A method for reducing the detection polarization effect of a three - electrode battery

Abstract

The present invention provides a method for reducing the polarization effect in the detection of a three - electrode battery, using Li at the plateau potential x Ti y O 12 to fabricate a reference electrode; the reference electrode has a current collector and a coating applied on the current collector, and the content of Li x Ti y O 12 in the mixed slurry of the coating is 85% - 95%, and the single - side coating areal density ≤ 10 mg / cm 2 ; the reference electrode tab has a tip, and the tab thickness ≤ 200 μm; and in the detection state, the three - electrode battery bears a pre - tightening force of ≥ 30 kg. The present invention effectively reduces the polarization potential of the three - electrode battery, improves the potential stability and the potential detection accuracy by optimizing the material, state, structure, and process of the reference electrode.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium-ion battery materials, and particularly relates to a method for reducing the polarization effect in the detection of three-electrode batteries. Background Art

[0002] With the development of fields such as electronic products and electric vehicles, on the basis of the development of lithium ions towards high energy density, the key indicators such as the fast charging ability and service life of lithium-ion batteries are gradually improved. At present, the methods for evaluating and analyzing the fast charging ability and cycle performance of lithium-ion batteries include destructive failure analysis methods (such as battery disassembly) and non-destructive in-situ detection methods (such as three-electrode technology). The problem with the destructive method is its irreversibility. The non-destructive method (three-electrode technology) implants a reference electrode inside the battery to perform in-situ and real-time testing to evaluate the battery performance. A three-electrode battery implants a reference electrode inside the battery. This electrode has a known constant potential and provides a standard potential for the research object. A certain current passes between the working electrode and the auxiliary electrode to achieve the polarization of the electrode; a very small current passes between the working electrode and the reference electrode for electrochemical measurement. The development of the three-electrode technology enables its wide application in aspects such as potential detection, impedance analysis, overcharge boundary determination (the lithium deposition potential is when the negative electrode relative to the Li / Li+ potential is equal to 0V), and failure mechanism research.

[0003] Electrode polarization is a phenomenon that occurs when a current passes through the battery, causing the electrode to deviate from the equilibrium electrode potential. The greater the current passing through the unit area of the electrode, the more serious the deviation from the equilibrium electrode potential. The factors affecting the polarization of lithium-ion batteries include the material itself, conductive agent, electrode thickness, etc. The changes in these apparent factors have different effects on the internal mechanisms such as the length of the diffusion channel of lithium ions in the battery, the movement resistance, and the insertion and extraction rate, thus forming polarization potentials of different degrees. The polarization potential is an important reason affecting the detection accuracy and stability of three-electrode batteries. Especially in the case of large currents required for fast charging in existing batteries, the detection stability is often greatly reduced due to the excessive polarization potential.

[0004] At present, the types of reference electrodes in lithium-ion batteries generally include metallic lithium, metallic lithium alloys, electrochemically active lithium-containing compounds, etc. as reference electrodes, and there are also various influencing factors for detection stability. When metallic lithium is used as the reference electrode, it is easy to react with the electrolyte, resulting in changes in the three-electrode potential, and further making the potential become unstable with aging. If metal wires such as copper wires are used to make a three-electrode battery, for example, lithium is electroplated on the surface of the copper wire using an electroplating lithium process, lithium ions are more likely to precipitate at the tips of the copper wire surface, resulting in uneven lithium plating or the formation of lithium dendrites. Uneven lithium plating makes it impossible to perform accurate potential calibration, and the formation of lithium dendrites easily causes the lithium dendrites to pierce the separator, resulting in phenomena such as unstable three-electrode potential; in addition, the specific surface area of metallic lithium on the copper wire surface is relatively large, which is easy to react with the electrolyte, resulting in dissolution or poor contact, and the service life is not long enough to meet the measurement requirements of the electrochemical potential during long-term cycling. When using a lithium-containing compound as the reference electrode, for example, an electrode made by pressing pure lithium titanate powder, its potential stability is difficult to control. Especially in a high-current mechanism, the polarization potential surges, the electrode shape design is unreasonable, and it is easy to produce a steric hindrance effect, resulting in artifacts in potential testing; in addition, the reference electrode is in contact through pressing, which easily leads to the problem of large three-electrode resistance. Summary of the Invention

[0005] The object of the present invention is to provide a method for reducing the detection polarization effect of a three-electrode battery. By optimizing the material, state, structure, and process of the reference electrode, the polarization potential of the three-electrode battery is effectively reduced, and the potential stability and potential detection accuracy ability are improved.

