A cross-pulling extraction process for lithium battery separators

Abstract

The present application relates to lithium battery diaphragm processing technical field, especially point to a kind of lithium battery diaphragm's transverse drawing extraction process.It solves the tensile strength and thermal shrinkage reduction of diaphragm and the problem of uneven thickness, uneven stress.It includes homogeneous solution preparation, melt extrusion, casting, synchronous bidirectional stretching, extraction drying, heat setting, winding and aging, slitting and other steps, film roll is aged to release its internal stress, through aging treatment to promote the dimensional stability of film roll, no longer change, after aging treatment, film roll can be cut according to the size required by customer, after cutting, the finished product is obtained.The present application can obtain the diaphragm with minimum thickness standard deviation, can effectively eliminate internal stress, effectively guarantee thermal shrinkage rate and best tensile resistance.

Description

Technical Field

[0001] This invention relates to the field of lithium battery separator processing technology, and in particular to a transverse stretching extraction process for lithium battery separators. Background Technology

[0002] Currently, with rapid economic development, the demand for energy in human life and production is constantly increasing. Energy is related to the world's economic and social development and affects human life. Fossil resources, represented by oil, natural gas, and coal, are the main energy base of countries around the world. According to scientific research estimates, the world's proven coal reserves can only supply human use for a little over 100 years, natural gas reserves can only continue to supply human use for a little over 70 years, and oil's exploitable reserves can only last for 50 to 60 years. In September 2020, my country announced that it would strive to reach peak carbon dioxide emissions before 2030 and achieve carbon neutrality before 2060. Due to the scarcity of non-renewable energy and the serious pollution problems caused by its use, renewable energy sources such as solar and wind power are gradually becoming the focus of human energy development. Because the development and utilization of clean energy is greatly constrained by environmental factors, the method of storing clean energy through energy storage devices is the focus and hot topic of energy development in today's society [4]. The emergence of lithium-ion batteries is a major breakthrough in the process of social development. It converts clean energy such as nuclear energy, wind energy, and hydropower into electrical energy and stores it. This method provides impetus for the transformation of the energy structure of human society.

[0003] Lithium-ion batteries, as efficient and safe energy storage and supply devices, are widely used in daily life. Whether it's communication tools or smart devices, they are indispensable, and the battery life of lithium-ion batteries largely determines the success or failure of digital products. Furthermore, with the rise of new energy electric vehicles and electric bicycles, the market demand for high-capacity lithium-ion batteries in transportation is constantly increasing.

[0004] As one of the four key materials in lithium-ion batteries, battery separators have attracted increasing attention from researchers. The quality of the separator affects the capacity, cycle performance, and safety of lithium-ion batteries, ultimately determining their overall performance. Therefore, developing and preparing high-quality separators with good mechanical properties, stable chemical properties, and the ability to meet the performance requirements of lithium-ion batteries holds great promise and has gradually become a key research focus.

[0005] Chinese patent CN1978037A uses supercritical extraction to remove a high-boiling-point, low-volatility first solvent, although no organic solvent is used in the extraction process.

[0006] However, the extractant used is a refrigerant containing hydrochlorofluorocarbons, which is not environmentally friendly. At the same time, the extraction efficiency is not high and the extraction process is stringent, making industrialization difficult. In the Chinese patent with publication number CN105655518A, light petroleum hydrogenation fractions are used to replace traditional high-boiling-point solvents. The solvent is removed by natural evaporation at high temperature during the stretching process. Although the process is environmentally friendly and has very low pollution, it is still very effective.

[0007] The problems are as follows:

[0008] 1. As the stretching temperature of biaxial stretching increases, the tensile strength of the diaphragm decreases and the thermal shrinkage decreases. This is because as the temperature increases, the molecular chain motion intensifies, and the molecules tend to become disordered, which facilitates disorientation and reduces the degree of orientation of the diaphragm, thereby reducing the tensile strength and thermal shrinkage of the diaphragm.

[0009] 2. As the stretching temperature increases, the defects of the diaphragm do not change significantly, but the defects such as deformation, sagging edges, and wrinkles increase. The movement of molecular chains intensifies, the disorientation is aggravated, and the diaphragm will exhibit uneven thickness due to the rearrangement of molecular chains.

[0010] 3. Uneven stress exists in different parts, making it more prone to appearance defects after aging. Summary of the Invention

[0011] This invention provides a transverse stretching extraction process for lithium battery separators. When the casting roller temperature is 25°C or higher, and the roller gap is 1.5 mm or less, the pore size distribution is relatively narrow. As the casting roller temperature increases, the roller gap increases, and the die lip opening increases, the thickness standard deviation first decreases and then increases. Setting the temperature to 25°C, 1.5 mm, and 1.3 mm respectively yields a separator with the smallest thickness standard deviation, effectively eliminating internal stress, effectively ensuring heat shrinkage rate, and having the best tensile strength. Setting the stretching ratio to 700%, the biaxial stretching temperature to 115°C, and the heat setting temperature to 122°C yields a separator with the best overall performance.

