A polymer blend membrane, its preparation method and application

Abstract

This application belongs to the field of lithium battery separator technology, and more specifically, relates to a polymer blend separator, its preparation method, and its application. The polymer blend separator provided in this application is prepared by a wet process using an electrochemically active polymer, polyethylene, and a pore-forming agent. Introducing the electrochemically active polymer enhances the electrolyte affinity of the separator, improves its ionic conductivity, promotes lithium-ion transport, increases the lithium-ion transference number, and improves the electrochemical performance of the battery while effectively inhibiting lithium dendrite growth and enhancing battery safety. Furthermore, the melting point of the electrochemically active polymer is lower than that of polyethylene but higher than the heat-setting temperature in the wet process. When the internal temperature of the battery rises, the electrochemically active polymer in the separator can melt and close the pores before the polyethylene shrinks, effectively preventing electrochemical reactions, reducing heat generation, preventing the spread of thermal runaway, and further improving battery safety.

Description

Technical Field

[0001] This application belongs to the field of lithium battery separator technology, and more specifically, relates to a polymer blend separator, its preparation method and application. Background Technology

[0002] Lithium-ion batteries have become one of the most popular rechargeable batteries due to their high energy density and long cycle life. Commercial lithium-ion batteries mainly consist of a positive electrode, a negative electrode, an electrolyte, and a separator. The separator plays a crucial role in isolating the positive and negative electrodes and providing a transport channel for lithium ions, making it a key component that determines the safety and electrochemical performance of lithium-ion batteries.

[0003] Commercial polyolefin separators exhibit significant resistance to lithium-ion migration, limiting improvements in battery electrochemical performance. Furthermore, their insufficient lithium-ion transport capacity and low ion transference number make it difficult to effectively suppress lithium dendrite growth, increasing the risk of short circuits and thermal runaway, thus restricting improvements in lithium battery safety. Coating the surface of polyolefin separators with hydrophilic, high-strength, or high-temperature-resistant polymers or ceramics such as polydopamine, polyacrylic acid, alumina, or silica can improve electrolyte affinity and enhance mechanical and thermal properties. However, this coating modification increases separator thickness and may even reduce porosity. Additionally, the polymer coating layer is prone to peeling, leading to increased internal resistance and limiting performance improvements.

[0004] The normal operating temperature of lithium batteries is typically between -20°C and 60°C. However, unpredictable mechanical, electrical, or thermal abuse during battery use can cause the internal temperature of the battery to rise. When the internal temperature rises to 70°C–90°C, the solid electrolyte interface layer between the electrode material and the electrolyte will rapidly decompose. The accompanying side reactions will release a large amount of heat, causing the internal temperature of the battery to rise further. If the electrochemical reaction cannot be effectively stopped, the continuous accumulation of heat will cause the internal temperature of the battery to rise rapidly, leading to electrolyte decomposition, polyolefin membrane melting, electrode decomposition, and finally thermal runaway, resulting in battery fire and explosion. Coating the surface of the polyolefin membrane with a low-melting-point polymer can give the membrane thermally closed-pore properties, but the low-melting-point polymer coating layer is prone to pore blockage and peeling, affecting the battery's cycle capacity and cycle stability.

[0005] Preparing membranes that combine high electrochemical performance and high safety is an urgent problem to be solved in this field. Summary of the Invention

[0006] In view of the shortcomings of the prior art, the purpose of this application is to provide a polymer blend separator, its preparation method and application, which aims to solve the problems of weak lithium-ion transport capacity and insufficient thermal safety of existing separators.

[0007] To achieve the above objectives, this application provides a polymer blend membrane, which is prepared by a wet process from an electrochemically active polymer, polyethylene, and a pore-forming agent;

[0008] The aforementioned electrochemically active polymers are one or more of maleic anhydride-grafted polyethylene, ethylene-maleic anhydride copolymer, ethylene-acrylic acid copolymer, sulfonated polyethylene, and benzenesulfonic acid-grafted polyethylene.

[0009] The melting point of the aforementioned electrochemically active polymer is lower than that of the aforementioned polyethylene and higher than the heat setting temperature in the aforementioned wet process, which allows the polymer blend membrane to maintain its pore structure intact during normal battery operation. When the internal temperature of the battery rises, the electrochemically active polymer in the polymer blend membrane can melt before the polyethylene shrinks, thus closing the membrane pore structure.

[0010] Preferably, the melting point of the above-mentioned electrochemically active polymer is 75°C to 115°C.

[0011] Preferably, the electrochemically active polymer contains electrochemically active groups, which are one or more of maleic anhydride structural units, acrylic acid structural units, sulfonic acid groups, and benzenesulfonic acid structural units; the mass percentage of each of the electrochemically active groups is independently 1% to 60%.

[0012] Preferably, the viscosity-average molecular weight of the polyethylene is 500,000 to 5,800,000.

[0013] Preferably, the pore-forming agent is one or more of liquid paraffin, decahydronaphthalene, di-n-octyl phthalate, dodecanol, peanut oil, and soybean oil.

[0014] Preferably, the mass ratio of the electrochemically active polymer, the polyethylene and the porogen is (1-10):10:(10-80).

