Evaluation method for performance of base membrane in diaphragm and application

Abstract

The invention provides a method for evaluating the performance of a base membrane in a diaphragm and application. The method comprises the following steps: S1, preparing at least two diaphragms with the same material and structure, and taking at least one diaphragm as an initial sample; s2, bonding a first bonding layer on the surface of the coating of the initial sample, stripping the first bonding layer to obtain a bare basement membrane, and testing the air permeability d0 of the bare basement membrane; s3, carrying out heating treatment, pressurizing treatment or electrolyte soaking treatment on the other diaphragm to obtain a sample to be detected; s4, bonding a second bonding layer on the surface of the coating of the sample to be tested, stripping the second bonding layer to obtain a base film to be tested, and testing the air permeability dt of the base film to be tested; and S5, calculating the affected degree of the to-be-measured base film according to the formula (I). According to the evaluation method, damage to the base membrane in the stripping process can be reduced, the accuracy of the evaluation result is improved, meanwhile, the influence of different environmental conditions on the performance of the base membrane can be quantified, and therefore a powerful scientific basis is provided for improving the performance of the diaphragm.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium-ion batteries, and in particular, to a method for evaluating the performance of a base film in a separator and its application. Background Art

[0002] As one of the core components of a lithium-ion battery, the separator has a decisive influence on the performance of the battery. The performance of the separator mainly depends on two aspects: the base film and the coating. The performance of the base film affects the mechanical properties, air permeability, liquid absorption capacity, and thermal stability of the separator, etc. The air permeability of the separator is closely related to that of the base film. The air permeability of the base film directly affects the air permeability of the separator, thereby affecting the performance of the battery. Therefore, the air permeability of the base film is an important index for evaluating the performance of the separator.

[0003] The coating of the separator usually consists of materials such as alumina, boehmite, polyvinylidene fluoride (PVDF), polymethyl methacrylate (PMMA), etc. These materials have good mechanical properties and insulating properties, which can ensure the safety of the lithium-ion battery and improve the performance of the battery. However, the covering effect of the coating makes it complicated to directly observe and test the pore structure and performance of the base film.

[0004] Currently, the existing methods for evaluating the performance of the base film mainly involve testing after removing the coating of the separator by solvent wiping. However, during the actual wiping process, it is easy to scrape the coating material into the pores of the base film or artificially change the pore structure of the base film, resulting in the evaluation data being unable to truly reflect the performance changes of the base film under the influence of different environmental conditions.

[0005] Therefore, researching and developing a method for evaluating the performance of the base film in a separator is of great significance for accurately evaluating the degree of influence of the base film in the separator under different environmental conditions. Summary of the Invention

[0006] The main purpose of the present invention is to provide a method for evaluating the performance of a base film in a separator and its application, so as to solve the problem that the existing evaluation methods cannot accurately evaluate the performance changes of the base film in the separator under the influence of various environmental conditions.

[0007] To achieve the above object, on the one hand, the present invention provides a method for evaluating the performance of the base film in a separator, the method comprising: Step S1, preparing at least two separators with the same material and structure, taking at least one of the separators as an initial sample, the separator including a base film layer and a coating applied on at least one surface of the base film layer; Step S2, bonding a first adhesive layer to the surface of the coating in the initial sample away from its base film layer, and then peeling off the first adhesive layer to obtain a bare base film, and testing the air permeability d0 of the bare base film; wherein, the peeling force between the first adhesive layer and the coating in the initial sample is greater than the peeling force between the coating in the initial sample and its base film layer; Step S3, performing heat treatment, pressure treatment, or electrolyte immersion treatment on another separator to obtain a sample to be tested; Step S4, bonding a second adhesive layer to the surface of the coating in the sample to be tested away from its base film layer, and then peeling off the second adhesive layer to obtain a base film to be tested, and testing the air permeability d of the base film to be tested t ; wherein, the peeling force between the second adhesive layer and the coating in the sample to be tested is greater than the peeling force between the coating in the sample to be tested and its base film layer; Step S5, calculating the affected degree of the base film to be tested according to formula (I), wherein, I is the affected degree of the base film to be tested, d t is the air permeability of the base film to be tested, and d0 is the air permeability of the bare base film.

[0008] In the method for evaluating the performance of the base film in the separator provided in the present application, the first adhesive layer and the second adhesive layer are respectively used to separate the coating in the initial sample from its base film layer (limiting the peeling force between the two to meet the above relationship), and to separate the coating in the sample to be tested from its base film layer (limiting the peeling force between the two to meet the above relationship), and a bare base film and a base film to be tested are obtained. Compared with the solvent rubbing method, using the above method can reduce the damage of the base film during the peeling process and reduce the influence of human factors on the pore structure of the base film, thereby improving the accuracy of the evaluation results. In Step S3, performing heat treatment, pressure treatment or electrolyte immersion treatment on the separator can simulate the conditions in the actual working environment of the lithium-ion battery, thereby comprehensively evaluating the performance changes of the base film under the influence of various environmental conditions. In Step S5, through formula (I), the affected degree of the base film to be tested under specific environmental conditions can be calculated, and by comparing the air permeabilities of the bare base film and the base film to be tested, the influence of different environmental conditions on the performance of the base film is quantified, thereby providing a strong scientific basis for improving the performance of the separator and the electrochemical performance and safety of the lithium-ion battery.

