High-temperature-resistant PI lithium battery porous diaphragm, preparation method thereof and secondary battery

Abstract

The invention relates to the technical field of batteries, in particular to a high-temperature-resistant PI lithium battery porous diaphragm, a preparation method thereof and a secondary battery. The preparation method comprises the following steps: contacting a first spiro compound, bicyclic orthoester, a dianhydride compound and a diamine compound, and carrying out polymerization reaction to obtain a polyamide acid solution; mixing the polyamide acid solution, a pore-forming agent, a second spiro compound and a photoinitiator, and carrying out phase separation pore-forming and photo-crosslinking reaction to obtain a porous polyamide acid film; and carrying out imidization treatment on the porous polyamide acid membrane to obtain the high-temperature-resistant PI lithium battery porous diaphragm. According to the preparation method, the spiro compound and the bicyclic orthoester are added in the polymerization reaction and are used as sites for subsequent photo-crosslinking curing, so that fixation of holes in the PI diaphragm and improvement of the mechanical property, the thermal stability and the processability of the material are facilitated, and therefore, the capacity, the cycle life and the safety of the lithium battery are improved, and the service life of the lithium battery is prolonged.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of batteries, in particular to a high-temperature-resistant PI lithium battery porous diaphragm and a preparation method thereof and a secondary battery. BACKGROUND

[0002] Lithium ion batteries are widely used in the fields of consumer electronics, electric vehicles and energy storage systems. The lithium battery diaphragm is an important component of the lithium battery and is one of the key materials with technical barriers in the industry chain. At present, the lithium battery diaphragm is usually made of a thin-film polyolefin porous film, which has good electronic insulation and resistance to electrolyte. However, the polyolefin film has the following disadvantages: (1) poor thermal stability, easy to decompose and deform at high temperature; (2) poor wettability to electrolyte, resulting in reduced rate performance and cycle performance of the battery; (3) low mechanical strength, easy to cause short circuit failure of the battery when foreign matter punctures.

[0003] As a high-performance polymer material, polyimide (PI) has excellent mechanical strength, thermal stability and chemical inertness, and can be used as a lithium battery diaphragm. The existing production methods of PI diaphragm mainly include electrospinning method and phase separation method. The diaphragm prepared by the electrospinning method has high porosity, but poor mechanical properties, low film forming efficiency and is not conducive to large-scale industrial production. The phase separation method is simple to operate and suitable for large-area film forming, but also has the following defects: (1) the linear structure of PI leads to insufficient mechanical properties of the diaphragm; (2) the thermal stability of the linear structure of PI still needs to be improved; (3) polyamide acid (PAA) is an intermediate product for preparing the PI diaphragm, the PAA wet film is soaked into a casting solution to form pores, and then the PAA film is peeled off from the base film after drying, and then the PAA film is wound into a roll to obtain the PAA film. Due to the further evaporation of the solvent, the thin PAA coating and the poor mechanical properties of the linear PAA, the pores in the PAA are prone to deformation and collapse during drying, which affects the uniformity of the pores and the pore size, and further affects the porosity of the diaphragm. In severe cases, due to the different volume changes of the PAA and the base film caused by heat, the PAA layer is damaged, and the diaphragm cannot be completely peeled off; (4) the diaphragm needs large porosity and good mechanical strength, but it is difficult to achieve both porosity and mechanical properties; (5) in order to be flat, a device providing a traction tension is used when the PAA film is wound, and the PAA film wound into a roll is subjected to stress, and the diaphragm is prone to damage when winding; (6) the PAA film wound into a roll is converted into a PI diaphragm by heating. During the imidization process, with the generation of amide bonds, the volume of the diaphragm is further shrunk, which leads to the shrinkage or collapse of the pores, resulting in the decrease of the porosity and the mechanical properties. In addition, PI material is a brittle polymer, and due to the use of the device providing the traction tension in the winding process, the PAA film wound into a roll has internal stress, which is prone to damage to the PI diaphragm wound into a roll.

[0004] Therefore, there is an urgent need to provide a high-temperature-resistant PI lithium battery porous separator and a preparation method thereof to solve the above-mentioned defects. SUMMARY

[0005] The present application aims to at least partly solve one of the technical problems in the related art. To this end, the present application provides a high-temperature-resistant PI lithium battery porous separator and a preparation method thereof, and a secondary battery. The preparation method facilitates the fixation of holes in the PI separator and the improvement of the mechanical properties, thermal stability and processing performance of the material by adding a spiro compound and a bicyclic orthoester in the polymerization reaction and using them as the sites for subsequent photocrosslinking curing, thereby improving the capacity, cycle life and safety of the lithium battery.

[0006] To this end, the present application provides a preparation method of a high-temperature-resistant PI lithium battery porous separator, which comprises the following steps: contacting a first spiro compound, a bicyclic orthoester, a diacid anhydride compound and a diamine compound to perform a polymerization reaction to obtain a polyamic acid solution; mixing the polyamic acid solution, a pore-forming agent, a second spiro compound and a photoinitiator to perform a phase separation pore-forming and photocrosslinking reaction to obtain a porous polyamic acid membrane; performing imidization treatment on the porous polyamic acid membrane to obtain the high-temperature-resistant PI lithium battery porous separator.

[0007] The preparation method provided by the present application introduces a spiro ring group and a bicyclic orthoester group into the main chain and the end group of polyamic acid in the polymerization reaction, respectively, and after forming a porous membrane through phase separation, the introduced spiro ring group and bicyclic orthoester group are used as crosslinking sites for photocrosslinking curing. Among them, the spiro compound and the bicyclic orthoester belong to expansion monomers, which will not shrink in volume during polymerization. The subsequent photocrosslinking with the second spiro compound further fixes the holes in the separator and improves the mechanical properties, thermal stability and processing performance of the PI separator, thereby improving the capacity, cycle life and safety of the lithium battery.

[0008] According to an embodiment of the present application, the molar ratio of the diacid anhydride compound to the total molar amount of the first spiro compound, the bicyclic orthoester and the diamine compound is 1:(0.95-1.05).

[0009] According to an embodiment of the present application, the molar ratio of the diamine compound, the first spiro compound and the bicyclic orthoester is 1:(0.01-0.1):(0.01-0.1).

[0010] According to an embodiment of the present application, the first spiro compound structure contains two amino groups.

[0011] According to embodiments of the present application, the first spiro compound includes at least one of 1,5,7,11-tetraoxaspiro[5,5]undecane-3,9-diamine, 1,4,6,9-tetraoxaspiro[4,4]nonane-2,7-dimethanamine, 1,5,7,11-tetraoxaspiro[5.5]undecane-3,9-dipropylamine.

[0012] According to embodiments of the present application, the bicyclic orthoester structure contains a single amino group.

[0013] According to embodiments of the present application, the bicyclic orthoester includes at least one of 2,6,7-trioxabicyclo[2.2.2]octan-1-ylmethanamine, 3-(2,6,7-trioxabicyclo[2.2.2]octan-1-yl)propan-1-amine, 4-(2,6,7-trioxabicyclo[2.2.2]octan-1-yl)butan-2-amine, 3-(4-methyl-2,6,7-trioxabicyclo[2.2.2]octan-1-yl)propan-1-amine, 4-(4-methyl-2,6,7-trioxabicyclo[2.2.2]octan-1-yl)butan-2-amine, 2,6,7-trioxabicyclo[2.2.2]octan-1-ethylamine, 1-(2,6,7-trioxabicyclo[2.2.2]octan-1-yl)propan-2-amine.

[0014] According to embodiments of the present application, the dianhydride compound includes at least one of pyromellitic dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride, 4,4'-oxybisphthalic anhydride, 3,3',4,4'-benzophenonetetracarboxylic dianhydride, 3,3',4,4'-triphenyl ether tetracarboxylic dianhydride, 3,3',4,4'-diphenyl sulfone tetracarboxylic dianhydride, bisphenol A type diether dianhydride.

[0015] According to embodiments of the present application, the diamine compound includes at least one of p-phenylenediamine, 4,4'-oxydianiline, 4,4'-diaminodiphenylmethane, 4,4'-diaminobiphenyl, 3,5'-diaminobenzoic acid, 4,4'-diaminobenzenesulfone, 1,3-bis(4-aminophenoxy)benzene, 1,4-bis(4-aminophenoxy)benzene, 4,4'-bis(4-aminophenoxy)biphenyl, 4,4'-bis(3-aminophenoxy)biphenyl.

