A split-layer structure ultra-large pore diaphragm and preparation method thereof

Abstract

The present invention provides a staggered structure ultra-large pore diaphragm and a preparation method thereof, belonging to the technical field of diaphragms. The present invention adopts a three-layer co-extrusion structure, and the middle layer adopts high molecular weight high density polyethylene, which is phase-separated from long-chain white oil and oxide solid electrolyte microspheres. The long-chain white oil has a higher processing temperature and is easier to control the dryness. The oxide solid electrolyte microspheres serve as ion-conducting carriers, and phase separation also occurs between the discrete phase and the continuous phase to form an ultra-large pore diaphragm. The wide distribution high-density polyethylene surface layer is soft and has good fluidity. The volatilization of the white oil causes thickness expansion, and a larger pore size and a higher porosity can be obtained.

Description

Technical Field

[0001] The invention belongs to the technical field of diaphragms, and in particular to a diaphragm with a staggered structure and a super-large aperture and a preparation method thereof. Background Art

[0002] As an important component of the battery, in addition to the positive electrode, negative electrode, and electrolyte, the battery separator plays a vital role in the battery. The battery separator material is an insulating film containing a large number of microporous structures, and its main components are insulating olefin polymer materials. The separator has two main functions: one is to isolate the positive and negative electrodes in the battery to prevent the two electrodes from directly contacting and short-circuiting. At the same time, it needs to be as thin as possible while ensuring safety to reduce the distance between the two electrodes and reduce the internal resistance of the battery; the second is to be able to store and maintain sufficient electrolyte. The microporous structure allows Li+ in the electrolyte to pass freely, realizing the rapid transmission of Li+ between the positive and negative electrodes. Therefore, the temperature resistance, wettability to the electrolyte, and adhesion to the electrode of the battery separator can directly affect the key performance of the lithium battery, such as capacity, cycle performance, and charge and discharge current density.

[0003] The mainstream preparation methods of battery separators are divided into three categories: dry uniaxial stretch separators, dry biaxial stretch separators, and wet biaxial stretch separators. With the development of semi-solid / solid state, new requirements are also put forward for separators, such as high porosity and large pore size, which are conducive to filling more electrolytes, and high ionic conductivity, which is conducive to the transmission of electrolytes. Those skilled in the art are in urgent need of developing a composite separator for semi-solid batteries and a preparation method thereof to meet the existing usage needs and performance requirements. Summary of the invention

[0004] In view of the above problems, the present invention aims to provide a staggered structure ultra-large pore diaphragm and a preparation method thereof.

[0005] The present invention is achieved through the following technical solutions:

[0006] The present invention provides the following solutions:

[0007] The present invention provides a staggered-layer structure ultra-large pore diaphragm, which is a three-layer co-extruded structure of surface layer / middle layer / surface layer. The surface layer adopts wide-distribution high-density polyethylene and short-chain white oil, and the middle layer adopts high-molecular-weight high-density polyethylene, long-chain white oil and oxide solid electrolyte microspheres. It is made by extruding cast sheets, and then longitudinally stretching, transversely stretching, drying, and pulling and winding.

[0008] The method for preparing the staggered-layer structure ultra-large pore diaphragm provided by the present invention comprises the following steps:

[0009] 40-50 parts of wide distribution high-density polyethylene and 50-60 parts of short-chain white oil are measured by an electronic scale, mixed in a mixing bin, and then put into a first twin-screw extruder; 30-40 parts of high molecular weight high-density polyethylene, 40-65 parts of long-chain white oil and 5-20 parts of oxide solid electrolyte microspheres are measured by an electronic scale, mixed in a mixing bin, and then put into a second twin-screw extruder; (2) adjusting the temperature of the first and second extruders to 160-200°C, filtering after melting, using the material extruded by the first twin-screw extruder as the surface layer, and the material extruded by the second twin-screw extruder as the middle layer, and then extruding through three layers The co-extrusion die is used for in-die compounding and then extrusion, and the die temperature is adjusted to 180-220°C; (3) the melt extruded through the die is cooled at a temperature of 50-80°C, and a three-layer composite thick sheet is obtained at a pulling speed of 20-50 m / min; (4) the thick sheet is first longitudinally stretched at a temperature of 80-120°C with a stretching ratio of 8.0-10.0, and then transversely stretched at a temperature of 100-130°C with a stretching ratio of 8.0-10.0 to obtain a stretched sheet; (5) the stretched sheet is dried and volatilized at a temperature of 110-140°C, and after pulling and thickness measurement, it is rolled up to obtain the staggered structure ultra-large pore diaphragm of the present invention.