[0012] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0013] A transverse stretching extraction process for lithium battery separators includes the following steps:

[0014] a) Preparation of homogeneous solution: Polyethylene is mixed with a high-boiling-point, low-molecular-weight diluent in a certain proportion, and then subjected to high temperature and screw shearing in an extruder to form a homogeneous solution. The number average molecular weight of ultra-high molecular weight polyethylene is generally 50 × 10⁻⁶. 6 ~200×10 6 The diluent has a high boiling point and is between g / mol;

[0015] b) Melt extrusion: The homogeneous solution is filtered through a precision filter and then extruded through a die into sheet-like melt;

[0016] c) Casting: The sheet-like melt is attached to the casting roller for cooling. During the cooling process, as the polymer crystallizes, the solubility of the diluent gradually decreases, thereby inducing liquid-liquid or solid-liquid phase separation. The diluent is dispersed in the polymer phase to form a film.

[0017] d) Synchronous biaxial stretching: The film is stretched in both longitudinal and transverse directions at the same time. Microscopically, the molecular chains and segments are oriented in the longitudinal and transverse directions respectively. Under a certain temperature, the crystallinity of the oriented molecular chains gradually increases.

[0018] e) Extraction and drying: Based on the principle of like dissolves like, a low-boiling-point organic solvent is used as an extractant to extract the diluent. Then, taking advantage of the low-boiling-point characteristic of the extractant, it is volatilized by heating and recycled.

[0019] 1) Cut the unextracted ultra-high molecular weight polyethylene / paraffin oil blend membrane into a diaphragm with a size of 880mm × 50mm. Use an electronic balance to measure the weight of the diaphragm. Connect the two ends of the diaphragm with a plastic sealing machine so that the path of the diaphragm around the surface of each wheel becomes a ring.

[0020] 2) The active spokes are connected to the drive motor. The clamping force is generated by adjusting the position of the active spokes up and down. The drive motor is turned on and the speed is adjusted to check the movement of the diaphragm.

[0021] 3) Add extractant of different concentrations (70%-100%) to the extraction tank to the set liquid level, set different diaphragm movement speeds (0-50mm / s), place the upper and lower wheel structure in the extraction tank and start timing;

[0022] 4) After the experiment is completed, turn off the drive motor and remove the diaphragm. Dry it with hot air until the extractant has completely evaporated. After cooling, weigh it, calculate the change in diaphragm extraction weight, and test the diaphragm.

[0023] f) Heat setting: The diaphragm is stretched and shaped again, while releasing internal stress to reduce the dimensional shrinkage of the diaphragm at high temperatures;

[0024] g) Rewinding: The diaphragm is smoothly rolled onto the spinning drum to form a membrane roll, which facilitates subsequent aging treatment;

[0025] h) Aging and Slitting: The crystallinity of polymers will not reach 100%, and the separator is no exception. Since the glass transition temperature of PE is around -68°C, while the amorphous part of the separator is above the glass transition temperature at room temperature, it will tend to deorient. The membrane roll is aged to release its internal stress. The aging treatment promotes the dimensional stability of the membrane roll and prevents it from changing. The aged membrane roll can be slitted according to the size required by the customer, and the finished product is obtained after slitting.

[0026] Furthermore, in step c), during the casting process, the temperature control medium of the casting roller is water, which mainly serves to uniformly cool the sheet-like melt; experiments were conducted at 10℃, 15℃, 20℃, 25℃, and 30℃ respectively; when conducting single-factor variable experiments on other parameters, the casting roller temperature was fixed at 25℃.

[0027] In step c), the size of the gap between the guide roller and the casting roller is called the guide roller gap. The function of the guide roller is to tightly adhere the sheet-like melt to the casting roller. Experiments were conducted using gaps of 0.5 mm, 1.0 mm, 1.5 mm, 2.0 mm, and 2.5 mm. When conducting single-factor variable experiments on other parameters, the guide roller gap was fixed at 1.5 mm.

[0028] Further, in step d), the stretch ratio refers to the multiple by which the length and width of the diaphragm increase after stretching relative to the length and width before stretching; the experiment was conducted at stretch ratios of 500%, 600%, and 700%, and when conducting single-factor variable experiments on other parameters, the stretch ratio was fixed at 700%;

[0029] In step d), the maximum temperature of the biaxial stretching test should not exceed the melting point of PE (135℃) to prevent the diaphragm from melting during the stretching process. The experiments were conducted at 110℃, 115℃, 120℃, 125℃ and 130℃ respectively. When conducting single-factor variable tests on other parameters, the biaxial stretching temperature was fixed at 120℃.

[0030] Furthermore, the sample was tested using DSC, and the melting point, crystallization temperature, and melting point data were obtained from the DSC curve.

[0031] According to the formula

[0032]

[0033] The crystallinity data were calculated.

[0034] In the formula:

[0035] — Crystallinity (%);

[0036] —Heat value of sample crystallization melting (J / g);

[0037] —Heat of melting (J / g) of 100% crystallization of a unit mass sample;

[0038] in The value is 293 J / g.

[0039] Furthermore, the thickness and uniformity of the diaphragm were measured:

[0040] Thickness: The online thickness is measured by an online thickness gauge. This device can perform continuous scanning and generate a continuous curve of the transverse thickness variation after a single scan and multiple scans.

