Polar polyolefin microsphere, preparation method therefor, and use thereof

Abstract

Provided in the present invention are a polar polyolefin microsphere, a preparation method therefor, and the use thereof. The polar polyolefin microsphere comprises a core layer and a shell layer covering the surface of the core layer. The core layer comprises a polyolefin and a derivative thereof and / or an ethylene-acrylate copolymer, and silicone oil having a polar functional group. The shell layer comprises a polar polymer having a binding capability. The polar polyolefin microsphere prepared in the present application has a core-shell structure, and a separator coating containing said polar polyolefin microsphere can endow secondary-battery separators with characteristics of low swelling and high hydrophilicity, and has excellent swelling resistance and liquid absorbility, thus further improving the wettability of secondary-battery separators with respect to electrolyte solutions.

Description

Polar polyolefin microspheres and their preparation method and application Technical Field

[0001] The present invention relates to the field of secondary batteries, and in particular to polar polyolefin microspheres, a preparation method and applications thereof. Background Art

[0002] In recent years, the main method to improve battery energy density and power density is to use cathode materials with higher energy density, such as lithium-rich materials, nickel-cobalt-manganese (NCM) ternary materials, and lithium cobalt oxide materials. For example, ternary materials have gradually transitioned from NCM111 to NCM532 and NCM622 systems, and are accelerating the transition to NCM811 systems. However, the higher the Ni content, the higher the reversible capacity of the material. However, the higher the Ni content, the worse the stability of the ternary material and the shorter the cycle life. Therefore, the road to increasing the capacity of ternary materials by increasing the Ni content has come to an end.

[0003] As a key material in lithium batteries, separators act as an electronic barrier, preventing direct contact between the positive and negative electrodes while allowing the free passage of lithium ions in the electrolyte. Therefore, separator modification can improve battery performance. Currently, commercial polyolefin separators suffer from limitations such as low porosity, poor wettability, and low liquid absorption, which severely restrict the migration rate of lithium-ion batteries.

[0004] In traditional technology, the main way to improve its wettability is to increase the polarity of the diaphragm, or to coat it with other organic / inorganic polymers, such as PVDF or ceramic materials. However, PVDF has a certain swelling effect on the electrolyte, which leads to a decrease in the effective electrolyte and a decrease in the ion transmission capacity, which in turn causes a decrease in the battery charge and discharge rate. Common ceramic coatings have a large specific surface energy, high surface activity, and their surfaces are generally hydrophilic. When applied to polyolefin diaphragms, since polyolefin itself is a hydrophobic material, the bonding strength between the two is low. During high-rate cyclic charge and discharge, the diaphragm coating will "shed powder" or ceramic powder will adhere to the positive and negative electrodes, affecting the performance of the ceramic diaphragm in lithium-ion batteries. In addition, due to the diversity of electrolyte formulas, the coating's swelling properties in the electrolyte vary, making it unsuitable for all electrolytes, resulting in poor rate performance in the discharge cycle.

[0005] Therefore, how to solve the problem of poor wettability of existing polyolefin separators to electrolyte solutions is a technical problem that urgently needs to be solved in this field.

[0006] Summary of the Invention

[0007] The main technical problem solved by the present invention is to provide a polar polyolefin microsphere which can effectively improve the wettability of polyolefin diaphragms and has a low swelling rate, as well as a preparation method and application thereof.

[0008] According to a first aspect, the present application provides a polar polyolefin microsphere, wherein the polar polyolefin microsphere comprises a core layer and a shell layer coated on the surface of the core layer;

[0009] The core layer comprises polyolefins and their derivatives and / or ethylene-acrylate copolymers and silicone oil having polar functional groups;

[0010] The shell layer includes a polar polymer having adhesive capabilities.

[0011] In an optional embodiment, the polar polyolefin microspheres meet at least one of the conditions (1)-(3):

[0012] (1) The polyolefin and its derivatives include at least one of polyethylene, polypropylene, oxidized polyethylene, polyethylene-polyethylene glycol block copolymer, 2-bromoisobutyl ester end-functionalized polyethylene, methyl styrene end-capped polypropylene, polypropylene grafted with maleic anhydride, polypropylene grafted with acrylonitrile, and polypropylene grafted with acrylic acid;

[0013] (2) The polar polymer having adhesive ability includes an acrylic copolymer;

[0014] (3) The silicone oil having polar functional groups includes at least one of amino silicone oil and hydroxy silicone oil.

