Composite diaphragm, preparation method thereof and secondary battery

Abstract

The invention belongs to the technical field of battery diaphragms, and provides a composite diaphragm and a preparation method thereof, and a secondary battery, the composite diaphragm comprises a base membrane and a gradient aperture coating arranged on at least one surface of the base membrane; the gradient aperture coating is prepared from a self-polymerized microporous polymer; the gradient aperture coating comprises a nanopore layer and a composite pore layer, and the nanopore layer is located between the base membrane and the composite pore layer; the nanopore layer comprises nanopores, and the composite pore layer comprises micron pores and nanopores which are interspersed with each other. Through the design of the gradient aperture coating, the composite diaphragm has an excellent blocking effect of transition metal cations and large-volume anions, meanwhile, the lithium ion transference number is high, the internal resistance is relatively low, the electrolyte wettability and the heat resistance are excellent, the cycle performance of the secondary battery comprising the composite diaphragm can be improved, and the service life of the secondary battery can be prolonged.

Description

Technical Field

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

[0002] A lithium-ion battery includes a positive electrode, a negative electrode, an electrolyte, and a separator. Among them, the separator is called the "third electrode" of the lithium-ion battery. It is located between the positive electrode and the negative electrode and is an important part of the battery. It provides a channel for the transmission of lithium ions while preventing direct contact between the positive and negative electrodes and avoiding short circuits. However, for the currently mainstream lithium-ion battery separators, whether it is a base film or a coated separator, their minimum pore size is often greater than 30 nm. Not only is the voltage breakdown resistance performance not ideal, but it also easily allows relatively large lithium salt anions and transition metal cations unstably released from the positive electrode (such as Ni 2+ , Mn 3+ , Fe 3+ etc.) to easily pass through, resulting in a low lithium ion transference number, and also bringing problems such as pollution of the negative electrode and a decrease in the battery cycle life.

[0003] CN103081172A discloses a separator which includes a plate-shaped porous substrate and a porous thin film coating formed on at least one surface of the plate-shaped porous substrate and containing crosslinked polyamide. The crosslinked polyamide is prepared by polymerizing a polyfunctional amine compound having at least two amino groups with a polyfunctional acyl halide of an aromatic compound having at least two acyl halide groups; a polyamide skin layer with a nano-scale pore size structure of about 0.3 - 1 nm is grown on the base film by means of interfacial polymerization, etc. The pore size of such a microporous membrane is small enough to have a good blocking effect on large-volume anions and multivalent metal ions. However, when it is used in a lithium-ion battery, it will cause too high an impedance for lithium ion permeation, an increase in internal resistance, and affect the performance of the battery.

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

[0005] Aiming at the deficiencies of the prior art, the purpose of the present invention is to provide a composite separator, a preparation method thereof, and a secondary battery. Through the design of a gradient pore size coating, the composite separator has an excellent blocking effect on transition metal cations and large-volume anions, while having a high lithium ion transference number, a low internal resistance, excellent electrolyte wettability and heat resistance, and can improve the cycle performance and working life of the secondary battery containing it.

[0006] To achieve the purpose of this invention, the present invention adopts the following technical solutions:

[0007] In a first aspect, the present invention provides a composite separator, which comprises a base film and a gradient pore size coating provided on at least one surface of the base film; the gradient pore size coating is prepared by self-polymerized microporous polymer; the gradient pore size coating comprises a nanopore layer and a composite pore layer, and the nanopore layer is located between the base film and the composite pore layer; the nanopore layer comprises nanopores, and the composite pore layer comprises interpenetrating micropores and nanopores, the pore diameters of the nanopores are each independently < 10 nm, and the maximum pore diameter on one side of the opening of the micropores is each independently > 0.1 μm.

[0008] The gradient pore size coating with a specific structure is introduced into the composite separator provided by the present invention. It is prepared by self-polymerized microporous polymer. The nanopore layer close to the base film only contains nanopores and has a relatively dense structure; the composite pore layer close to the outer surface contains both nanopore structures and micropore structures, and the two structures interpenetrate each other to form a specific porous structure, so that the composite separator has excellent barrier effects 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 separator has a high lithium ion transference number, a low internal resistance, ideal electrolyte wettability and heat resistance, enabling the secondary battery containing it to have excellent performance, especially improving the cycle performance and working life of the battery.

[0009] The following are the preferred technical solutions of the present invention, but do not 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 < 10 nm, and the pore diameters of the nanopores are each independently < 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, as well as the specific point values between the above point values. Due to space limitations and for the sake of simplicity, the present invention does not exhaustively list the specific point values included in the above range.

[0011] In the present invention, the "pore diameter" of the nanopore represents its average pore diameter.

[0012] In the present invention, the term "micropore" refers to a through-hole with a maximum pore diameter on one side of the opening > 0.1 μm, and its pore diameter rapidly narrows from the opening side to the nano-scale micropore size inside the coating. The maximum pore diameter on one side of the opening 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 point values between the above point values. Due to space limitations and for the sake of simplicity, the present invention does not exhaustively list the specific point values included in the above range.

