Preparation method of ultrathin high-strength lithium battery diaphragm

Abstract

The invention provides a preparation method of an ultrathin high-strength lithium battery diaphragm, and relates to the technical field of preparation of lithium battery diaphragms. The preparation method of the ultrathin high-strength lithium battery diaphragm comprises the following steps: S1, injecting polyethylene, a plasticizer and an antioxidant into an extruder for mixing and melting to obtain an extruded sheet; s2, the extruded sheet is attached to a cast sheet cooling roller through a tape casting method for cooling forming, and a cast sheet is obtained; s3, carrying out longitudinal stretching and first transverse stretching on the casting piece to obtain a film, and extracting and drying to obtain a precursor; and S4, carrying out secondary transverse stretching and heat setting on the precursor to obtain the ultrathin high-strength lithium battery diaphragm. According to the preparation method disclosed by the invention, through collaborative optimization of a graded longitudinal drawing process and a composite antioxidant system, dual improvement of the performance and the production stability of the ultrathin diaphragm is realized on the premise of not greatly increasing the production cost.

Description

Technical Field

[0001] This invention relates to the field of lithium battery separator preparation technology, and in particular to a method for preparing an ultrathin, high-strength lithium battery separator. Background Technology

[0002] With the increasing requirements for energy density and safety of lithium batteries, separator materials are constantly developing towards thinner and stronger designs. Ultra-thin designs can significantly improve the energy density of batteries, while high strength ensures that the separator is not easily broken or deformed under high-rate charge and discharge conditions. Currently, the mainstream production processes include wet and dry biaxial stretching processes. The biaxial stretching process stretches the base film in both longitudinal and transverse directions, so that the molecular chains are aligned in a biaxial orientation, thereby improving mechanical strength, porosity and thermal stability. Its key steps include: (1) Raw material melt extrusion: HDPE / PP is mixed with plasticizer (such as white oil) and then extruded by a twin-screw extruder; (2) Casting: The melt is extruded through a T-die and quickly cooled into a base film; (3) Longitudinal stretching (MD): Stretched 5-15 times after preheating; (4) Transverse stretching (TD): Stretched 5-20 times in a transverse stretching oven; (5) Heat setting and winding.

[0003] Although biaxial stretching technology is relatively mature, the following problems still exist when preparing ultra-high strength and ultra-thin membranes (e.g., below 5μm): On the one hand, when using high molecular weight PE raw materials, the processing capacity of twin-screw extruders is limited, requiring increased screw speed or barrel temperature to promote melting. This can lead to excessive degradation of HDPE raw materials, resulting in product strength not meeting expectations. On the other hand, the upper limit of the roller gear transmission ratio of traditional longitudinal stretching machines is usually 15:1. Due to the limitations of asynchronous production line equipment structure, the transverse stretching ratio can currently reach 20 times, while the longitudinal stretching ratio can usually only be maintained at around 15 times. Furthermore, to avoid excessive transverse stretching inlet tension leading to membrane breakage, a retraction process is required in the shaping zone during actual production, further reducing the effective longitudinal stretching ratio. Therefore, the actual longitudinal stretching is less than 15 times. This significant difference between longitudinal and transverse stretching ratios directly leads to uneven biaxial orientation of the membrane, resulting in significant differences in biaxial tensile strength, difficulty in improving puncture strength, and poor product performance consistency. While existing technologies have attempted to optimize membrane performance by adjusting process parameters or improving equipment, these methods have failed to effectively address the aforementioned problems. Therefore, a preparation method that can reduce raw material degradation rates and improve biaxial stretching uniformity is urgently needed. Summary of the Invention

[0004] To address the aforementioned issues, this invention provides a method for preparing an ultrathin, high-strength lithium battery separator. Through the synergistic optimization of a graded longitudinal stretching process and a composite antioxidant system, the performance and production stability of the ultrathin separator are both improved.

