A method for preparing a wet lithium battery separator

Abstract

The application discloses a kind of wet lithium battery diaphragm preparation methods, including longitudinal stretching step, longitudinal stretching step includes: (1) the feed side of longitudinal stretching machine preheating roller is provided with air knife, the airflow outlet of air knife covers the entire film in width, in the direction of film advancement, using the airflow of air knife obliquely sweeps film, so that film closely preheating roller surface;(2) infrared thermal imaging online detector for identifying the area of film and preheating roller is set on the discharge side of preheating roller, the real-time calculation of film and preheating roller is adhered, and the adhesion rate=actual effective close contact area÷diaphragm roller surface total contact area×100%;(3) when the adhesion rate is less than the control threshold value set, the air pressure of airflow outlet of air knife is adjusted, and the angle of airflow outlet obliquely swept by air knife is adjusted simultaneously until the adhesion rate reaches the control threshold value;(4) after the adhesion rate reaches the control threshold value, longitudinal stretching machine is stably operated, and diaphragm oil accumulation, longitudinal wrinkle defect is completely eliminated.

Description

Technical Field

[0001] This invention relates to a wet-process lithium battery separator preparation method, belonging to the technical field of wet-process lithium battery separator production technology. Background Technology

[0002] Longitudinal stretching is a crucial step in the wet-process lithium-ion battery separator manufacturing process. The preheating process during longitudinal stretching requires a high adhesion rate between the separator surface and the preheating roller; otherwise, the quality of the finished separator will be affected. In existing wet-process separator technologies, the longitudinal stretching preheating section generally relies on manual visual inspection to determine the adhesion between the separator and the preheating roller, lacking online quantitative detection methods. This results in a consistently low adhesion rate (less than 90%), leading to the accumulation of paraffin oil between the separator and the roller surface. Consequently, longitudinal wrinkles and edge oil accumulation defects appear after the separator is formed. Furthermore, the quality of the membrane adhesion relies on manual post-production sampling inspection, making real-time correction impossible. Defects can only be addressed after a batch of defects have occurred, resulting in a high defect rate and low production efficiency in mass production.

[0003] Furthermore, existing technologies rely solely on the traction force of the stretching rollers to achieve longitudinal stretching, resulting in stress concentration areas at the edges of the separator. This leads to uneven stress and heating on the membrane surface, disordered polyethylene molecular chain orientation, high residual stress in the finished product, significant warping deformation, and excessive thermal shrinkage at 105℃. This poses safety hazards such as lithium plating and overheating during cell assembly and cycling. Existing lithium battery separator manufacturing processes cannot achieve coordinated airflow equalization and control, nor can they actively alleviate localized stress on the membrane surface. Moreover, there is no standardized, automatically adjustable airflow parameter system. Different thicknesses of separators cannot be matched with corresponding adhesion and leveling airflow parameters, resulting in large batch performance variations. This makes it difficult to meet the stringent requirements of high-end power battery separators for consistent appearance and thermal dimensional stability. Summary of the Invention

[0004] The purpose of this invention is to provide a wet-process lithium battery separator preparation method to solve the technical problems of existing wet-process separators, such as reliance on manual judgment for longitudinal stretching, preheating, and bonding, low bonding rate, and frequent oil accumulation and wrinkles.

[0005] The present invention adopts the following technical solution: a wet process lithium battery separator preparation method, which includes a longitudinal stretching step, the longitudinal stretching step includes: (1) setting an air knife on the feeding side of the preheating roller of the longitudinal stretching machine, the airflow outlet of the air knife covers the entire film in width, and the airflow of the air knife is used to obliquely blow the film in the forward direction of the film, so that the film is tightly attached to the surface of the preheating roller; (2) setting an infrared thermal imaging online detector on the discharge side of the preheating roller for identifying the bonding area between the film and the preheating roller, and calculating the bonding rate between the film and the preheating roller in real time, the bonding rate = actual effective tight bonding area ÷ total contact area of ​​the separator roller surface × 100%; (3) when the bonding rate is less than the set control threshold, the air pressure of the airflow outlet of the air knife is increased, and the angle of oblique blowing of the airflow outlet of the air knife is adjusted until the bonding rate reaches the control threshold; (4) after the bonding rate reaches the control threshold, the longitudinal stretching machine operates stably.

