High-tensile-strength isolating membrane

Abstract

The utility model discloses an isolating membrane with high tensile strength, which comprises a polyolefin porous base material with a plurality of porous structures on the surface and inside; the reinforcing film is formed on at least one surface of the polyolefin porous base material and the inner walls of the plurality of porous structures; and an inorganic layer which contains a plurality of inorganic particles and a binder and is formed on the reinforcing film. The high-tensile-strength isolating membrane disclosed by the utility model has good mechanical strength.

Description

Technical Field

[0001] The utility model relates to a novel lithium battery separator, and particularly to a separator with good mechanical strength for lithium batteries. Background Art

[0002] A lithium battery consists of four main parts: a positive electrode material, a negative electrode material, a separator, and an electrolyte. The separator is a thin film with a microporous structure, mainly used to separate the positive and negative electrodes of the battery to prevent electrical contact between the electrodes from forming a short circuit, but allowing free ions to pass through. Therefore, the separator has a decisive influence on the discharge rate, energy density, cycle efficiency, and safety of lithium batteries.

[0003] The separator can be mainly prepared by two methods: dry stretching and wet stretching. The dry separator has high safety and low cost, so it is mostly used in large lithium iron phosphate power lithium batteries. The wet separator has a thin thickness, high porosity, and high pore size uniformity, which can provide better air permeability. However, the wet separator needs a coating layer to meet the safety requirements of thermal stability and tensile strength.

[0004] When aiming for high battery capacity, the separator is generally thinned to facilitate the easy movement of ions. However, during battery assembly and charge-discharge cycling, the separator itself needs to have a certain mechanical strength. Therefore, after the separator is thinned, the mechanical strength may decrease. Thus, in order to maintain insulation, ion permeability, etc., improving the mechanical strength of the separator is an important issue.

[0005] Therefore, it is necessary to further strengthen the mechanical strength of the separator after thinning to ensure the safety of the battery. Summary of the Utility Model

[0006] The purpose of the utility model is to provide a high tensile strength separator with good mechanical strength.

[0007] To achieve the above purpose, the utility model provides a high tensile strength separator, which includes a polyolefin porous substrate, a reinforcing film, and an inorganic layer. The polyolefin porous substrate has a plurality of porous structures on its surface and inside; the reinforcing film is formed on at least one surface of the polyolefin porous substrate and the inner walls of the plurality of porous structures; the inorganic layer includes a plurality of inorganic particles and a binder, and is formed on the reinforcing film.

[0008] As an optional technical solution, the reinforcing film is a composite film composed of titanium oxide or / and titanium hydroxide and hexamethyldisilazane.

[0009] As an optional technical solution, the polyolefin porous substrate is a single-layer polyethylene film or a single-layer polypropylene film.

[0010] As an alternative technical solution, the thickness of the polyolefin porous substrate ranges from 5 μm to 30 μm.

[0011] As an alternative technical solution, the porosity of the polyolefin porous substrate ranges from 30% to 70%.

[0012] As an alternative technical solution, the inorganic layer contains 1 to 20 weight percent of the binder and 80 to 99 weight percent of the plurality of inorganic particles, and the thickness of the inorganic layer ranges from 0.5 μm to 3 μm.

[0013] As an alternative technical solution, the inorganic layer is an inorganic particle-organic binder composite layer.

[0014] As an alternative technical solution, the plurality of inorganic particles in the inorganic layer are Mg(OH)2 particles, BaSO4 particles, BaTiO3 particles, HfO2 particles, SrTiO3 particles, SnO2 particles, CeO2 particles, MgO particles, NiO particles, CaO particles, ZnO particles, ZrO2 particles, SiO2 particles, Y2O3 particles, Al(OH)3 particles, Al2O3 particles, boehmite (AlOOH) particles, SiC particles, TiO2 particles or any combination thereof, and the size of the plurality of inorganic particles ranges from 0.1 μm to 3 μm.

[0015] As an alternative technical solution, the binder in the inorganic layer is an ethylene-vinyl acetate copolymer binder, a poly(meth)acrylate binder, a crosslinkable (meth)acrylic resin binder, a fluorine-based rubber binder, a styrene-butadiene rubber binder, a polyvinyl alcohol binder, a polyvinyl butyral binder, a polyvinylpyrrolidone binder, a poly-N-vinylacetamide binder, a polyvinylidene fluoride binder, a polyurethane binder or any combination thereof.

