A lithium battery separator and preparation method thereof

Abstract

The present application proposes a lithium battery separator, comprising a non-woven fabric substrate, wherein the interior of the non-woven fabric substrate is filled with modified nano-silica aerogel particles, and the outside of the non-woven fabric substrate is coated with a layer of PVDF resin slurry. The present application also proposes a method for preparing the above-mentioned lithium battery separator. Through the preparation method of the present application, the silica aerogel with a porous structure can be squeezed and fixed inside the non-woven fabric substrate, and the silica aerogel can be fixed and sealed by the coated slurry, which can greatly improve the thermal insulation performance of the separator and the performance parameters such as the adsorption rate of the electrolyte, and has better durability.

Description

Technical Field

[0001] The present application relates to the field of lithium batteries, and in particular to a lithium battery separator and a preparation method thereof. Background Art

[0002] A lithium battery separator is a thin polymer film that separates the positive and negative electrode materials in a battery. The main characteristic of a lithium battery separator is its microporous structure, which allows lithium ions to pass through while preventing the positive and negative electrode materials on either side of the separator from directly connecting and causing a short circuit.

[0003] Existing lithium battery separators are typically made from polypropylene (PP), polyethylene (PE), or composite materials of polypropylene (PP) and polyethylene (PE). Existing technologies typically create a microporous structure on the substrate through wet or dry processes. To improve the separator's high-temperature resistance, wettability, and ion permeability, coatings made from various slurries are often applied to both sides of the substrate.

[0004] For example, CN 116315458 B discloses a ceramic diaphragm, which includes a base membrane, which is a porous film; a first coating, which is arranged on at least one side surface of the base membrane, and the first coating includes a porous nano-composite ceramic, a dispersant, a thickener, a binder and a wetting agent; and a second coating, which is arranged on the surface of the first coating, and the second coating includes a porous nano-composite ceramic, a polymer, a dispersant, a thickener, a binder and a wetting agent.

[0005] CN 109860471 B discloses a polymer membrane comprising a polymer base membrane and a coating on the polymer base membrane, wherein the coating contains a binder comprising a vinylidene fluoride copolymer. This prior art employs a vinylidene fluoride copolymer as the binder; the base membrane is a commercially available PP film with an average pore size of 100 nm, and the coating formed after coating has an average pore size of 500 nm.

[0006] Generally speaking, the coating slurry applied to the surface of the base membrane has relatively high wettability, which improves the ability of the electrolyte to transport ions through the micropores of the separator. Therefore, these coating slurries should theoretically easily penetrate the microporous structure of the base membrane, potentially blocking some of the micropores, reducing the porosity and pore size of the separator, thereby affecting the ion permeability of the separator and reducing the performance of the lithium battery.

[0007] CN 104183867 B discloses a nano-single-ion conductor-coated lithium battery separator using a polyester non-woven fabric as a substrate. The separator is formed by modifying the non-woven fabric with single-ion conductor nanoparticles and a polymer. The non-woven fabric has a tortuous and porous interior, but the coating slurry has difficulty penetrating the interior, and modification of the base film is limited to the surface, resulting in poor durability of the separator.

[0008] CN 105514324 B discloses a PET non-woven fabric-based composite lithium-ion battery separator with nanopores. The separator comprises a hydrophilic PET non-woven fabric substrate and a polymer, wherein the polymer is polyvinyl alcohol (PVA) or sodium hydroxymethyl cellulose (SMC). The polymer is filled into the pores of the hydrophilic PET non-woven fabric using ultrasonication, vacuum impregnation, low-temperature drying, and subsequent hot roller pressing. However, in this prior art embodiment, the non-woven fabric is a very small piece of fabric measuring 10 cm x 10 cm. Ultrasonic dispersion is performed in a water bath under vacuum for one hour. Large-scale industrial production using such a small piece of fabric is not feasible. Moisture evaporates easily under vacuum conditions, and after heating for another hour, it is difficult for the polymer to penetrate the fabric. Furthermore, since the polyvinyl alcohol or SMC is filled into the pores of the non-woven fabric, it is difficult to determine whether the nanoparticles will shed after drying. Furthermore, since polyvinyl alcohol or SMC are readily soluble in water, they quickly dissolve in the electrolyte, making sustained performance difficult. Summary of the Invention

[0009] The technical problem to be solved by the present application is to provide a lithium battery separator and a preparation method thereof, so as to reduce or avoid the problems mentioned above.

