Method for improving thickness of dry-process lithium battery separator and dry-process lithium battery separator

Abstract

The application relates to the technical field of energy storage material preparation, and particularly discloses a method for improving the thickness of a dry-process lithium battery diaphragm and the obtained dry-process lithium battery diaphragm. The method for improving the thickness of the dry-process lithium battery diaphragm comprises the following steps: S1. selecting polypropylene resin as a base material; S2. feeding the polypropylene resin particles into an extruder; S3. regulating the die lip opening to prepare a thickness-compensated casting film; and S4. performing heat treatment and tensile setting on the thickness-compensated casting film to prepare a dry-process lithium battery diaphragm with uniform thickness distribution. The dry-process lithium battery diaphragm prepared by the method has uniform thickness distribution, and the appearance defects such as burst muscle and diagonal printing folds of the diaphragm during slitting and winding are greatly improved, so that the production yield and use quality of the diaphragm are improved.

Description

Technical Field

[0001] This invention relates to the field of energy storage material preparation technology, and in particular to a method for improving the thickness of dry-process lithium battery separators, and the obtained dry-process lithium battery separators. Background Technology

[0002] Lithium-ion batteries are one of the main battery technologies in consumer electronics, electric vehicles, and energy storage. Their rapid growth in demand for grid energy storage and other applications continues to push the energy limits of batteries. Researchers are exploring different solutions to address these challenges. As a key component of the battery system, the quality of the separator directly determines battery performance and safety. A high-performance separator should possess good mechanical properties and chemical stability. Strong mechanical properties can suppress short circuits caused by dendrites piercing the separator. Simultaneously, the separator should also have good liquid absorption, porosity, wettability, and high-temperature self-sealing properties. Currently, lithium-ion battery separators are mainly classified into polyolefin-based, non-woven fabric-based, and modified separators. Commercially available lithium-ion battery separators, especially polyolefin-based ones such as polyethylene and polypropylene separators, are widely used. Their raw materials are mainly polyethylene and polypropylene resins, and the manufacturing processes include dry and wet methods. Especially for the dry uniaxial stretching process, the prepared separators also have many problems. For example, the battery separators prepared by the dry process are thicker, with narrow and unevenly distributed pores. In particular, the uneven thickness distribution easily causes appearance defects such as bursting, creases, or wrinkles during slitting and winding, reducing the quality of the separator and increasing production and usage costs. It is evident that the defects of the existing dry uniaxial stretching production process can no longer meet the actual high-end requirements of lithium-ion batteries, and there is an urgent need to change this situation.

[0003] The main dry-process membrane manufacturing processes include dry biaxial stretching and dry uniaxial stretching. Uniaxial stretching, as one of the relatively mature membrane manufacturing processes, occupies a certain market share due to its advantages of simple and efficient process and low cost. However, the membranes produced are relatively thick, especially with uneven thickness distribution, which greatly limits its application range, leading to its main use in low- and mid-range products. Summary of the Invention

[0004] (a) Technical problems to be solved

[0005] The technical problem to be solved by this invention is to improve the thickness of dry-process lithium battery separators, making their thickness distribution uniform, so that the lithium battery separators will not have appearance defects such as cracking, diagonal creases, etc. during slitting and winding, thereby improving the production yield and quality of the separators.

[0006] (II) Technical Solution

[0007] To address the aforementioned technical problems, this invention provides a method for improving the thickness of dry-process separators in lithium batteries, comprising the following steps:

[0008] S1. Polypropylene resin is selected as the matrix material;

[0009] S2. The polypropylene resin particles, which form the matrix material, are fed into an extruder;

[0010] S3. Adjust the die lip opening to obtain a thickness-compensated cast film;

[0011] S4. The thickness-compensated cast film is heat-treated, stretched, and shaped to prepare a lithium battery separator with uniform thickness distribution.

[0012] In one embodiment of the present invention, the matrix material polypropylene resin satisfies at least one of the following characteristics (a)-(e):

[0013] (a) Mass-average molecular weight of 300,000-500,000 g·mol - 1;

[0014] (b) Crystallinity is 40-50%;

[0015] (c) Melting temperature is 160-180℃;

[0016] (d) Melt index is 0.8-3.0 g / 10 min;

[0017] (e) Ash content is 10-20 ppm.

[0018] In one embodiment of the present invention, in step S2, the extruder includes extruder A and extruder B, the temperature of each temperature zone of extruder A is set to 180-240°C, and the temperature of each temperature zone of extruder B is set to 180-220°C.

[0019] In one embodiment of the present invention, the temperature of each temperature zone of the die head of extruder A and extruder B is set to 200-250°C.

