Vacuum suction-based wet lithium battery diaphragm and preparation method and system thereof

Abstract

This invention discloses a wet-process lithium-ion battery separator based on vacuum forming technology, and its preparation method and system. The process involves mixing, plasticizing, extruding annular oil film preforms, vacuum forming, longitudinal stretching, extraction drying, heat setting, and film winding. The oil film after vacuum forming and cooling typically exhibits significant and uniform orientation in the transverse direction. This invention uses vacuum forming to achieve transverse stretching, which effectively solves the problems of product consistency and the setting of stretching ratios in each direction. The production equipment is simple, and the production process is easy to control, thus achieving excellent separator properties while simultaneously addressing product consistency issues.

Description

Technical Field

[0001] This invention relates to the field of lithium batteries, and in particular to a wet-process separator for lithium-ion batteries based on vacuum forming technology and its preparation method. Background Technology

[0002] In lithium-ion batteries, the primary function of the separator is to separate the positive and negative electrodes to prevent short circuits while allowing ion transfer. As a crucial component of lithium-ion batteries, the separator is one of the key internal components. The performance of the separator directly affects the battery's capacity, internal resistance, cycle life, and self-discharge, thus influencing its capacity, cycle life, and safety characteristics. A high-performance separator plays a vital role in improving the overall performance of the battery. Tensile strength / puncture strength of the separator is one important metric; higher separator strength better ensures battery safety.

[0003] Currently, the main preparation methods for commercially used lithium-ion battery separators include wet membrane fabrication and dry membrane fabrication.

[0004] Dry-stretched separators are divided into uniaxially stretched and biaxially stretched separators. Uniaxially stretched dry-stretched separators have more uniform pore size and better thermal stability; however, their TD (diagonal and diametrical) strength is insufficient, leading to tearing during battery winding. Biaxially stretched dry-stretched separators, in addition to possessing the general advantages of dry-stretching, have more balanced MD (matrix) and TD (diagonal) strength and good oxidation resistance. However, their thermal stability is inferior to uniaxially stretched separators, and their pore size is uneven, making them prone to large pores, posing safety hazards for battery use.

[0005] The preparation process of wet-process biaxially oriented membranes involves mixing liquid hydrocarbons or some small molecules with polyolefin resin, heating and melting them to form a homogeneous mixture, then cooling to separate the phases, pressing the mixture into a membrane sheet, heating the membrane sheet to near its melting point, and biaxially oriented the molecular chains. Finally, the mixture is held at this temperature for a certain period of time, and residual solvents are washed away with volatile substances to prepare an interconnected microporous membrane material. Generally, the wet-process biaxially oriented membrane exhibits superior biaxial tensile strength. Wet stretching is divided into two types: synchronous biaxial stretching and asynchronous biaxial stretching. Synchronous biaxial stretching is characterized by good product consistency, but it is not suitable for adjusting the strength in each direction. For example, Chinese patent application CN102544416A discloses a method for preparing a multilayer polyolefin battery cadmium film. This method uses wet synchronous biaxial stretching to prepare the separator. The extruded film is first rapidly cooled to form a first membrane bubble, then stretched and blown to form a second membrane bubble. After being dried with an extractant, it is stretched again. This method, through multiple cooling and shaping processes and multiple stretching, achieves structural stability, but the process is relatively complex. Furthermore, the simultaneous stretching and blowing results in uneven air pressure along the longitudinal direction during the blowing process, leading to poor orientation consistency in both the transverse and longitudinal directions. Asynchronous biaxial stretching generally includes several stages: feeding, casting, longitudinal stretching, transverse stretching, extraction, shaping, and slitting. It allows for easy adjustment of the strength in each direction, resulting in higher strength, but with slightly poorer consistency. Summary of the Invention

[0006] The main objective of this invention is to provide a novel method for preparing wet-process lithium-ion battery separators based on vacuum forming technology, in order to solve the technical problem that existing wet-process lithium-ion battery separator preparation methods cannot simultaneously achieve good product consistency and high anisotropic strength.

