Composite diaphragm and preparation method thereof, and secondary battery

Abstract

The present invention belongs to the technical field of battery separators and provides a composite separator, a preparation method thereof, and a secondary battery. The composite separator comprises a base membrane and a gradient pore size coating disposed on at least one surface of the base membrane; the gradient pore size coating is prepared from a self-polymerized microporous polymer; the gradient pore size coating comprises a nanoporous layer and a composite pore layer, the nanoporous layer being located between the base membrane and the composite pore layer; the nanoporous layer comprises nanopores, and the composite pore layer comprises interpenetrating micropores and nanopores. Through the design of the gradient pore size coating, the present invention enables the composite separator to have excellent barrier properties for transition metal cations and large-volume anions, while also having a high lithium ion migration number, low internal resistance, excellent electrolyte wettability and heat resistance, and can improve the cycle performance and service life of a secondary battery containing the composite separator.

Description

Technical Field

[0001] The present invention belongs to the technical field of battery separators, and in particular relates to a composite separator and a preparation method thereof, and a secondary battery. Background Art

[0002] Lithium-ion batteries consist of a positive electrode, a negative electrode, an electrolyte, and a separator. The separator is known as the "third electrode" of a lithium-ion battery. It is located between the positive and negative electrodes and is an important component of the battery. It prevents direct contact between the positive and negative electrodes and prevents short circuits while providing a channel for the transmission of lithium ions. However, the current mainstream lithium-ion battery separators, whether base membranes or coated separators, often have a minimum pore size greater than 30 nm. This not only has poor voltage breakdown resistance, but also easily causes larger lithium salt anions and unstable transition metal cations (such as Ni) to escape from the positive electrode. 2+ 、Mn 3+ 、Fe 3+ The lithium ion migration number is low, which also brings about the problem of negative electrode contamination and reduced battery cycle life.

[0003] CN103081172A discloses a diaphragm, which comprises a plate-like porous substrate and a porous thin film coating formed on at least one surface of the plate-like porous substrate and containing a cross-linked polyamide. The cross-linked polyamide is prepared by polymerizing a multifunctional amine compound having at least two amine groups with a multifunctional acyl halide of an aromatic compound having at least two acyl halide groups. A polyamide skin layer is grown on the base membrane by interfacial polymerization or the like, and has a nanoscale pore structure of approximately 0.3-1 nm. The pore size of such a microporous membrane is small enough to provide a good barrier effect for large-volume anions and multivalent metal ions. However, when used in lithium-ion batteries, it will result in excessively high lithium ion transmission impedance and increased internal resistance, affecting the performance of the battery.

[0004] Based on this, developing a battery separator that has good barrier effect on transition metal cations and large-volume anions, and has low internal resistance and high lithium ion migration number is an urgent problem to be solved in this field. Summary of the Invention

[0005] In view of the shortcomings of the prior art, the purpose of the present invention is to provide a composite diaphragm and its preparation method, and a secondary battery. Through the design of a gradient pore size coating, the composite diaphragm has an excellent barrier effect on transition metal cations and large-volume anions, and at the same time has a high lithium ion migration number, low internal resistance, excellent electrolyte wettability and heat resistance, and can improve the cycle performance and service life of the secondary battery containing it.

[0006] In order to achieve the purpose of the invention, the present invention adopts the following technical solutions:

[0007] In a first aspect, the present invention provides a composite diaphragm, comprising a base membrane and a gradient pore size coating disposed on at least one surface of the base membrane; the gradient pore size coating is prepared by a self-polymerized microporous polymer; the gradient pore size coating comprises a nanoporous layer and a composite pore layer, the nanoporous layer being located between the base membrane and the composite pore layer; the nanoporous layer comprises nanopores, the composite pore layer comprises micropores and nanopores interpenetrating each other, the pore size of the nanopores being independently <10 nm, and the maximum pore size on one side of the opening of the micropores being independently >0.1 μm.

[0008] The composite membrane provided by the present invention introduces a gradient pore size coating with a specific structure, which is prepared by self-polymerizing microporous polymer. The nanoporous layer close to the base membrane contains only nanopores and has a relatively dense structure; the composite porous layer close to the outer surface contains both nanopore structures and micronpore structures, and the two structures are interspersed with each other to form a specific porous structure, so that the composite membrane has an excellent barrier effect on transition metal ions and large-volume anions, effectively avoiding the pollution of the battery negative electrode caused by the dissolution of transition metal ions; at the same time, the composite membrane has a high lithium ion migration number, low internal resistance, and ideal electrolyte wettability and heat resistance, so that the secondary battery containing it has excellent performance, especially can improve the battery's cycle performance and service life.

[0009] The following are preferred technical solutions of the present invention, but are not intended to limit the technical solutions provided by the present invention. Through the following preferred technical solutions, the objectives and beneficial effects of the present invention can be better achieved and realized.

[0010] In the present invention, the term "nanopore" refers to a through hole with a pore diameter of less than 10 nm, and the pore diameter of each nanopore is independently less than 10 nm, for example, it can be 0.1 nm, 0.2 nm, 0.5 nm, 0.8 nm, 1 nm, 1.5 nm, 2 nm, 2.5 nm, 3 nm, 3.5 nm, 4 nm, 4.5 nm, 5 nm, 5.5 nm, 6 nm, 6.5 nm, 7 nm, 8 nm, 9 nm, 10 nm, and specific point values ​​between the above point values. Due to space limitations and for the sake of simplicity, the present invention no longer exhaustively lists the specific point values ​​included in the said range.

[0011] In the present invention, the "pore size" of the nanopores refers to the average pore size.

[0012] In the present invention, the term "micropore" refers to a through hole with a maximum pore diameter greater than 0.1 μm at one side of the opening, and the pore diameter rapidly narrows from the opening side to the interior of the coating to a nanoscale micropore size. The maximum pore diameter at the opening side of the micropore can be 0.2 μm, 0.5 μm, 0.8 μm, 1 μm, 1.5 μm, 2 μm, 2.5 μm, 3 μm, 3.5 μm, 4 μm, 4.5 μm, 5 μm, 5.5 μm, 6 μm, 6.5 μm, 7 μm, or 8 μm, as well as specific values ​​between the above points. Due to space limitations and for the sake of simplicity, the present invention does not exhaustively list the specific points included in the range.

[0013] In the present invention, the type of the base film (such as polyolefin, polyamide, polyester, fluoropolymer, etc.) and the molding process (such as dry method, wet method, extrusion, etc.) are not particularly limited. The present invention is applicable to conventional base films for separators of secondary batteries.

[0014] Preferably, the base film comprises a polyethylene (PE) base film, a polypropylene (PP) base film, or a polyolefin composite film. The polyolefin composite film refers to a multilayer composite porous film formed by stacking polypropylene (PP) and polyethylene (PE) in any order, for example, a PP-PE-PP three-layer composite porous film, a PP-PE two-layer composite porous film, or a PP-PP-PE-PP four-layer composite porous film.

[0015] Preferably, the thickness of the base film is 2-25 μm, for example, it can be 2 μm, 5 μm, 8 μm, 10 μm, 15 μm, 18 μm, 20 μm, 22 μm or 25 μm, as well as specific point values ​​between the above point values. Due to space limitations and for the sake of simplicity, the present invention no longer exhaustively lists the specific point values ​​included in the range, and 3-20 μm is further preferred.

[0016] The present invention does not specifically limit the average pore size and porosity of the basement membrane. However, based on considerations of mechanical strength, ion permeability, and electrochemical safety, a porosity of 30% to 65% is preferred, and examples include 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, or any two thereof. The average pore size is preferably in the range of 20-100 nm, and examples include 20 nm, 30 nm, 40 nm, 50 nm, 60 nm, 70 nm, 80 nm, 90 nm, 100 nm, or any two thereof. Furthermore, the pore structure of the basement membrane is formed by the interlaced interconnection of a plurality of fibrils, which are formed by stretching and orienting the polymer during the preparation process. The preparation method of the basement membrane is not specifically limited, and examples include stretching pore formation and phase separation.

