Binder, diaphragm and preparation method and application thereof

Abstract

The invention provides a binder, a diaphragm and a preparation method and application thereof. The adhesive comprises an ester polymer obtained by copolymerizing a polyol monomer and a polycarboxylic acid monomer, the ester polymer comprising a structure comprising the following units (I)-(III): # imgabs0 # wherein "-" represents a single bond, "*" represents a bond connection position, units (II) and (III) are connected at least via unit (I), R1 to R4 represent hydrogen atoms or monovalent organic groups, and R2 represents a hydrogen atom or a monovalent organic group, and R3 represents a hydrogen atom or a monovalent organic group, and R3 represents a hydrogen atom or a monovalent organic group, and R4 represents a hydrogen atom or a monovalent organic group, and R4 represents a hydrogen atom or a monovalent organic group. R5 represents a saturated or unsaturated hydrocarbon group, and the hydrocarbon group optionally has a substituent. According to the binder provided by the invention, the coating can better support the diaphragm at a high temperature, the heat resistance of the diaphragm is effectively improved, and meanwhile, the separation from inorganic particles at the high temperature is avoided.

Description

Technical Field

[0001] The present invention relates to an adhesive, a diaphragm, and a preparation method and application thereof, and in particular to an adhesive with high heat resistance and a diaphragm with high heat resistance containing the adhesive and applicable to a lithium secondary battery, and a preparation method thereof, and a secondary battery containing the diaphragm, belonging to the field of new energy. Background Art

[0002] The separator is an indispensable component of the lithium-ion battery system. It can prevent the short circuit between the positive and negative electrodes of the battery, ensure the safety of the battery, and provide an effective channel for the rapid migration of lithium ions. At present, polyolefin microporous membranes have been widely used in the lithium-ion battery industry due to their good chemical stability and good mechanical strength. However, due to the inherent characteristics of polyethylene (PE) separators and polypropylene (PP) separators, their melting points (Tm) are only about 135°C and 165°C, and severe thermal shrinkage will occur under overheating conditions, and high-temperature applications also have certain limitations. This will cause internal short circuits and potential explosion hazards.

[0003] In order to improve the thermal shrinkage performance and temperature resistance of the diaphragm, the common method is to apply ceramic coating on the surface of the diaphragm. In order to ensure the adhesion and uniformity of the coating, a binder is usually added to bond the ceramic particles and the base film.

[0004] Some documents disclose a ceramic coating diaphragm and its preparation method and application in lithium-ion batteries. By setting a symmetrical five-layer structure, setting a ceramic coating on the outside of the diaphragm, and using a chlorinated polyolefin coating to bond the ceramic coating to the surface of the base film, the thermal shrinkage performance of the diaphragm is improved. However, the adhesive can only achieve heat shrinkage resistance of 130℃ to 150℃, and has limited improvement on the thermal shrinkage performance of the diaphragm. In addition, the coating has low bonding strength and is prone to uneven shedding, which reduces the heat resistance of the diaphragm in the battery, causing the diaphragm to shrink thermally in a high temperature environment, and ultimately affects the safety of the battery.

[0005] Some documents disclose a method for preparing a lithium-ion composite diaphragm by multi-layer co-extrusion coating. The invention is coated with two functional coatings. Although it can improve the thermal safety and liquid absorption of the diaphragm to a certain extent and is environmentally friendly, it does not fundamentally solve the problems of poor wettability between aqueous bacterial cellulose slurry and aqueous ceramic slurry and non-polar polyolefin diaphragm, difficulty in uniform distribution of the coating, and poor adhesion between the two functional coatings and the polyolefin diaphragm.

[0006] It can be seen that although the art has conducted certain explorations and researches on obtaining diaphragm materials with high heat resistance and high wettability, it cannot be said to be sufficient and there is still room for further improvement. Summary of the invention

[0007] Problem that the invention aims to solve

[0008] As mentioned above, although the currently commonly used polyolefin separators have good chemical stability and good mechanical strength, their melting point is low, resulting in poor thermal shrinkage performance, which affects their safety in use.

[0009] In order to improve the thermal shrinkage performance of the diaphragm, the method commonly used in the prior art is to coat the ceramic particles on the surface of the diaphragm with a binder. However, after long-term practice, it is found that, on the one hand, the existing binders have limited improvement on the thermal shrinkage performance, and on the other hand, these binders have low viscosity and may cause the ceramic particles to fall off at high temperatures, ultimately affecting the safety of the battery. Furthermore, it is believed that this phenomenon may be related to the characteristics of the binder itself and the surface characteristics of the binder and the inorganic particles.

[0010] In addition, although the wettability of ceramic diaphragms has been improved, it cannot meet the increasing demand for fast charging of battery cells.

[0011] In order to solve the above problems, the present invention provides a binder, which includes a polyester polymer having a unit with a specific structure. The binder has a high glass transition temperature, flexibility and bonding strength, and the polymer has a high molecular weight, which can make the coating better support the diaphragm at high temperature, thereby effectively improving the heat resistance of the diaphragm. In addition, the appropriate flexibility can also avoid separation from the inorganic particles at high temperature.

[0012] In addition, the present invention also provides a separator for a secondary battery, wherein the separator comprises the binder of the present invention, and the use of the binder effectively improves the heat resistance and wettability of the separator.

[0013] In addition, the present invention also provides a method for preparing the diaphragm, which is simple, the raw materials are easy to obtain, and is suitable for industrial production.

