Testing device and testing method for porosity of battery electrode sheet

Abstract

The present invention discloses a test device for the porosity of a battery electrode sheet, which is used to test the porosity of the battery electrode sheet. The test device includes a container for containing a liquid and a weighing mechanism for respectively weighing the weight of the battery electrode sheet when it is completely immersed in the liquid and after it is dried. The liquid is an organic solvent that has a contact angle less than 90° with the battery electrode sheet, does not react with the electrolyte and lithium salt in the battery electrode sheet, and can dissolve the lithium salt in the battery electrode sheet. The present invention also discloses a test method for the porosity of a battery electrode sheet, which is applied to the test device for the porosity of a battery electrode sheet as described above, and includes steps S1 to S6. Step S3 is to take out the battery electrode sheet from the organic solvent, dry it, and then measure the weight m2 with the weighing mechanism; step S4 is to calculate the volume V1 according to the shape of the battery electrode sheet; step S6 is to calculate the porosity of the battery electrode sheet according to the formula, where ρ is the density of the organic solvent at room temperature.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium-ion batteries, and particularly relates to a test device and a test method for the porosity of battery electrodes. Background Art

[0002] At present, the new energy vehicle field has developed very rapidly, and its market share has been continuously increasing, leading to a rapid growth in the production and installed capacity of power lithium-ion batteries. However, in recent years, the frequent occurrence of electric vehicle combustion incidents has increased consumers' concerns about the safety of electric vehicles, reflecting the importance of battery safety. Battery failure analysis is one of the important methods to improve battery safety. Disassembling fresh batteries, cycled batteries or failed batteries, and performing work such as testing the porosity and thickness of electrodes and analyzing the composition and structure of electrode active substances are important contents of battery failure analysis.

[0003] The electrodes of lithium-ion batteries are generally porous electrode structures. Generally, the porosity refers to the ratio of the total volume of interconnected tiny pores in a porous medium to the total volume of the porous medium. The electrode porosity reflects the compactness between active material particles and is an important parameter affecting the lithium-ion transport performance in the electrode. If the porosity of the electrode is too low, the diffusion impedance of lithium ions in the electrode increases, resulting in a larger internal resistance of the battery.

[0004] However, the positive and negative electrodes of charged lithium-ion batteries have high reactivity and are very sensitive to moisture in the air, increasing the difficulty and danger of testing. For example, when the highly reactive negative electrode is exposed to an environment with high humidity, the active substances in the electrode react with the moisture in the air, which will damage the porous structure in the electrode and even cause a combustion reaction. The failed battery after cycling cannot be fully discharged, and there will still be a relatively large amount of remaining power in the battery, and the battery electrode still has high reactivity. In addition, the disassembled battery electrode contains a certain amount of electrolyte, and there are non-volatile lithium salts and other additive components in the electrolyte, which increases the difficulty of measuring the electrode porosity.

[0005] At present, the commonly used methods for testing the porosity of the test electrode are the mercury intrusion porosimetry and the organic solvent infiltration method. A mercury intrusion porosimeter is an instrument for measuring the physical properties of powders and solids through the mercury intrusion method. Principle: Mercury is non-wetting to most solid materials and requires an external pressure to enter the pores of the solid. The pore radius into which mercury is pressed is inversely proportional to the external pressure applied. The greater the external pressure, the smaller the pore radius that mercury can enter. By measuring the amount of mercury entering the pores under different external pressures, the pore volume of the corresponding pore size can be obtained, and finally the porosity of the electrode can be calculated. However, the method of using a mercury intrusion porosimeter to test the porosity of a highly active electrode has many operation steps, a long test time, and mercury has certain toxicity, which poses a certain hazard to the operator. After the test, both the sample and the waste mercury need to be treated. In addition, since mercury enters the pores of the sample under pressure (up to 30,000 psi) in the mercury intrusion method, the measured pores of the sample include ineffective pores that liquid cannot enter under normal pressure. Therefore, the measured conclusion deviates from the effective pore volume and porosity in the actual use environment of the lithium-ion battery.