[0017] In the present invention, the self-polymerized microporous polymer can be purchased from the market or prepared by methods known in the art; all self-polymerized microporous polymers known in the art are suitable for the present invention.

[0018] As a preferred technical solution of the present invention, the self-polymerized microporous polymer includes any one or a combination of at least two of porous aromatic organic framework polymers, intrinsic microporous polymers, conjugated microporous polymers, and hyper-crosslinked polymers, preferably any one or a combination of at least two of intrinsic microporous polymers (PIMs), conjugated microporous polymers, and hyper-crosslinked polymers.

[0019] In some embodiments, the intrinsically microporous polymer may include a polymer listed in the following literature: BaranMJ, CarringtonME, SahuS, BaskinA, SongJ, BairdMA, HanKS, MuellerKT,TeatSJ, MecklerSM, FuC, PrendergastD, HelmsBA. Diversity-orientedsynthesis of polymer membranes with ion solvation cages. Nature. 2021 Apr;592(7853):225-231. doi: 10.1038 / s41586-021-03377-7. Epub 2021 Apr 7. PMID:33828319. (BaranMJ, CarringtonME, SahuS, BaskinA, SongJ, BairdMA, HanKS, MuellerKT, TeatSJ, MecklerSM, FuC, PrendergastD, HelmsBA BA. Diversity-guided synthesis of ionic solvatable cage polymer membranes. Nature, 592(7853): 225–231. doi:10.1038 / s41586-021-03377-7. Epub ahead of print April 7, 2021. PMID: 33828319.

[0020] McKeown, NB, Budd, PM, Msayib, K., Ghanem, B., Microporous Polymer Material, US7690514B2, 2010.

[0021] Preferably, the intrinsic microporous polymer PIMs comprises a repeating unit (R AB ):

[0022] Formula (I);

[0023] Wherein, n is an integer between 5 and 10,000;

[0024] A is a monomer segment selected from any one of the following (A), (B), (C), (D), (E), (F), (G), (H), (I) or (j):

[0025] ;

[0026] B is a monomer segment selected from any one of the following (a), (b), (c), (d), (e), (f) or (g):

[0027] ;

[0028] Among them, each R 10 Each independently selected from (C 1-6 ) alkyl or H;

[0029] Each R 11 Each is independently selected from -CH2NR1R2 or H;

[0030] Each R 12 Each independently selected from -C(NOR 13 )N(R 14 )2 or -CN;

[0031] R1 and R2 are each independently (C 1-20 ) alkyl, (C 2-20 ) alkenyl, (C 2-20 ) alkynyl, (C 6-12 ) aryl, (C 3-8 ) cycloalkyl, (C 6-12 aryl)-C1-20 Alkyl, (C 3-8 Cycloalkyl)-C 1-20 Alkyl, (C 1-20 ) heteroalkyl, (C 1-20 ) haloalkyl, (C 1-20 ) haloalkoxy, 3 to 8 membered heterocyclyl, (3 to 8 membered heterocyclyl)-C 1-20 alkyl, 5- to 8-membered heteroaryl, heteroaryl-C 1-20 alkyl.

[0032] In some embodiments, each alkyl, alkenyl, and alkynyl group in R1 and R2 optionally and independently contains one or more heteroatoms independently selected from silicon, chalcogenides, and pnictides, and one or more atoms in R1 and R2 optionally and independently exist in an oxidized form, such as C=O, C=S, N=O, N=S, S=O, or S(O2).

[0033] In some embodiments, R1 and R2 may be optionally and independently substituted with one or more Z1.

[0034] In some embodiments, R1 and R2 can be taken together with the nitrogen atom to which they are attached to form a 3- to 8-membered heterocyclyl or a 5- to 8-membered heteroaryl, each of which is optionally substituted with one or more Z2.

[0035] The Z1 and the Z2 are each independently halogen, -OH, -NO2, -CN, (C 1-20 ) alkyl, (C 2-20 ) alkenyl, (C 2-20 ) alkynyl, (C 6-12 ) aryl, (C 3-8 ) cycloalkyl, (C 6-12 aryl)-C 1-20 Alkyl, (C 1-20 ) heteroalkyl, 3 to 8 membered heterocyclyl, 3 to 8 membered heteroaryl, (C 3-8 Cycloalkyl)-C 1-20 Alkyl, (3 to 8 membered heterocyclyl)-C 1-20 Alkyl, (5- to 8-membered heteroaryl)-C 1-20 Alkyl, (C 1-20 ) haloalkyl, (C 1-20 ) haloalkoxy, -OR6, -SR6, -S(O)R6, -S(O)2R6, SO2NR6NR7, NR6C(O)R7, NR6S(O)2R7, NR6C(O)NR7R8, NR6R7, CO2R6, -C(O)NR6R7 or -C(O)R6;

[0036] R3, R4, R5, R6, R7 and R8 are each independently (C 1-20 ) alkyl, (C2-20 ) alkenyl, (C 2-20 ) alkynyl, (C 6-12 ) aryl, (C 3-8 ) cycloalkyl, (C 6-12 aryl)-C 1-20 Alkyl, (C 3-8 Cycloalkyl)-C 1-20 Alkyl, (C 1-20 )heteroalkyl, 3 to 8 membered heterocyclyl, (3 to 8 membered heterocyclyl)-C 1-20 alkyl, 5- to 8-membered heteroaryl, or (5- to 8-membered heteroaryl)-C 1-20 alkyl.

[0037] In some embodiments, R4 and R5 together form (C 4-8 ) cycloalkyl, (C 6-12 ) aryl, 4- to 8-membered heterocyclyl, or 5- to 8-membered heteroaryl.

[0038] In some embodiments, R6 and R7 are taken together to form a 4- to 8-membered heterocyclyl or a 5- to 8-membered heteroaryl.

[0039] In some embodiments, R7 and R8 are taken together to form a 4- to 8-membered heterocyclyl or a 5- to 8-membered heteroaryl.

[0040] In some embodiments, each R 13 are independently H, (C 1-20 ) alkyl or (C 3-8 ) cycloalkyl, wherein alkyl and cycloalkyl are optionally and independently substituted with one or more Z3;

[0041] Each Z3 is independently halogen, -NO2, -CN, -OH, -SO3H, -NH2, (C 1-20 ) alkyl, (C 2-20 ) alkenyl, (C 2-20 ) alkynyl, (C 6-12 ) aryl, (C 3-8 ) cycloalkyl, (C 6-12 aryl)-C 1-20 Alkyl, (C 1–20 ) heteroalkyl, 3 to 8 membered heterocyclyl, 5 to 8 membered heteroaryl, (C 3-8 Cycloalkyl)-C 1–20 Alkyl, (5- to 8-membered heteroaryl)-C 1-20 Alkyl, (C 1-20 ) haloalkyl, (C 1-20) haloalkoxy, -OR6, -SR6, -S(O)R6, -S(O)2R6, SO2NR6NR7, NR6C(O)R7, NR6S(O)2R7, NR6C(O)NR7R8, NR6R7, CO2R6, -C(O)NR6R7 or -C(O)R6.

[0042] In some embodiments, each R 14 are independently H, (C 1-20 ) alkyl or (C 3-8 ) cycloalkyl.

[0043] In some embodiments, for example, the intrinsic microporous polymer may include an intrinsic microporous polymer PIM-1 (A unit is selected from (A) monomer segment, R 11 H, B unit is selected from (a) monomer segment, R 12 CN), intrinsic microporous polymer PIM-2 (A unit is selected from (A) monomer segment, R 11 H, B unit is selected from (g) monomer segment), intrinsic microporous polymer PIM-C1 (A unit is selected from (j) monomer segment, B unit is selected from (a) monomer segment, R 12 CN), pyridine-based intrinsic microporous polymer PIM-py (A unit is selected from (A) monomer segment, R 11 H, B unit is selected from (b) monomer segment, R 12 CN), cyclotricatechin-based self-microporous polymer (CTC-network-PIM), hexachlorohexaazatrinathyl self-microporous polymer (HATN-network-PIM), or a combination of at least two thereof.

