Composite material, composite battery diaphragm, preparation method and battery

Abstract

The invention belongs to the technical field of lithium ion battery diaphragms, and particularly relates to a composite material, a composite battery diaphragm, a preparation method and a battery, and the preparation method of the composite material comprises the following steps: mixing PBO fibers, a polymer monomer and a solvent, and reacting under the action of an initiator to obtain polymer grafted PBO fibers; the polymer monomer is maleic anhydride or acrylic acid; mixing the PBO fiber grafted with the polymer with PEO, and granulating to obtain particles; mixing the PBO fiber grafted with the polymer, PEO, particles and a flame retardant, and granulating to obtain a composite material; according to the invention, the compatibility between the materials is improved, the mechanical property and cycle number of the diaphragm are improved, the thermal closing performance of the diaphragm is improved, and the safety of the battery is improved.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery separator technology, specifically relating to a composite material, a composite battery separator, a preparation method, and a battery. Background Technology

[0002] Polypropylene (PP) is widely used in battery separators. However, its non-polar and hydrophobic properties limit its application in fields such as battery separators.

[0003] The thermal shut-off performance of the battery separator is crucial for ensuring battery safety, and it is mainly improved through the following methods: 1. Material selection and modification High-melting-point PP (melting point > 165℃) or PE (melting point > 130℃) is used as the base material, and thermal stability is improved through copolymerization modification. For example, the PP / PE / PP three-layer composite membrane combines the high-temperature resistance of PP and the low-temperature closed-cell characteristics of PE, and can achieve thermal shutdown by triggering the melting of the PE layer at 130℃.

[0004] Inorganic filler doping: Add ceramic particles such as alumina and silicon dioxide (content 5-20%) to reduce the thermal shrinkage rate of the diaphragm by more than 50% through physical barrier effect.

[0005] 2. Structural design optimization Multi-layer composite structure: The PP / PE / PP sandwich structure achieves rapid thermal response (closed-cell time < 1 second) through the middle PE layer, while the outer PP layer provides mechanical support.

[0006] Gradient pore design: By controlling the pore distribution through bidirectional stretching process, the porosity in the high-temperature region is reduced to below 30%, thus delaying the thermal runaway diffusion.

[0007] 3. Surface functionalization treatment Ceramic coating technology: Using PVDF binder to coat alumina slurry can reduce the thermal shrinkage rate of the diaphragm to less than 5% at 180℃, while improving electrolyte wettability.

[0008] Plasma treatment: Polar groups are introduced on the PP surface by oxygen plasma, which enhances the interfacial bonding with the ceramic coating and avoids high-temperature delamination. Summary of the Invention

[0009] The technical problem to be solved by the present invention is to provide a composite material, a composite battery separator, a preparation method and a battery, which improves the compatibility between materials, improves the mechanical properties and cycle number of the separator, improves the thermal shut-off performance of the separator and improves the safety of the battery.

[0010] This invention provides a method for preparing a composite material, in which PBO fibers, polymer monomers and solvents are mixed and reacted under the action of an initiator to obtain PBO fibers grafted with polymers, wherein the polymer monomers are maleic anhydride or acrylic acid. PBO fibers grafted with polymer and PEO are mixed and granulated to obtain granules; The grafted polymer PBO fibers, PEO, granules, and flame retardant are mixed and granulated to obtain a composite material.

[0011] Preferably, the solvent is N,N-dimethylformamide.

[0012] Preferably, the initiator is azobisisobutyronitrile.

[0013] The flame retardant is nano-silica.

[0014] Specifically, PBO fibers are impregnated in a mixed solution (solvent: N,N-dimethylformamide, containing 0.5-1.5 wt% initiator and 5-10 wt% polymer monomer) and reacted at 60°C. The mass of the PBO fibers is 10-20% of the weight of the mixed solution. After the reaction, the fibers are washed (using DMF for ultrasonic washing, followed by washing with ethanol and deionized water) and dried (at 60°C) to obtain PBO fibers grafted with the polymer. The reaction system is purged with an inert gas to remove oxygen and prevent the initiator from oxidizing and becoming ineffective.

