A slurry, a pole piece with a three-dimensional long-range conductive network structure, and its preparation process and application

Abstract

The present invention belongs to the technical field related to capacitors, and more particularly to a preparation process for a slurry, wherein nano lithium cobaltate, activated carbon, a conductive agent and polyvinylidene fluoride are mixed uniformly in a dry-mixed manner; kneading: a certain amount of N-methyl-2-pyrrolidone solvent is added to the primary mixture and mixed uniformly again; dispersion: a certain amount of N-methyl-2-pyrrolidone solvent is added to the mixture, and then an appropriate amount of OP-10, AES and PAS are added, and the slurry is mixed again. The present invention adopts a dry-mixing process, optimizes the addition step of each component, adds OP-10, AES and PAS uncharged nonionic surfactants at the end of the dispersion step, and can significantly reduce the interaction between van der Waals forces and hydrogen bonds between particles in the suspension, so that the stability and dispersion uniformity of the suspension are significantly improved, and finally constructs a three-dimensional long-range conductive network structure electrode.

Description

Technical Field

[0001] The present invention belongs to the technical field of electrochemical energy storage, and in particular relates to a slurry, a pole piece with a three-dimensional long-range conductive network structure, and a preparation process and application thereof. Background Art

[0002] Lithium-ion capacitors are a new type of electrochemical energy storage device with relatively higher specific power compared to lithium-ion batteries and relatively higher specific energy compared to supercapacitors. Therefore, lithium-ion capacitors, which combine the advantages of lithium-ion batteries and supercapacitors, have received widespread attention.

[0003] Since lithium-ion capacitors must have both high specific energy and high specific power, they place high demands on the electrode preparation process: (1) There cannot be too many closed pores and complex pore structures in the electrode coating; (2) The dispersion uniformity of the slurry must be relatively stable, otherwise it will affect the consistency of the electrode, the stability of continuous coating (resistivity, porosity, coating load) and the power characteristics of the capacitor. Therefore, in order for lithium-ion capacitors to quickly enter the market and be widely used in the market, it is necessary to conduct in-depth research on the electrode preparation process of lithium-ion capacitors to solve the above problems. Among them, the mixing and dispersion process of lithium-ion capacitor slurry determines the physical and chemical properties of the electrode, and therefore places high demands on the slurrying process. The slurrying process affects the distribution state of active and inactive substances in the electrode coating, pore distribution, pore structure, conductive network structure, coating stability, etc., thereby having a significant impact on the diffusion rate of lithium ions in the positive electrode, especially under high-rate charge and discharge conditions. Its impact on electrical performance exceeds 30%. However, the nano-lithium cobalt oxide (LCO) currently used in lithium-ion capacitors has poor dispersion stability and uniformity in the slurry suspension. This is a common problem in the industry, and there is currently no better process strategy. Summary of the Invention

[0004] Addressing the challenges of existing technologies, this invention optimizes the slurry mixing and dispersion process, resulting in a highly uniform and stable slurry dispersion process suitable for industrial production. This process allows the preparation of electrode sheets with a three-dimensional, long-range conductive network structure, and establishes a correlation between stirring strategy, slurry dispersion stability, electrode sheet conductive network structure, and the electrical performance of lithium-ion capacitors. Compared to traditional slurry preparation processes, the resulting electrode sheets exhibit stronger peel strength, lower resistivity, and an excellent three-dimensional conductive network structure.

[0005] In the first aspect, a slurry preparation process in this solution includes the following preparation steps:

[0006] S1. Prepare the raw materials for the slurry: including nano-lithium cobalt oxide, activated carbon, conductive agent, polyvinylidene fluoride and N-methyl-2-pyrrolidone solvent. The N-methyl-2-pyrrolidone solvent is divided into two parts for standby use.

[0007] S2. Mixing: uniformly mixing the nano-lithium cobalt oxide, activated carbon, conductive agent and polyvinylidene fluoride by dry mixing to obtain a primary mixture;

[0008] S3, kneading: adding a portion of the N-methyl-2-pyrrolidone solvent to the initial mixture obtained in S2 and mixing again to obtain a mixture;

[0009] S4, dispersion: add another portion of the N-methyl-2-pyrrolidone solvent to the mixture obtained in S3, mix well, then add appropriate amounts of dodecylphenol polyoxyethylene ether, fatty alcohol polyoxyethylene ether sodium sulfate and monolauryl phthalate, and mix again to obtain the slurry.