[0010] The present invention uses wide distribution high-density polyethylene in the surface layer. Compared with the ultra-high molecular weight high-density polyethylene used in traditional diaphragms, the wide distribution high-density polyethylene has a lower molecular weight and has both high molecular weight segments and low molecular weight segments. This makes the wide distribution high-density polyethylene have very good fluidity. On the one hand, it can improve the uniformity of the diaphragm surface, and can adopt lower extrusion and processing temperatures, which can reduce the volatilization of short-chain white oil before the drying stage. At the same time, better fluidity also makes it easier to expand in thickness when the short-chain white oil evaporates, improves the porosity and pore size of the diaphragm, and matches the pore size of the surface layer with the pore size of the middle layer. Short-chain white oil is a low-flash point volatile pore-forming agent, so it does not need to be removed by extraction, and can be formed by volatilization. Since the pore-forming method is a dynamic volatilization type rather than a static extraction type, a larger porosity and pore size can be obtained during the volatilization process.

[0011] Lithium lanthanum titanium oxide (Li 0.33 La 0.56 TiO3), referred to as LLTO; lithium lanthanum zirconium oxide (Li7La3Zr2O 12 ), referred to as LLZO; lithium aluminum titanium phosphate (Li 1.3 Al 0.3 Ti 1.7 (PO4)3) is referred to as LATP.

[0012] Furthermore, the thickness of the diaphragm is 12 to 40 μm, the porosity is not less than 65%, the average pore size is not less than 100 nm, the maximum pore size is not less than 200 nm, and the ionic conductivity is not less than 10 -3 S / cm, air permeability is not higher than 100s / 100mL, and liquid absorption rate is not lower than 30%. The thickness of the surface layer is 20% to 30% of the total thickness, and the thickness of the middle layer is 40% to 60% of the total thickness.

[0013] Furthermore, the weight ratio of high molecular weight high density polyethylene, long chain white oil and oxide solid electrolyte microspheres in the middle layer is 30-40:40-65:5-20, the weight average molecular weight of the high molecular weight high density polyethylene is 600,000, the polydispersity coefficient is less than 3, the melt index at 190°C is 0.1g / 10min, and the melting point is not less than 130°C; the initial boiling point of the long chain white oil in the surface layer is not less than 220°C, and the viscosity at 40°C is 40mm 2 / s, the oxide solid electrolyte microspheres are one or more of LATP, LLZO, and LLTO, and the particle size of the microspheres is 100nm; the weight ratio of wide-distribution high-density polyethylene and short-chain white oil in the surface layer is 40-50:50-60, the weight-average molecular weight of the wide-distribution high-density polyethylene is 200,000, the polydispersity coefficient is 12, the melt index at 190°C is 7g / 10min, and the melting point is not higher than 135°C; the flash point of the short-chain white oil is not higher than 80°C, and the viscosity at 40°C is 1mm 2 / s.

[0014] The staggered structure ultra-large pore diaphragm of the present invention adopts a drying and volatilization method to remove short-chain white oil from the surface layer and long-chain white oil from the middle layer. Different from the traditional extraction method to remove white oil, the drying and volatilization method can reduce the use of organic solvents such as dichloromethane, which avoids the environmental problems caused by organic solvents and reduces energy consumption. On the other hand, it can reduce the damage of extraction to the appearance and performance of the membrane. At the same time, the drying and volatilization method can make the membrane have a higher porosity and a larger pore size.