[0041] Thickness uniformity: The uniformity of thickness is characterized by calculating the standard deviation of the thickness value measured by the online thickness gauge and the transverse thickness variation curve.

[0042] The formula for calculating the standard deviation of thickness is:

[0043]

[0044] S—Standard deviation;

[0045] —Thickness (µm) at each test point;

[0046] —Total number of test points;

[0047] —Average thickness (um).

[0048] Furthermore, the porosity of the membrane is measured and calculated using the following formula:

[0049]

[0050] In the formula:

[0051] —The areal density of the diaphragm (g / m³) 2 ) ;

[0052] —Density of raw materials (g / cm³) 3 For polyethylene diaphragms =0.95 g / cm 3 h — the measured thickness of the diaphragm (jum);

[0053] In the experiment, a 10 cm x 10 cm membrane was cut, folded, and its weight was measured using an electronic balance. The displayed value was multiplied by 100 to obtain the areal density of the membrane. Substituting all the data into the formula yields the porosity of the membrane.

[0054] Further, the heat shrinkage rate of the diaphragm was measured, and the heat shrinkage performance of the diaphragm was evaluated using an imaging instrument. The test method referred to GB / T12027-2004. Using a utility knife, samples were taken at positions of 30cm, 110cm, 190cm, and 270cm in the transverse direction. After sampling, two vertical lines were drawn on the sample along the transverse (TD) and longitudinal (MD) directions using a ballpoint pen and a steel ruler. The length in the TD direction before heat shrinkage was measured with an imaging instrument and recorded as W, and the length in the MD direction was recorded as Lo. The oven temperature was set to 120℃. After the sample was treated in the oven for 1 hour, it was taken out and cooled to room temperature. The length of the sample in the TD direction W1 and the length of the sample in the MD direction Li after shrinkage were measured with an imaging instrument. The heat shrinkage rate was calculated using (2-4) and (2-5).

[0055] TD shrinkage rate % = (W - W1) / W × 100% (2-4)

[0056] MD shrinkage rate % = (Lo - Li) / Lo × 100% (2-5)

[0057] In the formula:

[0058] W — Initial mark spacing length in the TD direction (mm);

[0059] Lo — Initial mark interval length in MD direction (mm);

[0060] W1—Length of the markings after heating in the TD direction (mm);

[0061] Li — Length of the mark interval (mm) after heating in the MD direction.

[0062] Furthermore, in step e), as can be seen from the solvent diffusion mechanism, the double diffusion of the solvent and extractant is caused by the concentration difference. The concentration difference of the solvent inside and outside the membrane in the extraction system is:

[0063]

[0064] Where: m is the experimental extraction bath ratio. This is the initial concentration of the extractant. Let be the solvent concentration after an infinite amount of time. From the above formula, it can be seen that the solvent concentration difference inside and outside the diaphragm in the extraction system increases with the increase of the extraction bath ratio, and gradually approaches . Dichloromethane was used as the extractant for lithium-ion battery separators, and the extraction temperature was 20℃.

[0065] Furthermore, using dichloromethane as the extractant for lithium-ion battery separators, with an extraction bath ratio of 25 ml / g, the effect of different extraction temperatures (15-25℃) on the separator extraction results was investigated.

[0066] Furthermore, different concentrations (70%-100%) of dichloromethane solvent (with paraffin oil content of 0-30%) were prepared in the designed extraction tank. The diaphragm was immersed in the extraction tank for extraction, then removed, dried at high temperature, cooled, and tested.

[0067] The beneficial effects of this invention are:

[0068] 1. When the temperature of the casting roll is 25℃ or above, the aperture distribution is narrow when the gap between the rolls is 1.5mm or less. As the temperature of the casting roll increases, the gap between the rolls increases, and the die lip opening increases, the standard deviation of the thickness will first decrease and then increase. Setting the temperature to 25℃, 1.5 mm, and 1.3 mm respectively, a diaphragm with the smallest standard deviation of thickness, effective elimination of internal stress, effective guarantee of heat shrinkage rate, and best tensile strength can be obtained.

[0069] 2. Setting the stretch ratio to 700%, the biaxial stretching temperature to 115℃, and the heat setting temperature to 122℃ will yield a diaphragm with the best overall performance. Attached Figure Description

[0070] Figure 1 A schematic diagram illustrating the effect of different roller gaps on the uniformity of diaphragm thickness;

[0071] Figure 2 A schematic diagram showing the standard deviation of thickness for different roller gaps;

[0072] Figure 3 This is a schematic diagram showing the thickness test results using three sets of stretch ratios: 500%, 600%, and 700%.

[0073] Figure 4 A schematic diagram showing the standard deviation of thickness for different draw ratios;

[0074] Figure 5 This is a schematic diagram of online diaphragm thickness detection at different stretching temperatures;

[0075] Figure 6 A schematic diagram showing the standard deviation of thickness at different stretching temperatures;

[0076] Figure 7 A schematic diagram illustrating the effect of different heat setting temperatures on the uniformity of diaphragm thickness;

[0077] Figure 8 A schematic diagram showing the standard deviation of thickness at different setting temperatures;

[0078] Figure 9 A schematic diagram summarizing the porosity and air permeability of diaphragms prepared at different heat-setting temperatures;

[0079] Figure 10 The graph shows the changes in porosity and air permeability at different heat setting temperatures.