[0010] Furthermore, the mass ratio of the nano-lithium cobaltate, activated carbon, conductive agent, and polyvinylidene fluoride is 85:9:3:3.

[0011] Furthermore, the conductive agent includes a mixture of an SP conductive agent and a CNT conductive agent in a mass ratio of 2:1.

[0012] Furthermore, in S2, the nano-lithium cobaltate, activated carbon, conductive agent and polyvinylidene fluoride are mixed twice, with the time interval between the two additions being 30 to 60 minutes. The stirring rates during the additions are 30 to 50 rpm and 500 to 800 rpm, respectively. After all the powders are added to the stirrer, dry mixing is performed at a rate of 30 to 50 rpm and 4000 to 6000 rpm for 60 to 80 minutes.

[0013] Furthermore, in step S3, an appropriate amount of N-methyl-2-pyrrolidone solvent is added to the initial mixture of step S2, where the solid content is 70% to 75%. The mixture is first stirred at 30-50 rpm and then at 500-700 rpm for 10-30 minutes, and then at 70-90 rpm and then at 3000-4000 rpm for 4-7 hours. The solid content refers to the weight percentage of all powders in the mixture system in this step.

[0014] Furthermore, in S4, the remaining N-methyl-2-pyrrolidone solvent is added to the mixture obtained in S3 twice, at which time the final solid content is 50% to 55%, the interval between the two additions is 30 to 60 minutes, and the stirring speed during the addition is 70 to 90 rpm and 2000 to 3000 rpm.

[0015] Furthermore, after all the N-methyl-2-pyrrolidone solvent is added to S4, dodecylphenol polyoxyethylene ether, sodium fatty alcohol polyoxyethylene ether sulfate and monolauryl phthalate are added and dispersed at high speed at stirring rates of 50-70 rpm and 4000-5000 rpm for 3-5 hours.

[0016] Furthermore, the dodecylphenol polyoxyethylene ether, fatty alcohol polyoxyethylene ether sodium sulfate and monolauryl phthalate account for 0.4% of the solid content. In a second aspect, the present invention also provides an electrode having a three-dimensional long-range conductive network structure, wherein the electrode is obtained by uniformly coating the slurry prepared by the process described in the first aspect on aluminum foil, drying it, pressing it into sheets, and cutting it.

[0017] In a third aspect, the electrode having a three-dimensional long-range conductive network structure prepared by the present invention is used to prepare a lithium ion capacitor.

[0018] For lithium ion capacitors, the introduction of activated carbon in the positive electrode formula can bring out the performance of double-layer capacitance, thereby improving the specific power of hybrid capacitors. However, compared with positive electrode active materials such as lithium cobalt oxide, activated carbon (AC) and conductive agents (SP, CNT) have larger specific surface area, lower density, larger particle diameter and complex pore structure, which will greatly affect the preparation process and stability of electrode slurry. The present invention is mainly aimed at the technical defects of poor dispersion stability and uniformity of nano-lithium cobalt oxide (LCO) in slurry suspension in the prior art, and solves the problem of poor slurry stability and dispersion uniformity by optimizing the slurry preparation process, adding the selected non-ionic surfactant and determining the addition ratio. Therefore, on the basis of the battery type slurry preparation process, the present invention further studies the influence of the stirring process parameters of the composite slurry on its uniformity and stability, constructs the influence law of the slurry stirring process parameters on the slurry stability and consistency, and provides guidance for the preparation of nano-lithium cobalt oxide slurry.

[0019] The present invention addresses the shortcomings of the existing technology and optimizes the slurry mixing and dispersion process to obtain a highly uniform and stable positive electrode material dispersion process suitable for industrial production. By adding OP-10 (dodecylphenol polyoxyethylene ether), AES (fatty alcohol polyoxyethylene ether sodium sulfate), and PAS (monolauryl phthalate) as uncharged nonionic surfactants at the end of the dispersion step, the process has good wettability and mild surface activity, significantly reducing the van der Waals forces and hydrogen bond interactions between particles in the suspension, significantly improving the stability and dispersion uniformity of the suspension, and ultimately achieving the purpose of constructing a pole piece with a three-dimensional long-range conductive network structure.

[0020] The slurry prepared by the present invention is used to prepare pole pieces. The conductive agent forms a sponge-like three-dimensional conductive network structure on the pole piece surface and between the particle gaps, indicating that the slurry coating prepared under the process conditions does not undergo obvious migration and agglomeration of the conductive agent and the binder during the coating and drying process.