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

[0007] A method for reducing the detection polarization effect of a three-electrode battery, using Li x Ti y O 12 (3 ≤ x ≤ 8, 4 ≤ y ≤ 6, the value ranges of x and y are omitted hereinafter) to make the reference electrode; the reference electrode has a current collector and a coating coated on the current collector, and the content of Li x Ti y O 12 in the mixed slurry of the coating is 85% - 95%, and the single-sided coating surface density ≤ 10 mg / cm 2 ; the reference electrode tab has a tip, and the tab thickness ≤ 200 μm; and under the detection state, the three-electrode battery bears a pre-tightening force of ≥ 30 kg.

[0008] The reference electrode proposed by the present invention is an electrochemically active lithium-containing compound, and this reference electrode selects Li x Ti y O 12The electrode pair is pre - charged and discharged with metallic lithium and finally stopped at the plateau potential, which has a relatively constant electrode potential; subsequently, the Li x Ti y O 12 electrode is taken out and implanted into the battery as a reference electrode. The Li x Ti y O 12 electrode serves as the negative electrode of a conventional battery. Its voltage plateau has a stable potential (vs. Li / Li+), has high stability in the electrolyte, is not prone to reacting with the electrolyte, and the Li x Ti y O 12 electrode has high cycle stability, so the potential remains stable with the aging of the battery. Through the above - mentioned design of the electrode thickness and coating density, the reference electrode can have a relatively thick current collector and a relatively thin coating. At the same time, with the cooperation of the tip shape design and appropriate pre - tightening force, the three - electrode polarization effect is significantly reduced, especially the potential stability can still be effectively maintained under high - current mechanisms.

[0009] Among them, the Li x Ti y O 12 at the plateau potential is generally Li x Ti y O 12 in the charged state of 1.5% SOC - 85% SOC or the discharged state of 2.8% - 80% DOD. The plateau potential will have a slight displacement under different charge - discharge rates, but it tends to be stable at the middle position of the charge - discharge plateau. Therefore, it is preferably Li x Ti y O 12 . Figure 1 For the charge - discharge curve of the reference electrode made of Li x Ti y O 12 relative to the lithium - metal negative electrode, it can be seen that the potential plateau is slightly shortened at high charge rates, but it is still relatively stable in the optimized middle region. Taking the charge - discharge rate of 0.33C as an example, the plateau potential of the reference electrode relative to the lithium - metal negative electrode is between 1.57 - 1.58V in the charged state or 1.53 - 1.54V in the discharged state. Due to the existence of the polarization potential, there is an obvious voltage gap between the charge - discharge plateaus, and this gap increases at a high current of 0.3C. However, in the present invention, the degree of potential polarization under high current is greatly reduced.

[0010] Among them, the current collector of the reference electrode can be copper foil or aluminum foil. The thickness of the copper foil is 6 - 100 μm, preferably 15 - 100 μm; or the thickness of the aluminum foil is 12 - 100 μm, preferably 20 - 100 μm; the coating surface density of single-sided coating is 4 mg / cm 2 - 8 mg / cm 2 ; The thick current collector and thin coating reduce the resistance of the reference electrode to a certain extent, but the steric hindrance and contact resistance effects are inevitable; at the same time, due to the small coating surface density, it is beneficial to the rapid passage of ions, and the potential polarization is significantly reduced.

[0011] Among them, the mixed slurry of the coating includes Li x Ti y O 12 85% - 95%, preferably Li x Ti y O 12 90% - 95%, conductive carbon black (SP) 3% - 10%, styrene-butadiene latex (SBR) 1% - 10%; High content of Li x Ti y O 12 improves the contact efficiency between particles at low coating density, reduces resistance and polarization.

