CN113146911B

CN113146911B - High-temperature-resistant diaphragm, dry preparation method and application thereof

Info

Publication number: CN113146911B
Application number: CN202011045354.XA
Authority: CN
Inventors: 马忠龙; 秦士林; 赵晓宁; 郑晓醒
Original assignee: Svolt Energy Technology Co Ltd
Current assignee: Svolt Energy Technology Co Ltd
Priority date: 2020-09-28
Filing date: 2020-09-28
Publication date: 2022-06-17
Anticipated expiration: 2040-09-28
Also published as: CN113146911A

Abstract

The invention discloses a preparation method of a diaphragm, which comprises the following steps: 1) mixing the fiberizable polymer powder with the high-temperature-resistant polymer, and drawing the fiberizable polymer powder into fibers under the action of shearing force to obtain a mixture; 2) carrying out hot-pressing treatment on the mixture to a preset thickness to obtain a diaphragm; wherein the high-temperature-resistant polymer is a polymer which stably exists at the use temperature of the separator. Solves the problems that the existing PP and PE polyolefin materials are not high temperature resistant and have poor electrolyte wettability, and the existing diaphragm has high preparation cost.

Description

High-temperature-resistant diaphragm, dry preparation method and application thereof

Technical Field

The invention relates to the technical field of batteries, relates to a diaphragm, a preparation method and application thereof, and particularly relates to a high-temperature-resistant diaphragm, a dry preparation method thereof and a battery.

Background

With the rapid development of new energy automobiles, the requirements on the energy density and the safety performance of the energy storage device for the automobile are continuously improved. The lithium ion battery with the advantages of high specific energy, high specific power, good cyclicity, no environmental pollution and the like is considered as the best choice. It is expected that by 2025, the energy density of the power battery will reach over 500 Wh/kg. With the increasing of energy density of power batteries, the safety performance of the power batteries is also under more severe examination. In addition, in recent years, a large number of electronic products are popularized, and lithium batteries used as power supplies of the electronic products are more and more emphasized due to the advantages of light weight, small size, high working voltage, high energy density, high output power, high charging efficiency, no memory effect and the like. And thus the inevitable requirements for safety and performance of lithium batteries are also increasing.

The diaphragm is used as an important component of the lithium battery, and has the functions of isolating positive and negative pole pieces, preventing short circuit and providing a lithium ion transmission channel. Therefore, improving the safety performance of the battery and reducing the production cost of the diaphragm are one of the main targets of the future lithium ion battery research.

Currently, commercially available lithium battery separators all use polypropylene (PP) or Polyethylene (PE) as a base material, and have a film made of a single material and a multilayer separator formed by compounding PP/PE/PP. However, the PP and PE polyolefin materials have low surface energy and lyophobic property, so that the wettability of the diaphragm on electrolyte is poor, and the cycle life of the battery is influenced. In addition, the thermal deformation temperature of the two materials is low (the thermal deformation temperature of PP is 80-85 ℃, the thermal deformation temperature of PP is 100 ℃), and severe thermal shrinkage can occur when the temperature is too high, so that the diaphragm is not suitable for a high-temperature environment, meanwhile, the diaphragm is used as a vital component in the safe operation of the battery, the safety of the battery can be protected under special conditions, such as accidents and puncture, when the battery is abused, the partial damage or deformation of the diaphragm can directly cause the contact of a positive electrode and a negative electrode, and further, severe battery reaction can be caused to cause the ignition and explosion of the battery.

Therefore, in order to improve the safety of the lithium ion battery and ensure the safe and stable operation of the battery, a safer membrane system needs to be found.

CN101974828 discloses a technology for preparing polyimide nanofiber lithium ion battery separator by electrostatic spinning, the PI separator prepared by the technology has the characteristics of high porosity, high temperature resistance and the like, but the electrostatic spinning process is complex, the production efficiency is low, and the technology is not beneficial to industrial large-scale production.

CN101645497A prepares a polyamic acid film by mixing a polyamic acid solution and a pore-forming agent, then takes out the pore-forming agent from the film through a non-solvent, and finally prepares the porous polyimide lithium battery diaphragm through thermal imidization. Although the scheme relatively improves the production efficiency compared with electrostatic spinning, a plurality of steps of chemical reactions are required in the experimental process, wherein long-time stirring and drying are involved. And in addition, the solvent is harmful to experimenters and the surrounding environment in the process of using the solvent, and the general purpose of environmental protection of the lithium ion battery is not utilized.