[0044] Preferably, the conjugated microporous polymer includes any one or a combination of at least two of tetrakis[4-(9H-carbazole-9-yl)phenyl]methane conjugated microporous polymer, 1,3,5-tris(2-thienyl)-phenyl conjugated microporous polymer, 1,1,2,2-tetrakis(4-(9H-carbazole-9-yl)-[1,1'-biphenyl]-4-yl)vinyl conjugated microporous polymer, and poly(1,3,5-triethynylbenzene) conjugated microporous polymer.

[0045] Preferably, the hyper-crosslinked polymer comprises polylactic acid-poly(4-vinylbenzyl chloride-co-divinylbenzene) hyper-crosslinked polymer and / or hyper-crosslinked polyaniline.

[0046] Preferably, the number average molecular weight of the self-polymerized microporous polymer is 5000-100000, for example, it can be 8000, 10000, 15000, 20000, 25000, 30000, 35000, 40000, 45000, 50000, 55000, 60000, 65000, 70000, 75000, 80000, 85000, 90000, 95000 or 100000, as well as specific values ​​between the above points. Due to space limitations and for the sake of simplicity, the present invention does not exhaustively list the specific values ​​included in the range. Preferably, the number average molecular weight of the self-polymerized microporous polymer is 5000-45000.

[0047] Preferably, the D of the self-polymerized microporous polymer is 50 The particle size is 0.1-5 μm, for example, it can be 0.2 μm, 0.5 μm, 0.8 μm, 1 μm, 1.5 μm, 2 μm, 2.5 μm, 3 μm, 3.5 μm, 4 μm or 4.5 μm, as well as specific point values ​​between the above point values. Due to space limitations and for the sake of simplicity, the present invention no longer exhaustively lists the specific point values ​​included in the range.

[0048] As a preferred technical solution of the present invention, the volume percentage of nanopores in the gradient pore size coating is 5-55%, for example, it can be 8%, 10%, 12%, 15%, 18%, 20%, 22%, 25%, 28%, 30%, 32%, 35%, 38%, 40%, 42%, 45%, 48%, 50%, 52% or 54%, as well as specific point values ​​between the above point values. Due to space limitations and for the sake of simplicity, the present invention no longer exhaustively lists the specific point values ​​included in the range.

[0049] Preferably, the volume percentage of micropores in the gradient pore size coating is 25-75%, for example, it can be 28%, 30%, 32%, 35%, 38%, 40%, 42%, 45%, 48%, 50%, 52%, 55%, 58%, 60%, 62%, 65%, 68%, 70%, 72% or 74%, as well as specific point values ​​between the above point values. Due to space limitations and for the sake of simplicity, the present invention no longer exhaustively lists the specific point values ​​included in the range.

[0050] As a preferred technical solution of the present invention, the volume fraction of micropores in the gradient pore size coating is 25-75%, and the volume fraction of nanopores is 5-55%. This forms a specific gradient porous structure, which enables the composite membrane to have excellent transition metal ion and bulk anion barrier properties, a high lithium ion transference number, a low internal resistance, a suitable air permeability value, excellent electrolyte wettability, and heat resistance. If the volume fraction of nanopores is too low or the volume fraction of micropores is too high, the membrane's barrier effect on transition metal ions and bulk lithium salt anions will be affected, reducing the lithium ion transference number. If the volume fraction of nanopores is too high or the volume fraction of micropores is too low, the membrane's internal resistance will increase, which is not conducive to the performance of the battery.

[0051] Preferably, the porosity of the gradient pore size coating is 20-80%, for example, it can be 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70% or 75%, as well as specific point values ​​between the above point values. Due to space limitations and for the sake of simplicity, the present invention no longer exhaustively lists the specific point values ​​included in the range.

[0052] Preferably, the BET specific surface area of ​​the gradient pore size coating is 250-5000 m 2 / g, for example, 300 m 2 / g, 500 m 2 / g, 800 m 2 / g, 1000 m 2 / g, 1500 m 2 / g, 2000 m 2 / g, 2500 m 2 / g, 3000 m 2 / g, 3500 m 2 / g, 4000 m 2 / g or 4500 m 2 / g, as well as specific point values ​​between the above point values, due to space limitations and for the sake of brevity, the present invention no longer exhaustively lists the specific point values ​​included in the range.

[0053] For example, the BET specific surface area in the present invention is obtained by N2 adsorption testing.

[0054] For example, the volume percentage of micropores and nanopores in the gradient pore size coating of the present invention can be tested and calculated by the following method: ignoring the closed pores in the coating, assuming that all pore structures have at least one open end; the volume percentage of all pores (total pore volume) can be measured by gas adsorption method and calculated using BET specific surface area; , where Vg represents the total pore volume in mL / g, R is the average pore size in μm, and Sg is the specific surface area measured by the BET adsorption method in m 2 / g, the volume content of micropores (pore volume of micropores) can be calculated by coating density weighing method, and then the pore volume of nanopores can be obtained by subtracting the pore volume of micropores from the total pore volume. The volume ratio can be used to directly calculate the volume share.

[0055] As a preferred technical solution of the present invention, the coating amount of the gradient pore size coating is 0.1-1 g / m 2 , for example, it can be 0.2 g / m 2 , 0.3 g / m 2 , 0.4 g / m 2 , 0.5 g / m 2 , 0.6 g / m 2 , 0.7 g / m 2 , 0.8 g / m 2 or 0.9 g / m 2 , as well as specific point values ​​between the above point values, due to space limitations and for the sake of simplicity, the present invention no longer exhaustively lists the specific point values ​​included in the range.

[0056] Preferably, the thickness of the gradient pore size coating is 0.1-2 μm, for example, it can be 0.2 μm, 0.5 μm, 0.8 μm, 1 μm, 1.2 μm, 1.5 μm or 1.8 μm, as well as specific point values ​​between the above point values. Due to space limitations and for the sake of simplicity, the present invention no longer exhaustively lists the specific point values ​​included in the range.

[0057] Preferably, based on the total thickness of the gradient pore size coating as 100%, the thickness of the composite porous layer is 30-70%, for example, it can be 32%, 35%, 38%, 40%, 42%, 45%, 48%, 50%, 52%, 55%, 58%, 60%, 62%, 65% or 68%, as well as specific point values ​​between the above point values. Due to space limitations and for the sake of simplicity, the present invention no longer exhaustively lists the specific point values ​​included in the range.

[0058] Preferably, based on the total thickness of the gradient pore size coating as 100%, the thickness of the nanoporous layer is 30-70%, for example, it can be 32%, 35%, 38%, 40%, 42%, 45%, 48%, 50%, 52%, 55%, 58%, 60%, 62%, 65% or 68%, as well as specific point values ​​between the above point values. Due to space limitations and for the sake of simplicity, the present invention no longer exhaustively lists the specific point values ​​included in the range.

[0059] As a preferred technical solution of the present invention, in the gradient pore size coating, the thickness of the composite porous layer with a structure of interpenetrating micropores and nanopores accounts for 30-70%, and correspondingly, the thickness of the nanoporous layer with a relatively dense structure also accounts for 30-70%, thereby forming a specific gradient porous structure. This allows the composite separator to have a good barrier effect for transition metal ions and large-volume anions, a high lithium ion migration number, while also having a low internal resistance, suitable permeability, excellent electrolyte wettability, and heat resistance. If the thickness of the composite porous layer is too low, the lithium ion transmission impedance will increase, resulting in a high internal resistance of the separator. If the thickness of the composite porous layer is too high, the separator's barrier performance for transition metal ions and large-volume anions will decrease, and the permeability will increase.

