A quantitative analysis method for SEI film of negative electrode of lithium ion battery

Abstract

The invention discloses a quantitative analysis method for the SEI film of the negative electrode of a lithium ion battery, which relates to the technical field of lithium ion battery pole piece testing, and comprises the following steps: in a glove box, disassembling a formed experimental battery cell to obtain a negative pole piece, grinding at low temperature to obtain pole piece powder; in the glove box, weighing the pole piece powder and potassium bromide powder, mixing and tableting, and calculating the Li 2 CO 3 The absorbance of the characteristic peak; graphite and lithium carbonate standard are mixed in different proportions to obtain a standard mixed sample, the standard mixed sample is mixed with potassium bromide and pressed into a tablet, and a standard curve is drawn according to the infrared spectrum; Li 2 CO 3 The absorbance of the characteristic peak was compared with the standard curve to determine the Li 2 CO 3 The present invention uses infrared spectroscopy to determine the content of Li in the negative electrode. 2 CO 3 The content can be measured, and the growth of SEI film in the battery cell can be quantitatively analyzed. The growth changes of SEI film under different formation conditions can be tested to find the appropriate formation process conditions.

Description

Technical Field

[0001] The invention relates to the technical field of lithium ion battery pole piece testing, and in particular to a quantitative analysis method for a negative electrode SEI film of a lithium ion battery. Background Art

[0002] In the production process of lithium-ion batteries, the first charging step after the battery is filled is called formation. During the formation of lithium-ion batteries, the electrolyte solvent and lithium salt react with each other, forming a solid electrolyte interface film, namely SEI film, at the negative electrode of the lithium-ion battery. The SEI film is Li + An excellent conductor that can transport Li + It can also be used to deintercalate lithium; at the same time, it is a good electronic insulator, which can effectively prevent the co-intercalation of solvent molecules and thus damage the electrode material. However, the formation of the SEI film consumes part of the Li + , increasing the irreversible capacity of the first charge and discharge. The continuous growth of the SEI film is accompanied by the consumption of the electrolyte, which continuously causes capacity decay. In addition, the SEI film also increases the Li + The transmission impedance has an impact on the kinetic performance of the entire system. It can be seen that the existence and growth of the SEI film are largely related to the cycle performance and life of the battery cell. The growth of the SEI film is affected by factors such as the injection volume, charging voltage, and temperature. Therefore, it is crucial to control the thickness of the SEI film by controlling the formation conditions. However, the prior art lacks quantitative detection of the SEI film.

[0003] The composition of the SEI film is greatly affected by the composition of the electrolyte. For example, ether electrolytes tend to produce SEI films containing lithium alkoxides, carbonate electrolytes produce SEI films containing lithium carbonate, and methyl formate electrolytes produce SEI films containing lithium formate. Currently, most electrolyte main solvents are carbonate substances, and the corresponding battery SEI films basically contain Li2CO3.

[0004] Infrared spectroscopy has many advantages such as fast detection speed, simple pre-treatment, and high sensitivity. It belongs to absorption spectroscopy. In the infrared spectrum of lithium carbonate standard, at 1450cm -1 Centered around 1400cm -1 and 1500cm -1 There is a pair of symmetrical peaks nearby, which are [CO3] 2- Antisymmetric stretching vibration peak, 870 cm -1 Nearby is [CO3] 2- Out-of-plane bending vibration absorption peak. Lambert-Beer's law shows that there is a linear relationship between the absorbance of the absorption peak of a specific component and the concentration of the substance. If infrared spectroscopy can be used to quantify Li2CO3 in the negative electrode, it will be of great help in finding suitable formation conditions. Summary of the invention

[0005] Based on the technical problems existing in the background technology, the present invention proposes a quantitative analysis method for the SEI film of the negative electrode of a lithium-ion battery, which uses infrared spectroscopy to determine the Li2CO3 content in the negative electrode sheet, and then quantitatively analyzes the growth of the SEI film in the battery cell.

[0006] The present invention provides a method for quantitatively analyzing the SEI film of a negative electrode of a lithium ion battery, comprising the following steps:

[0007] S1, forming the experimental battery cell of the lithium-ion battery;

[0008] S2. In a glove box, the formed experimental battery cell is disassembled to obtain the negative electrode sheet, and then cryogenically ground to obtain the electrode sheet powder;

[0009] S3. In a glove box, weigh the electrode powder and potassium bromide powder, mix and grind to obtain a mixed powder;

[0010] S4, pressing the mixed powder into tablets, performing infrared spectrum analysis on the tablets, and calculating the absorbance of the characteristic peak of Li2CO3 in the sample spectrum;

[0011] S5. Mixing graphite and lithium carbonate standard products in different proportions to obtain standard mixed samples, then mixing the standard mixed samples with potassium bromide, grinding them at low temperature, pressing them into tablets, and performing infrared spectroscopy analysis on them, and drawing a standard curve based on the absorbance of the Li2CO3 characteristic peak in the spectrum and the lithium carbonate content in the standard mixed samples;

[0012] S6. Compare the absorbance of the characteristic peak of Li2CO3 in the sample spectrum in S4 with the standard curve to quantitatively analyze the content of Li2CO3 in the negative electrode.

[0013] Preferably, in S2, the active material on the disassembled negative electrode sheet is scraped and then cryogenically ground.

[0014] Preferably, cryogenic grinding is performed using a cryo-grinder; the parameters of the cryo-grinder are set as follows: 3 cycles, precooling for 10 min, running time for 2 min, and rate of 10 CPS.