[0007] To achieve the above objectives, the present invention provides a method for preparing a wet-process lithium battery separator based on vacuum forming technology, comprising the following steps:

[0008] S1, Annular oil film preform extrusion: According to the preset material ratio, the material is conveyed, mixed, melted and plasticized by the extruder, and the uniformly mixed melt is conveyed to the die head and extruded from the die head as an annular oil film preform;

[0009] S2, Vacuum Forming: An annular oil film preform is fed into a vacuum forming device for lateral stretching and cooling to obtain a laterally stretched oil film; wherein, the vacuum forming device includes a vacuum section sleeve and a vacuum system, the vacuum section sleeve includes a cylindrical sleeve with openings at both ends, and the side wall of the cylindrical sleeve has evenly arranged through holes, the through holes connecting to the vacuum system, the annular oil film preform is conveyed in the cylindrical sleeve along the axial direction, and the annular oil film preform is laterally stretched under the pressure difference between the internal air pressure and the external vacuum low pressure;

[0010] S3, Cutting and unfolding: The oil film obtained in step S2 is laterally cut along the conveying direction, and the cut oil film is unfolded to form a single-layer oil film.

[0011] S4, Longitudinal stretching: The oil film cut and unfolded in step S3 is preheated and then longitudinally stretched. The longitudinal stretching ratio is at least 2. The stretched oil film is then cooled.

[0012] S5, Extraction and Drying: The oil film obtained in step S4 is sent into an extraction tank containing an extractant. The paraffin oil is extracted from the microporous structure of the oil film. The extracted oil film is then sent into a drying oven for drying to evaporate the extractant on the oil film.

[0013] S6, Setting and winding: The oil film dried in step S5 is heat-set and then sent to the winding section to obtain the diaphragm product.

[0014] Optionally, the discharge end of the cylindrical sleeve is further provided with a herringbone plate extrusion die with a gap; step S2 further includes that the film bubble obtained after the annular oil film preform is stretched laterally is extruded into a double-layer sheet-like oil film through the gap of the herringbone plate extrusion die.

[0015] Optionally, the ratio of the inner diameter of the cylindrical sleeve to the diameter of the annular oil film blank is 1 to 2: 0.5 to 0.8.

[0016] Optionally, the thickness of a single layer of the double-layered sheet oil film obtained in step S2 is between 500 and 1500 μm.

[0017] Optionally, in step S2, the vacuum system extracts air from the cylindrical sleeve of the vacuum section sleeve to maintain the vacuum level between -50 and -70 kPa.

[0018] Optionally, in the longitudinal stretching of step S4, the preheating temperature is between 40-100℃ and the longitudinal stretching ratio is between 2-10; in the setting and winding of step S6, the heat setting temperature is 110-130℃; and the extractant in step S5 is dichloromethane.

[0019] Optionally, in step S3, the double-layered sheet-like oil film is laid flat on the cutting platform, and the bottom surface of the oil film is cut with a cutter with the blade facing upward. Then, the cut oil film is unfolded to form a single-layered oil film.

[0020] The present invention also provides a wet-process lithium battery separator based on vacuum forming process, which is prepared by the preparation method of wet-process lithium battery separator based on vacuum forming process as described in any of the above claims.

[0021] The present invention also provides a preparation system for a wet-process lithium battery separator based on vacuum forming process as described in any of the preceding claims. The preparation system includes an extruder, a vacuum forming device, a cutting mechanism, an extraction tank, a drying oven, a heat setting device, and a winding device arranged sequentially along the separator conveying direction, and a plurality of casting rollers disposed in the middle of the preparation system to provide traction force. The die of the extruder forms an annular outlet to extrude an annular oil film preform. The vacuum forming device includes a vacuum section sleeve and a vacuum pumping system. The vacuum section sleeve includes a cylindrical sleeve with openings at both ends. The side wall of the cylindrical sleeve has uniformly arranged through holes that connect to the vacuum pumping system. The annular oil film preform is conveyed within the cylindrical sleeve along the axial direction. Under the pressure difference between the annular oil film preform and the inner wall of the cylindrical sleeve and the interior of the annular oil film preform, the annular oil film preform is laterally stretched. The cutting mechanism includes a cutting platform and a cutter located at the starting end of the cutting platform, with the cutting edge of the cutter facing upward.

[0022] Optionally, the upper surface of the cutting platform is triangular, the cutter is fixed at one corner of the triangle, and the two sides of the cutting platform adjacent to the cutter are both arc-shaped sides that smoothly transition downwards.

