Lithium battery diaphragm casting cooling method, system and application thereof

Abstract

The present invention discloses a cooling method, system and application of a lithium battery diaphragm casting sheet. The cooling method comprises: (1) providing a battery diaphragm melt cast film sheet; (2) using a traction roller arranged in a cooling tank and below the liquid level of the coolant to make the cast film sheet directly enter the coolant to achieve simultaneous cooling of the front and back sides; (3) using a guide roller arranged in the cooling tank and below the liquid level of the coolant to pull the film sheet entering the coolant out of the cooling tank; (4) using a liquid removal roller arranged above the cooling tank to remove the coolant on the surface of the film sheet leaving the cooling tank, so as to obtain a diaphragm casting sheet with consistent structures on both sides. The method can make the cooling rate of the front and back sides of the battery diaphragm melt cast film sheet consistent, the cooling uniformity is better, and the cooling effect is better, so that the microstructure of the front and back sides of the film sheet is consistent and the flatness of the diaphragm is better.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium batteries, and in particular to a cooling method and system for a lithium battery diaphragm casting and applications thereof. Background Art

[0002] Lithium battery separators are thin microporous membranes with microporous structures formed by mixing polyethylene resin and paraffin oil, and then melt extruding, biaxially stretching and extracting. Among them, the process conditions of melt extrusion cast film are closely related to the formation of original micropores on both sides of the cast film. However, in related technologies, one side of the melt extrusion cast film is usually attached to a chilled roller for cooling, which makes the cooling rate on both sides of the cast film inconsistent, resulting in differences in the microstructure of the front and back sides of the stretched separator, which in turn leads to an uneven surface of the separator.

[0003] Therefore, the performance of the current lithium battery diaphragm casting cooling system still needs to be improved. Summary of the invention

[0004] The present invention aims to solve one of the technical problems in the related art at least to a certain extent.

[0005] This application is mainly based on the following issues:

[0006] In order to solve the problem of inconsistent cooling effect on both sides of the cast film, which leads to differences in the microstructure of the front and back sides of the diaphragm, there is a method in the battery diaphragm casting cooling system to achieve consistent cooling effect on both sides of the casting through the synchronous use of white oil in the cooling tank and the chilled roller, so as to obtain a similar appearance. However, in this technical solution, before the cast sheet enters the cooling tank, since only the roller surface is cooled by the chilled roller, and the back roller surface is cooled by the ambient air, it is impossible to achieve true synchronous cooling, and there is still a problem of inconsistent cooling rate on both sides of the cast sheet; in addition, cold water is used for cooling in the chilled roller, and white oil is used for cooling in the cooling tank. Due to the different heat transfer coefficients of the two cooling media, especially at low temperatures, the viscosity of white oil is very large and the fluidity is very poor, which makes it difficult to achieve a consistent cooling rate on the front and back sides of the cast sheet, and also affects the consistency of the structure of the front and back sides of the diaphragm.

[0007] To this end, an object of the present invention is to propose a method for cooling a lithium battery diaphragm casting that makes the cooling rate on both sides of the cast film sheet consistent, the cooling uniformity good, and the cooling effect good, so that the final lithium battery diaphragm has consistent microstructure on both sides and a relatively smooth diaphragm surface.

[0008] In a first aspect of the present invention, the present invention provides a method for cooling a lithium battery separator casting. According to an embodiment of the present invention, the method comprises:

[0009] (1) Providing a battery separator melt casting film;

[0010] (2) using a traction roller disposed in the cooling tank and below the surface of the cooling liquid to allow the cast film to directly enter the cooling liquid to achieve simultaneous cooling of both the front and back surfaces;

[0011] (3) using a guide roller disposed in the cooling tank and below the surface of the cooling liquid to pull the membrane in the cooling liquid out of the cooling tank;

[0012] (4) Using a liquid removal roller disposed above the cooling trough to remove the cooling liquid on the surface of the diaphragm leaving the cooling trough, so as to obtain a diaphragm casting with consistent structures on both sides.

[0013] The lithium battery diaphragm casting cooling method of the above-mentioned embodiment of the present invention can make the battery diaphragm melt from the beginning of extrusion to the completion of cooling, the cooling environment and the temperature of the cooling medium on both sides of the diaphragm are always consistent, the cast diaphragm is cooled in the air before entering the cooling liquid, and the air cooling environment on both sides of the diaphragm is consistent, and when the cast diaphragm enters the cooling liquid, since the traction roller and the guide roller are both placed in the cooling liquid, the surface temperature of the two is consistent with the cooling liquid temperature, and the cooling temperature of the front and back sides of the diaphragm is always consistent when the diaphragm moves in the cooling liquid. Therefore, the cooling rate on the front and back sides of the cast diaphragm can always maintain good consistency, the cooling uniformity is good, and the cooling effect is better, which effectively solves the problem of differences in the microstructure of the front and back sides of the diaphragm product after stretching due to inconsistent cooling rates on both sides of the cast diaphragm and the uneven membrane surface caused by this. The use of this cooling method in the preparation process of lithium battery diaphragms can significantly improve the quality of lithium battery diaphragms, so that the microstructure of the front and back sides of the final lithium battery diaphragm is consistent, the diaphragm surface is smoother, and the diaphragm performance is more stable.

[0014] In addition, the lithium battery diaphragm casting cooling method of the above embodiment of the present invention may also have the following additional technical features:

[0015] According to an embodiment of the present invention, the temperature of the cooling liquid is 15-120°C.

[0016] According to an embodiment of the present invention, the cooling liquid is water or paraffin oil.

[0017] According to an embodiment of the present invention, the paraffin oil used as the coolant is the same as the paraffin oil used in preparing the cast film.

[0018] According to an embodiment of the present invention, when the cooling temperature is 15-50° C., the coolant is water; when the cooling temperature is 50-120° C., the coolant is paraffin oil.

[0019] According to an embodiment of the present invention, the residence time of the cast film in the cooling liquid is 5 to 60 seconds.

[0020] In a second aspect of the present invention, the present invention provides a lithium battery diaphragm casting cooling system. According to an embodiment of the present invention, the cooling system comprises:

[0021] A cooling tank, wherein a coolant is provided in the cooling tank, and the cooling tank comprises a coolant inlet and a coolant outlet;

[0022] A traction device, the traction device comprising traction rollers arranged in pairs, the traction rollers are arranged in the cooling tank and below the liquid surface of the cooling liquid, the traction rollers arranged in pairs rotate towards each other and are suitable for traction of the cast film into the cooling liquid;

[0023] A guide device, the guide device comprising at least one guide roller, the guide roller being arranged in the cooling tank and below the liquid surface of the coolant, the guide roller being adapted to pull the membrane in the coolant away from the cooling tank;

[0024] The liquid removal device comprises at least one pair of liquid removal rollers arranged opposite to each other, the liquid removal rollers are arranged above the cooling tank, the liquid removal rollers arranged opposite to each other rotate towards each other and are suitable for removing the cooling liquid on the surface of the membrane leaving the cooling tank.