[0060] As a preferred technical solution of the present invention, the composite diaphragm includes the gradient pore size coating, a base membrane and a second coating, and the base membrane is located between the gradient pore size coating and the second coating.

[0061] As a preferred technical solution of the present invention, one surface of the base membrane is provided with the gradient pore size coating, and the other surface is provided with a second coating, which has excellent heat resistance and / or electrolyte wettability and / or adhesion, and can enable the composite diaphragm to achieve more excellent comprehensive effects in multiple performance aspects such as electrolyte wettability, heat resistance, adhesion, barrier properties for transition metal ions and large-volume anions, lithium ion migration number, internal resistance, etc.

[0062] Preferably, the second coating layer comprises an organic coating layer or an organic-inorganic composite coating layer.

[0063] Preferably, the organic matter in the organic coating or the organic-inorganic composite coating independently includes any one or a combination of at least two of polyamide, aramid, poly(meth)acrylate, (meth)acrylic acid-(meth)acrylate copolymer, polyimide, polyacrylonitrile, cellulose, chemically modified cellulose, polyvinylidene fluoride, polytetrafluoroethylene, and vinylidene fluoride-hexafluoropropylene copolymer.

[0064] Further preferably, the organic coating comprises an aramid coating.

[0065] Preferably, the inorganic substance in the organic-inorganic composite coating includes any one of BaTiO3, HfO2, SrTiO3, SnO2, CeO2, MgO, NiO, CaO, ZnO, ZrO2, SiO2, Y2O3, Al2O3, AlOOH, TiO2, AlN, Si3N4, BN, TiN, SiC, B4C, and TiC, or a combination of at least two of them.

[0066] More preferably, the organic-inorganic composite coating comprises a nanocellulose composite coating, a PI fiber composite coating or an aramid-inorganic composite coating. The organic-inorganic composite coating can be applied in layers of organic and inorganic materials, or can be applied after mixing in a slurry.

[0067] Preferably, the thickness of the second coating is 1-4 μm, for example, it can be 1.2 μm, 1.5 μm, 1.8 μm, 2 μm, 2.2 μm, 2.5 μm, 2.8 μm, 3 μm, 3.2 μm, 3.5 μm or 3.8 μm, as well as specific point values ​​between the above point values. Due to space limitations and for the sake of simplicity, the present invention no longer exhaustively lists the specific point values ​​included in the range.

[0068] Preferably, the Gurley permeability value of the composite membrane is less than 10000 s / 100 mL, for example, it can be 500 s / 100 mL, 1000 s / 100 mL, 1500 s / 100 mL, 2000 s / 100 mL, 2500 s / 100 mL, 3000 s / 100 mL, 3500 s / 100 mL, 4000 s / 100 mL, 4500 s / 100 mL, 5000 s / 100 mL, 5500 s / 100 mL, 6000 s / 100 mL, 6500 s / 100 mL, 7000 s / 100 mL, 7500 s / 100 mL, 8000 s / 100 mL, 8500 s / 100 mL, 9000 s / 100 mL, 10000 s / 100 mL, 11000 s / 100 mL, 12000 s / 100 mL, 13000 s / 100 mL, 14000 s / 100 mL, 15000 s / 100 mL, 16000 s / 100 mL, 17000 s / 100 mL, 18000 s / 100 mL, 19000 s / 100 mL, 20000 s / 100 mL, 21000 s / 100 mL, 22000 s / 100 mL, 23000 s / 100 mL, 24000 mL or 9500 s / 100 mL, as well as specific point values ​​between the above point values, due to space limitations and for the sake of brevity, the present invention will no longer exhaustively list the specific point values ​​included in the range.

[0069] Preferably, the surface resistance of the composite membrane is 0.5-2.7 Ω·cm 2 , for example, it can be 0.6 Ω·cm 2 , 0.8Ω·cm 2 , 1 Ω·cm 2 , 1.2 Ω·cm 2 , 1.4 Ω·cm 2 , 1.5 Ω·cm 2 , 1.6 Ω·cm 2 , 1.8 Ω·cm 2 , 1.9 Ω·cm 2 , 2 Ω·cm 2 , 2.2 Ω·cm 2 or 2.5 Ω·cm 2, and specific point values ​​between the above point values. Due to space limitations and for the sake of simplicity, the present invention no longer exhaustively lists the specific point values ​​included in the range, and further preferably 0.5-2.25 Ω·cm 2 .

[0070] In a second aspect, the present invention provides a method for preparing the composite diaphragm according to the first aspect, the preparation method comprising:

[0071] providing a self-polymerizing microporous polymer slurry comprising a combination of a self-polymerizing microporous polymer and a solvent;

[0072] Applying the self-polymerizing microporous polymer slurry to at least one surface of a base membrane, drying, and forming a gradient pore size coating to obtain the composite membrane;

[0073] The drying comprises a first stage of drying and a second stage of drying which are performed sequentially, and a speed of the first stage of drying is less than a speed of the second stage of drying.

[0074] In the preparation method provided by the present invention, a segmented drying process is adopted to obtain a gradient pore size coating, wherein the drying speed of the first segment is relatively slow, which is conducive to the self-assembly of nanopores, thereby forming a relatively dense nanoporous layer, and the drying speed of the second segment is relatively fast, which is conducive to the rapid volatilization and gasification of the solvent, which is beneficial to the formation of micron pores, and a composite pore layer with micron pores and nanopores interlaced with each other is obtained.

[0075] As a preferred technical solution of the present invention, the drying satisfies any one of the following a) or b):

[0076] a) The temperature of the first drying stage is less than or equal to the temperature of the second drying stage, and the air volume of the first drying stage is less than the air volume of the second drying stage;

[0077] b) The temperature of the first drying stage is less than the temperature of the second drying stage, and the air volume of the first drying stage is less than or equal to the air volume of the second drying stage.

[0078] Preferably, the drying time is 10-100 s, for example, 20 s, 30 s, 40 s, 50 s, 60 s, 70 s, 80 s or 90 s, as well as specific values ​​between the above points. Due to space limitations and for the sake of simplicity, the present invention no longer exhaustively lists the specific points included in the range, and 10-80 s is further preferred.

[0079] Preferably, the solvent includes any one of chloroform, tetrahydrofuran, 2-methyltetrahydrofuran, ethyl acetate, methyl ethyl ketone, cyclopentyl methyl ether, dichloromethane, dichloroethane, trichloroethylene, trichloroethane, tetrachloroethylene, carbon tetrachloride, N-methylpyrrolidone, dimethyl sulfoxide, N,N-dimethylformamide, N,N-dimethylacetamide, or a combination of at least two thereof.

[0080] As a preferred technical solution of the present invention, the boiling point of the solvent or the azeotropic point of the solvent mixture is less than 100°C, for example, it can be 30°C, 35°C, 40°C, 45°C, 50°C, 55°C, 60°C, 62°C, 65°C, 68°C, 70°C, 72°C, 75°C, 78°C, 80°C, 82°C, 85°C, 88°C, 90°C, 92°C, 95°C or 98°C, as well as specific point values ​​between the above point values. Due to space limitations and for the sake of simplicity, the present invention no longer exhaustively lists the specific point values ​​included in the said range.

[0081] Preferably, the method for preparing the self-polymerizing microporous polymer slurry comprises: uniformly mixing the self-polymerizing microporous polymer and a solvent to obtain the self-polymerizing microporous polymer slurry.

[0082] The mixing method is any mixing method known in the art, including but not limited to: any one of stirring, grinding, ball milling, roller milling, homogenization, and ultrasound, or a combination of at least two thereof.

[0083] Preferably, the solid content of the self-polymerizing microporous polymer slurry is 1-10%, for example, it can be 2%, 3%, 4%, 5%, 6%, 7%, 8% or 9%, as well as specific point values ​​between the above point values. Due to space limitations and for the sake of simplicity, the present invention no longer exhaustively lists the specific point values ​​included in the range.

