Composite separator, preparation method therefor, and use thereof

Abstract

A composite separator, a preparation method therefor, and a use thereof. The composite separator comprises a nanofiber membrane and a hydrophobic polymer resin layer, the nanofiber membrane comprising a nanofiber layer, and the hydrophobic polymer resin layer being modified on at least one surface of the nanofiber layer. The ratio of the average volumetric pore size of the hydrophobic polymer resin layer to the average volumetric pore size of the nanofiber membrane is 1.3-1.75. By means of controlling the average volumetric pore size of the hydrophobic polymer resin layer to be greater than the average volumetric pore size of the nanofiber membrane, and controlling the pore size ratio of the two to be within a specific range, the hydrophobic polymer resin layer can not only facilitate the discharge of moisture from the nanofiber membrane in a drying stage during preparation, but also effectively prevent the nanofiber membrane from absorbing moisture in the air, thereby allowing the moisture content of the composite separator to be relatively low. In addition, the pore size ratio of the two being within a specific range also helps prevent lithium ion conduction from being affected by pore blockage of the nanofiber membrane.

Description

Composite diaphragm and preparation method and application thereof TECHNICAL FIELD

[0001] The present application relates to the technical field of lithium ion batteries, for example, a composite diaphragm and preparation method and application thereof. BACKGROUND

[0002] In recent years, new energy electric vehicle fires and explosions have occurred frequently, causing people to pay close attention to and question the safety of power lithium ion batteries. One of the most core reasons is that the performance of the existing lithium ion battery diaphragm cannot meet the application requirements of high specific energy batteries. Power lithium ion batteries require higher safety performance, greater capacity, uniform performance for long-time stable output, and large-rate charging and discharging performance.

[0003] In order to improve the safety of lithium ion batteries and meet market demand, a new generation of nanofiber composite diaphragm has appeared, which has the characteristics of ultra-thin and ultra-heat-resistant and can meet the safety requirements of the battery. However, the biggest disadvantage of the nanofiber composite diaphragm is that its moisture content is too high, which cannot meet the low moisture content requirements of many customers, which limits its application range. As we all know, a higher moisture content in the battery will affect the performance of the battery capacity and the cycle life of the battery. As one of the main components of the battery, the control of the moisture content of the diaphragm is naturally the most important.

[0004] Therefore, in the art, it is desirable to develop a composite diaphragm with low moisture content.

[0005] SUMMARY

[0006] The following is a summary of the subject matter described in detail in this document. This summary is not intended to limit the scope of protection of the claims.

[0007] The present application provides a composite diaphragm and a preparation method and application thereof. The composite diaphragm provided by the present application has excellent bonding performance, and at the same time, the moisture content of the nanofiber membrane can be reduced by more than 20%, meeting the customer's control requirements for diaphragm moisture.

[0008] In a first aspect, the present application provides a composite diaphragm, which comprises a nanofiber membrane and a hydrophobic polymer resin layer, the nanofiber membrane comprises a nanofiber layer, and the hydrophobic polymer resin layer is modified to at least one surface of the nanofiber layer.

[0009] The ratio of the average volume pore size of the hydrophobic polymer resin layer to the average volume pore size of the nanofiber membrane is 1.3-1.75.

[0010] In a second aspect, the present application provides a preparation method of the composite diaphragm according to the first aspect, which comprises the following steps:

[0011] (1) mixing raw materials for preparing the hydrophobic polymer resin layer to obtain a slurry;

[0012] (2) coating the slurry obtained in step (1) on the surface of the nanofiber layer, curing, eluting the solvent, drying to obtain the composite separator.

[0013] In a third aspect, the application provides a composite separator as described in the first aspect for use in a battery.

[0014] Compared with the related art, the application has at least the following beneficial effects:

[0015] (1) In the application, by controlling the average volume pore size of the hydrophobic polymer resin layer to be greater than the average volume pore size of the nanofiber membrane and controlling the pore size ratio of the two within a specific range, the hydrophobic polymer resin layer can not only facilitate the discharge of water in the nanofiber membrane during the drying stage of the preparation process, but also effectively hinder the nanofiber membrane from absorbing water in the air, so that the moisture content of the composite separator is low. In addition, the pore size ratio of the two within a specific range also helps to avoid the blockage of the pore size of the nanofiber membrane and affect the lithium ion conduction.

[0016] (2) The composite separator provided by the application has excellent adhesion and low moisture content, excellent heat resistance, greatly improves the safety performance, high-temperature cycle stability, and needle puncture resistance of lithium ion batteries, and is suitable for 3C, EV, energy storage, and other application fields.

[0017] Other aspects can be apparent after reading and understanding the drawings and detailed description. BRIEF DESCRIPTION OF DRAWINGS

[0018] The accompanying drawings are used to provide a further understanding of the technical solutions of the present application, and constitute a part of the specification, and are used together with the embodiments of the present application to explain the technical solutions of the present application, and do not constitute a limitation on the technical solutions of the present application.

