Method for testing hole closing temperature of battery diaphragm

Abstract

The invention discloses a method for testing the hole closing temperature of a battery diaphragm, which is applied to the technical field of batteries, and comprises the following steps: clamping a diaphragm sample between protective films, putting the protective films into a drying box at a preset temperature, and carrying out heat preservation for a first preset time length; the protective film has no tension interference on the diaphragm sample; after a first preset duration is reached, taking out the diaphragm sample, and standing until the temperature reaches the room temperature; the air permeability of the diaphragm sample cooled to the room temperature is measured, and then the step of heating the drying box to the preset temperature until the air permeability of the diaphragm sample cooled to the room temperature is measured is repeatedly executed till the numerical value of the air permeability changes suddenly. The diaphragm sample is isolated and protected during heating by using the protective film without tension interference on the diaphragm sample, so that the diaphragm sample is not subjected to mechanical interference of the protective film in the heating process, the diaphragm sample is ensured not to influence closed pores due to stress in the heating process, and the accuracy of a measurement result is ensured.

Description

Technical Field

[0001] The invention relates to the technical field of batteries, and in particular to a method for testing the closed-cell temperature of a battery diaphragm. Background Art

[0002] In order to avoid over-discharge or over-charge of the battery due to improper use, a triple protection mechanism is set up in the single lithium-ion battery. First, a switch element is used. When the temperature inside the battery rises, its resistance value rises accordingly. When the temperature is too high, it will automatically stop supplying power. Second, the appropriate separator material is selected. When the temperature rises to a certain value, the micron-level micropores on the separator will automatically dissolve, so that lithium ions cannot pass through and the internal reaction of the battery stops. Third, a safety valve (the vent hole on the top of the battery) is set. When the internal pressure of the battery rises to a certain value, the safety valve automatically opens to ensure the safety of the battery.

[0003] Sometimes, although the battery itself has safety control measures, the control fails for some reason, there is a lack of a safety valve or the gas does not have time to be released through the safety valve, the internal pressure of the battery will rise sharply and cause an explosion.

[0004] To improve the performance of existing safety control technology, we must first improve the safety performance of lithium-ion battery cells, which is particularly important for large-capacity batteries. Choose a diaphragm with good thermal shutdown performance. The function of the diaphragm is to allow lithium ions to pass through while isolating the positive and negative electrodes of the battery. When the temperature rises, the diaphragm is closed before it melts, so that the internal resistance rises to 2000 ohms and the internal reaction stops.

[0005] At present, the separators used in lithium-ion batteries generally provide an additional function, which is thermal shutdown. The closed-cell temperature, an important parameter reflecting the heat resistance and thermal safety performance of the separator, refers to the temperature at which the separator is closed and a short circuit is formed inside the battery, preventing the internal temperature of the battery from further rising due to excessive internal current, which may cause safety hazards. This feature can provide an additional safety protection for lithium-ion batteries. Therefore, how to provide a test method that can accurately measure the closed-cell temperature of the separator is an urgent problem that technicians in this field need to solve. Summary of the invention

[0006] The purpose of the present invention is to provide a method for testing the closed-cell temperature of a battery diaphragm, which can accurately measure the closed-cell temperature of the diaphragm.

[0007] In order to solve the above technical problems, the present invention provides a method for testing the closed-cell temperature of a battery separator, comprising:

[0008] Cutting the diaphragm to be tested into diaphragm samples of preset sizes;

[0009] Heat the drying oven to the preset temperature;

[0010] The membrane sample is sandwiched between the protective film and placed in a drying oven at the preset temperature for a first preset time period; the protective film has no tension interference on the membrane sample;

[0011] After reaching the first preset time, taking out the membrane sample and leaving it to stand until the temperature reaches room temperature;

[0012] The air permeability of the membrane sample cooled to room temperature is measured, and then the current preset temperature is increased to the next preset temperature, and the steps of increasing the temperature of the drying oven to the preset temperature and measuring the air permeability of the membrane sample cooled to room temperature are repeated, at least until the value of the air permeability changes suddenly.

[0013] Optionally, the protective film comprises dry paper.

[0014] Optionally, the size of the protective film is larger than the size of the diaphragm sample.

[0015] Optionally, the thickness of the protective film is not greater than 20 μm.

[0016] Optionally, the unevenness of the surface of the protective film is no more than 0.5 mm.

