A flame-retardant diaphragm for lithium battery and preparation method thereof

Abstract

The present invention discloses a flame-retardant lithium battery separator and a preparation method thereof. The preparation method comprises the following steps: adding a SiO2 nano-phosphorus-containing flame retardant, a polyvinylidene fluoride polymer, and deionized water to a blender and stirring at a speed of r1 for a time of t1; adding a thickener and stirring at a speed of r2 for a time of t2; maintaining the speed of r2, adding a binder and stirring for a time of t3; adding a wetting agent and a defoaming agent and stirring at a speed of r3 for a time of t4 to obtain a flame-retardant gel coating, wherein r3 < r2 ≤ r1; preparing a base film for a lithium battery separator, uniformly coating the flame-retardant gel coating on one or both surfaces of the base film, and drying at 50-60°C for 4-5 hours to obtain a flame-retardant lithium battery separator. The addition of the phosphorus-containing SiO2 nano-flame retardant prevents the aggregation of SiO2 nanoparticles, while introducing an organic phosphorus flame retardant element to further improve the flame retardant performance. The SiO2 surface is grafted with organic long chains that act as surfactants, effectively improving the liquid absorption capacity of the separator.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium batteries, and in particular relates to a flame-retardant separator for a lithium battery and a preparation method thereof. Background Art

[0002] Since their development, lithium batteries have garnered significant attention from various industries and researchers. In recent years, their rapid development has expanded their applications to include electric vehicles and energy storage. As lithium batteries continue to improve their safety, so too have the demands placed on high-safety battery materials. Lithium-ion battery separators, as the third electrode in a battery, play a crucial role in battery safety. If separators can provide significant protection against thermal runaway, various safety issues can be avoided. This is the basis for the present invention. Summary of the Invention

[0003] In view of one of the above-mentioned technical problems, the present invention aims to provide a flame-retardant separator for lithium batteries and a preparation method thereof.

[0004] The technical solution of the present invention is:

[0005] One of the objects of the present invention is to provide a method for preparing a flame retardant separator for a lithium battery, comprising the following steps:

[0006] Preparation of flame-retardant gel coating: Add a formulated amount of SiO2 nano-phosphorus flame retardant, polyvinylidene fluoride polymer, and deionized water to a blender and stir at a speed of r1 for a time of t1; add a formulated amount of thickener and continue stirring at a speed of r2 for a time of t2; maintain the speed of r2 and add a formulated amount of binder and continue stirring for a time of t3; add a formulated amount of wetting agent and defoaming agent and stir at a speed of r3 for a time of t4 to obtain the flame-retardant gel coating; wherein r3 < r2 ≤ r1;

[0007] Coating of flame-retardant gel coating: prepare a base film for lithium battery separator, evenly coat the flame-retardant gel coating obtained above on one or both sides of the base film, and dry it at 50-60°C for 4-5h to obtain a lithium battery flame-retardant separator.

[0008] Preferably, in the step of preparing the flame retardant gel coating, the thickener and the binder can also be added simultaneously and stirred at a speed r2 for a time t5, wherein the time t5 is 60-120 minutes.

[0009] Preferably, the rotation speed r1 is 2000-3000 r / min, the rotation speed r2 is 1000-2000 r / min, and the rotation speed r3 is 500-800 r / min.

[0010] Preferably, the time t1 is 60-120 min, the time t2 is 30-60 min, the time t3 is 60-120 min, and the time t4 is 30-60 min.

[0011] Preferably, the thickness of the flame retardant gel coating applied on the base film is 1-4 μm.

[0012] Preferably, the flame retardant gel coating comprises the following raw materials in parts by weight:

[0013] 15-20 parts of polyvinylidene fluoride polymer, 5-10 parts of phosphorus-containing SiO2 nano flame retardant, 2-8 parts of thickener, 0.5-1.5 parts of binder, 0.1-0.8 parts of wetting agent, 0.5-1 parts of defoaming agent, and the rest is deionized water.

