阅读说明：本技术 聚丙烯隔膜及其制备方法和锂离子电池 (Polypropylene diaphragm, preparation method thereof and lithium ion battery ) 是由 田如辉 秦银银 颜鸿彬 张佳宁 刘孟权 杨雪梅 陈秀峰 于 2020-06-01 设计创作，主要内容包括：本发明涉及一种聚丙烯隔膜及其制备方法和锂离子电池。上述聚丙烯隔膜包括依次层叠的第一表层、中间层和第二表层,第一表层和第二表层的原料包括熔体流动指数为6.0g/10min～12.0g/10min、重均分子量为2.0×10～(5)～3.5×10～(5)的聚丙烯,中间层的原料包括熔体流动指数为0.3g/10min～1.0g/10min、重均分子量为3.5×10～(5)～6.0×10～(5)的聚丙烯。上述聚丙烯隔膜通过对聚丙烯原料的选择和膜层的设计,使得聚丙烯隔膜具有改善的机械强度、离子迁移性能和电解质润湿性能,对电性能的匹配程度较高。(The invention relates to a polypropylene diaphragm, a preparation method thereof and a lithium ion battery. The polypropylene diaphragm comprises a first surface layer, a second surface layer, a third surface layer and a fourth surface layer which are sequentially laminated,The raw materials of the first surface layer and the second surface layer comprise a melt flow index of 6.0g/10 min-12.0 g/10min and a weight average molecular weight of 2.0 multiplied by 10 5 ～3.5×10 5 The middle layer of the polypropylene comprises the raw materials with the melt flow index of 0.3g/10 min-1.0 g/10min and the weight average molecular weight of 3.5 multiplied by 10 5 ～6.0×10 5 The polypropylene of (1). The polypropylene diaphragm has improved mechanical strength, ion migration performance and electrolyte wettability through selection of polypropylene raw materials and design of the film layer, and has high matching degree of electrical performance.)

1. The polypropylene diaphragm is characterized by comprising a first surface layer, a middle layer and a second surface layer which are sequentially laminated, wherein the raw materials of the first surface layer and the second surface layer comprise a melt flow index of 6.0g/10 min-12.0 g/10min and a weight average molecular weight of 2.0 multiplied by 10⁵～3.5×10⁵The raw material of the middle layer comprises melt flow index of 0.3g/10 min-1.0 g/10min and weight average molecular weight of 3.5 multiplied by 10⁵～6.0×10⁵The polypropylene of (1).

2. The polypropylene separator according to claim 1, wherein the average pore diameter of the first skin layer and the second skin layer is 40nm to 80 nm.

3. The polypropylene membrane according to claim 1 or 2, wherein the polypropylene membrane has a pore tortuosity of 2.0-4.5.

4. The polypropylene separator according to claim 1, wherein the raw material of the intermediate layer is 20 to 70% by mass of the raw material of the polypropylene separator.

5. The polypropylene separator according to claim 4, wherein the raw material of the intermediate layer is 20 to 60% by mass.

6. The polypropylene separator according to claim 1, wherein the polypropylene separator has a total thickness of 5 to 25 μm.

7. The polypropylene separator according to claim 1, wherein the polypropylene separator has a porosity of 20 to 70%.

8. The preparation method of the polypropylene diaphragm is characterized by comprising the following steps:

respectively melting and plasticizing raw materials of a first surface layer, a middle layer and a second surface layer, and then sequentially laminating and co-extruding to form three layers of fluid, wherein the raw materials of the first surface layer and the second surface layer comprise a melt flow index of 6.0g/10 min-12.0 g/10min and a weight average molecular weight of 2.0 multiplied by 10⁵～3.5×10⁵The raw material of the middle layer comprises melt flow index of 0.3g/10 min-1.0 g/10min and weight average molecular weight of 3.5 multiplied by 10⁵～6.0×10⁵The polypropylene of (4);

drawing and cooling the three layers of fluid to obtain a three-layer nonporous precursor film;

annealing the three-layer nonporous precursor film; and

and stretching and pore-forming the three-layer nonporous precursor film after annealing treatment to obtain the polypropylene diaphragm.

9. The method for preparing a polypropylene separator according to claim 8, wherein the drawing speed is 60 to 150m/min, and the cooling temperature is 40 to 100 ℃.

10. The preparation method of the polypropylene separator as claimed in claim 8, wherein the annealing temperature is 120 ℃ to 165 ℃ and the annealing time is 0.1h to 24 h.

11. The method for preparing a polypropylene separator according to claim 8, wherein the temperature for the stretch pore-forming is 100 ℃ to 150 ℃, the speed for the stretch pore-forming is 3m/min to 15m/min, and the total magnification for the stretch pore-forming is 1.5 times to 3.0 times.

12. A lithium ion battery, which is characterized by comprising a positive electrode, a negative electrode, an electrolyte and a separator, wherein the separator is the polypropylene separator as defined in any one of claims 1 to 7 or the polypropylene separator prepared by the preparation method of the polypropylene separator as defined in any one of claims 8 to 11.

