A double-layer negative electrode sheet, a method for manufacturing the same, and a lithium-ion capacitor

Abstract

The application discloses a double-layer negative plate preparation method in the field of energy storage equipment. The preparation process comprises three main steps: firstly, negative electrode slurry A is prepared, hard carbon, conductive carbon black, natural graphite, single-walled carbon nanotube dispersion liquid and polyvinylidene fluoride are stirred and uniformly mixed in a dry room with a dew point of-30 to-40 DEG C according to a specific mass ratio, and 1-methyl-2-pyrrolidone is added and stirred uniformly; secondly, negative electrode slurry B is prepared, and the process is similar to that of A, but a part of the hard carbon is replaced by an HL mixture (mixed uniformly by hard carbon and lithium cobalt nitride according to a specific ratio); finally, a double-layer negative plate is prepared, a layer of negative electrode slurry A or B is coated on both sides of the copper current collector and first dried, another layer is coated and second dried, and finally, the double-layer negative plate is obtained through rolling and cutting. The double-layer negative plate design aims to solve the problems of low first effect of hard carbon and insufficient power density in the super-high rate constant-current discharge.

Description

Technical Field

[0001] This invention belongs to the field of energy storage devices, and specifically relates to a double-layer negative electrode sheet, its preparation method, and a lithium-ion capacitor. Background Technology

[0002] Lithium-ion capacitors possess advantages such as high specific energy and power performance, good rate discharge capability, low self-discharge rate, wide operating temperature range, and good cycle life, leading to their widespread application in military equipment. For example, in high-power microwave weapons, lithium-ion capacitors have been adopted as the main power source for microwave weapon emission to maintain high energy and power density. With the development of related military equipment technologies, the required energy storage power sources must possess characteristics such as high specific energy, high power, high safety and reliability, and a wide operating temperature range.

[0003] Commercially available lithium-ion capacitors (such as automotive lithium-ion capacitors) primarily use hard carbon (HC) and graphite as anode materials. Hard carbon, in particular, can achieve a specific capacity of around 1000 mAh / g in lithium-ion batteries, almost three times that of graphite. Furthermore, due to its 3D ion transfer pathways, hard carbon exhibits significantly better rate performance than graphite. The anode material of hard carbon also displays a low potential plateau, ≤0.2V (relative to Li). + Hard carbon exhibits considerable capacity at the surface area ratio ( / Li), enabling it to deliver high power density in lithium-ion batteries. However, the initial coulombic efficiency (ICE) of hard carbon is typically around 70%, and even lower for those with higher surface areas.

[0004] To address the aforementioned technical issues, patent application publication number "CN 114899354 A" discloses a multilayer negative electrode sheet, its preparation method, and a secondary battery. The multilayer negative electrode sheet in this patent includes an ultrathin bottom coating layer coated on the surface of a current collector, a first coating layer coated on the surface of the ultrathin bottom coating layer, and a second coating layer coated on the surface of the first coating layer. The first coating layer includes a first active material, which is graphite, soft carbon, or hard carbon. The second coating layer includes a second active material, which is also graphite, soft carbon, or hard carbon. The ultrathin bottom coating layer includes a third active material, which is carbon nanotubes. The multilayer negative electrode sheet provided by this patent combines high energy load and fast ion transfer performance, not only improving the battery's energy density but also effectively preventing lithium plating, thus improving battery safety, cycle performance, and rate performance. However, lithium-ion batteries based on this multilayer negative electrode sheet exhibit poor high-rate discharge performance and low initial coulombic discharge, making it unsuitable for aerospace, military, and high-power applications. Summary of the Invention

[0005] The present invention aims to provide a method for preparing a double-layer negative electrode to solve the problems of low initial efficiency of hard carbon and insufficient power density under ultra-high rate constant current discharge conditions.