[0014] Furthermore, the present invention also provides a secondary battery, wherein the secondary battery comprises the separator of the present invention.

[0015] Solutions for solving problems

[0016] The present invention first provides a binder, wherein the binder comprises an ester polymer, wherein the ester polymer is obtained by copolymerizing a polyol monomer and a polycarboxylic acid monomer, wherein the ester polymer comprises a structure formed by the following units (I) to (III):

[0017]

[0018] in,

[0019] "-" represents a single bond, "*" represents the position of the bond, and units (II) and (III) are connected at least through unit (I).

[0020] R 1 ~R 4 represents a hydrogen atom or a monovalent organic group,

[0021] R 5 It represents a saturated or unsaturated hydrocarbon group, and the hydrocarbon group may have a substituent.

[0022] According to the binder of the present invention, the unit (II) is derived from a diol monomer or a dibasic acid monomer; and the unit (III) is derived from a dicarboxylic acid monomer or a diol monomer.

[0023] According to the binder of the present invention, the molar ratio of the polyol monomer to the polycarboxylic acid monomer is 0.95:1 to 1.05:1, and the molar numbers of the polyol monomer and the polycarboxylic acid monomer are calculated based on the molar numbers of hydroxyl groups or carboxylic acid groups contained therein.

[0024] According to the binder of the present invention, the ester polymer at least partially has a linear structure formed by units (I) to (III);

[0025] Preferably, the ester polymer is composed of a linear structure formed by units (I) to (III).

[0026] According to the binder of the present invention, the unit (II) is derived from a diol monomer as shown in formula (II-a): the unit (III) is derived from a dicarboxylic acid monomer as shown in formula (III-a):

[0027]

[0028] in,

[0029] The R 1 ~R 4 are selected from hydrogen atoms, substituted or unsubstituted C1-C5 alkyl groups or ether-containing groups,

[0030] The R 5 It represents a C1-C10 saturated or unsaturated alkylene group.

[0031] In addition, the present invention also provides a secondary battery separator, wherein the separator comprises a base film and a coating layer coated on at least one side of the base film;

[0032] Wherein, the coating comprises the binder according to the present invention.

[0033] According to the separator of the present invention, the coating further comprises inorganic particles and optional auxiliary agents;

[0034] Preferably, calculated by weight, the binder in the coating accounts for 3-8%; the inorganic particles account for 85-95%; and the auxiliary agent accounts for 1-5%;

[0035] Preferably, the inorganic particles include ceramic particles, and the particle size Dv50 of the ceramic particles is 0.5 to 2 μm;

[0036] More preferably, the particle size Dv50 of the ceramic particles is 0.5 to 1.1 μm;

[0037] Preferably, the ceramic particles include at least one of boehmite, aluminum oxide, magnesium hydroxide, magnesium oxide, titanium dioxide, silicon dioxide, titanium oxide, barium titanate, zinc oxide, nickel oxide, magnesium fluoride, zirconium oxide, cerium oxide, and barium sulfate particles.

[0038] According to the diaphragm of the present invention, the base film has a thickness of 5 to 9 μm, and the coating has a thickness of 0.5 to 3 μm; and / or,

[0039] The base film includes at least one of a polyolefin base film, a polyimide base film, and a polyester base film.

[0040] Furthermore, the present invention also provides a method for preparing the diaphragm according to the present invention, which comprises the following steps:

[0041] The step of preparing the binder comprises copolymerizing the polyol monomer and the polycarboxylic acid monomer to obtain the binder;

[0042] A mixing step of mixing the binder, inorganic particles and optional auxiliary agents in a solvent to obtain a coating liquid;

[0043] The coating step is to coat the coating liquid on at least one surface of the base film and obtain the separator after drying.

[0044] In addition, the present invention also provides a secondary battery, which includes the diaphragm according to the present invention, and the secondary battery includes a power battery or an energy storage battery for an energy storage power station.

[0045] Effects of the Invention

[0046] 1) The adhesive provided by the present invention includes a polyester polymer having a specific structural unit (copolymerization of a diol monomer with a dicarboxylic acid monomer of a specific structure), wherein the specific structural unit (II) has abundant conjugated structural units, which can increase the glass transition temperature of the adhesive.

[0047] 2) The length of the polymer molecular chain can be increased by copolymerizing the dicarboxylic acid monomer and the diol monomer to obtain an ester copolymer, and the control of the structural unit (II) to ensure that the obtained ester polymer has a two-dimensional linear straight chain structure. Different from the binder with a complete three-dimensional network structure, the ester copolymer with a two-dimensional linear structure has better elasticity, and the diaphragm finally prepared can maintain excellent flexibility when facing working conditions under different temperature environments, and enhance the supporting effect of the diaphragm coating on the diaphragm at high temperature.

[0048] 3) In addition, in some preferred technical solutions, the diol monomers have abundant hydroxyl functional groups, which can form hydrogen bonds, thereby significantly enhancing the adhesion of the adhesive. DETAILED DESCRIPTION

[0049] The following is a detailed description of the present invention. The following description of the technical features is based on representative embodiments and specific examples of the present invention, but the present invention is not limited to these embodiments and specific examples. It should be noted that:

[0050] In this specification, the numerical range expressed using "a numerical value A to a numerical value B" means a range including the endpoints numerical values ​​A and B.

[0051] In the present specification, a numerical range expressed using "above" or "below" means a numerical range including the number.