[0016] According to embodiments of the present application, the temperature of the polymerization reaction is 0-50℃.

[0017] According to embodiments of the present application, the time of the polymerization reaction is 0.5-5 h.

[0018] According to an embodiment of the present application, the preparation method further comprises: contacting the first spiro compound, the bicyclic orthoester, the diacid anhydride compound, the diamine compound, and the solvent to perform a polymerization reaction to obtain a polyamic acid solution.

[0019] According to an embodiment of the present application, the mass percentage of the solvent in the polyamic acid solution is 50 wt%-95 wt%.

[0020] According to an embodiment of the present application, the solvent comprises at least one of N-methylpyrrolidone, N,N-dimethylformamide, and N,N-dimethylacetamide.

[0021] According to an embodiment of the present application, the mass ratio of the polyamic acid solution, the pore-forming agent, the second spiro compound, and the photoinitiator is 1:(0.05-0.3):(0.001-0.01):(0.001-0.01).

[0022] According to an embodiment of the present application, the pore-forming agent comprises at least one of dioctyl phthalate, polyvinylpyrrolidone-K17, polyvinylpyrrolidone-K30, polyethylene glycol 400, diethylene glycol, triethylene glycol, and glycerol.

[0023] According to an embodiment of the present application, the second spiro compound comprises at least one of 3,9-dihydroxymethyl-3',9'-diethyl-1,5,7,11-tetraoxaspiro[5,5]undecane, 1,5,7,11-tetraoxaspiro[5.5]undecane, 8,10,19,20-tetraoxatrispiro[5.2.2.5.2]eicosane, 2,2'-spirobis[1,3-benzodioxane], and 1,4,6,9-tetraoxaspiro[4.4]nonane.

[0024] According to an embodiment of the present application, the photoinitiator comprises at least one of 2-hydroxy-2-methyl-1-phenyl-1-propanone, 1-hydroxycyclohexyl phenyl ketone, 2,4,6-trimethylbenzoyl-diphenylphosphine oxide, 2-hydroxy-4'-(2-hydroxyethoxy)-2-methylpropiophenone, and 1,1'-(oxybis(4,1-phenylene))bis(2-hydroxy-2-methyl-1-propanone).

[0025] According to an embodiment of the present application, the temperature of the mixing is 0 ℃-30 ℃.

[0026] According to an embodiment of the present application, the mixing is performed in a light-proof and nitrogen atmosphere.

[0027] According to an embodiment of the present application, the preparation method further comprises: adding the pore-forming agent, the second spiro compound, and the photoinitiator to the polyamic acid solution, and stirring to obtain a casting solution.

[0028] According to an embodiment of the present application, the preparation method further comprises: coating the casting solution on the base film with a doctor blade under light-proof conditions to obtain a wet film, immersing the wet film in a coagulation bath to realize phase separation into pores, irradiating the polyamide acid film with ultraviolet light under nitrogen protection to initiate a photo-crosslinking reaction, drying the crosslinked film, and peeling off the film from the base film, and winding the film to obtain a wound porous polyamide acid film.

[0029] According to an embodiment of the present application, the base film includes, but is not limited to, a polyethylene terephthalate film, a polyethylene film, or a polypropylene film.

[0030] According to an embodiment of the present application, the release force of the base film is 5 g / mm-100 g / mm.

[0031] According to an embodiment of the present application, the coating thickness is 25 μm-200 μm.

[0032] According to an embodiment of the present application, the coagulation bath comprises water and alcohol.

[0033] According to an embodiment of the present application, the volume ratio of the water and alcohol is 1:(0.05-1).

[0034] According to an embodiment of the present application, the alcohol includes at least one of ethanol, methanol, and isopropyl alcohol.

[0035] According to an embodiment of the present application, the coagulation bath is prepared in one batch.

[0036] According to an embodiment of the present application, the coagulation bath is prepared in multiple batches and immersed sequentially.

[0037] According to an embodiment of the present application, the immersion time is 0.5 min-10 min for each coagulation bath.

[0038] According to an embodiment of the present application, the immersion temperature is 10 ℃-30 ℃.

[0039] According to an embodiment of the present application, the drying temperature is 50 ℃-100 ℃.

[0040] According to an embodiment of the present application, the drying time is 1 min-10 min.

[0041] According to an embodiment of the present application, the photo-crosslinking reaction temperature is 10 ℃-30 ℃.

[0042] According to an embodiment of the present application, the photo-crosslinking reaction time is 0.5 min-5 min.

[0043] According to an embodiment of the present application, the photo-crosslinking reaction light source includes ultraviolet light.

[0044] According to an embodiment of the present application, the energy of the ultraviolet light is 100 mJ / cm 2 -1500 mJ / cm 2 .

[0045] According to an embodiment of the present application, the temperature of the imidization treatment is 100 ℃-350 ℃.

[0046] According to an embodiment of the present application, the time of the imidization treatment is 0.5 h-5 h.

[0047] The second aspect of the present application provides a high-temperature-resistant PI lithium battery porous separator prepared by the preparation method according to the first aspect.

[0048] The high-temperature-resistant PI lithium battery porous separator can be prepared by using the preparation method provided by the present application, and the separator has the advantages of good mechanical properties and high thermal stability, and meanwhile, the separator has high porosity and complete pores.

[0049] According to an embodiment of the present application, the thickness of the high-temperature-resistant PI lithium battery porous separator is 5 μm-25 μm.

[0050] The third aspect of the present application provides a secondary battery, which comprises the high-temperature-resistant PI lithium battery porous separator prepared by the preparation method according to the first aspect or the high-temperature-resistant PI lithium battery porous separator according to the second aspect.

[0051] The secondary battery prepared by using the high-temperature-resistant PI lithium battery porous separator provided by the present application has excellent capacity, cycle life and safety.

[0052] The present application has the following beneficial effects compared with the prior art: (1) Excellent processing performance: the light cross-linking reaction of the spiro ring group and the bicyclic orthoester group in the PAA segment with the second spiro compound is carried out, and then the imidization treatment is carried out, so as to obtain the cross-linked high-temperature-resistant PI lithium battery porous separator. The mechanical properties of the cross-linked separator are improved, and the pores are fixed. In the subsequent drying, peeling and winding processes, the cross-linked separator is not easy to be damaged, and can withstand greater traction tension and peeling force. Cross-linking reduces the volume shrinkage effect of the PAA film during the imidization treatment, which is beneficial to the integrity of the pores and the separator.

[0053] (2) Excellent performance: the mechanical properties of the crosslinked high-temperature-resistant PI lithium battery porous diaphragm are improved. In the subsequent battery preparation process, the diaphragm in the roll needs to be unfolded again, and the crosslinked PI polymer can withstand greater traction tension. Crosslinking also increases the heat resistance of the high-temperature-resistant PI lithium battery porous diaphragm. At the same time, the integrity of the pores makes the diaphragm have good ion permeability, which can reduce the internal resistance of the battery; the ability to resist electrochemical corrosion has good stability in the electrolyte; good electrolyte wettability. The electrochemical performance and safety performance of the lithium battery are improved.

[0054] (3) Low process cost and wide applicability: the preparation process of introducing expansion monomers for crosslinking has low equipment requirements, and by adjusting the types and proportions of monomers, the performance requirements of different products can be met, and the method has wide applicability.

[0055] Additional aspects and advantages of the application will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following description, or can be learned by practice of the application. DETAILED DESCRIPTION

[0056] Embodiments of the present application are described in detail below. The embodiments described below are exemplary only, and are intended to explain the present application, and should not be understood as limiting the present application.

[0057] It should be noted that the terms "first", "second" are used only for descriptive purposes, and should not be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined as "first", "second" can explicitly or implicitly include one or more of the features. Further, in the description of the present application, unless otherwise specified, the meaning of "a plurality of" is two or more.

[0058] The endpoints of the ranges and any values disclosed herein are not limited to the precise values stated. The ranges or values should be construed to be approximations that allow for significant variation. Within each range, other points can be derived by combining the endpoints of the ranges, the endpoints of the ranges and individual points, and individual points with each other, to form one or more new ranges, which should be considered as being specifically disclosed herein.

