Method for testing electrochemical stability of secondary battery additive

Abstract

The invention provides a method for testing the electrochemical stability of a secondary battery additive, which comprises the following steps: S1, co-dissolving a high-molecular polymer easy to form a film and an additive in a good solvent to obtain a mixed solution, dispersing a conductive agent in the mixed solution, and drying the solvent to obtain a thin film; step S2, punching the film obtained in the step S1 into a wafer, and preparing a button cell; and S3, carrying out an electrochemical stability test on the button cell by using an electrochemical workstation, and observing the oxidation potential of an oxidation peak. According to the method for testing the electrochemical stability of the secondary battery additive, the high-molecular polymer easy to form a film and the additive difficult to form the film are mixed and dried through a co-film-forming method to prepare the film, and the electrochemical stability of the additive difficult to form the film can be simply and rapidly tested.

Description

Technical Field

[0001] The present invention relates to the technical field of secondary batteries, and particularly relates to a method for testing the electrochemical stability of secondary battery additives. Background Art

[0002] The surging demand for electric vehicles and the rapid consumption of lithium-ion batteries require lithium-ion batteries to have high energy density and resource sustainability. These requirements all call for the use of electroactive materials with high capacity and high loading in a fixed electrode volume. Various solutions with high loading and high energy density have higher requirements for the content of the main materials in the positive electrode slurry, resulting in an increase in the slurry viscosity, a decrease in the processable window, an increase in the coating defect rate, and thus an increase in the powder falling off of the electrode sheet, a decrease in the battery capacity, and a decline in the cycling performance. On the other hand, fast charging and discharging put higher demands on the dosage and dispersion of the conductive agent. Conductive agents with a large specific surface area and small particle size are difficult to process and disperse, which will seriously affect the internal resistance and electrochemical performance of the battery. Therefore, it is particularly important to screen a dispersant with excellent dispersion performance and good electrochemical stability for the dispersion processing of the positive electrode slurry and the dispersion distribution of the conductive agent in the slurry to ensure the rate performance and cycling performance of the battery.

[0003] Existing modified dispersants are all polymer solutions with relatively small molecular weights. After evaporating the solvent at high temperature, they cannot form a film for conventional electrochemical stability testing. Moreover, their usage scenarios are mostly in the positive electrode with a relatively high potential, and they are immersed in the electrolyte with the active material for a long time. Therefore, it is necessary to ensure that they do not react with the electrolyte and are not oxidized at high potential to prevent side reactions from having a negative impact on the electrical performance and safety performance of the battery cell.

[0004] Therefore, it is necessary to develop a method for testing the electrochemical stability of secondary battery additives, which can simply and quickly test the electrochemical stability of additives that are difficult to form a film. Summary of the Invention

[0005] The purpose of the present invention is to provide, in view of the deficiencies of the prior art, a method for testing the electrochemical stability of secondary battery additives, which can simply and quickly test the electrochemical stability of additives that are difficult to form a film.

[0006] To achieve the above purpose, the present invention adopts the following technical solutions:

[0007] A method for testing the electrochemical stability of secondary battery additives includes the following steps:

[0008] Step S1: Co-dissolve an easy-to-form-film polymer and an additive in a good solvent to obtain a mixed solution, disperse a conductive agent in the mixed solution, and dry the solvent to obtain a film;

[0009] Step S2: Punch the film obtained in Step S1 into circular pieces and prepare a button cell;

[0010] Step S3: Use an electrochemical workstation to conduct an electrochemical stability test on the button cell and observe the oxidation potential at which the oxidation peak appears.

[0011] Preferably, in step S1, the mass ratio of the easy-to-form polymer, additive, and conductive agent is (90 - 110):(1 - 5):(4 - 6).

[0012] Preferably, in step S1, the thin film preparation method is to pour and dry the mixed solution in a mold to form a thin film, or coat it on a current collector and dry it to form a thin film.

[0013] Preferably, in step S1, the oxidation potential of the easy-to-form polymer > 4V;

[0014] And / or, the easy-to-form polymer is at least one of polyvinylidene fluoride, polytetrafluoroethylene, polyvinylidene fluoride - hexafluoropropylene, polyvinylidene fluoride-based modified polymer, polyimide, and polyurethane.