[0012] Among them, the shape of the reference electrode sheet is a long and narrow strip with a tip. Its electrode width is 2 - 10 mm, the length ≤ 1 / 2 × the width of the soft-pack battery, and the tip angle ≤ 90°, preferably the tip angle ≤ 45°. The shape design of the reference electrode, especially the design of the tip, effectively reduces the steric hindrance effect generated by the reference electrode.

[0013] Among them, the three-electrode battery in the detection state can bear a pre-tightening force of 30 kg - 100 kg. Cooperating with the thin coating and the tip of the reference electrode, it effectively reduces the contact resistance between the electrodes, helps the rapid insertion and extraction of lithium ions, and avoids local deposition to form an irreversible polarization potential.

[0014] The beneficial effects of the present invention are: through the selection of the reference electrode material (Li x Ti y O 12 ), the optimization of the state (charged state, discharged state, test state), and the comprehensive adjustment of the structural process (mixing slurry, coating, size), a three-electrode battery with an electrode potential that can basically remain constant is obtained, with a small polarization effect and high current tolerance. Especially under high current passing, it still has a small polarization potential, improving the stability and accuracy of potential detection, enabling real-time in-situ monitoring of the actual working state of commercial batteries, and can be used as a battery potential sensor. Brief Description of the Drawings

[0015] The accompanying drawings, which form a part of the present invention, are used to provide a further understanding of the present invention. The schematic embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention. In the drawings:

[0016] Figure 1 are the charge-discharge curves of lithium titanate at different rates.

[0017] Figure 2 is a schematic diagram of the shape design of the reference electrode. Detailed implementation manners

[0018] The present invention will be further described below in conjunction with the accompanying drawings, but the protection scope of the present invention is not limited thereto.

[0019] I. Preparation of a three-electrode battery

[0020] The preparation of the three-electrode battery of the present invention can be carried out by the following steps:

[0021] Preparation of the reference electrode: In a dry environment such as a drying room or a glove box or a sealed environment, conventional negative electrode Li4Ti5O 12 powder is selected, and Li4Ti5O 12 85%-95% (preferably Li4Ti5O 12 90%-95%), SP 3%-10%, and SBR 1%-10% are mixed into a slurry, which is double-sided coated on 12-100 μm aluminum foil or 6-100 μm copper foil, and the single-sided coating areal density ≤ 10 mg / cm 2 , preferably 4-8 mg / cm 2 , and after being roll-pressed and die-cut, it is made into an electrode sheet of the required size (abbreviated as "Li x Ti y O 12 electrode sheet"). A Li x Ti y O 12 electrode sheet is paired with a lithium metal sheet, and a battery cell is assembled by stacking or winding with a separator, filled with an electrolyte and then encapsulated. The battery cell is pre-charged and discharged so that the Li x Ti y O 12 electrode sheet is charged to the platform potential range (the platform potential is about 1.57-1.58 V at 1.5% SOC-85% SOC in the charging state and about 1.53-1.54 V at 2.8%-80% DOD in the discharging state). Then the battery cell is disassembled, and the Li x Ti y O 12 electrode sheet is taken out and then cut into such as Figure 2The slender strip-shaped reference electrode with a tip shape as shown. The aluminum foil on the reference electrode plate and the aluminum adapter tab are welded together by soldering or ultrasonic welding, and the Li x Ti y O 12 electrode plate is completely wrapped to make the required reference electrode for standby.

[0022] Fabrication of the three-electrode battery: The following operations are completed in a dry environment such as a drying room or a glove box. After the small soft-pack fabrication process from slurry mixing - coating - rolling - punching - baking - stacking - tab cutting - tab welding - insulation testing - tab gluing, the fabricated three electrodes are slowly inserted between the positive and negative electrodes of the bare battery cell, encapsulated with an aluminum-plastic film, and then after encapsulation as required, the conventional processes such as liquid injection - sealing - shelving - formation - air extraction and sealing - air bag cutting - grading are carried out to fabricate the required three-electrode battery. Before the three-electrode test, the fabricated three-electrode battery is clamped with a stripping clamp (clamping force ≥ 30 kg).