[0016] The high tensile strength separator film of the present utility model has good mechanical strength.

[0017] The present utility model will be described in detail below in conjunction with the accompanying drawings and specific embodiments, but it is not intended to limit the present utility model. Description of the Drawings

[0018] Figure 1 It is an image of a scanning electron microscope (SEM) of a wet separator film for an inorganic coating in the prior art at a magnification of 40,000;

[0019] Figure 2 It is an image of a scanning electron microscope of an embodiment of the high tensile strength separator film of the present utility model at a magnification of 40,000. Detailed Description of the Embodiments

[0020] To further understand the purpose, structure, features and functions of the present utility model, a detailed description is provided below in conjunction with embodiments.

[0021] To make the description of the disclosed content of the present novelty more detailed and complete, the following provides an illustrative description of the implementation aspects and specific embodiments of the present utility model; however, this is not the only form for implementing or applying the specific embodiments of the present utility model. Each of the disclosed embodiments can be combined or substituted with each other under beneficial circumstances, or other embodiments can be added to one embodiment without further record or explanation.

[0022] The advantages, features and technical methods achieved by the present utility model will be described in more detail with reference to exemplary embodiments and will be more easily understood. Moreover, the present utility model can be implemented in different forms, so it should not be understood as being limited only to the embodiments described herein. On the contrary, for those with ordinary knowledge in the technical field to which the present utility model pertains, the provided embodiments will more thoroughly, comprehensively and completely convey the scope of the present utility model, and the present utility model will only be defined by the appended patent application scope.

[0023] Unless otherwise defined, all terms (including technical and scientific terms) and proper nouns used hereinafter are substantially the same as those generally understood by those skilled in the technical field to which the present utility model pertains. For example, those terms defined in a general dictionary should be understood as having a meaning consistent with the content of the relevant field, and unless clearly defined hereinafter, they will not be understood in an overly idealized or overly formal sense.

[0024] The present utility model discloses a high tensile strength separator film, which includes a polyolefin porous substrate, a reinforcing film and an inorganic layer. The surface and interior of the polyolefin porous substrate have a plurality of porous structures; the reinforcing film is formed on at least one surface of the polyolefin porous substrate and the inner walls of the plurality of porous structures; the inorganic layer includes a plurality of inorganic particles and a binder, and is formed on the reinforcing film.

[0025] The high tensile strength separator film of the present utility model forms a reinforcing film on the surface of the polyolefin porous substrate and the inner walls of the plurality of porous structures, which can increase the mechanical strength of the polyolefin porous substrate, and enhance the tensile strength, puncture strength and compression resistance of the separator film without increasing the total thickness of the separator film. Compared with a separator film with an inorganic coating of the same thickness, the high tensile strength separator film of the present utility model has higher tensile strength and puncture strength.

[0026] In one embodiment of the present utility model, the reinforcing film is a composite film composed of titanium oxide or / and titanium hydroxide and hexamethyldisilazane.

[0027] In one embodiment of the present utility model, the polyolefin porous substrate is a single-layer polyethylene film or a single-layer polypropylene film.

[0028] In an embodiment of the present utility model, the thickness of the polyolefin porous substrate ranges from 5 micrometers (μm) to 30 μm.

[0029] In an embodiment of the present utility model, the porosity of the polyolefin porous substrate ranges from 30% to 70%.

[0030] In an embodiment of the present utility model, the inorganic layer contains 1 to 20 weight percent of a binder and 80 to 99 weight percent of a plurality of inorganic particles. In a preferred embodiment of the present utility model, the polyolefin porous substrate having the reinforcing film may include an inorganic coating on one or both of its surfaces. In a preferred embodiment of the present utility model, the thickness of the inorganic coating ranges from 0.5 μm to 5 μm, preferably from 0.5 μm to 3 μm.

[0031] In an embodiment of the present utility model, the inorganic particles in the inorganic layer may be Mg(OH)2, BaSO4, BaTiO3, HfO2, SrTiO3, SnO2, CeO2, MgO, NiO, CaO, ZnO, ZrO2, SiO2, Y2O3, Al(OH)3, Al2O3, boehmite (AlOOH), SiC, TiO2, or any combination thereof. The applicable size of the inorganic particles ranges from 0.1 μm to 3 μm, preferably from 0.3 μm to 2 μm.