[0015] Preferably, in step S1, the molecular weight of the additive is 1000 - 20000;

[0016] And / or, the additive is at least one of ether ester dispersants, polyphosphate ester dispersants, polycyclic ring modified dispersants, amide modified dispersants, and polyacrylic acid-based oil dispersants.

[0017] Preferably, in step S1, the good solvent is at least one of N-methylpyrrolidone, dimethyl sulfoxide, N,N-dimethylformamide, dimethylacetamide, triethyl phosphate, sulfolane, and 3-cyclobutenesulfone;

[0018] And / or, the conductive agent is at least one of single-walled carbon nanotubes, multi-walled carbon nanotubes, conductive carbon black, acetylene black, conductive carbon fiber, conductive graphite, and graphene.

[0019] Preferably, in step S1, the drying temperature is 100 - 150 °C.

[0020] Preferably, in step S2, the assembly method of the button cell is: sequentially assemble the negative electrode case, gasket, stainless steel sheet, lithium sheet, wafer, stainless steel sheet, gasket, and positive electrode case in order, and inject electrolyte during the assembly process.

[0021] Preferably, in step S2, the diameter of the wafer is 10 - 30 mm.

[0022] Preferably, in step S3, the electrochemical stability test is LSV or CV test, the test voltage is 0 - 6V, and the scan rate is 0.1 - 0.5 mV / s.

[0023] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0024] (1) A method for testing the electrochemical stability of a secondary battery additive provided by the present invention can simply and quickly test the electrochemical stability of a difficult-to-form additive by mixing an easy-to-form polymer and a difficult-to-form additive through a co-film-forming method and then drying to obtain a film.

[0025] (2) In the testing method provided by the present invention, adding a conductive agent in the film can provide a conductive network to reduce the polarization of the polymer film, obtain a more accurate oxidation response potential, and the selected easy-to-form polymer is a material with an oxidation potential significantly higher than the working potential of lithium iron phosphate, which will not affect the test potential response of the difficult-to-form additive. Detailed Embodiments

[0026] To make the technical solutions and advantages of the present invention clearer, the present invention and its beneficial effects will be further described in detail below in conjunction with specific embodiments, but the embodiments of the present invention are not limited thereto.

[0027] According to the first aspect of the present application, the present application aims to provide a method for testing the electrochemical stability of a secondary battery additive, including the following steps:

[0028] Step S1: Co-dissolve an easy-to-form polymer and an additive in a good solvent to obtain a mixed solution, disperse a conductive agent in the mixed solution, and dry the solvent to obtain a film;

[0029] Step S2: Punch the film obtained in Step S1 into circular pieces to prepare a button cell;

[0030] Step S3: Use an electrochemical workstation to perform an electrochemical stability test on the button cell and observe the oxidation potential at which an oxidation peak appears.

[0031] Among them, in the present application, an easy-to-form polymer and a difficult-to-form additive are co-dissolved in a good solvent and then dried to form a film; this process is not only simple and fast, but also through the film-forming process, the additive can be evenly distributed in the polymer matrix to form a stable structure, thus more accurately reflecting the electrochemical stability of the additive. In addition, the introduction of the conductive agent plays a key role in this method. Since there is a conductive network in the battery, adding a conductive agent can better reflect the actual situation of the redox reaction of the battery, and the conductive agent can establish an effective conductive network, which is crucial for reducing the polarization effect of the polymer film. The polarization effect may cause deviation in the measured oxidation potential, and the conductive network can reduce this deviation, thereby improving the measurement accuracy.

[0032] In some embodiments, in step S1, the mass ratio of the film-forming polymer, the additive, and the conductive agent is (90 - 110):(1 - 5):(4 - 6), for example, it can be 90:1:4, 90:2:4, 90:1:6, 100:1:4, 100:5:4, 100:1:6, 100:5:6, 110:1:4, 110:1:6, 110:5:1, or 110:5:6. When the mass ratio of the additive is too high, it is difficult to form a film after drying.

[0033] In some embodiments, in step S1, the method for preparing the thin film is to pour and dry the mixed solution in a mold to form a thin film, or coat it on a current collector and dry it to form a thin film.