[0052] In this specification, the word "may" means both performing a certain process and not performing a certain process.

[0053] In this specification, the use of "optional" or "optional" indicates that certain substances, components, execution steps, application conditions and other factors are used or not used.

[0054] In this specification, the "normal temperature" or "room temperature" used means an indoor ambient temperature of "23±2°C".

[0055] In this specification, the term "plurality" refers to a number of 2 or more.

[0056] In this specification, the unit names used are all international standard unit names, and unless otherwise stated, the "%" used indicates weight or mass percentage.

[0057] In the present specification, the use of “substantially” or “essentially” means that the standard deviation from a theoretical model, theoretical data or target data is within a numerical range of 1%, preferably 0.8%, and more preferably 0.5%.

[0058] In this specification, when the terms “include” and / or “comprises” are used, they indicate the presence of features, steps, operations, devices, components and / or their combinations.

[0059] In this specification, the references to "some specific / preferred embodiments", "other specific / preferred embodiments", "embodiments", etc., mean that the specific elements (e.g., features, structures, properties and / or characteristics) described in connection with the embodiments are included in at least one embodiment described herein, and may or may not exist in other embodiments. In addition, it should be understood that the elements may be combined in various embodiments in any suitable manner.

[0060] The present invention mainly provides a binder, which effectively improves the glass transition temperature, flexibility and bonding strength of the binder by using a polyester obtained by using units (I) to (III) with a specific structure. When the binder is used in a diaphragm, it can effectively improve the heat resistance of the diaphragm while taking into account the flexibility.

[0061] The present invention is mainly obtained through the following insights:

[0062] In view of the problem of poor heat resistance of diaphragms, especially polyolefin diaphragms, the method commonly used in the prior art is to coat ceramic particles on the surface of the diaphragm by a binder. However, there are various problems with the binders used in the prior art, such as limited improvement in heat resistance and low bonding strength. In addition, some prior arts improve the bonding of the coating by constructing a three-dimensional network structure, improve the bonding strength, and improve the heat resistance. However, although the three-dimensional structure helps to improve the heat resistance, it also improves the rigidity of the cured product. Therefore, under high temperature conditions, due to the presence of characteristics such as thermal shrinkage, excessive rigidity will cause the cured resin to be more easily peeled off from the surface of the inorganic particles, which may also cause the diaphragm to fail. Through the research of the inventor, it was found that the binder of the two-dimensional network structure has better elasticity than the three-dimensional structure, and through the combined use of units of a specific structure, it can not only maintain bonding strength and have better flexibility, but also improve the supporting effect of the diaphragm coating on the diaphragm at high temperatures, therefore, taking into account heat resistance and flexibility, and can avoid the problem of peeling of the resin and inorganic particles at high temperatures that existed in the past.

[0063] <First aspect>

[0064] A first aspect of the present invention provides a binder, wherein the binder comprises an ester polymer, and the ester polymer of the present invention is obtained by condensation polymerization of a polyol monomer and a polycarboxylic acid monomer.

[0065] Specifically, the ester polymer of the present invention includes a structure formed by the following units (I) to (III):

[0066]

[0067] in,

[0068] "-" represents a single bond, "*" represents a bonding position, and units (II) and (III) are connected via at least unit (I).

[0069] Furthermore, in unit (II), R 1 ~R 4 represents a hydrogen atom or a monovalent organic group. Further, the monovalent organic group is not particularly limited in principle, and in some preferred embodiments, it can be an alkyl group or a group containing an ether group. For the alkyl group, for example, it can be a C1-C5 straight chain or branched alkyl group; for the group containing an ether group, for example, it can be a group containing an alkylene ether structure containing ethylene ether, propylene ether, etc. The presence of the ether group is beneficial to improving the adhesion of the coating.

[0070] For unit (II), R 5 represents a saturated or unsaturated hydrocarbon group, which optionally has a substituent. In principle, there is no particular limitation on such a hydrocarbon group, and for example, it may be an alkyl group, a hydrocarbon group containing an alkenyl group or an alkynyl group, a hydrocarbon group containing a phenyl group, etc. In some preferred embodiments, the hydrocarbon group may be a C1 to C10 hydrocarbon group.

[0071] In the present invention, unit (II) controls the connection position on the para structure of the benzene ring, so that the ester polymer is connected to the unit (I) and the unit (II) to form a two-dimensional linear straight chain structure. The ester polymer having this structure has good elasticity, can improve the flexibility of the binder, enhance the supporting effect of the coating on the diaphragm at high temperature, improve the flexibility of the diaphragm, and also improve the heat resistance of the diaphragm.

[0072] Furthermore, there is no particular restriction on the sources of the above units (I) to (III):

[0073] In some specific embodiments, the unit (II) may be derived from a diol monomer, and correspondingly, the unit (III) may be derived from a dicarboxylic acid monomer;

[0074] In some other specific embodiments, the unit (II) may be derived from a dicarboxylic acid monomer, and correspondingly, the unit (III) may be derived from a diol monomer;

[0075] Furthermore, the above-mentioned units (I) to (III) may form a linear structure, and the linear structure may be formed by connecting the units (II) and (III) via the unit (I).

[0076] The ester polymer of the present invention at least partially has a linear structure due to the presence of the above units (I) to (III). The introduction of the linear structure can improve the flexibility of the final coating and also provide good heat resistance due to the presence of the phenyl structure of unit (II).

