Process for the preparation of high-heat-stable ceramic-coated polyolefin separators

Abstract

The application discloses a preparation method of a high-thermal-stability ceramic-coated polyolefin diaphragm and relates to the technical field of polyolefin diaphragms.The polyolefin diaphragm is used as a base film, water-based ceramic slurry is uniformly coated on the base film, and a ceramic coating is formed on the base film after solidification, so that the thermal stability of the polyolefin diaphragm is improved by using inorganic ceramic particles contained in the ceramic coating, the polyolefin diaphragm has a low thermal shrinkage rate at high temperatures, short circuit caused by the contact between positive and negative electrodes is prevented, the use safety of lithium batteries is greatly improved, and fire and explosion are avoided.

Description

Technical fields:

[0001] This invention relates to the field of polyolefin membrane technology, and specifically to a method for preparing a high thermal stability ceramic-coated polyolefin membrane. Background technology:

[0002] In the structure of a lithium-ion battery, the separator is one of the key internal components. The performance of the separator determines the battery's interface structure, internal resistance, and other factors, directly affecting the battery's capacity, cycle life, and safety performance. The main function of the separator is to separate the positive and negative electrodes, preventing short circuits caused by contact between the electrodes. It also allows electrolyte ions to pass through. For lithium-ion batteries, since the electrolyte is an organic solvent system, separator materials resistant to organic solvents are required, typically high-strength, thin-film polyolefin porous membranes.

[0003] Because polyolefin materials have a low melting point, they are prone to shrinkage or even melting at high temperatures, causing contact between the positive and negative electrodes and resulting in a short circuit in the lithium battery. To improve the thermal stability of polyolefin separators, the art typically uses an adhesive to coat the surface of the polyolefin separator with a layer of inorganic ceramic particles, which then cures to form a ceramic coating on the polyolefin separator. Currently, adhesives are mainly organic polymer compounds. Although they have sufficient adhesive strength to achieve firm adhesion of inorganic ceramic particles to the separator, the adhesives have poor heat resistance and will shrink under high-temperature conditions, thus affecting the effect of the ceramic coating on improving the thermal stability of the separator. Summary of the Invention:

[0004] The technical problem to be solved by the present invention is to provide a method for preparing a high thermal stability ceramic-coated polyolefin separator, wherein an aqueous ceramic slurry is uniformly coated on the polyolefin separator to form a ceramic coating, thereby combining the flexibility of organic materials and the thermal stability of inorganic materials, improving the heat resistance and puncture strength of the separator, and thus improving the safety performance of the battery.

[0005] The technical problem to be solved by the present invention is achieved by the following technical solution:

[0006] The first objective of this invention is to provide a method for preparing a high thermal stability ceramic-coated polyolefin separator. The method involves first adding inorganic ceramic particles and a binder to water and mixing them evenly to obtain an aqueous ceramic slurry; then uniformly coating the aqueous ceramic slurry onto the polyolefin separator, and curing it to form a ceramic coating, thereby obtaining the ceramic-coated polyolefin separator.

[0007] In this process, inorganic ceramic particles provide high heat resistance, while the binder provides adhesion to maintain the structural integrity of the ceramic coating and the entire composite membrane.

[0008] Furthermore, the polyolefin separator is one of PP separator, PE separator, or PP / PE composite separator. PP separator or PE separator can be selected as the base membrane, or PP / PE composite separator can be selected as the base membrane.

[0009] Furthermore, the inorganic ceramic particles are at least one of alumina, boehmite, silicon dioxide, and titanium dioxide. Appropriate amounts of transition metal oxides or rare earth metal oxides such as cerium dioxide and zirconium dioxide may also be added.

[0010] Furthermore, the D50 particle size of the inorganic ceramic particles is 0.5–1.5 μm. It is necessary to control the particle size of the inorganic ceramic particles so that they adhere to the membrane without clogging the membrane pores.

[0011] Furthermore, the binder is sodium carboxymethyl cellulose or polyvinyl alcohol. Using a hydrophilic organic polymer as a binder and water as a solvent to prepare the water-based ceramic slurry not only solves the problems of increased cost and environmental pollution caused by solvent evaporation during curing associated with using organic solvents to prepare ceramic slurries, but also addresses the issue that resin-based binders, due to their poor adhesion, cannot bond sufficient inorganic ceramic particles to the diaphragm to significantly improve its thermal stability.