[0034] In some embodiments, in step S1, the oxidation potential of the film-forming polymer > 4V;

[0035] And / or, the film-forming polymer is at least one of polyvinylidene fluoride, polytetrafluoroethylene, polyvinylidene fluoride - hexafluoropropylene, polyvinylidene fluoride-based modified polymer, polyimide, and polyurethane. When the oxidation potential of the film-forming polymer is controlled to be higher than 4V, that is, higher than the working potential of the lithium iron phosphate material, the test potential response of the additive can be avoided from being affected.

[0036] In some embodiments, in step S1, the molecular weight of the additive is 1000 - 20000, for example, it can be 1000, 2000, 4000, 6000, 8000, 10000, 15000, or 20000;

[0037] And / or, the additive is at least one of ether ester dispersants, polyphosphate ester dispersants, polycyclic ring modified dispersants, amide modified dispersants, and polyacrylic acid oil-based dispersants.

[0038] In some embodiments, in step S1, the good solvent is at least one of N-methylpyrrolidone, dimethyl sulfoxide, N,N-dimethylformamide, dimethylacetamide, triethyl phosphate, sulfolane, and 3-cyclobutenesulfone;

[0039] And / or, the conductive agent is at least one of single-walled carbon nanotubes, multi-walled carbon nanotubes, conductive carbon black, acetylene black, conductive carbon fibers, conductive graphite, and graphene. Adding a conductive agent in the thin film can provide a conductive network to reduce the polarization of the polymer thin film and obtain a more accurate oxidation response potential.

[0040] In some embodiments, in step S1, the drying temperature is 100 - 150°C, for example, it can be 100°C, 200°C, 300°C, 400°C, or 500°C.

[0041] When the temperature is too low, the solvent cannot be dried and the film cannot be formed. When the temperature is too high, it will cause the oxidation and carbonization of the polymer, resulting in a change in composition and affecting the test results.

[0042] In some embodiments, in step S2, the assembly method of the button cell is as follows: the negative electrode case, gasket, stainless steel sheet, lithium sheet, separator, wafer, stainless steel sheet, gasket, and positive electrode case are assembled in sequence, and electrolyte is injected during the assembly process.

[0043] In some embodiments, in step S2, the diameter of the wafer is 10 - 30 mm, for example, it can be 10 mm, 12 mm, 14 mm, 16 mm, 18 mm, or 20 mm.

[0044] When the diameter of the wafer is too small, the separator will be crushed, and the lithium sheet will contact the stainless steel sheet, causing a short circuit. When the wafer is too large, it will affect the installation and sealing of the button cell.

[0045] In some embodiments, in step S3, the electrochemical stability test is LSV or CV test, the test voltage is 0 - 6V, and the scan rate is 0.1 - 0.5 mV / s.

[0046] To make the technical solutions and advantages of the present invention clearer, the present invention and its beneficial effects will be further described in detail below in combination with specific embodiments, but the embodiments of the present invention are not limited thereto.

[0047] Example 1

[0048] This example provides a method for testing the electrochemical stability of a secondary battery additive, including the following steps:

[0049] Step S1: Dissolve 10 g of a film - forming polymer (PVDF) and 0.1 g of an additive (polysorbate) in 200 g of a good solvent (N - methylpyrrolidone) to obtain a mixed solution. Disperse 0.4 g of a conductive agent (conductive carbon black) in the mixed solution and stir at a high speed of 1500 rpm for 2 h to obtain a colloidal solution. Coat a thin film evenly on the aluminum foil with a 200 - μm doctor blade, and dry it at 105°C for 12 h to obtain a thin film;

[0050] Step S2: Punch the thin film obtained in step S1 into wafers with a diameter of 12 mm. Assemble the negative electrode case, gasket, stainless steel sheet, lithium sheet, wafer, stainless steel sheet, gasket, and positive electrode case in sequence. After injecting electrolyte during the assembly process, a button cell is prepared;

[0051] Step S3: Use an electrochemical workstation to perform LSV test on the button cell, with a scanning range of 0 - 6V and a scan rate of 0.01 V / s, and observe the potential at which the oxidation peak appears.

[0052] Among them, the molecular weight of the additive is 2000, and the mass ratio of the film-forming polymer, additive, and conductive agent is 100:1:4.

[0053] Example 2

[0054] Different from Example 1, in this example, the additive is a phosphate ester dispersant (alkylphenol polyoxyethylene ether phosphate) with a molecular weight of 1000.