[0077] Furthermore, in some preferred embodiments of the present invention, the ester polymer of the present invention is substantially entirely composed of a linear structure formed by the above units (I) to (III).

[0078] Furthermore, the monomers used in forming the ester polymer of the present invention are not particularly limited as long as they can provide the above-mentioned units (I) to (III).

[0079] In some specific embodiments, the monomers of the present invention may include various polyol monomers and polycarboxylic acid monomers, and, as essential components, these monomers include diol monomers and dibasic acid carboxylic acid monomers to provide the above-mentioned units (I) to (III).

[0080] In some preferred embodiments of the present invention, from the perspective of easy availability and easy condensation polymerization, unit (II) can be provided by diol monomers. The diol monomers can be monomers having a structural formula as shown in formula (II-a):

[0081]

[0082] Among them, the R 1 ~R 4 are selected from hydrogen atoms, substituted or unsubstituted C1-C5 alkyl groups or ether-containing groups, preferably, R 1 ~R 4 It may be a hydrogen atom, or a methyl group, an ethyl group, a propyl group, a methoxy group, an ethoxy group, or the like.

[0083] In some specific embodiments, the diol monomers that can be specifically listed can be selected from one or more of the compounds shown in the following formulas: 3-[4-(hydroxymethyl)phenyl]propyl-2-en-1-ol (the compound shown in formula (II-1)), 3-[4-(hydroxymethyl)-2,5-dimethylphenyl]propyl-2-en-1-ol (the compound shown in formula (II-2)), and 3-[4-(hydroxymethyl)-2,3,5,6-dimethylphenyl]propyl-2-en-1-ol (the compound shown in formula (II-3)).

[0084]

[0085] Further, in some preferred embodiments of the present invention, the unit (III) can be provided by a dicarboxylic acid monomer, the structure of which is shown in formula (III-a):

[0086]

[0087] Among them, the R 5 represents a C1-C10 saturated or unsaturated alkylene group, preferably, R 5 The group is selected from C1-C10 saturated alkylene groups such as methylene, ethylene, propylene, isopropylene, butylene and hexylene.

[0088] The optional unsaturated hydrocarbon group can be a C3 to C10 unsaturated hydrocarbon group, specifically, it can be selected from a group containing a carbon-carbon double bond, a group containing a carbon-carbon triple bond, a benzene ring and other unsaturated hydrocarbon groups. In order to ensure the straight-chain structure of the ester polymer, preferably, when the unsaturated hydrocarbon group is a benzene ring, the carboxyl group is in the para position on the benzene ring.

[0089] In some specific embodiments, the dicarboxylic acid monomer may be selected from succinic acid, glutaric acid, adipic acid, and the like.

[0090] Furthermore, as the monomers used in forming the ester polymer of the present invention, in addition to the above-mentioned diol monomers and dicarboxylic acid monomers, other polyol monomers and polycarboxylic acid monomers may be used.

[0091] Furthermore, for the ester polymer of the present invention, when it is formed, the molar ratio of the polyol monomer to the polycarboxylic acid monomer used can generally be 0.95:1 to 1.05:1, and the molar numbers of the polyol monomer and the polycarboxylic acid monomer are calculated based on the molar numbers of hydroxyl groups or carboxylic acid groups contained therein, and can be controlled by the dosage ratio to obtain the desired molecular weight and the desired end group structure.

[0092] In some preferred embodiments of the present invention, the total molar content of the monomers providing the units (II) and (III) is 80 mol% or more, more preferably 90 mol% or more, and even more preferably 95 mol% or more, based on the total molar content of the monomers forming the ester polymer. In a further preferred embodiment, the ester polymer of the present invention is substantially obtained by condensation copolymerization of the monomers providing the units (II) and (III). Therefore, such a polyester polymer has at least a part or substantially all of a linear structure formed by the units (I) to (III).

[0093] In some specific embodiments of the present invention, the molecular weight of the ester polymer is 300,000 to 1.3 million Da, for example, 500,000 Da, 700,000 Da, 900,000 Da, 1.1 million Da, etc.

[0094] In some specific embodiments of the present invention, the glass transition temperature of the binder is above 120°C, preferably 120-210°C, for example, 140°C, 160°C, 180°C, 200°C, etc.

[0095] The present invention has no particular limitation on the preparation method of the ester polymer, and may include the following steps: mixing the polyol monomer and the polycarboxylic acid monomer (heating) and then subjecting them to condensation polymerization, and the polymerization may be optionally carried out in the presence of a catalyst or a solvent.

[0096] The present invention does not particularly limit the solvent, and it can be any organic solvent.

[0097] The polymerization time can be selected as needed. In some specific embodiments, the polymerization time is 8 to 12 hours, for example, 9 hours, 10 hours, 11 hours, etc.

[0098] The polymerization temperature can be selected as needed. In some specific embodiments, the polymerization temperature is 60-170°C, for example, 80°C, 100°C, 120°C, 140°C, 160°C, etc.

[0099] In addition, the binder of the present invention may optionally include other types of polymer binders in addition to the above-mentioned ester polymer. Preferably, the content of these other types of polymer binders may be less than 10% by mass of the total amount of the binder, preferably less than 5% by mass, and more preferably less than 3% by mass. Alternatively, the binder of the present invention is substantially formed by the above-mentioned ester polymer.

[0100] <Second Aspect>

[0101] A second aspect of the present invention provides a secondary battery separator, comprising a base film and a coating layer coated on at least one surface of the base film; wherein the coating layer comprises the binder described in the first aspect of the present invention.