[0025] The lithium battery diaphragm casting cooling system of the above embodiment of the present invention has at least the following advantages: 1. Simple structure; 2. The cooling environment and the temperature of the cooling medium on both sides of the battery diaphragm can be kept consistent from the beginning of extrusion to the completion of cooling of the battery diaphragm melt. Before the cast diaphragm enters the coolant, it is first cooled in the air, and the air cooling environment on both sides of the diaphragm is consistent. When the cast diaphragm enters the coolant, the traction roller and the guide roller are both placed in the coolant, and the surface temperature of the two is consistent with the coolant temperature. When the diaphragm moves in the coolant, the cooling temperature on both sides of the diaphragm is always consistent. Therefore, the use of the cooling system can achieve By cooling the melt cast film of the lithium battery separator, the cooling rate on both sides of the cast film can always maintain good consistency, the cooling uniformity is good, the cooling effect is better, and the problem of uneven film surface caused by the difference in microstructure on both sides of the separator product after stretching due to inconsistent cooling rates on both sides of the cast film is effectively solved; 3. The cooling system can be combined with the preparation process of the lithium battery separator, so as to significantly improve the quality of the lithium battery separator, so that the microstructure on both sides of the final lithium battery separator is consistent, the separator film surface is smoother, and the separator performance is more stable.

[0026] According to an embodiment of the present invention, the cooling liquid is water or paraffin oil.

[0027] According to an embodiment of the present invention, the pulling roller is arranged at the lower part of the die head of the melt extrusion device, and the distance between the die lip of the die head and the liquid surface of the cooling liquid is not more than 300 mm.

[0028] According to an embodiment of the present invention, the guide device includes at least two guide rollers, and the liquid removal device includes 1 to 2 pairs of liquid removal rollers.

[0029] According to an embodiment of the present invention, at least two of the guide rollers are arranged in a staggered manner at high and low levels below the liquid surface of the cooling liquid.

[0030] In the third aspect of the present invention, the present invention proposes a system for wet preparation of lithium battery separators. According to an embodiment of the present invention, the system for wet preparation of lithium battery separators includes the aforementioned lithium battery separator casting cooling system. Compared with the prior art, by using the above-mentioned lithium battery separator casting cooling system in the wet preparation process, the battery separator melt can be made to keep the cooling environment and the cooling medium temperature on both sides of the diaphragm consistent from the beginning of extrusion until the cooling is completed, which is more conducive to obtaining castings with consistent microstructures and smooth surfaces on both sides of the front and back, thereby significantly improving the uniformity and flatness of the final battery separator. As a result, the system for wet preparation of lithium battery separators is not only easy to operate, but also can obtain lithium battery separators with higher quality, stable performance and stronger competitiveness.

[0031] In the fourth aspect of the present invention, the present invention proposes a method for preparing a lithium battery separator by a wet process. According to an embodiment of the present invention, the method adopts the aforementioned system implementation of preparing a lithium battery separator by a wet process or includes the aforementioned lithium battery separator casting cooling method. Compared with the prior art, the preparation method is not only easy to operate and implement when cooling the diaphragm, but also has a better cooling effect. It has the advantages of simple and convenient operation, easy implementation, and easy industrialization. The lithium battery separator prepared by this method has consistent microstructures on both sides, a smoother membrane surface, and stable performance. Using it in lithium batteries is more conducive to improving the comprehensive performance of lithium batteries.

[0032] According to an embodiment of the present invention, a method for preparing a lithium battery separator by a wet process includes: (i) mixing a polyethylene resin with paraffin oil for melt mixing and casting to obtain a cast film; (ii) using the traction roller to allow the cast film to directly enter a cooling liquid to achieve simultaneous cooling of both the front and back sides; (iii) using the guide roller to pull the film entering the cooling liquid out of the cooling tank; (iv) using the liquid removal roller to remove the cooling liquid on the surface of the film leaving the cooling tank to obtain a separator casting with consistent structures on both sides; (v) biaxially stretching the separator casting to obtain a separator sheet; (vi) extracting the separator sheet to remove the paraffin oil in the separator sheet; and (vii) heat setting and winding the extracted separator sheet to obtain a lithium battery separator.

[0033] In the fifth aspect of the present invention, the present invention proposes a lithium battery separator, which is prepared by the method for preparing a lithium battery separator by the wet method described above or by the system for preparing a lithium battery separator by the wet method described above. Compared with the prior art, the microstructures of the front and back sides of the lithium battery separator are consistent, the membrane surface is smoother, and the performance is more stable, with advantages such as high product quality and strong inherent competitiveness. Using it in lithium batteries is more conducive to improving the comprehensive performance of lithium batteries, and the features and advantages described above for the lithium battery separator casting cooling system and the method for preparing a lithium battery separator by the wet method are also applicable to the lithium battery separator, which will not be repeated here.

[0034] Additional aspects and advantages of the present invention will be given in part in the following description and in part will be obvious from the following description, or will be learned through practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] The above and / or additional aspects and advantages of the present invention will become apparent and easily understood from the description of the embodiments in conjunction with the following drawings, in which:

[0036] Figure 1 The present invention is a flowchart of a method for cooling a lithium battery separator casting according to an embodiment of the present invention.

[0037] Figure 2 It is a schematic structural diagram of a lithium battery diaphragm casting cooling system according to an embodiment of the present invention.

[0038] Figure 3 It is a structural schematic diagram of a lithium battery diaphragm casting cooling system according to another embodiment of the present invention.

[0039] Figure 4 It is a structural schematic diagram of a lithium battery diaphragm casting cooling system according to another embodiment of the present invention.

[0040] Figure 5 The present invention is a flow chart of a method for preparing a lithium battery separator by a wet process according to an embodiment of the present invention.

[0041] Figure 6 The microstructure of the two sides of the lithium battery separator prepared according to Example 1 of the present invention ( Figure 5 A and B represent the two opposite surfaces of the diaphragm, respectively).

[0042] Figure 7 The microstructure of the two sides of the lithium battery separator prepared according to Example 2 of the present invention ( Figure 6 A and B represent the two opposite surfaces of the diaphragm, respectively).

[0043] Figure 8 The microstructure of the two sides of the lithium battery separator prepared according to Comparative Example 1 of the present invention ( Figure 7 A and B represent the two opposite surfaces of the diaphragm, respectively).