[0013] In the present invention, the type of the base film (such as polyolefin, polyamide, polyester, fluoropolymer, etc.) and the forming process (such as dry method, wet method, extrusion, etc.) are not specifically limited, and conventional separator base films that can be used for secondary batteries are applicable to the present invention.

[0014] Preferably, the base film includes a polyethylene (PE) base film, a polypropylene (PP) base film or a polyolefin composite film. The polyolefin composite film refers to a multi-layer composite porous film formed by laminating polypropylene (PP) and polyethylene (PE) in any order. For example, a three-layer composite porous film of PP-PE-PP, a two-layer composite porous film of PP-PE, or a four-layer composite porous film of PP-PP-PE-PP.

[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 does not exhaustively list the specific point values included in the above range. Further preferably, it is 3-20 μm.

[0016] The present invention does not particularly limit the average pore diameter and porosity of the base film. However, from the perspectives of mechanical strength, ion permeability and electrochemical safety, it is preferably to have a porosity between 30% and 65%, for example, it can be 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65% or a range composed of any two of them; the average pore diameter is preferably in the range of 20-100 nm, for example, it can be 20 nm, 30 nm, 40 nm, 50 nm, 60 nm, 70 nm, 80 nm, 90 nm, 100 nm or a range composed of any two of them. In addition, the pore structure of the base film is formed by the interlaced connection of several fibrils. The fibrils are formed by the stretching and orientation of the polymer during the preparation process. The preparation method of the base film is not particularly limited. For example, it can be a stretching pore-forming method, a phase separation method, etc.

[0017] In the present invention, the self-polymerized microporous polymer can be obtained through market channels or prepared by methods well-known in the art; all self-polymerized microporous polymers known in the art are applicable to 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, intrinsically microporous polymers, conjugated microporous polymers, and hypercrosslinked polymers, preferably any one or a combination of at least two of intrinsically microporous polymers (PIMs), conjugated microporous polymers, and hypercrosslinked polymers.

[0019] In some embodiments, the intrinsically microporous polymer may include the polymers listed in the following literature: Baran MJ, Carrington ME, Sahu S, Baskin A, Song J, Baird MA, Han KS, Mueller KT, Teat SJ, Meckler SM, Fu C, Prendergast D, Helms BA. Diversity-oriented synthesis 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. (Baran MJ, Carrington ME, Sahu S, Baskin A, Song J, Baird MA, Han KS, Mueller KT, Teat SJ, Meckler SM, Fu C, Prendergast D, Helms BA. Diversity-oriented synthesis of polymer membranes with ion solvation cages. Nature, Volume 592 (Issue 7853): pp. 225 - 231. doi: 10.1038 / s41586-021-03377-7. Epub ahead of print on April 7, 2021. PMID: 33828319);

[0020] McKeown, N.B., Budd, P.M., Msayib, K., Ghanem, B., Microporous Polymer Material, US7690514B2, 2010. (McKeown, N.B., Budd, P.M., Msayib, K., Ghanem, B., Microporous Polymer Material, US Patent No. US7690514B2, 2010.)

[0021] Preferably, the intrinsically microporous polymer PIMs comprise repeating units represented by the following formula (I) (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] wherein each R 10 is independently selected from (C 1-6 ) alkyl or H;

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

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

[0031] R1 and R2 are independently (C 1-20 ) alkyl, (C 2-20 ) alkenyl, (C 2-20 ) alkynyl, (C 6-12 ) aryl, (C3-8 ), 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 heterocyclic group, (3- to 8-membered heterocyclic group)-C 1-20 alkyl, 5- to 8-membered heteroaryl, heteroaryl-C 1-20 alkyl.

[0032] In some embodiments, each alkyl, alkenyl, and alkynyl in R1 and R2 optionally and independently contains one or more heteroatoms independently selected from silicon, chalcogenide, and pnictide, and one or more atoms in said 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 optionally and independently be substituted by one or more Z1.

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

[0035] Said Z1 and said 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 heterocyclic group, 3- to 8-membered heteroaryl, (C 3-8 cycloalkyl)-C 1-20 alkyl, (3- to 8-membered heterocyclic group)-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, (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, a 3- to 8-membered heterocyclic group, (a 3- to 8-membered heterocyclic group)-C 1-20 alkyl, a 5- to 8-membered heteroaryl, or (a 5- to 8-membered heteroaryl)-C 1-20 alkyl.

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

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

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

[0040] In some embodiments, each R 13 is independently H, (C 1-20 ), alkyl, or (C 3-8 ), cycloalkyl, where the 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, a 3- to 8-membered heterocyclic group, a 5- to 8-membered heteroaryl, (C 3-8 cycloalkyl)-C 1–20 alkyl, (a 5- to 8-membered heteroaryl)-C 1-20 alkyl, (C 1-20 ), haloalkyl, (C 1-20haloalkoxy, -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 is independently H, (C 1-20 alkyl) or (C 3-8 cycloalkyl).