[0102] Furthermore, the coating layer further comprises inorganic particles and optional auxiliary agents.

[0103] In addition, various optional functional additive components may be used in the coating as long as they do not hinder the realization of the technical effects of the present invention.

[0104] Inorganic particles

[0105] The inorganic particles described in the present invention are mainly provided as high temperature resistant materials in the coating. The use of inorganic particles can effectively improve the heat resistance of the diaphragm. In addition, the component can also play a role in improving the wettability and dimensional stability of the diaphragm.

[0106] The type of the inorganic particles can be selected as needed, and preferably various ceramic particles can be used, for example, at least one of boehmite, aluminum oxide, magnesium hydroxide, magnesium oxide, titanium dioxide, silicon dioxide, titanium oxide, barium titanate, zinc oxide, nickel oxide, magnesium fluoride, zirconium oxide, cerium oxide, and barium sulfate particles.

[0107] The particle size Dv50 of the inorganic particles or ceramic particles can be 0.5-2 μm. Preferably, the particle size Dv50 of the ceramic particles is 0.5-1.1 μm, for example, 0.6 μm, 0.8 μm, 1 μm, 1.2 μm, 1.5 μm, 1.8 μm, etc.

[0108] Additives

[0109] There is no particular limitation on other additives that can be used in the coating of the present invention. For example, the uniformity can be improved by adding a dispersant. Such a dispersant may include, for example, sodium dodecyl sulfate, phosphate ester, etc. In addition, the viscosity of the coating can be adjusted by using a thickener. The present invention does not make any particular limitation on the type of the thickener, which can be selected as needed. For example, it may include sodium carboxymethyl cellulose, etc.

[0110] Basement membrane

[0111] The base film of the present invention is provided as a basis for a separator.

[0112] There is no particular limitation on the type of the base film, and it may be a diaphragm base film commonly used in the art, for example, it may include at least one of a polyolefin base film, a polyimide base film, and a polyester base film.

[0113] In some preferred embodiments, the base film may be a porous film such as polyethylene, polypropylene, etc.

[0114] In some specific implementations, the base film has a thickness of 5 to 9 μm, for example, 6 μm, 7 μm, 8 μm, etc.

[0115] Coating and its composition

[0116] In the present invention, the thickness of the coating is 0.5 to 3 μm, for example, 0.8 μm, 1 μm, 1.5 μm, 2 μm, 2.5 μm, etc.

[0117] Regarding the composition of the coating of the present invention, in terms of improving heat resistance and electrical properties, the content of each component of the present invention can be arranged as follows:

[0118] The content of the binder is 3% to 8% of the total mass of the coating, for example, 4%, 5%, 6%, 7%, etc.

[0119] The content of the inorganic particles is 85% to 95% of the total mass of the coating, for example, 88%, 90%, 92%, 94%, etc.

[0120] The content of the auxiliary agent is 1-5% of the total mass of the coating, for example, it can be 2%, 3%, 4%, etc.

[0121] <Third Aspect>

[0122] The third aspect of the present invention provides a method for preparing the diaphragm according to the second aspect, comprising the following steps:

[0123] The step of preparing the binder comprises copolymerizing the diol monomer and the dicarboxylic acid monomer to obtain the binder;

[0124] a mixing step of mixing the binder, the inorganic particles and the optional auxiliary agent in a solvent to obtain a coating liquid;

[0125] The coating step is to coat the coating liquid on at least one surface of the base film and obtain the separator after drying.

[0126] The types and amounts of the polyol monomers, polycarboxylic acid monomers, inorganic particles, additives, and base film are the same as those in the first aspect and will not be described in detail herein.

[0127] In the step of preparing the binder, the copolymerization temperature is 50 to 70° C., preferably 60° C., and the copolymerization time can be 8 to 12 hours.

[0128] The present invention does not specifically limit the solvent, and the solvent may be selected as needed, for example, a commonly used organic solvent may be included.

[0129] The present invention does not specifically limit the drying temperature, and can be selected as needed, for example, it can be 30 to 50°C.

[0130] <Fourth Aspect>

[0131] The secondary battery described in the present invention includes various secondary batteries with ion conduction, and in particular, refers to lithium secondary batteries, including non-aqueous electrolyte lithium secondary batteries, semi-solid, quasi-solid lithium secondary batteries, etc.

[0132] The secondary battery of the present invention may be a power battery, that is, a battery used to provide power for transportation or vehicles, or a secondary battery used in energy storage equipment such as wind power, hydropower, solar power or traditional petrochemical energy power.

[0133] In some specific embodiments, the battery of the present invention appears and is used in a single form. In other specific embodiments, the battery of the present invention can be used in parallel or in series in any number of scales.

[0134] The lithium secondary battery of the present invention may include a positive electrode, a negative electrode, an electrolyte and the above-mentioned separator.

[0135] The positive electrode includes a current collector and a positive electrode active material and optionally used additives, etc. In principle, there is no particular limitation on the positive electrode active material, for example, it can be a lithium oxide doped with a transition metal, typically, it can be a ternary lithium positive electrode active material doped with nickel, manganese, and cobalt.

[0136] The negative electrode may include a current collector, a negative electrode active material, and optional auxiliary agents, etc. The negative electrode active material may include carbon-based materials and non-carbon-based materials.

