A heat-sealable microporous membrane based on surface grafting strategy and preparation method and application thereof

Abstract

The application provides a heat-closed pore separator based on a surface grafting strategy and a preparation method and application thereof, and belongs to the technical field of lithium ion battery separator materials.The application forms a polydopamine modification layer on the surface of a polyolefin film, introduces a bromine initiator on the surface of the polydopamine modification layer, obtains the polyolefin film containing the bromine initiator on the surface, grafts a heat-sensitive polymer layer on the surface of the polyolefin film containing the bromine initiator, and obtains the heat-closed pore separator based on the surface grafting strategy.The heat-closed pore separator based on the surface grafting strategy can improve the safety performance of the battery while maintaining the electrochemical performance of the battery, the heat-closed pore separator can realize the heat-closed pore function at high temperature, the battery can operate at a safe temperature, and the heat runaway phenomenon caused by overheating of the battery can be effectively prevented.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium-ion battery diaphragm materials, and in particular to a thermally closed-cell diaphragm based on a surface grafting strategy, and a preparation method and application thereof. Background Art

[0002] With the widespread use of lithium-ion batteries, safety concerns are becoming increasingly prominent. Under conditions such as overcharging, short circuiting, and overheating, battery temperatures can rise abnormally. High temperatures accelerate exothermic side reactions within the battery, leading to thermal runaway and potentially safety incidents. Lithium-ion batteries consist of a positive electrode material, a negative electrode material, a separator, an electrolyte, and a battery casing. As a key internal component, the separator not only affects the battery's capacity and cycle performance, but also its safety.

[0003] Currently, the most common safety separator is a three-layer polypropylene / polyethylene / polypropylene (PP / PE / PP) separator with thermal closure. Its operating principle is as follows: the PE layer with a lower melting point (~135°C) melts first, blocking ion transport channels; the PP layer with a higher melting point maintains structural stability, preventing internal short circuits between the electrodes. However, because the melting point of PP (~165°C) is close to that of PE, thermal inertia can easily cause the battery temperature to rise to the melting point of PP, leading to separator melting and battery short circuits. Therefore, conventional thermal closure separators are generally only suitable for battery applications with a slow temperature rise. To ensure adequate safety protection, the difference between the closure temperature and the melting temperature should be as large as possible. Against this background, the strategy of using polymer microspheres with lower melting points to modify separators has gradually developed. However, most polymers tend to agglomerate, making it difficult to control particle size and uniformly modify the separator, making their practical application difficult.

[0004] For example, patent CN 114221092 A discloses a gradient self-closed pore composite membrane. The gradient self-closed pore composite membrane comprises a sequentially arranged thermal stability coating, a base film, and a thermal stability coating. A closed pore coating is disposed between the base film and at least one side of the thermal stability coating. The closed pore coating comprises thermosensitive microspheres and ceramic particles. The thermal stability coating comprises polydopamine. The composite membrane exhibits gradient self-closed pore characteristics, which enable closed pores when the battery temperature rises, thereby preventing further temperature increases. Even if the temperature continues to rise and the base film melts, the thermal stability coating of the composite membrane can still maintain the dimensional stability of the composite membrane, achieving complete closed pores, preventing contact between the positive and negative electrodes, and further improving the safety performance of the battery. However, in the preparation method of the gradient self-closed pore composite membrane, due to the possibility of agglomeration between the thermosensitive microspheres, the uniformity of the coating cannot be guaranteed, making it difficult to ensure the battery's room temperature performance and high-temperature thermal shutdown effect.

[0005] In summary, how to prepare thermosensitive materials with a suitable melting point and that can be evenly dispersed on the surface of the diaphragm is an urgent problem that needs to be solved. Summary of the Invention

[0006] In response to the above-mentioned deficiencies in the existing technology, the present invention provides a thermally closed-pore diaphragm based on a surface grafting strategy, and a preparation method and application thereof. The thermally closed-pore diaphragm based on a surface grafting strategy is applied to lithium-ion batteries, which can achieve a thermally closed function at high temperatures while maintaining the electrochemical properties of the battery, allowing the battery to operate at a safe temperature.

[0007] To achieve the above purpose, the specific technical solutions of the present invention are as follows:

[0008] In the first aspect, the present invention provides a thermal closed-cell diaphragm based on a surface grafting strategy, wherein a polydopamine modified layer is formed on the surface of a polyolefin base film, a bromine initiator is introduced on the surface of the polydopamine modified layer to obtain a polyolefin film having a surface containing a bromine initiator, and a thermosensitive polymer layer is grafted on the surface of the polyolefin film containing the bromine initiator to obtain a thermal closed-cell diaphragm based on a surface grafting strategy.