[0006] Initially, the distance between the air outlet of the air knife and the diaphragm is 2-5 cm, the angle between the air outlet of the air knife and the surface of the diaphragm is 10°-15°, the air pressure at the air outlet of the air knife is 0.2-0.5 MPa, and the reference wind speed at the air outlet of the air knife is 25±0.5 m / s.

[0007] The control threshold for the bonding rate is greater than or equal to 98%, and the infrared thermal imaging online detector collects and calculates a set of bonding rate data every 0.3~0.8s.

[0008] When the bonding rate is less than 98%, increase the air pressure at the air knife outlet by 0.2 to 0.3 MPa and adjust the oblique blowing angle of the air knife outlet within the range of 10 to 20° until the bonding rate reaches 98%.

[0009] The length direction of the air knife is perpendicular to the forward direction of the diaphragm, and the length of the air knife is 700~900mm.

[0010] The feed side of the preheating roller of the longitudinal stretching machine is equipped with a pressure roller, the air knife air outlet faces the gap between the preheating roller and the pressure roller, a scraper is provided at the guide roller at the film inlet, and the air knife is installed between the pressure roller and the scraper.

[0011] The longitudinal stretching machine has air nozzles symmetrically arranged on both sides of the stretching roller to blow air onto the diaphragm. The air outlet of the air nozzle is 140~160mm away from the edge of the diaphragm. When the diaphragm is stretched longitudinally, the air nozzles on both sides of the diaphragm spray air simultaneously. The air pressure at the air outlet of the air nozzle is 0.3MPa~0.8MPa.

[0012] The total longitudinal stretching ratio is 5 to 15 times, and the stretching temperature is 70 to 130℃. When the stretching ratio is 5 to 8 times, the air pressure at the nozzle outlet is 0.3 to 0.45 MPa. When the stretching ratio is 9 to 15 times, the air pressure at the nozzle outlet is 0.5 to 0.8 MPa.

[0013] The nozzle includes a nozzle body, one end of which is a blowing head and the other end is connected to a tail air inlet base. The nozzle body and the tail air inlet base are connected by a fastening clamp. The tail air inlet base is equipped with a variable diameter quick-connect connector. The nozzle body adopts a bamboo joint structure and includes multiple interlocking bamboo joints.

[0014] The longitudinal stretching step is preceded by a melt extrusion step. The raw material used in the melt extrusion step is a mixture of polyethylene and paraffin oil. The molecular weight of the polyethylene raw material is 60-200, the mass ratio of polyethylene to paraffin oil is 1:(3-6), the melt extrusion temperature is 180-210℃, and the temperature of the casting roller during melt extrusion is 8-30℃.

[0015] The longitudinal stretching step is followed by a first transverse stretching, extraction, a second transverse stretching, heat setting, and winding step. The first transverse stretching ratio is 5 to 18 times and the temperature is 120 to 135°C. In the extraction step, dichloromethane is used as the extractant, the extraction temperature is 15 to 30°C, and the extraction time is 150 to 300 seconds. The second transverse stretching ratio is 1 to 3 times and the temperature is 120 to 140°C.

[0016] The total contact area of ​​the diaphragm roller surface, A_total, is equal to L × B, where L is the contact arc length of the diaphragm on the roller (mm), and B is the effective width of the diaphragm (mm). The formula for calculating the contact arc length is L = (360 - a) × π × R / 180, where R is the radius of the preheating roller, and a is the central angle (°) formed by connecting the diaphragm entry point and exit point to the center of the roller. a can be measured by an angle gauge.