[0017] Optionally, the difference between the current preset temperature and the next preset temperature is not greater than 1°C.

[0018] Optionally, the first preset duration is not less than 30 minutes.

[0019] Optionally, heating the drying oven to a preset temperature includes:

[0020] The drying oven is heated to a preset temperature and maintained for a second preset time period; the second preset time period is not less than 5 minutes.

[0021] Optionally, after cutting the diaphragm to be tested into diaphragm samples of a preset size, the method further includes:

[0022] The initial air permeability of the membrane sample was measured.

[0023] Optionally, after taking out the membrane sample and leaving it to stand until the temperature reaches room temperature, the method further comprises:

[0024] Determining whether the morphology of the membrane sample meets the measurement requirements of the air permeability tester;

[0025] The method of measuring the air permeability of the membrane sample cooled to room temperature comprises:

[0026] When the morphology of the membrane sample meets the measurement requirements of the air permeability tester, the air permeability of the membrane sample cooled to room temperature is measured by the air permeability tester.

[0027] A method for testing the closed-cell temperature of a battery diaphragm provided by the present invention comprises: cutting a diaphragm to be tested into diaphragm samples of a preset size; heating a drying oven to a preset temperature; sandwiching the diaphragm sample between protective films and placing it in a drying oven at the preset temperature for insulation for a first preset time; the protective film has no tension interference on the diaphragm sample; after the first preset time is reached, taking out the diaphragm sample and letting it stand until the temperature reaches room temperature; measuring the air permeability of the diaphragm sample cooled to room temperature, then raising the current preset temperature to the next preset temperature, and repeating the steps of heating the drying oven to the preset temperature to measuring the air permeability of the diaphragm sample cooled to room temperature, at least until the value of the air permeability changes suddenly.

[0028] By using a protective film that does not have tension interference on the diaphragm sample to isolate and protect the diaphragm sample during heating, the diaphragm sample will not be mechanically disturbed by the protective film during the heating process, thereby ensuring that the diaphragm sample will not be affected by the force on its closed pores during the heating process, thereby ensuring the accuracy of the measurement results. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] In order to more clearly illustrate the embodiments of the present invention or the technical solutions of the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0030] FIG1 is a flow chart of a method for testing the closed-cell temperature of a battery separator provided by an embodiment of the present invention;

[0031] FIG2 is a flow chart of a specific method for testing closed-cell temperature of a battery separator provided in an embodiment of the present invention;

[0032] FIG. 3 is a graph showing changes in air permeability of the membrane samples of Example 1 and Example 2 of the present invention as a function of temperature. DETAILED DESCRIPTION

[0033] The core of the present invention is to provide a method for testing the closed-cell temperature of a battery diaphragm. In the prior art, diaphragms for lithium-ion batteries are generally able to provide an additional function, which is thermal shutdown. The closed-cell temperature, an important parameter reflecting the heat resistance and thermal safety performance of the diaphragm, refers to the temperature at which the diaphragm is closed and a short circuit is formed inside the battery, preventing the internal temperature of the battery from further rising due to excessive internal current, which could cause safety hazards. This feature can provide an additional safety protection for lithium-ion batteries, but in the prior art, there is a large difference between the measurement of the closed-cell temperature of the diaphragm and the actual closed-cell temperature of the diaphragm, so how to provide a test method that can accurately measure the closed-cell temperature of the diaphragm is a problem that those skilled in the art urgently need to solve.

[0034] The present invention provides a method for testing the closed-cell temperature of a battery diaphragm, comprising: cutting the diaphragm to be tested into diaphragm samples of a preset size; heating a drying oven to a preset temperature; sandwiching the diaphragm sample between protective films and placing it in a drying oven at the preset temperature for insulation for a first preset time; the protective film has no tension interference on the diaphragm sample; after the first preset time is reached, taking out the diaphragm sample and letting it stand until the temperature reaches room temperature; measuring the air permeability of the diaphragm sample cooled to room temperature, then raising the current preset temperature to the next preset temperature, and repeating the steps of heating the drying oven to the preset temperature to measuring the air permeability of the diaphragm sample cooled to room temperature, at least until the value of the air permeability changes suddenly.

[0035] By using a protective film that does not have tension interference on the diaphragm sample to isolate and protect the diaphragm sample during heating, the diaphragm sample will not be mechanically disturbed by the protective film during the heating process, thereby ensuring that the diaphragm sample will not be affected by the force on its closed pores during the heating process, thereby ensuring the accuracy of the measurement results.

