Preparation method of lithium ion battery wet diaphragm

Abstract

The invention relates to a preparation method of a wet-process lithium battery diaphragm, which comprises the following steps: producing an oil film semi-finished product by using film extrusion and blow molding equipment, carrying out longitudinal stretching, extraction and heat setting procedures, and finally rolling. In the preparation process, the product appearance defect problem of the product is controlled and the thermal stability performance characteristic of the product is improved by controlling factor parameters such as the process temperature, the blow-up ratio and the traction ratio in the blow molding process.

Description

Technical Field

[0001] The invention belongs to the technical field of lithium ion batteries, and in particular relates to a method for preparing a lithium ion battery wet-process diaphragm. Background Art

[0002] With the increasing awareness of environmental protection and the increasing pressure on the living environment, the status of green and clean energy in the field of power energy is gradually improving. As an important direction in the field of energy conversion and energy storage, lithium-ion batteries occupy an important position in it. Lithium-ion batteries have the characteristics of high energy density, high operating voltage, long cycle life, low self-discharge rate, and green environmental protection, so they are widely used in electronic products, electric vehicles and other fields.

[0003] In lithium-ion batteries, the separator is mainly used to separate the positive and negative electrodes to prevent short circuits while allowing the transfer of ions. As an important component of lithium-ion batteries, the performance of the separator will directly affect the battery's capacity, internal resistance, cycle and self-discharge.

[0004] At present, the main preparation methods of lithium-ion battery separators in commercial use are wet film making and dry film making. The dry method is divided into uniaxial stretching and biaxial stretching, and the wet method is divided into two types. The synchronous biaxial stretching process is characterized by good product consistency, but it is not suitable for adjusting the product's strength in all directions; asynchronous biaxial stretching can easily adjust the strength in all directions, and has higher strength, but the consistency of longitudinal and transverse stretching is slightly poor, and the product's appearance defects and thermal stability performance also urgently need to be further improved. Summary of the invention

[0005] A method for preparing a wet-process lithium battery separator comprises the following steps:

[0006] a) Feeding and extruding: mixing the ultra-high molecular weight polyethylene with paraffin oil, and melt-mixing to form a uniform melt;

[0007] b) Transverse stretching: filtering the melt obtained in step a) and then stretching it transversely, the transverse stretching ratio is 11-15 (for example, it can be 11, 12, 13, 14, 15), and the transverse stretching temperature is 110-140° C.;

[0008] c) cooling and shaping the oil film obtained in step b);

[0009] d) longitudinal stretching: longitudinally stretching the oil film obtained in step c), with a longitudinal stretching ratio of 11-15 (for example, 11, 12, 13, 14, 15), a longitudinal stretching temperature of 20-140° C., and cooling to set;

[0010] e) Extraction and drying: The oil film obtained in step d) is subjected to extraction and heat setting to obtain a lithium-ion battery separator.

[0011] Furthermore, a) the melt mixing temperature is 190°C to 225°C.

[0012] Furthermore, b) the transverse stretching adopts a film blowing process: the melt obtained in step a) is filtered and then enters the air ring of the film blowing mechanism, and the air ring injects compressed air into the oil film parison through the blowing pipe head of the film blowing mechanism, thereby transversely stretching the oil film parison, and the annular oil film parison forms a tubular film bubble. During the film blowing process, the blowing ratio is 11-15, the film bubble is cooled, and then passed through a herringbone plate with gaps, and the film bubble is squeezed into a double-layer sheet oil film through the gaps of the herringbone plate.

[0013] The blowing ratio refers to the ratio between the diameter of the film bubble after blowing and the extrusion width of the extruder die head.

[0014] Furthermore, when the film blowing process is adopted, b) also includes the step of cooling the double-layered sheet oil film, cutting one side of the double-layered sheet oil film with a cutter, and unfolding the cut oil film to form a layer of oil film.

[0015] Furthermore, the cutting is to spread the double-layer sheet-like oil film flat on a cutting platform, and cut the bottom surface of the oil film with a cutting knife with the blade facing upward.

[0016] Furthermore, the transverse stretching ratio is equal to the longitudinal stretching ratio.

[0017] Furthermore, the transverse stretching ratio is 11-14, and the longitudinal stretching ratio is 11-14.

[0018] The film blowing process of the present invention can adopt the existing film blowing process in the prior art, or adopt the film blowing process described in the patent CN115149205A.

[0019] Furthermore, in the single-layer oil film obtained by transverse stretching or the double-layer flaky oil film obtained by the film blowing process, the thickness of the single-layer oil film is 1600-2000 μm.

[0020] Furthermore, c) the cooling temperature is 10-40°C.

[0021] Furthermore, d) the longitudinal stretching temperature is 110-140°C, and d) before the longitudinal stretching, a preheating process at 40-100°C is also included.

