CN113725555B

CN113725555B - Lithium ion battery diaphragm and preparation method thereof

Info

Publication number: CN113725555B
Application number: CN202111146537.5A
Authority: CN
Inventors: 王永琛; 朱华君; 王正伟
Original assignee: Xingheng Power Supply Chuzhou Co ltd
Current assignee: Xingheng Power Supply Chuzhou Co ltd
Priority date: 2021-09-28
Filing date: 2021-09-28
Publication date: 2023-03-10
Anticipated expiration: 2041-09-28
Also published as: CN113725555A

Abstract

The invention discloses a lithium ion battery diaphragm and a preparation method thereof. Firstly, preparing active carbon and PTC material into slurry, soaking a base membrane in the slurry, taking out and drying; and preparing the graphene oxide and the PTC material into slurry, coating the slurry on two surfaces of the base film, and drying to obtain the lithium ion battery diaphragm. The diaphragm has high liquid absorption and retention rate, can absorb gas generated in the battery, and inhibit the lithium precipitation of the negative electrode caused by bubbles, so that the battery has long cycle life; the existence of PTC material in the inside of the base film and the surface coating guarantees the heat conduction characteristic of the diaphragm, effectively guarantees the even distribution of the internal temperature of the battery, further improves the battery performance, and guarantees the safety performance of the battery against internal short circuit and thermal runaway caused by needling, lithium dendrite and the like.

Description

Lithium ion battery diaphragm and preparation method thereof

Technical Field

The invention relates to a lithium ion battery, in particular to a diaphragm for the lithium ion battery and a preparation method thereof, and particularly relates to a diaphragm for the lithium ion battery with long cycle life and high safety performance and a manufacturing method thereof.

Background

The lithium ion battery diaphragm is one of four key materials of the lithium ion battery, the performance of the diaphragm determines the interface structure, internal resistance and the like of the battery, the characteristics of the battery, such as capacity, circulation, safety performance and the like, are directly influenced, and the diaphragm with excellent performance has an important effect on improving the comprehensive performance of the battery. The separator has the main function of separating the positive electrode and the negative electrode of the battery, preventing the short circuit caused by the contact of the two electrodes and enabling electrolyte ions to pass through. In addition, since the electrolyte of the lithium ion battery contains an organic solvent, the separator must also have resistance to the organic solvent.

At present, the most commonly used separator material of the lithium ion battery is a polyolefin material, but the polyolefin material is nonpolar, so that the wettability of the polyolefin material to the electrolyte is very poor, and the electrochemical properties such as liquid absorption and retention rate of the electrolyte, ionic conductivity and the like are seriously influenced. In addition, the organic bulk material and porous structure of polyolefin cause problems of poor heat resistance and heat dissipation.

Chinese utility model patent CN206313005U discloses a battery separator, which is attached with a Positive Temperature Coefficient (PTC) material layer on a thermoplastic resin layer (polyethylene layer or polypropylene layer). When the battery is overheated due to internal short circuit or acupuncture, the internal resistance of the positive temperature coefficient material is sharply increased, and the current is cut off to prevent the battery from further generating heat, thereby improving the safety performance of the battery. Chinese invention patent CN102272977B discloses a membrane, which is provided with a porous coating layer composed of filler particles and a binder polymer on a non-woven base fabric, wherein the filler particles include conductive Positive Temperature Coefficient (PTC) particles. On one hand, the conductive PTC particles show proper conductivity, and on the other hand, the conductive PTC particles expand when overheated, so that the pore area of the porous coating is reduced, the conductivity of the PTC particles is reduced, and the electrochemical reaction is prevented from further proceeding when thermal runaway occurs, thereby improving the safety of the battery.

Although the technical scheme can improve the safety of the battery to a certain extent, the PTC material is generally inorganic salt, the specific surface is very low, so that the liquid absorption and retention performance of the diaphragm is not improved, and the PTC material has certain conductivity, so that the breakdown voltage resistance value of the diaphragm can be reduced. Since the coating is generally thin and the electrolyte channels are reserved, the content of the PTC material is not high, the inside of the base film is not fully utilized, and the base film cannot completely cope with thermal runaway.