[0014] Preferably, the polyvinylidene fluoride polymer includes one or more of polyvinylidene fluoride, vinylidene fluoride-tetrafluoroethylene copolymer and vinylidene fluoride-hexafluoroacrylic acid copolymer; the thickener is a sodium hydroxymethyl cellulose thickener, such as CMC-Na, CMC-Li, etc.; the binder is a polyacrylic acid binder, such as lithium polyacrylate, sodium polyacrylate, etc.; the wetting agent is an alkynol wetting agent, such as 1-pentynol, 1-hexynol, 1-heptynol, etc.

[0015] Preferably, the phosphorus-containing SiO2 nano flame retardant has SiO2 inorganic nanoparticles as the core and an organic long-chain phosphorus system forming a core-shell structure, and the structural formula is as follows:

[0016] ,in The structure is as follows .

[0017] Preferably, the phosphorus-containing SiO2 nano flame retardant is prepared by the following method:

[0018] 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide, aminopropyltriethoxysilane, and triethylamine were dissolved in dichloromethane in equimolar amounts, and the mixture was reacted in an ice-water bath for 2 hours. Then, CCl4 solution was added, and the reaction was continued for 8 hours. After evaporating the solvent, the mixture was washed with acetone and dried to obtain a concentrated solution. Equimolar amounts of the above concentrated solution and 2,3-epoxypropyltrimethylammonium chloride were weighed and dissolved in ethanol, and the mixture was reacted at 60°C for 48 hours to obtain Solution 1.

[0019] Take 5-10g of nano-SiO2 sol and add it to 100mL of deionized water for dilution. Ultrasonicate at room temperature for 30min-60min at an ultrasonic frequency of 200-10000Hz to uniformly disperse the nano-SiO2 sol in the deionized water. Then add 10-20g of the above-mentioned solution 1, stir and react for 12-36 hours. After the reaction is completed, filter and wash with 50-100mL of deionized water 3-5 times, and repeatedly filter and wash with 50-100mL of ethanol 4-5 times to obtain grafted SiO2 nanoparticles. The grafted SiO2 nanoparticles are the phosphorus-containing SiO2 nano flame retardant.

[0020] Another object of the present invention is to provide a flame-retardant lithium battery separator, comprising a base film and a flame-retardant gel coating coated on one or both sides of the base film in the thickness direction, wherein the flame-retardant gel coating is prepared by any of the preparation methods described above.

[0021] Compared with the prior art, the advantages of the present invention are:

[0022] The flame-retardant lithium battery separator of the present invention forms a flame-retardant gel coating by coating a polyolefin-based film with a flame-retardant and PVDF, thereby improving the separator's thermal stability and flame-retardant properties. The PVDF also enhances the bonding between the separator and the electrode. The present invention introduces a phosphorus-containing SiO2 nano-flame retardant and grafts an organic long-chain phosphorus system onto the SiO2 surface to form a core-shell flame-retardant material. The addition of this specially structured flame retardant prevents the aggregation of SiO2 nanoparticles. The introduction of an organic phosphorus flame-retardant element into the flame-retardant coating further enhances the separator's flame-retardant properties. The long organic chains grafted onto the SiO2 surface, acting as surfactants, effectively improve the separator's liquid absorption capacity. DETAILED DESCRIPTION

[0023] To make the objectives, technical solutions, and advantages of the present invention more clearly understood, the present invention will be further described in detail below in conjunction with specific embodiments and with reference to the accompanying drawings. It should be understood that these descriptions are merely exemplary and are not intended to limit the scope of the present invention. In addition, in the following description, descriptions of well-known structures and technologies are omitted to avoid unnecessary confusion of the concepts of the present invention.

