Biaxially oriented polypropylene film material for lithium battery diaphragm and preparation method of biaxially oriented polypropylene film material

Abstract

The invention discloses a biaxially oriented polypropylene film material for a lithium battery diaphragm and a preparation method of the biaxially oriented polypropylene film material, and relates to the field of preparation of lithium battery diaphragms. The composite film is composed of the following raw materials in percentage by weight: at least one substrate film layer, and the substrate film layer is prepared by melt blending, two-way stretching and heat setting of the raw materials comprising the following components: 85-98% of polypropylene resin; 1-10% of a pore-forming agent; 0.1 to 2% of an antioxidant; 0.1-3% of a beta crystal form nucleating agent; wherein the pore-forming agent is a polymer or an inorganic substance which is incompatible in polypropylene and is extracted and removed in a subsequent process. By optimizing material components, introducing a specific beta-crystal nucleating agent and a pore-forming agent and combining a specific two-way stretching process, a diaphragm product which is uniform in pore diameter, high in thermal stability and excellent in mechanical property is synchronously obtained, meanwhile, the technological process is simplified, and the production efficiency and the product consistency are improved.

Description

Technical Field

[0001] This invention relates to the field of lithium battery separator preparation, specifically to a biaxially oriented polypropylene film material for lithium battery separators and its preparation method. Background Technology

[0002] Lithium-ion batteries, as efficient and clean energy storage devices, have been widely used in consumer electronics, new energy vehicles, and energy storage power stations. The separator is one of the key internal components of a lithium-ion battery, its main function being to isolate the positive and negative electrodes to prevent short circuits while allowing lithium ions to pass freely. Polyolefin separators, especially biaxially oriented polypropylene films, have become the mainstream choice due to their excellent electrochemical stability, mechanical strength, and closed-cell characteristics.

[0003] Currently, the main technical routes for preparing polypropylene microporous membranes include dry uniaxial stretching and wet phase separation methods. The dry process primarily involves forming a β-crystal form in polypropylene, followed by micropore generation during stretching through crystal transformation. The wet process, on the other hand, involves mixing liquid hydrocarbons or other diluents into the polypropylene resin as pore-forming agents, which are then extracted and removed after stretching to create pores. Furthermore, to improve the membrane's thermal stability, existing technologies typically coat the polypropylene base membrane with an inorganic ceramic particle layer or add a large amount of inorganic filler during the preparation process as a pore-forming agent and supporting framework.

[0004] However, the aforementioned existing technologies have significant limitations. First, the traditional dry stretching process requires extremely high control over the crystal structure of polypropylene. Insufficient or uneven distribution of β-crystals leads to poor uniformity of the formed pore structure, affecting the consistency and rate performance of the battery. Second, while the wet process can achieve more uniform pores, it has a long process flow, high energy consumption, and the risk of residual organic solvents, posing a potential threat to battery safety. Third, whether adding a large amount of inorganic filler or subsequent ceramic coating, impurities may be introduced, reducing the inherent toughness of the separator and increasing the separator thickness, thus hindering the improvement of battery energy density. Furthermore, the interfacial bonding between the coating and the base film may also affect long-term cycle stability. Therefore, there is an urgent need in this field to develop a novel biaxially oriented polypropylene separator material that can balance excellent pore structure, high thermal stability, and good mechanical properties, along with an efficient and environmentally friendly preparation method. Summary of the Invention

[0005] This invention aims to overcome the aforementioned deficiencies of the prior art and provide a biaxially oriented polypropylene film material for lithium-ion battery separators and its preparation method. The main objective of this invention is to simultaneously obtain separator products with uniform pore size, high thermal stability, and excellent mechanical properties by optimizing material composition, introducing specific β-crystal nucleating agents and pore-forming agents, and combining them with a specific biaxially oriented process. This also simplifies the process flow and improves production efficiency and product consistency.

