High-thermal-stability diaphragm and preparation method thereof

Abstract

The invention discloses a high-thermal-stability diaphragm and a preparation method thereof, and belongs to the field of lithium battery diaphragms. The high-thermal-stability diaphragm comprises a base film and an inorganic coating, the inorganic coatings are coated on the surfaces of the two sides of the base film; the inorganic coating is prepared from the following raw materials in parts by mass: 10 to 15 parts of modified boron nitride nanofiber, 6 to 10 parts of polyimide-polysiloxane block copolymer, 3 to 6 parts of adhesive and 100 parts of water; polyethylene oxide is grafted on the surface of the modified boron nitride nanofiber. The modified boron nitride nanofibers can form a three-dimensional network structure on the surface of the base membrane, the polyimide-polysiloxane block copolymer can also provide a high-temperature rigid supporting effect for the diaphragm, shrinkage of the polyolefin base membrane at a high temperature is effectively inhibited, and polyethylene oxide is introduced to the surface, so that the high-temperature-resistant performance of the diaphragm is improved. The dispersity of the boron nitride nanofibers in a system can be improved, the binding force between the boron nitride nanofibers and a base membrane is improved, lithium ion migration can be promoted through contained ether oxygen bonds, and the ion transmission efficiency is improved.

Description

Technical Field

[0001] This invention relates to the field of lithium battery separators, and particularly to a high thermal stability separator and its preparation method. Background Technology

[0002] Lithium-ion batteries possess advantages such as high energy density, long cycle life, and environmental friendliness, leading to their widespread application in new energy fields, mobile electronic devices, and power systems. In particular, the booming development of the new energy vehicle market in recent years has accelerated the advancement of lithium-ion battery technology. To increase power output, multiple individual cells are often connected in series or parallel. Therefore, the safety of lithium-ion batteries is one of the primary key performance considerations for power batteries. The separator, as a major component of lithium-ion batteries that directly separates the positive and negative electrodes, directly affects battery safety.

[0003] Most existing lithium-ion battery separators use polyolefin membranes. Polyolefin membranes have high mechanical strength and resistance to acids, alkalis, and chemical reagents, making them a relatively ideal separator material. During the operation of a lithium-ion battery, rapid charging and discharging can accelerate the internal reaction rate, increasing the heat generated per unit time and causing the battery temperature to rise sharply. Because of this increased internal temperature, and given the generally low melting point of polyolefin membranes, problems such as membrane shrinkage and softening can occur. This can lead to direct contact between the positive and negative electrodes inside the battery, causing an internal short circuit. A short circuit can potentially lead to fires or explosions.

[0004] The thermal stability of the separator directly affects battery safety. Polyolefin separators with high thermal stability can maintain structural stability at high temperatures, preventing thermal runaway. To improve the thermal stability of polyolefin separators, coating modification is often performed. However, this results in a high degree of crystallinity on the surface of the modified polyolefin separator, significantly reducing the number of ion transport channels and lowering the battery's ionic conductivity. Consequently, the cycle performance and rate performance of lithium-ion batteries are affected. Therefore, obtaining a polyolefin separator with both high thermal stability and good ionic conductivity can improve lithium-ion battery safety while maintaining good electrochemical performance. Summary of the Invention

[0005] This invention provides a high thermal stability membrane and its preparation method, which can solve the problem of poor thermal stability of polyolefin membranes in the prior art.

[0006] In a first aspect, the present invention provides a high thermal stability membrane, the high thermal stability membrane comprising a base membrane and an inorganic coating; the inorganic coating is coated on both sides of the base membrane;

[0007] The inorganic coating comprises the following raw materials in parts by weight:

[0008] 10-15 parts of modified boron nitride nanofibers;

[0009] 6-10 parts of polyimide-polysiloxane block copolymer;

[0010] 3-6 parts adhesive;

[0011] 100 parts water;

[0012] The surface of the modified boron nitride nanofibers is grafted with polyethylene oxide.

