Method for analyzing thickness of seifilm of lithium ion battery negative electrode

Abstract

The application discloses a kind of analysis methods for the thickness of lithium ion battery negative electrode SEI film, comprising: obtaining sample to be measured and reference sample;The sample to be measured and reference sample are tested before processing;After the sample to be measured and reference sample are removed shell and tab after testing before processing, true volume test is carried out, and the true volume of the sample to be measured and reference sample V is obtained;Based on the true volume of the sample to be measured V2 and the true volume of reference sample V1, the thickness of battery negative electrode SEI film is calculated.The detection means of the SEI film thickness of battery pole piece is changed into the method of true volume test calculation from electron microscope observation method which requires higher equipment and personnel, greatly reduces test cost and test cycle, and can speed up research and development progress.

Description

Technical Field

[0001] This invention relates to the field of battery evaluation and processing technology, and in particular to a method for analyzing the thickness of the SEI film on the negative electrode of a lithium-ion battery. Background Technology

[0002] Lithium-ion batteries, with their advantages of high power and high energy density, are increasingly widely used in the electric vehicle industry and stationary energy storage devices. This places high demands on the quality, safety performance, and cycle life of lithium-ion batteries. A key factor affecting these performance characteristics is the solid electrolyte interphase (SEI) film formed on the negative electrode surface during the decomposition of the electrolyte. The SEI film forms on the negative electrode surface during the initial charge and discharge phase of the battery's formation process. A uniform and stable SEI film protects the electrolyte from further consumption by the negative electrode during subsequent decomposition and also prevents graphite shedding. The quality of the SEI film formation is a crucial determinant of the subsequent cycle performance of the lithium-ion battery. However, a thicker SEI film is not necessarily better. A thicker SEI film can lead to increased battery impedance and increased polarization at the end of charge and discharge cycles, resulting in reduced charge and discharge efficiency and poorer cycle performance. Therefore, major lithium-ion battery manufacturers are continuously improving their battery formation processes. The optimal formation process yields the optimal SEI film, which improves both battery electrochemical performance and production efficiency while reducing production costs.

[0003] The presence of a SEI film on graphite surfaces is a common phenomenon, and the SEI film thickens as the battery ages. The thickness of the SEI is typically 5–50 nm, but there may be some variation between different battery systems. Although the SEI film constitutes a very small portion of the battery, it plays a crucial role in improving battery stability, power performance, and safety. Therefore, the condition of the SEI film is necessary to analyze battery performance. Currently, the evaluation of the SEI film in lithium-ion batteries is mainly based on electron microscopy and analysis of battery cycle performance. Specific analysis of SEI film thickness is also limited to electron microscopy, which requires advanced equipment and experienced, technically skilled operators, resulting in significant testing costs. Summary of the Invention

[0004] The purpose of this invention is to address the problems in the prior art by providing an analytical method for the thickness of the SEI film on the negative electrode of a lithium-ion battery. By performing a true volume test on the electrode assembly after the battery has been fully formed and disassembled, and comparing it with the true volume of the unformed electrode assembly after electrolyte injection, the thickness of the SEI film on the battery electrode can be calculated. This method can be used to evaluate the state of the SEI film and to quickly optimize the battery formation process of new systems.

[0005] This invention is implemented as follows: a method for analyzing the thickness of the SEI film on the negative electrode of a lithium-ion battery, comprising:

[0006] S1. Obtain the test sample and the reference sample:

[0007] The test sample is a battery that has formed a stable SEI film before discharge, which has been discharged multiple times to the cutoff voltage and the discharge current decreases sequentially. It is left to stand for a first time before each discharge. The reference sample is a battery that has been injected with liquid in the same batch as the test sample and has not formed after being left to stand for a second time.

[0008] S2. Pre-testing treatment of the test sample and the reference sample:

[0009] After removing the bottom shell of the negative terminal of the battery of the test sample and the reference sample, each sample is sealed in a bag and centrifuged to allow the electrolyte in the battery to be thrown out into the bag under centrifugal force.

[0010] After the electrolyte is removed, the open end of the battery is immersed in a dehydrating solution and placed in a vacuum environment to remove the remaining electrolyte from the battery using a negative pressure vacuum method.

[0011] The battery is dried after being treated with negative pressure vacuum, causing the dehydration solution inside the battery to evaporate.

