Thermal closed pore diaphragm based on surface grafting strategy as well as preparation method and application of thermal closed pore diaphragm

Abstract

The invention provides a thermal closed pore diaphragm based on a surface grafting strategy as well as a preparation method and application thereof, and belongs to the technical field of lithium ion battery diaphragm materials. A polydopamine modification layer is formed on the surface of a polyolefin film, a bromine initiator is introduced to the surface of the polydopamine modification layer to obtain a polyolefin film with the surface containing the bromine initiator, and a thermosensitive polymer layer is grafted to the surface of the polyolefin film containing the bromine initiator to obtain the thermal closed pore diaphragm based on the surface grafting strategy. According to the surface grafting strategy-based thermal closed pore diaphragm provided by the invention, the safety performance of a battery is improved while the electrochemical performance of the battery is maintained, and the thermal closed pore diaphragm can realize a thermal closed pore function at a high temperature, so that the battery can run at a safe temperature, and a thermal runaway phenomenon caused by overheating of the battery is 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 thermal 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, their safety issues have become increasingly prominent. In the case of overcharging, short circuit, overheating, etc., the battery temperature will rise abnormally. At the same time, high temperature accelerates the internal exothermic side reaction rate, resulting in thermal runaway and causing safety accidents. Lithium-ion batteries are composed of positive electrode materials, negative electrode materials, diaphragms, electrolytes and battery shells. As one of the key inner components, the diaphragm not only affects the battery capacity and cycle performance, but also its safety performance.

[0003] At present, the common safety diaphragm is a three-layer diaphragm of polypropylene / polyethylene / polypropylene (PP / PE / PP) with thermal closure function. Its working principle is: the PE layer with a lower melting point (~135 ℃) melts first and blocks the ion transmission channel; while the PP layer with a higher melting point maintains its structural stability to avoid internal short circuits between electrodes. However, since the melting point of PP (~165 ℃) is close to the melting temperature of PE, under the action of thermal inertia, the battery temperature can easily rise to the melting temperature of PP, resulting in the melting of the diaphragm and the short circuit of the battery. Therefore, conventional thermal closure diaphragms are often only suitable for situations where the battery temperature rises slowly. In order to ensure that the diaphragm has sufficient safety protection function, the difference between the closure temperature and the melting temperature should be as large as possible. Based on the above background, the strategy of using polymer microspheres with a lower melting point to modify the diaphragm has gradually developed. However, most polymers are prone to agglomeration, it is difficult to control the particle size, and it is impossible to modify them evenly on the diaphragm, which makes their practical application difficult.

[0004] For example, patent CN 114221092 A discloses a gradient self-closed pore composite diaphragm, which includes a thermal stability coating, a base film and a thermal stability coating arranged in sequence, and a closed pore coating is arranged between the base film and at least one side of the thermal stability coating; the closed pore coating includes thermosensitive microspheres and ceramic particles; the thermal stability coating includes polydopamine, and the composite diaphragm has the characteristics of gradient self-closed pores, and can achieve closed pores when the battery temperature rises, thereby avoiding the continued increase in battery temperature; even if the temperature continues to rise and the base film melts, the thermal stability coating of the composite diaphragm can still maintain the stability of the composite diaphragm size, achieve complete closed pores, avoid contact between the positive and negative electrodes, and further improve the safety performance of the battery. In the preparation method of the gradient self-closed pore composite diaphragm, since the thermosensitive microspheres may agglomerate, the uniformity of the coating cannot be guaranteed, and it is difficult to ensure the normal temperature performance and high temperature thermal shutdown effect of the battery.