[0009] The overall concept of the present invention is as follows: The present invention provides a thermally closed-cell membrane based on a surface grafting strategy, wherein a polyolefin base membrane is modified with polydopamine to obtain a polydopamine-modified polyolefin membrane. The polydopamine-modified layer on the surface of the polyolefin membrane contains abundant hydroxyl functional groups, which can undergo a series of reactions. This property is utilized to undergo an esterification reaction with bromoisobutyryl bromide to obtain a polyolefin membrane containing a bromine initiator on the surface. A thermosensitive polymer monomer undergoes an atom transfer radical polymerization reaction on the surface of the polyolefin membrane containing a bromine initiator to graft the thermosensitive polymer layer, thereby obtaining a thermally closed-cell membrane based on the surface grafting strategy. When applied to lithium-ion batteries, this thermally closed-cell membrane based on the surface grafting strategy can achieve thermal closure at high temperatures while maintaining other electrical properties of the battery, allowing the battery to operate at a safe temperature.

[0010] Preferably, the polyolefin-based film is at least one of a polyethylene film, a polypropylene film, and a polypropylene-polyethylene-polypropylene film.

[0011] Preferably, the polyolefin-based membrane has a thickness of 14-25 μm and a pore size of 150-300 nm.

[0012] Preferably, the thermosensitive polymer layer is formed by in-situ polymerization of a thermosensitive polymer monomer on the surface of a polyolefin film containing a bromine initiator, and the thermosensitive polymer monomer used in the thermosensitive polymer layer includes one of ethylene, vinyl acetate, styrene and methyl acrylate.

[0013] Preferably, the thickness of the polydopamine modified layer is 3-5 μm.

[0014] Preferably, the thickness of the thermosensitive polymer layer is 6-10 μm.

[0015] In a second aspect, the present invention provides a method for preparing the thermally closed-cell diaphragm based on the surface grafting strategy, comprising the following steps:

[0016] S1. placing the polyolefin-based membrane in a buffer solution containing dopamine hydrochloride and reacting at room temperature for 16 to 30 hours to obtain a polydopamine-modified polyolefin membrane;

[0017] S2. Placing the polydopamine-modified polyolefin membrane in a tetrahydrofuran solution containing triethylamine, and dropwise adding a tetrahydrofuran solution containing bromoisobutyryl bromide under an inert gas atmosphere, reacting at -5-5°C for 2-3 hours, then warming to room temperature, and then continuing the reaction at room temperature for 16-30 hours to obtain a polyolefin membrane with a bromine initiator on its surface;

[0018] S3. Dissolve the thermosensitive polymer monomer in an organic solvent, then add N, N, N′, N′′, N′′-pentamethyldiethylenetriamine (PMDETA) and cuprous bromide (CuBr) to obtain a mixed solution, and deoxygenate the mixed solution. Add the mixed solution to a reaction system of a polyolefin membrane containing a bromine initiator on its surface, react at room temperature for 4 to 8 hours, wash, and dry to obtain a thermal closed-cell membrane based on a surface grafting strategy.

[0019] Preferably, in step S1, the concentration of dopamine hydrochloride in the buffer solution containing dopamine hydrochloride is 1-3 mg / mL.

[0020] Preferably, in step S1, the buffer solution containing dopamine hydrochloride is a Tris-HCl buffer solution containing dopamine hydrochloride.

[0021] Preferably, the concentration of the Tris-HCl buffer solution is 10 mM and the pH is 8.5.

[0022] Preferably, in step S2, in the tetrahydrofuran solution containing triethylamine, the volume ratio of triethylamine to tetrahydrofuran is 1:8 to 1:12.

[0023] Preferably, in step S2, in the tetrahydrofuran solution containing bromoisobutyryl bromide, the volume ratio of bromoisobutyryl bromide to tetrahydrofuran is 1:4~1:10.

[0024] Preferably, in step S3, the organic solvent is at least one of benzene, toluene, tetrahydrofuran, 2-methyltetrahydrofuran, ether, chloroform, and acetone.

[0025] Preferably, in step S3, the concentration of the polymer monomer in the mixed solution is 300-700 mM, the concentration of the N, N, N', N'', N''-pentamethyldiethylenetriamine is 100-200 mM, and the concentration of the cuprous bromide is 30-80 mM.

[0026] Preferably, in step S3, the mixed solution is added to the reaction system of the polyolefin film containing bromine-containing initiators after the reaction system of the polyolefin film containing bromine-containing initiators has been subjected to oxygen removal treatment.