[0006] The organic solvent infiltration method calculates the porosity by calculating the ratio of the total pore volume of the electrode to the true volume after being filled with the organic solvent. However, this method requires complex equipment such as a vacuum pump, a sealing instrument, and a true density meter. Common organic solvents have various problems that affect the test results and test efficiency, such as being difficult to evaporate and easily reacting with the residual electrolyte and lithium salt.

[0007] The foregoing description is for the purpose of providing general background information and does not necessarily constitute prior art. Summary of the Invention

[0008] The purpose of the present invention is to provide a test device and a test method for the porosity of a battery electrode with simple method, high efficiency, and accurate measurement.

[0009] The present invention provides a test device for the porosity of a battery electrode, which is used to test the porosity of the battery electrode. The test device includes a container for containing a liquid and a weighing mechanism for respectively weighing the weight of the battery electrode when it is completely immersed in the liquid and after it is taken out of the liquid and dried. The liquid is an organic solvent that has a contact angle less than 90° with the battery electrode, does not react with the electrolyte and lithium salt in the battery electrode, and can dissolve the lithium salt in the battery electrode.

[0010] Further, the organic solvent is one of propylene carbonate, dimethyl carbonate, and ethyl methyl carbonate.

[0011] Further, the weighing mechanism includes an electronic scale, a bracket fixed on the electronic scale, and a thin wire for suspending the battery electrode on the bracket.

[0012] The present invention also provides a method for testing the porosity of a battery electrode sheet, which is applied to the testing device for the porosity of the battery electrode sheet as described above, and includes steps S1 to S6. Step S1 is to cut the battery electrode sheet into a regular shape; Step S2 is to completely immerse the battery electrode sheet in the organic solvent in the container and use the weighing mechanism to measure the weight m1 of the battery electrode sheet in the organic solvent. The contact angle between the organic solvent and the battery electrode sheet is less than 90°, and it does not react with the electrolyte and lithium salt in the battery electrode sheet and can dissolve the lithium salt in the battery electrode sheet; Step S3 is to take out the battery electrode sheet from the organic solvent, dry it, and then use the weighing mechanism to measure the weight m2; Step S4 is to calculate the volume V1 according to the shape of the battery electrode sheet; Step S5 is to soak the battery electrode sheet in N-methylpyrrolidone solution to wash away the diaphragm, and only leave the foil. After drying, calculate its volume V2. Since the foil material is pure copper or pure aluminum with a known density, the foil volume V2 is the foil weight divided by the foil density; Step S6 is to calculate the porosity of the battery electrode sheet according to the formula where ρ is the density of the organic solvent at room temperature.

[0013] Further, the organic solvent in Step S2 is one of propylene carbonate, dimethyl carbonate, and ethyl methyl carbonate.

[0014] Further, Step S1 includes Step S11 and Step S12. Step S11 is to cut the battery electrode sheet into a regular shape; Step S12 is to use the same liquid as the organic solvent in Step S2 to wash the residual electrolyte and lithium salt in the battery electrode sheet.

[0015] Further, Step S12 is to immerse the battery electrode sheet in the organic solvent, change the solvent every hour, and repeat 3 times.

[0016] Further, the weighing mechanism includes a bracket, a thin wire, and an electronic scale. Step S2 includes Step S21 and Step S22. Step S21 is to fix the bracket on the electronic scale, connect one end of the thin wire to the bracket, and then zero the weight reading of the electronic scale; Step S22 is to tie the battery electrode sheet with the other end of the thin wire and immerse it in the organic solvent. When the battery electrode sheet is completely immersed in the organic solvent and does not contact the bottom and side walls of the container, record the reading m1 of the electronic scale.

[0017] Further, Step S3 is to take out the battery electrode sheet from the organic solvent, dry it for 12 h, and then record its weight m2.

[0018] Further, the step S1 is to cut a section of the battery electrode sheet in the shape of a cuboid; the step S4 is to measure the length, width and thickness of the battery electrode sheet and calculate the volume V1.