Technical Field

The invention relates to the field of diaphragms, in particular to a polypropylene diaphragm, a preparation method thereof and a lithium ion battery.

Background

The lithium ion battery is widely applied to industries such as new energy automobiles, energy storage power stations, electric tools, military industry and the like as new energy which can be produced and applied in large scale at present, and the lithium ion battery diaphragm is one of four main materials of the lithium ion battery and is very mature after decades of development. However, with the demand of increasing the energy density of lithium ion batteries, the thickness of the diaphragm is required to be thinner and thinner, from 32 micrometers to 25 micrometers and 20 micrometers, and then to 16 micrometers and 12 micrometers which are commonly used at present, the improvement of the energy density of the batteries by thinning the diaphragm is a future trend on the premise of ensuring the safety performance of the batteries.

For a dry-method diaphragm, the dry-method diaphragm is mainly a one-way stretching product, the mechanical strength is generally weak, and the problem generally needs to be improved in the diaphragm thinning process. The conventional method can improve the mechanical strength of the separator, but simultaneously can cause the reduction of the ion migration performance and the electrolyte wetting performance of the separator, so that the matching degree of the electrical performance is obviously reduced.

Disclosure of Invention

Based on the above, there is a need for a polypropylene separator, which can be prepared by a dry method and has improved mechanical strength, ion mobility and electrolyte wettability, so that the degree of matching of electrical properties is high.

In addition, a preparation method of the polypropylene diaphragm and a lithium ion battery are also provided.

The polypropylene diaphragm comprises a first surface layer, a middle layer and a second surface layer which are sequentially laminated, wherein the raw materials of the first surface layer and the second surface layer comprise a melt flow index of 6.0g/10 min-12.0 g/10min and a weight average molecular weight of 2.0 multiplied by 10⁵～3.5×10⁵The raw material of the middle layer comprises melt flow index of 0.3g/10 min-1.0 g/10min and weight average molecular weight of 3.5 multiplied by 10⁵～6.0×10⁵The polypropylene of (1).

In one embodiment, the first skin layer and the second skin layer have an average pore size of 40nm to 80 nm.

In one embodiment, the pore bending degree of the polypropylene diaphragm is 2.0-4.5.

In one embodiment, in the raw material of the polypropylene diaphragm, the mass percentage of the raw material of the middle layer is 20-70%.

In one embodiment, the mass percentage of the raw material of the intermediate layer is 20% to 60%.

In one embodiment, the polypropylene separator has a total thickness of 5 to 25 μm.

In one embodiment, the polypropylene separator has a porosity of 20% to 70%.

A preparation method of a polypropylene separator comprises the following steps:

respectively melting and plasticizing raw materials of a first surface layer, a middle layer and a second surface layer, and then sequentially laminating and co-extruding to form three layers of fluid, wherein the raw materials of the first surface layer and the second surface layer comprise a melt flow index of 6.0g/10 min-12.0 g/10min and a weight average molecular weight of 2.0 multiplied by 10⁵～3.5×10⁵The raw material of the intermediate layer comprises a melt flow index of 0.3g/10min to 1.0g/10min, and the weight average molecular weight is 3.5 multiplied by 10⁵～6.0×10⁵The polypropylene of (4);

drawing and cooling the three layers of fluid to obtain a three-layer nonporous precursor film;

annealing the three-layer nonporous precursor film; and

and stretching and pore-forming the three-layer nonporous precursor film after annealing treatment to obtain the polypropylene diaphragm.

In one embodiment, the drawing speed is 60-150 m/min, and the cooling temperature is 40-100 ℃.

In one embodiment, the temperature of the annealing treatment is 120-165 ℃, and the time of the annealing treatment is 0.1-24 h.

In one embodiment, the temperature of the stretching pore-forming is 100-150 ℃, the speed of the stretching pore-forming is 3-15 m/min, and the total multiplying power of the stretching pore-forming is 1.5-3.0 times.

A lithium ion battery comprises a positive electrode, a negative electrode, electrolyte and a diaphragm, wherein the diaphragm is the polypropylene diaphragm or the polypropylene diaphragm prepared by the preparation method of the polypropylene diaphragm.

The polypropylene diaphragm has the advantages that through the selection of polypropylene raw materials and the design of the film layer, the first surface layer and the second surface layer of the polypropylene diaphragm have large pore structures, the migration capacity of ions in electrolyte can be improved, the pore diameter of the middle layer is small, the wettability of the electrolyte and the diaphragm can be improved, and meanwhile, the mechanical strength is improved. Therefore, the first surface layer, the middle layer and the second surface layer are mutually matched, so that the polypropylene diaphragm has improved mechanical strength, ion migration performance and electrolyte wettability, and has higher matching degree of electrical performance.