[0022] Furthermore, e) the extraction is performed by using dichloromethane to remove the paraffin oil.

[0023] Furthermore, e) the heat setting temperature is 90-170°C, preferably, e) the heat setting temperature is 90-105°C.

[0024] The absolute value of the difference between the MD tensile strength and the TD tensile strength of the diaphragm prepared by the preparation method of the present invention is ≤60MPa, for example, it can be 53MPa, 51MPa, 31MPa, 15MPa, 9MPa. Optionally, the TD tensile strength of the diaphragm is greater than the MD tensile strength.

[0025] Furthermore, the TD tensile strength of the separator is greater than 200 MPA, optionally, for example, 208 MPA, 215 MPA, 217 MPA, 226 MPA, 256 MPA.

[0026] The beneficial effects of the present invention are:

[0027] The applicant of this patent discovered in patent CN115149205A that a specific stretching ratio can effectively solve the consistency problem of lithium battery separators. In the technical solution of the present invention, the transverse stretching ratio or blowing ratio is increased to 11-15, the longitudinal stretching ratio is increased to 11-15, and the transverse stretching and longitudinal stretching temperatures are increased to 110-140°C. On the basis of solving the consistency problem, the appearance defect problem can be further solved and the thermal stability of the product can be improved. The absolute value of the difference between the MD tensile strength and the TD tensile strength is controlled within 60MPa. DETAILED DESCRIPTION

[0028] The present invention is further explained below with reference to the examples, but the examples do not limit the present invention in any form.

[0029] Embodiment 1:

[0030] The lithium-ion battery separator of this embodiment includes the following components: 22 parts of UHMWPE and 78 parts of paraffin oil.

[0031] a) In this embodiment, the mixture is melt-mixed in an extruder to obtain a mixed melt, and the temperature of the melt-mixing is 190° C.;

[0032] b) using the equipment shown in CN115149205A, the mixed melt passes through a filter and enters a right-angle die head, is extruded from the die head and enters an air ring for blowing, the stretching temperature is 110°C, 0.3-7Mpa compressed air is used to blow the parison, the transverse stretching ratio (i.e., the blowing ratio) is 11, the cooling section is between 10-40°C, and it is made to fit the mold; the oil film is cooled, and compressed into a sheet through a herringbone plate; at this time, the oil film is two layers, and one side is cut open with a cutter on one side, and then expanded into a layer of oil film;

[0033] c) cooling the oil film obtained in step b) to set the shape, the cooling temperature is 40° C.;

[0034] d) subsequently longitudinally stretching the product at a ratio of 11 and a temperature of 110° C.;

[0035] e) After longitudinal stretching, the membrane is placed in an extraction tank to remove the pore-forming agent in the membrane and dried with dichloromethane; the membrane is placed in a heat setting area for slight stretching at a temperature of 105°C; and finally rolled up to obtain a lithium-ion battery.

[0036] Embodiment 2:

[0037] The lithium-ion battery separator of this embodiment includes the following components: 20 parts of UHMWPE and 80 parts of paraffin oil.

[0038] The process flow of this embodiment includes the following processes: material feeding → mixing and plasticizing extrusion → blow molding → extraction and drying → heat setting → film winding.

[0039] a) In this embodiment, the mixture is melt-mixed in an extruder to obtain a mixed melt, and the melt-mixing temperature is 225° C.;

[0040] b) using the equipment shown in CN115149205A, the mixed melt passes through a filter and enters a right-angle die head, is extruded from the die head and enters an air ring for blowing, the mold temperature is 140°C, 0.3-7Mpa compressed air is used to blow the parison, the transverse stretching ratio (i.e., the blowing ratio) is 15, the cooling section is between 10-40°C, and it is made to fit the mold; the oil film is cooled, and compressed into a sheet through a herringbone plate; at this time, the oil film is two layers, and one side is cut open with a cutter on one side, and then expanded into a layer of oil film;

[0041] c) cooling the oil film obtained in step b) to set the shape, the cooling temperature is 40° C.;

[0042] d) subsequently longitudinally stretching the film, with a longitudinal stretching ratio of 15 and a stretching temperature of 110° C.;

[0043] e) After longitudinal stretching, the membrane is placed in an extraction tank to remove the pore-forming agent in the membrane and dried with dichloromethane; the membrane is placed in a heat setting area for slight stretching at a temperature of 90°C; and finally rolled up to obtain a lithium-ion battery.

[0044] Embodiment 3:

[0045] A lithium-ion battery separator was prepared according to the method of Example 1, except that the transverse stretching ratio was adjusted to 12 and the longitudinal stretching ratio was adjusted to 12.