[0005] To address the aforementioned technical problems, in a first aspect, the present invention provides a method for preparing an ultrathin, high-strength lithium battery separator, comprising the following steps: S1. Polyethylene, plasticizer, and antioxidant are injected into an extruder and mixed and melted to obtain an extruded sheet; wherein the antioxidant includes antioxidant 1010 and antioxidant 168; S2. The extruded sheet is applied to a casting cooling roller by a casting method and cooled and shaped to obtain a casting sheet; S3. The cast sheet is stretched longitudinally and then stretched transversely for the first time to obtain a thin film, which is then extracted and dried to obtain a precursor. The longitudinal stretching includes: performing two longitudinal stretching processes using a graded longitudinal stretching method, wherein the stretching ratio of the first longitudinal stretching process is 1.2-1.72 times, the stretching ratio of the second longitudinal stretching process is 10-15 times, and the total stretching ratio of the longitudinal stretching is 15-20 times. S4. The precursor is stretched laterally for the second time, then heat-set and cut to obtain the ultra-thin high-strength lithium battery separator.

[0006] This invention utilizes a staged longitudinal stretching technique to break down traditional one-time high-ratio stretching into two stages: a first pre-stretch (1.2-1.72 times) and a second main stretch (10-15 times). This achieves a gradual orientation of the molecular chains at a temperature of 105-110℃. The pre-stretching stage uses relatively low stress to initially detangle the molecular chains and form oriented crystal nuclei. The main stretching stage, driven by a high-precision servo, completes the high-ratio stretching based on the pre-oriented structure. This segmented control method overcomes the limitations of equipment on the longitudinal stretching ratio, significantly reduces the film breakage rate, and improves the MD / TD strength ratio and thickness uniformity.

[0007] This invention employs a specific ratio of composite antioxidants, forming a dual protection mechanism of free radical capture and hydroperoxide decomposition, effectively inhibiting molecular chain degradation during processing. This synergistic optimization of process and materials enables the prepared membrane to achieve ultra-thinness while possessing excellent mechanical strength, thermal stability, and electrochemical performance.

[0008] Preferably, in step S1, the antioxidant is a mixture of antioxidant 1010 and antioxidant 168 in a mass ratio of 1-2:1.

[0009] Preferably, the mass ratio of the polyethylene, antioxidant and plasticizer is 18:0.036-0.054:81.946-81.964.

[0010] Preferably, in step S1, the extrusion temperature of the extruder is 200-205℃, the die temperature is 180-210℃, the extrusion rate is 200-500kg / h, and the screw speed is 50-150rpm.

[0011] Preferably, in step S1, the ratio of the extrusion amount to the screw speed is 3-4.

[0012] Preferably, in step S2, the temperature of the casting cooling roller is 16-25℃, and the linear speed of the casting cooling roller is 3-4m / min.

[0013] Preferably, in step S3, a stretching roller is provided in the preheating zone of the longitudinal stretching equipment, with a roller speed ratio of 100%-120%. The longitudinal stretching includes: performing a first longitudinal stretching treatment in the preheating zone at 105-110°C with a stretching ratio of 1.5 times; performing a second longitudinal stretching treatment in the stretching zone at 105-110°C with a stretching ratio of 12 times; and the total longitudinal stretching stretching ratio is 18 times.

[0014] Preferably, in step S3, the first transverse stretching is performed at 115-120°C with a stretching ratio of 14-16 times.

[0015] Preferably, in step S4, the second transverse stretching is performed at 129-133°C with a stretching ratio of 1.4-1.6 times.

[0016] Preferably, the plasticizer is selected from at least one of white oil or dioctyl terephthalate.

[0017] Secondly, the present invention also provides an ultrathin high-strength lithium battery separator, which is prepared by the above-described method for preparing an ultrathin high-strength lithium battery separator.

[0018] Thirdly, the present invention also provides a longitudinal stretching device, comprising a preheating zone, a stretching zone and a shaping zone connected in sequence, wherein the preheating zone comprises a second preheating roller (L2), a third preheating roller (L3), a fourth preheating roller (L4), a fifth stretching roller (L5), a sixth stretching roller (L6), a seventh stretching roller (L7), an eighth preheating roller (L8) and a ninth preheating roller (L9) connected in sequence.

