Method for analyzing components and structure of SEI (solid electrolyte interface) membrane of lithium ion battery

Abstract

The invention relates to the technical field of batteries, in particular to a lithium ion battery SEI membrane component and structure analysis method. Comprising the following steps: 1, preprocessing a to-be-detected battery and obtaining a negative plate; 2, in-situ gas production monitoring of SEI membrane decomposition of the negative plate; and 3, determining the components of the SEI membrane according to the type of produced gas. The invention provides a lithium ion battery SEI membrane component and structure analysis method based on an acid titration test. The reaction speed of the acid and the inner and outer layer components of the SEI membrane can be effectively controlled by regulating and controlling the concentration of the acid solution, meanwhile, the components and the structure of the SEI membrane can be effectively distinguished and represented by combining in-situ gas production and gas component test results, and the problem that an existing SEI membrane test method is low in accuracy is solved.

Description

Technical Field

[0001] The present invention relates to the technical field of batteries, and in particular to a method for analyzing SEI membrane components and structures of lithium-ion batteries. Background Art

[0002] The solid electrolyte interface (SEI) film is an interfacial film formed on the surface of the electrode material after the electrolyte undergoes redox decomposition during the initial charging process of a lithium-ion battery. It has the characteristics of ion conduction and electronic insulation. An SEI film with excellent performance can effectively inhibit side reactions between the battery electrode material and the electrolyte, thereby improving the performance of the lithium-ion battery. Due to the complex formation process of the SEI film and the extreme difficulty of characterization and testing, the current understanding of the characteristics of the SEI film remains at the stage of experimental observation and model conjecture. In-depth research is needed to quantitatively analyze the composition and structure of the SEI film in order to specifically regulate the electrode material, electrolyte type, and formation conditions to improve the electrochemical performance of lithium-ion batteries.

[0003] Currently, the main methods for detecting SEI films include X-ray photoelectron spectroscopy (XPS), Raman spectroscopy (Raman), atomic force microscopy (AFM), etc. XPS uses X-rays to excite the surface electrons of the sample and analyzes the binding energy to determine the composition and chemical state of the organic and inorganic components in the SEI film, but it requires an ultra-high vacuum detection environment and may be affected by Ar. + Sputtering (depth profiling) introduces artifacts; Raman can distinguish between graphitized / non-graphitized carbon and Li2O2 to determine the thickness of the SEI film, but the detection is interfered by the fluorescence background and has low sensitivity to amorphous phases; AFM etches the surface to detect the thickness of the SEI film, but is easily affected by surface roughness. With the continuous development of these technologies, hole models, multilayer models, and mosaic models have been proposed to explain the structure of the SEI film. However, there is still a lack of reliable characterization methods for accurate and effective characterization of the SEI film, which limits our in-depth understanding of the SEI film. Summary of the Invention

[0004] The purpose of the present invention is to overcome the shortcomings and defects of the prior art and to provide a method for analyzing the components and structure of the SEI film of a lithium ion battery.

[0005] In order to achieve the above objectives, this application adopts the following solutions:

[0006] A method for analyzing SEI film components of a lithium-ion battery, characterized by comprising the following steps:

[0007] Step 1: Pre-treatment of the battery to be tested and acquisition of the negative electrode sheet;

[0008] Step 2: In-situ gas production monitoring of SEI film decomposition on the negative electrode;

[0009] Step 3: Determine the components of the SEI film based on the type of gas produced.

[0010] In the third step, the gas type is CO2, and the SEI film component is alkyl lithium carbonate or Li2CO3. Preferably, the alkyl lithium carbonate is (CH2OCO2Li)2 or HOCH2CH2OCO2Li. Because the content of alkyl lithium carbonate in the SEI film is much higher than that of Li2CO3, CO2 mainly comes from alkyl lithium carbonate.