[0019] The test device for the porosity of the battery electrode sheet provided by the present invention uses the Archimedes drainage method to measure the true volume of the battery electrode sheet, has low requirements for the test equipment, few steps, high test efficiency, and basically does not damage the morphology and structure of the battery electrode sheet sample; uses an organic solvent as the wetting reagent to isolate moisture in the air and avoid violent reactions between the battery electrode sheet and water vapor; the organic solvent has a small contact angle with the battery electrode sheet and has a good wetting effect on the battery electrode sheet; the organic solvent has good volatility, and the wetted battery electrode sheet is easy to dry quickly; the organic solvent does not react with the electrolyte and lithium salt in the battery electrode sheet, and can dissolve the lithium salt to avoid the influence of lithium salt precipitation on the porosity test. Selecting one of propylene carbonate, dimethyl carbonate, and ethyl methyl carbonate as the organic solvent perfectly meets the above requirements. The test method for the porosity of the battery electrode sheet provided by the present invention avoids the influence of cleaning solution residue on the data by cleaning the battery electrode sheet with the same organic solvent as the wetting solution before the test; the organic solvent has good compatibility with the electrolyte and lithium salt, can remove the residual electrolyte and lithium salt in the battery electrode sheet sample, and improves the measurement accuracy; by using an N-methylpyrrolidone solution to clean the membrane sheet and then calculating the remaining foil volume and subtracting it in the subsequent calculation, it avoids the error caused by the foil with inconvenient thickness measurement in the porosity calculation and affects the accuracy of the calculation result. Description of the Drawings

[0020] Figure 1 It is a schematic structural diagram of the test device for the porosity of the battery electrode sheet in the embodiment of the present invention. Detailed Embodiments

[0021] The following combines the drawings and embodiments to further describe in detail the specific embodiments of the present invention. The following embodiments are used to illustrate the present invention, but are not used to limit the scope of the present invention.

[0022] Please refer to Figure 1 , the embodiment of the present invention provides a test device for the porosity of a battery electrode sheet, including a container 20, a battery electrode sheet 30, an organic solvent 40, a weighing mechanism, a steel ruler (not shown), and a glove box (not shown). The weighing mechanism includes a thin line 10, a bracket 50, and an electronic scale 60. The steel ruler, the thin line 10, the container 20, the battery electrode sheet 30, the organic solvent 40, the bracket 50, and the electronic scale 60 are all located in the glove box, and the glove box meets the humidity requirements or is protected by an inert gas.

[0023] The organic solvent 40 is contained in the container 20. The contact angle between the organic solvent 40 and the battery electrode sheet 30 is less than 90°, and it does not react with the electrolyte and lithium salt in the battery electrode sheet 30 and can dissolve the lithium salt in the battery electrode sheet 30. In this embodiment, the organic solvent 40 is propylene carbonate. In other embodiments, the organic solvent can also be dimethyl carbonate or ethyl methyl carbonate.

[0024] The electronic scale 60 is placed on the plane inside the glove box. The bracket 50 is fixed on the electronic scale 60. One end of the thin line is tied to the bracket 50, and the other end is tied to the battery electrode sheet 30 to suspend it. The electronic scale 60 is used to measure the weight of the battery electrode sheet 30 when it is completely immersed in the organic solvent 40 and does not contact the bottom and side walls of the container, and the weight of the battery electrode sheet 30 after it is taken out of the organic solvent 40 and dried. The steel ruler is used to measure the length, width and height dimensions of the battery electrode sheet 30.

[0025] The test device for the porosity of the battery electrode sheet provided by the present invention uses the Archimedes drainage method to measure the true volume of the battery electrode sheet 30. It has low requirements for test equipment, few steps, high test efficiency, and basically does not damage the morphology and structure of the battery electrode sheet 30 sample; uses the organic solvent 40 as the wetting reagent to isolate the moisture in the air and avoid violent reaction between the battery electrode sheet 30 and water vapor; the contact angle between the organic solvent 40 and the battery electrode sheet 30 is small, and the wetting effect on the battery electrode sheet 30 is good; the organic solvent 40 has good volatility, and the wetted battery electrode sheet 30 is easy to dry quickly; the organic solvent 40 does not react with the electrolyte and lithium salt in the battery electrode sheet 30 and can dissolve the lithium salt, avoiding the influence of lithium salt precipitation on the porosity test. Selecting propylene carbonate as the organic solvent 40 perfectly meets the above requirements.