Chinese patent application CN109786623A discloses a polymer coated membrane, which is a composite membrane obtained by coating graphene oxide or graphene on a base membrane. The existence of the coating layer improves the liquid absorption and retention rate, the ionic conductivity and the heat resistance of the diaphragm, but the graphene oxide or the graphene cannot inhibit the internal short circuit such as acupuncture or lithium dendrite in principle, and in addition, the graphene has too strong conductivity, so that the breakdown voltage resistance value of the diaphragm is easily reduced.

Disclosure of Invention

The invention aims to provide a lithium ion battery diaphragm which has high liquid absorption and retention rate, absorbs gas generated in a battery, prolongs the cycle life of the battery, inhibits lithium precipitation, has high heat resistance and excellent heat dissipation performance, and improves the safety performance of the battery. Another object of the present invention is to provide a method for preparing such a separator.

In order to achieve the purpose of the invention, the technical scheme adopted by the invention is as follows: a lithium ion battery diaphragm comprises a base film, wherein the base film contains activated carbon and a Positive Temperature Coefficient (PTC) material, coatings are arranged on the surfaces of the two sides of the base film, and the coatings contain graphene oxide and the PTC material.

According to the preferred technical scheme, the activated carbon is the currently commercialized common mesoporous activated carbon, and the particle size D50 is generally 2-50 nm. The specific surface of the mesoporous activated carbon is larger than that of graphene oxide, the conductivity of the activated carbon tends to be insulating and lower than that of the graphene oxide, the activated carbon and the base film are made of non-polar materials, and the base film is used as a diaphragm main body, so that the mesoporous activated carbon is more suitable for being used in the base film compared with the graphene oxide, and the electric insulating property of the diaphragm is guaranteed. Another reason is that activated carbon has a lower compaction density than graphene oxide, so activated carbon is used inside the diaphragm and graphene oxide is used as the coating layer on the surface of the diaphragm. The specific surface area of the mesoporous activated carbon is about 3000 to 4000m ² Per g, 2600m graphene oxide ² The liquid absorption and retention rate of the mesoporous activated carbon is higher than that of graphene oxide, gas generated in the battery can be adsorbed, lithium precipitation caused by bubbles on a negative electrode is prevented, and the cycle life and the safety performance are improved.

In the technical scheme, the thickness of the coating on any surface of the base film is 1-4 microns, the coating process is difficult due to the excessively thin coating, the breakdown voltage resistance is affected due to the excessively thick coating, and the capability of lithium ions passing through the diaphragm is affected due to the excessively thick coating.

In the technical scheme, the particle size D50 of the Positive Temperature Coefficient (PTC) material is less than 0.1 μm, the positive temperature coefficient material is one or more of niobium titanate, tantalum titanate, strontium titanate, barium titanate and lead titanate, and the materials are commonly used PTC materials. The existence of the PTC material ensures that the internal resistance can be increased rapidly to cut off the current when thermal runaway is caused by acupuncture or lithium dendrite and the like, and the PTC material has high safety performance. Since the pore diameter of the separator is generally several tens of nanometers, the particle diameter D50 of the PTC material should be less than 0.1 μm to be able to enter the separator.

The number of the graphene oxide layers is 6-15, and the particle size D50 is less than or equal to 0.1 mu m. The too thin graphene oxide layer such as 1~5 is expensive and highly conductive, and is not suitable for coating on the surface of the diaphragm. And the graphene oxide with too large number of layers and too large particle size has small specific surface area, which is not beneficial to absorbing and retaining liquid.

In the above technical solution, the base film is selected from one of a polyolefin film, a polyolefin composite film, a polyimide film, and a non-woven microporous film, and these separators are common commercial separators. The thickness of the current thick diaphragm for the lithium battery is about 24 μm, and in order to pursue the improvement of energy density, the diaphragm is gradually thinner, so the thickness of the base film is less than or equal to 30 μm. The polyolefin can be selected from polyethylene or polypropylene, which is a commonly used separator material.