[0024] The method for preparing a flame-retardant lithium battery separator according to an embodiment of the present invention comprises the following steps:

[0025] Preparation of flame retardant gel coating: The flame retardant gel coating includes, by weight, 15-20 parts of polyvinylidene fluoride polymer, 5-10 parts of phosphorus-containing SiO2 nano flame retardant, 2-8 parts of thickener, 0.5-1.5 parts of binder, 0.1-0.8 parts of wetting agent, 0.5-1 parts of defoaming agent, and the rest is deionized water. The above-mentioned formula amount of SiO2 nano-phosphorus flame retardant, polyvinylidene fluoride polymer and deionized water are added to a mixer and stirred at a speed of r1 (2000-3000 r / min) for a time of t1 (60-120 min); the formula amount of thickener is added and stirred at a speed of r2 (1000-2000 r / min) for a time of t2 (30-60 min), and the formula amount of binder is added while maintaining the speed of r2 and stirring for a time of t3 (60-120 min); the formula amount of wetting agent and defoaming agent are added and stirred at a speed of r3 (500-800 r / min) for a time of t4 (30-60 min) to obtain the flame retardant gel coating; wherein r3<r2≤r1;

[0026] Coating of flame-retardant gel coating: prepare a base film for lithium battery separator, and evenly coat the flame-retardant gel coating obtained above on one or both sides of the base film (coating thickness is 1-4 μm), and dry it at 50-60°C for 4-5 hours to obtain a lithium battery flame-retardant separator.

[0027] The phosphorus-containing SiO2 nano flame retardant in the embodiment of the present invention has SiO2 inorganic nanoparticles as the core and an organic long-chain phosphorus system to form a core-shell structure, and the structural formula is as follows:

[0028] ,in The structure is as follows This special structure can inhibit the aggregation of nanoparticles, while the synergistic effect of SiO2 and organic phosphorus increases the flame retardant effect.

[0029] Phosphorus-containing SiO2 nano flame retardant is prepared by the following method:

[0030] 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide, aminopropyltriethoxysilane, and triethylamine were dissolved in dichloromethane in equimolar amounts, and the mixture was reacted in an ice-water bath for 2 hours. Then, CCl4 solution was added, and the reaction was continued for 8 hours. After evaporating the solvent, the mixture was washed with acetone and dried to obtain a concentrated solution. Equimolar amounts of the above concentrated solution and 2,3-epoxypropyltrimethylammonium chloride were weighed and dissolved in ethanol, and the mixture was reacted at 60°C for 48 hours to obtain Solution 1.

[0031] 5-10g of nano-SiO2 sol is diluted in 100mL of deionized water and ultrasonicated at room temperature for 30-60min at a frequency of 200-10,000Hz to uniformly disperse the nano-SiO2 sol in the deionized water. 10-20g of the above-mentioned solution 1 is then added and stirred for a reaction of 12-36 hours. After the reaction is complete, the mixture is filtered and washed 3-5 times with 50-100mL of deionized water and then repeatedly filtered and washed 4-5 times with 50-100mL of ethanol to obtain grafted SiO2 nanoparticles. The grafted SiO2 nanoparticles are phosphorus-containing SiO2 nano-flame retardants. The present invention introduces a phosphorus-containing SiO2 nano-flame retardant and grafts an organic long-chain phosphorus system onto the SiO2 surface to form a core-shell flame retardant material. The addition of this special flame retardant prevents the aggregation of SiO2 nanoparticles. Furthermore, the introduction of an organic phosphorus flame retardant element into the flame retardant coating further improves the flame retardant properties of the diaphragm. The surface of SiO2 is grafted with organic long chains that act as surfactants, which can effectively improve the liquid absorption capacity of the membrane.

[0032] The polyvinylidene fluoride polymer in the embodiments of the present invention includes one or more of polyvinylidene fluoride (preferably polyvinylidene fluoride with a molecular weight of 100,000 purchased from Solvay Specialty Polymers (Changshu) Co., Ltd.), a vinylidene fluoride-tetrafluoroethylene copolymer, and a vinylidene fluoride-hexafluoroacrylic acid copolymer. The thickener is a sodium hydroxymethylcellulose thickener, such as CMC-Na and CMC-Li. The binder is a polyacrylic acid binder, such as lithium polyacrylate and sodium polyacrylate. The wetting agent is an alkynol wetting agent, such as 1-pentynol, 1-hexynol, and 1-heptynol.