[0017] The beneficial effects of this invention are: This invention utilizes an air knife to obliquely sweep the surface of the film, causing the film to adhere to the preheating roller. The adhesion rate is detected in real time by an infrared thermal imaging online detector, realizing real-time quantification and automatic fine-tuning of the film adhesion status, and stably controlling the adhesion rate between the film surface and the roller surface to be above 98%, thus eliminating oil accumulation and wrinkles from the source.

[0018] Air nozzles are arranged on both sides of the stretching roller. These nozzles blow air onto the membrane surface, causing it to flatten under the influence of airflow. This, combined with the air knife at the inlet of the preheating roller, creates a two-stage airflow system. This system establishes a three-field coupled control system encompassing temperature, tensile stress, and airflow fields, uniformly dispersing tensile stress on the membrane surface and reducing residual stress within the diaphragm. This solves the problems of stress concentration and uneven molecular chain orientation caused by single-roller stretching in existing technologies. Simultaneously, the synergistic effect of the air knife and nozzles improves the diaphragm's warpage and heat shrinkage performance, enhancing the batch appearance and dimensional stability of the diaphragm.

[0019] In summary, the present invention has the following advantages: (1) The online quantitative detection of the bonding rate using infrared thermal imaging and the automatic adjustment of PLC closed loop are used to replace the traditional manual visual inspection. The bonding rate of the membrane surface is steadily increased from less than 90% to more than 98%, completely eliminating appearance defects such as oil accumulation, longitudinal wrinkles, and edge warping of the diaphragm. (2) The two-stage airflow synergistic structure of "patch air knife + double-sided air nozzle" constructs a three-field coupled control system of heat-force-flow, which evenly disperses the tensile stress of the membrane surface, making the polyethylene molecular chain orientation uniform, and the tensile strength within the width of the diaphragm has no obvious difference between the edge and the center. (3) It effectively reduces the residual stress of the diaphragm molding, and the warping deformation of the finished product is ≤0.5mm / m. After heat treatment at 105℃×1h, the longitudinal heat shrinkage is ≤1.0% and the transverse heat shrinkage is ≤0.5%, which is more than 40% lower than the heat shrinkage index of the traditional process. It greatly improves the thermal dimensional stability of the diaphragm and avoids the safety hazards of lithium plating and overheating of the battery cell from the source. (4) The entire airflow control system can directly modify existing wet-process diaphragm mass production equipment without replacing the main unit, resulting in low technical modification costs. It is fully compatible with the original PLC control system and is suitable for large-scale continuous production of 5μm to 9μm ultra-thin diaphragms. (5) Full-process automatic monitoring and real-time correction reduce manual intervention, decrease the frequency of diaphragm breakage and shutdown, significantly reduce the diaphragm defect rate, and simultaneously improve mass production efficiency and batch consistency. Attached Figure Description

[0020] Figure 1 This is a schematic diagram of the longitudinal stretching step in a wet-process lithium battery separator preparation method according to an embodiment of the present invention; Figure 2 yes Figure 1 Schematic diagram of the structure of the central air nozzle; Figure 3 This is a schematic diagram showing the contact state of the diaphragm on the surface of the preheating roller.

[0021] In the figure: 1-preheating roller, 2-air knife, 3-stretching roller, 4-air nozzle, 4.1-air nozzle body, 4.2-tail air inlet base, 4.3-fastening clamp, 4.4-reducing quick connector, 4.5-blowing head, 5-pressure roller, 6-guide roller, 7-scraper, 8-infrared thermal imaging detector, a-central angle. Detailed Implementation

[0022] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments.

[0023] The wet process method for preparing lithium battery separators of the present invention includes the steps of melt extrusion, longitudinal stretching, primary transverse stretching, extraction, secondary transverse stretching, heat setting, and winding.

[0024] The raw material used in the melt extrusion step is a mixture of polyethylene and paraffin oil. The molecular weight of the polyethylene raw material is 600,000 to 2,000,000. The mass ratio of polyethylene to paraffin oil is 1:(3 to 6). The melt extrusion temperature is 180 to 210°C. The temperature of the casting roller during melt extrusion is 8 to 30°C.