[0015] Furthermore, the preparation method comprises mixing the components of the surface layer and the middle layer in proportion and then subjecting the mixture to three-layer co-extrusion, and after casting, longitudinally stretching, transversely stretching, drying, and pulling and winding the three-layer co-extruded diaphragm.

[0016] Beneficial effects of the present invention:

[0017] Compared with the prior art, the super-large pore size diaphragm with staggered structure provided by the present invention solves the problem that the traditional diaphragm has low porosity and small pore size, and when applied to semi-solid / solid batteries, the interface internal resistance is large and cannot accommodate solid electrolytes, and the ionic conductivity and high-power charge and discharge performance of the battery are further improved. The present invention uses a high-density high-molecular-weight polyethylene / oxide solid electrolyte microsphere / long-chain white oil system to form a multiphase structure, and forms solid / liquid phase separation and continuous phase / discontinuous phase phase separation during the casting and stretching process, thereby obtaining a super-large pore size diaphragm with staggered structure having different pore sizes and pore structures. Adding oxide solid electrolyte microspheres as a carrier for electrolyte transmission matches the pore size of the diaphragm, which plays a role in improving the ionic conductivity of the diaphragm. At the same time, the pore-forming agent is removed by drying and volatilization, and a more fluid wide-distribution high-density polyethylene and a more volatile short-chain white oil are used to ensure that the short-chain white oil can be completely volatilized, and the volatilization method also expands the pore size and increases the porosity. DETAILED DESCRIPTION

[0018] The present invention is described below with specific examples, but is not intended to be limiting of the present invention.

[0019] Raw materials: 100nm LLZO inorganic solid electrolyte Ningbo Fengli New Energy LLZO-G3-100; high molecular weight high density polyethylene has a weight average molecular weight of 1 million, a polydispersity coefficient of <3, a melt index of 0.1g / 10min at 190°C, a melting point of not less than 130°C, an initial distillation point of long-chain white oil of not less than 220°C, and a viscosity of 40mm at 40°C 2 / s, the weight average molecular weight of the broad distribution high density polyethylene is 300,000, the polydispersity coefficient is 12, the melt index at 190°C is 7g / 10min, the melting point is not higher than 135°C, the flash point of the short chain white oil is not higher than 80°C, and the viscosity at 40°C is 1mm 2 / s. Example 1

[0020] (1) 40 parts of wide distribution high-density polyethylene and 60 parts of short-chain white oil are measured by an electronic scale, mixed in a mixing bin, and then put into a first twin-screw extruder; 30 parts of high molecular weight high-density polyethylene, 65 parts of long-chain white oil and 5 parts of oxide solid electrolyte microspheres are measured by an electronic scale, mixed in a mixing bin, and then put into a second twin-screw extruder;

[0021] (2) The temperature of the first and second extruders is adjusted to 180°C. After melting, the material extruded by the first twin-screw extruder is filtered as the surface layer, and the material extruded by the second twin-screw extruder is used as the middle layer. After compounding in the three-layer co-extrusion die, the die temperature is adjusted to 200°C.

[0022] (3) The melt extruded through the die is cooled at a temperature of 70°C, and a three-layer composite thick sheet is obtained at a pulling speed of 30 m / min; the thick sheet is first longitudinally stretched at a temperature of 100°C with a stretching ratio of 9.0, and then transversely stretched at a temperature of 120°C with a stretching ratio of 9.0 to obtain a stretched sheet; the stretched sheet is dried and volatilized at a temperature of 130°C, and after pulling and thickness measurement, it is rolled up to obtain the staggered structure ultra-large pore diaphragm of the present invention, wherein the thickness of the surface layer is 20% of the total thickness, and the thickness of the middle layer is 60% of the total thickness. Example 2

[0023] The same method as in Example 1 is used, except that in step (1), 50 parts of broad distribution high density polyethylene and 50 parts of short chain white oil are used. Example 3

[0024] The same method as in Example 2 is used, except that in step (1), 40 parts of high molecular weight high density polyethylene, 55 parts of long chain white oil and 5 parts of oxide solid electrolyte microspheres are used. Example 4