In order to achieve another object of the present invention, the present invention provides a method for preparing a lithium ion battery separator, comprising the steps of:

(1) Adding a binder and a solvent into a pre-stirring tank, wherein the solid content is less than 5%, and completely dissolving to obtain a first mixture; gradually adding activated carbon and positive temperature coefficient material powder into the first mixture, stirring and dispersing to ensure that the solid content is 10 +/-5%, and obtaining second mixture slurry; soaking the base film in the second mixture slurry, taking out after complete soaking, and drying to obtain the base film containing the activated carbon and the positive temperature coefficient material;

(2) Adding the binder and the solvent into a pre-stirring tank, and completely dissolving to obtain a third mixture; gradually adding graphene oxide and positive temperature coefficient material powder into the third mixture, stirring and dispersing to ensure that the solid content is 40-60%, and obtaining fourth mixture slurry; and (3) coating the fourth mixture slurry on two surfaces of the base film treated in the step (1), and drying to obtain the lithium ion battery diaphragm.

In the technical scheme, in the step (1), when the activated carbon and the positive temperature coefficient material powder are added into the first mixture, the stirring speed is 10-50 rpm in the feeding process, and the dispersion speed is 1000-5000 rpm after the feeding is finished; in the step (2), when the graphene oxide and the positive temperature coefficient material powder are added into the third mixture, the stirring speed is 10-50 rpm in the feeding process, and the dispersion speed is 1000-5000 rpm after the feeding is finished; the coating speed is 1-100 m/min, and the coating is dried at 80-100 ℃.

In the technical scheme, the solvent is N-methyl pyrrolidone, and the binder is polyvinylidene fluoride.

Due to the application of the technical scheme, compared with the prior art, the invention has the following advantages:

1. the diaphragm of the invention is composed of a base film and coatings on two sides of the base film, active carbon and PTC materials are added into the base film, graphene oxide and PTC materials are added into the coatings, and the lithium ion battery diaphragm with long cycle life and high safety performance is obtained through a specific structure composition;

2. in terms of performance, the invention ensures the liquid absorption and retention rate of the diaphragm through the existence of mesoporous activated carbon and graphene oxide, and can adsorb gas generated in the battery, inhibit lithium precipitation of the negative electrode caused by bubbles, and finally enable the battery to have long cycle life;

3. in the aspect of safety, the PTC materials in the inner part and the surface coating of the base film ensure the safety performance of the battery against internal short circuit and thermal runaway caused by needle punching, lithium dendrite and the like.

4. The existence of the mixture in the base film and the existence of the surface coating improve the heat resistance and heat dissipation performance of the diaphragm.

Drawings

FIG. 1 is a schematic cross-sectional view of a septum in accordance with an embodiment of the present invention.

Detailed Description

The invention will be better understood from the following description of embodiments thereof, with reference to the accompanying drawings. However, the embodiments of the present invention may be modified in various ways, and the following embodiments are only intended to provide a clearer and more specific description to those skilled in the art.

The first embodiment is as follows:

referring to fig. 1, which is a cross-sectional view of a lithium ion battery separator, a base film 11 is arranged in the middle layer, surface coatings 12 are arranged on two sides of the base film 11, and the base film 11 and the surface coatings 12 on the two sides form a whole separator.

The base film can be polyolefin film (such as polyethylene and polypropylene), polyimide film, or non-woven microporous film, and the thickness of the base film can be selected from 10-20 μm.

The base film contains Active Carbon (AC) and a Positive Temperature Coefficient (PTC) material, and the coating contains Graphene Oxide (GO) and the PTC material.

This example uses a polypropylene (PP) based film as a benchmark for membrane performance comparison. Barium titanate is used as the positive temperature coefficient material.