[0015] Preferably, the weight ratio of PBO fibers to PEO in the grafted polymer is 1:3-5 (preferably 1:4); the granulation is performed by twin-screw extrusion granulation.

[0016] This invention provides a composite material, characterized in that it is prepared using the aforementioned preparation method.

[0017] This invention provides a method for preparing a composite battery separator, which is prepared by electrospinning or coating. The steps of the electrospinning method are as follows: The base film material is dissolved to obtain the shell solution. The composite material was dissolved to obtain a core layer solution. The core layer solution is placed inside the shell layer solution, and coaxial electrospinning is performed to obtain a spun diaphragm. The spun separator is dried (vacuum dried at 100°C) and annealed (annealed at 120°C) to obtain a composite battery separator; Before drying, the spinning diaphragm can also be soaked in ethanol to remove residual solvent.

[0018] The steps of the coating method are as follows: The composite material is dissolved in an organic solvent to obtain a coating solution, which is then coated onto a base film and dried to obtain a composite battery separator.

[0019] Preferably, the base film is a PP film.

[0020] Preferably, in the shell solution, the solvent is a mixture of dichloromethane, chloroform, acetone and DMSO (the volume ratio of dichloromethane, chloroform, acetone and dimethyl sulfoxide (DMSO) is 90:95:10:5, and the solution is dissolved by stirring at 140°C); in the core solution, the solvent is hexafluoroisopropanol. The solvent in the coating solution is tetrahydrofuran or DMF; before coating the base film with the coating solution, the base film is subjected to corona treatment (power set to 1-5kW, treatment time 20-30s, electrode-base film spacing 10-15mm).

[0021] This invention provides a composite battery separator, which is prepared using the aforementioned preparation method.

[0022] This invention provides a battery, including the aforementioned composite battery separator.

[0023] The beneficial effects of this invention are that it significantly improves the interfacial compatibility of PEO and PBO through a synergistic modification method of chemical grafting and physical blending, while maintaining the original performance advantages of both. In the PP / PEO-g-PBO composite separator prepared from this composite material, the PEO layer completely melts and blocks the micropores at 71°C, causing the composite separator to rapidly thermally shut down at 71°C. This allows the lithium-ion battery to quickly interrupt the reaction and prevent further increases in internal battery temperature. Furthermore, the addition of PBO makes the separator less prone to melting at temperatures above 120°C, ensuring battery safety at high temperatures.

[0024] PBO introduces carboxyl groups through grafting with monomers MAH or AA, forming a hydrogen bond network with the ether bonds of PEO, thus reducing interfacial tension. Nano-SiO2 synergistically enhances interfacial stress transfer with PEO through silanol groups. In this invention, PEO enhances ion transport, PBO provides mechanical support, and the PP matrix of the base film exhibits thermal stability.

[0025] The composite battery separator of this invention exhibits high thermal stability. PBO possesses extremely high thermal stability (decomposition temperature > 600℃) and a rigid molecular chain structure, effectively suppressing thermal shrinkage at high temperatures. This invention reduces interfacial impedance by leveraging the strong polar groups (-O-) of PEO to form a strong interaction with lithium salts (such as LiPF6) in the electrolyte.

[0026] The present invention exhibits high mechanical strength and anti-dendritic properties. The tensile strength (525 MPa) and modulus (20 GPa) of PBO are significantly higher than those of PP, effectively blocking lithium dendrite penetration. The composite separator retains 95% of its capacity after 500 cycles, while the pure PP separator only retains 80%. The flexibility of PEO alleviates the stress caused by electrode volume expansion during cycling and reduces interfacial side reactions.