[0023] Using the above method, Examples 1 - 4 are selected for further electrical performance evaluation. And Comparative Examples 1 - 4 are further set. Among them, in Comparative Example 1, the original Li x Ti y O 12 electrode plate is used, in Comparative Example 2, an electrode plate without a tip shape is selected, in Comparative Example 3, a high-content Li x Ti y O 12 is used for slurry mixing and high-density coating, and in Comparative Example 4, the pre-tightening force during the test is cancelled. The three-electrode batteries selected for electrical performance evaluation are listed in Table 1.

[0024] Table 1

[0025]

[0026] II. Performance evaluation of the three-electrode battery

[0027] The performance test methods for the reference electrode and the three-electrode battery are as follows:

[0028] (1) Potential stability detection:

[0029] The fabricated reference electrode is subjected to terminal voltage detection using a voltage and internal resistance tester. The detection method is to assemble a half-cell by using the Li x Ti y O 12 electrode plate and a fresh lithium sheet through a separator and inject an appropriate amount of electrolyte. The original Li x Ti y O 12The electrode and the fresh lithium sheet are assembled into a half-cell through a separator and an appropriate amount of electrolyte is injected; the potential stability of the above half-cell is tested, and three parallel samples are tested for each group of half-cells.

[0030] (2) Internal resistance test:

[0031] The fabricated three-electrode cell is subjected to EIS impedance test. The three-electrode cell is adjusted to 50% SOC state, and the test frequency is selected as 30 mHz - 100 kHz, and the amplitude is 10 mV for the three-electrode impedance spectrum test. Specifically, during the full-cell test, the positive electrode is connected to the working electrode and the working induction electrode in the EIS device, and the negative electrode is connected to the counter electrode and the reference electrode; during the positive electrode test, the positive electrode is connected to the working electrode and the induction electrode, the negative electrode is connected to the counter electrode, and the lithium reference electrode is connected to the reference electrode in the EIS; for the negative electrode test, the positive electrode is connected to the counter electrode, the negative electrode is connected to the working electrode and the working induction electrode, and the lithium reference electrode is connected to the reference electrode in the EIS.

[0032] (3) Current tolerance test (polarization voltage test):

[0033] The fabricated three-electrode cells are charged at different rates of 0.3C, 1C, 2C, 3C, etc. in a high-precision Neware charge and discharge cabinet until the cut-off voltage of the battery is 4.3V (at this time, the full-cell voltage is denoted as V cell , and the negative electrode-reference electrode voltage is denoted as V an ), then left standing for 4 h (at this time, the full-cell voltage is denoted as E cell , and the negative electrode-reference electrode voltage is denoted as E an ). At the same time, a voltage data collector is used to detect the negative electrode and reference electrode voltages to obtain the negative electrode-reference electrode voltage relaxation curve. The following formula is used to calculate the proportion of the reference electrode polarization potential: δV an-R / δE an-R =(V an-R -E an-R ) / (V cwll -E cell ).

[0034] Table 2 shows the test results of the potential stability of the reference electrode state. The potential standard deviation data results show that the potential stability of the reference electrode in the plateau potential range is relatively good. Compared with the original electrode (0.0665), the potential standard deviations of the examples are all greatly reduced. And the potential stability of the three-electrode cell fabricated with the 50% SOC state Li x Ti y O 12 electrode is the smallest compared with the potential standard deviation of the original Li x Ti y O 12 electrode. According to the experimental data, the standard deviation result <0.005 is selected, combined withFigure 1 It can be seen that the state of the reference electrode can be maintained relatively stable when the charging rate is low, whether it is in the charging state of 1.5% SOC - 85% SOC or the discharging state of 2.8% DOD - 80% DOD.

