Method for analyzing film thickness of lithium ion battery

Abstract

The application discloses a kind of lithium ion battery film thickness analysis method, comprising: obtaining sample to be measured and reference sample;The sample to be measured is the battery that has formed stable film before discharge for multiple times to cut-off voltage and discharge current decreases in turn;The reference sample is the winding pole group of the same batch battery with the sample to be measured;The true density test of pole powder is carried out to the sample to be measured and reference sample, and the true density of the pole powder of both is obtained;Film thickness is calculated based on the true density of pole powder.The lithium ion battery film thickness analysis method of the application can obtain the film thickness of battery pole piece by comparing the volume change of battery pole piece active material before and after cycle, and can be obtained by simple mathematical calculation, and negative electrode material, electrolyte, etc.can be screened by the film formation state of different negative electrode materials, and it is also auxiliary to battery system development.

Description

Technical Field

[0001] This invention relates to the field of battery film formation analysis technology, and in particular to a method for analyzing the film thickness of lithium-ion batteries. Background Technology

[0002] The biggest problem currently hindering the development of lithium-ion batteries lies in battery capacity, especially the capacity of the negative electrode material. A crucial factor affecting lithium-ion battery performance is the solid electrolyte interphase (SEI) film formed on the negative electrode surface during the initial charge and discharge phase of the battery's formation. A uniform and stable SEI film protects the electrolyte from further decomposition during the subsequent negative electrode process and also prevents graphite shedding. The quality of the SEI film formation is a key determinant of the subsequent cycle performance of lithium-ion batteries. At a microscopic level, the surface of the positive electrode particles also has a very thin film called the cathode electrolyte interphase (CEI) film, the thickness of which is difficult to measure. A fully formed SEI film possesses high lithium-ion conductivity and negligible electronic conductivity, along with a certain degree of flexibility and strength. 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 constantly improving the battery formation process. The optimal formation process can obtain the optimal SEI film, which can improve the electrochemical performance of the battery on the one hand, and increase production efficiency and reduce production costs on the other hand.

[0003] The presence of an 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 variations between different battery systems. Although the SEI content in a battery is extremely low, it plays a crucial role in improving battery stability, power performance, and safety. Therefore, analyzing the thickness of the SEI film in lithium-ion batteries is significant for evaluating battery performance. Currently, the evaluation of the SEI film in lithium-ion batteries mainly relies on electron microscopy equipment and analysis of battery cycle performance. Specific analysis of SEI film thickness is also limited to electron microscopy, especially when analyzing the SEI film on the surface of the cathode material. This requires high-level equipment capabilities and skilled operators, resulting in substantial testing costs. Summary of the Invention

[0004] The purpose of this invention is to address the problems in the prior art by providing a method for analyzing the film thickness of lithium-ion batteries.

[0005] This invention is implemented as follows: a method for analyzing the film thickness of lithium-ion batteries, comprising the following steps:

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

[0007] The test sample is a battery that has formed a stable film before discharge, having been discharged multiple times to the cutoff voltage with the discharge current decreasing sequentially; the reference sample is a wound electrode assembly of batteries from the same batch as the test sample.

[0008] S1: Perform true density testing of the powder on the test sample and the reference sample.

[0009] Immerse the electrode sheet to be tested in a dehydrated solution to clean the residual electrolyte on the surface of the electrode sheet and then air dry it; weigh the electrode sheet of the same polarity in the reference sample and the dried electrode sheet to be tested, and record the mass m1 and m2 respectively; after weighing, take the electrode powder on the two electrode sheets and test the true density of the electrode powder to obtain the true density ρ1 and ρ2 of the electrode powder of the two.

[0010] S3: Calculate the film thickness d = [(m2-m) / ρ2-(m1-m) / ρ1] / (m1-m) / S based on the true density ρ1 and ρ2 of the electrode powder, where m is the mass of the electrode foil to be tested and S is the specific surface area of ​​the coating material on the electrode to be tested.