[0006] The method for preparing a double-layer negative electrode in this scheme includes the following steps:

[0007] Step 1: Preparation of negative electrode slurry A: Hard carbon (HC), conductive carbon black (SP), natural graphite (KS-6), single-walled carbon nanotube dispersion, and polyvinylidene fluoride (PVDF) are mixed in a dry room with a dew point of -30 to -40°C at a mass ratio of (92-96):(2.0-0.82):(2.18-1.05):(0.18-0.13):(3.64-2.0) to obtain kneaded material A; then 1-methyl-2-pyrrolidone (NMP) is added and stirred evenly to adjust the viscosity to 3000-6000 mPa·s to obtain negative electrode slurry A;

[0008] Step 2: Preparation of negative electrode slurry B: The HL mixture, conductive carbon black (SP), natural graphite (KS-6), single-walled carbon nanotube dispersion, and polyvinylidene fluoride (PVDF) are mixed in a drying room with a dew point of -30 to -40°C at a mass ratio of (92-96):(2.0-0.82):(2.18-1.05):(0.18-0.13):(3.64-2.0) to obtain kneaded material B; then 1-methyl-2-pyrrolidone (NMP) is added and stirred evenly to adjust the viscosity to 3000-6000 mPa·s to obtain negative electrode slurry B; the HL mixture is composed of HC (hard carbon) and LCN (lithium cobalt nitride) in a mass ratio of m LCN / m (HC+LCN) =0-10% by mass to obtain;

[0009] Step 3: Preparation of the double-layer negative electrode sheet: Two layers of negative electrode slurry are coated on both sides of the copper current collector. The two negative electrode slurries are negative electrode slurry A and negative electrode slurry B, respectively. The two negative electrode slurries are different. If the first layer is negative electrode slurry A, then the second layer is negative electrode slurry B, and vice versa. Specifically: First, a layer of negative electrode slurry A or negative electrode slurry B is coated on both sides of the copper current collector and dried for the first time at a temperature of 80-120℃. Then, another layer of negative electrode slurry is coated on both sides of the copper current collector, and then dried a second time to obtain the negative electrode coating. Finally, the negative electrode coating is rolled and cut to obtain the double-layer negative electrode sheet.

[0010] Furthermore, in step one and / or step two, a vacuum mixer is used to mix the materials thoroughly.

[0011] Furthermore, in step three, a coating machine is used to coat the negative electrode slurry onto the copper current collector.

[0012] Furthermore, in step three, the initial drying time is 30–60 minutes.

[0013] Furthermore, in step three, during the secondary drying, vacuum drying is adopted. Specifically, after coating another layer of negative electrode slurry on both sides of the copper current collector, it is continuously vacuum dried in a vacuum dryer for 18 to 24 hours with a vacuum degree ≤ -0.095 MPa.

[0014] Furthermore, in step three, a roller press is used to roll the negative electrode coating.

[0015] Furthermore, in step three, a slitting machine is used to slit the rolled negative electrode coating.

[0016] Experimental verification shows that the bilayer negative electrode obtained by the above preparation method, when assembled with a lithium cobalt oxide positive electrode to form a lithium-ion capacitor, exhibits high rate capability and capacity, solving the problems of low initial efficiency and insufficient power density under ultra-high rate constant current discharge conditions associated with hard carbon capacitors. Therefore, this application also claims protection for the bilayer negative electrode prepared by the above preparation method, and for lithium-ion capacitors comprising the above bilayer negative electrode. The lithium-ion capacitor of this application can effectively meet the application requirements of aerospace, military, and high-power applications, and can also be used in commercial fields such as automobiles. Attached Figure Description

[0017] Figure 1 Scanning electron microscope images of the monolayer A electrode and monolayer B electrode prepared in Examples 3-4;

[0018] Figure 2 Comparison of resistivity of lithium-ion capacitor electrode sheets prepared in Examples 1-4;

[0019] Figure 3 AC impedance test diagrams of lithium-ion capacitor electrodes prepared in Examples 1-4;

[0020] Figure 4 Rate performance test charts of lithium-ion capacitors prepared in Examples 1-4;

[0021] Figure 5 The lithium-ion capacitor prepared in Example 1 underwent a 1600A pulse discharge curve.