[0009] Further, the adhesion strength of the first adhesive layer and the second adhesive layer is independently ≥800 N / m, preferably 800 - 1000 N / m.

[0010] Compared with other ranges, limiting the adhesion strength of the first adhesive layer and the second adhesive layer within the above range is beneficial to reducing the damage to the base film during the peeling process, improving the integrity of the obtained bare base film and the base film to be tested, and thus beneficial to improving the accuracy of the evaluation results.

[0011] Further, the materials of the first adhesive layer and the second adhesive layer are each independently selected from tapes.

[0012] Compared with other types, using tapes as the materials of the first adhesive layer and the second adhesive layer is beneficial to reducing the damage to the base film during the peeling process, and at the same time beneficial to reducing the evaluation cost and improving the evaluation efficiency.

[0013] Further, in step S2, after placing the first adhesive layer on the surface of the coating in the initial sample that is away from its base film layer, a first rolling treatment is performed on the surface of the first adhesive layer; preferably, the pressure of the first rolling treatment is 0.1 - 1 MPa, and the time is 1 - 10 s.

[0014] Performing a first rolling treatment on the surface of the first adhesive layer and limiting the pressure and time of the first rolling treatment within the above range are beneficial to improving the adhesion effect between the first adhesive layer and the coating in the initial sample, reducing the damage to the base film during the peeling process, improving the integrity of the bare base film, and thus beneficial to improving the accuracy of the evaluation.

[0015] Further, in step S4, after placing the second adhesive layer on the surface of the coating in the sample to be tested that is away from its base film layer, a second rolling treatment is performed on the surface of the second adhesive layer; preferably, the pressure of the second rolling treatment is 0.1 - 1 MPa, and the time is 1 - 10 s.

[0016] Performing a second rolling treatment on the surface of the second adhesive layer and limiting the pressure and time of the second rolling treatment within the above range are beneficial to improving the adhesion effect between the second adhesive layer and the coating in the sample to be tested, reducing the damage to the base film during the peeling process, improving the integrity of the base film to be tested, and thus beneficial to improving the accuracy of the evaluation.

[0017] Further, in step S2, the ratio of the peeling force between the first adhesive layer and the coating in the initial sample to the peeling force between the coating in the initial sample and its base film layer is (1.5 - 20):1.

[0018] Compared with other ranges, limiting the ratio of the peeling force between the first adhesive layer and the coating in the initial sample to the peeling force between the coating in the initial sample and its base film layer within the above range is beneficial to improving the separation effect between the coating in the initial sample and its base film layer, improving the integrity of the bare base film, and thus beneficial to improving the accuracy of the evaluation results.

[0019] Further, in step S4, the ratio of the peeling force between the second adhesive layer and the coating in the sample to be measured to the peeling force between the coating in the sample to be measured and its base film layer is (1.5 - 20):1.

[0020] Compared with other ranges, limiting the ratio of the peeling force between the second adhesive layer and the coating in the sample to be measured to the peeling force between the coating in the sample to be measured and its base film layer within the above range is beneficial to improving the separation effect between the coating in the sample to be measured and its base film layer, improving the integrity of the base film to be measured, and thus beneficial to improving the accuracy of the evaluation result.

[0021] Further, in step S3, the temperature of the heat treatment is ≥60°C, preferably 65 - 120°C, and the time is 0.5 - 2 h.

[0022] Compared with other ranges, limiting the temperature and time of the heat treatment within the above range is beneficial to evaluating the performance changes of the base film under different temperature conditions, and beneficial to improving the accuracy and comprehensiveness of the evaluation result.

[0023] Further, in step S3, the pressure of the pressure treatment is >0 MPa, preferably 1 - 10 MPa, and the time is 5 - 180 s.

[0024] Compared with other ranges, limiting the pressure and time of the pressure treatment within the above range is beneficial to evaluating the performance changes of the base film under different pressure conditions, and beneficial to improving the accuracy and comprehensiveness of the evaluation result.

[0025] Further, the electrolyte immersion treatment includes immersion treatment and drying treatment; preferably, the temperature of the immersion treatment is 15 - 60°C, and the time is 1 - 72 h; preferably, the temperature of the drying treatment is 60 - 80°C, and the time is 2 - 24 h.

[0026] Compared with other ranges, limiting the temperature and time of the immersion treatment and the temperature and time of the drying treatment within the above range is beneficial to simulating the electrolyte contact process and the natural drying process of the lithium-ion battery during actual operation, and beneficial to improving the accuracy and comprehensiveness of the evaluation result.

[0027] Further, in step S1, in the separator, the thickness of the coating is 1 - 10 μm, and the thickness of the base film layer is 5 - 16 μm; preferably, the coating includes a ceramic layer, and the material of the ceramic layer is selected from one or more of the group consisting of alumina, boehmite, lithium titanium aluminum phosphate, silicon dioxide, and zinc oxide. The above evaluation method provided by the present application is applicable to separators of different specifications.

[0028] To achieve the above object, another aspect of the present invention also provides an application of the above evaluation method for the performance of the base film in the separator provided by the present application in the production of lithium-ion batteries.

