WO2023045384A1

WO2023045384A1 - High-porosity, high-permeability lithium ion battery base film and preparation method therefor, and lithium ion battery

Info

Publication number: WO2023045384A1
Application number: PCT/CN2022/095883
Authority: WO
Inventors: 董秋春; 白耀宗; 刘杲珺; 张绪杰; 薛山; 刘志刚; 吴奇阳; 朱滕辉; 史新明; 马源; 郑蕾; 甘珊珊
Original assignee: 中材锂膜有限公司
Priority date: 2021-09-26
Filing date: 2022-05-30
Publication date: 2023-03-30
Also published as: CN113972435B; CN113972435A

Abstract

Provided in the present invention are a high-porosity, high-permeability lithium ion battery base film and a preparation method therefor, and a lithium ion battery. The method comprises: mixing a diluent with a polyolefin resin; heating and melting same, and then forming a uniform mixture; cooling the mixture to achieve phase separation; pressing to obtain a diaphragm and heating the diaphragm to a temperature close to the melting point; performing first two-way synchronous or stepwise stretching to orient the molecular chains, followed by maintaining the temperature for a certain time period; eluting a residual solvent by using a volatile substance; and finally, by means of second transverse stretching and shaping, preparing an interpenetrating microporous film material. The present invention solves the problem of simultaneously improving the air permeability value and porosity of a lithium ion battery diaphragm, and the air permeability is enhanced by 20-200% while maintaining the high porosity and thickness of a product.

Description

A kind of high porosity, highly gas permeable lithium-ion battery base film and its preparation method and lithium-ion battery

Priority information

This disclosure requests the priority of the Chinese patent application submitted to the State Intellectual Property Office of China on September 26, 2021, with the patent application number 202111131104.2, and the application name "A method for preparing a high-porosity, high-breathability lithium-ion battery base film" rights, and the entire contents of which are incorporated by reference in this disclosure.

technical field

The present disclosure relates to the field of lithium battery diaphragms, in particular to a high-porosity, highly breathable lithium-ion battery base film, a preparation method thereof, and a lithium-ion battery.

Background technique

The separator plays the role of electronic isolation in lithium-ion batteries, preventing direct contact between the positive and negative electrodes, allowing lithium ions in the electrolyte to pass through freely. At the same time, the separator also plays a vital role in ensuring the safe operation of the battery. The performance of the separator directly affects Battery internal resistance, discharge capacity, cycle life and battery safety performance, the lithium battery separator is the part with the highest technical barriers among the four major materials. With the development of high-performance power batteries, there is a strong demand for high-rate, start-stop power batteries, and the development of suitable separators is urgently needed.

Increasing the pores of the separator can effectively reduce the average transmission distance of lithium ions in the battery, allowing lithium ions to come and go freely between the cathode and anode, greatly reducing the transmission resistance of lithium ions, and increasing the battery rate. At present, there are high-porosity diaphragms (porosity 38-45%) and ultra-high-porosity diaphragms (porosity 45-55%). At the same time, the gas permeability of the lithium battery separator can indirectly characterize the pore size of the separator, and then characterize its safety in battery applications.

However, there is a contradiction between increasing the air permeability and porosity of lithium-ion battery separators at the same time. The reason is that the current process mainly adjusts these two properties through stretching process and heat treatment, and the stretching process and heat treatment process are limited to adjusting the pore size, but they are improving. Porosity cannot improve air permeability at the same time, resulting in the risk of short circuit and self-discharge in current high-porosity separators. How to balance high rate and safety is a big challenge. The development of high-porosity and high-breathability separators can effectively solve this problem.

Contents of the invention

The purpose of the present disclosure is to provide a lithium-ion battery base film with high porosity and high air permeability, a preparation method thereof, and a lithium-ion battery. To solve the seemingly contradictory problem of increasing the air permeability and porosity of the lithium-ion battery separator at the same time, while maintaining the high porosity and thickness of the product, it also increases the air permeability by 20-200%. The lithium ion battery prepared from the base film has high safety.

In order to achieve the above object, the first aspect of the present disclosure provides a high-porosity, high-breathability lithium-ion battery base film, the porosity of the high-porosity, high-breathability lithium-ion battery base film is 30% to 60%, and the air permeability is 120～300sec/100cc, air permeability (sec/100cc)/thickness (μm)>20sec/100cc/μm, and air permeability (sec/100cc)/porosity>100sec/100cc, average pore size is 10～60nm, each The number of pores contained in the square micron basement membrane is more than 300.

According to an embodiment of the present disclosure, the porosity of the high-porosity and highly breathable lithium-ion battery base film is 30-45%, the air permeability value is 125-250sec/100cc, and the air permeability (sec/100cc)/thickness (μm) is 20.1~30sec/100cc/μm, and the air permeability (sec/100cc)/porosity is 300~700sec/100cc, the average pore size is 20~50nm, and the number of micropores per square micron base membrane is 300~2000 .

The second aspect of the present disclosure provides a method for preparing a base film of a lithium-ion battery with high porosity and high air permeability, the method comprising the following steps:

(1) Extrusion: mix the diluent with the polyolefin resin to obtain a diluent-polyolefin resin mixed system, and in the diluent-polyolefin resin mixed system, the mass ratio of the polyolefin resin is 20% to 35% , the diluent-polyolefin resin mixed system is heated and melted by the extruder to form a uniform mixed melt; the polyolefin resin is polyethylene with an average molecular weight of 300,000 to 5 million or a polyethylene with an average molecular weight of 300,000 to 5 million A mixture with other resins, in the mixture, polyethylene with an average molecular weight of 300,000-5 million accounts for 50-100% by mass;

(2) Forming: Extrude the mixed melt obtained in step (1) to the casting roll to cool down, carry out phase separation, cool and form, and obtain a diaphragm;

(3) Biaxial stretching: heat the membrane prepared in step (2) to a temperature close to the melting point, carry out bidirectional simultaneous stretching or bidirectional stepwise stretching to orient the molecular chains, and then keep warm for 6 to 60 seconds to obtain a biaxially stretched film. Diaphragm whose stretching area is increased to 16-160 times; the area free shrinkage of the obtained diaphragm at 25°C for 1 hour is ≤20%.