[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 view of the deficiencies of the above-mentioned prior art, the present invention provides a thermal closed-cell diaphragm based on a surface grafting strategy and a preparation method and application thereof. The thermal closed-cell diaphragm based on a surface grafting strategy is applied to a lithium-ion battery, which can achieve a thermal closed-cell function at high temperature while maintaining the electrochemical properties of the battery, thereby 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 modification layer is formed on the surface of a polyolefin base film, a bromine initiator is introduced on the surface of the polydopamine modification layer to obtain a polyolefin film having a bromine initiator on the surface, 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 idea of ​​the present invention is as follows: the thermal closed-cell diaphragm based on the surface grafting strategy provided by the present invention uses polydopamine to modify the polyolefin base film 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, and this characteristic is utilized to undergo an esterification reaction with bromoisobutyryl bromide to obtain a polyolefin membrane containing a bromine initiator on the surface; the thermosensitive polymer monomer undergoes an atom transfer radical polymerization reaction on the surface of the polyolefin membrane containing a bromine initiator on the surface to graft the thermosensitive polymer layer to obtain a thermal closed-cell diaphragm based on the surface grafting strategy. The thermal closed-cell diaphragm based on the surface grafting strategy is applied to lithium-ion batteries, which can achieve a thermal closed-cell function at high temperatures while maintaining other electrical properties of the battery, so that the battery can 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 on the surface, 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 thermal 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, reacting at room temperature for 16 to 30 hours, and obtaining a polydopamine-modified polyolefin membrane;

[0017] S2, placing the polydopamine-modified polyolefin membrane in a tetrahydrofuran solution containing triethylamine, and under an inert gas atmosphere, dropping a tetrahydrofuran solution containing bromoisobutyryl bromide, reacting at -5 to 5 °C for 2 to 3 h, then warming to room temperature, and then continuing to react at room temperature for 16 to 30 h, to obtain a polyolefin membrane containing a bromine initiator on the 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 into a reaction system of a polyolefin membrane containing a bromine initiator on the 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.

[0019] Preferably, in step S1, in the buffer solution containing dopamine hydrochloride, the concentration of 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 1:8-1:12.

[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, 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.

[0026] Preferably, in step S3, before the mixed solution is added to the reaction system of the polyolefin membrane containing bromine initiator on the surface, the reaction system of the polyolefin membrane containing bromine initiator on the surface has been subjected to deoxygenation treatment.

[0027] Preferably, the preparation method of the thermal closed-cell diaphragm based on the surface grafting strategy is as follows:

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

[0029] S2, placing the polydopamine-modified polyolefin membrane in a tetrahydrofuran solution containing triethylamine, slowly dripping a tetrahydrofuran solution containing bromoisobutyryl bromide under argon protection, reacting at -5~5 ℃ for 1~3 h, and then continuing to react at room temperature for 16~30 h, to obtain a polyolefin membrane with a bromine initiator on the surface;

[0030] S3. Dissolve the thermosensitive polymer monomer in an organic solvent, add ligand N, N, N′, N′′, N′′-pentamethyldiethylenetriamine (PMDETA) and catalyst CuBr to obtain a mixed solution, and remove oxygen from the mixed solution by bubbling argon for 30 min. Add a polyolefin membrane containing a bromine initiator on the surface into another reaction container, evacuate the container and fill it with argon to displace the oxygen in the system. Under argon atmosphere, add the deoxygenated mixed solution to the reaction system of the deoxygenated polyolefin membrane containing a bromine initiator on the surface, react at room temperature under argon protection for 4 to 8 h, wash, and blow dry to obtain a thermal closed-cell diaphragm based on a surface grafting strategy.

[0031] In a third aspect, the present invention provides the use of the thermally closed pore diaphragm or the thermally closed pore diaphragm prepared by the method in a lithium ion battery.

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

[0033] Compared with the prior art, the present invention is beneficial in that:

[0034] (1) The thermally closed-cell separator based on the surface grafting strategy provided by the present invention is applied to lithium-ion batteries. It can achieve the thermally closed-cell function at high temperatures while maintaining good electrochemical performance of the battery, so that the battery can operate at a safe temperature.