[0080] Figure 11 This is a schematic diagram of the transverse stretching extraction process for the lithium battery separator. Detailed Implementation

[0081] To facilitate understanding by those skilled in the art, the present invention will be further described below with reference to embodiments and accompanying drawings. The content mentioned in the embodiments is not intended to limit the present invention.

[0082] like Figure 1-11 As shown, a transverse stretching extraction process for a lithium battery separator includes the following steps:

[0083] a) Preparation of homogeneous solution: Polyethylene is mixed with a high-boiling-point, low-molecular-weight diluent in a certain proportion, and then subjected to high temperature and screw shearing in an extruder to form a homogeneous solution. The number average molecular weight of ultra-high molecular weight polyethylene is generally 50 × 10⁻⁶. 6 ~200×10 6 The diluent has a high boiling point and is between g / mol;

[0084] b) Melt extrusion: The homogeneous solution is filtered through a precision filter and then extruded through a die into sheet-like melt;

[0085] c) Casting: The sheet-like melt is attached to the casting roller for cooling. During the cooling process, as the polymer crystallizes, the solubility of the diluent gradually decreases, thereby inducing liquid-liquid or solid-liquid phase separation. The diluent is dispersed in the polymer phase to form a film.

[0086] d) Synchronous biaxial stretching: The film is stretched in both longitudinal and transverse directions at the same time. Microscopically, the molecular chains and segments are oriented in the longitudinal and transverse directions respectively. Under a certain temperature, the crystallinity of the oriented molecular chains gradually increases.

[0087] e) Extraction and drying: Based on the principle of like dissolves like, a low-boiling-point organic solvent is used as an extractant to extract the diluent. Then, taking advantage of the low-boiling-point characteristic of the extractant, it is volatilized by heating and recycled.

[0088] 1) Cut the unextracted ultra-high molecular weight polyethylene / paraffin oil blend membrane into a diaphragm with a size of 880mm × 50mm. Use an electronic balance to measure the weight of the diaphragm. Connect the two ends of the diaphragm with a plastic sealing machine so that the path of the diaphragm around the surface of each wheel becomes a ring.

[0089] 2) The active spokes are connected to the drive motor. The clamping force is generated by adjusting the position of the active spokes up and down. The drive motor is turned on and the speed is adjusted to check the movement of the diaphragm.

[0090] 3) Add extractant of different concentrations (70%-100%) to the extraction tank to the set liquid level, set different diaphragm movement speeds (0-50mm / s), place the upper and lower wheel structure in the extraction tank and start timing;

[0091] 4) After the experiment is completed, turn off the drive motor and remove the diaphragm. Dry it with hot air until the extractant has completely evaporated. After cooling, weigh it, calculate the change in diaphragm extraction weight, and test the diaphragm.

[0092] f) Heat setting: The diaphragm is stretched and shaped again, while releasing internal stress to reduce the dimensional shrinkage of the diaphragm at high temperatures;

[0093] g) Rewinding: The diaphragm is smoothly rolled onto the spinning drum to form a membrane roll, which facilitates subsequent aging treatment;

[0094] h) Aging and Slitting: The crystallinity of polymers will not reach 100%, and the separator is no exception. Since the glass transition temperature of PE is around -68°C, while the amorphous part of the separator is above the glass transition temperature at room temperature, it will tend to deorient. The membrane roll is aged to release its internal stress. The aging treatment promotes the dimensional stability of the membrane roll and prevents it from changing. The aged membrane roll can be slitted according to the size required by the customer, and the finished product is obtained after slitting.

[0095] In this embodiment, during step c), the temperature control medium of the casting roller is water, which mainly serves to uniformly cool the sheet-like melt. The experiments were conducted at 10℃, 15℃, 20℃, 25℃, and 30℃ respectively. When conducting single-factor variable experiments on other parameters, the temperature of the casting roller was fixed at 25℃.

[0096] In step c), the size of the gap between the guide roller and the casting roller is called the guide roller gap. The function of the guide roller is to tightly adhere the sheet-like melt to the casting roller. Experiments were conducted using gaps of 0.5 mm, 1.0 mm, 1.5 mm, 2.0 mm, and 2.5 mm. When conducting single-factor variable experiments on other parameters, the guide roller gap was fixed at 1.5 mm.

[0097] The effect of different roller gaps on diaphragm thickness uniformity is shown in the results. Figure 1 ,

[0098]

[0099] Depend on Figure 1 The standard deviation of thickness for different roller gaps can be calculated from the data. Figure 2,

[0100] Depend on Figure 2 It can be seen that as the gap between the rollers increases, the standard deviation of thickness first decreases and then increases, reaching its optimum at 1.5 mm. This is because when the gap between the rollers is small, the friction between the melt and the rollers increases, causing the melt to be stretched.