(4) Extraction and drying: Use volatile substances to elute the residual diluent in the biaxially stretched film in step (3), and then dry it at 25-80°C for 10-100s; 1h area free shrinkage ≤ 60%.

(5) Transverse stretching: the dried film obtained in step (4) is transversely stretched, and the transverse stretch ratio is 1.2 to 1.8 times;

(6) Shaping: heat-treat the film after transverse stretching in step (5), and shrink the area by 10-50% to obtain a porous film, that is, a high-porosity, high-breathability lithium-ion battery base film.

According to an embodiment of the present disclosure, the diluent in step (1) is a C18-C30 hydrocarbon mixture;

Preferably, the diluent is a C18-C30 liquid hydrocarbon mixture with a molecular weight of 200-600;

Further preferably, the diluent is paraffin oil.

According to an embodiment of the present disclosure, the other resins described in step (1) are mainly one or more of low-density polyethylene, polypropylene, polyethylene oxide, polyvinyl acetate, and polybutene with an average molecular weight of less than 300,000 the mix of.

According to an embodiment of the present disclosure, the average molecular weight W _m (unit: ten thousand) of the polyolefin resin in step (1)/the biaxial stretching ratio (times) in step (3)>1 (ten thousand).

According to an embodiment of the present disclosure, the surface temperature of the casting roll in step (2) is 10-60°C.

According to an embodiment of the present disclosure, the biaxial stretching temperature in step (3) is T ₁ , and the melting point temperature of the polyolefin resin is T _m -70°C < T1 < the melting point temperature of the polyolefin resin is T _m -5°C.

According to an embodiment of the present disclosure, the volatile substance in step (4) is dichloromethane.

According to the embodiment of the present disclosure, step (4) uses volatile substances to elute the residual solvent in the biaxially stretched film in step (3) and the drying process, the film tension F per unit width is controlled as follows: 1kgf<F<80kgf .

According to an embodiment of the present disclosure, the transverse stretching temperature in step (5) is T ₂ , T ₁ +5°C<T ₂ <T ₁ +70°C.

According to an embodiment of the present disclosure, the heat treatment temperature in step (6) is 0-15° C. higher than the transverse stretching temperature T ₂ in step (5).

According to an embodiment of the present disclosure, the heat treatment time in step (6) is 10-60s.

The third aspect of the present disclosure: provide a high-porosity, highly-breathable lithium-ion battery base film prepared by the preparation method described in the second aspect of the present disclosure, the porosity of the high-porosity, highly-breathable lithium-ion battery base film is 30 ~60%, air permeability value is 120~300sec/100cc, air permeability (sec/100cc)/thickness (μm)>20sec/100cc/μm, and air permeability (sec/100cc)/porosity>100sec/100cc, average The pore size is 10-60nm, and the number of pores contained in each square micron basement membrane is more than 300.

According to an embodiment of the present disclosure, the porosity of the high-porosity and highly breathable lithium-ion battery base film is 30-45%, the air permeability value is 125-250sec/100cc, and the air permeability (sec/100cc)/thickness (μm) is 20-30sec/100cc/μm, and the air permeability (sec/100cc)/porosity is 300-700sec/100cc, the average pore size is 20-50nm, 2000 > the number of pores contained in the base membrane per square micron > 300 .

The fourth aspect of the present disclosure: provide a lithium-ion battery, including the high-porosity, highly gas-permeable lithium-ion battery base film described in the first aspect of the present disclosure or the high-porosity, Highly breathable lithium-ion battery base film.

Compared with the prior art, the technical solution of the present invention has the following beneficial effects:

By adopting the method for preparing the base film of the lithium-ion battery with high porosity and high air permeability of the present invention, the air permeability is increased by 20-200% at the same time under the premise of maintaining the porosity and thickness of the base film. It can effectively improve the safety of high-performance power batteries (batteries with high rate, high energy density, start-stop power, etc.), and avoid risks such as short circuit and self-discharge.

Additional aspects and advantages of the disclosure will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the disclosure.

Description of drawings

The accompanying drawings are used to provide a further understanding of the present disclosure, and constitute a part of the description, together with the following specific embodiments, are used to explain the present disclosure, but do not constitute a limitation to the present disclosure. In the attached picture:

Fig. 1 is a schematic diagram of a method for preparing a base film of a lithium-ion battery with high porosity and high gas permeability according to the present invention.

Detailed ways

Specific embodiments of the present disclosure will be described in detail below in conjunction with the accompanying drawings. It should be understood that the specific embodiments described here are only used to illustrate and explain the present disclosure, and are not intended to limit the present disclosure.

In the first aspect of the present disclosure, there is provided a base film of a lithium-ion battery with high porosity and high air permeability, the porosity of the base film is 30-60%, for example, the porosity is 30%, 31%, ... 59%, 60%; And the air permeability value of the base film is 120～300sec/100cc, for example, the air permeability value is 120sec/100cc, 121sec/100cc...299sec/100cc, 300sec/100cc; and the air permeability of the base film (sec/100cc) /Thickness (μm)>20sec/100cc/μm, preferably 20.1～30sec/100cc/μm, such as 21sec/100cc/μm, 25sec/100cc/μm...29sec/100cc/μm, 30sec/100cc/μm; and breathable Degree (sec/100cc)/porosity (%)>1(sec/100cc)/% (or 100sec/100cc), preferably 7(sec/100cc)/% (or 700sec/100cc)>air permeability (sec/100cc )/porosity (%)>2(sec/100cc)/% (or 200sec/100cc), such as 3(sec/100cc)/%, 4(sec/100cc)/%, 5(sec/100cc)/% ...6(sec/100cc)/%, 7(sec/100cc)/%; and the base film has pores with an average pore diameter of 10-60nm, for example, the average pore diameter of the pores is 10nm, 12nm...58nm, 60nm , and the number of pores per square micron basement membrane (also known as the number of pores per square micron) ≥ 300, preferably 300-2000, for example 301, 302... 1000, 2000.

The porosity, air permeability, thickness, average pore size and the number of micropores per square micrometer of the base film can be detected by methods well known to those skilled in the art.

