solution through the polymerization reaction, the polyamic acid can be subjected to an amine neutralization reaction in the same container to form a polyamic acid-amine salt.

[0131] In some embodiments of this application, in step (2), the bis-tertiary amine may be selected from at least one of N,N,N′,N′-tetramethyl-hexanediamine, N,N,N′,N′-tetraethyl-butanediamine, N,N,N',N'-tetraethyl-ethylenediamine, 4,4-bipyridine, 2,2-bipyridine, 4-diethylaminopyridine, N,N'-diethyl-piperazine, N,N'-dimethyl-piperazine, 1,4-diazabicyclo[2.2.2]octane, N,N'-dimethyl-N,N'-dicyclohexylhexanediamine, N-butylimidazole, N-ethylimidazole, and N,N'-dimethyl-N,N'-dibenzylpropanediamine.

[0132] In some embodiments of this application, in step (2), when adding a tertiary amine to the polyamic acid solution, the molar amount of the tertiary amine is 10% to 100% of the molar amount of the carboxylic acid groups in the polyamic acid solution, preferably 30% to 80%.

[0133] In some embodiments of this application, in step (2), the bis-tertiary amine is dissolved in an organic solvent selected from toluene, xylene, dichlorobenzene, ethanol, isopropanol, n-butanol, cyclohexanol, ethylene glycol, ethylene glycol monoethyl ether, ethylene glycol dimethyl ether, 2,2′-dimethoxyethyl ether, acetone, acetonitrile, methyl ethyl ketone, cyclohexanone, tetrahydrofuran, dioxane, N-methylpyrrolidone, N-cyclohexylpyrrolidone, N,N-dimethylacetamide, cresol, m-cresol, ethyl acetate, γ-butyrolactone, or mixtures thereof.

[0134] In some embodiments of this application, in step (2), a surfactant and / or nucleating agent are also added to the polyamic acid solution to promote optimal phase separation formation.

[0135] In some embodiments of this application, step (3) can be performed by drying or immersing the coating in a poor solvent to cure it into a polyamic acid-amine salt film with a cross-linked structure; alternatively, a combination of drying followed by immersion in a poor solvent can be used. The film curing process using a combination of drying and immersion in a poor solvent is beneficial for improving certain methodological properties of lithium-ion battery separators, such as air permeability.

[0136] In some embodiments of this application, step (3) can be performed by drying the coating to cure it into a polyamic acid-amine salt film. For example, the coating can be dried in a drying oven at a temperature of 50 to 100 degrees Celsius for 0.1 to 5 hours.

[0137] In some embodiments of this application, step (3) can be performed by immersing the coating in a poor solvent to cure it into a polyamic acid-amine salt film. The poor solvent can be selected from organic solvents or water with a boiling point below 200°C. For example, it can be at least one of water, methanol, ethanol, n-propanol, isopropanol, n-butanol, ethylene glycol, ethylene glycol monomethyl ether, dimethoxyethyl ether, acetone, methyl ethyl ketone, tetrahydrofuran, dioxane, and mixtures thereof.

[0138] In some embodiments of this application, the imidization method includes thermal imidization or chemical imidization.

[0139] In some embodiments of this application, the polyimide is selected from one of aliphatic polyimide, aromatic polyimide, aliphatic copolyimide, aromatic copolyimide, aliphatic polyimide block, aliphatic polyimide graft copolymer, aromatic polyimide block, and aromatic polyimide graft copolymer.

[0140] In some embodiments of this application, a thermal imidization method can be used to prepare the battery separator. This method requires only relatively mild conditions to transform the polyamic acid-amine salt film precursor into the final polyimide film. The mild thermal imidization temperature conditions are beneficial for maintaining the integrity of the micropores in the polyimide film and ensuring high porosity. Furthermore, in some embodiments of this application, a stepwise heating method or a gradient heating method is used for imidization to control the micropore size distribution within a narrow range. The stepwise heating method can be: heating at 80–150°C for 0.1–10 hours; or heating at 150–250°C for 0.1–5 hours. The gradient heating method uses a heating rate of 0.1–5°C / min.

[0141] In some embodiments of this application, a chemical imidization method can be used to prepare the battery separator. The chemical imidization method involves converting a cross-linked polyamic acid-amine salt film into a polyimide film using a dehydrating agent. The dehydrating agent mentioned in this application refers to a compound that can react with water, preferably from acid anhydrides, acyl chlorides, phosphorus halides, or carboimides. Specifically, the dehydrating agent includes, but is not limited to, acetic anhydride, trifluoroacetic anhydride, acetyl chloride, phosphorus trichloride, and dicyclohexylcarboimide. The chemical imidization process can be carried out by forming a solution of the dehydrating agent in an inert solvent. The molar amount of the dehydrating agent is 1 to 10 times the amount of carboxyl groups in the polyamic acid. The inert solvent is an organic solvent that neither reacts with the dehydrating agent nor is compatible with the solvent used in the polymerization. Examples of solvents include n-hexane, cyclohexane, octane, toluene, xylene, dichlorobenzene, butyl ether, methyl tert-butyl ether, ethylene glycol dimethyl ether, cyclohexanone, butanone, perfluoroalkanes, perfluoroalkyl ethers, dioxane, tetrahydrofuran, acetonitrile, butyl acetate, and γ-butyrolactone. These inert solvents facilitate the penetration of dehydrating agents into the polyamic acid-amine salt film and simultaneously replace residual solvents in the film, reducing capillary tension in the micropores and contributing to the preservation of the microporous structure. Chemical imidization can be performed at room temperature or at elevated temperatures. Appropriate heating can accelerate chemical imidization and improve the degree of imidization, which is valuable in applications. The preferred chemical imidization temperature is 50–160 °C.