[0020] In one embodiment of the present invention, the temperature of each temperature zone in the flow channel of extruder A and extruder B is set to 210-230°C.

[0021] The temperature control settings for the extruder and casting die described above take into account the melting point (melting range) and melt index of the selected matrix material to ensure that the polypropylene resin particles can fully reach a molten state and have good flowability, thereby guaranteeing the quality of the prepared cast film. The screw speed settings of each part of the extruder can be adjusted appropriately according to actual operation to ensure smooth production of the cast film; this invention does not impose any limitations on these settings.

[0022] In one embodiment of the present invention, the mass of the matrix material fed into extruder A accounts for 40-60% of the total mass of the matrix material fed into extruder A and extruder B.

[0023] In one embodiment of the present invention, the matrix material fed into extruder A and the matrix material fed into extruder B can be the same or different.

[0024] In one embodiment of the present invention, the thickness of the thickness-compensated cast film is distributed such that its thickness gradually decreases from the middle to both sides along the width direction of the film.

[0025] In one embodiment of the present invention, in step S3, the die lip opening is adjusted as follows: the opening of the middle part of the entire die lip is 3.0-3.1 mm, and the opening of other areas is 2.6-2.8 mm. By adjusting the die lip opening, a thickness-compensated cast film can be obtained. The thickness of this cast film gradually decreases from the middle to both sides along the width direction of the film. That is, the cross-section of the cast film is an "arch" shape with a thicker middle area and thinner sides. The thickness of the thicker middle area is about 0.1-0.5 μm thicker than the thickness of the thinner sides.

[0026] Studies have found that the opening degree of the casting die lip affects the thickness distribution of the cast film, and consequently the thickness uniformity of the manufactured lithium battery separator. Therefore, controlling the die lip opening degree is crucial. This invention, during the preparation of the lithium battery separator, achieves thickness compensation of the cast film by controlling the opening degree of the casting die lip, resulting in a cast film with an "arched" cross-section. After further stretching and pore formation, the overall thickness distribution of the resulting dry-process lithium battery separator product is more uniform. This invention significantly improves the appearance defects such as cracking and diagonal creases that occur during slitting and winding of the dry-process lithium battery separator, thereby improving the separator production yield and usage quality.

[0027] The present invention also provides a dry-process lithium battery separator, which is made by the method described in the present invention.

[0028] This invention also provides an application of a dry-process lithium battery separator in lithium battery manufacturing. Attached Figure Description

[0029] Figure 1 This is a schematic diagram of the existing dry uniaxial stretching process;

[0030] Figure 2 This is a side view of the casting die head. The arrow in the figure points to point A, which is the die lip position.

[0031] Figure 3 This is a dense thickness trend diagram of the cast film obtained after thickness compensation in Embodiment 1 of the present invention;

[0032] Figure 4This is a dense thickness trend diagram of the cast film obtained after thickness compensation in Embodiment 2 of the present invention;

[0033] Figure 5 This is a graph showing the dense thickness trend of the cast film obtained in the comparative example of this invention.

[0034] Figure 6 This is a view of the wound-up appearance of the lithium battery separator prepared in Example 1 of the present invention.

[0035] Figure 7 This is a view of the wound appearance of the lithium battery separator prepared in Embodiment 2 of the present invention;

[0036] Figure 8 This is a wound-up appearance diagram of the lithium battery separator prepared in the comparative example of the present invention. Detailed Implementation

[0037] The specific embodiments of the present invention will be described in further detail below with reference to the accompanying drawings and examples. The following examples are for illustrative purposes only and are not intended to limit the scope of the invention.

[0038] The polypropylene resin particles used in the following examples and comparative examples were purchased from Daehan Oil & Chemical Trading Co., Ltd., South Korea.

[0039] Example 1

[0040] This embodiment provides a method for improving the thickness of dry-process lithium battery separators, and a dry-process lithium battery separator with a thickness of 18 μm (D1802) was prepared. The preparation adopts the existing dry uniaxial stretching process, and the process diagram is shown in [reference needed]. Figure 1 As shown, the preparation process includes the following steps:

[0041] S1. Polypropylene resin particles are fed into the extruder as the matrix material.

[0042] The polypropylene resin particles fed into extruder A have the following characteristics: mass average molecular weight of approximately 500,000 g·mol / L. -1 It has a crystallinity of 47%, a melting temperature of 167℃, a melt index of 0.86 g / 10 min, and an ash content of 12 ppm.

[0043] The polypropylene resin particles fed into extruder B have the following characteristics: mass average molecular weight approximately 400,000 g·mol⁻¹ -1 It has a crystallinity of 48%, a melting temperature of 168℃, a melt index of 2.13 g / 10 min, and an ash content of 14 ppm.