[0026] This embodiment also provides a method for testing the porosity of a battery electrode sheet, which is applied to the above-mentioned test device. The method includes steps S1 to S6. Step S1 includes step S11 and step S12. Step S11 is to cut the battery electrode sheet 30 into a regular shape. In this embodiment, it is a cuboid. In other embodiments, it can also be cut into a cylinder, a triangular prism or other shapes that are convenient for calculating the outer contour volume.

[0027] Step S12 is to clean the residual electrolyte and lithium salt in the battery electrode sheet 30 with the propylene carbonate organic solvent 40. Specifically, the battery electrode sheet 30 is immersed in the propylene carbonate organic solvent 40, and the solvent is replaced every once in a while, and repeated many times. There is residual electrolyte and lithium salt inside the battery electrode sheet 30. Conventional drying methods can remove the electrolyte organic solvent, but it is difficult to remove the residual lithium salt, increasing the test error of the electrode sheet porosity. The propylene carbonate organic solvent 40 has good compatibility with the electrolyte and lithium salt and can remove the residual electrolyte and lithium salt in the electrode sheet sample. In other embodiments, dimethyl carbonate and ethyl methyl carbonate also have high solubility in lithium salt and can replace propylene carbonate.

[0028] Step S2 includes step S21 and step S22. Step S21 is to fix the bracket 50 on the electronic scale 60, connect one end of the thin wire 10 to the bracket 50, and then clear the weight reading of the electronic scale 60. Step S22 is to pour a certain amount of propylene carbonate organic solvent 40 into the container 20, tie the other end of the thin wire 10 to the battery electrode plate 30 and immerse it in the propylene carbonate organic solvent 40 in the container 20. When the battery electrode plate 30 is completely immersed in the propylene carbonate organic solvent 40 and does not contact the bottom and side walls of the container 20, record the reading m1 after the reading of the electronic scale 60 is stable.

[0029] According to the Young equation, the contact angle is the angle at the three-phase junction of solid, liquid, and gas, from the solid-liquid interface through the liquid interior to the gas interface, usually denoted by θ, and can be regarded as the result of the balance of the three interfacial tensions at the three-phase junction. The contact angle can be used as an important index to evaluate wettability. Generally, the smaller the contact angle, the better the wettability. Conventionally, θ > 90° indicates non-wetting; θ < 90° indicates wetting. Taking the sample of the negative electrode battery electrode plate 30 as an example, the contact angle test result of the propylene carbonate organic solvent 40 on the battery electrode plate 30 is 8.261°, and its contact angle is much smaller than 90°, proving that its wettability to the electrode plate is very good. Moreover, using the propylene carbonate organic solvent 40 as the wetting reagent can isolate the moisture in the air and prevent the battery electrode plate 40 from reacting violently with water vapor. The propylene carbonate organic solvent 40 has good volatility, and the wetted battery electrode plate 40 is easy to dry quickly.

[0030] The contact angles of dimethyl carbonate and ethyl methyl carbonate with the battery electrode plate 30 are also much smaller than 90°, and they can replace propylene carbonate. For other reagents with good wettability to the battery electrode plate 30, such as ethylene glycol, hexadecane, or N-methylpyrrolidone, some are not easy to evaporate, affecting the test efficiency, some will react with the residual electrolyte and lithium salt in the battery electrode plate 30, and some cannot dissolve the residual lithium salt, resulting in errors in the test results.

[0031] Step S3 is to take out the battery electrode plate 30 from the propylene carbonate organic solvent 40, air-dry it for 12 h, and then record the reading of the electronic scale as m2. In other embodiments, the soaking and cleaning time, cleaning times, and drying time of the battery electrode plate 30 can be adjusted according to actual needs. Step S4 is to measure the length, width, and thickness of the battery electrode plate 30 and calculate the volume V1. Since the shape of the battery electrode plate 30 cut is a cuboid, the volume V is equal to the length multiplied by the width multiplied by the thickness.

[0032] The battery electrode sheet 30 includes a foil (not shown) and a diaphragm (not shown) covering the foil. Since this method actually calculates the porosity of the diaphragm, the volume of the foil needs to be deducted. Step S5 is to soak the battery electrode sheet in an N-methylpyrrolidone (NMP) solution for 1 h, wash off the active material layer (i.e., the diaphragm) on the surface, leaving only the shiny foil, and calculate its volume V2 after drying. Since the foil material is pure copper (negative electrode) or pure aluminum (positive electrode) with a known density, the foil volume V2 is the foil weight divided by the foil density.