[0142] The polyimide film formed by the method of this application typically has an imidization degree of more than 90%, and most preferably 100% imidization.

[0143] In some embodiments of this application, the imidization process involves peeling the polyamic acid-amine salt film from the carrier plate surface and then fixing the film edges to a hollow frame. For example, existing equipment used in polyimide film industrial production, such as pin plates and chain clamps, can be used to hold and maintain the film edges, enabling a more convenient imidization process and helping to prevent shrinkage of the film during imidization. Furthermore, this will better match existing polyimide film production technologies and equipment, enabling continuous roll-to-roll production of microporous polyimide films.

[0144] Battery separator

[0145] Secondly, this application provides a battery separator, which is a battery separator prepared according to the method described in the first aspect of this application.

[0146] In some embodiments of this application, the polyimide film prepared has a narrower pore size distribution, resulting in superior mechanical properties. Specifically, the polyimide film has an average pore size of 1 nm to 500 nm, a micropore porosity of 30% to 60%, a thickness of 10 μm to 20 μm, a tensile strength greater than 100 MPa, and an elongation at break greater than 50%.

[0147] Secondary batteries

[0148] Thirdly, this application provides a secondary battery, which includes a battery separator prepared according to the method described in the first aspect of this application.

[0149] In some embodiments of this application, the secondary battery may be a lithium-ion secondary battery, a lithium metal secondary battery, a sodium-ion secondary battery, etc. Taking a lithium-ion secondary battery as an example, it may include a positive electrode, a negative electrode, a separator membrane spaced between the positive and negative electrode, and an electrolyte. The separator membrane is the battery separator prepared by the method described in the first aspect of this application.

[0150] In some embodiments of this application, the positive electrode of a lithium-ion secondary battery includes a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector. In the positive electrode, the positive active material layer may be disposed on one or both surfaces of the positive current collector.

[0151] Those skilled in the art can choose a suitable method to prepare the positive electrode sheet. For example, it may include the following steps: mixing positive active material, binder and conductive agent to form a slurry, and then coating it onto the positive current collector.

[0152] The specific type of positive electrode active material is not particularly limited, as long as it can satisfy the requirements for lithium ion insertion and extraction. The positive electrode active material can be a layered structure material, allowing lithium ions to diffuse in two-dimensional space, or a spinel structure, allowing lithium ions to diffuse in three-dimensional space. Preferably, the positive electrode active material can be selected from one or more of lithium transition metal oxides, compounds obtained by adding other transition metals, non-transition metals, or non-metals to lithium transition metal oxides. Specifically, the positive electrode active material is preferably selected from one or more of lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, and lithium phosphates with an olivine structure.

[0153] Among them, the general formula for lithium phosphates with an olivine structure can be LiFe. 1-x-y Mn x M' y PO4, 0≤x≤1, 0≤y<1, 0≤x+y≤1, M' is selected from one or more transition metal elements or non-transition metal elements other than Fe and Mn, and M' is preferably selected from one or more of Cr, Mg, Ti, Al, Zn, W, Nb, and Zr. Preferably, the lithium-containing phosphate with olivine structure is selected from one or more of lithium iron phosphate, lithium manganese phosphate, and lithium manganese iron phosphate.

[0154] Lithium transition metal oxides are selected from LiCoO2, LiMnO2, LiNiO2, LiMn2O4, and LiNi x Co y Mn 1-x-y O2, LiNi x Co y Al 1-x-y O2, LiNi x Mn 2-x One or more of O4, wherein 0 < x < 1, 0 < y < 1, and 0 < x + y < 1. Preferably, the lithium transition metal oxide is selected from LiCoO2 and LiNi. 1 / 3 Co 1 / 3 Mn 1 / 3 O2, LiNi 0.5 Co 0.2 Mn 0.3 O2, LiNi 0.6 Co 0.2 Mn 0.2 O2, LiNi 0.8 Co 0.1 Mn 0.1 O2, LiNi 0.8 Co 0.15 Mn 0.05 O2, LiNi 0.8 Co 0.15 Al 0.05O2, LiNi 0.5 Mn 1.5 One or more of O4 and LiMn2O4.

[0155] In the positive electrode sheet, the positive electrode active material layer may further include a conductive agent and a binder. The type and content of the conductive agent and binder are not specifically limited and can be selected according to actual needs. The binder typically includes a fluorinated polyolefin binder. Water is generally a good solvent for the fluorinated polyolefin binder, meaning it typically has good solubility in water. For example, the fluorinated polyolefin binder may be, but is not limited to, polyvinylidene fluoride (PVDF), PVDF copolymers, or their modified derivatives (e.g., modified with carboxylic acid, acrylic acid, acrylonitrile, etc.). The mass percentage content of the binder in the positive electrode material layer may be limited because the binder itself has poor conductivity, thus the amount of binder cannot be too high. Preferably, the mass percentage content of the binder in the positive electrode active material layer is less than or equal to 2 wt% to obtain a lower electrode impedance. The conductive agent of the positive electrode sheet can be any conductive agent suitable for secondary batteries in the art, such as, but not limited to, one or more combinations of acetylene black, conductive carbon black, carbon fiber (VGCF), carbon nanotubes (CNT), Ketjen black, etc. The weight of the conductive agent can account for 1 wt% to 10 wt% of the total mass of the positive electrode material layer. More preferably, the weight ratio of the conductive agent to the positive electrode active material in the positive electrode sheet is greater than or equal to 1.5:95.5.