[0084] Preferably, the method for preparing the composite diaphragm further comprises the step of coating an organic coating or an organic-inorganic composite coating on the surface of the base membrane opposite to the gradient pore size coating.

[0085] Specifically, the steps for preparing the organic-inorganic composite coating include: (1) adding raw materials including inorganic particles and an adhesive into a first solvent and mixing them to obtain a first coating slurry;

[0086] (2) The first coating slurry is placed on the surface of the base membrane opposite to the gradient pore size coating, and after drying, the composite membrane is obtained.

[0087] In some embodiments, the first solvent comprises water and / or ethanol.

[0088] In some embodiments, at least one of a dispersant, a wetting agent, and a thickener may be added to the first coating slurry in step (1).

[0089] In some embodiments, the solid content of the first coating slurry is ≤50%, for example, 50%, 45%, 40%, 35%, 30%, 25%, 20%, 15%, 10% or a range consisting of any two thereof.

[0090] In the present invention, the coating method can be any coating method known in the art, including but not limited to: any one or a combination of at least two of extrusion coating, spray coating, roller coating, dip coating, gravure coating, slit coating, blade coating, comma coating, rod coating, spin coating, and screen printing, preferably extrusion coating.

[0091] Preferably, the temperature of the first drying stage is 20-50°C, for example, it can be 22°C, 25°C, 28°C, 30°C, 32°C, 35°C, 38°C, 40°C, 42°C, 45°C or 48°C, as well as specific point values ​​between the above point values. Due to space limitations and for the sake of simplicity, the present invention no longer exhaustively lists the specific point values ​​included in the range.

[0092] Preferably, the first drying time is 5-40 s, for example, 8 s, 10 s, 12 s, 15 s, 18 s, 20 s, 22 s, 25 s, 28 s, 30 s, 32 s, 35 s or 38 s, as well as specific point values ​​between the above point values. Due to space limitations and for the sake of simplicity, the present invention no longer exhaustively lists the specific point values ​​included in the range.

[0093] Preferably, the air volume of the first drying stage is 0-5 m 3 / min, for example, it can be 0.5 m 3 / min, 1 m 3 / min, 1.5 m 3 / min, 2 m 3 / min, 2.5 m 3 / min, 3 m 3 / min, 3.5 m 3 / min, 4 m 3 / min or 4.5 m 3 / min, as well as specific point values ​​between the above point values, due to space limitations and for the sake of brevity, the present invention no longer exhaustively lists the specific point values ​​included in the range.

[0094] Preferably, the temperature of the second drying stage is 50-80°C, for example, it can be 52°C, 55°C, 58°C, 60°C, 62°C, 65°C, 68°C, 70°C, 72°C, 75°C or 78°C, as well as specific point values ​​between the above point values. Due to space limitations and for the sake of simplicity, the present invention no longer exhaustively lists the specific point values ​​included in the range.

[0095] Preferably, the second drying time is 5-40 s, for example, 8 s, 10 s, 12 s, 15 s, 18 s, 20 s, 22 s, 25 s, 28 s, 30 s, 32 s, 35 s or 38 s, as well as specific point values ​​between the above point values. Due to space limitations and for the sake of simplicity, the present invention no longer exhaustively lists the specific point values ​​included in the range.

[0096] Preferably, the air volume of the second drying stage is 5-30 m 3 / min, for example, it can be 8 m 3 / min, 10 m 3 / min, 12 m 3 / min, 15 m 3 / min, 18 m 3 / min, 20 m 3 / min, 22 m 3 / min, 25 m 3 / min or 28 m 3 / min, as well as specific point values ​​between the above point values, due to space limitations and for the sake of brevity, the present invention no longer exhaustively lists the specific point values ​​included in the range.

[0097] In another aspect, the present invention provides a composite diaphragm as described in the first aspect and a use of the composite diaphragm prepared by the preparation method as described in the second aspect in a secondary battery.

[0098] In a third aspect, the present invention provides a secondary battery comprising a positive electrode sheet, a negative electrode sheet, and a separator arranged between the positive electrode sheet and the negative electrode sheet, wherein the separator is the composite separator described in the first aspect or the composite separator prepared by the preparation method described in the second aspect.

[0099] Preferably, the secondary battery comprises a lithium-ion battery.

[0100] Preferably, the gradient pore size coating in the composite separator is located on the side close to the negative electrode sheet.

[0101] As a preferred technical solution of the present invention, in a secondary battery (lithium-ion battery), the gradient pore coating is located on the side closest to the negative electrode, fully accommodating Li ions during discharge and facilitating their desolvation, thereby ensuring low discharge resistance. Furthermore, the pore structure of the gradient pore coating provides sufficient barrier properties for transition metal ions such as Ni, Fe, and Mn, as well as lithium salt anions, effectively preventing contamination of the battery's negative electrode by the dissolution of transition metal ions. Furthermore, preferably, a second coating with high electrolyte wettability is provided on the other surface of the composite separator. This coating is located on the side closest to the positive electrode, ensuring sufficient electrolyte wetting and accommodating transition metal ions released by the disintegration of the positive electrode, trapping them in the nanopore structure of the gradient pore coating on the other side, thereby ensuring low charge resistance. Through the design of the composite separator, the present invention enables secondary batteries (lithium-ion batteries) containing it to exhibit excellent performance, particularly improving the battery's cycle performance and operating life.

[0102] Compared with the prior art, the present invention has the following beneficial effects:

[0103] The composite membrane provided by the present invention, through the design of a specifically structured gradient pore size coating, provides excellent barrier properties against transition metal ions and bulky lithium salt anions, effectively preventing contamination of the battery's negative electrode by the dissolution of transition metal ions. Furthermore, the composite membrane exhibits a high lithium ion transference number, low internal resistance, excellent electrolyte wettability, heat resistance, and lithium ion permeability, along with a suitable air permeability value. This enables secondary batteries incorporating the membrane to exhibit superior performance, particularly significantly improving the battery's cycle performance and operating life. BRIEF DESCRIPTION OF THE DRAWINGS

[0104] Figure 1 A schematic structural diagram of a composite diaphragm provided in a specific embodiment;

[0105] Figure 2 This is a SEM image of the composite membrane provided in Example 1, with a magnification of 10,000 times. DETAILED DESCRIPTION

[0106] The technical solution of the present invention is further described below by way of specific embodiments. It should be understood by those skilled in the art that the embodiments are merely to help understand the present invention and should not be regarded as specific limitations of the present invention.

[0107] As used herein, the terms "comprises," "including," "having," "containing" or any other variations thereof, are intended to cover a non-exclusive inclusion. For example, a composition, process, method, article, or apparatus that comprises a listed element is not necessarily limited to only those elements but may include other elements not expressly listed or inherent to such composition, process, method, article, or apparatus.

[0108] In the present invention, features defined as "first" or "second" may explicitly or implicitly include one or more of the features, and are used to distinguish and describe features, without regard to order or importance. In the description of the present invention, unless otherwise specified, "plurality" means two or more.

[0109] In one embodiment, the structural diagram of the composite membrane is as follows: Figure 1 As shown, it comprises a gradient pore size coating, a base membrane, and a second coating layer arranged in sequence, wherein the base membrane is located between the gradient pore size coating and the second coating layer. The gradient pore size coating is prepared from a self-polymerized microporous polymer; the gradient pore size coating comprises a nanoporous layer and a composite porous layer, wherein the nanoporous layer is located between the base membrane and the composite porous layer; the nanoporous layer comprises nanopores, and the composite porous layer comprises micropores and nanopores interpenetrating each other, wherein the pore size of each nanopore is independently less than 10 nm, and the maximum pore size on one side of the opening of each micropore is independently greater than 0.1 μm.

[0110] The composite diaphragm and the preparation method thereof of the present invention will be described in detail below using a number of embodiments as examples, but the composite diaphragm and the preparation method thereof are not limited to these embodiments.