[0025] The longitudinal stretching steps include: (1) as follows Figure 1 As shown, an air knife 2 is installed on the feed side of the preheating roller 1 of the longitudinal stretching machine. The airflow outlet of the air knife 2 covers the entire diaphragm in width. The length of the air knife 2 is perpendicular to the forward direction of the diaphragm, and the length of the air knife 2 is 800mm. A pressure roller is provided on the feed side of the preheating roller of the longitudinal stretching machine. The airflow outlet of the air knife faces the gap between the preheating roller and the pressure roller. A pressure roller 5 is provided on the feed side of the preheating roller 1 of the longitudinal stretching machine. A scraper 7 is provided at the guide roller 6 at the diaphragm inlet. The air knife 2 is installed between the pressure roller 5 and the scraper 7. From top to bottom, the sequence is: pressure roller 5, preheating roller 1, air knife 2, scraper 7, guide roller 6. The air knife 2 is located below the pressure roller 5 and above the scraper 7, so it will not be contaminated by oil on the roller surface. The airflow is blown obliquely into the gap between the diaphragm and the pressure roller 5, pressing the diaphragm against the surface of the preheating roller 1. The reference wind speed of the airflow outlet of the air knife 2 is 25±0.5m / s. The distance between the air outlet of the air knife 2 and the diaphragm is 2-5 cm, and there is an angle of 10°-15° between the air outlet of the air knife 2 and the surface of the diaphragm. In the forward direction of the diaphragm, the airflow of the air knife 2 is used to sweep the diaphragm obliquely, so that the diaphragm is in close contact with the surface of the preheating roller 1.

[0026] (2) An infrared thermal imaging detector 8 is installed on the discharge side of the preheating roller 1 to collect the film surface temperature distribution and calculate the bonding rate. An infrared thermal imaging online detector 8 is set on the discharge side of the preheating roller 1 to identify the bonding area between the film and the preheating roller 1, and calculates the bonding rate between the film and the preheating roller 1 in real time. The bonding rate = actual effective tight bonding area ÷ total contact area of ​​the diaphragm roller surface × 100%; the control threshold of the bonding rate is 98%, and the infrared thermal imaging online detector collects and calculates a set of bonding rate data every 0.5s.

[0027] (3) When the bonding rate is less than the control threshold, increase the air pressure at the air outlet of the air knife 2, and at the same time adjust the angle of the airflow of the air knife 1 to blow obliquely until the bonding rate reaches the control threshold.

[0028] (4) After the bonding rate reaches the control threshold, the longitudinal stretching machine operates stably. When the bonding rate is lower than 98%, the air pressure at the air knife outlet is increased by 0.2 to 0.3 MPa, and the oblique blowing angle at the air knife outlet is adjusted within the range of 10 to 20° until the bonding rate reaches 98%.

[0029] The longitudinal stretching machine has air nozzles 4 symmetrically arranged on both sides of the stretching roller 3 for blowing air onto the diaphragm. The air outlet of the air nozzle 4 is 150mm from the edge of the diaphragm. During longitudinal stretching, the air nozzles 4 on both sides of the diaphragm simultaneously eject airflow, and the air pressure at the outlet of the air nozzle 4 is 0.3MPa to 0.8MPa. The total longitudinal stretching ratio is 5 to 15 times, and the stretching temperature is 70 to 130℃. When the stretching ratio is 5 to 8 times, the air pressure at the outlet of the air nozzle 4 is 0.3 to 0.45MPa; when the stretching ratio is 9 to 15 times, the air pressure at the outlet of the air nozzle is 0.5 to 0.8MPa.

[0030] In the first transverse stretching step, the first transverse stretching ratio is 5 to 18 times and the temperature is 120 to 135℃; in the extraction step, dichloromethane is used as the extractant, the extraction temperature is 15 to 30℃, and the extraction time is 150 to 300s; in the second transverse stretching step, the second transverse stretching ratio is 1 to 3 times and the temperature is 120 to 140℃.