[0011] If the gas type is CH4, the SEI film component is CH3Li;

[0012] If the gas type is C2H2, the SEI film component is Li2C2;

[0013] If the gas type is C2H4, the SEI film component is C2H3Li;

[0014] If the gas type is C2H6, the SEI film component is C2H5Li;

[0015] If the gas type is C3H6, the SEI film component is C3H5Li;

[0016] If the gas type is H2, the SEI film component is Li or Li x C6; where X≤1.

[0017] The first step includes the following steps: 1.1) fully discharging the battery to be tested; 1.2) disassembling the battery to be tested to obtain the negative electrode sheet, and placing it in a sealed gas collection bag; 1.3) testing the gas collection bag containing the negative electrode sheet of the battery to be tested as a test sample.

[0018] The specific steps of step 1.1) are: first discharge the battery to the cut-off voltage at 0.5C, let it rest for 30 minutes, then discharge it again to the cut-off voltage at 0.1C, let it rest for 30 minutes, and then discharge it to the cut-off voltage at 0.05C again.

[0019] In the second step, an acid solution is used to completely decompose the SEI film; in the third step, a high-sensitivity gas chromatography is used to test the gas composition of the sample; preferably, the acid solution is an aqueous solution of one or a mixture of at least two of hydrochloric acid, sulfuric acid, and nitric acid.

[0020] The present invention also includes a method for analyzing the SEI membrane structure of a lithium-ion battery, comprising the method for analyzing the components of the SEI membrane of a lithium-ion battery;

[0021] In the second step, a low-concentration acid solution of appropriate concentration is used to decompose the SEI film, and samples are taken at appropriate sampling intervals;

[0022] In the third step, high-sensitivity gas chromatography is used to test the gas composition of the samples and draw a relationship diagram between gas production components and gas content-reaction time. The SEI film structure of the lithium-ion battery is analyzed based on the relationship diagram between gas production components and gas content-reaction time.

[0023] The SEI film is decomposed using a low-concentration acid solution of appropriate concentration, which specifically includes the following steps:

[0024] Step 2.1) preparing a high-concentration acid solution with a concentration of 2-12 M and a low-concentration acid solution with a concentration of 0.5-1 M, preferably a 1 M low-concentration acid solution;

[0025] Step 2.2) Add 10-20 mL of a high-concentration acid solution and 10-20 mL of a low-concentration acid solution to two of the test samples, respectively, and monitor the gas production in situ;

[0026] After the gas production in step 2.3) stabilizes, compare the gas production of the two test samples in step 2.2). If the gas production is the same, it indicates that the low-concentration acid solution can completely decompose the SEI film; then use the same low-concentration acid solution as in step 2.2) to test the test sample; if the gas production is different, it indicates that the reaction is incomplete, and increase the concentration of the low-concentration acid solution until the gas production is the same as that of the high-concentration acid solution;

[0027] In step 2.4), the SEI film is decomposed using the appropriate low-concentration acid solution obtained in step 2.3).

[0028] The method for determining the sampling interval is as follows: based on the two inflection points of the gas production-reaction time relationship diagram in the in-situ monitoring data, the reaction of the SEI film components is divided into three stages, and sampling is performed according to different stages.

[0029] In the first stage of the reaction between the acid solution and the SEI membrane components, gas is taken from the air bag valve every 5-10 minutes; in the second stage of the reaction between the acid solution and the SEI membrane components, gas is taken every 10-30 minutes; in the third stage of the reaction between the acid solution and the SEI membrane components, gas is taken every 30-60 minutes.

[0030] In the third step, in the relationship diagram of gas production components and gas content-reaction time, if two gas components appear and the contents of the two gases show a trend of first increasing and then tending to be stable with the extension of reaction time, it proves that the SEI film is a double-layer structure; if multiple gas components appear and the contents of multiple gases show a trend of first increasing and then tending to be stable with the extension of reaction time, it proves that the SEI film is a multi-layer structure; if multiple gas components appear but the contents of multiple gases show a trend of increasing at the same time with the extension of reaction time, it proves that the SEI film is a mosaic structure composed of multiple components.