[0013] Preferably, the base film includes any one of polyethylene base film, polypropylene base film, polyethylene / polypropylene double-layer co-extruded film, and polyethylene / polypropylene / polyethylene multilayer co-extruded film.

[0014] Preferably, the adhesive comprises one or a combination of two of polyvinylidene fluoride and polytetrafluoroethylene.

[0015] By adopting the above technical solution, an inorganic coating is coated on the surface of a polyolefin-based film. The inorganic coating contains modified boron nitride nanofibers. The boron nitride nanofibers themselves have extremely high thermal decomposition temperature and thermal conductivity. After being uniformly dispersed in the inorganic coating, they form a three-dimensional network structure and are coated on the surface of the base film. This can effectively inhibit the shrinkage of the polyolefin-based film at high temperatures, maintain the structural and dimensional stability of the base film at high temperatures, and prevent internal short circuits in the battery. At the same time, the high thermal conductivity of the boron nitride nanofibers can disperse the heat generated by the battery during long-term cycling, avoiding local overheating or heat accumulation that could lead to thermal runaway.

[0016] The unique nanofiber morphology of boron nitride nanofibers does not block the original pores of polyolefin-based membranes. After modification, the surface of boron nitride nanofibers contains polyethylene oxide segments. On the one hand, their hydrophilicity can enhance the electrolyte wettability of the membrane, thereby reducing interfacial impedance. On the other hand, the polyethylene oxide segments contain a large number of ether oxygen bonds, which can form coordination with lithium ions, thereby promoting lithium ion migration and improving ion transport efficiency.

[0017] Furthermore, the inorganic coating also includes a polyimide-polysiloxane block copolymer. The polyimide segments possess excellent thermal stability, which can improve the overall glass transition temperature and thermal decomposition temperature. Moreover, after copolymerization with the flexible polysiloxane blocks, it can enhance the overall flexibility, thereby jointly suppressing the risk of brittle fracture of the inorganic coating at high temperatures. Simultaneously, the hydrophilic channels formed by the polyimide-polysiloxane block copolymer can promote lithium-ion transport, improve ionic conductivity, and maintain the base film's good rate performance and cycling stability.

[0018] Furthermore, the polar groups such as amino and ether groups in the polyimide-polysiloxane block copolymer can form hydrogen bonds with the polyethylene oxide on the surface of the modified boron nitride nanofibers, thereby improving the bonding force of the modified boron nitride nanofibers in the inorganic coating system, enhancing the adhesion of the inorganic coating, and reducing the risk of boron nitride nanofiber detachment during cycling. Simultaneously, the mesoporous structure formed by the nanofiber network and microphase separation of both materials can form hierarchical channels, maintaining good porosity, improving the wettability of the membrane and electrolyte, and thus increasing ionic conductivity.

[0019] Preferably, the raw materials for the modified boron nitride nanofibers include boron nitride nanofibers, an epoxy silane coupling agent, and an ethylene oxide monomer in a mass-to-volume ratio of 1g:(3-3.5)mL:(1.2-1.5)g.

[0020] Preferably, the boron nitride nanofibers have a diameter of 20–100 nm and a length of 40–60 μm.

[0021] Preferably, the epoxy silane coupling agent includes one or more combinations of 3-glycidoxypropyltrimethoxysilane, 3-glycidoxypropyltriethoxysilane, 3-(2,3-epoxypropoxypropyl)methyldiethoxysilane and 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane.

[0022] Preferably, the modified boron nitride nanofibers are prepared according to the following method:

[0023] An epoxy silane coupling agent is added to an acidic solution to adjust the pH value to 3-4. The temperature is raised to 90-100℃ and the reaction is carried out for 1-2 hours. Then boron nitride nanofibers are added and ultrasonically dispersed for 10-20 minutes. The mixture is stirred at 90-100℃ for 2-4 hours. Finally, the pretreated boron nitride nanofibers are obtained after washing and drying.

