Extremely thin and high-strength lithium ion battery separator and preparation method therefor

Abstract

An extremely thin and high-strength lithium ion battery separator and a preparation method therefor. The preparation method for the extremely thin and high-strength lithium ion battery separator comprises the following steps: mixing a nucleating agent, ultra-high molecular weight polyethylene powder, an antioxidant, and a pore-forming agent, and uniformly stirring to obtain a premixed raw material; injecting the premixed raw material into an extruder, heating to obtain a thermodynamic single-phase melt, and carrying out melt casting to obtain a crystallized cast sheet, wherein the ratio of the linear speed of a chill roll in the melt casting to the flow rate at which the thermodynamic single-phase melt is subjected to melt casting is 5-50; and carrying out longitudinal stretching and first transverse stretching on the crystallized cast sheet, carrying out extraction, washing and drying, and carrying out second transverse stretching, retraction and heat setting to obtain the extremely thin and high-strength lithium ion battery separator. By using the ultra-high molecular weight polyethylene powder, nucleating agent heterogeneous crystallization, and high-magnification casting and stretching, the multiple effects allow the crystallinity of a thin film to be improved, and the thinning of the separator is realized, so that the high-strength and extremely thin lithium ion battery separator is prepared.

Description

Ultra-thin high-strength lithium-ion battery separator and preparation method thereof Technical Field

[0001] The present invention belongs to the technical field of batteries, and in particular relates to an ultra-thin and high-strength lithium-ion battery separator and a preparation method thereof. Background Art

[0002] Lithium-ion battery separators are crucial components of lithium batteries. They serve as an insulating layer, separating the positive and negative electrode materials and providing a pathway for lithium ion migration. With the continuous innovation of separator manufacturing technology, the trend toward thinner and more functional separators has become mainstream. Thinner lithium-ion battery separators have lower internal resistance, higher ionic conductivity, and occupy less volume within the lithium-ion cell, making them ideal for high-rate, high-capacity lithium-ion batteries. However, as separators become thinner, their puncture resistance decreases, compromising safety. This poses a significant bottleneck, leading to serious safety incidents in battery applications. Currently, the mainstream separator product thickness in the industry ranges from 7 to 12 μm. Ultra-thin separators with a thickness below 7 μm are considered ultra-thin. Ultra-thin separators have very low needle puncture strengths, generally below 3N, with strengths per micron within 0.5N / μm. Therefore, overcoming this strength bottleneck is the key to achieving thinner and lighter separators.

[0003] Summary of the Invention

[0004] In view of the shortcomings of the prior art, the present invention aims to provide an extremely thin and high-strength lithium-ion battery separator.

[0005] Another object of the present invention is to provide a method for preparing the above-mentioned ultra-thin and high-strength lithium-ion battery separator, which achieves a double improvement in product crystallinity by high-ratio stretching of melt casting and the addition of a nucleating agent, while initially achieving a thinning of the battery separator thickness.

[0006] The purpose of the present invention is achieved through the following technical solutions.

[0007] An ultra-thin, high-strength lithium-ion battery separator is prepared from a nucleating agent, an ultra-high molecular weight polyethylene powder, an antioxidant, and a pore-forming agent. The ratio of the nucleating agent, the ultra-high molecular weight polyethylene powder, the antioxidant, and the pore-forming agent is (0.1-1): (15-35): (0.01-0.1): (65-85) by mass. The nucleating agent is a mixture of pimelic acid and calcium stearate. The ratio of pimelic acid to calcium stearate is (0.5-4): (1-4.5), preferably (0.8-3.8): (1.2-4.2), by mass.

[0008] In the above technical solution, 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) by mass.

[0009] In the above technical solution, the weight average molecular weight of the ultra-high molecular weight polyethylene powder is 4 to 5 million.

[0010] In the above technical solution, the pore-forming agent is white oil and / or dioctyl terephthalate (DOTP).

[0011] In the above technical solution, the antioxidant is pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate (antioxidant 1010).

[0012] The method for preparing the ultra-thin, high-strength lithium-ion battery separator comprises the following steps:

[0013] Step 1: mixing a nucleating agent, ultra-high molecular weight polyethylene powder, an antioxidant, and a pore-forming agent, and stirring until uniform to obtain a premixed raw material;

[0014] In step 1, the stirring temperature is 60-110° C., the stirring speed is 40-60 RPM, and the stirring time is 20-40 min.

