WO2023024588A1

WO2023024588A1 - High-pressure-resistance separator for lithium ion battery and preparation method therefor

Info

Publication number: WO2023024588A1
Application number: PCT/CN2022/092789
Authority: WO
Inventors: 庄志; 彭锟; 杨宏彪; 蔡裕宏; 冶成良; 程跃
Original assignee: 江西省通瑞新能源科技发展有限公司
Priority date: 2021-08-25
Filing date: 2022-05-13
Publication date: 2023-03-02
Also published as: CN113871791B; CN113871791A

Abstract

The present application relates to the field of lithium battery separator manufacturing, and in particular to a high-pressure-resistance separator for a lithium ion battery and a preparation method therefor. In the present application, in a preparation process of a separator, stretching processes in a length direction and a width direction of the separator are set, and a stretching process perpendicular to a separator surface direction, i.e., a thickness direction, is also added, such that the separator is stretched in the length direction, the width direction, and the thickness direction, crystal chain growth in the thickness direction is increased, and the separator having excellent compression resistance is finally obtained and can adapt to a cold pressing or hot pressing process in a production process, thereby reducing the deformation of the separator, and delaying an aging process of the separator in use.

Description

A high pressure-resistant separator for lithium ion battery and preparation method thereof

Cross References to Related Applications

This application claims the priority of the Chinese patent application with the application number 202110981224.5 and the title "a high pressure-resistant separator for lithium-ion batteries and its preparation method" submitted to the State Intellectual Property Office of China on August 25, 2021. The entire contents are incorporated by reference in this application.

technical field

The present application relates to the field of lithium battery diaphragm manufacturing, in particular to a high pressure-resistant diaphragm for lithium ion batteries and a preparation method thereof.

Background technique

Lithium-ion battery separator is a necessary component in lithium-ion batteries. It is a thin film with microporosity, which plays the role of isolating the positive and negative electrodes of lithium-ion batteries and preventing the internal short circuit of the battery. At the same time, another part of the lithium-ion battery separator is An important role is to ensure that lithium ions can pass through quickly to complete the electrochemical reaction inside the lithium-ion battery. The various characteristics of the separator determine the performance of the lithium-ion battery, such as cycle performance, specific energy and battery safety performance, so improving the performance of the separator plays a significant role in the development of lithium-ion batteries.

PE material is a conventional material for making lithium-ion battery separators. PE separators have the advantages of high porosity, low gas permeability, chemical resistance, and non-toxicity. At present, the production process of lithium-ion battery separator is mainly dry method and wet method. The production of wet-process diaphragms is extruded after melting PE powder and plasticizer at high temperature, and finally formed into a film after stretching in the length direction and width direction. The stretching process can be roughly divided into two types: asynchronous stretching and synchronous stretching.

The asynchronous stretching process is divided into three steps: MD stretching, TD1 stretching and TD2 stretching. The MD stretching process is completed on the roller, and the stretching of the film is completed by the speed difference between the front and rear rollers. The MD stretching ratio is about 4-9. , the roll temperature is about 100-120°C; TD1 stretching is completed in the oven, only stretching in the TD direction, and the stretching ratio is about 4-12; TD2 stretching is also carried out in the oven, only stretching in the TD direction, stretching The magnification is small, about 1-3, and it will retract a part after stretching to the maximum width, about 5%-20%.

The simultaneous stretching process is divided into two steps: SBS and HS. SBS is bidirectional stretching, that is, stretching in the MD direction and TD direction at the same time, and the bidirectional stretching ratio is about 4-10. The HS step is similar to the TD2 stretching of the asynchronous stretching line. Only the TD direction is stretched, and the stretching ratio is small, about 1-3. After stretching to the maximum width, it will retract a part, about 5%-20%.

In the production process of lithium-ion batteries, there will be a process of hot pressing or cold pressing on the pole piece and the diaphragm. At this time, the diaphragm is between the positive and negative pole pieces. The hot pressing or cold pressing process will generate a certain pressure on the diaphragm; During the use of ion batteries, the separator directly contacts the positive and negative electrodes in the battery. The surface of the positive and negative electrodes is composed of positive and negative electrode materials and binders. After the battery has been cycled for a certain period of time, the materials of the positive and negative electrodes will expand. , A certain amount of gas will also be produced in the continuous electrochemical reaction, so whether it is a simple button battery, a wound cylindrical battery or a laminated square battery, after a period of electrochemical reaction, the pole pieces will Generate pressure on the diaphragm, which will accelerate the aging of the diaphragm and change or close the micropore morphology in the diaphragm, thereby affecting the cycle performance of the battery and increasing the safety risk of the battery. Therefore, a diaphragm with high pressure resistance It has a positive improvement effect on battery safety.