[0019] FIG. 1 is an SEM image of the composite separator provided in Example 5 (magnification is 50K). DETAILED DESCRIPTION

[0020] In order for those skilled in the art to better understand the scheme of the present application, the present application is further described in detail as follows.

[0021] As mentioned above, in order to improve the safety of lithium ion batteries and meet market demand, a new generation of nanofiber composite separator has appeared, which has the characteristics of ultra-thin and ultra-heat resistance, and improves the battery end pin test pass rate, and meets the safety requirements of the battery. However, the biggest disadvantage of the nanofiber composite separator is that its moisture content is high. As we all know, the moisture content in the battery will affect the performance of the battery capacity and the cycle life of the battery, so it cannot meet the low moisture content requirements of many customers. Even if drying treatment is performed, it is difficult to significantly reduce the moisture content, which is mainly related to the structure and material properties of nanofiber, thereby limiting its application range.

[0022] Therefore, as one of the main components of the battery, the moisture content of the separator needs to be controlled.

[0023] The embodiment of the present application provides a composite separator, which comprises a nanofiber membrane and a hydrophobic polymer resin layer, the nanofiber membrane comprises a nanofiber layer, and the hydrophobic polymer resin layer is modified to at least one surface of the nanofiber membrane.

[0024] The ratio of the average volume pore size of the hydrophobic polymer resin layer to the average volume pore size of the nanofiber membrane is 1.3-1.75.

[0025] For example, the ratio of the average volume pore size of the hydrophobic polymer resin layer to the average volume pore size of the nanofiber membrane can be 1.3, 1.32, 1.34, 1.36, 1.38, 1.4, 1.42, 1.44, 1.46, 1.48, 1.5, 1.52, 1.54, 1.56, 1.58, 1.6, 1.62, 1.64, 1.66, 1.68, 1.7, 1.72, 1.74, 1.75 or a range consisting of any two of them.

[0026] According to the research and analysis of the applicant, by controlling the average volume pore size of the hydrophobic polymer resin layer to be greater than the average volume pore size of the nanofiber membrane, and controlling the volume pore size ratio of the two within a certain range, the hydrophobic polymer resin layer can not only facilitate the water in the nanofiber membrane to be discharged in the drying stage of the preparation process, but also effectively prevent the nanofiber membrane from absorbing water in the air, so that the moisture content of the composite separator is low. In addition, the pore size ratio of the two within a certain range also helps to avoid the pore size blockage of the nanofiber layer affecting the lithium ion conduction.

[0027] It is to be noted that the "surface" refers to two surfaces of a film or layer that are largest in area and oppositely arranged, and the "at least one surface" refers to at least one of the two surfaces. The "volume pore size" refers to the pore size of the layer or film in volume, and the "surface pore size" refers to the pore size of the layer or film on its outer surface (i.e., the surface on the side away from the nanofiber membrane in the thickness direction).

[0028] In some embodiments, the hydrophobic polymer resin layer has a three-dimensional network structure, and the three-dimensional network structure increases the tortuosity of the pore channels of the hydrophobic polymer resin layer, further increasing the difficulty of water absorption, and hindering the nanofiber membrane from absorbing water in the air.

[0029] In some embodiments, the surface minimum pore size (D min ) of the hydrophobic polymer resin layer is 20-30 nm, the surface average pore size (D avg ) is 40-50 nm, and the surface maximum pore size (D max ) is 60-70 nm.

[0030] The surface minimum pore size (D min ) can be 20 nm, 21 nm, 22 nm, 23 nm, 24 nm, 25 nm, 26 nm, 27 nm, 28 nm, 29 nm, 30 nm, or a range formed by any two of them, the surface average pore size (D avg ) can be 40 nm, 41 nm, 42 nm, 43 nm, 44 nm, 45 nm, 46 nm, 47 nm, 48 nm, 49 nm, 50 nm, or a range formed by any two of them, and the surface maximum pore size (D max ) can be 60 nm, 61 nm, 62 nm, 63 nm, 64 nm, 65 nm, 66 nm, 67 nm, 68 nm, 69 nm, 70 nm, or a range formed by any two of them.

[0031] The surface pore size of the hydrophobic polymer resin layer is within the above range, which can make the pore size of the hydrophobic polymer resin layer exhibit a non-uniform structure feature, thereby balancing the improvement of the water discharge efficiency in the nanofiber membrane and the hindering of the nanofiber membrane from absorbing water in the air, thereby reducing the overall moisture of the composite separator while effectively improving the lithium ion transmission efficiency.

[0032] In some embodiments, the thickness of the hydrophobic polymer resin layer is 0.1-0.4 μm, for example, 0.1 μm, 0.12 μm, 0.14 μm, 0.16 μm, 0.18 μm, 0.2 μm, 0.22 μm, 0.24 μm, 0.26 μm, 0.28 μm, 0.3 μm, 0.32 μm, 0.34 μm, 0.36 μm, 0.38 μm, 0.4 μm, or a range defined by any two of them.