[0011] The method for analyzing the film thickness of lithium-ion batteries of the present invention can obtain the film thickness of battery electrodes by comparing the volume changes of active materials before and after cycling through simple mathematical calculations. This includes the thickness of the SEI film on the negative electrode surface and the thickness of the CEI film on the positive electrode surface. Furthermore, it can screen negative electrode materials and electrolytes by different film formation states, which is also of auxiliary significance for battery system development. Attached Figure Description

[0012] Figure 1 This is a flowchart illustrating the method for analyzing the film thickness of lithium-ion batteries according to an embodiment of the present invention. Detailed Implementation

[0013] 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 embodiments of the present invention. Obviously, the described embodiments are some embodiments of the present invention, but not all embodiments.

[0014] See Figure 1 As shown, a method for analyzing the film thickness of a lithium-ion battery includes the following steps:

[0015] S1: Obtain the test sample and reference sample

[0016] The test sample is a battery that has formed a stable film before discharge, having been discharged multiple times to the cutoff voltage with the discharge current decreasing sequentially; the reference sample is a wound electrode assembly of batteries from the same batch as the test sample.

[0017] S1: Perform true density testing of the powder on the test sample and the reference sample.

[0018] Immerse the electrode sheet to be tested in a dehydrated solution to clean the residual electrolyte on the surface of the electrode sheet and then air dry it; weigh the electrode sheet of the same polarity in the reference sample and the dried electrode sheet to be tested, and record the mass m1 and m2 respectively; after weighing, take the electrode powder on the two electrode sheets and test the true density of the electrode powder to obtain the true density ρ1 and ρ2 of the electrode powder of the two.

[0019] S3: Calculate the film thickness d = [(m2-m) / ρ2-(m1-m) / ρ1] / (m1-m) / S based on the true density ρ1 and ρ2 of the electrode powder, where m is the mass of the electrode foil to be tested and S is the specific surface area of ​​the coating material on the electrode to be tested.

[0020] In some embodiments, the steps for obtaining the sample to be tested and discharging the sample are as follows: First, discharge the sample with a first discharge current to the cutoff voltage and then let it stand for a preset time; then discharge the sample with a second discharge current to the cutoff voltage and let it stand for a preset time; finally, discharge the sample with a third discharge current to the cutoff voltage and then use the battery as the sample to be tested. The number of discharges can be set according to specific circumstances and is not limited to this set range, as long as the battery is completely discharged. Generally, the battery can be completely discharged through three different discharges.

[0021] In some embodiments, during multiple discharges, such as three times, 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. Excessive discharge current may result in incomplete discharge, while insufficient current will waste time. The specific current can be set according to the specific circumstances and is not limited to this time range, as long as the battery is fully discharged.

[0022] In some embodiments, the preset resting time is 15-60 minutes. The preset resting time can be set according to specific circumstances and is not limited to this time range, as long as the battery is fully discharged.

[0023] In some embodiments, the dehydrating solution is a dimethyl carbonate (DMC) solution. However, the dehydrating solution can also be other dehydrating solutions available in the prior art, and is not limited to DMC.

[0024] In this embodiment of the invention, the film may include a positive electrode CEI film or a negative electrode SEI film, and the electrode to be tested may be a positive electrode or a negative electrode. That is to say, the film thickness analysis method of the present invention can be used to analyze the film thickness of the negative electrode of a battery or to analyze the film thickness of the positive electrode of a battery.

[0025] In this embodiment of the invention, the true density of the electrode powder can be obtained by using a true density tester to test the true density of the electrode powder.

[0026] 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.

[0027] 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, it is cycled multiple times at room temperature before testing, charged at a constant current of 0.5C = 2.4A until the voltage reaches 4.25V, then charged at a constant voltage of 4.25V until the current drops to 0.05C = 0.24A, and then discharged at 1C = 4.8A until the voltage reaches 2.5V.

