High-safety battery diaphragm and preparation method thereof

Abstract

The invention provides a high-safety battery diaphragm and a preparation method thereof. The diaphragm rupture area of the diaphragm in a hot nailing experiment at the temperature of 300 DEG C is smaller than 6 mm < 2 >, and the diaphragm rupture area in the hot nailing experiment at the temperature of 400 DEG C is smaller than 10 mm < 2 >. According to the present invention, the secondary stretching is changed into the transverse stretching and the longitudinal stretching through the process improvement, such that the short circuit area expansion of the diaphragm in the cell puncture experiment can be effectively avoided, the high thermal stability and high safety diaphragm can be achieved, and the further cell thermal runaway can be avoided.

Description

Technical Field

[0001] The present disclosure relates to the technical field related to battery separators, and in particular to a high-safety battery separator and a preparation method thereof. Background Art

[0002] With the widespread use of lithium-ion batteries in daily life, their safety is receiving increasing attention. The separator, a crucial component of lithium-ion batteries, separates the positive and negative electrodes. If the separator is damaged due to process defects (dust, burrs, lithium dendrites, etc.), external forces such as compression or needle puncture, causing a short circuit between the positive and negative electrodes, the battery could catch fire, explode, or engage in other dangerous situations. Therefore, as the first barrier to battery safety, the safety of the separator is crucial to the overall safety of the battery.

[0003] The cell needle penetration test is a key test for evaluating battery short-circuit safety. During the penetration test, the steel needle squeezes the cell, causing a local short circuit and releasing heat, which can cause the separator to melt and shrink. The extent of the separator damage caused by external force determines the initial short-circuit heat generation. This initial heat generation and the thermal stability of the separator determine the initial expansion rate of the short-circuit area. As the short-circuit area expands, the ohmic heat generated by the short circuit increases sharply, inducing other heat-generating side reactions within the battery. The coupling of these various heats and the separator short-circuit area ultimately causes the battery to enter an uncontrollable self-heating phase and thermal runaway. Therefore, when a battery is subjected to external compression or needle penetration, minimizing the extent of separator surface damage can effectively prevent large-scale short circuits and is an important means of improving battery needle penetration safety. Therefore, the deformation and rupture of the separator under external force, as well as the expansion of the separator puncture area at different temperatures, are crucial to preventing thermal runaway and ensuring cell safety.

[0004] Patent publication CN115377608A of the People's Republic of China describes a method for manufacturing a thermally stable composite separator for lithium batteries. By adding a diluent and a nucleating agent, a highly thermally stable PE-PP hybrid separator can be produced. The resulting hybrid separator maintains a low closed-cell temperature while also increasing the membrane rupture temperature. Patent publication CN108346766A of the People's Republic of China describes a thermally stable lithium-ion battery separator made from a copolymerized polyimide or a blended polyimide, and its preparation method.

[0005] However, there is currently no patent that improves the thermal stability of the separator from the base film itself. In view of this, the present disclosure is proposed. Summary of the Invention

[0006] Under the current state of the art, the thermal safety performance of lithium battery base membranes is poor. Therefore, the present disclosure provides the diaphragm with high battery cell safety performance by adjusting the rupture area of ​​the diaphragm at a specific temperature.

[0007] The first aspect of the present disclosure provides a highly safe battery separator, wherein the membrane rupture area is less than 6 mm in a hot nail puncture test at 300°C. 2 In the hot nail puncture test at 400℃, the broken membrane area is less than 10mm 2 .

[0008] In the embodiment of the present disclosure, the film rupture area is less than or equal to 5.6 mm in the hot nail puncture test at 300°C. 2 In the hot nail puncture test at 400℃, the broken film area is less than or equal to 9.8mm 2 .

[0009] In the embodiment of the present disclosure, the film rupture area in the hot nail puncture test at 300°C is 4.0 mm 2 Up to 5.6mm 2 In the hot nail puncture test at 400℃, the membrane rupture area is 8.0mm 2 Up to 9.8mm 2 .