Disclosure of Invention

In view of the above problems in the prior art, the present invention aims to provide a separator, a preparation method and a use thereof, and in particular, to a high temperature resistant separator, a dry preparation method thereof and a battery. Solves the problems that the existing PP and PE polyolefin materials are not high temperature resistant and have poor electrolyte wettability, and the existing diaphragm has high preparation cost.

The applicant provides a composite diaphragm and a preparation method thereof in a prior application (application number: 202010723671.6), wherein a polymer network with rich pores is constructed by utilizing a thermoplastic polymer to be fiberized under high-speed shearing and is used for dispersing fillers, but the composite diaphragm does not have high-temperature resistance, and when an electric core works in an extremely high-temperature environment, the polymer can reach the glass transition temperature and deform, such as bending, curling and the like, so that the positive electrode and the negative electrode of the electric core are in contact short circuit, and risks, such as combustion, explosion and the like, are caused. This patent adopts PI class high temperature resistant polymer material as diaphragm main part material, has improved the temperature application scope of diaphragm greatly, has effectively improved the safety in utilization of electric core.

In order to achieve the purpose, the invention adopts the following technical scheme:

in a first aspect, the present invention provides a method for preparing a separator, the method comprising the steps of:

(1) mixing the fiberizable polymer powder with the high-temperature-resistant polymer, and drawing the fiberizable polymer powder into fibers under the action of shearing force to obtain a mixture;

(2) carrying out hot-pressing treatment on the mixture obtained in the step (1) to a preset thickness to obtain a diaphragm;

wherein the high-temperature-resistant polymer is a polymer which stably exists at the use temperature of the separator.

The invention provides a dry film-making technology for preparing a diaphragm, in particular to a self-supporting diaphragm, which is prepared by stirring a fiberizable polymer and a high-temperature-resistant polymer at a high speed under a dry condition, drawing and fiberizing the fiberizable polymer under a shearing action force, and hot-pressing to form a film, wherein polymer fibers converted from the fiberizable polymer are randomly lapped to form a polymer network structure with rich pores in the hot-pressing film-forming process, and high-temperature-resistant polymer particles are dispersed and bonded on the polymer network structure to obtain the diaphragm. The existence of high-temperature resistant polymeric ions improves the service temperature of the diaphragm, avoids the defect of low hot melting temperature of the conventional PP and PE diaphragms, and greatly enriches the types of selectable materials of the diaphragm (the conventional wet process is only suitable for PE, and most of PP adopts a dry process). The method can adjust the material proportion and the hot pressing parameter to adjust the porosity of the diaphragm in the preparation process.

Specifically, the dry mixing is carried out in the step (1), the fiberizable polymer and the high-temperature resistant polymer are stirred and mixed at a high speed according to a certain proportion to prepare dry powder with certain viscosity, and in the high-speed stirring process, the fiberizable polymer is drawn and fiberized to bond the high-temperature resistant polymer particles and endow the powder with certain viscosity. Step (2) hot-pressing to prepare a membrane, namely pressing the powder mixed by the dry method into a membrane material with a certain thickness in a certain hot-pressing mode; in the hot pressing process, the wiredrawing and fiberizing polymers are mutually overlapped and extruded to move to form a network structure, and meanwhile, the bonding effect among all substances is increased.

In the present invention, the form of the high-temperature resistant polymer as the raw material is not limited, and may be, for example, a powder of the high-temperature resistant polymer. The following is a preferred technical solution of the present invention, but not a limitation to the technical solution provided by the present invention, and the technical objects and advantageous effects of the present invention can be better achieved and achieved by the following preferred technical solution.

The type of the fiberizable polymer powder in the present invention is not particularly limited, and any fiberizable polymer powder can be used as long as it can be fiberized by the action of high-speed stirring. Including but not limited to at least one of Polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), Styrene Butadiene Rubber (SBR), and polyacrylic acid (PAA), preferably PTFE.

