Separator and preparation method therefor, and method for determining cross-linking degree of separator

Abstract

A separator and a preparation method therefor, and a method for determining the cross-linking degree of a separator. The separator has a TMA membrane rupture temperature of greater than or equal to 170°C, a tensile strength of greater than or equal to 2000 kgf / cm2, and a ductility of greater than or equal to 100%. The preparation method for the separator comprises the following steps: sequentially subjecting a polyolefin base material to dip-coating and a light treatment, so as to obtain a separator, wherein a dip-coating solution for dip-coating comprises a photo-initiator, a synergist and an organic solvent. The separator has a higher membrane rupture temperature and higher high-temperature strength. In the preparation method for the separator, by using a method of using the photo-initiator and the synergist in combination, the initiation efficiency of the photo-initiator is improved, the cross-linking degree of the separator is improved, and the residue of the photo-initiator is reduced, thereby improving the performance of the separator.

Description

A diaphragm and its preparation method and method for judging the crosslinking degree of the diaphragm

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to Chinese patent application number 2023117116146 filed with the Chinese Patent Office on December 13, 2023, entitled “A diaphragm, a method for preparing the same, and a method for determining the degree of cross-linking of the diaphragm,” the entire contents of which are incorporated herein by reference. Technical Field

[0003] The present disclosure relates to the technical field of diaphragms, and in particular to a diaphragm and a preparation method thereof, and a method for determining the crosslinking degree of the diaphragm. Background Art

[0004] As a key component of lithium batteries, the diaphragm is located between the positive and negative electrodes. It isolates the positive and negative electrodes, preventing internal short circuits in the battery, while allowing lithium ions to pass freely to ensure battery charging and discharging. Polyolefin diaphragms have become one of the commercial diaphragm materials due to their high strength, good acid and alkali resistance and solvent resistance, and electrochemical stability. Polyolefin materials have become an important raw material for commercial diaphragms due to their low cost, high chemical stability, and good electrochemical stability. However, polyolefin materials have a relatively low melting point and are prone to melting and rupturing when the battery overheats abnormally, causing an internal short circuit in the battery, which is dangerous and seriously threatens the user's life. Therefore, increasing the diaphragm rupture temperature can improve the safety of lithium batteries.

[0005] In order to improve the safety performance of batteries, the methods used in the prior art include: (1) doping or blending of multiple resins, for example, patent No. CN111244369A provides a battery separator, including a polyolefin porous membrane, which includes polyethylene and polypropylene resins. In the case of containing inorganic particles, the membrane rupture temperature is above 180°C. The control of the blending process and the cost increase caused by polypropylene are both disadvantages of this process; (2) Siloxane grafted polyethylene modification and cross-linking, for example, patent No. CN105576172A, patent No. CN111108627A, patent No. CN111108628A and patent No. CN111081949A disclose a method for preparing a cross-linked separator with increased membrane rupture temperature, by adding an initiator, a siloxane cross-linking agent and a catalyst during the preparation process of the polyethylene separator, thereby preparing a siloxane cross-linked separator. However, this method will inevitably produce gel points during the preparation process, causing abnormal membrane surface of the diaphragm, and the cross-linking process will cause severe and uneven shrinkage.

[0006] In the existing technology, in order to increase the film rupture temperature, it is necessary to add more photoinitiators during the preparation process of the diaphragm. However, excessive use of photoinitiators will bring many problems, such as more migrants, reduced weather resistance, insufficient coating thickness, and increased costs; at the same time, the degree of cross-linking is not high, and the film rupture temperature is difficult to further increase.

[0007] The dip coating process disclosed in the patent with publication number CN115995655A and the patent with publication number CN115377610A can achieve complete cross-linking or partial cross-linking treatment of polyolefin porous diaphragm materials in a more convenient process to obtain products with excellent heat resistance. However, the reactants used in the dip coating process have fluidity and concentration changes. In rapid continuous production, continuous small changes in reaction conditions, material concentration and infiltration amount will inevitably affect product quality. Using this process requires finding a method that can conveniently evaluate and detect the effect of membrane cross-linking treatment in real time, and obtain better products based on the detection structure of this method.

[0008] Application Contents

[0009] One object of the present disclosure is to provide a diaphragm having a high rupture temperature and high-temperature strength.

[0010] Another object of the present disclosure is to provide a method for preparing the above-mentioned diaphragm, which can achieve better deep curing and reduce the residual photoinitiator.