[0015] In an optional embodiment, the acrylic ester copolymer includes a copolymer of acrylic ester and at least one of acrylic acid, acrylonitrile, acrylamide, and oxidized polyolefin.

[0016] In an optional embodiment, the polar polyolefin microspheres further contain an emulsifier;

[0017] Optionally, in the polar polyolefin microspheres, the mass percentage of the emulsifier is 1%-10%;

[0018] Optionally, the emulsifier includes at least one of fatty alcohol polyoxyethylene ether, isomeric alcohol polyoxyethylene ether, sodium lauryl sulfate, sodium dodecylbenzene sulfonate, Tween and Span.

[0019] In an optional embodiment, the silicone oil having polar functional groups and the polar polymer having adhesive ability are connected through covalent bonds or non-covalent bonds.

[0020] In an optional embodiment, at least one of the conditions (1)-(4) is satisfied:

[0021] (1) In the core layer, the mass percentage of the polyolefin and its derivatives and / or ethylene-acrylate copolymer is ≥50% by mass; the mass percentage of the silicone oil having polar functional groups is 2%-50% by mass;

[0022] (2) The mass ratio of the core layer to the shell layer is 90:10-99:1;

[0023] (3) The particle size D50 of the polar polyolefin microspheres is 100 nm-6 μm;

[0024] (4) The polar polyolefin microspheres satisfy 1.5≤D90 / D50≤2.5.

[0025] According to a second aspect, the present application provides a method for preparing polar polyolefin microspheres, comprising the following steps:

[0026] mixing and emulsifying silicone oil having a polar functional group and an emulsifier to obtain a preformed emulsion;

[0027] Mixing polyolefins and their derivatives and / or ethylene-acrylate copolymers, an emulsifier and water at 100-200° C. to obtain a water-in-oil emulsion;

[0028] The prefabricated emulsion is dripped into the water-in-oil emulsion until the water-in-oil emulsion undergoes phase inversion, and then a polar polymer with adhesive ability is added to prepare polar polyolefin microspheres.

[0029] In an optional embodiment, the preparation method satisfies at least one of the conditions (1)-(5):

[0030] (1) In the prefabricated emulsion, the content of the silicone oil is 5-50% wt, and the content of the emulsifier is 1-5% wt;

[0031] (2) In the water-in-oil emulsion, the content of the polyolefin and its derivatives and / or ethylene-acrylate copolymer is 50-90%wt, and the content of the emulsifier is 1-20%wt;

[0032] (3) The amount of the polar polymer added is 1-5 wt% of the polyolefin and its derivatives and / or ethylene-acrylate copolymer;

[0033] (4) The stirring speed when the silicone oil and the emulsifier are mixed is 50-200 rpm;

[0034] (5) The stirring speed when mixing the polyolefin and its derivatives and / or ethylene-acrylate copolymer, emulsifier and water is 100-2000 rpm.

[0035] According to a third aspect, the present application provides a coating slurry, comprising the polar polyolefin microspheres described above or the polar polyolefin microspheres prepared by the above preparation method.

[0036] According to a fourth aspect, the present application provides a secondary battery separator, comprising a separator substrate and a separator coating attached to at least one surface of the separator substrate;

[0037] The diaphragm coating comprises the polar polyolefin microspheres described above or the polar polyolefin microspheres prepared by the above-mentioned preparation method;

[0038] Alternatively, the diaphragm coating is prepared using the above-mentioned coating slurry.

[0039] The beneficial effects of the present application are that the polar polyolefin microspheres prepared in the present application are a core-shell structure. Through the membrane coating containing polar polyolefin microspheres, the secondary battery membrane can be given the characteristics of low swelling and high lyophilicity, and has excellent anti-swelling and liquid absorption properties, thereby improving the wettability of the secondary battery membrane to the electrolyte solution. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] FIG1 is a schematic diagram of the structure of polyolefin microspheres in one embodiment of the present application;

[0041] FIG2 is a SEM image of polyolefin microspheres in one embodiment of the present application.