Preferably, the fiberizable polymer powder in step (1) is high-temperature resistant fiberizable polymer powder which is stable at the use temperature of the separator. The polymer which can be fiberized and has high temperature resistance is adopted to be matched with the high temperature resistant polymer for use, and the service temperature of the diaphragm is improved under the synergistic effect.

In the present invention, the "use temperature of the separator" means: the temperature of the environment where the diaphragm is located is generally-40 ℃ to 300 ℃.

Preferably, the high temperature resistant polymer in step (1) comprises any one or a combination of at least two of polyimide PI, polyethylene terephthalate PET, polyamide PA, polyacrylonitrile PAN, polymetaphenylene isophthalamide PMIA, polyparaphenylene terephthalamide PPTA, polyvinyl alcohol or polyparaphenylene benzobisoxazole PBO, such as PI and PET, PI and PA, PI and PAN, PI and PNIA, PI, PET and PPTA, PET, PA and PBO, PAN, PMIA and PPTA, and the like, preferably PI. The PI has good high-temperature resistance and excellent electrolyte wettability, and can effectively improve the liquid absorption rate and the liquid retention rate of the diaphragm.

Preferably, in the mixture of step (1), the mass ratio of the high temperature resistant polymer to the fiberizable polymer powder is 70:30-98:2, such as 70:30, 75:25, 77:23, 80:20, 85:15, 90:10, 92:8 or 95:5, etc., preferably 90:10-95: 5. Within the above range, a high temperature resistant separator having excellent properties can be obtained while satisfying the requirements of forming a polymer network structure and exerting a good adhesive effect thereof.

Preferably, in the mixture of step (1), the fiberizable polymer powder is PTFE and is present in an amount of 5% to 10% (e.g., 5%, 6%, 7%, 8%, 9%, 10%, etc.); the high temperature resistant polymer is PI with a content of 90% -95% (e.g. 90%, 92%, 94%, 94.5% or 95%). The higher the PTFE content in the mixture, the higher the mechanical strength of the membrane, but at the same time, the lower the liquid absorption and porosity of the membrane. Within the above preferable range, good mechanical properties, liquid absorption properties and high porosity can be combined more favorably.

The types of the fiberizable polymer powder are different, the preferable addition amount is different, and the addition amount of the fiberizable polymer powder with better high-temperature resistance is wider in selectable range; the fiberizable polymer powder with poor high temperature resistance is preferably added in a small amount on the premise of forming a polymer network structure and exerting good bonding effect.

Preferably, the method for drawing the fiberizable polymer powder into the fibers under the action of the shearing force in the step (1) comprises the following steps: at least one of high-speed stirring, screw extrusion and air flow crushing, and a mashing machine, a high-speed dispersing machine, a twin-screw extruder, an air flow crusher, and the like may be used as the equipment.

Preferably, the manner in which the fiberizable polymer powder is drawn into fibers under the action of the shear force in step (1) is high speed agitation at a speed of > 1000rpm (e.g., 2000rpm, 3000rpm, 4000rpm, 5000rpm, 6000rpm, 7000rpm, 8000rpm, 9000rpm, 10000rpm, 12000rpm, 12500rpm, 13500rpm, 15000rpm, 16500rpm, 18000rpm, 20000rpm, 21000rpm, 22500rpm, 26000rpm, 28000rpm, etc.). However, the present invention is not limited to the above-mentioned embodiments, and other embodiments for achieving the object of fiberization are also applicable to the present invention.

Preferably, the high speed stirring speed in step (1) is 8000rpm-25600rpm, such as 8000rpm, 9000rpm, 10000rpm, 11000rpm, 12000rpm, 13000rpm, 15000rpm, 17500rpm, 18500rpm, 20000rpm, 21000rpm, 22500rpm, 23500rpm, 25000rpm or 25530rpm, etc., preferably 17000rpm-21000 rpm.

Preferably, the high-speed stirring and mixing time in the step (1) is 2min to 2h, such as 2min, 5min, 10min, 15min, 17min, 20min, 25min, 28min, 30min, 40min, 50min, 1h, 1.1h, 1.2h, 1.3h, 1.5h, 1.8h or 2h, etc., preferably 10min to 30 min.

