Method for detecting residues of negative electrode anti-cracking auxiliary agent in battery and judging influence of negative electrode anti-cracking auxiliary agent

Abstract

The invention belongs to the field of battery detection, and particularly relates to a method for detecting residues of a negative electrode anti-cracking auxiliary agent in a battery and judging the influence of the negative electrode anti-cracking auxiliary agent. The detection method comprises the following steps: 1) preparing a negative plate, wherein the negative plate comprises a current collector and a negative active layer arranged on the surface of the current collector; a negative electrode anti-cracking auxiliary agent is contained in the negative electrode active material layer; 2) drying the negative plate; 3) assembling the negative plate into a battery; 4) drying the battery; and 5) carrying out EIS test on the battery. According to the method, the non-free EC can be quickly and efficiently detected. The verification period is greatly shortened, and other ester negative electrode anti-cracking agents can also be used in parallel by the method.

Description

Technical Field

[0001] The invention belongs to the field of battery detection, and in particular relates to a method for detecting the residue of a negative electrode anti-cracking aid in a battery and judging the influence of the negative electrode anti-cracking aid. Background Art

[0002] Power lithium-ion battery technology is increasingly used in people's daily life and work. One of the keys to the successful application of lithium-ion batteries lies in the preparation of reversible lithium-ion battery negative electrode sheets that can be embedded and removed. At present, the market has higher and higher requirements for the energy density of lithium batteries. When designing, the surface density of the negative electrode is getting higher and higher, and the processing difficulty is increasing. The appearance of the electrode sheets produced by the existing coating equipment and parameters does not meet the design requirements. This leads to the phenomenon of "curling" and "cracking" of the foil at the edge of the negative electrode sheet after coating and drying. These cracking phenomena have become the weak points of the disintegration of the negative electrode structure during the use of lithium batteries. In order to solve this problem, more and more types of negative electrode anti-cracking additives have emerged. The basic requirements of anti-cracking additives are: solving the problem of cracking of negative electrode sheets, less residue, simple processing, and no effect on battery performance.

[0003] At present, the most commonly used anti-cracking additives include NMP (N-methylpyrrolidone), EC (ethylene carbonate), various alcohols, esters, etc. The drying process can dry out most of the additives. Through gas chromatography (GC) or liquid chromatography (HPLC) detection, the battery process usually formulates a residual amount standard to control the residual amount. If the amount of additive added increases, it is necessary to adjust the drying process so that the residual amount remains at the original level, and it is considered that the impact on the battery remains unchanged, but the actual situation is often that the increase in the amount of additives added will affect the battery performance. This shows that the residual additives detected by the chromatogram are not the real residual amount. Some chemical bonds in the additives may have undergone chemical reactions, resulting in the inability to be detected by chromatography. And this kind of additive that undergoes chemical bond reactions can be called a non-free additive. So far, the basic means of detecting the impact of anti-cracking additives on battery performance are: residual amount test, test electrode resistance, battery DCIR growth rate and cycle performance test. The test means either have too one-sided cause analysis or too long test cycle, which affects the screening progress. This invention uses EC as an anti-cracking agent for the negative electrode and discovers a time-saving and easy-to-operate detection method for qualitatively detecting the presence of non-free EC and the depth of its impact on the battery. Summary of the invention

[0004] The purpose of the present invention is to overcome the shortcomings of the prior art and provide a method for detecting the residual negative electrode anti-cracking aid in a battery and judging the influence of the negative electrode anti-cracking aid.

[0005] To achieve the above object, the technical solution adopted by the present invention is:

[0006] A method for inspecting the negative electrode anti-cracking agent residue in a battery comprises the following steps: 1) preparing a negative electrode sheet, wherein the negative electrode sheet comprises a current collector and a negative electrode active layer arranged on the surface of the current collector; the negative electrode active material layer comprises the negative electrode anti-cracking agent; 2) drying the negative electrode sheet; 3) assembling the negative electrode sheet into a battery; 4) drying the battery; 5) performing an EIS test on the battery.

[0007] In step 4), after the battery is dried, the negative electrode additive is subjected to chromatographic detection until the residual additive content is ≤20ppm.

[0008] In step 2), the negative electrode sheet is dried in a vacuum at 100-150°C for 6-24 hours, preferably at 130°C for 10 hours. In step 4), the battery is dried in a vacuum with nitrogen at 80-100°C for 12-36 hours, preferably at 90°C for 24 hours.