[0156] In the positive electrode sheet, the type of positive current collector is not specifically limited and can be selected according to actual needs. The positive current collector is typically a layer, and is usually a structure or component capable of collecting current. The positive current collector can be any material suitable for use as a positive current collector in electrochemical energy storage devices. For example, the positive current collector can be, but is not limited to, metal foil, and more specifically, can be, but is not limited to, nickel foil and aluminum foil.

[0157] In some embodiments of this application, the negative electrode of the secondary battery typically includes a negative current collector and a negative active material layer located on the surface of the negative current collector, wherein the negative active material layer typically includes a negative active material. The negative active material can be any material suitable for lithium-ion secondary batteries, for example, it can be one or more combinations of, but not limited to, graphite, soft carbon, hard carbon, carbon fiber, mesophase carbon microspheres, silicon-based materials, tin-based materials, lithium titanate, or other metals capable of forming alloys with lithium. Specifically, the graphite can be selected from one or more combinations of artificial graphite, natural graphite, and modified graphite; the silicon-based material can be selected from one or more combinations of elemental silicon, silicon oxide compounds, silicon-carbon composites, and silicon alloys; and the tin-based material can be selected from one or more combinations of elemental tin, tin oxide compounds, and tin alloys.

[0158] The negative electrode current collector is typically a structure or component that collects current. It can be any material suitable for use as a negative electrode current collector in a lithium secondary battery, for example, it can be, but is not limited to, metal foil, and more specifically, copper foil. Furthermore, the negative electrode sheet can also be a lithium sheet.

[0159] In some embodiments of this application, the electrolyte of the secondary battery can be any electrolyte suitable for secondary batteries in the art. For example, the electrolyte typically includes an electrolyte and a solvent. Taking a lithium-ion secondary battery as an example, the electrolyte typically includes a lithium salt. More specifically, the lithium salt can be an inorganic lithium salt and / or an organic lithium salt, specifically including but not limited to one or more combinations of LiPF6, LiBF4, LiN(SO2F)2 (abbreviated as LiFSI), LiN(CF3SO2)2 (abbreviated as LiTFSI), LiClO4, LiAsF6, LiB(C2O4)2 (abbreviated as LiBOB), and LiBF2C2O4 (abbreviated as LiDFOB). For another example, the concentration of the electrolyte can be 0.8 mol / L to 1.5 mol / L. The solvent can be any solvent suitable for electrolytes of lithium-ion secondary batteries in the art. The solvent of the electrolyte is usually a non-aqueous solvent and can be an organic solvent. Specifically, it can be one or more combinations of ethylene carbonate, propylene carbonate, butene carbonate, pentene carbonate, dimethyl carbonate, diethyl carbonate, dipropyl carbonate, methyl ethyl carbonate, or their halogenated derivatives, including but not limited to ethylene carbonate, propylene carbonate, butene carbonate, pentene carbonate, dimethyl carbonate, diethyl carbonate, dipropyl carbonate, methyl ethyl carbonate, or their halogenated derivatives.

[0160] In some embodiments of this application, the method for preparing the secondary battery should be known to those skilled in the art. For example, the positive electrode, separator and negative electrode can each be a layer, which can be cut to the target size and stacked in sequence, or wound to the target size to form a cell, and can be further combined with electrolyte to form a secondary battery.

[0161] Figure 9 A perspective view of a secondary battery according to a specific embodiment of this application is shown. Figure 10 yes Figure 9 The exploded view of the secondary battery is shown. (See also...) Figure 9 and Figure 10 The secondary battery 5 (hereinafter referred to as battery cell 5) according to this application includes an outer packaging 51, an electrode assembly 52, a top cover assembly 53, and an electrolyte (not shown). The electrode assembly 52 is housed within the casing 51, and the number of electrode assemblies 52 is not limited and can be one or more.

[0162] It should be noted that, Figure 9 The battery cell 5 shown is a can-type battery, but this application is not limited to this. The battery cell 5 can be a pouch-type battery, that is, the casing 51 is replaced by a metal plastic film and the top cover assembly 53 is removed.

[0163] Battery Module

[0164] A fourth aspect of this application provides a battery module comprising the secondary battery described in the third aspect of this application. In some embodiments, the secondary battery can be assembled into a battery module, and the number of secondary batteries contained in the battery module can be multiple, the specific number of which can be adjusted according to the application and capacity of the battery module. Figure 11 This is a 3D view of battery module 4 as an example. (See reference) Figure 11 In the battery module 4, multiple secondary batteries 5 can be arranged sequentially along the length of the battery module 4. Of course, they can also be arranged in any other manner. Furthermore, the multiple secondary batteries 5 can be fixed in place by fasteners. Optionally, the battery module 4 may also include a housing with a receiving space in which the multiple secondary batteries 5 are received.