[0101] Uneven force distribution leads to a larger thickness standard deviation. When the gap between the rollers is too large, the melt cannot fill the gap between the casting roller and the roller, resulting in air between the melt and the casting roller, as well as between the melt and the roller. The presence of air affects the uniformity of melt cooling, thus leading to poor thickness uniformity.

[0102] In this embodiment, in step d), the stretch ratio refers to the multiple by which the length and width of the diaphragm increase after stretching relative to the length and width before stretching; the experiment was conducted at stretch ratios of 500%, 600%, and 700%, and the stretch ratio was fixed at 700% when conducting single-factor variable experiments on other parameters;

[0103] Experiments were conducted using three sets of tensile ratios: 500%, 600%, and 700%. The online thickness measurement results are shown below. Figure 3 ,

[0104]

[0105] Depend on Figure 4 The data can be used to calculate the standard deviation of thickness for different stretch ratios.

[0106] See Figure 4 ;

[0107]

[0108] Depend on Figure 4 It is evident that the uniformity of membrane thickness increases with increasing draw ratio. As the draw ratio increases, the orientation of the membrane increases, ensuring sufficient stretching at every position in the transverse direction, including the thicker portions of the membrane, thereby improving thickness uniformity. Furthermore, increasing the draw ratio leads to a higher stretching speed, preventing the molecular chains from deorienting quickly enough during the rapid orientation process, thus reducing the degree of thickness uniformity degradation caused by uneven stress release.

[0109] In step d), the maximum temperature of the biaxial stretching test should not exceed the melting point of PE (135℃) to prevent the diaphragm from melting during the stretching process. The experiments were conducted at 110℃, 115℃, 120℃, 125℃ and 130℃ respectively. When conducting single-factor variable tests on other parameters, the biaxial stretching temperature was fixed at 120℃.

[0110] In this embodiment, DSC is used to test the sample. Based on the DSC curve, the melting point, crystallization temperature, and melting point data can be obtained.

[0111] According to the formula

[0112]

[0113] The crystallinity data were calculated.

[0114] In the formula:

[0115] — Crystallinity (%);

[0116] —Heat value of sample crystallization melting (J / g);

[0117] —Heat of melting (J / g) of 100% crystallization of a unit mass sample;

[0118] in The value is 293 J / g.

[0119] See Figure 5 Online thickness detection of diaphragms at different stretching temperatures;

[0120]

[0121] See Figure 6 The standard deviation of thickness at different stretching temperatures was calculated.

[0122]

[0123] As the stretching temperature of biaxial stretching increases, the thickness uniformity of the diaphragm deteriorates. This is because stress hardening is more likely to occur during low-temperature stretching. That is, once a section is stretched to uniformity, the deformation tends to stabilize and no longer changes, while the remaining sections continue to be stretched, eventually resulting in uniform thickness across the entire diaphragm. Simultaneously, lower stretching temperatures are less conducive to molecular chain disorientation, thus resulting in better diaphragm thickness uniformity.

[0124] In this embodiment, the thickness and thickness uniformity of the diaphragm are measured:

[0125] Thickness: The online thickness is measured by an online thickness gauge. This device can perform continuous scanning and generate a continuous curve of the transverse thickness variation after a single scan and multiple scans.

[0126] Thickness uniformity: The uniformity of thickness is characterized by calculating the standard deviation of the thickness value measured by the online thickness gauge and the transverse thickness variation curve.

[0127] The formula for calculating the standard deviation of thickness is:

[0128]

[0129] S—Standard deviation;

[0130] —Thickness (µm) at each test point;

[0131] —Total number of test points;

[0132] —Average thickness (um).

[0133] See Figure 7 The effect of different heat setting temperatures on the uniformity of diaphragm thickness was analyzed.

[0134] See Figure 8 The standard deviation of thickness at different setting temperatures was analyzed.

[0135] It can be seen that as the heat setting temperature increases, the standard deviation of thickness increases, meaning the uniformity of membrane thickness deteriorates. This is because at higher heat setting temperatures, the movement of molecular chains and chain segments intensifies, and the degree of disorientation increases in different parts of the membrane in the lateral direction. However, the degree of disorientation varies, resulting in poorer uniformity of membrane thickness.

[0136] In this embodiment, the porosity of the membrane is measured. The porosity is calculated using a formula, as follows:

[0137]

[0138] In the formula:

[0139] —The areal density of the diaphragm (g / m³) 2 ) ;

[0140] —Density of raw materials (g / cm³) 3 For polyethylene diaphragms =0.95 g / cm 3 h — the measured thickness of the diaphragm (jum);

[0141] In the experiment, a 10 cm x 10 cm membrane was cut, folded, and its weight was measured using an electronic balance. The displayed value was multiplied by 100 to obtain the areal density of the membrane. Substituting all the data into the formula yields the membrane's porosity. Different heat-setting temperatures result in different pore sizes and distributions in the membrane, leading to variations in porosity and air permeability.

[0142] The porosity and air permeability of diaphragms prepared at different heat-setting temperatures are summarized in [see...]. Figure 9 .

[0143] A summary of the changes in porosity and air permeability at different heat-setting temperatures is available in the following graphs. Figure 10 .