[0027] Preferably, the method for preparing the thermal closed-pore separator based on the surface grafting strategy is as follows:

[0028] S1, placing a polyolefin-based film into a Tris-HCl (10 mM, pH=8.5) buffer solution containing dopamine hydrochloride, and reacting at room temperature in air for 24 h to obtain a polydopamine-modified polyolefin film;

[0029] S2, placing the polydopamine-modified polyolefin film into a tetrahydrofuran solution containing triethylamine, slowly dropping a tetrahydrofuran solution containing bromoisobutyryl bromide under the condition of argon protection, and reacting at-5-5 ℃ for 1-3 h, and then continuing to react at room temperature for 16-30 h to obtain a polyolefin film containing bromine-containing initiators on the surface;

[0030] S3, dissolving a thermal polymer monomer in an organic solvent, adding a ligand N, N, N', N'', N''-pentamethyldiethylenetriamine (PMDETA) and a catalyst CuBr to obtain a mixed solution, removing oxygen in the mixed solution by argon bubbling for 30 min; adding the polyolefin film containing bromine-containing initiators on the surface to another reaction container, removing oxygen in the system by vacuum pumping and argon replacement, adding the mixed solution from which oxygen has been removed to the reaction system of the polyolefin film containing bromine-containing initiators on the surface from which oxygen has been removed under an argon atmosphere, and reacting at room temperature under the protection of argon for 4-8 h, washing, and blowing dry to obtain a thermal closed-pore separator based on the surface grafting strategy.

[0031] In a third aspect, the present application provides an application of the thermal closed-pore separator or the thermal closed-pore separator prepared by the method in a lithium ion battery.

[0032] In a fourth aspect, the present application provides a lithium ion battery, wherein the separator used is the thermal closed-pore separator or the thermal closed-pore separator prepared by the method.

[0033] Compared with the prior art, the present application has the following advantages:

[0034] (1) The heat-closed pore separator based on the surface grafting strategy applied in the lithium ion battery can realize the heat-closed pore function at high temperature while maintaining good electrochemical performance of the battery, so that the battery can operate at a safe temperature.

[0035] (2) The heat-closed pore separator based on the surface grafting strategy provided by the application introduces a polydopamine modification layer, which can improve the thermal stability of the separator, so that the separator is less likely to collapse at high temperature and maintain the stability of the structure. BRIEF DESCRIPTION OF DRAWINGS

[0036] Figure 1 The structure schematic diagram of the heat-closed pore separator based on the surface grafting strategy provided by the application is shown in the figure, wherein 1 is a polyolefin film, 2 is a polydopamine modification layer, and 3 is a heat-sensitive polymer layer.

[0037] Figure 2 The charge-discharge curve of the LiFePO4||Li button cell assembled by the separator prepared in Example 5 at 25 DEG C and 60 DEG C is shown in a, and the charge-discharge curve of the LiFePO4||Li button cell assembled by the separator prepared in Example 5 at 25 DEG C and 110 DEG C is shown in b.

[0038] Figure 3 The charge-discharge curve of the LiFePO4||Li button cell assembled by the separator prepared in Comparative Example 1 at 25 DEG C and 110 DEG C is shown in a, and the charge-discharge curve of the LiFePO4||Li button cell assembled by the separator prepared in Comparative Example 1 at 25 DEG C and 110 DEG C is shown in b.

[0039] Figure 4 The charge-discharge curve of the LiFePO4||Li button cell assembled by the separator prepared in Comparative Example 2 at 25 DEG C is shown in a. DETAILED DESCRIPTION

[0040] The technical solutions of the application will be described clearly and completely below. Obviously, the described embodiments are only some of the embodiments of the application, not all the embodiments. Based on the embodiments in the application, all other embodiments obtained by those skilled in the art without creative labor are within the scope of protection of the application.

[0041] The application provides a heat-closed pore separator based on a surface grafting strategy. A polydopamine modification layer is formed on the surface of a polyolefin-based film, a bromine initiator is introduced on the surface of the polydopamine modification layer, a polyolefin film containing a bromine initiator is obtained, and a heat-sensitive polymer layer is grafted on the surface of the polyolefin film containing the bromine initiator to obtain a heat-closed pore separator based on a surface grafting strategy.

[0042] In some examples, the polyolefin-based film is at least one of a polyethylene film, a polypropylene film, and a polypropylene-polyethylene-polypropylene film.

[0043] In some examples, the polyolefin-based membrane has a thickness of 14-25 μm and a pore size of 150-300 nm.

[0044] In some examples, the polydopamine modified layer has a thickness of 3-5 μm.

[0045] In some examples, the thermosensitive polymer layer has a thickness of 6-10 μm.