[0042] Reference numerals: core layer 1 , silicone oil 1 a , polyolefin and its derivatives and / or ethylene-acrylate copolymer 1 b , shell layer 2 . DETAILED DESCRIPTION

[0043] The present invention will be further described in detail below by means of specific embodiments in conjunction with the accompanying drawings. Similar elements in different embodiments are numbered with associated similar elements. In the following embodiments, many detailed descriptions are provided to enable the present application to be better understood. However, those skilled in the art will readily appreciate that some of the features may be omitted in different circumstances, or may be replaced by other elements, materials, or methods. In some cases, some operations related to the present application are not shown or described in the specification. This is to avoid the core portion of the present application being overwhelmed by excessive descriptions, and for those skilled in the art, it is not necessary to describe these related operations in detail. They will fully understand the related operations based on the description in the specification and the general technical knowledge in the art.

[0044] In addition, the features, operations, or characteristics described in the specification may be combined in any appropriate manner to form various embodiments. Furthermore, the steps or actions in the method description may be reordered or adjusted in a manner readily apparent to those skilled in the art. Therefore, the various sequences in the specification and drawings are provided solely for the purpose of clearly describing a particular embodiment and are not intended to be mandatory, unless otherwise specified.

[0045] The serial numbers assigned to components herein, such as "first," "second," etc., are used solely to distinguish the objects being described and do not convey any sequential or technical meaning. References to "connection" and "coupling" herein, unless otherwise specified, include both direct and indirect connections (couplings).

[0046] Polyolefins are among the most commonly used plastic materials due to their chemical stability. However, their non-polar properties also limit their further applications. For example, their adhesion, compatibility, toughness, adhesion, surface properties (such as dyeability and printability), and rheological properties. Industrial applications mainly rely on post-polymerization functionalization of polyolefins, which often lack selectivity under harsh conditions due to free radical processes causing chain scission and crosslinking. Therefore, how to impart polarity to olefin materials has always been a difficult problem in the industry.

[0047] To this end, the present application provides a polar polyolefin microsphere, which has a core-shell structure, as shown in Figure 1, wherein the core layer 1 is a co-emulsification product of polyolefins and their derivatives and / or ethylene-acrylate copolymers 1b and silicone oil 1a having polar functional groups. For example, as shown in Figure 1, the silicone oil 1a having polar functional groups is wrapped in the outer layer of polyolefins and their derivatives and / or ethylene-acrylate copolymers 1b; the above-mentioned shell layer 2 is a polar polymer with adhesive ability coated on the periphery of the core layer, wherein the silicone oil 1a and the polar polymer with adhesive ability are connected by covalent bonds or non-covalent bonds. Exemplarily, the above-mentioned polar polymer with adhesive ability can be but is not limited to a polyacrylate copolymer, which is grafted onto the side chain of the silicone oil 1a.

[0048] In these core-shell polar polyolefin microspheres, the two-component core layer, composed of polyolefins and their derivatives and / or ethylene-acrylate copolymers and a silicone oil with polar functional groups, exhibits excellent compatibility. This improves the compatibility of the polyolefins and their derivatives and / or ethylene-acrylate copolymers with polar substances, thereby ensuring a stable connection with the polar polymer in the shell. Compared to traditional separator coating materials, these polar polyolefin microspheres have a lower swelling rate in electrolytes and a more suitable surface energy. When used in separator coatings, they can improve the separator's electrolyte wettability while maintaining a low swelling rate.

[0049] In an optional design, the mass percentage of the polyolefin and its derivatives and / or ethylene-acrylate copolymer 1b in the core layer 1 is ≥50% by mass.

[0050] In an optional design, the content of the silicone oil 1b having polar functional groups in the core layer 1 is 2%-50% by mass. For example, the mass percentage of the silicone oil can be 20%, 30%, 40% or 50%; or within the range of any of the above values.