[0044] Fig. 9 The microstructure of the two sides of the lithium battery separator prepared according to Comparative Example 2 of the present invention ( Figure 8 A and B represent the two opposite surfaces of the diaphragm, respectively).

[0045] Reference numerals:

[0046] 100: cooling tank; 101: cooling liquid; 110: cooling liquid inlet; 120: cooling liquid outlet; 210: pulling roller; 300: die head; 410: guide roller; 510: liquid removal roller. DETAILED DESCRIPTION

[0047] The embodiments of the present invention are described in detail below. The embodiments described below are exemplary and are only used to explain the present invention, and should not be construed as limiting the present invention. If no specific techniques or conditions are specified in the embodiments, the techniques or conditions described in the literature in this area or the product instructions are used. The reagents or instruments used that do not specify the manufacturer are all conventional products that can be obtained commercially.

[0048] In the first aspect of the present invention, the present invention provides a method for cooling a lithium battery separator casting. According to an embodiment of the present invention, Figures 1 to 3 As shown, the method includes: (1) providing a battery diaphragm melt cast film; (2) using a traction roller 210 disposed in a cooling tank 100 and below the liquid surface of a coolant 101 to allow the cast film to directly enter the coolant 101 to achieve simultaneous cooling of the front and back sides; (3) using a guide roller 410 disposed in the cooling tank 100 and below the liquid surface of the coolant 101 to pull the film entering the coolant 101 out of the cooling tank 100; (4) using a liquid removal roller 510 disposed above the cooling tank 100 to remove the coolant on the surface of the film leaving the cooling tank 100, so as to obtain a diaphragm casting with consistent structures on both sides. The method can make the cooling rate of the front and back sides of the battery diaphragm melt cast film consistent, the cooling uniformity is better, and the cooling effect is better, so that the microstructure of the front and back sides of the film is consistent and the flatness of the diaphragm is better. It should be noted that the "front and back sides" of the diaphragm or membrane described in the present invention refer to two surfaces of the diaphragm or membrane arranged opposite to each other, that is, one side of the diaphragm or membrane and the other side corresponding to the one side; it can also be understood that with the direction of one side of the diaphragm or membrane away from the diaphragm or membrane body as the top, the front and back sides refer to the upper surface and lower surface of the diaphragm or membrane.

[0049] Reference below Figures 1 to 4 The cooling method for the lithium battery diaphragm casting sheet of the above embodiment of the present invention is described in detail.

[0050] According to a specific embodiment of the present invention, polyethylene resin and paraffin oil can be mixed for melt mixing and film casting to obtain a battery separator melt casting film, wherein the amount of paraffin oil added can account for 25wt% to 30wt% of the polyethylene resin, specifically 25wt%, 26wt%, 27wt%, 28wt%, 29wt%, 30wt%, etc., thereby making the microstructure of the front and back sides of the final lithium battery separator more consistent and the membrane surface smoother. Among them, the melt mixing and film casting can be carried out in a melt extrusion device, such as a twin-screw extruder. In addition, the specific type of the die 300 of the melt extrusion device is not particularly limited, and those skilled in the art can flexibly select according to actual conditions. For example, the die 300 of the melt extrusion device can be a hanger-type die, a T-type die, etc., as long as it can meet the needs of film casting.

[0051] According to another specific embodiment of the present invention, Figure 2 It is understood that the die head 300 of the melt extrusion device can be located above the traction roller 210, and the distance between the die lip of the die head 300 and the liquid surface of the cooling liquid 101 can be no more than 300 mm, for example, it can be 100-300 mm, 30 mm, 50 mm, 80 mm, 120 mm, 150 mm, 180 mm, 200 mm, 250 mm or 280 mm. The inventor found that if the distance between the die head and the liquid surface of the cooling liquid is too long, the retention time of the cast film in the air is too long, so that the cooling time of the cast film is increased, thereby reducing the cooling efficiency of the cast film; at the same time, the long retention time of the cast film in the ambient air will also cause the initial cooling rate of the melt to be unstable, resulting in a decrease in production stability; in addition, if the distance between the die head and the liquid surface of the cooling liquid is too long, the melt has low strength at high temperature and is relatively soft before it is fully cooled and solidified, which will cause poor stability of the film, thereby affecting the stability of the thickness and width of the thick film. When the distance between the die head and the surface of the coolant is within the above range, the cooling efficiency of the cast film can be better improved, the stability of the melt film formation can be enhanced, and a guarantee can be provided for the cast film to stably enter the cooling tank for cooling.

[0052] According to another specific embodiment of the present invention, the type of coolant used in the present invention and the temperature of the coolant are not particularly limited, and those skilled in the art can flexibly select according to actual conditions, for example, the coolant can be water or paraffin oil; the cooling temperature of the coolant can be 15-120°C, specifically 15°C, 25°C, 40°C, 50°C, 75°C, 100°C, 115°C, 120°C, etc. Specifically, the coolant in the cooling tank can select a suitable cooling medium according to the needs of the diaphragm casting. For example, when the diaphragm casting requires cooling to a low temperature (15-50°C), since the viscosity of paraffin oil at low temperatures is relatively large and the fluidity is poor, water can be selected as the cooling medium, and when the diaphragm casting requires cooling to a high temperature (50-120°C), paraffin oil can be selected as the cooling medium. Thus, the applicability of the cooling system can be further improved, and the cooling requirements of different diaphragm castings can be met. In addition, when paraffin oil is selected as the coolant, the paraffin oil used as the coolant can be the same as the paraffin oil used in preparing the cast film. In this way, not only the cooling effect can be guaranteed, but also new impurity components will not be introduced when cooling the cast film.

[0053] According to another specific embodiment of the present invention, the residence time of the cast film in the cooling liquid can be 5 to 60 s, for example, it can be 5s, 6s, 7s, 8s, 9s, 10s, 15s, 20s, 25s, 30s, 35s, 40s, 45s or 50s, etc. The temperature of the melt-extruded battery diaphragm melt cast film is generally higher than 200°C. If the residence time of the cast film in the cooling liquid is too short, it is difficult to ensure that the expected cooling temperature can be reached. If the residence time in the cooling liquid is too long, the cooling efficiency will be affected. By controlling the residence time of the cast film in the cooling liquid to be within the above range, the cooling efficiency can be guaranteed, the cooling effect can be improved, and it is more conducive to improving the consistency of the microstructure on the front and back sides of the lithium battery diaphragm and the flatness of the diaphragm membrane surface.