[0010] In an embodiment of the present disclosure, the thickness is greater than or equal to 10 μm.

[0011] The second aspect of the present disclosure provides a method for preparing a high-safety battery separator, comprising: mixing a polyolefin and a porogen and then melt-extruding the mixture to form a cast sheet; biaxially stretching the sheet for the first time; extracting and drying the sheet; biaxially stretching the sheet for the second time; and heat setting.

[0012] In embodiments of the present disclosure, the polyolefin accounts for 20 wt % to 40 wt % of the total weight of the polyolefin and the porogen.

[0013] In an embodiment of the present disclosure, the second biaxial stretching has a longitudinal stretching ratio of 1 to 1.5 and a transverse stretching ratio of 1 to 2.

[0014] In an embodiment of the present disclosure, the stretching temperature of the second biaxial stretching is 40°C to 120°C.

[0015] In an embodiment of the present disclosure, the total average molecular weight of the polyolefin is 400,000 to 2.5 million.

[0016] In the embodiment of the present disclosure, the extrusion feed rate is 230 kg / h to 300 kg / h, and the melt temperature is 180° C. to 250° C.

[0017] The main steps of the current wet process for battery separators are: feeding, tape casting, stretching, extraction, drying, secondary transverse stretching, shaping, and winding. Usually, the secondary stretching after extraction and drying is limited to the transverse direction, and the molecular chains in the longitudinal direction are relatively fixed and cannot be effectively relaxed. The present disclosure changes the secondary stretching to stretching in both the transverse and longitudinal directions, and limits the stretching ratio to achieve the manufacture of a highly thermally stable and highly safe separator. In other words, the separator disclosed in the present disclosure can effectively prevent the expansion of the short-circuit area in the battery cell puncture test, thereby avoiding further thermal runaway of the battery cell. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 This is a time-dependent temperature / voltage curve of the diaphragm after acupuncture of Example 1;

[0019] Figure 2 This is a time-dependent temperature / voltage curve of the diaphragm after acupuncture in Example 2;

[0020] Figure 3 This is a time-dependent temperature / voltage curve of the diaphragm after acupuncture of Example 3;

[0021] Figure 4 This is a time-dependent temperature / voltage curve of the diaphragm after acupuncture of Example 4;

[0022] Figure 5 This is a time-dependent temperature / voltage curve of the diaphragm of Comparative Example 1 after acupuncture;

[0023] Figure 6 The time-dependent temperature / voltage curve of the diaphragm after acupuncture of Comparative Example 2 is shown; and

[0024] Figure 7 This is a time-dependent temperature / voltage curve of the diaphragm of Comparative Example 3 after acupuncture. DETAILED DESCRIPTION

[0025] The following describes the specific embodiments of the present invention in detail with reference to the accompanying drawings. It should be understood that the specific embodiments described herein are only used to illustrate and explain the present invention and are not intended to limit the present invention.

[0026] The endpoints of the ranges and any values ​​disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoints of each range, the endpoints of each range and individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered to be specifically disclosed herein.

[0027] The present disclosure achieves the manufacture of high-safety battery separators by adjusting the process means. The manufactured battery separators have a small membrane rupture area in the thermal needle puncture test, which can effectively improve the thermal safety performance of the battery.

[0028] The embodiments of the present disclosure are implemented as follows:

[0029] A high-safety battery separator with a rupture area of ​​less than 6mm in a hot nail puncture test at 300°C 2 In the hot nail puncture test at 400℃, the broken membrane area is less than 10mm 2 .