[0024] Pretreated boron nitride nanofibers were dispersed in toluene solution, ethylene oxide and an initiator were added, and the mixture was stirred and dispersed under a nitrogen atmosphere. The temperature was then raised to 80-85°C and reacted for 20-24 hours. Finally, the mixture was washed and dried to obtain the final product.

[0025] More preferably, the acidic solution includes one or more combinations of aqueous hydrochloric acid, aqueous nitric acid, and aqueous sulfuric acid.

[0026] More preferably, the initiator includes one or more combinations of ammonium persulfate, potassium persulfate, and azobisisobutyronitrile; the amount of initiator added is 5 to 10% of the mass of the ethylene oxide monomer.

[0027] By adopting the above technical solution, the modified boron nitride nanofibers of the present invention first introduce epoxy groups as reactive sites on the surface of boron nitride nanofibers using an epoxy silane coupling agent, and then polymerize polyethylene oxide in situ under the action of an initiator to obtain modified boron nitride nanofibers.

[0028] The unique fibrous structure of boron nitride nanofibers can form an interpenetrating three-dimensional network framework on the substrate membrane surface. Compared to two-dimensional structures such as typical boron nitride nanosheets, the fibrous structure can more effectively mechanically anchor the substrate membrane, inhibiting its melting and shrinkage at high temperatures. Furthermore, the fibrous structure provides a continuous thermal conductivity path for the membrane, improving thermal conductivity and rapidly dispersing localized hot spots to prevent thermal runaway. Moreover, compared to boron nitride nanosheets or microparticles, the nanofiber structure is less likely to clog the pores of the substrate membrane, thus maximizing the preservation of the membrane's porosity, improving electrolyte wettability, and enhancing ionic conductivity.

[0029] After modification, the polyethylene oxide segments contain a large number of ether oxygen bonds, which can form a weak coordination with lithium ions in the electrolyte, reduce the lithium ion migration resistance, and improve ionic conductivity. In addition, the hydrophilicity of polyethylene oxide can help reduce the contact angle between the membrane and the electrolyte, thereby allowing electrolyte penetration and reducing interfacial impedance.

[0030] Furthermore, the long-chain structure of polyethylene oxide can prevent the aggregation of boron nitride nanofibers on the surface of boron nitride nanofibers by utilizing the steric hindrance effect, thereby improving the uniformity of boron nitride nanofiber distribution on the base membrane surface. In addition, polyethylene oxide has good compatibility with the base membrane, which can improve the interfacial bonding force between boron nitride nanofibers and the base membrane, reduce the risk of inorganic coating peeling off, and thus extend the cycle life of the membrane.

[0031] Preferably, the raw materials for the polyimide-polysiloxane block copolymer include 4,4'-(hexafluoroisopropene)phthalic anhydride, 4,4'-diaminodiphenyl ether, and amino-terminated polydimethylsiloxane in a mass ratio of 1:(0.4-0.45):(0.15-0.25).

[0032] Preferably, under a nitrogen atmosphere, 4,4'-diaminodiphenyl ether is added to a solvent and stirred to dissolve. Then, 4,4'-(hexafluoroisopropene) phthalic anhydride is added and dispersed at room temperature for 30-40 minutes. The temperature is then increased to 140-150°C and stirred for 1-2 hours. Next, a terminal amino polydimethylsiloxane toluene solution is added, and the temperature is increased to 160-180°C. The reaction is continued for 3-4 hours. Finally, the product is obtained after precipitation and drying.

[0033] More preferably, the solvent includes N,N-dimethylacetamide.

[0034] More preferably, the concentration of the amino-terminated polydimethylsiloxane toluene solution is 3–5 g / mL.