[0029] The evaluation method for the performance of the base film in the separator provided in this application can simulate the conditions in the actual working environment of a lithium-ion battery, comprehensively evaluate the performance changes of the base film under the influence of various environmental conditions, and at the same time reduce the damage to the base film during the separation process of the coating and the base film layer, reduce the influence of human factors on the pore structure of the base film, and improve the accuracy of the evaluation results; through the above formula (I), the degree of influence of the base film to be measured under specific environmental conditions can be calculated, and by comparing the air permeability of the bare base film and the base film to be measured, the influence of different environmental conditions on the performance of the base film is quantified. Applying the above evaluation method in the production of lithium-ion batteries can provide a strong scientific basis for improving the performance of the separator, enhancing the electrochemical performance and safety of lithium-ion batteries, thereby enabling targeted improvement of the separator performance, enhancing the electrochemical performance and service safety of lithium-ion batteries, and the above evaluation method provided in this application is simple to operate and has high accuracy, and is suitable for large-scale rapid testing. Detailed implementation manners

[0030] It should be noted that, without conflict, the embodiments in this application and the features in the embodiments can be combined with each other. The present invention will be described in detail below with reference to the embodiments.

[0031] As described in the background art, the existing evaluation methods have the problem that they cannot accurately evaluate the performance changes of the base film in the separator under different environmental conditions. To solve the above technical problems, a first aspect of this application provides an evaluation method for the performance of the base film in the separator, and the evaluation method includes: Step S1, prepare at least two separators with the same material and structure, and use at least one of the separators as the initial sample. The separator includes a base film layer and a coating applied on at least one surface of the base film layer; Step S2, bond a first adhesive layer on the surface of the coating in the initial sample away from its base film layer, and then peel off the first adhesive layer to separate the coating in the initial sample from its base film layer to obtain a bare base film, and measure the air permeability d0 of the bare base film; wherein, the peeling force between the first adhesive layer and the coating in the initial sample is greater than the peeling force between the coating in the initial sample and its base film layer; Step S3, perform heat treatment, pressure treatment, or electrolyte immersion treatment on another separator to obtain a sample to be measured; Step S4, bond a second adhesive layer on the surface of the coating in the sample to be measured away from its base film layer, and then peel off the second adhesive layer to separate the coating in the sample to be measured from its base film layer to obtain a base film to be measured, and measure the air permeability d of the base film to be measured t ; wherein, the peeling force between the second adhesive layer and the coating in the sample to be measured is greater than the peeling force between the coating in the sample to be measured and its base film layer; Step S5, calculate the degree of influence of the base film to be measured according to formula (I), wherein, I is the degree of influence of the base film to be measured, d t is the air permeability of the base film to be measured, and d0 is the air permeability of the bare base film.

[0032] It should be noted that the air permeability in this application refers to the time required for 100 mL of gas to pass through a unit area of the base film, with the unit of s / 100 mL; the larger the value of the air permeability, the slower the rate of gas passing through the base film, the greater the air permeability resistance, and the more serious the blockage of the pore structure of the base film. The degree of influence on the base film to be measured in this application reflects the degree of change in the air permeability of the base film after being treated under specific environmental conditions (heat treatment, pressure treatment, or electrolyte immersion treatment), directly reflects the degree of change in the pore structure of the base film, and indirectly reflects the degree of change in the electrochemical properties, thermal stability, mechanical strength, and other properties of the base film. The larger the value of the degree of influence on the base film to be measured, the more obvious the performance degradation under the corresponding environmental conditions.

[0033] In the above evaluation method provided by this application, the first adhesive layer and the second adhesive layer are respectively used to separate the coating in the initial sample from its base film layer (limiting the peel force between the two to meet the above relationship), separate the coating in the sample to be measured from its base film layer (limiting the peel force between the two to meet the above relationship), and obtain the bare base film and the base film to be measured. Compared with the solvent rubbing method, using the above method can reduce the damage to the base film during the peeling process and reduce the influence of human factors on the pore structure of the base film, thereby improving the accuracy of the evaluation results. In step S3, heat treatment, pressure treatment, or electrolyte immersion treatment of the separator can simulate the conditions in the actual working environment of the lithium-ion battery, so as to comprehensively evaluate the performance changes of the base film under the influence of various environmental conditions. In step S5, through formula (I), the degree of influence on the base film to be measured under specific environmental conditions can be calculated. By comparing the air permeability of the bare base film and the base film to be measured, the influence of different environmental conditions on the performance of the base film is quantified, providing a strong scientific basis for improving the performance of the separator and enhancing the electrochemical performance and safety of the lithium-ion battery.

[0034] In a preferred embodiment, the adhesion strength of the first adhesive layer and the second adhesive layer is independently ≥800 N / m. The adhesion strength of the first adhesive layer and the second adhesive layer includes but is not limited to the above range. Limiting it within the above range is beneficial to making the coating in the initial sample separate from its base film layer more easily and fully, making the coating in the sample to be measured separate from its base film layer more easily and fully, at the same time being beneficial to reducing the damage to the base film during the peeling process, being beneficial to improving the integrity of the obtained bare base film and the base film to be measured, and thus being beneficial to improving the accuracy of the evaluation results.