[0031] Compared with the prior art, the present invention has the following beneficial effects:

[0032] This paper proposes a method for analyzing the SEI membrane composition and structure in lithium-ion batteries based on acid titration testing. By adjusting the concentration of the acid solution, the reaction rate between the acid and the inner and outer components of the SEI membrane can be effectively controlled. Combined with in-situ gas production and gas composition testing, the SEI membrane's composition and structure can be effectively distinguished and characterized, addressing the low accuracy of existing SEI membrane testing methods. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 This is the in-situ monitoring data of the reaction between the high and low concentration acid solutions and the negative electrode sheet in Examples 1 and 2 of the present invention;

[0034] Figure 2 This is a graph showing the relationship between the gas components and gas content and the reaction time in Example 2 of the present invention;

[0035] Figure 3 This is a graph showing the relationship between the gas components and gas content and the reaction time in Example 3 of the present invention;

[0036] Figure 4 This is a graph showing the relationship between the gas production components and gas content and the reaction time in Example 4 of the present invention. DETAILED DESCRIPTION

[0037] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0038] Example 1

[0039] In this embodiment, the battery to be disassembled is a fresh battery after formation, and the battery capacity is 2.8 Ah.

[0040] The method for analyzing SEI film components of lithium-ion batteries comprises the following steps:

[0041] Step 1: Pre-treatment of the battery to be tested and acquisition of the negative electrode sheet

[0042] Step 1.1) Fully discharge the battery under test at 0.5C to 2.5V, let it rest for 30 minutes, then discharge it again at 0.1C to 2.5V, let it rest for 30 minutes, and then discharge it again at 0.05C to 2.5V;

[0043] Step 1.2) Place a commercial Beekman Bio single-valve aluminum foil gas sampling bag and the battery to be tested in a glove box. Disassemble the battery in the glove box, clean the negative electrode sheet with DMC, and air-dry it. Weigh 5g of the negative electrode sheet and place it in the gas sampling bag. The bag is sealed to obtain the test sample.

[0044] Step 1.3) For the same battery to be tested, prepare three negative electrode sheet test samples and then take the samples out of the glove box.

[0045] Step 2: In-situ gas production monitoring of SEI film decomposition of negative electrode

[0046] Step 2.1) Prepare 15 mL of 2 M and 1 M hydrochloric acid solutions respectively;

[0047] Step 2.2) Sample 1 was taken and 15 mL of 2M hydrochloric acid solution was injected into the gas sampling bag valve using a syringe. The gas production from the reaction between the hydrochloric acid solution and the negative electrode sheet was monitored in situ using the Archimedean displacement method. The Archimedean displacement method measures the change in buoyancy of the sample in the liquid and divides the change in buoyancy by the density of the liquid to calculate the change in sample volume, i.e., the gas production.

[0048] Take sample 2 and inject 15 mL of 1 M hydrochloric acid solution from the gas sampling bag valve using a syringe. Similarly, use the Archimedean displacement method to monitor the gas production from the reaction between the hydrochloric acid solution and the negative electrode in situ.

[0049] Step 2.3) Record the relationship between the gas production and reaction time of 2M and 1M hydrochloric acid solutions and the negative electrode sheet. When the gas production no longer changes with the reaction time, it proves that the reaction is complete. Figure 1 After about 400 minutes of reaction between the 2M and 1M acid solutions and the negative electrode, the gas production stabilized at 3.6 mL, proving that hydrochloric acid solutions of different concentrations reacted completely with the components in the SEI film.

[0050] Step 2.4) Select 1 M hydrochloric acid solution to decompose the SEI film of sample 3; inject 15 mL of 1 M hydrochloric acid solution from the air bag valve, and collect the gas in the air bag into coded commercial vacuum blood collection tubes according to the gas sampling interval.