[0012] Furthermore, the amount of binder used is 10-15% of the weight of the inorganic ceramic particles. If the amount of binder is too small, there will be too few inorganic ceramic particles bonded to the diaphragm, which will not achieve the technical effect of substantially improving the thermal stability of the diaphragm; if the amount of binder is too large, the ceramic coating will contain too much binder, which is also not conducive to improving the thermal stability of the diaphragm.

[0013] Furthermore, the curing is either room temperature curing or heat curing. Using water as the solvent for preparing the ceramic slurry allows for air drying and curing at room temperature, or heat curing, thus shortening the curing time.

[0014] Furthermore, the heating and curing temperature is 80–120°C. Curing can also be carried out at a temperature below 80°C, but the curing time will be longer.

[0015] Furthermore, the thickness of the ceramic coating is 2–4 μm. The ceramic coating can be prepared on only one side of the diaphragm, or it can be prepared on both sides of the diaphragm.

[0016] A second objective of this invention is to provide a high thermal stability ceramic-coated polyolefin separator prepared by the aforementioned preparation method.

[0017] A third objective of this invention is to provide the application of the high thermal stability ceramic-coated polyolefin separator in lithium batteries.

[0018] The beneficial effects of this invention are as follows: This invention uses a polyolefin separator as the base membrane, and uniformly coats the base membrane with an aqueous ceramic slurry. After curing, a ceramic coating is formed on the base membrane. The inorganic ceramic particles contained in the ceramic coating are used to improve the thermal stability of the polyolefin separator, so that the polyolefin separator has a low thermal shrinkage rate at high temperature, preventing short circuits caused by positive and negative electrode contact, greatly improving the safety of lithium battery use, and avoiding fire and explosion. Detailed implementation method:

[0019] To make the technical means, creative features, objectives and effects of this invention easier to understand, the invention will be further described below with reference to specific embodiments.

[0020] The raw materials used in the embodiments and comparative examples of this invention are described below:

[0021] The PP membrane has a thickness of 25μm, an air permeability of 350sec / 100mL, and a porosity of 39%.

[0022] The PE membrane has a thickness of 20μm, an air permeability of 200sec / 100mL, and a porosity of 40%.

[0023] The D50 particle size of alumina is 0.5–1.0 μm, and the specific surface area is 4–8 m². 2 / g;

[0024] Boehmite has a D50 particle size of 0.5–1.4 μm and a specific surface area of ​​4–9 m². 2 / g.

[0025] Example 1

[0026] The preparation method of the ceramic-coated polyolefin membrane in this embodiment is as follows: First, alumina and sodium carboxymethyl cellulose are added to water, with the amount of sodium carboxymethyl cellulose being 10% of the weight of alumina. After mixing evenly, an aqueous ceramic slurry is obtained. Then, the aqueous ceramic slurry is uniformly coated onto the PP membrane and cured at 110°C to form a ceramic coating with a thickness of 3μm.

[0027] Example 2

[0028] The preparation method of the ceramic-coated polyolefin membrane in this embodiment is as follows: first, alumina and sodium carboxymethyl cellulose are added to water, and the amount of sodium carboxymethyl cellulose is 12% of the weight of alumina. After mixing evenly, an aqueous ceramic slurry is obtained. Then, the aqueous ceramic slurry is uniformly coated on the PE membrane and cured at 100°C to form a ceramic coating with a thickness of 4μm.

[0029] Example 3

[0030] The preparation method of the ceramic-coated polyolefin membrane in this embodiment is as follows: first, alumina and polyvinyl alcohol are added to water, and the amount of polyvinyl alcohol is 10% of the weight of alumina. After mixing evenly, an aqueous ceramic slurry is obtained. Then, the aqueous ceramic slurry is uniformly coated on the PP membrane and cured at 80°C to form a ceramic coating with a thickness of 3μm.

