Ultra-thin, high-strength lithium-ion battery separator and its manufacturing method

Abstract

An ultra-thin, high-strength lithium-ion battery separator and a manufacturing method thereof, the manufacturing method for an ultra-thin, high-strength lithium-ion battery separator includes the steps of: mixing a nucleating agent, ultra-high molecular weight polyethylene powder, an antioxidant, and a pore-forming agent and stirring until homogeneous to obtain a pre-mixed raw material; injecting the pre-mixed raw material into an extruder and heating it to obtain a thermodynamic single-phase melt, and melt-casting the pre-mixed raw material to obtain a crystallized cast sheet, wherein the ratio of the linear velocity of the quench roll to the melt-casting velocity of the thermodynamic single-phase melt is 5 to 50 in the melt-casting; and longitudinally stretching and a first transverse stretching of the crystallized cast sheet, followed by extraction, washing, drying, a second transverse stretching, shrinking, and heat setting to obtain an ultra-thin, high-strength lithium-ion battery separator. By using ultra-high molecular weight polyethylene powder, heterogeneous nucleation of the nucleating agent, and high-magnification flow-stretching, the crystallinity of the film is improved in multiple ways, while at the same time achieving a thinner separator, resulting in the production of a high-strength, ultra-thin lithium-ion battery separator.

Description

[Technical Field]

[0001] The present invention relates to the field of battery technology, and more particularly to an ultra-thin, high-strength lithium-ion battery separator and a method for manufacturing the same. [Background technology]

[0002] Lithium-ion battery separators are an essential component of lithium batteries. They function as an insulating layer, separating the cathode and anode materials while providing a path for lithium ion migration. As separator manufacturing technology advances, the trend toward lighter, thinner, and more functional separators is becoming mainstream. Thinner separators offer lower internal resistance, higher ionic conductivity, and reduced volume and space in lithium-ion cells, making them ideal for high-rate, high-capacity lithium batteries. However, the resulting reduced thickness of lightweight and thin separators reduces their puncture resistance, potentially leading to serious safety issues in battery applications. This presents a significant bottleneck. Currently, the mainstream separator product in the separator industry is 7–12 μm thick. Thicknesses less than 7 μm are called ultra-thin separators. Ultra-thin separators have a very low pin puncture strength of approximately 3 N or less, meaning that their strength per micrometer is within 0.5 N / μm. Therefore, the question of how to overcome the strength bottleneck of ultra-thin separators must be solved in order to make them lighter and thinner. Summary of the Invention

[0003] In view of the deficiencies of the prior art, the present invention aims to provide an ultra-thin, high-strength lithium-ion battery separator.

[0004] Another object of the present invention is to provide a method for producing the above-mentioned ultra-thin, high-strength lithium ion battery separator, which achieves a double improvement in the crystallinity of the product and at the same time realizes a thin battery separator by high-ratio stretching in melt casting and the addition of a nucleating agent.

[0005] The object of the present invention is achieved by the following aspects.

[0006] An ultra-thin, high-strength lithium-ion battery separator made from a nucleating agent, ultra-high molecular weight polyethylene powder, an antioxidant, and a pore-forming agent, wherein the ratio of the nucleating agent, ultra-high molecular weight polyethylene powder, antioxidant, and pore-forming agent, in parts by mass, is (0.1-1):(15-35):(0.01-0.1):(65-85), wherein the nucleating agent is a mixture of pimelic acid and calcium stearate, and the ratio of the pimelic acid to the calcium stearate, in parts by mass, is (0.5-4):(1-4.5), preferably (0.8-3.8):(1.2-4.2).

[0007] In the above aspect, the ratio of the nucleating agent, ultra-high molecular weight polyethylene powder, antioxidant and pore-forming agent is (0.3-1):(20-30):(0.01-0.05):(73-75) in parts by mass.

[0008] In the above aspect, the weight average molecular weight of the ultra-high molecular weight polyethylene powder is 4,000,000 to 5,000,000.

[0009] In the above embodiment, the pore-forming agent is white oil and / or dioctyl terephthalate (DOTP).