[0036] In order to enable those skilled in the art to better understand the scheme of the present invention, the present invention is further described in detail below in conjunction with the accompanying drawings and specific implementation methods. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0037] Please refer to FIG. 1 , which is a flow chart of a method for testing closed-cell temperature of a battery separator provided in an embodiment of the present invention.

[0038] Referring to FIG. 1 , in an embodiment of the present invention, a method for testing the closed-cell temperature of a battery separator includes:

[0039] S101: Cut the diaphragm to be tested into diaphragm samples of preset sizes.

[0040] In this step, the membrane to be tested needs to be cut first, and a membrane of a preset size is cut out as a membrane sample. The size of the membrane sample is not specifically limited in this embodiment and can be determined according to the size of the drying oven and the membrane air permeability meter used subsequently.

[0041] It should be noted that the cutting tool used in this step should be sharp enough to maintain the flatness and integrity of the membrane sample and avoid damage to the structure of the membrane sample during the cutting process.

[0042] S102: Raise the temperature of the drying oven to a preset temperature.

[0043] The drying oven used in this step is usually an adjustable temperature drying oven, that is, the heating temperature can be adjusted according to the setting, so that the membrane sample can be heated at different temperatures to measure its closed-cell temperature later.

[0044] In this step, the drying oven needs to be controlled to raise the temperature inside to a preset temperature and maintain it for a certain period of time, so that the membrane sample can be placed in the drying oven for heating later. The drying oven used in this embodiment can be a vacuum drying oven, and the specific type of the drying oven can be set according to actual conditions, and is not specifically limited here.

[0045] S103: sandwiching the membrane sample between the protective films and placing it in a drying oven at a preset temperature for a first preset time.

[0046] In this embodiment, the protective film has no tension interference on the diaphragm sample. That is, the protective film will not interfere with the deformation of the diaphragm sample, such as shrinkage, curling, etc., when the diaphragm sample is heated in a drying oven, that is, the diaphragm sample can be deformed freely between the protective films due to temperature changes, thereby not interfering with the closed-cell behavior of the diaphragm sample.

[0047] Specifically, in this embodiment, the protective film may specifically include dry paper, that is, in this embodiment, the membrane sample may be placed between dry paper and heated in a drying oven. The protective film mainly plays a certain role of isolation and protection, such as preventing foreign particles from falling on the surface of the membrane sample, etc. Preferably, the size of the protective film is usually larger than the size of the membrane sample, ensuring that the protective film can completely cover the membrane sample when the membrane sample is sandwiched between the protective films, avoiding exposure of the membrane sample, thereby increasing the isolation and protection performance of the protective film.

[0048] In this step, the diaphragm sample needs to be sandwiched between the protective films and placed in a drying oven at a preset temperature for insulation for a first preset time. That is, the diaphragm sample is heated by the drying oven at a preset temperature, and the insulation duration reaches the first preset time to ensure that the diaphragm sample has been sufficiently heated and has sufficient time to close the pores after the preset temperature reaches the closing temperature.

[0049] S104: After reaching the first preset time, take out the membrane sample and let it stand until the temperature reaches room temperature.

[0050] In this step, the heated membrane sample is taken out from the drying oven and allowed to stand at room temperature, which is usually 20° C., so as to facilitate subsequent measurement of the heated membrane sample.

[0051] S105: measuring the air permeability of the membrane sample cooled to room temperature.

[0052] After this step, the current preset temperature is increased to the next preset temperature, and the steps of increasing the temperature of the drying oven to the preset temperature and measuring the air permeability of the membrane sample cooled to room temperature are repeated, at least until the air permeability value suddenly changes.

[0053] In this step, an air permeability tester is required to test the above-mentioned membrane sample to measure its air permeability. After the air permeability is measured, it is necessary to record it so that the closed-cell temperature of the membrane sample can be determined based on the recorded air permeabilities. The above-mentioned air permeability tester can use an existing air permeability tester, and its specific structure can refer to the prior art, which will not be described in detail here.

[0054] After this step, it is necessary to increase the current preset temperature set in the above S102 during this round of processing to the next preset temperature, and repeat the above steps S102 to S105. That is, in this embodiment, the temperature in the drying oven will be gradually increased, and the diaphragm sample will be heated after the temperature in the drying oven is increased each time, and the air permeability of the heated diaphragm sample will be measured. When the value of the air permeability changes suddenly, it means that the diaphragm sample has closed pores at this temperature. Therefore, when the value of the air permeability changes suddenly, the preset temperature of the corresponding drying oven is the closed pore temperature of the diaphragm to be tested.