[0165] battery pack

[0166] A fifth aspect of this application provides a battery pack comprising the battery modules described in the fourth aspect of this application. In some embodiments, the battery modules can be assembled into a battery pack, and the number of battery modules contained in the battery pack can be adjusted according to the application and capacity of the battery pack. Figure 12 This is a 3D view of battery pack 1 as an example. Figure 13 yes Figure 12 An exploded view of the battery pack shown. (Refer to...) Figure 12 and Figure 13 The battery pack 1 may include a battery box and multiple battery modules 4 disposed within the battery box. The battery box includes an upper box 2 and a lower box 3, with the upper box 2 covering the lower box 3 to form a closed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the battery box.

[0167] Electrical appliances

[0168] A sixth aspect of this application provides an electrical device comprising a secondary battery as described in the third aspect of this application, a battery module as described in the fourth aspect of this application, or a battery pack as described in the fifth aspect of this application. The secondary battery, battery module, or battery pack can be used as a power source for the electrical device or as an energy storage unit for the electrical device. The electrical device may be, but is not limited to, mobile devices (e.g., mobile phones, laptops, etc.), electric vehicles (e.g., pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc.

[0169] The electrical device can be equipped with a secondary battery, battery module, or battery pack according to its usage requirements.

[0170] Figure 14 A schematic diagram of an electrical device according to a specific embodiment of this application is shown. This electrical device can be a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. To meet the high power and high energy density requirements of the lithium-ion secondary battery (i.e., the secondary battery of this application), a battery pack or battery module can be used.

[0171] Another example of an electrical device could be a mobile phone, tablet computer, laptop computer, etc. Such devices typically require a slim and lightweight design and can utilize the rechargeable battery described in this application as their power source.

[0172] Those skilled in the art will understand that the various limitations or preferred ranges for the selection of components, component content and physicochemical properties of materials in the different embodiments of this application mentioned above can be arbitrarily combined, and the various embodiments obtained by such combinations are still within the scope of this application and are considered as part of the disclosure of this specification.

[0173] Unless otherwise specified, the various parameters mentioned in this specification have common meanings known in the art and can be measured using methods known in the art. For example, they can be tested according to the methods given in the embodiments of this application. In addition, the preferred ranges and options of various parameters given in the various preferred embodiments can be combined arbitrarily, and all such combinations are considered to be within the scope of disclosure of this application.

[0174] The advantages of this application are further illustrated below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of this application.

[0175] Preparation of polyimide battery separator

[0176] Example 1

[0177] 10.012 g of diphenyl ether diamine (ODA) and 120 mL of N-methylpyrrolidone were added to a 500 mL three-necked round-bottom flask equipped with a nitrogen inlet and a mechanical stirrer. After complete dissolution, 10.906 g of pyromellitic dianhydride (PMDA) was added. The reaction mixture was stirred at room temperature for 24 hours to form a very viscous polyamic acid solution. 8.66 g of N,N,N′,N′-tetramethyl-hexanediamine was dissolved in 30 mL of cyclohexanone, and this solution was added to the polyamic acid solution through a dropping funnel to obtain a polyamic acid-amine salt solution, wherein the structural formula (I) of the polyamic acid-amine salt is shown in Table 1. The solution was coated onto the surface of a glass plate using a coater and then placed in a 60 °C oven for drying for 5 h. The film was then heated to 130 °C and held for 5 h, followed by heating to 250 °C for 2 h to obtain the polyimide battery separator PMDA-ODA.

[0178] Example 2

[0179] 5.407 g of p-phenylenediamine (PPD) and 160 mL of N-methylpyrrolidone were added to a 500 mL three-necked round-bottom flask equipped with a nitrogen inlet and a mechanical stirrer. After complete dissolution, 14.711 g of biphenyl dianhydride (BPDA) was added. The reaction mixture was stirred at room temperature for 24 hours to form a very viscous polyamic acid solution. 8.65 g of N,N,N′,N′-tetraethyl-butanediamine was dissolved in 50 mL of n-butanol, and this solution was added to the polyamic acid solution through a dropping funnel to obtain a polyamic acid-amine salt solution, wherein the structural formula (I) of the polyamic acid-amine salt is shown in Table 1. This solution was coated onto the surface of a glass plate using a coater and dried in an oven at 80°C for 4 h. Then it was immersed in methyl ethyl ketone and kept for 1 h. The film was then transferred to isophorone and heated to 220°C and kept for 4 h. Remove and dry to obtain polyimide battery separator BPDA-PPD.

[0180] Example 3

[0181] 10.615 g of 3,3-dimethyl-4,4-biphenyldiamine (TB) and 165 mL of N,N′-dimethylacetamide were added to a 500 mL three-necked round-bottom flask equipped with a nitrogen inlet and a mechanical stirrer. After complete dissolution, 16.111 g of benzophenone tetracarboxylic dianhydride (BTDA) was added. The reaction mixture was stirred at room temperature for 24 hours to form a very viscous polyamic acid solution. 12.49 g of 4,4-bipyridine was dissolved in 50 mL of N-methylpyrrolidone, and this solution was added to the polyamic acid solution through a dropping funnel to obtain a polyamic acid-amine salt solution, wherein the structural formula (I) of the polyamic acid-amine salt is shown in Table 1. This solution was coated onto the surface of a glass plate using a coater. It was placed in an oven at 100 °C for drying for 1 h. Then, the temperature was gradually increased to 250 °C at a heating rate of 2 °C / min and heated for 2 h to obtain the polyimide battery separator TB-BTDA.