[0011] Another object of the present disclosure is to provide a method for determining the degree of cross-linking of the above-mentioned diaphragm, which can quickly analyze the degree of cross-linking of the diaphragm; through optimized processes and modified formulas, a diaphragm with excellent heat resistance and a certain degree of yellowness is obtained, and the yellowness of the diaphragm is correlated with the degree of cross-linking, which can evaluate and detect the effect of the diaphragm cross-linking treatment in real time online, becoming an important parameter for process control.

[0012] In order to achieve the above-mentioned purpose of the present disclosure, the following technical solutions are adopted:

[0013] The present disclosure provides a diaphragm having a TMA rupture temperature of ≥170°C and a tensile strength of ≥2000 kgf / cm 2 , elongation ≥100%.

[0014] Further, it includes at least one of the following features (1) to (4);

[0015] (1) The TMA rupture temperature of the diaphragm is 195-260°C;

[0016] (2) The puncture strength of the diaphragm is ≥40 gf / μm;

[0017] (3) The yellowness index of the diaphragm is 1% to 5%;

[0018] (4) The cross-linking degree of the diaphragm is 1% to 75%.

[0019] Furthermore, the TMA membrane rupture temperature of the diaphragm is 214-260°C.

[0020] Furthermore, the TMA membrane rupture temperature of the diaphragm is 220-260°C.

[0021] Furthermore, it includes at least one of the following features (1) to (3):

[0022] (1) The yellowness index of the separator is 1 to 1.5, corresponding to a cross-linking degree of 1% to 45%;

[0023] (2) The yellowness index of the separator is 1.5 to 4, corresponding to a cross-linking degree of 45% to 55%;

[0024] (3) The yellowness index of the separator is 4 to 5, and the corresponding cross-linking degree is 55% to 75%.

[0025] The present disclosure also provides a method for preparing the above-mentioned diaphragm, comprising the following steps:

[0026] The polyolefin substrate is dip-coated and light-treated in sequence to obtain the diaphragm; wherein the dipping solution of the dipping coating comprises a photoinitiator, a synergist and an organic solvent.

[0027] The photoinitiator includes at least one of benzophenone, thioxanthone photoinitiator and photoinitiator 819;

[0028] Furthermore, the photoinitiator includes a thioxanthone photoinitiator.

[0029] Further, it includes at least one of the following features (1) to (3);

[0030] (1) The synergist includes ethyl 4-dimethylaminobenzoate;

[0031] (2) The mass ratio of the photoinitiator to the synergist is 1:(1-10);

[0032] (3) The mass ratio of the photoinitiator to the organic solvent is 1:(20-1000).

[0033] The present disclosure also provides a method for determining the cross-linking degree of the separator as described above, wherein the cross-linking degree of the separator is obtained according to the yellowing degree of the separator;

[0034] or,

[0035] The cross-linking degree of the separator is obtained according to the content of the S element in the separator.

[0036] Furthermore, the yellowness index of the diaphragm is 0-1, and the corresponding cross-linking degree is <1%; the yellowness index of the diaphragm is 1-1.5, and the corresponding cross-linking degree is 1%-45%; the yellowness index of the diaphragm is 1.5-4, and the corresponding cross-linking degree is 45%-55%; the yellowness index of the diaphragm is 4-5, and the corresponding cross-linking degree is 55%-75%.

[0037] Furthermore, the content of S element in the diaphragm is 90-300 ppm, and the corresponding cross-linking degree is 50%-62%; the content of S element in the diaphragm is 300-500 ppm, and the corresponding cross-linking degree is 62%-70%; the content of S element in the diaphragm is 500-700 ppm, and the corresponding cross-linking degree is 70%-73%; the content of S element in the diaphragm is 700-1000 ppm, and the corresponding cross-linking degree is 73%-75%. DETAILED DESCRIPTION

[0038] In some embodiments of the present disclosure, a diaphragm is provided, wherein the diaphragm has a TMA rupture temperature of ≥170°C and a tensile strength of ≥2000 kgf / cm 2 , elongation is ≥100%.

[0039] The diaphragm provided by the present disclosure has a higher membrane rupture temperature and high-temperature strength.

[0040] In some embodiments of the present disclosure, the TMA membrane rupture temperature of the separator is 195-260°C.