The high-porosity and high-permeability lithium-ion battery base film of the present disclosure also improves the air permeability by 20-200% on the premise of maintaining the porosity and thickness of the base film. The diaphragm has high porosity and high air permeability, which is conducive to improving the safety of high-performance power batteries (batteries with high rate, high energy density, start-stop power, etc.), and avoiding risks such as short circuit and self-discharge.

According to a specific embodiment of the present disclosure, the porosity of the high-porosity and highly breathable lithium-ion battery base film is 30-45%, the air permeability value is 125-250sec/100c, and the air permeability (sec/100c)/thickness ( μm) is 20.1-30sec/100c/μm, and the air permeability (sec/100c)/porosity is 300-700sec/100c, the average pore size is 20-50nm, and the number of micropores per square micron base film is 300 ~2000 pieces.

According to the present disclosure, the high-porosity and high-breathability lithium-ion battery base film has a uniform thickness, for example, the thickness can be 4-16 μm, such as 4 μm, 5 μm, 7 μm, 9 μm, 12 μm, 14 μm...15 μm, 16 μm.

According to the present disclosure, in order to further improve the performance of the base film, one or both sides of the base film may have a ceramic coating. Wherein, the material of the ceramic coating can be selected from aluminum oxide, silicon oxide, calcium oxide, magnesium oxide, chromium oxide, titanium oxide, zinc oxide, calcium hydroxide, aluminum hydroxide, magnesium hydroxide, silicon carbide, sulfuric acid At least one of barium, zeolite and boehmite. The ceramic coating of the above material can further improve the heat resistance of the base film, and the prepared lithium ion battery has better safety.

The preparation method of the high-porosity and high-breathability lithium-ion battery base film can be to mix liquid hydrocarbons or some small molecular substances (paraffin oil) with polyolefin resin, heat and melt to form a uniform mixture, and then cool down for phase separation. Cool the membrane, then heat the membrane to a temperature close to the melting point, perform the first two-way synchronous or step-by-step stretching to orient the molecular chains, and then keep it warm for a certain period of time, elute the residual solvent with volatile substances, and finally pass through the first The interpenetrating microporous membrane material is prepared by secondary transverse stretching and shaping.

The second aspect of the present disclosure provides a method for preparing a base film of a lithium-ion battery with high porosity and high air permeability. As shown in FIG. 1 , the method includes the following steps:

(1) Extrusion: mix the diluent with the polyolefin resin to obtain a diluent-polyolefin resin mixed system, in the diluent-polyolefin resin mixed system, the mass ratio of the polyolefin resin is 20-35%, The diluent-polyolefin resin mixed system is heated and melted by the extruder to form a uniform mixed melt;

The polyolefin resin is a polyethylene with an average molecular weight of 300,000 to 5 million or a mixture of polyethylene with an average molecular weight of 300,000 to 5 million and other resins. In the mixture, the proportion of polyethylene with an average molecular weight of 300,000 to 5 million is 50-100%;

step 1)

In this step, in the diluent-polyolefin resin mixed system, the mass ratio of the polyolefin resin is 20-35%, for example, the mass of the polyolefin resin accounts for 2% of the mass of the diluent-polyolefin resin mixed system. 20%, 21%... 34%, 35%.

Wherein, the polyolefin resin is polyethylene with an average molecular weight (viscosity average molecular weight) of 300,000 to 5 million, for example, the viscosity average molecular weight of the polyethylene is 300,000, 350,000, 400,000, 450,000, 500,000...100 10,000, 2 million, 3 million, 4 million, 5 million, according to a specific embodiment of the present disclosure, the viscosity average molecular weight of the polyethylene is between 300,000 and 3 million. When the molecular weight of polyethylene is too low, the air permeability value of the product under high stretch ratio is low, and when the molecular weight of polyethylene is too high, the product processing is more difficult.

Or, the polyolefin resin is a mixture of polyethylene with an average molecular weight (viscosity average molecular weight) of 300,000 to 5 million and other resins, and in the mixture, the proportion of polyethylene with an average molecular weight of 300,000 to 5 million is 50 to 500,000. 100%, for example, the mass of polyethylene with an average molecular weight of 300,000 to 5 million accounts for 50%, 51%, ... 99%, 100% of the mass of the mixture of polyethylene and other resins, according to a specific embodiment of the present disclosure , in the mixture, the mass proportion of polyethylene with an average molecular weight of 300,000 to 5,000,000 is 70 to 100%. When the molecular weight of high-density polyethylene is too low, the air permeability of the product obtained at a high stretch ratio is low, and when the molecular weight of high-density polyethylene is too high, the product processing is more difficult.

According to the present disclosure, the other resin is mainly a mixture of one or more of low-density polyethylene, polypropylene, polyethylene oxide, polyvinyl acetate, and polybutene with an average molecular weight of less than 300,000. The above-mentioned other resins can improve the processing performance and adjust the physical properties of the product.

According to the present disclosure, the diluent may be a C18-C30 hydrocarbon mixture. According to a specific embodiment of the present disclosure, the diluent is a C18-C30 liquid hydrocarbon mixture with a molecular weight of 200-600, such as the diluent For paraffin oil.

step (2)

According to the present disclosure, the object of the present disclosure can be achieved when the surface temperature of the casting roll is relatively low. According to a specific embodiment of the present disclosure, the surface temperature of the casting roll is 10-60°C, for example, the casting roll Roller surface temperature is 10°C, 20°C...50°C, 60°C.

After cooling and forming, the diaphragm is obtained.

step (3)

In this step, the prepared film is heated to a temperature close to the melting point, and biaxially stretched (biaxial simultaneous stretching or bidirectional stepwise stretching) to orient the molecular chains. According to the present disclosure, the bidirectional stretching temperature is T ₁ (°C): T _m -70°C <T ₁ <T _m -5°C, T _m is the melting point temperature of polyolefin resin (°C), for example, T _m is 135±2°C, T ₁ is 65±2°C～130± 2°C, for example, _T1 is 70°C, 75°C...120°C, 125°C. When the above relationship is satisfied, it is ensured that the diaphragm can be stretched, the molecular chains have a certain orientation, and the air permeability and porosity are easy to adjust; when the biaxial stretching temperature is too low, the stretching process of the diaphragm is easy to tear, and the stretching is extremely uneven. When the biaxial stretching temperature is too high, the degree of molecular chain orientation is low, and the air permeability and porosity are not easy to adjust.