[0015] Step 2: injecting the premixed raw material into an extruder and heating it to obtain a thermodynamic single-phase melt, and melt-casting the thermodynamic single-phase melt to obtain a crystalline cast sheet, wherein the ratio of the linear velocity of the chill roll in the melt casting to the flow velocity of the thermodynamic single-phase melt during melt casting is 5 to 50, the unit of the linear velocity is m / min, and the unit of the flow velocity is m / min;

[0016] 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.

[0017] In step 2, the temperature of the chill roller in the melt casting is 15-25°C.

[0018] In step 2, the extruder is a twin-screw extruder, the screw speed of the twin-screw extruder is 100-240RPM, and the screw length-diameter ratio is 56-68.

[0019] In step 2, the temperature of the thermodynamically single-phase melt during melt casting is 180-220°C.

[0020] Step 3: The crystalline cast sheet is longitudinally stretched and first transversely stretched to obtain a film, washed, and dried to obtain a precursor, and the precursor is transversely stretched a second time, retracted, and heat-set to obtain an extremely thin and high-strength lithium-ion battery separator.

[0021] In step 3, the stretching ratios of the longitudinal stretching and the first transverse stretching are 5 to 15 times respectively, and the longitudinal stretching is performed at a temperature of 80 to 120° C. and a speed of 25 to 250% / s.

[0022] In step 3, the first transverse stretching is performed at a temperature of 110 to 140° C. and a speed of 10 to 150% / s.

[0023] In step 3, the extraction and washing is carried out in an extractant at 25 to 55° C. for 60 to 240 seconds. The extractant is a dichloromethane solution, and the concentration of dichloromethane in the dichloromethane solution is greater than 99%.

[0024] In step 3, the drying temperature is 40 to 110° C., and the drying time is 10 to 40 seconds.

[0025] In step 3, the second transverse stretching is performed at 120-150° C. at a speed of 2-30% / s to a stretching ratio of 1.2 to 2 times.

[0026] In step 3, the shrinking is performed by heating and shrinking at a temperature of 120 to 150° C. at a speed of 0.5 to 5% / s, and the heating shrinkage ratio is 5 to 25%.

[0027] In step 3, the heat setting temperature is 80-120° C., and the heat setting time is 5-50 seconds.

[0028] Compared with the prior art, the present invention has the following beneficial effects:

[0029] 1. The present invention provides a method for preparing an ultra-thin, high-strength lithium-ion battery separator. This method utilizes ultra-high molecular weight polyethylene powder, nucleating agent heterogeneous crystallization, and high-rate cast stretching to enhance the crystallinity of the film while simultaneously thinning the separator, thereby producing a high-strength, ultra-thin lithium-ion battery separator.

[0030] 2. The ultra-thin and 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 puncture strength of 0.83 to 1.02 N / um. The ionic conductivity is more than 1.2 times higher than that of conventional films (Comparative Example 1), and the puncture strength exceeds 0.8 N / um, which is more than 60% higher than that of conventional films. The separator is both light and thin and safe, and has good application prospects and economic benefits. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] FIG1 is a SEM of a cross section of an ultra-thin, high-strength lithium-ion battery separator prepared in Example 2;

[0032] FIG2 is a SEM of a cross section of the ultra-thin, high-strength lithium-ion battery separator prepared in Example 3. DETAILED DESCRIPTION

[0033] The technical solution of the present invention is further described below with reference to specific embodiments.

[0034] % / s: The unit of stretching or retraction speed, which refers to the ratio of the diaphragm to stretch or retract at its original size per unit time (1S).

[0035] Heating shrinkage ratio: heating shrinkage ratio = 1-(shrinkage size / original size).

[0036] The raw materials and their manufacturers involved in the following examples are as follows:

[0037] Nucleating agent: a mixture of pimelic acid and calcium stearate, purchased from Chengdu West Asia Chemical Co., Ltd.

[0038] Ultra-high molecular weight polyethylene: Hankook Chemical Industry Co., Ltd., South Korea;

[0039] Antioxidant: 1010, Tianjin Li'anlong New Materials Co., Ltd.

[0040] White oil: 50#, Zhejiang Zhengxin Petroleum Technology Co., Ltd.