[0111] In the following examples, the specific information of the materials used are shown below, and other reagents for which the preparation methods are not specified are all commercially available chemicals.

[0112] (1) Self-polymerized microporous polymer

[0113] PIM-1: brand HWG58338, purchased from Beijing Huawei Ruike Chemical Co., Ltd., number average molecular weight of 38500;

[0114] PIM-py: purchased from Sigma-Aldrich, with a number average molecular weight of 25,800.

[0115] (2) Basement membrane

[0116] A polyethylene microporous membrane with a thickness of 9 μm and a porosity of 42% (Xingyuan Material, model: SW509I).

[0117] (3) Inorganic particles

[0118] Alumina, XKA243 from Xinke Zhonglian New Materials (Changzhou) Co., Ltd.

[0119] (4) Adhesive

[0120] Polyacrylate adhesive, Hunan Gaorui Power Materials Co., Ltd. GR-506.

[0121] Example 1

[0122] A composite diaphragm comprises a gradient pore size coating, a base membrane and a second coating, wherein the base membrane is located between the gradient pore size coating and the second coating; the second coating comprises inorganic particles and an adhesive; the gradient pore size coating is prepared using PIM-1 and comprises a nanoporous layer and a composite pore layer, wherein the nanoporous layer is located between the base membrane and the composite pore layer, the nanoporous layer comprises nanopores, and the composite pore layer comprises micropores and nanopores interpenetrating each other.

[0123] The preparation method of the composite diaphragm is as follows:

[0124] (1) Slurry preparation: PIM-1 was added to a solvent (a mixed solvent of chloroform and tetrahydrofuran, with a mass ratio of 1:1) and slurried using a single-paddle stirrer at 60 rpm for 8 h to obtain a self-polymerized microporous polymer slurry with a solid content of 5%;

[0125] The raw materials including aluminum oxide and polyacrylate adhesive are dispersed in a first solvent (deionized water) and stirred evenly to obtain a first coating slurry having a solid content of 35% and an aluminum oxide content of 32%.

[0126] (2) Coating and drying: Use extrusion coating equipment to coat the self-polymerized microporous polymer slurry obtained in step (1) on one side of the base film. After coating, the first drying stage and the second drying stage are carried out in sequence. The temperature of the first drying stage is 25°C, the time is 20 s, and the air volume is 3 m 3 / min, the second stage drying temperature is 50℃, the time is 20 s, and the air volume is 10 m 3 / min, thereby forming a gradient pore size coating;

[0127] The first coating slurry was coated on the surface opposite to the gradient pore size coating and then dried to form a second coating with a thickness of 2 μm, thereby obtaining the composite membrane. Specific information is shown in Table 1, and the test results are shown in Table 3.

[0128] Example 2

[0129] A composite diaphragm comprises a gradient pore size coating, a base membrane and a second coating, wherein the base membrane is located between the gradient pore size coating and the second coating; the gradient pore size coating is prepared using PIM-1 and comprises a nanoporous layer and a composite pore layer, wherein the nanoporous layer is located between the base membrane and the composite pore layer, the nanoporous layer comprises nanopores, and the composite pore layer comprises micropores and nanopores interpenetrating each other.

[0130] The preparation method of the composite diaphragm is as follows:

[0131] (1) Slurry preparation: PIM-1 was added to chloroform and slurried using a single-paddle agitator at 60 rpm for 8 h to obtain a self-polymerized microporous polymer slurry with a solid content of 3%;

[0132] (2) Coating and drying: Use extrusion coating equipment to coat the self-polymerized microporous polymer slurry obtained in step (1) on one side of the base film. After coating, the first drying stage and the second drying stage are carried out in sequence. The temperature of the first drying stage is 40°C, the time is 5 s, and the air volume is 5 m 3 / min, the second stage drying temperature is 60℃, the time is 40s, and the air volume is 15m 3 / min, thereby forming a gradient pore size coating; the preparation method of the second coating is the same as that of Example 1 to obtain the composite diaphragm, the specific information of which is shown in Table 1, and the test results are shown in Table 3.

[0133] Examples 3-13, Comparative Examples 1-4

[0134] A composite diaphragm and a preparation method thereof. Specific information is shown in Table 1 and Table 2. Items not shown in Table 1 and Table 2 are the same as those in Example 1. Test results are shown in Table 3.

[0135] Table 1

[0136]

[0137] Table 2

[0138]

[0139] Table 3

[0140]

[0141] In Table 2, the drying method of Comparative Example 4 is as follows: the self-polymerizing microporous polymer slurry is coated according to the same method as in Example 1, and after coating, it is dried at 50°C for 30 min, then heated to 80°C and dried for 40 min, and then heated to 100°C and dried for 30 min to form a self-polymerizing microporous polymer coating; other materials and process parameters of Comparative Example 4 are the same as in Example 1.

[0142] In the above table, the test method of the composite diaphragm is as follows:

[0143] (1) Micromorphology

[0144] Scanning electron microscopy (SEM, Zeiss Gemini 3) was used to test the micromorphology of the gradient pore size coating in the composite membrane. The SEM image of the composite membrane provided in Example 1 is shown in FIG. Figure 2As shown in the figure, it can be seen that the area on one side close to the basement membrane forms a relatively dense structure containing only nanopores, namely the nanoporous layer; the area on one side close to the outer surface forms a composite porous layer containing both nanopores and micropore structures, and the two structures are interspersed with each other, thereby constructing a gradient pore size coating.

[0145] (2) Average pore size of nanopores

[0146] The average nanopore diameter can be measured using a Bruker Dimension Icon atomic force microscope (AFM) using a 2 nm silicon nitride probe on a SCANASYST-AIR, selecting the adhesion channel and a 15 nm scan range. The average nanopore diameter can be obtained by dimensioning the multiple shaded micropore regions in the resulting image.

[0147] (3) Maximum pore diameter on one side of the micropore opening

[0148] The sample surface was observed by scanning with an SEM at 20,000 times magnification. The multiple shadowed micropore areas in the obtained image were dimensioned to obtain the maximum pore diameter on one side of the micropore opening.

[0149] (4) Porosity

[0150] Weigh a sample of a certain area. If the true density ρ of the self-polymerized microporous polymer is known, the average thickness dc of the gradient pore size coating can be measured using a ruler after taking a cross-sectional image through SEM. The average loading Ac can be measured by weighing, which is the weight of the composite membrane minus the weight of the base membrane and other coatings. The coating porosity can be further calculated = .

[0151] (5) BET specific surface area

[0152] A resin scraper was used to scrape the coating debris of the diaphragm sample of the present invention, 10 mg of the debris was weighed and added to the sample tube, and the specific surface area was tested by the nitrogen adsorption BET method with reference to GBT39713-2020 using a JW-BK112 specific surface and pore size analyzer.

[0153] (6) Volume percentage of nanopores and micropores in gradient pore size coatings

[0154] Ignore closed pores in the coating and assume that all pore structures have at least one open end; the volume fraction of all pores (total pore volume) can be measured by gas adsorption and calculated using the BET surface area; , where Vg represents the total pore volume in mL / g, R is the average pore size in μm, and Sg is the specific surface area measured by the BET adsorption method in m 2 / g, the volume content of micropores (pore volume of micropores) is calculated by coating density weighing method, and then the pore volume of nanopores is obtained by subtracting the pore volume of micropores from the total pore volume. The volume ratio can be used to directly calculate the volume share.

[0155] (7) Thickness of gradient pore size coating and thickness of composite pore layer

[0156] After taking a cross-sectional view with SEM, the average thickness of each coating such as the gradient pore size coating and the composite pore layer can be measured using a ruler.

[0157] (8) Coating amount

[0158] The value can be obtained by subtracting the mass of the diaphragm per unit area before and after coating.

[0159] (9) Gurley air permeability value

[0160] The air permeability value of the composite membrane was obtained by referring to the method specified in the national standard GB / T 36363-2018 "Polyolefin separators for lithium-ion batteries".