[0033] By controlling the thickness of the hydrophobic polymer resin layer, and only with ultra-thin thickness, the battery cycle performance can be effectively improved while ensuring the air moisture absorption is hindered. It can be understood that the air permeability of the composite separator after modification of the hydrophobic polymer resin layer is 30 sec / 100 ml or less relative to the nanofiber membrane.

[0034] The type of hydrophobic polymer resin in the hydrophobic polymer resin layer is not particularly limited in the present application, and the polymer resin containing a hydrophobic group (a group having no affinity for water, insoluble in water, or very small water solubility) in the related art can be selected according to the needs.

[0035] In some embodiments, the hydrophobic polymer resin in the hydrophobic polymer resin layer contains any one of fluorine (-F), siloxane group, phenyl group, long-chain alkyl group (alkyl group with 6 or more main chain carbon atoms), cyano group, or a combination of at least two of them in the molecular chain of the hydrophobic polymer resin;

[0036] In some embodiments, the hydrophobic polymer resin includes any one of polyvinylidene fluoride homopolymer, vinylidene fluoride-hexafluoropropylene copolymer, siloxane-modified polyimide, long-chain alkyl group (alkyl group with 6 or more main chain carbon atoms)-modified polyimide, fluoromethyl-modified polyimide, cyano-modified polyimide, or a combination of at least two of them.

[0037] The hydrophobic polymer resin layer can form a combined coating on the nanofiber layer, effectively preventing the nanofiber layer from absorbing moisture in the air, and providing a sufficient hydrophobic layer on the surface of the nanofiber layer to ensure that the final moisture content is low, and will not have any impact on the heat resistance and electrolyte wettability of the nanofiber layer. In addition, the hydrophobic polymer resin layer and the nanofiber layer can form an interface layer that allows one-way diffusion of moisture, which blocks moisture and cooperatively reduces moisture.

[0038] In the embodiments of the present application, the nanofiber membrane can be directly purchased through commercial channels or prepared by referring to the related art, unless otherwise specified.

[0039] In some embodiments, the nanofiber membrane has an average volume pore size of 40-50 nm, for example, 40 nm, 41 nm, 42 nm, 43 nm, 44 nm, 45 nm, 46 nm, 47 nm, 48 nm, 49 nm, 50 nm, or a range defined by any two of them. In this way, the water discharge efficiency in the nanofiber membrane can be improved, thereby reducing the overall moisture of the composite separator while effectively improving the lithium ion transmission efficiency.

[0040] Although the thickness of the nanofiber membrane is not particularly limited, the nanofiber membrane can optionally have a thickness of between 1 μm and 50 μm, and further optionally between 3 μm and 20 μm. If the substrate has a thickness of less than 1 μm, it is difficult to maintain mechanical properties. If the substrate has a thickness of greater than 50 μm, it severely affects the electrochemical properties.

[0041] In some embodiments, the nanofiber membrane includes a base film and a nanofiber layer disposed on at least one surface of the base film.

[0042] Specifically, the nanofiber layer includes one or more of organic nanofibers and inorganic nanofibers, wherein the organic nanofibers can include at least one of cellulose nanofibers, aramid nanofibers, polypropylene nanofibers, etc., and the inorganic nanofibers can include at least one of carbon nanotubes, nanosilver wires, boron nitride nanowires, copper hydroxide nanowires, hydroxyapatite nanowires, attapulgite, etc. It can be understood that nanofibers refer to linear materials with a certain length-diameter ratio (length-diameter ratio ≥ 10) with a nanometer scale diameter (diameter less than 1000 nm) and a larger length.

[0043] Specifically, the base film can be a porous film commonly used in the art, and exemplarily, the base film can be a polymer porous film, or a polymer porous film provided with a coating layer. The polymer porous film can be a polyethylene porous film (PE), a polypropylene porous film (PP), a polymethacrylate porous film (PMMA), a polybutylene (PB), or a multilayer composite separator thereof. The multilayer composite separator refers to a multilayer composite porous film formed by stacking the above-mentioned polymer porous films in any order, for example, a PP-PE-PP three-layer composite separator, a PP-PE double-layer composite separator, a PP-PP-PE-PP four-layer composite separator, a PE-PP-PB three-layer composite film, a PP-PMMA-PP three-layer composite film, etc.

[0044] The polymeric porous membrane provided with a coating layer refers to a polymeric porous membrane provided with a coating layer on at least one surface. The coating layer is not particularly limited and can be selected according to requirements and is commonly used in the art, for example, can be inorganic particles including alumina, boehmite, magnesium oxide, magnesium hydroxide, etc., or a coating layer including at least one of organic polymers such as polyimide, PMMA, PVDF, etc. For example, the polyolefin porous membrane provided with a coating layer can be an alumina-coated polyolefin porous membrane, a boehmite-coated polyolefin porous membrane, a polyimide-coated polyolefin porous membrane, a PMMA-coated polyolefin porous membrane, a PVDF-coated polyolefin porous membrane; the polyolefin porous membrane provided with a coating layer can also be a polyolefin porous membrane coated with a mixture of polyimide and alumina, a polyolefin porous membrane coated with a mixture of PVDF and alumina, a porous membrane coated with a mixture of PMMA and alumina, etc.