Heat preservation after stretching, the heat preservation time can be 6-60s, such as 6s, 7s...59s, 60s.

After heat preservation, a film with a biaxially stretched area increased to 16 to 160 times (ie, biaxial stretch ratio) is obtained, that is, the biaxial stretch ratio (longitudinal stretch ratio × transverse stretch ratio) is 16 to 160 times, such as biaxial stretch ratio The magnification ratio is 16 times, 17 times... 159 times, 160 times.

According to the present disclosure, the average molecular weight of the polyolefin resin W _m (unit: 10,000)/the biaxial stretching ratio (times) in step (3)>1 (10,000/times), according to a specific embodiment of the present disclosure, the Said polyolefin resin viscosity-average molecular weight W _m (10,000)/biaxial stretching ratio (times) is 1.1 to 5.0, preferably 3.0 to 5.0, when the above relationship is satisfied, under the same stretching ratio, other physical properties remain unchanged. In some cases, it is easy to increase the air permeability value. If the W _m /biaxial stretching ratio is less than 1, at the same stretching ratio, other physical properties will not change much, and the air permeability adjustment space will be small.

The area free shrinkage of the obtained diaphragm at 25°C for 1h is ≤20%. According to a specific embodiment of the present disclosure, the area free shrinkage of the diaphragm at 25°C for 1h is 1% to 20%, for example, the area of the diaphragm is free at 1h at 25°C. The amount of shrinkage is 20%, 19%, ... 5%, 1%. When the amount of shrinkage is higher than 20%, the consistency of the transverse physical properties of the product is poor.

MD or TD stretch ratio=length after stretching/length before stretching; area free shrinkage=(area before retraction-area after retraction)/area before retraction.

step (4)

According to the present disclosure, the volatile substance eluted may be dichloromethane.

The drying condition can be 10-100s at 25-80°C, for example, the drying temperature is 25°C, 26°C...79°C, 80°C, and the drying time is 10s, 11s...99s, 100s.

According to the present disclosure, during the process of eluting the residual solvent in the biaxially stretched film with volatile substances and the drying process, the film tension F per unit width is controlled to be: 1kgf<F<80kgf. According to a specific embodiment of the present disclosure, 10kgf<F<60kgf, for example, F is 2kgf, 3kgf...78kgf, 79kgf, when F is too low, the elution process is easy to break the membrane, and when F is too high, the lateral shrinkage is severe, and the consistency of lateral physical properties is poor.

The film tension F per unit width is controlled by the speed difference between the rollers before and after elution, and can be measured by an online tension device.

The area free shrinkage of the obtained diaphragm at 105°C for 1h is ≤60%. According to a specific embodiment of the present disclosure, the free area shrinkage of the diaphragm at 105°C for 1h is 10-60%, for example, the free area shrinkage of the diaphragm at 105°C for 1h 10%, 20% ... 50%, 60%, when the shrinkage is higher than 60%, the lateral consistency of the product is poor.

step (5)

In this step, the transverse stretching ratio is 1.2 to 1.8 times, for example, the bidirectional stretching ratio is 1.2 times, 1.3 times...1.7 times, 1.8 times.

According to the present disclosure, the transverse stretching temperature is T ₂ (°C): T ₁ +5°C<T ₂ <T ₁ +70°C, T ₁ is the biaxial stretching temperature (°C), for example, T ₁ is 120±2 ℃, _T2 is 125±2℃～190±2℃, for example, _T2 is 125℃, 130℃...180℃, 190℃. When the above relationship is satisfied, it is easy to stretch and adjust the physical properties. When the transverse stretching temperature is too low, the stretching is difficult, easy to tear during the stretching process, and the stretching is uneven. When the transverse stretching temperature is too high, the fiber structure of the product is easy. Destruction, poor physical properties.

step (6)

According to the present disclosure, the heat treatment temperature after transverse stretching is 0-15°C higher than the transverse stretching temperature T ₂ in step (5), for example, 1°C, 2°C...10°C, 15°C higher.

According to the present disclosure, the heat treatment time is 10-60s, for example, the heat treatment time is 10s, 11s...59s, 60s.

After heat treatment, the area shrinks back by 10-50%. According to a specific embodiment of the present disclosure, the area shrinks after heat treatment by 11-30%. %, 30%.

After shrinkage, a high-porosity, high-breathability lithium-ion battery base film is obtained.

The base film prepared by the disclosed method has high air permeability and porosity at the same time, while maintaining high porosity and thickness, the air permeability is increased by 20-200%, improving the safety of lithium-ion batteries.

The third aspect of the present disclosure provides a high-porosity, high-breathability lithium-ion battery base film prepared by the preparation method described in the second aspect of the present disclosure, the porosity of the high-porosity, high-breathability lithium-ion battery base film is 30 ~60%, air permeability value is 120~300sec/100cc, air permeability (sec/100cc)/thickness (μm)>20sec/100cc/μm, and air permeability (sec/100c)/porosity>100sec/100cc, average The pore size is 10-60nm, and the number of micropores contained in each square micron basement membrane is more than 300.

According to an embodiment of the present disclosure, the porosity of the high-porosity and highly breathable lithium-ion battery base film is 30-45%, the air permeability value is 125-250sec/100cc, and the air permeability (sec/100cc)/thickness (μm) is 20～30sec/100cc/μm, and the air permeability (sec/100cc)/porosity is 300～700sec/100cc, the average pore size is 20～50nm, 2000>The number of micropores contained in the base film per square micron>300 .

The fourth aspect of the present disclosure provides a lithium-ion battery, including the high-porosity, highly-permeable lithium-ion battery base film described in the first aspect of the present disclosure or the high-porosity, Highly breathable lithium-ion battery base film.

The present disclosure will be described below with reference to specific embodiments. It should be noted that these embodiments are only illustrative and do not limit the present disclosure in any way.