[0137] The carbon-based materials include graphite materials (natural graphite, artificial graphite and intermediate carbon spheres) and other carbon-based materials (hard carbon, soft carbon and graphene); the non-carbon-based materials can be subdivided into titanium-based materials, silicon-based materials, tin-based materials, nitrides and metallic lithium, etc.

[0138] Furthermore, in principle, there is no particular limitation on the electrolyte solution that can be used in the secondary battery of the present invention.

[0139] In some specific embodiments, the electrolyte comprises an electrolyte and a non-aqueous solvent.

[0140] The present invention does not particularly limit the type of the non-aqueous solvent, as long as it is a non-aqueous solvent commonly used as a non-aqueous electrolyte solution.

[0141] In some specific embodiments, the non-aqueous solvent can be selected from one or more of cyclic carbonate solvents, linear carbonate solvents, ether solvents, ester solvents, and ketone solvents.

[0142] The cyclic carbonate solvent can be selected from ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate (BC); the linear carbonate solvent can be selected from diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), ethyl methyl carbonate (EMC), methyl propyl carbonate (MPC), methyl trifluoroethyl carbonate (FEMC), di(2,2,2-trifluoroethyl) carbonate (DFDEC), etc. The ester solvent can be selected from methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, methyl butyrate, ethyl butyrate, and methyl pivalate, etc.; the ether solvent can be selected from dibutyl ether, tetrahydrofuran, 2-methyltetrahydrofuran, 1,4-dioxane, 1,2-dimethoxyethane, ethylene glycol dimethyl ether (DME), ethylene glycol diethyl ether (DEE), dioxane (DX), dioxolane (DOL), etc.; the ketone solvent can be selected from polymethyl vinyl ketone, etc. These non-aqueous solvents can be used alone or in the form of a mixture of two or more.

[0143] In some preferred embodiments, the non-aqueous solvent can be selected from one or more of ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), ethyl acetate, propyl acetate, ethyl propionate, propyl ester, methyl butyrate, and ethyl butyrate.

[0144] The electrolyte of the present invention may generally be various lithium salts. The present invention does not specifically limit the type of lithium salt, and may be a lithium salt commonly used in the art. In some specific embodiments, the lithium salt may be selected from one or more salts formed by lithium ions and the following anions: PF 6 - , BF 4 - , Cl - Br - ,I - , ClO 4 - , AsF 6 - 、SiF 6 2- 、AlCl 4 - , B(C 2 O 4 ) 2 - , CH 3 CO 2 - CF 3 SO 3 - 、N(CF 3 SO2 ) 2 - 、N(FSO 2 ) 2 - , C(CF 2 SO 2 ) 3 - , C 2 BF 2 O 4 - .

[0145] In principle, there is no particular limitation on other functional additives that can be used in the electrolyte of the present invention. For example, the use of some additives can promote film formation.

[0146] Examples of such additives include ethylene carbonate (VC), lithium difluorophosphate (LiPO 2 F 2 ), fluoroethylene carbonate (FEC), boron-containing additives, sulfur-containing additives or oxalate-containing additives, etc. Among them, the boron-containing additive can be selected from lithium tetrafluoroborate (LiBF 4 ), trimethylsilyl phosphate (TMSP), trimethylsilyl borate (TMSB), etc.; the sulfur-containing additive can be selected from 1,3-propane sultone (1,3PS), 1,4-butane sultone (1,4-BS), 2,4-butane sultone (2,4-BS), 1,3-propylene sultone (PST), vinyl sulfate (DTD), methylene disulfonate (MMDS), vinyl sulfite (ES), etc.; the oxalate-containing additive can be selected from lithium difluorooxalatoborate (LiDFOB), lithium bisoxalatoborate (LiBOB), lithium tetrafluorooxalatophosphate (LiTFOP), lithium difluorobisoxalatophosphate (LiDFOP), etc. These additives can be used alone or in the form of a mixture of two or more.

[0147] Example

[0148] The embodiments of the present invention will be described in detail below in conjunction with the examples, but those skilled in the art will appreciate that the following examples are only used to illustrate the present invention and should not be considered to limit the scope of the present invention. If no specific conditions are specified in the examples, they are carried out according to normal conditions or the conditions recommended by the manufacturer. If the manufacturer is not specified for the reagents or instruments used, they are all conventional products that can be obtained commercially.

[0149] Example 1

[0150] (1) Synthesis method of the binder: 3-[4-(Hydroxymethyl)phenyl]propyl-2-en-1-ol and succinic acid were added to a beaker at a molar ratio of 1:1, respectively, and stirred at 60°C for 10 hours to allow the alcohol and the carboxylic acid to undergo an esterification reaction, and the novel binder was obtained by purification.

[0151] (2) Preparation method of the diaphragm: 90% by mass of aluminum oxide, 0.5% of sodium dodecyl sulfate (dispersant), 2% of sodium carboxymethyl cellulose (thickener), 7% of a novel binder and 0.5% of a phosphate ester are uniformly dispersed in water to obtain a uniform ceramic coating liquid, which is then coated on a polyethylene film and dried at 40° C. to form a composite diaphragm. The coating has a thickness of 2 μm.

[0152] Example 2

[0153] (1) Synthesis method of binder: 3-[4-(hydroxymethyl)-2,5-dimethylphenyl]propyl-2-en-1-ol and glutaric acid were added into a beaker at a molar ratio of 1:1, respectively, and stirred at 60°C for 10 hours to allow the alcohol and the carboxylic acid to undergo an esterification reaction, and a novel binder was obtained by purification.