Contents of the invention

The inventors of the present application have found that the preparation process of the wet diaphragm at the present stage does not have a stretching process perpendicular to the direction of the membrane surface, and the diaphragm can only be stretched in the width and length directions. The crystallization of the diaphragm during the stretching process The direction is relatively single, and the growth of crystal chains in the thickness direction is less, which cannot effectively control the compression resistance of the finished diaphragm, and the deformation resistance is poor when compressed, and the aging resistance of the diaphragm is poor.

This application provides a process for stretching the diaphragm in the thickness direction during the production process of the diaphragm, that is, in the production process of the diaphragm, not only synchronous stretching or asynchronous stretching is carried out, but also a stretching process in the thickness direction of the diaphragm is added to increase the thickness The growth of the crystal chain in the direction improves the compressive performance. Referring to the TD direction and MD direction commonly used in the diaphragm industry, this application defines the thickness direction as the VD direction for the sake of clear and concise description in the following.

The application provides a method for preparing a high pressure-resistant lithium-ion battery separator, the method may include:

Polyethylene, polypropylene and other raw materials are mixed with pore-forming agents, melted and extruded by an extruder at high temperature to form a film, and after synchronous MD stretching or synchronous SBS stretching, the MD stretched diaphragm enters the oven to start the first stage A stretch in the TD direction.

Mix polyethylene, polypropylene and other raw materials with pore-forming agent, melt and extrude through an extruder at high temperature to form a film, stretch in asynchronous MD direction or synchronous SBS direction, and then stretch in VD direction.

Optionally, after MD stretching in the asynchronous stretching process, VD direction stretching is performed after the first TD (ie TD1) direction stretching is completed;

Optionally, after MD stretching in the asynchronous stretching process, VD direction stretching is performed after completing the second TD (ie TD2) direction stretching;

Optionally, after MD stretching in the asynchronous stretching process, stretching in the VD direction is performed after the first stretching in the TD direction and the second stretching in the TD direction;

Optionally, after the simultaneous stretching process completes the bidirectional stretching, stretching in the TD direction is carried out, and then stretching in the VD direction is carried out after the TD stretching is completed;

Optionally, the VD stretching device may be a stretching guide roll, and the surface of the stretching guide roll is filled with a negative pressure inside the pores;

Optionally, the air pressure inside the stretching guide roller is adsorption air pressure, and the air pressure range of the stretching guide roller can be 10kpa to 80kpa;

Optionally, the stretching guide rollers can be placed in a TD stretching oven, the stretching guide rollers can be distributed on the upper and lower surfaces of the diaphragm, and the two stretching guide rollers corresponding to the upper and lower can be a group, and a single group of stretching guide rollers Guide roller spacing can be adjustable;

Optionally, the number of groups of stretching guide rollers can be set in multiple groups as required;

Optionally, pores are evenly distributed on the surface of the guide roller, the diameter of the pores may be 0.1 mm to 5 mm, and the spacing of the pores may be 1 mm to 5 mm.

The beneficial effects of the present application are at least: stretching the surface of the diaphragm in the VD direction by means of negative pressure, etc., increasing the crystal chain growth in the VD direction, enhancing the deformation resistance of the diaphragm, and adapting to the cold pressing or hot pressing process in the production process , reduce the deformation of the diaphragm, and can also adapt to the complex chemical environment inside the cell, delaying the aging process of the diaphragm during use.

Description of drawings

Fig. 1 is the schematic diagram of drawing guide roller structure in the present application;

Figure 2 is a simulation diagram of the VD stretching process in this application.

Detailed ways

In order to further understand the present application, the preferred solutions of the present application are described below in combination with specific embodiments, so as to facilitate those skilled in the art to understand the present application.

Embodiment 1 (asynchronous stretching process: stretching in the VD direction after the first TD stretching)

This case provides the preparation method of the above-mentioned high pressure-resistant lithium-ion battery separator, including the following steps:

1. Mix raw materials such as polyethylene and polypropylene with a pore-forming agent, melt and extrude through an extruder at high temperature to form a film, and stretch in the MD direction;

2. The diaphragm after step 1 enters the first TD direction stretching oven, the temperature in the oven is 100°C to 160°C, and the stretching ratio is 120% to 1200%;

3. After the diaphragm reaches the maximum width through the stretching process, it enters the stretching guide roller in the VD direction. The suction pressure inside the stretching guide roller is 10kpa to 80kpa, and the distance between the two guide rollers is consistent with the thickness of the diaphragm.

