Method for evaluating electrolyte resistance of polypropylene film and aluminum-plastic composite film

Abstract

This invention provides a method for evaluating the electrolyte resistance of polypropylene membranes, comprising the following steps: (1) immersing the polypropylene membrane sample to be tested in an electrolyte, the electrolyte including an organic solvent and lithium hexafluorophosphate; (2) removing the sample to be tested, placing it in an oven for baking, cooling, cleaning, and drying; (3) cutting the sample to be tested into pieces, placing them in a container, and sealing the container; (4) placing the container in an oven for baking; (5) using a syringe to extract gas from the container, and then using a gas chromatograph to analyze the extracted gas, comparing the peak area corresponding to the retention time of the solvent contained in the electrolyte, and characterizing the electrolyte resistance of the polypropylene membrane by the size of the peak area. This evaluation method can evaluate the electrolyte resistance of polypropylene membranes independently, and can evaluate the electrolyte resistance of polypropylene membranes in advance before the production of aluminum-plastic composite membranes, providing clearer indicators for the electrolyte corrosion resistance of polypropylene membranes, which helps to promote the improvement of upstream materials, and the detection method is simple.

Description

Technical Field

[0001] This invention relates to the field of electrolyte resistance testing technology for aluminum-plastic composite films, and particularly to a method for evaluating the electrolyte resistance of polypropylene films and aluminum-plastic composite films. Background Technology

[0002] Aluminum-plastic composite film (referred to as aluminum-plastic film) is one of the five major materials for lithium-ion batteries, serving as the outer packaging material for soft-pack lithium batteries. It is mainly used as a packaging material for soft-pack lithium batteries and is applied in soft-pack battery packaging in fields such as 3C digital products, energy storage, and power.

[0003] Currently, commercially available dry-process aluminum-plastic composite films mainly consist of an outer protective layer, a middle aluminum foil layer, and an inner heat-sealing layer, bonded together by adhesives. The outer protective layer primarily protects the middle aluminum foil layer from scratches; the middle aluminum foil layer mainly acts as a barrier, preventing moisture intrusion and blocking oxygen; the inner heat-sealing layer (usually CPP) is an electrolyte-resistant layer, primarily preventing leaked electrolyte from corroding the aluminum foil layer. Therefore, the electrolyte resistance of the aluminum-plastic composite film is crucial to the performance of lithium-ion batteries, especially their safety. Currently, most aluminum-plastic composite film manufacturers use immersion methods to test the electrolyte resistance of their films, such as the immersion method disclosed in Chinese patent CN 108613919A. However, this testing method requires the aluminum-plastic composite film to be manufactured before evaluation, which is time-consuming. Therefore, the testing efficiency is low, and the electrolyte resistance of the main material, CPP, cannot be determined before the finished product is produced, making it impossible to make a preliminary assessment. In practice, the polypropylene film (CPP), as the inner heat-sealing layer, is mainly used to prevent leaked electrolyte from corroding the aluminum foil layer. Therefore, the better the corrosion resistance of the polypropylene film to electrolyte, the better the electrolyte resistance performance of the aluminum-plastic composite film. However, the above-mentioned immersion method fails to achieve the purpose of independently evaluating the electrolyte resistance performance of the polypropylene film, and aluminum-plastic composite film manufacturers cannot provide clear electrolyte resistance performance indicators to upstream polypropylene film manufacturers.

[0004] Therefore, it is necessary to provide a method for evaluating the electrolyte resistance of polypropylene films, to evaluate the quality of the electrolyte resistance of polypropylene films independently, and to preliminarily evaluate the electrolyte resistance of aluminum-plastic composite films by testing the electrolyte resistance of polypropylene films, so that aluminum-plastic composite film manufacturers can put forward clear electrolyte resistance performance indicators to upstream polypropylene film manufacturers. Summary of the Invention

[0005] In view of the above problems, the purpose of this invention is to provide a method for evaluating the electrolyte resistance of polypropylene membranes and aluminum-plastic composite membranes. This method can evaluate the electrolyte resistance of polypropylene membranes independently, allowing for pre-assessment of the electrolyte resistance of polypropylene membranes before the production of aluminum-plastic composite membranes. It provides more specific indicators for the electrolyte corrosion resistance of polypropylene membranes, and the testing method is simple. Furthermore, it can also preliminarily assess the electrolyte resistance of aluminum-plastic composite membranes using this polypropylene membrane, contributing to the improvement of upstream materials.