[0051] In an optional design, the mass ratio of the core layer 1 to the shell layer 2 is 90:10-99:1. For example, the mass ratio of the core layer 1 to the shell layer 2 is 90:10, 93:7, 96:4, or 99:1, or any range thereof. When the mass ratio of the core layer 1 to the shell layer 2 is within this range, the mass proportion of the swellable polymer in the shell layer is relatively small, and the swelling rate of the polar polyolefin microspheres is relatively low.

[0052] In an optional design, the particle size D50 of the polar polyolefin microspheres is 100 nm-6 μm. For example, the particle size D50 of the polar polyolefin microspheres is 100 nm, 200 nm, 300 nm, 500 nm, 1 μm, 2 μm, 3 μm, 4 μm, 5 μm, 6 μm or any range thereof.

[0053] In an optional design, the polar polyolefin microspheres satisfy 1.5 ≤ D90 / D50 ≤ 2.5. For example, the D90 / D50 ratio of the polar polyolefin microspheres can be 1.5, 1.7, 1.9, 2.1, 2.3, 2.5, or any range thereof. Polar polyolefin microspheres with a D90 / D50 ratio within this range have a narrow particle size distribution, resulting in better uniformity in the separator coating.

[0054] In the embodiments disclosed in the present application, the polar polyolefin microspheres further contain an emulsifier; for example, in an optional design, the mass percentage of the emulsifier in the polar polyolefin microspheres is 1%-10%.

[0055] For the convenience of explaining the present application, the present application further provides a method for preparing polar polyolefin microspheres, which comprises the following steps:

[0056] Step S1: A silicone oil having a polar functional group and an emulsifier are uniformly mixed and emulsified to obtain a preformed emulsion. In this step, the silicone oil having a polar group can be, but is not limited to, amino silicone oil or hydroxy silicone oil. The silicone oil content is 1-50% by weight, the emulsifier content is 1-5% by weight, and the balance is solvent.

[0057] In the specific implementation of this step, silicone oil having polar functional groups is added into a stirring kettle, and then an emulsifier is added thereto, and the mixture is stirred and mixed at a speed of 50-200 rpm to form a prefabricated emulsion as component A.

[0058] Step S2: Mixing polyolefins and their derivatives and / or ethylene-acrylate copolymers, an emulsifier, and water at 100-200°C to form a water-in-oil emulsion. In this step, the polyolefins and their derivatives and / or ethylene-acrylate copolymers comprise 50-90% by weight, and the emulsifier comprises 1-20% by weight.

[0059] In the specific implementation of this step, the above raw materials are added to a high-temperature and high-pressure reactor, and then the reactor is heated to 100-200°C to melt the polyolefin and its polymer, and then the above raw materials are stirred at a rate of 100-2000 rpm to obtain an oil-in-water emulsion as component B.

[0060] Among them, the polyolefin and its derivatives can be at least one of but not limited to polyethylene, polypropylene, oxidized polyethylene, polyethylene-polyethylene glycol block copolymer, 2-bromoisobutyl ester end-functionalized polyethylene, methyl styrene end-capped polypropylene, polypropylene grafted with maleic anhydride, polypropylene grafted with acrylonitrile and polypropylene grafted with acrylic acid.

[0061] Step S3: The preformed emulsion is dripped into the water-in-oil emulsion until the water-in-oil emulsion undergoes phase inversion. A polar polymer with adhesive properties (e.g., an acrylate copolymer) is then added to produce polar polyolefin microspheres. In this step, the acrylate copolymer is added in an amount of 1-5 wt% based on the weight of the polyolefin and its derivatives and / or the ethylene-acrylate copolymer.

[0062] During the specific implementation of this step, component A is slowly added dropwise to a high-temperature and high-pressure reactor to mix component A with component B. During the addition process, the dropping speed needs to be controlled to ensure that the temperature of the reaction materials in the reactor does not change, until the emulsion in the reactor undergoes a phase inversion and becomes an oil-in-water emulsion. At this time, a polar polymer with adhesive ability is added to the reactor, and then the temperature is quickly cooled to room temperature to obtain a polar polypropylene microsphere structure.