[0030] First, the hot nail penetration test may include, but is not limited to: piercing the battery separator with a steel nail with a maximum diameter of no more than 4 mm, a needle tip length of no more than 2 cm, and a needle tip diameter of approximately 0.2 mm, heated to 300°C or 400°C; and observing the battery separator under a microscope and calculating the rupture area. The hot nail penetration test preferably includes: piercing the battery separator 0.1 mm with a steel nail with a maximum diameter of no more than 4 mm, a needle tip length of no more than 2 cm, and a needle tip diameter of approximately 0.2 mm, heated to 300°C or 400°C at a rate of 5mm / s to 20mm / s and staying for 5 to 20 seconds; and observing the battery separator under a microscope and calculating the rupture area. The hot nail penetration test more preferably includes: piercing the battery separator 0.1 mm with a steel nail with a maximum diameter of no more than 4 mm, a needle tip length of no more than 2 cm, and a needle tip diameter of approximately 0.2 mm, heated to 300°C or 400°C at a rate of 10mm / s and staying for 10 seconds; and observing the battery separator under a microscope and using the connected software to measure the rupture diameter and calculate the rupture area.

[0031] In addition, the membrane rupture area in the hot nail puncture test at 300°C is preferably less than or equal to 5.6mm 2 , more preferably 4.0mm 2 , 4.2mm 2 , 4.4mm 2 , 4.6mm 2 , 4.8mm 2 , 5.0mm 2 , 5.2mm 2 , 5.4mm 2 , 5.6mm 2 The film rupture area in the hot nail puncture test at 400°C is preferably less than or equal to 9.8 mm 2 , more preferably 8.0mm 2 , 8.2mm 2 , 8.4mm 2 , 8.6mm 2 , 8.8mm 2 , 9.0mm2 , 9.2mm 2 , 9.4mm 2 , 9.6mm 2 , 9.8mm 2 Any value or the range between any two values ​​in .

[0032] Moreover, the thickness of the diaphragm is not limited to being greater than or equal to 10μm, and is preferably any value among 10μm, 10.2μm, 10.4μm, 10.6μm, 10.8μm, 11μm, 11.2μm, 11.4μm, 11.5μm, 11.6μm, 11.8μm, 12μm, 12.2μm, 12.4μm, 12.6μm, 12.8μm, 13μm, or the range between any two values.

[0033] The following preparation method can be used to prepare the separator of the above embodiment, but can also be used to prepare other separators and should not be limited in any way:

[0034] First, polyolefin is mixed with a porogen and then melt-extruded to form a cast sheet. The polyolefin may be, but is not limited to, one or more of polyethylene and polypropylene. The total average molecular weight of the polyolefin can be, but is not limited to, 400,000 to 2.5 million, preferably 400,000, 450,000, 500,000, 550,000, 600,000, 650,000, 700,000, 750,000, 800,000, 850,000, 900,000, 950,000, 1,050,000, 1,100,000, 1,150,000, 1,200,000, 1,250,000, 1,350,000, 1,400,000, 1,500,000, 1,550,000, 1,600,000, 1,650,000, 1,700,000, 1,750,000, 1,800,000, 1,850,000, 1,900,000, 1,950,000, 2,000,000, 2,050,000, 2,100,000, 2,150,000, 2,200,000, 2,250,000, 2,300,000, 2,350,000, 2,400,000, 2,450,000, or the range between any two of these values. The polyolefin may include, but is not limited to, polyolefins of various molecular weights, preferably including polyolefins with a molecular weight of 100,000 to 400,000, polyolefins with a molecular weight of 500,000 to 800,000, and polyolefins with a molecular weight of 1.3 million to 1.6 million. More preferably, polyolefins with a molecular weight of 100,000, 150,000, 200,000, 250,000, 300,000, 350,000, and 400,000, or any range between any two of these values; polyolefins with a molecular weight of 500,000, 550,000, 600,000, 650,000, 700,000, 750,000, and 800,000, or any range between any two of these values; and polyolefins with a molecular weight of 1.3 million, 1.35 million, 1.4 million, 1.45 million, 1.5 million, 1.55 million, and 1.6 million, or any range between any two of these values. The porogen may be, but is not limited to, one or more of white oil, paraffin oil, and polyethylene glycol. In addition, the polyolefin may account for, but is not limited to, 20 wt % to 40 wt % of the total weight of the polyolefin and the porogen, preferably any one of 20 wt %, 21.5 wt %, 23 wt %, 24.5 wt %, 26 wt %, 27.5 wt %, 29 wt %, 30.5 wt %, 32 wt %, 33.5 wt %, 35 wt %, 36.5 wt %, 38 wt %, 39.5 wt %, and 40 wt %, or a range between any two of these values. In addition, the extrusion feeding rate may be, but is not limited to, 230 kg / h to 300 kg / h, preferably any one of 230 kg / h, 240 kg / h, 250 kg / h, 260 kg / h, 270 kg / h, 280 kg / h, 290 kg / h, and 300 kg / h, or the range between any two values; the melt temperature may be, but is not limited to, 180°C to 250°C, preferably any one of 180°C, 185°C, 190°C, 195°C, 200°C, 205°C, 210°C, 215°C, 220°C, 225°C, 230°C, 235°C, 240°C, 245°C, and 250°C, or the range between any two values.Moreover, the width of the cast sheet during formation can be but is not limited to 300 mm to 1500 mm, preferably any value among 300 mm, 400 mm, 500 mm, 600 mm, 700 mm, 800 mm, 900 mm, 1000 mm, 1100 mm, 1200 mm, 1300 mm, 1400 mm, 1500 mm, or the range between any two values.