[0054] According to another specific embodiment of the present invention, the pulling speed of the pulling roller 210 and the guide roller 410 in the present invention is not particularly limited, and those skilled in the art can select according to actual needs. For example, the pulling speed of the pulling roller 210 and the guide roller 410 can be 2 to 8 m / min, specifically 2 m / min, 4 m / min, 6 m / min or 8 m / min, etc. When the pulling speed is within this range, the cast film can be stretched and thinned to a certain extent to obtain a cast thick film of desired thickness, thereby better improving the cooling efficiency and cooling effect of the cast film, and also helping to improve the consistency of the microstructure on both sides of the lithium battery separator and the flatness of the separator surface.

[0055] According to another specific embodiment of the present invention, the thickness of the battery separator melt-cast film provided in the present invention can be 500-3000 μm, for example, it can be 750 μm, 1000 μm, 1250 μm, 1500 μm, 1750 μm, 2000 μm, 2250 μm, 2500 μm or 2750 μm, etc. The inventor found that since the thickness of the diaphragm prepared by the wet preparation process is usually between 5 and 25 μm, after being cast into a thick sheet by cast extrusion, it is necessary to perform longitudinal and transverse stretching at a certain ratio to orient the polyethylene resin in the transverse and longitudinal directions to obtain good mechanical properties. If the thickness of the cast film is too thin, on the one hand, it is easy to have the risk of tensile fracture, and on the other hand, the subsequent stretching ratio cannot be too large, otherwise it is easy to have the risk of tensile fracture. However, when the stretching ratio is low, the tensile strength of the prepared diaphragm is low, which affects the performance of the diaphragm; if the thickness of the cast film is too thick, the cooling efficiency of the cast film decreases, and the crystallinity of the polyethylene resin in the casting stage is too large, resulting in increased difficulty in subsequent longitudinal stretching; in addition, if the cast film is too thick, in order to obtain a product of the target thickness, a larger stretching ratio is required, and higher requirements are put forward for the stretching equipment, otherwise the thickness of the product is too thick, which will reduce the battery capacity when used, which is not desired by the battery manufacturer. In the present invention, by controlling the thickness of the cast film to be within the above range, the cast film is not only difficult to break, but also can obtain the desired stretching and orientation in the subsequent process, thereby ensuring the good physical properties and application of the diaphragm.

[0056] According to another specific embodiment of the present invention, referring to Figure 2 It is understood that the traction roller 210 may include an active roller and a passive roller used in conjunction with the active roller. The pressure between the active roller and the passive roller may be no more than 0.4 MPa, for example, 0.1 MPa, 0.2 MPa, 0.3 MPa or 0.4 MPa, etc., wherein the passive roller may be fitted to the active roller according to the thickness of the cast film so as to apply a certain traction force to the cast film. When the traction roller stops working, the active roller and the passive roller may be in a separated state. When the cast film is pulled, the roller spacing between the active roller and the passive roller may be no more than the thickness of the cast film, thereby providing a certain traction force and producing an appropriate stretching and thinning effect on the cast film. Preferably, the roller spacing range between the two applies a pressure of no more than 0.4 MPa to the cast film, which can further facilitate the effective traction of the cast film.

[0057] According to another specific embodiment of the present invention, referring to Figure 3 or Figure 4It is understood that at least two guide rollers 410 can be used to pull the film entering the cooling liquid 101 out of the cooling tank 100. The at least two guide rollers 410 can also be arranged horizontally or in a staggered manner below the liquid surface of the cooling liquid, thereby increasing the movement path of the cast film in the cooling liquid and extending the cooling time, thereby further improving the cooling effect of the cast film, so that the cast film can be fully cooled. Further, referring to Figure 4 It is understood that the number of guide rollers 410 can also be an odd number ( Figure 4 It is shown that the number of guide rollers is 3. Those skilled in the art can understand that the number of guide rollers can be not only 3, but also 5, 7, 9, etc.). An odd number of guide rollers can be arranged in a high and low staggered manner under the liquid surface of the coolant. It is preferred that the first guide roller and the last guide roller are both located near the bottom of the cooling trough, which can further help to extend the length of the cooling path and improve the cooling effect of the cast film.

[0058] According to another specific embodiment of the present invention, 1 to 2 pairs of liquid removal rollers 510 disposed opposite to each other may be used to remove the coolant on the surface of the diaphragm leaving the cooling trough 100. Preferably, two pairs of liquid removal rollers 510 may be included. The liquid removal rollers 510 are preferably placed above the cooling trough 100, thereby further improving the liquid removal effect, thereby not only reducing the coolant residue on the surface of the diaphragm casting, but also allowing the removed coolant to flow back into the cooling trough, thereby realizing the recycling of the coolant.

[0059] According to another specific embodiment of the present invention, the internal structures of the traction roller 210 and the guide roller 410 used in the present invention are not particularly limited, and those skilled in the art can make a selection according to actual needs. For example, the two can be independently hollow or solid structures. It should be noted that the traction roller and guide roller used in the present invention do not require built-in coolant.

[0060] In summary, the lithium battery diaphragm casting cooling method of the above embodiment of the present invention has at least the following advantages: 1. The battery diaphragm melt can be extruded from the beginning to the completion of cooling, and the cooling environment and the cooling medium temperature on both sides of the diaphragm are always consistent. The cast diaphragm is cooled in the air before entering the coolant, and the air cooling environment on both sides of the diaphragm is consistent. When the cast diaphragm enters the coolant, since the traction roller and the guide roller are both placed in the coolant, the surface temperature of the two is consistent with the coolant temperature. When the diaphragm moves in the coolant, the cooling temperature on both sides of the diaphragm is always consistent. Therefore, the cooling rate on both sides of the cast diaphragm can always maintain good consistency, the cooling uniformity is good, and the cooling effect is better, which effectively solves the problem of differences in the microstructure of the front and back sides of the diaphragm product after stretching due to inconsistent cooling rates on both sides of the cast diaphragm and the resulting difference in the microstructure of the front and back sides of the diaphragm product after stretching. 1. The problem of uneven membrane surface; 2. The cooling method is used in the preparation process of lithium battery diaphragms to significantly improve the quality of lithium battery diaphragms, so that the microstructures of the front and back sides of the final lithium battery diaphragm are consistent, the membrane surface is smoother, and the diaphragm performance is more stable; 3. The cooling method has low requirements for cooling equipment, the required cooling system has a simple structure and strong practicality, and the cooling medium can be adjusted according to the cooling temperature of the cast diaphragm. For example, when the diaphragm casting is required to be cooled to low temperature, water can be selected as the cooling medium, and when the diaphragm casting is required to be cooled to medium and high temperature, paraffin oil can be selected as the cooling medium. Water and paraffin oil have high cost performance and play a very important role in the field; 4. The cooling method can return the coolant attached to the surface of the diaphragm to the cooling tank for continued use, thereby reducing the waste of coolant and saving resources.