[0012] S3. After removing the outer shell and tabs from the pre-processed test sample and the reference sample, perform a true volume test to obtain the true volume V2 of the test sample and the true volume V1 of the reference sample.

[0013] S4. Based on the true volume V2 of the sample to be tested and the true volume V1 of the reference sample, calculate the thickness d of the SEI film of the battery negative electrode: d=(V2-V1) / (m*S), where m is the mass of the active material of the battery negative electrode sheet and S is the specific surface area of ​​the battery negative electrode material.

[0014] This invention changes the method for detecting the SEI film thickness of battery electrodes from electron microscopy observation, which requires high levels of equipment and personnel, to a method of true volume measurement and calculation. This greatly reduces testing costs and testing cycle, and can accelerate the research and development process. In addition, by collecting SEI film thickness data of battery electrodes after formation, the quality of the formation process can be evaluated, providing data support for process improvement. Attached Figure Description

[0015] Figure 1 This is a schematic flowchart illustrating the method for analyzing the thickness of the SEI film on the negative electrode of a lithium-ion battery according to the present invention. Detailed Implementation

[0016] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below in conjunction with the present invention. Obviously, the described embodiments are some embodiments of the present invention, but not all embodiments.

[0017] See Figure 1 As shown, a method for analyzing the thickness of the SEI film on the negative electrode of a lithium-ion battery includes:

[0018] S1. Obtain the test sample and the reference sample:

[0019] The test sample is a battery that has formed a stable SEI film before discharge, which has been discharged multiple times to the cutoff voltage and the discharge current decreases sequentially. It is left to stand for a first time before each discharge. The reference sample is a battery that has been injected with liquid in the same batch as the test sample and has not formed after being left to stand for a second time.

[0020] S2. Pre-testing treatment of the test sample and the reference sample:

[0021] After removing the bottom shell of the negative terminal of the battery of the test sample and the reference sample, each sample is sealed in a bag and placed in a centrifuge, so that the electrolyte in the battery is thrown out into the bag under centrifugal force.

[0022] After the electrolyte is removed, the open end of the battery is immersed in a dehydrating solution and placed in a vacuum environment to remove the remaining electrolyte from the battery using a negative pressure vacuum method.

[0023] After the battery has been treated with negative pressure vacuum, it is placed in an oven to allow the dehydration solution inside the battery to evaporate.

[0024] S3. After removing the outer shell and tabs from the pre-processed test sample and the reference sample, perform a true volume test to obtain the true volume V2 of the test sample and the true volume V1 of the reference sample.

[0025] S4. Based on the true volume V2 of the sample to be tested and the true volume V1 of the reference sample, calculate the thickness d of the SEI film of the battery negative electrode: d=(V2-V1) / (m*S), where m is the mass of the active material of the battery negative electrode sheet and S is the specific surface area of ​​the battery negative electrode material.

[0026] In some embodiments, the steps for obtaining the sample to be tested and discharging the sample are as follows: discharging to the cutoff voltage with a first discharge current, allowing it to stand for a first time, then discharging to the cutoff voltage with a second discharge current, allowing it to stand for a second time, and then discharging to the cutoff voltage with a third discharge current, which is then used as the sample to be tested. Generally, when discharging a battery, using this sequentially decreasing method can achieve a more complete discharge of the battery. Typically, a complete discharge of the battery can be achieved through three interval discharges.

[0027] Preferably, in some embodiments, the first discharge current is 0.5C-1C, the second discharge current is 0.2C-0.3C, and the third discharge current is 0.02-0.1C. When the battery is discharging, this method of successively reducing the discharge current can achieve more complete battery discharge, ensuring safety during battery dissection and sample consistency. Generally speaking, the battery can be completely discharged through three interval discharges.

[0028] In some embodiments, preferably, the first settling time is 15-60 minutes, and the second settling time is 2-4 hours. The first and second settling times can be selected within this range. Generally, when discharging the battery multiple times, it should be done at intervals, usually within the range of 15-60 minutes. Generally, the settling time after the battery is filled with electrolyte is 2-4 hours. During this time, the electrolyte in the battery can fully wet the battery, and the battery can meet the standards for testing and use.

[0029] In some embodiments, preferably, the centrifugation is carried out in a centrifuge with a speed of 3000-5000 rpm for 30-60 minutes. In this invention, a centrifuge is used to remove the electrolyte from the battery. The speed and rotation time of the centrifuge can be set according to specific circumstances, and can be lower or higher than the specified speed range. The rotation time is determined according to the speed, with the principle of removing the electrolyte from the battery to the required degree. It can be selected within a publicly available range, or it can be outside this range, and is not limited thereto.