[0035] It should be noted that the adhesion strength of the first adhesive layer and the second adhesive layer in this application is measured by method 1 recorded in GB / T 2792-2014 "Test Method for Peel Strength of Adhesive Tapes": the test method for the 180° peel strength of adhesive tapes from stainless steel.

[0036] In order to more fully separate the coating in the initial sample from its base film layer, more fully separate the coating in the sample to be tested from its base film layer, while further reducing the damage to the base film during the peeling process and further improving the integrity of the obtained bare base film and the base film to be tested, thereby further improving the accuracy of the evaluation results. Preferably, the adhesion strengths of the first adhesive layer and the second adhesive layer are independently 800 - 1000 N / m. Specifically, the adhesion strengths of the first adhesive layer and the second adhesive layer can be independently 800 N / m, 850 N / m, 900 N / m, 950 N / m or 1000 N / m.

[0037] In order to further reduce the damage to the base film during the peeling process, further improve the integrity of the bare base film and the base film to be tested, and at the same time, in order to reduce the evaluation cost and improve the evaluation efficiency, in a preferred embodiment, the materials of the first adhesive layer and the second adhesive layer independently include but are not limited to tapes.

[0038] In a preferred embodiment, in step S2, after placing the first adhesive layer on the surface of the coating in the initial sample that is away from its base film layer, a first rolling treatment is performed on the surface of the first adhesive layer. Compared with other methods, using the above method is beneficial to improving the adhesion effect between the first adhesive layer and the coating in the initial sample, thereby facilitating more fully separating the coating in the initial sample from its base film layer, reducing the damage to the base film during the peeling process, improving the integrity of the bare base film, and thus improving the accuracy of the evaluation.

[0039] In a preferred embodiment, in step S4, after placing the second adhesive layer on the surface of the coating in the sample to be tested that is away from its base film layer, a second rolling treatment is performed on the surface of the second adhesive layer. Compared with other methods, using the above method is beneficial to improving the adhesion effect between the second adhesive layer and the coating in the sample to be tested, thereby facilitating more fully separating the coating in the sample to be tested from its base film layer, reducing the damage to the base film during the peeling process, improving the integrity of the base film to be tested, and thus improving the accuracy of the evaluation.

[0040] In order to further improve the bonding effect between the first adhesive layer and the coating in the initial sample, so as to more fully separate its coating from the base film layer, further improve the bonding effect between the second adhesive layer and the coating in the sample to be tested, so as to more fully separate its coating from the base film layer, and at the same time, in order to further reduce the damage to the base film during the peeling process and further improve the accuracy of the evaluation, in a preferred embodiment, the pressures of the first rolling treatment and the second rolling treatment are independently 0.1 - 1 MPa, and the times are independently 1 - 10 s. Preferably, the pressures of the first rolling treatment and the second rolling treatment are independently 0.3 - 0.6 MPa, and the times are independently 1 - 5 s.

[0041] In a preferred embodiment, in step S2, the ratio of the peeling force between the first adhesive layer and the coating in the initial sample to the peeling force between the coating in the initial sample and its base film layer is (1.5 - 20):1. The ratio of the peeling force between the first adhesive layer and the coating in the initial sample to the peeling force between the coating in the initial sample and its base film layer includes but is not limited to the above range. Limiting it within the above range is beneficial to improving the separation effect between the coating in the initial sample and its base film layer, beneficial to inhibiting the damage to the pore structure of the base film during the peeling process, beneficial to improving the integrity of the bare base film, and thus beneficial to improving the accuracy of the evaluation result. Specifically, the ratio of the peeling force between the first adhesive layer and the coating in the initial sample to the peeling force between the coating in the initial sample and its base film layer can be 1.5:1, 2:1, 5:1, 10:1 or 20:1.

[0042] In a preferred embodiment, in step S4, the ratio of the peeling force between the second adhesive layer and the coating in the sample to be tested to the peeling force between the coating in the sample to be tested and its base film layer is (1.5 - 20):1. The ratio of the peeling force between the second adhesive layer and the coating in the sample to be tested to the peeling force between the coating in the sample to be tested and its base film layer includes but is not limited to the above range. Limiting it within the above range is beneficial to improving the separation effect between the coating in the sample to be tested and its base film layer, beneficial to inhibiting the damage to the pore structure of the base film during the peeling process, beneficial to improving the integrity of the base film to be tested, and thus beneficial to improving the accuracy of the evaluation result. Specifically, the ratio of the peeling force between the second adhesive layer and the coating in the sample to be tested to the peeling force between the coating in the sample to be tested and its base film layer can be 1.5:1, 2:1, 5:1, 10:1 or 20:1.

[0043] In order to simulate the working state of a lithium-ion battery in different temperature environments, in a preferred embodiment, in step S3, the temperature of the heat treatment ≥ 60°C, preferably 65 - 120°C, and the time is 0.5 - 2 h. The temperature and time of the heat treatment include but are not limited to the above range. Limiting it within the above range is beneficial to evaluating the performance changes of the base film under different temperature conditions and beneficial to improving the accuracy and comprehensiveness of the evaluation result.

