Preparation and evaluation method of lithium-ion battery negative electrode slurry

Abstract

The present invention discloses a method for preparing negative electrode slurry for lithium-ion batteries. Starting with incoming materials, negative electrode materials with suitable viscosity are selected, and then dry-mixed to prepare the slurry. During the slurry preparation process, the slurrying process is optimized to improve the uniformity of the dispersion of the various components in the slurry. The present invention also provides a method for evaluating negative electrode slurry for lithium-ion batteries, which tests the slurry viscosity and solids content. The slurry is then coated to form electrode sheets, which are then subjected to SEM and EDS testing to evaluate the performance of the negative electrode slurry.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium-ion battery electrode slurry, and particularly relates to a preparation method and an evaluation method of lithium-ion battery negative electrode slurry. Background Art

[0002] Lithium-ion batteries generally consist of positive electrodes, negative electrodes, and separators. During electrode preparation, electrode materials (active materials, such as lithium cobalt oxide, lithium titanate, and graphite), conductive agents, binders, and solvents are first combined to form an electrode slurry. This slurry is then coated and dried as required to form the electrode sheets. The performance of lithium-ion battery electrode slurry has a significant impact on battery performance. The more evenly the components in the electrode slurry are dispersed, the better the processing properties of the electrode sheet. This also allows for a more uniform impedance distribution across the electrode, allowing the active material to play a more effective role during charge and discharge, thereby improving the performance of the lithium-ion battery. Currently, most lithium-ion batteries use an oil-based positive electrode slurry and an aqueous negative electrode slurry.

[0003] Aqueous anode slurry is generally prepared using a mixing process. The mixing process can be categorized into wet and dry mixing. The wet mixing method primarily involves three steps: sizing, dry mixing, and batch addition. First, sodium carboxymethyl cellulose (CMC) is dissolved to create a slurry. The anode active material (such as graphite) and conductive agent are then mixed and added to the slurry, stirring to form a non-Newtonian fluid-like slurry. Finally, a binder (such as SBR) is added to the slurry, and the slurry viscosity is adjusted to within the process range (e.g., 3000-5000 mPa·s). The slurry is then coated to form the electrode sheets. The wet mixing slurry production cycle is long, with each reaction kettle requiring 7-10 hours. Furthermore, the reactor needs to be frequently opened and closed during the batching and addition process, which can lead to dust in the air entering the slurry, causing contamination, or the slurry sticking to the walls, increasing losses. Furthermore, if the slurry is not completely dissolved during the vacuum degassing process, the slurry delivery ratio will change, resulting in decreased battery performance.

[0004] Dry mixing does not require separate gluing. First, the negative electrode active material, conductive agent, and CMC are mixed and dispersed, then the glue is added, and finally the binder (such as SBR) is added. The slurry viscosity is adjusted to the process range (such as 3000-5000mPa·s). The dry mixing equipment has strong mechanical energy and shear friction, which can fully disperse the negative electrode active material, conductive agent, and CMC and interact with each other, so that the conductive agent is evenly coated on the surface of the active material particles, preventing small particles from secondary agglomeration and sedimentation in the negative electrode slurry. It also prevents dust in the air from contaminating the slurry. Therefore, the negative electrode slurry produced by dry mixing is more uniform and stable, and the slurry preparation time is shortened to 3.5-5 hours compared to wet mixing, which can effectively improve the slurry preparation efficiency. However, dry mixing has higher equipment requirements, which increases the slurry preparation cost.

[0005] Regarding dry mixing, technical researchers have conducted extensive research on feeding, mixing, and stirring methods, aiming to improve the uniformity of various raw materials in the negative electrode slurry and enhance the overall performance of lithium-ion batteries. Patent application number 201810977615.8, entitled "Process for Preparing Negative Electrode Slurry for Lithium-ion Batteries," involves dry-mixing the main graphite material, conductive agent, and thickening stabilizer all at once before dispersing them. Parameters such as kneading solid content, kneading time, and dispersion time are optimized, resulting in a negative electrode slurry with a solid content increase of 5% to 10%, which is beneficial for improving the slurry's coating performance. Patent application number 201710914647.9, entitled "Method for Preparing Negative Electrode Slurry for Lithium-ion Batteries," involves adding graphite in two stages, wetting the powder with solvent twice, and stirring at high viscosity in the early stages, followed by low viscosity stirring in the later stages. This ensures uniform mixing of the slurry components, with a slurry preparation time of 145 to 185 minutes. This further shortens the preparation time of the negative electrode slurry and improves production controllability.