[0063] The polar polymer with adhesive ability may be, but is not limited to, an acrylate copolymer. For example, the acrylate copolymer is a copolymer of at least one of acrylic acid, acrylonitrile, acrylamide, and oxidized polyolefin with acrylate.

[0064] In the embodiments disclosed in the present application, the emulsifier used in the above steps S1 and S2 is at least one of fatty alcohol polyoxyethylene ether, isomeric alcohol polyoxyethylene ether, sodium lauryl sulfate, sodium dodecylbenzene sulfonate, Tween and Span.

[0065] This application creatively designs multi-component materials through emulsification to prepare polar polyolefin microspheres with a core-shell structure. Unmodified polyolefins are difficult to make into water-based materials and have very poor stability in water. At the same time, polyolefins have very poor compatibility with polar substances and have obvious phase interfaces. The silicone oil with polar functional groups in the two-component core can be used as a compatibilizer to improve the compatibility of polyolefins with polar substances. Therefore, it can effectively impart stability to polyolefin materials in water and improve the adhesion of polyolefin materials. At the same time, the polar polyolefin microspheres can obtain polymer coatings of different polarities by regulating the polarity and ratio of monomers to cope with electrolytes of different systems, thereby achieving the effect of high lyophilicity and low swelling, thereby improving battery rate performance.

[0066] The morphology of the polar polyolefin microspheres prepared by the method of the present application is shown in FIG2 . As can be seen from FIG2 , the particle size D50 of the polar polyolefin microspheres is 100 nm-6 μm.

[0067] On the basis of the above, the present application further provides a coating slurry, which can be used for secondary battery separators. Specifically, the coating slurry includes the above-mentioned polar polyolefin microspheres; and other functional components.

[0068] In some embodiments, the coating slurry comprises the following components by weight: 5-50 parts of polar polyolefin microspheres, 0.1-15 parts of a binder, 0.1-5 parts of a dispersant, and 0-5 parts of a wetting agent.

[0069] In a more specific embodiment, the coating slurry comprises the following components by mass percentage: 5-50% polar polyolefin microspheres, 0.1-15% adhesive, 0.1-5% dispersant, and 0-5% wetting agent.

[0070] Among them, the polar polyolefin microspheres are the main body of the diaphragm coating, which are used to give the battery diaphragm high lyophilicity and low swelling effects; the above-mentioned adhesive is used to give the diaphragm coating a basic adhesion. For example, the above-mentioned adhesive is at least one of polyacrylonitrile, SBR (styrene-butadiene latex), sodium hydroxymethyl cellulose, polyvinyl alcohol, polyacrylic acid, polyacrylate, and polyacrylamide; the above-mentioned dispersant is used to enable the polar polyolefin microspheres to be fully dispersed in the solvent. For example, the above-mentioned dispersant is at least one of polyacrylate ammonium salt, sodium hexametaphosphate, and sodium pyrophosphate; the above-mentioned wetting agent is used to improve the wetting properties and surface tension of the diaphragm coating. For example, the above-mentioned wetting agent is at least one of alkyl sulfate, sulfonate, polyoxyethylene alkylphenol ether, and polyoxyethylene fatty alcohol ether.

[0071] Based on the above, the present application further provides a secondary battery separator, which includes a separator substrate and a separator coating, wherein the separator coating is applied to at least one side of the separator substrate. The separator coating includes the aforementioned polar polyolefin microspheres or is made using the aforementioned coating slurry.

[0072] For example, the diaphragm substrate may be a polyolefin material with a thickness of 2-25 μm, and the diaphragm coating may have a thickness of 0.5-6 μm. The diaphragm coating may be applied to the surface of the diaphragm substrate by spraying, roller coating, or spot coating.

[0073] In order to illustrate the effects of this application, this application also provides the following more specific embodiments.

[0074] Example 1:

[0075] Step S1: preparing component A, mixing and emulsifying 10% wt of amino silicone oil and 5% wt of emulsifier at a rotation speed of 50-200 rpm to obtain a prefabricated emulsion.