[0043] In some embodiments, illustratively, the intrinsically microporous polymer may include intrinsically microporous polymer PIM-1 (the A unit is selected from (A) monomer segments, R 11 is H, the B unit is selected from (a) monomer segments, R 12 is CN), intrinsically microporous polymer PIM-2 (the A unit is selected from (A) monomer segments, R 11 is H, the B unit is selected from (g) monomer segments), intrinsically microporous polymer PIM-C1 (the A unit is selected from (j) monomer segments, the B unit is selected from (a) monomer segments, R 12 is CN), pyridyl-based intrinsically microporous polymer PIM-py (the A unit is selected from (A) monomer segments, R 11 is H, the B unit is selected from (b) monomer segments, R 12 is CN), any one or a combination of at least two of cyclotriveratrylene-based intrinsically microporous polymer (CTC-network-PIM) and hexaazatriphenylenehexacarbonitrile-based intrinsically microporous polymer (HATN-network-PIM).

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

[0045] Preferably, the hypercrosslinked polymer includes poly(lactic acid)-poly(4-vinylbenzyl chloride-co-divinylbenzene) hypercrosslinked polymer and / or hypercrosslinked polyaniline.

[0046] Preferably, the number-average molecular weight of the self-polymerized microporous polymer is 5,000 - 100,000, for example, it can be 8,000, 10,000, 15,000, 20,000, 25,000, 30,000, 35,000, 40,000, 45,000, 50,000, 55,000, 60,000, 65,000, 70,000, 75,000, 80,000, 85,000, 90,000, 95,000 or 100,000, as well as the specific point values between the above point values. Due to space limitations and for the sake of brevity, the present invention does not exhaustively list the specific point values included in the said range. Preferably, the number-average molecular weight of the self-polymerized microporous polymer is 5,000 - 45,000.

[0047] Preferably, the D of the self-polymerized microporous polymer 50 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 the specific point values between the above point values. Due to space limitations and for the sake of brevity, the present invention does not exhaustively list the specific point values included in the said range.

[0048] As a preferred technical solution of the present invention, the volume percentage content of nano-pores 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 the specific point values between the above point values. Due to space limitations and for the sake of brevity, the present invention does not exhaustively list the specific point values included in the said range.

[0049] Preferably, the volume percentage content of micro-pores 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 the specific point values between the above point values. Due to space limitations and for the sake of brevity, the present invention does not exhaustively list the specific point values included in the said range.

[0050] As a preferred technical solution of the present invention, the volume ratio of the micron pores in the gradient pore size coating is 25-75%, and the volume ratio of the nano pores is 5-55%. Thus, a specific gradient porous structure is formed, enabling the composite separator to have excellent barrier effects on transition metal ions and large-volume anions, a relatively high lithium ion transference number, a relatively low internal resistance, an appropriate air permeability value, excellent electrolyte wettability, and heat resistance. If the volume ratio of the nano pores is too low or the volume ratio of the micron pores is too high, the barrier effect of the separator on transition metal ions and large-volume lithium salt anions will be affected, and the lithium ion transference number will be reduced. If the volume ratio of the nano pores is too high or the volume ratio of the micron pores is too low, the internal resistance of the separator 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 brevity, the present invention does not exhaustively list the specific point values included in the above range.

[0052] Preferably, the BET specific surface area of the gradient pore size coating is 250-5000 m 2 / g, for example, it can be 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 does not exhaustively list the specific point values included in the above range.

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

[0054] Exemplarily, the volume percentage content of the micron pores and nano pores 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 ratio of all pores (total pore volume) can be measured by gas adsorption method and calculated using the BET specific surface area; , where Vg represents the total pore volume in mL / g, R is the average pore diameter 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 the coating density weighing method. Subtracting the pore volume of micropores from the total pore volume gives the pore volume of nanopores. The volume ratio can be directly calculated using the pore volume ratio.

[0055] As a preferred technical solution of the present invention, the coating amount of the gradient pore diameter 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 , and the specific point values between the above point values. Due to space limitations and for the sake of brevity, the present invention does not exhaustively list the specific point values included in the above range.

[0056] Preferably, the thickness of the gradient pore diameter 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, and the specific point values between the above point values. Due to space limitations and for the sake of brevity, the present invention does not exhaustively list the specific point values included in the above range.

[0057] Preferably, based on the total thickness of the gradient pore diameter coating being 100%, the thickness of the composite pore 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%, and the specific point values between the above point values. Due to space limitations and for the sake of brevity, the present invention does not exhaustively list the specific point values included in the above range.

[0058] Preferably, based on the total thickness of the gradient pore diameter coating being 100%, the thickness of the nanopore 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%, and the specific point values between the above point values. Due to space limitations and for the sake of brevity, the present invention does not exhaustively list the specific point values included in the above range.

[0059] As a preferred technical solution of the present invention, in the gradient pore size coating, the thickness proportion of the composite pore layer with an interpenetrating structure of micro-pores and nano-pores is 30-70%, and correspondingly, the thickness proportion of the relatively dense nano-pore layer is also 30-70%, thereby forming a specific gradient porous structure, so that while the composite separator has good barrier effects on transition metal ions and large-volume anions and a high lithium ion transference number, it has a lower internal resistance, an appropriate air permeability value, excellent electrolyte wettability and heat resistance. If the thickness proportion of the composite pore layer is too low, the impedance of lithium ion permeation will increase, resulting in a higher internal resistance of the separator; if the thickness proportion of the composite pore layer is too high, the barrier performance of the separator to transition metal ions and large-volume anions will decline, and the air permeability value will increase.