[0023] The lithium-ion battery wet-process separator provided by this invention is prepared by a process of mixing, plasticizing, extruding annular oil film preforms, vacuum forming, longitudinal stretching, extraction and drying, heat setting, and film winding. In this invention, after vacuum forming at a vacuum level of at least -10 kPa and cooling and setting, the microporous structure is basically formed, although paraffin oil still occupies the micropores. When the vacuum level used in the vacuum forming method is -10 to -80 kPa, the transverse tensile strength of the lithium-ion battery separator product is improved to a certain extent, with a stretch ratio between 3 and 8. The oil film after vacuum forming and cooling is usually significantly oriented laterally, with uniform orientation and good consistency. This invention uses vacuum forming to achieve transverse stretching, which can effectively solve the problems of product consistency and the setting of stretch ratios in each direction. The production equipment of this invention is simple, and the production process is easy to control, thus achieving excellent physical properties of the separator while effectively solving the product consistency problem. Attached Figure Description

[0024] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the structures shown in these drawings without creative effort.

[0025] Figure 1 This is a schematic diagram of a wet-process lithium battery separator preparation system based on vacuum air forming process in one embodiment of the present invention.

[0026] Figure 2 This is a schematic diagram of a vacuum forming device according to one embodiment of the present invention;

[0027] Figure 3 This is an enlarged schematic diagram of the cutting mechanism in one embodiment of the present invention.

[0028] The objectives, features, and advantages of this invention will be further explained in conjunction with the embodiments and with reference to the accompanying drawings. Detailed Implementation

[0029] The technical solutions in this embodiment will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0030] It should be noted that all directional indicators (such as up, down, left, right, front, back, etc.) in this embodiment are only used to explain the relative positional relationship and movement of each component in a specific posture (as shown in the attached figure). If the specific posture changes, the directional indicator will also change accordingly.

[0031] Furthermore, in this invention, descriptions involving "first," "second," etc., are for descriptive purposes only and should not be construed as indicating or implying their relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this invention, "a plurality of" means at least two, such as two, three, etc., unless otherwise explicitly specified.

[0032] In this invention, unless otherwise explicitly specified and limited, the terms "connection," "fixed," etc., should be interpreted broadly. For example, "fixed" can mean a fixed connection, a detachable connection, or an integral part; it can mean a mechanical connection or an electrical connection; it can mean a direct connection or an indirect connection through an intermediate medium; it can mean the internal communication of two components or the interaction between two components, unless otherwise explicitly limited. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0033] Furthermore, the technical solutions of the various embodiments of the present invention can be combined with each other, but only if they are feasible for those skilled in the art. If the combination of technical solutions is contradictory or cannot be implemented, it should be considered that such combination of technical solutions does not exist and is not within the scope of protection claimed by the present invention.

[0034] It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

[0035] Please combine Figure 1-3 As shown, a wet-process lithium battery separator preparation system 100 based on vacuum forming technology is provided in one embodiment of the present invention. It includes an extruder 1, a vacuum forming device 2, a cutting mechanism 3, an extraction tank 4, a drying oven (not shown), a heat setting device 6, and a winding device 7 arranged sequentially along the separator conveying direction. Multiple casting rollers 8 are disposed in the middle of the preparation system 100 to provide traction force. The die of the extruder 1 has an annular outlet to extrude an annular oil film preform. The vacuum forming device 2 includes a vacuum section sleeve 2. 1. A vacuum system (not shown in the figure) is included. The vacuum section sleeve 21 includes a cylindrical sleeve 211 with openings at both ends. Uniformly arranged through holes 22 are provided on the side wall of the cylindrical sleeve 211, and the through holes 22 connect to the vacuum system. The annular oil film blank is conveyed within the cylindrical sleeve 211 along its axial direction. Under the pressure difference between the inside of the annular oil film blank and the inner wall of the cylindrical sleeve 211, the annular oil film blank undergoes transverse stretching. Transverse stretching refers to stretching in the radial direction of the annular oil film blank; similarly, longitudinal stretching, as described later, refers to stretching in the axial direction of the annular oil film blank.

[0036] Those skilled in the art can reasonably arrange the casting rollers 8 among the various sub-devices of the preparation system 100 as needed to provide traction force; only one casting roller 8 is shown in the figure, but multiple sets of casting rollers are shown. The specific embodiments mainly describe the technical features that solve the technical problem that the present invention actually intends to solve. The preparation of wet lithium battery separators overlaps with the prior art and can be selected by those skilled in the art as needed, so these parts are not described in detail; however, this does not mean that the technical solution of the self-heating pyrolysis recovery device in the present invention is incomplete.

[0037] Please combine them together Figure 3 The cutting mechanism 3 includes a cutting platform 31 and a cutter 32 located at the starting end of the cutting platform 31, with the blade of the cutter 32 facing upwards. Optionally, the upper surface of the cutting platform 31 is triangular, and the cutter 32 is fixed at one corner of the triangle. The two adjacent sides of the cutting platform 31 and the cutter 32 are both curved sides that smoothly transition downwards.