[0015] This application also provides a method for preparing the above-mentioned polymer blend membrane, comprising the following steps:

[0016] S1. The above-mentioned electrochemically active polymer, polyethylene and pore-forming agent are mixed and then cast or molded to obtain a blended gel sheet;

[0017] S2. The above-mentioned blended gel sheet is biaxially stretched to obtain a blended oil film;

[0018] S3. Extract, wash, dry and heat-set the oil film of the above blend to obtain a polymer blend diaphragm.

[0019] Preferably, in step S1, the mixing temperature is 140℃~180℃, the rotation speed is 20rpm~120rpm, and the time is 10min~40min.

[0020] Preferably, the thickness of the above-mentioned blended gel sheet is 0.5 mm to 2 mm.

[0021] Preferably, in step S2, the stretching ratio in the transverse and longitudinal directions of the above biaxial stretching is 4×4 to 10×10, such as 4×4, 5×5, 5×8, 6×6, 8×8, and 10×10.

[0022] Preferably, the temperature for biaxial stretching is 70°C to 105°C, which is lower than the melting point of the electrochemically active polymer.

[0023] Preferably, in step S3, the reagents used for extraction and washing are one or more of n-hexane and dichloromethane, which can effectively remove pore-forming agents.

[0024] Preferably, in step S3, the heat setting temperature is 70°C to 105°C, which is lower than the melting point of the electrochemically active polymer.

[0025] Preferably, the heat setting time is 1 min to 3 min.

[0026] This application also provides a battery separator material comprising the above-mentioned polymer blend separator.

[0027] In summary, compared with the prior art, the technical solutions conceived in this application have the following main technical advantages:

[0028] (1) The polymer blend separator provided in this application is prepared by a wet process using electrochemically active polymers, polyethylene and pore-forming agents. By introducing electrochemically active polymers such as maleic anhydride-grafted polyethylene, ethylene-maleic anhydride copolymer, ethylene-acrylic acid copolymer, sulfonated polyethylene and benzenesulfonic acid-grafted polyethylene into the separator, the separator has lithium-ion conduction characteristics similar to solid electrolytes. This can enhance the electrolyte affinity of the separator, effectively improve the ionic conductivity of the separator, promote lithium-ion transport, increase the lithium-ion transference number, improve the electrochemical performance of the battery, and effectively inhibit lithium dendrite growth, thereby improving the safety of the battery. Furthermore, the melting point of the aforementioned electrochemically active polymer is lower than that of polyethylene but higher than the heat-setting temperature in the wet process. During normal battery operation, the electrochemically active polymer in the separator does not melt, and the separator's pore structure remains intact. When the internal temperature of the battery rises to near the thermal runaway temperature, the electrochemically active polymer in the separator can melt before the polyethylene skeleton shrinks, thereby closing the separator's pore structure, effectively preventing electrochemical reactions, reducing heat generation, preventing the spread of thermal runaway, and further improving battery safety. The polymer blend separator provided in this application combines high electrochemical performance and high safety, and has broad application prospects.

[0029] (2) This application employs a blending-biaxial stretching wet process to prepare a polymer blend separator that combines high electrochemical performance and high safety, solving the problems of easy pore blockage and peeling of traditional coated polymer coatings. The preparation method provided in this application has the advantages of simple process and high production efficiency, which facilitates large-scale production and application. At the same time, the polymer blend separator prepared and applied to batteries can effectively improve the cycle performance and cycle stability of batteries, and has high safety. Attached Figure Description

[0030] Figure 1 These are scanning electron microscope (SEM) images of the polymer blend membrane prepared in Example 1 of this application before and after thermal closure at room temperature and at 120°C, respectively. Content (a) is before thermal closure at room temperature and content (b) is after thermal closure at 120°C.

[0031] Figure 2 These are scanning electron microscope images of the membranes prepared in Comparative Examples 1 and 2 of this application, wherein content (a) is Comparative Example 1 and content (b) is Comparative Example 2.

[0032] Figure 3 This is a scanning electron microscope image of the polymer blend membrane prepared in Comparative Example 3 of this application at room temperature;

[0033] Figure 4 These are images of the blended gel sheet prepared in Comparative Example 4 of this application during the stretching process;

[0034] Figure 5 The lithium metal || lithium iron phosphate battery obtained by assembling the separator prepared in Example 1 and Comparative Example 1 of this application is the cycle performance at 1C. Detailed Implementation

[0035] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application.

[0036] Commercially available polyolefin separators suffer from poor electrolyte affinity and insufficient lithium-ion transport capacity, resulting in low ionic conductivity and low lithium-ion transference numbers. Furthermore, commercially available polyolefin separators lack thermally closed-cell properties. When the battery ambient temperature approaches the melting point of polyolefins (polyethylene has a melting point of approximately 135°C), the commercially available polyolefin separator melts and shrinks, leading to direct contact between the positive and negative electrodes, which can cause short circuits and thermal runaway, resulting in poor battery safety. Therefore, this application provides a polymer blend separator prepared by a wet process from an electrochemically active polymer, polyethylene, and a pore-forming agent.