[0031] The core control of this invention focuses on the longitudinal stretching section, comprising two mutually cooperating and complementary core control systems: a patch air knife infrared bonding rate closed-loop control subsystem and a dual-sided air nozzle three-field coupling shaping subsystem, as detailed below: (1) Infrared online bonding rate closed-loop control system for patch air knife: Patch air knife 2 is fully covered on the feeding side of the preheating roller of the longitudinal stretching machine. The air knife 2 is 800mm long. The air knife 2 is installed in the middle position between the pressure roller and the scraper. The distance between the air outlet of the air knife 2 and the film is controlled to be 2-5cm. The angle between the air knife 2 and the film surface is 10°-15°. The air flow of the air knife 2 blows the diaphragm obliquely. The air knife 2 is equipped with an independent pressure regulating fan, wind speed sensor and wind pressure frequency converter. The signal is connected to the PLC system (not shown in the attached figure). The reference wind speed of the air knife 2 is constant at 25±0.5m / s. The wind pressure is adjustable in the range of 0.2MPa to 0.5MPa. The film is tightly attached to the surface of the preheating roller 1 by the air flow of the air knife 2 blowing the film obliquely. An infrared thermal imaging online detector is set up on the discharge side of the preheating roller 1. The infrared thermal imaging online detector detects the temperature difference between the bonding area and the gap area to identify the effective bonding area and calculates the bonding rate in real time according to the formula. The PLC system has a preset bonding rate control threshold of 98%. It collects a set of bonding rate data every 0.5 seconds and automatically adjusts the closed loop: when the bonding rate is lower than 98%, the air pressure is slightly increased and the blowing angle of the air knife 2 is adjusted until the bonding rate reaches 98%; when the bonding rate meets the standard, the current parameters are maintained to ensure stable operation and eliminate defects such as oil accumulation and wrinkles in the bonding quality.

[0032] The infrared thermal imaging detector has a built-in pixel area calculation algorithm, which can automatically and in real-time identify and calculate the effective bonding area between the diaphragm and the preheating roller without manual intervention. The calculation principle is to distinguish the bonding area from the gap area based on the thermal resistance temperature difference between the preheating roller, PE film, and interlayer air / paraffin oil: tightly bonded areas have smooth heat transfer and higher film surface temperature, resulting in bright pixels; oil accumulation and warped / suspended areas have severe heat insulation and lower film temperature, resulting in dark pixels. The calculation steps are: 1. Median filtering to reduce noise in the original infrared thermal image; 2. Using OTU adaptive temperature difference thresholding to perform image binary segmentation, dividing the bonding foreground pixels into bonding foreground pixels and gap background pixels; 3. Built-in edge shielding module to remove interference from bare roller edges and invalid pixels on the frame; 4. Connectivity component algorithm to count the total number of effective bonding pixels; 5. Calculate the actual bonding area according to the previously calibrated pixel physical size, and output the bonding rate in real time: Bonding rate = Actual effective tightly bonded area ÷ Total contact area of ​​the diaphragm and roller surface × 100%. The total contact area of ​​the diaphragm roller surface, A_total, is equal to L × B, where L is the contact arc length of the diaphragm on the roller (mm), and B is the effective width of the diaphragm (mm). The formula for calculating the contact arc length is L = (360 - a) × π × R / 180, where R is the radius of the preheating roller, and a is the central angle (°) formed by connecting the diaphragm entry point and exit point to the center of the roller. a can be measured by an angle gauge.

[0033] The entire measurement error is controlled within ±1%, and a calculation is completed every 0.5 seconds, with the data uploaded to the PLC to achieve digital closed-loop control of the fit status. Infrared thermal imaging detectors can include FOTRIC's 600C series, Guide's AT series online temperature measuring instruments, Shenzhen Junfeng Optoelectronic Technology's FLC series measuring light curtains, Wuhan Huajingkang Optoelectronic's HJKIR, and the HJ-IR online fixed thermal imaging system, etc.