[0161] (10) Thermal shrinkage

[0162] The thermal shrinkage rates of the composite membrane in the MD direction (longitudinal direction) and TD direction (transverse direction) were obtained according to the method specified in the national standard GB / T 36363-2018 "Polyolefin separators for lithium-ion batteries", and the average value was taken as the thermal shrinkage rate of the composite membrane; wherein the heat treatment temperature of the oven was 150°C and the heat treatment time was 1 h.

[0163] (11) Surface resistance

[0164] Cut four composite membrane samples with a diameter of 45 mm from a flat surface, immerse the samples in an electrolyte (1 mol / L LiPF6 electrolyte, solvents are ethylene carbonate EC, ethyl methyl carbonate EMC, and dimethyl carbonate DMC in a volume ratio of 1:1:1) and seal and immerse for 30 min; pour the above electrolyte into a surface resistance test fixture; place 1, 2, 3, and 4 membranes in the fixture for testing respectively; use the number of membrane layers as the horizontal axis and the membrane resistance as the vertical axis for linear fitting, and calculate the slope and fit of the straight line. When the fit is greater than 0.999, the slope is the surface resistance of the membrane, expressed in Ω·cm 2 .

[0165] (12) Mn 3+ Ion permeation concentration (1 h)

[0166] The transition metal ion Mn was tested using an H-type electrolytic cell. 3+ The transmittance of Mn 3+ The permeation concentration after 1 hour is as follows:

[0167] In a glove box, a glass H-type electrolytic cell was used, with the left side being the high-concentration side (simulating the positive electrode side) connected to the counter electrode (Pt wire), and the right side being the low-concentration side (simulating the negative electrode side) connected to the working electrode (glassy carbon) and the reference electrode (lithium wire). A 10 cm diameter circular diaphragm sample was used to separate the two cells in the middle, with the diaphragm facing the same direction as in the battery. Equal amounts of electrolyte (1 mol / L LiPF6 electrolyte, with EC, EMC, and DMC in a volume ratio of 1:1:1) were added to both sides. Before the test, manganese acetylacetonate (III) salt was added to the left side and stirred to dissolve, so that the Mn 3+ The concentration is 50 mmol / L, and then LiPF6 is added to the right side and stirred to dissolve. + The concentration was 150 mmol / L. The three electrodes were connected to an electrochemical workstation and CV tests were performed at a rate of 0.2 V / s in the range of 4.0-1.5 V. One hour later, 5 mL of liquid sample was taken from the right side and digested with acid before ICP analysis of Mn 3+ concentration.

[0168] (13) Li ion migration number

[0169] The Li symmetric button cell was used for determination by constant potential polarization method, as follows:

[0170] A 2032-type Li / diaphragm / Li-type symmetric button cell was assembled using the same electrolyte (1 mol / L LiPF6 electrolyte, solvents were EC, EMC, and DMC in a volume ratio of 1:1:1). A potential difference ΔV = 10 mV was applied using an electrochemical workstation, and the initial current I0 and steady-state current Is were recorded. At the same time, the EIS curves were scanned in the initial state and steady state, and the interfacial impedance values ​​R0 and Rs in the initial state and after polarization, as well as the Li ion transference number (t + ) can be calculated using the following formula: .

[0171] (14) Assembly of lithium-ion batteries and performance testing

[0172] The preparation of lithium-ion batteries includes the following steps:

[0173] 1) The positive electrode active material nickel-cobalt-manganese ternary material (nickel-cobalt-manganese mass ratio = 8:1:1), the conductive agent conductive carbon black SP, and the binder (polyvinylidene fluoride, PVDF) at a mass ratio of 90:3:3 were added to the solvent (N-methylpyrrolidone, NMP) and mixed evenly to form a positive electrode slurry. The slurry was then coated onto a 10 μm aluminum foil current collector. The slurry was then dried in an oven at 95°C and rolled on a roller press to obtain a positive electrode sheet for later use.

[0174] 2) Add artificial graphite (the negative electrode active material), acetylene black (the conductive agent), and a binder (styrene-butadiene emulsion, SBR) to water in a mass ratio of 90:5:5 and disperse evenly to form a negative electrode slurry. The slurry is then coated onto a 10 μm thick copper foil current collector. After drying in an oven at 85°C, the slurry is rolled on a roller press to obtain a negative electrode sheet for later use.

[0175] 3) The composite diaphragm is placed between the positive electrode sheet and the negative electrode sheet for stacking and packaging, wherein the second coating layer of the composite diaphragm faces the positive electrode sheet and the gradient aperture coating layer faces the negative electrode sheet, and then an electrolyte (1 mol / L LiPF6 electrolyte, that is, the electrolyte includes lithium salt LiPF6 with a concentration of 1 mol / L, and the solvent is EC, EMC and DMC in a volume ratio of 1:1:1) is injected to obtain a battery.

[0176] Battery cycle performance: The battery's 100th cycle capacity retention rate at 60°C is taken as the cycle performance result. The specific test method includes the following steps: charging at a constant current and constant voltage of 0.8 C, discharging at a constant current of 1 C, measuring the first round of discharge capacity, cycling 100 times under this condition, testing the discharge capacity after 100 cycles, and calculating the ratio of the discharge capacity after 100 cycles to the first round of discharge capacity as the capacity retention rate after 100 cycles at 60°C.

[0177] According to the above test data, the composite membrane provided by the present invention has an excellent barrier effect on transition metal ions and large-volume lithium salt anions through the design of a gradient pore size coating with a specific structure. The transition metal ion transmittance is tested using an H-type electrolytic cell, and Mn 3+ The ion permeation concentration after 1 hour is ≤0.59 mmol / L, and preferably ≤0.4 mmol / L, which is more than 2 times lower than that of base membranes and ceramic coating membranes. Furthermore, the composite membrane has a high lithium ion transference number, measured using a Li symmetric button cell and constant potential polarization method, with a Li ion transference number greater than 0.3, and preferably ≥0.4. Furthermore, the composite membrane has low impedance, excellent electrolyte wettability, and sufficient Li ion permeability, as well as low thermal shrinkage and excellent heat resistance, resulting in significantly improved cycle performance and service life for lithium-ion batteries using it.

[0178] In addition, compared with the diaphragm with an aluminum oxide coating on only one side in Comparative Example 1, the Gurley permeability value of the composite diaphragm provided by the present invention is increased, but its permeability value can still meet the performance requirements of the secondary battery.

[0179] Although the composite membrane provided in Comparative Examples 2-4 is provided with a coating formed by a self-polymerized microporous polymer, the coating does not have the gradient pore structure defined in the present invention, resulting in at least one of its transition metal cation barrier property, lithium ion migration number, air permeability, and internal resistance being significantly insufficient, making it difficult to meet the high performance requirements of secondary battery membranes.

[0180] The applicant declares that while the above-described embodiments illustrate the composite separator, preparation method thereof, and secondary battery of the present invention, the present invention is not limited to the above-described process steps, nor does it imply that the present invention must rely on the above-described process steps for implementation. Persons skilled in the art should understand that any improvements to the present invention, equivalent substitutions for the raw materials used, addition of auxiliary components, and selection of specific methods, etc., fall within the scope of protection and disclosure of the present invention.

Claims

1. A composite diaphragm, characterized in that: The composite membrane comprises a base membrane and a gradient pore size coating provided on at least one surface of the base membrane; the gradient pore size coating is prepared by self-polymerizing microporous polymer; The gradient pore size coating comprises a nanoporous layer and a composite pore layer, wherein the nanoporous layer is located between the basement membrane and the composite pore layer; The nanoporous layer includes nanopores, and the composite porous layer includes micropores and nanopores interpenetrating each other, wherein the pore diameter of each nanopore is independently less than 10 nm, and the maximum pore diameter on one side of the opening of each micropore is independently greater than 0.1 μm; The preparation method of the gradient aperture coating includes segmented drying, wherein the segmented drying includes a first segment drying and a second segment drying performed sequentially, and the speed of the first segment drying is less than the speed of the second segment drying.