[0046] Embodiment 4:

[0047] A lithium-ion battery separator was prepared according to the method of Example 1, except that the transverse stretching ratio was adjusted to 13 and the longitudinal stretching ratio was adjusted to 13.

[0048] Embodiment 5:

[0049] A lithium-ion battery separator was prepared according to the method of Example 1, except that the transverse stretching ratio was adjusted to 14 and the longitudinal stretching ratio was adjusted to 14.

[0050] Comparative Example 1:

[0051] The lithium-ion battery separator of this embodiment includes the following components: 22 parts of UHMWPE and 78 parts of paraffin oil.

[0052] The process flow of this embodiment includes the following processes: material feeding → mixing and plasticizing extrusion → die casting → longitudinal stretching → transverse stretching 1 → extraction and drying → transverse stretching 2 → film winding.

[0053] In this embodiment, the mixture is melt-mixed in an extruder to obtain a mixed melt, and the temperature of the melt-mixing is 190° C.

[0054] The mixed melt passes through a filter, enters a die head and is cast onto a cooling roller, the temperature of which is controlled between 10-40°C, and then is longitudinally stretched, the stretching temperature is controlled at 110°C, the stretching ratio is 11, and after stretching and cooling;

[0055] Enter the transverse stretching zone 1, the temperature is set at 110°C, and the stretching ratio is 11; enter the extraction tank to remove the pore-forming agent in the membrane and dry its dichloromethane; place the membrane in the transverse stretching zone 2 for slight stretching, and the temperature is set at 105°C; finally, roll it up to obtain a lithium-ion battery separator.

[0056] Comparative Example 2:

[0057] The lithium-ion battery separator of this embodiment includes the following components: 20 parts of UHMWPE and 80 parts of paraffin oil.

[0058] The process flow of this embodiment includes the following processes: material feeding → mixing and plasticizing extrusion → die casting → longitudinal stretching → transverse stretching 1 → extraction and drying → transverse stretching 2 → film winding.

[0059] In this embodiment, the mixture is melt-mixed in an extruder to obtain a mixed melt, and the temperature of the melt-mixing is 190° C.

[0060] After the mixed melt passes through the filter, it enters the die head and is cast onto a cooling roller, the temperature of which is controlled between 10-40°C, and then it is longitudinally stretched, the stretching temperature is 110°C, the stretching ratio is 11, and after stretching and cooling; it enters the transverse stretching zone 1, the temperature is set between 110°C, and the transverse stretching ratio is 11; it enters the extraction tank to remove the pore-forming agent in the membrane and dry its dichloromethane; the membrane is placed in the transverse stretching zone 2 for small stretching, the temperature is set at 105°C; and finally it is rolled up to obtain a lithium-ion battery separator.

[0061] Comparative Example 3:

[0062] A lithium-ion battery separator was prepared according to the method of Example 1, except that the longitudinal stretching (MDO stretching) ratio was adjusted to 5, and the transverse stretching (TDO) stretching ratio was adjusted to 5.

[0063] Comparative Example 4:

[0064] A lithium ion battery separator was prepared according to the method of Comparative Example 1, except that the MDO stretching ratio was adjusted to 13 and the TDO stretching ratio was adjusted to 10.

[0065] Comparative Example 5:

[0066] A lithium ion battery separator was prepared according to the method of Example 1, except that the MDO stretching ratio was adjusted to 17 and the TDO stretching ratio was adjusted to 17.

[0067] Tensile strength in MD and TD directions: The tensile strength of the lithium ion battery separators prepared in Examples 1-5 and Comparative Examples 1-5 was tested respectively:

[0068] (1) Tensile strength test method:

[0069] The test was carried out in accordance with the provisions of GB / T 1040.3-2006. A sample of 2.5 cm × 20 cm was cut and marked as the diaphragm in MD / TD direction. The sample was tested using a tensile testing machine (Jinan Sike Testing Technology Co., Ltd., TSL-1002). The sample was fixed between the upper and lower clamps of the tensile testing machine (the distance between the clamps was 100 ± 5 mm). It was ensured that the sample was flat and wrinkle-free, vertical and not skewed. The tensile speed was 250 mm / min. Based on the width and thickness of the sample, the test was performed 3 times and the average value was taken as the tensile strength (MPa) in the MD / TD direction.

[0070] (2) Shrinkage detection method:

[0071] Test according to the requirements of GB / T36363-2018. Cut a sample of 10cm×10cm, mark the transverse TD and longitudinal MD on the sample, measure the transverse and longitudinal widths with a fully automatic image measuring projector (Kunshan Gaopin Precision Instrument Co., Ltd., GP-300C), clamp the sample between two sealed A4 papers, place the sample in a 105℃ oven for 1h, and measure the transverse and longitudinal widths with a fully automatic image measuring projector after the sample returns to room temperature. Measure 3 times and take the average value:

[0072] MD heat shrinkage rate (%) = (MD length before heating - MD length after heating) ÷ MD length before heating × 100;

[0073] TD heat shrinkage rate (%) = (TD length before heating - TD length after heating) ÷ TD length before heating × 100;

[0074] (3) Thickness test method:

[0075] The test was carried out with reference to the requirements of GB / T6672-2001 "Mechanical measurement method for thickness of plastic film and sheet".