[0035] Next, the cast sheet is subjected to a first biaxial stretching. The first biaxial stretching can be, but is not limited to, synchronous stretching or asynchronous stretching. "Synchronous stretching" means that longitudinal and transverse stretching are performed simultaneously, while "asynchronous stretching" means that longitudinal and transverse stretching are not performed simultaneously. Asynchronous stretching is preferably performed in the longitudinal direction first and then in the transverse direction, or in the transverse direction first and then in the longitudinal direction, and more preferably in the longitudinal direction first and then in the transverse direction. Regardless of whether synchronous stretching or asynchronous stretching is adopted, the stretching temperature of the first double-draw stretching may be, but is not limited to, 50°C to 150°C, preferably 50°C, 55°C, 60°C, 65°C, 70°C, 75°C, 80°C, 85°C, 90°C, 95°C, 100°C, 105°C, 110°C, 115°C, 120°C, 125°C, 130°C, 135°C, 140°C, 145°C, or the range between any two values. The maximum stretch ratio in the longitudinal direction of the first double-draw stretching may be, but is not limited to, 1 to 2, preferably 1 to 2. The maximum stretching ratio in the transverse direction of the first double-draw stretching can be, but is not limited to, 4 to 15, preferably 4, 4.5, 5, 5.5, 6, 6.5, 7, 7.5, 8, 8.5, 9, 9.5, 10, 10.5, 11, 11.5, 12, 12.5, 13, 13.5, 14, 14.5, 15, or the range between any two values.

[0036] The flakes are then extracted and dried. Extraction can wash away the oil components in the flakes, and can be performed by, but not limited to, an extraction tank, preferably containing dichloromethane.

[0037] The cast sheet is then subjected to a second biaxial stretching. The second biaxial stretching may also be, but is not limited to, synchronous stretching or asynchronous stretching. The asynchronous stretching is preferably performed by first stretching in the longitudinal direction and then in the transverse direction, or first stretching in the transverse direction and then in the longitudinal direction, and more preferably by first stretching in the transverse direction and then in the longitudinal direction. Regardless of whether synchronous stretching or asynchronous stretching is adopted, the stretching temperature of the second double-stretching may be, but not limited to, 40°C to 120°C, preferably any value among 40°C, 45°C, 50°C, 55°C, 60°C, 65°C, 70°C, 75°C, 80°C, 85°C, 90°C, 95°C, 100°C, 105°C, 110°C, 115°C, and 120°C, or a range between any two values; the longitudinal stretching ratio of the second double-stretching may be, but not limited to, 1 to 1.5, preferably any value among 1, 1.1, 1.2, 1.3, 1.4, and 1.5, or a range between any two values; the transverse stretching ratio of the second double-stretching may be, but not limited to, 1 to 2, preferably any value among 1, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, and 2, or a range between any two values.