[0044] In order to simulate the working state of a lithium-ion battery in different pressure environments, in a preferred embodiment, in step S3, the pressure of the pressure treatment > 0 MPa, preferably 1 - 10 MPa, and the time is 5 - 180 s. The pressure and time of the pressure treatment include but are not limited to the above range. Limiting it within the above range is beneficial to evaluating the performance changes of the base film under different pressure conditions and beneficial to improving the accuracy and comprehensiveness of the evaluation result.

[0045] In order to simulate the electrolyte contact process and natural drying process during the actual operation of a lithium-ion battery, and further improve the accuracy and comprehensiveness of the evaluation results, in a preferred embodiment, the electrolyte immersion treatment includes immersion treatment and drying treatment.

[0046] In a preferred embodiment, the temperature of the immersion treatment is 15 - 60 °C, and the time is 1 - 72 h. The temperature and time of the immersion treatment include but are not limited to the above ranges. Limiting them within the above ranges is beneficial to improving the authenticity of simulating the contact process between the separator and the electrolyte, thereby facilitating the evaluation of the performance changes of the base film under different electrolyte immersion conditions, and is beneficial to improving the accuracy and comprehensiveness of the evaluation results.

[0047] In order to improve the authenticity of simulating the actual working environment of a lithium-ion battery and further improve the accuracy of the evaluation results, in a preferred embodiment, the temperature of the drying treatment is 60 - 80 °C, and the time is 2 - 24 h.

[0048] In order to further improve the authenticity of simulating the contact process between the separator and the electrolyte, and thus further improve the accuracy of the evaluation results, in a preferred embodiment, the separator is immersed in an electrolyte, which includes a lithium salt, a solvent, and an optional additive. Among them, the lithium salt includes but is not limited to one or more of the group consisting of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium bis(trifluoromethylsulfonyl)imide (LiTFSI), lithium difluorophosphate (LiDFP), and lithium difluoro(oxalato)borate (LiDFOB); the molar concentration of the lithium salt is 0.1 - 2 mol / L; the solvent includes but is not limited to one or more of the group consisting of ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), and propylene carbonate (PC); the additive includes but is not limited to one or more of the group consisting of vinylene carbonate (VC), trimethyl phosphate (TMP), fluoroethylene carbonate (FEC), and triethyl phosphate (TEP).

[0049] The above evaluation method provided by this application is applicable to diaphragms of different specifications. In a preferred embodiment, in step S1, the thickness of the coating in the diaphragm is 1-10 μm, and the thickness of the base film layer is 5-16 μm. In a preferred embodiment, the coating includes a ceramic layer, and the materials of the ceramic layer include, but are not limited to, one or more of the group consisting of alumina, boehmite, and lithium aluminum titanium phosphate (LATP); the materials of the base film layer include, but are not limited to, one or more of the group consisting of polyethylene (PE), polypropylene (PP), polyethylene terephthalate (PET), and polyimide (PI). Optionally, the coating further includes a polymer material layer, and the polymer material layer is disposed on the surface of the ceramic layer away from the base film layer, and the materials of the polymer material layer include, but are not limited to, polyvinylidene fluoride (PVDF) and / or polymethyl methacrylate (PMMA).

[0050] In order to evaluate the degree of influence of the coating in the diaphragm under different environmental conditions, so as to more intuitively compare the degree of influence of the coating and the base film under different environmental conditions, and further improve the comprehensiveness of the evaluation results, in a preferred embodiment, step S1 further includes testing the air permeability d of the initial sample c , and step S3 further includes testing the air permeability d of the sample to be tested m ; step S5 further includes calculating the degree of influence of the coating in the sample to be tested according to formula (II), where I' is the degree of influence of the coating in the sample to be tested, d m is the air permeability of the sample to be tested, d t is the air permeability of the base film to be tested, d c is the air permeability of the initial sample, and d0 is the air permeability of the bare base film. Compared with other methods, the above method can more accurately evaluate the degree of influence of the coating under different environmental conditions.

[0051] The second aspect of this application also provides an application of the above evaluation method for the performance of the base film in the diaphragm provided by this application in the production of lithium-ion batteries.

[0052] The evaluation method for the performance of the base film in the diaphragm provided by this application can simulate the conditions in the actual working environment of a lithium-ion battery, comprehensively evaluate the performance changes of the base film under the influence of various environmental conditions, and at the same time reduce the damage to the base film during the separation process of the coating and the base film layer, reduce the influence of human factors on the pore structure of the base film, and improve the accuracy of the evaluation results; through the above formula (I), the degree of influence of the base film to be measured under specific environmental conditions can be calculated, and by comparing the air permeability of the bare base film and the base film to be measured, the influence of different environmental conditions on the performance of the base film is quantified. Applying the above evaluation method in the production of lithium-ion batteries can provide a strong scientific basis for improving the performance of the diaphragm, enhancing the electrochemical performance and safety of lithium-ion batteries, thereby enabling targeted improvement of the diaphragm performance, enhancing the electrochemical performance and service safety of lithium-ion batteries, and the above evaluation method provided by this application is simple to operate, highly accurate, and suitable for large-scale rapid testing.

[0053] The following further describes this application in detail with specific embodiments, and these embodiments should not be construed as limiting the scope claimed by this application.