[0055] Specifically, the thickness of the protective film in this embodiment is usually not more than 20μm, to ensure that the temperature in the drying oven can be transferred to the diaphragm sample as soon as possible to heat the diaphragm sample. The thickness of the above-mentioned protective film can be 2μm, 5μm, 7μm, 9μm, 10μm, 13μm, 15μm, 17μm, etc., which are not specifically limited here. In order to avoid the protective film from affecting the diaphragm sample when it is in contact with the diaphragm sample, especially during the heating process, the unevenness of the surface of the protective film in this embodiment is usually required to be no more than 0.5mm, that is, the unevenness of the surface of the protective film is usually required to be between 0 and 0.5mm, to avoid the protective film from affecting the structure of the surface of the diaphragm sample when it is in contact with the diaphragm sample.

[0056] In order to ensure the accuracy of the measurement in this embodiment, the difference between the current preset temperature and the next preset temperature is usually not greater than 1°C. That is, the drying oven performs the above steps of heating the diaphragm sample and measuring its air permeability after each temperature increase of up to 1°C, so as to accurately obtain the closed-cell temperature of each diaphragm sample. In this embodiment, the above first preset time is not less than 30 minutes. That is, when using a drying oven to heat the diaphragm sample, it is usually necessary to keep it warm at the preset temperature for at least 30 minutes to ensure that the drying oven fully heats the diaphragm sample, and when the preset temperature is not less than the closed-cell temperature, the diaphragm sample undergoes closed-cell operation.

[0057] In the present embodiment, the position where the above-mentioned air permeability value suddenly changes is usually the inflection point in the curve composed of various air permeability values ​​measured with the preset temperature rising. Determining whether the air permeability value suddenly changes can be done by drawing a curve with various air permeability values ​​measured with the preset temperature rising, judging the inflection point based on the slope of the curve at different positions, determining the inflection point position as the position where the air permeability value suddenly changes, and determining the preset temperature of the inflection point as the closed-cell temperature of the diaphragm to be tested; it can also be that when recording each air permeability value, the difference between adjacent air permeability values ​​or a certain temperature range, such as the difference between air permeability values ​​within 5°C, is calculated. When the above-mentioned difference changes greatly, for example, the latter difference is 3 to 5 times the former difference, then the preset temperature corresponding to the latter difference is determined to be the closed-cell temperature; it can also be that when recording each air permeability value, the preset temperature corresponding to the first air permeability value greater than the target permeability, such as the first air permeability value greater than 1000 s / 100cc, is used as the temperature at which the air permeability value suddenly changes, and the temperature is determined as the closed-cell temperature. The specific standards for determining the sudden change in the air permeability value can be set according to actual conditions and are not specifically limited here.

[0058] It should be noted that, in this embodiment, when measuring the closed-cell temperature, the instruments used are a simple temperature-adjustable drying oven and a conventional membrane air permeability tester, so that accurate measurement of the closed-cell temperature of the membrane to be tested can be achieved under the premise of low requirements on the instrument.

[0059] The present embodiment provides a method for testing the closed-cell temperature of a battery diaphragm. The diaphragm sample is isolated and protected during heating by using a protective film that does not interfere with the diaphragm sample through tension. The diaphragm sample will not be mechanically disturbed by the protective film during the heating process, thereby ensuring that the closed-cell diaphragm sample will not be affected by the force during the heating process, thereby ensuring the accuracy of the measurement result.

[0060] The specific contents of the method for testing the closed-cell temperature of a battery separator provided by the present invention will be described in detail in the following invention embodiments.

[0061] Please refer to FIG. 2 , which is a flow chart of a specific battery separator closed-cell temperature testing method provided in an embodiment of the present invention.

[0062] Referring to FIG. 2 , in an embodiment of the present invention, a method for testing the closed-cell temperature of a battery separator includes:

[0063] S201: Cutting the diaphragm to be tested into diaphragm samples of preset sizes.

[0064] This step is basically the same as S101 in the above embodiment. Please refer to the above embodiment for details, and no further description will be given here.

[0065] S202: Measure the initial air permeability of the membrane sample.

