CN113871791A

CN113871791A - High-pressure-resistance diaphragm for lithium ion battery and preparation method thereof

Info

Publication number: CN113871791A
Application number: CN202110981224.5A
Authority: CN
Inventors: 庄志; 彭锟; 杨宏彪; 蔡裕宏; 冶成良; 程跃
Original assignee: Jiangxi Tongrui New Energy Technology Development Co ltd
Current assignee: Jiangxi Tongrui New Energy Technology Development Co ltd
Priority date: 2021-08-25
Filing date: 2021-08-25
Publication date: 2021-12-31
Anticipated expiration: 2041-08-25
Also published as: CN113871791B; WO2023024588A1

Abstract

The invention relates to the field of lithium battery diaphragm manufacturing, in particular to a high-pressure-resistance diaphragm for a lithium ion battery and a preparation method thereof. In the invention, in the process of preparing the diaphragm, not only are stretching processes in the length direction and the width direction of the diaphragm arranged, but also a stretching process perpendicular to the film surface direction, namely the thickness direction, is additionally arranged, so that the diaphragm is stretched in the length direction, the width direction and the thickness direction, the growth of crystal chains in the thickness direction is increased, and the diaphragm with excellent compression resistance is finally obtained, and the diaphragm can adapt to the cold pressing or hot pressing process in the production process, reduce the deformation of the diaphragm and delay the aging process of the diaphragm in use.

Description

High-pressure-resistance diaphragm for lithium ion battery and preparation method thereof

Technical Field

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

Background

The lithium ion battery diaphragm is a necessary component in the lithium ion battery, is a film with micropores, plays a role in isolating the positive electrode and the negative electrode of the lithium ion battery and preventing the internal short circuit of the battery, and simultaneously has another important role in ensuring that lithium ions can rapidly pass through the lithium ion battery diaphragm so as to complete the electrochemical reaction inside the lithium ion battery. Various properties of the separator determine various performances of the lithium ion battery, such as cycle performance, specific energy and safety performance of the battery, so that the improvement of the performance of the separator has a remarkable effect on various developments of the lithium ion battery.

The PE material is a conventional material for manufacturing the lithium ion battery diaphragm, and the PE diaphragm has the advantages of high porosity, low air permeability, chemical reagent resistance, no toxicity and the like. At present, the production process of the lithium ion battery diaphragm mainly comprises a dry method and a wet method. In the production of the wet-process diaphragm, PE powder and a plasticizer are melted at high temperature and then extruded, and finally a film is formed by stretching in the length direction and the width direction, wherein the stretching process can be roughly divided into asynchronous stretching and synchronous stretching.

The asynchronous stretching process comprises three steps of MD stretching, TD1 stretching and TD2 stretching, wherein the MD stretching process is finished on a roller, the stretching of the film is finished by the speed difference of a front roller and a rear roller, the MD stretching multiplying power is about 4-9, and the roller temperature is about 100-120 ℃; TD1 stretching is completed in an oven, only TD direction stretching is carried out, and the stretching ratio is about 4-12; TD2 stretching was also performed in an oven, with TD stretching only, with a small stretch ratio of about 1-3, and a partial shrinkage of about 5% -20% after stretching to the maximum width.

The synchronous stretching process comprises two steps of SBS and HS, wherein SBS is bidirectional stretching, namely, MD and TD directions are simultaneously stretched, and the bidirectional stretching ratio is about 4-10. HS step is similar to asynchronous stretch line TD2 stretch, stretching only in the TD direction, with a low stretch ratio of about 1-3, and a partial shrinkage of about 5% -20% after stretching to maximum width.