[0054] It should be noted that an air permeability detector (Prises International Trade (Shanghai) Co., Ltd., Gurley4340) was used to test the air permeability of the initial samples, bare base films, samples to be tested, and base films to be measured in all the examples and comparative examples of this application, and the test results are shown in Table 1.

[0055] Example 1

[0056] An evaluation method for the performance of the base film in a diaphragm includes the following steps:

[0057] (1) Prepare two diaphragms with the same material and structure. The diaphragm includes a base film layer and an alumina layer coated on one surface of the base film layer. Among them, the material of the base film layer is polyethylene, and its weight-average molecular weight is 8×10 5 g / mol, the thickness of the base film layer is 12 μm, and the thickness of the alumina layer is 2 μm. Take one of the diaphragms as the initial sample and test its air permeability d c ;

[0058] (2) Paste a tape with an adhesion strength of 800 N / m on the surface of the coating of the above initial sample away from the base film layer, perform a rolling treatment on it for 3 s under a pressure of 0.6 MPa, and then peel off the tape to separate the coating in the initial sample from its base film layer to obtain a bare base film. Among them, the ratio of the peeling force between the tape and the coating in the initial sample to the peeling force between the coating and the base film layer in the initial sample is 2:1; test the air permeability d0 of the bare base film.

[0059] (3) Heat another diaphragm in step (1) at a temperature of 80 °C for 1 h, and after natural cooling to room temperature, obtain a sample to be tested, and test the air permeability d of the sample to be tested. m ;

[0060] (4) Stick a tape with an adhesion strength of 800 N / m on the surface of the coating of the sample to be tested obtained in step (3) on the side far from the base film layer, perform a rolling treatment on it at a pressure of 0.6 MPa for 3 s, and then peel off the tape to separate the coating in the sample to be tested from its base film layer to obtain a base film to be tested, wherein the ratio of the peeling force between the tape and the coating in the sample to be tested to the peeling force between the coating in the sample to be tested and its base film layer is 2:1; test the air permeability d of the base film to be tested. t ;

[0061] (5) Calculate the affected degree I of the base film to be tested according to the above formula (I); calculate the affected degree I' of the coating in the sample to be tested according to the above formula (II), and the calculation results are shown in Table 1.

[0062] Example 2

[0063] The difference from Example 1 is that in step (3), another diaphragm in step (1) is heated at a temperature of 120 °C for 1 h; the remaining steps are the same as those in Example 1.

[0064] Example 3

[0065] The difference from Example 1 is that in step (3), another diaphragm in step (1) is pressurized at 25 °C and 1.5 MPa for 60 s; the remaining steps are the same as those in Example 1.

[0066] Example 4

[0067] The difference from Example 1 is that in step (3), another diaphragm in step (1) is pressurized at 25 °C and 4.5 MPa for 60 s; the remaining steps are the same as those in Example 1.

[0068] Example 5

[0069] The difference from Example 1 is that in step (3), at 25 °C, another diaphragm in step (1) is immersed in an electrolyte solution (1 mol / L LiPF6, the solvent is a combination of EC, EMC, and DMC, and the volume ratio of EC, EMC, and DMC is 1:1:1) for 24 h, taken out and washed once with ethanol, and then dried at 60 °C for 12 h to obtain a sample to be tested; the remaining steps are the same as those in Example 1.

[0070] Example 6

[0071] The difference from Example 1 is that: the separator prepared in step (1) includes a polymer material layer, an alumina layer, and a base film layer that are sequentially stacked. Among them, the material of the base film layer is polyethylene, and its weight-average molecular weight is 1×10 6 g / mol, the thickness of the base film layer is 12 μm, the thickness of the alumina layer is 2 μm, the material of the polymer material layer is polyvinylidene fluoride (PVDF), and its weight-average molecular weight is 4×10 5 g / mol, the thickness of the polymer material layer is 1 μm, and the remaining steps are the same as those in Example 1.

[0072] Example 7

[0073] The difference from Example 1 is that: in step (4), a tape with an adhesion strength of 1000 N / m is used to peel the coating in the sample to be tested, and the ratio of the peeling force between the tape and the coating in the sample to be tested to the peeling force between the coating in the sample to be tested and its base film layer is 10:1, and the remaining steps are the same as those in Example 1.

[0074] Example 8

[0075] The difference from Example 1 is that: in step (4), a tape with an adhesion strength of 650 N / m is used to peel the coating in the sample to be tested, and the ratio of the peeling force between the tape and the coating in the sample to be tested to the peeling force between the coating in the sample to be tested and its base film layer is 2:1, and the remaining steps are the same as those in Example 1.

[0076] Example 9

[0077] The difference from Example 1 is that: in step (4), after the tape is pasted on the surface of the coating in the sample to be tested that is far from the base film layer, no rolling treatment is performed, and the tape is directly peeled off to separate the coating in the sample to be tested from its base film layer, and the remaining steps are the same as those in Example 1.

[0078] Example 10

[0079] The difference from Example 1 is that: in step (4), the pressure of the rolling treatment is 1 MPa and the time is 2 s, and the remaining steps are the same as those in Example 1.