[0066] In this step, before heating the membrane sample, the membrane sample is measured for air permeability as the initial air permeability using an air permeability tester. The initial air permeability can be used as one of the criteria for determining whether a sudden change occurs in the subsequent air permeability value. For example, when determining whether a sudden change occurs in the air permeability value, the first air permeability value that is ten times greater than the initial air permeability value can be determined as the starting point of the sudden change in the air permeability value.

[0067] In this embodiment, no matter when the air permeability of the membrane sample is measured, including the initial air permeability, the air permeability of the membrane sample can be more accurately measured by measuring multiple times and then taking the average value. For example, the air permeability of the membrane sample is measured three times, and the average value of the three values ​​is taken as the actual air permeability of the membrane sample.

[0068] S203: Raise the temperature of the drying oven to a preset temperature and maintain it for a second preset time period.

[0069] In this embodiment, the second preset time is not less than 5 minutes. In order to ensure that the drying box has been heated to the preset temperature, this step will keep the drying box at the preset temperature for a second preset time after the drying box displays that the temperature has been heated to the preset temperature. The second preset time is usually not less than 5 minutes to ensure that the temperature inside the drying box is heated to the preset temperature.

[0070] S204: sandwiching the membrane sample between the protective films and placing it in a drying oven at a preset temperature for a first preset time.

[0071] This step is basically the same as S103 in the above embodiment. Please refer to the above embodiment for details, which will not be described again. It should be noted that in this embodiment, membrane samples of various materials can be heated at the same time.

[0072] S205: Determine whether the morphology of the membrane sample meets the measurement requirements of the air permeability tester.

[0073] In this step, it is necessary to first determine whether the morphology of the diaphragm sample meets the measurement requirements of the permeability tester. Only after it meets the requirements will its permeability be measured in the subsequent steps. In this step, it is necessary to observe the changes in the appearance of the diaphragm sample, including its shrinkage and curling. When the shrinkage and curling are too large, it may make it impossible to measure. What's more, when the preset temperature is too high, it may cause carbonization of the diaphragm sample. At this time, since the appearance of the diaphragm sample has changed significantly, its performance has obviously changed significantly, so there is no need to measure its closed-cell temperature.

[0074] S206: When the morphology of the membrane sample meets the measurement requirements of the air permeability tester, the air permeability of the membrane sample cooled to room temperature is measured by the air permeability tester.

[0075] In this step, the air permeability of the membrane sample is measured after the appearance of the membrane sample does not change significantly. The specific measurement of the air permeability has been described in detail in the above embodiments, and will not be repeated here.

[0076] The present embodiment provides a method for testing the closed-cell temperature of a battery diaphragm. The diaphragm sample is isolated and protected during heating by using a protective film that does not interfere with the diaphragm sample through tension. The diaphragm sample will not be mechanically disturbed by the protective film during the heating process, thereby ensuring that the closed-cell diaphragm sample will not be affected by the force during the heating process, thereby ensuring the accuracy of the measurement result.

[0077] Please refer to FIG. 3 , which is a graph showing changes in air permeability of the membrane samples of Example 1 and Example 2 of the present invention as a function of temperature.

[0078] Referring to FIG. 3 , two specific embodiments are provided below, both of which measure the closed-cell temperature of the diaphragm to be measured through the above process.

[0079] Example 1

[0080] The first step is to take a 16μm thick PP (polypropylene) material diaphragm and a 16μm thick PE (polyethylene) ceramic-coated diaphragm and cut them into a size of 400mm×190mm.

[0081] The second step is to open the membrane air permeability meter, test the air permeability of the two material membranes respectively, and record the data (test three times and take the average of the three values).

[0082] Step 3: Heat the drying oven to the preset temperature of 100°C (temperature rises by 1°C each time the test is conducted) and maintain the temperature constant for 5 minutes.

[0083] Step 4: sandwich the cut samples of the two materials between flat dry papers, put them in a drying oven at a preset temperature, and keep them warm for 30 minutes.

[0084] Step 5. After the specified time, take out the diaphragm samples, let them stand at room temperature, and observe the changes in the appearance (shrinkage and curling) of the two types of diaphragm samples.

[0085] Step 6: Test the air permeability of the two materials and record the data (test three times and take the average of the three values). Repeat the cycle from step 3 until the air permeability value suddenly changes and the test is terminated. Some specific test values ​​are shown in Table 1 below.