[0055] The others are the same as in Example 1 and will not be elaborated here.

[0056] Example 3

[0057] Different from Example 1, in this example, the additive is a polycyclic modified dispersant (polyvinylpyrrolidone) with a molecular weight of 5000.

[0058] The others are the same as in Example 1 and will not be elaborated here.

[0059] Example 4

[0060] Different from Example 1, in this example, the additive is an amide modified dispersant (polyamide) with a molecular weight of 3000.

[0061] The others are the same as in Example 1 and will not be elaborated here.

[0062] Example 5

[0063] Different from Example 1, in this example, the additive is a polyacrylic acid oil dispersant (acrylic acid polymerization derivative modified with lauryl acrylate) with a molecular weight of 30000.

[0064] The others are the same as in Example 1 and will not be elaborated here.

[0065] Example 6

[0066] Different from Example 1, in this example, the film-forming polymer is PTFE.

[0067] The others are the same as in Example 1 and will not be elaborated here.

[0068] Example 7

[0069] Different from Example 1, in this example, the mass of the conductive agent is 0.6 g, and among them, the mass ratio of the film-forming polymer, additive, and conductive agent is 100:1:6.

[0070] The others are the same as in Example 1 and will not be elaborated here.

[0071] Example 8

[0072] Different from Example 1, in this example, the additive is 0.2 g, and the mass ratio of the film-forming polymer, additive, and conductive agent is 100:2:5.

[0073] Others are the same as in Example 1 and will not be elaborated here.

[0074] Example 9

[0075] Different from Example 1, in this example, the additive is 0.5 g, and the mass ratio of the film-forming polymer, additive, and conductive agent is 100:5:5.

[0076] Others are the same as in Example 1 and will not be elaborated here.

[0077] Example 10

[0078] Different from Example 1, in this example, the additive is 0.5 g, the conductive agent is 0.6 g, and the mass ratio of the film-forming polymer, additive, and conductive agent is 100:5:6.

[0079] Others are the same as in Example 1 and will not be elaborated here.

[0080] Example 11

[0081] Different from Example 1, in this example, the drying temperature is 200 °C, and the wafer diameter is 30 mm.

[0082] Others are the same as in Example 1 and will not be elaborated here.

[0083] Comparative Example 1

[0084] Different from Example 1, in this comparative example, the additive is 1 g, and the mass ratio of the film-forming polymer, additive, and conductive agent is 100:10:5.

[0085] Others are the same as in Example 1 and will not be elaborated here.

[0086] Comparative Example 2

[0087] Different from Example 1, in this comparative example, no conductive agent is added.

[0088] Others are the same as in Example 2 and will not be elaborated here.

[0089] Comparative Example 3

[0090] Different from Example 1, in this comparative example, the film-forming polymer is styrene-butadiene rubber.

[0091] Others are the same as in Example 1 and will not be elaborated here.

[0092] Comparative Example 4

[0093] Different from Example 1, the drying temperature in this comparative example is 90°C.

[0094] The others are the same as in Example 1 and will not be elaborated here.

[0095] The measured oxidation potential (V) results of the examples and comparative examples are recorded in Table 1 as follows.

[0096] Table 1

[0097] Number Oxidation potential (V) Example 1 5.3 Example 2 5.2 Example 3 5.1 Example 4 4.7 Example 5 5.2 Example 6 5.2 Example 7 5.0 Example 8 5.1 Example 9 4.9 Example 10 4.8 Example 11 5.1 Comparative Example 1 Failed to form a film after drying Comparative Example 2 5.4 Comparative Example 3 4.1 Comparative Example 4 4.6

[0098] From the data comparison of Examples 1-11 and Comparative Examples 1-4 in Table 1, it can be seen that the test method of an electrochemical stabilizer for secondary battery additives provided by this application can detect the electrochemical stability of difficult-to-form-film additives.

[0099] From the data comparison of Examples 1-5, it can be seen that the phosphoric acid ester additives have the highest oxidation potential and the highest electrochemical stability, while the polyamide additives have the lowest oxidation potential and the lowest electrochemical stability.

[0100] From the data comparison of Examples 1, 7-10 and Comparative Example 1, it can be seen that when the mass ratio of the additive increases, the oxidation potential will decrease; when the mass ratio of the additive is too large, it cannot be dried and formed into a film normally, resulting in the inability to coat and prepare the electrode sheet normally or it will dissolve in the electrolyte and affect the test results.