[0060] As a preferred technical solution of the present invention, the composite separator includes the gradient pore size coating, a base film and a second coating, and the base film 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 film 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 adhesiveness, and can enable the composite separator to achieve a more excellent comprehensive effect in terms of multiple properties such as electrolyte wettability, heat resistance, adhesiveness, barrier properties to transition metal ions and large-volume anions, lithium ion transference number, and internal resistance.

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

[0063] Preferably, the organic substances in the organic coating or the organic-inorganic composite coating independently include 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, vinylidene fluoride-hexafluoropropylene copolymer.

[0064] More preferably, the organic coating includes an aramid coating.

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

[0066] Further preferably, the organic-inorganic composite coating includes a nanocellulose composite coating, a PI fiber composite coating, or an aramid-inorganic composite coating. The organic-inorganic composite coating can be coated with organic and inorganic layers separately, or can be coated 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 the specific point values between the above point values. Due to space limitations and for the sake of simplicity, the specific point values included in the scope of the present invention are not exhaustively listed herein.

[0068] Preferably, the Gurley air permeability value of the composite separator < 10000 s / 100 mL, for example, it can be 500 s / 100mL, 1000 s / 100 mL, 1500 s / 100 mL, 2000 s / 100 mL, 2500 s / 100 mL, 3000 s / 100 mL, 3500s / 100 mL, 4000 s / 100 mL, 4500 s / 100 mL, 5000 s / 100 mL, 5500 s / 100 mL, 6000 s / 100mL, 6500 s / 100 mL, 7000 s / 100 mL, 7500 s / 100 mL, 8000 s / 100 mL, 8500 s / 100 mL, 9000s / 100 mL, or 9500 s / 100 mL, as well as the specific point values between the above point values. Due to space limitations and for the sake of simplicity, the specific point values included in the scope of the present invention are not exhaustively listed herein.

[0069] Preferably, the surface resistance of the composite separator 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 the specific point values between the above point values. Due to space limitations and for the sake of brevity, the present invention does not exhaustively list the specific point values included in the range. Further preferably, it is 0.5 - 2.25 Ω·cm 2 .

[0070] In a second aspect, the present invention provides a method for preparing a composite separator as described in the first aspect, and the preparation method includes:

[0071] Providing a self-polymerized microporous polymer slurry, the self-polymerized microporous polymer slurry including a combination of a self-polymerized microporous polymer and a solvent;

[0072] Coating the self-polymerized microporous polymer slurry on at least one surface of a base film, drying to form a gradient pore size coating, and obtaining the composite separator;

[0073] The drying includes a first-stage drying and a second-stage drying carried out in sequence, and the speed of the first-stage drying < the speed of the second-stage drying.

[0074] In the preparation method provided by the present invention, a gradient pore size coating is obtained by a segmented drying process. The speed of the first-stage drying is slower, which helps the self-assembly of nanopores, thereby forming a relatively dense nanopore layer. The speed of the second-stage drying is faster, which helps the rapid volatilization and gasification of the solvent, is conducive to the formation of micron pores, and obtains a composite pore layer in which micron pores and nanopores are interpenetrated.

[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-stage drying ≤ the temperature of the second-stage drying, and the air volume of the first-stage drying < the air volume of the second-stage drying;

[0077] b) The temperature of the first-stage drying < the temperature of the second-stage drying, and the air volume of the first-stage drying ≤ the air volume of the second-stage drying.

[0078] Preferably, the drying time is 10 - 100 s. For example, it can be 20 s, 30 s, 40 s, 50 s, 60 s, 70 s, 80 s, or 90 s, and the specific point values between the above point values. Due to space limitations and for the sake of brevity, the present invention does not exhaustively list the specific point values included in the range. Further preferably, it is 10 - 80 s.

[0079] Preferably, the solvent includes any one or a combination of at least two 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, and N,N-dimethylacetamide.

[0080] As a preferred technical solution of the present invention, the boiling point of the solvent or the azeotropic point of the solvent mixture < 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 the specific point values between the above point values. Due to space limitations and for the sake of simplicity, the present invention does not exhaustively list the specific point values included in the range.

[0081] Preferably, the preparation method of the self-polymerized microporous polymer slurry includes: mixing the self-polymerized microporous polymer and the solvent uniformly to obtain the self-polymerized microporous polymer slurry.

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

[0083] Preferably, the solid content of the self-polymerized microporous polymer slurry is 1-10%, for example, it can be 2%, 3%, 4%, 5%, 6%, 7%, 8% or 9%, as well as the specific point values between the above point values. Due to space limitations and for the sake of simplicity, the present invention does not exhaustively list the specific point values included in the range.

[0084] Preferably, the preparation method of the composite separator further includes the step of coating an organic coating or an organic-inorganic composite coating on the surface of the base film opposite to the gradient pore size coating.

[0085] Specifically, the preparation steps of the organic-inorganic composite coating include: (1) adding raw materials including inorganic particles and adhesives to a first solvent and mixing to obtain a first coating slurry;

[0086] (2) disposing the first coating slurry on the surface of the base film opposite to the gradient pore size coating, and after drying, obtaining the composite separator.