[0059] In order to make the present application more easily understood, certain technical and scientific terms are specifically defined below. Unless otherwise clearly defined elsewhere in this document, all other technical and scientific terms used herein have the meanings commonly understood by one of ordinary skill in the art to which the present application belongs.

[0060] In this document, the term "comprising" or "including" is an open-ended expression, i.e. it includes the indicated content, but does not exclude other aspects.

[0061] In this document, the terms "optionally," "optional," or "may" generally mean that the subsequently described event or circumstance can or cannot occur or that the described event or circumstance is or can be present, and the description includes instances where the event or circumstance occurs and instances where it does not.

[0062] The term "expansion monomer" is a class of monomer substances that can produce expansion or reduce volume shrinkage in the polymerization process (the volume of the monomer usually shrinks after polymerization). The expansion monomers mainly include spiro orthocarbonate (SOC), spiro orthoester (SOE) and bicyclo orthoester (BOE) and the like.

[0063] According to an embodiment of the present application, the first aspect of the present application provides a preparation method of a high-temperature-resistant PI lithium battery porous diaphragm, comprising the following steps: (1) Contacting a first spiro compound, a bicyclo orthoester, a dianhydride compound and a diamine compound to perform a polymerization reaction to obtain a polyamic acid solution.

[0064] In step (1), in addition to using common dianhydride compounds and diamine compounds, a first spiro compound and a bicyclo orthoester are additionally used to participate in the synthesis of PAA, and spiro groups and bicyclo orthoester groups are introduced into the chain segments and the ends of PAA, respectively, which will serve as crosslinking points for the subsequent photocrosslinking reaction.

[0065] Specifically, in the polymerization reaction, each polymerization monomer is randomly polymerized.

[0066] According to a specific embodiment of the present application, the molar ratio of the dianhydride compound to the total molar amount of the first spiro compound, the bicyclo orthoester and the diamine compound is 1:(0.95-1.05), and as some specific examples, the molar ratio of the dianhydride compound to the total molar amount of the first spiro compound, the bicyclo orthoester and the diamine compound can be 1:0.95, 1:1, 1:1.05, etc.

[0067] According to a specific embodiment of the present application, the molar ratio of the diamine compound, the first spiro compound and the bicyclo orthoester is 1:(0.01-0.1):(0.01-0.1), and as some specific examples, the molar ratio of the diamine compound, the first spiro compound and the bicyclo orthoester can be 1:0.01:0.01, 1:0.01:0.1, 1:0.1:0.01, 1:0.1:0.1, etc.

[0068] According to specific embodiments of the present application, the first spiro compound structure contains two amino groups. The class of the first spiro compound is not particularly limited, as some specific examples, the first spiro compound includes at least one of 1,5,7,11-tetraoxaspiro[5,5]undecane-3,9-diamine (CAS: 1379509-69-9), 1,4,6,9-tetraoxaspiro[4,4]nonane-2,7-dimethylhydrazine (CAS: 1379509-68-8), 1,5,7,11-tetraoxaspiro[5.5]undecane-3,9-dipropylamine (CAS: 118432-48-7).

[0069] Specifically, the two amino groups in the first spiro compound can react with a dianhydride compound, thereby introducing a spiro ring group into the PAA segment.

[0070] According to specific embodiments of the present application, the bicyclic orthoester structure contains a single amino group. The class of the bicyclic orthoester is not particularly limited, as some specific examples, the bicyclic orthoester includes at least one of 2,6,7-trioxabicyclo[2.2.2]octan-1-ylmethylamine (CAS: 1893887-68-7), 3-(2,6,7-trioxabicyclo[2.2.2]octan-1-yl)propane-1-amine (CAS: 1896790-05-8), 4-(2,6,7-trioxabicyclo[2.2.2]octan-1-yl)butan-2-amine (CAS: 1892374-20-7), 3-(4-methyl-2,6,7-trioxabicyclo[2.2.2]octan-1-yl)propan-1-amine (CAS: 1893947-48-2), 4-(4-methyl-2,6,7-trioxabicyclo[2.2.2]octan-1-yl)butan-2-amine (CAS: 1893258-44-0), 2,6,7-trioxabicyclo[2.2.2]octan-1-ethylamine (CAS: 1893887-72-3), 1-(2,6,7-trioxabicyclo[2.2.2]octan-1-yl)propan-2-amine (CAS: 1894564-22-7).

[0071] Specifically, the single amino group in the bicyclic orthoester can react with a dianhydride compound to generate a capping group, introducing a bicyclic orthoester group to the end of PAA.

[0072] According to a specific embodiment of the present application, the kind of the dianhydride compound is not particularly limited, and as some specific examples, the dianhydride compound includes at least one of pyromellitic dianhydride (CAS: 89-32-7), 3,3',4,4'-diphenyltetracarboxylic dianhydride (CAS: 2420-87-3), 4,4'-oxydiphthalic anhydride (CAS: 1823-59-2), 3,3',4,4'-benzophenonetetracarboxylic dianhydride (CAS: 2421-28-5), 3,3',4,4'-triphenyl ether tetracarboxylic dianhydride (CAS: 17828-53-4), 3,3',4,4'-diphenyl sulfone tetracarboxylic dianhydride (CAS: 2540-99-0), and bisphenol A type diether dianhydride (CAS: 38103-06-9).

[0073] According to a specific embodiment of the present application, the kind of the diamine compound is not particularly limited, and as some specific examples, the diamine compound includes at least one of p-phenylenediamine (CAS: 106-50-3), 4,4'-oxydianiline (CAS: 101-80-4), 4,4'-diaminodiphenylmethane (CAS: 101-77-9), 4,4'-diaminobiphenyl (CAS: 92-87-5), 3,5'-diaminobenzoic acid (CAS: 535-87-5), 4,4'-diaminobenzenesulfone (CAS: 80-08-0), 1,3-bis(4-aminophenoxy)benzene (CAS: 2479-46-1), 1,4-bis(4-aminophenoxy)benzene (CAS: 3491-12-1), 4,4'-bis(4-aminophenoxy)biphenyl (CAS: 13080-85-8), and 4,4'-bis(3-aminophenoxy)biphenyl (CAS: 105112-76-3).

[0074] According to a specific embodiment of the present application, the temperature of the polymerization reaction is 0 ℃-50 ℃, and as some specific examples, the temperature of the polymerization reaction can be 0 ℃, 10 ℃, 20 ℃, 30 ℃, 40 ℃, 50 ℃, etc.

[0075] According to a specific embodiment of the present application, the time of the polymerization reaction is 0.5 h-5 h, and as some specific examples, the time of the polymerization reaction can be 0.5 h, 1 h, 2 h, 3 h, 4 h, 5 h, etc.

[0076] According to a specific embodiment of the present application, the preparation method further includes: contacting the first spiro compound, the bicyclic orthoester, the dianhydride compound, the diamine compound, and the solvent to perform a polymerization reaction to obtain a polyamic acid solution.

[0077] According to a specific embodiment of the present application, the mass percentage of the solvent in the polyamic acid solution is 50 wt%-95 wt%, and as some specific examples, the mass percentage of the solvent in the polyamic acid solution can be 50 wt%, 60 wt%, 70 wt%, 80 wt%, 90 wt%, 95 wt%, etc.

[0078] According to a specific embodiment of the present application, the type of the solvent is not particularly limited, and as some specific examples, the solvent includes at least one of N-methylpyrrolidone, N,N-dimethylformamide, and N,N-dimethylacetamide.

[0079] According to a specific embodiment of the present application, the specific operation of step (1) can be: dissolving the diamine compound, the first spiro compound, and the bicyclo-orthoester in the solvent, adding the diacid anhydride compound under nitrogen protection, and stirring until the reaction is complete.

[0080] (2) Mixing the polyamic acid solution, the pore-forming agent, the second spiro compound, and the photoinitiator, and obtaining a porous polyamic acid membrane through phase separation pore formation and photocrosslinking reaction.