[0144] Depend on Figure 10 It can be seen that as the heat setting temperature increases, the membrane porosity decreases and the air permeability increases. This is because as the heat setting temperature increases, the existing large pores shrink and become smaller, and the small pores may close, resulting in a reduction in the number of pores, thereby reducing the porosity and increasing the air permeability.

[0145] In this embodiment, the heat shrinkage rate of the diaphragm is measured, and the heat shrinkage performance of the diaphragm is evaluated using an imaging instrument. The test method refers to GB / T12027-2004. Using a utility knife, samples are taken at positions of 30cm, 110cm, 190cm, and 270cm in the transverse direction. After sampling, two vertical lines are drawn on the sample along the transverse (TD) and longitudinal (MD) directions using a ballpoint pen and a steel ruler. The length of the TD direction before heat shrinkage is measured by the imaging instrument and recorded as W, and the length of the MD direction is recorded as Lo. The oven temperature is set to 120℃. After the sample is treated in the oven for 1 hour, it is taken out and cooled to room temperature. The length of the sample in the TD direction W1 and the length of the sample in the MD direction Li after shrinkage are measured by the imaging instrument. The heat shrinkage rate is calculated using (2-4) and (2-5).

[0146] TD shrinkage rate % = (W - W1) / W × 100% (2-4)

[0147] MD shrinkage rate % = (Lo - Li) / Lo × 100% (2-5)

[0148] In the formula:

[0149] W — Initial mark spacing length in the TD direction (mm);

[0150] Lo — Initial mark interval length in the MD direction (mm);

[0151] W1—Length of the markings after heating in the TD direction (mm);

[0152] Li — Length of the mark interval (mm) after heating in the MD direction.

[0153] In this embodiment, in step e), as can be seen from the solvent diffusion mechanism, the double diffusion of the solvent and extractant is caused by the concentration difference. The concentration difference of the solvent inside and outside the membrane in the extraction system is:

[0154]

[0155] Where: m is the experimental extraction bath ratio. This is the initial concentration of the extractant. Let be the solvent concentration after an infinite amount of time. From the above formula, it can be seen that the solvent concentration difference inside and outside the diaphragm in the extraction system increases with the increase of the extraction bath ratio, and gradually approaches . Dichloromethane was used as the extractant for lithium-ion battery separators, and the extraction temperature was 20℃.

[0156] In this embodiment, dichloromethane was used as the extractant for lithium-ion battery separators, and the extraction bath ratio was 25 ml / g. The effect of different extraction temperatures (15-25℃) on the separator extraction results was studied.

[0157] In this embodiment, dichloromethane solvent of different concentrations (70%-100%) (with paraffin oil content of 0-30% in the solvent) is prepared in the designed extraction tank. The diaphragm is immersed in the extraction tank for extraction, then taken out, dried at high temperature, cooled, and tested.

[0158] The effects of the stretching ratio, stretching temperature, and heat setting temperature on diaphragm performance in the biaxial stretching process were analyzed. Increasing the stretching ratio in the biaxial stretching process ensures sufficient stretching at every transverse position of the diaphragm. Simultaneously, the stretching speed increases with the stretching ratio, preventing the molecular chains from disorienting and thus avoiding uneven thickness uniformity caused by uneven shrinkage. With increasing stretching speed, micropores are rapidly generated and uniformly enlarged, ensuring a narrower pore size distribution. Decreasing the stretching temperature in the biaxial stretching process easily leads to stress hardening, resulting in uniform stretching of the diaphragm. High-temperature stretching is unfavorable for crystal nucleation but favorable for crystal growth, thus hindering the formation of small micropores and resulting in a wider pore size distribution. Increasing the heat setting temperature promotes molecular chain disorientation, but varying degrees of disorientation lead to poor thickness uniformity. Excessively high heat setting temperatures can cause micropore shrinkage or even closure, resulting in a wider pore size distribution.

[0159] Since the above three key parameters significantly affect the thermal shrinkage of the separator, considering all the properties of the separator, a stretching ratio of 700%, a stretching temperature of 115℃, and a setting temperature of 122℃ are considered optimal. Among these, a stretching ratio of 700% is considered optimal for biaxial stretching. This is because the maximum adjustable stretching ratio of the stretching machine used in this study is 700%, and also because when the stretching ratio reaches 700%, the thermal shrinkage is close to the upper limit required for separators in general lithium battery designs. The effects of casting roller temperature, roller gap, biaxial stretching temperature, and heat setting temperature on the separator thickness uniformity and pore size distribution were investigated in detail, leading to the following conclusions:

[0160] (1) Introducing ultra-high molecular weight polyethylene into polyethylene base material can improve thickness uniformity and narrow pore size distribution, increase the strength of the diaphragm, improve the microstructure of the diaphragm, and significantly reduce the proportion of defective black spots and crystal points.

[0161] (2) The temperature of the casting roller and the gap between the rollers can control the pore size distribution of the diaphragm. When the temperature of the casting roller is 25℃ or above, the pore size distribution is narrow when the gap between the rollers is 1.5mm or below. As the temperature of the casting roller increases, the gap between the rollers increases, and the die lip opening increases, the standard deviation of the thickness will first decrease and then increase. When set to 25℃, 1.5 mm, and 1.3 mm respectively, a diaphragm with the smallest standard deviation of thickness, which can effectively eliminate internal stress, effectively ensure the heat shrinkage rate, and has the best tensile strength can be obtained.