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

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

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

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

[0091] Preferably, the temperature of the first-stage drying is 20 - 50 °C, such as 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. For the sake of brevity and limited space, the present invention does not exhaustively list the specific point values included in the above range.

[0092] Preferably, the time of the first-stage drying is 5 - 40 s, such as 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. For the sake of brevity and limited space, the present invention does not exhaustively list the specific point values included in the above range.

[0093] Preferably, the air volume of the first-stage drying is 0 - 5 m 3 / min, such as 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. For the sake of brevity and limited space, the present invention does not exhaustively list the specific point values included in the above range.

[0094] Preferably, the temperature of the second-stage drying is 50 - 80 °C, such as 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. For the sake of brevity and limited space, the present invention does not exhaustively list the specific point values included in the above range.

[0095] Preferably, the time for the second-stage drying is 5 - 40 s, and can be, 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 brevity, the specific point values included in the scope of the present invention are not exhaustively listed herein.

[0096] Preferably, the air volume for the second-stage drying is 5 - 30 m 3 / min, and can be, for example, 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 specific point values included in the scope of the present invention are not exhaustively listed herein.

[0097] On the other hand, the present invention provides an application of the composite separator as described in the first aspect or the composite separator 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, which includes a positive electrode sheet, a negative electrode sheet, and a separator disposed between the positive electrode sheet and the negative electrode sheet, and the separator is the composite separator as described in the first aspect or the composite separator prepared by the preparation method as described in the second aspect.

[0099] Preferably, the secondary battery includes 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 the secondary battery (lithium-ion battery), the gradient pore size coating is located on the side close to the negative electrode sheet, which can fully accommodate Li ions during the discharge process and facilitate their desolvation, ensuring a low discharge resistance; at the same time, the pore size structure of the gradient pore size coating has a sufficient blocking effect on transition metal ions such as Ni, Fe, Mn, etc. and lithium salt anions, effectively avoiding the pollution of the battery negative electrode caused by the dissolution of transition metal ions. Further preferably, a second coating is provided on the other surface of the composite separator, which has high electrolyte wettability and is located on the side close to the positive electrode sheet, which can ensure the full wetting of the electrolyte, fully accommodate the transition metal ions released by the breakdown on the positive electrode side, and intercept them in the nanopore structure of the gradient pore size coating on the other side, and also ensure a low charging resistance. Through the design of the composite separator, the secondary battery (lithium-ion battery) containing it has excellent performance, especially can improve the cycle performance and service life of the battery.

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

[0103] In the composite separator provided by the present invention, through the design of the gradient pore size coating with a specific structure, the composite separator has an excellent blocking effect on transition metal ions and large-volume lithium salt anions, effectively avoiding the pollution of the battery negative electrode caused by the dissolution of transition metal ions. At the same time, the composite separator has a high lithium ion transference number, a low internal resistance, excellent electrolyte wettability, heat resistance and lithium ion permeability, and an appropriate air permeability value, making the secondary battery containing it have excellent performance, especially can significantly improve the cycle performance and service life of the battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0104] Figure 1 Schematic diagram of the structure of the composite separator provided for a specific embodiment;

[0105] Figure 2 SEM image of the composite separator provided for Example 1, with a magnification of 10,000 times. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0106] The technical solutions of the present invention will be further described below through specific embodiments. Those skilled in the art should understand that the embodiments are only for helping to understand the present invention and should not be regarded as specific limitations on the present invention.

[0107] As used herein, the terms "comprising", "including", "having", "containing" or any other variation thereof are intended to cover non-exclusive inclusion. For example, a composition, step, method, article or device containing the recited elements is not necessarily limited to those elements, but may also include other elements not expressly listed or elements inherent to such composition, step, method, article or device.

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

[0109] In a specific embodiment, the structural schematic diagram of the composite separator is as Figure 1 shown, which includes a gradient pore size coating, a base film, and a second coating arranged in sequence, and the base film is located between the gradient pore size coating and the second coating. The gradient pore size coating is prepared by self-polymerized microporous polymer; the gradient pore size coating includes a nanopore layer and a composite pore layer, and the nanopore layer is located between the base film and the composite pore layer; the nanopore layer includes nanopores, the composite pore layer includes interpenetrating micropores and nanopores, the pore diameter of the nanopores is independently <10 nm, and the maximum pore diameter on one side of the opening of the micropores is independently >0.1 μm.

[0110] Hereinafter, multiple embodiments will be taken as examples to elaborate on the composite separator and its preparation method of the present invention, but the composite separator and its preparation method are not limited to these embodiments.

[0111] In the following embodiments, the specific information of the materials used is as shown below, and other reagents whose preparation methods are not specified are commercially available chemicals.

[0112] (1) Self-polymerized microporous polymer

[0113] PIM-1: Grade HWG58338, purchased from Beijing Huawei Ruike Chemical Co., Ltd., with a number average molecular weight of 38,500;

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

[0115] (2) Base film

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

[0117] (3) Inorganic particles

[0118] Aluminum oxide, XKA243 of Xinke Zhonglian New Materials (Changzhou) Co., Ltd.