In the production process of the lithium ion battery, the process of hot pressing or cold pressing of the pole pieces and the diaphragm exists, the diaphragm is positioned between the positive pole piece and the negative pole piece, and certain pressure is generated on the diaphragm in the process of hot pressing or cold pressing; in the use process of the lithium ion battery, the diaphragm is in direct contact with the positive electrode and the negative electrode in the battery, the surfaces of the positive electrode and the negative electrode are composed of positive electrode materials, negative electrode materials and adhesives, after the battery is circulated for a certain time, the positive electrode materials and the negative electrode materials can expand, and a certain amount of gas can be generated in continuous electrochemical reaction, so that no matter the battery is a simple button battery, a winding cylindrical battery or a laminated square battery, after the electrochemical reaction for a period of time, the pole piece can generate pressure on the diaphragm, the pressure can accelerate the aging of the diaphragm, and the shape of micropores in the diaphragm is changed or closed, so that the cycle performance of the battery is influenced, and the safety risk of the battery is increased, therefore, the diaphragm with high pressure resistance has a positive improvement effect on the safety of the battery.

Disclosure of Invention

The existing wet diaphragm preparation process does not have a stretching process perpendicular to the direction of the diaphragm surface, the diaphragm can be stretched only in the width direction and the length direction, the diaphragm has single crystal form direction in the stretching process, crystal chains grow less in the thickness direction, the compression resistance of the finished diaphragm cannot be effectively controlled, the deformation resistance is poor when the diaphragm is compressed, and the ageing resistance of the diaphragm is poor. The invention provides a process for stretching a diaphragm in the thickness direction in the diaphragm production process, namely, in the diaphragm production process, synchronous stretching or asynchronous stretching is carried out, the stretching process in the thickness direction of the diaphragm is additionally arranged, the growth of crystal chains in the thickness direction is increased, and the compression resistance is improved. The thickness direction is defined herein as the VD direction with reference to the TD direction and the MD direction commonly used in the separator industry for clarity and conciseness hereinafter.

The technical scheme of the application is as follows:

mixing raw materials such as polyethylene, polypropylene and the like with a pore-forming agent, carrying out high-temperature melt extrusion through an extruder to form a film, carrying out synchronous MD (MD) direction stretching or synchronous SBS (styrene butadiene styrene) direction stretching, and then enabling a diaphragm stretched through the MD to enter an oven to start primary TD direction stretching.

Mixing raw materials such as polyethylene, polypropylene and the like with a pore-forming agent, performing high-temperature melting extrusion through an extruder to form a film, performing asynchronous MD (machine direction) stretching or synchronous SBS (styrene butadiene styrene) stretching, and then performing VD (vacuum degassing) stretching.

Preferably, after the asynchronous stretching process is performed by MD stretching, VD-direction stretching is performed after the first TD (i.e., TD1) direction stretching is completed;

preferably, after the asynchronous stretching process is performed by MD stretching, VD-direction stretching is performed after the second TD (i.e., TD2) direction stretching is performed;

preferably, after the asynchronous stretching process is subjected to MD stretching, stretching in the VD direction is performed after the first TD-direction stretching and the second TD-direction stretching are completed;

preferably, after the synchronous stretching process finishes the biaxial stretching, the TD-direction stretching is carried out, and then the VD-direction stretching is carried out after the TD-direction stretching is finished;

preferably, the VD stretching device is a stretching guide roller, and negative pressure is introduced into the surface of the stretching guide roller and distributed throughout the air holes;

preferably, the air pressure in the guide roller is adsorption air pressure, and the air pressure range is 10-80 kpa;

preferably, the guide rollers are arranged in the TD stretching oven and distributed on the upper surface and the lower surface of the diaphragm, two rollers corresponding to the upper surface and the lower surface of the diaphragm form a group, and the distance between the single group of guide rollers is adjustable;

preferably, the number of the guide roller groups can be provided with a plurality of groups as required;

preferably, the guide roller surface is uniformly distributed with air holes, the diameter of the air holes is 0.1-5mm, and the distribution distance of the air holes is 1-5 mm.