[0162] (3) Increasing the stretch ratio, decreasing the stretching temperature, and decreasing the heat setting temperature reduce the thickness deviation, average pore size, and pore size distribution of the separator, but also increase thermal shrinkage. Currently, the industry requires that the thermal shrinkage rate of lithium battery separators at 120℃ / h be ≤5% in the longitudinal direction and ≤3.5% in the transverse direction. Taking all factors into consideration, setting the stretch ratio to 700%, the biaxial stretching temperature to 115℃, and the heat setting temperature to 122℃ will yield the separator with the best overall performance.

[0163] All technical features in this embodiment can be freely combined according to actual needs.

[0164] It should be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such process, method, article, or apparatus.

[0165] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

[0166] The above embodiments are preferred implementations of the present invention. In addition, other implementations are also included. Any obvious substitutions without departing from the concept of the present invention are within the protection scope of the present invention.

Claims

1. A transverse stretching extraction process for lithium battery separators, characterized in that, Includes the following steps: a) Preparation of homogeneous solution: Polyethylene is mixed with a high-boiling-point, low-molecular-weight diluent in a certain proportion, and then subjected to high temperature and screw shearing in an extruder to form a homogeneous solution. The number average molecular weight of ultra-high molecular weight polyethylene is generally 50 × 10⁻⁶. 6 ~200×10 6 The diluent has a high boiling point and is between g / mol; b) Melt extrusion: The homogeneous solution is filtered through a precision filter and then extruded through a die into sheet-like melt; c) Casting: The sheet-like melt is attached to the casting roller and cooled. During the cooling process, the solubility of the diluent gradually decreases as the polymer crystallizes, thereby inducing liquid-liquid or solid-liquid phase separation. The diluent is dispersed in the polymer phase, forming a film. In step c) casting, water is used as the temperature control medium for the casting roller, and the gap between the roller and the casting roller and the die lip opening are set as process control parameters. The process parameters are: casting roller temperature set to 25°C, roller gap set to 1.5 mm, and die lip opening set to 1.3 mm. d) Synchronous biaxial stretching: The film is stretched in both longitudinal and transverse directions at the same time. Microscopically, the molecular chains and segments are oriented in the longitudinal and transverse directions respectively. Under a certain temperature, the crystallinity of the oriented molecular chains gradually increases. e) Extraction and drying: Based on the principle of like dissolves like, a low-boiling-point organic solvent is used as an extractant to extract the diluent. Then, taking advantage of the low-boiling-point characteristic of the extractant, it is volatilized by heating and recycled. 1) Cut the unextracted ultra-high molecular weight polyethylene / paraffin oil blend membrane into a diaphragm with a size of 880mm×50mm. Use an electronic balance to measure the weight of the diaphragm. Connect the two ends of the diaphragm with a plastic sealing machine so that the path of the diaphragm around the surface of each wheel becomes a ring. 2) The active spokes are connected to the drive motor. The clamping force is generated by adjusting the position of the active spokes up and down. The drive motor is turned on and the speed is adjusted to check the movement of the diaphragm. 3) Add an extractant with a concentration of 70%-100% to the extraction tank until the set liquid level is reached, set the diaphragm movement speed to 0-50 mm / s, place the upper and lower wheel structure in the extraction tank and start timing; 4) After the experiment is completed, turn off the drive motor and remove the diaphragm. Dry it with hot air until the extractant has completely evaporated. After cooling, weigh it, calculate the change in diaphragm extraction weight, and test the diaphragm. f) Heat setting: The diaphragm is stretched and shaped again, while releasing internal stress to reduce the dimensional shrinkage of the diaphragm at high temperatures; g) Rewinding: The diaphragm is smoothly rolled onto the drum to form a membrane roll, which facilitates subsequent aging treatment; h) Aging and Slitting: The crystallinity of polymers will not reach 100%, and the separator is no exception. Since the glass transition temperature of PE is around -68°C, while the amorphous part of the separator is above the glass transition temperature at room temperature, it will tend to deorient. The membrane roll is aged to release its internal stress. The aging treatment promotes the dimensional stability of the membrane roll and prevents it from changing. The aged membrane roll can be slitted according to the size required by the customer, and the finished product is obtained after slitting.

2. The transverse stretching extraction process for lithium battery separators as described in claim 1, characterized in that, In step c), during the casting process, the temperature control medium of the casting roller is water, which mainly serves to uniformly cool the sheet-like melt. The experiments were conducted at 10℃, 15℃, 20℃, 25℃, and 30℃ respectively. When conducting single-factor variable experiments on other parameters, the casting roller temperature was fixed at 25℃. In step c), the size of the gap between the guide roller and the casting roller is called the guide roller gap. The function of the guide roller is to tightly adhere the sheet-like melt to the casting roller. The experiments were conducted with gaps of 0.5 mm, 1.0 mm, 1.5 mm, 2.0 mm, and 2.5 mm. When conducting single-factor variable experiments on other parameters, the guide roller gap was fixed at 1.5 mm.