Drawings

FIG. 1 is a process flow diagram of a method of making a polypropylene separator according to one embodiment;

fig. 2 is a sectional SEM image of the polypropylene separator prepared in example 1.

Detailed Description

In order that the invention may be more fully understood, reference will now be made to the following description taken in conjunction with the accompanying drawings. The detailed description sets forth the preferred embodiments of the invention. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete.

Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. The terminology used in the description of the invention herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention.

The polypropylene separator of one embodiment includes a first skin layer, an intermediate layer, and a second skin layer stacked in this order. The polypropylene diaphragm is a three-layer diaphragm with a symmetrical structure.

Wherein the melt flow index of the polypropylene raw materials of the first surface layer and the second surface layer is 6.0g/10 min-12.0 g/10min, and the weight average molecular weight is 2.0 multiplied by 10⁵～3.5×10⁵. In one embodiment, the polypropylene raw materials of the first skin layer and the second skin layer have a melt flow index of 6g/10min, 7g/10min, 8g/10min, 9g/10min, 10g/10min, 11g/10min, or 12g/10 min. The polypropylene raw materials of the first skin layer and the second skin layer have a weight average molecular weight of 2.0 × 10⁵、2.2×10⁵、2.5×10⁵、2.8×10⁵、3.0×10⁵、3.2×10⁵Or 3.5X 10⁵。

The melt flow index of the polypropylene raw material of the middle layer is 0.3g/10 min-1.0 g/10min, and the weight average molecular weight is 3.5 multiplied by 10⁵～6.0×10⁵. In one embodiment, the polypropylene of the middle layer has a melt flow index of 0.3g/10min, 0.4g/10min, 0.5g/10min, 0.6g/10min, 0.7g/10min, 0.8g/10min, 0.9g/10min, or 1.0g/10 min. The weight average molecular weight of the polypropylene of the middle layer is 3.5X 10⁵、3.8×10⁵、4.0×10⁵、4.2×10⁵、4.5×10⁵、5.0×10⁵、5.2×10⁵、5.5×10⁵、5.8×10⁵Or 6.0X 10⁵。

In the embodiment, the polypropylene raw materials used for the first surface layer and the second surface layer have higher melt flow indexes, so that the processing difficulty is reduced, and the production efficiency is improved. And through the raw material selection of the first surface layer, the middle layer and the second surface layer, the defect of poor mechanical strength caused by a macroporous structure of the outer layer is avoided, so that the polypropylene diaphragm has improved mechanical strength, ion migration performance and electrolyte wettability, and the matching degree of the diaphragm to the electrical property is improved.

Preferably, the first skin layer and the second skin layer are the same composition. The polypropylene of first top layer and second top layer constitutes the same, therefore the performance is similar, and under the same process conditions, the rete structure that obtains is similar to make above-mentioned polypropylene diaphragm need not to do the ectonexine when using and distinguish, it is more convenient to use, and can avoid the interior nexine to misplace the safety risk that leads to.

The average pore diameter of the first surface layer is the same as that of the second surface layer, and is larger than that of the intermediate layer. Specifically, the average pore diameter of the first surface layer and the second surface layer ranges from 40nm to 80 nm. The average pore diameter of the intermediate layer is 20nm to 35 nm. The first surface layer and the second surface layer have an average pore diameter within the above range, have a large pore structure, and are capable of improving ion mobility and electrolyte wettability. And the pore diameter of the intermediate layer is smaller, so that the mechanical strength of the diaphragm can be improved, and the self-discharge performance of the battery can be improved.

Specifically, the pore bending degree of the polypropylene diaphragm is 2.0-4.5. Preferably, the pore bending degree of the polypropylene diaphragm is 2.5-4.0. The inventor finds through a large number of experiments that too much pore tortuosity reduces the ion mobility, while too little pore tortuosity affects the wettability of the electrolyte, especially the liquid retention. And the pore curvature can enable the polypropylene diaphragm to have good electrolyte wettability and ion migration capability.

Specifically, in the raw materials of the polypropylene diaphragm, the mass percentage of the polypropylene raw material of the middle layer is 20-70%. Further, the mass percentage of the polypropylene raw material of the middle layer is 20-60%. Furthermore, the mass percentage of the polypropylene raw material of the middle layer is 20-50%. In one embodiment, the raw material of the intermediate layer is 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, or 70% by mass. By controlling the ratio of the polypropylene raw material in the middle layer, the mechanical strength and the surface resistance of the polypropylene diaphragm can reach better performances.

Further, the mass percentages of the raw materials of the first surface layer and the second surface layer are the same.

In the present embodiment, the number of layers of the polypropylene separator is not limited to three, and may be six or nine. And when the polypropylene diaphragm is six layers, taking the first surface layer, the middle layer and the second surface layer which are sequentially laminated as a film layer unit, and laminating the two film layer units to obtain the six-layer polypropylene diaphragm. And when the polypropylene diaphragm is nine layers, taking the first surface layer, the middle layer and the second surface layer which are sequentially stacked as a film layer unit, and stacking the three film layer units to obtain the nine-layer polypropylene diaphragm.