The invention has the beneficial effects that: the diaphragm is subjected to surface stretching in the VD direction by means of negative pressure and the like, the crystal chain growth in the VD direction is increased, the anti-deformation capacity of the diaphragm is enhanced, the diaphragm can adapt to the cold pressing or hot pressing process in the production process, the diaphragm deformation is reduced, the diaphragm can also adapt to the complex chemical environment in the battery core, and the aging process of the diaphragm during use is delayed.

Drawings

FIG. 1 is a schematic view of the structure of a stretching guide roller according to the present invention;

FIG. 2 is a diagram of a VD drawing process simulation in accordance with the present invention.

Detailed Description

For a further understanding of the invention, preferred embodiments of the invention are described below in conjunction with the detailed description to facilitate an understanding of the invention by those skilled in the art.

Example 1 (asynchronous stretching Process: first TD stretching followed by VD stretching)

The present embodiment provides a method for preparing the high compressive resistance lithium ion battery separator, which comprises the following steps:

1. mixing raw materials such as polyethylene, polypropylene and the like with a pore-forming agent, performing high-temperature melt extrusion through an extruder to form a film, and stretching in the MD direction;

2. the diaphragm obtained in the step 1 enters a first TD direction stretching oven, the temperature in the oven is 100-160 ℃, and the stretching ratio is 120-1200%;

3. after the diaphragm is stretched to the maximum width, the diaphragm enters a VD direction stretching guide roller, the adsorption air pressure in the stretching guide roller is 10-80kpa, and the distance between the two guide rollers is consistent with the thickness of the diaphragm.

4. And (3) continuously carrying out the processes of extraction, secondary TD direction stretching and the like on the diaphragm, and finally rolling and slitting to obtain the high-pressure-resistance lithium ion battery diaphragm.

Example 2 (asynchronous stretching Process, second TD stretch followed by VD stretch)

The method comprises the following steps:

1. mixing raw materials such as polyethylene, polypropylene and the like with a pore-forming agent, performing high-temperature melt extrusion through an extruder to form a film, and performing MD (machine direction) stretching, TD (time division) stretching and extraction;

2. the diaphragm obtained in the step 1 enters a secondary TD direction stretching oven, the temperature in the oven is 100-160 ℃, and the stretching ratio is 120-1200%;

3. after the diaphragm is stretched to the maximum width in the stretching process, the diaphragm enters a VD direction stretching guide roller, the adsorption air pressure in the stretching guide roller is 10-80kpa, and the distance between the two guide rollers is consistent with the thickness of the diaphragm;

after the adsorption and stretching in the VD direction is finished, the diaphragm retracts from the TD direction, and the retraction ratio is 0-30%;

6. and (4) continuously carrying out other processes on the diaphragm subjected to the steps, and finally rolling and slitting to obtain the high-pressure-resistance lithium ion battery diaphragm.

Example 3 (asynchronous stretching Process, VD stretching after both the first and second TD stretching)

The method comprises the following steps:

4. the diaphragm obtained in the step enters a secondary TD direction stretching oven, the temperature in the oven is 100-160 ℃, and the stretching ratio is 120-1200%;

5. after the diaphragm is stretched to the maximum width in the stretching process, the diaphragm enters a VD direction stretching guide roller, the adsorption air pressure in the stretching guide roller is 10-80kpa, and the distance between the two guide rollers is consistent with the thickness of the diaphragm;

after the VD direction adsorption and stretching is finished, the diaphragm retracts in the TD direction, and the retraction ratio is 0-30%;

7. and (4) continuing other production processes of the diaphragm after the steps, and finally rolling and slitting to obtain the high-pressure-resistance lithium ion battery diaphragm.

Example 4 (synchronous stretching Process)

The method comprises the following steps:

2. the membrane obtained in the step 1 enters a bidirectional stretching oven, the temperature in the oven is 100 ℃ and 160 ℃, and the bidirectional stretching ratio is 120-1200%;

4. and (3) continuously carrying out the processes of extraction, secondary TD direction stretching, third TD direction stretching and the like on the diaphragm, and finally rolling and slitting to obtain the high-pressure-resistance lithium ion battery diaphragm.