TABLE 1 comparison of the properties of the diaphragms made of various materials

Diaphragm

Total thickness of

Liquid absorption rate

Liquid retention rate

Ionic conductivity

Withstand voltage value

Penetration rate of acupuncture

PP

18μm

30%

20%

1.50μS/cm

1000V

50%

PP+AC

18μm

200%

80%

1.72μS/cm

900V

47%

PP+GO

18μm

180%

70%

1.65μS/cm

850V

46%

PP+PTC

18μm

26%

18%

1.47μS/cm

1400V

90%

PP+PTC+AC

18μm

200%

85%

1.70μS/cm

1350V

91%

AC/PP/AC

20μm

100%

40%

1.60μS/cm

950V

49%

GO/PP/GO

20μm

95%

38%

1.58μS/cm

900V

48%

PTC/PP/PTC

20μm

28%

19%

1.49μS/cm

1200V

60%

PTC+GO/PP/PTC+GO

20μm

90%

35%

1.57μS/cm

1150V

61%

PTC+GO/PP+PTC+AC/PTC+GO

20μm

210%

90%

1.80μS/cm

1500V

100%

The performance of the different material separators was compared as shown in table 1. Pure polypropylene PP basement membrane is as benchmark characteristic, after mixing active carbon AC or oxidation graphite alkene GO respectively wherein, because AC and GO have high specific surface area, so improved PP's imbibition liquid retention rate and ionic conductivity, AC specific surface area is higher than GO, so former imbibition liquid retention rate and ionic conductivity are a little higher. However, since the conductive layer has a certain conductivity as compared with the insulating layer, the withstand voltage and the puncture rate are slightly lowered. Since inorganic PTC is widely used to improve the needling performance, when AC and PTC materials are mixed into PP, the liquid absorption and retention rate, ionic conductivity, withstand voltage, and needling passage rate are greatly improved as compared with pure PP.

After the surface of the PP is coated with AC or GO respectively, the liquid absorption and retention rate and the ionic conductivity of the diaphragm are improved, but the withstand voltage value and the needling passing rate are slightly reduced. After the surface of the PP is coated with the PTC, the penetration rate of needling can be improved similarly to the mixing of the PTC in the PP, the pressure resistance value is also improved due to the existence of inorganic salt, and on the other hand, the liquid absorption and retention rate and the ionic conductivity are slightly reduced due to the low specific surface area of the PTC.

After PTC and AC are mixed in the PP basal membrane and PTC and GO are coated on the surface of the PP basal membrane, the liquid absorption and retention rate, the ionic conductivity, the withstand voltage value and the needling passage rate of the membrane are optimal, and the liquid absorption and the needling passage rate are close to theoretical limit values.

Example two:

referring to example one, a 20 μm separator was formed by mixing PTC and AC into an 18 μm thick PP-based film and coating a mixture of PTC and GO each 1 μm on both sides of the surface. The diaphragm is adopted to respectively assemble two monomer battery cores, one positive electrode adopts a 622 ternary positive system, the negative electrode adopts an artificial graphite system, 10 multiplying power type square aluminum shell monomer battery cores of 43Ah are assembled, and the energy density is about 180Wh/kg (the model of the square battery core is 2614891); the other positive electrode adopts a 811 ternary positive system, the negative electrode adopts an artificial graphite system, 10 capacity type square aluminum shell monomer battery cells of 52Ah are assembled, and the energy density is about 210Wh/kg (the model of the square battery cell is 2614891). Similarly, pp base films with the thickness of 20 μm are respectively organized into two monomer battery cells, wherein one positive electrode adopts a 622 ternary positive system, the negative electrode adopts an artificial graphite system, 10 multiplying power type square aluminum shell monomer battery cells with the height of 43Ah are assembled, and the energy density is about 180Wh/kg (the model of the square battery cell is 2614891); the other positive electrode adopts a 811 ternary positive system, the negative electrode adopts an artificial graphite system, 10 capacity type square aluminum shell monomer battery cells of 52Ah are assembled, and the energy density is about 210Wh/kg (the model of the square battery cell is 2614891).

