A positive electrode sheet, a method for manufacturing the same, and use thereof

Abstract

The application specifically discloses a positive electrode sheet and a preparation method and application thereof. The positive electrode sheet comprises a current collector and a composite active material layer arranged on at least one surface of the current collector, wherein the composite active material layer comprises a first active material layer and a second active material layer arranged in sequence, and the second active material layer is arranged on the surface of the first active material layer away from the current collector; the first active material layer comprises a first conductive agent, and the first conductive agent comprises carbon black and multi-walled carbon nanotubes; the second conductive agent comprises single-walled carbon nanotubes and vapor-grown carbon fibers, and the vapor-grown carbon fibers have a hollow structure; and the aspect ratio of the single-walled carbon nanotubes is greater than that of the multi-walled carbon nanotubes. The application has the advantages of making lithium batteries have both high energy density and excellent rate performance.

Description

Technical Field

[0001] This invention relates to the field of lithium battery technology, and in particular to a positive electrode sheet, its preparation method, and its application. Background Technology

[0002] With the development of the new energy vehicle industry, in order to meet the market demand for new energy vehicles that combine long driving range and fast charging performance, it is necessary to develop power batteries with high energy density and high rate performance. Currently, increasing the electrode coating density is an effective way to improve battery energy density. However, as the electrode coating density increases, the rate performance of the battery will show a downward trend, making it difficult for lithium batteries to achieve both high energy density and excellent fast charging performance.

[0003] Current technology often employs a double-layer cathode design to simultaneously improve battery energy density and fast-charging performance. However, due to the greater thickness of the double-layer cathode, electrons travel a longer distance, resulting in weaker electron transport in the electrode layer farther from the current collector, negatively impacting the rate performance of lithium batteries. Furthermore, the electrolyte's wettability to the active material layer near the current collector decreases, hindering the full capacity utilization of thick-electrode lithium batteries.

[0004] Therefore, there is an urgent need to provide a lithium battery that can balance high energy density and excellent rate performance. Summary of the Invention

[0005] In order to enable lithium batteries to achieve both high energy density and excellent rate performance, this application provides a positive electrode sheet, its preparation method, and its application.

[0006] In a first aspect, this application provides a positive electrode sheet, which adopts the following technical solution:

[0007] A positive electrode includes a current collector and a composite active material layer disposed on at least one surface of the current collector. The composite active material layer includes a first active material layer and a second active material layer stacked sequentially, and the second active material layer is disposed on the surface of the first active material layer away from the current collector.

[0008] The first active material layer includes a first conductive agent, which includes carbon black and multi-walled carbon nanotubes; the second conductive agent includes single-walled carbon nanotubes and vapor-grown carbon fibers, which have a hollow structure.

[0009] Vapor-grown carbon fiber is a fibrous carbon material produced by thermal desorption in hydrogen and other oxygen-containing gases using low-carbon hydrocarbons as raw materials and transition metals or other ultrafine particles (catalysts) as crystal nuclei. It consists of irregular short fibers. Compared to multi-walled carbon nanotubes, single-walled carbon nanotubes exhibit better dispersion performance in cathode slurries. Applying single-walled carbon nanotubes as a conductive agent in the second active material layer, which is located far from the current collector, can effectively improve the electron transport capacity of the second active material layer, thereby enhancing the rate performance of lithium batteries.

[0010] The combination of carbon black and multi-walled carbon nanotubes in the first active material layer has two main benefits. First, the conductive network structure formed by the multi-walled carbon nanotubes and carbon black can form point-to-point contact with the active material, which is beneficial for the adsorption of electrolyte by the active material layer and prevents the degradation of ion and electron transport performance in the first active material layer near the current collector, thereby improving the rate performance of the lithium battery. Second, because the layers of multi-walled carbon nanotubes can easily become trap centers during the preparation process, capturing various defects, the walls of multi-walled carbon nanotubes are usually covered with small hole-like defects. Mixing carbon black with multi-walled carbon nanotubes can, to some extent, compensate for the defects on the surface of multi-walled carbon nanotubes and further improve the conductivity of the first active material layer near the current collector.

[0011] The combination of single-walled carbon nanotubes and vapor-grown carbon fibers in the second active material layer can, on the one hand, construct a conductive network in the electrode that facilitates electron transport; on the other hand, the addition of vapor-grown carbon fibers can improve the adsorption of electrolyte by the second active material layer, allowing more electrolyte to enter the second active material layer. Furthermore, the multi-walled carbon nanotubes in the first active material layer near the current collector also have a hollow structure, enabling the first active material layer, which is far from the electrolyte, to adsorb and wet the electrolyte in the second active material layer, thereby improving the degree of electrolyte wetting of the electrode and thus improving the kinetic performance of the battery during charge and discharge, which is beneficial to improving the cycle performance and energy density of the lithium battery.