[0006] To achieve the above objectives, the present invention provides a method for evaluating the electrolyte resistance of polypropylene membranes, comprising the following steps:

[0007] (1) Provide a polypropylene membrane sample to be tested and immerse the sample in the electrolyte;

[0008] (2) Take out the sample to be tested after step (1), put it into an oven to bake, take it out to cool, clean and dry;

[0009] (3) Cut the sample to be tested after step (2) into sample pieces, put the sample pieces into a container, and seal the container;

[0010] (4) Place the sealed container in an oven for baking;

[0011] (5) The gas in the container is extracted using a syringe, and the extracted gas is analyzed by a gas chromatograph. The peak area corresponding to the retention time of the solvent contained in the electrolyte is compared, and the size of the peak area is used to characterize the electrolyte resistance of the polypropylene membrane.

[0012] In the technical solution of this invention, the polypropylene membrane sample is immersed in an electrolyte. Because the organic solvent in the electrolyte has a swelling effect on the polypropylene membrane, and because the electrolyte contains hydrofluoric acid, this swelling is more pronounced under the strong penetrating effect of hydrofluoric acid. The adsorption amount varies due to differences in swelling capacity. After immersion in the electrolyte, the polypropylene membrane reaches its boiling point during baking, and the adsorbed solvent is further released into the container. A fixed volume of gas is extracted and analyzed using a gas chromatograph. The gas chromatogram identifies the peak area corresponding to the retention time, and the peak area corresponds to the adsorption amount. The size of the peak area characterizes the electrolyte resistance of the polypropylene membrane. Therefore, by analyzing the gas chromatogram, recording the retention time and corresponding peak area of ​​the solvent in the electrolyte, the size of the peak area determines the electrolyte resistance of the polypropylene membrane. A larger peak area indicates poorer electrolyte resistance, providing an important reference for evaluating the electrolyte resistance of polypropylene membranes independently.

[0013] In some embodiments, the thickness of the polypropylene film is 30-80 μm. For example, the thickness of the polypropylene film may be, but is not limited to, 30 μm, 40 μm, 50 μm, 60 μm, 70 μm, or 80 μm.

[0014] In some embodiments, the size of the polypropylene film sample to be tested can be cut according to the test, for example, the length and width of the sample to be tested are 300×300mm or 400×400mm, but not limited thereto.

[0015] In some embodiments, the concentration of lithium hexafluorophosphate is 1 mol / L. For example, lithium hexafluorophosphate is added to an organic solvent to prepare an electrolyte with a lithium hexafluorophosphate molar concentration of 1 mol / L. Furthermore, the free acid content of the electrolyte is no higher than 50 ppm; excessively high levels of free acid can severely corrode the sample and affect the accuracy of the test.

[0016] In some embodiments, the sample to be tested is immersed in an electrolyte and baked for 12-36 hours, preferably 24 hours. Further, the sample to be tested is heated simultaneously during immersion at a temperature of 85±2℃.

[0017] In some embodiments, the organic solvent is selected from at least one of ethylene carbonate, dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, ethyl acetate, and methyl acetate. These organic solvent systems are commonly used organic solvents and exhibit strong response signals in gas chromatography analysis, which helps to identify differences. As an example, the organic solvent is either dimethyl carbonate or methyl ethyl carbonate. Of course, the organic solvent can also be a mixture of dimethyl carbonate and methyl ethyl carbonate, or a mixture of ethylene carbonate and dimethyl carbonate. Furthermore, the organic solvent can also be a mixture of ethylene carbonate, dimethyl carbonate, and methyl ethyl carbonate, or a mixture of ethylene carbonate, dimethyl carbonate, and diethyl carbonate.

[0018] In some embodiments, the boiling point of the organic solvent is below 130°C. If an organic solvent with a particularly high boiling point is used, it is difficult to vaporize and is easy to liquefy after being taken out of the oven, resulting in low accuracy when sampling with a syringe.

[0019] In some embodiments, in step (2), the sample to be tested is placed in an oven at 85±2℃ and baked for 24±0.5h.

[0020] In some embodiments, in step (2), after baking, the sample to be tested is taken out, cooled, and the residual electrolyte on the surface of the sample to be tested is washed with clean water and wiped dry with a lint-free cloth.

[0021] In some embodiments, the size of the sample can be determined according to the experimental design. The sample size should not be too small, otherwise the absolute content of the detected solvent components will be low and difficult to distinguish. Conversely, if the sample size is too large, the test results will all be high and difficult to distinguish. As an example, the length and width of the sample are 300×300mm or 400×400mm, but it is not limited to this.

[0022] In some embodiments, in step (4), the container is placed in an oven at 130±2℃ and baked for 30 minutes. This temperature is higher than the boiling point of the solvent, which is beneficial for drying.

[0023] In some embodiments, in step (5), 1 mL of gas is drawn from the container using a 5 mL syringe.

