Method for testing the ion resistance of a separator

Abstract

The application discloses a kind of diaphragm ion resistivity test methods, comprising the following steps: two pieces of same size foil, diaphragm to be measured and target electrolyte are made into only one layer diaphragm button cell;Pre-tightening force is applied to the prepared button cell;Linear sweep voltammetry in electrochemical workstation is used to test the linear voltammogram of button cell;According to the ion resistance and layer number curve diagram of different layer number diaphragm button cell, the calculation method of diaphragm ion resistivity is: ρ=k×S÷d, wherein the slope k is the ion resistance of the diaphragm, S is aluminum foil area, and d is diaphragm thickness;Through the test method of the application, it is not necessary to test the meaningless diaphragm electronic resistance in lithium battery, but directly test diaphragm ion resistivity, and the method is more accurate and can meet actual demand.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, specifically to a method for testing the ionic resistivity of a separator. Background Technology

[0002] Lithium-ion batteries consist of positive and negative electrode materials, a separator, and an electrolyte. As one of the four main materials in a lithium battery, the conductivity of the separator directly affects the battery's performance. Inside the battery, during charging, lithium ions pass through the separator pores from the positive electrode to the negative electrode; during discharging, lithium ions pass through the separator pores from the negative electrode to the positive electrode. No electrons pass through the separator during this entire process, therefore, the ionic resistivity of the separator needs to be carefully evaluated.

[0003] Currently, there are two main methods to evaluate the resistance of the diaphragm. One is to test the diaphragm conductivity, which has the advantage of being simple to test, but the disadvantage is that the measured value and the actual value are very different. This is because the diaphragm thickness is very small, so even small fluctuations in the thickness direction will affect the diaphragm conductivity. In addition, the flatness of the electrode plate and the degree of contact between the electrode plate and the diaphragm will also affect the accuracy of the diaphragm conductivity test. The second method is to test the diaphragm surface resistance, but the disadvantage is that there is currently no dedicated equipment to test the diaphragm surface resistance.

[0004] To address the challenge of lacking dedicated equipment for resistance testing, patents CN104678173A, CN102998534A, and CN108614012A have designed devices for testing surface resistance. These devices utilize two flat electrodes to test the resistance of the diaphragm in the electrolyte. Their advantage lies in their ability to directly characterize the surface resistance of the diaphragm in the electrolyte, thus directly serving the design, research and development, and application of diaphragms.

[0005] Patent CN104678173A directly selects the impedance at a frequency of 100KHz when testing AC resistance. While this simplifies the test, it does not clearly explain the basis for selecting this frequency. Furthermore, the equipment designed in this patent cannot be completely sealed, and there is a possibility that the electrode liquid may come into contact with air and water, affecting the accuracy of the test results.

[0006] The actual test value of patent CN102998534A is the ohmic resistance of the diaphragm. When testing the ohmic resistance of diaphragms with different numbers of layers, it is impossible to calculate the contact resistance between the diaphragms by calculating the slope, which causes the test value to be higher than the actual value.

[0007] Patent CN108614012A uses an electrolyte aqueous solution as the conductive material of the diaphragm and uses the resistance of the diaphragm in an aqueous solution environment to evaluate the resistance of the diaphragm in an electrolyte environment. Its accuracy needs to be evaluated. Summary of the Invention

[0008] The purpose of this invention is to provide a method for testing the ionic resistivity of a membrane, so as to solve the problems mentioned in the background art.

[0009] To achieve the above objectives, the present invention provides the following technical solution: a method for testing the ionic resistivity of a membrane, comprising the following steps:

[0010] Step 1: Use one of copper foil, aluminum foil, and lithium foil as the test electrode and working electrode;

[0011] Step 2: Combine two identical foil sheets, the membrane to be tested, and the target electrolyte to form a button cell with only one membrane layer;

[0012] Step 3: Apply pre-tightening force to the prepared button cell;

[0013] Step 4: Measure the linear voltammetry curve of the coin line using the linear sweep voltammetry method in the electrochemical workstation;

[0014] Step 5: The polarization resistance of the coin cell can be calculated based on the curve obtained in Step 4;

[0015] Step 6: Repeat steps 1-5 to prepare coin cells with n separator layers, and calculate the polarization resistance of the coin cell with this number of separator layers.