Example 1

The diluent in this embodiment is paraffin oil, and the polyolefin resin in this embodiment is polyethylene (manufacturer: Celanese, model: GURX223) with an average molecular weight (viscosity average) of 1.5 million.

(1) Extrusion: Paraffin oil is mixed with polyolefin resin (polyethylene) to obtain a diluent (paraffin oil)-polyolefin resin mixed system, and in the diluent-polyolefin resin mixed system, the mass ratio of polyethylene 20%, the diluent-polyolefin resin mixed system is heated and melted by the extruder to form a uniform mixed melt;

(2) Forming: the mixed melt obtained in (1) flows out to the casting roll for cooling, carries out phase separation, cooling and forming, and makes a diaphragm;

The surface temperature of the casting roll described in this embodiment is 10°C.

(3) Biaxial stretching: heating the membrane prepared in (2) to a temperature close to the melting point, performing biaxial stretching to orient the molecular chains, followed by heat preservation for 12 seconds, and obtaining a membrane whose area has been increased to 49 times through biaxial stretching; The area free shrinkage of the obtained film at 25° C. for 1 hour was 10%.

This embodiment adopts two-way simultaneous stretching, and the biaxial stretching temperature _T1 is 118°C. The melting point _Tm of polyethylene used in this embodiment is 135±2°C, and _T1 meets the melting point temperature of polyolefin resin _Tm -70°C<T ₁ < polyolefin resin melting point temperature T _m -5°C.

Present embodiment step (1) average molecular weight W _m of polyethylene is 1,500,000, and step (3) longitudinal stretching ratio is 700%, and transverse stretching ratio is 700%, and the diaphragm area of biaxial stretching increases to 7 * 7= 49 times, the average molecular weight W _m (unit: ten thousand)/biaxial stretch ratio is 150/49=3.06.

(4) Extraction and drying: Use dichloromethane to elute the residual diluent in the biaxially stretched film in step (3), and then dry it at 45°C for 23s; The tension F is: 30kgf<F<60kgf. The area free shrinkage of the obtained film at 105° C. for 1 hour was 28%.

(5) Transverse stretching: the dried film obtained in step (4) is transversely stretched; the transverse stretch ratio is 1.5 times.

In this embodiment, the transverse stretching temperature T ₂ is 127°C, which satisfies T ₁ +5°C<T ₂ <T ₁ +70°C.

(6) Shaping: heat treatment at 130° C. for 20 s after transverse stretching in step (5), the area shrinks by 16%, and a porous film is obtained, which is the high-safety lithium-ion battery base film of the present invention.

The performance parameters of the lithium-ion battery base film prepared in this embodiment are:

The porosity is 32.8%, the air permeability value is 173sec/100cc, the thickness is 7.0μm, the air permeability/thickness is 24.7sec/100cc/μm, and the air permeability/porosity=173/32.8=5.3(sec/100cc)/% , the average pore size is 35nm, and the number of pores per square micron basement membrane is 300-800.

Example 2

(1) extrusion: paraffin oil is mixed with polyolefin resin (polyethylene) to obtain a diluent-polyolefin resin mixed system, and in the described diluent-polyolefin resin mixed system, the mass ratio of polyethylene is 20%, The diluent-polyolefin resin mixed system is heated and melted by the extruder to form a uniform mixed melt;

(2) Molding: flow the mixed melt obtained in (1) to the casting roll to cool down, carry out phase separation, cool and form, and make a diaphragm;

The surface temperature of the casting roll described in this embodiment is 15°C.

(3) Biaxial stretching: heating the membrane prepared in (2) to a temperature close to the melting point, performing biaxial stretching to orient the molecular chains, followed by heat preservation for 8 seconds, and obtaining a membrane whose area has been increased to 81 times through biaxial stretching; The area free shrinkage of the obtained film at 25° C. for 1 hour was 13%.

This embodiment adopts two-way simultaneous stretching, and the biaxial stretching temperature _T1 is 120°C. The melting point _Tm of polyethylene used in this embodiment is 135±2°C, and _T1 meets the melting point temperature of polyolefin resin _Tm -70°C<T ₁ < polyolefin resin melting point temperature T _m -5°C.

Present embodiment step (1) average molecular weight W _m of polyethylene is 1,500,000, and step (3) longitudinal stretching ratio is 900%, and transverse stretching ratio is 900%, and the diaphragm area of biaxial stretching increases to 9 * 9= 81 times, the average molecular weight W _m (unit: ten thousand)/biaxial stretch ratio is 150/81=1.85.

(4) Extraction and drying: Use dichloromethane to elute the residual diluent in the biaxially stretched film in step (3), and then dry it at 45°C for 16s; The tension F is: 30kgf<F<60kgf. The area free shrinkage of the obtained film at 105° C. for 1 hour was 30%.

In this embodiment, the transverse stretching temperature T ₂ is 129°C, which meets T ₁ +5°C<T ₂ <T ₁ +70°C.

(6) Shaping: heat treatment at 131° C. for 16 s after transverse stretching in step (5), the area shrinks by 18%, and a porous film is obtained, which is the high-safety lithium-ion battery base film of the present invention.

The porosity is 33.2%, the air permeability value is 156sec/100cc, the thickness is 6.7μm, the air permeability/thickness is 23.3sec/100cc/μm, and the air permeability/porosity=156/33.2=4.7(sec/100cc)/% , the average pore size is 37nm, and the number of pores per square micron basement membrane is 300-700.

Comparative example 1

The diluent in this embodiment is paraffin oil, and the polyolefin resin in this embodiment is polyethylene (manufacturer: asahiKASEI, model: VH035) with an average molecular weight (viscosity average) of 600,000. The equipment used is a wet-process diaphragm of Sinoma Lithium Membrane Co., Ltd. Wire.

The present embodiment step (1) polyethylene average molecular weight W _m is 600,000, and step (3) longitudinal stretching ratio is 900%, and transverse stretching ratio is 900%, and the film area of biaxial stretching increases to 9 * 9= 81 times, the average molecular weight W _m (unit: 10,000)/biaxial stretch ratio is 60/81=0.74.