[0010] To solve the above technical problems, the present application proposes a lithium battery separator, comprising a non-woven fabric substrate, wherein the interior of the non-woven fabric substrate is filled with modified nano-silica aerogel particles, and the outer side of the non-woven fabric substrate is coated with a layer of PVDF resin slurry.

[0011] Preferably, the non-woven fabric substrate has a thickness of 5-100 μm and a porosity of 50-90%.

[0012] Preferably, the particle size of the modified nano-silica aerogel particles is 10-20 nm.

[0013] Preferably, the thickness of the PVDF resin adhesive layer is 1-3 μm.

[0014] The present application also proposes a method for preparing a lithium battery separator, comprising the following steps: uniformly mixing modified nano-silica aerogel particles with paraffin wax heated to a liquid state, and cooling the mixture into a paste-like mixture; applying the paste-like mixture to both sides of a non-woven fabric substrate, and repeatedly scraping the paste-like mixture with a scraper to fill the pores of the non-woven fabric substrate until the surface fibers of the non-woven fabric substrate are free of the paste-like mixture; coating the surface of the non-woven fabric substrate after scraping; drying the non-woven fabric substrate coated with the PVDF resin slurry to solidify the PVDF resin slurry; clamping the non-woven fabric substrate with a PVDF resin slurry layer between two layers of oil-absorbing paper, and squeezing the outside of the two layers of oil-absorbing paper with a heated roller; spraying and rinsing the extruded separator with hot water at a temperature of 75-80°C, and obtaining the lithium battery separator after drying.

[0015] Preferably, the content of the modified nano-silica aerogel in the paste mixture is 5wt%-10wt%.

[0016] Preferably, 0.1-0.2 wt% of a dye is further added to the paste mixture, and the color of the dye is different from the color of the non-woven fabric substrate.

[0017] Preferably, the preparation method further comprises the step of surface-modifying the nano-silica aerogel particles, comprising: adding 10-20 parts by weight of the nano-silica aerogel particles to 100-200 parts by weight of a polyvinyl alcohol aqueous solution, stirring continuously for 60-120 minutes to fully disperse the particles; then subjecting the dispersed mixture to freeze-drying spraying to obtain surface-modified nano-silica aerogel particles, and screening particles with a particle size of 10-20 nm for later use.

[0018] Through the preparation method of the present application, the porous silica aerogel can be squeezed and fixed inside the non-woven fabric substrate, and the silica aerogel can be fixed and sealed by the coated slurry, which can greatly improve the thermal insulation performance of the diaphragm and the adsorption rate of the electrolyte and other parameters, and has better durability. DETAILED DESCRIPTION

[0019] In order to have a clearer understanding of the technical features, purposes and effects of this application, the specific implementation methods of this application are now described in detail.

[0020] In view of the problems existing in the prior art, the present application proposes a lithium battery separator, comprising a non-woven fabric substrate, the interior of the non-woven fabric substrate is filled with modified nano-silica aerogel particles, and the outer surface of the non-woven fabric substrate is coated with a layer of PVDF resin slurry. The non-woven fabric substrate can be made of any existing non-woven fabric material that can be used for lithium batteries. For example, it is preferred to use a non-woven fabric with a thickness of 5-100 μm and a porosity of 50-90% purchased or prepared. The non-woven fabric can be made of high-density materials such as PE, PP, PET, PET / cellulose, etc., and can be prepared by any process, including but not limited to hydroentanglement, heat sealing, spunbonding, meltblowing, etc.

[0021] The PVDF resin slurry layer can be formed by curing any slurry with PVDF resin as a binder suitable for the lithium battery field in the prior art. Flame retardants, ceramic powders and other reinforcing materials can be added to the slurry as needed. The thickness of the PVDF resin slurry layer is preferably 1-3 μm.