[0033] In some alternative embodiments of the present invention, in the step of preparing the flame retardant gel coating, the thickener and the binder may be added simultaneously and stirred at a speed r2 (1000-2000 r / min) for a time t5 (60-120 min).

[0034] Another object of the present invention is to provide a flame-retardant lithium battery separator, comprising a base film and a flame-retardant gel coating coated on one or both sides of the base film in the thickness direction, wherein the flame-retardant gel coating is prepared by any of the preparation methods described above.

[0035] The base film in the embodiments of the present invention can be made of any polyolefin material familiar to those skilled in the art, such as polyethylene (PE) or polypropylene (PP). Regarding the base film thickness, if the base film is too thin, while the separator can provide greater electron and ion transport rates, it may accelerate metal deposition and increase the risk of perforation. If the base film is too thick, while it can ensure internal isolation between the positive and negative electrodes and reduce the risk of short circuits and thermal runaway, the transport rate is significantly reduced, increasing the battery's internal resistance. Therefore, taking all factors into consideration, the base film thickness in the embodiments of the present invention is preferably 5-15 μm. Within this base film thickness range, the battery exhibits high power and energy density, low internal resistance, good cycle life, and safety. The coating layer on at least one side (preferably both sides) of the base film in the embodiments of the present invention can further enhance the mechanical strength of the separator and extend the battery's cycle life. However, excessive coating thickness can also affect the battery's power and energy density. Therefore, taking all factors into consideration, the coating thickness in the embodiments of the present invention is preferably 1-4 μm. In the embodiment of the present invention, a flame retardant coating is formed by coating a flame retardant and PVDF on a polyolefin base film, thereby improving the thermal stability and flame retardant properties of the diaphragm, and improving the bonding ability between the diaphragm and the pole piece through PVDF.

[0036] More specific embodiments are as follows:

[0037] Example 1

[0038] The method for preparing a flame-retardant lithium battery separator according to an embodiment of the present invention comprises the following steps:

[0039] In parts by weight, 15 parts of polyvinylidene fluoride (molecular weight 1.1 million), 5 parts of phosphorus-containing SiO2 nano flame retardant (prepared by the preparation method in the above embodiment), and 70 parts of deionized water were first added to a blender and stirred at a speed of 2000 r / min to achieve uniformity. Then, 8 parts of thickener were added and stirred for 60 minutes. After that, the speed was adjusted to 500 r / min, 1.0 part of binder and 0.5 part of defoamer were added, and the mixture was stirred for 30 minutes to obtain a flame-retardant gel coating.

[0040] (2) The flame retardant gel coating was evenly coated on one side of the polyethylene porous membrane with a coating thickness of 4 μm, and dried in a 50°C environment for 6 hours to obtain a flame retardant separator for lithium batteries.

[0041] Example 2

[0042] The method for preparing a flame-retardant lithium battery separator according to an embodiment of the present invention comprises the following steps:

[0043] In parts by weight, 18 parts of polyvinylidene fluoride (molecular weight 1.1 million), 10 parts of phosphorus-containing SiO2 nano flame retardant, and 70 parts of deionized water were first added to a blender and stirred at a speed of 2000 r / min to achieve uniformity; then 8 parts of thickener were added and stirred for 60 minutes; then the speed was adjusted to 500 r / min, 1.0 part of binder and 0.5 part of defoamer were added, and stirred for 30 minutes to obtain a flame retardant gel coating;

[0044] The flame retardant gel coating was evenly coated on one side surface of the polyethylene porous membrane with a coating thickness of 4 μm, and dried in an environment of 50° C. for 6 hours to obtain a flame retardant separator for a lithium battery.

[0045] Example 3

[0046] The method for preparing a flame-retardant lithium battery separator according to an embodiment of the present invention comprises the following steps:

[0047] The flame retardant gel coating in Example 2 was evenly coated on both sides of the polyethylene porous membrane with a coating thickness of 2 μm, and dried in an environment of 50° C. for 6 hours to obtain a flame retardant separator for a lithium battery.