[0182] Example 4

[0183] 16.012 g of 3,3-ditrifluoromethyl-4,4-biphenyldiamine (TFMB) and 200 mL of N,N′-dimethylacetamide were added to a 500 mL three-necked round-bottom flask equipped with a nitrogen inlet and a mechanical stirrer. After complete dissolution, 15.511 g of diphenyl ether tetracarboxylic dianhydride (ODPA) was added. The reaction mixture was stirred at room temperature for 24 hours to form a very viscous polyamic acid solution. 10.51 g of 4-diethylaminopyridine was dissolved in 50 mL of N,N-dimethylacetamide, and this solution was added to the polyamic acid solution through a dropping funnel to obtain a polyamic acid-amine salt solution, wherein the structural formula (I) of the polyamic acid-amine salt is shown in Table 1. This solution was coated onto the surface of a glass plate using a coater. It was then placed in an oven at 100 °C for drying for 2 hours. The membrane was then immersed in a dichlorobenzene solution containing 52.3g of acetic anhydride and heated at 160°C for 1 hour before being removed to obtain the polyimide battery separator TFMB-ODPA.

[0184] Example 5

[0185] 9.9135 g of diaminodiphenylmethane (MDA) and 190 mL of N,N′-dimethylacetamide were added to a 500 mL three-necked round-bottom flask equipped with a nitrogen inlet and a mechanical stirrer. After complete dissolution, 22.212 g of 4,4-hexafluoroisopropylphthalic anhydride (6FDA) was added. The reaction mixture was stirred at room temperature for 24 hours to form a very viscous polyamic acid solution. 4.49 g of 1,4-diazabicyclo[2.2.2]octane was dissolved in 50 mL of ethylene glycol, and this solution was added to the polyamic acid solution through a dropping funnel. The polyamic acid-amine salt structure (I) is shown in Table 1. The solution was coated onto the surface of a glass plate using a coater. It was then placed in an oven at 100 °C and dried for 3 hours. The membrane was then immersed in a xylene solution of 42.0 g of dicyclohexylcarboimide and heated at 140 °C for 2 hours before being removed to obtain the polyimide battery separator MDA-6FDA.

[0186] Example 6

[0187] 12.415 g of diaminodiphenyl sulfone (DDS) and 250 mL of N,N′-dimethylformamide were added to a 500 mL three-necked round-bottom flask equipped with a nitrogen inlet and a mechanical stirrer. After complete dissolution, 22.916 g of 4,4'-(4,4'-isopropyldiphenoxy)bis(phthalic anhydride) (BPADA) was added. The reaction mixture was stirred at room temperature for 24 hours to form a very viscous polyamic acid solution. 11.26 g of N,N′-dimethyl-N,N′-dibenzylpropanediamine was dissolved in 30 mL of N,N-dimethylacetamide, and this solution was added to the polyamic acid solution through a dropping funnel to obtain a polyamic acid-amine salt solution, wherein the structural formula (I) of the polyamic acid-amine salt is shown in Table 1. This solution was coated onto the surface of a glass plate using a coater. It was then placed in an oven at 60 °C for drying for 4 hours. The membrane was then immersed in a butanone solution containing 63.0 g of trifluoroacetic anhydride and heated at 120 °C for 4 hours before being removed to obtain the polyimide battery separator DDS-BPADA.

[0188] Comparative Example 1 (Polyamic acid precipitation phase inversion forms a porous membrane)

[0189] 10.012 g of diphenyl ether diamine (ODA) and 120 mL of N-methylpyrrolidone were added to a 500 mL three-necked round-bottom flask equipped with a nitrogen inlet and a mechanical stirrer. After complete dissolution, 10.906 g of pyromellitic dianhydride (PMDA) was added. The reaction mixture was stirred at room temperature for 24 hours to form a very viscous polyamic acid solution. This solution was coated onto a glass plate using a coater and then immersed in water for 24 hours to completely exchange the solvent and achieve phase inversion precipitation. After drying, the pre-film was placed in an oven at 80 °C for 8 hours. Then, the film was heated to 350 °C and held for 24 hours to obtain a polyimide film PMDA-ODA.

[0190] Comparative Example 2 (Polyamate precipitation phase inversion forms a porous membrane)

[0191] 10.012 g of diphenyl ether diamine (ODA) and 120 mL of N-methylpyrrolidone were added to a 500 mL three-necked round-bottom flask equipped with a nitrogen inlet and a mechanical stirrer. After complete dissolution, 10.906 g of pyromellitic dianhydride (PMDA) was added. The reaction mixture was stirred at room temperature for 24 hours to form a very viscous polyamic acid solution. A 30 mL solution of triethylamine (10.12 g) in cyclohexanone was added to the reaction mixture through a dropping funnel to obtain a polyamic acid-amine salt solution. This solution was coated onto a glass plate using a coater and then immersed in water for 24 hours to completely exchange the solvent and achieve phase inversion precipitation. After drying, the pre-existing membrane was placed in an oven at 80 °C for 8 hours. Then, the membrane was heated to 150 °C and held for 24 hours to obtain a polyimide film PMDA-ODA.

[0192] Table 1. Molecular formulas of polyamic acid-amine salts and polyimide battery separators synthesized in Examples 1-6

[0193]

[0194]

[0195]

[0196] Performance testing of polyimide battery separators

[0197] In the following examples and comparative examples, the testing instruments and conditions used to perform performance tests on the samples are as follows:

[0198] 1. Microscopic morphological characterization:

[0199] The microstructure of the gold-sprayed microporous polyimide film was observed using an XL30 field emission environmental scanning electron microscope (SEM) from FEI Corporation, USA.