[0016] Step 7: According to the calculation in Step 6, as the number of membrane layers increases, the resistance of the coin cell also increases. This means that the increase is in the ion resistance of the membrane, that is, the ion resistance is the increase of each layer. Therefore, the increase in polarization resistance is the ion resistance of the membrane.

[0017] Step 8: According to the curve of ion resistance versus number of layers for button cells with different number of separators, the calculation method of separator ion resistivity is: ρ=k×S÷d=, where the slope k is the ion resistance of the separator, S is the aluminum foil area, and d is the separator thickness.

[0018] Furthermore, based on the above method, the ionic resistivity of the diaphragm can be tested under different temperatures, pressures, and electrolytes.

[0019] Preferably, the foil material in step 1 has a thickness of 10-50 μm and needs to be cut into a circle with a diameter of 12-16 mm.

[0020] Preferably, the button cell battery in step 2 is CR2032 or CR2025.

[0021] The button cell needs to be filled with spring clips, gaskets, test electrodes, separators, and working electrodes in sequence, followed by the addition of electrolyte, and finally sealed using a button cell sealing machine.

[0022] The diameter of the battery separator should be slightly larger than the foil by 2-4 mm; the battery electrolyte can be in excess, preferably less than or equal to 2 ml.

[0023] Preferably, the initial preload in step 3 is between 100 and 1000 kgf.

[0024] Preferably, in step 4, the test voltage range is between -2V and 2V, and the scan rate is 1-10mV / s, thereby obtaining a linear volt-ampere curve.

[0025] Preferably, the method for calculating the polarization resistance of the coin cell in step 5 can be obtained directly by the testing software, or it can be obtained by calculating the slope of the polarization region of the linear volt-ampere curve, which is the reciprocal of the polarization resistance.

[0026] Preferably, the number of membrane layers n in step 6 can be 2-8 layers.

[0027] Compared with the prior art, the beneficial effects of the present invention are:

[0028] 1. The testing method of this invention eliminates the need to test the meaningless electronic resistance of the separator in lithium batteries, and instead directly tests the ionic resistivity of the separator. This method is more accurate and better meets actual needs.

[0029] 2. This invention uses coin cell evaluation to assess the ionic resistivity of the membrane, simplifying the preparation process;

[0030] 3. This invention can be tested under different temperatures, pressures, and target electrolyte environments to evaluate the ion resistance of the membrane under different conditions, thus better serving the evaluation and application of membrane materials. Attached Figure Description

[0031] Figure 1 This is a graph showing the relationship between the ion resistance of the membrane and the number of layers in this invention;

[0032] Figure 2 This is a graph showing the ionic resistivity of the diaphragm at different temperatures according to the present invention. Detailed Implementation

[0033] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0034] Example 1

[0035] Step 1: Select two aluminum foils as the test electrode and working electrode for the diaphragm impedance, respectively. Their thickness is 13μm and their diameter is 12mm.

[0036] Step 2: Two aluminum foils of the same size, the separator to be tested (12μm PE base film), and the target electrolyte are used to make a button cell with only one separator layer. The separator size of the symmetrical cell is 16mm. The amount of electrolyte added is 2ml.

[0037] Step 3: Apply a pre-tightening force to the button cell prepared in Step 2, setting the initial pre-tightening force to 200 kgf;

[0038] Step 4: Test the linear volt-ampere curve of the battery at room temperature, with a test voltage range of -1.5V to 1.5V and a scan rate of 5mV / s;

[0039] Step 5: Plot the curve obtained in Step 4 to calculate the polarization resistance of the coin cell;

[0040] Step 6: Repeat steps 1-5 to test the linear volt-ampere curves of coin cells with 2, 3, and 4 separator layers respectively, in order to calculate the polarization resistance of coin cells with different separator layers. The specific calculation results are shown in Table 1.

[0041] Table 1: Ion resistance of symmetrical cells with different numbers of separator layers

[0042] Polarization resistance / mΩ 0.079 0.23 0.368 0.436

[0043] Step 7: Observing Table 1, we can see that the resistance of the coin cell increases with the increase of the number of membrane layers. This indicates that the increase is in the ionic resistance of the membrane, meaning the ionic resistance is the increase per layer. Therefore, the increase in polarization resistance is the membrane ionic resistance.