[0035] (2) The introduction of a polydopamine modification layer into the thermal closed-cell diaphragm based on the surface grafting strategy provided by the present invention can improve the thermal stability of the diaphragm, making it less likely to collapse at high temperatures and maintaining the stability of the structure. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 This is a schematic diagram of the structure of the thermal closed-cell diaphragm based on the surface grafting strategy provided by the present invention; wherein 1 is a polyolefin membrane, 2 is a polydopamine modified layer, and 3 is a thermosensitive polymer layer;

[0037] Figure 2 a is the cycle performance diagram of the LiFePO4||Li button cell assembled with the diaphragm prepared in Example 5 at 25°C and 60°C, Figure 2 b is the charge and discharge curves of the diaphragm-assembled LiFePO4||Li button cell prepared in Example 5 at 25°C and 110°C;

[0038] Figure 3 The charge and discharge curves of the LiFePO4||Li button cell assembled with the diaphragm prepared in Comparative Example 1 at 25°C and 110°C;

[0039] Figure 4 The charge and discharge curves of the LiFePO4||Li button cell assembled with the diaphragm prepared in Comparative Example 2 at 25°C. DETAILED DESCRIPTION

[0040] The technical solution of the present invention will be described clearly and completely below. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0041] The present invention provides a thermal closed-cell diaphragm based on a surface grafting strategy, wherein a polydopamine modification layer is formed on the surface of a polyolefin base film, a bromine initiator is introduced on the surface of the polydopamine modification layer to obtain a polyolefin film having a bromine initiator on the surface, and a thermosensitive polymer layer is grafted on the surface of the polyolefin film containing the bromine initiator to obtain the thermal closed-cell diaphragm based on the 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 thickness of the polydopamine modified layer is 3-5 μm.

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

[0046] The method for preparing the thermal 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, reacting at room temperature for 16 to 30 hours, and obtaining a polydopamine-modified polyolefin membrane;

[0048] S2, placing the polydopamine-modified polyolefin membrane in a tetrahydrofuran solution containing triethylamine, and under an inert gas atmosphere, dropping a tetrahydrofuran solution containing bromoisobutyryl bromide, reacting at -5 to 5 °C for 2 to 3 h, then warming to room temperature, and then continuing to react at room temperature for 16 to 30 h, to obtain a polyolefin membrane containing a bromine initiator on the 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 into a reaction system of a polyolefin membrane containing a bromine initiator on the 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 step S3, 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-cell diaphragm based on a surface grafting strategy, the preparation steps are as follows:

[0056] Step 1: Place a polyethylene membrane (thickness 20 μm, pore size 200 nm) 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 polyethylene membrane modified with polydopamine;

[0057] Step 2: Place the polydopamine-modified polyethylene membrane in a solution containing triethylamine (2 mL) and tetrahydrofuran (18 mL). Under argon protection, slowly drip a tetrahydrofuran (10 mL) solution containing bromoisobutyryl bromide (1.5 mL) in an ice bath. Maintain the ice bath reaction for 2 h, then remove the ice bath, warm to room temperature, and react at room temperature for 24 h to obtain a polyethylene membrane with a bromine initiator on the surface.

[0058] Step 3: Dissolve 500 mM vinyl acetate in toluene, add N, N, N′, N′′, N′′-pentamethyldiethylenetriamine (PMDETA, 160 mM) and catalyst CuBr (60 mM) to the reaction system, and remove oxygen from the system by bubbling argon for 30 min; add a polyethylene film containing a bromine initiator into another glass reaction bottle, evacuate and fill with argon to replace the oxygen in the system, and transfer the above solution containing the monomer and the catalyst to the deoxygenated polyethylene film reaction system containing the bromine initiator under an argon atmosphere. 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.

[0059] The schematic diagram of the structure of the thermal closed-cell diaphragm provided in this embodiment is as follows Figure 1 shown.

[0060] Example 2

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

[0062] Step 1: Place a polyethylene membrane (thickness 20 μm, pore size 200 nm) 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 polyethylene membrane modified with polydopamine;

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

[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 in 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 in the system, and under an argon atmosphere, transfer the above solution containing the monomer and the catalyst to the deoxygenated polyethylene film reaction system containing a bromine initiator, and 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 thermal closed-cell diaphragm based on a surface grafting strategy, the preparation steps are as follows:

[0067] Step 1: Place a polyethylene membrane (thickness 20 μm, pore size 200 nm) 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 polyethylene membrane modified with polydopamine;

[0068] Step 2: Place the polydopamine-modified polyethylene membrane in a solution containing triethylamine (2 mL) and tetrahydrofuran (18 mL). Slowly drip a solution of bromoisobutyryl bromide (2 mL) in tetrahydrofuran (10.5 mL) under argon protection in an ice bath. Maintain the reaction in the ice bath 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 the surface is obtained.