[0046] The method for preparing the thermally closed-cell diaphragm based on the surface grafting strategy comprises the following steps:

[0047] S1. placing the polyolefin-based membrane in a buffer solution containing dopamine hydrochloride and reacting at room temperature for 16 to 30 hours to obtain a polydopamine-modified polyolefin membrane;

[0048] S2. Placing the polydopamine-modified polyolefin membrane in a tetrahydrofuran solution containing triethylamine, and dropwise adding a tetrahydrofuran solution containing bromoisobutyryl bromide under an inert gas atmosphere, reacting at -5-5°C for 2-3 hours, then warming to room temperature, and then continuing the reaction at room temperature for 16-30 hours to obtain a polyolefin membrane with a bromine initiator on its surface;

[0049] S3. Dissolve the thermosensitive polymer monomer in an organic solvent, then add N, N, N′, N′′, N′′-pentamethyldiethylenetriamine (PMDETA) and cuprous bromide (CuBr) to obtain a mixed solution, and deoxygenate the mixed solution. Add the mixed solution to a reaction system of a polyolefin membrane containing a bromine initiator on its surface, react at room temperature for 4 to 8 hours, wash, and dry to obtain a thermal closed-cell diaphragm based on a surface grafting strategy.

[0050] In some examples, in step S1, the concentration of dopamine hydrochloride in the buffer solution containing dopamine hydrochloride is 1-3 mg / mL.

[0051] In some examples, in step S2, in the tetrahydrofuran solution containing triethylamine, the volume ratio of triethylamine to tetrahydrofuran is 1:8~1:12; in the tetrahydrofuran solution containing bromoisobutyryl bromide, the volume ratio of bromoisobutyryl bromide to tetrahydrofuran is 1:4~1:10.

[0052] In some examples, in step S3, the organic solvent is at least one of benzene, toluene, tetrahydrofuran, 2-methyltetrahydrofuran, ether, chloroform, and acetone.

[0053] In some examples, in the mixed solution, the concentration of the polymer monomer is 300-700 mM, the concentration of the N, N, N', N", N"-pentamethyldiethylenetriamine is 100-200 mM, and the concentration of the cuprous bromide is 30-80 mM.

[0054] Example 1

[0055] A thermal closed-pore separator based on a surface grafting strategy, the preparation steps are as follows:

[0056] Step 1: Put the polyethylene film (thickness of 20 μm, pore size of 200 nm) into a Tris-HCl (10 mM, pH=8.5) buffer solution containing 2 mg / mL of dopamine hydrochloride, and shake or stir the reaction in air at room temperature for 24 h to obtain a polydopamine-modified polyethylene film;

[0057] Step 2: Put the polydopamine-modified polyethylene film into a solution containing triethylamine (2 mL) and tetrahydrofuran (18 mL), slowly drop the solution containing isobutyryl bromide (1.5 mL) in tetrahydrofuran (10 mL) under argon protection conditions, ice bath, maintain ice bath reaction for 2 h, remove ice water bath, and rise to room temperature, after reaction at room temperature for 24 h, obtain a polyethylene film containing bromine initiator on the surface;

[0058] Step 3: Dissolve 500 mM of vinyl acetate in toluene, add N, N, N', N", N"-pentamethyldiethylenetriamine (PMDETA, 160 mM) and catalyst CuBr (60 mM) to the reaction system, and bubble argon for 30 min to remove oxygen in the system; In another glass reaction bottle, add the polyethylene film containing bromine initiator, vacuum and replace with argon to remove oxygen in the system, under argon atmosphere, transfer the above solution containing monomer and catalyst to the deoxygenated polyethylene film containing bromine initiator reaction system, react at room temperature under argon protection, after 6 h of reaction time, take out the film to expose to air to terminate the reaction, then wash with a large amount of ultrapure water, dry with nitrogen, and obtain a thermal closed-pore separator with polyvinyl acetate uniformly grafted on the surface.

[0059] The structural schematic diagram of the thermal closed-pore separator provided in the embodiment is shown in Figure 1 .

[0060] Example 2

[0061] A thermal closed-pore separator based on a surface grafting strategy, the preparation steps are as follows:

[0062] Step 1: Place a polyethylene membrane (20 μm thick, 200 nm pore size) in a Tris-HCl (10 mM, pH 8.5) buffer solution containing 2 mg / mL dopamine hydrochloride. Oscillate or stir in air at room temperature for 24 h to obtain a polydopamine-modified polyethylene membrane.

[0063] Step 2: Place the polydopamine-modified polyethylene membrane in a solution containing triethylamine (1.8 mL) and tetrahydrofuran (18.2 mL). Under argon protection, slowly drop a solution of bromoisobutyryl bromide (1.5 mL) in tetrahydrofuran (10 mL) in an ice bath. Maintain the ice bath reaction for 2 h, then remove the ice bath and warm to room temperature. After reacting at room temperature for 24 h, a polyethylene membrane with a bromine initiator on its surface is obtained.