[0038] The working principle of the above-mentioned wet lithium battery separator preparation system 100 based on vacuum forming process will be explained below with reference to a method for preparing a wet lithium battery separator based on vacuum forming process provided by the present invention.

[0039] The method for preparing a wet-process lithium battery separator based on vacuum forming technology includes the following steps:

[0040] S1, Annular oil film preform extrusion: According to the preset material ratio, the material is conveyed, mixed, melted and plasticized by the extruder, and the uniformly mixed melt is conveyed to the die head and extruded from the die head as an annular oil film preform.

[0041] It is understandable that preparing an oil film preform refers to using an extruder to complete the functions of conveying, mixing, melting, and plasticizing materials, conveying the melt to the die head, setting a suitable temperature and speed, and extruding a preform of appropriate thickness and diameter from the die head.

[0042] Specifically, step S1 is a feeding, mixing, and melt extrusion step. The material can be fed through a feeding system. The material at this stage includes ultra-high molecular weight olefin polymers with a molecular weight of 200,000 to 3 million, one or a mixture of paraffin oil, phthalate esters, and glycerides. The extruder is used to convey, mix, melt, and plasticize the material from the feeding system. The extruder conveys the uniformly mixed melt to the die head and extrudes an annular oil film preform from the die head.

[0043] S2, Vacuum Forming: An annular oil film preform is fed into a vacuum forming device for lateral stretching and cooling to obtain a laterally stretched oil film; wherein, the vacuum forming device includes a vacuum section sleeve and a vacuum pumping system, the vacuum section sleeve includes a cylindrical sleeve with openings at both ends, and the side wall of the cylindrical sleeve has evenly arranged through holes, the through holes connecting to the vacuum pumping system, the annular oil film preform is conveyed in the cylindrical sleeve along the axial direction of the cylindrical sleeve, and the annular oil film preform is laterally stretched under the pressure difference between the internal air pressure and the external vacuum low pressure.

[0044] Please combine them together Figure 2Specifically, the vacuum section sleeve 21 includes the cylindrical sleeve 211, and the interior of the cylindrical sleeve 211 forms a mold inner core cavity groove arranged along the axial direction of the vacuum section sleeve 21; a plurality of through holes 22 are opened on the side wall of the cylindrical sleeve 211 as external vacuum holes; the annular oil film preform extruded by the die head of the extruder 1 enters the mold inner core cavity groove of the vacuum section sleeve 21 and begins to be vacuumed, maintaining a certain vacuum degree. Since there is a certain amount of air inside the annular oil film preform, under the condition that there is a certain vacuum degree outside the annular oil film preform, a pressure difference is formed, the inside expands outward, forming a certain degree of lateral stretching, and the expanded film bubble is pulled out from the mold inner core cavity groove.

[0045] In this embodiment, the annular oil film preform is stretched laterally by vacuum forming. In the axial direction of the cylindrical sleeve 211, the vacuum suction force at each position is the same, and the resulting internal and external pressure difference is consistent, which makes the annular oil film preform have good uniformity in lateral stretching, thereby achieving good orientation performance.

[0046] Under a vacuum degree of at least -10 kPa, annular oil film preforms are vacuum-formed and cooled for shaping. The microporous structure of the oil film is essentially formed, although paraffin oil still occupies the micropores. The oil film after vacuum forming and cooling exhibits significant lateral orientation. Different stretching ratios correspond to different diameter core grooves in the annular oil film preform; specifically, the larger the diameter of the core groove, the greater the stretching ratio and the higher the required vacuum degree. As the vacuum degree and core diameter increase, the stretching ratio increases, and the lateral tensile strength of the lithium-ion battery separator gradually increases. This invention uses vacuum forming to achieve lateral stretching, which can effectively solve the problems of product consistency and setting the stretching ratio in each direction.

[0047] In this embodiment, the transverse stretching ratio of the annular oil film preform is 2-14, preferably 3-8. The transverse stretching ratio refers to the ratio of the diameter of the oil film after transverse stretching in vacuum forming to the diameter of the annular oil film preform before stretching.

[0048] In a preferred embodiment, the vacuum section sleeve 21 further includes an outer sleeve 212, which is fitted onto the outside of the cylindrical sleeve 211. The outer sleeve 212 also has through holes 23, through which the vacuum system connects to the through hole 22 to evacuate the interior of the cylindrical sleeve 211. By using the outer sleeve 212, only a few through holes 23 are needed to connect the vacuum system to achieve vacuuming of the cylindrical sleeve 211, thus simplifying the pipe design and evacuation structure.