[0045] In the embodiments of the present application, the modification method of the hydrophobic polymer resin layer is not particularly limited and can be achieved by the compounding method in the related art, for example, the slurry coating method, the film layer hot pressing compounding method, etc. It can be understood that the composite separator can be obtained by coating the slurry containing the hydrophobic polymer resin on one side of the nanofiber layer of the commercially available nanofiber membrane; or the modification of the hydrophobic polymer resin layer can be performed during the preparation of the nanofiber membrane, for example, the nanofiber slurry is coated on both surfaces of the base film, the nanofiber membrane is obtained after curing, and then the slurry containing the hydrophobic polymer resin is coated on the side of the nanofiber layer to obtain the composite separator, or for example, the slurry containing the hydrophobic polymer resin is first coated on at least one surface of the base film to form a hydrophobic polymer resin modification layer, and then the nanofiber slurry is coated on at least one surface of the hydrophobic polymer resin modification layer and the base film to obtain a nanofiber coating layer, and finally the hydrophobic polymer resin layer is hot compounded on the surface of the nanofiber coating layer, so as to obtain the composite separator in which the hydrophobic polymer resin layer is modified on both surfaces of the nanofiber layer. The nanofiber slurry can be the slurry in the related art or commercially available slurry.

[0046] In some embodiments, the hydrophobic polymer resin layer is modified on the surface of the side of the nanofiber layer away from the base film.

[0047] In some embodiments, the hydrophobic polymer resin layer can be prepared from the following raw materials.

[0048] The raw materials for preparing the hydrophobic polymer resin layer include the following components in terms of weight percentage:

[0049] hydrophobic polymer resin 2% to 10%;

[0050] a pore-expanding agent 0.05% to 3%;

[0051] The remainder is a solvent.

[0052] As an embodiment of the present application, by limiting the raw material for preparing the hydrophobic polymer resin layer, in the raw material for preparing the hydrophobic polymer resin layer, the hydrophobic polymer resin can reduce the water content at a lower concentration (2% to 10%), without affecting the air permeability, heat resistance, and electrolyte wettability of the nanofiber layer, and can provide certain adhesion.

[0053] In some embodiments, the amount of the hydrophobic polymer resin in the raw material for preparing the hydrophobic polymer resin layer can be 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, or a range consisting of any two of them, in terms of weight percentage.

[0054] In some embodiments, the amount of the pore-expanding agent in the raw material for preparing the hydrophobic polymer resin layer can be 0.05%, 0.08%, 0.1%, 0.3%, 0.5%, 0.8%, 1%, 1.2%, 1.4%, 1.6%, 1.8%, 2%, 2.2%, 2.4%, 2.6%, 2.8%, 3%, or a range consisting of any two of them, in terms of weight percentage.

[0055] Controlling the amount of the pore-expanding agent in this range can better control the pore size of the hydrophobic polymer resin layer and make the pore structure distribution more uniform, thereby achieving water discharge and blocking water absorption in the nanofiber membrane, reducing the overall moisture of the composite separator, and effectively improving the lithium ion transmission efficiency.

[0056] In some embodiments, the amount of the solvent in the raw material for preparing the hydrophobic polymer resin layer can be 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, or a range consisting of any two of them, in terms of weight percentage.

[0057] In some embodiments, the hydrophobic polymer resin contains any one or a combination of at least two of strong hydrophobic groups such as a fluorine group (-F), a siloxane group, a phenyl group, a long-chain alkyl group (an alkyl group with 6 or more carbon atoms in the main chain), a cyano group, etc. in the molecular chain.

[0058] In some embodiments, the hydrophobic polymer resin comprises any one of or a combination of at least two of polyvinylidene fluoride (PVDF) homopolymer, polyvinylidene fluoride-hexafluoropropylene copolymer [P(VDF-HFP)], siloxane-modified polyimide, long-chain alkyl (alkyl with 6 or more main chain carbon atoms)-modified polyimide, fluoromethyl-modified polyimide, cyano-modified polyimide.

[0059] In some embodiments, the preparation raw material of the hydrophobic polymer resin layer further comprises a wetting agent, and the wetting agent accounts for 0.3% to 1% in terms of percentage by weight, for example, can be 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1% or a range consisting of any two of them. The wetting agent can improve the uniformity of the distribution of the hydrophobic polymer resin layer on the surface of the nanofiber membrane, better cover the surface of the nanofiber membrane, and block the moisture absorption in the nanofiber membrane.

[0060] In the embodiments of the present application, the types of the reaming agent and the wetting agent are not particularly limited, and the commonly used reaming agents and wetting agents in the related art can be selected according to the needs.