[0032] In an embodiment of the present utility model, the binder in the inorganic layer may be ethylene-vinyl acetate copolymer (EVA), poly(meth)acrylate, crosslinkable (meth)acrylic resin, fluorine rubber, styrene-butadiene rubber (SBR), polyvinyl alcohol (PVA), polyvinyl butyral (PVB), polyvinylpyrrolidone (PVP), poly-N-vinylacetamide, polyvinylidene fluoride (PVDF), polyurethane, or any combination thereof.

[0033] In an embodiment of the present utility model, the reinforcing film can be formed by sequentially applying an aqueous solution containing 0.1 wt% to 5 wt% of a titanium alkoxide solution, 0.1 wt% to 5 wt% of hexamethyldisilazane, and 30 wt% to 70 wt% of alcohol to the polyolefin porous substrate to form the reinforcing film on at least one surface of the polyolefin porous substrate and the inner walls of the plurality of porous structures.

[0034] In an embodiment of the present utility model, the titanium alkoxide used in forming the reinforcing film may be a methanol titanate solution, an ethanol titanate solution, an isopropanol titanate solution, a tert-butanol titanate solution, or any combination thereof, and preferably isopropanol titanate, and the solvent of the titanium alkoxide solution is methanol, ethanol, isopropanol, or any combination thereof.

[0035] In an embodiment of the present utility model, the aqueous solution of alcohol used in forming the reinforcing film may be an aqueous solution of methanol, an aqueous solution of ethanol, an aqueous solution of isopropanol, an aqueous solution of ethoxyethanol, an aqueous solution of allyl alcohol, an aqueous solution of ethylene glycol, or any combination thereof. In a preferred embodiment of the present utility model, the aqueous solution of alcohol for strengthening treatment is an aqueous solution of methanol, an aqueous solution of ethanol, or an aqueous solution of isopropanol, or any combination thereof. In a preferred embodiment of the present utility model, the aqueous solution of alcohol is preferably an aqueous solution of 40 to 60 wt% alcohol.

[0036] In an embodiment of the present utility model, the reinforcing film may further include additives such as tackifiers, antistatic agents, flame retardants, antioxidants, or surface modifiers as required.

[0037] In a specific implementation manner of an embodiment of the present utility model, a polyethylene porous substrate with a thickness of 9 μm (porosity 48%) can be used. It is immersed in a titanium isopropoxide solution obtained by uniformly mixing 196.4 g of 99.5% anhydrous ethanol, 1.6 g of titanium isopropoxide, and 2 g of hexamethyldisilazane for 1 minute. After removing the excess liquid beads with a scraper, it is then immersed in an aqueous ethanol solution obtained by uniformly mixing 98.4 g of deionized water, 98.4 g of 95% ethanol, 3 g of poly-N-vinylacetamide (PNVA GE191-107), and 0.13 g of polyacrylate (BM-950B). The polyethylene porous substrate is taken out and dried at 80 °C to form a reinforcing film on the surface of the polyethylene porous substrate and the inner wall of the porous structure. Then, an inorganic particle coating solution obtained by uniformly mixing 38 g of alumina (CQ-030EN, purchased from Shandong Guoci Functional Materials Co., Ltd., China), 3.4 g of polyacrylate, 0.68 g of ammonium polyacrylate (BYK-154, solid content 42%, purchased from BKY Company, Germany), 0.13 g of polyether-modified silicone surfactant (BYK-349, purchased from BKY Company, Germany), and 52.9 g of deionized water is coated on the above-mentioned polyethylene porous substrate with a reinforcing film and dried to form an inorganic layer with a thickness of 2 μm on both sides of the polyethylene porous substrate, and a high tensile strength separator film with a total thickness of 13 μm is formed.