[0024] In some embodiments, the column temperature of the gas chromatograph is set at 50-90°C, and the injection outlet temperature is set at 90-200°C.

[0025] Accordingly, this invention also provides a method for evaluating the electrolyte resistance of aluminum-plastic composite films, using the aforementioned method for evaluating the electrolyte resistance of polypropylene films. Before the production of aluminum-plastic composite films, the electrolyte resistance of the aluminum-plastic composite films using this polypropylene film can be preliminarily assessed, allowing aluminum-plastic composite film manufacturers to provide clear electrolyte resistance performance indicators to upstream polypropylene film manufacturers, thus contributing to the improvement of upstream materials. Detailed Implementation

[0026] The following are preferred embodiments of the present invention. It should be noted that those skilled in the art can make several improvements and modifications without departing from the principle of the present invention, and these improvements and modifications are also considered to be within the protection scope of the present invention.

[0027] Example 1

[0028] A method for evaluating the electrolyte resistance of polypropylene membranes, comprising the following steps:

[0029] (1) Provide a three-layer cast acid-modified polypropylene film (CPP) of type A (purchased from the market) with a thickness of 40μm, cut into 300×300mm test samples (referred to as polypropylene film sample No. 1), roll up the test sample and put it into a polytetrafluoroethylene bottle, add electrolyte and submerge the polypropylene film test sample, immerse the test sample in the electrolyte, tighten the bottle cap, put it in an oven and heat it at a temperature of 85±2℃;

[0030] Electrolyte preparation: First, mix ethylene carbonate, dimethyl carbonate and diethyl carbonate in a ratio of 1:1:1 (mass ratio) to obtain a mixed solution. Then, add lithium hexafluorophosphate to the mixed solution to prepare an electrolyte with a lithium hexafluorophosphate concentration of 1 mol / L. The free acid in the electrolyte should not exceed 50 ppm.

[0031] (2) Take out the sample to be tested after step (1), put it in an oven at 85±2℃ and bake for 24±0.5h. After taking it out and cooling it, wash the surface of the residual electrolyte with clean water and wipe the surface liquid dry with a lint-free cloth.

[0032] (3) Cut the sample to be tested after step (2) into a piece with a length and width of 300×300mm, put the piece into a clean conical flask and seal it with a stopper;

[0033] (4) Place the conical flask in an oven at 130±2℃ and bake for 30 minutes;

[0034] (5) Use a 5mL syringe to extract 1mL of gas from the conical flask, and then use a gas chromatograph to analyze the extracted gas. Set the column temperature of the gas chromatograph to 50-90℃ and the injection outlet temperature to 90-200℃.

[0035] Gas chromatographic analysis was performed, and the peak areas corresponding to the retention times of the electrolyte containing solvent were recorded. The results are shown in Table 1.

[0036] Examples 2-4

[0037] BD type 40μm three-layer cast acid modified polypropylene film (CPP) was purchased from the market. Electrolyte resistance test was performed on the BD type CPP. The BD type CPP samples were designated as polypropylene film samples No. 2-4. The test method was the same as in Example 1. The test results are shown in Table 1.

[0038] Application Example 1

[0039] The preparation of aluminum-plastic composite film is as follows:

[0040] Provide 40μm 8021O state aluminum foil, gravure-coated on both sides of the aluminum foil with PACCAR SG-L700, and then baked in an oven at 150℃ for 10s.

[0041] The nylon film is laminated with the matte surface of passivated aluminum foil. The nylon film used is a 25μm biaxially oriented nylon film, and a two-component polyurethane adhesive is used for lamination, with an adhesive application rate of 4-5g / m. 2 Curing time is 2-3 days at 50-80℃.

[0042] Polypropylene film was laminated with passivated aluminum foil. The polypropylene film used was a 40μm three-layer cast acid-modified polypropylene film (CPP, sample No. 1 polypropylene film). The lamination was carried out at 160℃ to obtain an aluminum-plastic composite film (denoted as aluminum-plastic composite film No. 1).

[0043] Application Example 2-4

[0044] Application Examples 2-4 are basically the same as Application Example 1, except that in Application Examples 2-4, polypropylene film samples 2-4 are used to prepare aluminum-plastic composite films 2-4. The preparation method is the same as in Application Example 1, and will not be described here again.

[0045] The electrolyte resistance of aluminum-plastic composite films No. 1-4 was tested. The test results are shown in Table 1. The test method is as follows:

[0046] Cut the aluminum-plastic composite film into 15mm×100mm strips and place them vertically in a polytetrafluoroethylene bottle. Add electrolyte (the same electrolyte as in Example 1), tighten the cap, and bake in an oven at 85±2℃ for 24±0.5h. After cooling, wash the surface with clean water to remove any residual electrolyte and wipe the surface dry with a lint-free cloth. Test the peel strength between CPP and aluminum foil, which is the electrolyte resistance of the aluminum-plastic composite film.