[0027] The battery of this invention boasts high safety. When the internal temperature rises abnormally, reaching the thermal shutdown trigger temperature of 71°C for the separator, the PEO pores close, shutting down the ion conduction channels and preventing further electrochemical reactions inside the battery. This effectively reduces the risk of thermal runaway and prevents the battery from catching fire or exploding. Furthermore, the addition of PBO makes the separator less prone to melting at temperatures above 120°C, thus ensuring battery safety at high temperatures to a certain extent.

[0028] This invention grafts PBO onto a polymer, mixes it with PEO, and granulates it to obtain PEO-g-MAH-PBO, which serves as a compatibilizer. This compatibilizer is then mixed with PBO-g-MAH and PEO, and granulated to obtain a PEO-g-PBO composite material. This composite material is then coaxially electrospun with a PP base film, so that the PEO-g-PBO composite material is located in the core layer and the PP material is located in the shell layer.

[0029] PBO and PBO-g-MAH exhibit ultra-high thermal stability (PBO decomposition temperature > 600°C), significantly reducing the thermal shrinkage rate of the core layer at high temperatures. In a single PEO-g-MAH-PBO compatibilizer, PEO has a higher proportion and is prone to softening and deformation at high temperatures, resulting in a large thermal shrinkage rate of the membrane. By mixing the compatibilizer with PBO-g-MAH and PEO and granulating the mixture, a PEO-g-PBO composite material is obtained, which possesses both high thermal stability and good mechanical properties and cycle life.

[0030] Blended electrospinning involves directly mixing PP, PEO, and PBO before spinning. This process suffers from poor compatibility; PP is non-polar and hydrophobic, PEO is polar and hydrophilic, and PBO contains strong intermolecular hydrogen bonds, making phase separation a likely consequence of their blending. Compared to blended electrospinning, the core advantage of coaxial electrospinning "PP shell + PEO-g-PBO core" composite fibers lies in: the coaxial structure avoids direct blending of PP, PEO, and PBO; the core layer retains the polarity and transport properties of PEO, as well as the high modulus and strength of PBO; the PP shell provides hydrophobicity (contact angle >90°), preventing the PEO core layer from absorbing water and swelling, extending its service life, and providing mechanical protection. Attached Figure Description

[0031] Figure 1 This is a comparison chart of DSC curves.

[0032] Figure 2 This is a comparison chart of stress-strain curves.

[0033] Figure 3 This is a comparison chart of 1C loop data. Detailed Implementation

[0034] The present invention will now be described in detail with reference to specific embodiments, but the scope of protection of the present invention is not limited thereto.

[0035] Example 1 A method for preparing a PEO-g-PBO composite material includes the following steps: 1) 15g of PBO fiber was impregnated in 200 mL of a mixed solution. The solvent of the mixed solution was DMF (N,N-dimethylformamide), containing 1wt% AIBN (azobisisobutyronitrile) and 8wt% MAH (maleic anhydride). The reaction was carried out at 60℃ for 5 hours. The fiber was ultrasonically washed three times with DMF (to remove unreacted MAH and residual AIBN), then washed twice each with ethanol and deionized water, and finally vacuum dried at 60℃ for 4 hours to obtain PBO fiber grafted with polymaleic anhydride, PBO-g-MAH (nitrogen gas was passed through the reaction system to remove oxygen and prevent AIBN from oxidizing and becoming ineffective).

[0036] 2) Take 1g of grafted polymaleic anhydride PBO fiber PBO-g-MAH and mix it with 4g of PEO (Mw=6000). Extrude and granulate using a twin-screw extruder at an extrusion temperature of 180-200℃ (twin-screw extrusion parameters: screw speed 200-300rpm, feed section 180℃ → melt section 185-195℃ → die head 200℃, residence time 2-3min). Water-cool and pelletize the extrudate to obtain PEO-g-MAH-PBO.