[0051] In the in-situ monitoring data of the reaction between 1M hydrochloric acid solution and the negative electrode sheet, the relationship between gas production and reaction time shows that there are two inflection points in the change of gas production with reaction time, proving that the reaction between low-concentration hydrochloric acid solution and SEI film components goes through three reaction stages, and different gas sampling intervals are used in different stages:

[0052] In the first stage of the reaction between the acid solution and the SEI membrane components, gas is taken from the air bag valve every 5 minutes; in the second stage of the reaction between the acid solution and the SEI membrane components, gas is taken every 30 minutes; in the third stage of the reaction between the acid solution and the SEI membrane components, gas is taken every 50 minutes.

[0053] Step 3: Determine the components of the SEI film based on the detection of the gas components collected in the second step; use high-sensitivity gas chromatography to test the gas components collected in the vacuum tube. The components of the SEI film can be determined based on the types of gas components. The corresponding relationship between the two is shown in Table 1. The reaction formula is as follows:

[0054] RLi+H + →Li + +RH

[0055] Li2C2+2H + →2Li + +C2H2

[0056] Li2CO3+2H + →H2O+2Li + +CO2

[0057] (CH2OCO2Li)2+2H + →(CH2OH)2+2Li + +2CO2

[0058] HOCH2CH2OCO2Li+H + →(CH2OH)2+Li + +CO2

[0059] Li+H + →Li + +1 / 2H2

[0060] Li x C6+xH + →xLiOH+C6+x / 2H2(x≤1)

[0061] Wherein RLi is CH3Li, C2H3Li, C2H5Li and C3H5Li.

[0062] Table 1 shows the correspondence between SEI film components and gas products.

[0063] Table 1

[0064]

[0065] The gases detected in the gas composition test results include CO2, C2H2, C2H6 and H2. It can be determined that the components of the SEI film are alkyl lithium carbonate, Li2C2, and C2H5Li. H2 is produced by the reaction of active lithium in the negative electrode material with acid.

[0066] Example 2-4 is a method for analyzing the SEI film structure of a lithium-ion battery.

[0067] Example 2

[0068] In this embodiment, the battery to be disassembled is a fresh battery after formation, and the battery capacity is 2.8 Ah.

[0069] Step 1: Pre-treatment of the battery to be tested and acquisition of the negative electrode sheet

[0070] Step 1.1) Fully discharge the battery under test at 0.5C to 2.5V, let it rest for 30 minutes, then discharge it again at 0.1C to 2.5V, let it rest for 30 minutes, and then discharge it again at 0.05C to 2.5V;

[0071] Step 1.2) Place a commercial Beekman Bio single-valve aluminum foil gas sampling bag and the battery to be tested in a glove box. Disassemble the battery in the glove box, clean the negative electrode sheet with DMC, and air-dry it. Weigh 5g of the negative electrode sheet and place it in the gas sampling bag. The bag is sealed to obtain the test sample.

[0072] Step 1.3) For the same battery to be tested, prepare three negative electrode sheet test samples, named Sample 1, Sample 2, and Sample 3, and then remove the samples from the glove box.

[0073] Step 2: In-situ gas production monitoring of SEI film decomposition of negative electrode

[0074] Step 2.1) Prepare 15 mL of 2 M and 1 M hydrochloric acid solutions respectively;

[0075] Step 2.2) Take sample 1 and inject 15 mL of 2M hydrochloric acid solution through the valve of the gas sampling bag using a syringe. Use the Archimedean displacement method to monitor the gas production generated by the reaction between the hydrochloric acid solution and the negative electrode in situ;

[0076] Take sample 2 and inject 15 mL of 1 M hydrochloric acid solution from the gas sampling bag valve using a syringe. Similarly, use the Archimedean displacement method to monitor the gas production from the reaction between the hydrochloric acid solution and the negative electrode in situ.

[0077] Step 2.3) Record the relationship between the gas production and reaction time of 2M and 1M hydrochloric acid solutions and the negative electrode sheet. When the gas production no longer changes with the reaction time, it proves that the reaction is complete. Figure 1 After about 400 minutes of reaction between the 2M and 1M acid solutions and the negative electrode, the gas production stabilized at 3.6 mL, proving that hydrochloric acid solutions of different concentrations reacted completely with the components in the SEI film.