[0154] (2) Preparation method of the diaphragm: 89% by mass of aluminum oxide, 0.5% of sodium dodecyl sulfate (dispersant), 2.5% of sodium carboxymethyl cellulose (thickener), 7.5% of a novel binder and 0.5% of a phosphate ester are uniformly dispersed in water to obtain a uniform ceramic coating liquid, which is then coated on a polypropylene base film and dried at 40° C. to form a composite diaphragm. The coating thickness is 1 μm.

[0155] Example 3

[0156] (1) Synthesis method of the binder: 3-[4-(hydroxymethyl)-2,3,5,6-dimethylphenyl]propyl-2-en-1-ol and adipic acid were added into a beaker at a molar ratio of 1:1, respectively, and stirred at 60° C. for 10 hours to allow the alcohol and the carboxylic acid to undergo an esterification reaction, and a novel binder was obtained by purification.

[0157] (2) Preparation method of the diaphragm: 89% by mass of aluminum oxide, 0.5% of sodium dodecyl sulfate (dispersant), 3% of sodium carboxymethyl cellulose (thickener), 7% of a novel binder and 0.5% of a phosphate ester are uniformly dispersed in water to obtain a uniform ceramic coating liquid, which is then coated on a non-woven fabric base film and dried at 40° C. to form a composite diaphragm. The coating thickness is 3 μm.

[0158] Example 4

[0159] (1) Synthesis method of the binder: 3-[4-(Hydroxymethyl)phenyl]propyl-2-en-1-ol and succinic acid were added to a beaker at a molar ratio of 1:1, respectively, and stirred at 60°C for 10 hours to allow the alcohol and the carboxylic acid to undergo an esterification reaction, and the novel binder was obtained by purification.

[0160] (2) Preparation method of the diaphragm: 89% by mass of aluminum oxide, 1% of sodium dodecyl sulfate (dispersant), 2% of sodium carboxymethyl cellulose (thickener), 7% of a novel binder and 0.1% of a phosphate ester are uniformly dispersed in water to obtain a uniform ceramic coating liquid, which is then coated on a polyimide-based film and dried at 40° C. to form a composite diaphragm. The coating thickness is 2 μm.

[0161] Comparative Example 1

[0162] 90% by mass of aluminum oxide, 0.5% of sodium dodecyl sulfate (dispersant), 2% of sodium carboxymethyl cellulose (thickener), 7% of polyacrylate and 0.5% of phosphate ester were uniformly dispersed in water to obtain a uniform ceramic coating liquid, which was then coated on a polyethylene film and dried at 40°C to form a composite diaphragm. The coating thickness was 2 μm.

[0163] Comparative Example 2

[0164] A uniform ceramic coating liquid is obtained by uniformly dispersing 89% of aluminum oxide, 0.5% of sodium dodecyl sulfate (dispersant), 2.5% of sodium carboxymethyl cellulose (thickener), 7.5% of polyacrylate and 0.5% of phosphate in water, and then coated on a polypropylene base film and dried at 40°C to form a composite diaphragm. The coating thickness is 1 μm.

[0165] Comparative Example 3

[0166] A uniform ceramic coating liquid is obtained by uniformly dispersing 89% of aluminum oxide, 0.5% of sodium dodecyl sulfate (dispersant), 3% of sodium carboxymethyl cellulose (thickener), 7% of polyacrylic acid and 0.5% of phosphate in water, and then coated on a non-woven fabric base film and dried at 40°C to form a composite diaphragm. The coating thickness is 3 μm.

[0167] Comparative Example 4

[0168] 89% of aluminum oxide, 0.5% of sodium dodecyl sulfate (dispersant), 3% of sodium carboxymethyl cellulose (thickener), 7% of 1,4-styrene butadiene rubber and 0.5% of phosphate ester were uniformly dispersed in water to obtain a uniform ceramic coating liquid, which was then coated on a non-woven fabric base film and dried at 40°C to form a composite diaphragm. The coating thickness was 3 μm.

[0169] Comparative Example 5

[0170] (1) Synthesis method of the binder: Dimethylenediol and thiophene-2,5-dicarboxylic acid are added into a beaker at a molar ratio of 1:1, respectively, and stirred at 60° C. for 10 hours to allow the alcohol and the carboxylic acid to undergo an esterification reaction, and the binder is obtained by purification.

[0171] (2) Preparation method of the diaphragm: 89% by mass of aluminum oxide, 1% of sodium dodecyl sulfate (dispersant), 2% of sodium carboxymethyl cellulose (thickener), 7% of a binder and 0.1% of a phosphate ester are uniformly dispersed in water to obtain a uniform ceramic coating liquid, which is then coated on a polyimide-based film and dried at 40° C. to form a composite diaphragm. The coating has a thickness of 2 μm.

[0172] Performance Testing

[0173] 1. Diaphragm heat shrinkage test: Cut the diaphragm into 10×10 pieces, cover with A4 paper, place in an oven, heat at 180℃ for 1h, and measure the heat shrinkage of the diaphragm. The results are shown in Table 1.

[0174] 2. Diaphragm rupture temperature test: Thermomechanical analyzer method is used. The specific steps are as follows: Take a 20-30mm diaphragm sample and use a thermomechanical analyzer to record the diaphragm length versus temperature curve until the diaphragm breaks. Find the temperature point where the diaphragm length increases instantly, which is the rupture temperature. The results are shown in Table 1.