[0037] 3) Mix 10 g PBO-g-MAH, 15 g PEO (Mw=6000), 2.5 g PEO-g-MAH-PBO and 0.5 g SiO2, and co-blend them in a twin-screw extruder at an extrusion temperature of 180-200℃ (the relevant parameters for twin-screw extrusion are the same as in step 2). Water-cool the extrudate and pelletize it to obtain the PEO-g-PBO composite material.

[0038] Example 2 Example 2 differs from Example 1 in that acrylic acid (AA) is used instead of MAH to obtain a PEO-g-PBO composite material. Everything else is the same as in Example 1.

[0039] Example 3 A method for preparing a composite battery separator includes the following steps: 1) Weigh 1g of polypropylene (PP) granules (Mw=20000-40000), add 10mL of a mixture (the mixture includes dichloromethane, chloroform, acetone and DMSO, and dichloromethane, chloroform, acetone and dimethyl sulfoxide (DMSO) are mixed in a volume ratio of 90:95:10:5), and stir magnetically in an oil bath at 140°C for 5 hours until completely dissolved to obtain a shell PP solution.

[0040] 2) Weigh 0.4g of the PEO-g-PBO composite material prepared in Example 1 and dissolve it in 10mL of hexafluoroisopropanol (HFIP). Stir magnetically for 6 hours, and then disperse it at 10000 rpm for 30 minutes using a high-speed homogenizer to obtain a uniformly dispersed suspension, i.e., the core layer solution.

[0041] 3) Electrospinning: The shell PP solution obtained in step 1) and the core solution obtained in step 2) are injected into two 5ml flat-mouth syringes respectively. Using a coaxial spinning needle (shell diameter 1.5mm, core diameter 0.6mm), the shell syringe (containing shell PP solution) is connected to the shell channel and delivered through a PTFE conduit. The core syringe (containing core solution) is connected to the core channel. The heating temperature is set to 120°C, the shell is connected to the positive electrode, the core is connected to the negative electrode, and the voltage is 20KV. Fibers are collected on a roller receiver (diameter 10cm) covered with aluminum foil at a receiving distance of 12cm to obtain the spinning diaphragm.

[0042] 4. Post-treatment: Solvent extraction: Soak the spinning diaphragm in ethanol for 24 h to remove residual HFIP.

[0043] Heat treatment: Vacuum drying at 100℃ for 12 h, followed by annealing at 120℃ for 2 h (160℃ below the melting point of PP) to promote PP crystallization (crystallinity increased from 50% to 70%) and PBO segment orientation, thereby improving the thermal shrinkage rate. Cooling yields the PP / PEO-g-PBO composite battery separator.

[0044] Example 4 A method for preparing a composite battery separator includes the following steps: 1) The PP base film is surface treated by corona treatment, with a power of 3kW and a treatment time of 30s. The electrode-base film spacing is 10-15mm to obtain the treated PP base film.

[0045] 2) Disperse 2 g of the PEO-g-PBO composite material from Example 1 in 20 mL of THF tetrahydrofuran or DMF to obtain a coating solution.

[0046] 3) The coating liquid is applied to the surface-treated PP base film by scraping.

[0047] 4) After drying in a ventilated environment at room temperature for 2 hours, transfer it to a vacuum oven at 40°C and continue drying for 6 hours to obtain the composite battery separator.

[0048] Comparative Example 1 2g PP, 1g PEO and 1g PBO were directly mixed in 30mL decahydronaphthalene and magnetically stirred in an oil bath at 140℃ for 5 hours.

[0049] Because PBO is completely insoluble and incompatible with the system, a uniform spinning solution cannot be formed. Forced uniaxial electrospinning fails to produce a film.

[0050] Comparative Example 2 A method for preparing a composite material includes the following steps: 1) Weigh 1g of PEO (Mw=300000) and dissolve it in 10mL of ethanol / water mixed solvent (ethanol and water volume ratio is 1:1).