[0078] Step 2.4) Select 1 M hydrochloric acid solution to decompose the SEI film of sample 3; inject 15 mL of 1 M hydrochloric acid solution from the air bag valve, and collect the gas in the air bag into coded commercial vacuum blood collection tubes according to the gas sampling interval.

[0079] In the in-situ monitoring data of the reaction between 1M hydrochloric acid solution and the negative electrode sheet, the relationship between gas production and reaction time shows that there are two inflection points in the change of gas production with reaction time, proving that the reaction between low-concentration hydrochloric acid solution and SEI film components goes through three reaction stages, and different gas sampling intervals are used in different stages:

[0080] In the first stage of the reaction between the acid solution and the SEI membrane components, gas is taken from the air bag valve every 5 minutes; in the second stage of the reaction between the acid solution and the SEI membrane components, gas is taken every 30 minutes; in the third stage of the reaction between the acid solution and the SEI membrane components, gas is taken every 50 minutes.

[0081] Step 3: Determine the structure of the SEI film

[0082] The components of the SEI film are determined by detecting the gas components collected in the second step, using the same method as in Example 1. Then, a graph is drawn with the reaction time of hydrochloric acid and SEI film as the horizontal axis and the gas components and gas content as the vertical axis. Figure 2 ; At the beginning of the reaction, a large amount of CO2 is immediately produced, proving that the outermost layer of the SEI film is alkyl lithium carbonate and has the highest content; secondly, as the reaction time increases, the content of C2H2 and C2H6 increases, but at the same time it is still accompanied by an increase in CO2. The change trends of the three are the same, proving that the inner layer structure of SEI is a mosaic structure composed of three components: alkyl lithium carbonate, Li2C2, and C2H5Li.

[0083] Example 3:

[0084] In this embodiment, the battery to be disassembled is a battery after high-temperature storage, and the battery capacity is 2 Ah.

[0085] Step 1: Pre-treatment of the battery to be tested and acquisition of the negative electrode sheet

[0086] Step 1.1) Fully discharge the battery under test at 0.5C to 2.5V, let it rest for 30 minutes, then discharge it again at 0.1C to 2.5V, let it rest for 30 minutes, and then discharge it again at 0.05C to 2.5V;

[0087] Step 1.2) Place a commercial Beekman Bio single-valve aluminum foil gas sampling bag and the battery to be tested in a glove box. Disassemble the battery in the glove box, clean the negative electrode sheet with DMC, and air-dry it. Weigh 5g of the negative electrode sheet and place it in the gas sampling bag. The bag is sealed to obtain the test sample.

[0088] Step 1.3) For the same battery to be tested, prepare three negative electrode sheet test samples, named Sample 1, Sample 2, and Sample 3, and then remove the samples from the glove box.

[0089] Step 2: In-situ gas production monitoring of SEI film decomposition of negative electrode

[0090] Step 2.1) Prepare 15 mL of 4 M and 1 M hydrochloric acid solutions respectively;

[0091] Step 2.2) Take sample 1 and inject 15 mL of 4M hydrochloric acid solution through the valve of the gas sampling bag using a syringe. Use the Archimedean displacement method to monitor the gas production generated by the reaction between the hydrochloric acid solution and the negative electrode in situ;

[0092] Take sample 2 and inject 15 mL of 1 M hydrochloric acid solution from the gas sampling bag valve using a syringe. Similarly, use the Archimedean displacement method to monitor the gas production from the reaction between the hydrochloric acid solution and the negative electrode in situ.

[0093] Step 2.3) Record the relationship between the gas production and reaction time of the 4M and 1M hydrochloric acid solutions with the negative electrode. When the gas production no longer changes with the reaction time, the reaction is complete. After the 4M and 1M acid solutions react with the negative electrode for 30 minutes and 350 minutes, respectively, the gas production stabilizes at 5.7 mL, indicating that the hydrochloric acid solutions of different concentrations have completely reacted with the components in the SEI film.