[0025] The same method as in Example 2 is used, except that in step (1), 40 parts of high molecular weight high density polyethylene, 40 parts of long chain white oil and 20 parts of oxide solid electrolyte microspheres are used. Example 5

[0026] Raw materials: 50 nm LATP inorganic solid electrolyte Ningbo Fengli New Energy LATP-N2-50; high molecular weight high density polyethylene with a weight average molecular weight of 1 million, a polydispersity coefficient of <3, a melt index of 0.1 g / 10 min at 190 ° C, a melting point of not less than 130 ° C, an initial distillation point of long-chain white oil of not less than 220 ° C, and a viscosity of 40 mm at 40 ° C 2 / s, the weight average molecular weight of the broad distribution high density polyethylene is 200,000, the polydispersity coefficient is 12, the melt index at 190°C is 10g / 10min, the melting point is not higher than 135°C, the flash point of the short chain white oil is not higher than 80°C, and the viscosity at 40°C is 1mm 2 / s.

[0027] (1) According to the weight percentage, 50 parts of wide distribution high-density polyethylene and 50 parts of short-chain white oil are measured by electronic scale, mixed in a mixing bin, and then put into a first twin-screw extruder; 40 parts of high molecular weight high-density polyethylene, 40 parts of long-chain white oil and 20 parts of LATP oxide solid electrolyte microspheres are measured by electronic scale, mixed in a mixing bin, and then put into a second twin-screw extruder; (2) The temperature of the first and second extruders is adjusted to 160°C, and the melt is filtered, and the material extruded by the first twin-screw extruder is used as the surface layer, and the material extruded by the second twin-screw extruder is used as the surface layer. As the middle layer, it is extruded after being compounded in a three-layer co-extrusion die, and the die temperature is adjusted to 180°C; (3) the melt extruded through the die is cooled at a temperature of 50-80°C, and a three-layer composite thick sheet is obtained at a pulling speed of 20m / min; (4) the thick sheet is first longitudinally stretched at a temperature of 80°C with a stretching ratio of 8.0, and then transversely stretched at a temperature of 100°C with a stretching ratio of 8.0 to obtain a stretched sheet; (5) the stretched sheet is dried and volatilized at a temperature of 110-140°C, and after pulling and thickness measurement, it is rolled up to obtain the staggered structure ultra-large pore diaphragm of the present invention. Example 6

[0028] Raw materials: LLZO inorganic solid electrolyte Ningbo Fengli New Energy LLZO-G3-100; high molecular weight high density polyethylene has a weight average molecular weight of 600,000, a polydispersity coefficient of <3, a melt index of 2g / 10min at 190°C, a melting point of not less than 130°C, an initial distillation point of long-chain white oil of not less than 220°C, and a viscosity of 80mm at 40°C 2 / s, the weight average molecular weight of the broad distribution high density polyethylene is 300,000, the polydispersity coefficient is 20, the melt index at 190°C is 10g / 10min, the melting point is not higher than 135°C, the flash point of the short chain white oil is not higher than 80°C, and the viscosity at 40°C is 5mm 2 / s.

[0029] According to the weight percentage, 40 parts of wide distribution high-density polyethylene and 60 parts of short-chain white oil are measured by electronic scale, mixed in a mixing bin, and then put into a first twin-screw extruder; 30 parts of high molecular weight high-density polyethylene, 65 parts of long-chain white oil and 5 parts of LLZO oxide solid electrolyte microspheres are measured by electronic scale, mixed in a mixing bin, and then put into a second twin-screw extruder; (2) the temperature of the first and second extruders is adjusted to 200°C, and the melt is filtered, and the material extruded by the first twin-screw extruder is used as the surface layer, and the material extruded by the second twin-screw extruder is used as the surface layer. The intermediate layer is extruded after being compounded in a three-layer co-extrusion die, and the die temperature is adjusted to 220°C; (3) the melt extruded through the die is cooled at a temperature of 50 to 80°C, and a three-layer composite thick sheet is obtained at a pulling speed of 50 m / min; (4) the thick sheet is first longitudinally stretched at a temperature of 120°C with a stretching ratio of 10.0, and then transversely stretched at a temperature of 30°C with a stretching ratio of 10.0 to obtain a stretched sheet; (5) the stretched sheet is dried and volatilized at a temperature of 140°C, and after pulling and thickness measurement, it is rolled up to obtain the staggered structure ultra-large pore diaphragm of the present invention.