[0035] By adopting the above technical solution, the polyimide-polysiloxane block copolymer includes rigid polyimide segments and flexible polydimethylsiloxane segments. The polyimide can provide rigid support at high temperatures, and the polysiloxane segments can buffer thermal stress through the movement of the segments at high temperatures, thereby inhibiting the brittle fracture of the coating at high temperatures and improving the overall thermal stability of the diaphragm.

[0036] Furthermore, the polar groups contained in polyimide can synergistically form hydrophilic channels with polyethylene oxide segments to promote lithium ion transport, thereby improving ion transport efficiency.

[0037] However, adding only polyimide can lead to brittle shrinkage at high temperatures due to a lack of flexible buffering, which can negatively impact the thermal and cycling stability of the membrane. Furthermore, the absence of hydrophobic polysiloxane segments in polyimide results in a dense porous coating structure between it and the modified boron nitride nanofibers, significantly reducing the membrane's porosity and consequently decreasing ion flux. In contrast, the polyimide-polysiloxane block copolymer, after block copolymerization, allows the flexible segments of the polysiloxane to compensate for the rigidity of the polyimide, maintaining high thermal stability while improving the membrane's elongation at break.

[0038] Furthermore, the microphase separation structure formed by the block copolymer can synergistically modify boron nitride nanofibers, improve thermal and mechanical stability, and also form multi-level channels to increase lithium-ion flux and suppress the puncture effect of lithium dendrites on the separator, thereby improving the cycle life of lithium-ion batteries.

[0039] Secondly, the present invention provides a method for preparing a high thermal stability diaphragm, characterized by comprising the following process steps:

[0040] S1. Weigh the corresponding mass fractions of the raw material for the inorganic coating, add the polyimide-polysiloxane block copolymer and adhesive to water, stir and disperse, then add the modified boron nitride nanofibers, and continue to mix evenly to obtain the inorganic slurry;

[0041] S2. An inorganic slurry is coated on both sides of the base membrane and dried to form an inorganic coating, resulting in a membrane with high thermal stability.

[0042] The beneficial effects of this invention are:

[0043] 1. The high thermal stability diaphragm provided by this invention comprises a base membrane and an inorganic coating. The inorganic coating includes modified boron nitride nanofibers, which, after being uniformly dispersed in the inorganic coating, form a three-dimensional network structure coated on the surface of the base membrane, thereby effectively inhibiting the shrinkage of the polyolefin base membrane at high temperatures. Furthermore, after modification treatment, polyethylene oxide is introduced onto the surface, which can improve the dispersibility of boron nitride nanofibers in the system, enhance the bonding force with the base membrane, and the contained ether oxygen bonds can also promote lithium ion migration and improve ion transport efficiency.

[0044] 2. The inorganic coating of the present invention also includes a polyimide-polysiloxane block copolymer. The polyimide can provide high-temperature rigid support for the membrane, and the introduction of flexible polysiloxane segments can compensate for the problem of high-temperature brittle fracture caused by excessive rigidity. The formed hydrophilic channels can promote lithium-ion transport and improve ionic conductivity. It can also work synergistically with modified boron nitride nanofibers. On the one hand, it can improve the bonding force of boron nitride nanofibers in the inorganic coating and reduce the risk of detachment. On the other hand, it can also work synergistically to form multi-level channels, maintain good porosity of the base membrane, and improve ionic conductivity. Its polar groups can also improve the wettability of the membrane and the electrolyte. Detailed Implementation

[0045] The specific embodiments of the present invention will be described in detail below, but it should be understood that the scope of protection of the present invention is not limited to the specific embodiments.

[0046] Preparation Example 1

[0047] Preparation Example 1-1: A modified boron nitride nanofiber was prepared according to the following method:

[0048] 30 mL of 3-glycidoxypropyltriethoxysilane was added to an aqueous sulfuric acid solution to adjust the pH of the solution to 3.5. The temperature was raised to 90 °C and the reaction was carried out for 1 h. Then, 10 g of boron nitride nanofibers (average diameter 60 nm, average length 50 μm) were added. After ultrasonic dispersion for 20 min, the mixture was stirred at 90 °C for 3 h. Finally, the pretreated boron nitride nanofibers were obtained after washing and drying.