[0094] Step 2.4) Select 1 M hydrochloric acid solution to decompose the SEI film of sample 3; inject 15 mL of 1 M hydrochloric acid solution from the air bag valve, and collect the gas in the air bag into coded commercial vacuum blood collection tubes according to the gas sampling interval.

[0095] In the in-situ monitoring data of the reaction between 1M hydrochloric acid solution and the negative electrode sheet, the relationship between gas production and reaction time shows that there are two inflection points in the change of gas production with reaction time, proving that the reaction between low-concentration hydrochloric acid solution and SEI film components goes through three reaction stages, and different gas sampling intervals are used in different stages:

[0096] In the first stage of the reaction between the acid solution and the SEI membrane components, gas is taken from the air bag valve every 5 minutes; in the second stage of the reaction between the acid solution and the SEI membrane components, gas is taken every 30 minutes; in the third stage of the reaction between the acid solution and the SEI membrane components, gas is taken every 50 minutes.

[0097] Step 3: Determine the structure of the SEI film

[0098] The components of the SEI film are determined by detecting the gas components collected in the second step, using the same method as in Example 1. Then, a graph is drawn with the reaction time of hydrochloric acid and SEI film as the horizontal axis and the gas components and gas content as the vertical axis. Figure 3 At the beginning of the reaction, a large amount of CO2 is immediately produced, indicating that the outermost layer of the SEI membrane is composed of alkyl lithium carbonate and has the highest content. Subsequently, C2H6 appears and its content gradually increases, indicating that the inner layer is C2H5Li. In summary, the SEI membrane has a double-layer structure with an outer layer of alkyl lithium carbonate and an inner layer of C2H5Li.

[0099] Example 4:

[0100] In this embodiment, the battery to be disassembled is a battery that has been cycled to a capacity retention rate of 60% SOH, and the battery capacity is 2 Ah.

[0101] Step 1: Pre-treatment of the battery to be tested and acquisition of the negative electrode sheet

[0102] Step 1.1) Fully discharge the battery under test at 0.5C to 2.5V, let it rest for 30 minutes, then discharge it again at 0.1C to 2.5V, let it rest for 30 minutes, and then discharge it again at 0.05C to 2.5V;

[0103] Step 1.2) Place a commercial Beekman Bio single-valve aluminum foil gas sampling bag and the battery to be tested in a glove box. Disassemble the battery in the glove box, clean the negative electrode sheet with DMC, and air-dry it. Weigh 5g of the negative electrode sheet and place it in the gas sampling bag. The bag is sealed to obtain the test sample.

[0104] Step 1.3) For the same battery to be tested, prepare three negative electrode sheet test samples, named Sample 1, Sample 2, and Sample 3, and then remove the samples from the glove box.

[0105] Step 2: In-situ gas production monitoring of SEI film decomposition of negative electrode

[0106] Step 2.1) Prepare 15 mL of 3 M and 1 M hydrochloric acid solutions respectively;

[0107] Step 2.2) Take sample 1 and inject 15 mL of 3M hydrochloric acid solution through the valve of the gas sampling bag using a syringe. Use the Archimedean displacement method to monitor the gas production generated by the reaction between the hydrochloric acid solution and the negative electrode in situ;

[0108] Take sample 2 and inject 15 mL of 1 M hydrochloric acid solution from the gas sampling bag valve using a syringe. Similarly, use the Archimedean displacement method to monitor the gas production from the reaction between the hydrochloric acid solution and the negative electrode in situ.

[0109] Step 2.3) Record the relationship between the gas production and reaction time of the 3M and 1M hydrochloric acid solutions and the negative electrode sheet. When the gas production no longer changes with the reaction time, the reaction is complete.

[0110] After about 300 minutes of reaction between the 3M and 1M acid solutions and the negative electrode sheet, the gas production stabilized at 7.4 mL, proving that the hydrochloric acid solutions of different concentrations reacted completely with the components in the SEI film.