[0015] Preferably, the powder particle size for tableting is ≥ 200 mesh.

[0016] In the present invention, during the establishment of the standard curve, the ratio of graphite and lithium carbonate standards in the standard sample can be adjusted according to the absorbance of the Li2CO3 characteristic peak in the sample spectrum to ensure that the Li2CO3 content in the sample is within the range of the standard curve to ensure the accuracy of the analysis results.

[0017] Beneficial effects: The present invention uses infrared spectroscopy to determine the Li2CO3 content in the negative electrode plate, and then quantitatively analyzes the growth of the SEI film in the battery cell, and by testing the growth changes of the SEI film under different formation conditions, further explores the appropriate formation process conditions in order to optimize the cycle performance and life of the battery.

[0018] During the detection process, low-temperature grinding and mixing are used to fully mix graphite with materials such as lithium carbonate and make them mixed evenly, so as to make up for the local defects of spectral analysis and make the analysis results more accurate. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 This is an infrared spectrum of the negative electrode sample in the embodiment of the present invention;

[0020] Figure 2 The characteristic peak of Li2CO3 in the embodiment of the present invention is 1432cm -1 Relationship between the absorbance and the lithium carbonate content in the standard mixed sample. DETAILED DESCRIPTION

[0021] In the following examples, infrared spectroscopy was performed using a ThermoFisher Nicolet iS50 Fourier transform infrared spectrometer.

[0022] The technical solution of the present invention is described in detail below through specific embodiments.

[0023] Example

[0024] (1) The experimental lithium iron phosphate battery cell was formed at 20°C, 0.33C, and a cut-off voltage of 3.5V.

[0025] (2) In the glove box, weigh 40±10 mg of the disassembled negative electrode sheet and place it in a grinding jar. Set the parameters of the cryo-grinder and place the sample for grinding.

[0026] (3) After grinding, the grinding jar was transferred to a glove box, and 3 mg of the ground negative electrode powder and 200 mg of potassium bromide powder were placed in a new grinding jar for grinding.

[0027] (4) The mixed powder obtained after grinding is placed in a tabletting mold, pressed into tablets on a tablet press, and subjected to infrared spectroscopy analysis. The spectrum is as follows: Figure 1 As shown, calculate the absorbance of the characteristic peak of Li2CO3 in the sample spectrum;

[0028] (5) Graphite and lithium carbonate standard were mixed in different proportions to obtain standard samples, and then the standard samples were mixed with potassium bromide, ground at low temperature, pressed into tablets, and subjected to infrared spectroscopy analysis. According to the characteristic peak of Li2CO3 at 1432 cm in the spectrum, -1The standard curve is drawn by comparing the absorbance of the sample with the lithium carbonate content in the standard mixed sample; the standard curve is as follows: Figure 2 As shown;

[0029] (6) Compare the absorbance of the characteristic peak of Li2CO3 in the sample spectrum with the standard curve to quantitatively analyze the content of Li2CO3 in the negative electrode.

[0030] By comparison, the content of Li2CO3 in the negative electrode sheet in this embodiment is 2.13%.

[0031] In the present invention, the SEI film under other formation conditions can also be quantitatively analyzed, such as the SEI film formed at 30°C, 0.33C, and a cutoff voltage of 3.5V. By testing the growth changes of the SEI film under different formation conditions, the appropriate formation process conditions can be explored to optimize the cycle performance and life of the battery.

[0032] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any technician familiar with the technical field can make equivalent replacements or changes according to the technical scheme and inventive concept of the present invention within the technical scope disclosed by the present invention, which should be covered by the protection scope of the present invention.

Claims

1. A quantitative analysis method for the SEI film of a negative electrode of a lithium ion battery, characterized in that: The following steps are involved: S1, forming the experimental cells of the lithium-ion battery; S2. In a glove box, the formed experimental battery cell is disassembled to obtain the negative electrode sheet, and then cryogenically ground to obtain the electrode sheet powder; S3. In a glove box, weigh the electrode powder and potassium bromide powder, mix and grind to obtain a mixed powder; S4, pressing the mixed powder into tablets, performing infrared spectrum analysis on the tablets, and calculating the absorbance of the characteristic peak of Li2CO3 in the sample spectrum; S5. Mixing graphite and lithium carbonate standard products in different proportions to obtain standard mixed samples, then mixing the standard mixed samples with potassium bromide, grinding them at low temperature, pressing them into tablets, and performing infrared spectroscopy analysis on them, and drawing a standard curve based on the absorbance of the Li2CO3 characteristic peak in the spectrum and the lithium carbonate content in the standard mixed samples; S6. Compare the absorbance of the characteristic peak of Li2CO3 in the sample spectrum in S4 with the standard curve to quantitatively analyze the content of Li2CO3 in the negative electrode.

2. The quantitative analysis method of the negative electrode SEI film of a lithium ion battery according to claim 1, characterized in that: In S2, the active material on the disassembled negative electrode sheet is scraped and then cryogenically ground.

3. The quantitative analysis method of the negative electrode SEI film of a lithium ion battery according to claim 1, characterized in that: Cryogenic grinding was performed using a cryo-grinder; the parameters of the cryo-grinder were set as follows: 3 cycles, 10 min precooling, 2 min run time, and a rate of 10 CPS.

4. The quantitative analysis method of the negative electrode SEI film of a lithium ion battery according to claim 1, characterized in that: The powder particle size for tableting is ≥200 mesh.

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