[0049] The pretreated boron nitride nanofibers obtained above were dispersed in 250 mL of toluene solution, 13 g of ethylene oxide and 1 g of potassium persulfate were added, and the mixture was stirred and dispersed under a nitrogen atmosphere. The temperature was then raised to 80 °C and reacted for 24 h. Finally, the mixture was washed and dried to obtain the final product.

[0050] Preparation Example 1-2, a modified boron nitride nanofiber, differs from Preparation Example 1-1 only in that the amount of ethylene oxide added is 12g.

[0051] Preparation Examples 1-3: A modified boron nitride nanofiber, differing from Preparation Example 1-1 only in that the amount of 3-glycidoxypropyltriethoxysilane added is 35 mL; and the amount of ethylene oxide added is 15 g.

[0052] Preparation Examples 1-4: A modified boron nitride nanofiber, differing from Preparation Example 1-1 only in that the amount of ethylene oxide added is 10g.

[0053] Preparation Examples 1-5: A modified boron nitride nanofiber, differing from Preparation Example 1-1 only in that the amount of ethylene oxide added is 18g.

[0054] Preparation Examples 1-6: A modified boron nitride nanofiber was prepared according to the following method:

[0055] 30 mL of 3-glycidoxypropyltriethoxysilane was added to an aqueous sulfuric acid solution to adjust the pH of the solution to 3.5. The temperature was raised to 90 °C and the reaction was carried out for 1 h. Then, 10 g of boron nitride nanofibers (average diameter 60 nm, average length 50 μm) were added. After ultrasonic dispersion for 20 min, the mixture was stirred at 90 °C for 3 h. Finally, the modified boron nitride nanofibers were obtained after washing and drying.

[0056] Preparation Examples 1-7: A modified boron nitride nanosheet, which differs from Preparation Example 1-1 only in that an equal amount of boron nitride nanosheets are used to replace boron nitride nanofibers.

[0057] Preparation Example 2

[0058] Preparation Example 2-1: A polyimide-polysiloxane block copolymer was prepared according to the following method:

[0059] Under a nitrogen atmosphere, 4 g of 4,4'-diaminodiphenyl ether was added to 100 mL of N,N-dimethylacetamide and stirred until dissolved. Then, 10 g of 4,4'-(hexafluoroisopropene)phthalic anhydride was added and stirred at room temperature for 30 min. The temperature was then raised to 140 °C and stirred for 2 h. Next, a 4 g / mL solution of terminal amino polydimethylsiloxane toluene was added (1.5 g of terminal amino polydimethylsiloxane was added). The temperature was raised to 170 °C and stirred for another 4 h. Finally, the product was obtained after precipitation and drying.

[0060] Preparation Example 2-2 is a polyimide-polysiloxane block copolymer, which differs from Preparation Example 2-1 only in that the amount of 4,4'-diaminodiphenyl ether added is 4.5 g and the amount of amino-terminated polydimethylsiloxane added is 2.5 g.

[0061] Preparation Examples 2-3: A polyimide was prepared according to the following method:

[0062] Under a nitrogen atmosphere, 4 g of 4,4'-diaminodiphenyl ether was added to 100 mL of N,N-dimethylacetamide and stirred until dissolved. Then, 10 g of 4,4'-(hexafluoroisopropene)phthalic anhydride was added and stirred to disperse at room temperature for 30 min. The temperature was then increased to 140 °C and stirred for 2 h. The temperature was then increased to 170 °C and stirred for 3 h. Finally, the product was obtained after precipitation and drying.

[0063] Example

[0064] Example 1: A membrane with high thermal stability was prepared according to the following process steps:

[0065] S1. Add 8 parts of the polyimide-polysiloxane block copolymer prepared in Preparation Example 2-1 and 4 parts of polyvinylidene fluoride adhesive to 100 parts of water, stir and disperse, then add 12 parts of the modified boron nitride nanofibers prepared in Preparation Example 1-1, and continue to mix evenly to obtain an inorganic slurry.