[0048] Example 4

[0049] The method for preparing a flame-retardant lithium battery separator according to an embodiment of the present invention comprises the following steps:

[0050] In parts by weight, 10 parts of phosphorus-containing SiO2 nano flame retardant and 70 parts of deionized water were added to a blender and stirred uniformly at a speed of 2000 r / min; 18 parts of thickener were then added and stirred for 60 minutes; the speed was then adjusted to 500 r / min, 1.0 part of binder and 0.5 part of defoamer were added, and the mixture was stirred for 30 minutes to obtain a flame-retardant gel coating;

[0051] The flame retardant gel coating was evenly coated on one side surface of the polyethylene porous membrane with a coating thickness of 4 μm, and dried in an environment of 50° C. for 6 hours to obtain a flame retardant separator material for a lithium battery.

[0052] Example 5

[0053] The method for preparing a flame-retardant lithium battery separator according to an embodiment of the present invention comprises the following steps:

[0054] By weight, 10 parts of phosphorus-containing SiO2 nano-flame retardant and 70 parts of deionized water were added to a blender and stirred at 2000 r / min for 2 hours. Then, 8 parts of thickener, 1.0 part of binder, and 0.5 parts of defoamer were added and stirred at 500 r / min for 30 minutes to obtain a flame-retardant gel coating, designated as solution 2.

[0055] In parts by weight, 18 parts of polyvinylidene fluoride (molecular weight 1.1 million, purchased from Solvay Specialty Polymers), 8 parts of thickener, 1.0 part of binder, 0.5 parts of defoamer and 60 parts of deionized water were added into a blender and stirred evenly at a speed of 1000 r / min for 2 hours to form a uniform solution 3.

[0056] Solution 2 was coated on one side of a commercial polyolefin separator. After drying, solution 3 was coated on the other side of the commercial polyolefin separator. The coating thickness was 2 μm and the drying temperature was 50° C.

[0057] Comparative Example 1

[0058] On the basis of Example 1, the phosphorus-containing SiO2 nano flame retardant was removed and only polyvinylidene fluoride was coated, while the other materials and their amounts remained unchanged.

[0059] Comparative Example 2

[0060] On the basis of Example 1, the phosphorus-containing SiO2 nano flame retardant was replaced with SiO2 sol, and the other materials and amounts remained unchanged.

[0061] The flame retardant lithium battery separators prepared in Examples 1 to 5 and Comparative Examples 1 to 2 were subjected to relevant basic physical property tests, and the obtained data are shown in Table 1 below:

[0062] Table 1

[0063]

[0064] It should be noted that the test methods for the longitudinal shrinkage rate, transverse shrinkage rate, puncture strength and limiting oxygen index in Table 1 above are all based on existing national standards, industry standards or enterprise standards. No specific description or limitation is given, and those skilled in the art can easily understand and implement them.

[0065] As can be seen from Table 1, the lithium battery separator materials of Examples 1 to 5 of the present invention all exhibit superior flame retardancy compared to Comparative Examples 1 and 2. Furthermore, a comparison between the various Examples shows that increasing the amount of SiO2 nano-flame retardant in the coating slurry improves the flame retardancy while maintaining the same coating thickness. For the same coating amount, the difference in flame retardancy between coating on one or both sides of the separator is not particularly noticeable.

[0066] The specified directions in this specific embodiment are only for the purpose of facilitating the description of the positional relationship and the coordination relationship between the various components. The above is only a preferred embodiment of the present invention. The scope of protection of the present invention is not limited to the above embodiment. All technical solutions under the concept of the present invention fall within the scope of protection of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications without departing from the principle of the present invention should also be considered as the scope of protection of the present invention.