[0061] Based on the same inventive concept, in the second aspect of the present invention, the present invention proposes a lithium battery diaphragm casting cooling system. According to an embodiment of the present invention, reference is made to Figures 2 to 4It is understood that the cooling system includes: a cooling tank 100, a traction device (not shown), a guide device (not shown) and a liquid removal device (not shown), wherein a coolant 101 is provided in the cooling tank 100, and the cooling tank 100 includes a coolant inlet 110 and a coolant outlet 120; the traction device includes a pair of traction rollers 210, which are arranged in the cooling tank 100 and are located below the liquid level of the coolant 101, and the pair of traction rollers 210 rotate in opposite directions and are suitable for moving the cast film The film is pulled into the cooling liquid 101; the guide device includes at least one guide roller 410, which is arranged in the cooling tank 100 and below the liquid surface of the cooling liquid 101, and the guide roller 410 is suitable for pulling the film in the cooling liquid 101 away from the cooling tank 100; the liquid removal device includes at least one pair of liquid removal rollers 510 arranged opposite to each other, and the liquid removal rollers 510 are arranged above the cooling tank 100, and the liquid removal rollers 510 arranged opposite to each other rotate in opposite directions and are suitable for removing the cooling liquid on the surface of the film leaving the cooling tank 100. The system has a simple structure, and can cool down the front and back sides of the battery diaphragm melt casting film at the same time and keep the cooling rate of the front and back sides of the film consistent, so as to obtain a diaphragm casting sheet with consistent microstructure on both sides and a smooth surface, which can be widely used in the wet preparation process of lithium battery diaphragms and improve the quality of diaphragms.

[0062] According to some specific embodiments of the present invention, the type of coolant 101 used in the cooling system of the lithium battery diaphragm casting and the temperature of the coolant are not particularly limited. Those skilled in the art can flexibly select according to actual conditions. For example, the coolant 101 can be water or paraffin oil; the cooling temperature of the coolant 101 can be 15-120°C, specifically 15°C, 25°C, 40°C, 50°C, 75°C, 100°C, 115°C, 120°C, etc. Specifically, the coolant 101 in the cooling tank 100 can select a suitable cooling medium according to the needs of the diaphragm casting. For example, when the diaphragm casting requires cooling to a low temperature (15-50°C), since the viscosity of paraffin oil at low temperatures is relatively large and the fluidity is poor, water can be selected as the cooling medium, and when the diaphragm casting requires cooling to a high temperature (50-120°C), paraffin oil can be selected as the cooling medium. Thus, the applicability of the cooling system can be further improved, and the cooling requirements of different diaphragm castings can be met. In addition, when paraffin oil is selected as the coolant, the paraffin oil used as the coolant can be the same as the paraffin oil used to prepare the cast film. In this way, not only the cooling effect can be guaranteed, but also new impurity components will not be introduced when cooling the cast film.

[0063] According to some further specific embodiments of the present invention, the battery separator raw material can be melt-mixed and cast to obtain a cast film, wherein the melt-mixed and cast can be carried out in a melt extrusion device, such as a twin-screw extruder. In addition, the specific type of the die 300 of the melt extrusion device is not particularly limited, and those skilled in the art can flexibly select it according to actual conditions. For example, the die 300 of the melt extrusion device can be a hanger-type die, a T-type die, etc., as long as it can meet the needs of cast film. Further, the traction roller 210 can be arranged at the lower part of the die 300 of the melt extrusion device, and the distance between the die lip of the die 300 and the liquid surface of the coolant 101 can be no more than 300mm, for example, it can be 100-300mm, 30mm, 50mm, 80mm, 120mm, 150mm, 180mm, 200mm, 250mm or 280mm, etc. The inventors found that if the distance between the die head and the cooling liquid surface is too long, the cast film will stay in the air for too long, which will increase the cooling time of the cast film and reduce the cooling efficiency of the cast film. At the same time, the long residence time of the cast film in the ambient air will also lead to unstable cooling rate of the melt in the early stage, resulting in decreased production stability. In addition, if the distance between the die head and the cooling liquid surface is too long, the melt has low strength at high temperature and is relatively soft before being fully cooled and solidified, which will cause poor stability of the film and further affect the thickness and width stability of the thick film. When the distance between the die head and the cooling liquid surface is within the above range, the cooling efficiency of the cast film can be better improved, the stability of the melt film formation can be enhanced, and a guarantee can be provided for the stable entry into the cooling tank for cooling the casting.

[0064] According to some further specific embodiments of the present invention, Figure 2 It is understood that the traction roller 210 may include an active roller and a passive roller used in conjunction with the active roller. The pressure between the active roller and the passive roller may be no greater than 0.4 MPa, for example, 0.1 MPa, 0.2 MPa, 0.3 MPa or 0.4 MPa, etc., wherein the passive roller may fit to the active roller according to the thickness of the cast film so as to apply a certain traction force to the cast film. When the traction roller stops working, the active roller and the passive roller may be in a separated state. When the cast film is pulled, the roller spacing between the active roller and the passive roller may be no greater than the thickness of the cast film, thereby providing a certain traction force and producing an appropriate stretching and thinning effect on the cast film. Preferably, the roller spacing range between the two applies a pressure of no more than 0.4 MPa to the cast film, which can further facilitate the effective traction of the cast film.

[0065] According to another specific embodiment of the present invention, referring to Figure 3 or Figure 4It is understood that the guide device may include at least two guide rollers 410, and at least two guide rollers 410 may be arranged horizontally or in a staggered manner below the liquid surface of the coolant 101, thereby increasing the movement path of the cast film in the coolant and extending the cooling time, thereby further improving the cooling effect of the cast film, so that the cast film can be fully cooled. Figure 4 It is understood that the number of guide rollers 410 can also be an odd number ( Figure 4 It is shown that the number of guide rollers is 3. Those skilled in the art can understand that the number of guide rollers can be not only 3, but also 5, 7, 9, etc.). An odd number of guide rollers can be arranged in a high and low staggered manner under the liquid surface of the coolant. It is preferred that the first guide roller and the last guide roller are both located near the bottom of the cooling trough, which can further help to extend the length of the cooling path and improve the cooling effect of the cast film.

[0066] According to another specific embodiment of the present invention, the liquid removal device may include 1 to 2 pairs of liquid removal rollers 510, preferably two pairs of liquid removal rollers 510, and the liquid removal rollers 510 are preferably placed above the cooling tank 100, thereby further improving the liquid removal effect, thereby not only reducing the coolant residue on the surface of the membrane, but also allowing the removed coolant to flow back into the cooling tank, thereby realizing the recycling of the coolant.