[0041] DOTP: analytical grade, Shandong Lanfan Chemical Co., Ltd.

[0042] The instruments and their model information involved in the following embodiments are as follows:

[0043] Diaphragm production line: MZSTER, South Korea, width 4m, line speed 50m / min;

[0044] Mahr thickness gauge: Model C1216, manufactured by Mahr Precision Measuring Instruments Co., Ltd., Germany;

[0045] Analytical balance: FA2204, Shanghai Sheyan Instrument Co., Ltd.

[0046] Water pressure meter: AAQ-5, PMI Instruments, USA;

[0047] Conductivity tester: Model S2021, Jiangsu Chuanyuan Technology Co., Ltd.

[0048] Puncture strength tester: Model DY05, Jinan Sike Testing Technology Co., Ltd.

[0049] Implementation principle: The preparation method of the present invention causes heterogeneous crystallization when the polyethylene and the pore-forming agent are cooled and phase-separated during the casting stage, thereby improving the crystallinity of the crystalline casting sheet; at the same time, when casting the melt, a chilled roller is used to stretch the crystalline casting sheet at a high ratio, so that the polyethylene molecules are regularly arranged longitudinally, further improving the crystallinity of the crystalline casting sheet. The multiple effects improve the crystallinity and simultaneously achieve thinning of the diaphragm, thereby producing an extremely thin and high-strength lithium-ion battery diaphragm.

[0050] In the technical solution of the present invention, during the melt casting process, the thermodynamic single-phase melt is attached to the roller surface of the chill roller in the form of a sheet to obtain a crystalline casting sheet. The chill roller stretches the crystalline casting sheet at an ultra-high ratio, which can make the polyethylene molecular chains in the melt casting state quickly and regularly arrange along the stretching direction, and oriented crystallization occurs. During the cooling process of the chill roller, the polyethylene and the pore-forming agent undergo solid-liquid phase separation due to the temperature dropping below the phase separation temperature line, and the polyethylene forms crystals. The addition of the nucleating agent can accelerate the crystallization of the polyethylene with the nucleating agent as the heterogeneous crystal nucleus, thereby improving the crystallinity of the casting sheet. The higher the crystallinity, the greater the strength of the product.

[0051] The pore-forming agent has better compatibility with the extractant, so that the pore-forming agent in the film is extracted, and the extractant is dried and volatilized in a drying room, and the volume space occupied by the pore-forming agent in the film is replaced to form pores.

[0052] The second transverse stretching is a small ratio stretching, which further improves the crystallinity of the film, solidifies the microporous structure of the film, and increases the strength of the film through the dual effects of heating and molecular orientation. In addition, heating and shrinking can eliminate the internal stress generated during the stretching process and improve the thermal stability of the film.

[0053] In the following examples, the thickness, porosity, ionic conductivity and needle penetration strength test methods are all based on the national standard "Polyolefin Separator for Lithium Ion Batteries" (GB / T 36363-2018); the pore size is tested using a water pressure tester.

[0054] Examples 1 to 4

[0055] An ultra-thin, high-strength lithium-ion battery separator is prepared from a nucleating agent, an ultra-high molecular weight polyethylene powder, an antioxidant, and a pore-forming agent. The ratio of the nucleating agent, the ultra-high molecular weight polyethylene powder, the antioxidant, and the pore-forming agent is X, calculated by weight. The nucleating agent is a mixture of pimelic acid and calcium stearate, and the ratio of pimelic acid to calcium stearate is Y, calculated by weight. The ultra-high molecular weight polyethylene powder has a weight-average molecular weight of 400. The pore-forming agent is white oil. The antioxidant is pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate (antioxidant 1010).