[0047] Table 1 Performance Test Results

[0048]

[0049]

[0050] As shown in Table 1, the electrolyte resistance of aluminum-plastic composite film No. 1 is less than 6N / 15mm, while the electrolyte resistance of aluminum-plastic composite films No. 2-4 is greater than 6N / 15mm. According to the immersion test to determine the electrolyte resistance ≥6N / 15mm, the electrolyte resistance of aluminum-plastic composite film No. 1 does not meet the requirement, while aluminum-plastic composite films No. 2-4 all meet the requirement. Furthermore, the electrolyte resistance of aluminum-plastic composite film No. 4, No. 3, and No. 2 increases sequentially.

[0051] The data in Table 1 also show that the total peak area gradually increases in polypropylene membrane samples 2, 3, 4, and 1. Based on actual observations, sample 1 exhibits the worst electrolyte resistance, while samples 4, 3, and 2 all demonstrate good electrolyte resistance, with the resistance increasing sequentially. This is consistent with the electrolyte resistance of aluminum-plastic composite films made from the corresponding polypropylene membranes. Therefore, this method is a relatively economical and simple evaluation method for both aluminum-plastic film and polypropylene film manufacturers to evaluate the electrolyte resistance of polypropylene membranes.

[0052] In summary, when the peak area of ​​solvent residue in the polypropylene membrane is controlled to be <100,000 μV·min, the electrolyte resistance of the polypropylene membrane combined with aluminum foil meets the requirements. Preferably, the peak area of ​​solvent residue in the polypropylene membrane is less than 60,000 μV·min, and more preferably, the peak area of ​​solvent residue in the polypropylene membrane is less than 30,000 μV·min, resulting in a polypropylene membrane with excellent electrolyte resistance.

[0053] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to preferred embodiments, it is not limited to those listed in the embodiments. Those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A method for evaluating the electrolyte resistance of a polypropylene membrane, characterized in that, Including the following steps: (1) Provide a polypropylene membrane sample to be tested, and immerse the sample to be tested in an electrolyte, wherein the electrolyte includes an organic solvent and lithium hexafluorophosphate; (2) Take out the sample to be tested after step (1), put it into an oven to bake, take it out to cool, clean and dry; (3) Cut the sample to be tested after step (2) into sample pieces, put the sample pieces into a container, and seal the container; (4) Place the sealed container in an oven for baking; (5) The gas in the container is extracted using a syringe, and the extracted gas is analyzed by a gas chromatograph. The peak area corresponding to the retention time of the solvent contained in the electrolyte is compared. The size of the peak area is used to characterize the electrolyte resistance of the polypropylene membrane. If the peak area is <100000μV.min, the electrolyte resistance of the polypropylene membrane after being combined with aluminum foil can meet the requirements.

2. The method for evaluating the electrolyte resistance of polypropylene membrane according to claim 1, characterized in that, The thickness of the polypropylene film is 30-80 μm.

3. The method for evaluating the electrolyte resistance of polypropylene membrane according to claim 1, characterized in that, The concentration of lithium hexafluorophosphate is 1 mol / L.

4. The method for evaluating the electrolyte resistance of polypropylene membrane according to claim 1, characterized in that, The organic solvent is selected from at least one of ethylene carbonate, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, ethyl acetate, and methyl acetate.

5. The method for evaluating the electrolyte resistance of polypropylene membrane according to claim 1, characterized in that, The organic solvent has a boiling point below 130°C.

6. The method for evaluating the electrolyte resistance of polypropylene membrane according to claim 1, characterized in that, In step (2), the sample to be tested is placed in an oven at 85±2℃ and baked for 24±0.5h.

7. The method for evaluating the electrolyte resistance of polypropylene membrane according to claim 1, characterized in that, In step (4), the container is placed in an oven at 130±2℃ and baked for 30 minutes.

8. The method for evaluating the electrolyte resistance of polypropylene membrane according to claim 1, characterized in that, In step (5), 1 mL of gas is drawn from the container using a 5 mL syringe.

9. The method for evaluating the electrolyte resistance of polypropylene membrane according to claim 1, characterized in that, The sample to be tested is heated while being immersed in water at a temperature of 85±2℃.

10. A method for evaluating the electrolyte resistance of an aluminum-plastic composite film, characterized in that, The evaluation was conducted using the method described in any one of claims 1-9 for evaluating the electrolyte resistance of the polypropylene membrane.

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