[0019] Compared with the prior art, the present invention has the following beneficial effects: This invention achieves a dual improvement in the performance and production stability of ultrathin separators without significantly increasing production costs by employing a graded longitudinal stretching process and a composite antioxidant system. It solves problems such as severe molecular chain degradation and uneven biaxial stretching in traditional lithium battery separator preparation, and is particularly suitable for the industrial production of ultrathin separators below 5 micrometers. Attached Figure Description

[0020] Figure 1 This is a schematic diagram of the lithium battery separator preparation process in one embodiment of the present invention; Figure 2 A schematic diagram of an existing, unmodified longitudinal stretching device. Figure 3 This is a schematic diagram of the modified longitudinal stretching device structure in one embodiment of the present invention. Detailed Implementation

[0021] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the specific embodiments will be briefly introduced below. Obviously, the embodiments described below are some embodiments of the present invention. For those skilled in the art, other embodiments can be obtained based on these embodiments without creative effort.

[0022] Where specific experimental steps or conditions are not specified in the embodiments, they can be performed according to the conventional experimental steps or conditions described in the literature in this field. All raw materials or instruments used are commercially available conventional products, including but not limited to those used in the embodiments of this application.

[0023] Example 1 This embodiment modifies the longitudinal stretching equipment by replacing the original L5 preheating roller with three Φ150mm small-diameter stretching rollers. See the structural diagram of the longitudinal stretching equipment before modification. Figure 2 For details, please refer to the structural diagram of the modified longitudinal tensioning equipment. Figure 3 .

[0024] The modified longitudinal stretching equipment includes a preheating zone, a stretching zone, and a shaping zone connected in sequence. The preheating zone includes a second preheating roller (L2), a third preheating roller (L3), a fourth preheating roller (L4), a fifth stretching roller (L5), a sixth stretching roller (L6), a seventh stretching roller (L7), an eighth preheating roller (L8), and a ninth preheating roller (L9) connected in sequence.

[0025] Its process flow is as follows: The raw material is injected into the extruder for plasticization and melting; the molten fluid is extruded through the die head, and the extruded melt is cast and cooled on the cooling roller to form a casting sheet. Then, the casting sheet is preheated and stretched for the first time in the preheating zone of the longitudinal stretching equipment by passing through three preheating rollers, three small diameter stretching rollers, and two preheating rollers in sequence through the traction device. Then, it is stretched for the second time in the stretching zone of the longitudinal stretching equipment by passing through seven stretching rollers. Then, it is heat-set in the longitudinal stretching equipment by passing through four shaping rollers. Then, it enters the transverse stretching equipment for preheating, stretching, and shaping. Finally, it is trimmed and wound up to obtain the ultra-thin high-strength lithium battery separator.

[0026] Example 2 This embodiment provides a method for preparing an ultrathin, high-strength lithium battery separator, which is produced using a modified longitudinal stretching equipment and includes the following steps: Step 1: High-molecular-weight polyethylene powder (Daehan Oil & Chemical VH200U), antioxidants, and white oil are injected into a twin-screw extruder in a ratio of 18:0.036 (antioxidant 1010):0.018 (antioxidant 168):81.946. The barrel temperature is 210℃, the die width is 900mm, the die temperature is 180℃, the extrusion rate is 210kg / h, and the screw speed is 60rpm (the ratio of extrusion rate to screw speed is 3.5), resulting in a sheet. The sheet is then calendered onto a casting cooling roller for cooling and shaping. The casting cooling roller temperature is 12℃, and the linear speed is 3.33m / min, resulting in a cast sheet.

[0027] Step 2, move the original preheating zone L5 of the longitudinal stretching equipment (e.g., Figure 2 (As shown) The preheating roller was modified into three smaller diameter stretching rollers (such as...) Figure 3 As shown), the roller diameter is Φ150mm, and the roller speed ratio setting range is 120%.