[0022] Figure 6 for Figure 5 A magnified view of a portion of the image. Detailed Implementation

[0023] The following detailed description illustrates the specific implementation method:

[0024] Example 1:

[0025] (1) 85.707 g of HC (hard carbon), SP (conductive carbon black), KS-6 (natural graphite), single-walled carbon nanotube dispersion, and PVDF (polyvinylidene fluoride) were weighed in a mass ratio of 94:1.7:1.85:0.15:2.3. The material was stirred at 1000 r / min for 10 min in a planetary vacuum mixer in a drying room with a dew point of -35℃. 137 g of 1-methyl-2-pyrrolidone (NMP) organic solvent was added. The mixture was stirred at 1000 r / min and 1200 r / min for 15 min and 10 min respectively in the planetary vacuum mixer to obtain a uniform and stable negative electrode slurry A. Then, the negative electrode slurry A was coated on both sides of the copper current collector using a small coating machine with a coating thickness of 15 μm. The coating was then dried in an oven at 80℃ for 30 min to obtain the A electrode sheet.

[0026] (2) Mix HC (hard carbon) and LCN according to m LCN / m (HC+LCN) =5% by mass, and thoroughly mixed in a dry powder mixer at 800 r / min for 8 min to obtain HC and LCN mixed powder material (hereinafter referred to as HL mixture); 85.707 g of HL mixture, SP, KS-6, single-walled carbon nanotube dispersion, and PVDF were weighed in a mass ratio of 94:1.7:1.85:0.15:2.3, and the material was stirred in a planetary vacuum mixer at 1000 r / min for 10 min in a drying room with a dew point of -35℃, and 137 g of [unclear text] was added. 1-Methyl-2-pyrrolidone (NMP) organic solvent was used to stir the anode slurry B in a planetary vacuum mixer at 1000 rpm for 15 min, followed by 1200 rpm for 10 min. This yielded a uniform and stable negative electrode slurry B. The negative electrode slurry B was then coated onto both sides of the A electrode sheet using a small coating machine to a thickness of 15 μm. The coated sheet was then dried in a vacuum drying oven at 120℃ (vacuum degree -0.095 MPa) for 12 h to obtain the AB electrode sheet, with a total coating thickness of 30 μm. Finally, the AB negative electrode sheet was rolled to a certain thickness using a roller mill, and then slitting the AB negative electrode sheet into rectangular electrode sheets of 300*57 mm using a slitting machine, forming the double-layer AB negative electrode sheet, or AB electrode.

[0027] (3) Assemble the AB electrode prepared in (2) with the corresponding lithium cobalt oxide positive electrode to form a soft-pack lithium-ion capacitor. The entire assembly process is carried out in a dry room with strict humidity control (dew point -35℃, the same below). Use an electrolyte system with an EC:DMC:EMC volume ratio of 1:1:1 containing 1mol / L LiPF6, and an NKK ceramic separator. Assemble the lithium-ion capacitor using a winding method. After assembly, seal it with an 80μm aluminum plastic mold and bake it in a vacuum oven at 80℃ for 12 hours before adding the electrolyte. During the assembly of the lithium-ion capacitor, ensure that there are no operational errors, that the positive and negative electrodes do not come into contact to avoid short circuits, and that there are no air bubbles between the separator and the electrode. Seal tightly to isolate air and avoid affecting the performance of the lithium-ion capacitor.

[0028] Example 2:

[0029] The difference between Example 2 and Example 1 is only that: during coating, negative electrode slurry B is first coated and dried to obtain B electrode sheet, and then negative electrode slurry A is coated on B electrode sheet and dried, rolled and cut to obtain double-layer BA negative electrode sheet, i.e. BA electrode.

[0030] Example 3:

[0031] (1) Weigh 85.707 g of HC (hard carbon), SP (conductive carbon black), KS-6 (natural graphite), single-walled carbon nanotube dispersion, and PVDF (polyvinylidene fluoride) in a mass ratio of 94:1.7:1.85:0.15:2.3. In a drying room with a dew point of -35℃, stir the material at 1000 r / min for 10 min in a planetary vacuum mixer. Add 137 g of 1-methyl-2-pyrrolidone (NMP) organic solvent. First, stir the mixture at 1000 r / min in the planetary vacuum mixer. Stir at 100 rpm for 15 minutes, then at 1200 rpm for 10 minutes to obtain a uniform and stable negative electrode slurry A. Then, use a small coating machine to coat the negative electrode slurry A onto both sides of the copper current collector, with a total coating thickness of 30 μm. Dry it in a vacuum drying oven at 120℃ for 12 hours to obtain a single-layer A electrode sheet. Finally, use a roller press to roll the single-layer A electrode sheet to a certain thickness, and then use a slitting machine to cut the single-layer electrode sheet into rectangular electrode sheets of 300*57 mm, i.e., single-layer A electrodes.