[0049] Optionally, the discharge end of the cylindrical sleeve 211 is also provided with a herringbone plate extrusion die 24 with a gap; step S2 further includes that the film bubble obtained after the annular oil film blank is stretched laterally is flattened by the gap 241 of the herringbone plate extrusion die 24, that is, extruded into a double-layer sheet oil film, and then cooled after forming.

[0050] Optionally, the ratio of the inner diameter of the cylindrical sleeve to the diameter of the annular oil film blank is 3 to 8:1.

[0051] Optionally, the thickness of a single layer of the double-layered sheet oil film obtained in step S2 is between 500 and 1500 μm.

[0052] Optionally, in step S2, the vacuum system extracts air from the cylindrical sleeve of the vacuum section sleeve to maintain the vacuum level between -50 and -70 kPa.

[0053] S3, Cutting and unfolding: The oil film obtained in step S2 is laterally cut along the conveying direction, and the cut oil film is unfolded to form a single-layer oil film.

[0054] The specific steps may include: after cooling the double-layer sheet oil film obtained in step S2, cutting one side of the double-layer sheet oil film with a cutter. Since the oil film in the subsequent process is in an unfolded state and is transferred and pulled on the casting roller 8, the oil film will be unfolded after cutting to form a single layer of oil film.

[0055] In this embodiment,

[0056] It is understandable that when the preparation system 100 is started for the first time, the double-layer sheet oil film that has been cut for the first time needs to be manually or by external force to unfold it and be pulled on the subsequent casting rollers until the final winding section can continuously wind it to provide traction. In the subsequent process, the unfolding in step S3 does not need to be achieved manually or by external force.

[0057] In a specific example, a double-layered sheet-like oil film can be laid flat on a cutting platform, and the bottom surface of the oil film can be cut with a cutter with the blade facing upwards. Since the double-layered oil film is compressed into a two-layered sheet structure by the herringbone plate and cooled, the two layers of oil film are not completely compacted at this time. The cutter cuts in the middle of the lower layer of the double-layered sheet-like oil film. The middle position of the lower layer of the double-layered sheet-like oil film is cut into a slightly wider slit at the front end of the cutting platform 31 by the cutter 32 on the platform. Under the action of the casting roller 8, the cut double-layered sheet-like oil film is gradually unfolded. At the same time, along the oil film transmission direction, the cutting platform 31 gradually becomes larger, and the lower layer of the double-layered sheet-like oil film wraps around the arc edge of the cutting platform 31, which can further prevent the oil film from being scratched.

[0058] S4, Longitudinal stretching: The oil film cut and unfolded in step S3 is preheated and then longitudinally stretched. The longitudinal stretching ratio is at least 2. The stretched oil film is then cooled.

[0059] In the longitudinal stretching section, the single-layer oil film is first preheated at a temperature between 40-100℃ and a stretching ratio between 2-10. The oil film is then stretched longitudinally using different speed ratios of the rollers and cooled. At this point, the microporous structure is basically formed.

[0060] The longitudinal stretch ratio refers to the ratio of the longitudinal length of the oil film before and after stretching.

[0061] S5, Extraction and Drying: The oil film obtained in step S4 is sent into an extraction tank containing an extractant. The paraffin oil is extracted from the microporous structure of the oil film. The extracted oil film is then sent into a drying oven for drying to evaporate the extractant on the oil film.

[0062] After longitudinal stretching and cooling, the thin oil film with a microporous structure formed needs to enter an extraction tank containing an extractant for thorough extraction, extracting the paraffin oil from the microporous structure. After the extraction process, the diaphragm enters a drying oven for drying, allowing the extractant remaining on the product to fully evaporate.

[0063] S6, Setting and winding: The oil film dried in step S5 is heat-set and then sent to the winding section to obtain the diaphragm product.

[0064] Specifically, the dried oil film undergoes heat setting at a process temperature of 110-130℃ to accelerate the volatilization of the extractant, eliminate thermal stress, and reduce thermal shrinkage. It then proceeds to the winding stage.

[0065] Optionally, those skilled in the art will understand that during the heat setting process, a certain amount of shrinkage will occur, at which point a small amount of stretching or a certain amount of tensile tension can be provided.