[0200] 2. Heat resistance:

[0201] The TGA-2 thermogravimetric analyzer (TGA), manufactured by PerkinElmer, USA, operates at a heating rate of 10°C / min under a nitrogen atmosphere. The specific testing procedure is as follows: Place the sample in the heating chamber of the analyzer, which is under a nitrogen atmosphere. Heate the sample from 25°C at a heating rate of 10°C / min until the sample weight loss reaches 5 wt%. Record the heating temperature at this point.

[0202] 3. Glass transition temperature:

[0203] The static thermomechanical analyzer (TMA Q400) was used in a nitrogen atmosphere with a heating rate of 5℃ / min and a temperature range of 40℃~400℃.

[0204] 4. Porosity test:

[0205] Porosity was tested using the n-butanol absorption method. After drying, the sample was cut into approximately 20mm × 20mm square specimens. The thickness and actual side length were measured using a digital micrometer and vernier calipers, and the volume V was calculated. The weight M0 was then measured. The specimens were then immersed in n-butanol and soaked for 2 hours at room temperature and in air. After removal, the adsorbed n-butanol liquid on the surface was absorbed, and the weight M0 was measured. t The porosity of the membrane can be calculated using the following formula:

[0206]

[0207] In the formula: ρ is the density of n-butanol, 0.809 g / cm³. 3 .

[0208] 5. Mechanical properties:

[0209] Mechanical property testing machine (Instron-1121 model), tensile rate 5 mm / min, sample strip size 50 mm × 10 mm, the test result is the average value of 10 test sample strips.

[0210] Battery manufacturing

[0211] 1. Preparation of positive electrode sheet

[0212] LiNi, the positive electrode active material 0.5 Co 0.2 Mn 0.3 O2 (NCM523), conductive carbon black (Super P), and binder polyvinylidene fluoride (PVDF) are mixed evenly in an appropriate amount of solvent N-methylpyrrolidone (NMP) at a mass ratio of 96.2:2.7:1.1 to obtain a positive electrode slurry. The positive electrode slurry is coated onto the positive electrode current collector aluminum foil, and the positive electrode sheet is obtained through processes such as drying, cold pressing, slitting, and cutting.

[0213] 2. Negative electrode preparation

[0214] Artificial graphite (anode active material), carbon black (Super P) (conductive agent), styrene-butadiene rubber (SBR) (binder), and sodium carboxymethyl cellulose (CMC-Na) (mass ratio 96.4:0.7:1.8:1.1) are mixed evenly in an appropriate amount of deionized water to obtain a cathode slurry. The cathode slurry is then coated onto a copper foil (cathode current collector), and the cathode sheet is obtained through processes such as drying, cold pressing, slitting, and cutting.

[0215] 3. Separating membrane

[0216] The separator membrane was prepared using the separator membranes prepared in Examples 1-6 and Comparative Examples 1-2.

[0217] 4. Preparation of electrolyte

[0218] Ethyl carbonate (EC) and ethyl methyl carbonate (EMC) were mixed at a mass ratio of 30:70 to obtain an organic solvent. The fully dried electrolyte salt LiPF6 was dissolved in the above mixed solvent at a concentration of 1.0 mol / L. After mixing evenly, an electrolyte solution was obtained.

[0219] 5. Preparation of secondary batteries

[0220] The positive electrode, separator, and negative electrode are stacked in sequence, with the separator acting as a separator between the positive and negative electrodes. The electrode assembly is then wound up. The electrode assembly is placed in an outer package, and the prepared electrolyte is injected into the dried secondary battery. After vacuum sealing, settling, formation, and shaping, the secondary battery is obtained.

[0221] Battery performance test

[0222] 1. Hot box test

[0223] At 25°C, the secondary batteries prepared in the examples and comparative examples were charged at a constant current rate of 1C to the charging cutoff voltage of 4.2V, and then charged at a constant voltage until the current was ≤0.05C, and left to stand for 5 minutes. Then, each battery was tested in a DHG-9070A DHG series high temperature oven with fixtures, and the temperature was increased from room temperature to 80°C±2°C at a rate of 5°C / min and held for 30 minutes. Then, the temperature was increased at a rate of 5°C / min, and the temperature was held for 30 minutes for every 5°C increase until the cell failed. The temperature at which the cell began to fail was recorded.

[0224] 2. Thermal spread performance

[0225] At 25°C, the secondary batteries prepared in the examples and comparative examples were charged at a constant current rate of 1C to the charging cutoff voltage of 4.2V, and then charged at a constant voltage until the current was ≤0.05C. The batteries were then left to stand for 10 minutes. A metal heating plate was then attached tightly to the surface of the first cell, the clamp was tightened, and a 3mm heat insulation pad was added between the clamp and the cell. The battery was heated at a constant temperature of 200°C until thermal runaway occurred. The time when thermal runaway occurred was recorded.

[0226] 3. Shallow puncture test

[0227] At 25°C, take several secondary batteries prepared in each example and comparative example (each example and comparative example has several batteries).

[0228] Step 1: Charge the battery at a constant current rate of 1C until the charging cutoff voltage is 4.2V, then charge it at a constant voltage until the current is ≤0.05C, and let it stand for 10 minutes; peel off the outer shell of the battery, clamp it with a clamp, and then use a 1mm steel needle to puncture the secondary battery at a speed of 0.1mm / s until thermal runaway occurs. Record the puncture depth as L0.