[0028] Test Example 1

[0029] In this embodiment, the test sample is an 18650 cylindrical lithium-ion experimental battery, the negative electrode material is graphite negative electrode material, and the designed capacity is 3Ah.

[0030] Battery 1 under test: Cycled 8 times at room temperature, charged at a constant current of 0.5C = 1.5A until the voltage reaches 4.2V, then charged at a constant voltage of 4.2V until the current drops to 0.05C = 0.15A, and discharged at 1C = 3A until the voltage reaches 2.5V. Based on the battery discharge capacity, the capacity of battery 1 under test is 2.98Ah, and the capacity retention rate is 99%.

[0031] The battery testing equipment is a charge / discharge instrument, specifically the Arbin BT2000 charge / discharge system.

[0032] Step 1: Obtain the test sample and the reference sample

[0033] 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. The winding electrode group from the same batch as the test sample was used as the reference sample.

[0034] Step 2: Perform true density testing on the test sample and the reference sample.

[0035] Step 1) Use a battery dissection tool to dissect the sample obtained in the first step and remove its negative electrode.

[0036] Step 2) Take a 100mL beaker, pour 70mL of dehydrated dimethyl carbonate (DMC) into the beaker, and immerse the battery negative electrode obtained in Step 1) directly in the DMC solution. After cleaning the residual electrolyte on the electrode surface, let it air dry.

[0037] Step 3) Weigh the negative electrode sheet in the reference sample and the negative electrode sheet obtained in Step 2), and record the masses as m1 = 2.04 g and m2 = 2.14 g. After weighing, scrape off the electrode powder from both negative electrode sheets and test the true density of the electrode material using a true density meter. The true densities of the two electrode materials are ρ1 = 2.24 g / cm³. 3 ρ2=2.27g / cm 3 .

[0038] Step 3: Calculation of SEI film thickness

[0039] During the battery design phase, the mass of the negative electrode copper foil, m = 0.53 g, and the specific surface area of ​​the negative electrode material, S = 2.25 m², can be obtained. 2 / g, then the SEI film thickness d=[(2.14-0.53) / 2.27-(2.04-0.53) / 2.24] / (2.04-0.53) / 2.25*1000=10.3nm.

[0040] Battery details are shown in Table 1.

[0041] Table 1. Detailed information of sample 1 battery under test.

[0042]

[0043] Test Example 2

[0044] In this embodiment, the test sample is a 21700 cylindrical lithium-ion experimental battery, and the negative electrode material is a silicon-carbon negative electrode.

[0045] Battery 2 under test: After the battery formation is completed, it is stored at 60°C for one month. It is charged at a constant current of 0.5C = 2.4A until the voltage reaches 4.25V, and then charged at a constant voltage of 4.25V until the current drops to 0.05C = 0.24A. It is then discharged at 1C = 4.8A until the voltage reaches 2.5V. Based on the battery discharge capacity, the capacity of battery 2 under test is 4.8Ah, and the capacity retention rate is 98%.

[0046] The battery testing equipment is a charge / discharge instrument, specifically the Arbin BT2000 charge / discharge system.

[0047] Step 1: Obtain the test sample and the reference sample

[0048] 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. The winding electrode group from the same batch as the test sample was used as the reference sample.

[0049] Step 2: Perform true density testing of the powder on the test sample and the reference sample.

[0050] Step 1). Use a battery dissection tool to dissect the sample obtained in the first step and remove its negative electrode.

[0051] Step 2). Take a 100mL beaker, pour 70mL of dehydrated dimethyl carbonate (DMC) into the beaker, and immerse the battery negative electrode obtained in Step 1) directly in the DMC solution. After cleaning the residual electrolyte on the electrode surface, let it air dry.

[0052] Step 3). Weigh the negative electrode sheet from the reference sample and the negative electrode sheet obtained in Step 2), and record the masses as m1 = 16.81g and m2 = 18.41g respectively. After weighing, scrape off 6g of electrode powder from both negative electrode sheets and test the true density of the electrode material using a true density meter. The true densities of the two electrode materials are ρ1 = 2.16g / cm³. 3 ρ2=2.13g / cm 3 .