In the invention, the purpose of high-speed shearing is to realize uniform mixing of materials on one hand and more importantly to realize fiberization by drawing fiberizable polymers under the action of ultra-strong high-speed dispersion on the other hand. If the shearing speed is too low, on one hand, the mixing time is too long, the time cost is increased, and on the other hand, the fiberization effect is not ideal; if the shearing speed is too high, the loss of the equipment is large, the service life of the equipment is reduced, and the performance of the temperature-sensitive raw material is possibly deteriorated due to the heat generation problem.

Preferably, the high-speed stirring and mixing in step (1) is pulse stirring, and the gap time is 0-30min, such as 0.5min, 1min, 3min, 5min, 10min, 15min, 20min, 25min or 30 min.

The form of the hot pressing treatment in step (2) is not limited in the present invention, and for example, a roller press may be used to perform hot rolling at a certain temperature.

Preferably, the hot pressing treatment in step (2) is performed at a temperature of 25 ℃ to 300 ℃, for example, 25 ℃, 30 ℃, 35 ℃, 40 ℃, 50 ℃, 65 ℃, 80 ℃, 100 ℃, 115 ℃, 130 ℃, 150 ℃, 180 ℃, 200 ℃, 220 ℃, 230 ℃, 240 ℃ or 250 ℃, etc., preferably at a temperature of 60 ℃ to 250 ℃, and more preferably at a temperature of 180 ℃ to 220 ℃.

Preferably, during the hot pressing process in step (2), the hot pressing speed is 5rpm to 50rpm, such as 5rpm, 7rpm, 10rpm, 15rpm, 20rpm, 30rpm, 35rpm, 40rpm, or 50 rpm. Preferably 5rpm to 10 rpm.

Preferably, the predetermined thickness of step (2) is 15um-70um, such as 15um, 20um, 25um, 30um, 35um, 40um, 45um, 50um, 55um, 60um, 65um or 70um, etc.

As a further preferred technical solution of the method of the present invention, the method comprises the steps of:

(1) mixing PTFE and PI, and performing pulse type stirring at the speed of more than or equal to 10000rpm, wherein the gap time is 0-30min and does not contain 0, and the pulse type stirring time is 2min-2h to obtain a uniformly mixed mixture, wherein in the mixture, the mass ratio of PI is 70-98%, and the mass ratio of PTFE is 2-30%;

(2) and (2) carrying out hot pressing treatment on the mixture obtained in the step (1), wherein the hot pressing temperature is 60-250 ℃, the hot pressing speed is 5-50 rpm, and the thickness is 15-70 um, so as to obtain the self-supporting diaphragm.

Polyimide (PI) is one of polymers with good comprehensive performance, has excellent thermal stability, can effectively avoid the problems of melting and thermal shrinkage of a diaphragm, greatly improves the high-temperature safety performance of the battery, has excellent electrolyte wettability compared with the conventional polyolefin material, and can effectively improve the rate capability and the cycle life of the lithium ion battery.

The preferable technical scheme adopts PI as a high-temperature-resistant polymer, the PI has excellent electrolyte wettability, the washing liquid rate and the liquid retention rate of the diaphragm can be effectively improved, and the PI and the PTFE both have high-temperature-resistant characteristics, so that the working temperature range of the battery is enlarged, and the safety characteristic of the battery is enhanced.

In a second aspect, the present invention provides a separator prepared by the method of the first aspect, the separator comprising: a network structure formed by overlapping polymer fibers converted from a fiberizable polymer, and a high temperature resistant polymer dispersed and bonded in the network structure.

In a third aspect, the present invention provides a battery comprising the separator of the second aspect.

Compared with the prior art, the invention has the following beneficial effects:

the invention is different from the preparation methods and technologies of wet preparation, dry stretching, electrostatic spinning and the like of other lithium battery diaphragms, two polymers, namely fiberizable polymer powder and high-temperature-resistant polymer, are selected as material main bodies to be matched for use, the diaphragm is prepared by a dry method under the action of ultra-strong high-speed dispersion, the fiberizable polymer powder is subjected to high-speed shearing and wire drawing fiberized and is mixed with the high-temperature-resistant polymer, fibers formed by subsequent hot-pressing treatment and wire drawing are randomly lapped to form a network structure, and the high-temperature-resistant polymer is dispersed and bonded in the network structure. The structure is beneficial to the diaphragm to obtain proper porosity, good mechanical property and high temperature resistance. The method has simple preparation process and is convenient for commercialization and amplification; multiple steps of material mixing, homogenate, coating, drying and the like are not needed in the preparation process, and the whole process does not involve the use of a solvent, so that the material cost is reduced, the green and environment-friendly effects are effectively achieved, and the problems that the processes such as an electrostatic spinning process and wet polyamic acid conversion are not easy to industrialize and are not environment-friendly are solved.