[0033] During the production process, the foil will be stretched to a certain extent and its thickness will change, that is, the factory size of the foil raw material is not the actual value. Moreover, the thickness of the foil is relatively thin, generally 6-14 microns, and it is difficult to accurately measure the thickness of the foil with tools such as micrometers and vernier calipers, resulting in a large error. Of course, in other embodiments, if the thickness change of the foil during the production process is not considered, in step S5, a clean foil with the same length and width as the measured sample electrode sheet can also be cut for weighing, or if the calculation accuracy is not considered, the foil volume can be directly calculated by size measurement.

[0034] Step S6 is to calculate the porosity of the battery electrode sheet 30 according to the formula where ρ is the density of the propylene carbonate organic solvent 40 at room temperature. The principle referred to in this method is the Archimedes' drainage method, that is, for the same object, the larger the volume in the liquid, the greater the buoyancy force. Since there are pores on the battery electrode sheet 30, its actual volume in the liquid is smaller than that of a solid cuboid, so the buoyancy force it receives is also smaller than that of a solid cuboid. In the formula, m2 - m1 can obtain the buoyancy force of the battery electrode sheet 30 in the liquid, and dividing by the liquid density gives the actual volume of the discharged liquid, that is, the actual volume of the battery electrode sheet 30. Subtracting the actual volume of the foil V2 from it, the actual volume of the diaphragm is obtained. Dividing the actual volume of the diaphragm by the outer contour volume of the diaphragm (the difference between the outer contour volume of the battery electrode sheet 30 and the foil volume) gives the ratio of the actual volume to the outer contour volume. Subtracting this volume ratio from 1 can obtain the porosity of the diaphragm.

[0035] This embodiment also provides parameter groups and control groups with different parameters for verification tests. The first group follows the above method. Among them, when cleaning the battery electrode sheet 30 with the propylene carbonate organic solvent 40, the solvent is replaced every 1 hour and repeated three times. The only difference between the second group and the first group is that the solvent is replaced every 0.5 hour during the soaking and cleaning process. The only difference between the third group and the first group is that the solvent is replaced every 2 hours during the soaking and cleaning process. The only difference between the fourth group and the first group is that the solvent is replaced every 3 hours during the soaking and cleaning process. The only difference between the fifth group and the first group is that the battery electrode sheet 30 is soaked and cleaned with ethanol. The only difference between the sixth group and the first group is that the battery electrode sheet 30 is not cleaned with a solvent.

[0036] Meanwhile, a control group was also set up and tested by mercury intrusion method. The specific steps were as follows: After disassembling the charged lithium-ion battery in the glove box, highly reactive battery electrodes were obtained. Take a section of the positive or negative battery electrode 30 and place it in the glove box, and let it stand for 12 hours until the residual electrolyte in the electrode has basically evaporated. Use a steel ruler to measure the length and width of the battery electrode 30 sample, and use a micrometer to measure the total thickness of the battery electrode 30. The thickness of the battery electrode 30 needs to subtract the thickness of the foil, and calculate the volume V1 of the battery electrode 30 = length * width * thickness. Use a mercury intrusion instrument to measure the total pore volume V2 of the battery electrode 30. The mercury intrusion instrument needs to be placed in an environment with a relative humidity of less than 5% to prevent the electrode sample from reacting violently with the moisture in the air. Calculate the porosity of the battery electrode 30 according to the formula, porosity

[0037] Repeat the above 7 groups of test groups for multiple tests and record the data. The data are shown in Table 1 (the sample is the positive electrode) and Table 2 (the sample is the negative electrode) below. It can be seen from the comparison between the sixth group and the control group that the experimental results of this embodiment are close to those of the mercury intrusion method, and the experimental results have good parallelism, indicating the reliability of this test method. It can be seen from the comparison between the fifth and sixth groups and the first, second, third, and fourth groups that if the battery electrode 30 is not cleaned with the propylene carbonate organic solvent 40, there will be more lithium salt residues in the battery electrode 30, reducing the internal pores of the battery electrode 30 and resulting in a lower measured porosity result. It can be seen from the comparison between the first group and the second, third, and fourth groups that soaking and cleaning the battery electrode 30 with the propylene carbonate organic solvent 40 for more than one hour each time can effectively remove the residual lithium salt in the battery electrode 30 and ensure the accuracy of the measurement result.