4. After the above steps, the separator continues to go through the process of extraction, second TD stretching, etc., and is finally wound and cut to form a high-pressure lithium-ion battery separator.

Embodiment 2 (asynchronous stretching process, carry out VD direction stretching after the second TD stretching)

Include the following steps:

1. Mix raw materials such as polyethylene and polypropylene with a pore-forming agent, melt and extrude through an extruder at high temperature to form a film, stretch in the MD direction, stretch in the first TD direction and extract;

2. The diaphragm after step 1 enters the second TD direction stretching oven, the temperature in the oven is 100°C to 160°C, and the stretching ratio is 120% to 1200%;

3. After the diaphragm reaches the maximum width through the stretching process, it enters the stretching guide roller in the VD direction. The suction pressure inside the stretching guide roller is 10kpa to 80kpa, and the distance between the two guide rollers is consistent with the thickness of the diaphragm;

5. After the adsorption and stretching in the VD direction is completed, the diaphragm retracts from the TD direction, and the retraction ratio is 0% to 30%;

6. After the above steps, the separator continues to go through other processes, and finally is wound and cut to form a high-pressure lithium-ion battery separator.

Embodiment 3 (asynchronous stretching process, after the first and second TD stretching, all carry out VD direction stretching)

Include the following steps:

4. After the above steps, the diaphragm enters the second TD direction stretching oven, the temperature in the oven is 100°C to 160°C, and the stretching ratio is 120% to 1200%;

5. After the diaphragm reaches the maximum width through the stretching process, it enters the stretching guide roller in the VD direction. The internal adsorption pressure of the stretching guide roller is 10kpa to 80kpa, and the distance between the two guide rollers is consistent with the thickness of the diaphragm;

6. After the adsorption and stretching in the VD direction is completed, the diaphragm will retract in the TD direction, and the retraction ratio is 0% to 30%;

7. After the above steps, the separator continues to other production processes, and finally it is wound and cut to form a high-pressure lithium-ion battery separator.

Embodiment 4 (synchronous stretching process)

Include the following steps:

2. The diaphragm after step 1 enters the bidirectional stretching oven, the temperature in the oven is 100°C to 160°C, and the bidirectional stretching ratio is 120% to 1200%;

3. After the diaphragm reaches the maximum width through the stretching process, it enters the stretching guide roller in the VD direction. The adsorption air pressure in the stretching guide roller is 10kpa to 80kpa, and the distance between the two guide rollers is consistent with the thickness of the diaphragm;

4. After the above steps, the separator continues to go through extraction, second stretching in TD direction, third stretching in TD direction, etc., and finally winding and cutting to form a lithium-ion battery separator with high compression resistance.

The above is only the preferred implementation of the application, it should be pointed out that for those of ordinary skill in the art, without departing from the concept of the application, some improvements and modifications can also be made, and these improvements and modifications should also be considered For the scope of protection of this application.

The diaphragm of the example and the diaphragm not stretched in the VD direction were used as comparative examples to perform performance comparison tests. Comparative examples 1-3 were diaphragms that were not stretched in the VD direction by the asynchronous stretching process, and comparative example 4 was not stretched by the synchronous stretching process. Separator stretched in the VD direction.

The test method for compression set is:

1) Superimpose 16 layers of films and cut them into 40mm*60mm size samples;

2) Test the thickness data of five points at the four corners and the central position of the above-mentioned sample;

3) Place the above sample in the structure of "A4 paper/PET/sample/PET/A4 paper";

4) The above structure is placed in a compressor for compression, the temperature of the compressor is set to 70°C, the pressure is 7.4MPa, and the duration is 10s;

5) After the compression is completed, measure the thickness of five points of the sample, and calculate the compression deformation ratio: compression deformation ratio = (thickness before compression - thickness after compression) / thickness before compression, and take the average value as the compression ratio in the VD direction of the diaphragm. Table 1. Diaphragm Performance Comparison Table 1

Result description: when adopting the asynchronous stretching process, compared with the sample of the comparative example, the thickness, air permeability, shrinkage rate and tensile strength of the example sample are not much different, but the VD direction compression ratio of the example (average value 3.3%) is significantly lower than Comparative example (average value 8.5%), the compressive performance of the diaphragm of the embodiment is better, and the acupuncture strength of the embodiment is slightly higher than that of the comparative example; when the simultaneous stretching process is adopted, compared with the comparative example 4, the thickness, air permeability , shrinkage rate and tensile strength are not much different, but the compression ratio in the VD direction is lower, the compression resistance is better, and the acupuncture strength is higher, which is more conducive to production line use.