[0038] The high tensile strength separator film of the present utility model forms a reinforcing film on at least one surface of the polyolefin porous substrate and the inner wall of the plurality of porous structures, which can increase the mechanical strength of the polyolefin porous substrate, and strengthen the tensile strength, puncture strength, and compression resistance of the separator film without increasing the total thickness of the separator film. As Figure 2 shown, Figure 2 is an image of a scanning electron microscope (SEM) at a magnification of 40,000 times of an embodiment of the high tensile strength separator film of the present utility model. Relative Figure 1In the prior art of the wet isolation film image for inorganic coatings, the high tensile strength isolation film of the present utility model has a reinforced film that can increase the inner wall thickness of the porous structure, so the mechanical strength of the polyolefin porous substrate can be increased. In one embodiment, taking a 9μm polyolefin porous substrate and an isolation film with a 2μm inorganic coating on both sides as an example, the tensile degrees in the longitudinal (MD) direction and the transverse (TD) direction of those without the reinforced film are both less than 1400Kgf / cm 2 and the puncture strength is less than 400gf, while the tensile degrees in the longitudinal (MD) direction and the transverse (TD) direction of the isolation film of the present utility model are both increased to be greater than 1450Kgf / cm 2 , preferably greater than 1500Kgf / cm 2 and the puncture strength is greater than 400gf, preferably greater than 430gf.

[0039] Furthermore, the high tensile strength isolation film of the present utility model has a compression resistance greater than 90% after being compressed under a load of 88Kgf / cm 2 for 30 seconds, providing enhanced compression resistance to inhibit the deformation of the isolation film holes due to stress and maintaining the ionic conductivity in the thickness direction. And after compression, the holes of the isolation film do not undergo excessive deformation due to stress and can still maintain a certain air permeability, that is, the air permeability (Gurley) decreases by less than 40%, which can avoid the increase of the internal resistance or internal pressure of the battery caused by stress during use.

[0040] In this article, the so-called "compression resistance" is calculated according to the following formula from the initial thickness (T1) of the isolation film and the compressed thickness (T2) measured after being held under a load of 88Kgf / cm 2 for 30 seconds. Sample compression rate (%) = (T1 - T2) / T1 × 100; Compression resistance = 100% - Sample compression rate %.

[0041] The so-called "decrease in air permeability (Gurley, sec / 100c.c.) after compression" means that the physical structure of the isolation film is damaged after being held under a load of 88Kgf / cm 2 for 30 seconds, resulting in a decrease in air permeability. The calculation method is that the air permeability before compression is G1 and the air permeability after compression is G2, then the decrease in air permeability after compression (%) = (1 - G1 / G2) × 100%.

[0042] In this article, the tensile strength test of the isolation film is carried out according to the ASTM D882-09 standard. The tested isolation film is cut into pieces with a width of 10mm and a length of ≧150mm along the longitudinal (MD) and transverse (TD) directions respectively, and stretched at a rate of 500mm / min using a universal tensile machine. After obtaining the maximum load value at the moment of specimen fracture, it is divided by the cross-sectional area of the isolation film (specimen width × substrate thickness) to calculate the tensile strengths of the isolation film in the longitudinal (MD) and transverse (TD) directions respectively.

[0043] In this article, for the puncture resistance (gf) test of the separator membrane, a tensile testing machine (MSG-5, purchased from Kato Tech, Japan) was used to measure the puncture strength. A round-headed stainless steel needle with a needle diameter of 1 mm and an R angle of 0.5 mm was used, and the test speed was 100 ± 10 mm / min to puncture the sample to be tested, and the maximum applied force (gf) required to pierce the separator membrane to be tested was recorded.

[0044] Of course, the present utility model may also have many other embodiments. Without departing from the spirit and essence of the present utility model, those skilled in the art can make various corresponding changes and deformations according to the present utility model, but these corresponding changes and deformations should all fall within the protection scope of the appended claims of the present utility model.

Claims

1. A high tensile strength separator film, characterized in that, Comprising: A polyolefin porous substrate having a plurality of porous structures on its surface and inside; A reinforcing film formed on at least one surface of the polyolefin porous substrate and the inner walls of the plurality of porous structures; And An inorganic layer comprising a binder layer and a plurality of inorganic particles dispersed in the binder layer, and formed on the reinforcing film.

2. The high tensile strength separator film according to claim 1, characterized in that, The polyolefin porous substrate is a single-layer polyethylene film or a single-layer polypropylene film.

3. The high tensile strength separator film according to claim 1, characterized in that, The thickness of the polyolefin porous substrate is between 5 µm and 30 µm.

4. The high tensile strength separator film according to claim 1, characterized in that, The porosity of the polyolefin porous substrate is between 30% and 70%.

5. The high tensile strength separator film according to claim 1, characterized in that, The thickness of the inorganic layer is between 0.5 µm and 3 µm.

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