[0080] Example 11

[0081] The difference from Example 1 is that: in step (4), the pressure of the rolling treatment is 0.05 MPa and the time is 1 s, and the remaining steps are the same as those in Example 1.

[0082] Comparative Example 1

[0083] The difference from Example 1 is as follows: The coating of the separator is erased by the solvent wiping method. Specifically, in step (2), 10 mL of absolute ethanol is used to wipe the coating of the initial sample, exposing the base film of the initial sample to obtain a bare base film, and the air permeability d0 of the bare base film is measured; in step (4), 10 mL of absolute ethanol is used to wipe the coating of the sample to be tested, exposing the base film of the sample to be tested to obtain a base film to be tested, and the air permeability d of the base film to be tested is measured. t The remaining steps are the same as those in Example 1.

[0084] Comparative Example 2

[0085] The difference from Example 1 is as follows: In step (4), the ratio of the peeling force between the tape and the coating in the sample to be tested to the peeling force between the coating in the sample to be tested and its base film layer is 0.5:1, resulting in incomplete peeling of the coating in the sample to be tested from its base film layer. The remaining steps are the same as those in Example 1.

[0086] Table 1

[0087] <![CDATA[d c (s / 100mL)]]> <![CDATA[d0(s / 100mL)]]> <![CDATA[d m (s / 100mL)]]> <![CDATA[d t (s / 100mL)]]> I(％) I’(％) Example 1 203 174 260 230 32.2 3.4 Example 2 203 174 322 291 67.2 6.9 Example 3 203 174 221 191 9.8 3.4 Example 4 203 174 287 254 46.0 13.8 Example 5 203 174 210 180 3.4 3.4 Example 6 216 174 272 203 16.7 64.3 Example 7 204 173 262 229 32.4 6.5 Example 8 203 184 261 240 30.4 10.5 Example 9 205 189 260 244 29.1 0 Example 10 204 174 260 229 31.6 3.4 Example 11 203 182 261 238 30.8 9.5 Comparative Example 1 203 314 260 489 55.7 106.3 Comparative Example 2 203 186 260 241 29.6 11.8

[0088] Comparing Examples 1 and 2, it can be seen the influence of the heating treatment temperature on the base film and coating in the separator. Among them, compared with the coating, the base film is more affected, and the higher the heating treatment temperature, the greater the value of the influence degree on the base film. This indicates that the heat resistance of the base film is lower than that of the ceramic coating.

[0089] Comparing Examples 3 and 4, it can be seen the influence of the pressure in the pressing treatment on the base film and coating in the separator. Among them, compared with the coating, the base film is more affected, and the greater the pressure in the pressing treatment, the greater the value of the influence degree on the base film. This indicates that the pressure resistance of the base film is lower than that of the ceramic coating.

[0090] From Example 5, it can be seen the influence of the electrolyte immersion treatment on the base film and coating in the separator. Among them, the affected degrees of the base film and the coating are quite the same, and both are only 3.4%. This indicates that there is no obvious reaction between the base film and the ceramic coating in the electrolyte.

[0091] The above evaluation method provided by the present application is applicable to separators of different specifications. Comparing Examples 1 and 6, it can be seen that the air permeability d of the sample to be tested in Example 6 m is larger. This is because the separator in Example 6 includes a polymer material layer, an alumina layer, and a base film layer stacked in sequence. The glass transition temperature of the polymer material layer is relatively low. Therefore, when it is subjected to the heating treatment, the polymer material layer is likely to melt, causing the pore structure on the surface of the separator to be blocked, resulting in an increase in the air permeability d of the sample to be tested. m become larger.

[0092] The above embodiments of the present invention achieve the following technical effects:

[0093] Comparing Example 1 and Comparative Example 1, it can be seen that the air permeability d0 of the bare base film and the air permeability d of the base film to be tested in Comparative Example 1 are both relatively large. This is because during the process of solvent erasing the coating in Comparative Example 1, a part of the coating will be scratched into the pores of the base film, blocking the pore structure of the base film, resulting in data distortion. It can be seen that compared with the test method using solvent erasure, the above evaluation method provided by the application can reduce the damage of the base film during the peeling process, reduce the influence of human factors on the pore structure of the base film, improve the accuracy of the evaluation results. At the same time, by calculating the affected degree of the base film to be tested under specific environmental conditions through formula (I), the influence of different environmental conditions on the performance of the base film can be quantified, so as to more intuitively judge the performance change of the base film under the influence of different environmental conditions. t Comparing Example 1 and Comparative Example 2, it can be seen that limiting the magnitude relationship between the peeling force between the second adhesive layer and the coating in the sample to be tested and the peeling force between the coating in the sample to be tested and its base film layer within the above range of the present application can separate the coating in the sample to be tested from its base film layer, obtaining a complete base film to be tested, thereby improving the accuracy of the evaluation results.

[0094] Comparing Example 1, 7 and 8, it can be seen that compared with other ranges, limiting the adhesion strength of the second adhesive layer within the above range is beneficial to reducing the damage of the base film during the peeling process, beneficial to improving the integrity of the obtained base film to be tested, and thus beneficial to improving the accuracy of the evaluation results.