[0051] Weigh 1g of PBO nanofibers and 0.2g of SiO2, and slowly add the above solution under ultrasonic oscillation (ultrasonic oscillation conditions: power 200W, frequency 40kHz, temperature 30℃). Sonicate for 1 hour to form a uniform dispersion, i.e., coating solution.

[0052] 2) Fix a PP film with a thickness of 20μm onto the base of the doctor blade, pour the above coating liquid onto the film surface, and use a doctor blade to control the wet film thickness to 200μm.

[0053] 3) After drying in the air at room temperature for 2 hours, transfer to a 50°C vacuum oven and continue drying for 6 hours to obtain the PEO-PBO composite material.

[0054] Comparative Example 3 The difference between Comparative Example 3 and Example 3 is that the PEO-PBO composite material obtained in Comparative Example 2 is used to replace the PEO-g-PBO composite material in step 2) of Example 3 to obtain a PP / PEO-PBO membrane. Everything else is the same as in Example 3.

[0055] Example 5 Differential scanning calorimetry (DSC) is a widely used thermal analysis technique. Using a differential scanning calorimeter, the glass transition temperature (Tg) of a material can be obtained, thereby assessing its compatibility. Completely compatible blends: The molecular chains of the two polymers diffuse into each other, forming a homogeneous system with only one broadened Tg, located between the Tg of the two pure polymers. Incompatible blends: The two components are thermodynamically incompatible, forming macroscopic phase separation; two independent, sharp Tgs are retained in the DSC.

[0056] 1g of the PEO-PBO composite material from Comparative Example 2 (i.e. Figure 1 PEO-g-PBO (1g PEO (Mw=300000) and a 10μm PP membrane were used as comparative examples, and were used together with 1g of the PEO-g-PBO sample from Example 1 of this invention for DSC testing. The DSC testing conditions were Ar gas protection, the test temperature range was 40 ℃ to 400 ℃, and the heating rate was set to 3 ℃ / min. The results are as follows. Figure 1 As shown.

[0057] like Figure 1 As shown, the green curve contains two independent melting peaks, corresponding to the characteristic melting points of pure PEO and pure PBO, respectively. The presence of these two peaks directly indicates that PEO and PBO do not form effective interactions in the blend system, their molecular chains separate, and their compatibility is poor.

[0058] Grafting modification involves chemically grafting PBO onto the PEO backbone, forming a copolymer with PEO as the backbone and PBO as the side chain, with the molecules connected by chemical bonds. For example... Figure 1 As shown, only a single melting peak appears in the DSC red curve. After grafting, PEO and PBO are chemically bonded, and the intermolecular interactions are significantly enhanced, resulting in a homogeneous system and reflecting improved compatibility. This demonstrates that the grafted system has significantly better compatibility than the physically blended system.

[0059] Traditional PP membranes rely primarily on the melting of PP (approximately 160-170℃) for thermal shut-off, resulting in relatively high shut-off temperatures. Figure 1 As shown, the curve for PP shows a melting peak around 165℃. PEO is a semi-crystalline polymer with a low glass transition temperature; the melting peak of PEO on the DSC corresponds to a temperature of 71℃, much lower than that of PP. When the temperature rises to 80℃ or higher, the amorphous region of PEO in the PP / PEO-g-PBO composite battery separator gradually softens, while the crystalline region partially melts. This causes the separator pores to shrink or partially close, effectively shutting off ions and ensuring battery safety.

[0060] Example 6 A PP membrane with a thickness of 10 μm was used as a comparative example to test the mechanical properties of the PP / PEO-g-PBO composite battery separator and the PP membrane in Example 3.

[0061] Figure 2The stress-strain curves of the PP / PEO-g-PBO composite battery separator and the PP separator at 60℃ and 3MPa are shown. The stress-strain curve of the PP separator shows a low tensile strength of only 31MPa. However, the stress-strain curve of the PP / PEO-g-PBO composite battery separator shows a higher tensile strength of 461 MPa, which is likely due to the increased intermolecular forces caused by PBO. Therefore, the PP / PEO-g-PBO composite battery separator can meet the mechanical performance requirements of lithium-ion battery separators.