Specifically, the total thickness of the polypropylene separator is 5 to 25 μm. In one embodiment, the polypropylene separator has a total thickness of 5 μm, 8 μm, 10 μm, 12 μm, 15 μm, 16 μm, 20 μm, or 25 μm.

Experiments prove that the polypropylene diaphragm has the following properties: the Gurley number is 30sec/100 mL-800 sec/100mL, the porosity is 20% -70%, and the longitudinal tensile strength is more than 1700kgf/cm²The thermal shrinkage at 105 ℃ is less than or equal to 3 percent, and the puncture strength per unit thickness is more than or equal to 20 g/mu m. Further, the porosity of the polypropylene diaphragm is 30-60%.

One conventional art discloses an improved low electrical impedance microporous battery separator membrane comprising: extruding polypropylene having a melt flow index of less than 1.0g/10 minutes to form a single layer nonporous precursor film, stretching the nonporous polypropylene precursor film in a machine direction to form a semi-porous intermediate film having a puncture strength of >350gf and a TD elongation of > 600%, stretching the semi-porous intermediate film in a transverse direction, and stretching using a stretch ratio of 15% to 400%, preferably using a stretch ratio of 25% to 100%, to form a microporous separator. When the polypropylene with the melt flow index less than 1.0g/10min is adopted in the method, the production efficiency of the product is obviously influenced to a certain extent by a step-by-step multi-time stretching process method.

In addition, a three-layer co-extrusion dry method PP-PE-PP diaphragm is disclosed in the technology, the diaphragm is specifically of an A-B-C asymmetric structure, the raw material of the A film layer is 2.0-6.0 g/10min in melt index, and the weight average molecular weight is 2 multiplied by 10⁵～3.5×10⁵The raw material of the film layer B is 0.2-1.0 g/10min of melt index and 1 multiplied by 10 of weight average molecular weight⁵～2.5×10⁵The high-density polyethylene of (1) has a C film layer made of a raw material with a melt index of 0.3-1.0 g/10min and a weight average molecular weight of 4 x 10⁵～6×10⁵When the polypropylene is used, the A film layer is used as an inner layer, the C film layer is used as an outer layer and faces to a negative electrode, the A film layer is used as the inner layer, the aperture is large, the ion migration capacity is strong, and the C film layer is used as the outer layer, the aperture is small, the contact angle is small, and the wettability to electrolyte is good. Although the asymmetric structure improves the defect of the mechanical property of the large hole to a certain extent, the product needs to be distinguished by polypropylene on the inner layer and the outer layer when in use, and safety accidents are easily caused by misplacement. And the melt flow index of the raw materials adopted by the diaphragm is still lower, so that the processing difficulty is increased.

Therefore, the conventional polypropylene separator cannot balance the mechanical strength, ion mobility, and electrolyte wettability of the separator at a relatively thin thickness, so that the application of the separator is limited.

In addition, compared with a polypropylene/polyethylene/polypropylene composite diaphragm, the polypropylene composite diaphragm of the embodiment has no composite polyethylene layer because of different performances of polypropylene and polyethylene, has poor compatibility during composite molding, and cannot give consideration to synchronous casting and synchronous stretching pore-forming, so that the technical defect that the uniform composite diaphragm with high porosity is obtained by adding other additives exists, the technical difficulty is high, and the processing cost is high. And under the condition of the same melt flow index, the pore diameter of the polyethylene diaphragm is larger than that of the polypropylene diaphragm, so that the self-discharge phenomenon of the battery is easily caused. Therefore, in the embodiment, polypropylene with different melt flow indexes and molecular weights is used as a raw material, the problem of poor compatibility during product compounding is solved, the compatibility of the obtained polypropylene diaphragm is better, the process difficulty is low, and the aperture of the middle layer is smaller.

The polypropylene separator of the present embodiment has at least the following advantages:

(1) the polypropylene diaphragm is formed by the dry-process multilayer co-extrusion of polypropylene diaphragm with improved mechanical strength, ion migration performance and electrolyte wettability through the selection of polypropylene materials and the structural design of the polypropylene diaphragm and the matching use of polypropylene raw materials with different melt indexes.

(2) The first surface layer and the second surface layer of the polypropylene diaphragm are of a macroporous structure with symmetrical structure, and the polypropylene diaphragm does not need to be distinguished by an inner layer and an outer layer when in use, so that the polypropylene diaphragm is simple and convenient, and the product safety risk is avoided.

(3) The polypropylene diaphragm is matched with polypropylene raw materials with different melt indexes, so that the consumption of the raw materials with low melt flow index is less, the processing difficulty of the product is further reduced by using the polypropylene material with high melt flow index, the hole structure of the composite product is ensured by matching use, and the product can meet the performance requirement.

