A pitch-coated separator for lithium-ion batteries and a method of making the same

Abstract

The present application belongs to the technical field of lithium ion battery materials, and particularly relates to a pitch-coated separator for a lithium ion battery and a preparation method thereof. The method comprises the following steps: S1, preparation of an emulsifier aqueous solution: dispersing an emulsifier in deionized water, stirring uniformly, and obtaining the emulsifier aqueous solution; S2, preparation of a pitch emulsion: adding high-temperature pitch to the emulsifier aqueous solution, stirring uniformly, and obtaining the pitch emulsion; and S3, preparation of the pitch-coated separator: coating the pitch emulsion on a base film and drying, and obtaining the pitch-coated separator. The present application selects high-temperature pitch as a coating material, utilizes the strong adhesion and low water absorption of the high-temperature pitch to improve the peeling strength of the separator coating and reduce the water absorption of the separator.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery material technology, and specifically relates to an asphalt-coated separator for lithium-ion batteries and its preparation method. Background Technology

[0002] With the increasing demands for energy density and safety in power lithium-ion battery materials for new energy vehicles, the requirements for the heat resistance, mechanical properties, and electrolyte wettability of lithium-ion battery separators are also becoming more stringent. The water content, tensile strength, and heat shrinkage deformation temperature of the separator are crucial indicators related to the safety performance of lithium-ion batteries. Traditional polyolefin porous membranes cannot simultaneously meet the requirements for both electrical performance and safety, necessitating separator modification.

[0003] Currently, a common modification method is to coat the membrane surface with a functional layer, such as a ceramic layer, to simultaneously meet the requirements of electrical and safety performance. Although binders are added during the preparation of the ceramic coating slurry, the adhesion between the ceramic coating and the base membrane layer is still poor, and ceramic delamination is prone to occur. Moreover, the small-diameter ceramic particles and binders present in the coating can cause it to absorb water, which deteriorates the electrical and safety performance.

[0004] Chinese patent CN113659286A discloses a method for preparing boehmite slurry and boehmite-coated separators. This patent involves mixing boehmite with a solvent, adding a dispersant to obtain a dispersion, then adding a thickener and a binder sequentially to the dispersion to adjust the pH, and finally adding a surfactant to obtain the boehmite slurry. The slurry obtained through this stepwise preparation process exhibits good dispersibility, does not settle, and has good viscosity stability; it is a low-moisture slurry. The boehmite-coated separator prepared from it has a coating thickness of 1-4 μm, meeting the technical requirement of a water content below 500 ppm for use in assembled batteries. However, the adhesion between the boehmite coating and the base film layer is still relatively poor, affecting its electrical and safety performance.

[0005] Chinese patent CN106221480A discloses a lithium-ion battery separator coating, its preparation method, and its application. The battery separator coating is applied to the surface of a lithium-ion battery separator using a spin-coating method. It consists of a modified SiO2 core, which is coated with a polymer copolymer via emulsion polymerization to obtain a core-shell polymer-coated silica nanoparticle emulsion, thickened with hydroxyethyl cellulose. The polymer copolymer is a copolymer of polystyrene / polybutyl acrylate or polystyrene / polymethyl acrylate. The coating prepared by this invention provides dual protection for the lithium-ion battery separator, offering both low-temperature thermal shutdown and high-temperature shrinkage resistance. However, it still suffers from problems such as easy delamination and water absorption. Summary of the Invention

[0006] To address the aforementioned technical problems, the present invention provides the following technical solutions.

[0007] A method for preparing an asphalt-coated separator for lithium-ion batteries includes the following steps:

[0008] S1, Preparation of emulsifier aqueous solution: Disperse the emulsifier in deionized water and stir evenly to obtain emulsifier aqueous solution;

[0009] S2, Preparation of asphalt emulsion: High-temperature asphalt is added to an emulsifier aqueous solution and stirred until homogeneous to obtain asphalt emulsion;

[0010] S3, Preparation of asphalt-coated diaphragm: Asphalt emulsion is coated onto a base film and dried to obtain an asphalt-coated diaphragm.

[0011] In the above-mentioned method for preparing an asphalt-coated separator for lithium-ion batteries, in S2, the softening point of the high-temperature asphalt is 150℃-250℃, and the particle size D50 is 0.8-1.5μm.