[0031] Example 4

[0032] The preparation method of the ceramic-coated polyolefin membrane in this embodiment is as follows: first, boehmite and polyvinyl alcohol are added to water, and the amount of polyvinyl alcohol is 15% of the weight of boehmite. After mixing evenly, an aqueous ceramic slurry is obtained. Then, the aqueous ceramic slurry is uniformly coated on the PE membrane and cured at 120°C to form a ceramic coating with a thickness of 4μm.

[0033] Example 5

[0034] The preparation method of the ceramic-coated polyolefin membrane in this embodiment is as follows: Boehmite and sodium carboxymethyl cellulose are added to water first, and the amount of sodium carboxymethyl cellulose is 15% of the weight of boehmite. After mixing evenly, an aqueous ceramic slurry is obtained. The aqueous ceramic slurry is then uniformly coated onto the PE membrane and cured at 120°C to form a ceramic coating with a thickness of 4μm.

[0035] The fourth objective of this invention is to provide a method for preparing an adhesive that replaces sodium carboxymethyl cellulose and polyvinyl alcohol. This method not only achieves uniform and firm adhesion of inorganic ceramic particles to the diaphragm, but also provides good heat resistance and does not reduce the effect of the ceramic coating on improving the thermal stability of the diaphragm.

[0036] Further, the adhesive is prepared by adding allyl dihexylchlorosilane and ethanol to toluene, heating the reaction until the allyl dihexylchlorosilane reacts completely, then adding an azo initiator dropwise, maintaining the temperature after the addition is complete, and after the reaction is finished, recovering toluene and unreacted ethanol by vacuum distillation to obtain the adhesive.

[0037] Preferably, the number-average molecular weight of the binder is 3000-4000. If the molecular weight is too small, it will affect the adhesion of the inorganic ceramic particles to the diaphragm; if the molecular weight is too large, it will affect the uniformity of dispersion of the inorganic ceramic particles in the ceramic slurry, making it impossible for the inorganic ceramic particles to adhere uniformly to the diaphragm.

[0038] Preferably, the molar ratio of allyl dihexylchlorosilane to ethanol is 1:(1.2-1.5). Under conditions of excess ethanol, allyl dihexylchlorosilane is allowed to react completely, converting the chlorine substituent in the allyl dihexylchlorosilane structure to an ethoxy group.

[0039] Preferably, the azo initiator is one of azobisisobutyronitrile, azobisisoheptanenitrile, and dimethyl azobisisobutyrate, and the amount used is 0.5-1% of the weight of allyl dihexylchlorosilane. Other oil-soluble initiators, such as organic peroxide initiators, may also be used.

[0040] The preparation principle of the adhesive described in this invention is as follows: allyl dihexylchlorosilane undergoes a substitution reaction with ethanol to obtain allyl (ethoxy) dihexylsilane, and polyallyl (ethoxy) dihexylsilane is obtained by polymerization using allyl (ethoxy) dihexylsilane as a monomer. Polyallyl (ethoxy) dihexylsilane is then used as an adhesive.

[0041] Example 6

[0042] The preparation method of the ceramic-coated polyolefin separator in this embodiment is the same as in Example 5, except that sodium carboxymethyl cellulose is replaced with the same mass of polyallyl(ethoxy)dihexylsilane. The preparation method of polyallyl(ethoxy)dihexylsilane is as follows:

[0043] Allyl dihexylchlorosilane and ethanol were added to toluene in a molar ratio of 1:1.2. The mixture was heated to 80°C and reacted until the allyl dihexylchlorosilane was completely reacted. Then, dimethyl azobisisobutyrate was added dropwise at a concentration of 0.5% of the weight of allyl dihexylchlorosilane. After the addition was complete, the mixture was kept at the same temperature for the reaction to proceed. Once the reaction was complete, toluene and unreacted ethanol were recovered by vacuum distillation to obtain polyallyl(ethoxy)dihexylsilane with a number average molecular weight of 3200.