Referring to fig. 1, a method for preparing a polypropylene separator according to an embodiment includes the following steps:

step S110: and respectively melting and plasticizing the raw materials of the first surface layer, the middle layer and the second surface layer, and then sequentially laminating and co-extruding to form three layers of fluid.

Wherein, the three-layer fluid structure is a first surface layer, a middle layer and a second surface layer.

Specifically, the raw materials of the first surface layer and the second surface layer comprise a melt flow index of 6.0g/10 min-12.0 g/10min and a weight average molecular weight of 2.0 multiplied by 10⁵～3.5×10⁵The middle layer of the polypropylene comprises the raw materials with the melt flow index of 0.3g/10 min-1.0 g/10min and the weight average molecular weight of 3.5 multiplied by 10⁵～6.0×10⁵The polypropylene of (1).

In step S110, the raw materials of the surface layer and the intermediate layer are melted and plasticized in different extruders. The raw materials on the surface layer are uniformly distributed into two different runners through the sub-runners. The raw materials of the intermediate layer can be combined in the die cavity by a distributor to form a structure of a first surface layer, an intermediate layer and a second surface layer. Or two layers of flow separated from the surface layer before extrusion of the die head through the cavity-divided die head are merged with the raw material flow of the middle layer to form a structure of a first surface layer, the middle layer and a second surface layer.

Specifically, the processing temperature of the raw materials of the first surface layer and the second surface layer is 170-250 ℃. The processing temperature of the raw material of the middle layer is 170-250 ℃.

Step S120: and drawing and cooling the three layers of fluid to obtain the three layers of nonporous precursor films.

Wherein the traction speed is 60 m/min-150 m/min, and the cooling temperature is 40-100 ℃. Specifically, the three layers of fluid are drawn and cooled by cooling rollers.

Specifically, the three-layer fluid may be drawn and cooled by an extrusion blow molding method or a casting method in step S120. The methods of extrusion blow molding and casting may be methods commonly used in the art and will not be described herein.

Step S130: and annealing the three-layer nonporous precursor film.

Wherein the temperature of the annealing treatment is 120-165 ℃, and the time of the annealing treatment is 0.1-24 h.

Step S140: and stretching and pore-forming the three-layer nonporous precursor film after annealing treatment to obtain the polypropylene diaphragm.

Specifically, in step S140, the temperature for drawing and pore-forming is 100 ℃ to 150 ℃. The stretching pore-forming speed is 3m/min to 15 m/min. The total multiplying power of the stretching pore-forming is 1.5-3.0 times.

The preparation method of the polypropylene diaphragm obtains the polypropylene diaphragm through extrusion, cooling forming, annealing and stretching pore-forming treatment. The polypropylene diaphragm has improved mechanical strength, ion migration performance and electrolyte wettability.

Specifically, the average pore size of the first skin layer and the second skin layer is the same and larger than the average pore size of the intermediate layer. Specifically, the average pore diameter of the first surface layer and the second surface layer ranges from 40nm to 80 nm. The average pore diameter of the intermediate layer is 20nm to 35 nm. The first surface layer and the second surface layer have an average pore diameter within the above range, have a large pore structure, and can improve the mobility of ions in the electrolytic solution. And the aperture of the middle layer is smaller, so that the wettability of the electrolyte and the diaphragm can be improved, and the mechanical strength of the diaphragm is improved.

Specifically, the pore bending degree of the polypropylene diaphragm is 2.0-4.5. Preferably, the pore bending degree of the polypropylene diaphragm is 2.5-4.0. The inventor finds through a large amount of experiments that too much pore bending reduces the mobility of ions, and too little pore bending affects the wettability of the electrolyte. The pore curvature enables the polypropylene diaphragm to have good electrolyte wettability and ion migration capability.

Specifically, in the raw materials of the polypropylene diaphragm, the mass percentage of the polypropylene raw material of the middle layer is 20-70%. Further, the mass percentage of the polypropylene raw material of the middle layer is 20-60%. Furthermore, the mass percentage of the polypropylene raw material of the middle layer is 20-50%. In one embodiment, the raw material of the intermediate layer is 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, or 70% by mass. By controlling the ratio of the polypropylene raw material in the middle layer, the mechanical strength and the surface resistance of the polypropylene diaphragm can reach better performances.

Further, the mass percentages of the raw materials of the first surface layer and the second surface layer are the same.

In the present embodiment, the number of layers of the polypropylene separator is not limited to three, and may be six or nine. And when the polypropylene diaphragm is six layers, taking the first surface layer, the middle layer and the second surface layer which are sequentially laminated as a film layer unit, and laminating the two film layer units to obtain the six-layer polypropylene diaphragm. And when the polypropylene diaphragm is nine layers, taking the first surface layer, the middle layer and the second surface layer which are sequentially stacked as a film layer unit, and sequentially stacking the three film layer units to obtain the nine-layer polypropylene diaphragm. Among them, the step of laminating the film layer unit may be a step commonly used in the art.