[0032] (2) Assemble the prepared monolayer A electrode from (1) with the corresponding lithium cobalt oxide positive electrode to form a soft-pack lithium-ion capacitor. The assembly process is carried out in a dry room with strict humidity control. An electrolyte system containing 1 mol / L LiPF6 with a volume ratio of EC:DMC:EMC of 1:1:1 is used. The separator is an NKK ceramic separator. The lithium-ion capacitor is assembled using a winding method. After assembly, it is sealed in an aluminum mold and placed in a vacuum oven at 80°C for 12 hours before being injected with electrolyte. During the assembly of the lithium-ion capacitor, it is essential to ensure that there are no operational errors, that the positive and negative electrodes do not come into contact to avoid short circuits, and that there are no air bubbles between the separator and the electrode. The seal must be tight to isolate air and prevent any impact on the performance of the lithium-ion capacitor.

[0033] Example 4:

[0034] (1) Mix HC (hard carbon) and LCN according to m LCN / m (HC+LCN) =5% by mass, and thoroughly mixed in a dry powder mixer at 800 r / min for 8 min to obtain HC and LCN mixed powder material (hereinafter referred to as HL mixture); 85.707 g of HL mixture, SP, KS-6, single-walled carbon nanotube dispersion, and PVDF were weighed in a mass ratio of 94:1.7:1.85:0.15:2.3. The material was stirred in a planetary vacuum mixer at 1000 r / min for 10 min in a drying room with a dew point of -35℃. 137 g of 1-methyl-2-pyrrolidone (NMP) organic solvent was added. The agent is first stirred at 1000 r / min for 15 min in a planetary vacuum mixer, and then stirred at 1200 r / min for 10 min to obtain a uniform and stable negative electrode slurry B. Then, a small coating machine is used to coat the negative electrode slurry B on both sides of the copper current collector, with a total coating thickness of 30 μm. The coating is then placed in a vacuum drying oven at 120℃ for 12 h to obtain a single-layer B electrode sheet. Finally, a roller mill is used to roll the single-layer B electrode sheet to a certain thickness, and then a slitting machine is used to slit the single-layer B electrode sheet into rectangular electrode sheets of 300*57 mm, i.e., single-layer B electrodes.

[0035] (2) Assemble the prepared monolayer B electrode sheet from (1) with the corresponding lithium cobalt oxide positive electrode sheet into a soft-pack lithium-ion capacitor. The assembly process is carried out in a dry room with strict humidity control. An electrolyte system containing 1 mol / L LiPF6 with a volume ratio of EC:DMC:EMC of 1:1:1 is used. The separator is an NKK ceramic separator. The lithium-ion capacitor is assembled using a winding method. After assembly, it is sealed in an aluminum mold and placed in a vacuum oven at 80°C for 12 hours before being injected with electrolyte. During the assembly of the lithium-ion capacitor, it is essential to ensure that there are no operational errors, that the positive and negative electrodes do not come into contact to avoid short circuits, and that there are no air bubbles between the separator and the electrode sheet. The seal must be tight to isolate air and prevent any impact on the performance of the lithium-ion capacitor.

[0036] In Examples 1, 2, and 4, LCN is Li 2.6 Co 0.4 N.

[0037] Electrochemical performance testing

[0038] The capacity of the soft-pack lithium-ion capacitor was tested using a 0.2C rate, with the charge and discharge voltage limited to 2.5–4.2V.

[0039] The initial coulombic efficiencies of the lithium-ion capacitors prepared in Examples 1-4 are shown in the table below:

[0040]

[0041] Principle: Utilizing a multi-layer coating process, a lithium supplement agent is added to the double-layer structure to form a lithium supplement layer in the vertical direction of the electrode. This not only improves the conductivity of the positive electrode but also alleviates the problems of severe lithium supplement agent gas generation and difficult venting. Simultaneously, it constructs a thick electrode with a porosity gradient distribution, resulting in higher conductivity. The assembled lithium-ion capacitor exhibits higher initial efficiency and excellent electrochemical performance.