Claims

1. A method for preparing a flame-retardant separator for a lithium battery, characterized in that: The following steps are involved: Preparation of flame-retardant gel coating: adding a formulated amount of SiO2 nano-phosphorus flame retardant, polyvinylidene fluoride polymer, and deionized water to a blender at a speed of r1 and stirring for a time of t1; adding a formulated amount of thickener at a speed of r2 and continuing stirring for a time of t2; adding a formulated amount of binder while maintaining the speed of r2 and continuing stirring for a time of t3; adding a formulated amount of wetting agent and defoaming agent and stirring at a speed of r3 and a time of t4 to obtain the flame-retardant gel coating; Where r3<r2≤r1; Coating of flame-retardant gel coating: preparing a base film for lithium battery separator, uniformly coating the flame-retardant gel coating obtained above on one or both sides of the base film, and drying at 50-60° C. for 4-5 hours to obtain a flame-retardant separator for lithium battery; The phosphorus-containing SiO2 nano flame retardant has SiO2 inorganic nanoparticles as the core and an organic long-chain phosphorus system to form a core-shell structure; The phosphorus-containing SiO2 nano flame retardant is prepared by the following method: 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide, aminopropyltriethoxysilane, and triethylamine were dissolved in dichloromethane in equimolar amounts, and the mixture was reacted in an ice-water bath for 2 hours. Then, CCl4 solution was added, and the reaction was continued for 8 hours. After evaporating the solvent, the mixture was washed with acetone and dried to obtain a concentrated solution. Equimolar amounts of the above concentrated solution and 2,3-epoxypropyltrimethylammonium chloride were weighed and dissolved in ethanol, and the mixture was reacted at 60°C for 48 hours to obtain Solution 1. Take 5-10g of nano-SiO2 sol and add it to 100mL of deionized water for dilution. Ultrasonicate at room temperature for 30min-60min at an ultrasonic frequency of 200-10000Hz to uniformly disperse the nano-SiO2 sol in the deionized water. Then add 10-20g of the above-mentioned solution 1, stir and react for 12-36 hours. After the reaction is completed, filter and wash with 50-100mL of deionized water 3-5 times, and repeatedly filter and wash with 50-100mL of ethanol 4-5 times to obtain grafted SiO2 nanoparticles. The grafted SiO2 nanoparticles are the phosphorus-containing SiO2 nano flame retardant.

2. The preparation method according to claim 1, characterized in that In the preparation step of the flame retardant gel coating, the thickener and the binder can also be added simultaneously and then stirred at a speed r2 for a time t5, wherein the time t5 is 60-120 minutes.

3. The preparation method according to claim 1 or 2, characterized in that The rotation speed r1 is 2000-3000 r / min, the rotation speed r2 is 1000-2000 r / min, and the rotation speed r3 is 500-800 r / min.

4. The preparation method according to claim 3, characterized in that The time t1 is 60-120 minutes, the time t2 is 30-60 minutes, the time t3 is 60-120 minutes, and the time t4 is 30-60 minutes.

5. The preparation method according to claim 1, characterized in that The thickness of the flame retardant gel coating applied on the base film is 1-4 μm.

6. The preparation method according to claim 1, characterized in that The flame retardant gel coating comprises the following raw materials in parts by weight: 15-20 parts of polyvinylidene fluoride polymer, 5-10 parts of phosphorus-containing SiO2 nano flame retardant, 2-8 parts of thickener, 0.5-1.5 parts of binder, 0.1-0.8 parts of wetting agent, 0.5-1 parts of defoaming agent, and the rest is deionized water.

7. The preparation method according to claim 1 or 6, characterized in that The polyvinylidene fluoride polymer includes one or more of polyvinylidene fluoride, vinylidene fluoride-tetrafluoroethylene copolymer and vinylidene fluoride-hexafluoroacrylic acid copolymer; the thickener is a sodium hydroxymethyl cellulose thickener; the binder is a polyacrylic acid binder; and the wetting agent is an acetylene alcohol wetting agent.

8. A flame retardant separator for a lithium battery, characterized in that: The invention comprises a base film and a flame retardant gel coating coated on one side or both sides of the base film in the thickness direction. The flame retardant gel coating is prepared by the preparation method according to any one of claims 1 to 7.

Images