[0119] (4) Adhesive

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

[0121] Example 1

[0122] A composite separator, which comprises a gradient pore size coating, a base film and a second coating, wherein the base film 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 by PIM-1 and comprises a nanopore layer and a composite pore layer, the nanopore layer is located between the base film and the composite pore layer, the nanopore layer comprises nanopores, and the composite pore layer comprises interpenetrating micropores and nanopores.

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

[0124] (1)Pulp making: Add PIM-1 into a solvent (a mixed solvent of chloroform / tetrahydrofuran, and the mass ratio of the two is 1:1), and use a single stirring paddle type stirrer to make pulp, stir at 60 rpm for 8 h to obtain a self-polymerized microporous polymer slurry with a solid content of 5%;

[0125] Disperse raw materials including alumina and polyacrylate adhesives in a first solvent (deionized water), stir evenly to obtain a first coating slurry with a solid content of 35% and an alumina mass content of 32%.

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

[0127] Coat the first coating slurry on the surface on the opposite side of the gradient pore size coating and then dry it to form a second coating with a thickness of 2 μm, obtaining the composite separator. The specific information is shown in Table 1, and the test results are shown in Table 3.

[0128] Example 2

[0129] A composite separator, which comprises a gradient pore size coating, a base film and a second coating, wherein the base film is located between the gradient pore size coating and the second coating; the gradient pore size coating is prepared by PIM-1 and comprises a nanopore layer and a composite pore layer, the nanopore layer is located between the base film and the composite pore layer, the nanopore layer comprises nanopores, and the composite pore layer comprises interpenetrating micropores and nanopores.

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

[0131] (1) Pulping: Add PIM-1 into chloroform and pulp it using a single agitator paddle mechanism, stirring 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 an extrusion coating device to coat the self-polymerized microporous polymer slurry obtained in step (1) on one side of the base film. After coating, perform the first-stage drying and the second-stage drying in sequence. The temperature of the first-stage drying is 40 °C, the time is 5 s, and the air volume is 5 m 3 / min. The temperature of the second-stage drying is 60 °C, the time is 40 s, and the air volume is 15 m 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 separator, and its specific information 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 separator and its preparation method, the 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, and the 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: Coat the self-polymerized microporous polymer slurry according to the same method as in Example 1. After coating, dry it at 50 °C for 30 min, then raise the temperature to 80 °C and dry for 40 min, and continue to raise the temperature to 100 °C and dry for 30 min to form a self-polymerized microporous polymer coating; Other materials and process parameters of Comparative Example 4 are the same as those of Example 1.

[0142] In the foregoing table, the test method for the composite separator is as follows:

[0143] (1) Microscopic morphology

[0144] Use a scanning electron microscope (SEM, Zeiss ZEISS Gemini 3) to test the microscopic morphology of the gradient pore size coating in the composite separator. Among them, the SEM image of the composite separator provided in Example 1 is as 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, a nanoporous layer; the area on one side close to the outer surface forms a composite pore 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 pore size of the nanopores can be measured using a Bruker Dimension Icon atomic force microscope, using a 2 nm silicon nitride probe of SCANASYST-AIR, selecting the adhesion channel, and selecting a 15 nm scanning range. The average pore size of the nanopores can be obtained by dimensioning the multiple shadowed micropore areas in the image results.

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

[0148] The sample surface was scanned and observed using SEM at 20,000 times magnification, and multiple shadowed micropore areas in the obtained image results were dimensioned to obtain the maximum pore size 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 by a ruler after taking a cross-sectional view by SEM. The average load 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 off 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 according to GBT39713-2020 using a JW-BK112 specific surface and pore size analyzer from Jingwei Gaobo.

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

[0154] Ignore the 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 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 the micropores (the pore volume of the micropores) is calculated by the coating density weighing method. Then, the pore volume of the nanopores can be obtained by subtracting the pore volume of the micropores from the total pore volume. The volume ratio can be directly calculated using the pore volume ratio.

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

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

[0157] (8) Coating amount

[0158] It can be obtained by subtracting the mass of the separator per unit area before and after coating.

[0159] (9) Gurley air permeability value

[0160] Refer to the method specified in the national standard GB / T 36363-2018 "Polyolefin Separator for Lithium-ion Batteries" to obtain the air permeability value of the composite separator.

[0161] (10) Thermal shrinkage rate

[0162] Refer to the method specified in the national standard GB / T 36363-2018 "Polyolefin Separator for Lithium-ion Batteries" to obtain the thermal shrinkage rates in the MD direction (longitudinal direction) and TD direction (transverse direction) of the composite separator, and take the average value as the thermal shrinkage rate of the composite separator; among them, the heat treatment temperature of the oven is 150 °C and the heat treatment time is 1 h.