[0028] Step 3: The cast sheet is pulled along the rollers of the longitudinal stretching machine and subjected to the first longitudinal stretching at 105°C in the preheating zone, with a stretching ratio of 1.5 times. Then, a second longitudinal stretching is performed at 105°C in the stretching zone, with a stretching ratio of 12 times. The total longitudinal stretching ratio is 18 times, resulting in a longitudinally highly oriented thin film. The longitudinally highly oriented thin film is then held by the clamps of the transverse stretching machine and stretched to 15 times at 115°C, resulting in a transversely highly oriented film. The film is then pulled by the extraction roller and immersed in a dichloromethane solution (concentration > 99 wt%) at 16°C for extraction. It is then pulled by the hot roller to the drying room and dried at 60°C, shrinking by 15%, to obtain the precursor. The precursor is held in the clamp of the transverse stretching machine and stretched to 1.57 times in an oven at 129°C. At this time, the transverse stretching ratio is 15*1.57*(1-15%)≈20 times. Finally, it is heat-treated by hot roller traction at 100°C for 20 seconds for heat setting. It is then pulled along the roller and wound up by the winding roller to obtain the ultra-thin high-strength lithium battery separator.

[0029] Example 3 This embodiment provides a method for preparing an ultrathin, high-strength lithium battery separator, which is produced using a modified longitudinal stretching equipment and includes the following steps: Step 1: High-molecular-weight polyethylene powder (Daihan Oil & Chemical VH200U), antioxidant, and white oil are injected into a twin-screw extruder at a mass ratio of 18:0.036 (antioxidant 1010):0.018 (antioxidant 168):81.946. The barrel temperature is 210℃, the die width is 900mm, the die temperature is 200℃, the extrusion rate is 210kg / h, and the screw speed is 60rpm (the ratio of extrusion rate to screw speed is 3.5), resulting in a sheet. The sheet is then calendered onto a casting cooling roller for cooling and shaping. The casting cooling roller temperature is 12℃, and the linear speed is 3m / min, resulting in a cast sheet.

[0030] Step 2, move the original preheating zone L5 of the longitudinal stretching equipment (e.g., Figure 2 (As shown) The preheating roller was modified into three smaller diameter stretching rollers (such as...) Figure 3 As shown), the roller diameter is Φ150mm, and the roller speed ratio is set within the range of 100-120%.

[0031] Step 3: The cast sheet is pulled along the rollers of the longitudinal stretching machine and subjected to the first longitudinal stretching at 107°C in the preheating zone, with a stretching ratio of 1.6 times. Then, it is subjected to the second longitudinal stretching at 108°C in the stretching zone, with a stretching ratio of 12.5 times. The total longitudinal stretching ratio is 20 times, resulting in a longitudinally highly oriented thin film. The longitudinally highly oriented thin film is then held by the clamps of the transverse stretching machine and stretched to 15 times at 118°C, resulting in a transversely highly oriented film. The film is then pulled by the extraction roller and immersed in a dichloromethane solution (concentration > 99 wt%) at 16°C for extraction. It is then pulled by the hot roller to the drying room and dried at 60°C, shrinking by 15%, to obtain the precursor. The precursor is held in the clamp of the transverse stretching machine and stretched to 1.57 times in an oven at 130°C. At this time, the transverse stretching ratio is 16*1.57*(1-15%)≈21.35 times. Finally, it is heat-treated by hot roller traction at 100°C for 20 seconds for heat setting. It is then pulled along the roller and wound up by the winding roller to obtain the ultra-thin high-strength lithium battery separator.

[0032] Example 4 This embodiment provides a method for preparing an ultrathin, high-strength lithium battery separator, which is produced using a modified longitudinal stretching equipment and includes the following steps: Step 1: Polyethylene powder (Daihan Oil & Chemical VH200U), antioxidant, and white oil are injected into a twin-screw extruder at a mass ratio of 18:0.018 (antioxidant 1010):0.018 (antioxidant 168):81.964. The barrel temperature is 205℃, the die width is 900mm, the die temperature is 210℃, the extrusion rate is 210kg / h, and the screw speed is 60rpm (the ratio of extrusion rate to screw speed is 3.5), resulting in a sheet. The sheet is then calendered onto a casting cooling roller for cooling and shaping. The casting cooling roller temperature is 12℃, and the linear speed is 3m / min, resulting in a cast sheet.