[0038]

[0039] Table 1

[0040]

[0041] Table 2

[0042] The test device for the porosity of the battery electrode provided by the present invention measures the true volume of the battery electrode 30 by the Archimedes drainage method. It has low requirements for the test equipment, few steps, high test efficiency, and basically does not damage the morphology and structure of the battery electrode 30 sample; an organic solvent 40 is used as the infiltration reagent to isolate moisture in the air and avoid violent reactions between the battery electrode 30 and water vapor; the organic solvent 40 has a small contact angle with the battery electrode 30 and has a good infiltration effect on the battery electrode 30; the organic solvent 40 has good volatility, and the infiltrated battery electrode 30 is easy to dry quickly; the organic solvent 40 does not react with the electrolyte and lithium salt in the battery electrode 30, and can dissolve the lithium salt to avoid the influence of lithium salt precipitation on the porosity test. Propylene carbonate is selected as the organic solvent 40, which perfectly meets the above requirements. The test method for the porosity of the battery electrode provided by the present invention avoids the influence of cleaning solution residue on the data by cleaning the battery electrode 30 with the same organic solvent 40 as the infiltration solution before the test; the organic solvent 40 has good compatibility with the electrolyte and lithium salt, can remove the residual electrolyte and lithium salt in the battery electrode 30 sample, and improve the measurement accuracy; by using an N-methylpyrrolidone solution to clean the diaphragm and then calculating the remaining foil volume and subtracting it in the subsequent calculation, it avoids the error caused by the foil with inconvenient thickness measurement in the porosity calculation of the battery electrode 30 and affects the accuracy of the calculation result.

[0043] In the drawings, for clarity, the dimensions and relative dimensions of layers and regions are exaggerated. It should be understood that when an element such as a layer, region, or substrate is referred to as "formed on", "disposed on", or "located on" another element, the element can be directly disposed on the other element, or there can also be an intermediate element. In contrast, when an element is referred to as "directly formed on" or "directly disposed on" another element, there is no intermediate element.

[0044] In this document, unless otherwise clearly specified and defined, the terms "mounted", "connected", and "connected" should be understood in a broad sense. For example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be directly connected, or indirectly connected through an intermediate medium, and can be the internal communication of two elements. For those of ordinary skill in the art, the specific meanings of the above terms can be understood according to specific situations.

[0045] In this document, the term "comprising", "including", or any other variation thereof is intended to cover non-exclusive inclusion. In addition to including the listed elements, it may also include other elements not specifically listed.

[0046] In this text, the orientation or positional relationship indicated by terms such as "upper", "lower", "front", "rear", "left", "right", "top", "bottom", "inner", "outer", "vertical", "horizontal", etc. is based on the orientation or positional relationship shown in the drawings, and is only for the sake of clarity in expressing the technical solution and convenience in description. Therefore, it should not be construed as a limitation to the present invention.

[0047] In this text, the sequential adjectives "first", "second", etc. used to describe elements are only for differentiating elements with similar attributes, and do not mean that the elements so described must be in a given order, or subject to limitations such as time, space, rank, or others.

[0048] In this text, unless otherwise specified, the meanings of "a plurality of" and "several" are two or more.

[0049] As described above, it is only the specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present invention can easily think of changes or substitutions, which should all be covered within the protection scope of the present invention. Therefore, the protection scope of the present invention should be subject to the protection scope of the claims.

[0050] The technical features of the above-described embodiments can be combined arbitrarily. For the sake of brevity in description, not all possible combinations of the technical features in the above-described embodiments are described. However, as long as there is no contradiction in the combination of these technical features, it should be considered as within the scope described in this specification.