The porosity is 34.3%, the air permeability value is 103sec/100cc, the thickness is 7.1μm, the air permeability/thickness is 14.5sec/100cc/μm, and the air permeability/porosity=103/34.3=3.00, the average pore size is 40nm, The base membrane contains 200-500 holes per square micron.

Comparative example 2

The diluent in this embodiment is paraffin oil, the polyolefin resin in this embodiment is polyethylene with an average molecular weight of 900,000 (manufacturer: asahiKASEI, model: UH650), and the equipment used is a wet-process diaphragm line from Sinoma Lithium Membrane Co., Ltd.

(1) extrusion: paraffin oil is mixed with polyolefin resin (polyethylene) to obtain a diluent-polyolefin resin mixed system, and in the diluent-polyolefin resin mixed system, the mass ratio of polyethylene is 17%, The diluent-polyolefin resin mixed system is heated and melted by the extruder to form a uniform mixed melt;

(3) Biaxial stretching: heating the membrane prepared in (2) to a temperature close to the melting point, performing biaxial stretching to orient the molecular chains, followed by heat preservation for 12 seconds, and obtaining a membrane whose area has been increased to 81 times through biaxial stretching; The area free shrinkage of the obtained film at 25° C. for 1 hour was 10%.

Present embodiment step (1) average molecular weight W _m of polyethylene is 900,000, and step (3) longitudinal stretching ratio is 900%, and transverse stretching ratio is 900%, and the diaphragm area of biaxial stretching increases to 9 * 9= 81 times, the average molecular weight W _m (unit: ten thousand)/biaxial stretch ratio is 90/81=1.11.

The porosity is 34.2%, the air permeability value is 95sec/100cc, the thickness is 7.1μm, the air permeability/thickness is 13.4sec/100cc/μm, and the air permeability/porosity=95/34.2=2.8, the average pore size is 41nm, The base membrane contains 200-500 holes per square micron.

Example 3

(1) extrusion: paraffin oil is mixed with polyolefin resin (polyethylene) to obtain a diluent-polyolefin resin mixed system, and in the diluent-polyolefin resin mixed system, the mass ratio of polyethylene is 25%, The diluent-polyolefin resin mixed system is heated and melted by the extruder to form a uniform mixed melt;

The surface temperature of the casting roll described in this embodiment is 12°C.

(3) Biaxial stretching: heating the membrane prepared in (2) to a temperature close to the melting point, performing biaxial stretching to orient the molecular chains, followed by heat preservation for 13 seconds, and obtaining a membrane whose area has been increased to 49 times through biaxial stretching; The area free shrinkage of the obtained film at 25° C. for 1 hour was 12%.

(4) Extraction and drying: Use dichloromethane to elute the residual diluent in the biaxially stretched film in step (3), and then dry it at 35°C for 21s; The tension F is: 30kgf<F<60kgf. The area free shrinkage of the obtained film at 105° C. for 1 hour was 26%.

(5) Transverse stretching: The dried film obtained in step (4) is subjected to transverse stretching; the transverse stretching ratio is 1.42 times.

In this embodiment, the transverse stretching temperature T ₂ is 125°C, which meets T ₁ +5°C<T ₂ <T ₁ +70°C.

(6) Shaping: heat treatment at 129° C. for 20 s after transverse stretching in step (5), shrink the area by 15%, and obtain a porous film, which is the high-safety lithium-ion battery base film of the present invention.

The porosity is 40%, the air permeability value is 220sec/100cc, the thickness is 9.2μm, the air permeability/thickness is 24sec/100cc/μm, and the air permeability/porosity=220/40=5.5(sec/100cc)/%, The average pore size is 30nm, and the number of pores contained in each square micron of the base membrane is 400-800.

Example 4

The diluent of this embodiment is paraffin oil, and the polyolefin resin of this embodiment is the polyethylene (manufacturer: AsahiKASEI, model: UH650) and other resin polypropylene (manufacturer: Yangzi Petrochemical, model: F401) mixture, in which, polyethylene with an average molecular weight of 900,000 accounts for 95% by mass, and the other resins are polypropylene, which accounts for 5% by mass.

(1) Extrusion: Paraffin oil is mixed with polyolefin resin to obtain a diluent-polyolefin resin mixed system. In the diluent-polyolefin resin mixed system, the mass ratio of polyolefin resin is 30%, and the diluent-polyolefin resin mixed system is 30%. After the polyolefin resin mixing system is heated and melted by the extruder, a uniform mixed melt is formed;

(3) Biaxial stretching: heating the membrane prepared in (2) to a temperature close to the melting point, performing biaxial stretching to orient the molecular chains, followed by heat preservation for 10 s, and obtaining a membrane whose area has been increased to 64 times through biaxial stretching; The area free shrinkage of the obtained film at 25° C. for 1 hour was 14%.

This embodiment adopts two-way simultaneous stretching, and the biaxial stretching temperature is _T1 is 121 ° C, the melting point T _m of polyethylene used in this embodiment is 135 ± 2 ° C, and _T1 conforms to the melting point temperature of polyolefin resin T _m -70 ° C < T ₁ < polyolefin resin melting point temperature T _m -5°C.

Present embodiment step (1) average molecular weight W _m of polyethylene is 900,000, and step (3) longitudinal stretching ratio is 800%, and transverse stretching ratio is 800%, and the diaphragm area of bidirectional stretching increases to 8 * 8= 64 times, the average molecular weight W _m (unit: 10,000)/biaxial stretch ratio is 90/64=1.1.

(4) Extraction and drying: Use dichloromethane to elute the residual diluent in the biaxially stretched film in step (3), and then dry it at 38°C for 18s; The tension F is: 40kgf<F<60kgf. The area free shrinkage of the obtained film at 105° C. for 1 hour was 28%.

(5) Transverse stretching: the dried film obtained in step (4) is transversely stretched; the transverse stretch ratio is 1.48 times

In this embodiment, the transverse stretching temperature T ₂ is 126°C, which meets T ₁ +5°C<T2<T ₁ +70°C.

(6) Shaping: heat treatment at 130° C. for 15 s after transverse stretching in step (5), the area shrinks by 18%, and a porous film is obtained, which is the high-safety lithium-ion battery base film of the present invention.