[0061] It can be understood that the reaming agent is a kind of liquid reagent which has poor compatibility with the hydrophobic polymer resin and good compatibility with the solvent.

[0062] In some embodiments, the reaming agent comprises any one of or a combination of at least two of water, polyethylene glycol (PEG), ethanol and the like.

[0063] In some embodiments, the wetting agent is an oily wetting agent. It can be understood that the oily wetting agent refers to a kind of wetting agent which is compatible with organic solvents.

[0064] In some embodiments, the wetting agent comprises any one of or a combination of at least two of polyether siloxane copolymer, silicone twin structure copolymer, polyacrylate copolymer, polyether-modified silicone oil copolymer or polyoxyethylene alkylamine copolymer.

[0065] In the embodiments of the present application, the reaming agent and the wetting agent can be directly purchased through commercial channels unless otherwise specified.

[0066] The wetting agent described in the present application can provide excellent dynamic wetting effect in the process of preparing the composite separator, can quickly wet the nanofiber membrane substrate, strengthen the effective coating of the hydrophobic polymer resin layer on the surface of the nanofiber membrane, and strengthen the adhesion effect of the resin layer and the nanofiber membrane during the drying process, so as to make the hydrophobic end of the hydrophobic polymer resin layer face outward, hinder the moisture from entering the nanofiber membrane, and thus reduce the overall moisture of the composite separator.

[0067] In some embodiments, the solvent comprises any one or a combination of at least two of acetone, N-methylpyrrolidone, N,N-dimethylacetamide (DMAC).

[0068] In some embodiments, the present application provides a preparation method of the composite separator, comprising the following steps:

[0069] (1) mixing a preparation raw material of the hydrophobic polymer resin layer to obtain a slurry;

[0070] (2) coating the slurry obtained in step (1) on the surface of the nanofiber layer, curing, eluting the solvent, drying to obtain the composite separator.

[0071] In the embodiments of the present application, the coating method can be carried out by using existing coating methods, such as dip coating, spray coating, roller printing, micro-gravure coating, slot coating, and inclined plate coating.

[0072] In some embodiments, the coating method in step (2) comprises spray coating.

[0073] In some embodiments, the coating method in step (2) comprises spray coating by using ultrasonic atomization method.

[0074] In some embodiments, the present application provides an application of the composite separator in a battery.

[0075] In some embodiments, the battery comprises a lithium ion battery.

[0076] The technical solutions of the present application are further illustrated by specific embodiments. Those skilled in the art should understand that the embodiments are only used to help understand the present application and should not be regarded as specific limitations of the present application.

[0077] Unless otherwise specified, the information of the nanofiber membrane used in the embodiments and comparative examples of the present application is as follows:

[0078] Nanofiber membrane: model: SU810J99L, purchased from Shenzhen Xingyuan Material Technology Co., Ltd.

[0079] Nanofiber membrane: model: SU306J55C, purchased from Shenzhen Xingyuan Material Technology Co., Ltd.

[0080] Example 1

[0081] In the present embodiment, a composite separator is provided, which comprises a nanofiber membrane and a hydrophobic polymer resin layer, the nanofiber membrane comprises a base film and a nanofiber layer arranged on both surfaces of the base film, and the hydrophobic polymer resin layer is arranged on the surface of the nanofiber layer away from the base film.

[0082] The preparation raw material of the hydrophobic polymer resin layer comprises the following components in percentage by weight:

[0083] The hydrophobic polymer resin is polyvinylidene fluoride homopolymer (Arkema), the pore expander is deionized water, the wetting agent is modified organosiloxane (BASF), and the solvent is N-methyl pyrrolidone.

[0084] The preparation method of the composite diaphragm comprises the following steps:

[0085] (1) mixing the preparation raw material of the hydrophobic polymer resin layer to obtain a slurry;

[0086] (2) coating the slurry obtained in step (1) on the surface of the nanofiber layer, curing at 25°C, eluting the solvent, drying to obtain the composite diaphragm.

[0087] Example 2

[0088] In this embodiment, a composite diaphragm is provided, which comprises a nanofiber membrane and a hydrophobic polymer resin layer, the nanofiber membrane comprises a base film and a nanofiber layer arranged on both surfaces of the base film, and the hydrophobic polymer resin layer is arranged on the surface of the nanofiber layer away from the base film.

[0089] The preparation raw material of the hydrophobic polymer resin layer comprises the following components in percentage by weight:

[0090] The hydrophobic polymer resin is polyvinylidene fluoride homopolymer (Arkema), the pore expander is deionized water, the wetting agent is modified organosiloxane (BASF), and the solvent is N-methyl pyrrolidone.

[0091] The preparation method of the composite diaphragm comprises the following steps:

[0092] (1) mixing the preparation raw material of the hydrophobic polymer resin layer to obtain a slurry;

[0093] (2) coating the slurry obtained in step (1) on the surface of the nanofiber layer, curing at 25°C, eluting the solvent, drying to obtain the composite diaphragm.