[0076] Step S2, preparing component B, mixing HDPE (high-density polyethylene) 1% ionic emulsifier and water in a reactor to make the solid content of the reactants reach 50%; then heating the reactor to 150° C. and stirring at a speed of 1000 rpm to melt the reactants and form an oil-in-water emulsion.

[0077] Step S3, then slowly dripping component A into the reactor to mix component A with component B, and the dripping speed is slow to ensure that the temperature of the reactor does not change until phase inversion occurs, and then adding acrylate-acrylic acid copolymer accounting for 5% of the mass of polyolefins and their derivatives and / or ethylene-acrylate copolymers to react, and finally quickly cooling to room temperature (25±5°C) to obtain polar polyolefin microspheres with a core-shell structure.

[0078] The particle size D50 of the polar polyolefin microspheres is 500 nm. The polar polyolefin microspheres are used to prepare a slurry for the diaphragm coating. The specific mass percentages are as follows: 10% polar polyolefin microspheres, 5% adhesive, 0.1% dispersant, and 0.01% wetting agent. The slurry for the diaphragm coating is coated on a PP diaphragm substrate by roller coating to obtain a highly lyophilic and low-swelling secondary battery diaphragm.

[0079] Example 2:

[0080] Polar polyolefin microspheres and diaphragm coatings were prepared according to the method and parameters of Example 1.

[0081] The difference is that, in this embodiment, when preparing component A in step S1, the solid content of the amino silicone oil is 20 wt % and the stirring rate is 1400 rpm.

[0082] The particle size D50 of the polar polyolefin microspheres finally obtained was 200 nm. The polar polyolefin microspheres were then used to prepare a separator coating and a secondary battery separator according to the proportions in Example 1.

[0083] Example 3:

[0084] Polar polyolefin microspheres and diaphragm coatings were prepared according to the method and parameters of Example 1.

[0085] The difference is that in this embodiment, when preparing component A in step S1, the solid content of the amino silicone oil is 1 wt%; in step S2, the stirring rate is 400 rpm; and in step S3, the acrylic ester copolymer added is an acrylic ester-acrylonitrile copolymer accounting for 3% of the mass of HDPE.

[0086] The particle size D50 of the polar polyolefin copolymer finally obtained was 1.2 μm. The polar polyolefin microspheres were then used to prepare a separator coating and a secondary battery separator according to the proportions in Example 1.

[0087] Example 4:

[0088] Polar polyolefin microspheres and diaphragm coatings were prepared according to the method and parameters of Example 1.

[0089] The difference is that in this embodiment, 5 wt% of hydroxy silicone oil is used when preparing component A in step S1; in step S2, the stirring rate is 400 rpm; and in step S3, the acrylic ester copolymer added is an acrylate-acrylic acid copolymer accounting for 10% of the mass of HDPE.

[0090] The particle size D50 of the polar polyolefin copolymer finally obtained was 300 nm. The polar polyolefin microspheres were then used to prepare a separator coating and a secondary battery separator according to the proportions in Example 1.

[0091] Embodiment 5:

[0092] Polar polyolefin microspheres and diaphragm coatings were prepared according to the method and parameters of Example 1.

[0093] The difference is that in this embodiment, when preparing component A in step S1, 1 wt% hydroxy silicone oil is used; in step S2, the stirring rate is 400 rpm; and in step S3, the acrylic ester copolymer added is an acrylic ester-acrylic acid copolymer accounting for 3% of the mass of HDPE.

[0094] The particle size D50 of the polar polyolefin copolymer finally obtained was 1.1 μm. The polar polyolefin microspheres were then used to prepare a separator coating and a secondary battery separator according to the proportions in Example 1.

[0095] Example 6:

[0096] The various properties of the secondary battery separators prepared in Examples 1 to 5 above were tested.

[0097] The peel strength test method includes the following steps:

[0098] The secondary battery separator and electrode prepared above were cut into samples with a width of 20 mm and a length of 80 mm, and pressed on a hot press. One end of the secondary battery separator and one end of the electrode were fixed on a universal tensile testing machine, and peeled at a constant speed of 10 mm / min and 180°C. The experiment was repeated 5 times and the average value was taken.