[0037] The aforementioned electrochemically active polymers are one or more of maleic anhydride-grafted polyethylene, ethylene-maleic anhydride copolymer, ethylene-acrylic acid copolymer, sulfonated polyethylene, and benzenesulfonic acid-grafted polyethylene.

[0038] The melting point of the aforementioned electrochemically active polymer is lower than that of polyethylene and higher than the heat setting temperature in the aforementioned wet process. This allows the polymer blend membrane to maintain its pore structure intact during normal battery operation. When the internal temperature of the battery rises, the electrochemically active polymer in the polymer blend membrane can melt before the polyethylene shrinks, thus closing the membrane pore structure.

[0039] The polymer blend separator provided in this application has a single-layer porous structure. The electrochemically active polymer enhances the electrolyte affinity of the separator, effectively improving its ionic conductivity. It also promotes lithium-ion transport, increases the lithium-ion transference number, and enhances the battery's electrochemical performance while effectively suppressing lithium dendrite growth, thus improving battery safety. Furthermore, the electrochemically active polymer in the separator does not melt during normal battery operation, maintaining the intact pore structure. When the internal temperature of the battery rises to near thermal runaway, the electrochemically active polymer melts before the polyethylene skeleton shrinks, causing the polymer blend separator's pore structure to close. This effectively prevents electrochemical reactions, reduces heat generation, and prevents the spread of thermal runaway, significantly improving battery safety. This separator combines high electrochemical performance with high safety.

[0040] In some embodiments, the melting point of the electrochemically active polymer is 75°C to 115°C.

[0041] In some embodiments, the electrochemically active polymer contains electrochemically active groups, which are one or more of maleic anhydride structural units, acrylic acid structural units, sulfonic acid groups, and benzenesulfonic acid structural units. The mass percentage of each of the electrochemically active groups is independently 1% to 60%, which can increase the lithium-ion transport capacity of the separator, improve the ionic conductivity and lithium-ion transference number of the separator, and at the same time meet the operating temperature requirements of the separator. During normal battery operation, the pore structure of the separator remains intact and will not melt and close the pores.

[0042] In some embodiments, the above-mentioned polyethylene has a viscosity-average molecular weight of 500,000 to 5.8 million, which can play a skeleton role in the diaphragm, meet the requirements of the biaxial stretching process in the wet process for the tensile properties of the material, ensure the normal progress of biaxial stretching, maintain the structural stability of the diaphragm, and has good thermal properties and excellent mechanical properties.

[0043] In some embodiments, the pore-forming agent is one or more of liquid paraffin, decahydronaphthalene, di-n-octyl phthalate, dodecanol, peanut oil, and soybean oil.

[0044] In some embodiments, the mass ratio of the electrochemically active polymer, polyethylene and pore-forming agent is (1-10):10:(10-80), and the components are mutually soluble to form a homogeneous composite system, which makes the prepared blended gel sheet have good extensibility and can be prepared by biaxial stretching process to obtain a polymer blended membrane with a single-layer porous structure, high ionic conductivity, high lithium ion transference number and moderate porosity.

[0045] This application also provides a method for preparing the above-mentioned polymer blend membrane, comprising the following steps:

[0046] S1. The above-mentioned electrochemically active polymer, polyethylene and pore-forming agent are mixed and then cast or molded to obtain a blended gel sheet;

[0047] S2. The above-mentioned blended gel sheet is biaxially stretched to obtain a blended oil film;

[0048] S3. Extract, wash, dry and heat-set the oil film of the above blend to obtain a polymer blend diaphragm.

[0049] In some embodiments, in step S1, the mixing temperature is 140°C to 180°C, the rotation speed is 20 rpm to 120 rpm, and the time is 10 min to 40 min.

[0050] In some embodiments, the thickness of the above-mentioned blended gel sheet is 0.5 mm to 2 mm.

[0051] In some embodiments, in step S2, the biaxial stretching can be biaxial synchronous stretching or biaxial asynchronous stretching, with a stretching ratio of 4×4 to 10×10 in the transverse and longitudinal directions, for example, but not limited to, 4×4, 5×5, 5×8, 6×6, 8×8, and 10×10. In some embodiments, the biaxial stretching temperature is 70°C to 105°C, which is lower than the melting point of the electrochemically active polymer.

[0052] In some embodiments, in step S3, the reagents used for extraction and washing are one or more of n-hexane and dichloromethane, which can effectively remove pore-forming agents from the membrane.

[0053] In some embodiments, in step S3, the drying temperature is higher than the boiling point of the washing reagent and lower than the melting point of the electrochemically active polymer, for example, 40°C to 70°C, and the drying time can be 10 min to 30 min. Those skilled in the art can adjust the drying temperature and time according to the actual situation, all of which are within the protection scope of this application.

[0054] In some embodiments, in step S3, the heat-setting temperature is 70°C to 105°C, which is lower than the melting point of the electrochemically active polymer. This eliminates residual stress inside the biaxially stretched separator and prevents the separator structure from collapsing. Experiments have shown that when the heat-setting temperature is too low, the separator structure collapses, making it unsuitable for use in batteries.