[0012] In the above-mentioned method for preparing an asphalt-coated separator for lithium-ion batteries, the mass fraction of high-temperature asphalt in the asphalt emulsion is 40%-60%.

[0013] In the above-mentioned method for preparing an asphalt-coated separator for lithium-ion batteries, in S1, the emulsifier is a cationic quaternary ammonium salt.

[0014] In the above-mentioned method for preparing an asphalt-coated separator for lithium-ion batteries, in S1, the pH of the emulsifier aqueous solution is 8-10, and the mass fraction of the emulsifier is 0.4%-0.8%, and the emulsifier is trimethyloctadecylammonium bromide.

[0015] In the above-mentioned method for preparing an asphalt-coated separator for lithium-ion batteries, in step S3, the base membrane is a polyethylene separator with a thickness of 8μm-15μm.

[0016] In the above-mentioned method for preparing an asphalt-coated separator for lithium-ion batteries, in step S3, the asphalt emulsion is coated by micro-gravure printing, spraying, or dot coating.

[0017] In the above-mentioned method for preparing an asphalt-coated separator for lithium-ion batteries, in step S3, the thickness of the coating of the asphalt emulsion on the base film is 1-3 μm.

[0018] In the above-mentioned method for preparing an asphalt-coated separator for lithium-ion batteries, in step S3, the drying temperature is 40-55℃.

[0019] The asphalt-coated separator for lithium-ion batteries is prepared according to the above-described method. Compared with existing technologies, the advantages of this invention are:

[0020] 1) High-temperature asphalt emulsion has a certain viscosity and can bond with the base film without the need for binder. During use, the asphalt coating is not easy to fall off the base film.

[0021] 2) Because asphalt does not absorb water and the coating does not contain binders, the entire asphalt coating does not easily absorb water, resulting in a low moisture content in the coated diaphragm. Detailed Implementation

[0022] The present invention will now be described in further detail with reference to specific embodiments.

[0023] Example 1

[0024] Trimethyloctadecylammonium bromide was selected as the emulsifier and dispersed in water to obtain an aqueous emulsifier solution with a mass fraction of 0.4% and a pH of 8.0. Then, high-temperature asphalt with a softening point of 160℃ and a particle size D50 of 1.0 μm (60% by mass) was added to the emulsifier solution and stirred to disperse the mixture. Finally, a 2 μm thick asphalt coating was applied to a 10 μm thick polyethylene diaphragm using a dot-coating method and dried at 50℃ to obtain an asphalt-coated diaphragm. The thermal shrinkage properties, moisture content, and peel strength of the coated diaphragm were then tested.

[0025] Example 2

[0026] Trimethyloctadecylammonium bromide was selected as the emulsifier and dispersed in water to obtain an aqueous emulsifier solution with a mass fraction of 0.6% and a pH of 9.0. Then, high-temperature asphalt with a softening point of 150℃ and a particle size D50 of 0.8 μm (40% by mass) was added to the emulsifier solution and stirred to disperse the mixture. Finally, a 1 μm thick asphalt coating was applied to an 8 μm thick polyethylene diaphragm using a microgravure process and dried at 40℃ to obtain an asphalt-coated diaphragm. The thermal shrinkage properties, moisture content, and peel strength of the coated diaphragm were then tested.

[0027] Example 3

[0028] Trimethyloctadecylammonium bromide was selected as the emulsifier and dispersed in water to obtain an aqueous emulsifier solution with a mass fraction of 0.8% and a pH of 10. Then, high-temperature asphalt with a softening point of 230℃ and a particle size D50 of 1.5μm (50% by mass) was added to the emulsifier solution and stirred to disperse the mixture. Finally, a 3μm thick asphalt coating was applied to a 15μm thick polyethylene diaphragm by spraying and dried at 55℃ to obtain an asphalt-coated diaphragm. The thermal shrinkage properties, moisture content, and peel strength of the coated diaphragm were then tested.