[0041] In some embodiments of the present disclosure, the TMA rupture temperature of the diaphragm is 214-260°C; further, the TMA rupture temperature of the diaphragm is 220-260°C; further, 230-258°C.

[0042] In some embodiments of the present disclosure, the membrane has a needle punch strength of ≥40 gf / um.

[0043] In some embodiments of the present disclosure, the yellowness index of the separator is 1% to 5%.

[0044] In some embodiments of the present disclosure, the cross-linking degree of the separator is 1% to 75%.

[0045] In some embodiments of the present disclosure, the yellowness index of the separator is 1 to 1.5, corresponding to a cross-linking degree of 1% to 45%;

[0046] The yellowness index of the separator is 1.5 to 4, corresponding to a cross-linking degree of 45% to 55%;

[0047] The yellowness index of the separator is 4 to 5, and the corresponding cross-linking degree is 55% to 75%.

[0048] The diaphragm disclosed in the present invention has a certain yellowness, and the yellowness of the diaphragm is correlated with the degree of cross-linking. The cross-linking treatment effect of the diaphragm can be evaluated and detected online in real time, becoming an important parameter for process control.

[0049] In some embodiments of the present disclosure, a method for preparing the above-mentioned diaphragm is also provided, comprising the following steps:

[0050] The polyolefin substrate is dip-coated and light-treated in sequence to obtain a diaphragm; wherein the dipping solution comprises a photoinitiator, a synergist and an organic solvent.

[0051] The present invention transfers a photoinitiator and a synergist into the micropores and the surface of a polyolefin substrate in a solution state, and achieves cross-linking through light treatment to produce a diaphragm with excellent performance.

[0052] The preparation method of the diaphragm disclosed in the present invention adopts the combined use of photoinitiators and enhancers, which can not only reduce costs, but also greatly improve the initiation efficiency of the photoinitiator, achieve better deep curing, greatly increase the cross-linking degree of the diaphragm, reduce the residual photoinitiator, and increase the membrane rupture temperature and high-temperature strength of the diaphragm.

[0053] In some embodiments of the present disclosure, the photoinitiator includes at least one of benzophenone, thioxanthone photoinitiator, and photoinitiator 819; further, the photoinitiator includes a thioxanthone photoinitiator.

[0054] In some embodiments of the present disclosure, the thioxanthone-based photoinitiator includes 2-Isopropylthioxanthone (ITX).

[0055] In some embodiments of the present disclosure, the synergist includes ethyl 4-dimethylaminobenzoate (EDAB).

[0056] In some embodiments of the present disclosure, the polyolefin substrate includes at least one of polyethylene, polypropylene, polybutene, polyhexene, ethylene propylene copolymer, ethylene butene copolymer, and ethylene hexene copolymer.

[0057] In some embodiments of the present disclosure, the parameters of the polyolefin substrate are: thickness of 0.5 to 20 μm, TMA film rupture temperature ≤ 152°C, impedance film rupture temperature ≤ 152°C, crosslinking degree ≤ 0.5%, porosity 20% to 60%, air permeability ≤ 400s / 100cc, pore size 20 to 80 nm, needle puncture strength ≥ 35 gf / μm, and thermal shrinkage rate at 130°C ≤ 25%.

[0058] In some embodiments of the present disclosure, the mass ratio of the photoinitiator to the synergist is 1:(1-10); typically but not limiting, for example, the mass ratio of the photoinitiator to the synergist can be 1:1, 1:2, 1:3, 1:4, 1:5, 1:6, 1:7, 1:8, 1:9, 1:10 or a range consisting of any two thereof.

[0059] In some embodiments of the present disclosure, the mass ratio of the photoinitiator to the organic solvent is 1:(20-1000); typically but not limitatively, for example, the mass ratio of the photoinitiator to the organic solvent can be 1:20, 1:100, 1:200, 1:300, 1:400, 1:500, 1:600, 1:700, 1:800, 1:900, 1:1000 or a range of any two thereof.

[0060] In some embodiments of the present disclosure, in the dipping solution, the content of the photoinitiator is 0.1wt% to 1wt%, and the content of the synergist is 0.05wt% to 0.5wt%; further, in the dipping solution, the content of the photoinitiator is 0.5wt% to 1wt%, and the content of the synergist is 0.25wt% to 0.5wt%.