[0111] Step 2.4) Select 1 M hydrochloric acid solution to decompose the SEI film of sample 3; inject 15 mL of 1 M hydrochloric acid solution from the air bag valve, and collect the gas in the air bag into coded commercial vacuum blood collection tubes according to the gas sampling interval.

[0112] In the in-situ monitoring data of the reaction between 1M hydrochloric acid solution and the negative electrode sheet, the relationship between gas production and reaction time shows that there are two inflection points in the change of gas production with reaction time, proving that the reaction between low-concentration hydrochloric acid solution and SEI film components goes through three reaction stages, and different gas sampling intervals are used in different stages:

[0113] In the first stage of the reaction between the acid solution and the SEI membrane components, gas is taken from the air bag valve every 10 minutes; in the second stage of the reaction between the acid solution and the SEI membrane components, gas is taken every 30 minutes; in the third stage of the reaction between the acid solution and the SEI membrane components, gas is taken every 50 minutes.

[0114] Step 3: Determine the structure of the SEI film

[0115] The components of the SEI film are determined by detecting the gas components collected in the second step, using the same method as in Example 1. Then, a graph is drawn with the reaction time of hydrochloric acid and SEI film as the horizontal axis and the gas components and gas content as the vertical axis. Figure 4 ; During the reaction time of 0-50min, a large amount of CO2 was produced and then tended to be stable, proving that the outermost layer of the SEI film was alkyl lithium carbonate; secondly, C2H2 increased and tended to be stable after 140min of reaction, proving that the second layer of the SEI film was Li2C2; subsequently, CH4, C2H4, and C2H6 increased almost simultaneously but at a lower content, proving that the third layer of the SEI film was a complex of CH3Li, C2H3Li, and C2H5Li; in summary, it was proved that the structure of SEI was a multi-layer structure, and the third layer was a mosaic structure.

[0116] In summary, this invention proposes a method for analyzing the composition and structure of the SEI membrane in lithium-ion batteries based on acid titration testing. By regulating the concentration of the acid solution, the reaction rate between the acid and the inner and outer components of the SEI membrane can be effectively controlled. Combined with in-situ gas production and gas composition testing, the SEI membrane's composition and structure can be effectively distinguished and characterized, addressing the low accuracy of existing SEI membrane testing methods.

[0117] The basic principles, main features and advantages of the present invention are shown and described above. It is obvious to those skilled in the art that the present invention is not limited to the details of the above exemplary embodiments and that the present invention can be implemented in other specific forms without departing from the spirit or basic features of the present invention.

[0118] The embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being defined by the appended claims rather than the foregoing description, and all changes that come within the meaning and range of equivalents of the claims are therefore intended to be embraced therein.

[0119] In addition, it should be understood that although this specification is described in terms of implementation methods, not every implementation method contains only one independent technical solution. This narrative method of the specification is only for the sake of clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in each embodiment can also be appropriately combined to form other implementation methods that can be understood by those skilled in the art.

Claims

1. A method for analyzing SEI film components of a lithium-ion battery, characterized in that: The steps include: Step 1: Pre-treatment of the battery to be tested and acquisition of the negative electrode sheet; Step 2: In-situ gas production monitoring of SEI film decomposition on the negative electrode; Step 3: Determine the components of the SEI film based on the type of gas produced.

2. The method for analyzing SEI film components of a lithium-ion battery according to claim 1, wherein: In the third step, the gas type is CO2, and the SEI film component is alkyl lithium carbonate or Li2CO3; preferably, the alkyl lithium carbonate is (CH2OCO2Li)2, or HOCH2CH2OCO2Li; If the gas type is CH4, the SEI film component is CH3Li; If the gas type is C2H2, the SEI film component is Li2C2; If the gas type is C2H4, the SEI film component is C2H3Li; If the gas type is C2H6, the SEI film component is C2H5Li; If the gas type is C3H6, the SEI film component is C3H5Li; If the gas type is H2, the SEI film component is Li or Li x C6; where X≤1.