Specifically, the total thickness of the polypropylene separator is 5 to 25 μm. In one embodiment, the polypropylene separator has a total thickness of 5 μm, 8 μm, 10 μm, 12 μm, 15 μm, 16 μm, 20 μm, or 25 μm.

Experiments prove that the polypropylene diaphragm has the following properties: the Gurley number is 30sec/100 mL-800 sec/100mL, the porosity is 20% -70%, and the longitudinal tensile strength is more than 1700kgf/cm²The thermal shrinkage at 105 ℃ is less than or equal to 3 percent, and the puncture strength per unit thickness is more than or equal to 20 g/mu m. Further, the porosity of the polypropylene diaphragm is 30-60%.

The polypropylene microporous diaphragm prepared by the method has the structure of the first surface layer, the middle layer and the second surface layer, so that the surface layer structure with a large surface pore structure and the middle layer structure for improving the mechanical strength are prepared while the production efficiency is achieved, the ion migration capability of electrolyte is improved through the large surface pores while the mechanical strength of the diaphragm is improved, and the wettability of the electrolyte and the diaphragm is improved through the middle small pore structure, so that the polypropylene microporous diaphragm has the improved mechanical strength, ion migration performance and electrolyte wettability.

The following are specific examples. The following examples are merely illustrative of the present invention and should not be construed as limiting thereof.

Example 1

The polypropylene separator of the present embodiment specifically comprises the following raw materials: the raw materials of the first surface layer and the second surface layer have melt flow index of 8.0g/10min and weight average molecular weight of 3.0 × 10⁵The middle layer is made of polypropylene, the melt flow index of the raw material of the middle layer is 0.5g/10min, and the weight average molecular weight is 5.0 multiplied by 10⁵The polypropylene of (1). The mass percentage of the raw material of the intermediate layer is 50%, and the mass percentages of the raw materials of the first surface layer and the second surface layer are 25% and 25%, respectively.

The preparation process of the polypropylene separator of the embodiment is specifically as follows:

(1) extruding: the raw materials of the first surface layer and the middle layer are respectively melted and plasticized in different extruders, and two fluids are converged in the die head through the three-cavity structure of the shunt channel and the die head to form a flow structure of the first surface layer, the middle layer and the second surface layer. Wherein the extruder processing temperature of the raw material of the first surface layer is 200 deg.c and the extruder processing temperature of the raw material of the middle layer is 210 deg.c.

(2) Forming a nonporous precursor film: and drawing and cooling the three-layer fluid of the first surface layer, the middle layer and the second surface layer flowing out of the die head by using cooling rolls, wherein the temperature of the cooling rolls is 80 ℃, and the drawing speed is 100m/min, so that the three-layer nonporous precursor film is obtained.

(3) Annealing: and annealing the three-layer nonporous precursor film at 135 ℃ for 12 hours.

(4) Stretching and pore-forming: and directly carrying out high-temperature stretching pore-forming on the annealed three-layer nonporous precursor film to obtain the polypropylene diaphragm. Wherein the high-temperature stretching temperature is 140 ℃, the stretching speed is 6m/min, and the total stretching multiplying power is 2.2 times.

Example 2

The polypropylene separator of the present embodiment specifically comprises the following raw materials: the raw materials of the first surface layer and the second surface layer have melt flow index of 6.0g/10min and weight average molecular weight of 3.3 × 10⁵The middle layer is made of polypropylene, the melt flow index of the raw material of the middle layer is 0.8g/10min, and the weight average molecular weight is 4.5 multiplied by 10⁵The polypropylene of (1). The mass percentage of the raw material of the intermediate layer is 50%, and the mass percentages of the raw materials of the first surface layer and the second surface layer are 25% and 25%, respectively.

The preparation process of the polypropylene separator of this example is the same as that of the polypropylene separator of example 1, and is not described herein again.

Example 3

The polypropylene separator of the present embodiment specifically comprises the following raw materials: the raw materials of the first surface layer and the second surface layer have melt flow index of 12.0g/10min and weight average molecular weight of 2.0 × 10⁵The polypropylene of (1). The material of the middle layer has melt flow index of 0.3g/10min and weight average molecular weight of 5.5 × 10⁵The polypropylene of (1). The mass percentage of the raw material of the intermediate layer is 50%, and the mass percentages of the raw materials of the first surface layer and the second surface layer are 25% and 25%, respectively.

The preparation process of the polypropylene separator of this example is the same as that of the polypropylene separator of example 1, and is not described herein again.

Example 4

The polypropylene separator of the present embodiment specifically comprises the following raw materials: the raw materials of the first surface layer and the second surface layer have melt flow index of 6.0g/10min and weight average molecular weight of 3.3 × 10⁵The polypropylene of (1). The material of the middle layer has melt flow index of 0.8g/10min and weight average molecular weight of 4.5 × 10⁵The polypropylene of (1). The mass percentage of the raw material of the middle layer is 20%, and the mass percentages of the raw materials of the first surface layer and the second surface layer are 40% and 40%, respectively.