[0009] The battery after drying in step 4) is charged and stored at high temperature.

[0010] Adjust the load to 50% SOC-100% SOC; preferably 100% SOC.

[0011] The conditions for high temperature storage are storage at 60°C for ≥ 2 days.

[0012] The EIS test method in step 5) is a three-electrode battery test, and the test mode is a two-electrode mode test or a three-electrode mode test.

[0013] The R SEI +R CT The larger the arc, the more anti-cracking additive remains.

[0014] The present invention also includes a method for judging the influence of the negative electrode anti-cracking aid, using the method for testing the negative electrode anti-cracking aid residue in the battery, wherein the EIS test in step 5) adopts a three-electrode battery test, and by comparing the R values ​​of the EIS test curves of the positive electrode reference and the negative electrode reference with different contents of the anti-cracking aid. SEI +R CT The amplitude of the arc change determines the impact on the positive electrode and the negative electrode.

[0015] R of the positive reference relative to the negative reference SEI +R CT If the arc changes greatly, it indicates that the negative electrode anti-cracking aid has a greater impact on the positive electrode sheet. Conversely, it indicates that the negative electrode anti-cracking aid has a greater impact on the negative electrode sheet.

[0016] Compared with the prior art, the present invention has the following beneficial effects:

[0017] The technical solution of the present application can monitor the residual amount of the anti-cracking aid (hereinafter referred to as the aid) inside the battery that cannot be detected by chromatography. As a preferred form, the battery can be charged and / or stored at high temperature to make the detection results more obvious. The technical solution of the present application uses the EIS test method to use the battery film formation impedance (R SEI ) and charge transfer resistance (R CT ) changes, monitoring the presence of non-free additives. At the same time, through the test results of this method, the impact of different addition amounts on the impedance of the battery can be measured, thereby determining the proportion of additives in the negative electrode formula. Finally, through this detection method, the positive reference and negative reference of the three-electrode battery are tested separately, and the impact on the positive and negative electrodes of the battery can be deeply analyzed. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 The two-electrode mode AC impedance test result diagram of different amounts of additives;

[0019] Figure 2 The three-electrode mode AC impedance test results for different amounts of additives;

[0020] Figure 3 The AC impedance test results of the two-electrode mode at different storage temperatures with different amounts of additives are shown;

[0021] Figure 4 The AC impedance test results of the two-electrode mode with different amounts of additives and different storage times are shown;

[0022] Figure 5 The graph shows the AC impedance test results of the two-electrode mode at different storage SOCs with different amounts of additives. DETAILED DESCRIPTION

[0023] In order to enable those skilled in the art to better understand the technical solution of the present invention, the present invention is further described in detail below in conjunction with the accompanying drawings and the best embodiments.

[0024] A method for testing the residual negative electrode anti-cracking aid in a battery, comprising the following steps:

[0025] 1) Prepare a negative electrode sheet, the negative electrode sheet includes a current collector and a negative electrode active layer arranged on the surface of the current collector; the negative electrode active material layer contains a negative electrode anti-cracking aid; this method is verified in a medium-nickel high-voltage ternary system, but is not limited to iron lithium, manganese iron lithium and other systems. Different proportions of EC are added as an anti-cracking aid during the negative electrode homogenization process to make a pole sheet. The proportion of the negative electrode anti-cracking aid in this method is 0%, 1%, and 5% of the main material, and the added amount can be adjusted appropriately according to actual needs. The negative electrode anti-cracking aid of this application can serve as a guide and reference for the amount of EC added in the battery negative electrode formula. This method can also be used in parallel with other ester negative electrode anti-cracking aids.

[0026] 2) drying the negative electrode sheet; the negative electrode sheet is dried under vacuum at 100-150° C. for 6-24 hours; preferably, drying at 130° C. for 10 hours is used as an exemplary description;

[0027] 3) Assembling the negative electrode sheets into batteries; performing conventional processes such as liquid injection, exhaust, formation, and aging;

[0028] 4) Dry the battery; vacuum and nitrogen, at 80-100°C, for 12-36h; preferably 90°C, for 24h. This is an exemplary description, and the drying process can be adjusted appropriately. Ensure that all EC is dried out. Through liquid chromatography testing, it was detected that the residual content of additives in the negative electrode sheets of each group was ≤20ppm, as shown in Table 1. It can be considered that there is almost no residual free EC or the residual level is equivalent.