[0030] Comparative Example 1

[0031] The same method as in Example 1 is used, except that in step (1), no ion-conducting polymer matrix is ​​added to the intermediate layer, which is 30 parts of high molecular weight high density polyethylene and 70 parts of long chain white oil.

[0032] Comparative Example 2

[0033] The same method as in Example 1 is used, except that step (1) only includes the surface layer.

[0034] Comparative Example 3

[0035] The same method as in Example 1 is used, except that step (1) only includes the intermediate layer.

[0036] The results of the porosity, average pore size, maximum pore size, air permeability, and liquid absorption rate of the separators prepared in Examples 1 to 4 and Comparative Examples 1 to 3 are shown in Table 1 below.

[0037] Table 1 Test results of the diaphragms of Examples 1 to 4 and Comparative Examples 1 to 3

[0038]

[0039] It can be concluded from the test results of the embodiments and comparative examples shown in Table 1 that the super-large pore size diaphragm with staggered structure provided by the present invention can effectively improve the porosity, average pore size and maximum pore size of the diaphragm, high ionic conductivity, and high air permeability and liquid absorption rate. Compared with the prior art, the super-large pore size diaphragm with staggered structure provided by the present invention solves the problem that the traditional diaphragm has low porosity and small pore size, and cannot accommodate solid electrolytes when applied to semi-solid / solid batteries, and further improves the ionic conductivity and high-power charge and discharge performance of the battery. The present invention uses a high-density high-molecular-weight polyethylene / oxide solid electrolyte microsphere / long-chain white oil system to form a multiphase structure, and forms solid / liquid phase separation and continuous phase / discontinuous phase separation during the casting and stretching process, thereby obtaining a super-large pore size diaphragm with staggered structure having different pore sizes and pore structures. The addition of oxide solid electrolyte microspheres as a carrier for electrolyte transmission matches the pore size of the diaphragm, which plays a role in improving the ionic conductivity of the diaphragm. At the same time, the pore-forming agent is removed by drying and volatilization. At the same time, more fluid wide-distribution high-density polyethylene and more volatile short-chain white oil are used to ensure that the short-chain white oil can evaporate completely. At the same time, the volatilization method also expands the pore size and increases the porosity.

[0040] Note: The above-mentioned split-layer structure super-large aperture diaphragm was cut into A4 size and tested for various performances. The test items are as follows:

[0041] (1) Average thickness

[0042] Use a micrometer to measure the thickness of the sample at different locations and calculate the average value.

[0043] (2) Porosity

[0044] The average density of the sample was tested using an electronic balance, and the porosity of the sample was calculated based on the theoretical density. Three samples were tested and their average value was calculated.

[0045] (3) Aperture

[0046] The pore size of the samples was tested using a pore size analyzer. Three samples were tested and their average pore size and maximum pore size were calculated.

[0047] (4) Air permeability

[0048] The air permeability of the five-layer co-extruded microporous membrane was tested using an air permeability tester. Five samples were tested and their average value was calculated.

[0049] (5) Liquid absorption rate

[0050] The liquid absorption rate of the diaphragm is tested by weighing method. First, the mass of the diaphragm is recorded after it is completely dried. Then, the completely dried diaphragm is immersed in electrolyte for 24 hours. After the surface electrolyte is wiped dry, the weight of the diaphragm is recorded again. The difference between the two records is the liquid absorption rate of the diaphragm. Test 5 samples and calculate the average value.

[0051] (6) Ionic conductivity

[0052] The diaphragm was assembled into a button cell, the electrolyte was 1M dimethyl lithium hexafluorophosphate solution, and the ionic conductivity of the diaphragm was tested.