[0094] Example 3

[0095] In this embodiment, a composite diaphragm is provided, which comprises a nanofiber membrane and a hydrophobic polymer resin layer, the nanofiber membrane comprises a base film and a nanofiber layer arranged on both surfaces of the base film, and the hydrophobic polymer resin layer is arranged on the surface of the nanofiber layer away from the base film.

[0096] The preparation raw material of the hydrophobic polymer resin layer comprises the following components in percentage by weight:

[0097] The hydrophobic polymer resin is long-chain alkyl modified polyimide (Shenzhen Yan Yi), the pore expander is PEG (polyethylene glycol, Mw = 1000 g / mol), the wetting agent is polyether modified polysiloxane wetting agent (Hunan Guangxin), and the solvent is N-methyl pyrrolidone.

[0098] The preparation method of the composite diaphragm comprises the following steps:

[0099] (1) mixing the preparation raw material of the hydrophobic polymer resin layer to obtain a slurry;

[0100] (2) coating the slurry obtained in step (1) on the surface of the nanofiber layer, curing at 40°C, eluting the solvent, and drying to obtain the composite diaphragm.

[0101] Example 4

[0102] In this embodiment, a composite diaphragm is provided, which comprises a nanofiber membrane and a hydrophobic polymer resin layer, the nanofiber membrane comprises a base film and nanofiber layers arranged on both surfaces of the base film, and the hydrophobic polymer resin layer is arranged on the surface of the nanofiber layer away from the base film.

[0103] The preparation raw material of the hydrophobic polymer resin layer comprises the following components in percentage by weight:

[0104] The hydrophobic polymer resin is long-chain alkyl modified polyimide (Shenzhen Yan Yi), the pore expander is ethanol, the wetting agent is modified organosiloxane (BASF), and the solvent is acetone.

[0105] The preparation method of the composite diaphragm comprises the following steps:

[0106] (1) mixing the preparation raw material of the hydrophobic polymer resin layer to obtain a slurry;

[0107] (2) coating the slurry obtained in step (1) on the surface of the nanofiber layer, curing at 30°C, eluting the solvent, and drying to obtain the composite diaphragm.

[0108] Example 5

[0109] In this embodiment, a composite diaphragm is provided, which comprises a nanofiber membrane and a hydrophobic polymer resin layer, the nanofiber membrane comprises a base film and nanofiber layers arranged on both surfaces of the base film, and the hydrophobic polymer resin layer is arranged on the surface of the nanofiber layer away from the base film.

[0110] The preparation raw materials of the hydrophobic polymer resin layer include the following components in percentage by weight:

[0111] The hydrophobic polymer resin is long-chain alkyl modified polyimide (Shenzhen Yan Yi), the reaming agent is PEG (polyethylene glycol, Mw = 1000 g / mol), the wetting agent is modified organosiloxane (BASF), and the solvent is N, N-dimethylacetamide (DMAC).

[0112] The preparation method of the composite diaphragm includes the following steps:

[0113] (1) mixing the preparation raw materials of the hydrophobic polymer resin layer to obtain a slurry;

[0114] (2) coating the slurry obtained in step (1) on the surface of the nanofiber layer, curing at 15°C, eluting the solvent, and drying to obtain the composite diaphragm.

[0115] The composite diaphragm provided in the embodiment is characterized by SEM, as shown in FIG. 1, it can be seen that the hydrophobic polymer resin layer has a three-dimensional network structure, and the surface is distributed with microporous structure.

[0116] Comparative Example 1

[0117] In the comparative example, a composite diaphragm is provided, which includes a nanofiber membrane and a hydrophobic polymer resin layer, the nanofiber membrane includes a base film and a nanofiber layer arranged on both surfaces of the base film, and the hydrophobic polymer resin layer is arranged on the surface of the nanofiber layer away from the base film.

[0118] The preparation raw materials of the hydrophobic polymer resin layer include the following components in percentage by weight:

[0119] The hydrophobic polymer resin is long-chain alkyl modified polyimide (Shenzhen Yan Yi), the reaming agent is ethanol, the wetting agent is modified organosiloxane (BASF), and the solvent is N-methyl pyrrolidone.

[0120] The preparation method of the composite diaphragm refers to Example 1.

[0121] Comparative Example 2

[0122] In the comparative example, a composite diaphragm is provided, which includes a nanofiber membrane and a hydrophobic polymer resin layer, the nanofiber membrane includes a base film and a nanofiber layer arranged on both surfaces of the base film, and the hydrophobic polymer resin layer is arranged on the surface of the nanofiber layer away from the base film.

[0123] The raw material for preparing the hydrophobic polymer resin layer comprises the following components in terms of percentage by weight:

[0124] The hydrophobic polymer resin is PVDF-HFP (Kynar), the pore expander is deionized water, the wetting agent is modified organosiloxane (BASF), and the solvent is N-methyl pyrrolidone.