2. The composite diaphragm according to claim 1, characterized in that The volume percentage of nanopores in the gradient pore size coating is 5-55%; And / or, the volume percentage of micropores in the gradient pore size coating is 25-75%; and / or, the porosity of the gradient pore size coating is 20-80%; And / or, the BET specific surface area of ​​the gradient pore size coating is 250-5000 m 2 / g.

3. The composite diaphragm according to claim 1, characterized in that The coating amount of the gradient pore size coating is 0.1-1g / m 2 ; and / or, the gradient pore size coating has a thickness of 0.1-2 μm; And / or, based on the total thickness of the gradient pore size coating being 100%, the thickness of the composite porous layer is 30-70%.

4. The composite diaphragm according to claim 1, characterized in that The self-polymerized microporous polymer includes any one of an intrinsic microporous polymer, a conjugated microporous polymer, and a hyper-crosslinked polymer, or a combination of at least two of them.

5. The composite diaphragm according to claim 4, characterized in that The intrinsic microporous polymer comprises a repeating unit represented by the following formula (I): Formula (I); Wherein, n is an integer between 5 and 10,000; A is a monomer segment selected from any one of the following (A), (B), (C), (D), (E), (F), (G), (H), (I) or (j): ； ； B is a monomer segment selected from any one of the following (a), (b), (c), (d), (e), (f) or (g): ； ； ； Among them, each R 10 Each independently selected from (C 1-6 ) alkyl or H; Each R 11 Each is independently selected from -CH2NR1R2 or H; Each R 12 Each independently selected from -C(NOR 13 )N(R 14 )2 or -CN; R1 and R2 are each independently (C 1-20 ) alkyl, (C 2-20 ) alkenyl, (C 2-20 ) alkynyl, (C 6-12 ) aryl, (C 3-8 ) cycloalkyl, (C 6-12 Aryl)-C 1-20 Alkyl, (C 3-8 Cycloalkyl)-C 1-20 Alkyl, (C 1-20 ) heteroalkyl, (C 1-20 ) haloalkyl, (C 1-20 ) haloalkoxy, 3 to 8 membered heterocyclyl, (3 to 8 membered heterocyclyl)-C 1-20 alkyl, 5- to 8-membered heteroaryl, heteroaryl-C 1-20 alkyl; or, each alkyl, alkenyl and alkynyl group in R1 and R2 optionally and independently contains one or more heteroatoms independently selected from silicon, chalcogenides and pnictides, and one or more atoms in R1 and R2 are optionally and independently present in an oxidized form; or R1 and R2 may be optionally and independently substituted with one or more Z1; or, R1 and R2 may be taken together with the nitrogen atom to which they are attached to form a 3- to 8-membered heterocyclyl or a 5- to 8-membered heteroaryl, each of which is optionally substituted with one or more Z2; The Z1 and the Z2 are each independently halogen, -OH, -NO2, -CN, (C 1-20 ) alkyl, (C 2-20 ) alkenyl, (C 2-20 ) alkynyl, (C 6-12 ) aryl, (C 3-8 ) cycloalkyl, (C 6-12 Aryl)-C 1-20 Alkyl, (C 1-20 ) heteroalkyl, 3 to 8 membered heterocyclyl, 3 to 8 membered heteroaryl, (C 3-8 Cycloalkyl)-C 1-20 Alkyl, (3 to 8 membered heterocyclyl)-C 1-20 Alkyl, (5- to 8-membered heteroaryl)-C 1-20 Alkyl, (C 1-20 ) haloalkyl, (C 1-20 ) haloalkoxy, -OR6, -SR6, -S(O)R6, -S(O)2R6, SO2NR6NR7, NR6C(O)R7, NR6S(O)2R7, NR6C(O)NR7R8, NR6R7, CO2R6, -C(O)NR6R7 or -C(O)R6; R3, R4, R5, R6, R7 and R8 are each independently (C 1-20 ) alkyl, (C 2-20 ) alkenyl, (C 2-20 ) alkynyl, (C 6-12 ) aryl, (C 3-8 ) cycloalkyl, (C 6-12 Aryl)-C 1-20 Alkyl, (C 3-8 Cycloalkyl)-C 1-20 Alkyl, (C 1-20 )heteroalkyl, 3 to 8 membered heterocyclyl, (3 to 8 membered heterocyclyl)-C 1-20 alkyl, 5- to 8-membered heteroaryl, or (5- to 8-membered heteroaryl)-C 1-20 alkyl; Each R 13 Each independently is H, (C 1-20 ) alkyl or (C 3-8 ) cycloalkyl, wherein alkyl and cycloalkyl are optionally and independently substituted with one or more Z3; Each Z3 is independently halogen, -NO2, -CN, -OH, -SO3H, -NH2, (C 1-20 ) alkyl, (C 2-20 ) alkenyl, (C 2-20 ) alkynyl, (C 6-12 ) aryl, (C 3-8 ) cycloalkyl, (C 6-12 aryl)-C 1-20 Alkyl, (C 1–20 ) heteroalkyl, 3 to 8 membered heterocyclyl, 5 to 8 membered heteroaryl, (C 3-8 Cycloalkyl)-C 1–20 Alkyl, (5- to 8-membered heteroaryl)-C 1-20 Alkyl, (C 1-20 ) haloalkyl, (C 1-20 ) haloalkoxy, -OR6, -SR6, -S(O)R6, -S(O)2R6, SO2NR6NR7, NR6C(O)R7, NR6S(O)2R7, NR6C(O)NR7R8, NR6R7, CO2R6, -C(O)NR6R7 or -C(O)R6; Each R 14 Each independently is H, (C 1-20 ) alkyl or (C 3-8 ) cycloalkyl.

6. The composite diaphragm according to claim 1, characterized in that The composite membrane comprises the gradient pore size coating, a base membrane and a second coating, wherein the base membrane is located between the gradient pore size coating and the second coating; And / or, the second coating layer includes an organic coating layer or an organic-inorganic composite coating layer; And / or, the thickness of the second coating layer is 1-4 μm.

7. A method for preparing a composite diaphragm according to claim 1, characterized in that: The preparation method comprises: providing a self-polymerizing microporous polymer slurry comprising a combination of a self-polymerizing microporous polymer and a solvent; Applying the self-polymerizing microporous polymer slurry to at least one surface of a base membrane, drying, and forming a gradient pore size coating to obtain the composite membrane; The drying comprises a first stage of drying and a second stage of drying which are performed sequentially, and a speed of the first stage of drying is less than a speed of the second stage of drying.

8. The preparation method according to claim 7, characterized in that The drying satisfies any one of the following a) or b): a) The temperature of the first drying stage is less than or equal to the temperature of the second drying stage, and the air volume of the first drying stage is less than the air volume of the second drying stage; b) The temperature of the first drying stage is less than the temperature of the second drying stage, and the air volume of the first drying stage is less than or equal to the air volume of the second drying stage.

9. The preparation method according to claim 7, characterized in that The boiling point of the solvent is less than 100°C; And / or, the first drying stage has a temperature of 20-50°C, a time of 5-40 s, and an air volume of 0-5 m 3 / min; And / or, the second drying stage has a temperature of 50-80°C, a time of 5-40 s, and an air volume of 5-30 m 3 / min.

10. A secondary battery, characterized in that: The secondary battery includes a positive electrode sheet, a negative electrode sheet, and a separator disposed between the positive electrode sheet and the negative electrode sheet. The separator is the composite separator according to any one of claims 1 to 6 or a composite separator prepared by the preparation method according to any one of claims 7 to 9.

11. The secondary battery according to claim 10, wherein The gradient pore size coating in the composite diaphragm is located on a side close to the negative electrode sheet.

Images