[0064] Step 3: Dissolve 500 mM ethylene in toluene, add N, N, N′, N′′, N′′-pentamethyldiethylenetriamine (PMDETA, 160 mM) and catalyst CuBr (60 mM) to the reaction system, bubble argon for 30 min to remove oxygen from the system, add a polyethylene film containing a bromine initiator to another glass reaction bottle, evacuate and fill with argon to replace and remove oxygen from the system, and under an argon atmosphere, transfer the above solution containing the monomer and catalyst to the deoxygenated polyethylene film reaction system containing a bromine initiator. React at room temperature under argon protection. After the reaction time reaches 6 h, take out the membrane and expose it to the air to terminate the reaction. Then wash it with a large amount of ultrapure water and blow dry it with nitrogen to obtain a thermal closed-cell diaphragm with polyethylene uniformly grafted on the surface.

[0065] Example 3

[0066] A thermally closed-cell membrane based on a surface grafting strategy is prepared in the following steps:

[0067] Step 1: Place a polyethylene membrane (20 μm thick, 200 nm pore size) in a Tris-HCl (10 mM, pH 8.5) buffer solution containing 2 mg / mL dopamine hydrochloride. Oscillate or stir in air at room temperature for 24 h to obtain a polydopamine-modified polyethylene membrane.

[0068] Step 2: Place the polydopamine-modified polyethylene membrane in a solution containing triethylamine (2 mL) and tetrahydrofuran (18 mL). Under argon protection, slowly drop a solution of bromoisobutyryl bromide (2 mL) in tetrahydrofuran (10.5 mL) in an ice bath. Maintain the ice bath reaction for 2 h, then remove the ice bath and warm to room temperature. After reacting at room temperature for 24 h, a polyethylene membrane with a bromine initiator on its surface is obtained.

[0069] Step 3: 500 mM of vinyl acetate was dissolved in toluene, N, N, N', N", N"- pentamethyldiethylenetriamine (PMDETA, 160 mM) and catalyst CuBr (60 mM) were added into the reaction system, argon was bubbled for 30 min to remove oxygen in the system, in another glass reaction bottle, polyethylene film containing bromine initiator was added, vacuum was applied and argon was filled to replace oxygen in the system, under argon atmosphere, the above solution containing monomer and catalyst was transferred to the polyethylene film reaction system containing bromine initiator which had been deoxygenated, reaction was carried out at room temperature under argon protection, after 6 h of reaction time, the film was taken out to terminate the reaction by exposure to air, then it was washed with a large amount of ultrapure water and dried with nitrogen to obtain a heat-closed pore separator with polyvinyl acetate uniformly grafted on the surface.

[0070] Example 4

[0071] A heat-closed pore separator based on surface grafting strategy, the preparation steps are as follows:

[0072] Step 1: Polypropylene film (thickness of 20 μm, pore size of 200 nm) was placed in a Tris-HCl (10 mM, pH=8.5) buffer solution containing 2 mg / mL dopamine hydrochloride, and reacted at room temperature under air oscillation or stirring for 24 h to obtain a polydopamine-modified polypropylene film;

[0073] Step 2: The polydopamine-modified polypropylene film was placed in a solution containing triethylamine (2 mL) and tetrahydrofuran (18 mL), under argon protection, a solution containing bromoisobutyryl bromide (1.5 mL) in tetrahydrofuran (10 mL) was slowly added dropwise under ice bath, the ice bath was removed after 2 h of reaction, and the temperature was raised to room temperature, after 24 h of reaction at room temperature, a polypropylene film containing bromine initiator on the surface was obtained;

[0074] Step 3: 600 mM of methyl acrylate was dissolved in toluene, N, N, N', N", N"- pentamethyldiethylenetriamine (PMDETA, 160 mM) and catalyst CuBr (60 mM) were added into the reaction system, argon was bubbled for 30 min to remove oxygen in the system, in another glass reaction bottle, polypropylene film containing bromine initiator was added, vacuum was applied and argon was filled to replace oxygen in the system, under argon atmosphere, the above solution containing monomer and catalyst was transferred to the polypropylene film reaction system containing bromine initiator which had been deoxygenated, reaction was carried out at room temperature under argon protection, after 6 h of reaction time, the film was taken out to terminate the reaction by exposure to air, then it was washed with a large amount of ultrapure water and dried with nitrogen to obtain a heat-closed pore separator with polyacrylic acid methyl ester uniformly grafted on the surface.

[0075] Example 5

[0076] A thermally closed-cell membrane based on a surface grafting strategy is prepared in the following steps:

[0077] Step 1: Place a polypropylene membrane (20 μm thick, 200 nm pore size) in a Tris-HCl (10 mM, pH 8.5) buffer solution containing 2 mg / mL dopamine hydrochloride and shake or stir in air at room temperature for 24 h to obtain a polydopamine-modified polypropylene membrane.