[0069] Step 3: Dissolve 500 mM vinyl acetate 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 in 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 in the system, and under an argon atmosphere, transfer the above solution containing the monomer and the catalyst to the deoxygenated polyethylene film reaction system containing a bromine initiator, and 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.

[0070] Example 4

[0071] A thermal closed-cell diaphragm based on a surface grafting strategy, the preparation steps are as follows:

[0072] Step 1: Place a polypropylene membrane (thickness 20 μm, pore size 200 nm) 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;

[0073] Step 2: Place the polydopamine-modified polypropylene membrane in a solution containing triethylamine (2 mL) and tetrahydrofuran (18 mL). Under argon protection, slowly drip a tetrahydrofuran (10 mL) solution containing bromoisobutyryl bromide (1.5 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 membrane with a bromine initiator on the surface is obtained.

[0074] Step 3: Dissolve 600 mM methyl acrylate 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 in the system, add polypropylene membrane containing bromine initiator to another glass reaction bottle, evacuate and fill with argon to replace and remove oxygen in the system, and transfer the above solution containing monomer and catalyst to the deoxygenated polypropylene membrane reaction system containing bromine initiator under argon atmosphere. 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 polymethyl acrylate uniformly grafted on the surface.

[0075] Example 5

[0076] A thermal closed-cell diaphragm based on a surface grafting strategy, the preparation steps are as follows:

[0077] Step 1: Place a polypropylene membrane (thickness 20 μm, pore size 200 nm) 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 drip a tetrahydrofuran (10 mL) solution containing bromoisobutyryl bromide (1.5 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 membrane with a bromine initiator on the 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 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 the catalyst to the deoxygenated polypropylene membrane reaction system containing a bromine initiator, and 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 thermal closed-cell diaphragm based on a surface grafting strategy, the preparation steps are as follows:

[0082] Step 1: Place a polypropylene membrane (thickness 20 μm, pore size 200 nm) 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;

[0083] Step 2: Place the polydopamine-modified polypropylene membrane in a solution containing triethylamine (2 mL) and tetrahydrofuran (18 mL). Under argon protection, slowly drip a tetrahydrofuran (10 mL) solution containing bromoisobutyryl bromide (1.5 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 membrane with a bromine initiator on the 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 the catalyst to the deoxygenated polypropylene membrane reaction system containing a bromine initiator, and 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 thermal closed-cell diaphragm based on a surface grafting strategy, the preparation steps are as follows:

[0087] Step 1: Place a polypropylene-polyethylene-polypropylene membrane (thickness 20 μm, pore size 200 nm) 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-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). Slowly drip a tetrahydrofuran (9.5 mL) solution containing bromoisobutyryl bromide (2 mL) in an ice bath under argon protection. Maintain the ice bath reaction for 2 h, then remove the ice bath, warm to room temperature, and react at room temperature for 24 h to obtain a polypropylene-polyethylene-polypropylene membrane with a bromine initiator on the surface.

[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 transfer the above solution containing monomers and catalyst to the deoxygenated polypropylene-polyethylene-polypropylene membrane reaction system containing bromine initiator under argon atmosphere, react at room temperature under argon protection, and 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 thermal closed-cell diaphragm based on a surface grafting strategy, the preparation steps are as follows:

[0092] Step 1: Place a polypropylene-polyethylene-polypropylene membrane (thickness 20 μm, pore size 200 nm) 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-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). Slowly drip a tetrahydrofuran (9.5 mL) solution containing bromoisobutyryl bromide (2 mL) in an ice bath under argon protection. Maintain the ice bath reaction for 2 h, then remove the ice bath, warm to room temperature, and react at room temperature for 24 h to obtain a polypropylene-polyethylene-polypropylene membrane with a bromine initiator on the surface.