[0067] It should be noted that the lithium battery diaphragm casting sheet cooling system of the above-mentioned embodiment of the present invention and the above-mentioned lithium battery diaphragm casting sheet cooling method are proposed based on the same inventive concept, and the technical features and effects described for the above-mentioned lithium battery diaphragm casting sheet cooling method are also applicable to the lithium battery diaphragm casting sheet cooling system, which will not be described one by one here.

[0068] In summary, the lithium battery diaphragm casting cooling system of the above embodiment of the present invention has at least the following advantages: 1. Simple structure; 2. The cooling environment and cooling medium temperature on both sides of the diaphragm can be kept consistent from the beginning of extrusion of the battery diaphragm melt until the cooling is completed. The cast diaphragm is cooled in the air before entering the coolant, and the air cooling environment on both sides of the diaphragm is consistent. When the cast diaphragm enters the coolant, the traction roller and the guide roller are both placed in the coolant, and the surface temperature of the two is consistent with the coolant temperature. When the diaphragm moves in the coolant, the cooling temperature on both sides of the diaphragm is always consistent. Therefore, the use of the cooling system The system implements cooling of the melt cast film of the lithium battery separator, which can make the cooling rate on the front and back sides of the cast film always maintain good consistency, with good cooling uniformity and better cooling effect, and effectively solves the problem of differences in the microstructure of the front and back sides of the separator product after stretching and the uneven membrane surface caused by the inconsistent cooling rate on both sides of the cast film; 3. The cooling system can be combined with the preparation process of the lithium battery separator, so as to significantly improve the quality of the lithium battery separator, so that the microstructure of the front and back sides of the final lithium battery separator is consistent, the separator membrane surface is smoother, and the separator performance is more stable.

[0069] In the third aspect of the present invention, the present invention proposes a system for wet preparation of lithium battery separators. According to an embodiment of the present invention, the system for wet preparation of lithium battery separators includes the aforementioned lithium battery separator casting cooling system. Compared with the prior art, by using the above-mentioned lithium battery separator casting cooling system in the wet preparation process, the battery separator melt can be made to be extruded from the beginning until the cooling is completed, and the cooling environment and the temperature of the cooling medium on both sides of the diaphragm are always consistent, which is more conducive to obtaining castings with consistent microstructures and smooth surfaces on both sides of the front and back, thereby significantly improving the uniformity and flatness of the final battery separator. As a result, the system for wet preparation of lithium battery separators is not only easy to operate, but also can obtain lithium battery separators with higher quality, stable performance and stronger competitiveness. It should be noted that the technical features and effects described for the above-mentioned lithium battery separator casting cooling system are also applicable to the system for wet preparation of lithium battery separators, and will not be repeated here one by one.

[0070] In the fourth aspect of the present invention, the present invention proposes a method for preparing a lithium battery separator by a wet process. According to an embodiment of the present invention, the method adopts the aforementioned system implementation of preparing a lithium battery separator by a wet process or includes the aforementioned lithium battery separator casting cooling method. Compared with the prior art, the preparation method is not only easy to operate and implement when cooling the diaphragm, but also has a better cooling effect. It has the advantages of simple and convenient operation, easy implementation, and easy industrialization. The lithium battery separator prepared by this method has consistent microstructures on both sides, a smoother membrane surface, and stable performance. Using it in lithium batteries is more conducive to improving the comprehensive performance of lithium batteries.

[0071] According to an embodiment of the present invention, referring to Figure 5 It is understood that the method for preparing a lithium battery separator by a wet process may specifically include:

[0072] (i) Mixing polyethylene resin and paraffin oil, performing melt kneading and casting to form a film, thereby obtaining a cast film sheet

[0073] According to an embodiment of the present invention, the addition amount of paraffin oil can account for 25wt% to 30wt% of the polyethylene resin, specifically 25wt%, 26wt%, 27wt%, 28wt%, 29wt%, 30wt%, etc., thereby making the microstructures of the front and back sides of the final lithium battery separator more consistent and the membrane surface smoother. Among them, the melt mixing and cast film forming can be carried out in a melt extrusion device, such as a twin-screw extruder. In addition, it can be understood that the specific process of melt mixing and cast film forming is not particularly limited, as long as a cast film can be obtained in the end. Those skilled in the art can flexibly choose, and will not be elaborated here.

[0074] According to some specific embodiments of the present invention, the thickness of the cast film may be 500-3000 μm, for example, 750 μm, 1000 μm, 1250 μm, 1500 μm, 1750 μm, 2000 μm, 2250 μm, 2500 μm or 2750 μm. The inventor found that since the thickness of the diaphragm prepared by the wet preparation process is usually between 5 and 25 μm, after being cast into a thick sheet by cast extrusion, it is necessary to perform longitudinal and transverse stretching at a certain ratio to orient the polyethylene resin in the transverse and longitudinal directions to obtain good mechanical properties. If the thickness of the cast film is too thin, on the one hand, it is easy to have the risk of tensile fracture, and on the other hand, the subsequent stretching ratio cannot be too large, otherwise it is easy to have the risk of tensile fracture. However, when the stretching ratio is low, the tensile strength of the prepared diaphragm is low, which affects the performance of the diaphragm; if the thickness of the cast film is too thick, the cooling efficiency of the cast film decreases, and the crystallinity of the polyethylene resin in the casting stage is too large, resulting in increased difficulty in subsequent longitudinal stretching; in addition, if the cast film is too thick, in order to obtain a product of the target thickness, a larger stretching ratio is required, and higher requirements are put forward for the stretching equipment, otherwise the thickness of the product is too thick, which will reduce the battery capacity when used, which is not desired by the battery manufacturer. In the present invention, by controlling the thickness of the cast film to be within the above range, the cast film is not only difficult to break, but also can obtain the desired stretching and orientation in the subsequent process, thereby ensuring the good physical properties and application of the diaphragm.

[0075] (ii) Using a traction roller to allow the cast film to directly enter the cooling liquid to achieve simultaneous cooling of both the front and back sides. According to the embodiments of the present invention, the traction speed of the traction roller in the present invention is not particularly limited, and those skilled in the art can select it according to actual needs. For example, the traction speed of the traction roller can be 2 to 8 m / min, specifically 2 m / min, 4 m / min, 6 m / min or 8 m / min, etc. When the traction speed is within this range, the cast film can be stretched and thinned to a certain extent to obtain a cast thick film of the desired thickness, thereby better improving the cooling efficiency and cooling effect of the cast film, and also helping to improve the consistency of the microstructure of the front and back sides of the lithium battery separator and the flatness of the separator membrane surface.