[0163] (11) Surface resistance

[0164] Cut a total of 4 composite separator samples with a diameter of 45 mm at a flat position. Immerse the samples in the electrolyte (1 mol / L LiPF6 electrolyte, and the solvent is ethylene carbonate EC, ethyl methyl carbonate EMC, and dimethyl carbonate DMC with a volume ratio of 1:1:1) and seal and soak for 30 min; pour the above electrolyte into the surface resistance test fixture; place 1, 2, 3, and 4 separators in the fixture for testing respectively; make a linear fit with the number of separator layers as the abscissa and the separator resistance as the ordinate, and find the slope and goodness of fit of the straight line. When the goodness of fit is greater than 0.999, the slope at this time is the surface resistance of the separator, and the unit is Ω·cm 2

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

[0166] Use an H-type electrolytic cell to test the transmittance of the transition metal ion Mn 3+ of, specifically test the permeation concentration of Mn 3+ after 1 h, and the method is as follows:

[0167] In a glove box, a glass H-shaped electrolytic cell was used. The left side was the high-concentration side (simulating the positive electrode side) and was connected to the counter electrode (Pt wire). The right side was the low-concentration side (simulating the negative electrode side) and was connected to the working electrode (glassy carbon) and the reference electrode (lithium wire). The middle was separated by a 1-layer circular diaphragm sample with a diameter of 10 cm, and the orientation of the diaphragm was the same as that in the battery; an equal amount of electrolyte (1 mol / L LiPF6 electrolyte, with the solvent being EC, EMC, and DMC in a volume ratio of 1:1:1) was added to both sides. Before the test, manganese(III) acetylacetonate salt was added to the left side and stirred to dissolve, so that the Mn 3+ concentration on the left side was 50 mmol / L, and then LiPF6 was added to the right side and stirred to dissolve so that the Li + concentration was 150 mmol / L. The above three electrodes were connected to an electrochemical workstation, and a CV test was performed at a rate of 0.2 V / s in the range of 4.0 - 1.5 V. After one hour, 5 mL of liquid sample was taken from the right side, acid digested, and then the Mn 3+ concentration was measured by ICP.

[0168] (13) Li ion transference number

[0169] A Li symmetric coin cell was used and measured by the potentiostatic polarization method as follows:

[0170] Assemble a 2032-type Li / diaphragm / Li-type symmetric coin cell, using the same type of electrolyte (1 mol / L LiPF6 electrolyte, with the solvent being EC, EMC, and DMC in a volume ratio of 1:1:1); apply a potential difference ΔV = 10 mV with an electrochemical workstation, record the initial current I0 and the steady-state current Is, and simultaneously scan the EIS curves at the initial state and the steady state and read out the initial-state and polarized interfacial impedance values R0 and Rs. The Li ion transference number (t + ) can be calculated by the following formula: .

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

[0172] The preparation of the lithium-ion battery 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) were added to the solvent (N-methylpyrrolidone, NMP) in a mass ratio of 90:3:3 and mixed evenly to make a positive electrode slurry, which was coated on a 10-μm aluminum foil current collector; then it was dried in an oven at 95 °C and rolled on a rolling press to obtain a positive electrode sheet for standby.

[0174] 2) The artificial graphite of the negative electrode active material, acetylene black as the conductive agent, and a binder (styrene-butadiene rubber emulsion, SBR) are added to water in a mass ratio of 90:5:5 and dispersed evenly to form a negative electrode slurry. The slurry is coated on a copper foil current collector with a thickness of 10 μm, dried in an oven at 85°C, and then roll-pressed by a roll press to obtain a negative electrode sheet for standby.

[0175] 3) The composite separator is placed between the positive electrode sheet and the negative electrode sheet for lamination and encapsulation. The second coating of the composite separator faces the positive electrode sheet, and the gradient pore size coating faces the negative electrode sheet. Then, an electrolyte (1 mol / L LiPF6 electrolyte, that is, the electrolyte includes the lithium salt LiPF6 with a concentration of 1 mol / L, and the solvent is EC, EMC, and DMC with a volume ratio of 1:1:1) is injected to obtain a battery.

[0176] The cycle performance of the battery: The cycle capacity retention rate of the battery at the 100th cycle at 60°C is used as the result of the cycle performance. The specific test method includes the following steps: Constant current and constant voltage charging at 0.8 C, constant current discharging at 1 C, the first-round discharge capacity is measured, cycled 100 times under this condition, the discharge capacity after 100 cycles is tested, and the ratio of the discharge capacity after 100 cycles to the first-round discharge capacity is calculated as the capacity retention rate of the 100-week cycle at 60°C.

[0177] According to the foregoing test data, in the composite separator provided by the present invention, through the design of the gradient pore size coating with a specific structure, the composite separator has an excellent barrier effect on transition metal ions and large-volume lithium salt anions. The H-type electrolytic cell is used to test the transition metal ion transmittance. The transmittance concentration of Mn ions after 1 h ≤ 0.59 mmol / L, and in the preferred scheme ≤ 0.4 mmol / L. Compared with the base film, the ceramic-coated film, etc., it is reduced by more than 2 times. At the same time, the lithium ion transference number of the composite separator is high. The lithium ion transference number measured by the Li symmetric button battery using the potentiostatic polarization method > 0.3, and in the preferred scheme ≥ 0.4. At the same time, the composite separator has a low impedance, excellent electrolyte wettability, sufficient lithium ion permeability, a low thermal shrinkage rate, and good heat resistance, enabling the lithium ion battery using it to have significantly improved cycle performance and service life. 3+ In addition, compared with the separator with only an alumina coating on one side in Comparative Example 1, the Gurley air permeability value of the composite separator provided by the present invention has increased, but its air permeability value can still meet the performance requirements of secondary batteries.