[0056] The method for preparing the ultra-thin, high-strength lithium-ion battery separator comprises the following steps:

[0057] Step 1: Mix a nucleating agent, ultra-high molecular weight polyethylene powder, an antioxidant, and a pore-forming agent in a premixing tank, and stir at 70° C. and 50 RPM for 30 minutes until uniform, to obtain a premixed raw material (the ultra-high molecular weight polyethylene powder is swollen at this time);

[0058] Step 2: injecting the premixed raw materials into the feed port of a co-rotating twin-screw extruder with a screw aspect ratio of 64 at a uniform speed through a pneumatic diaphragm pump, heating the extruder at 200° C. for 45 seconds at a speed of 170 RPM in the twin-screw extruder to obtain a thermodynamic single-phase melt, extruding the 200° C. thermodynamic single-phase melt along a melt pipeline through a T-die for melt casting, and attaching the sheet-like thermodynamic single-phase melt to a chill roll at 20° C. for quenching to obtain a crystalline casting sheet, wherein the ratio of the linear velocity of the chill roll to the flow velocity of the thermodynamic single-phase melt during melt casting is A, the unit of the linear velocity is m / min, and the unit of the flow velocity is m / min;

[0059] Step 3, longitudinally stretching the crystallized cast sheet: the crystallized cast sheet is pulled along the roller of the longitudinal stretching machine, and stretched to 10 times (i.e., the stretching ratio is 10 times) at a speed of 100% / s at 110°C to obtain a thin film with a high degree of longitudinal orientation; the thin film with a high degree of longitudinal orientation is then clamped by a horizontal stretching machine clamp, and stretched to 10 times (i.e., the stretching ratio is 10 times) at a speed of 80% / s in a 125°C oven to obtain a thin film with a high degree of transverse orientation, and the film is pulled by an extraction roller and immersed in a 25°C dichloromethane solution (dichloromethane in the dichloromethane solution) The precursor was clamped by a horizontal stretching machine clamp, stretched to 1.5 times at a speed of 15% / s in an oven at 130°C, and then heated and shrunk (transverse width shrinkage) at a speed of 2% / s at 135°C, with a heating shrinkage (transverse width shrinkage) ratio of 15%, and finally heated and hauled on a hot roller at 100°C for 20s for heat setting, and then hauled along a roller and wound by a winder to obtain an extremely thin and high-strength lithium-ion battery separator.

[0060] Table 1

[0061] Figures 1 and 2 show SEM cross-sectional images of the ultra-thin, high-strength lithium-ion battery separators prepared in Examples 2 and 3. The nucleating agent addition weight fraction was gradually increased from 0.4 to 1.2, and the nucleating agent ratio was varied from pimelic acid less than calcium stearate, to pimelic acid equal to calcium stearate, and finally to pimelic acid greater than calcium stearate, to find the nucleating agent addition amount and ratio that maximized crystallinity.

[0062] Comparative Example 1

[0063] A lithium-ion battery separator is prepared from ultra-high molecular weight polyethylene powder, an antioxidant and a pore-forming agent. The ratio of the ultra-high molecular weight polyethylene powder, the antioxidant and the pore-forming agent is 25:0.02:75 in parts by mass. The ultra-high molecular weight polyethylene powder has a weight-average molecular weight of 4 million, the pore-forming agent is white oil, and the antioxidant is pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate (antioxidant 1010).

[0064] The preparation method of the above-mentioned lithium-ion battery separator is basically the same as the preparation method of the ultra-thin and high-strength lithium-ion battery separator in Example 1. The only difference is that in this comparative example, the ratio of the linear velocity of the chilled roller to the flow rate of the thermodynamic single-phase melt during melt flow casting is 1 and step 1 in this comparative example does not contain a nucleating agent.

[0065] Comparative Example 2

[0066] A lithium ion battery separator, the same as that of Example 4.

[0067] The preparation method of the above-mentioned lithium-ion battery separator is basically the same as the preparation method of the ultra-thin high-strength lithium-ion battery separator in Example 4. The only difference is that in this comparative example, the ratio of the linear velocity of the chilled roller to the flow rate of the thermodynamic single-phase melt during melt flow is 1.

[0068] Comparative Example 3

[0069] A lithium ion battery separator, the same as that of Comparative Example 1.

[0070] The preparation method of the above-mentioned lithium-ion battery separator is basically the same as the preparation method of the lithium-ion battery separator in Comparative Example 1. The only difference is that in this comparative example, the ratio of the linear velocity of the chilled roller to the flow rate of the thermodynamic single-phase melt during melt flow is 50.

[0071] Table 2 shows the thickness, porosity, average pore size, ionic conductivity (tested at 25°C), and needle puncture strength characteristics of the ultra-thin, high-strength lithium-ion battery separators prepared in Examples 1-4 and Comparative Examples 1-3. The ultra-thin, high-strength lithium-ion battery separators prepared in Examples 1-4 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 needle puncture strength of 0.83 to 1.02 N / μm.