The preparation process of the polypropylene separator of this example is the same as that of the polypropylene separator of example 1, and is not described herein again.

Example 5

The polypropylene separator of the present embodiment specifically comprises the following raw materials: the raw materials of the first surface layer and the second surface layer have melt flow index of 6.0g/10min and weight average molecular weight of 3.3 × 10⁵The polypropylene of (1). The material of the middle layer has melt flow index of 0.8g/10min and weight average molecular weight of 4.5 × 10⁵The polypropylene of (1). The mass percentage of the raw material of the intermediate layer is 35%, and the mass percentages of the raw materials of the first surface layer and the second surface layer are 32.5% and 32.5%, respectively.

The preparation process of the polypropylene separator of this example is the same as that of the polypropylene separator of example 1, and is not described herein again.

Comparative example 1

The polypropylene separator of comparative example 1 was specifically prepared from the following raw materials: the raw materials of the first surface layer, the second surface layer and the middle layer all adopt a melt flow index of 2.0g/10min and a weight average molecular weight of 3.5 multiplied by 10⁵The polypropylene of (1).

The preparation process of the polypropylene separator of comparative example 1 is the same as that of the polypropylene separator of example 1, and thus, a detailed description thereof is omitted.

Comparative example 2

The polypropylene separator of comparative example 2 was specifically prepared from the following raw materials: the raw materials of the first surface layer and the second surface layer have melt flow index of 4.0g/10min and weight average molecular weight of 3.0 × 10⁵The polypropylene of (1). The material of the middle layer has melt flow index of 0.5g/10min and weight average molecular weight of 5.0 × 10⁵Polypropylene (A) ofAn alkene. The mass percentage of the raw material of the intermediate layer is 50%, and the mass percentages of the raw materials of the first surface layer and the second surface layer are 25% and 25%, respectively.

The preparation process of the polypropylene separator of comparative example 2 is the same as that of the polypropylene separator of example 1, and thus, a detailed description thereof is omitted.

Comparative example 3

The polypropylene separator of comparative example 3 was specifically prepared from the following raw materials: the raw materials of the first surface layer and the second surface layer have melt flow index of 14.0g/10min and weight average molecular weight of 3.0 × 10⁵The polypropylene of (1). The material of the middle layer has melt flow index of 0.5g/10min and weight average molecular weight of 5.0 × 10⁵The polypropylene of (1). The mass percentage of the raw material of the intermediate layer is 50%, and the mass percentages of the raw materials of the first surface layer and the second surface layer are 25% and 25%, respectively.

The preparation process of the polypropylene separator of comparative example 3 is the same as that of the polypropylene separator of example 1, and thus, a detailed description thereof is omitted.

Comparative example 4

The polypropylene separator of comparative example 4 was specifically prepared from the following raw materials: the raw materials of the first surface layer and the second surface layer have melt flow index of 8.0g/10min and weight average molecular weight of 3.0 × 10⁵The polypropylene of (1). The material of the middle layer has melt flow index of 1.5g/10min and weight average molecular weight of 5.0 × 10⁵The polypropylene of (1). The mass percentage of the raw material of the intermediate layer is 50%, and the mass percentages of the raw materials of the first surface layer and the second surface layer are 25% and 25%, respectively.

The preparation process of the polypropylene separator of comparative example 4 is the same as that of the polypropylene separator of example 1, and thus, a detailed description thereof is omitted.

The properties of the raw materials of the respective layers of the polypropylene separators of the above examples and comparative examples are specifically shown in table 1 below:

TABLE 1 Properties of the raw materials of the respective layers of the examples and comparative examples

The following are test sections:

1. the cross section of the polypropylene separator prepared in example 5 was tested using a Scanning Electron Microscope (SEM), and a cross-sectional SEM image as shown in fig. 2 was obtained.

2. The polypropylene separators prepared in examples 1 to 5 and comparative examples 1 to 4 were subjected to the following performance tests:

(1) thickness: reference is made to the provisions of GB/T6672-2001, wherein the resolution of the thickness gauge should not be greater than 0.1 μm, and no less than 3 points are measured at equal distances in the width direction, and the average value is taken.

(2) Porosity: the length, width and thickness of the diaphragm were measured as specified in GB/T6673-2001 and GB/T6672-2001, the mass of the sample was weighed with an analytical balance having a resolution of 0.0001g, and the porosity was calculated according to the following formula

P＝(1-m/(L×b×d×ρ))×100％

In the formula: p is the porosity of the separator in%; m is the mass of the diaphragm and is given in g; l is the length of the diaphragm in cm; b is the width of the diaphragm in cm; d is the thickness of the diaphragm in um; rho is the density of the raw material and has the unit of g/cm²。

(3) Tensile strength: the test was carried out as specified in GB/T1040.3-2006 using a type 2 specimen having a width of (15. + -. 0.1) mm, an initial distance between the grips (100. + -.5) mm and a test speed of (250. + -.10) mm/min.