[0033] Step 2, move the original preheating zone L5 of the longitudinal stretching equipment (e.g., Figure 2 (As shown) The preheating roller was modified into three smaller diameter stretching rollers (such as...) Figure 3 As shown), the roller diameter is Φ150mm, and the roller speed ratio is set within the range of 100%-120%.

[0034] Step 3: The cast sheet is pulled along the rollers of the longitudinal stretching machine and subjected to the first longitudinal stretching at 110°C in the preheating zone, with a stretching ratio of 1.6 times. Then, it is subjected to the second longitudinal stretching at 110°C in the stretching zone, with a stretching ratio of 12.5 times. The total longitudinal stretching ratio is 20 times, resulting in a longitudinally highly oriented thin film. The longitudinally highly oriented thin film is then held by the clamps of the transverse stretching machine and stretched to 16 times at 120°C, resulting in a transversely highly oriented film. The film is then pulled by the extraction roller and immersed in a dichloromethane solution (concentration > 99 wt%) at 16°C for extraction. It is then pulled by the hot roller to the drying room and dried at 60°C, shrinking by 15%, to obtain the precursor. The precursor is held in the clamp of the transverse stretching machine and stretched to 1.57 times in an oven at 133°C. At this time, the transverse stretching ratio is 16*1.57*(1-15%)≈21.35 times. Finally, it is heat-treated by hot roller traction at 100°C for 20 seconds for heat setting. It is then pulled along the roller and wound up by the winding roller to obtain the ultra-thin high-strength lithium battery separator.

[0035] Comparative Example 1 The longitudinal stretching machine used in this comparative example was an unmodified device. The entire longitudinal stretching process was performed only once in the stretching zone, and no antioxidants were added. Other parameters and methods were the same as in Example 2. The specific steps are as follows: This embodiment provides a method for preparing an ultrathin, high-strength lithium battery separator, comprising the following steps: Step 1: Polyethylene powder (Daihan Oil Chemical VH200U) and white oil are injected into a twin-screw extruder at a mass ratio of 18:82. The barrel temperature is 210℃, the die width is 900mm, the die temperature is 200℃, the extrusion rate is 210kg / h, and the screw speed is 60rpm (the ratio of extrusion rate to screw speed is 3.5), resulting in a sheet. The sheet is then calendered onto a casting cooling roller for cooling and shaping. The casting cooling roller has a temperature of 12℃ and a linear speed of 4m / min, resulting in a cast sheet.

[0036] Step 2: The cast sheet is drawn along the rollers of the longitudinal stretching machine and subjected to a second longitudinal stretching at 108°C in the stretching zone, with a stretching ratio of 15 times, to obtain a longitudinally highly oriented thin film. The longitudinally highly oriented thin film is then held by the clamps of the transverse stretching machine and stretched to 14 times at 115°C to obtain a transversely highly oriented film. The film is then drawn by the extraction roller and immersed in a dichloromethane solution (concentration > 99wt%) at 16°C for extraction washing. It is then drawn by the hot roller to an drying oven at 60°C for drying, shrinking by 15% to obtain the precursor. The precursor is held by the clamps of the transverse stretching machine and stretched to 1.57 times in an oven at 129°C. At this point, the transverse stretching ratio is approximately 14 * 1.57 * (1 - 15%) ≈ 18.7 times. Finally, it undergoes heat treatment at 100°C for 20 seconds to heat set, and is then drawn along the rollers and wound by the winding roller to obtain the ultra-thin high-strength lithium battery separator.

[0037] Comparative Example 2 No antioxidants were added during the preparation of this comparative example, and other parameters and methods were the same as in Example 2.

[0038] Comparative Example 3 The longitudinal stretching machine used in this comparative example is an unmodified device. The entire longitudinal stretching process is performed only once in the stretching zone. Other parameters and methods are the same as in Example 2.

[0039] Comparative Example 4 The antioxidant used in this comparative example was antioxidant 1010, and other parameters and methods were the same as in Example 2.

[0040] Performance testing Molecular weight degradation rate: Molecular weight degradation was tested using an Ubbelohde viscometer based on the intrinsic viscosity method.