[0029] Table 1

[0030]

[0031] 5) Perform EIS test on the battery. In order to shorten the production cycle, this method uses soft-pack batteries to make three-electrode wound batteries. Users can make square, soft-pack, stacked, and wound batteries according to actual needs. The three-electrode battery uses traditional copper wire production technology to bury the acid-treated copper wire in the center of the positive and negative electrodes of the middle electrode of the electrode group. Then weld the copper wire to the transfer tab (the tab can be made of copper strip or copper nickel-plated strip).

[0032] The three-electrode battery needs to be plated with lithium on the copper wire: the three-electrode battery is charged with a small current lithium plating of the positive electrode / reference and the negative electrode / reference, respectively, with a current range of 10uA to 20uA, and each lithium plating time is 15h to 20h. The three-electrode battery sample preparation is considered complete until the resistance of the positive electrode / reference and the negative electrode / reference no longer decreases.

[0033] In order to make the test results more obviously different, the battery is subjected to load adjustment and / or high-temperature storage, and the load is adjusted to 50% SOC-100% SOC; preferably 100% SOC; the high-temperature storage condition is storage at 60°C for ≥2d.

[0034] Determine the test conditions using the following steps:

[0035] (1) Take two sample batteries with 0% and 1% EC addition from steps 1) to 4) above, store them at 60°C for 2 days, and use 25°C as a comparison. Charge the batteries to 100% SOC and perform a two-electrode mode EIS test (the battery tested in this step is a battery charged to 50% SOC). Figure 3 As shown, the storage time is the same, 0% additive content battery, after storage at 60℃ R SEI +R ct The semicircular area (R) is slightly larger than that of the storage at 25°C. However, when the EC addition amount is 1%, the semicircular area (R) is more obvious when the storage is at 60°C than that at 25°C. SEI +R CT ) increase phenomenon, in order to detect the difference more clearly, choose 60℃ storage condition.

[0036] (2) Take 4 sample batteries with 0% and 1% EC addition from steps 1) to 4) above, store them at 60°C and 100% SOC for 0, 1, 2, and 5 days, and perform 100% SOC two-electrode mode EIS test after storage. Figure 4 As shown in Figure 2, when the storage time at 60℃ is ≥2 days, the sample with 1% EC addition can show differences. Compared with 0% EC, under the same storage and test conditions, the R SEI +R ct The trend of change is not obvious. Select a storage time of at least 2 days.

[0037] (3) Take three sample batteries with EC contents of 0% and 1% after the above steps 1) to 4), and adjust the batteries to 0% SOC, 50% SOC, and 100% SOC respectively. Set the storage time to 2 days, and perform a two-electrode mode EIS test on the stored batteries (the battery tested in this step is a battery adjusted to 50% SOC). Figure 5 As shown, the R of the battery at 100% SOC for 0% and 1% EC addition SEI +R ct Maximum, 1% content EC battery 100% SOC, R SEI +R ct Larger than the 0% EC battery, in order to detect the difference more clearly, 100% SOC condition storage is selected.

[0038] Take the batteries with different EC addition amounts (0%, 1%, 5%) after the above steps 1) to 4), store them at 100% SOC at 60°C for 2 days, and perform two-electrode mode EIS test on the stored batteries ( Figure 1 Shown), three-electrode mode EIS test ( Figure 2 , Figure 1 It shows that although the experimental components control the EC residue at a relatively low level (through liquid chromatography test, the residual content of additives in the negative electrode sheets of each group is detected to be ≤20ppm), as the amount of EC added increases, the R SEI +R CT The greater the impact, the greater the SEI +R CT The reason for the increase in the arc is still due to the addition of EC, and this part of EC has no obvious correlation with the residual EC detected in the chromatogram.

[0039] Figure 2 It shows that for batteries with different EC addition amounts, the RSEI of the positive electrode reference increases with the increase of EC addition amount, while the RSE I+RCT of the negative electrode reference does not increase with the increase of EC content. Figure 1 The EIS test can confirm the existence of non-free EC. According to the three-electrode battery test, non-free EC leads to the R SEI +R CT The change is greater than that of the negative electrode R SEI+ R CT This indicates that the effect of non-free EC on the film formation of the positive electrode is greater than that of the negative electrode, and it is necessary to consider the reaction principle of non-free EC from the perspective of positive electrode film formation.