[0099] The swelling rate test method includes the following steps:

[0100] Place the secondary battery separator in a culture dish, bake it in an oven at 80°C for 24 hours, take it out and cool it to room temperature in a desiccator, and weigh it immediately. Repeat this operation until the mass is constant (the weight error does not exceed 2%). At this time, the sample is mostly in powder or colloidal form; then take a small amount of dried secondary battery separator sample (for example, 0.1g) and place it in a beaker (if the secondary battery separator is in colloidal form, cut it into 5cm*5cm size film); prepare the electrolyte solvent according to the ratio of EC:DEC:DMC=1:1:1; pour 100mL of the prepared electrolyte into the beaker and seal it; after 24 hours, separate the swollen secondary battery separator by filtration; by weighing the mass of the swollen secondary battery separator, calculate the swelling rate of the material according to the following formula (1): Swelling rate (%) = (Ws-Wd) / Wd×100 (1)

[0101] Wherein, Ws is the initial weight of the secondary battery separator before being immersed in the electrolyte, and Wd is the mass of the secondary battery separator after swelling in the electrolyte solvent.

[0102] The ionic conductivity test method includes the following steps:

[0103] Symmetrical cells were made using inert stainless steel electrodes for testing. As the number of separator layers increases, the battery resistance also increases, and there is a linear relationship. The corresponding slope is the separator resistance. The separator ion conductivity is calculated using the following formula (2): σS = d / (RS × A × 10) (2)

[0104] Wherein, σS is the ionic conductivity of the separator, in mS / cm; d is the thickness of the separator, in μm, measured by a thickness gauge; RS is the separator resistance, in Ω; A is the effective area of ​​the separator in a symmetrical battery, which is 6 cm 2 ; 10 is the dimension conversion ratio.

[0105] The specific test results are shown in the following table:

[0106] As can be seen from the table above, the diaphragm coating containing polar polyolefin microspheres has good adhesion and a smaller contact angle than the diaphragm substrate, indicating that the secondary battery diaphragm coated with the diaphragm coating has good hydrophilicity. At the same time, compared with the diaphragm substrate, the swelling rate and ion conductivity of the secondary battery diaphragm coated with the diaphragm coating are improved to a certain extent. The swelling rate of the polar polyolefin microspheres of Examples 1 to 5 is significantly lower than that of PVDF. The improvement in ionic conductivity shows that the polar polyolefin microspheres of Examples 1 to 5 can effectively improve the electrolyte wettability of the secondary battery diaphragm and improve the ion conductivity performance of the secondary battery diaphragm.

[0107] The secondary battery separator provided in the present application has the characteristics of low swelling and high lyophilicity through the separator coating containing polar polyolefin microspheres, has excellent anti-swelling and liquid absorption properties, and has broad application prospects in the field of secondary batteries.

[0108] The above examples are used to illustrate the present invention, which are only used to help understand the present invention and are not intended to limit the present invention. Those skilled in the art can make several simple deductions, modifications or substitutions based on the concept of the present invention.

Claims

1. A polar polyolefin microsphere, characterized in that: The polar polyolefin microspheres include a core layer and a shell layer coated on the surface of the core layer; The core layer comprises polyolefins and their derivatives and / or ethylene-acrylate copolymers, wherein the polyolefins and their derivatives comprise at least one of polyethylene, polypropylene, oxidized polyethylene, polyethylene-polyethylene glycol block copolymer, 2-bromoisobutyl ester end functionalized polyethylene, methyl styrene end capped polypropylene, polypropylene grafted with maleic anhydride, polypropylene grafted with acrylonitrile and polypropylene grafted with acrylic acid; and, Silicone oil having a polar functional group, wherein the silicone oil having a polar functional group comprises at least one of amino silicone oil and hydroxy silicone oil; In the core layer, the mass percentage of the polyolefin and its derivatives and / or ethylene-acrylate copolymer is ≥50% by mass; the mass percentage of the silicone oil having polar functional groups is 2%-50% by mass; The shell layer includes a polar polymer with bonding ability, and the polar polymer with bonding ability includes an acrylic ester copolymer.