[0012] Therefore, when the positive electrode of this application is used in lithium batteries, it enables lithium batteries to achieve both high energy density and excellent rate performance.

[0013] Preferably, the single-walled carbon nanotubes include at least one of acid-treated single-walled carbon nanotubes and alkali-treated single-walled carbon nanotubes.

[0014] Acid-treated and alkali-treated single-walled carbon nanotubes exhibit better dispersion performance than untreated single-walled carbon nanotubes. This facilitates the formation of a conductive network in the second active material layer, which is far from the current collector, further enhancing electron transport performance in the electrode and improving the rate performance and cycle performance of lithium batteries.

[0015] Preferably, the acid-treated single-walled carbon nanotubes are prepared by the following steps: the single-walled carbon nanotubes are mixed with acid solution, stirred for 1-2 hours, heated at 85-95°C for 2-3 hours, and the resulting solid product is washed after solid-liquid separation to obtain the acid-treated single-walled carbon nanotubes.

[0016] Preferably, the acid solution comprises nitric acid and sulfuric acid in a volume ratio of 2-3:1, wherein the concentration of the nitric acid is 8-12 mol / L and the concentration of the sulfuric acid is 8-12 mol / L.

[0017] The alkali-treated single-walled carbon nanotubes are prepared by the following steps: the single-walled carbon nanotubes are mixed with an alkali solution, stirred for 1-2 hours, heated at 85-95°C for 2-3 hours, and the solid product is washed after solid-liquid separation to obtain the alkali-treated single-walled carbon nanotubes.

[0018] Preferably, the alkaline solution comprises a sodium hydroxide solution with a concentration of 5-8 mol / L.

[0019] The acid and alkali treatment methods used in this application are simple, low-cost, and easy to industrialize.

[0020] Preferably, the aspect ratio of the single-walled carbon nanotube is greater than that of the multi-walled carbon nanotube; the aspect ratio of the multi-walled carbon nanotube is 3-500:1; and the aspect ratio of the single-walled carbon nanotube is 30-1000:1.

[0021] In this application, the aspect ratio of single-walled carbon nanotubes (SWCNTs) is greater than that of multi-walled carbon nanotubes (MWCNTs). Therefore, the resistivity of SWCNTs is lower than that of MWCNTs, which is beneficial for forming a good conductive network in the second active material layer far from the current collector. Simultaneously, the higher aspect ratio of SWCNTs results in more contact points with the active material in the active material layer, further improving the conductivity of the second active material layer far from the current collector. This maintains the conductivity of both the second and first active material layers at a relatively high level, facilitating electron conduction between the current collector and the composite active material layer, thereby improving the rate performance of the lithium battery. When the aspect ratios of MWCNTs and SWCNTs are controlled within the aforementioned range, the electron transport capability of the second active material layer far from the current collector can be adjusted to a level similar to that of the first active material layer close to the current collector, avoiding difficulties in electron transport in the active material layer far from the current collector, thus improving the rate performance of the lithium battery.

[0022] Preferably, the length of the single-walled carbon nanotube is greater than the length of the multi-walled carbon nanotube; the length of the multi-walled carbon nanotube is 5-60 μm, and the length of the single-walled carbon nanotube is 10-200 μm.

[0023] By controlling the length of single-walled carbon nanotubes to be greater than the diameter of multi-walled carbon nanotubes, single-walled carbon nanotubes can more easily overlap and form a network structure in the second active material layer far from the current collector, which helps to improve the electron transport capability of the second active material layer and improve the rate performance of lithium batteries.

[0024] Preferably, the carbon black has a particle size of 10-60 nm.

[0025] Carbon black that meets the above value range can help to compensate for defects on the surface of multi-walled carbon nanotubes and form point-to-point contact with the active material in the first active material layer, thereby improving the conductivity of the first active material layer. When the carbon black particle size is too small or too large, it cannot effectively repair the defects on the surface of multi-walled carbon nanotubes, which will reduce the conductivity of the first active material layer to a certain extent.

[0026] Preferably, the length of the vapor-grown carbon fiber is 10-100 μm, and the aspect ratio of the vapor-grown carbon fiber is 300-800:1.

[0027] Preferably, the content of the first conductive agent in the first active material layer is 0.5wt%-1.8wt%; and / or, the content of the second conductive agent in the second active material layer is 0.8wt%-2wt%.

[0028] By controlling the amounts of the first and second conductive agents in the first and second active material layers, respectively, it is beneficial to form an overlapping conductive network in the first and second active material layers, so that the conductivity of the second active material layer can be kept at a similar level to that of the first active material layer, thereby improving the rate performance of the lithium battery.

[0029] Preferably, the mass ratio of carbon black to multi-walled carbon nanotubes in the first conductive agent is 1.5-2:1; and the mass ratio of vapor-grown carbon fiber to single-walled carbon nanotubes in the second conductive agent is 1-1.5:1.