[0072] It can be seen from Table 2 that with the increase of the ratio of the linear speed of the chilled roller to the flow rate of the thermodynamic single-phase melt during melt casting, the porosity and pore size of the ultra-thin high-strength lithium-ion battery separator gradually increase, and the ionic conductivity of the ultra-thin high-strength lithium-ion battery separator gradually increases, which is more than 1.2 times higher than that of the film that has not been melt-cast and stretched at a high ratio. The needle puncture strength exceeds 0.8N / um, which is more than 60% higher than that of the film that has not been melt-cast and stretched at a high ratio. At the same time, as the amount of nucleating agent added gradually increases within the scope of the technical solution of the present invention, the needle puncture strength of the film gradually increases, but it is significantly reduced after exceeding the range specified by the technical solution of the present invention. The main reason is that the addition of the nucleating agent can increase the crystallinity of the film, thereby increasing the strength of the film. However, when added in excess, the nucleating agent, as a foreign matter in the system, destroys the system balance of the homogeneous melt, which in turn makes the crystallinity of the film smaller and the strength lower.

[0073] It can be seen from Comparative Examples 1 to 3 that when only a nucleating agent is added or only melt casting high-ratio stretching is performed, although the ionic conductivity and needle puncture strength of the product can be improved to a certain extent, it is far less significant than the synergistic effect of the nucleating agent and melt casting high-ratio stretching.

[0074] Table 2

[0075] The above is an exemplary description of the present invention. It should be noted that, without departing from the core of the present invention, any simple deformation, modification or other equivalent replacement that can be made by other skilled in the art without expending creative labor falls within the scope of protection of the present invention.

Claims

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

2. The ultra-thin high-strength lithium-ion battery separator according to claim 1, characterized in that: The weight average molecular weight of the ultra-high molecular weight polyethylene powder is 4 to 5 million.

3. The ultra-thin high-strength lithium-ion battery separator according to claim 1 or 2, characterized in that: The pore-forming agent is white oil and / or dioctyl terephthalate.

4. The ultra-thin high-strength lithium-ion battery separator according to claim 3, characterized in that: The antioxidant is pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate].

5. The method for preparing an ultra-thin high-strength lithium-ion battery separator according to any one of claims 1 to 4, characterized in that: The following steps are involved: 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 premixed 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 crystalline cast sheet, wherein the ratio of the linear velocity of the chilled roller in the melt casting to the flow velocity of the thermodynamic single-phase melt during melt casting is 5 to 50, the unit of the linear velocity is m / min, and the unit of the flow velocity is m / min; Step 3, the crystalline cast sheet is longitudinally stretched and first transversely stretched to obtain a film, washed, dried to obtain a precursor, and the precursor is transversely stretched a second time, shrunk, and heat-set to obtain an extremely thin and high-strength lithium-ion battery separator.

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

7. The preparation method according to claim 5, characterized in that: 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 chilled roller in the melt casting is 15 to 25° C.; In step 2, the temperature of the thermodynamic single-phase melt during melt casting is 180-220°C.

8. The preparation method according to claim 5, characterized in that: In step 3, the stretching ratios of the longitudinal stretching and the first transverse stretching are 5 to 15 times respectively, the longitudinal stretching is at 80 to 120° C. and at a speed of 25 to 250% / s; the first transverse stretching is at 110 to 140° C. and at a speed of 10 to 150% / s; In step 3, the second transverse stretching is performed at 120-150° C. at a speed of 2-30% / s to 1.2-2 times; In step 3, the shrinking is performed by heating and shrinking at a temperature of 120 to 150° C. at a speed of 0.5 to 5% / s, and the heating and shrinking ratio is 5 to 25%.

9. The preparation method according to claim 5, characterized in that: In step 3, the extraction and washing is performed in an extractant at 25 to 55° C. for 60 to 240 seconds, the extractant is a dichloromethane solution, and the concentration of dichloromethane in the dichloromethane solution is greater than 99%.

10. The preparation method according to claim 5, characterized in that: In step 3, the drying temperature is 40 to 110° C., and the drying time is 10 to 40 seconds. In step 3, the heat setting temperature is 80-120° C., and the heat setting time is 5-50 seconds.

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