[0066] S2. An inorganic slurry is coated on both sides of the polypropylene base membrane, and after drying, an inorganic coating is formed with a thickness of 3 μm, resulting in a membrane with high thermal stability.

[0067] Example 2: A membrane with high thermal stability was prepared according to the following process steps:

[0068] S1. Add 10 parts of the polyimide-polysiloxane block copolymer prepared in Preparation Example 2-1 and 3 parts of polyvinylidene fluoride adhesive to 100 parts of water, stir and disperse, then add 10 parts of the modified boron nitride nanofibers prepared in Preparation Example 1-1, and continue to mix evenly to obtain an inorganic slurry.

[0069] S2. An inorganic slurry is coated on both sides of the polypropylene base membrane, and after drying, an inorganic coating is formed with a thickness of 3 μm, resulting in a high thermal stability diaphragm.

[0070] Example 3: A membrane with high thermal stability was prepared according to the following process steps:

[0071] S1. Add 6 parts of the polyimide-polysiloxane block copolymer prepared in Preparation Example 2-2 and 6 parts of polyvinylidene fluoride adhesive to 100 parts of water, stir and disperse, then add 15 parts of the modified boron nitride nanofibers prepared in Preparation Example 1-1, and continue to mix evenly to obtain an inorganic slurry.

[0072] S2. An inorganic slurry is coated on both sides of the polypropylene base membrane, and after drying, an inorganic coating is formed with a thickness of 3 μm, resulting in a high thermal stability diaphragm.

[0073] Example 4, a membrane with high thermal stability, differs from Example 1 only in that the modified boron nitride nanofibers prepared in Example 1-1 are replaced with an equal amount of the modified boron nitride nanofibers prepared in Examples 1-2.

[0074] Example 5, a membrane with high thermal stability, differs from Example 1 only in that the modified boron nitride nanofibers prepared in Example 1-1 are replaced with an equal amount of the modified boron nitride nanofibers prepared in Examples 1-3.

[0075] Example 6, a membrane with high thermal stability, differs from Example 1 only in that the modified boron nitride nanofibers prepared in Example 1-1 are replaced with an equal amount of the modified boron nitride nanofibers prepared in Examples 1-4.

[0076] Example 7, a membrane with high thermal stability, differs from Example 1 only in that the modified boron nitride nanofibers prepared in Example 1-1 are replaced with an equal amount of the modified boron nitride nanofibers prepared in Examples 1-5.

[0077] Comparative Example

[0078] Comparative Example 1, a membrane with high thermal stability, differs from Example 1 only in that the amount of modified boron nitride nanofibers prepared in Preparation Example 1-1 is 5 parts.

[0079] Comparative Example 2, a membrane with high thermal stability, differs from Example 1 only in that the amount of modified boron nitride nanofibers prepared in Preparation Example 1-1 added is 20 parts.

[0080] Comparative Example 3, a membrane with high thermal stability, differs from Example 1 only in that the modified boron nitride nanofibers prepared in Example 1-1 are replaced with an equal amount of the modified boron nitride nanofibers prepared in Examples 1-6.

[0081] Comparative Example 4, a membrane with high thermal stability, differs from Example 1 only in that an equal amount of boron nitride nanofibers are used to replace the modified boron nitride nanofibers prepared in Preparation Example 1-1.

[0082] Comparative Example 5, a membrane with high thermal stability, differs from Example 1 only in that the modified boron nitride nanofibers prepared in Example 1-1 are replaced with an equal amount of modified boron nitride nanosheets prepared in Examples 1-7.

[0083] Comparative Example 6, a membrane with high thermal stability, differs from Example 1 only in that an equal amount of polyimide prepared in Preparation Examples 2-3 is used to replace the polyimide-polysiloxane block copolymer prepared in Preparation Example 2-1.