3. The transverse stretching extraction process for the lithium battery separator as described in claim 1, characterized in that, In step d), the stretch ratio refers to the multiple by which the length and width of the diaphragm increase after stretching relative to the length and width before stretching; the experiment was conducted at stretch ratios of 500%, 600%, and 700%, and the stretch ratio was fixed at 700% when conducting single-factor variable experiments on other parameters; In step d), the maximum temperature of the biaxial stretching test should not exceed the melting point of PE, 135°C, to prevent the diaphragm from melting during the stretching process. The tests were conducted at 110°C, 115°C, 120°C, 125°C, and 130°C. When conducting single-factor variable tests on other parameters, the biaxial stretching temperature was fixed at 120°C.

4. The transverse stretching extraction process for the lithium battery separator as described in claim 1, characterized in that, The sample was tested using DSC, and the melting point, crystallization temperature, and melting point data can be obtained from the DSC curve. According to the formula ； The crystallinity data were calculated. In the formula: — Crystallinity (%); —Heat value of sample crystallization melting (J / g); —Heat of melting (J / g) of 100% crystallization of a unit mass sample; in The value is 293 J / g.

5. The transverse stretching extraction process for the lithium battery separator as described in claim 4, characterized in that, Measuring the thickness and uniformity of the diaphragm: Thickness: The online thickness is measured by an online thickness gauge. This device can perform continuous scanning and generate a continuous change curve of the transverse thickness after a single scan and multiple scans. Thickness uniformity: The uniformity of thickness is characterized by calculating the standard deviation of the thickness value measured by the online thickness gauge and the transverse thickness variation curve. The formula for calculating the standard deviation of thickness is: ； S—Standard deviation; —Thickness (µm) at each test point; —Total number of test points; —Average thickness (um).

6. The transverse stretching extraction process for the lithium battery separator as described in claim 5, characterized in that, The porosity of the membrane is measured and calculated using a formula, as follows: ； In the formula: —The areal density of the diaphragm (g / m³) 2 ) ; —Density of raw materials (g / cm³) 3 For polyethylene diaphragms =0.95 g / cm 3 h — the measured thickness of the diaphragm (jum); In the experiment, a 10 cm x 10 cm membrane was cut, folded, and its weight was measured using an electronic balance. The displayed value was multiplied by 100 to obtain the areal density of the membrane. Substituting all the data into the formula yields the porosity of the membrane.

7. The transverse stretching extraction process for the lithium battery separator as described in claim 1, characterized in that, The heat shrinkage rate of the diaphragm was measured, and the heat shrinkage performance of the diaphragm was evaluated using an imaging instrument. The test method referred to GB / T12027-2004. Using a utility knife, samples were taken at positions of 30cm, 110cm, 190cm, and 270cm in the transverse direction. After sampling, two vertical lines were drawn on the sample along the transverse (TD) and longitudinal (MD) directions using a ballpoint pen and a steel ruler. The length of the TD direction before heat shrinkage was measured with an imaging instrument and recorded as W, and the length of the MD direction was recorded as Lo. The oven temperature was set to 120℃. After the sample was treated in the oven for 1 hour, it was taken out and cooled to room temperature. The length of the sample in the TD direction W1 and the length of the sample in the MD direction Li after shrinkage were measured with an imaging instrument. The heat shrinkage rate was calculated using (2-4) and (2-5). TD shrinkage rate % = (W - W1) / W × 100% (2-4) MD shrinkage rate % = (Lo - Li) / Lo × 100% (2-5) In the formula: W — Initial mark spacing length in the TD direction (mm); Lo — Initial mark interval length in the MD direction (mm); W1—Length of the markings after heating in the TD direction (mm); Li — Length of the mark interval (mm) after heating in the MD direction.

8. The transverse stretching extraction process for the lithium battery separator as described in claim 1, characterized in that, In step e), according to the solvent diffusion mechanism, the double diffusion of the solvent and extractant is caused by the concentration difference; the concentration difference of the solvent inside and outside the membrane in the extraction system is: ； Where: m is the experimental extraction bath ratio. This is the initial concentration of the extractant. Let be the solvent concentration after an infinite amount of time. From the above formula, it can be seen that the solvent concentration difference inside and outside the diaphragm in the extraction system increases with the increase of the extraction bath ratio, and gradually approaches . Dichloromethane was used as the extractant for lithium-ion battery separators, and the extraction temperature was 20℃.

9. The transverse stretching extraction process for the lithium battery separator as described in claim 1, characterized in that, Dichloromethane was used as the extractant for lithium-ion battery separators, and the extraction bath ratio was 25 ml / g. The effect of extraction temperature (15-25℃) on the separator extraction results was studied.

10. The transverse stretching extraction process for the lithium battery separator as described in claim 9, characterized in that, A 70%-100% concentration of dichloromethane solvent is prepared in the designed extraction tank. The paraffin oil content in the dichloromethane solvent is 0-30%. The diaphragm is immersed in the extraction tank for extraction, then removed, dried at high temperature, cooled, and tested.

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