[0006] The above-mentioned dry stirring methods are all operated directly on the incoming materials. In fact, in order to improve the dispersion uniformity of the negative electrode slurry, starting from the incoming materials, screening the incoming materials, selecting suitable active materials, and then optimizing the dry stirring method can further improve the performance of the slurry. In addition, for the evaluation of the stirring process, the existing technology usually evaluates the viscosity and solid content of the slurry in combination with the electrical performance of the finished battery cell. This evaluation method can objectively reflect the performance of the slurry, but there is a certain hysteresis and the evaluation cycle is long. Summary of the Invention

[0007] To address the shortcomings of the prior art, the present invention provides a method for preparing negative electrode slurry for lithium-ion batteries. Starting with incoming materials, negative electrode materials with appropriate viscosity are selected, and then dry-mixed to prepare the slurry. During the slurry preparation process, the slurrying process is optimized to improve the uniformity of the dispersion of the various components in the slurry. The present invention also provides a method for evaluating negative electrode slurry for lithium-ion batteries, testing the slurry viscosity and solids content; the slurry is then coated to form electrode sheets, which are then tested to evaluate the performance of the negative electrode slurry.

[0008] The technical effects to be achieved by the present invention are achieved through the following solutions:

[0009] The present invention provides a method for preparing a negative electrode slurry for a lithium ion battery, comprising the following steps:

[0010] S01: Establishing a relationship model between the sedimentation coefficient of the negative electrode material of a lithium-ion battery and the viscosity of the fluid prepared therefrom as shown in Formula I;

[0011] S02: In the relationship model, the sedimentation coefficient first decreases and then tends to be stable with the increase of viscosity. The viscosity range corresponding to the stable sedimentation coefficient is selected, and the particle size of the negative electrode material is selected according to the viscosity range. Then, the negative electrode material is selected according to the particle size to prepare the negative electrode slurry;

[0012] S03: mixing and stirring the negative electrode material selected in S02 with the conductive agent and thickener;

[0013] S04: adding a solvent and a thickener to the mixture obtained in S03, and stirring and mixing;

[0014] S05: adding a binder to the mixture obtained in S04, adjusting the viscosity of the mixture to a process range, and obtaining a negative electrode slurry;

[0015] w=aμ 3 +bμ 2 +cμ+d formula Ⅰ;

[0016] In formula I, w is the sedimentation coefficient of the negative electrode material; μ is the viscosity of the fluid prepared with the negative electrode material; a, b, c, and d are fitting coefficients.

[0017] Furthermore, the method for selecting the negative electrode material according to the viscosity range is: for different batches of negative electrode materials, the batch with the smallest sedimentation coefficient corresponding to the viscosity range is selected as the negative electrode material.

[0018] Furthermore, the negative electrode material is graphite or lithium titanate.

[0019] Furthermore, the sedimentation coefficient of the negative electrode material in S01 is calculated and determined by Stokes' formula.

[0020] Furthermore, the conductive agent is one or more of Super P, acetylene black, Super S, KS-6, KS-15, SFG-6, SFG-15, Ketjen black, CNTs, VGCF, and graphene; the binder is one or more of SBR, polyvinylidene fluoride copolymer, polyimide, polyacrylic acid, and sodium alginate.

[0021] Furthermore, the thickener is sodium carboxymethyl cellulose.

[0022] Furthermore, the solvent is water.

[0023] Furthermore, the mass ratio of the thickener in S02 to the thickener in S03 is 1:(0.4~2.5).

[0024] Preferably, the mass ratio of the thickener in S02 to the thickener in S03 is 1:1.5.

[0025] The present invention starts with the raw materials of the negative electrode material and screens the raw materials of the negative electrode material. First, the sedimentation coefficient of the raw materials of the negative electrode material is determined according to the particle size of the raw materials of the negative electrode material in combination with the Stokes sedimentation velocity formula. Then, a relationship model between the sedimentation coefficient of the negative electrode material and the viscosity is established. In the process of establishing the relationship model, the viscosity and the sedimentation coefficient are fitted. The fitting result is that the result of the cubic polynomial fitting has the highest reliability, and the correlation coefficient R 2 Greater than 0.99, therefore, the present invention adopts cubic polynomial fitting to establish the relationship model between the sedimentation coefficient and viscosity of the negative electrode material, which can objectively and accurately reflect the relationship between the sedimentation coefficient and viscosity of the negative electrode material.