[0022] Silica aerogel is a low-density material with a porous, disordered, nanoscale continuous network structure. Its specific surface area is much larger than that of ordinary silica, and it has excellent thermal insulation properties. Adding it to non-woven fabrics can greatly improve the heat resistance and flame retardancy of the diaphragm. Due to its large specific surface area, it can also adsorb electrolyte ions by filling the pores of the non-woven fabric, thereby improving the ability of electrolyte ions to enter the non-woven fabric and reducing the internal resistance of the diaphragm. Of course, due to its large specific surface area, commonly used phosphate coupling agents, silane coupling agents (such as vinyl triethoxysilane, vinyl trimethoxysilane, vinyl tri (β-methoxyethoxy) silane, etc.) are difficult to disperse silica aerogel. To prevent agglomeration, the present invention applies surface modification to silica aerogel particles. The specific steps are as follows: 10-20 parts by weight of nano-silica aerogel particles are added to 100-200 parts by weight of a polyvinyl alcohol aqueous solution and stirred for 60-120 minutes to fully disperse the particles; the dispersed mixture is then freeze-dried and spray-dried to obtain surface-modified nano-silica aerogel particles, which are then screened to obtain particles with a particle size of 10-20 nm for later use. The polyvinyl alcohol content in the polyvinyl alcohol aqueous solution is 10-15% by weight.

[0023] Furthermore, the present application proposes a preparation method for the above-mentioned lithium battery separator. Specifically, the preparation method of the present application includes the following steps.

[0024] First, the modified nano-silica aerogel particles are uniformly mixed with paraffin wax heated to a liquid state and cooled to form a paste-like mixture. Preferably, a paraffin wax raw material that is a paste at room temperature is used and heated to a liquid state to facilitate uniform mixing of the modified nano-silica aerogel particles. After uniform mixing, the mixture is allowed to cool naturally to a paste-like state, where the particles are fixedly dispersed in the paste-like mixture and can be stored for long periods of time at low temperatures. In a specific embodiment, the content of the modified nano-silica aerogel in the paste-like mixture is 5wt%-10wt%.

[0025] The paste mixture can then be applied to both sides of the non-woven fabric substrate and repeatedly scraped with a scraper to fill the pores of the non-woven fabric substrate with the paste mixture until the surface fibers of the non-woven fabric substrate are free of the paste mixture. In another embodiment, to facilitate observation of the absence of residual paste on the surface fibers of the non-woven fabric substrate, the paste mixture preferably further comprises 0.1-0.2 wt% of a dye having a color different from that of the non-woven fabric substrate. The dye can be any organic or inorganic dye that can be used to dye paraffin wax.

[0026] Afterwards, the PVDF resin slurry is coated on the surface of the non-woven fabric substrate after the scraping. The slurry coating operation can be carried out by scraping, spin coating or spraying, and the scraping process is preferably used. For example, the thickness of the slurry layer can be controlled by controlling the height of the scraper.

[0027] After that, the non-woven fabric substrate coated with PVDF resin slurry is dried to solidify the PVDF resin slurry. Of course, the solidification temperature of PVDF resin slurry is higher than the melting point of paraffin wax. Therefore, while drying and solidifying, the paraffin wax in the non-woven fabric substrate will partially melt and seep out from the microporous structure of the non-woven fabric substrate. The PVDF resin slurry that has not yet solidified outside the micropores attached to the surface of the non-woven fabric substrate will be taken away by the seeping paraffin wax to expose the micropores on the surface. At the same time, the space originally occupied by paraffin wax inside the non-woven fabric substrate becomes hollow due to the seepage of paraffin wax, and the nano-silica aerogel mixed with paraffin wax is retained in these hollows. The coated PVDF resin slurry can lock the fibers on the surface of the non-woven fabric substrate by adhesion to prevent the internal silica aerogel from easily leaking out.

[0028] Then, a non-woven fabric substrate with a PVDF resin slurry layer is sandwiched between two layers of oil-absorbing paper and squeezed on the outside of the two layers of oil-absorbing paper with a heated roller. Through heating and extrusion, the paraffin remaining in the non-woven fabric substrate can be melted and squeezed out and absorbed with oil-absorbing paper (the heating temperature is higher than the melting temperature of the paraffin). The cavity inside the non-woven fabric substrate, which is enlarged by the paraffin wax, is squeezed to reduce its diameter to the designed range. At the same time, the non-woven fabric is compressed to be thinned to reduce the overall thickness of the lithium battery. While thinning, the length of the internal channel of the non-woven fabric substrate is also increased to increase the breakdown voltage of the non-woven fabric, which is beneficial to increase the charging speed of the lithium battery. The outer PVDF resin slurry layer is deformed by extrusion, and the deformation stress can prevent the thickness of the non-woven fabric substrate from rebounding.