[0010] In the above embodiment, the antioxidant is pentaerythritol tetrakis[3-[3,5-di(tert-butyl)-4-hydroxyphenyl]propionate] (antioxidant 1010).

[0011] The method for manufacturing the ultra-thin, high-strength lithium-ion battery separator includes the following steps:

[0012] In step 1, a nucleating agent, an ultra-high molecular weight polyethylene powder, an antioxidant, and a pore-forming agent are mixed and stirred until homogeneous to obtain a pre-mixed raw material; In step 1, the stirring temperature is 60 to 110° C., the stirring rotation speed is 40 to 60 RPM, and the stirring time is 20 to 40 minutes.

[0013] In step 2, the premixed raw material is injected into an extruder and heated to obtain a thermodynamic single-phase melt, and the thermodynamic single-phase melt is melt-cast to obtain a crystallized cast sheet, wherein in the melt-casting, the ratio of the linear velocity of the quench roll to the melt-casting velocity of the thermodynamic single-phase melt is 5 to 50, the linear velocity is in m / min, and the flow velocity is in m / min; In step 2, the heating time is 30 to 60 seconds, and the heating temperature is 170 to 230°C, preferably 170 to 210°C.

[0014] In step 2, the temperature of the quench roll in the melt casting is 15 to 25°C.

[0015] In step 2, the extruder is a twin-screw extruder having a screw rotation speed of 100-240 RPM and a screw aspect ratio of 56-68.

[0016] In step 2, the temperature of the thermodynamic single-phase melt to be melt-cast is 180 to 220°C.

[0017] In step 3, the crystallized casting sheet is stretched longitudinally and transversely for the first time to obtain a film, which is then extracted, washed, and dried to obtain a precursor, which is then stretched transversely for a second time, shrunk, and heat-set to obtain an ultra-thin, high-strength lithium-ion battery separator.

[0018] In step 3, the longitudinal stretching and the first transverse stretching are each performed at a stretching ratio of 5 to 15, and the longitudinal stretching is performed at a temperature of 80 to 120° C. and a speed of 25 to 250% / s.

[0019] In step 3, the first transverse stretching is performed at 110 to 140° C. and at a speed of 10 to 150% / s.

[0020] In step 3, the extraction and washing is carried out in an extractant at 25 to 55° C. for 60 to 240 seconds, and the extractant is a methylene chloride solution having a methylene chloride concentration of >99%.

[0021] In step 3, the drying temperature is 40 to 110° C., and the drying time is 10 to 40 seconds.

[0022] In step 3, the second transverse stretching is performed at 120 to 150° C. at a speed of 2 to 30% / s until the stretching reaches 1.2 to 2 times its original size.

[0023] In step 3, the shrinkage is performed by heating at a temperature of 120 to 150° C. at a rate of 0.5 to 5% / s, and the heat shrinkage ratio is 5 to 25%.

[0024] In step 3, the heat setting temperature is 80 to 120° C., and the heat setting time is 5 to 50 seconds.

[0025] The present invention has advantageous effects compared to the prior art.

[0026] 1. The present invention provides a method for producing an ultra-thin, high-strength lithium-ion battery separator, which uses ultra-high molecular weight polyethylene powder, heterogeneous nucleation of a nucleating agent, and high-magnification flow-stretching to improve the crystallinity of the film in multiple ways, while also achieving a thinner separator, thereby producing a high-strength, ultra-thin lithium-ion battery separator.