[0076] (iii) Using a guide roller to pull the membrane into the coolant and leave the cooling tank. According to an embodiment of the present invention, the pulling speed of the guide roller can be consistent with that of the pulling roller, which is more conducive to obtaining a battery separator with uniform structure, consistent microstructure on both sides, and a flat membrane surface.

[0077] (iv) removing the coolant on the surface of the film leaving the cooling tank by a liquid removal roller, and obtaining a diaphragm casting sheet with consistent structures on both sides. According to an embodiment of the present invention, after cooling and lowering the temperature, the cast film is pulled out of the cooling tank by a guide roller, and the coolant attached to the surface of the cast film can be removed by a liquid removal roller, and the removed coolant is returned to the cooling tank, thereby reducing the waste of coolant and saving resources.

[0078] (v) Biaxially stretching the diaphragm casting to obtain a diaphragm sheet. According to an embodiment of the present invention, the biaxial stretching may include transverse stretching and longitudinal stretching. It is understood that the specific process of biaxial stretching is not particularly limited and can be flexibly selected by those skilled in the art, which will not be described in detail herein.

[0079] (vi) Extracting the membrane slice to remove the paraffin oil in the membrane slice. It is understandable that the specific process of extracting the membrane slice is not particularly limited, as long as the paraffin oil in the membrane slice can be removed. Those skilled in the art can flexibly select, and will not be described in detail here.

[0080] (vii) heat setting and rolling the extracted membrane sheet to obtain a lithium battery membrane. It is understood that the specific process of heat setting and rolling is not particularly limited, as long as a lithium battery membrane can be obtained, and those skilled in the art can flexibly select, and will not be described in detail here.

[0081] According to a specific embodiment of the present invention, referring to Figure 4It is understood that polyethylene resin and paraffin oil can be melt-mixed and cast into film through a twin-screw extruder to obtain a cast film; the cast film is extruded from the die head 300 in the twin-screw extruder, and is cast into a cooling tank 100 by using a traction roller 210 corresponding to the die lip of the die head 300 of the melt extrusion device for double-sided cooling, and then is pulled away from the cooling tank 100 by a guide roller 410, and the coolant on the cast film after cooling is removed by a liquid removal roller 510 to obtain a diaphragm casting; the diaphragm casting is biaxially stretched step by step first longitudinally and then transversely to obtain a diaphragm sheet; the diaphragm sheet is extracted to remove the paraffin oil from the diaphragm sheet; the extracted diaphragm sheet is heat-set and rolled up at high temperature to obtain a lithium battery diaphragm.

[0082] It should be noted that the technical features and effects described for the above-mentioned system for wet-process preparation of lithium battery separators and the above-mentioned method for cooling lithium battery separator castings are also applicable to the method for wet-process preparation of lithium battery separators, and will not be described one by one here.

[0083] In the fifth aspect of the present invention, the present invention proposes a lithium battery separator, which is prepared by the above-mentioned wet method for preparing lithium battery separators or by the above-mentioned system for preparing lithium battery separators by wet method. Compared with the prior art, the microstructures of the front and back sides of the lithium battery separator are consistent, the membrane surface of the separator is smoother, and the performance is more stable, with the advantages of high product quality and strong inherent competitiveness. Using it in lithium batteries is more conducive to improving the comprehensive performance of lithium batteries, and the above-mentioned features and advantages described for the lithium battery separator casting cooling system and the wet method for preparing lithium battery separators are also applicable to the lithium battery separator, which will not be repeated here.

[0084] The present invention is described below with reference to specific embodiments. It should be noted that these embodiments are merely illustrative and do not limit the present invention in any way.

[0085] Example 1

[0086] Preparation of lithium battery diaphragm: polyethylene resin and paraffin oil are melt-kneaded in a mass ratio of 25:75 through a twin-screw extruder to obtain a melt with a temperature of 210°C; the melt is extruded from a die head in the twin-screw extruder to form a film, and is cast into a cooling tank by a traction roller for double-sided cooling, wherein the temperature of the cooling tank is 30°C, the coolant is water, and the distance between the die head and the liquid level of the coolant is 200 mm; the film is pulled out of the cooling tank by a guide roller, and the coolant on the cast film after cooling is removed by a liquid removal roller to obtain a diaphragm casting; the diaphragm casting is first biaxially stretched longitudinally and then transversely to obtain a diaphragm sheet, wherein the longitudinal stretching temperature is 95°C, the transverse stretching temperature is 120°C, and the stretching ratio is 7×7 times; the diaphragm sheet is extracted to remove the paraffin oil of the diaphragm sheet; the extracted diaphragm sheet is subjected to high-temperature heat setting and winding to obtain a lithium battery diaphragm, wherein the setting temperature is 130°C.

[0087] in conclusion: Figure 6 This is a microstructure diagram of the front and back sides of the lithium battery separator prepared in Example 1 (wherein side A represents one surface of the lithium battery separator, and side B represents the other surface corresponding to side A). It can be seen from the figure that the microstructure of the front and back sides of the prepared lithium battery separator is consistent.

[0088] Example 2

[0089] Preparation of lithium battery diaphragm: polyethylene resin and paraffin oil are melt-mixed in a mass ratio of 30:70 through a twin-screw extruder to obtain a melt with a temperature of 215°C; the melt is extruded from a die head in the twin-screw extruder to form a film, and is cast into a cooling tank by a traction roller for double-sided cooling, wherein the temperature of the cooling tank is 80°C, the coolant is paraffin oil, and the distance between the die head and the liquid level of the coolant is 300 mm; the film is pulled out of the cooling tank by a guide roller, and the coolant on the cast film after cooling is removed by a liquid removal roller to obtain a diaphragm casting; the diaphragm casting is first biaxially stretched longitudinally and then transversely to obtain a diaphragm sheet, wherein the longitudinal stretching temperature is 98°C, the transverse stretching temperature is 123°C, and the stretching ratio is 8×8 times; the diaphragm sheet is extracted to remove the paraffin oil of the diaphragm sheet; the extracted diaphragm sheet is subjected to high-temperature heat setting and winding to obtain a lithium battery diaphragm, wherein the setting temperature is 133°C.

[0090] in conclusion: Figure 7 This is a microstructure diagram of the front and back sides of the lithium battery separator prepared in Example 2 (wherein side A represents one surface of the lithium battery separator, and side B represents the other surface corresponding to side A). It can be seen from the figure that the microstructure of the front and back sides of the prepared lithium battery separator is consistent.