[0078] Step 2: Place the polydopamine-modified polypropylene membrane in a solution containing triethylamine (2 mL) and tetrahydrofuran (18 mL). Under argon protection, slowly add a solution of bromoisobutyryl bromide (1.5 mL) in tetrahydrofuran (10 mL) in an ice bath. Maintain the ice bath reaction for 2 h, then remove the ice bath and warm to room temperature. After reacting at room temperature for 24 h, a polypropylene membrane with a bromine initiator on its surface is obtained.

[0079] Step 3: Dissolve 500 mM vinyl acetate in toluene, add N, N, N′, N′′, N′′-pentamethyldiethylenetriamine (PMDETA, 120 mM) and catalyst CuBr (60 mM) to the reaction system, bubble argon for 30 min to remove oxygen from the system, add a polypropylene membrane containing a bromine initiator to another glass reaction bottle, evacuate and fill with argon to replace and remove oxygen from the system, and under an argon atmosphere, transfer the above solution containing the monomer and catalyst to the deoxygenated polypropylene membrane reaction system containing a bromine initiator. React at room temperature under argon protection. After the reaction time reaches 6 h, take out the membrane and expose it to the air to terminate the reaction. Then wash it with a large amount of ultrapure water and blow dry it with nitrogen to obtain a thermal closed-cell diaphragm with polyvinyl acetate uniformly grafted on the surface.

[0080] Example 6

[0081] A thermally closed-cell membrane based on a surface grafting strategy is prepared in the following steps:

[0082] Step 1: Place a polypropylene membrane (20 μm thick, 200 nm pore size) in a Tris-HCl (10 mM, pH 8.5) buffer solution containing 2 mg / mL dopamine hydrochloride. Oscillate or stir in air at room temperature for 24 hours to obtain a polydopamine-modified polypropylene membrane.

[0083] Step 2: Place the polydopamine-modified polypropylene membrane in a solution containing triethylamine (2 mL) and tetrahydrofuran (18 mL). Under argon protection, slowly add a solution of bromoisobutyryl bromide (1.5 mL) in tetrahydrofuran (10 mL) in an ice bath. Maintain the ice bath reaction for 2 h, then remove the ice bath and warm to room temperature. After reacting at room temperature for 24 h, a polypropylene membrane with a bromine initiator on its surface is obtained.

[0084] Step 3: Dissolve 500 mM ethylene in toluene, add N, N, N′, N′′, N′′-pentamethyldiethylenetriamine (PMDETA, 160 mM) and catalyst CuBr (50 mM) to the reaction system, bubble argon for 30 min to remove oxygen in the system, add a polypropylene membrane containing a bromine initiator to another glass reaction bottle, evacuate and fill with argon to replace and remove oxygen in the system, and under an argon atmosphere, transfer the above solution containing the monomer and catalyst to the deoxygenated polypropylene membrane reaction system containing a bromine initiator. React at room temperature under argon protection. After the reaction time reaches 6 h, take out the membrane and expose it to the air to terminate the reaction. Then wash it with a large amount of ultrapure water and blow dry it with nitrogen to obtain a thermal closed-cell diaphragm with polyethylene uniformly grafted on the surface.

[0085] Example 7

[0086] A thermally closed-cell membrane based on a surface grafting strategy is prepared in the following steps:

[0087] Step 1: Place a polypropylene-polyethylene-polypropylene membrane (20 μm thick, 200 nm pore size) in a Tris-HCl (10 mM, pH 8.5) buffer solution containing 2 mg / mL dopamine hydrochloride. Oscillate or stir the solution in air at room temperature for 24 h to obtain a polydopamine-modified polypropylene-polyethylene-polypropylene membrane.

[0088] Step 2: Place the polydopamine-modified polypropylene-polyethylene-polypropylene membrane in a solution containing triethylamine (2 mL) and tetrahydrofuran (18 mL). Under argon protection, slowly dropwise add a solution of bromoisobutyryl bromide (2 mL) in tetrahydrofuran (9.5 mL) in an ice bath. Maintain the ice bath reaction for 2 h, then remove the ice bath and warm to room temperature. After reacting at room temperature for 24 h, a polypropylene-polyethylene-polypropylene membrane with a bromine initiator on its surface is obtained.

[0089] Step 3: Dissolve 500 mM vinyl acetate in toluene, add N, N, N′, N′′, N′′-pentamethyldiethylenetriamine (PMDETA, 200 mM) and catalyst CuBr (80 mM) to the reaction system, bubble argon for 30 min to remove oxygen in the system, add polypropylene-polyethylene-polypropylene membrane containing bromine initiator into another glass reaction bottle, evacuate and fill with argon to replace and remove oxygen in the system, and under argon atmosphere, transfer the above solution containing monomer and catalyst to the deoxygenated polypropylene-polyethylene-polypropylene membrane reaction system containing bromine initiator. React at room temperature under argon protection. After the reaction time reaches 6 h, take out the membrane and expose it to the air to terminate the reaction. Then wash it with a large amount of ultrapure water and blow dry it with nitrogen to obtain a thermal closed-cell diaphragm with polyvinyl acetate uniformly grafted on the surface.