[0081] In step (2), in addition to the pore-forming agent, a photoinitiator and a second spiro compound are also introduced, and the second spiro compound will form a crosslinked segment in the subsequent photocuring crosslinking reaction. During phase separation, the polymer-lean phase forms pores, and the polymer-rich phase forms a skeleton, and the photoinitiator and the second spiro compound remain in the polymer-rich phase and undergo ring-opening polymerization with the spiro ring group and the bicyclo-orthoester group in the PAA through photo-initiation. After the photocuring crosslinking reaction, a crosslinked PAA porous membrane is obtained. Since the expansion monomer (i.e., the spiro compound and the bicyclo-orthoester) does not cause volume shrinkage during polymerization, the collapse of the pores caused by the shrinkage of the polymer skeleton during crosslinking is reduced, thereby facilitating the porosity and pore integrity of the separator. After crosslinking, the pores of the PAA are fixed. The mechanical properties of the polymer are also improved, and the pores are not easily collapsed during drying, and the PAA membrane is not easily damaged. In the subsequent winding process, a device providing tension is used, and the crosslinked PAA membrane has a greater tensile strength than the uncrosslinked PAA membrane, and can withstand greater internal stress, so that the wound PAA membrane is not easily damaged. In addition, compared with thermal initiation crosslinking, photocuring crosslinking does not require heating, and the heat effect during polymerization is small, which does not cause the volume of the wet film to change dramatically and wrinkle, and is beneficial to maintaining the flatness of the film.

[0082] According to a specific embodiment of the present application, the mass ratio of the polyamic acid solution, the pore-forming agent, the second spiro compound, and the photoinitiator is 1:(0.05-0.3):(0.001-0.01):(0.001-0.01), and as some specific examples, the mass ratio of the polyamic acid solution, the pore-forming agent, the second spiro compound, and the photoinitiator can be 1:0.05:0.001:0.001, 1:0.3:0.01:0.01, etc.

[0083] According to a specific embodiment of the present application, the kind of the pore-forming agent is not particularly limited, and as some specific examples, the pore-forming agent includes at least one of dioctyl phthalate, polyvinylpyrrolidone-K17, polyvinylpyrrolidone-K30, polyethylene glycol 400, diethylene glycol, triethylene glycol, and glycerol.

[0084] According to a specific embodiment of the present application, the kind of the second spiro compound is not particularly limited, and as some specific examples, the second spiro compound includes at least one of 3,9-dihydroxymethyl-3',9'-diethyl-1,5,7,11-tetraoxaspiro[5,5]undecane (CAS: 65282-19-1), 1,5,7,11-tetraoxaspiro[5.5]undecane (CAS: 24472-02-4), 8,10,19,20-tetraoxatrispiro[5.2.2.5.2]eicosane (CAS: 57565-08-9), 2,2'-spirobis[1,3-benzodioxane] (CAS: 181-82-8), and 1,4,6,9-tetraoxaspiro[4.4]nonane (CAS: 24471-99-6).

[0085] Specifically, the second spiro compound does not contain an amino group and only participates in the subsequent photocuring cross-linking reaction.

[0086] According to a specific embodiment of the present application, the kind of the photoinitiator is not particularly limited, and as some specific examples, the photoinitiator includes at least one of 2-hydroxy-2-methyl-1-phenyl-1-propanone (photoinitiator 1173, CAS No.: 7473-98-5), 1-hydroxycyclohexyl phenyl ketone (photoinitiator 184, CAS No.: 947-19-3), 2,4,6-trimethylbenzoyl-diphenylphosphine oxide (photoinitiator TPO, CAS No.: 75980-60-8), 2-hydroxy-4'-(2-hydroxyethoxy)-2-methylpropiophenone (photoinitiator 2959, CAS No.: 106797-53-9), and 1,1'-(oxybis(4,1-phenylene))bis(2-hydroxy-2-methyl-1-propanone) (photoinitiator 160, CAS No.: 71868-15-0).

[0087] According to a specific embodiment of the present application, the temperature of the mixing is 0 ℃-30 ℃, and as some specific examples, the temperature of the mixing can be 0 ℃, 10 ℃, 20 ℃, 30 ℃, etc.

[0088] According to a specific embodiment of the present application, the specific operation of step (2) can be: under the conditions of light shielding and nitrogen atmosphere, adding the pore-forming agent, the second spiro compound and the photoinitiator into the PAA solution, stirring uniformly to obtain a casting solution; under the condition of light shielding, coating the casting solution on the base film by using a doctor blade to obtain a wet film, immersing the wet film in a coagulation bath to realize phase separation and pore formation, and then irradiating the PAA film by using ultraviolet light under the protection of nitrogen to initiate the photocrosslinking reaction, drying the crosslinked film, peeling off the film from the base film, and winding to obtain a porous PAA film in a roll.

[0089] According to a specific embodiment of the present application, the type of the base film is not particularly limited, and as some specific examples, it includes but is not limited to polyethylene terephthalate (PET) film, polyethylene (PE) film and polypropylene (PP) film.

[0090] According to a specific embodiment of the present application, the release force of the base film is 5 g / mm-100 g / mm, and as some specific examples, the release force of the base film can be 5 g / mm, 10 g / mm, 20 g / mm, 50 g / mm, 100 g / mm, etc.

[0091] According to a specific embodiment of the present application, the thickness of the coating is 25 μm-200 μm, and as some specific examples, the thickness of the coating can be 25 μm, 50 μm, 100 μm, 150 μm, 200 μm, etc.

[0092] According to a specific embodiment of the present application, the components of the coagulation bath include water and alcohol.

[0093] According to a specific embodiment of the present application, the volume ratio of the water and the alcohol is 1:(0.05-1), and as some specific examples, the volume ratio of the water and the alcohol can be 1:0.05, 1:0.1, 1:0.2, 1:0.5, 1:1, etc. The type of the alcohol is not particularly limited, and as some specific examples, it includes but is not limited to ethanol, methanol and isopropanol.

[0094] According to a specific embodiment of the present application, the coagulation bath is prepared in one kind.

[0095] According to a specific embodiment of the present application, the coagulation bath is prepared in multiple kinds and is immersed in sequence.

[0096] According to specific embodiments of the present application, the soaking time is 0.5 min - 10 min for each coagulation bath, as some specific examples, the soaking time is 0.5 min, 1 min, 2 min, 3 min, 4 min, 5 min, 6 min, 7 min, 8 min, 9 min, 10 min, etc. for each coagulation bath.

[0097] According to specific embodiments of the present application, the soaking temperature is 10 ℃ - 30 ℃, as some specific examples, the soaking temperature can be 10 ℃, 15 ℃, 20 ℃, 25 ℃, 30 ℃, etc.

[0098] According to specific embodiments of the present application, the drying temperature is 50 ℃ - 100 ℃, as some specific examples, the drying temperature can be 50 ℃, 60 ℃, 70 ℃, 80 ℃, 90 ℃, 100 ℃, etc. The drying time is 1 min - 10 min, as some specific examples, the drying time can be 1 min, 2 min, 3 min, 5 min, 8 min, 10 min, etc.

[0099] According to specific embodiments of the present application, the light crosslinking reaction temperature is 10 ℃ - 30 ℃, as some specific examples, the light crosslinking reaction temperature can be 10 ℃, 15 ℃, 20 ℃, 25 ℃, 30 ℃, etc.

[0100] According to specific embodiments of the present application, the light crosslinking reaction time is 0.5 min - 5 min, as some specific examples, the light crosslinking reaction time can be 0.5 min, 1 min, 2 min, 3 min, 4 min, 5 min, etc.

[0101] According to specific embodiments of the present application, the light source of the light crosslinking reaction is not particularly limited, including but not limited to ultraviolet light.

[0102] According to specific embodiments of the present application, the energy of the ultraviolet light is 100 mJ / cm 2 - 1500 mJ / cm 2 , as some specific examples, the energy of the ultraviolet light can be 100 mJ / cm 2 , 200 mJ / cm 2 , 300 mJ / cm 2 , 500 mJ / cm 2 , 1000 mJ / cm 2 , 1500 mJ / cm 2 , etc.

[0103] (3) performing imidization treatment on the porous polyamide acid membrane to obtain the high-temperature-resistant PI lithium battery porous diaphragm.

[0104] In step (3), since the PAA membrane has been crosslinked, the volume shrinkage effect caused by the imidization treatment will be reduced, so that the PAA is not easy to break during the imidization treatment. In the use of the rolled PI diaphragm, the PI diaphragm needs to be unrolled again, and the crosslinked PI polymer can withstand greater traction tension.

[0105] According to a specific embodiment of the present application, the temperature of the imidization treatment is 100-350 ℃, and as some specific examples, the temperature of the imidization treatment can be 100 ℃, 200 ℃, 300 ℃, 350 ℃, etc. Specifically, a gradient heating mode can be used.