Based on the above, the diaphragm stretched in the VD direction has more excellent compression resistance, which can adapt to the cold or hot pressing process in the production process, reduce the deformation of the diaphragm, and delay the aging process of the diaphragm during use. Significant improvement, the addition of VD direction stretching process in the production process of the diaphragm will bring unexpected outstanding effects and effectively improve the performance of the diaphragm.

Industrial Applicability

The application provides a high pressure-resistant separator for lithium ion batteries and a preparation method thereof. In the process of preparing the diaphragm, not only the stretching process in the length direction and width direction of the diaphragm is provided, but also the stretching process perpendicular to the film surface direction, that is, the thickness direction, so that the diaphragm is stretched in the length direction, width direction and thickness direction. The stretching effect increases the crystal chain growth in the thickness direction, and finally obtains a diaphragm with excellent compression resistance, which can adapt to the cold or hot pressing process in the production process, reduce the deformation of the diaphragm, and delay the aging process of the diaphragm during use.

In addition, it can be understood that the high pressure-resistant separator for lithium-ion batteries of the present application and the preparation method thereof are reproducible and can be used in various industrial applications. For example, the high pressure-resistant diaphragm for lithium ion batteries of the present application and the preparation method thereof can be used in the field of lithium battery diaphragm manufacturing.

Claims

A method for preparing a high-compression-resistant separator for a lithium-ion battery is characterized in that, in the production process, the separator is not only stretched in the length and width directions, but also stretched in the thickness direction by using a stretching device in the thickness direction.
The preparation method of a high-compression-resistant separator for a lithium-ion battery according to claim 1, wherein the separator is stretched in the length and width directions, and is also stretched in the thickness direction using a thickness direction stretching device. Stretching includes: mixing polyethylene, polypropylene and a pore-forming agent, extruding through an extruder at high temperature to form a film, and after synchronous longitudinal stretching or synchronous bidirectional stretching, the length-stretched diaphragm enters an oven Start the first widthwise stretch.
The preparation method of a high-compression-resistant separator for a lithium-ion battery according to claim 1, wherein the separator is stretched in the length and width directions, and is also stretched in the thickness direction using a thickness direction stretching device. Stretching includes: mixing polyethylene, polypropylene and a pore-forming agent, extruding through an extruder at high temperature to form a film, and stretching in the thickness direction after asynchronous longitudinal stretching or synchronous bidirectional stretching.
According to the preparation method of a kind of lithium-ion battery high compressive resistance diaphragm according to any one of claims 1 to 3, it is characterized in that, the stretching device in the thickness direction is a stretching guide roller, and the stretching guide roller There are air holes all over the surface and negative pressure inside.
The preparation method of a high-pressure-resistant separator for a lithium-ion battery according to claim 4, wherein the stretching guide roller is placed in a TD stretching oven, and the stretching guide roller is distributed above and below the separator On the surface, the upper and lower corresponding two stretching guide rollers form a group, and the distance between a single set of stretching guide rollers in the thickness direction of the diaphragm is adjustable.
The preparation method of a high-compression-resistant diaphragm for lithium-ion batteries according to claim 4, wherein the stretching guide rollers are placed in an SBS stretching oven, and the stretching guide rollers are distributed above and below the diaphragm On the surface, the upper and lower corresponding two stretching guide rollers form a group, and the distance between a single set of stretching guide rollers in the thickness direction of the diaphragm is adjustable.
According to claim 5 or 6, a preparation method of a high-pressure-resistant separator for lithium-ion batteries, characterized in that the distance between a single set of stretching guide rollers is consistent with the thickness of the separator.
The method for preparing a high-compression-resistant separator for lithium-ion batteries according to claim 5 or 6, wherein the number of sets of the stretching guide rollers is set to multiple sets.
The preparation method of a high-compression-resistant diaphragm for lithium-ion batteries according to any one of claims 4 to 8, wherein the air pressure inside the stretching guide roller is an adsorption air pressure, and the stretching guide roller The air pressure of the rollers ranges from 10kpa to 80kpa.
A method for preparing a high-compression diaphragm for lithium-ion batteries according to any one of claims 4 to 9, wherein the diameter of pores on the surface of the stretching guide roller is 0.1 mm to 5 mm, and the distribution of pores is 0.1 mm to 5 mm. The spacing is 1mm to 5mm.
A diaphragm produced by the method for preparing a high-pressure-resistant diaphragm for a lithium-ion battery according to any one of claims 1 to 10.
The diaphragm according to claim 11, wherein the compression ratio in the thickness direction of the diaphragm is 3% to 4%.