The capacity type batteries adopting the PTC + GO/PP + PTC + AC/PTC + GO diaphragms can be cycled for more than 2000 times until the capacity is attenuated to 80% of the initial capacity, the rate type batteries can be cycled for more than 4000 times until the initial capacity is 80%, and the cycle water-jumping phenomenon is not found in 20 batteries. The capacity type battery adopting the pp base film after 20 mu m generally circulates for about 1500 times, namely, the capacity is attenuated to 80 percent of the initial capacity, and 1 cell circulates to generate the water jump phenomenon and the rapid attenuation condition; the multiplying power type battery generally circulates for about 3000 times, namely, the multiplying power type battery is attenuated to 80% of the initial capacity, and 2 battery cores circulate to generate a water jumping phenomenon. Because of the existence of AC and GO in the PTC + GO/PP + PTC + AC/PTC + GO diaphragm, the gas generated by the decomposition of the battery in the circulating process due to the existence of constant water and the electrolyte can be adsorbed by the AC and GO, so that the battery has longer cycle life compared with a base film. The lithium ion battery avoids lithium precipitation caused by excessive lithium ion insertion at the edge of the bubble due to gas accumulation on the surface of the negative electrode, and finally avoids the phenomenon of circulating water jump and even the internal short circuit thermal runaway caused by lithium dendrites.

Claims

1. A lithium ion battery separator, comprising a base film, characterized in that: the base film contains active carbon and a positive temperature coefficient material, the surfaces of the two sides of the base film are provided with coatings, and the coatings contain graphene oxide and the positive temperature coefficient material.

2. The lithium ion battery separator according to claim 1, wherein: the active carbon is mesoporous active carbon, and the particle size D50 is 2-50 nm.

3. The lithium ion battery separator according to claim 1, wherein: the thickness of the coating on any surface of the base film is 1-4 mu m.

4. The lithium ion battery separator according to claim 1, wherein: the particle size D50 of the positive temperature coefficient material is less than 0.1 mu m, and the positive temperature coefficient material is one or a mixture of more of niobium titanate, tantalum titanate, strontium titanate, barium titanate and lead titanate.

5. The lithium ion battery separator according to claim 1, wherein: the number of the graphene oxide layers is 6-15, and the particle size D50 is less than or equal to 0.1 mu m.

6. The lithium ion battery separator according to claim 1, wherein: the base film is selected from one of a polyolefin film, a polyolefin composite film, a polyimide film and a non-woven microporous film, and the thickness of the base film is less than or equal to 30 mu m.

7. The preparation method of the lithium ion battery separator of any one of claims 1 to 6, characterized by comprising the steps of:

(1) Adding a binder and a solvent into a pre-stirring tank, and completely dissolving to obtain a first mixture; gradually adding activated carbon and positive temperature coefficient material powder into the first mixture, stirring and dispersing to ensure that the solid content is 10 +/-5%, and obtaining second mixture slurry; soaking the base film in the second mixture slurry, taking out after complete soaking, and drying to obtain the base film containing the activated carbon and the positive temperature coefficient material;

(2) Adding the binder and the solvent into a pre-stirring tank, and completely dissolving to obtain a third mixture; gradually adding graphene oxide and positive temperature coefficient material powder into the third mixture, stirring and dispersing to ensure that the solid content is 40-60%, and obtaining fourth mixture slurry; and (4) coating the fourth mixture slurry on two sides of the base film treated in the step (1), and drying to obtain the lithium ion battery diaphragm.

8. The method for preparing a lithium ion battery separator according to claim 7, wherein: in the step (1), when the activated carbon and the positive temperature coefficient material powder are added into the first mixture, the stirring speed is 10-50 rpm in the feeding process, and the dispersion speed is 1000-5000 rpm after the feeding is finished; in the step (2), when the graphene oxide and the positive temperature coefficient material powder are added into the third mixture, the stirring speed is 10-50 rpm in the feeding process, and the dispersion speed is 1000-5000 rpm after the feeding is finished; the coating speed is 1-100 m/min, and the coating is dried at 80-100 ℃ after the coating is finished.

9. The method for preparing a lithium ion battery separator according to claim 7, wherein: the solvent is N-methyl pyrrolidone, and the binder is polyvinylidene fluoride.