[0026] In the aforementioned relationship model, the sedimentation coefficient first decreases and then stabilizes with increasing viscosity. Based on this model, a viscosity range with a stable sedimentation coefficient is selected, and graphite raw material with a particle size range corresponding to this viscosity range is then selected. The negative electrode material selected within this viscosity range has a stable sedimentation coefficient, meaning that during the slurry preparation process, the slurry at the same location in the slurry kettle has good uniformity. Therefore, selecting the negative electrode material based on the aforementioned relationship model between sedimentation coefficient and viscosity can improve the uniformity of the slurry at the same location in the slurry kettle.

[0027] On the other hand, in the actual process of preparing the negative electrode slurry, the negative electrode material may be provided by different suppliers, or different batches provided by the same supplier. Therefore, the raw materials of the negative electrode material will have certain differences. Under the same process conditions in the slurrying process, different raw materials may cause uneven slurry due to their own different properties. Materials with a large sedimentation coefficient are prone to sedimentation, and the slurry will be layered up and down, resulting in abnormalities in subsequent processing steps, such as the surface density of the electrode after coating does not meet the specifications. The negative electrode material is the main component in the negative electrode slurry. If the sedimentation coefficient of the negative electrode material is too large, it is not conducive to its uniform dispersion in the slurry. Selecting raw materials with a small sedimentation coefficient as the negative electrode material is easy to fully mix with other components in the slurry during the stirring process, which is beneficial to improve the uniformity of the slurry. Therefore, for different batches of negative electrode materials, the batch with the smallest sedimentation coefficient within the above-mentioned viscosity range is selected as the negative electrode material.

[0028] According to the sedimentation coefficient and viscosity model of the present invention, the negative electrode material raw materials can be screened to ensure that the slurry at the same position in the slurry kettle is evenly dispersed. At the same time, on this basis, raw materials with a small sedimentation coefficient are selected to prepare the negative electrode slurry, which can effectively improve the uniformity of the overall slurry.

[0029] After selecting a suitable negative electrode material, the interaction between the negative electrode material and other slurry components (such as the conductive agent, binder, and thickener) is another key factor in improving slurry uniformity. During the negative electrode slurry preparation process, the present invention adds the thickener in two steps, controlling the ratio of the two additions to enhance its interaction with the negative electrode material and the conductive agent. In the first step, the thickener is dry-mixed with the negative electrode material and the conductive agent. This solid-phase mixing occurs with high stirring intensity. The mechanical forces of stirring (friction, collision, extrusion, crushing, etc.) thoroughly break up and uniformly mix the three materials. Simultaneously, due to the interparticle forces, the conductive agent is fully adsorbed on the surface of the negative electrode material, preventing agglomeration of the conductive agent in the slurry. In the second step, the solvent and thickener are added, and stirring and mixing are continued. This is a coexistent solid-liquid mixing. Under stirring, the slurry is evenly dispersed by centrifugation, creating and maintaining a solid-liquid suspension, enhancing mass transfer between the solid and liquid phases, and ensuring full dispersion of the components within the slurry. The addition ratio (mass ratio) of the first and second thickeners is controlled to 1: (0.4~2.5). Within this range, the slurry is uniform, the surface density of the electrode after slurry coating is stable, and the negative electrode material has a large and uniform adsorption of the thickener.

[0030] The present invention also provides a method for evaluating lithium-ion battery negative electrode slurry: preparing battery negative electrode slurry according to the above preparation method, sampling and testing viscosity and solid content; then coating and drying to make pole pieces, and performing SEM testing and EDS testing on the pole pieces.

[0031] Furthermore, the uniformity and stability of the negative electrode slurry are evaluated based on the viscosity test results, the solid content test results, and the SEM test results; and the adsorption performance of the negative electrode material in the negative electrode slurry is evaluated based on the EDS test results.