[0178] Although the composite separators provided in Comparative Examples 2-4 are provided with a coating formed by self-polymerized microporous polymers, this coating does not have the gradient pore size structure defined by the present invention, resulting in at least one of the transition metal cation barrier property, lithium ion transference number, air permeability, and internal resistance being significantly insufficient, and it is difficult to meet the high-performance requirements of secondary battery separators.

[0179] ​

[0180] The applicant declares that the present invention uses the above embodiments to illustrate the composite separator, its preparation method, and the secondary battery of the present invention. However, the present invention is not limited to the above process steps, that is, it does not mean that the present invention must rely on the above process steps to be implemented. Those skilled in the art should understand that any improvement to the present invention, the equivalent substitution of the raw materials selected for the present invention, the addition of auxiliary components, the selection of specific methods, etc. all fall within the protection scope and the disclosure scope of the present invention.

Claims

1. A composite separator, characterized in that, The composite separator includes a base film and a gradient pore size coating provided on at least one surface of the base film; the gradient pore size coating is prepared from a self-polymerized microporous polymer; The gradient pore size coating includes a nanopore layer and a composite pore layer, and the nanopore layer is located between the base film and the composite pore layer; The nanopore layer includes nanopores, and the composite pore layer includes interpenetrating micropores and nanopores. The pore diameters of the nanopores are each independently < 10 nm, and the maximum pore diameter on one side of the opening of the micropores is each independently > 0.1 μm.

2. The composite separator according to claim 1, wherein The volume percentage content of nanopores in the gradient pore size coating is 5-55%; and / or, the volume percentage content 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 separator according to claim 1, wherein The coating amount of the gradient aperture coating is 0.1-1 g / m 2 ; and / or, the thickness of the gradient pore size coating is 0.1-2 μm; and / or, based on the total thickness of the gradient pore size coating being 100%, the thickness of the composite pore layer is 30-70%; 4. The composite separator according to claim 1, characterized in that, The self-polymerized microporous polymer includes any one or a combination of at least two of an intrinsically microporous polymer, a conjugated microporous polymer, and a hypercrosslinked polymer.

5. The composite separator according to claim 4, wherein, The intrinsically microporous polymer contains repeating units 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): ； wherein each R 10 is independently selected from (C 1-6 )alkyl or H; Each R 11 is independently selected from -CH2NR1R2 or H; Each R 12 is 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 heterocyclic group, (3- to 8-membered heterocyclic group)-C 1-20 alkyl, 5- to 8-membered heteroaryl, heteroaryl-C 1-20 alkyl; or, each of the alkyl, alkenyl, and alkynyl groups 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 said R1 and R2 are optionally and independently present in oxidized form; or R1 and R2 may optionally and independently be substituted by one or more Z1; or, R1 and R2 may together with the nitrogen atom to which they are attached form a 3- to 8-membered heterocyclic group or a 5- to 8-membered heteroaryl, each of which is optionally substituted by one or more Z2; Said Z1 and said Z2 are each independently a 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, a 3- to 8-membered heterocyclic group, a 3- to 8-membered heteroaryl, (C 3-8 cycloalkyl)-C 1-20 alkyl, (3- to 8-membered heterocyclic group)-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 heterocyclic group, (3- to 8-membered heterocyclic group)-C 1-20 alkyl, 5- to 8-membered heteroaryl, or (5- to 8-membered heteroaryl)-C 1-20 alkyl; Each R 13 is independently H, (C 1-20 )alkyl or (C 3-8 )cycloalkyl, where the alkyl and cycloalkyl are optionally and independently substituted by one or more Z3; Each Z3 is independently a 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 heterocyclic group, 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 is independently H, (C 1-20 )alkyl or (C 3-8 )cycloalkyl.

6. The composite separator according to claim 1, wherein The composite separator includes the gradient pore size coating, a base film, and a second coating, and the base film is located between the gradient pore size coating and the second coating; and / or, the second coating includes an organic coating or an organic-inorganic composite coating; and / or, the thickness of the second coating is 1-4 μm.

7. A method for preparing a composite separator as claimed in claim 1, characterized in that, The preparation method includes: providing a self-polymerized microporous polymer slurry, which includes a combination of a self-polymerized microporous polymer and a solvent; coating the self-polymerized microporous polymer slurry on at least one surface of the base film, and drying to form a gradient pore size coating, thereby obtaining the composite separator; The drying includes a first-stage drying and a second-stage drying carried out in sequence, and the speed of the first-stage drying < the speed of the second-stage 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-stage drying ≤ the temperature of the second-stage drying, and the air volume of the first-stage drying < the air volume of the second-stage drying; b) The temperature of the first-stage drying < the temperature of the second-stage drying, and the air volume of the first-stage drying ≤ the air volume of the second-stage drying.

9. The preparation method according to claim 7, characterized in that The boiling point of the solvent < 100°C; And / or, the temperature of the first-stage drying is 20 - 50°C, the time is 5 - 40 s, and the air volume is 0 - 5 m 3 / min; And / or, the temperature of the second-stage drying is 50-80°C, the time is 5-40 s, and the air volume is 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, and the separator is the composite separator according to any one of claims 1-6 or the composite separator prepared by the preparation method according to any one of claims 7-9.

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

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