The method of the invention can simplify the process, improve the consistency of the diaphragm, and save the cost, and the process is easy to control.

Drawings

Fig. 1 is a photograph of a liquid absorption test of the separator prepared in example 1 of the present invention.

Fig. 2 is an optical photograph of the separator prepared in example 1 of the present invention.

Detailed Description

The technical scheme of the invention is further explained by the specific implementation mode in combination with the attached drawings.

Example 1

In a room temperature environment, weighing PI powder and PTFE powder according to a mass ratio of 70:30, placing the powder in a high-speed dispersion machine, performing pulse stirring at 20000rpm for 5min, placing the mixed material in a roller press, rolling the powder into a self-supporting diaphragm at the temperature of 200 ℃ and the rotation speed of 10 rpm.

Example 2

In a room temperature environment, weighing PI powder and PTFE powder according to a mass ratio of 90:5, placing the powder in a high-speed dispersion machine, performing pulse stirring at 20000rpm for 5min, placing the mixed material in a roller press, rolling the powder into a self-supporting diaphragm at the temperature of 200 ℃ and the rotation speed of 10 rpm.

Example 3

In a room temperature environment, weighing PI powder and PTFE powder according to a mass ratio of 80:20, placing the powder in a high-speed dispersion machine, performing pulse stirring at 10000rpm for 30min, placing the mixed material in a roller press, rolling the powder into a self-supporting diaphragm at the temperature of 180 ℃ and the rotating speed of 5 rpm.

Example 4

In a room temperature environment, weighing PI powder and PTFE powder according to a mass ratio of 75:25, placing the powder in a high-speed dispersion machine, performing pulse stirring at 17500rpm for 20min, placing the mixed material in a roller press, and rolling the powder into a self-supporting diaphragm at the temperature of 135 ℃ and the rotating speed of 3 rpm.

Example 5

Weighing PMIA powder and PTFE powder according to a mass ratio of 75:25 in a room temperature environment, placing the mixture in a high-speed dispersion machine, performing pulse stirring at 17500rpm for 20min, placing the mixture in a roller press, and rolling the powder into a self-supporting diaphragm at the temperature of 135 ℃ and the rotating speed of 3 rpm.

Example 6

In a room temperature environment, weighing PI powder, PET powder and PTFE powder according to a mass ratio of 80:5:15, placing the weighed materials in a high-speed dispersion machine, performing pulse type stirring for 60min at 8000rpm, placing the mixed materials in a roller press, and rolling the powder into a self-supporting diaphragm at 90 ℃ and 2 rpm.

Example 7

In a room temperature environment, weighing PI powder and PVDF powder according to a mass ratio of 98:2, placing the powder in a high-speed dispersion machine, performing pulse stirring at 18000rpm for 10min, placing the mixed material in a roller press, rolling the powder into a self-supporting diaphragm at the temperature of 170 ℃ and the rotating speed of 8 rpm.

Example 8

The difference from example 1 is that the mass ratio of the PI powder to the PTFE powder is 50: 50.

Example 9

The difference from example 1 is that the mass ratio of the PI powder to the PTFE powder is 99: 1.

Comparative example 1

Celgard PP monolayer membranes were purchased ex-situ.

Comparative example 2

The difference from example 1 is that no PTFE powder was added, and only PI powder was used (the amount used was the same as the total mass of PI powder and PTFE powder in example 1).

Comparative example 3

The difference from example 1 is that no PI powder was added, and only PTFE powder was used (the amount used was the same as the total mass of PI powder and PTFE powder in example 1).

And (3) testing:

the separators of the respective examples and comparative examples were tested for mechanical strength by tensile rate, and for thickness, porosity, liquid absorption rate, liquid retention rate and 150 ℃ high temperature resistance, respectively, and the results are shown in Table 1.