[0006] To achieve the above objectives, the present invention provides the following technical solution: a biaxially oriented polypropylene film material for lithium battery separators, characterized in that it comprises at least one matrix film layer, wherein the matrix film layer is made from raw materials containing the following components through melt blending, biaxial stretching and heat setting: polypropylene resin: 85-98%; pore-forming agent: 1-10%; antioxidant: 0.1-2%; β-crystal nucleating agent: 0.1-3%; wherein the pore-forming agent is a polymer or inorganic substance that is incompatible with polypropylene and is extracted and removed in subsequent processes.

[0007] This invention also provides a method for preparing biaxially oriented polypropylene film material for lithium battery separators, comprising the following steps: S1: According to the proportions described in claim 1, polypropylene resin, pore-forming agent, antioxidant, and β-crystal nucleating agent are melt-blended to prepare masterbatch or mixture; S2: The masterbatch or mixture obtained in step S1 is melt-plasticized through an extruder and extruded through a die to form a cast sheet; S3: The cast sheet is subjected to longitudinal stretching and transverse stretching, wherein the longitudinal stretching temperature is 100-130°C and the stretching ratio is 4-6; the transverse stretching temperature is 130-155°C and the stretching ratio is 7-10; S4: The biaxially stretched film is subjected to heat setting treatment at a temperature of 140-165°C; S5: The heat-set film is subjected to pore-forming agent extraction and drying treatment to obtain a biaxially oriented polypropylene film with a microporous structure.

[0008] In summary, the present invention has the following main beneficial effects:

[0009] (1) Pore optimization: It improves the pore consistency of the separator, enhances the ionic conductivity and charge / discharge rate performance of the lithium battery, and fundamentally overcomes the problem of uneven pore structure caused by poor crystal form control in traditional dry process.

[0010] (2) Process upgrade: By using specific solvents to selectively extract polymers or inorganic pore-forming agents, the risk of residual harmful solvents such as hydrocarbons is avoided, ensuring the safety and environmental friendliness of the battery, while ensuring the precise control of the pore structure.

[0011] (3) Performance synergy: By designing asymmetric or symmetric multilayer structures, the optimal distribution of porosity and mechanical strength can be achieved in different functional layers, which not only ensures excellent electrolyte wettability, but also enhances the puncture strength and toughness of the diaphragm, avoiding the problems of interface bonding and energy density reduction caused by additional coating or the addition of a large amount of inorganic fillers. Attached Figure Description

[0012] Figure 1 This is a flowchart illustrating the preparation process of the present invention. Detailed Implementation

[0013] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain the present invention, and should not be construed as limiting the present invention.

[0014] This invention provides a biaxially oriented polypropylene film material for lithium battery separators. The biaxially oriented polypropylene film material is composed of the following raw materials by weight percentage, and the pore-forming agent is a polymer or inorganic substance that is incompatible with polypropylene and is extracted and removed in subsequent processes:

[0015] Polypropylene resin: 85-98%;

[0016] Pore-forming agent: 1-10%;

[0017] Antioxidant: 0.1-2%;

[0018] β-crystal nucleating agent: 0.1-3%.

[0019] This formulation range was obtained through extensive experimental verification, achieving a significant improvement in membrane performance. A specific ratio of β-crystal nucleating agent was used to effectively regulate the crystallization behavior of polypropylene. Combined with precisely controlled biaxial stretching temperature and stretching ratio, and heat setting treatment at 140-165℃, a microporous structure with uniform pore size distribution was formed. The basic formulation and process achieved synergistic effects through optimized components.

[0020] In a preferred embodiment, the present invention employs a three-layer composite structure, wherein the pore-forming agent content in the middle layer (8-10%) is significantly higher than that in the surface layer (1-3%). Through co-extrusion and simultaneous biaxial stretching processes, combined with segmented heat setting and extraction with an ethanol-acetone mixed solvent, the diaphragm possesses high porosity (45-50%), excellent puncture strength (>5.0N), and low heat shrinkage (<2.5%), resulting in a more concentrated and uniform pore size distribution.