[0042] The above descriptions are merely embodiments of the present invention, and common knowledge regarding specific structures and characteristics is not elaborated upon here. It should be noted that those skilled in the art can make various modifications and improvements without departing from the structure of the present invention, and these should also be considered within the scope of protection of the present invention. These modifications and improvements will not affect the effectiveness of the present invention or the practicality of the patent. The scope of protection claimed in this application should be determined by the content of its claims, and the specific embodiments described in the specification can be used to interpret the content of the claims.

Claims

1. A method for preparing a double-layer negative electrode, characterized in that: Includes the following steps: Step 1: Preparation of negative electrode slurry A: Hard carbon, conductive carbon black, natural graphite, single-walled carbon nanotube dispersion, and polyvinylidene fluoride are mixed in a drying room with a dew point of -30 to -40°C at a mass ratio of (92-96):(2.0-0.82):(2.18-1.05):(0.18-0.13):(3.64-2.0) to obtain kneaded material A; then 1-methyl-2-pyrrolidone is added and stirred evenly to adjust the viscosity to 3000-6000 mPa•s to obtain negative electrode slurry A; Step 2, Preparation of Negative Electrode Slurry B: The HL mixture, conductive carbon black, natural graphite, single-walled carbon nanotube dispersion, and polyvinylidene fluoride were mixed in a drying room with a dew point of -30 to -40°C at a mass ratio of (92–96):(2.0–0.82):(2.18–1.05):(0.18–0.13):(3.64–2.0) to obtain kneaded material B; then 1-methyl-2-pyrrolidone (NMP) was added and stirred until homogeneous, adjusting the viscosity to 3000–6000 mPa•s to obtain negative electrode slurry B; the HL mixture was prepared by mixing HC and LCN in a ratio of m... LCN / m (HC+LCN) =5%~10% by mass to be obtained by mixing. LCN refers to lithium cobalt nitride and HC refers to hard carbon. Step 3: Preparation of the double-layer negative electrode sheet: Two layers of negative electrode slurry are coated on both sides of the copper current collector. The two negative electrode slurries are negative electrode slurry A and negative electrode slurry B, respectively. The two negative electrode slurries are different. If the first layer is negative electrode slurry A, then the second layer is negative electrode slurry B, and vice versa. Specifically: First, a layer of negative electrode slurry A or negative electrode slurry B is coated on both sides of the copper current collector and dried for the first time at a temperature of 80~120°C. Then, another layer of negative electrode slurry is coated on both sides of the copper current collector, and then dried a second time to obtain the negative electrode coating. Finally, the negative electrode coating is rolled and cut to obtain the double-layer negative electrode sheet.

2. The method for preparing a double-layer negative electrode according to claim 1, characterized in that: In step one and / or step two, a vacuum mixer is used to mix the materials thoroughly.

3. The method for preparing a double-layer negative electrode according to claim 2, characterized in that: In step three, a coating machine is used to coat the negative electrode slurry onto the copper current collector.

4. The method for preparing a double-layer negative electrode according to claim 3, characterized in that: In step three, the initial drying time is 30–60 minutes.

5. The method for preparing a double-layer negative electrode according to claim 4, characterized in that: In step three, during the secondary drying, vacuum drying is used. Specifically, after coating another layer of negative electrode slurry on both sides of the copper current collector, it is continuously vacuum dried in a vacuum dryer for 18~24h, with a vacuum degree ≤-0.095 MPa.

6. The method for preparing a double-layer negative electrode according to claim 5, characterized in that: In step three, a roller press is used to roll the negative electrode coating.

7. The method for preparing a double-layer negative electrode according to claim 6, characterized in that: In step three, a slitting machine is used to slit the rolled negative electrode coating.

8. A double-layer negative electrode, characterized in that: It is prepared by the preparation method according to any one of claims 1 to 7.

9. A lithium-ion capacitor, characterized in that: It includes the double-layer negative electrode sheet as described in claim 8.

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