[0038] Finally, heat setting is performed. Heat setting stabilizes and solidifies the structure of the cast sheet. The heat setting temperature can be, but is not limited to, 60°C to 140°C, preferably any one of 60°C, 65°C, 70°C, 75°C, 80°C, 85°C, 90°C, 95°C, 100°C, 105°C, 110°C, 115°C, 120°C, 125°C, 130°C, 135°C, and 140°C, or any range between any two of these values. Furthermore, after heat setting, the sheet can be rolled up for later storage.

[0039] The present disclosure is illustrated by the following examples:

[0040] Example 1

[0041] Three PE materials with molecular weights of 250,000, 650,000, and 1.45 million were mixed with white oil and melt-extruded. The total molecular weight of the PE material was 1.15 million, and the weight ratio of the PE material to the white oil was 26% by weight. The extrusion feed rate was 260 kg / h, the melt temperature was between 190°C and 230°C, and the resulting cast sheet had a width of 900 mm. The sheet was first stretched longitudinally at a stretch ratio of 1.5, then stretched transversely, with a maximum stretch ratio of 7.5. The oil component was washed away in an extraction tank, with dichloromethane as the extraction solvent. After drying, the sheet was first stretched transversely and then longitudinally, with a stretch ratio of 1.6 for the transverse stretching and a stretch ratio of 1.2 for the longitudinal stretching. Finally, it was heat-set and wound at a heat-setting temperature between 60°C and 140°C, with a product thickness of 12 μm.

[0042] Example 2

[0043] Three PE materials with molecular weights of 250,000, 650,000, and 1.45 million were mixed with white oil and melt-extruded. The total molecular weight of the PE material was 1.3 million, and the weight ratio of the PE material to the white oil was 24.5% by weight. The extrusion feed rate was 260 kg / h, and the melt temperature was between 190°C and 230°C. The resulting cast sheet had a width of 900 mm. The sheet was first stretched longitudinally at a draw ratio of 1.5, then stretched transversely, with a maximum draw ratio of 7.6. The oil component was washed away in an extraction tank, where the extraction solvent was dichloromethane. After drying, the sheet was first stretched transversely and then longitudinally, with a draw ratio of 1.5 and a draw ratio of 1.2. Finally, it was heat-set and wound at a heat-setting temperature between 60°C and 140°C, and the product thickness was 12 μm.

[0044] Example 3

[0045] The same as Example 1, the difference is that the stretching process after extraction is synchronous stretching.

[0046] Example 4

[0047] The method is the same as Example 1, except that the stretching process before extraction and the stretching process after extraction are both synchronous stretching.

[0048] Comparative Example 1

[0049] The same as Example 1, except that the maximum stretching ratio of the transverse stretching before extraction is 11, and the stretching process after extraction only includes transverse stretching without longitudinal stretching.

[0050] Comparative Example 2

[0051] The same as Example 2, except that the maximum stretching ratio of the transverse stretching before extraction is 12, and the stretching process after extraction only includes transverse stretching without longitudinal stretching.

[0052] Comparative Example 3

[0053] The same as Example 1, except that the extrusion feeding speed is 220 kg / h.

[0054] The specific operation of the hot nail penetration test is as follows:

[0055] A steel nail with a maximum diameter of no more than 4mm is heated to a maximum of 400°C and pierced through the battery separator by 0.1mm from a distance of 30mm at a rate of 10mm / s. The nail is then observed under a digital microscope to measure the diameter of the diaphragm hole and calculate the area of ​​the hole.

[0056] The specific operation of the battery cell puncture test is as follows:

[0057] 1. A soft-pack battery is made by winding or lamination process, wherein the positive electrode material used is a ternary or lithium iron phosphate material, wherein the nickel component in the ternary positive electrode material is less than 0.5, and the maximum capacity of the battery is 2Ah.