[0090] Example 8

[0091] A thermally closed-cell membrane based on a surface grafting strategy is prepared in the following steps:

[0092] Step 1: Place a polypropylene-polyethylene-polypropylene membrane (20 μm thick, 200 nm pore size) in a Tris-HCl (10 mM, pH 8.5) buffer solution containing 2 mg / mL dopamine hydrochloride. Oscillate or stir the solution in air at room temperature for 24 hours to obtain a polydopamine-modified polypropylene-polyethylene-polypropylene membrane.

[0093] Step 2: Place the polydopamine-modified polypropylene-polyethylene-polypropylene membrane in a solution containing triethylamine (2 mL) and tetrahydrofuran (18 mL). Under argon protection, slowly dropwise add a solution of bromoisobutyryl bromide (2 mL) in tetrahydrofuran (9.5 mL) in an ice bath. Maintain the ice bath reaction for 2 h, then remove the ice bath and warm to room temperature. After reacting at room temperature for 24 h, a polypropylene-polyethylene-polypropylene membrane with a bromine initiator on its surface is obtained.

[0094] Step 3: Dissolve 700 mM methyl acrylate monomer in toluene, add N, N, N′, N′′, N′′-pentamethyldiethylenetriamine (PMDETA, 180 mM) and catalyst CuBr (70 mM) to the reaction system, bubble argon for 30 min to remove oxygen in the system, add polypropylene-polyethylene-polypropylene membrane containing bromine initiator into another glass reaction bottle, evacuate and fill with argon to replace and remove oxygen in the system, and under argon atmosphere, transfer the above solution containing monomer and catalyst to the deoxygenated polypropylene-polyethylene-polypropylene membrane reaction system containing bromine initiator. React at room temperature under argon protection. After the reaction time reaches 6 h, take out the membrane and expose it to the air to terminate the reaction. Then wash it with a large amount of ultrapure water and blow dry it with nitrogen to obtain a thermal closed-cell diaphragm with methyl acrylate uniformly grafted on the surface.

[0095] Comparative Example 1

[0096] A thermally closed-cell diaphragm, the preparation steps are as follows:

[0097] The polypropylene membrane was placed in a Tris-HCl (10 mM, pH=8.5) buffer solution containing 2 mg / ml dopamine hydrochloride, and the reaction was shaken or stirred in air at room temperature for 24 hours to obtain a thermal closed-pore membrane.

[0098] Comparative Example 2

[0099] A thermally closed-cell diaphragm, the preparation steps are as follows:

[0100] Step 1: Place the polypropylene membrane in a Tris-HCl (10 mM, pH = 8.5) buffer solution containing 2 mg / ml dopamine hydrochloride and shake or stir in air at room temperature for 24 hours to obtain a polydopamine-modified polypropylene membrane;

[0101] Step 2: uniformly coat a layer of polyvinyl acetate on the surface of the polydopamine-modified polyolefin membrane to obtain a thermally closed-cell membrane (the thermally closed-cell membrane obtained in this comparative example has the same thickness as the thermally closed-cell membrane in Example 5 in which polyvinyl acetate is uniformly grafted on the surface).

[0102] The separators prepared in Examples 1 to 8 and Comparative Examples 1 to 2 were used to assemble LiFePO4||Li button batteries, and their cycle stability and charge-discharge performance were tested at 25°C, 60°C, and 110°C. The experimental results show that the thermally closed-cell separator based on the surface grafting strategy of the present invention has good thermal stability and achieves complete pore closure at high temperatures. The lithium-ion battery using the thermally closed-cell separator based on the surface grafting strategy of the present invention has excellent cycle stability and safety performance. Figure 2Figure a shows the cycle stability of the LiFePO4||Li button battery assembled with the diaphragm prepared in Example 5 at 25°C and 60°C. As can be seen from the figure, after 100 cycles at 25°C, the capacity retention rate exceeds 95%, and the battery capacity is relatively stable; after 100 cycles at 60°C, the capacity decreases significantly compared to that at 25°C, but the capacity retention rate still exceeds 80%. Figure 2 Figure b shows the charge and discharge curves of the diaphragm-assembled LiFePO4||Li button battery prepared in Example 5 at 25°C and 110°C. As can be seen from the figure, the charge and discharge performance of the battery is stable at 25°C; while at 110°C, the micropores of the thermally closed-pore diaphragm are closed, and lithium ions cannot pass through the diaphragm to conduct between the positive and negative electrodes, blocking the current path inside the battery and effectively preventing thermal runaway caused by overheating of the battery.