[0055] In some embodiments, the heat setting time in step S3 is 1 min to 3 min.

[0056] The polymer blend separator prepared in this application does not melt during normal battery operation. When the internal temperature of the battery rises to near thermal runaway, the electrochemically active polymer can melt before the polyethylene skeleton shrinks, thereby closing the separator pore structure, preventing thermal runaway from spreading, and improving battery safety.

[0057] This application also provides a battery separator material, including the above-mentioned polymer blend separator.

[0058] In some embodiments, the battery separator material is a lithium battery separator material.

[0059] It should be understood that materials of the same or similar type, model, quality, properties, or function as the reagents and instruments used in the following embodiments can be used to implement this application. Unless otherwise specified, the experimental methods used in the following embodiments are conventional methods. Unless otherwise specified, the materials and reagents used in the following embodiments are commercially available.

[0060] The following are examples and comparative examples:

[0061] Example 1

[0062] The method for preparing the polymer blend membrane provided in this embodiment includes the following steps:

[0063] (1) Ethylene-acrylic acid copolymer (melting point 103℃, mass percentage of acrylic structural unit 6.5%, brand name SK Primacor 1321), polyethylene (viscosity average molecular weight 1.1 million, brand name Mitsui HI-ZEXMILLION 150M), and liquid paraffin were mixed in a mass ratio of 2.5:10:30 in a mixer with a chamber temperature of 160℃ and a shear rate of 50rpm for 10min. Then, after molding and cooling, a blend gel sheet with a thickness of 1.0mm was obtained.

[0064] (2) The above-mentioned blend gel sheet was subjected to biaxial synchronous stretching using a biaxial stretching machine at a temperature of 100℃ and a stretching ratio of 6×6 in the transverse and longitudinal directions to obtain a blend oil film.

[0065] (3) The oil film of the above blend was fully extracted and washed with dichloromethane, dried at 40°C for 20 min and then fixed in a clamping mold. It was then heat-set at 100°C for 2 min to obtain a polymer blend membrane with a single-layer porous structure.

[0066] The prepared polymer blend membrane was placed in a 60℃ oven for 15 min. The surface morphology of the membrane before heat closure at room temperature and after heat closure at 60℃ was observed using field emission scanning electron microscopy. It was found that the pores on the membrane surface did not melt. Figure 1 As shown in content (a), the above-mentioned separator can meet the working temperature requirements and maintains its pore structure intact during normal battery operation, without melting and closing the pores. The above-mentioned polymer blend separator was heat-sealed in a 120°C oven for 15 minutes, and the surface morphology of the separator after heat-sealing was observed using a field emission scanning electron microscope, as shown... Figure 1 As shown in section (b), after a 120°C heat-sealing treatment, the pores on the surface of the polymer blend separator completely disappear, indicating that the polymer blend separator has a heat-sealing function. When the internal temperature of the battery rises, the electrochemically active polymer in the separator can melt before the polyethylene skeleton shrinks, thereby closing the separator pore structure, effectively preventing electrochemical reactions, reducing heat generation, preventing the spread of thermal runaway, and improving battery safety.

[0067] Example 2

[0068] The method for preparing the polymer blend membrane provided in this embodiment includes the following steps:

[0069] (1) Ethylene-acrylic acid copolymer (melting point 77℃, mass percentage of acrylic structural unit 20.5%, brand name SK Primacor 5980I), polyethylene (viscosity average molecular weight 1.1 million, brand name Mitsui HI-ZEXMILLION 150M) and liquid paraffin were mixed in a mass ratio of 2.5:10:30 in a mixer with a chamber temperature of 160℃ and a shear rate of 50rpm for 10min. Then, after molding and cooling, a blend gel sheet with a thickness of 1.0mm was obtained.

[0070] (2) The above-mentioned blend gel sheet was subjected to biaxial synchronous stretching using a biaxial stretching machine at a temperature of 75℃ and a stretching ratio of 6×6 in the transverse and longitudinal directions to obtain a blend oil film.

[0071] (3) The oil film of the above blend was fully extracted and washed with dichloromethane, dried at 50°C for 10 min and then fixed in a clamping mold. It was then heat-set at 70°C for 2 min to obtain a polymer blend membrane with a single-layer porous structure.

[0072] The prepared polymer blend membrane was placed in a 120°C oven for heat sealing treatment for 15 minutes. It was found that the pores on the surface of the membrane completely disappeared, indicating that the polymer blend membrane has heat sealing function.

[0073] Example 3

[0074] The method for preparing the polymer blend membrane provided in this embodiment includes the following steps:

[0075] (1) Ethylene-acrylic acid copolymer (melting point 104℃, mass percentage of acrylic structural unit 3.0%, brand name SK Primacor 3150), polyethylene (viscosity average molecular weight 5.8 million, brand name Mitsui HI-ZEXMILLION 630M), liquid paraffin and peanut oil were mixed in an extruder with a cavity temperature of 170℃ and a shear rate of 20rpm for 30min at a mass ratio of 1:10:8:2. Then, after casting and cooling, a blend gel sheet with a thickness of 1.6mm was obtained.