Claims

1. A test device for the porosity of a battery electrode sheet, which is used to test the porosity of the battery electrode sheet. The battery electrode sheet includes a foil and a diaphragm covering the surface of the foil, and is characterized in that, It includes a container for holding a liquid and a weighing mechanism for respectively weighing the weight of the battery electrode plate when it is completely immersed in the liquid and after it is taken out of the liquid and dried. The liquid is an organic solvent that has a contact angle with the battery electrode plate less than 90°, does not react with the electrolyte and lithium salt in the battery electrode plate, and can dissolve the lithium salt in the battery electrode plate, so as to determine the porosity of the diaphragm according to the ratio of the measured actual volume of the diaphragm to the difference between the outer contour volume of the battery electrode plate and the volume of the foil. Among them, the actual volume of the diaphragm is determined according to the difference between the actual volume of the battery electrode plate and the actual volume of the foil. The actual volume of the battery electrode plate is determined according to the difference between the weight m2 measured by the weighing mechanism after being taken out of the organic solvent and dried and the weight m1 measured when the battery electrode plate is completely immersed in the organic solvent in the container, and the density of the liquid. The organic solvent is propylene carbonate.

2. The test device for the porosity of the battery electrode sheet according to claim 1, characterized in that, The weighing mechanism includes an electronic scale, a bracket fixed on the electronic scale, and a thin wire for suspending the battery electrode plate on the bracket.

3. A method for testing the porosity of a battery electrode sheet, which is applied to the testing device for the porosity of the battery electrode sheet according to any one of claims 1 to 2, and is characterized in that, Including steps S1 to S6. Step S1 is to cut the battery electrode sheet into a regular shape; Step S2 is to completely immerse the battery electrode sheet in the organic solvent in the container and use the weighing mechanism to measure the weight m1 of the battery electrode sheet in the organic solvent. The contact angle between the organic solvent and the battery electrode sheet is less than 90°, and it does not react with the electrolyte and lithium salt in the battery electrode sheet and can dissolve the lithium salt in the battery electrode sheet; Step S3 is to take out the battery electrode sheet from the organic solvent, dry it, and then use the weighing mechanism to measure the weight m2; Step S4 is to calculate the volume V1 according to the shape of the battery electrode sheet; Step S5 is to soak the battery electrode sheet in an N-methylpyrrolidone solution to clean off the diaphragm of the battery electrode sheet, leaving only the foil of the battery electrode sheet. After drying, calculate its volume V2. Since the foil material is pure copper or pure aluminum with a known density, the foil volume V2 is the foil weight divided by the foil density; Step S6 is to calculate the porosity of the diaphragm of the battery electrode sheet according to the formula where ρ is the density of the organic solvent at room temperature; The organic solvent is propylene carbonate.

4. The method for testing the porosity of a battery electrode sheet according to claim 3, characterized in that The step S1 includes step S11 and step S12. The step S11 is to cut the battery electrode plate into a regular shape; the step S12 is to clean the residual electrolyte and lithium salt in the battery electrode plate with the same liquid as the organic solvent in the step S2.

5. The method for testing the porosity of the battery electrode sheet according to claim 4, characterized in that, The step S12 is to immerse the battery electrode plate in the organic solvent and replace the solvent every hour, repeating 3 times.

6. The method for testing the porosity of the battery electrode sheet according to claim 3, wherein The weighing mechanism includes a bracket, a thin wire and an electronic scale. The step S2 includes step S21 and step S22. The step S21 is to fix the bracket on the electronic scale, connect one end of the thin wire to the bracket, and then zero the weight reading of the electronic scale; the step S22 is to tie the battery electrode plate with the other end of the thin wire and immerse it in the organic solvent, and record the reading m1 of the electronic scale when the battery electrode plate is completely immersed in the organic solvent and does not contact the bottom and side walls of the container.

7. The method for testing the porosity of a battery electrode sheet according to claim 3, characterized in that, The step S3 is to take out the battery electrode plate from the organic solvent, dry it for 12 h, and then record its weight m2.

8. The method for testing the porosity of a battery electrode sheet according to claim 3, characterized in that, The step S1 is to cut a section of the battery electrode plate in the shape of a cuboid; the step S4 is to measure the length, width and thickness of the battery electrode plate and calculate the volume V1.

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