[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 transfer the above solution containing monomer and catalyst to the deoxygenated polypropylene-polyethylene-polypropylene membrane reaction system containing bromine initiator under argon atmosphere, react at room temperature under argon protection, and 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 thermal 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 carried out by shaking or stirring in air at room temperature for 24 hours to obtain a thermal closed-pore membrane.

[0098] Comparative Example 2

[0099] A thermal 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 h to obtain a polydopamine-modified polypropylene membrane;

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

[0102] The membranes 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 at 25°C, 60°C, and 110°C were tested. The experimental results show that the thermally closed-cell membrane based on the surface grafting strategy of the present invention has good thermal stability and achieves complete closure at high temperatures. The lithium-ion battery using the thermally closed-cell membrane based on the surface grafting strategy of the present invention has excellent cycle stability and safety performance. Among them, Figure 2a is the cycle stability of the LiFePO4||Li button cell assembled with the separator prepared in Example 5 at 25 ℃ and 60 ℃, Figure 2 It can be seen from a that 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 drop is more obvious than that at 25 °C, but the capacity retention rate is still over 80%. Figure 2 b is the charge and discharge curves of the LiFePO4||Li button cell assembled with the diaphragm prepared in Example 5 at 25 ℃ and 110 ℃. Figure 2 b It can be seen that the charge and discharge performance of the battery is stable at 25 °C; at 110 °C, the micropores of the thermal closed-cell separator are closed, and lithium ions cannot pass through the separator 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 cell assembled with the diaphragm prepared in Comparative Example 1 at 25°C and 110°C are shown in Table 1. Figure 3 It can be seen that the discharge capacity of the battery does not decrease significantly at 110 °C, indicating that the diaphragm 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 cell assembled with the diaphragm prepared in Comparative Example 2 at 25°C are shown in Table 1. Figure 4 It can be seen that the capacity of the battery at 25°C is lower.

[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 in the above embodiments. Within the scope of the claims and technical concept of the present invention, the technical solution of the present invention can be modified and changed in many simple ways, and these simple modifications all belong to the protection scope of the present invention.

Claims

1. A thermally closed-cell diaphragm 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.

2. A thermally closed-cell diaphragm based on a 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. The thermally closed-cell diaphragm based on the surface grafting strategy according to claim 1, characterized in that: The thermosensitive polymer monomer used in the thermosensitive polymer layer includes one of ethylene, vinyl acetate, styrene and methyl acrylate.

4. A method for preparing a thermally closed-cell diaphragm 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, reacting at room temperature, and obtaining a polydopamine-modified polyolefin membrane; S2, placing the polydopamine-modified polyolefin membrane in a tetrahydrofuran solution containing triethylamine, and under an inert gas atmosphere, dropwise adding a tetrahydrofuran solution containing bromoisobutyryl bromide, 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 containing 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 into the reaction system of the polyolefin membrane containing bromine initiator on the surface, reacted at room temperature, washed and dried to obtain a thermal closed-cell diaphragm based on the surface grafting strategy.

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

6. The method for preparing a thermally closed-cell diaphragm based on a surface grafting strategy according to claim 4, 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.

7. The method for preparing a thermally closed-cell diaphragm based on a surface grafting strategy according to claim 4, characterized in that: In the step S2, the reaction time at -5 to 5°C is 1 to 3 h, and the reaction time at room temperature is 16 to 30 h.

8. The method for preparing a thermally closed-cell diaphragm based on a surface grafting strategy according to claim 4, 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 cuprous bromide is 30-80 mM.

9. Use of the thermally closed-cell membrane according to any one of claims 1 to 3 or the thermally closed-cell membrane prepared by the method according to any one of claims 4 to 8 in lithium-ion batteries.

10. A lithium ion battery, characterized in that: The diaphragm used is the thermally closed-cell diaphragm as described in any one of claims 1 to 3 or the thermally closed-cell diaphragm prepared by the method as described in any one of claims 4 to 8.

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