[0032] After the negative electrode slurry is prepared, to evaluate its performance, samples are taken from different locations in the slurry kettle (e.g., top, middle, and bottom layers) and their viscosity and solids content are measured over time to determine the slurry's uniformity and stability. The prepared negative electrode slurry is then used for coating to form negative electrode sheets. SEM analysis of the negative electrode sheets is performed to observe their morphology, and EDS analysis is performed to observe the distribution of elements on the surface of the sheets. This also assesses the adsorption performance of the negative electrode material on other components in the slurry.

[0033] The present invention has the following advantages:

[0034] 1. The present invention starts with the raw materials of the negative electrode material. By establishing the relationship between the sedimentation coefficient and viscosity of the negative electrode material, the negative electrode material with a stable sedimentation coefficient is selected as the raw material to prepare the negative electrode slurry.

[0035] 2. In the present invention, during the preparation of the negative electrode slurry, the thickener is added in two steps and the mass ratio of the thickener added in the two steps is set, which is beneficial to improving the uniformity and stability of the slurry and preventing the conductive agent from agglomerating in the slurry.

[0036] 3. The negative electrode slurry evaluation method provided by the present invention can control the performance of the slurry in real time by testing the viscosity and solid content of the slurry, as well as the electrode after the slurry is coated, further improving the controllability of slurry production. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 This is a graph showing the relationship between the viscosity of the graphite raw material and the sedimentation coefficient in the present invention.

[0038] Figure 2 1 is a graph showing the relationship between the viscosity of Example 1 and Comparative Example 1 of the present invention and the change over time.

[0039] Figure 3 This is a graph showing the relationship between the solid content of Example 1 and Comparative Example 1 of the present invention and the change over time.

[0040] Figure 4 The graph is a relationship between the viscosity of Example 1 and Comparative Example 2 of the present invention and the change over time.

[0041] Figure 5 This is a graph showing the relationship between the solid content of Example 1 and Comparative Example 2 of the present invention and the change over time.

[0042] Figure 6 This is an SEM image of the negative electrode sheet of Example 1 of the present invention.

[0043] Figure 7 This is the EDS image of the negative electrode sheet of Example 1 of the present invention.

[0044] Figure 8 This is an SEM image of the negative electrode sheet of Example 2 of the present invention.

[0045] Figure 9 This is the EDS image of the negative electrode sheet of Example 2 of the present invention.

[0046] Figure 10 This is the SEM image of the negative electrode sheet of Comparative Example 1 in the present invention.

[0047] Figure 11 This is the EDS image of the negative electrode sheet of Comparative Example 1 in the present invention.

[0048] Figure 12 This is the SEM image of the negative electrode sheet of Comparative Example 2 in the present invention.

[0049] Figure 13 This is the EDS image of the negative electrode sheet of Comparative Example 2 in the present invention. DETAILED DESCRIPTION

[0050] The present invention will be described in detail below with reference to the accompanying drawings and embodiments.

[0051] 1. Screening of negative electrode material raw materials.

[0052] Three batches of graphite raw materials were selected, labeled A, B, and C. According to the particle size of the graphite raw materials, the sedimentation coefficient was determined in combination with the Stokes sedimentation velocity formula. Then, a relationship model between the sedimentation coefficient and viscosity of the negative electrode material was established. The relationship model between the sedimentation coefficient and viscosity of the graphite raw materials A, B, and C is shown in the attached figure. Figure 1 As shown, the relationship model of batch A graphite is w=-0.0024μ 3 +0.0398μ 2 -0.2345μ+0.6215, correlation coefficient R 2 =0.9984; the relationship model of batch B graphite is w=-0.0014μ 3 +0.0232μ 2 -0.1369μ+0.3629, correlation coefficient R 2 =0.9987; the relationship model of batch C graphite is w=-0.0032μ 3 +0.0533μ 2 -0.3144μ+0.8332, correlation coefficient R 2 =0.9989. The sedimentation coefficient and viscosity model provided in the present invention have a high fitting correlation for the three batches of graphite A, B, and C, and can accurately reflect the relationship between the sedimentation coefficient and viscosity of the three batches of graphite.

[0053] By the attached Figure 1 As shown in the figure, the sedimentation coefficients of the three batches of graphite A, B, and C decrease with the increase of viscosity and finally tend to be stable. The viscosity range for the stable sedimentation coefficient is 6000~7000 mPa·s. At the same time, within this viscosity range, the sedimentation coefficient of batch B graphite is the smallest among the three batches of graphite A, B, and C. Therefore, batch B graphite is selected as the negative electrode material for the embodiment experiment.