[0066] The lithium-ion battery wet-process separator provided by this invention is prepared using a process of mixing, plasticizing, extruding annular oil film preform → vacuum forming → longitudinal stretching → extraction and drying → heat setting → film winding. In this invention, after vacuum forming and cooling at a vacuum level of at least -10 kPa, the microporous structure is basically formed, although paraffin oil still occupies the micropores. When the vacuum level used in the vacuum forming method is -10 to -80 kPa, the transverse tensile strength of the lithium-ion battery separator product is improved to a certain extent, with a stretch ratio between 3 and 8. According to this invention, the oil film after vacuum forming and cooling is usually significantly oriented in the transverse direction. As the vacuum level and the inner core diameter increase, the transverse tensile strength of the lithium-ion battery separator gradually increases. This invention uses vacuum forming to achieve transverse stretching, which can better solve the problems of product consistency and the setting of stretch ratios in all directions. Compared with the wet process, the production equipment of this invention is simple, and the production process is easy to control. Therefore, while achieving excellent physical properties of the separator, it also effectively solves the product consistency problem.

[0067] The invention will be further described below with reference to specific application examples. The equipment used in Examples 1-5 is as follows: Figure 1 As shown, the membrane preparation method includes the following steps in sequence: material feeding and extrusion, vacuum forming, cutting and unfolding, longitudinal stretching, extraction and drying, and shaping and winding. The blown film cooling step is described in detail below. Figure 3 It is carried out in the equipment.

[0068] Example 1:

[0069] The lithium-ion battery separator of this embodiment includes the following components: 21 parts of UHMWPE and 79 parts of paraffin oil.

[0070] The process flow of this embodiment includes: material feeding → mixing, plasticizing and extrusion → vacuum forming → longitudinal stretching → extraction and drying → heat setting → film winding.

[0071] In this embodiment, the mixture is melt-mixed in an extruder to obtain a mixed melt at a temperature of 192°C. The mixed melt is then filtered and fed into an annular die head. After extrusion, it enters a vacuum section where a vacuum level is maintained at -60 kPa. The ratio of the inner diameter of the cylindrical sleeve within the vacuum section to the diameter of the extruder die head is 8:1. The oil film is cooled and compressed into a sheet shape through a herringbone extrusion die. At this point, the oil film consists of two layers. One side is cut open with a cutter, allowing it to unfold into a single layer. The sheet then enters a longitudinal stretching zone for longitudinal stretching, with a stretching ratio controlled at 7 and a temperature of 80°C. After stretching, it is cooled and shaped. Next, it enters an extraction tank to remove the pore-forming agent from the membrane and dries the dichloromethane. The membrane is then placed in a heat-setting zone for slight stretching at a temperature of 132°C. Finally, it is wound up to obtain a lithium-ion battery.

[0072] Example 2:

[0073] The lithium-ion battery separator of this embodiment comprises the following components: 25 parts of UHMWPE and 75 parts of paraffin oil.

[0074] The process flow of this embodiment includes: material feeding → mixing, plasticizing and extrusion → vacuum forming → longitudinal stretching → extraction and drying → heat setting → film winding.

[0075] In this embodiment, the mixture is melt-mixed in an extruder to obtain a mixed melt at a temperature of 192°C. The mixed melt is then filtered and fed into an annular die head. After extrusion, it enters a vacuum section where a vacuum of -50 kPa is applied. The ratio of the inner diameter of the cylindrical sleeve within the vacuum section to the diameter of the extruder die head is 7:1. The oil film is cooled and compressed into a sheet shape using a herringbone plate. At this point, the oil film consists of two layers. One side is cut open with a cutter, allowing it to unfold into a single layer. The film then enters a longitudinal stretching zone for longitudinal stretching, with a stretch ratio controlled at 7 and a temperature of 80°C. After stretching, it is cooled and shaped. Next, it enters an extraction tank to remove the pore-forming agent from the membrane and dries the dichloromethane. The membrane sheet is then placed in a heat-setting zone for slight stretching at a temperature of 132°C. Finally, it is wound up to obtain a lithium-ion battery.

[0076] Example 3:

[0077] The lithium-ion battery separator was prepared according to the method of Example 1, except that the vacuum degree was adjusted and maintained at -40 kPa. The inner diameter of the cylindrical sleeve in the vacuum section was 180 cm, the diameter ratio of the cylindrical sleeve to the extruder die head was 6:1, and the longitudinal stretching ratio was 7.