[0106] According to a specific embodiment of the present application, the time of the imidization treatment is 0.5-5 h, and as some specific examples, the time of the imidization treatment can be 0.5 h, 1 h, 2 h, 3 h, 4 h, 5 h, etc.

[0107] According to a specific embodiment of the present application, the imidization treatment comprises: placing the rolled PAA membrane in a high-temperature environment under nitrogen protection and gradient heating, holding at T1 temperature for 0.5-2 h, holding at T2 temperature for 0.5-2 h, holding at T3 temperature for 0.5-2 h, and after cooling, obtaining the high-temperature-resistant PI lithium battery porous diaphragm; wherein T3>T2>T1.

[0108] According to a specific embodiment of the present application, the imidization treatment is carried out in an inert atmosphere, for example, can be carried out in a nitrogen atmosphere.

[0109] According to an embodiment of the present application, the second aspect of the present application provides a high-temperature-resistant PI lithium battery porous diaphragm obtained by the preparation method according to the first aspect.

[0110] The preparation method provided by the present application can be used to prepare the high-temperature-resistant PI lithium battery porous diaphragm, which has the advantages of good mechanical properties and high thermal stability, and at the same time, the diaphragm has high porosity and complete pores.

[0111] According to a specific embodiment of the present application, the thickness of the high-temperature-resistant PI lithium battery porous diaphragm is 5-25 μm, and as some specific examples, the thickness of the high-temperature-resistant PI lithium battery porous diaphragm can be 5 μm, 10 μm, 15 μm, 20 μm, 25 μm, etc.

[0112] According to the third aspect of the present application, a secondary battery is provided, which comprises the high-temperature-resistant PI lithium battery porous separator prepared by the preparation method according to the first aspect or the high-temperature-resistant PI lithium battery porous separator according to the second aspect.

[0113] The secondary battery prepared by using the high-temperature-resistant PI lithium battery porous separator provided by the present application has excellent capacity, cycle life and safety.

[0114] According to the specific embodiments of the present application, the secondary battery further comprises a positive electrode sheet and a negative electrode sheet; the positive electrode sheet comprises a positive electrode current collector and a positive electrode active material layer arranged on at least one side surface of the positive electrode current collector, and the positive electrode active material layer comprises a positive electrode active material. The positive electrode active material comprises at least one of lithium cobaltate, lithium manganate, lithium nickel manganate, lithium iron phosphate and nickel cobalt manganese ternary material. Generally, a battery comprises a positive electrode sheet, a negative electrode sheet, an electrolyte and a separator. During the charging and discharging process of the battery, active ions are embedded and extracted between the positive electrode sheet and the negative electrode sheet. The electrolyte plays a role in conducting ions between the positive electrode sheet and the negative electrode sheet. The separator is arranged between the positive electrode sheet and the negative electrode sheet, mainly playing a role in preventing short circuit between the positive electrode and the negative electrode, and at the same time allowing ions to pass through.

[0115] In some embodiments of the present application, the positive electrode current collector can comprise a metal foil or a composite positive electrode current collector. For example, the metal foil can be an aluminum foil. The composite positive electrode current collector can comprise a high polymer material base layer and a metal layer formed on at least one side surface of the high polymer material base layer, for example, the composite positive electrode current collector can be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, etc.) on a high polymer material base material (such as a polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT) or the like).

[0116] In some embodiments of the present application, the positive electrode active material layer can further optionally comprise a conductive agent. As an example, the conductive agent can comprise at least one of super conductive carbon, conductive carbon black SP, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, graphene and carbon nanofibers.

[0117] In some embodiments of the present application, the positive electrode active material layer can further optionally comprise a binder. As an example, the binder can comprise at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer and fluorine-containing acrylic ester resin.

[0118] In some embodiments of the present application, the positive electrode sheet can be prepared by dispersing the above-mentioned components for preparing the positive electrode sheet, such as the positive electrode active material, the conductive agent, and the binder, in a solvent (e.g., N-methylpyrrolidone, NMP) to form a positive electrode slurry; coating the positive electrode slurry on a positive electrode current collector; and drying, cold-pressing, and the like to obtain the positive electrode sheet.

[0119] The negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer disposed on at least one surface of the negative electrode current collector, the negative electrode active material layer including a negative electrode active material.

[0120] In some embodiments of the present application, the negative electrode current collector can be a metal foil or a composite current collector. For example, as the metal foil, a copper foil can be used. The composite current collector can include a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer. The composite current collector can be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, etc.) on a polymer material base layer (e.g., a base layer of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), etc.).

[0121] In some embodiments of the present application, the negative electrode active material can be a negative electrode active material known in the art. For example, the negative electrode active material can include at least one of natural graphite, artificial graphite, soft carbon, hard carbon, mesocarbon microbeads, nanocarbon, elemental silicon, silicon oxide compounds, silicon-carbon composites, silicon alloys, elemental tin, tin oxide compounds, tin-carbon composites, tin alloys, and at least one of lithium titanate.

[0122] In some embodiments of the present application, the negative electrode active material layer can further include a binder. The binder can include at least one of styrene butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

[0123] In some embodiments of the present application, the negative electrode active material layer can further include a conductive agent. The conductive agent can include at least one of super conductive carbon, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0124] In some embodiments of the present application, the negative electrode active material layer can further include other additives, such as a thickening agent (e.g., sodium carboxymethyl cellulose (CMC-Na)) and the like.

[0125] In some embodiments of the present application, the negative electrode active material layer can further include other additives, such as a dispersant (e.g., Electro-Rite BD040) and the like.

[0126] In some embodiments of the present application, the negative electrode sheet can be prepared by dispersing the above-mentioned components for preparing the negative electrode sheet, such as the negative electrode active material, the dispersing agent, the conductive agent, the binder, and the thickening agent, in a solvent (e.g., water) to form a negative electrode slurry; coating the negative electrode slurry on a negative electrode current collector, and then drying, cold-pressing, and the like to obtain the negative electrode sheet.

[0127] In some embodiments of the present application, the separator is wound with the positive electrode sheet and the negative electrode sheet to obtain a lithium ion secondary battery.

[0128] The schemes of the present application will be explained below in conjunction with examples. Those skilled in the art will understand that the following examples are only used to illustrate the present application and should not be regarded as limiting the scope of the present application. If no specific techniques or conditions are specified in the examples, the techniques or conditions described in the literature in the art or according to the product instructions are used. If no manufacturer of the reagents or instruments is specified, all are conventional products that can be obtained commercially.

[0129] Example 1 The present example provides a high-temperature-resistant PI lithium battery porous separator and a preparation method thereof, the preparation method comprising the following steps: PAA synthesis: 82.62 g of 4,4'-diaminodiphenyl ether (diamine compound), 4.36 g of 1,5,7,11-tetraoxaspiro[5,5]undecane-3,9-diamine (first spiro compound), and 3.33 g of 2,6,7-trioxabicyclo[2.2.2]oct-1-ylmethylamine (bicyclic orthoester) were dissolved in 800 g of NMP solvent, 100 g of pyromellitic dianhydride (dianhydride compound) was added under nitrogen protection at 10 ℃, and the reaction was stirred for 3 h until completion to obtain an NMP solution containing PAA; Preparation of casting solution: 150 g of dioctyl phthalate (pore-forming agent), 5 g of 1,5,7,11-tetraoxaspiro[5.5]undecane (second spiro compound), and 5 g of 2-hydroxy-2-methyl-1-phenyl-1-propanone (photoinitiator) were added to 1 kg of the NMP solution containing PAA under light-proof, 10 ℃, and nitrogen protection conditions, and stirred uniformly to obtain a casting solution; Wet film coating: the casting solution was coated on a PET base film with a release force of 50 g / mm using a doctor blade under light-proof conditions, and the wet film thickness was 100 μm; Preparation of coagulation bath: coagulation bath 1 was prepared in a volume ratio of V 水 :V 乙醇 =1:1, and coagulation bath 2 was prepared in a volume ratio of V 水 :V 乙醇 =2:1; Phase separation pore formation: the wet film was sequentially immersed in coagulation bath 1 for 5 min and then coagulation bath 2 for 5 min at 25 ℃ in the dark; Photocuring crosslinking: the PAA film was irradiated with 1000 mJ / cm 2 of UV light at 25 ℃ under nitrogen protection for 1 min; Peeling off the PET film: after the crosslinked film was dried at 80 ℃ for 3 min, the PAA film was peeled off from the PET base film, and then wound to obtain a rolled PAA film; Imidization treatment: the rolled PAA film was placed in a high-temperature environment under nitrogen protection and gradually heated, and then kept at 150 ℃ for 30 min, at 200 ℃ for 30 min, and at 300 ℃ for 30 min. After cooling, a high-temperature-resistant PI lithium battery porous separator was obtained.