[0062] Example 7 To verify the impact of the separator on battery performance, pouch cells were assembled using different separators for testing. The separators used were the PP / PEO-g-PBO composite battery separator from Example 3 of this invention, the PP / PEO / PBO separator from Comparative Example 3, and PP with a thickness of 10 μm. Pouch cells were assembled using lithium iron phosphate as the positive electrode, graphite as the negative electrode, and 1 mol / L LiPF6 EC and DMC (volume ratio 1.1-3) as the electrolyte. Electrochemical tests were conducted at 25°C and a 1C rate.

[0063] Figure 3 This paper compares the cycling performance of two different separator materials (PP / PEO-g-PBO composite separator, PP membrane, and PP / PEO / PBO separator) in pouch cells at 1C rate. The horizontal axis represents the number of cycles (0-150 cycles), and the vertical axis represents the discharge specific capacity (mAh / g). The PP / PEO-g-PBO composite separator exhibits the best initial capacity and capacity retention at 1C rate. This is likely because the grafting enhances the compatibility between PEO and PBO, resulting in a more stable separator structure that better utilizes the functionality of PEO and PBO, thus slowing down battery capacity decay.

[0064] Those skilled in the art should understand that the discussion of any of the above embodiments is merely exemplary and is not intended to imply that the scope of protection of this application is limited to these examples; within the framework of this application, the technical features of the above embodiments or different embodiments can also be combined, the steps can be implemented in any order, and there are many other variations of different aspects of one or more embodiments of this application as described above, which are not provided in detail for the sake of brevity.

[0065] One or more embodiments in this application are intended to cover all such substitutions, modifications, and variations that fall within the broad scope of this application. Therefore, any omissions, modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of one or more embodiments in this application should be included within the protection scope of this application.

Claims

1. A method of producing a composite material, characterized by, Mixing PBO fiber, polymer monomer and solvent, under the action of initiator, the reaction, the grafted polymer PBO fiber, the polymer monomer is maleic anhydride or acrylic acid; Mixing grafted polymer PBO fiber and PEO, granulation, to get particles; Mixing grafted polymer PBO fiber, PEO, particles and flame retardant, granulation, to get composite material.

2. The production method according to claim 1, wherein The solvent is N, N-dimethylformamide.

3. The production method according to claim 1, wherein The initiator is azobisisobutyronitrile.

4. The production method according to claim 1, wherein The weight ratio of the grafted polymer PBO fiber and PEO is 1:3-5; the granulation is double screw extrusion granulation.

5. A composite material, characterized by, Prepared by the preparation method of any one of claims 1-4.

6. A method of preparing a composite battery separator, characterized by, Prepared by electrospinning method or coating method; The steps of the electrospinning method are: Dissolving base film material to get shell layer solution, Dissolving the composite material of claim 5 to get core layer solution, Placing the core layer solution inside the shell layer solution, and performing coaxial electrospinning to get spinning membrane, Drying and annealing the spinning membrane to get composite battery separator; The steps of the coating method are: Dissolving the composite material of claim 5 in organic solvent to get coating liquid, coating the coating liquid on the base film, drying to get composite battery separator.

7. The production method according to claim 6, wherein the production method is characterized by, The base film is PP film.

8. The production method according to claim 6, wherein In the shell layer solution, the solvent is a mixture of dichloromethane, chloroform, acetone and DMSO; in the core layer solution, the solvent is hexafluoroisopropanol; In the coating liquid, the solvent is tetrahydrofuran or DMF; before coating the coating liquid on the base film, the base film is subjected to corona treatment.

9. A composite battery separator characterized by, Prepared by the preparation method of any one of claims 6-8.

10. A battery characterized by, Including the composite battery separator of claim 9.

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