The foregoing is only a preferred embodiment of the present invention, and it should be noted that, for those skilled in the art, several modifications and decorations can be made without departing from the concept of the present invention, and these modifications and decorations should also be regarded as the protection scope of the present invention.

The performance comparison test is carried out on the diaphragms of the examples and diaphragms which are not subjected to VD direction stretching as comparative examples, the comparative examples 1 to 3 are diaphragms which are not subjected to VD direction stretching in the asynchronous stretching process, and the comparative example 4 is a diaphragm which is not subjected to VD direction stretching in the synchronous stretching process.

The test method of the compression deformation comprises the following steps:

1) superposing and cutting the 16 layers of films into sample wafers with the size of 40mm by 60 mm;

2) testing the thickness data of five points at four corners and the center of the sample wafer;

3) placing the sample in a structure of 'A4 paper/PET/sample/PET/A4 paper';

4) the structure is placed in a compressor for compression, the temperature of the compressor is set to be 70 ℃, the pressure is 7.4MPa, and the duration time is 10 s;

5) and (3) testing the thickness of the sample piece at five points after the compression is finished, and calculating the compression deformation ratio: the compression deformation ratio (thickness before compression-thickness after compression)/thickness before compression, and the average value is the compression ratio in the VD direction of the diaphragm. TABLE 1 comparison of Membrane Properties TABLE 1

The results describe: when the asynchronous stretching process is adopted, compared with a comparative sample, the thickness, the air permeability, the shrinkage and the tensile strength of a sample in an embodiment are not greatly different, but the compression ratio (average value 3.3%) in the VD direction of the embodiment is obviously lower than that of the comparative sample (average value 8.5%), the compression resistance of a diaphragm in the embodiment is better, and the needling strength of the embodiment is slightly higher than that of the comparative sample; when the synchronous stretching process is adopted, compared with the comparative example 4, the thickness, the air permeability, the shrinkage rate and the tensile strength of the embodiment 4 are not greatly different, but the compression ratio in the VD direction is lower, the compression resistance is better, the needling strength is higher, and the production line is more favorably used.

Bearing said, the tensile diaphragm of VD direction has more excellent compressive property, and the cold pressing or the hot pressing process of adaptable production in-process reduces diaphragm deformation, delays the ageing process when diaphragm uses, and acupuncture intensity also has obvious promotion simultaneously, adds the tensile technology of VD direction in diaphragm production process, can bring unexpected outstanding effect, can effectively improve diaphragm performance.

Claims

1. A preparation method of a high-pressure-resistance diaphragm for a lithium ion battery is characterized in that the diaphragm is stretched in the length direction and the width direction in the production process, and further stretched in the thickness direction by using a thickness direction stretching device.

2. The method according to claim 1, wherein the drawing means in the thickness direction is a drawing guide roller having pores distributed over the surface and a negative pressure applied to the inside.

3. The method according to claim 2, wherein the guide rollers are disposed in the TD stretching oven and distributed on the upper and lower surfaces of the separator, the two rollers corresponding to each other are a set, and the distance between the guide rollers in a single set is adjustable along the thickness direction of the separator.

4. The method according to claim 2, wherein the guide rollers are disposed in the SBS stretching oven and distributed on the upper and lower surfaces of the separator, the two corresponding upper and lower rollers are in one group, and the distance between the guide rollers in a single group is adjustable in the thickness direction of the separator.

5. The method according to claim 2, wherein the pressure inside the guide roll is an adsorption pressure ranging from 10 kpa to 80 kpa.

6. The method of claim 2, wherein the diameter of the pores on the surface of the guide roll is 0.1-5mm, and the pore distribution distance is 1-5 mm.

7. A separator produced according to the method of any one of claims 1-6.

8. The membrane of claim 7, wherein the compression ratio in the thickness direction of the membrane is 3% to 4%.