[0078] Example 4:

[0079] The lithium-ion battery separator was prepared according to the method of Example 1, except that the vacuum degree was adjusted and maintained at -30 kPa. The inner diameter of the cylindrical sleeve in the vacuum section was 150 cm, the diameter ratio of the cylindrical sleeve to the extruder die head was 5:1, and the longitudinal stretching ratio was 7.

[0080] Example 5:

[0081] The lithium-ion battery separator was prepared according to the method of Example 1, except that the vacuum degree was adjusted and maintained at -20 kPa. The inner diameter of the cylindrical sleeve in the vacuum section was 120 cm, the diameter ratio of the cylindrical sleeve extruder die head was 4:1, and the longitudinal stretching ratio was 7.

[0082] Comparative Example 1:

[0083] The lithium-ion battery separator of this embodiment includes the following components: 21 parts of UHMWPE and 79 parts of paraffin oil.

[0084] The process flow of this embodiment includes: material feeding → mixing, plasticizing and extrusion → die casting → longitudinal stretching → transverse stretching 1 → extraction and drying → transverse stretching 2 → film winding.

[0085] In this embodiment, the mixture is melt-mixed in an extruder to obtain a mixed melt at a temperature of 192°C. The mixed melt is then filtered and cast onto a cooling roller through a die, with the temperature controlled at 25°C. It is then longitudinally stretched at a temperature of 80°C and a stretching ratio of 7. After stretching and cooling, it enters the transverse stretching zone 1 at a temperature of 120°C and a stretching ratio of 8. The membrane then enters an extraction tank to remove the pore-forming agent and dry its dichloromethane. The membrane is then placed in the transverse stretching zone 2 for minor stretching at a temperature of 132°C. Finally, it is wound up to obtain a lithium-ion battery separator.

[0086] Comparative Example 2:

[0087] The lithium-ion battery separator of this embodiment comprises the following components: 25 parts of UHMWPE and 75 parts of paraffin oil.

[0088] The process flow of this embodiment includes: material feeding → mixing, plasticizing and extrusion → die casting → longitudinal stretching → transverse stretching 1 → extraction and drying → transverse stretching 2 → film winding.

[0089] In this embodiment, the mixture is melt-mixed in an extruder to obtain a mixed melt at a temperature of 192°C. The mixed melt is then filtered and cast onto a cooling roller at a temperature controlled at 25°C. It is then longitudinally stretched at a temperature controlled at 80°C with a stretching ratio of 7. After stretching and cooling, it enters the transverse stretching zone 1 at a temperature between 110-140°C with a stretching ratio of 7. The membrane then enters an extraction tank to remove the pore-forming agent and dry its dichloromethane. The membrane is then placed in the transverse stretching zone 2 for minor stretching at a temperature of 132°C. Finally, it is wound up to obtain a lithium-ion battery separator.

[0090] Comparative Example 3:

[0091] The lithium-ion battery separator was prepared according to the method of Comparative Example 1, except that the MDO stretching ratio was adjusted to 7 and the TDO stretching ratio was adjusted to 6.

[0092] Comparative Example 4:

[0093] The lithium-ion battery separator was prepared according to the method of Comparative Example 1, except that the MDO stretching ratio was adjusted to 7 and the TDO stretching ratio was adjusted to 5.

[0094] Comparative Example 5:

[0095] The lithium-ion battery separator was prepared according to the method of Comparative Example 1, except that the MDO stretching ratio was adjusted to 7 and the TDO stretching ratio was adjusted to 4.

[0096] Tensile strength tests in the MD and TD directions: The lithium-ion battery separators prepared in Examples 1-5 and Comparative Examples 1-5 were subjected to tensile strength tests, respectively.

[0097]

[0098] Based on the above experimental results, it can be seen that, with a constant transverse tensile ratio, the larger the longitudinal tensile ratio, the greater the longitudinal tensile strength and the smaller the transverse tensile strength; and with a constant longitudinal tensile ratio, the larger the transverse tensile ratio, the higher the transverse tensile strength.

[0099] The above are merely preferred embodiments of the present invention and are not intended to limit the scope of protection of the present invention. Any equivalent structural or procedural transformations made based on the description and drawings of the present invention, or direct or indirect applications in other related technical fields, are similarly included within the scope of protection of the present invention.