[0076] (2) The above-mentioned blend gel sheet was subjected to biaxial synchronous stretching using a biaxial stretching machine at a temperature of 100℃ and a stretching ratio of 8×8 in the transverse and longitudinal directions to obtain a blend oil film.

[0077] (3) The oil film of the above blend was fully extracted and washed with n-hexane, dried at 70°C for 10 min and then fixed in a clamping mold. It was then heat-set at 95°C for 2 min to obtain a polymer blend membrane with a single-layer porous structure.

[0078] The prepared polymer blend membrane was placed in a 120°C oven for heat sealing treatment for 15 minutes. It was found that the pores on the surface of the membrane completely disappeared, indicating that the polymer blend membrane has heat sealing function.

[0079] Example 4

[0080] The method for preparing the polymer blend membrane provided in this embodiment includes the following steps:

[0081] (1) Ethylene-acrylic acid copolymer (melting point 75℃, mass percentage of acrylic structural unit 20.0%, brand name SK Primacor 5990I), polyethylene (viscosity average molecular weight 3.3 million, brand name Mitsui HI-ZEXMILLION 320MU), and di-n-octyl phthalate were mixed in an extruder with a cavity temperature of 160℃ and a shear rate of 80rpm for 20min at a mass ratio of 3:10:25. After casting and cooling, a blend gel sheet with a thickness of 0.8mm was obtained.

[0082] (2) The above-mentioned blend gel sheet was subjected to biaxial synchronous stretching using a biaxial stretching machine at a temperature of 75℃ and a stretching ratio of 6×6 in the transverse and longitudinal directions to obtain a blend oil film.

[0083] (3) The oil film of the above blend was fully extracted and washed with dichloromethane, dried at 50°C for 10 min and then fixed in a clamping mold. It was then heat-set at 70°C for 1 min to obtain a polymer blend membrane with a single-layer porous structure.

[0084] The prepared polymer blend membrane was placed in a 120°C oven for heat sealing treatment for 15 minutes. It was found that the pores on the surface of the membrane completely disappeared, indicating that the polymer blend membrane has heat sealing function.

[0085] Example 5

[0086] The method for preparing the polymer blend membrane provided in this embodiment includes the following steps:

[0087] (1) Ethylene-maleic anhydride copolymer (melting point 108℃, melt index 25g / 10min (test conditions 190℃ / 2.16kg), maleic anhydride structural unit mass percentage 1.0%~3.0%, brand DOW FUSABONDM603), polyethylene (viscosity average molecular weight 5.8 million, brand Mitsui HI-ZEX MILLION 630M), and soybean oil were mixed in a mass ratio of 5:10:40 in a mixer with a chamber temperature of 180℃ and a shear rate of 30rpm for 30min. Then, after molding and cooling, a blend gel sheet with a thickness of 0.8mm was obtained.

[0088] (2) The above-mentioned blend gel sheet was subjected to biaxial synchronous stretching using a biaxial stretching machine at a temperature of 105℃ and a stretching ratio of 4×4 in the transverse and longitudinal directions to obtain a blend oil film.

[0089] (3) The oil film of the above blend was fully extracted and washed with n-hexane, dried at 70°C for 10 min and then fixed in a clamping mold. It was then heat-set at 100°C for 1 min to obtain a polymer blend membrane with a single-layer porous structure.

[0090] The prepared polymer blend membrane was placed in a 120°C oven for heat sealing treatment for 15 minutes. It was found that the pores on the surface of the membrane completely disappeared, indicating that the polymer blend membrane has heat sealing function.

[0091] Example 6

[0092] The method for preparing the polymer blend membrane provided in this embodiment includes the following steps:

[0093] (1) Reference (Wu Xiangguo. Polyethylene wax grafting modification and its application. Qingdao University of Science and Technology, 2021) Maleic anhydride grafted polyethylene (melting point 110℃, maleic anhydride grafting mass percentage 3.4%) was prepared. Then, maleic anhydride grafted polyethylene, polyethylene (viscosity average molecular weight 500,000, brand name Mitsui HI-ZEX MILLION 030S) and decahydronaphthalene were placed in an extruder with a cavity temperature of 140℃ and a shear rate of 20rpm at a mass ratio of 10:10:50 and mixed for 40min. After casting and cooling, a blend gel sheet with a thickness of 1.4mm was obtained.

[0094] (2) The above-mentioned blend gel sheet was subjected to biaxial asynchronous stretching using a biaxial stretching machine at a temperature of 105℃ and a stretching ratio of 8×8 in the transverse and longitudinal directions to obtain a blend oil film.

[0095] (3) The above blend oil film was fully extracted and washed with n-hexane, dried at 70°C for 10 min and then fixed in a clamping mold. It was then heat-set at 100°C for 1 min to obtain a polymer blend membrane with a single-layer porous structure.

[0096] The prepared polymer blend membrane was placed in a 120°C oven for heat sealing treatment for 15 minutes. It was found that the pores on the surface of the membrane completely disappeared, indicating that the polymer blend membrane has heat sealing function.