(4) Puncture strength: reference is made to the provisions of GB/T6672-2001, with a load cell resolution of 0.1N, a puncture needle diameter of 1.0mm, a sample holding jig internal diameter of 10mm, a septum flattened and clamped in the jig, the puncture is performed at a rate of (100. + -. 10) mm/min, no less than 3 points are measured, and the values are averaged.

(5) Average pore diameter: mean pore size data were obtained using PMI instrument measurements, pore size expressed in nm.

(6) Liquid absorption and retention: cutting a diaphragm sample with the size of 150mm multiplied by 150mm, and markingAnd weigh m₁Then the sample is unfolded (to fully absorb the electrolyte) and soaked in the electrolyte for 1 h. Then taking out the sample, wiping the electrolyte on the surface of the diaphragm with dust-free cloth until the granular electrolyte can not be seen by naked eyes, and weighing the weight m of the wiped sample₂The liquid absorption rate was calculated as follows: liquid absorption rate ═ m₂-m₁)/m₁×100％；

Spreading the weighed diaphragm, standing for 1h, and weighing the sample weight m₃The liquid retention rate is calculated according to the following formula: retention rate ═ m₃-m₁)/m₁×100％。

(7) Ionic conductivity: a sample preparation mold is adopted to punch a sample diaphragm into a sample with the thickness of 50mm multiplied by 50mm, 4 pieces of the sample diaphragm are taken and placed in electrolyte for sealed soaking for 1 hour, then 1-4 layers of diaphragms are sequentially placed, the resistance value is tested, the number of the diaphragm layers is used as the abscissa, the diaphragm resistance is used as the ordinate to make a curve, the slope of the curve is obtained, and the ion conductivity can be characterized.

(8) Hole tortuosity: the degree of pore bending can be determined by the following equation: τ { (Rm × ∈)/(ρ × t) }^1/2Wherein τ is tortuosity and Rm is membrane resistance (Ω. cm)²) ε represents a porosity (%), ρ represents a resistivity (Ω · cm) of the electrolyte solution, and t represents a film thickness (μm). In this context, 1M LiBF was used at 20 ℃₄1, 2-propylene carbonate/ethylene carbonate (mass ratio: 1/1) as an electrolyte, and ρ was 2.663 × 10^-2Ω·cm。

(9) Surface resistance: measuring the resistance value of a layer of diaphragm by the ion conductivity test method (7), and calculating by the following formula: rm is R/S, where Rm is membrane resistance (Ω. cm)²) R is the resistance value (omega) of the diaphragm, S is the area (m) of the diaphragm to be tested²) (ii) a The surface layer resistance is the surface resistance value of the surface layer of the composite diaphragm.

The test results of the polypropylene separators prepared in the above examples 1 to 5 and comparative examples 1 to 4 are shown in the following table 2.

Table 2 comparison of performance of polypropylene separators of examples and comparative examples

As can be seen from Table 2 above, the polypropylene separators prepared in examples 1 to 5 all had MD tensile strengths of more than 1700Kgf/cm²Even up to 2100Kgf/cm²And has high mechanical strength. And the ion conductivity is higher, and the ion migration capability is better. The pore diameter is appropriate, the liquid absorption rate is high, and the electrolyte wettability is good. Therefore, the polypropylene diaphragm prepared in the embodiment has improved mechanical strength, ion migration performance and electrolyte wettability, and has high matching degree on electrical performance.

The polypropylene with the fused fingers of 2.0 is used in the comparative example 1, and the obtained composite diaphragm has much smaller aperture and larger pore bending degree than the composite diaphragm using different fused fingers in the example 1, so that the composite diaphragm has poor liquid absorption and retention property, low ionic conductivity and large surface resistance value.

Comparative example 2 a polypropylene having a melt index of 4.0 was used for the surface layer, and the resulting composite separator had a smaller pore diameter than example 1, poor liquid absorption and retention properties, and a large degree of pore tortuosity, resulting in low ionic conductivity and a large sheet resistance.

In comparative examples 3 and 4, polypropylene with higher melt index was used in the surface layer and the intermediate layer, respectively, so that the pore structure was too large, and the mechanical strength of the composite separator was significantly reduced although good liquid absorption and retention properties were obtained.

The technical features of the embodiments described above may be arbitrarily combined, and for the sake of brevity, all possible combinations of the technical features in the embodiments described above are not described, but should be considered as being within the scope of the present specification as long as there is no contradiction between the combinations of the technical features.

The above-mentioned embodiments only express several embodiments of the present invention, and the description thereof is more specific and detailed, but not construed as limiting the scope of the invention. It should be noted that, for a person skilled in the art, several variations and modifications can be made without departing from the inventive concept, which falls within the scope of the present invention. Therefore, the protection scope of the present patent shall be subject to the appended claims.