[0029] Finally, the extruded separator is rinsed with hot water at a temperature of 75-80°C and dried to obtain the lithium battery separator. The hot water spray rinse can remove paraffin and other water-soluble substances in the non-woven fabric substrate to prevent them from dissolving into the electrolyte during use of the lithium battery, thereby improving the consistency and durability of the lithium battery.

[0030] Examples 1-3 and Comparative Examples 1-3

[0031] Lithium battery separators were prepared using the parameters in the following table, wherein silica was used in Comparative Examples 1-3 to replace the silica aerogel used in Examples 1-3, and Comparative Examples 1-3 were not modified. The silica in Comparative Examples 1-3 was dispersed into the non-woven fabric substrate using ultrasonic dispersion of a gel aqueous solution.

[0032]

[0033] The parameters of the membranes of Examples 1-3 and Comparative Examples 1-3 are shown in the following table.

[0034]

[0035] Further, by immersing the diaphragm in a high-temperature electrolyte for aging experiments, the capacity retention rates of the diaphragms of Examples 1-3 were 140%, 143% and 148% of the capacity retention rates of Comparative Examples 1-3, respectively, indicating that the diaphragm of the present application has better durability.

[0036] Those skilled in the art should understand that although this application is described in terms of multiple embodiments, not each embodiment contains only one independent technical solution. This description is only for the sake of clarity. Those skilled in the art should understand the description as a whole and understand the scope of protection of this application by considering the technical solutions involved in each embodiment as being combinable into different embodiments.

[0037] The above description is only an illustrative embodiment of the present application and is not intended to limit the scope of the present application. Any equivalent changes, modifications and combinations made by any person skilled in the art without departing from the concept and principle of the present application shall fall within the scope of protection of the present application.

Claims

1. A method for preparing a lithium battery separator, wherein the lithium battery separator comprises a non-woven fabric substrate, wherein the interior of the non-woven fabric substrate is filled with modified nano-silica aerogel particles, and the outer surface of the non-woven fabric substrate is coated with a layer of PVDF resin slurry; characterized in that: The preparation method comprises the following steps: uniformly mixing modified nano-silica aerogel particles with paraffin wax heated to a liquid state, and cooling the mixture into a paste-like mixture; applying the paste-like mixture to both sides of a non-woven fabric substrate, and repeatedly scraping the paste-like mixture with a scraper to fill the pores of the non-woven fabric substrate until the surface fibers of the non-woven fabric substrate are free of the paste-like mixture; coating the surface of the non-woven fabric substrate after scraping; drying the non-woven fabric substrate coated with the PVDF resin slurry to solidify the PVDF resin slurry; sandwiching the non-woven fabric substrate with the PVDF resin slurry layer between two layers of oil-absorbing paper, and squeezing the outer sides of the two layers of oil-absorbing paper with a heated roller; The extruded separator is sprayed and rinsed with hot water at a temperature of 75-80° C., and dried to obtain the lithium battery separator.

2. The preparation method according to claim 1, wherein The content of the modified nano-silica aerogel in the paste mixture is 5 wt%-10 wt%.

3. The preparation method according to claim 1, wherein 0.1-0.2 wt% of a dye is further added to the paste mixture, and the color of the dye is different from the color of the non-woven fabric substrate.

4. The preparation method according to claim 1, further comprising the step of surface modification of the nano-silica aerogel particles, comprising: 10-20 parts by weight of nano-silica aerogel particles are added to 100-200 parts by weight of polyvinyl alcohol aqueous solution and stirred continuously for 60-120 minutes to fully disperse; the dispersed mixture is then freeze-dried and sprayed to obtain surface-modified nano-silica aerogel particles, which are then screened to obtain particles with a particle size of 10-20 nm for later use.

5. A lithium battery separator, characterized in that: The lithium battery separator is prepared by the preparation method according to any one of claims 1 to 4.

6. The lithium battery separator according to claim 5, characterized in that The non-woven fabric substrate has a thickness of 5-100 μm and a porosity of 50-90%.

7. The lithium battery separator according to claim 5, characterized in that The particle size of the modified nano-silicon dioxide aerogel particles is 10-20 nm.

8. The lithium battery separator according to claim 5, wherein The thickness of the PVDF resin slurry layer is 1-3 μm.

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