[0044] The mass ratio of polypropylene resin fed into extruder A and extruder B is A:B = 53:47.

[0045] S2. Perform melt extrusion and casting.

[0046] In this step, the temperature of each zone of extruder A is set at 180-240℃, the temperature of each zone of extruder B is set at 180-220℃, the temperature of each zone of the flow channel of extruder A and extruder B is set at 210-230℃, and the temperature of each zone of the die head of extruder A and extruder B is set at 200-250℃.

[0047] The material exiting extruders A and B enters the casting material pipeline, and the outlet of the casting material pipeline is the casting die head section, whose side view is shown below. Figure 2 As shown, the opening of the casting die lip A is adjusted as follows: the opening of the middle part of the entire die lip is 3.0 mm, and the opening of other areas is 2.8 mm. A casting film with thickness compensation is obtained by casting.

[0048] S3. The thickness-compensated cast film is heat-treated, stretched, and shaped to prepare a lithium battery separator with uniform thickness distribution, namely a 18μm (D1802) lithium battery separator. Finally, the separator is slit and wound up, and the surface and appearance quality of the wound separator are observed.

[0049] The heat treatment and shaping processes are consistent with existing diaphragm preparation processes. The difference lies in the stretching process, where the oven temperature is increased by 1-2°C compared to existing diaphragm preparation processes.

[0050] The dense thickness trend diagram of the thickness-compensated cast film obtained in this embodiment is shown in the figure. Figure 3 The appearance of the wound lithium battery separator prepared in this embodiment is shown in the figure below. Figure 6 .

[0051] Example 2

[0052] This embodiment provides a method for improving the thickness of dry-process lithium-ion battery separators, resulting in a dry-process lithium-ion battery separator with a thickness of 18 μm (D1803). The preparation employs an existing dry-process uniaxial stretching process, and the process diagram is shown below. Figure 1 As shown, the preparation process includes the following steps:

[0053] S1. Polypropylene resin particles are fed into the extruder as the matrix material.

[0054] The polypropylene resin particles fed into extruder A have the following characteristics: mass average molecular weight of approximately 500,000 g·mol / L. -1 It has a crystallinity of 47%, a melting temperature of 167℃, a melt index of 0.86 g / 10 min, and an ash content of 12 ppm.

[0055] The polypropylene resin particles fed into extruder B have the following characteristics: mass average molecular weight approximately 400,000 g·mol⁻¹ -1It has a crystallinity of 48%, a melting temperature of 168℃, a melt index of 2.13 g / 10 min, and an ash content of 14 ppm.

[0056] The mass ratio of polypropylene resin fed into extruder A and extruder B is A:B = 53:47.

[0057] S2. Perform melt extrusion and casting.

[0058] In this step, the temperature of each zone of extruder A is set at 180-240℃, the temperature of each zone of extruder B is set at 180-220℃, the temperature of each zone of the flow channel of extruder A and extruder B is set at 210-230℃, and the temperature of each zone of the die head of extruder A and extruder B is set at 200-250℃.

[0059] The material exiting extruders A and B enters the casting material pipeline, and the outlet of the casting material pipeline is the casting die head section, whose side view is shown below. Figure 2 As shown, the opening of the casting die lip A is adjusted as follows: the opening of the middle part of the entire die lip is 3.1 mm, and the opening of other areas is 2.8 mm. A casting film with thickness compensation is obtained by casting.

[0060] S3. The thickness-compensated cast film is heat-treated, stretched, and shaped to prepare a lithium battery separator with uniform thickness distribution, namely a 18μm (D1803) lithium battery separator. Finally, the separator is slit and wound up, and the surface and appearance quality of the wound separator are observed.

[0061] The heat treatment and shaping processes are consistent with existing diaphragm preparation processes. The difference lies in the stretching process, where the oven temperature is increased by 1-2°C compared to existing diaphragm preparation processes.

[0062] The dense thickness trend diagram of the thickness-compensated cast film obtained in this embodiment is shown in the figure. Figure 4 The appearance of the wound lithium battery separator prepared in this embodiment is shown in the figure below. Figure 7 .

[0063] Comparative Example

[0064] This comparative example provides a method for preparing a dry-process lithium-ion battery separator with a thickness of 18 μm (D1800), using existing processes. The specific steps are as follows:

[0065] S1. Polypropylene resin particles are fed into the extruder as the matrix material.

[0066] The polypropylene resin particles fed into extruder A have the following characteristics: mass average molecular weight of approximately 500,000 g·mol / L. -1It has a crystallinity of 47%, a melting temperature of 167℃, a melt index of 0.86 g / 10 min, and an ash content of 12 ppm.