[0095] Comparing Example 1, 9 to 11, it can be seen that performing the second rolling treatment on the surface of the second adhesive layer and limiting the pressure and time of the second rolling treatment within the above range are beneficial to improving the adhesion effect between the second adhesive layer and the coating in the sample to be tested, beneficial to reducing the damage of the base film during the peeling process, improving the integrity of the base film to be tested, and thus beneficial to improving the accuracy of the evaluation.

[0096] It should be noted that the terms "first", "second", etc. in the description and claims of the present application are used to distinguish similar objects, and do not necessarily have to be used to describe a specific order or sequence. It should be understood that such terms can be interchanged under appropriate circumstances, so that the embodiments of the present application described herein can be implemented in an order other than those described herein, for example.

[0097] The above are only the preferred embodiments of the present invention, and are not used to limit the present invention. For those skilled in the art, the present invention can have various modifications and changes. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

[0098] The above are only the preferred embodiments of the present invention, and are not used to limit the present invention. For those skilled in the art, the present invention can have various modifications and changes. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A method for evaluating the performance of a base film in a diaphragm, characterized in that: The evaluation method includes: Step S1, preparing at least two diaphragms with the same material and structure, and taking at least one of the diaphragms as an initial sample, wherein the diaphragm comprises a base film layer and a coating coated on at least one side of the base film layer; Step S2, bonding a first adhesive layer to a surface of the coating in the initial sample away from its base film layer, then peeling off the first adhesive layer to obtain a bare base film, and testing the air permeability d0 of the bare base film; wherein the peeling force between the first adhesive layer and the coating in the initial sample is greater than the peeling force between the coating in the initial sample and its base film layer; Step S3, heating, pressurizing, or immersing the other diaphragm in an electrolyte to obtain a sample to be tested; Step S4, bonding the second adhesive layer to the surface of the coating layer in the sample to be tested away from the base film layer, then peeling off the second adhesive layer to obtain the base film to be tested, and testing the air permeability d of the base film to be tested. t ; wherein the peeling force between the second adhesive layer and the coating in the sample to be tested is greater than the peeling force between the coating in the sample to be tested and its base film layer; Step S5, calculating the degree of influence of the base film to be tested according to formula (I), Wherein, I is the degree of influence of the basement membrane to be tested, d t is the air permeability of the base film to be tested, and d0 is the air permeability of the bare base film.

2. The method for evaluating the performance of the base film in the diaphragm according to claim 1, characterized in that: The adhesion strength of the first adhesive layer and the second adhesive layer is independently ≥ 800 N / m, preferably 800-1000 N / m; Preferably, the materials of the first adhesive layer and the second adhesive layer are each independently selected from adhesive tapes.

3. The method for evaluating the performance of the base film in the diaphragm according to claim 1 or 2, characterized in that: In the step S2, after placing the first adhesive layer on the surface of the coating layer in the initial sample away from the base film layer, the surface of the first adhesive layer is subjected to a first rolling process; Preferably, in step S4, after placing the second adhesive layer on the surface of the coating layer in the sample to be tested on a side away from the base film layer thereof, a second rolling process is performed on the surface of the second adhesive layer.

4. The method for evaluating the performance of the base film in the diaphragm according to claim 3, characterized in that: The pressure of the first roll pressing treatment and the second roll pressing treatment are each independently 0.1 to 1 MPa, and the time is each independently 1 to 10 seconds.

5. The method for evaluating the performance of the base film in the diaphragm according to any one of claims 1 to 4, characterized in that: In step S2, the ratio of the peeling force between the first adhesive layer and the coating in the initial sample to the peeling force between the coating in the initial sample and its base film layer is (1.5-20):1; Preferably, in step S4, the ratio of the peeling force between the second adhesive layer and the coating in the sample to be tested to the peeling force between the coating in the sample to be tested and its base film layer is (1.5-20):

1.

6. The method for evaluating the performance of the base film in the diaphragm according to claim 1, characterized in that: In step S3, the temperature of the heating treatment is ≥60°C, preferably 65-120°C, and the time is 0.5-2h.

7. The method for evaluating the performance of the base film in the diaphragm according to claim 1, characterized in that: In the step S3, the pressure of the pressurization treatment is greater than 0 MPa, preferably 1 to 10 MPa, and the time is 5 to 180 seconds.

8. The method for evaluating the performance of the base film in the diaphragm according to claim 1, characterized in that: The electrolyte immersion treatment includes immersion treatment and drying treatment; Preferably, the soaking treatment is carried out at a temperature of 15 to 60°C and for a time of 1 to 72 hours; Preferably, the drying process is carried out at a temperature of 60 to 80° C. and for a time of 2 to 24 hours.

9. The method for evaluating the performance of the base film in the diaphragm according to any one of claims 1 to 8, characterized in that: In the step S1, in the diaphragm, the thickness of the coating layer is 1 to 10 μm, and the thickness of the base film layer is 5 to 16 μm; Preferably, the coating layer comprises a ceramic layer, and the material of the ceramic layer is selected from one or more of the group consisting of aluminum oxide, boehmite, lithium aluminum titanium phosphate, silicon dioxide and zinc oxide.

10. Use of the method for evaluating the performance of a base film in a separator according to any one of claims 1 to 9 in the production of lithium-ion batteries.

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