[0097] Example 7

[0098] The method for preparing the polymer blend membrane provided in this embodiment includes the following steps:

[0099] (1) Reference (Polymer Engineering and Science, 2013, 54(11):2522-2530) Sulfonated polyethylene was prepared, and then sulfonated polyethylene (melting point of 104℃, mass percentage of sulfonic acid group of about 13%), polyethylene (viscosity average molecular weight of 5.8 million, brand name Mitsui HI-ZEX MILLION 630M), liquid paraffin and di-n-octyl phthalate were placed in an extruder with a cavity temperature of 160℃ and a shear rate of 100rpm and mixed for 20min. After casting and cooling, a blend gel sheet with a thickness of 2.0mm was obtained.

[0100] (2) The above-mentioned blend gel sheet was subjected to biaxial asynchronous stretching using a biaxial stretching machine at a temperature of 100℃ and a stretching ratio of 10×10 in the transverse and longitudinal directions to obtain a blend oil film.

[0101] (3) The oil film of the above blend was fully extracted and washed with dichloromethane, dried at 40°C for 20 min and then fixed in a clamping mold. It was then heat-set at 95°C for 3 min to obtain a polymer blend membrane with a single-layer porous structure.

[0102] The prepared polymer blend membrane was placed in a 120°C oven for heat sealing treatment for 15 minutes. It was found that the pores on the surface of the membrane completely disappeared, indicating that the polymer blend membrane has heat sealing function.

[0103] Example 8

[0104] The method for preparing the polymer blend membrane provided in this embodiment includes the following steps:

[0105] (1) Sulfonated polyethylene (same as in Example 7), polyethylene (viscosity average molecular weight of 500,000, brand name Mitsui HI-ZEXMILLION 030S), and dodecanol were mixed in a mass ratio of 10:10:70 in a mixer with a chamber temperature of 150°C and a shear rate of 60 rpm for 30 min. Then, after molding and cooling, a blend gel sheet with a thickness of 1.6 mm was obtained.

[0106] (2) The above-mentioned blend gel sheet was subjected to biaxial asynchronous stretching using a biaxial stretching machine at a temperature of 100℃ and a stretching ratio of 8×8 in the transverse and longitudinal directions to obtain a blend oil film.

[0107] (3) The oil film of the above blend was fully extracted and washed with n-hexane, dried at 70°C for 10 min and then fixed in a clamping mold. It was then heat-set at 95°C for 2 min to obtain a polymer blend membrane with a single-layer porous structure.

[0108] The prepared polymer blend membrane was placed in a 120°C oven for heat sealing treatment for 15 minutes. It was found that the pores on the surface of the membrane completely disappeared, indicating that the polymer blend membrane has heat sealing function.

[0109] Example 9

[0110] The method for preparing the polymer blend membrane provided in this embodiment includes the following steps:

[0111] (1) Sulfonated polyethylene (same as in Example 7), polyethylene (viscosity average molecular weight of 1.1 million, brand name Mitsui HI-ZEXMILLION 150M), liquid paraffin, and decahydronaphthalene were mixed in an extruder with a cavity temperature of 150°C and a shear rate of 30 rpm for 25 min at a mass ratio of 6:10:40:10. After casting and cooling, a blend gel sheet with a thickness of 0.5 mm was obtained.

[0112] (2) The above-mentioned blend gel sheet was subjected to biaxial synchronous stretching using a biaxial stretching machine at a temperature of 100℃ and a stretching ratio of 5×5 in the transverse and longitudinal directions to obtain a blend oil film.

[0113] (3) The above blend oil film was fully extracted and washed using a mixed solvent of n-hexane and dichloromethane with a mass ratio of 1:1. After drying at 70°C for 20 min, it was fixed in a clamping mold and heat-set at 95°C for 2 min to obtain a polymer blend membrane with a single-layer porous structure.

[0114] The prepared polymer blend membrane was placed in a 120°C oven for heat sealing treatment for 15 minutes. It was found that the pores on the surface of the membrane completely disappeared, indicating that the polymer blend membrane has heat sealing function.

[0115] Example 10

[0116] The method for preparing the polymer blend membrane provided in this embodiment includes the following steps:

[0117] (1) Reference (International Journal of Polymeric Materials, 2004, 53(12): 1027-1043) Styrene sulfonic acid grafted polyethylene was prepared. Then, styrene sulfonic acid grafted polyethylene (melting point of 106℃, mass percentage of styrene sulfonic acid structural unit of about 60%), polyethylene (viscosity average molecular weight of 2 million, brand name MitsuiMILPELON XM330) and liquid paraffin were placed in an extruder with a cavity temperature of 180℃ and a shear rate of 120rpm at a mass ratio of 8:10:60 and mixed for 20min. After casting and cooling, a blend gel sheet with a thickness of 1.4mm was obtained.

[0118] (2) The above-mentioned blend gel sheet was subjected to biaxial asynchronous stretching using a biaxial stretching machine at a temperature of 100℃ and a stretching ratio of 8×8 in the transverse and longitudinal directions to obtain a blend oil film.