[0030] In some embodiments, preferably, the method of removing residual electrolyte from the battery using negative pressure vacuum involves using a vacuum pump to maintain a pressure in a vacuum environment within the range of -90 to -100 kPa for a predetermined time, then slowly restoring the pressure to atmospheric pressure. This pressure-relief cycle is repeated multiple times. In this invention, the electrolyte in the battery is discharged using negative pressure vacuum. The negative pressure value in the environment can be set according to the situation, selected within a publicly available range, or may not be within that range; it is not limited thereto.

[0031] In some embodiments, preferably, the pressure application and depressurization cycle is 8-10 times, and the constant pressure predetermined time is 5-10 minutes. As mentioned above, in this invention, the electrolyte in the battery is discharged under negative pressure by vacuuming. The negative pressure time of vacuuming and the pressure application and depressurization cycle in the environment can be set according to the situation, and can be selected within the publicly available range, or may not be within this range, and are not limited thereto.

[0032] In some embodiments, the oven temperature for battery processing is preferably 40-50°C, and the drying time is 4-6 hours. In this invention, the dehydration solution in the battery is further treated by drying to ensure complete evaporation. Therefore, the drying time and temperature can be selected within this range or outside of it, as long as the dehydration solution in the battery can be completely evaporated.

[0033] In some embodiments, the dehydrating solution is preferably a DMC solution, but other solutions may also be used, and are not limited thereto.

[0034] In some embodiments, the bag is preferably made of aluminum-plastic composite, but other packaging materials can also be used, and it is not limited to this.

[0035] In this embodiment of the invention, since the sample to be tested is in a storage state or has undergone some other tests before testing, it is necessary to charge and discharge it once before testing to ensure that the battery is normal or reaches the desired state for testing. The sample to be tested needs to be pre-treated by charging and discharging before testing.

[0036] That is, the sample to be tested is pre-treated by charging and discharging to reach a predetermined test state before testing. For example, taking a 21700 cylindrical lithium-ion experimental battery as an example, after the battery is formed, it is cycled multiple times at room temperature before testing. First, it is charged at a constant current of 0.5C = 2.4A until the voltage reaches 4.2V. Then, it is charged at a constant voltage of 4.2V until the current drops to 0.05C = 0.24A. Finally, it is discharged at 1C = 4.8A until the voltage reaches 2.5V.

[0037] Test Example 1

[0038] In this embodiment, the test sample is a 21700 cylindrical lithium-ion experimental battery with a 1C capacity of 4.8Ah;

[0039] Battery 1 under test: Cycled 100 times at room temperature, charged at a constant current of 0.5C = 2.4A until the voltage reaches 4.2V, then charged at a constant voltage of 4.2V until the current drops to 0.05C = 0.24A, and discharged at 1C = 4.8A until the voltage reaches 2.5V. Based on the battery discharge capacity, the capacity of battery 1 under test at this time is 4.8Ah, and the capacity retention rate is 98.7%.

[0040] The battery testing equipment is a charge / discharge instrument, specifically the Arbin BT2000 charge / discharge system.

[0041] Step 1: Obtain the test sample and the reference sample:

[0042] The battery under test 1 was fully discharged at 0.5C to the cutoff voltage of 2.5V, left to stand for 30 minutes, then discharged at 0.2C to the cutoff voltage of 2.5V, left to stand for 30 minutes, and then discharged at 0.05C to the cutoff voltage of 2.5V. This group was used as the test sample.

[0043] Batteries from the same batch as the test sample, with wound electrode groups assembled and injected with liquid, and left to stand for 2 hours but not formed, were used as reference samples.

[0044] Step 2: Pre-testing of the test samples and reference samples

[0045] Step 1). In the drying workshop, the test sample and reference sample obtained in the first step are dissected. Using battery dissection tools, the bottom shell of the negative terminal of the battery is removed. The battery is sealed in an aluminum-plastic bag and placed in a centrifuge at 5000 rpm for 30 minutes. The electrolyte in the battery is then thrown out into the aluminum-plastic bag under centrifugal force.

[0046] Step 2). Take a 100mL beaker, pour 30mL of dehydrated dimethyl carbonate (DMC) into the beaker, immerse the open end of the battery obtained in Step 1) directly in the DMC solution, and then place the beaker in a vacuum environment.