[0053] The above is only a preferred embodiment of the present invention and is not intended to limit the protection scope of the present invention. All equivalent changes and modifications made according to the content of the present invention are included in the patent scope of the present invention.

Claims

1. A split-layer structure ultra-large pore diaphragm, characterized in that: The staggered structure ultra-large pore diaphragm is a three-layer co-extruded structure of surface layer / middle layer / surface layer; the surface layer raw materials include wide distribution high-density polyethylene and short-chain white oil, and the weight ratio of wide distribution high-density polyethylene and short-chain white oil is 40-50:50-60; the flash point of the short-chain white oil is not higher than 80°C, and the viscosity at 40°C is 1-5mm 2 / s, the weight average molecular weight of the wide distribution high-density polyethylene is 200,000 to 300,000, the polydispersity coefficient is 12 to 20, the melt index at 190°C is 7 to 10g / 10min, and the melting point is not higher than 135°C; the intermediate layer raw materials include high molecular weight high-density polyethylene, long-chain white oil and oxide solid electrolyte microspheres, the initial distillation point of the long-chain white oil is not lower than 220°C, and the viscosity at 40°C is 40 to 80mm 2 / s; the staggered structure ultra-large pore diaphragm is made by extruding a cast sheet, and then longitudinally stretching, transversely stretching, drying, and pulling and rolling, wherein the temperature in the drying stage is 110-140°C.

2. A method for preparing a staggered structure ultra-large pore diaphragm as claimed in claim 1, characterized in that: The following steps are involved: (1) By weight, 40 to 50 parts of wide distribution high-density polyethylene and 50 to 60 parts of short-chain white oil are weighed by an electronic scale, mixed in a mixing bin, and then put into a first twin-screw extruder; 30 to 40 parts of high molecular weight high-density polyethylene, 40 to 65 parts of long-chain white oil and 5 to 20 parts of oxide solid electrolyte microspheres are weighed by an electronic scale, mixed in a mixing bin, and then put into a second twin-screw extruder; (2) adjusting the temperature of the first and second extruders to 160-200°C, filtering after melting, using the material extruded by the first twin-screw extruder as the surface layer, and the material extruded by the second twin-screw extruder as the middle layer, extruding after compounding in a three-layer co-extrusion die, and adjusting the die temperature to 180-220°C; (3) The melt extruded through the die is cooled at a temperature of 50 to 80° C., and a three-layer composite thick sheet is obtained at a pulling speed of 20 to 50 m / min; (4) The thick sheet is firstly longitudinally stretched at a temperature of 80 to 120° C. with a stretching ratio of 8.0 to 10.0, and then transversely stretched at a temperature of 100 to 130° C. with a stretching ratio of 8.0 to 10.0 to obtain a stretched sheet; (5) The stretched sheet is dried and volatilized at a temperature of 110 to 140° C., and after being pulled and thickness measured, it is rolled up to obtain the staggered structure ultra-large pore diaphragm.

3. The method for preparing a staggered structure ultra-large pore diaphragm according to claim 2, characterized in that: The thickness of the surface layer is 20% to 30% of the total thickness, and the thickness of the intermediate layer is 40% to 60% of the total thickness.

4. The method for preparing a staggered structure ultra-large pore diaphragm according to claim 2, characterized in that: The weight ratio of high molecular weight high density polyethylene, long chain white oil and oxide solid electrolyte microspheres in the intermediate layer is 30-40:40-65:5-20.

5. The method for preparing a staggered structure ultra-large pore diaphragm according to claim 2, characterized in that: The high molecular weight high density polyethylene has a weight average molecular weight of 600,000 to 1,000,000, a polydispersity coefficient of less than 3, a melting index of 0.1 to 2 g / 10 min at 190° C., and a melting point of not less than 130° C.

6. The method for preparing a staggered structure ultra-large pore diaphragm according to claim 2, characterized in that: The oxide solid electrolyte microspheres are one or more of LATP and LLZO, and the particle size of the microspheres is 50-100 nm.

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