[0021] This invention also provides the above-mentioned biaxially oriented polypropylene film material for lithium battery separators and its preparation method, such as... Figure 1 As shown, the method includes the following steps:

[0022] S1: According to the proportions described in claim 1, polypropylene resin, pore-forming agent, antioxidant and β-crystal nucleating agent are melt-blended to prepare masterbatch or mixture; this step aims to achieve uniform dispersion of each component, form full fusion of β-crystal nucleating agent and matrix, and lay the material foundation for subsequent molding.

[0023] S2: The masterbatch or mixture obtained in step S1 is melted and plasticized through an extruder, and then extruded through a die to form a casting sheet. The purpose of this step is to obtain a precast casting sheet with uniform thickness and dense structure through melt plasticization and extrusion molding, so as to ensure the stability of the subsequent stretching process.

[0024] S3: The casting is subjected to longitudinal and transverse stretching. The longitudinal stretching temperature is 100-130°C and the stretching ratio is 4-6. The transverse stretching temperature is 130-155°C and the stretching ratio is 7-10. The purpose of this step is to induce β-phase transformation and form microporous structure by biaxial stretching. Longitudinal stretching lays the foundation for molecular chain orientation, and transverse stretching further expands the porosity and improves mechanical properties.

[0025] S4: Heat set the biaxially stretched film at a temperature of 140-165°C. The purpose of this step is to eliminate internal stress, stabilize the crystal structure, reduce the thermal shrinkage rate of the diaphragm, and ensure dimensional stability through heat setting.

[0026] S5: The heat-set film is subjected to pore-forming agent extraction and drying to obtain a biaxially oriented polypropylene film with a microporous structure. The purpose of this step is to selectively remove the pore-forming agent to form interconnected microporous channels, while eliminating solvent residue through drying, ultimately obtaining a membrane product with stable performance.

[0027] As a preferred embodiment of the method of the present invention, a simultaneous biaxial stretching process is adopted, in which longitudinal stretching ratio of 5:1 and transverse stretching ratio of 8:1 are simultaneously carried out at 125±5℃. This scheme completes biaxial orientation in one step, effectively improving production efficiency and avoiding stress concentration problems that may be caused by step stretching, resulting in a more uniform micropore distribution and reducing the pore size variation coefficient to below 15%.

[0028] As another preferred embodiment of the method of the present invention, a three-layer co-extrusion structure is adopted, with a surface layer containing 2% pore-forming agent and an intermediate layer containing 8%. Functional zoning is achieved through differentiated design: the surface layer ensures mechanical strength (puncture strength ≥5.2N), and the intermediate layer provides high porosity (≥48%). This structure employs a two-stage heating process of 155℃ / 160℃ during the heat setting stage, further optimizing the pore size distribution.

[0029] In another preferred embodiment of the method of the present invention, polyvinyl alcohol is selected as the pore-forming agent, and extraction is carried out at 65°C in combination with an ethanol-water mixed solvent (volume ratio 3:1). This system achieves green and environmentally friendly production, with an extraction efficiency of over 98%, and the solvent can be recycled and reused, significantly reducing VOC emissions during the production process while ensuring the accuracy of pore size control.

[0030] To more fully illustrate the technical effects of the present invention, the following comparisons are made through specific embodiments and comparative examples, supplemented by experimental data.

[0031] Example 1: Basic Formula and Process

[0032] This embodiment provides a single-layer biaxially oriented polypropylene film material for lithium battery separators, the raw material composition of which, by weight percentage, is as follows:

[0033] Polypropylene resin: 92%;

[0034] Pore-forming agent (polyvinyl alcohol): 5%;

[0035] Antioxidant (pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate]): 0.5%;

[0036] β-crystal nucleating agent (arylamide compounds): 2.5%;

[0037] The polypropylene resin is isotactic polypropylene, and its melt flow rate is 2.5 g / 10 min at 230℃ / 2.16 kg.

[0038] The preparation method includes the following steps:

[0039] The components are mixed evenly in a high-speed mixer according to the above proportions, and then melt-blended and granulated at 200°C using a twin-screw extruder to prepare masterbatch.