Fig. 1 is a photograph of a liquid absorption test of the separator prepared in example 1. Fig. 2 is an optical photograph of the separator prepared in example 1.

TABLE 1

As can be seen from the above table, the separator of the present invention has high porosity, liquid absorption rate and liquid retention rate,

it is understood from the comparison of examples 8 to 9 with example 1 that when the content of PTFE is too high, the porosity, liquid absorption rate and liquid retention rate of the separator are all decreased, and the elongation is increased, whereas when the content of PI is too high, the porosity, liquid absorption rate and liquid retention rate of the separator are all increased, and the elongation is decreased.

As can be seen from comparison of comparative example 1 with example 1, the separator made of PTFE and PI-based high temperature resistant materials did not deform and did not burn at 150 ℃, whereas the conventional PP separator had been melt-crimped and burned at 150 ℃.

As is clear from comparison between comparative example 2 and example 1, the separator mainly exhibits PI material characteristics, has high porosity and liquid absorption rate, but has poor mechanical strength and low elongation rate, only when PI powder and no PTFE binder are present.

As can be seen from comparison between comparative example 3 and example 1, when only PTFE and no PI material are used, the separator mainly exhibits characteristics of the PTFE material and has high mechanical strength, but the porosity and the liquid absorption rate are low.

The applicant states that the present invention is illustrated in detail by the above examples, but the present invention is not limited to the above detailed methods, i.e. it is not meant that the present invention must rely on the above detailed methods for its implementation. It should be understood by those skilled in the art that any modification of the present invention, equivalent substitutions of the raw materials of the product of the present invention, addition of auxiliary components, selection of specific modes, etc., are within the scope and disclosure of the present invention.

Claims

1. A dry preparation method of a diaphragm is characterized by comprising the following steps:

in the mixture in the step (1), the fiberizable polymer powder is PTFE, and the mass percentage is 5-10%; the high-temperature resistant polymer is PI, and the mass percentage is 90-95%;

the method for drawing the fiberizable polymer powder into fibers under the action of the shearing force in the step (1) is high-speed stirring, and the high-speed stirring speed is 8000-25600 rpm;

the preset thickness of the step (2) is 15-70 μm;

wherein the high-temperature-resistant polymer is a polymer stably existing at the use temperature of the separator;

the fiberizable polymer powder is stable at the use temperature of the diaphragm;

the diaphragm includes: a network structure formed by mutually lapping polymer fibers converted from a fiberizable polymer and a high temperature resistant polymer dispersed and bonded in the network structure;

the network structure is formed by drawing the fiberizable polymer powder under the action of shearing force to form fibers and mutually lapping the fibers in the hot pressing process.

2. The dry-process preparation method according to claim 1, wherein the high-speed stirring speed in the step (1) is 17000rpm-21000 rpm.

3. The dry preparation method according to claim 1, wherein the time for the high-speed stirring and mixing in step (1) is 2min-2 h.

4. The dry-process preparation method according to claim 3, wherein the time for the high-speed stirring and mixing in the step (1) is 10-30 min.

5. The dry preparation method according to claim 1, wherein the high-speed stirring and mixing in step (1) is pulse stirring, and the gap time is 0-30 min.

6. The method of claim 1, wherein the hot pressing process of step (2) is hot rolling.

7. The dry-process preparation method according to claim 1, wherein the temperature of the hot-pressing treatment in the step (2) is 25 ℃ to 300 ℃.

8. The dry-process preparation method according to claim 1, wherein the temperature of the hot-pressing treatment in the step (2) is 60 ℃ to 250 ℃.

9. The dry-process preparation method according to claim 1, wherein the temperature of the hot-pressing treatment in the step (2) is 180 ℃ to 220 ℃.

10. The dry-process preparation method according to claim 1, wherein the hot-pressing speed during the hot-pressing treatment in step (2) is 5rpm to 50 rpm.

11. The dry-process preparation method according to claim 1, wherein the hot-pressing speed during the hot-pressing treatment in step (2) is 5rpm to 10 rpm.

12. A separator prepared by the method of any one of claims 1 to 11.

13. A battery comprising the separator of claim 12.