[0021] The impedance spectrum of the electrode prepared with the obtained slurry has a higher slope in the low-frequency region, showing relatively good capacitance performance, indicating that the electrode ion diffusion resistance prepared by this process is relatively small. Combining the above advantages, the electrode presents relatively optimal pulse power performance and peel strength, and the overall performance of the electrode is improved.

[0022] The method of the present invention establishes a correlation between dry-mixing and kneading strategies, slurry dispersion stability, electrode sheet conductive network structure, and the electrical performance of lithium-ion capacitors. Compared to traditional slurry preparation processes, the electrode sheets prepared from the slurry of the present invention exhibit strong peel strength, low resistivity, and an excellent three-dimensional conductive network structure. This electrode sheet with a three-dimensional long-range conductive network structure has excellent ionic and electronic conductivity and demonstrates good ion migration rate. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 From left to right are the contact angle test results of the slurries obtained by the three processes of Comparative Example 1, Example 1 and Comparative Example 2;

[0024] Figure 2 From left to right are SEM images of electrodes prepared using the slurries obtained by the three processes of Comparative Example 1, Comparative Example 2 and Example 1;

[0025] Figure 3 The impedance test graphs of the electrode sheets prepared by the slurries obtained by the three processes of Example 1, Comparative Example 1 and Comparative Example 2 are shown. In the figure, strategy A corresponds to Example 1, strategy B corresponds to Comparative Example 1, and strategy C corresponds to Comparative Example 2.

[0026] Figure 4 The pulse power test of the electrode sheets prepared by the slurries obtained by the three processes of Example 1, Comparative Example 1 and Comparative Example 2; in the figure, strategy A corresponds to Example 1, strategy B corresponds to Comparative Example 1, and strategy C corresponds to Comparative Example 2;

[0027] Figure 5 These are the peeling force test curves of the electrodes prepared using the slurries obtained by the three processes of Example 1, Comparative Example 1 and Comparative Example 2. In the figure, strategy A corresponds to Example 1, strategy B corresponds to Comparative Example 1, and strategy C corresponds to Comparative Example 2. DETAILED DESCRIPTION

[0028] The following is further described in detail through specific implementation methods:

[0029] In the following process, all stirring steps are carried out using a planetary stirrer.

[0030] Example 1: A process for preparing a slurry, comprising the following steps:

[0031] S1. Prepare the raw materials for the slurry: The raw materials include nano-lithium cobalt oxide (LCO), activated carbon (AC), a conductive agent (including a mixture of SP conductive agent and CNT conductive agent with a mass ratio of 2:1), polyvinylidene fluoride (PVDF) and N-methyl-2-pyrrolidone (NMP) solvent. All materials are divided into two parts for standby use, among which the mass ratio of LCO, AC, conductive agent and polyvinylidene fluoride is 85:9:3:3.

[0032] S2. Dry mixing: LCO, activated carbon (AC), conductive agent and PVDF were added to the blender twice for dry mixing. The time interval between the two additions was 30 minutes. The stirring speeds during the addition were 30 rpm and 500 rpm respectively. After all the powders were added to the blender, dry mixing was carried out at a speed of 30 rpm and 4000 rpm for 60 minutes.

[0033] S3. Kneading: Add NMP to the mixed powder, at which point the solid content of the mixture is 70%. Stir at 30 rpm and 500 rpm for 10 min, then at 70 rpm and 3000 rpm for 4 h.

[0034] S4, dispersion: the remaining NMP solvent is added twice, the interval between the two times is 30min, and the stirring speed (S.S.) is 70rpm, 3000rpm during the feeding. After the NMP solvent is fully added, the final solid content is 50%. Then, an appropriate amount of nonionic surfactant is added to the dispersion liquid. The nonionic surfactant is a mixed component of OP-10 (dodecylphenol polyoxyethylene ether), AES (fatty alcohol polyoxyethylene ether sodium sulfate) and PAS (monolauryl phthalate). The nonionic surfactant accounts for 0.4% of the solid content and is dispersed at a high speed of 70rpm, 4000rpm for 3h.

[0035] Comparative Example 1: Process flow of wet mixing and stirring strategy for making glue:

[0036] (1) Gel preparation: Dissolve all PVDF in NMP. Stir at 30 rpm and 500 rpm for 15 min, then at 45 rpm and 2000 rpm for 4 h.