[0101] From the data comparison of Examples 1, 3 and Comparative Example 3, it can be seen that when changing the easy-to-form-film polymer, it will not affect the test results of the additive; when the oxidation potential of the easy-to-form-film polymer decreases, it will cause the overall potential of the glue film to decrease and decompose, covering the decomposition potential of the additive and making it impossible to judge the stability.

[0102] From the data comparison of Examples 1, 4, it can be seen that when the mass ratio of the conductive agent increases, the oxidation potential will decrease.

[0103] From the data comparison of Example 1 and Comparative Example 2, it can be seen that when no conductive agent is added, the oxidation potential will increase.

[0104] According to the disclosure and teachings of the above specification, those skilled in the art to which the present invention pertains can also make changes and modifications to the above embodiments. Therefore, the present invention is not limited to the above specific embodiments, and any obvious improvements, substitutions or variations made by those skilled in the art on the basis of the present invention fall within the protection scope of the present invention. In addition, although some specific terms are used in this specification, these terms are only for convenience of description and do not constitute any limitation to the present invention.

Claims

1. A test method for the electrochemical stability of additives for secondary batteries, characterized in that It includes the following steps: Step S1: Co-dissolve an easy-to-form-film polymer and an additive in a good solvent to obtain a mixed solution, disperse a conductive agent in the mixed solution, and dry the solvent to obtain a film; Step S2: Punch the film obtained in Step S1 into wafers and prepare button cells; Step S3: Use an electrochemical workstation to perform an electrochemical stability test on the button cell and observe the oxidation potential at which the oxidation peak appears.

2. The test method for the electrochemical stability of the secondary battery additive according to claim 1, characterized in that In Step S1, the mass ratio of the easy-to-form-film polymer, the additive, and the conductive agent is (90 - 110):(1 - 5):(4 - 6).

3. The test method for the electrochemical stability of the secondary battery additive according to claim 1, characterized in that In Step S1, the film preparation method is to pour and dry the mixed solution in a mold to form a film, or coat it on a current collector and dry it to form a film.

4. The test method for the electrochemical stability of the secondary battery additive according to claim 1, wherein In Step S1, the oxidation potential of the easy-to-form-film polymer > 4V; And / or, the easy-to-form-film polymer is at least one of polyvinylidene fluoride, polytetrafluoroethylene, polyvinylidene fluoride-hexafluoropropylene, polyvinylidene fluoride-based modified polymer, polyimide, and polyurethane.

5. The test method for the electrochemical stability of the secondary battery additive according to claim 1, wherein In Step S1, the molecular weight of the additive is 1000 - 20000; And / or, the additive is at least one of ether ester dispersants, polyphosphate dispersants, polycyclic ring modified dispersants, amide modified dispersants, and polyacrylic acid oil dispersants.

6. The test method for the electrochemical stability of the secondary battery additive according to claim 1, characterized in that, In Step S1, the good solvent is at least one of N-methylpyrrolidone, dimethyl sulfoxide, N,N-dimethylformamide, dimethylacetamide, triethyl phosphate, sulfolane, and 3-cyclobutenesulfone; And / or, the conductive agent is at least one of single-walled carbon nanotubes, multi-walled carbon nanotubes, conductive carbon black, acetylene black, conductive carbon fibers, conductive graphite, and graphene.

7. The testing method for the electrochemical stability of the secondary battery additive according to claim 1, characterized in that, In Step S1, the drying temperature is 100 - 150°C.

8. The method for testing the electrochemical stability of the secondary battery additive according to claim 1, wherein In Step S2, the assembly method of the button cell is: sequentially assemble the negative electrode case, gasket, stainless steel sheet, lithium sheet, wafer, stainless steel sheet, gasket, and positive electrode case in order, and inject electrolyte during the assembly process.

9. The test method for the electrochemical stability of the secondary battery additive according to claim 1, wherein, In Step S2, the diameter of the wafer is 10 - 30 mm.

10. The test method for the electrochemical stability of the secondary battery additive according to claim 1, characterized in that, In Step S3, the electrochemical stability test is LSV or CV test, the test voltage is 0 - 6V, and the scan rate is 0.1 - 0.5 mV / s.