[0229] Step 2: Take another new battery prepared by the battery preparation method of this embodiment, and perform a nail penetration test according to the conditions in step 1), wherein the nail penetration depth is L1 = L0 - 1 mm, and observe whether the secondary battery immediately experiences thermal runaway; if the secondary battery does not immediately experience thermal runaway, observe it at rest, and record the minimum nail penetration depth at which thermal runaway occurs after the secondary battery is nailed, as well as the interval time from nail penetration to thermal runaway.

[0230] Step 3: If the secondary battery still experiences thermal runaway at a needle penetration depth of L1, take another new battery prepared by the battery preparation method of this embodiment and perform the needle penetration test again according to the conditions in Step 1, where the needle penetration depth is L2 = L1 - 1 mm, and observe whether the secondary battery immediately experiences thermal runaway.

[0231] This process continues until the secondary battery does not immediately experience thermal runaway at a certain needle depth. Observations are then conducted, and the minimum needle depth at which thermal runaway occurs after the secondary battery is punctured, as well as the interval between needle puncture and the occurrence of thermal runaway, are recorded.

[0232] Test results

[0233] Scanning electron microscope images of the polyimide battery separators prepared in Examples 1-6 are attached. Figures 1-6 As shown. Scanning electron microscope images of the polyimide battery separators prepared in Comparative Examples 1 and 2 are attached. Figures 7-8 As shown.

[0234] The performance test data of the polyimide battery separators prepared in Examples 1-6 and Comparative Examples 1-2 are shown in Table 2.

[0235] Table 3 shows the cell performance test data of lithium secondary batteries prepared using the battery separators of Examples 1-6 and Comparative Examples 1-2.

[0236] Table 2 Performance parameters of polyimide porous membranes in the examples and comparative examples

[0237]

[0238] Table 3. Electrical performance tests of the secondary batteries in the examples and comparative examples.

[0239]

[0240] As shown in Tables 2 and 3, the microporous polyimide film formed by imidization using cross-linked polyamic acid-amine salts as raw materials exhibits significant mechanical property advantages compared to polyamic acid-amine salts with linear structures. The micropores formed in the prepared polyimide film have a narrower pore size distribution, resulting in superior mechanical properties. Furthermore, using the polyimide film prepared in this application as a separator in secondary batteries also significantly improves the performance of the battery cell.

[0241] Based on the disclosure and teachings of the above specification, those skilled in the art can make changes and modifications to the above embodiments. Therefore, this application is not limited to the specific embodiments disclosed and described above, and some modifications and changes to this application should also fall within the protection scope of the claims of this application. Furthermore, although some specific terms are used in this specification, these terms are only for convenience of explanation and do not constitute any limitation on this application.

Claims

1. A method for preparing a battery separator, characterized in that, The battery separator includes a polyimide porous membrane as shown in formula (II), which is prepared by an imidization reaction of a polyamic acid-amine salt with a cross-linked structure as shown in formula (I). in, A and B are independently selected from C6~C 30 Aromatic groups, C3~C 20 Alicyclic groups, C3~C 30 Heterocyclic groups, C1~C 30 One of the aliphatic groups; The substituents in A and B are independently selected from C1 to C2. 20 Alkyl, C1~C 20 Fluoroalkyl, carboxyl, nitro, C1~C 10 One of alkoxy, acetyl, and hydroxyl groups; M1-Y-M2 has a bis-tertiary amine structure; The structure of the bis-tertiary amine is R1R2N-Z-NR3R4; Among them, R1, R2, R3, and R4 may be the same or different; R1, R2, R3, and R4 are independently selected from C1 to C2. 40 Alkyl, C3~C 20 Alicyclic groups, C3~C 30 Heterocyclic groups or C5~C 30 One of the aromatic groups; Z is selected from C1~C 40 Alkyl, C3~C 20 Alicyclic groups, C3~C 30 Heterocyclic groups and C5~C 30 One of the aromatic groups; n is selected from 10 2 ~10 5 .

2. The method for preparing the battery separator according to claim 1, characterized in that, The two tertiary amine bonds in M1-Y-M2 are respectively bonded to the two -COOH groups in formula (I).

3. The method for preparing the battery separator according to claim 1, characterized in that, The M1-Y-M2 includes at least one of the following structural formulas: Among them, R5, R6, R7, R8, R9, R 10 R 11 R 12 R 13 Selected independently from C1~C 40 Alkyl, C3~C 20 Alicyclic groups, C3~C 30 Heterocyclic groups and C5~C 30 One of the aromatic groups.

4. The method for preparing the battery separator according to claim 1, characterized in that, The battery separator includes an aromatic polyimide film of formula (IV), wherein the aromatic polyimide film of formula (IV) is prepared by an imidization reaction of an aromatic polyamic acid-amine salt with a cross-linked structure as shown in formula (III). in, Selected from: One of them; R′ is selected from: One of them; R′′ is selected from: One of them; -Ar2- is selected from: One of them; X and X′ are each independently selected from one of the following groups: hydrogen, alkyl, cycloalkyl, aryl, fluoroalkyl, hydroxyl, alkoxy, phenoxy, cyano, nitro, amino, acetamino, ester, acyl, halogen, and carboxyl. The M1-Y-M2 has a bis-tertiary amine structure; n is selected from 10 2 ~10 5 .