[0054] 2. Preparation and evaluation of negative electrode slurry.

[0055] Example 1

[0056] Graphite, Super P, SBR, sodium carboxymethyl cellulose (CMC), and deionized water were weighed in 96 parts by mass, 1.3 parts by mass, 0.7 parts by mass, 2 parts by mass, and 100 parts by mass, respectively, as raw materials for preparing the negative electrode slurry. The graphite was the graphite from batch B described above.

[0057] Preparation steps of negative electrode slurry:

[0058] Step 1: Mix 96 parts of graphite, 1.3 parts of Super P, and 0.8 parts of CMC and stir for 1.5 hours.

[0059] Step 2: Add 100 parts of deionized water and 1.2 parts of CMC to the mixture obtained in step 1, and stir and mix for 2 hours; control the temperature of the mixture to be 24±2°C.

[0060] Step 3: Add 0.7 parts of SBR to the mixture obtained in step 2, adjust the viscosity of the mixture to 3000±200 mPa·s, and obtain a negative electrode slurry; control the slurry temperature to 24±2°C.

[0061] After the slurry preparation was completed, the slurry at the upper, middle and lower positions of the slurry in the slurry kettle (labeled as Example 1a, Example 1b and Example 1c respectively) was taken as experimental samples. 50 samples were taken from each position and their viscosity was measured over time. The average viscosity of the 50 samples at each position was taken as the viscosity at that position. The viscosity test data is shown in the attached figure. Figure 2 (Attached Figure 4 ) as shown.

[0062] After the slurry preparation was completed, the slurries at the upper, middle and lower positions of the slurry in the slurry kettle (labeled as Example 1a, Example 1b and Example 1c, respectively) were taken as experimental samples. 50 samples were taken from each position and their solid content was measured over time. The average solid content of the 50 samples at each position was taken as the solid content of that position. The solid content test data is shown in the attached figure. Figure 3 (Attached Figure 5 ) as shown.

[0063] After the slurry is prepared, it is coated and dried to form the negative electrode. The electrode piece at a distance of 10 to 20 mm from the edge of the prepared electrode piece is taken as the experimental sample and tested by SEM and EDS to observe the electrode piece morphology and the distribution of elements on the electrode piece surface. The SEM test (5000 times) is shown in the attached picture. Figure 6 As shown, EDS test pictures are as attached Figure 7 The EDS test image shows the distribution of sodium on the electrode surface. The weight percentage and atomic percentage of each element tested by EDS are shown in Table 1.

[0064] Example 2

[0065] Graphite, Super P, SBR, sodium carboxymethyl cellulose (CMC), and deionized water were weighed in 96 parts by mass, 1.3 parts by mass, 0.7 parts by mass, 2 parts by mass, and 100 parts by mass, respectively, as raw materials for preparing the negative electrode slurry. The graphite was the graphite from batch B described above.

[0066] Preparation steps of negative electrode slurry:

[0067] Step 1: Mix 96 parts of graphite, 1.3 parts of Super P, and 1 part of CMC and stir for 1.5 hours.

[0068] Step 2: Add 100 parts of deionized water and 1 part of CMC to the mixture obtained in step 1, and stir and mix for 2 hours; control the temperature of the mixture to be 24±2°C.

[0069] Step 3: Add 0.7 parts of SBR to the mixture obtained in step 2, adjust the viscosity of the mixture to 3000±200 mPa·s, and obtain a negative electrode slurry; control the slurry temperature to 24±2°C.

[0070] The difference between Example 2 and Example 1 is that 1 part of CMC is added in the first step and 1 part is added in the second step.

[0071] After the slurry is prepared, it is coated and dried to form the negative electrode. The electrode piece at a distance of 10 to 20 mm from the edge of the prepared electrode piece is taken as the experimental sample and tested by SEM and EDS to observe the electrode piece morphology and the distribution of elements on the electrode piece surface. The SEM test (5000 times) is shown in the attached picture. Figure 8 As shown, EDS test pictures are as attached Figure 9 The EDS test image shows the distribution of sodium on the electrode surface. The weight percentage and atomic percentage of each element tested by EDS are shown in Table 1.