The porosity is 45%, the air permeability value is 250sec/100cc, the thickness is 9.3μm, the air permeability/thickness is 27sec/100cc/μm, and the air permeability/porosity=250/39=6.4(sec/100cc)/%, The average pore size is 25nm, and the number of pores contained in each square micron of the base membrane is 500-1000.

Example 5

(1) extrusion: paraffin oil is mixed with polyolefin resin (polyethylene) to obtain a diluent-polyolefin resin mixed system, and in the diluent-polyolefin resin mixed system, the mass ratio of polyethylene is 27%, The diluent-polyolefin resin mixed system is heated and melted by the extruder to form a uniform mixed melt;

(3) Biaxial stretching: heating the membrane prepared in (2) to a temperature close to the melting point, carrying out biaxial stretching to orient the molecular chains, followed by heat preservation for 13 seconds, and obtaining a membrane whose area has been increased to 42 times through biaxial stretching; The area free shrinkage of the obtained film at 25° C. for 1 hour was 12%.

Present embodiment step (1) average molecular weight W _m of polyethylene is 1,500,000, and step (3) longitudinal stretching ratio is 700%, and transverse stretching ratio is 600%, and the diaphragm area of biaxial stretching increases to 7 * 6= 42 times, the average molecular weight W _m (unit: ten thousand)/biaxial stretch ratio is 150/42=3.57.

(4) Extraction and drying: Use dichloromethane to elute the residual diluent in the biaxially stretched film in step (3), and then dry it at 42°C for 21s; The tension F is: 30kgf<F<60kgf. The area free shrinkage of the obtained film at 105° C. for 1 hour was 28%.

(6) Shaping: heat treatment at 127° C. for 20 seconds after transverse stretching in step (5), shrink the area by 15%, and obtain a porous film, which is the high-safety lithium-ion battery base film of the present invention.

The porosity is 45%, the air permeability value is 220sec/100cc, the thickness is 9.1μm, the air permeability/thickness is 24sec/100cc/μm, and the air permeability/porosity=200s/40=4.9(sec/100cc)/%, The average pore size is 27nm, and the number of pores contained in each square micron basement membrane is 400-900.

Comparative example 3

(1) Extrusion: Paraffin oil is mixed with polyolefin resin to obtain a diluent-polyolefin resin mixed system. In the diluent-polyolefin resin mixed system, the mass ratio of polyolefin resin is 17%, and the diluent-polyolefin resin mixed system is 17%. After the polyolefin resin mixing system is heated and melted by the extruder, a uniform mixed melt is formed;

(3) Biaxial stretching: heating the membrane prepared in (2) to a temperature close to the melting point, performing biaxial stretching to orient the molecular chains, followed by heat preservation for 10 s, and obtaining a membrane whose area has been increased to 81 times through biaxial stretching; The area free shrinkage of the obtained film at 25° C. for 1 hour was 14%.

Present embodiment step (1) average molecular weight W _m of polyethylene is 900,000, and step (3) longitudinal stretching ratio is 900%, and transverse stretching ratio is 900%, and the diaphragm area of biaxial stretching increases to 9 * 9= 81 times, the average molecular weight W _m (unit: 10,000)/biaxial stretch ratio is 90/81=1.1.

In this embodiment, the transverse stretching temperature T ₂ is 126°C, which satisfies T ₁ +5°C<T ₂ <T ₁ +70°C.

The porosity is 39%, the air permeability value is 145sec/100cc, the thickness is 9.1μm, the air permeability/thickness is 15.9sec/100cc/μm, and the air permeability/porosity=145/39=3.7(sec/100cc)/% , the average pore size is 45nm, and the number of pores per square micron basement membrane is 200-400.

Comparative example 4

(1) extrusion: paraffin oil is mixed with polyolefin resin (polyethylene) to obtain a diluent-polyolefin resin mixed system, and in the described diluent-polyolefin resin mixed system, the mass ratio of polyethylene is 16%, The diluent-polyolefin resin mixed system is heated and melted by the extruder to form a uniform mixed melt;

The surface temperature of the casting roll described in this embodiment is 20°C.

(3) Biaxial stretching: heating the membrane prepared in (2) to a temperature close to the melting point, performing biaxial stretching to orient the molecular chains, followed by heat preservation for 13 seconds, and obtaining a membrane whose area has been increased to 49 times through biaxial stretching; The area free shrinkage of the obtained film at 25° C. for 1 hour was 18%.

(4) Extraction and drying: Use dichloromethane to elute the residual diluent in the biaxially stretched film in step (3), and then dry it at 35°C for 21s; The tension F is: 30kgf<F<60kgf. The area free shrinkage of the obtained film at 105°C for 1 hour was 30%.

The porosity is 40%, the air permeability value is 125sec/100cc, the thickness is 9.0μm, the air permeability/thickness is 13.8sec/100cc/μm, and the air permeability/porosity=125/40=3.1(sec/100cc)/% , the average pore size is 46nm, and the number of pores per square micron basement membrane is 200-400.

test case 1

The pore size, porosity, thickness, areal density, air permeability, tensile strength, elongation at break, puncture strength, and thermal shrinkage of the base films prepared in the test examples and comparative examples are shown in Table 1.

The pore size and porosity are tested by BET specific surface area tester under liquid nitrogen conditions. The calculation method of average pore size is: select the adsorption data according to the BET specific surface area test results, and use the BJH function method to obtain the average pore size results through software.

The thickness adopts a circular arc surface measuring head with a diameter of 12mm and a measurement accuracy of 0.1μm. The MahrMillimar C1216 thickness gauge takes a sample of 100mm×100mm and the test pressure is 0.5N; respectively test five points of the sample, including four corners and a central point , the test result takes the mean of five points.

The areal density is tested with a high-precision electronic balance instrument at normal temperature and pressure. The test method is: cut a porous diaphragm with a length of 10 cm and a width of 10 cm, weigh it, and divide it by the area of the base film to obtain the areal density.

The air permeability adopts the digital type Wangyan type breather with a measurement accuracy of 1s, the diameter of the measuring head is 30mm, the inlet pressure is 0.25MPa, the test pressure is 0.05MPa, and the test time is 3s; sampling size: 50mm×width value.