[0029] Example 4

[0030] Trimethyloctadecylammonium bromide was selected as the emulsifier and dispersed in water to obtain an aqueous emulsifier solution with a mass fraction of 0.5% and a pH of 9.5. Then, high-temperature asphalt with a softening point of 180℃ and a particle size D50 of 1.2 μm (55% by mass) was added to the emulsifier solution and stirred to disperse the mixture. Finally, a 2 μm thick asphalt coating was applied to a 10 μm thick polyethylene diaphragm by spraying and dried at 50℃ to obtain an asphalt-coated diaphragm. The thermal shrinkage properties, moisture content, and peel strength of the coated diaphragm were then tested.

[0031] Comparative Example 1

[0032] The raw materials for preparing the boehmite slurry include 100 parts boehmite, 5 parts dispersant, 20 parts thickener, 3 parts binder, 5 parts surfactant, and 9 parts water. First, boehmite with a particle size D50 of 1 μm is mixed with water, and then ammonium polyacrylate dispersant is added to obtain a dispersion. Sodium carboxymethyl cellulose thickener and polyacrylate binder are added sequentially to the dispersion to adjust the pH to 9.5. Then, sodium dodecyl sulfate surfactant is added and mixed to obtain the boehmite slurry. The slurry is sprayed onto a 12 μm thick polyethylene membrane, resulting in a 2 μm coating thickness, and dried at 45°C to obtain a boehmite-coated membrane. The heat shrinkage properties, moisture content, and peel strength of the coated membrane are then tested.

[0033] Test case

[0034] The thermal shrinkage properties, moisture content, and coating peel strength of the coated diaphragms prepared in Examples 1-4 and Comparative Example 1 were tested, and the results are shown in the table below.

[0035]

[0036] Compared to Comparative Example 1, in Examples 1-4, the high-temperature asphalt coating, being non-absorbent and without added binders in the coating slurry, exhibits a significantly lower water content compared to the boehmite-coated diaphragm. Simultaneously, the high-temperature asphalt slurry, due to its strong viscosity, demonstrates higher adhesion to the base film, resulting in a significantly improved peel strength for the asphalt coating compared to boehmite, making the asphalt-coated diaphragm less prone to detachment during use. The lower water content and higher coating peel strength make the asphalt-coated diaphragm superior to the boehmite-coated diaphragm in terms of processing and electrical properties.

[0037] The specific embodiments described herein are merely illustrative examples of the spirit of the invention. Those skilled in the art to which this invention pertains may make various modifications or additions to the described specific embodiments or use similar methods to substitute them, without departing from the spirit of the invention.

Claims

1. A method for preparing an asphalt-coated separator for lithium-ion batteries, characterized in that, Includes the following steps: S1, Preparation of emulsifier aqueous solution: Disperse the emulsifier in deionized water and stir evenly to obtain emulsifier aqueous solution; S2, Preparation of asphalt emulsion: High-temperature asphalt with a softening point of 150℃-250℃ and a particle size D50 of 0.8-1.5μm is added to an emulsifier aqueous solution and stirred to obtain an asphalt emulsion. The mass fraction of high-temperature asphalt in the asphalt emulsion is 40%-60%. S3, Preparation of asphalt-coated diaphragm: Asphalt emulsion is coated onto a base film and dried to obtain an asphalt-coated diaphragm. The thickness of the coating of the asphalt emulsion on the base film is 1-3 μm.

2. The method for preparing an asphalt-coated separator for lithium-ion batteries according to claim 1, characterized in that, In S1, the emulsifier is a cationic quaternary ammonium salt.

3. The method for preparing an asphalt-coated separator for lithium-ion batteries according to claim 2, characterized in that, In S1, the pH of the emulsifier aqueous solution is 8-10, and the mass fraction of the emulsifier is 0.4%-0.8%. The emulsifier is trimethyloctadecylammonium bromide.

4. The method for preparing an asphalt-coated separator for lithium-ion batteries according to claim 1, characterized in that, In S3, the base membrane is a polyethylene diaphragm with a thickness of 8μm-15μm.

5. The method for preparing an asphalt-coated separator for lithium-ion batteries according to claim 1, characterized in that, In S3, the methods for applying asphalt emulsion are micro-gravure, spraying, or dot coating.

6. The method for preparing an asphalt-coated separator for lithium-ion batteries according to claim 1, characterized in that, In S3, the drying temperature is 40-55℃.

7. The asphalt-coated separator prepared by the method for preparing an asphalt-coated separator for lithium-ion batteries according to any one of claims 1-6.

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