[0072] Comparative Example 1

[0073] The difference between Comparative Example 1 and Example 1 is that the graphite raw material is not screened, and the graphite obtained by evenly mixing three batches of A, B, and C is used as the negative electrode material.

[0074] Graphite, Super P, SBR, sodium carboxymethyl cellulose (CMC), and deionized water were weighed in mass fractions of 96 parts, 1.3 parts, 0.7 parts, 2 parts, and 100 parts, respectively, as raw materials for preparing negative electrode slurry.

[0075] Preparation steps of negative electrode slurry:

[0076] Step 1: Mix 96 parts of graphite, 1.3 parts of Super P, and 0.8 parts of CMC and stir for 1.5 hours.

[0077] Step 2: Add 100 parts of deionized water and 1.2 parts of CMC to the mixture obtained in step 1, and stir and mix for 2 hours; control the temperature of the mixture to be 24±2°C.

[0078] Step 3: Add 0.7 parts of SBR to the mixture obtained in step 2, adjust the viscosity of the mixture to 3000±200 mPa·s, and obtain a negative electrode slurry; control the slurry temperature to 24±2°C.

[0079] After the slurry preparation was completed, the slurries at the upper, middle and lower positions of the slurry in the slurry kettle (labeled as comparative example 1a, comparative example 1b and comparative example 1c respectively) were taken as experimental samples. 50 samples were taken from each position and their viscosity was measured over time. The average viscosity of the 50 samples at each position was taken as the viscosity at that position. The viscosity test data is shown in the attached figure. Figure 2 shown.

[0080] After the slurry preparation was completed, the slurries at the upper, middle and lower positions of the slurry in the slurry kettle (labeled as comparative example 1a, comparative example 1b and comparative example 1c respectively) were taken as experimental samples. 50 samples were taken from each position and their solid content was measured over time. The average solid content of the 50 samples at each position was taken as the solid content of the position. The solid content test data is shown in the attached figure. Figure 3 shown.

[0081] After the slurry is prepared, it is coated and dried to form the negative electrode. The electrode piece at a distance of 10 to 20 mm from the edge of the prepared electrode piece is taken as the experimental sample and tested by SEM and EDS to observe the electrode piece morphology and the distribution of elements on the electrode piece surface. The SEM test (5000 times) is shown in the attached picture. Figure 10 As shown, EDS test pictures are as attached Figure 11 The EDS test image shows the distribution of sodium on the electrode surface. The weight percentage and atomic percentage of each element tested by EDS are shown in Table 1.

[0082] Comparative Example 2

[0083] The difference between Comparative Example 2 and Example 1 is that CMC is added in one step.

[0084] Graphite, Super P, SBR, sodium carboxymethyl cellulose (CMC), and deionized water were weighed in 96 parts by mass, 1.3 parts by mass, 0.7 parts by mass, 2 parts by mass, and 100 parts by mass, respectively, as raw materials for preparing the negative electrode slurry. The graphite was the graphite from batch B described above.

[0085] Preparation steps of negative electrode slurry:

[0086] Step 1: Mix 96 parts of graphite, 1.3 parts of Super P, and 2 parts of CMC and stir for 1.5 hours.

[0087] Step 2: Add 100 parts of deionized water to the mixture obtained in step 1, and stir and mix for 2 hours; control the temperature of the mixture to be 24±2°C.

[0088] Step 3: Add 0.7 parts of SBR to the mixture obtained in step 2, adjust the viscosity of the mixture to 3000±200 mPa·s, and obtain a negative electrode slurry; control the slurry temperature to 24±2°C.

[0089] After the slurry preparation was completed, the slurries at the upper, middle and lower positions of the slurry in the slurry kettle (labeled as Comparative Example 2a, Comparative Example 2b and Comparative Example 2c, respectively) were taken as experimental samples. 50 samples were taken from each position and their viscosity was measured over time. The average viscosity of the 50 samples at each position was taken as the viscosity at that position. The viscosity test data is shown in the attached figure. Figure 4 shown.

[0090] After the slurry preparation was completed, the slurries at the upper, middle and lower positions of the slurry in the slurry kettle (labeled as Comparative Example 2a, Comparative Example 2b and Comparative Example 2c, respectively) were taken as experimental samples. 50 samples were taken from each position and their solid content was measured over time. The average solid content of the 50 samples at each position was taken as the solid content of that position. The solid content test data is shown in the attached figure. Figure 5 shown.