[0035] Table 3 shows the test results for verifying the resistance of the reference electrode. The smaller the measured resistance, the smaller the polarization and the higher the potential stability. In Table 3, the sum of the three - electrode impedance results in Example 1 and the difference between the measurements of the two electrodes of the battery is ≤ 3%, while the sum of the impedance results in Comparative Example 3 and the difference between the measurements of the two electrodes of the battery is about 6%. This proves that optimizing the design of the Li x Ti y O 12 electrode can effectively reduce the internal resistance of the reference electrode. A decrease in internal resistance means an increase in the ion passing rate, which is beneficial to reducing the polarization effect.

[0036] Table 4 shows the comparison of the polarization tests of the three - electrode battery. By testing the potential changes under different current excitation conditions, the polarization characteristics of the three - electrode battery are evaluated. Under different current excitation conditions, the test results show that compared with other examples, the embodiment of the present invention has a smaller polarization potential, and at a high charging rate (3C), the polarization potential does not increase significantly, indicating that the reference electrode design of the present invention has good current - resistant ability and excellent potential stability under higher current mechanism conditions.

[0037] Table 2 Comparison of potential stability detection

[0038] Parallel Sample 1 (V) Parallel Sample 2 (V) Parallel Sample 3 (V) Mean Potential Potential Standard Deviation Example 1 1.5767 1.5761 1.5764 1.5764 0.0002 Example 2 1.5473 1.5491 1.5491 1.5485 0.0008 Example 3 1.5704 1.5702 1.5703 1.5707 0.0002 Example 4 1.5501 1.5520 1.5522 1.5515 0.0007 Comparative Example 1 1.6080 1.7520 1.6140 1.6140 0.0665

[0039] Table 3 Electrochemical impedance values of three - electrode

[0040]

[0041]

[0042] Table 4 Comparison of polarization potential tests of three - electrode battery

[0043]

[0044] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention shall be included within the protection scope of the present invention.

Claims

1. A method to reduce the polarization effect of three-electrode battery detection, using Li at a platform potential x Ti y O 12 , 3≤x≤8, 4≤y≤6, to prepare a reference electrode; the reference electrode comprises a current collector and a coating coated on the current collector, wherein the mixed slurry of the coating contains Li x Ti y O 12 The content is 85%-95%, and the single-sided coating density is 4 mg / cm 2 -8mg / cm 2 ; The reference electrode pole piece has a pointed tip, and the pole piece thickness is ≤200μm; and the three-electrode battery bears a preload of 30kg-100kg under the detection state; the reference electrode pole piece is in the shape of a slender strip with a pointed tip, and its electrode width is 2-10mm, and the length is ≤1 / 2×the width of the soft-pack battery.

2. The method for reducing the polarization effect in the detection of a three - electrode battery according to claim 1, wherein, The Li at the platform potential x Ti y O 12 , is Li4Ti5O in the charged state of 1.5% SOC - 85% SOC or the discharged state of 2.8% - 80% DOD 12 .

3. The method for reducing the polarization effect in the detection of a three - electrode battery according to claim 2, characterized in that, The Li at the plateau potential x Ti y O 12 , is Li4Ti5O in a charged state of 30% SOC - 65% SOC or a discharged state of 30% - 65% DOD 12 .

4. The method for reducing the polarization effect in the detection of a three - electrode battery according to claim 1, characterized in that, The current collector of the reference electrode is made of copper foil or aluminum foil, with the copper foil having a thickness of 6 - 100 μm or the aluminum foil having a thickness of 12 - 100 μm.

5. The method for reducing the polarization effect in the detection of a three - electrode battery according to claim 4, characterized in that, The thickness of the copper foil is 15 - 100 μm, or the thickness of the aluminum foil is 20 - 100 μm.

6. The method for reducing the polarization effect in the detection of a three-electrode battery according to claim 1, characterized in that, The mixed slurry of the coating includes Li x Ti y O 12 90% - 95%, SP 3% - 10%, SBR 1% - 10%.

7. The method for reducing the polarization effect in the detection of a three - electrode battery according to claim 1, characterized in that, The tip angle of the reference electrode plate is ≤ 90°.

8. The method for reducing the polarization effect in the detection of a three - electrode battery according to claim 7, wherein, The tip angle of the reference electrode plate is ≤ 45°.

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