[0061] Excessive concentration of a single component can easily lead to problems such as pore plugging. The present disclosure utilizes a combination of a photoinitiator and a synergist to not only reduce formulation costs but also achieve better deep curing, reduce photoinitiator residues, and especially the introduction and residue of elements other than C, H, and O. It can also increase the film break temperature and reduce the required photoinitiator concentration.

[0062] In some embodiments of the present disclosure, the organic solvent includes at least one of methanol, ethanol, dichloromethane, acetone, ethyl ether, petroleum ether, n-hexane, dimethylformamide, dimethylacetamide, and methylpyrrolidone.

[0063] In some embodiments of the present disclosure, a method for preparing a polyolefin substrate comprises the following steps:

[0064] The polyolefin and paraffin oil are mixed and then subjected to banburying, mixing, extrusion, cooling and casting, stretching and extraction to obtain a polyolefin substrate.

[0065] In some embodiments of the present disclosure, the mass ratio of polyolefin to paraffin oil is (2-4):1.

[0066] In some embodiments of the present disclosure, the polyolefin includes at least one of polyethylene, polypropylene, polybutene, polyhexene, ethylene propylene copolymer, ethylene butene copolymer, and ethylene hexene copolymer.

[0067] In some embodiments of the present disclosure, the dipping time is 5 to 30 seconds.

[0068] In some embodiments of the present disclosure, the light source for the light treatment is ultraviolet light with a wavelength of 100 to 400 nm, and the light treatment time is 0.1 to 600 s.

[0069] In some embodiments of the present disclosure, a lithium battery is provided, comprising the above-mentioned separator.

[0070] The diaphragm in lithium batteries has a low rupture temperature and poor safety. When the battery overheats abnormally, it is prone to melting and rupture, causing an internal short circuit in the battery. The diaphragm disclosed in this disclosure has a higher rupture temperature, which helps improve the safety performance of lithium batteries.

[0071] In some embodiments of the present disclosure, a method for determining the cross-linking degree of the above-mentioned separator is also provided, comprising: obtaining the cross-linking degree of the separator according to the yellowing degree of the separator;

[0072] or,

[0073] The crosslinking degree of the separator is obtained according to the content of the S element in the separator.

[0074] Yellowing is the phenomenon of a material turning yellow under natural sunlight, ultraviolet light, or due to heat, oxygen, stress, trace moisture, impurities, or improper processing. When a material absorbs light energy, carbon-carbon or carbon-hydrogen bonds in the molecular chain break down at the site of absorption. Diaphragms, such as cross-linked polyolefins, are exposed to ultraviolet light during the manufacturing process, generating free radicals that cause yellowing. The degree of yellowing can be used to determine the degree of crosslinking in the separator.

[0075] Cross-linking polyolefins typically requires the addition of various photoinitiators to polymerize the polyolefin. Photoinitiators contain elements such as O, S, and P that are not present in polyolefins, and the amount of photoinitiator affects the degree of cross-linking. By characterizing the content of a specific element in the photoinitiator, the photoinitiator content can be calculated, and thus the degree of cross-linking of the polyolefin can be determined.

[0076] The yellowness index (YI) refers to the degree to which a colorless, transparent, translucent, or near-white polymer material deviates from white, or appears yellowish. Standard "C" illuminant, as specified by the International Commission on Illumination (CIE), is used to illuminate the material. The tristimulus values ​​(X, Y, and Z) of the material's color are measured and calculated using the following formula: Y1 = 100 (1.28X - 1.06Z) / Y.

[0077] In some embodiments of the present disclosure, the yellowness index of the diaphragm is 0-1, and the corresponding cross-linking degree is <1%; the yellowness index of the diaphragm is 1-1.5, and the corresponding cross-linking degree is 1%-45%; the yellowness index of the diaphragm is 1.5-4, and the corresponding cross-linking degree is 45%-55%; the yellowness index of the diaphragm is 4-5, and the corresponding cross-linking degree is 55%-75%.

[0078] If the yellowness index of the diaphragm is greater than 1.5, it can be judged that the diaphragm has a certain degree of cross-linking. If the yellowness index is greater than 4, it indicates that the degree of cross-linking is very high.

[0079] In some embodiments of the present disclosure, an integrating sphere spectrophotometer is used to measure the yellowness index of the membrane.