[0084] Comparative Example 7, a membrane with high thermal stability, differs from Example 1 only in that the inorganic coating does not contain the polyimide-polysiloxane block copolymer prepared in Preparation Example 2-1.

[0085] Performance testing

[0086] 1. Thermal stability test: According to the relevant records in ISO 14616-1997 "Heat shrinkage films of polyethylene, ethylene copolymers and mixtures thereof - Determination of shrinkage stress", the thermal shrinkage rate of the diaphragms obtained in the examples and comparative examples was measured under the following conditions: constant temperature 140°C, holding time 30 min.

[0087] 2. Electrochemical Testing: Commercial lithium cobalt oxide cathode material, acetylene black, and PVDF binder were uniformly mixed at a mass ratio of 85:10:5, coated onto aluminum foil, and dried in a vacuum drying oven at 120°C to prepare the cathode sheet. The dried sheet was then cut into 10mm round pieces for later use. Pure lithium metal sheets were used as the anode. The membranes obtained in the examples and comparative examples were used as the membrane material. A 1mol / L solution of LiPF6 in vinyl carbonate (EC)-dimethyl carbonate (DMC) (volume ratio 1:1) was used as the electrolyte. Electrochemical testing was conducted with water and oxygen contents both below 1×10⁻⁶. -6 The CR2032 test battery was assembled inside a glove box protected by an argon atmosphere.

[0088] At room temperature and 2.75V, a constant current of 14mAg was used for charge-discharge cycle testing, with 100 cycles to test the retention rate of battery capacity before and after the test.

[0089] The results of the above experiments are shown in Table 1:

[0090] Table 1 Performance test results

[0091]

[0092] According to Table 1, combined with Example 1 and Comparative Example 4, it can be seen that the thermal shrinkage rate of Comparative Example 4 increased and the capacity retention rate decreased, indicating that the thermal stability and cycle stability of Comparative Example 4 decreased. The reason is that the boron nitride nanofibers in Comparative Example 4 were not modified, which will affect the dispersibility of boron nitride nanofibers in the inorganic coating. The agglomeration of boron nitride nanofibers can easily lead to local heat concentration in the diaphragm, resulting in thermal runaway and decreased cycle performance.

[0093] Combining Example 1 and Comparative Example 5, it can be seen that the thermal shrinkage rate of Comparative Example 5 is increased and the capacity retention rate is reduced, indicating that the thermal stability and cycle stability of Comparative Example 4 are reduced. The reason is that the same modified boron nitride nanosheets used in Comparative Example 5 cannot form a three-dimensional network of thermally conductive materials compared to nanofibers, resulting in reduced thermal stability. Moreover, the nanosheets will block the pores of the base film, leading to a decrease in ionic conductivity and cycle performance.

[0094] Based on Examples 1, 6, and 7, it can be seen that the thermal shrinkage rate and capacity retention rate of Comparative Examples 6 and 7 increased, indicating a decrease in thermal stability and cycle stability. This is because Comparative Example 6 only added polyimide, which was not modified with polysiloxane blocks. The polyimide molecular chain segments are highly rigid, making them prone to brittle fracture at high temperatures, leading to defects such as microcracks in the inorganic coating and difficulty in suppressing dendrite puncture, thus reducing cycle stability and thermal stability. Comparative Example 7 did not add polyimide-polysiloxane block copolymer, lacking the supporting effect of polyimide on high-temperature thermal stability, resulting in decreased thermal stability and reduced cycle performance.

[0095] The above-disclosed embodiments are merely a few specific examples of the present invention. However, the embodiments of the present invention are not limited thereto, and any variations that can be conceived by those skilled in the art should fall within the protection scope of the present invention.