[0125] The preparation method of the composite separator is as in Example 1.

[0126] Comparative Example 3

[0127] In the present comparative example, a composite separator is provided, which comprises a nanofiber membrane and a hydrophobic polymer resin layer, the nanofiber membrane comprises a base film and nanofiber layers arranged on both surfaces of the base film, and the hydrophobic polymer resin layer is arranged on the surface of the nanofiber layer away from the base film.

[0128] The raw material for preparing the hydrophobic polymer resin layer comprises the following components in terms of percentage by weight:

[0129] The hydrophobic polymer resin is PVDF-HFP (Kynar), the pore expander is deionized water, the wetting agent is modified organosiloxane (BASF), and the solvent is N-methyl pyrrolidone.

[0130] The preparation method of the composite separator is as in Example 1.

[0131] Comparative Example 4

[0132] In the present comparative example, a composite separator is provided, which comprises a nanofiber membrane and a hydrophobic polymer resin layer, the nanofiber membrane comprises a base film and nanofiber layers arranged on both surfaces of the base film, and the hydrophobic polymer resin layer is arranged on the surface of the nanofiber layer away from the base film.

[0133] The raw material for preparing the hydrophobic polymer resin layer comprises the following components in terms of percentage by weight:

[0134] The hydrophobic polymer resin is PVDF-HFP (Kynar), the pore expander is PVP, the wetting agent is modified organosiloxane (BASF), and the solvent is N-methyl pyrrolidone.

[0135] The preparation method of the composite separator is as in Example 1.

[0136] The composite separators provided in the examples and comparative examples are tested for performance, and the testing method is as follows:

[0137] (1) Water content of nanofiber membrane and composite separator

[0138] The sample to be tested was left to stand in an environment with humidity of 1% or less and dew point of -30°C or less for 2 hours. 0.15 g of the sample was weighed into a sample bottle and sealed. A Karl Fischer moisture tester was used for the test, with baking conditions set to 120°C for 10 minutes. The test data was recorded after the test.

[0139] The water reduction amount was calculated using the following formula:

[0140] Water reduction amount = (water content of nanofiber membrane - water content of composite separator) / water content of nanofiber membrane * 100%.

[0141] (2) Average pore diameter

[0142] The following (a) and (b) were measured using a PMI instrument (Gore Co., Ltd., Model CFP-1500AE) with a galwick (25°C surface tension 15.9 dyn / cm) as the wetting liquid at 25°C.

[0143] (a) is the pressure-flow curve in the state of being immersed in the test liquid;

[0144] (b) is the pressure-flow curve at 1 / 2 of the flow rate measured in the dry state;

[0145] The average pore diameter D of the porous base material was calculated using the following formula (1) based on the pressure P at the intersection of the curves of (a) and (b):

[0146] where D: represents the average pore diameter (nm), r: represents the surface tension of the test liquid (N / m), P: represents the pressure at the intersection shown above (Pa), and θ: represents the contact angle of the sample film with the test liquid (°).

[0147] (3) Surface pore diameter (D min , D avg , D max ) of hydrophobic polymer resin layer

[0148] The surface of the composite separator was observed using a scanning electron microscope (SEM) (S-5500, manufactured by Hitachi High-Technologies Corporation) at a magnification of 20,000 times, the image was introduced into a computer, the SEM image was binarized using image processing software ImageJ, an image in which the void portion was black and the structure portion was white was obtained, and each hole was fitted into a circular shape, and the diameter was measured. When the void portion and the structure portion could not be clearly binarized due to a difference in contrast within the analyzed image, the void portion was first painted black and then the image was processed, the hole was fitted into a circular shape, and the minimum value, the average value, and the maximum value of the diameter were calculated. The value rounded off to the first digit after the decimal point was used as the hole diameter.

[0149] (4) Change in gas permeability

[0150] The gas permeability of the nanofiber membrane and the composite separator was tested according to the standard "GB-T 36363-2018 Polyolefin Separator for Lithium Ion Batteries", and the change in gas permeability was calculated according to the following formula:

[0151] Change in gas permeability = gas permeability of composite separator - gas permeability of nanofiber membrane.

[0152] (5) Ion conductivity

[0153] A circular separator piece with a diameter of 47 mm was cut from the composite separator, and then it was placed between two electrodes (the electrode area was not greater than the separator piece area). The separator piece was completely saturated with battery electrolyte; the resistance R (Ω) of the separator piece was measured by probe AC impedance technology. Then, the ion conductivity σ (unit: S·cm -1 ) of the separator saturated with electrolyte was calculated according to the following formula, and the results are shown in Table 1: σ = d / (R×S),

[0154] In the formula, σ represents the ion conductivity of the sample, with a unit of S·cm -1 ; R represents the slope of the change in the resistance (Ω) of the separator piece with the number of layers; d represents the thickness of the separator piece, with a unit of μm; and S represents the test area of the electrode, with a unit of cm 2 .