[0053] Step 3: Calculation of SEI film thickness

[0054] During the battery design phase, the mass of the negative electrode copper foil, m = 5.25 g, and the specific surface area of ​​the negative electrode material, S = 1.89 m², were obtained. 2 / g, then the SEI film thickness d=[(18.41-5.25) / 2.13-(16.81-5.25) / 2.16] / (16.81-5.25) / 1.89*1000=37.5nm.

[0055] Battery details are shown in Table 2.

[0056] Table 2. Detailed information on sample 2 batteries under test.

[0057]

[0058] This invention changes the method for detecting battery film thickness from electron microscopy observation, which requires high levels of equipment and personnel, to a true density test and calculation method, which greatly reduces testing costs and testing cycle and accelerates the research and development process. In addition, by collecting battery film thickness data after formation, the quality of the formation process and the selection of active materials can be evaluated.

[0059] 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 for analyzing the film thickness of lithium-ion batteries, characterized in that, Including the following steps: S1: Obtain the test sample and the reference sample; The test sample is a battery that has formed a stable film before discharge, having been discharged multiple times to the cutoff voltage with the discharge current decreasing sequentially; the reference sample is a wound electrode assembly of batteries from the same batch as the test sample. S1: Perform true density testing of the powder on the test sample and the reference sample; Immerse the electrode to be tested in a dehydrated solution to clean the residual electrolyte on the surface of the electrode and then air dry it. Weigh the same polarity electrode sheet in the reference sample and the dried electrode sheet to be tested, and record the mass m1 and m2 respectively; after weighing, take the electrode powder on the two electrode sheets and test the true density of the electrode powder to obtain the true density ρ1 and ρ2 of the electrode powder of the two. S3: Calculate the film thickness d = [(m2-m) / ρ2 - (m1-m) / ρ1] / (m1-m) / S based on the true density ρ1 and ρ2 of the electrode powder, where m is the mass of the electrode foil to be tested and S is the mass specific surface area of ​​the material coated on the electrode to be tested.

2. The method for analyzing the film thickness of lithium-ion batteries according to claim 1, characterized in that, The steps for obtaining the sample to be tested and discharging the sample are as follows: First, discharge the battery to the cutoff voltage with the first discharge current and let it stand for a preset time. Then, discharge the battery to the cutoff voltage with the second discharge current and let it stand for a preset time. Finally, discharge the battery to the cutoff voltage with the third discharge current and use the battery as the test sample.

3. The method for analyzing the film thickness of lithium-ion batteries according to claim 2, characterized in that, 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.

4. The method for analyzing the film thickness of lithium-ion batteries according to claim 2, characterized in that, The preset settling time is 15-60 minutes.

5. The method for analyzing the film thickness of lithium-ion batteries according to claim 1, characterized in that, The dehydrated solution is a dimethyl carbonate (DMC) solution.

6. The method for analyzing the film thickness of lithium-ion batteries according to claim 1, characterized in that, The membrane includes a positive electrode CEI membrane or a negative electrode SEI membrane.

7. The method for analyzing the film thickness of lithium-ion batteries according to claim 1, characterized in that, The electrode to be tested is either a positive electrode or a negative electrode.

8. The method for analyzing the film thickness of lithium-ion batteries according to claim 1, characterized in that, The true density of the powder was obtained by testing the true density of the powder using a true density meter.

9. The method for analyzing the film thickness of lithium-ion batteries according to claim 1, characterized in that, The sample to be tested was charged and discharged using a charge-discharge instrument.

10. The method for analyzing the film thickness of a lithium-ion battery according to claim 1, characterized in that, Before testing, the sample to be tested undergoes a charge-discharge pretreatment process to bring it to a predetermined test state.

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