Claims

1. A high thermal stability diaphragm, characterized in that, The high thermal stability diaphragm comprises a base membrane and an inorganic coating; the inorganic coating is coated on both sides of the base membrane; The inorganic coating comprises the following raw materials in parts by weight: 10-15 parts of modified boron nitride nanofibers; 6-10 parts of polyimide-polysiloxane block copolymer; 3-6 parts adhesive; 100 parts water; The modified boron nitride nanofibers are grafted with polyethylene oxide on their surface.

2. The high thermal stability diaphragm according to claim 1, characterized in that, The raw materials for the modified boron nitride nanofibers include boron nitride nanofibers, an epoxy silane coupling agent, and an ethylene oxide monomer in a mass-to-volume ratio of 1g:(3-3.5)mL:(1.2-1.5)g.

3. The high thermal stability diaphragm according to claim 2, characterized in that, The boron nitride nanofibers have a diameter of 20–100 nm and a length of 40–60 μm.

4. The high thermal stability diaphragm according to claim 2, characterized in that, The epoxy silane coupling agent includes one or more combinations of 3-glycidoxypropyltrimethoxysilane, 3-glycidoxypropyltriethoxysilane, 3-(2,3-epoxypropoxypropyl)methyldiethoxysilane and 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane.

5. The high thermal stability diaphragm according to claim 2, characterized in that, The modified boron nitride nanofibers were prepared according to the following method: An epoxy silane coupling agent is added to an acidic solution to adjust the pH value to 3-4. The temperature is raised to 90-100℃ and the reaction is carried out for 1-2 hours. Then boron nitride nanofibers are added and ultrasonically dispersed for 10-20 minutes. The mixture is stirred at 90-100℃ for 2-4 hours. Finally, the pretreated boron nitride nanofibers are obtained after washing and drying. Pretreated boron nitride nanofibers were dispersed in toluene solution, ethylene oxide and an initiator were added, and the mixture was stirred and dispersed under a nitrogen atmosphere. The temperature was then raised to 80-85°C and reacted for 20-24 hours. Finally, the mixture was washed and dried to obtain the final product.

6. The high thermal stability diaphragm according to claim 1, characterized in that, The raw materials for the polyimide-polysiloxane block copolymer include 4,4'-(hexafluoroisopropene)phthalic anhydride, 4,4'-diaminodiphenyl ether, and amino-terminated polydimethylsiloxane in a mass ratio of 1:(0.4-0.45):(0.15-0.25).

7. The high thermal stability diaphragm according to claim 6, characterized in that, The polyimide-polysiloxane block copolymer was prepared according to the following method: Under a nitrogen atmosphere, 4,4'-diaminodiphenyl ether is added to a solvent and stirred until dissolved. Then, 4,4'-(hexafluoroisopropene) phthalic anhydride is added and dispersed at room temperature for 30-40 minutes. The temperature is then increased to 140-150°C and stirred for 1-2 hours. Next, a terminal amino polydimethylsiloxane toluene solution is added, and the temperature is increased to 160-180°C. The reaction is continued for 3-4 hours. Finally, the product is obtained after precipitation and drying.

8. The high thermal stability diaphragm according to claim 1, characterized in that, The base film includes any one of polyethylene base film, polypropylene base film, polyethylene / polypropylene double-layer co-extruded film, and polyethylene / polypropylene / polyethylene multilayer co-extruded film.

9. The high thermal stability diaphragm according to claim 1, characterized in that, The adhesive includes one or a combination of two of polyvinylidene fluoride and polytetrafluoroethylene.

10. A method for preparing a high thermal stability diaphragm according to any one of claims 1 to 9, characterized in that, The process includes the following steps: S1. Weigh the corresponding mass fractions of the raw material for the inorganic coating, add the polyimide-polysiloxane block copolymer and adhesive to water, stir and disperse, then add the modified boron nitride nanofibers, and continue to mix evenly to obtain the inorganic slurry; S2. An inorganic slurry is coated on both sides of the base membrane and dried to form an inorganic coating, resulting in a membrane with high thermal stability.

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