[0091] After the slurry is prepared, it is coated and dried to form the negative electrode. The electrode piece at a distance of 10 to 20 mm from the edge of the prepared electrode piece is taken as the experimental sample and tested by SEM and EDS to observe the electrode piece morphology and the distribution of elements on the electrode piece surface. The SEM test (5000 times) is shown in the attached picture. Figure 12 As shown, EDS test pictures are as attached Figure 13 The EDS test image shows the distribution of sodium on the electrode surface. The weight percentage and atomic percentage of each element tested by EDS are shown in Table 1.

[0092] Table 1

[0093]

[0094] 3. Results analysis.

[0095] By the attached Figure 2 It can be seen that in Example 1, the viscosity of the slurry at the upper, middle and lower positions in the slurry kettle does not change much over time, and the viscosity of the three positions at the same time point is not much different. This shows that the overall uniformity of the slurry in Example 1 is high and it has high stability. In Comparative Example 1, the viscosity of the slurry at the upper, middle and lower positions in the slurry kettle fluctuates greatly over time, which shows that the stability of the slurry in Comparative Example 1 is poor; at the same time, the viscosity of the three positions is quite different at the same time point, which shows that the overall uniformity of the slurry in Comparative Example 1 is poor. It shows that screening the graphite raw materials can effectively improve the uniformity and stability of the negative electrode slurry. From the attached Figure 4It can be seen that the viscosity of the slurry at the top, middle, and bottom positions in the slurry kettle in Comparative Example 2 fluctuates greatly over time, indicating that the slurry stability in Comparative Example 2 is poor. At the same time, the viscosity at the three positions varies greatly at the same time point, indicating that the overall uniformity of the slurry in Comparative Example 2 is poor. This shows that the step-by-step addition of CMC can effectively improve the uniformity and stability of the negative electrode slurry.

[0096] By the attached Figure 3 It can be seen that in Example 1, the solid content of the slurry at the upper, middle and lower positions in the slurry kettle changes relatively evenly over time, and the solid content of the three positions at the same time point is not much different. This shows that the overall uniformity of the slurry in Example 1 is relatively high and it has relatively high stability. In Comparative Example 1, the solid content of the slurry at the upper, middle and lower positions in the slurry kettle fluctuates greatly over time, which shows that the stability of the slurry in Comparative Example 1 is relatively poor; at the same time, the solid content of the three positions is quite different at the same time point, which shows that the overall uniformity of the slurry in Comparative Example 1 is relatively poor. It shows that screening the graphite raw materials can effectively improve the uniformity and stability of the negative electrode slurry. From the attached Figure 5 It can be seen that the solid contents at the top, middle, and bottom positions of the slurry in Comparative Example 2 fluctuate greatly over time, indicating that the slurry in Comparative Example 2 has poor stability. At the same time, the solid contents at the three positions vary greatly at the same time point, indicating that the overall uniformity of the slurry in Comparative Example 2 is poor. This shows that the step-by-step addition of CMC can effectively improve the uniformity and stability of the negative electrode slurry.

[0097] By the attached Figure 6 , Attachment Figure 8 , Attachment Figure 10 , Attachment Figure 12 It can be seen that in Comparative Example 1, there are sharp objects on the graphite surface, and the conductive agent and thickener are distributed more here, which makes the slurry particles easy to settle, showing that the slurry uniformity and stability are poor, which will directly affect the stability of the coating surface density and ultimately affect the performance of the battery. In Example 1, Example 2, and Comparative Example 2, the slurry particles have better roundness, and the conductive agent and thickener are distributed more evenly, which can improve the stability of the coating surface density and improve the battery performance. This shows that screening graphite raw materials can improve the stability of the slurry, thereby improving the performance of the battery.

[0098] As shown in Table 1, compared with Comparative Example 1, the weight percentage of sodium element in the electrode of Example 1 and Example 2 is higher, indicating that the graphite in Example 1 and Example 2 has a stronger adsorption effect on CMC. Figure 7 , Attachment Figure 9 and attached Figure 11EDS test images show that compared to Comparative Example 1, the graphite surfaces in Examples 1 and 2 have more and more uniform points (i.e., sodium elements), further demonstrating that the graphites in Examples 1 and 2 have a stronger and more uniform adsorption of CMC than the graphite in Comparative Example 1, which is beneficial for improving the performance of the negative electrode material (graphite). Furthermore, the graphite in Example 1 has a stronger and more uniform adsorption of CMC than that in Example 2, indicating that the CMC addition ratio in Example 1 is more conducive to its adsorption.