[0080] In some embodiments of the present disclosure, the content of S element in the diaphragm is 90-300 ppm, and the corresponding cross-linking degree is 50%-62%; the content of S element in the diaphragm is 300-500 ppm, and the corresponding cross-linking degree is 62%-70%; the content of S element in the diaphragm is 500-700 ppm, and the corresponding cross-linking degree is 70%-73%; the content of S element in the diaphragm is 700-1000 ppm, and the corresponding cross-linking degree is 73%-75%.

[0081] In some embodiments of the present disclosure, the content of S element in the separator is measured by inductively coupled plasma spectrometry (ICP) or X-ray fluorescence spectrometry.

[0082] The method for obtaining the cross-linking degree of the diaphragm according to the content of the S element in the diaphragm is applicable to the diaphragm using a thioxanthone photoinitiator during the preparation process.

[0083] Example 1

[0084] The method for preparing the diaphragm provided in this embodiment comprises the following steps:

[0085] S1. Mixing polyolefin and paraffin oil in a mass ratio of 3:1, performing banburying in sequence, kneading at 230° C. for 10 min, extruding, and cooling the cast sheet to obtain an oil-containing substrate. The oil-containing substrate is biaxially stretched (stretching ratio of 7 times in both directions), and extracted to obtain a substrate;

[0086] Among them, the thickness of the substrate is 8.6μm, the TMA membrane breaking temperature is 152℃, the impedance method membrane breaking temperature is 152℃, the cross-linking degree is 0.4%, and the porosity is 34%;

[0087] The polyolefin is polyethylene, the base material has an air permeability of 146s / 100cc, a pore size of 45nm, a needle punch strength per unit thickness of 45.9gf / μm, a thermal shrinkage rate MD at 130°C of 16%, and a thermal shrinkage rate TD at 130°C of 12.3%.

[0088] S2. Mix 2-isopropylthioxanthone, ethyl 4-dimethylaminobenzoate and dichloromethane to obtain a dipping solution; dip-coat the substrate with the dipping solution to ensure full contact for 5 seconds. After drying, irradiate with ultraviolet light for 10 seconds to obtain a diaphragm;

[0089] In the dipping solution, the content of 2-isopropylthioxanthone is 0.1 wt %, and the content of ethyl 4-dimethylaminobenzoate is 0.05 wt %.

[0090] Example 2

[0091] The preparation method of the diaphragm provided in this embodiment refers to that in Example 1, except that in step S2, the content of 2-isopropylthioxanthone in the dipping solution is 0.5 wt %, and the content of ethyl 4-dimethylaminobenzoate is 0.25 wt %.

[0092] Example 3

[0093] The preparation method of the diaphragm provided in this embodiment refers to that in Example 1, except that in step S2, the content of 2-isopropylthioxanthone in the dipping solution is 1 wt %, and the content of ethyl 4-dimethylaminobenzoate is 0.5 wt %.

[0094] Example 4

[0095] The method for preparing the diaphragm provided in this embodiment comprises the following steps:

[0096] S1. Mixing polyolefin and paraffin oil in a mass ratio of 3:1, performing banburying in sequence, kneading at 230° C. for 10 min, extruding, and cooling the cast sheet to obtain an oil-containing substrate. The oil-containing substrate is biaxially stretched (stretching ratio of 7 times in both directions), and extracted to obtain a substrate;

[0097] Among them, the thickness of the substrate is 8.6μm, the TMA membrane breaking temperature is 152℃, the impedance method membrane breaking temperature is 152℃, the cross-linking degree is 0.4%, and the porosity is 34%;

[0098] The polyolefin is polypropylene, the base material has an air permeability of 90s / 100cc, a pore size of 80nm, a needle punch strength per unit thickness of 43.5gf / μm, a thermal shrinkage rate MD at 130°C of 16.7%, and a thermal shrinkage rate TD at 130°C of 11%.

[0099] S2. Mix 2-isopropylthioxanthone, ethyl 4-dimethylaminobenzoate and dichloromethane to obtain a dipping solution; dip-coat the substrate with the dipping solution to ensure full contact for 5 seconds. After drying, irradiate with ultraviolet light for 10 seconds to obtain a diaphragm;

[0100] In the dipping solution, the content of 2-isopropylthioxanthone is 0.5 wt %, and the content of ethyl 4-dimethylaminobenzoate is 0.25 wt %.