Tensile strength is measured by an intelligent electronic tensile testing machine with a measurement accuracy of 0.001N. The tensile strength and elongation at break of the sample are measured by an intelligent electronic tensile testing machine. A sample of size 2.5cm×20cm is taken, and the distance between the clamps is (100±5 )mm, the test speed is (250±10)mm/min.

The puncture strength is measured by a pressure tester (KES-G5) with a measurement accuracy of 0.001gf; sampling size: 50mm×width value; the needle shape is a hemisphere with Ф=1.0mm, and the needle running speed is 1mm/s.

The heat shrinkage is heated by an electric blast drying oven, and the dimensional change rate is measured by a ruler at 105°C for 1 hour.

Table 1

It can be seen that by adopting the preparation method of the base film of the lithium-ion battery with high porosity and high air permeability of the present invention, the air permeability is increased by 20-200% at the same time under the premise of maintaining the porosity and thickness of the base film.

The performance comparison between the prior art and the embodiment is shown in Table 2.

Table 2

Note: The number of micropores contained in each square micron basement membrane is counted by the number of pores on the scanning electron microscope pictures.

Claims

A high-porosity, high-breathability lithium-ion battery base film, characterized in that the porosity of the high-porosity, high-breathability lithium-ion battery base film is 30-60%, the air permeability is 120-300sec/100cc, and the air permeability (sec/100cc)/thickness (μm) > 20sec/100cc/μm, and air permeability (sec/100cc)/porosity > 100sec/100cc, the average pore size is 10-60nm, and the number of pores per square micron base membrane Number ≥ 300.
The highly porous and highly breathable lithium-ion battery base film according to claim 1, wherein the porosity of the high-porosity, highly breathable lithium-ion battery base film is 30-45%, and the air permeability value is 125-250sec /100cc, air permeability (sec/100cc)/thickness (μm) is 20.1～30sec/100cc/μm, and air permeability (sec/100cc)/porosity is 300～700sec/100cc, average pore size is 20～50nm, The number of micropores contained in each square micron base film is 300-2000.
A method for preparing a high-porosity, highly breathable lithium-ion battery base film, characterized in that the method comprises the following steps:

(1) Extruding: mixing the diluent with the polyolefin resin to obtain a diluent-polyolefin resin mixed system, in which the mass ratio of the polyolefin resin is 20 to 35% in the diluent-polyolefin resin mixed system; The diluent-polyolefin resin mixed system is heated and melted by the extruder to form a uniform mixed melt;

The polyolefin resin is polyethylene with an average molecular weight of 300,000 to 5 million or a mixture of polyethylene with an average molecular weight of 300,000 to 5 million and other resins. In the mixture, polyethylene with an average molecular weight of 300,000 to 5 million accounts for The ratio is 50-100%;

(2) Forming: Extrude the mixed melt obtained in step (1) to the casting roll to cool down, carry out phase separation, cool and form, and obtain a diaphragm;

(3) Biaxial stretching: heat the membrane prepared in step (2) to a temperature close to the melting point, perform bidirectional simultaneous stretching or bidirectional stepwise stretching to orient the molecular chains, and then keep warm for 6 to 60 seconds to obtain bidirectional stretching Diaphragm with a magnification of 16 to 160 times; the area free shrinkage of the obtained diaphragm at 25°C for 1 hour is ≤20%;

(4) Extraction and drying: Use volatile substances to elute the residual diluent in the biaxially stretched film in step (3), and then dry it at 25-80°C for 10-100s; 1h area free shrinkage ≤ 60%;

(5) Transverse stretching: the dried film obtained in step (4) is transversely stretched, and the transverse stretch ratio is 1.2 to 1.8 times;

(6) Shaping: heat-treating the transversely stretched film in step (5), shrinking its area by 10-50% to obtain a porous film.
The preparation method according to claim 3, characterized in that, in step (1), the diluent is a C18-C30 hydrocarbon mixture.
The preparation method according to claim 4, characterized in that, in step (1), the diluent is a C18-C30 liquid hydrocarbon mixture with a molecular weight of 200-600.
The preparation method according to claim 5, characterized in that, in step (1), the diluent is paraffin oil.
The preparation method according to claim 3, characterized in that, in step (1), the other resins are low-density polyethylene, polypropylene, polyethylene oxide, polyvinyl acetate, poly One or more mixtures of butenes.
The preparation method according to claim 3, characterized in that, the polyolefin resin average molecular weight W m (10,000) in the step (1)/the biaxial stretching ratio in the step (3)>1.
The preparation method according to claim 3, characterized in that, in step (2), the surface temperature of the casting roll is 10-60°C.
The preparation method according to claim 3, characterized in that, in step (3), the biaxial stretching temperature is T 1 (°C), polyolefin resin melting point temperature T m -70°C < T 1 < polyolefin resin Melting temperature Tm -5°C.
The preparation method according to claim 3, characterized in that, in step (4), during the process of eluting volatile substances with volatile substances in the biaxially stretched film and the drying process, the unit width is controlled Membrane tension F is: 1kgf<F<80kgf.
The preparation method according to claim 10, characterized in that, in step (5), the transverse stretching temperature is T 2 (°C), T 1 +5°C<T 2 <T 1 +70°C.
The preparation method according to claim 12, characterized in that the heat treatment temperature in step (6) is 0-15°C higher than the transverse stretching temperature T2 in step (5), and the heat treatment time is 10-60s.
The high-porosity, highly gas-permeable lithium-ion battery base film prepared by the preparation method described in any one of claims 3-13;

The porosity of the high-porosity and highly breathable lithium-ion battery base film is 30-60%, the air permeability value is 120-300sec/100cc, and the air permeability (sec/100cc)/thickness (μm)>20sec/100cc/μm, And the air permeability (sec/100cc)/porosity>100sec/100cc, the average pore size is 10-60nm, and the number of pores contained in each square micron of the base film is more than or equal to 300.
A lithium-ion battery, characterized in that it comprises the high-porosity, highly-permeable lithium-ion battery base film according to any one of claims 1-2 or prepared by the preparation method described in any one of claims 3-13 Highly porous, highly breathable lithium-ion battery base membrane.