3. The method for analyzing SEI film components of a lithium-ion battery according to claim 1, wherein: The first step includes the following steps: 1.1) fully discharging the battery to be tested; 1.2) disassembling the battery to be tested to obtain the negative electrode sheet, and placing it in a sealed gas collection bag; 1.3) testing the gas collection bag containing the negative electrode sheet of the battery to be tested as a test sample.

4. The method for analyzing SEI film components of a lithium-ion battery according to claim 3, wherein: The specific steps of step 1.1) are: first discharge the battery to the cut-off voltage at 0.5C, let it rest for 30 minutes, then discharge it again to the cut-off voltage at 0.1C, let it rest for 30 minutes, and then discharge it to the cut-off voltage at 0.05C again.

5. The method for analyzing SEI film components of a lithium-ion battery according to claim 1, wherein: In the second step, an acid solution is used to completely decompose the SEI film; and sampling is performed at appropriate sampling intervals; preferably, the acid solution is an aqueous solution of one or a mixture of at least two of hydrochloric acid, sulfuric acid, and nitric acid.

6. The method for analyzing SEI film components of a lithium-ion battery according to claim 5, characterized in that: The SEI film is decomposed using a low-concentration acid solution of appropriate concentration, which specifically includes the following steps: Step 2.1) preparing a high-concentration acid solution with a concentration of 2-12 M and a low-concentration acid solution with a concentration of 0.5-1 M respectively; Step 2.2) Add 10-20 mL of a high-concentration acid solution and 10-20 mL of a low-concentration acid solution to two of the test samples, respectively, and monitor the gas production in situ; After the gas production in step 2.3) stabilizes, compare the gas production of the two test samples in step 2.2). If the gas production is the same, it indicates that the low-concentration acid solution can completely decompose the SEI film; then use the same low-concentration acid solution as in step 2.2) to test the test sample; if the gas production is different, it indicates that the reaction is incomplete, and increase the concentration of the low-concentration acid solution until the gas production is the same as that of the high-concentration acid solution; In step 2.4), the SEI film is decomposed using the appropriate low-concentration acid solution obtained in step 2.3).

7. The method for analyzing SEI film components of a lithium-ion battery according to claim 5 or 6, characterized in that: The method for determining the sampling interval is: based on the two inflection points of the gas production-reaction time relationship diagram in the in-situ monitoring data, the reaction of the SEI membrane component and the acid solution is divided into three stages, and sampling is performed according to different stages; preferably, in the first stage of the reaction between the acid solution and the SEI membrane component, gas is taken from the air bag valve every 5-10 minutes; in the second stage of the reaction between the acid solution and the SEI membrane component, gas is taken every 10-30 minutes; and in the third stage of the reaction between the acid solution and the SEI membrane component, gas is taken every 30-60 minutes.

8. The method for analyzing SEI film components of a lithium-ion battery according to claim 1, wherein: In the third step, high-sensitivity gas chromatography is used to test the gas composition of the sample.

9. A method for analyzing the SEI film structure of a lithium-ion battery, characterized in that: A method for analyzing SEI film components of a lithium-ion battery comprising the steps of any one of claims 1 to 8; In the third step, high-sensitivity gas chromatography is used to test the gas composition of the samples and draw a relationship diagram between gas production components and gas content-reaction time. The SEI film structure of the lithium-ion battery is analyzed based on the relationship diagram between gas production components and gas content-reaction time.

10. The method for analyzing the SEI film structure of a lithium-ion battery according to claim 9, characterized in that: In the third step, in the relationship diagram of gas production components and gas content-reaction time, if two gas components appear and the contents of the two gases show a trend of first increasing and then tending to be stable with the extension of reaction time, it proves that the SEI film is a double-layer structure; if multiple gas components appear and the contents of multiple gases show a trend of first increasing and then tending to be stable with the extension of reaction time, it proves that the SEI film is a multi-layer structure; if multiple gas components appear but the contents of multiple gases show a trend of increasing at the same time with the extension of reaction time, it proves that the SEI film is a mosaic structure composed of multiple components.

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