[0041] Tensile strength: The tensile strength was tested using an electronic universal testing machine in accordance with the testing standard GB / T 36363-2018.

[0042] Puncture strength: Tested according to the GB / T 36363-2018 standard using an electronic puncture strength testing machine. The puncture strength was tested, and the data are shown in Table 1.

[0043] Table 1 As can be seen from Table 1, this invention achieves a dual improvement in the performance and production stability of ultrathin diaphragms without significantly increasing production costs by employing a graded longitudinal stretching process and a composite antioxidant system through synergistic optimization.

[0044] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A method for preparing an ultrathin, high-strength lithium battery separator, characterized in that, Includes the following steps: S1. Polyethylene, plasticizer, and antioxidant are injected into an extruder and mixed and melted to obtain an extruded sheet; wherein the antioxidant includes antioxidant 1010 and antioxidant 168; S2. The extruded sheet is applied to a casting cooling roller by a casting method and cooled and shaped to obtain a casting sheet; S3. The cast sheet is stretched longitudinally and then stretched transversely for the first time to obtain a thin film, which is then extracted and dried to obtain a precursor. The longitudinal stretching includes: performing two longitudinal stretching processes using a graded longitudinal stretching method, wherein the stretching ratio of the first longitudinal stretching process is 1.2-1.72 times, the stretching ratio of the second longitudinal stretching process is 10-15 times, and the total stretching ratio of the longitudinal stretching is 15-20 times. S4. The precursor is stretched laterally for the second time, then heat-set and cut to obtain the ultra-thin high-strength lithium battery separator.

2. The method for preparing the ultrathin high-strength lithium battery separator as described in claim 1, characterized in that, In step S1, the antioxidant is composed of antioxidant 1010 and antioxidant 168 in a mass ratio of (1-2):

1.

3. The method for preparing the ultrathin high-strength lithium battery separator as described in claim 2, characterized in that, In step S1, the extrusion temperature of the extruder is 200-205℃, the die temperature is 180-210℃, the extrusion rate is 200-500kg / h, and the screw speed is 50-150rpm.

4. The method for preparing the ultrathin high-strength lithium battery separator as described in claim 3, characterized in that, In step S1, the ratio of the extrusion amount to the screw speed is 3-4.

5. The method for preparing the ultrathin high-strength lithium battery separator as described in claim 1, characterized in that, In step S2, the temperature of the casting cooling roller is 16-25℃, and the linear speed of the casting cooling roller is 3-4m / min.

6. The method for preparing the ultrathin high-strength lithium battery separator as described in claim 1, characterized in that, In step S3, a stretching roller is provided in the preheating zone of the longitudinal stretching equipment, with a roller speed ratio of 100%-120%. The longitudinal stretching includes: a first longitudinal stretching treatment in the preheating zone at 105-110℃ with a stretching ratio of 1.5 times; a second longitudinal stretching treatment in the stretching zone at 105-110℃ with a stretching ratio of 12 times; and a total longitudinal stretching stretching ratio of 18 times.

7. The method for preparing the ultrathin high-strength lithium battery separator as described in claim 1, characterized in that, In step S3, the first transverse stretching is performed at 115-120℃ with a stretching ratio of 14-16 times.

8. The method for preparing the ultrathin high-strength lithium battery separator as described in claim 1, characterized in that, In step S4, the second transverse stretching is performed at 129-133℃ with a stretching ratio of 1.4-1.6 times.

9. An ultra-thin, high-strength lithium battery separator, characterized in that, It is prepared by the method for preparing ultrathin high-strength lithium battery separator according to any one of claims 1-8.

10. A longitudinal stretching apparatus for the preparation method of the ultrathin high-strength lithium battery separator according to any one of claims 1-8, comprising a preheating zone, a stretching zone, and a shaping zone connected in sequence, characterized in that, The preheating zone includes a second preheating roller (L2), a third preheating roller (L3), a fourth preheating roller (L4), a fifth stretching roller (L5), a sixth stretching roller (L6), a seventh stretching roller (L7), an eighth preheating roller (L8), and a ninth preheating roller (L9) connected in sequence.

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