[0034] Under the action of the air knife airflow, the floating oil in the gap between the film rollers is quickly discharged and the warped edges of the casting are flattened. The occurrence rate of local oil accumulation at the inlet is reduced from 5-30% in the existing technology to less than 0.5%. The full width bonding rate of the film is stable in the range of 98% to 99.5%, and the oil interlayer in the roller area is almost completely eliminated. The warped edges and single-sided arching of the film entering the roller are flattened in an instant, and the amount of initial micro-wrinkles generated in the preheating section is reduced by more than 90%.

[0035] (2) Dual-sided air nozzle three-field coupled shaping subsystem: Air nozzles are symmetrically arranged on both sides of the stretching roller 3 of the longitudinal stretching machine. The air nozzles 4 are aligned with the stress concentration area of ​​the membrane surface. The air outlet of the air nozzle 4 is fixed at 150mm from the edge of the diaphragm to ensure that the airflow acts uniformly on the entire membrane surface. The outlet air pressure of the air nozzle 4 is dynamically adjusted in the range of 0.3MPa to 0.8MPa. The air pressure is matched with 0.3 to 0.45MPa for low stretching ratios of 5 to 8 times and 0.5 to 0.8MPa for high stretching ratios of 9 to 15 times. In this process, the upper and lower stretching rollers are responsible for pulling the diaphragm and providing the core stretching power. At the same time, the air nozzles on the left and right sides blow air out synchronously, which promotes the uniform extension of the diaphragm under high temperature environment and ensures the uniformity of the longitudinal stretching effect. This system can respond to the micro deformation of the membrane surface in real time and achieve uniform distribution of tensile stress.

[0036] like Figure 2 As shown, the nozzle 4 includes a nozzle body 4.1. One end of the nozzle body 4.1 is a blowing head 4.5, and the other end is connected to a tail air inlet base 4.2. The nozzle body 4.1 is 120mm long, and the blowing head 4.5 has an air hole diameter of 3mm. The nozzle body 4.1 and the tail air inlet base 4.2 are connected by a fastening clamp 4.3. The tail air inlet base 4.2 is equipped with a reducing quick-connect fitting 4.4. The fastening clamp 4.3 is a 40-type stainless steel clamp, and the reducing quick-connect fitting 4.4 uses a 10-to-8 conversion air hose. Both the blowing head 4.5 and the reducing quick-connect fitting 4.4 have a 2-point internal thread standard interface. The nozzle body 4.1 adopts a bamboo-joint structure, featuring multiple interlocking bamboo joints that automatically position themselves after bending, requiring no auxiliary fixation. The nozzle body 4.4 employs a hollow, straight-through air passage with no abrupt diameter reduction in the inner cavity, resulting in minimal airflow loss and strong, uniform air output. The bamboo-joint interlocking structure provides stable damping and allows for 360° adjustment of the blowing angle in any direction, ensuring it remains secure and does not spring back after positioning. It is equipped with a variable-diameter quick-connect fitting and a fastening clamp for double fixation, enabling long-term stable use in 0–8 bar compressed air conditions without leakage or pipe detachment. Both ends of the nozzle have universal 2-point internal thread interfaces, offering strong compatibility and allowing replacement of nozzles, extension tubes, solenoid valves, and other accessories.

[0037] The air nozzle 4 and the air knife 2 work together to form a three-field coupled control of heat, force and flow: the preheating roller 1 provides a uniform temperature field, the stretching roller 2 provides a longitudinal tensile principal stress field, and the air knife 2 and the double-sided air nozzles combine to form a bidirectional airflow field; the airflow continuously smooths the micro-wrinkles on the film surface and disperses the edge stress concentration points. Under the high temperature stretching environment of 70 to 130°C, it guides the polyethylene molecular chains to be highly ordered along the principal stress direction, balances the overall stress distribution of the film surface, and reduces the residual stress after molding.