[0044] Example 7

[0045] The preparation method of the ceramic-coated polyolefin separator in this embodiment is the same as in Example 5, except that sodium carboxymethyl cellulose is replaced with the same mass of polyallyl(ethoxy)dihexylsilane. The preparation method of polyallyl(ethoxy)dihexylsilane is as follows:

[0046] Allyl dihexylchlorosilane and ethanol were added to toluene in a molar ratio of 1:1.5. The mixture was heated to 80°C and reacted until the allyl dihexylchlorosilane was completely reacted. Then, dimethyl azobisisobutyrate was added dropwise at a concentration of 1% of the weight of allyl dihexylchlorosilane. After the addition was complete, the mixture was kept at the same temperature for the reaction to proceed. Once the reaction was complete, toluene and unreacted ethanol were recovered by vacuum distillation to obtain polyallyl(ethoxy)dihexylsilane with a number average molecular weight of 3400.

[0047] Example 8

[0048] The preparation method of the ceramic-coated polyolefin separator in this embodiment is the same as in Example 5, except that sodium carboxymethyl cellulose is replaced with the same mass of polyallyl(ethoxy)dihexylsilane. The preparation method of polyallyl(ethoxy)dihexylsilane is as follows:

[0049] Allyl dihexylchlorosilane and ethanol were added to toluene in a molar ratio of 1:1.5. The mixture was heated to 80°C and reacted until the allyl dihexylchlorosilane was completely reacted. Then, dimethyl azobisisobutyrate was added dropwise at a concentration of 1% of the weight of allyl dihexylchlorosilane. After the addition was complete, the mixture was kept at the same temperature for the reaction to proceed. Once the reaction was complete, toluene and unreacted ethanol were recovered by vacuum distillation to obtain polyallyl(ethoxy)dihexylsilane with a number average molecular weight of 3800.

[0050] Comparative Example 1

[0051] The preparation method of the ceramic-coated polyolefin separator in this comparative example is the same as that in Example 5, except that sodium carboxymethyl cellulose is replaced with the same mass of acrylic resin, and the preparation method of acrylic resin is the same as that in Example 1 of patent CN 113583532A.

[0052] The transverse and longitudinal thermal shrinkage rates of the ceramic-coated polyolefin separators prepared in Examples 1-8 and Comparative Example 1 were tested under test conditions of 160℃ / 1h, and the results are shown in Table 1.

[0053] Table 1 Thermal shrinkage properties of ceramic-coated polyolefin separators

[0054]

[0055] As can be seen from the data in Table 1, the present invention can significantly improve the thermal stability of the diaphragm by using polyallyl(ethoxy)dihexylsilane as a binder, so that the ceramic coating formed on the diaphragm surface can better exert its effect on improving the heat resistance of the diaphragm.

[0056] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of this invention is defined by the appended claims and their equivalents.

Claims

1. A method for preparing a high thermal stability ceramic-coated polyolefin separator, characterized in that: First, inorganic ceramic particles and binder are added to water and mixed evenly to obtain an aqueous ceramic slurry. Then, the aqueous ceramic slurry is evenly coated onto a polyolefin membrane and cured to form a ceramic coating, thus obtaining a ceramic-coated polyolefin membrane. The adhesive is prepared by adding allyl dihexylchlorosilane and ethanol to toluene, heating the mixture until the allyl dihexylchlorosilane reacts completely, then adding an azo initiator dropwise. After the addition is complete, the mixture is kept at a constant temperature for further reaction. Once the reaction is complete, the toluene and unreacted ethanol are recovered by vacuum distillation to obtain the adhesive. The number average molecular weight of the adhesive is 3000-4000.

2. The preparation method according to claim 1, characterized in that: The polyolefin separator is one of PP separator, PE separator, or PP / PE composite separator.

3. The preparation method according to claim 1, characterized in that: The inorganic ceramic particles are at least one of alumina, boehmite, silicon dioxide, and titanium dioxide.

4. The preparation method according to claim 3, characterized in that: The inorganic ceramic particles have a D50 particle size of 0.5~1.5μm.

5. The preparation method according to claim 1, characterized in that: The amount of the binder is 10-15% of the weight of the inorganic ceramic particles.

6. The preparation method according to claim 1, characterized in that: The curing process is either room temperature curing or heat curing.

7. The preparation method according to claim 1, characterized in that: The thickness of the ceramic coating is 2~4μm.

8. A high thermal stability ceramic-coated polyolefin separator prepared by the preparation method according to any one of claims 1 to 7.

9. The application of the high thermal stability ceramic-coated polyolefin separator as described in claim 8 in lithium batteries.

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