[0067] The polypropylene resin particles fed into extruder B have the following characteristics: mass average molecular weight approximately 400,000 g·mol⁻¹ -1 It has a crystallinity of 48%, a melting temperature of 168℃, a melt index of 2.13 g / 10 min, and an ash content of 14 ppm.

[0068] The mass ratio of polypropylene resin fed into extruder A and extruder B is A:B = 53:47.

[0069] S2. Perform melt extrusion and casting.

[0070] In this step, the temperature of each zone of extruder A is set at 180-240℃, the temperature of each zone of extruder B is set at 180-220℃, the temperature of each zone of the flow channel of extruder A and extruder B is set at 210-230℃, and the temperature of each zone of the die head of extruder A and extruder B is set at 200-250℃.

[0071] The material exiting extruders A and B enters the casting material pipeline, and the outlet of the casting material pipeline is the casting die head section, whose side view is shown below. Figure 2 As shown, the opening of the casting die lip A is adjusted so that the opening of the entire casting die lip is 2.8 mm, and a casting film is obtained by casting.

[0072] S3. The cast film is heat-treated, stretched, and shaped to prepare a lithium battery separator, namely a lithium battery separator with a thickness of 18μm (D1800). Finally, it is slit and wound up, and the surface and appearance quality of the wound separator are observed.

[0073] The thickness trend of the cast film prepared in this comparative example is shown in the graph. Figure 5 The appearance of the wound lithium battery separator prepared in this comparative example is shown in the following figure. Figure 8 .

[0074] Depend on Figures 3-4 It can be seen that the overall thickness of the cast films prepared in Examples 1 and 2 shows a trend of "thin at both ends and thick in the middle", which indicates that the thickness compensation was successfully implemented. Figure 5 The results show that although the overall thickness values ​​of the cast films prepared in the comparative model are somewhat different, they are basically consistent in thickness and no thickness compensation is performed.

[0075] Depend on Figures 6-8 It can be seen that the lithium battery separators prepared in Examples 1 and 2 have a smooth and even appearance with no defects. However, the lithium battery separator prepared in the comparative example has obvious diagonal wrinkles and defects such as deformation and wavy surface.

[0076] It should be noted that the various embodiments in this specification are described in a progressive manner, and the same or similar parts between the various embodiments can be referred to mutually. Each embodiment focuses on describing the differences from other embodiments. For the embodiments of the method, relevant parts can be referred to the description of the device embodiments (as appropriate). The present invention is not limited to the specific steps and structures described above and shown in the figures. Furthermore, for the sake of brevity, detailed descriptions of known methods and techniques are omitted here.

[0077] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the technical principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method of improving the thickness of a dry method separator for a lithium battery, characterized by, Including the following steps: S1. Polypropylene resin particles are selected as the matrix material; S2. The polypropylene resin particles are fed into an extruder; The extruder includes extruder A and extruder B. The temperature of each temperature zone of extruder A is set to 180-240℃, and the temperature of each temperature zone of extruder B is set to 180-220℃. The temperature of each temperature zone of the die head of extruder A and extruder B is set to 200-250℃. The temperature of each temperature zone of the runner of extruder A and extruder B is set to 210-230℃. The mass of the matrix material fed into extruder A accounts for 40-60% of the total mass of matrix material fed into extruder A and extruder B. S3. Adjust the die lip opening to obtain a thickness-compensated cast film; the die lip opening is adjusted as follows: the opening in the middle of the die lip is 3.0-3.1 mm, and the opening in other areas is 2.6-2.8 mm. S4. The thickness-compensated cast film is heat-treated and stretched to obtain a lithium battery separator with uniform thickness distribution.

2. The method according to claim 1, characterized in that, The polypropylene resin particles satisfy at least one of the following characteristics (a)-(e): (a) a mass average molecular weight of 300,000 to 500,000 g-mol 1 ; (b) Crystallinity is 40-50%; (c) The melting temperature is 160-180 ℃; (d) Melt index is 0.8-3.0 g / 10min; (e) Ash content is 10-20 ppm.

3. The method of claim 1, wherein, The matrix material fed into extruder A may be the same as or different from the matrix material fed into extruder B.

4. The method according to claim 1, characterized in that, The thickness of the cast film after thickness compensation is distributed in a way that gradually decreases from the middle to both sides along the width direction of the film.

5. A dry-process lithium battery separator, made by the method described in any one of claims 1-4.

6. The application of the dry-process lithium battery separator as described in claim 5 in the preparation of lithium batteries.

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