[0103] Figure 3 The charge and discharge curves of the LiFePO4||Li button battery assembled with the diaphragm prepared in Comparative Example 1 at 25 ℃ and 110 ℃ are shown in Figure 1. Figure 3 It can be seen that the discharge capacity of the battery does not decrease significantly at 110 °C, indicating that the separator cannot achieve closed pores and shut down the electrode reaction at high temperatures, and therefore cannot prevent thermal runaway of the battery at high temperatures.

[0104] Figure 4 The charge and discharge curves of the LiFePO4||Li button battery assembled with the diaphragm prepared in Comparative Example 2 at 25°C are shown in Figure 2. Figure 4 It can be seen that the capacity of the battery is lower at 25°C.

[0105] The above specific embodiments describe the implementation of the present invention in detail, but the present invention is not limited to the specific details of the above embodiments. Within the scope of the claims and technical concept of the present invention, various simple modifications and changes can be made to the technical solution of the present invention, and these simple modifications all fall within the scope of protection of the present invention.

Claims

1. A thermally closed-cell membrane based on a surface grafting strategy, characterized in that: A polydopamine modified layer is formed on the surface of a polyolefin base film, a bromine initiator is introduced into the surface of the polydopamine modified layer to obtain a polyolefin film containing a bromine initiator on the surface, and a thermosensitive polymer layer is grafted onto the surface of the polyolefin film containing the bromine initiator to obtain a thermal closed-cell diaphragm based on a surface grafting strategy; the thermosensitive polymer layer is formed by in-situ polymerization of a thermosensitive polymer monomer on the surface of the polyolefin film containing the bromine initiator on the surface, and the thermosensitive polymer monomer used in the thermosensitive polymer layer includes one of ethylene, vinyl acetate, styrene, and methyl acrylate.

2. The thermally closed-cell membrane based on the surface grafting strategy according to claim 1, characterized in that: The polyolefin-based film is at least one of a polyethylene film, a polypropylene film, and a polypropylene-polyethylene-polypropylene film.

3. A method for preparing a thermally closed-cell membrane based on a surface grafting strategy, characterized in that: The following steps are involved: S1, placing the polyolefin-based membrane in a buffer solution containing dopamine hydrochloride, and reacting at room temperature to obtain a polydopamine-modified polyolefin membrane; S2. placing the polydopamine-modified polyolefin membrane in a tetrahydrofuran solution containing triethylamine, and dropwise adding a tetrahydrofuran solution containing bromoisobutyryl bromide under an inert gas atmosphere, reacting at -5 to 5°C for a certain period of time, then warming to room temperature, and then continuing the reaction at room temperature to obtain a polyolefin membrane with a bromine initiator on the surface; S3, dissolving the thermosensitive polymer monomer in an organic solvent, then adding N, N, N', N'', N''-pentamethyldiethylenetriamine and cuprous bromide to obtain a mixed solution, and deoxygenating the mixed solution; The mixed solution is added to the reaction system of a polyolefin membrane containing a bromine initiator on the surface, reacted at room temperature, washed, and dried to obtain a thermal closed-cell diaphragm based on a surface grafting strategy; the thermosensitive polymer monomer includes one of ethylene, vinyl acetate, styrene, and methyl acrylate.

4. The method for preparing a thermally closed-cell membrane based on a surface grafting strategy according to claim 3, characterized in that: In the buffer solution containing dopamine hydrochloride, the concentration of dopamine hydrochloride is 1-3 mg / mL.

5. The method for preparing a thermally closed-cell membrane based on a surface grafting strategy according to claim 3, characterized in that: In the tetrahydrofuran solution containing triethylamine, the volume ratio of triethylamine to tetrahydrofuran is 1:8-1:12; in the tetrahydrofuran solution containing bromoisobutyryl bromide, the volume ratio of bromoisobutyryl bromide to tetrahydrofuran is 1:4-1:

10.

6. The method for preparing a thermally closed-cell membrane based on a surface grafting strategy according to claim 3, characterized in that: In step S2, the reaction time at -5 to 5°C is 1 to 3 hours, and the reaction time at room temperature is 16 to 30 hours.

7. The method for preparing a thermally closed-cell membrane based on a surface grafting strategy according to claim 3, characterized in that: In the mixed solution, the concentration of the polymer monomer is 300-700 mM, the concentration of the N, N, N', N'', N''-pentamethyldiethylenetriamine is 100-200 mM, and the concentration of the cuprous bromide is 30-80 mM.

8. Use of the thermally closed pore membrane according to claim 1 or 2, or the thermally closed pore membrane prepared by the method according to any one of claims 3 to 7, in a lithium ion battery.

9. A lithium-ion battery, characterized in that: The diaphragm used is the thermally closed-cell diaphragm according to claim 1 or 2, or the thermally closed-cell diaphragm prepared by the method according to any one of claims 3 to 7.

Images