[0038] The diaphragm enters the stretching roller traction area, and the bamboo-joint air nozzles on both sides are activated, matching the corresponding air pressure according to the longitudinal stretching ratio. The stretching roller completes the longitudinal stretching at a ratio of 5 to 15 times and a temperature of 70 to 130°C. The air knife and air nozzles work synchronously throughout the process, balancing the heat and stress on the membrane surface. This significantly reduces diaphragm warpage, with the finished product warpage ≤0.5mm / m, far lower than the warpage of existing technologies (average 1.5 to 3mm / m). Thermal dimensional stability (under 105°C × 1h conditions): longitudinal thermal shrinkage ≤1.0%, transverse ≤0.5%, a reduction of more than 40% compared to existing technologies. After longitudinal stretching, the diaphragm is fed into a transverse stretching section, with the system continuously monitoring and dynamically fine-tuning in real time.

[0039] The test method for warpage is as follows: The test is conducted according to GB / T 25257-2010, "Method for Determination of Warpage of Optical Functional Thin Films." Samples are taken 2-3 layers from the outer layer of the film roll and cut into 20cm × 20cm specimens. The samples need to be equilibrated for 2-8 hours in an environment of 23℃±2℃ and 50±5% relative humidity. The warped side of the specimen is placed upwards on a horizontal platform, and the height of the warp on all four sides (in mm) is measured using a steel ruler. The maximum value is taken as the warpage of the specimen. Five sets of specimens are tested, and the warpage is divided by the specimen width to obtain the warpage amount in this application.

[0040] Heat shrinkage rate test method: The test shall be conducted in accordance with the requirements of GB / T36363-2018. Cut a 10cm×10cm sample and mark the transverse (TD) and longitudinal (MD) on the sample. Measure the transverse and longitudinal widths using a fully automatic image measuring projector (Kunshan Gaopin Precision Instrument Co., Ltd., GP-300C). Clamp the sample between two sealed A4 sheets of paper and place it in a 105℃ oven for 1 hour. After the sample returns to room temperature, measure the transverse and longitudinal widths again using the fully automatic image measuring projector. Take three measurements and average the results.

[0041]

[0042] The system integrates a wind pressure transmitter, wind speed sensor, infrared thermal imaging detector, and tension sensor, with all sensor data connected to the PLC control cabinet and touch screen. Automatic pop-up alarms are triggered when the bonding rate, wind pressure, wind speed, or nozzle wind pressure exceed thresholds; minor deviations are automatically adjusted without machine shutdown. All process parameters are automatically stored, enabling full traceability of the diaphragm process for each roll. The entire system is compatible with existing wet-process diaphragm production line PLC control systems, requiring low technical upgrade investment, and enabling uninterrupted automated mass production control, reducing the proportion of broken film and defective products.

[0043] In summary, this invention employs longitudinal stretching preheating and bonding, along with dual-sided airflow synergistic forming. Utilizing an oblique blowing air knife on the membrane, and employing an online infrared thermal imaging detector to monitor and calculate the bonding rate in real time, a closed-loop correction of the bonding rate is achieved. Combined with dual-sided air nozzles on the membrane surface, uniform flow and slow release are implemented, ensuring real-time control of the membrane surface-to-preheating roller surface bonding rate to ≥98%. This completely eliminates defects such as oil accumulation and wrinkles on the membrane surface, balances the overall longitudinal tensile stress of the separator, and simultaneously improves the longitudinal mechanical strength and thermal dimensional stability of the separator. This invention is applicable to the longitudinal stretching and forming process of 5μm~9μm ultra-thin power battery wet-process separators.

Claims

1. A method for preparing a wet-process lithium battery separator, comprising a longitudinal stretching step, characterized in that, The longitudinal stretching steps include: (1) setting an air knife on the feed side of the preheating roller of the longitudinal stretching machine, with the air outlet of the air knife covering the entire film in width, and using the airflow of the air knife to sweep the film obliquely in the forward direction of the film, so that the film is tightly attached to the surface of the preheating roller; (2) setting an infrared thermal imaging online detector on the discharge side of the preheating roller to identify the area of ​​the film and the preheating roller to calculate the adhesion rate of the film and the preheating roller in real time, the adhesion rate = actual effective tight adhesion area ÷ total contact area of ​​the diaphragm roller × 100%; (3) when the adhesion rate is less than the set control threshold, the air pressure of the air outlet of the air knife is increased, and the angle of the oblique sweeping of the air outlet of the air knife is adjusted until the adhesion rate reaches the control threshold; (4) after the adhesion rate reaches the control threshold, the longitudinal stretching machine operates stably.