[0101] Example 5

[0102] The preparation method of the diaphragm provided in this embodiment refers to that of Example 4, except that, in step S1, the thickness of the substrate is 8.6 μm, the TMA membrane rupture temperature is 152°C, the impedance rupture temperature is 152°C, the cross-linking degree is 0.4%, and the porosity is 34%;

[0103] The polyolefin is an ethylene propylene copolymer, the base material has an air permeability of 245s / 100cc, a pore size of 30nm, a needle punch strength per unit thickness of 45gf / μm, a thermal shrinkage MD at 130°C of 15%, and a thermal shrinkage TD at 130°C of 12%.

[0104] Example 6

[0105] The preparation method of the diaphragm provided in this embodiment refers to that in Example 5, except that 2-isopropylthioxanthone is replaced by benzophenone.

[0106] Example 7

[0107] The preparation method of the diaphragm provided in this embodiment refers to that of Example 5, except that 2-isopropylthioxanthone is replaced by photoinitiator 819.

[0108] Comparative Example 1

[0109] The preparation method of the diaphragm provided in this comparative example comprises the following steps:

[0110] The polyolefin and paraffin oil were mixed in a mass ratio of 3:1, and then subjected to banburying, kneading at 230°C for 10 minutes, extrusion, and cooling to form a sheet to obtain an oil-containing substrate. The oil-containing substrate was biaxially stretched (stretching ratio of 7 times in both directions) and extracted to obtain a diaphragm.

[0111] Among them, the thickness of the separator is 8.6μm, the TMA membrane rupture temperature is 152℃, the impedance method membrane rupture temperature is 152℃, the cross-linking degree is 0.4%, and the porosity is 34%;

[0112] The polyolefin is polyethylene, the membrane has an air permeability of 146s / 100cc, a pore size of 45nm, a needle puncture strength per unit thickness of 45.9gf / μm, a thermal shrinkage rate MD at 130°C of 16%, and a thermal shrinkage rate TD at 130°C of 12.3%.

[0113] Comparative Example 2

[0114] The preparation method of the diaphragm provided in this comparative example comprises the following steps:

[0115] S1. Mixing polyolefin and paraffin oil in a mass ratio of 3:1, performing banburying in sequence, kneading at 230° C. for 10 min, extruding, and cooling the cast sheet to obtain an oil-containing substrate. The oil-containing substrate is biaxially stretched (stretching ratio of 7 times in both directions), and extracted to obtain a substrate;

[0116] Among them, the thickness of the substrate is 8.6μm, the TMA membrane breaking temperature is 152℃, the impedance method membrane breaking temperature is 152℃, the cross-linking degree is 0.4%, and the porosity is 34%;

[0117] The polyolefin is polyethylene, the base material has an air permeability of 245s / 100cc, a pore size of 30nm, a needle punch strength per unit thickness of 45gf / μm, a heat shrinkage MD at 130°C of 15%, and a heat shrinkage TD at 130°C of 12%.

[0118] S2. Mix 2-isopropylthioxanthine and dichloromethane to obtain a dipping solution; dip-coat the above-mentioned dipping solution with the substrate to fully contact it for 5 seconds. After drying, irradiate with ultraviolet light for 10 seconds to obtain a diaphragm;

[0119] Wherein, in the dipping coating, the content of 2-isopropylthioxanthone is 0.5 wt %.

[0120] Comparative Example 3

[0121] The preparation method of the diaphragm provided in this comparative example refers to comparative example 2, except that, in the dip coating, the content of 2-isopropylthioxanthone is 1.5 wt %.

[0122] Comparative Example 4

[0123] The preparation method of the diaphragm provided in this comparative example refers to comparative example 2, except that the content of 2-isopropylthioxanthone in the dip coating is 3 wt %.

[0124] Test Example 1

[0125] The properties of the diaphragms prepared in Examples 1 to 7 and Comparative Examples 1 to 4 were tested, and the results are shown in Table 1.

[0126] Table 1

[0127] The performance of the diaphragms prepared in Example 2 and Comparative Example 1 was tested at different temperatures, and the results are shown in Table 2.

[0128] Table 2

[0129] Test Example 2

[0130] The test results of light transmittance and yellowness index of the diaphragms prepared in Examples 1 to 7 and Examples 1 to 4 are shown in Table 3.

[0131] Table 3

[0132] The contents of each element in Example 1, Example 2, Example 6, Example 7, Comparative Example 1 and Comparative Example 2 are shown in Table 4.

[0133] Table 4

[0134] In summary, the diaphragm provided by the present disclosure has a higher membrane rupture temperature and high-temperature strength.