[0044] Step 8: Then, plot the polarization resistance of the coin cell with different numbers of separator layers as shown in the figure. Figure 1 The slope of the curve shown is the ionic resistance of the membrane. Therefore, the ionic resistivity ρ = k × S ÷ d = 0.1209 * 1.13 / 0.012 = 0.0114 Ωm.

[0045] Example 2

[0046] Based on Example 1, keeping everything else unchanged, the linear current-voltage curves of coin cells with different numbers of separator layers were tested at -10℃, 0℃, 10℃, and 45℃ respectively;

[0047] The polarization resistance of coin cells with different numbers of separator layers was calculated at -10℃, 0℃, 10℃ and 45℃ respectively. The ion resistance of the separator versus the number of layers was plotted at -10℃, 0℃, 10℃ and 45℃ respectively. The ion resistivity of the separator at this temperature was calculated according to the ion resistance calculation formula, as shown in Table 2.

[0048] Table 2: Ionic resistivity of the diaphragm at different temperatures

[0049] Ionic resistivity / Ωm 0.0577 0.0389 0.0245 0.0114 0.0075

[0050] Furthermore, drawing as Figure 2 The membrane ionic resistivity versus temperature curve shown shows that the relationship between membrane ionic resistivity and temperature conforms to the Arrhenius expression, lnρ=kT+A (where ρ is the membrane ionic resistivity, k is a constant, T is the Kelvin temperature, and A is a constant), which is consistent with thermodynamic laws.

[0051] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for testing the ionic resistivity of a membrane, characterized in that: Includes the following steps: Step 1: Use one of copper foil, aluminum foil, and lithium foil as the test electrode and working electrode; Step 2: Combine two identical foil sheets, the membrane to be tested, and the target electrolyte to form a button cell with only one membrane layer; Step 3: Apply pre-tightening force to the prepared button cell; Step 4: Measure the linear voltammetry curve of the coin line using the linear sweep voltammetry method in the electrochemical workstation; Step 5: Calculate the polarization resistance of the coin cell based on the curve obtained in Step 4; Step 6: Repeat steps 1-5 to prepare coin cells with n separator layers, and calculate the polarization resistance of the coin cell with this number of separator layers. Step 7: According to the calculation in Step 6, as the number of membrane layers increases, the resistance of the coin cell also increases. This means that the increase is in the ion resistance of the membrane, that is, the ion resistance is the increase of each membrane layer. Therefore, the increase in polarization resistance is the membrane ion resistance. Step 8: According to the curve of ion resistance versus number of layers for button cells with different number of separators, the calculation method of separator ion resistivity is: ρ=k×S÷d, where the slope k is the ion resistance of the separator, S is the aluminum foil area, and d is the separator thickness.

2. The method for testing the ionic resistivity of a membrane according to claim 1, characterized in that: The above method was used to test the ionic resistivity of the diaphragm at different temperatures, pressures, and electrolytes.

3. The method for testing the ionic resistivity of a membrane according to claim 1, characterized in that: In step 1, copper foil, aluminum foil, and lithium foil are all foil materials with a thickness of 10-50 μm. They need to be cut into circles with a diameter of 12-16 mm.

4. The method for testing the ionic resistivity of a membrane according to claim 1, characterized in that: The button cell battery specifications mentioned in step 2 are CR2032 or CR2025; The button cell needs to be filled with spring clips, gaskets, test electrodes, separators, and working electrodes in sequence, followed by the addition of electrolyte, and finally sealed using a button cell sealing machine. The diameter of the battery separator should be slightly larger than that of the foil by 2-4 mm. The battery electrolyte is less than or equal to 2 ml.

5. The method for testing the ionic resistivity of a membrane according to claim 1, characterized in that: The initial preload in step 3 is between 100 and 1000 kgf.

6. The method for testing the ionic resistivity of a membrane according to claim 1, characterized in that: In step 4, the test voltage range is between -2V and 2V, and the scan rate is 1-10mV / s, thereby obtaining a linear volt-ampere curve.

7. The method for testing the ionic resistivity of a membrane according to claim 1, characterized in that: The method for calculating the polarization resistance of the coin cell in step 5 can be obtained directly from the testing software, or it can be obtained by calculating the slope of the polarization region of the linear volt-ampere curve, which is the reciprocal of the polarization resistance.

8. The method for testing the ionic resistivity of a membrane according to claim 1, characterized in that: The number of membrane layers n in step 6 is 2-8.

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