[0027] 2. The ultra-thin, high-strength lithium-ion battery separator prepared by the present invention has a thickness of 0.54 to 5.26 μm, a porosity of 45.6 to 51.5%, an average pore size of 42.1 to 50.2 nm, an ionic conductivity of 2.47 to 3.24 ms / cm, and a pin puncture strength of 0.83 to 1.02 N / μm. The ionic conductivity is improved by more than 1.2 times compared to the conventional film (Comparative Example 1), and the pin puncture strength is over 0.8 N / μm in both cases, which is more than 60% improved compared to the conventional film. Therefore, the separator is lightweight, thin, and safe, and has relatively good application prospects and economic benefits. [Brief explanation of the drawings]

[0028] [Figure 1] 1 is an SEM of a cross section of an ultra-thin, high-strength lithium-ion battery separator produced in Example 2. [Figure 2] 1 is an SEM of a cross section of an ultra-thin, high-strength lithium-ion battery separator produced in Example 3. DETAILED DESCRIPTION OF THE INVENTION

[0029] The technical solution of the present invention will be further explained below based on specific examples.

[0030] % / s: A unit of stretching or shrinking speed, which indicates the rate of stretching or shrinking of the separator from its original dimensions within a unit time (1 s).

[0031] Heat shrinkage ratio: Heat shrinkage ratio = 1 - (dimension after shrinkage / original dimension).

[0032] The raw materials and manufacturer information for the following examples are as follows:

[0033] Nucleating agent: a mixture of pimelic acid and calcium stearate purchased from Chengdu Xiya Chemical Co., Ltd. Ultra-high molecular weight polyethylene: Korea Daehan Yuhh Industrial Co., Ltd.; Antioxidant: 1010, Tianjin Li'anlong New Materials Co., Ltd. White oil: 50#, Zhejiang Zhengxin Petroleum Technology Co., Ltd. DOTP: Analytical reagent, Shandong Bluefan Chemical Co., Ltd.

[0034] The devices and their model information according to the following examples are as follows:

[0035] Separator production line: MZSTER, Korea, width 4m, linear speed 50m / min, Mahl thickness gauge: C1216 type, Mahl Precision Measuring Instruments Co., Ltd., Germany Analytical balance: FA2204 type, Shanghai Shayan Equipment Co., Ltd. Water pressure gauge: AAQ-5 type, PMI Instruments, USA Conductivity measuring device: S2021 type, Jiangsu Chuanyuan Technology Co., Ltd. Puncture strength measuring device: DY05 type, Jinan Sike Testing Technology Co., Ltd.

[0036] Mechanism of operation: In the manufacturing method of the present invention, when the polyethylene and pore-forming agent are cooled and phase-separated during the casting stage, heterogeneous nucleation occurs, resulting in a high crystallinity of the crystallized cast sheet. At the same time, when the melt is cast, a quenching roll is used to stretch the crystallized cast sheet at a high ratio, which causes the polyethylene molecules to be regularly aligned vertically, further increasing the crystallinity of the crystallized cast sheet. This improves the crystallinity in multiple ways, and enables the separator to be made thinner, resulting in the production of an ultra-thin, high-strength lithium-ion battery separator.

[0037] In the technical solution of the present invention, in the melt casting process, a thermodynamic single-phase melt is applied in the form of a sheet to the roll surface of a quench roll to obtain a crystallized casting sheet. When the crystallized casting sheet is stretched at an ultra-high ratio by the quench roll, the polyethylene molecular chains in the melt-cast state are rapidly aligned in the stretching direction, resulting in oriented crystals. When cooled by the quench roll, the polyethylene and the pore-forming agent undergo solid-liquid phase separation when cooled below the phase separation temperature line, and become polyethylene crystals. The addition of a nucleating agent accelerates the crystallization of polyethylene with the nucleating agent as a heterogeneous nucleus, thereby increasing the crystallinity of the casting sheet. The higher the crystallinity, the greater the strength of the product.

[0038] Porosity-forming agents have good compatibility with the extractant, and when the pore-forming agent in the film is extracted and then dried and volatilized in a drying chamber, the volume and space occupied by the pore-forming agent in the film is replaced, forming pores.

[0039] In the second transverse stretching, the film is stretched at a lower ratio. The dual effects of heating and molecular orientation further increase the film's crystallinity, harden the film's pore structure, and increase its strength. In addition, heat shrinkage relieves the internal stress created during stretching, improving the film's thermal stability.