[0037] (2) Mixing: Add LCO, CNTs, and SP to the glue solution all at once. Stir at 30 rpm and 500 rpm for 10 min, then knead at 70 rpm and 3000 rpm for 4 h.

[0038] (3) Dispersion: Add NMP (final solid content is 50%) to the above slurry and disperse at high speeds of 60 rpm and 3000 rpm for 3 h.

[0039] Comparative Example 2: Process flow of wet mixing and stirring strategy for making glue:

[0040] (1) Gel preparation: Dissolve all PVDF in NMP. Stir at 40 rpm and 700 rpm for 25 min, then at 50 rpm and 3000 rpm for 5 h.

[0041] (2) Mixing: Add LCO, CNTs, and SP to the glue solution all at once. Stir at 40 rpm and 600 rpm for 20 min, then knead at 60 rpm and 2500 rpm for 5 h.

[0042] (3) Dispersion: Add NMP (final solid content is 53%) to the above slurry and disperse at high speeds of 70 rpm and 4000 rpm for 3 h.

[0043] Figure 1 From left to right are the contact angle test results of the slurries obtained by the three processes of Comparative Example 1, Example 1 and Comparative Example 2. Figure 1 It can be seen that the slurry suspension prepared in Example 1 has the smallest contact angle on the aluminum foil surface, at 63.5°, demonstrating the best wettability. Better wettability results in a stronger bond between the slurry and the current collector, and a lower surface tension effect on the slurry coating during drying. This significantly reduces binder migration and thick edges during baking. Therefore, the suspension prepared in Example 1 is more conducive to coating stability during electrode preparation.

[0044] Comparative Example 3: The slurry preparation process differs from that of Comparative Example 1 in that, after NMP is added and mixed in the dispersion step, the same nonionic surfactant as in Example 1 is added.

[0045] Example 2: Preparation of pole pieces. The slurries obtained in Example, Comparative Example 1, Comparative Example 2, and Comparative Example 3 were used to prepare pole pieces using the same method. The specific preparation method is as follows:

[0046] The prepared slurry was evenly coated on a 12 μm aluminum foil, and then dried in a vacuum drying oven at 140° C. for 72 h, and then pressed into sheets using a sheet press. Finally, the electrode sheet was cut into electrode sheets.

[0047] Figure 2 From left to right in the figure are SEM images of the pole pieces prepared by the slurries obtained by the three processes of Comparative Example 1, Comparative Example 2 and Example 1. Figure 2It can be seen that the electrode surface active material particles prepared with the slurries of Comparative Examples 1 and 2 have many large pores between them, and no long-range conductive network structure coated with a conductive agent has been formed between the particles. The pore structure on the electrode surface is not conducive to the rapid transmission of electrons and cannot meet the performance requirements of high-current pulse discharge. In the electrode prepared with the slurry of Example 1, the conductive agent forms a sponge-like three-dimensional conductive network structure on the electrode surface and between the particles. That is, the slurry coating prepared under these process conditions does not undergo significant migration or agglomeration of the conductive agent and binder during the coating and drying process.

[0048] from Figure 3 It can be seen that the electrode impedance spectrum prepared by the slurry of Example 1 has a higher slope in the low-frequency region, showing relatively good capacitance performance, indicating that the electrode ion diffusion resistance prepared by this process is relatively small.

[0049] from Figure 4 It can be seen that the electrode prepared from the slurry of Example 1 exhibits the highest pulse power (4312.5W), while the electrode prepared from Comparative Examples 1 and 2 have relatively poor pulse power performance. Instantaneous high-power pulse discharge is mainly determined by a combination of indicators such as electrode porosity and electrode resistivity. The electrode prepared from the slurry of Example 1 exhibits the best pulse power performance.

[0050] The present invention subjected the electrode sheets prepared from the slurries of Example 1, Comparative Example 3, and Comparative Example 4 to resistivity tests, limiting pulse power tests, and peel strength tests. The test results are shown in Tables 1, 2, and 3. As can be seen from the test results, the electrode sheet prepared from the slurry of Example 1 exhibits relatively superior overall performance compared to the electrode sheet prepared from the slurry of Comparative Example 3.