[0119] (3) The oil film of the above blend was fully extracted and washed with dichloromethane, dried at 40°C for 20 min and then fixed in a clamping mold. It was then heat-set at 95°C for 1 min to obtain a polymer blend membrane with a single-layer porous structure.

[0120] The prepared polymer blend membrane was placed in a 120°C oven for heat sealing treatment for 15 minutes. It was found that the pores on the surface of the membrane completely disappeared, indicating that the polymer blend membrane has heat sealing function.

[0121] Comparative Example 1

[0122] The preparation method of the diaphragm provided in this comparative example is the same as that in Example 1, except that no electrochemically active polymer is used in step (1), resulting in a polyethylene diaphragm. The surface morphology of the polyethylene diaphragm was observed using a field emission scanning electron microscope, as shown below. Figure 2 As shown in content (a).

[0123] Comparative Example 2

[0124] The method for preparing the diaphragm provided in this comparative example includes the following steps:

[0125] Using di-n-octyl phthalate as a pore-forming agent and tetrahydrofuran as a solvent, an ethylene-acrylic acid copolymer (same as in Example 1) was dissolved in a mixture of di-n-octyl phthalate and tetrahydrofuran to prepare a coating solution. The mass ratio of di-n-octyl phthalate to tetrahydrofuran in the mixture was 1:1. This coating solution was then coated onto the surface of a commercial polyethylene separator. After solvent evaporation, washing, and drying, an ethylene-acrylic acid copolymer-coated polyethylene separator was obtained. The surface morphology of the coated modified polyethylene separator was observed using field emission scanning electron microscopy. Figure 2 As shown in content (b).

[0126] Comparative Example 3

[0127] The preparation method of the diaphragm provided in this comparative example is the same as that in Example 1, wherein the heat setting temperature in step (3) is 110°C, and a polymer blend diaphragm is obtained.

[0128] The above-mentioned polymer blended separator was heat-sealed in a 120°C oven for 15 minutes. The surface morphology of the separator before heat sealing at room temperature and after heat sealing at 120°C was observed using a field emission scanning electron microscope. Figure 3 As shown, after the above-mentioned separator was heat-sealed at 120°C for 15 minutes, the pores on the surface of the separator did not change significantly. That is, when the battery experiences thermal runaway, the above-mentioned polymer blend separator does not have the ability to melt and close the pores, and does not meet the requirements for battery safety.

[0129] Comparative Example 4

[0130] The preparation method of the diaphragm provided in this comparative example is the same as that in Example 5, wherein the mass ratio of ethylene-maleic anhydride copolymer, polyethylene and soybean oil in step (1) is 8:7:40.

[0131] Experiments revealed that during the biaxial synchronous stretching of the blend gel sheets using a biaxial stretching machine, cracking occurred (e.g., Figure 4 It is impossible to prepare a blend oil film with a complete morphology.

[0132] The ionic conductivity of the above-mentioned membrane was tested using the AC impedance method, and the lithium-ion transference number of the above-mentioned membrane was measured using a combination of steady-state current and AC impedance.

[0133] The above-mentioned separator was assembled into a lithium metal / lithium iron phosphate battery. The assembly process was as follows: In a glove box, the prepared separator was placed between the lithium metal negative electrode and the lithium iron phosphate positive electrode, and 30 μL of electrolyte was added to assemble a CR2032 coin cell. The lithium metal negative electrode had a diameter of 15 mm, a thickness of 1 mm, and a purity of 99.95%; the lithium iron phosphate positive electrode had a diameter of 12 mm and a lithium iron phosphate loading of 10.5 mg·cm³. -2 The electrolyte contains ethylene carbonate and diethyl carbonate in a volume ratio of 1:1, and the lithium salt, lithium hexafluorophosphate, has a concentration of 1 mol·L⁻¹. -1 The film-forming additive, fluoroethylene carbonate, had a mass percentage of 10%. The battery performance was tested using a CT2001A Blue Electric testing system from Wuhan Blue Electric Electronics Co., Ltd., with a test voltage range of 2.5V to 3.8V and a temperature of 30℃. The test results are shown in Table 1. The cycle performance of the lithium metal / lithium iron phosphate battery at 1C is shown in Table 1. Figure 5 .

[0134] Table 1 shows the performance of the separators prepared in the examples and comparative examples, and the performance of the assembled batteries.

[0135] performance Example 1 Comparative Example 1 Comparative Example 2 Lithium-ion transference number 0.47 0.31 0.42 <![CDATA[Ionic conductivity (mS·cm -1 )]]> 0.61 0.51 0.10 <![CDATA[Initial discharge specific capacity (mAh·g -1 )]]> 157.2 155.4 / <![CDATA[Discharge specific capacity after 200 cycles (mAh·g -1 )]]> 86.5 38.3 / Capacity retention after 200 cycles 55.0％ 24.6％ / Coulomb efficiency for 200 cycles 99.2％ 99.5％ /

[0136] Figure 1 Content (a), Figure 1 Content (b) shows scanning electron microscope (SEM) images of the polymer blend separator prepared in Example 1 before and after thermal closure at room temperature and at 120°C, respectively. It can be seen that the polymer blend separator has a uniform porous structure before thermal closure, and the pores on the surface of the polymer blend separator completely disappear after heat treatment. The reason for this is likely that the ethylene-acrylic acid copolymer melts, leading to the closure of the pores in the polymer blend separator. The polymer blend separator prepared in this application has a thermal pore-closing function, which is beneficial to improving the thermal safety of the battery.