[0040] In the following examples, the test methods for thickness, porosity, ionic conductivity, and pin puncture strength were all in accordance with the national standard "Polyolefin Separators for Lithium Ion Batteries" (GB / T 36363-2018), and the pore size was measured using a water pressure gauge.

[0041] Examples 1 to 4 An ultra-thin, high-strength lithium-ion battery separator made from a nucleating agent, ultra-high molecular weight polyethylene powder, an antioxidant, and a pore-forming agent, wherein the ratio of the nucleating agent, ultra-high molecular weight polyethylene powder, antioxidant, and pore-forming agent, in parts by mass, is X, wherein the nucleating agent is a mixture of pimelic acid and calcium stearate, and the ratio of the pimelic acid to the calcium stearate, in parts by mass, is Y, the weight-average molecular weight of the ultra-high molecular weight polyethylene powder is 400, the pore-forming agent is white oil, and the antioxidant is pentaerythritol tetrakis[3-[3,5-di(tert-butyl)-4-hydroxyphenyl]propionate] (antioxidant 1010).

[0042] The manufacturing method of the ultra-thin and high-strength lithium ion battery separator includes the following steps:

[0043] In step 1, the nucleating agent, ultra-high molecular weight polyethylene powder, antioxidant, and pore-forming agent were mixed in a pre-mixing stirring tank and stirred uniformly at 70°C and 50 RPM for 30 minutes to obtain a pre-mixed raw material (at this time, the ultra-high molecular weight polyethylene powder swelled).

[0044] In step 2, the premixed raw materials were injected at a constant rate into the feed port of a co-rotating twin-screw extruder with a screw aspect ratio of 64 using an air diaphragm pump. The extruder was then heated to 200 °C for 45 s at 170 RPM to obtain a thermodynamic single-phase melt. The 200 °C thermodynamic single-phase melt was extruded through a T-die along the melt line and quenched onto a 20 °C quench roll to obtain a crystallized casting sheet. The ratio of the linear velocity of the quench roll to the melt-casting velocity of the thermodynamic single-phase melt was defined as A, where the linear velocity was expressed in m / min and the flow velocity was expressed in m / min.

[0045] In step 3, the crystallized cast sheet was longitudinally stretched. The crystallized cast sheet was taken along the rolls of the longitudinal stretching machine and stretched 10 times at 110°C and a speed of 100% / s (i.e., a stretching ratio of 10 times) to obtain a thin film highly oriented in the longitudinal direction. Furthermore, the thin film highly oriented in the longitudinal direction was clamped between the clips of the transverse stretching machine and stretched 10 times at a speed of 80% / s in an oven at 125°C (i.e., a stretching ratio of 10 times) to obtain a film highly oriented in the transverse direction. The film was taken up by an extraction roll, immersed in a methylene chloride solution at 25°C (methylene chloride concentration in the methylene chloride solution >99 wt%) for 120 seconds, washed, taken up by a heated roll into a drying chamber, and dried at 60°C for 25 seconds to obtain a precursor. The precursor was then clamped with the clips of a transverse stretching machine and stretched 1.5 times at a speed of 15% / s in an oven at 130°C, and then heat-shrunk (transverse width shrinkage) at 135°C and a speed of 2% / s, with a heat-shrinkage (transverse width shrinkage) ratio of 15%. Finally, the precursor was taken up by a heater roller at 100°C, heat-treated for 20 seconds to be heat-set, taken up along the roller, and wound up by a winding roller to obtain an ultra-thin, high-strength lithium-ion battery separator.

[0046] Table 1 [Table 1]

[0047] 1 and 2 are SEM images of the cross sections of the ultra-thin, high-strength lithium-ion battery separators produced in Examples 2 and 3. The mass parts of the nucleating agent added were varied from 0.4 to 1.2, and the blending ratio of the nucleating agent was changed stepwise from a ratio of less pimelic acid than calcium stearate, to an equal ratio of pimelic acid and calcium stearate, to a ratio of more pimelic acid than calcium stearate, to investigate the amount of nucleating agent added and blending ratio at which the degree of crystallinity was maximized.