[0051] Table 1 Electrode resistivity test

[0052] Group Internal resistance (mΩ) Comparative Example 3 1.21 Example 1 1.03

[0053] Table 2 Pole piece limit pulse power test

[0054] Group Power (W, 2000A) Comparative Example 3 3910.4 Example 1 4312.5

[0055] Table 3 Pole peeling strength test

[0056] Group N Comparative Example 3 0.78 Example 1 1.92

[0057] The present invention tests and analyzes the bonding strength between the electrode coating and the current collector. The peeling effect is as follows Figure 5As shown in the figure below, the peeling force data results show that the electrode prepared by the slurry of Example 1 shows a relatively maximum peeling force of 1.92N, while the electrode prepared by the slurry of Comparative Example 3 has a peeling force less than the peeling force of the electrode prepared by the slurry of Example 1.

[0058] Example 3: Preparation of soft-pack lithium ion capacitors:

[0059] The assembly of the soft-pack laminated lithium-ion capacitor is carried out in a drying room. First, 20 lithium cobalt oxide / activated carbon pole pieces (pole pieces prepared from the slurry obtained in Example 1) and 21 hard carbon pole pieces (N:P=1.3, hard carbon mass fraction of 94%) are taken as positive and negative electrodes respectively. The diaphragm uses a polypropylene diaphragm, and the electrolyte is an organic electrolyte (1 mol / L LiPF6, EC:PC:DMC volume ratio of 1:1:1). Then, the soft-pack lithium-ion capacitor is laminated, and the shape is designed to be rectangular.

Claims

1. A process for preparing a slurry, characterized in that The preparation steps included are as follows: S1. Prepare the raw materials for the slurry: including nano-lithium cobalt oxide, activated carbon, conductive agent, polyvinylidene fluoride and N-methyl-2-pyrrolidone solvent. The N-methyl-2-pyrrolidone solvent is divided into two parts for standby use. S2. Mixing: uniformly mixing the nano-lithium cobalt oxide, activated carbon, conductive agent and polyvinylidene fluoride by dry mixing to obtain a primary mixture; S3, kneading: adding a portion of the N-methyl-2-pyrrolidone solvent to the initial mixture obtained in S2 and mixing again to obtain a mixture; S4, dispersion: add another portion of the N-methyl-2-pyrrolidone solvent to the mixture obtained in S3, mix well, add dodecylphenol polyoxyethylene ether, sodium fatty alcohol polyoxyethylene ether sulfate and monolauryl phthalate, and disperse at a high speed of 50-70 rpm and 4000-5000 rpm for 3-5 hours; the dodecylphenol polyoxyethylene ether, sodium fatty alcohol polyoxyethylene ether sulfate and monolauryl phthalate account for 0.4% of the solid content.

2. The process for preparing a slurry according to claim 1, wherein: The mass ratio of the nano-lithium cobaltate, activated carbon, conductive agent and polyvinylidene fluoride is 85:9:3:

3.

3. The process for preparing a slurry according to claim 2, characterized in that: The conductive agent is obtained by mixing an SP conductive agent and a CNT conductive agent in a mass ratio of 2:

1.

4. The process for preparing a slurry according to claim 1, wherein: In S2, the nano-lithium cobaltate, activated carbon, conductive agent and polyvinylidene fluoride are mixed twice, with the time interval between the two additions being 30 to 60 minutes. The stirring rates during the additions are 30 to 50 rpm and 500 to 800 rpm, respectively. After all the powders are added to the stirrer, dry mixing is carried out at a rate of 30 to 50 rpm and 4000 to 6000 rpm for 60 to 80 minutes.

5. The process for preparing a slurry according to claim 4, characterized in that: In the above-mentioned S3, an appropriate amount of N-methyl-2-pyrrolidone solvent is added to the initial mixture of S2, at which time the solid content is 70% to 75%, and the mixture is first stirred at 30 to 50 rpm and 500 to 700 rpm for 10 to 30 minutes, and then stirred at 70 to 90 rpm and 3000 to 4000 rpm for 4 to 7 hours.

6. The process for preparing a slurry according to claim 5, characterized in that: In the step S4, the remaining N-methyl-2-pyrrolidone solvent is added to the mixture obtained in step S3 in two additions, at which point the final solid content is 50% to 55%, the interval between the two additions is 30 to 60 minutes, and the stirring speeds during the additions are 70 to 90 rpm and 2000 to 3000 rpm.

7. A slurry, characterized in that: The invention is prepared by the preparation process according to any one of claims 1 to 6.

8. A pole piece having a three-dimensional long-range conductive network structure, characterized in that: The electrode is obtained by uniformly coating the slurry described in claim 7 on aluminum foil, drying it, and then pressing and cutting it.

9. The electrode having a three-dimensional long-range conductive network structure according to claim 8 is used in the preparation of a lithium ion capacitor.

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