[0137] Figure 2 Content (a), Figure 2Content (b) shows scanning electron microscope (SEM) images of the membranes prepared in Comparative Examples 1 and 2, respectively. It can be seen that the pure polyethylene membrane prepared by the biaxial stretching method (Comparative Example 1) has a porous structure similar to that of Example 1, while the surface pore structure of the membrane prepared by the surface coating method (Comparative Example 2) is covered by the coating layer. Electrochemical impedance spectroscopy results (Table 1) show that the lithium-ion transference number and ionic conductivity of the polyethylene / ethylene-acrylic acid copolymer blend membrane prepared in Example 1 are significantly higher than those of the pure polyethylene membrane (Comparative Example 1). This may be because the introduction of the electrochemically active polymer can increase the polarity of the membrane, reduce the resistance to ion migration, and promote lithium-ion migration, thereby increasing the lithium-ion transference number and ionic conductivity of the membrane. The ionic conductivity of the polymer blend membrane prepared in Example 1 is significantly higher than that of Comparative Example 2. The reason for this may be that the coating layer of the membrane prepared in Comparative Example 2 is dense, which severely blocks the membrane pore structure, thus hindering ion movement and migration. The electrochemical impedance results are consistent with the morphological results observed by electron microscopy, indicating that compared with traditional coating modification, the single-layer membrane prepared by blending combined with biaxial stretching has better pore connectivity, which is beneficial to reducing lithium ion transport resistance and ensuring that the membrane has high ionic conductivity.

[0138] The modified polyethylene separator prepared in Comparative Example 2 had excessive impedance, leading to severe battery polarization and inability to operate normally; therefore, it was not assembled into a battery for performance testing. (See Table 1 and...) Figure 5 It can be seen that the lithium metal / lithium iron phosphate battery assembled using the separator prepared in Example 1 has a higher initial discharge specific capacity at 1C than that of Comparative Example 1. This may be because the polymer blend separator prepared in Example 1 has a strong affinity with the electrolyte and a low degree of battery polarization. After cycling the assembled batteries for 200 cycles, it was found that the discharge specific capacity and capacity retention rate of the battery assembled in Example 1 were significantly higher than those of Comparative Example 1. The reason for this may be that the polymer blend separator prepared in Example 1 can increase the lithium-ion transference number, inhibit the growth of lithium dendrites, and extend the cycle life of the battery.

[0139] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A polymer blend separator, characterized by, The polymer blend separator is prepared by a wet process from an electrochemically active polymer, polyethylene and a porogen; The electrochemically active polymer is one or more of maleic anhydride grafted polyethylene, ethylene-maleic anhydride copolymer, ethylene-acrylic acid copolymer, sulfonated polyethylene and benzene sulfonic acid grafted polyethylene; The melting point of the electrochemically active polymer is 75-115°C, and the temperature of heat setting in the wet process is 70-105°C; and the melting point of the electrochemically active polymer is lower than the melting point of the polyethylene and higher than the temperature of heat setting in the wet process; The mass ratio of the electrochemically active polymer, the polyethylene and the porogen is (1-10):10:(10-80).

2. The polymer blend separator of claim 1, wherein The electrochemically active polymer contains electrochemically active groups, which are one or more of maleic anhydride structural units, acrylic acid structural units, sulfonic acid groups and benzene sulfonic acid structural units; the mass percentage content of the electrochemically active groups is independently 1-60%.

3. The polymer blend separator of claim 1, wherein The viscosity average molecular weight of the polyethylene is 500-580 million; and / or, The porogen is one or more of liquid paraffin, decaline, di-n-octyl phthalate, dodecanol, peanut oil and soybean oil.

4. A method of producing a polymer blend separator according to any one of claims 1 to 3, characterized by, The method comprises the following steps: S1, mixing the electrochemically active polymer, polyethylene and porogen, and then casting or molding to obtain a blend gel sheet; S2, bidirectional stretching the blend gel sheet to obtain a blend oil film; the temperature of the bidirectional stretching is 70-105°C and is lower than the melting point of the electrochemically active polymer; S3, extracting, washing, drying and heat setting the blend oil film to obtain a polymer blend separator; the temperature of the heat setting is 70-105°C and is lower than the melting point of the electrochemically active polymer.

5. The production method according to claim 4, wherein In step S1, the mixing temperature is 140-180°C, the rotation speed is 20-120 rpm and the time is 10-40 min; The thickness of the blend gel sheet is 0.5-2 mm.

6. The production method according to claim 4, wherein In step S2, in the bidirectional stretching, the stretching ratio of the transverse direction to the longitudinal direction is 4×4-10×10.

7. The production method according to claim 4, wherein In step S3, the time of heat setting is 1-3 min.

8. A battery separator material characterized by, The polymer blend separator according to any one of claims 1-3.

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