[0175] 3. Wettability: The wettability is characterized by the contact angle. The specific test method is as follows: stick the diaphragm on a glass slide, add 10 μL 1 mol / L LiTFSI 1wt% LiNO 3 DOL DME electrolyte, the contact angle is measured by contact angle meter. The results are shown in Table 1.

[0176] Table 1 Diaphragm formulations and performance test results of embodiments and comparative examples

[0177]

[0178]

[0179] From the comparison of Examples 1 to 4 in Table 1, it can be seen that when conventional adhesives are used in Comparative Examples 1 to 4, the heat resistance is poor, it is difficult to maintain good stability at 180°C, and it is difficult to obtain a diaphragm having both excellent heat resistance and wettability. The diaphragms prepared in Examples 1 to 4 have a low thermal shrinkage rate and a film rupture temperature of 183 to 210°C, have excellent heat resistance, and a small contact angle of only less than 17°, so they have good wettability.

[0180] From the comparison between Comparative Example 5 and Comparative Examples 1 to 4, it can be seen that although the thermal shrinkage rate of the binder formed by forming a three-dimensional network structure in Comparative Example 5 is lower, the film rupture temperature is not significantly improved, and the contact angle reaches 28°. Although the heat resistance and wettability are improved compared with the diaphragms prepared with traditional binders (Comparative Examples 1 to 4), the improvement effect is very limited compared with Example 1. The diaphragm prepared by the present invention (Example 1) has a significant improvement effect on heat resistance and wettability.

[0181] It should be noted that, although the technical solution of the present invention is introduced with specific examples, those skilled in the art will appreciate that the present invention should not be limited thereto.

[0182] The embodiments of the present invention have been described above, and the above description is exemplary, not exhaustive, and is not limited to the disclosed embodiments. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the described embodiments. The selection of terms used herein is intended to best explain the principles of the embodiments, practical applications, or technical improvements in the market, or to enable other persons of ordinary skill in the art to understand the embodiments disclosed herein.

Claims

1. A binder, characterized in that: The binder comprises an ester polymer, which is obtained by copolymerizing a polyol monomer and a polycarboxylic acid monomer, wherein the ester polymer comprises a structure formed by the following units (I) to (III): in, "-" indicates a single bond, "*" indicates the position of the bond, and units (II) and (III) are connected at least through unit (I). R1 to R4 represent a hydrogen atom or a monovalent organic group, R5 represents a saturated or unsaturated hydrocarbon group, which may have a substituent.

2. The adhesive according to claim 1, characterized in that The unit (II) is derived from a diol monomer or a dibasic acid monomer; and the unit (III) is derived from a dicarboxylic acid monomer or a diol monomer.

3. The adhesive according to claim 1 or 2, characterized in that: The molar ratio of the polyol monomer to the polycarboxylic acid monomer is 0.95:1 to 1.05:1, and the molar numbers of the polyol monomer and the polycarboxylic acid monomer are calculated based on the molar numbers of hydroxyl groups or carboxylic acid groups contained therein.

4. The adhesive according to any one of claims 1 to 3, characterized in that The ester polymer at least partially has a linear structure formed by units (I) to (III); Preferably, the ester polymer is composed of a linear structure formed by units (I) to (III).

5. The adhesive according to any one of claims 1 to 4, characterized in that: The unit (II) is derived from a diol monomer as shown in formula (II-a): The unit (III) is derived from a dicarboxylic acid monomer as shown in formula (III-a): in, R1 to R4 are selected from hydrogen atoms, substituted or unsubstituted C1 to C5 alkyl groups or ether-containing groups, The R5 represents a C1-C10 saturated or unsaturated alkylene group.

6. A secondary battery separator, characterized in that: The diaphragm includes a base film and a coating coated on at least one side of the base film; Wherein, the coating comprises the binder according to any one of claims 1 to 5.

7. The diaphragm according to claim 6, characterized in that The coating also includes inorganic particles and optional additives; Preferably, calculated by weight, the binder in the coating accounts for 3-8%; the inorganic particles account for 85-95%; and the auxiliary agent accounts for 1-5%; Preferably, the inorganic particles include ceramic particles, and the particle size Dv50 of the ceramic particles is 0.5 to 2 μm; More preferably, the particle size Dv50 of the ceramic particles is 0.5 to 1.1 μm; Preferably, the ceramic particles include at least one of boehmite, aluminum oxide, magnesium hydroxide, magnesium oxide, titanium dioxide, silicon dioxide, titanium oxide, barium titanate, zinc oxide, nickel oxide, magnesium fluoride, zirconium oxide, cerium oxide, and barium sulfate particles.

8. The diaphragm according to claim 6 or 7, characterized in that: The base film has a thickness of 5 to 9 μm, and the coating has a thickness of 0.5 to 3 μm; and / or, The base film includes at least one of a polyolefin base film, a polyimide base film, and a polyester base film.

9. A method for preparing a diaphragm according to any one of claims 6 to 9, characterized in that: The following steps are involved: The step of preparing the binder comprises copolymerizing the polyol monomer and the polycarboxylic acid monomer to obtain the binder; A mixing step of mixing the binder, inorganic particles and optional auxiliary agents in a solvent to obtain a coating liquid; In the coating step, the coating liquid is coated on at least one surface of the base film, and the separator is obtained after drying.

10. A secondary battery, characterized in that: Comprising the diaphragm according to any one of claims 6 to 9, the secondary battery comprises a power battery or an energy storage battery for an energy storage power station.

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