[0155] The performance test results are shown in Table 1.

[0156] Table 1

[0157] As can be seen from Table 1, the composite separator provided by the present application can reduce the moisture content of the nanofiber membrane by more than 20% (29%-42%) to 1305 ppm or less by modifying the hydrophobic polymer resin on the surface of the nanofiber membrane and controlling the average volume pore size of the hydrophobic polymer resin layer and the average volume pore size of the nanofiber membrane to be 1.3-1.75; while the composite separators provided by the comparative examples all have significantly higher moisture content.

[0158] Applicants declare that the composite separator of the present application, its preparation method and application are illustrated by the above examples, but the present application is not limited to the above examples, i.e. it does not mean that the present application must rely on the above examples to be implemented. It should be understood by those skilled in the art that any improvement of the present application, equivalent replacement of the raw materials selected by the present application, addition of auxiliary ingredients, selection of specific methods, etc. all fall within the protection scope and disclosure scope of the present application.

Claims

1. A composite separator comprising a nanofiber membrane including a nanofiber layer and a hydrophobic polymer resin layer modified to at least one surface of the nanofiber layer. A ratio of an average pore size of the hydrophobic polymer resin layer to an average pore size of the nanofiber membrane is 1.3 to 1.

75.

2. The composite separator of claim 1, wherein, The hydrophobic polymer resin layer has a three-dimensional network structure. Optionally, the hydrophobic polymer resin layer has a minimum pore size of 20 to 30 nm, an average pore size of 40 to 50 nm, and a maximum pore size of 60 to 70 nm. Optionally, the hydrophobic polymer resin layer has a thickness of 0.1 to 0.4 μm. Optionally, the composite separator has a gas permeability increase of 30 sec / 100 ml or less with respect to the nanofiber membrane.

3. The composite separator according to claim 1 or 2, wherein The hydrophobic polymer resin of the hydrophobic polymer resin layer has a molecular chain including any one of a fluorine, a siloxane group, a phenyl group, a long-chain alkyl group, and a cyano group or a combination of at least two thereof. Optionally, the hydrophobic polymer resin includes any one of a polyvinylidene fluoride homopolymer, a vinylidene fluoride-hexafluoropropylene copolymer, a siloxane-modified polyimide, a long-chain alkyl-modified polyimide, a fluoromethyl-modified polyimide, and a cyano-modified polyimide or a combination of at least two thereof.

4. The composite separator of any one of claims 1-3, wherein, The nanofiber membrane has an average pore size of 40 to 50 nm. Optionally, the nanofiber membrane includes a base membrane, and the nanofiber layer is provided to at least one surface of the base membrane. Optionally, the hydrophobic polymer resin layer is modified to a surface of the nanofiber layer away from the base membrane.

5. The composite separator of any one of claims 1-4, wherein, The raw material for preparing the hydrophobic polymer resin layer includes the following components in terms of weight percentage: a hydrophobic polymer resin: 2% to 10%; a pore expander: 0.05% to 3%; and the remainder being a solvent.

6. The composite separator of claim 5, wherein, The hydrophobic polymer resin has a molecular chain including any one of a fluorine, a siloxane group, a phenyl group, a long-chain alkyl group, and a cyano group or a combination of at least two thereof. Optionally, the hydrophobic polymer resin includes any one of a polyvinylidene fluoride homopolymer, a vinylidene fluoride-hexafluoropropylene copolymer, a siloxane-modified polyimide, a long-chain alkyl-modified polyimide, a fluoromethyl-modified polyimide, and a cyano-modified polyimide or a combination of at least two thereof.

7. The composite separator of claim 5 or 6, wherein, The pore expander includes any one of water, polyethylene glycol, and ethanol or a combination of at least two thereof. Optionally, the raw material for preparing the hydrophobic polymer resin layer further includes a wetting agent, and the wetting agent is 0.3% to 1% in terms of weight percentage. Optionally, the wetting agent is an oily wetting agent. Optionally, the wetting agent includes any one of a polyether siloxane-based copolymer, a silicone double structure-based copolymer, a polyacrylate-based copolymer, a polyether-modified silicone oil copolymer, and a polyoxyethylene alkylamine-based copolymer or a combination of at least two thereof. Optionally, the solvent comprises any one or a combination of at least two of acetone, N-methylpyrrolidone, and N,N-dimethylacetamide. 8.A method for preparing the composite separator according to any one of claims 1-7, comprising the following steps: (1) mixing raw materials for preparing a hydrophobic polymer resin layer to obtain a slurry; (2) coating the slurry obtained in step (1) on a surface of the nanofiber layer, curing, eluting a solvent, and drying to obtain the composite separator.

9. The method of claim 8, wherein the composite separator is prepared by the steps of: The coating method in step (2) comprises spraying. Optionally, the coating method in step (2) comprises spraying by ultrasonic atomization. 10.Use of the composite separator according to any one of claims 1-7 in a battery. Optionally, the battery comprises a lithium ion battery.

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