[0076] Use a 10cm×10cm mold to cut 5 square specimens of the porous substrate layer and the battery separator with a coating along the TD direction for testing. If the TD direction is less than 10cm, take 10cm in the MD direction. At this time, the specimen is not square. Use a Mahr thickness gauge (Mahr, C1202) to measure the four corners and the middle point of the specimen. The average value of the above 5 points is used as the thickness value of a single specimen. Take the test average of the 5 specimens as the thickness of the porous substrate layer and the thickness of the battery separator. The thickness of the coating = the thickness of the battery separator - the thickness of the porous substrate layer.

[0077]

[0078] It can be seen from the above embodiments that the diaphragm prepared by the preparation method of the present application has a larger MD tensile strength, a larger TD tensile strength, and a smaller thermal shrinkage rate (MD or TD thermal shrinkage rate). In particular, when the transverse stretching ratio is the same as the longitudinal stretching ratio, and satisfies 10≤transverse stretching ratio≤15, 10≤longitudinal stretching ratio≤15, the prepared diaphragm has a larger TD tensile strength.

[0079] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A method for preparing a wet-process lithium battery separator, characterized in that: It includes the following steps: a) Feeding and extruding: mixing the ultra-high molecular weight polyethylene with paraffin oil, and melt-mixing to form a uniform melt; b) Transverse stretching: filtering the melt obtained in step a) and then stretching it transversely, with a transverse stretching ratio of 11-15 and a transverse stretching temperature of 110-140° C.; c) cooling and shaping the oil film obtained in step b); d) longitudinal stretching: longitudinally stretching the oil film obtained in step c) at a longitudinal stretching ratio of 11-15 and a longitudinal stretching temperature of 20-140° C., followed by cooling and setting; e) Extraction and drying: The oil film obtained in step d) is subjected to extraction and heat setting to obtain a lithium-ion battery separator.

2. The method for preparing a wet-process lithium battery separator according to claim 1, characterized in that: a) The melt mixing temperature is 190°C to 225°C.

3. The method for preparing a wet-process lithium battery separator according to claim 1, characterized in that: b) The transverse stretching adopts a film blowing process: the melt obtained in step a) is filtered and then enters the air ring of the film blowing mechanism. The air ring injects compressed air into the oil film parison through the blowing pipe head of the film blowing mechanism, thereby transversely stretching the oil film parison, and the annular oil film parison forms a tubular film bubble. During the film blowing process, the blowing ratio is 11-15, the film bubble is cooled, and then passed through a herringbone plate with gaps, and the film bubble is squeezed into a double-layer sheet oil film through the gaps of the herringbone plate.

4. The method for preparing a wet-process lithium battery separator according to claim 3, characterized in that: When the blown film process is adopted, b) also includes the steps of cooling the double-layered sheet oil film, cutting one side of the double-layered sheet oil film with a cutter, and unfolding the cut oil film to form a layer of oil film.

5. The method for preparing a wet-process lithium battery separator according to claim 4, characterized in that: The cutting is to lay the double-layer sheet-like oil film flat on a cutting platform and cut the bottom surface of the oil film with a cutting knife with the blade facing upward.

6. The method for preparing a wet-process lithium battery separator according to claim 1 or 3, characterized in that: In the single-layer oil film obtained by transverse stretching or the double-layer flaky oil film obtained by the film blowing process, the thickness of the single-layer oil film is 1600-2000μm.

7. The method for preparing a wet-process lithium battery separator according to claim 1, characterized in that: c) The cooling temperature is 10-40°C.

8. The method for preparing a wet-process lithium battery separator according to claim 1, characterized in that: d) the longitudinal stretching temperature is 110-140°C, and d) before the longitudinal stretching, a preheating process at 40-100°C is also included.

9. The method for preparing a wet-process lithium battery separator according to claim 1, characterized in that: e) the extraction is to remove the paraffin oil by dichloromethane extraction; e) the heat setting temperature is 90-170°C, preferably, e) the heat setting temperature is 90-105°C.

10. The diaphragm prepared by the method for preparing a wet-process lithium battery diaphragm according to claim 1, characterized in that: The TD tensile strength of the separator is greater than 200 MPa; the absolute value of the difference between the MD tensile strength and the TD tensile strength of the separator is ≤60 MPa.

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