[0086] Example 2

[0087] The first step is to take a 20μm thick PP (polypropylene) material diaphragm and a 12μm thick PE (polyethylene) ceramic-coated diaphragm and cut them into a size of 400mm×190mm.

[0088] The second step is to open the membrane air permeability meter, test the air permeability of the two material membranes respectively, and record the data (test three times and take the average of the three values).

[0089] Step 3: Heat the drying oven to the preset temperature of 100°C (temperature rises by 1°C each time the test is conducted) and maintain the temperature constant for 5 minutes.

[0090] Step 4: sandwich the cut samples of the two materials between flat dry papers, put them in a drying oven at a preset temperature, and keep them warm for 30 minutes.

[0091] Step 5. After the specified time, take out the diaphragm samples, let them stand at room temperature, and observe the changes in the appearance (shrinkage and curling) of the two types of diaphragm samples.

[0092] Step 6: Test the air permeability of the two materials and record the data (test three times and take the average of the three values). Repeat the cycle from step 3 until the air permeability value suddenly changes and the test is terminated. Some specific test values ​​are shown in Table 1.

[0093] Table 1: Raw data of closed cell temperature of tested diaphragm samples

[0094] It can be seen from the above table that the closed-cell temperature of diaphragms of the same material but different thicknesses is the same, and the thickness mainly affects the air permeability of the same material.

[0095] The various embodiments in this specification are described in a progressive manner, and each embodiment focuses on the differences from other embodiments. The same or similar parts between the various embodiments can be referenced to each other.

[0096] Finally, it should be noted that, in this article, relational terms such as first and second, etc. are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the terms "include", "comprise" or any other variants thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device. In the absence of further restrictions, the elements defined by the sentence "comprise a ..." do not exclude the presence of other identical elements in the process, method, article or device including the elements.

[0097] The above is a detailed introduction to a method for testing the closed-cell temperature of a battery diaphragm provided by the present invention. This article uses specific examples to illustrate the principles and implementation methods of the present invention. The description of the above embodiments is only used to help understand the method of the present invention and its core idea. It should be pointed out that for ordinary technicians in this technical field, without departing from the principles of the present invention, the present invention can also be improved and modified in a number of ways, and these improvements and modifications also fall within the scope of protection of the claims of the present invention.

Claims

1. A method for testing the closed-cell temperature of a battery separator, characterized in that: include: Cutting the diaphragm to be tested into diaphragm samples of preset sizes; Heat the drying oven to the preset temperature; The membrane sample is sandwiched between the protective film and placed in a drying oven at the preset temperature for a first preset time; the protective film has no tension interference on the membrane sample; After reaching the first preset time, taking out the membrane sample and leaving it to stand until the temperature reaches room temperature; The air permeability of the membrane sample cooled to room temperature is measured, and then the current preset temperature is increased to the next preset temperature, and the steps of increasing the temperature of the drying oven to the preset temperature and measuring the air permeability of the membrane sample cooled to room temperature are repeated, at least until the value of the air permeability changes suddenly.

2. The method according to claim 1, characterized in that The protective film includes dry paper.

3. The method according to claim 2, characterized in that The size of the protective film is larger than that of the diaphragm sample.

4. The method according to claim 2, characterized in that: The thickness of the protective film is not greater than 20 μm.

5. The method according to claim 2, characterized in that: The unevenness of the surface of the protective film is no greater than 0.5 mm.

6. The method according to claim 1, characterized in that The difference between the current preset temperature and the next preset temperature is no greater than 1°C.

7. The method according to claim 1, characterized in that The first preset time length is not less than 30 minutes.

8. The method according to claim 1, characterized in that Heating the oven to a preset temperature involves: The drying oven is heated to a preset temperature and maintained for a second preset time period; the second preset time period is not less than 5 minutes.

9. The method according to claim 1, characterized in that: After the diaphragm to be tested is cut into diaphragm samples of preset sizes, the following steps are also included: The initial air permeability of the membrane sample was measured.

10. The method according to claim 1, characterized in that After taking out the membrane sample and leaving it to stand until the temperature reaches room temperature, the method further comprises: Determining whether the morphology of the membrane sample meets the measurement requirements of the air permeability tester; The method of measuring the air permeability of the membrane sample cooled to room temperature comprises: When the morphology of the membrane sample meets the measurement requirements of the air permeability tester, the air permeability of the membrane sample cooled to room temperature is measured by the air permeability tester.

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