[0048] Comparative Example 1 A lithium-ion battery separator made from ultra-high molecular weight polyethylene powder, an antioxidant, and a pore-forming agent, wherein the ratio of the ultra-high molecular weight polyethylene powder, the antioxidant, and the pore-forming agent was 25:0.02:75 in parts by mass, wherein the weight-average molecular weight of the ultra-high molecular weight polyethylene powder was 4,000,000, the pore-forming agent was white oil, and the antioxidant was pentaerythritol tetrakis[3-[3,5-di(tert-butyl)-4-hydroxyphenyl]propionate] (antioxidant 1010).

[0049] The method for producing the lithium-ion battery separator was almost the same as the method for producing the ultra-thin, high-strength lithium-ion battery separator in Example 1, except that the ratio of the linear velocity of the quenching roll to the flow velocity of the melt-casting of the thermodynamic single-phase melt in this comparative example was set to 1, and no nucleating agent was used in Step 1.

[0050] Comparative Example 2 This is a lithium ion battery separator similar to that of Example 4.

[0051] The method for producing the lithium ion battery separator was almost the same as the method for producing the ultra-thin, high-strength lithium ion battery separator of Example 4, except that the ratio of the linear velocity of the quenching roll to the flow velocity of the melt-casting of the thermodynamic single-phase melt in this comparative example was set to 1.

[0052] Comparative Example 3 This is a lithium ion battery separator similar to that of Comparative Example 1.

[0053] The method for producing the lithium ion battery separator was almost the same as the method for producing the ultra-thin, high-strength lithium ion battery separator of Comparative Example 1, except that the ratio of the linear velocity of the quenching roll to the flow velocity of the melt-casting of the thermodynamic single-phase melt in this comparative example was set to 50.

[0054] The thickness, porosity, average pore size, ionic conductivity (measured at 25°C), and pin puncture strength properties of the ultra-thin, high-strength lithium-ion battery separators produced in Examples 1 to 4 and the lithium-ion battery separators produced in Comparative Examples 1 to 3 are shown in Table 2. The ultra-thin, high-strength lithium-ion battery separators produced in Examples 1 to 4 had a thickness of 0.54 to 5.26 μm, a porosity of 45.6 to 51.5%, an average pore size of 42.1 to 50.2 nm, an ionic conductivity of 2.47 to 3.24 ms / cm, and a pin puncture strength of 0.83 to 1.02 N / μm.

[0055] As can be seen from Table 2, as the ratio of the linear velocity of the quench roll to the melt-casting velocity of the thermodynamic single-phase melt increased, the porosity and pore size of the ultra-thin, high-strength lithium-ion battery separators increased. The ionic conductivity of the ultra-thin, high-strength lithium-ion battery separators increased, improving by more than 1.2 times compared to films not subjected to high-magnification melt-casting. The pin puncture strength of all films exceeded 0.8 N / μm, improving by more than 60% compared to films not subjected to high-magnification melt-casting. Furthermore, as the amount of nucleating agent added increased within the scope of the technical solution of the present invention, the film's pin puncture strength also increased. However, a significant decrease was observed when the amount exceeded the range specified in the technical solution of the present invention. The main reason for this is thought to be that the addition of a nucleating agent increases the film's crystallinity, further improving film strength. However, excessive addition of the nucleating agent, a foreign substance in the system, disrupts the balance of the homogeneous melt system, lowering the film's crystallinity and reducing its strength.

[0056] Comparative Examples 1 to 3 revealed that when only a nucleating agent is added or when only high-magnification melt-casting stretching is performed, the ionic conductivity and pin puncture strength of the product are improved to some extent, but the synergistic effect of the nucleating agent and high-magnification melt-casting stretching is not as significant.

[0057] Table 2 [Table 2]

[0058] The present invention has been described above by way of example, but it goes without saying that any simple variations or modifications, or equivalent substitutions that can be made by those skilled in the art without resorting to original ingenuity, fall within the technical scope of the present invention, provided that they do not deviate from the gist of the present invention.