Claims

1. A method for the production of wet lithium battery separators based on a vacuum thermoforming process, characterized by, Including the following steps: S1, Annular oil film preform extrusion: According to the preset material ratio, the material is conveyed, mixed, melted and plasticized by the extruder, and the uniformly mixed melt is conveyed to the die head and extruded from the die head as an annular oil film preform; S2, Vacuum Forming: An annular oil film preform is fed into a vacuum forming device for lateral stretching and cooling to obtain a laterally stretched oil film; wherein, the vacuum forming device includes a vacuum section sleeve and a vacuum system, the vacuum section sleeve includes a cylindrical sleeve with openings at both ends, and the side wall of the cylindrical sleeve has evenly arranged through holes, the through holes connecting to the vacuum system, the annular oil film preform is conveyed in the cylindrical sleeve along the axial direction, and the annular oil film preform is laterally stretched under the pressure difference between the internal air pressure and the external vacuum low pressure; S3, Cutting and unfolding: The oil film obtained in step S2 is laterally cut along the conveying direction, and the cut oil film is unfolded to form a single-layer oil film. S4, Longitudinal stretching: The oil film cut and unfolded in step S3 is preheated and then longitudinally stretched. The longitudinal stretching ratio is at least 2. The stretched oil film is then cooled. S5, Extraction and Drying: The oil film obtained in step S4 is sent into an extraction tank containing an extractant. The paraffin oil is extracted from the microporous structure of the oil film. The extracted oil film is then sent into a drying oven for drying to evaporate the extractant on the oil film. S6, Setting and winding: The oil film dried in step S5 is heat-set and then sent to the winding section to obtain the diaphragm product. The cylindrical sleeve is provided with a herringbone plate extrusion die with a gap at the discharge end; step S2 also includes that the annular oil film blank obtained after transverse stretching is extruded into a double-layer sheet-like oil film through the gap of the herringbone plate extrusion die.

2. The method for preparing a wet lithium battery separator based on a vacuum thermoforming process according to claim 1, characterized in that, The ratio of the inner diameter of the cylindrical sleeve to the diameter of the annular oil film blank is (3~8):

1.

3. The method of claim 1, wherein the vacuum-forming process is a wet lithium battery separator production process. In step S2, the vacuum system extracts air from the cylindrical sleeve of the vacuum section sleeve to maintain the vacuum level between -50 and -70 kPa.

4. The method of claim 1, wherein the vacuum-forming process is a wet lithium battery separator production process. In the longitudinal stretching of step S4, the preheating temperature is between 40-100℃ and the longitudinal stretching ratio is between 2-10; in the setting and winding of step S6, the heat setting temperature is 110-130℃.

5. The method of claim 1, wherein the vacuum-forming process is a wet lithium battery separator production process. The thickness of a single layer of the double-layered sheet-like oil film obtained in step S2 is between 500 and 1500 μm.

6. The method of claim 1, wherein the vacuum-forming process is a wet lithium battery separator production process. In step S3, the double-layered sheet-like oil film is laid flat on the cutting platform, and the bottom surface of the oil film is cut with a cutting blade facing upwards. Then, the cut oil film is unfolded to form a single-layered oil film.

7. A wet lithium battery separator based on a vacuum thermoforming process, characterized in that, The wet-process lithium battery separator is prepared using any one of the vacuum forming processes as described in any one of claims 1-6.

8. A preparation system for a method of preparing a wet-process lithium battery separator based on vacuum forming process as described in any one of claims 1-6, characterized in that, The preparation system includes an extruder, a vacuum forming device, a cutting mechanism, an extraction tank, a drying oven, a heat setting device, and a winding device arranged sequentially along the diaphragm conveying direction, and multiple casting rollers arranged in the middle of the preparation system to provide traction force; the die head of the extruder forms an annular outlet to extrude an annular oil film preform; the vacuum forming device includes a vacuum section sleeve and a vacuum pumping system, the vacuum section sleeve includes a cylindrical sleeve with openings at both ends, and the side wall of the cylindrical sleeve has uniformly arranged through holes, the through holes connecting to the vacuum pumping system, the annular oil film preform is conveyed in the cylindrical sleeve along the axial direction of the cylindrical sleeve, and under the pressure difference between the annular oil film preform and the inner wall of the cylindrical sleeve and the interior of the annular oil film preform, the annular oil film preform is stretched laterally; the cutting mechanism includes a cutting platform and a cutter located at the starting end of the cutting platform, the cutting edge of the cutter is set upward.

9. The production system according to claim 8, characterized in that The upper surface of the cutting platform is triangular, and the cutter is fixed at one corner of the triangle. The two sides of the cutting platform adjacent to the cutter are both arc-shaped sides that smoothly transition downwards.

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