[0099] As shown in Table 1, compared with Comparative Example 2, the weight percentage of sodium element in the electrode of Example 1 and Example 2 is higher, indicating that the graphite in Example 1 and Example 2 has a stronger adsorption effect on CMC. Figure 7 , Attachment Figure 9 and attached Figure 13 The EDS test images show that compared with Comparative Example 2, there are more and more uniform points (i.e., sodium elements) on the graphite surface in Examples 1 and 2, further illustrating that the adsorption of CMC by graphite is stronger and more uniform when CMC is added in two steps than when CMC is added in one step.

[0100] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the embodiments of the present invention rather than to limit them. Although the embodiments of the present invention are described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the embodiments of the present invention can still be modified or replaced by equivalents, and these modifications or equivalent replacements cannot cause the modified technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing a negative electrode slurry for a lithium ion battery, characterized in that: The steps include: S01: Establishing a relationship model between the sedimentation coefficient of the negative electrode material of a lithium-ion battery and the viscosity of the fluid prepared therefrom as shown in Formula I; S02: In the relationship model, the sedimentation coefficient first decreases and then tends to be stable with the increase of viscosity. The viscosity range corresponding to the stable sedimentation coefficient is selected, and the particle size of the negative electrode material is selected according to the viscosity range. Then, the negative electrode material is selected according to the particle size to prepare the negative electrode slurry; S03: mixing and stirring the negative electrode material selected in S02 with the conductive agent and thickener; S04: adding a solvent and a thickener to the mixture obtained in S03, and stirring and mixing; S05: adding a binder to the mixture obtained in S04, adjusting the viscosity of the mixture to a process range, and obtaining a negative electrode slurry; w = aμ 3 + bμ 2 + cμ + d Equation I; In formula I, w is the sedimentation coefficient of the negative electrode material; μ is the viscosity of the fluid prepared with the negative electrode material; a, b, c, and d are fitting coefficients; The particle size of the negative electrode material is selected according to the viscosity range, and further the method of selecting the negative electrode material according to the particle size is: for different batches of negative electrode materials, the batch with the smallest sedimentation coefficient corresponding to the viscosity range is selected as the negative electrode material.

2. The method for preparing a negative electrode slurry for a lithium-ion battery according to claim 1, wherein: The negative electrode material is graphite or lithium titanate.

3. The method for preparing a negative electrode slurry for a lithium-ion battery according to claim 1, wherein: The sedimentation coefficient of the negative electrode material described in S01 is determined by calculating the Stokes equation.

4. The method for preparing a negative electrode slurry for a lithium-ion battery according to claim 1, wherein: The conductive agent is one or more of Super P, acetylene black, Super S, KS-6, KS-15, SFG-6, SFG-15, Ketjen black, CNTs, VGCF, and graphene; the binder is one or more of SBR, polyvinylidene fluoride copolymer, polyimide, polyacrylic acid, and sodium alginate.

5. The method for preparing a negative electrode slurry for a lithium-ion battery according to claim 1, wherein: The thickener is sodium carboxymethyl cellulose; the solvent is water.

6. The method for preparing a negative electrode slurry for a lithium-ion battery according to claim 1, wherein: The mass ratio of the thickener in S02 to the thickener in S03 is 1:(0.4~2.5).

7. The method for preparing a negative electrode slurry for a lithium-ion battery according to claim 1, wherein: The mass ratio of the thickener in S02 to the thickener in S03 is 1:1.

5.

8. A method for evaluating anode slurry for lithium-ion batteries, characterized in that: A negative electrode slurry is prepared according to the preparation method according to any one of claims 1 to 7, and a sample is taken to test the viscosity and solid content; then, the slurry is coated and dried to form a pole piece, and the pole piece is subjected to SEM testing and EDS testing.

9. The method for evaluating anode slurry for lithium-ion batteries according to claim 8, wherein: The uniformity and stability of the negative electrode slurry are evaluated based on the viscosity test results, the solid content test results, and the SEM test results; and the adsorption performance of the negative electrode material in the negative electrode slurry is evaluated based on the EDS test results.

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