[0058] 2. After fully charging the soft-pack battery, place it in an explosion-proof box and pierce the battery to half its depth with a steel needle to simulate the situation where a metal foreign body penetrates into the battery. The diameter of the steel needle is 5 mm.

[0059] 3. Detect the temperature fluctuation of the battery in the puncture area until the temperature returns to room temperature, and observe whether the battery has thermal runaway phenomena such as fire and smoke. At the same time, detect the voltage change of the battery to evaluate the degree of thermal runaway of batteries equipped with different diaphragms during foreign object puncture.

[0060] Table 1 shows the membrane rupture areas of Examples 1 to 4 and Comparative Examples 1 to 3 at 300° C. and 400° C. This shows that Examples 1 to 4 can effectively improve the heat resistance of polyethylene membranes, and the manufactured polyethylene membranes have high thermal safety performance.

[0061] Table 1

[0062]

[0063] Please see Figures 1 to 7 The following are time-dependent temperature / voltage curves after acupuncture for Examples 1 to 4 and Comparative Examples 1 to 3, respectively. As can be seen from the figures, the peak temperatures after acupuncture for Examples 1 to 4 were all around 100°C, with no signs of fire or smoke, and relatively slow voltage decay. In contrast, the peak temperatures after acupuncture for Comparative Examples 1 to 3 reached 362.6°C, with noticeable signs of fire and smoke. This demonstrates that Examples 1 to 4 achieve the beneficial effect of preventing the expansion of the short-circuit area during the cell acupuncture test, thereby preventing further heat generation.

[0064] The above contents involving common knowledge are not described in detail and can be understood by those skilled in the art.

[0065] The above descriptions are merely specific embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention shall be included within the scope of protection of the present invention. The technical scope of the present invention is not limited to the contents of the specification and must be determined according to the scope of the claims.

Claims

1. A high-safety battery separator, characterized in that: In the hot nail puncture test at 300℃, the film rupture area is less than 6mm 2 In the hot nail puncture test at 400℃, the broken membrane area is less than 10mm 2 .

2. The high-safety battery separator according to claim 1, characterized in that: In the hot nail puncture test at 300℃, the film rupture area is less than or equal to 5.6mm 2 In the hot nail puncture test at 400℃, the broken film area is less than or equal to 9.8mm 2 .

3. The high-safety battery separator according to claim 1, characterized in that: The film rupture area in the hot nail puncture test at 300°C is 4.0 mm 2 Up to 5.6mm 2 In the hot nail puncture test at 400℃, the membrane rupture area is 8.0mm 2 Up to 9.8mm 2 .

4. The high-safety battery separator according to claim 1, characterized in that: The thickness is greater than or equal to 10 μm.

5. A method for preparing a high-safety battery separator, characterized in that: include: The polyolefin and the porogen are mixed and then melt-extruded to form a cast sheet; biaxially stretching the cast sheet for the first time; Extraction and drying; biaxially stretching the cast sheet for a second time; as well as Heat setting.

6. The method for preparing a high-safety battery separator according to claim 5, characterized in that: The polyolefin accounts for 20 wt % to 40 wt % of the total mass of the polyolefin and the porogen.

7. The method for preparing a high-safety battery separator according to claim 5, characterized in that: The second biaxial stretching has a longitudinal stretching ratio of 1 to 1.5 and a transverse stretching ratio of 1 to 2.

8. The method for preparing a high-safety battery separator according to claim 7, characterized in that: The stretching temperature of the second biaxial stretching is 40°C to 120°C.

9. The method for preparing a high-safety battery separator according to claim 5, characterized in that: The total average molecular weight of the polyolefin is 400,000 to 2.5 million.

10. The method for preparing a high-safety battery separator according to claim 5, characterized in that: The extrusion feeding speed is 230kg / h to 300kg / h, and the melting temperature is 180°C to 250°C.

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