[0091] Comparative Example 1

[0092] Preparation of lithium battery separator: polyethylene resin and paraffin oil are melt-kneaded in a mass ratio of 25:75 through a twin-screw extruder to obtain a melt with a temperature of 210°C; the melt is extruded from a die head in the twin-screw extruder into a cast film, and the film is cooled by a cooling roller, wherein the temperature of the cooling roller is 30°C, to obtain a separator casting; the separator casting is subjected to step-by-step biaxial stretching first longitudinally and then transversely to obtain a separator sheet, wherein the longitudinal stretching temperature is 95°C, the transverse stretching temperature is 120°C, and the stretching ratio is 7×7 times; the separator sheet is extracted to remove the paraffin oil from the separator sheet; the extracted separator sheet is subjected to high-temperature heat setting and winding to obtain a lithium battery separator, wherein the setting temperature is 130°C.

[0093] in conclusion: Figure 8 This is a microstructure diagram of the front and back sides of the lithium battery separator prepared in Comparative Example 1 (wherein side A represents one surface of the lithium battery separator, and side B represents the other surface corresponding to side A). It can be seen from the figure that the microstructures of the front and back sides of the prepared lithium battery separator are obviously different.

[0094] Comparative Example 2

[0095] Preparation of lithium battery separator: polyethylene resin and paraffin oil are melt-kneaded in a mass ratio of 30:70 through a twin-screw extruder to obtain a melt with a temperature of 215°C; the melt is extruded from a die head in the twin-screw extruder into a cast film, and the film is cooled by a cooling roller, wherein the temperature of the cooling roller is 80°C, to obtain a separator casting; the separator casting is first biaxially stretched longitudinally and then transversely to obtain a separator sheet, wherein the longitudinal stretching temperature is 98°C, the transverse stretching temperature is 123°C, and the stretching ratio is 8×8 times; the separator sheet is extracted to remove the paraffin oil from the separator sheet; the extracted separator sheet is heat-set and rolled at high temperature to obtain a lithium battery separator, wherein the setting temperature is 133°C.

[0096] in conclusion: Fig. 9 This is a microstructure diagram of the front and back sides of the lithium battery separator prepared in Comparative Example 2 (wherein side A represents one surface of the lithium battery separator, and side B represents the other surface corresponding to side A). It can be seen from the figure that the microstructures of the front and back sides of the prepared lithium battery separator are obviously different.

[0097] Based on Examples 1 to 2 and Comparative Examples 1 to 2, it can be seen that the lithium battery diaphragm obtained by using the lithium battery diaphragm casting cooling system of the above embodiment of the present invention has good microscopic consistency on both sides and the diaphragm surface is flat.

[0098] In the description of this specification, the description with reference to the terms "one embodiment", "some embodiments", "example", "specific example", or "some examples" etc. means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described may be combined in any one or more embodiments or examples in a suitable manner. In addition, those skilled in the art may combine and combine the different embodiments or examples described in this specification and the features of the different embodiments or examples, without contradiction.

[0099] Although the embodiments of the present invention have been shown and described above, it is to be understood that the above embodiments are exemplary and are not to be construed as limitations of the present invention. A person skilled in the art may change, modify, replace and vary the above embodiments within the scope of the present invention.

Claims

1. A lithium battery diaphragm casting cooling system, characterized in that: include: A cooling tank, wherein a coolant is provided in the cooling tank, the cooling tank comprises a coolant inlet and a coolant outlet, and the coolant is paraffin oil; A traction device, the traction device comprising traction rollers arranged in pairs, the traction rollers are arranged in the cooling tank and below the liquid surface of the cooling liquid, the traction rollers arranged in pairs rotate towards each other and are suitable for traction of the cast film into the cooling liquid; A guide device, the guide device comprising at least one guide roller, the guide roller being arranged in the cooling tank and below the liquid surface of the coolant, the guide roller being adapted to pull the membrane in the coolant away from the cooling tank; A liquid removal device, the liquid removal device comprising at least one pair of liquid removal rollers disposed opposite to each other, the liquid removal rollers being disposed above the cooling tank, the liquid removal rollers disposed opposite to each other rotating towards each other and being adapted to remove the coolant on the surface of the membrane leaving the cooling tank; The traction roller is arranged at the lower part of the die head of the melt extrusion device, and the distance between the die lip of the die head and the liquid surface of the cooling liquid is not more than 300 mm; Among them, the lithium battery diaphragm casting cooling method includes: (1) Providing a battery separator melt casting film; (2) using a traction roller disposed in a cooling tank and below the liquid level of a coolant to allow the cast film to directly enter the coolant to achieve simultaneous cooling of both the front and back sides, wherein the coolant is paraffin oil, and the paraffin oil used as the coolant is the same as the paraffin oil used to prepare the cast film; (3) using a guide roller disposed in the cooling tank and below the surface of the cooling liquid to pull the membrane in the cooling liquid out of the cooling tank; (4) Using a liquid removal roller disposed above the cooling trough to remove the cooling liquid on the surface of the diaphragm leaving the cooling trough, so as to obtain a diaphragm casting with consistent structures on both sides.

2. The lithium battery diaphragm casting sheet cooling system according to claim 1, characterized in that: The guide device includes at least two guide rollers, and the liquid removal device includes 1 to 2 pairs of liquid removal rollers; Alternatively, at least two of the guide rollers are arranged in a staggered manner at high and low levels below the liquid surface of the cooling liquid.

3. The lithium battery diaphragm casting sheet cooling system according to claim 1, characterized in that: The residence time of the cast film in the cooling liquid is 5 to 60 seconds.

4. A system for preparing a lithium battery separator by a wet process, characterized in that: The invention comprises a lithium battery diaphragm casting sheet cooling system as claimed in any one of claims 1 to 3.

5. A method for preparing a lithium battery separator by a wet process, characterized in that: The system described in claims 1 to 3 is used for implementation.

6. The method according to claim 5, characterized in that include: (i) mixing polyethylene resin and paraffin oil for melt mixing and casting to obtain a cast film; (ii) using the traction roller to allow the cast film to directly enter the cooling liquid to achieve simultaneous cooling of both the front and back sides; (iii) using the guide roller to pull the film in the cooling liquid out of the cooling tank; (iv) removing the coolant on the surface of the diaphragm leaving the cooling tank by the liquid removal roller, so as to obtain a diaphragm casting sheet with consistent structures on both sides; (v) biaxially stretching the diaphragm cast sheet to obtain a diaphragm sheet; (vi) extracting the membrane slice to remove paraffin oil from the membrane slice; (vii) heat setting and rolling the extracted separator sheet to obtain a lithium battery separator.

7. A lithium battery separator, characterized in that: It is prepared by the method described in any one of claims 5-6 or by the system for preparing lithium battery separator by wet method described in claim 6.

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