[0135] The preparation method of the diaphragm disclosed herein greatly improves the initiation efficiency of the photoinitiator, increases the cross-linking degree, reduces the residual photoinitiator, and thus increases the membrane rupture temperature and high-temperature strength of the diaphragm by using a photoinitiator and a synergist in combination.

[0136] The method provided in the present disclosure for determining the degree of cross-linking of the diaphragm can quickly analyze the degree of cross-linking of the diaphragm according to the degree of yellowing of the diaphragm or the content of S element in the diaphragm; there is a correlation between the yellowness and the degree of cross-linking of the diaphragm disclosed in the present disclosure, and the effect of the cross-linking treatment of the diaphragm can be evaluated and detected online in real time, providing important parameters for process control. Industrial Applicability

[0137] The present disclosure provides a diaphragm and a method for preparing the same, as well as a method for determining the crosslinking degree of the diaphragm. The diaphragm disclosed herein has a higher rupture temperature and high-temperature strength.

Claims

1. A diaphragm, characterized in that: The TMA film breaking temperature of the diaphragm is ≥170°C, and the tensile strength is ≥2000kgf / cm 2 , elongation ≥100%.

2. The diaphragm according to claim 1, characterized in that The method comprises at least one of the following features (1) to (4); (1) The TMA membrane breaking temperature of the diaphragm is 195-260°C; (2) The puncture strength of the diaphragm is ≥40 gf / um; (3) The yellowness index of the diaphragm is 1% to 5%; (4) The cross-linking degree of the separator is 1% to 75%.

3. The diaphragm according to claim 1, characterized in that The TMA film breaking temperature of the diaphragm is 214-260°C.

4. The diaphragm according to claim 1, characterized in that The TMA film breaking temperature of the diaphragm is 220-260°C.

5. The diaphragm according to claim 2, characterized in that The method comprises at least one of the following features (1) to (3); (1) The yellowness index of the separator is 1 to 1.5, and the corresponding cross-linking degree is 1% to 45%; (2) The yellowness index of the separator is 1.5 to 4, and the corresponding cross-linking degree is 45% to 55%; (3) The yellowness index of the separator is 4 to 5, and the corresponding cross-linking degree is 55% to 75%.

6. The method for preparing a diaphragm according to any one of claims 1 to 5, characterized in that: The steps include: The polyolefin substrate is dip-coated and light-treated in sequence to obtain the diaphragm; wherein the dipping solution of the dipping coating comprises a photoinitiator, a synergist and an organic solvent.

7. The method for preparing a diaphragm according to claim 6, characterized in that: The photoinitiator includes at least one of benzophenone, thioxanthone photoinitiator and photoinitiator 819 .

8. The method for preparing a diaphragm according to claim 6, characterized in that: The photoinitiator includes a thioxanthone photoinitiator.

9. The method for preparing a diaphragm according to claim 6, characterized in that: The method comprises at least one of the following features (1) to (3); (1) The synergist includes ethyl 4-dimethylaminobenzoate; (2) The mass ratio of the photoinitiator to the synergist is 1:(1-10); (3) The mass ratio of the photoinitiator to the organic solvent is 1:(20-1000).

10. The method for determining the degree of crosslinking of the separator according to any one of claims 1 to 5, characterized in that: According to the yellowing degree of the separator, the cross-linking degree of the separator is obtained; or, The separator is cross-linked according to the content of the S element in the separator.

11. The method according to claim 10, characterized in that The yellowness index of the diaphragm is 0-1, and the corresponding cross-linking degree is less than 1%; the yellowness index of the diaphragm is 1-1.5, and the corresponding cross-linking degree is 1%-45%; the yellowness index of the diaphragm is 1.5-4, and the corresponding cross-linking degree is 45%-55%; the yellowness index of the diaphragm is 4-5, and the corresponding cross-linking degree is 55%-75%.

12. The method according to claim 10, characterized in that The content of S element in the diaphragm is 90-300 ppm, and the corresponding cross-linking degree is 50%-62%; the content of S element in the diaphragm is 300-500 ppm, and the corresponding cross-linking degree is 62%-70%; the content of S element in the diaphragm is 500-700 ppm, and the corresponding cross-linking degree is 70%-73%; the content of S element in the diaphragm is 700-1000 ppm, and the corresponding cross-linking degree is 73%-75%.

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