[0130] The specific reaction process of this embodiment can refer to the following reaction formula. Since the monomers in the polymerization reaction are randomly polymerized, the monomer letter subscript in the reaction formula only represents the repeating unit.

[0131]

[0132] Example 2 The difference between this embodiment and Example 1 is that the types of the first spiro compound and the bicyclic orthoester are changed, and the preparation process is as follows: PAA synthesis: 82.62 g of 4,4'-diaminodiphenyl ether (diamine compound), 6.29 g of 1,5,7,11-tetraoxaspiro[5.5]undecane-3,9-dipropylamine (first spiro compound), and 3.97 g of 3-(2,6,7-trioxabicyclo[2.2.2]octan-1-yl)propane-1-amine (bicyclic orthoester) were dissolved in 800 g of NMP solvent. Under nitrogen protection at 10 ℃, 100 g of pyromellitic dianhydride (dianhydride compound) was added, and stirred for 3 h until the reaction was complete to obtain an NMP solution containing PAA; Preparation of casting solution: under the conditions of light protection, 10 ℃, and nitrogen protection, 150 g of dioctyl phthalate (pore forming agent), 5 g of 1,5,7,11-tetraoxaspiro[5.5]undecane (second spiro compound), and 5 g of 2-hydroxy-2-methyl-1-phenyl-1-propanone (photoinitiator) were added to 1 kg of the NMP solution containing PAA, and stirred uniformly to obtain a casting solution; Wet film coating: the casting solution was coated on a PET base film with a release force of 50 g / mm using a doctor blade under light protection, and the wet film thickness was 100 μm; Preparation of coagulation bath: the volume ratio was V 水 :V 乙醇=1:1 to prepare coagulation bath 1, the volume ratio is V 水 : V 乙醇 =2:1 to prepare coagulation bath 2; Phase separation into pores: under the condition of 25℃ and light shielding, the wet film is sequentially immersed in coagulation bath 1 for 5 min, and then immersed in coagulation bath 2 for 5 min; Photo-curing cross-linking: under the protection of nitrogen, the PAA film is irradiated with ultraviolet light with energy of 1000 mJ / cm 2 for 1 min at 25℃; Peeling PET film: after the cross-linked film is dried at 80℃ for 3 min, the PAA film is peeled from the PET base film, and then the PAA film is wound to obtain a coiled PAA film; Imidization treatment: under the protection of nitrogen, the coiled PAA film is placed in a high-temperature environment for gradient heating, and then kept at 150℃ for 30 min, kept at 200℃ for 30 min, and kept at 300℃ for 30 min. After cooling, a high-temperature-resistant PI lithium battery porous separator is obtained.

[0133] Example 3 The difference between this example and Example 1 is that the type of the second spiro compound is changed, and the preparation process is as follows: PAA synthesis: 82.62 g of 4,4'-diaminodiphenyl ether (diamine compound), 4.36 g of 1,5,7,11-tetraoxaspiro[5,5]undecane-3,9-diamine (first spiro compound), and 3.33 g of 2,6,7-trioxabicyclo[2.2.2]oct-1-ylmethylamine (bicyclic orthoester) are dissolved in 800 g of NMP solvent. Under the protection of nitrogen at 10℃, 100 g of pyromellitic dianhydride (dianhydride compound) is added, and stirred for 3 h until the reaction is complete to obtain an NMP solution containing PAA; Preparation of casting solution: under the conditions of light shielding, 10℃, and nitrogen protection, 150 g of dioctyl phthalate (pore forming agent), 5 g of 1,4,6,9-tetraoxaspiro[4.4]nonane (second spiro compound), and 5 g of 2-hydroxy-2-methyl-1-phenyl-1-propanone (photoinitiator) are added to 1 kg of NMP solution containing PAA, and stirred uniformly to obtain a casting solution; Wet film coating: under the condition of light shielding, the casting solution is coated on a PET base film with release force of 50 g / mm by using a doctor blade, and the wet film thickness is 100 μm; Preparation of coagulation bath: V 水 : V 乙醇 =1:1 to prepare coagulation bath 1, the volume ratio is V 水 : V 乙醇 =2:1 to prepare coagulation bath 2; Phase separation and pore formation: the wet film was sequentially immersed in coagulation bath 1 for 5 min and then in coagulation bath 2 for 5 min at 25 °C in the dark; Photocuring crosslinking: the PAA film was irradiated with 1000 mJ / cm 2 of UV light at 25 °C under nitrogen protection for 1 min; Peeling off the PET film: after the crosslinked film was dried at 80 °C for 3 min, the PAA film was peeled off from the PET substrate, and then wound to obtain a coiled PAA film; Imidization treatment: the coiled PAA film was placed in a high-temperature environment and gradually heated under nitrogen protection, and then kept at 150 °C for 30 min, at 200 °C for 30 min, and at 300 °C for 30 min. After cooling, a high-temperature-resistant PI lithium battery porous separator was obtained.

[0134] Example 4 The difference between this example and Example 1 is that the types of diamine compound and diacid anhydride compound are changed, and the preparation process is as follows: PAA synthesis: 53.45 g of 4,4'-diaminobiphenyl (diamine compound), 3.07 g of 1,5,7,11-tetraoxaspiro[5,5]undecane-3,9-diamine (first spiro compound), and 2.34 g of 2,6,7-trioxabicyclo[2.2.2]oct-1-ylmethylamine (bicyclic orthoester) were dissolved in 750 g of NMP solvent. Under nitrogen protection at 10 °C, 100 g of 4,4'-oxydiphthalic anhydride (diacid anhydride compound) was added, and stirred for 3 h until the reaction was complete to obtain an NMP solution containing PAA; Preparation of casting solution: under the conditions of light protection, 10 °C, and nitrogen protection, 150 g of dioctyl phthalate (pore-forming agent), 5 g of 1,5,7,11-tetraoxaspiro[5.5]undecane (second spiro compound), and 5 g of 2-hydroxy-2-methyl-1-phenyl-1-propanone (photoinitiator) were added to 1 kg of the NMP solution containing PAA, and stirred uniformly to obtain a casting solution; Wet film coating: the casting solution was coated on a PET base film with a release force of 50 g / mm using a doctor blade under light protection, and the wet film thickness was 100 μm; Preparation of coagulation bath: coagulation bath 1 was prepared in a volume ratio of V 水 :V 乙醇 =1:1, and coagulation bath 2 was prepared in a volume ratio of V 水 :V 乙醇 =2:1; Phase separation and pore formation: the wet film was sequentially immersed in coagulation bath 1 for 5 min and then in coagulation bath 2 for 5 min at 25 °C in the dark; Photocuring crosslinking: Under nitrogen protection, energy of 1000 mJ / cm at 25°C 2 The PAA film was irradiated with UV light for 1 min; Peeling off the PET film: After the cross-linked film was dried at 80°C for 3 minutes, the PAA film was peeled off from the PET substrate and wound up to obtain a roll of PAA film. Imidization treatment: Under nitrogen protection, the rolled PAA membrane was placed in a high-temperature environment and the temperature was increased gradually, maintaining at 150 °C for 30 min, 200 °C for 30 min, and 300 °C for 30 min. After cooling, a high-temperature resistant PI lithium battery porous membrane was obtained.