5. The method for preparing the battery separator according to claim 4, characterized in that, X and X′ are each independently selected from one of -H, -Br, -Cl, -F, -NO2, -CN, -H, -CH3, -CH2CH3, -CH2CH2CH3, isopropyl, isobutyl, tert-butyl, cyclopentyl, cyclohexyl, phenyl, and naphthyl.

6. The method for preparing the battery separator according to claim 4, characterized in that, Ar1 is selected from one of benzene, biphenyl, pyridine, naphthalene, binaphthalene, diphenyl ether, diphenyl sulfone, benzophenone, diphenylpropane, diphenylhexafluoropropane, diphenylsilane, phenyl benzoate, and phenylbenzamide; Ar2 is selected from one of benzene, biphenyl, pyridine, naphthalene, binaphthalene, diphenyl ether, diphenyl sulfone, benzophenone, diphenylpropane, diphenylhexafluoropropane, diphenylsilane, phenyl benzoate, and phenylbenzamide.

7. The method for preparing the battery separator according to any one of claims 1 to 6, characterized in that, The method for preparing the polyamic acid-amine salt film with a cross-linked structure includes the following steps: Step (1): Polymerize dianhydride and diamine in a polar solvent to prepare a polyamic acid solution; Step (2): Add a tertiary amine to the polyamic acid solution to obtain a solution containing the polyamic acid-amine salt, and coat the solution onto the surface of the carrier plate to form a coating containing the polyamic acid-amine salt; Step (3): Dry the coating or immerse the coating in a poor solvent to cure the coating and obtain the polyamic acid-amine salt film with cross-linked structure.

8. The method for preparing the battery separator according to claim 7, characterized in that, In step (1), the polar solvent is selected from at least one of N,N′-dimethylformamide, N,N′-dimethylacetamide, N-methylpyrrolidone, dimethyl sulfoxide, and a mixture of tetrahydrofuran / methanol.

9. The method for preparing the battery separator according to claim 7, characterized in that, In step (2), the bis-tertiary amine comprises at least one of the following structural formulas: Among them, R5, R6, R7, R8, R9, R 10 R 11 R 12 R 13 Selected independently from C1~C 40 Alkyl, C3~C 20 Alicyclic groups, C3~C 30 Heterocyclic groups and C5~C 30 One of the aromatic groups; The tertiary amine is selected from N,N,N′,N′-tetramethylhexanediamine, N,N,N′,N′-tetraethylbutanediamine, N,N,N′,N′-tetraethylethylenediamine, 4,4-bipyridine, 2,2-bipyridine, 4-diethylaminopyridine, and N,N′-dimethyl... Piperazine, N,N′-diethylpiperazine, 1,4-diazabicyclo[2.2.2]octane, N,N′-dimethyl-N,N′-dicyclohexylhexanediamine, N-butylimidazole, N-ethylimidazole, N,N′-dimethyl At least one of N,N′-dibenzylpropanediamine.

10. The method for preparing the battery separator according to claim 7, characterized in that, In step (2), the molar amount of the bis-tertiary amine is 10% to 100% of the molar amount of the carboxylic acid groups in the polyamic acid solution.

11. The method for preparing the battery separator according to claim 10, characterized in that, The molar amount of the tertiary amine is 30% to 80% of the molar amount of carboxylic acid groups in the polyamic acid solution.

12. The method for preparing the battery separator according to claim 7, characterized in that, Step (2) further includes adding a surfactant and / or a nucleating agent to the polyamic acid solution.

13. The method for preparing the battery separator according to claim 7, characterized in that, In step (3), the temperature for drying the coating is 50℃~100℃ and the drying time is 0.1 hours~5 hours.

14. The method for preparing the battery separator according to claim 7, characterized in that, In step (3), the undesirable solvent is selected from organic solvents with a boiling point below 200°C or water; the undesirable solvent is selected from at least one of water, methanol, ethanol, n-propanol, isopropanol, n-butanol, ethylene glycol, ethylene glycol monomethyl ether, dimethoxyethyl ether, acetone, methyl ethyl ketone, tetrahydrofuran, dioxane and mixtures thereof.

15. The method for preparing the battery separator according to claim 1, characterized in that, The imidization method includes thermal imidization or chemical imidization.

16. The method for preparing the battery separator according to claim 1, characterized in that, The polyimide is selected from one of aliphatic polyimide, aromatic polyimide, aliphatic copolyimide, aromatic copolyimide, aliphatic polyimide block, aliphatic polyimide graft copolymer, aromatic polyimide block, and aromatic polyimide graft copolymer.

17. A battery separator, characterized in that, The separator is prepared by the method for preparing a battery separator according to any one of claims 1 to 16.

18. A secondary battery, characterized in that, The battery separator prepared by the method according to any one of claims 1 to 16 includes the battery separator prepared according to any one of claims 1 to 16.

19. A battery module, characterized in that, Includes the secondary battery according to claim 18.

20. A battery pack, characterized in that, This includes the secondary battery according to claim 18 or the battery module according to claim 19.

21. An electrical device, comprising an electric vehicle, characterized in that, Includes the secondary battery according to claim 17, the battery module according to claim 19, or the battery pack according to claim 20, wherein the secondary battery, the battery module, or the battery pack serves as the power source of the electrical device or the energy storage unit of the electrical device.

Images