[0047] Step 3). Turn on the vacuum device to bring the pressure inside the device to about -90 kPa, maintain the pressure for 10 minutes, and then slowly restore the pressure to atmospheric pressure. Repeat this pressure application and depressurization cycle 10 times to clean the remaining electrolyte in the battery.

[0048] Step 4). Place the battery obtained in step 3) in an oven at 40°C for 6 hours to allow the DMC inside the battery to completely evaporate.

[0049] Step 3: Perform full volume testing on the test sample and the reference sample.

[0050] The battery obtained in step two was completely dissected, removing the outer steel casing and the exposed electrode tabs. The electrode assembly was then placed in a true volume analyzer and tested according to the equipment operating procedures to obtain the true volume data of the electrode assembly. The true volume of the sample to be tested was V2 = 19.621 cm³. 3 The true volume of the reference sample is V1 = 18.930 cm³. 3 .

[0051] Step 4: Calculation of SEI film thickness

[0052] During the battery design phase, given the mass and specific surface area of ​​the positive and negative electrode active materials, the mass of the negative electrode active material is calculated to be m = 12.288 g, and the specific surface area of ​​the negative electrode material is calculated to be S = 1.42 m². 2 / g, SEI film thickness d=(19.621-18.930) / (12.288*1.42)*1000=39.6nm. Detailed battery information is shown in Table 1.

[0053] Table 1

[0054]

[0055] Test Example 2

[0056] In this embodiment, the test sample is an 18650 cylindrical lithium-ion experimental battery with a 1C capacity of 3.0Ah;

[0057] Battery 2 under test: After formation, the battery was cycled 5 times, charged at a constant current of 0.5C = 1.5A until the voltage reached 4.2V, and then charged at a constant voltage of 4.2V until the current dropped to 0.05C = 0.15A. It was then discharged at 1C = 3A until the voltage reached 2.5V. Based on the battery discharge capacity, the capacity of battery 2 under test at this time is 3.0Ah, and the capacity retention rate is 100%.

[0058] The battery testing equipment is a charge / discharge instrument, specifically the Arbin BT2000 charge / discharge system.

[0059] Step 1: Obtain the test sample and the reference sample

[0060] Battery 2 to be tested was fully discharged at 0.5C to the cutoff voltage of 2.5V, left to stand for 30 minutes, then discharged at 0.2C to the cutoff voltage of 2.5V, left to stand for 30 minutes, and then discharged at 0.05C to the cutoff voltage of 2.5V. This group was used as the test sample. The winding electrode group of the same batch as the test sample was assembled and injected with liquid. The battery was left to stand for 4 hours but was not formed as the reference sample.

[0061] Step 2: Pre-testing of the test samples and reference samples

[0062] Step 1). In the drying workshop, the test sample and reference sample obtained in the first step are dissected. Using battery dissection tools, the bottom shell of the negative terminal of the battery is removed. The battery is sealed in an aluminum-plastic bag and placed in a centrifuge at 3000 rpm for 60 minutes. The electrolyte in the battery is then thrown out into the aluminum-plastic bag under centrifugal force.

[0063] Step 2. Take a 100mL beaker, pour 20mL of dehydrated dimethyl carbonate (DMC) into the beaker, immerse the open end of the battery obtained in step 1) directly in the DMC solution, and then place the beaker in a vacuum environment.

[0064] Step 3). Turn on the vacuum device to bring the pressure inside the device to -100 kPa, maintain the pressure for 5 minutes, and then slowly restore the pressure to atmospheric pressure. Repeat this pressure application and depressurization cycle 8 times to clean the remaining electrolyte in the battery.

[0065] Step 4). Place the battery obtained in Step 3) in an oven at 50°C for 4 hours to allow the DMC inside the battery to completely evaporate.

[0066] Step 3: Perform full volume testing on the test sample and the reference sample.

[0067] The battery obtained in step two was completely dissected, removing the outer steel casing and the exposed electrode tabs. The electrode assembly was then placed in a true volume analyzer and tested according to the equipment operating procedures to obtain the true volume data of the electrode assembly. The true volume of the sample to be tested was V² = 15.46 cm³. 3 The true volume of the reference sample is V1 = 15.21 cm³. 3 .