[0135] Comparative Example 1 This comparative example provides a polyimide separator and a preparation method thereof, comprising the following steps: PAA synthesis: 91.8 g of 4,4'-diaminodiphenyl ether (diamine compound) was dissolved in 800 g of NMP solvent. Under nitrogen protection at 10 °C, 100 g of pyromellitic dianhydride (dianhydride compound) was added and stirred for 3 h until the reaction was complete to obtain an NMP solution containing PAA. Prepare the casting solution: Add 150 g of dioctyl phthalate (pore-forming agent) to 1 kg of NMP solution containing PAA in a dark place at 10°C under nitrogen protection and stir evenly to obtain the casting solution. Wet film coating: The casting solution was coated on a PET base film with a release force of 50 g / mm using a doctor blade. The wet film thickness was 100 μm. Prepare coagulation bath: V 水 :V 乙醇 =1:1 to prepare coagulation bath 1, the volume ratio is V 水 :V 乙醇 =2:1 to prepare coagulation bath 2; Phase separation pore formation: At 25 °C, the wet film was immersed in coagulation bath 1 for 5 min and then immersed in coagulation bath 2 for 5 min. Peeling off the PET film: After the cross-linked film was dried at 80°C for 3 minutes, the PAA film was peeled off from the PET substrate and wound up to obtain a roll of PAA film. Imidization treatment: Under nitrogen protection, the rolled PAA membrane was placed in a high-temperature environment and the temperature was increased gradually, maintaining at 150 °C for 30 min, 200 °C for 30 min, and 300 °C for 30 min. After cooling, a polyimide membrane was obtained.

[0136] Comparative Example 2 The comparative example provides a polyimide separator and a preparation method thereof, comprising the following steps: PAA synthesis: 82.62 g of 4,4'-diaminodiphenyl ether (diamine compound), 4.36 g of 1,5,7,11-tetraoxaspiro[5,5]undecane-3,9-diamine (first spiro compound), and 3.33 g of 2,6,7-trioxabicyclo[2.2.2]oct-1-ylmethylamine (bicyclic orthoester) were dissolved in 800 g of NMP solvent, and 100 g of pyromellitic dianhydride (dianhydride compound) was added under nitrogen protection at 10°C. Stirring for 3 h until the reaction was complete. An NMP solution containing PAA was obtained; Preparation of casting solution: under the conditions of light shielding, 10°C, and nitrogen protection, 150 g of dioctyl phthalate (pore-forming agent) was added to 1 kg of the NMP solution containing PAA, and stirred uniformly to obtain a casting solution; Wet film coating: under light shielding conditions, the casting solution was coated on a PET base film with a release force of 50 g / mm using a doctor blade, and the wet film thickness was 100 μm; Preparation of coagulation bath: a coagulation bath 1 was prepared in a volume ratio of V 水 :V 乙醇 =1:1, and a coagulation bath 2 was prepared in a volume ratio of V 水 :V 乙醇 =2:1; Phase separation and pore formation: under the condition of 25°C, the wet film was sequentially immersed in the coagulation bath 1 for 5 min, and then immersed in the coagulation bath 2 for 5 min; Peeling of PET film: after the crosslinked film was dried at 80°C for 3 min, the PAA film was peeled off from the PET substrate, and after a winding process, a coiled PAA film was obtained; Imidization treatment: under nitrogen protection, the coiled PAA film was placed in a high-temperature environment for gradient heating, and was kept at 150°C for 30 min, at 200°C for 30 min, and at 300°C for 30 min. After cooling, a polyimide separator was obtained.

[0137] Test example The separators prepared in the above examples and comparative examples were subjected to the following performance tests: (1) Finished product thickness test: a Shanghai Liulv CH-1-ST thousandth table film thickness tester was used.

[0138] (2) Tensile strength and elongation at break test: an INSTRON tensile testing machine was used. The sample was prepared into a sample test with a width of 3 cm and a length of about 10 cm, and the pulling speed was 2 mm / s.

[0139]

[0140] (3) Puncture strength test: The sample was cut into a square of 10 cm x 10 cm, and the maximum load of the steel needle puncture was recorded.

[0141] (4) Air permeability test: A GURLEY air permeability timer was used. The time for 100 ml of gas to penetrate the membrane was measured.

[0142] (5) Porosity test: The mass method was used for measurement. The membrane sample was soaked in a n-butanol solution for 4 h, and after saturation, the sample was taken out and the excess n-butanol on the surface was absorbed with filter paper. The porosity was calculated by comparing the weight difference before and after soaking.

[0143] (6) Contact angle test: A compounded electrolyte was used for testing, and the electrolyte was ethylene carbonate: methyl ethyl carbonate: diethyl carbonate = 3:5:2 (volume ratio).

[0144] (7) Thermal shrinkage rate test: The sample was cut into a square of 10 cm x 10 cm, and placed in a 200 ℃ nitrogen-protected environment for 1 h. The area change rate was measured.

[0145] The performance test data of the membranes prepared in Examples 1-4 and Comparative Examples 1-2 are shown in Table 1.

[0146] Table 1

[0147] Batteries were prepared using the membranes prepared in Examples 1-4 and Comparative Examples 1-2, and the preparation method of the batteries was as follows: Positive electrode preparation: Under nitrogen protection, 2 parts of PVDF, 60 parts of lithium iron phosphate and 1 part of conductive carbon black SP were uniformly dispersed in 37 parts of NMP according to the mass percentage, and the slurry was stirred to obtain a positive electrode slurry. The slurry was coated on an aluminum foil, and the obtained positive electrode was placed in a vacuum drying oven at 100 ℃ for drying for 30 min, and after rolling, a positive electrode sheet was obtained; Negative electrode preparation: 97 parts of artificial graphite, 0.1 parts of dispersant (Electro-Rite BD040), 1 parts of SBR binder, 1 parts of carbon nanotube and 1 parts of carboxymethyl cellulose sodium (CMC-Na) were uniformly dispersed into water according to the mass percentage, and the slurry was stirred to obtain a negative electrode slurry. The slurry was coated on a copper foil, and the obtained negative electrode was placed in a vacuum drying oven at 100 ℃ for drying for 30 min, and a negative electrode sheet was obtained; The membranes prepared in Examples 1-4 and Comparative Examples 1-2 were each wound with the positive and negative electrode sheets to obtain 26650 cylindrical lithium ion secondary batteries.

[0148] The secondary batteries prepared above were tested, and the test method was as follows, and the test results are shown in Table 2: Cyclic test was carried out at 25℃, the voltage range was 1.5-3.0 V, the current density was 0.5C, 1C and 2C respectively, and the cycle was 100 times. Discharge retention rate = discharge capacity of the 100th ÷ discharge capacity of the 1st × 100%.

[0149] Table 2

[0150] Result analysis: From the data of Table 1 and Table 2, it can be seen that the thickness of the separators prepared in Examples 1-4 and Comparative Examples 1-2 is the same or not much different. The mechanical properties of the separators in Examples 1-4 are improved by crosslinking reaction, which is specifically manifested in that the tensile strength of Examples 1-4 is greater than that of the non-crosslinked Comparative Examples, and the elongation at break thereof is greater than that of the Comparative Examples, indicating that the toughness of the separators prepared in Examples is also improved. At the same time, the puncture resistance thereof is also better than that of the non-crosslinked Comparative Examples.

[0151] Through the comparison of the measured values of air permeability, the air permeability of the Examples is better than that of the Comparative Examples. Gas is easy to pass through the pores, which indirectly indicates that ions are more easily passed through the separators of the Examples, which is beneficial to reduce the internal resistance of the battery. At the same time, it indicates that the pores of the Examples remain intact during the processes of drying, peeling, winding and imidization treatment, and do not appear to collapse, break and block, etc. The pore structure can be effectively fixed by crosslinking reaction, which is beneficial to the subsequent processing procedures and also beneficial to the performance of the separator. In addition, the measurement of porosity also supports the above view.

[0152] The contact angle test shows that after the ring-opening crosslinking of the spiro compound and the bicyclic orthoester, a flexible segment is formed, which enhances the electrophilic electrolyte properties of the separator, is beneficial to the infiltration of the electrolyte, and improves the cycle performance of the battery.

[0153] The measurement of thermal shrinkage rate shows that the shrinkage rate of Examples 1-4 after crosslinking is less than that of Comparative Examples 1-2 without crosslinking, indicating that the crosslinking reaction can improve the heat resistance of the separator material.

[0154] Through the cycle test of the battery, the performance of Examples 1-4 at 0.5C, 1C and 2C rates is better than that of Comparative Examples 1-2, indicating that the improved preparation method of the separator can effectively improve the performance of the lithium battery.