Claims

1. An ultra-thin, high-strength lithium-ion battery separator, the separator being prepared from a nucleating agent, ultra-high molecular weight polyethylene powder, an antioxidant, and a pore-forming agent, wherein the ratio of the nucleating agent, ultra-high molecular weight polyethylene powder, antioxidant, and pore-forming agent, in parts by mass, is (0.1-1):(15-35):(0.01-0.1):(65-85), and wherein the nucleating agent is a mixture of pimelic acid and calcium stearate, and the ratio of the pimelic acid to the calcium stearate, in parts by mass, is (0.5-4):(1-4.5).

2. 2. The ultra-thin, high-strength lithium-ion battery separator according to claim 1, wherein the weight-average molecular weight of the ultra-high molecular weight polyethylene powder is 4,000,000 to 5,000,000.

3. 3. The ultra-thin, high-strength lithium ion battery separator according to claim 1, wherein the pore-forming agent is white oil and / or dioctyl terephthalate.

4. 4. The ultra-thin, high-strength lithium ion battery separator of claim 3, wherein the antioxidant is pentaerythritol tetrakis.

5. A method for producing the ultra-thin, high-strength lithium-ion battery separator according to any one of claims 1 to 4, Step 1: Mixing a nucleating agent, an ultra-high molecular weight polyethylene powder, an antioxidant, and a pore-forming agent and stirring until uniform to obtain a pre-mixed raw material; Step 2: injecting the premixed raw material into an extruder, heating it to obtain a thermodynamic single-phase melt, and melt-casting the thermodynamic single-phase melt to obtain a crystallized cast sheet, wherein in the melt-casting, a ratio of a linear velocity of a quench roll to a flow rate of the thermodynamic single-phase melt is 5 to 50, and the unit of the linear velocity is m / min, and the unit of the flow rate is m / min; Step 3: longitudinally stretching the crystallized casting sheet and then transversely stretching it a first time to obtain a film, which is then extracted, washed, and dried to obtain a precursor, which is then transversely stretched a second time, shrunk, and heat-set to obtain an ultra-thin, high-strength lithium-ion battery separator; A method for manufacturing an ultra-thin, high-strength lithium-ion battery separator, comprising:

6. 6. The method according to claim 5, wherein in step 1, the stirring temperature is 60 to 110°C, the stirring rotation speed is 40 to 60 RPM, and the stirring time is 20 to 40 min, and in step 2, the extruder is a twin-screw extruder having a screw rotation speed of 100 to 240 RPM and a screw aspect ratio of 56 to 68.

7. In step 2, the heating time is 30 to 60 seconds, and the heating temperature is 170 to 230°C, preferably 170 to 210°C; In step 2, the temperature of the quench roll in the melt casting is 15 to 25°C, 6. The method according to claim 5, wherein in step 2, the temperature of the thermodynamic single-phase melt to be melt-cast is 180 to 220°C.

8. In step 3, the longitudinal stretching and the first transverse stretching have stretching ratios of 5 to 15, the longitudinal stretching is performed at a temperature of 80 to 120°C and a speed of 25 to 250% / s, and the first transverse stretching is performed at a temperature of 110 to 140°C and a speed of 10 to 150% / s; In step 3, the second transverse stretching is performed at 120 to 150°C at a speed of 2 to 30% / s until the stretching ratio reaches 1.2 to 2 times, 6. The method according to claim 5, wherein in step 3, the shrinkage is performed by heating at a temperature of 120 to 150°C at a rate of 0.5 to 5% / s, and the heat shrinkage ratio is 5 to 25%.

9. 6. The method according to claim 5, wherein in step 3, the extraction and washing is performed in an extractant at 25 to 55°C for 60 to 240 seconds, and the extractant is a methylene chloride solution having a methylene chloride concentration of greater than 99%.

10. In step 3, the drying temperature is 40 to 110°C, and the drying time is 10 to 40 seconds.

6. The method according to claim 5, wherein in step 3, the heat setting temperature is 80 to 120° C. and the heat setting time is 5 to 50 seconds.

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