[0040] The masterbatch is fed into a single-screw extruder, melted and plasticized at 210°C, extruded through a T-die, and cooled and shaped by a cooling roller at 25°C to obtain a cast sheet with a thickness of 800μm.

[0041] The casting is first subjected to longitudinal stretching: at 115°C, longitudinal stretching is performed at a stretching ratio of 5:1.

[0042] Next, perform transverse stretching: at 145℃, perform transverse stretching at a stretching ratio of 8:1.

[0043] The biaxially stretched film was heat-set at 155°C for 15 seconds.

[0044] Finally, the membrane was immersed in an ethanol solution at 70°C to extract and remove the pore-forming agent, and then dried with hot air at 50°C to obtain a single-layer biaxially oriented polypropylene microporous membrane with a thickness of 12 μm.

[0045] Example 2: Multilayer Composite Structure

[0046] This embodiment provides a three-layer composite structure biaxially oriented polypropylene film material, the raw material composition of which, by weight percentage, is as follows:

[0047] Surface layer (two layers):

[0048] Polypropylene resin: 95%;

[0049] Pore-forming agent (silica): 2%;

[0050] Antioxidant (tris[2,4-di-tert-butylphenyl]phosphite): 0.5%;

[0051] β-crystal nucleating agent (rare earth complex): 2.5%;

[0052] Intermediate layer:

[0053] Polypropylene resin: 88%;

[0054] Pore-forming agent (polyethylene glycol): 10%;

[0055] Antioxidant (tris[2,4-di-tert-butylphenyl]phosphite): 0.5%;

[0056] β-crystal nucleating agent (rare earth complex): 1.5%;

[0057] The preparation method includes the following steps:

[0058] Masterbatch for the surface layer and intermediate layer were prepared separately.

[0059] Three extruders were used in conjunction with multi-layer co-extrusion dies to co-extrude the three-layer melt at 205°C. The resulting three-layer composite casting was formed by cooling rollers and obtained with a thickness of 1000μm.

[0060] The synchronous bidirectional stretching method is adopted, and the longitudinal ratio is 5:1 and the transverse ratio is 8:1 at 125℃.

[0061] Heat set at 160°C for 20 seconds.

[0062] The pore-forming agent was removed by extraction with acetone at 60°C, and the membrane was dried with hot air at 55°C to obtain a three-layer composite microporous membrane with a thickness of 15 μm.

[0063] Example 3: Implementation methods with different stretching sequences

[0064] The raw material composition in this embodiment is the same as in Embodiment 1, but the stretching step adopts the sequence of first stretching laterally and then stretching longitudinally:

[0065] First, perform transverse stretching at 140℃ with a stretching ratio of 7:1;

[0066] Then, longitudinal stretching was performed at 110℃ with a stretching ratio of 5:1.

[0067] The other steps are the same as in Example 1;

[0068] Example 4: Implementation methods with different extraction solvents

[0069] The raw material composition in this embodiment is the same as in Example 1, but the following different solvent combinations are used in the extraction step:

[0070] Extraction was performed using xylene at 80°C;

[0071] Extraction was performed using chloroform at 40°C;

[0072] Extraction was carried out at 60°C using a mixed solvent of ethanol and acetone (volume ratio 1:1).

[0073] Comparative example: Traditional dry process

[0074] To demonstrate the advantages of this invention, a comparative example is provided:

[0075] The traditional dry uniaxial stretching process is adopted, without adding β-crystal nucleating agent, and β crystals are formed only through thermal phase separation. Uniaxial stretching is carried out at 120℃ with a stretching ratio of 6:1.

[0076] Performance testing

[0077] The performance of the diaphragms prepared in the above embodiments and comparative examples was tested, and the results are shown in the table below:

[0078] Test Project Example 1 Example 2 Example 3 Comparative Example Porosity (%) 45 48 43 38 Pore ​​size distribution (μm) 0.05-0.15 0.04-0.12 0.06-0.16 0.08-0.25 Puncture intensity (N) 4.5 5.2 4.2 3.1 Heat shrinkage rate (90℃, 1h) 2.5% 2.1% 2.8% 8.5% Ionic conductivity (mS / cm) 1.2 1.3 1.1 0.8

[0079] Test results show that the membranes prepared in the various embodiments of the present invention are significantly superior to the comparative examples in terms of pore structure, mechanical properties, thermal stability and ionic conductivity.