2. The wet-process lithium battery separator preparation method according to claim 1, characterized in that: Initially, the distance between the air outlet of the air knife and the diaphragm is 2-5 cm, the angle between the air outlet of the air knife and the surface of the diaphragm is 10°-15°, the air pressure at the air outlet of the air knife is 0.2-0.5 MPa, and the reference wind speed at the air outlet of the air knife is 25±0.5 m / s.

3. The wet-process lithium battery separator preparation method according to claim 1, characterized in that: The control threshold for the bonding rate is greater than or equal to 98%, and the infrared thermal imaging online detector collects and calculates a set of bonding rate data every 0.3~0.8s.

4. The wet-process lithium battery separator preparation method according to claim 3, characterized in that: When the bonding rate is less than 98%, increase the air pressure at the air knife outlet by 0.2 to 0.3 MPa and adjust the oblique blowing angle of the air knife outlet within the range of 10 to 20° until the bonding rate reaches 98%.

5. The wet-process lithium battery separator preparation method according to claim 1, characterized in that: The length direction of the air knife is perpendicular to the forward direction of the diaphragm, and the length of the air knife is 700~900mm.

6. The wet-process lithium battery separator preparation method according to claim 1, characterized in that: The feed side of the preheating roller of the longitudinal stretching machine is equipped with a pressure roller, the air knife air outlet faces the gap between the preheating roller and the pressure roller, and a scraper is provided at the guide roller at the film inlet. The air knife is installed between the pressure roller and the scraper.

7. The wet-process lithium battery separator preparation method according to claim 1, characterized in that: The longitudinal stretching machine has air nozzles symmetrically arranged on both sides of the stretching roller to blow air onto the diaphragm. The air outlet of the air nozzle is 140~160mm away from the edge of the diaphragm. When the diaphragm is stretched longitudinally, the air nozzles on both sides of the diaphragm spray air simultaneously. The air pressure at the air outlet of the air nozzle is 0.3MPa~0.8MPa.

8. The wet-process lithium battery separator preparation method according to claim 7, characterized in that: The total longitudinal stretching ratio is 5 to 15 times, and the stretching temperature is 70 to 130℃. When the stretching ratio is 5 to 8 times, the air pressure at the nozzle outlet is 0.3 to 0.45 MPa. When the stretching ratio is 9 to 15 times, the air pressure at the nozzle outlet is 0.5 to 0.8 MPa.

9. The wet-process lithium battery separator preparation method according to claim 7, characterized in that: The nozzle includes a nozzle body, one end of which is a blowing head and the other end is connected to a tail air inlet base. The nozzle body and the tail air inlet base are connected by a fastening clamp. The tail air inlet base is equipped with a variable diameter quick-connect connector. The nozzle body adopts a bamboo joint structure and includes multiple interlocking bamboo joints.

10. The wet-process lithium battery separator preparation method according to claim 1, characterized in that: The longitudinal stretching step is preceded by a melt extrusion step. The raw material used in the melt extrusion step is a mixture of polyethylene and paraffin oil. The molecular weight of the polyethylene raw material is 60-200, the mass ratio of polyethylene to paraffin oil is 1:(3-6), the melt extrusion temperature is 180-210℃, and the temperature of the film after melt extrusion is 8-30℃. And / or, the longitudinal stretching step is followed by a first transverse stretching, extraction, a second transverse stretching, heat setting and winding step in sequence. The first transverse stretching ratio is 5 to 18 times and the temperature is 120 to 135°C. In the extraction step, dichloromethane is used as the extractant, the extraction temperature is 15 to 30°C and the extraction time is 150 to 300 seconds. The second transverse stretching ratio is 1 to 3 times and the temperature is 120 to 140°C.

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