[0040] EC, as a commonly used negative electrode anti-cracking agent, can solve the cracking and curling problem of the negative electrode sheet by adjusting the addition amount in the negative electrode formula. The residual amount of EC is usually detected by a chromatographic detection method, and a detection standard is formulated for residual detection. The present invention points out that even if the EC in the electrode sheets with different addition amounts is regulated at the same level (chromatographic detection standard) by tightening the drying process, the performance of the battery cell still changes significantly. This change proves that EC, as a negative electrode anti-cracking agent, will react with the active substance in the negative electrode to generate non-free EC compounds. This compound cannot be detected by chromatography, but the detection method of the present invention can effectively detect the presence of non-free EC, and the non-free EC content can be roughly judged by comparison.

[0041] Compared with conventional evaluation additives, this method requires at least testing the cycle performance of the battery, which has a long cycle and slow effect. This method has been proven through experiments that at 60°C high temperature and 100% SOC, non-free EC can be detected quickly and efficiently. The verification cycle is greatly shortened, and this method can also be used in parallel with other ester negative electrode anti-cracking agents.

[0042] The above method can be used as a guide and reference for the amount of EC added in the battery negative electrode formula. This method can also be used in parallel for other ester negative electrode anti-cracking agents.

[0043] Through the split EIS test of the positive electrode / reference and negative electrode / reference of the three electrodes, it is possible to analyze whether the non-free EC has a greater impact on the performance of the positive electrode sheet or the negative electrode sheet of the battery. This provides technical support for in-depth research on the mechanism of the impact of non-free EC on battery electrodes. This method can also be used in parallel to promote the use of other ester negative electrode anti-cracking agents.

[0044] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A method for testing the residual negative electrode anti-cracking agent in a battery, characterized in that: The method comprises the following steps: 1) preparing a negative electrode sheet, wherein the negative electrode sheet comprises a current collector and a negative electrode active layer arranged on the surface of the current collector; the negative electrode active material layer comprises a negative electrode anti-cracking aid; 2) drying the negative electrode sheet; 3) assembling the negative electrode sheet into a battery; 4) drying the battery; and 5) performing an EIS test on the battery.

2. The method for testing the negative electrode anti-cracking additive residue in a battery according to claim 1, characterized in that: In step 4), after drying the battery, the negative electrode additive is chromatographically detected until the residual additive content is ≤ 20ppm.

3. The method for testing the negative electrode anti-cracking additive residue in a battery according to claim 1, characterized in that: In step 2), the negative electrode sheet is dried in a vacuum at 100-150°C for 6-24 hours, preferably at 130°C for 10 hours. In step 4), the battery is dried in a vacuum with nitrogen at 80-100°C for 12-36 hours, preferably at 90°C for 24 hours.

4. The method for testing the negative electrode anti-cracking agent residue in a battery according to claim 1, characterized in that: The battery after drying in step 4) is charged and stored at high temperature.

5. The method for testing the negative electrode anti-cracking additive residue in a battery according to claim 4, characterized in that: Adjust the load to 50% SOC-100% SOC; preferably 100% SOC.

6. The method for testing the negative electrode anti-cracking additive residue in a battery according to claim 1, characterized in that: In step 4), the high temperature storage condition is 60° C. for ≥ 2 days.

7. The method for testing the negative electrode anti-cracking agent residue in a battery according to claim 1, characterized in that: The EIS test method in step 5) is a three-electrode battery test, and the test mode is a two-electrode mode test or a three-electrode mode test.

8. The method for inspecting the negative electrode anti-cracking agent residue in a battery according to claim 7, characterized in that: The R SEI +R CT The larger the arc, the more anti-cracking additive remains.

9. A method for judging the influence of a negative electrode anti-cracking aid, characterized in that: The method for inspecting the negative electrode anti-cracking aid residue in the battery according to claims 1 to 9, wherein the EIS test in step 5) adopts a three-electrode battery test, and the R of the EIS test curves of the positive electrode reference and the negative electrode reference with different contents of the anti-cracking aid are compared. SEI +R CT The amplitude of the arc change determines the impact on the positive electrode and the negative electrode.

10. The method for judging the influence of the negative electrode anti-cracking aid according to claim 9, characterized in that: R of the positive reference relative to the negative reference SEI +R CT If the arc changes greatly, it indicates that the negative electrode anti-cracking aid has a greater impact on the positive electrode sheet. Conversely, it indicates that the negative electrode anti-cracking aid has a greater impact on the negative electrode sheet.

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