2. The polar polyolefin microspheres according to claim 1, characterized in that The acrylic ester copolymer includes a copolymer of at least one of acrylic acid, acrylonitrile and acrylamide and acrylic ester.

3. The polar polyolefin microspheres according to claim 1 or 2, characterized in that: The polar polyolefin microspheres also contain an emulsifier.

4. The polar polyolefin microspheres according to claim 3, characterized in that: In the polar polyolefin microspheres, the mass percentage of the emulsifier is 1%-10%.

5. The polar polyolefin microspheres according to claim 3, characterized in that: The emulsifier includes at least one of fatty alcohol polyoxyethylene ether, isomeric alcohol polyoxyethylene ether, sodium dodecyl sulfate, sodium dodecylbenzene sulfonate, Tween and Span.

6. The polar polyolefin microspheres according to claim 1 or 2, characterized in that: The silicone oil with polar functional groups and the polar polymer with bonding ability are connected through covalent bonds or non-covalent bonds.

7. The polar polyolefin microspheres according to claim 1 or 2, characterized in that: Satisfy at least one of the following conditions (1)-(3): (1) The mass ratio of the core layer to the shell layer is 90:10-99:1; (2) The particle size D50 of the polar polyolefin microspheres is 100 nm-6 μm; (3) The polar polyolefin microspheres satisfy 1.5≤D90 / D50≤2.

5.

8. A method for preparing polar polyolefin microspheres, characterized in that: The following steps are involved: The silicone oil having a polar functional group and an emulsifier are mixed and emulsified to obtain a preformed emulsion; Mixing polyolefins and their derivatives and / or ethylene-acrylate copolymers, an emulsifier and water at 100-200° C. to obtain a water-in-oil emulsion; wherein, in terms of mass percentage, the mass percentage of the polyolefins and their derivatives and / or ethylene-acrylate copolymers is ≥50%; and the mass percentage of the silicone oil having polar functional groups is 2%-50%; The polyolefin and its derivatives include at least one of polyethylene, polypropylene, oxidized polyethylene, polyethylene-polyethylene glycol block copolymer, 2-bromoisobutyl ester end functionalized polyethylene, methyl styrene end capped polypropylene, polypropylene grafted with maleic anhydride, polypropylene grafted with acrylonitrile and polypropylene grafted with acrylic acid; The silicone oil having polar functional groups includes at least one of amino silicone oil and hydroxy silicone oil; The prefabricated emulsion is dripped into the water-in-oil emulsion until the water-in-oil emulsion undergoes phase inversion, and then a polar polymer with bonding ability is added to prepare polar polyolefin microspheres; the polar polymer with bonding ability includes an acrylic copolymer.

9. The preparation method according to claim 8, characterized in that: The preparation method satisfies at least one of the conditions (1) to (5): (1) In the prefabricated emulsion, the content of the silicone oil is 5-50%wt, and the content of the emulsifier is 1-5%wt; (2) In the water-in-oil emulsion, the content of the polyolefin and its derivatives and / or ethylene-acrylate copolymer is 50-90%wt, and the content of the emulsifier is 1-20%wt; (3) The amount of the polar polymer added is 1-5% by weight of the polyolefin and its derivatives and / or ethylene-acrylate copolymer; (4) The stirring speed when the silicone oil and the emulsifier are mixed is 50-200 rpm; (5) the polyolefin and its derivatives and / or ethylene-acrylate copolymer, emulsifier and water mixture The appropriate stirring speed is 100-2000 rpm.

10. A coating slurry, characterized in that: The invention comprises the polar polyolefin microspheres as claimed in any one of claims 1 to 7 or the polar polyolefin microspheres prepared by the preparation method as claimed in any one of claims 8 to 9.

11. A secondary battery separator, characterized in that: A diaphragm substrate and a diaphragm coating attached to at least one surface of the diaphragm substrate; The diaphragm coating comprises the polar polyolefin microspheres according to any one of claims 1 to 7 or the polar polyolefin microspheres prepared by the preparation method according to any one of claims 8 to 9; Alternatively, the diaphragm coating is prepared using the coating slurry as claimed in claim 10.

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