[0068] Step 4: Calculation of SEI film thickness

[0069] During the battery design phase, given the mass and specific surface area of ​​the positive and negative electrode active materials, we can determine that the mass of the negative electrode active material is m = 10.4042 g and the specific surface area of ​​the negative electrode material is S = 2.25 m². 2 If the SEI film thickness is d = (15.46 - 15.21) / (10.4024 * 2.25) * 1000 = 10.7 nm.

[0070] Battery details are shown in Table 2.

[0071] Table 2

[0072]

[0073] This invention changes the method for detecting the SEI film thickness of battery electrodes from electron microscopy observation, which requires high levels of equipment and personnel, to a method of true volume measurement and calculation. This greatly reduces testing costs and testing cycle, and can accelerate the research and development process. In addition, by collecting SEI film thickness data of battery electrodes after formation, the quality of the formation process can be evaluated, providing data support for process improvement.

[0074] The above description is only a preferred embodiment of the present invention. It should be noted that, for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method of analyzing the thickness of the SEI film of a lithium-ion battery negative electrode, characterized by, The method comprises the steps of: S1. obtaining a test sample and a reference sample: The test sample is a battery with a stable SEI film formed before discharge, which is discharged to the cut-off voltage with a discharge current decreasing in turn, and is static for a first time before each discharge; the reference sample is a battery of the same batch as the test sample, which is not formed after being static for a second time; S2. test pretreatment of the test sample and the reference sample: After removing the negative end bottom shell of the battery of the test sample and the reference sample, each is sealed with a bag and then centrifuged to make the electrolyte in the battery be thrown out into the bag under the action of centrifugation; The opening end of the battery after the electrolyte throwing treatment is immersed in a water-removing solution and placed in a vacuumable environment, and the residual electrolyte in the battery is removed by using a negative pressure vacuum method; The battery after negative pressure vacuum treatment is dried to volatilize the water-removing solution in the battery; S3. true volume test of the test sample and the reference sample after removing the shell and the tab, to obtain the true volume V2 of the test sample and the true volume V1 of the reference sample; S4. based on the true volume V2 of the test sample and the true volume V1 of the reference sample, the thickness d of the SEI film of the battery negative electrode is calculated: d=(V2-V1) / (m*S),m is the mass of the active material of the battery negative electrode, and S is the specific surface area of the battery negative material.

2. The method of claim 1, wherein the method is characterized by: The test sample is obtained, and the test sample is discharged by the following steps: discharged to the cut-off voltage at a first discharge rate, static for a first time, discharged to the cut-off voltage at a second discharge rate, static for a first time, and discharged to the cut-off voltage at a third discharge rate, then as a test sample.

3. The method of claim 2, wherein the method is characterized by: The first discharge rate is 0.5C-1C, the second discharge rate is 0.2C-0.3C, and the third discharge rate is 0.02-0.1C.

4. The method of claim 1, wherein the method is used to analyze the SEI film thickness of a lithium-ion battery anode. The static first time is 15-60 min, and the static second time is 2-4h.

5. The method of claim 1, wherein the method is used to analyze the thickness of SEI film of a lithium-ion battery anode. The centrifugation is carried out in a centrifuge, and the speed of the centrifuge is 3000-5000rpm, and the rotation time is 30-60min.

6. The method of claim 1, wherein the method is used to analyze the thickness of the SEI film of a lithium-ion battery anode. The method of removing the residual electrolyte in the battery by using a negative pressure vacuum method is that the pressure in the vacuumable environment is in the range of-100 to-90kPa by using a vacuum device, and the pressure is kept constant for a predetermined time, and then the pressure is slowly recovered to atmospheric pressure, and the pressure and pressure relief cycle is repeated for multiple times.

7. The method of claim 6, wherein the method is used to analyze the thickness of the SEI film of a lithium-ion battery anode. The pressure and pressure relief cycle is 8-10 times, and the constant pressure for a predetermined time is 5-10min.

8. The method of claim 1, wherein the method is used to analyze the thickness of the SEI film of a lithium-ion battery anode. The temperature for drying the battery is 40-50℃, and the drying time is 4-6h.

9. The method for analyzing the thickness of the SEI film on the negative electrode of a lithium-ion battery according to claim 1, characterized in that, The water-removing solution is a DMC solution.

10. The method of claim 1, wherein the method is used to analyze the thickness of the SEI film of a lithium-ion battery anode. The bag is an aluminum plastic bag.

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