[0155] In the description of the specification, the description of the terms "one embodiment", "some embodiments", "an example", "a specific example", or "some examples" etc. means that the specific features, structures, materials or characteristics described in connection with the embodiment or example are contained in at least one embodiment or example of the present application. In the specification, the illustrative description of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any suitable manner in any one or more embodiments or examples. In addition, the person skilled in the art can combine and combine the different embodiments or examples described in the specification and the features of the different embodiments or examples without contradiction.

[0156] Although the embodiments of the present application have been shown and described above, it is understood that the above-described embodiments are exemplary and are not to be construed as limiting the present application, and those skilled in the art can make changes, modifications, replacements and variations to the above-described embodiments within the scope of the present application.

Claims

1. A method for preparing a high-temperature resistant PI lithium battery porous diaphragm, characterized in that: The preparation method comprises the following steps: contacting a first spiro compound, a bicyclic orthoester, a dianhydride compound, and a diamine compound to carry out a polymerization reaction to obtain a polyamic acid solution; The polyamic acid solution, the pore-forming agent, the second spiro compound and the photoinitiator are mixed to obtain a porous polyamic acid membrane through phase separation pore formation and photocrosslinking reaction; The porous polyamic acid membrane is subjected to imidization treatment to obtain the high-temperature resistant PI lithium battery porous separator.

2. The preparation method according to claim 1, characterized in that The ratio of the molar amount of the dianhydride compound to the total molar amount of the first spiro compound, the bicyclic orthoester, and the diamine compound is 1:(0.95-1.05); Optionally, the molar ratio of the diamine compound, the first spiro compound, and the bicyclic orthoester is 1:(0.01-0.1):(0.01-0.1).

3. The preparation method according to any one of claims 1 to 2, characterized in that The first spiro compound structure contains two amino groups; Optionally, the first spiro compound comprises at least one of 1,5,7,11-tetraoxaspiro[5,5]undecane-3,9-diamine, 1,4,6,9-tetraoxaspiro[4,4]nonane-2,7-dimethylamine, and 1,5,7,11-tetraoxaspiro[5.5]undecane-3,9-dipropylamine; Optionally, the bicyclic orthoester structure contains a single amino group; Optionally, the bicyclic orthoester comprises at least one of 2,6,7-trioxabicyclo[2.2.2]octan-1-ylmethylamine, 3-(2,6,7-trioxabicyclo[2.2.2]octan-1-yl)propan-1-amine, 4-(2,6,7-trioxabicyclo[2.2.2]octan-1-yl)butan-2-amine, 3-(4-methyl-2,6,7-trioxabicyclo[2.2.2]octan-1-yl)propan-1-amine, 4-(4-methyl-2,6,7-trioxabicyclo[2.2.2]octan-1-yl)butan-2-amine, 2,6,7-trioxabicyclo[2.2.2]octan-1-ethylamine, and 1-(2,6,7-trioxabicyclo[2.2.2]octan-1-yl)propan-2-amine; Optionally, the dianhydride compound includes at least one of pyromellitic dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride, 4,4'-oxydiphthalic anhydride, 3,3',4,4'-benzophenonetetracarboxylic dianhydride, 3,3',4,4'-triphenylbisether tetracarboxylic dianhydride, 3,3',4,4'-diphenylsulfone tetracarboxylic dianhydride, and bisphenol A diether dianhydride; Optionally, the diamine compound includes at least one of p-phenylenediamine, 4,4'-diaminodiphenyl ether, 4,4'-diaminodiphenylmethane, 4,4'-diaminobiphenyl, 3,5'-diaminobenzoic acid, 4,4'-diaminophenylsulfone, 1,3-bis(4-aminophenoxy)benzene, 1,4-bis(4-aminophenoxy)benzene, 4,4'-bis(4-aminophenoxy)biphenyl, and 4,4'-bis(3-aminophenoxy)biphenyl.

4. The preparation method according to any one of claims 1 to 3, characterized in that The polymerization reaction temperature is 0°C-50°C; Optionally, the polymerization reaction time is 0.5 h-5 h; Optionally, the preparation method further comprises: contacting the first spiro compound, the bicyclic orthoester, the dianhydride compound, the diamine compound, and the solvent to perform a polymerization reaction to obtain a polyamic acid solution; Optionally, the mass proportion of the solvent in the polyamic acid solution is 50 wt%-95 wt%; Optionally, the solvent includes at least one of N-methylpyrrolidone, N,N-dimethylformamide, and N,N-dimethylacetamide.

5. The preparation method according to any one of claims 1 to 4, characterized in that The mass ratio of the polyamic acid solution, the pore-forming agent, the second spiro compound, and the photoinitiator is 1:(0.05-0.3):(0.001-0.01):(0.001-0.01); Optionally, the pore-forming agent includes at least one of dioctyl phthalate, polyvinyl pyrrolidone-K17, polyvinyl pyrrolidone-K30, polyethylene glycol 400, diethylene glycol, triethylene glycol, and glycerol; Optionally, the second spiro compound includes at least one of 3,9-dihydroxymethyl-3',9'-diethyl-1,5,7,11-tetraoxaspiro[5,5]undecane, 1,5,7,11-tetraoxaspiro[5.5]undecane, 8,10,19,20-tetraoxatrispiro[5.2.2.5.2]eicosane, 2,2'-spirobis[1,3-benzodioxane], and 1,4,6,9-tetraoxaspiro[4.4]nonane; Optionally, the photoinitiator includes at least one of 2-hydroxy-2-methyl-1-phenyl-1-propanone, 1-hydroxycyclohexyl phenyl ketone, 2,4,6-trimethylbenzoyl-diphenylphosphine oxide, 2-hydroxy-4'-(2-hydroxyethoxy)-2-methylpropiophenone, and 1,1'-(oxybis(4,1-phenylene))bis(2-hydroxy-2-methyl-1-propanone); Optionally, the mixing temperature is 0°C-30°C.

6. The preparation method according to any one of claims 1 to 5, characterized in that The mixing is carried out in a dark and nitrogen atmosphere; Optionally, the preparation method further comprises: adding a pore-forming agent, a second spiro compound and a photoinitiator to the polyamic acid solution, stirring uniformly to obtain a casting solution; Optionally, the preparation method further comprises: coating the casting solution on the base film with a doctor blade under light-proof conditions to obtain a wet film, immersing the wet film in a coagulation bath to achieve phase separation and pore formation, then irradiating the polyamic acid film with ultraviolet light under nitrogen protection to induce a photocrosslinking reaction, drying the crosslinked film, peeling it from the base film, and winding it to obtain a rolled porous polyamic acid film; Optionally, the base film includes but is not limited to polyethylene terephthalate film, polyethylene film or polypropylene film; Optionally, the base film has a release force of 5 g / mm-100 g / mm; Optionally, the coating has a thickness of 25 μm-200 μm; Optionally, the coagulation bath comprises water and alcohol; Optionally, the volume ratio of water to alcohol is 1:(0.05-1); Optionally, the alcohol comprises at least one of ethanol, methanol, and isopropanol; Optionally, the coagulation bath is prepared as one; Optionally, the coagulation baths are prepared in multiple ways and immersed in sequence; Optionally, the soaking time is 0.5 min to 10 min in each coagulation bath; Optionally, the soaking temperature is 10°C-30°C; Optionally, the drying temperature is 50°C-100°C; Optionally, the drying time is 1 min-10 min; Optionally, the temperature of the photocrosslinking reaction is 10°C-30°C; Optionally, the photocrosslinking reaction time is 0.5 min-5 min; Optionally, the light source for the photocrosslinking reaction includes ultraviolet light; Optionally, the energy of the ultraviolet light is 100 mJ / cm 2 -1500 mJ / cm 2 .

7. The preparation method according to any one of claims 1 to 6, characterized in that The temperature of the imidization treatment is 100°C-350°C; Optionally, the imidization treatment time is 0.5 h-5 h.

8. A high-temperature resistant PI lithium battery porous diaphragm obtained according to the preparation method according to any one of claims 1 to 7.

9. The high temperature resistant PI lithium battery porous diaphragm according to claim 8, characterized in that The thickness of the high-temperature resistant PI lithium battery porous diaphragm is 5 μm-25 μm.

10. A secondary battery, characterized in that: The secondary battery comprises the high-temperature resistant PI lithium battery porous diaphragm obtained by the preparation method according to any one of claims 1 to 7 or the high-temperature resistant PI lithium battery porous diaphragm according to any one of claims 8 to 9.

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