[0080] In summary, through formulation and process innovation, a lithium battery separator with uniform microporous structure, excellent thermal stability and high mechanical strength was successfully prepared, while the process was simplified and the production was made cleaner.

[0081] Although embodiments of the present invention have been shown and described, these specific embodiments are merely explanations of the invention and are not intended to limit it. The specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. After reading this specification, those skilled in the art may make modifications, substitutions, and variations to the embodiments as needed without departing from the principles and spirit of the invention, but such modifications, substitutions, and variations are protected by patent law as long as they are within the scope of the claims of the present invention.

Claims

1. A biaxially oriented polypropylene film material for lithium battery separators, characterized in that, It includes at least one substrate film layer, which is formed by melt blending, biaxial stretching and heat setting of raw materials comprising the following components: Polypropylene resin: 85-98%; Pore-forming agent: 1-10%; Antioxidant: 0.1-2%; β-crystal nucleating agent: 0.1-3%; The pore-forming agent is a polymer or inorganic substance that is incompatible with polypropylene and is extracted and removed in a subsequent process.

2. The biaxially oriented polypropylene film material for lithium battery separators according to claim 1, characterized in that, The pore-forming agent is selected from one or a combination of several of polyvinyl alcohol, polyethylene glycol, polyethylene oxide, silicon dioxide, and calcium carbonate.

3. The biaxially oriented polypropylene film material for lithium battery separators according to claim 1, characterized in that, The polypropylene resin is standard polypropylene, and its melt flow rate is 1.0-5.0 g / 10 min at 230°C / 2.16 kg.

4. The biaxially oriented polypropylene film material for lithium battery separators according to claim 1, characterized in that, The β-crystal nucleating agent is one of aryl amide compounds, rare earth complexes, or quinacridone pigments.

5. The biaxially oriented polypropylene film material for lithium battery separators according to any one of claims 1 to 4, characterized in that, The thin film material is a multilayer composite structure, including at least two matrix thin film layers, and the type and / or content of the pore-forming agent in at least one layer is different from that in the other layers.

6. The biaxially oriented polypropylene film material for lithium battery separators according to claim 5, characterized in that, The multilayer composite structure is a three-layer symmetrical structure, wherein the pore-forming agent content in the middle layer is higher than that in the two outer layers.

7. A method for preparing a biaxially oriented polypropylene film material for lithium battery separators as described in any one of claims 1 to 6, characterized in that, Includes the following steps: S1: According to the proportions described in claim 1, polypropylene resin, pore-forming agent, antioxidant and β-crystal nucleating agent are melt-blended to prepare masterbatch or mixture; S2: The masterbatch or mixture obtained in step S1 is melted and plasticized through an extruder, and then extruded through a die to form a casting sheet; S3: The casting sheet is subjected to longitudinal and transverse stretching. The longitudinal stretching temperature is 100-130°C and the stretching ratio is 4-6. The transverse stretching temperature is 130-155°C and the stretching ratio is 7-10. S4: Heat-set the biaxially stretched film at a temperature of 140-165°C. S5: The heat-set film is subjected to pore-forming agent extraction and drying treatment to obtain a biaxially oriented polypropylene film with a microporous structure.

8. The method according to claim 7, characterized in that, In step S2, when preparing a multilayer composite film, co-extrusion technology is used to combine the melts of different ratios and then extrude them from the die.

9. The method according to claim 7, characterized in that, In step S3, the order of longitudinal stretching and transverse stretching is either longitudinal stretching followed by transverse stretching, or transverse stretching followed by longitudinal stretching, or bidirectional stretching simultaneously.

10. The method according to claim 7, characterized in that, In step S5, the extraction of the pore-forming agent is carried out using an organic solvent at 40-80°C, wherein the organic solvent is selected from one or more of ethanol, acetone, xylene or chloroform.

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