[0030] By controlling the mass ratio of carbon black to multi-walled carbon nanotubes in the first conductive agent, carbon black can compensate for the defects on the surface of multi-walled carbon nanotubes and improve the conductivity of the conductive network in the first active material layer. By controlling the mass ratio of single-walled carbon nanotubes to vapor-grown carbon fibers in the second conductive agent, the continuity of the conductive network in the second active material layer can be improved while the electrolyte absorption of the second active material layer can be improved, thereby improving the conductivity of the composite active material layer and promoting the specific energy utilization of the lithium battery.

[0031] Preferably, the first active material layer further includes a first active material, and the second active material layer further includes a second active material, wherein the mass ratio of the first active material to the second active material is 1:9-9:1.

[0032] Preferably, the first active material and the second active material are each independently selected from at least one of lithium iron phosphate, lithium manganese iron phosphate, lithium cobalt oxide, lithium manganese oxide, and ternary cathode materials.

[0033] Preferably, the particle size D50 of the first active material is 0.8-1.4 μm; and the particle size D50 of the second active material is 1-1.6 μm.

[0034] The positive electrode sheet in this application can be used in various types of lithium batteries and improve the rate performance and cycle performance of lithium batteries.

[0035] Secondly, this application provides a method for preparing a positive electrode sheet, which adopts the following technical solution:

[0036] A method for preparing a positive electrode sheet includes the following steps:

[0037] S1: Prepare a first active slurry from raw materials including a first active material and a first conductive agent; prepare a second active slurry from raw materials including a second active material and a second conductive agent;

[0038] S2: The first active slurry is coated onto at least one surface of the current collector and then dried, followed by coating with the second active slurry and drying to obtain the positive electrode sheet; or, the first active slurry and the second active slurry are simultaneously coated onto at least one surface of the current collector and dried to obtain the positive electrode sheet.

[0039] Preferably, the coating process in S2 is carried out using a double-layer coating device.

[0040] Thirdly, this application provides a lithium-ion battery, which adopts the following technical solution:

[0041] A lithium-ion battery, the lithium-ion battery comprising a positive electrode, the positive electrode being a positive electrode as described above or a positive electrode prepared by the method described above. Attached Figure Description

[0042] Figure 1 This is a schematic diagram of the structure of the positive electrode sheet in this application.

[0043] Figure 2 The figures show the fast charging cycle performance test results of lithium batteries in Examples 1, 10, and Comparative Example 1 of this application.

[0044] Explanation of reference numerals in the attached figures:

[0045] 10. Current collector; 20. Composite active material layer; 201. First active material layer; 202. Second active material layer. Detailed Implementation

[0046] To better understand and implement this invention, the technical solution of the present invention will be clearly and completely described below in conjunction with embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0047] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein in the description of the invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention.

[0048] Unless otherwise stated, all numerical values ​​for the amounts of expressed components, reaction conditions, etc., used in the specification and claims are to be understood as being modified by the term "about". Therefore, unless otherwise indicated, the numerical parameters set forth herein are approximate values ​​that can be varied to obtain the desired performance.

[0049] The word “and / or” as used in this article refers to one or all of the elements mentioned.

[0050] The terms "include" and "contain" as used in this article cover both cases where only the mentioned elements exist and cases where there are other unmentioned elements in addition to the mentioned elements.

[0051] All percentages in this invention are weight percentages, unless otherwise stated.

[0052] Unless otherwise stated, the terms “a,” “an,” “an,” and “the” as used in this specification are intended to include “at least one” or “one or more.” For example, “a component” refers to one or more components, and therefore more than one component may be considered and may be employed or used in the implementation of the described embodiments.

[0053] Example

[0054] Example 1

[0055] 1. Preparation of positive electrode sheet

[0056] The positive electrode sheet is prepared using the following steps:

[0057] S1: Lithium iron phosphate (LiFePO4), a first active material with a particle size D50 of 1.1 μm, PVDF binder, and a first conductive agent are mixed in a mass ratio of 97.4 wt% : 0.9 wt% : 1.7 wt%. The first conductive agent consists of 0.55 wt% carbon black (particle size 10-20 nm) and 0.35 wt% multi-walled carbon nanotubes (fiber length 8-15 μm, aspect ratio 10-20:1). The mixture is then mixed with NMP to obtain a first mixed slurry (the solid content of the first mixed slurry is 60%). The mixture is stirred under vacuum until homogeneous to obtain the first active slurry.

[0058] A second mixed slurry was obtained by mixing lithium iron phosphate (a second active material with a particle size D50 of 1.4 μm), PVDF binder, and a second conductive agent in a mass ratio of 97.3 wt% : 1.7 wt% : 1 wt%. The second conductive agent consisted of 0.5 wt% vapor-grown carbon fibers (fiber length of 20-40 μm, aspect ratio of 30-60:1) and 0.5 wt% single-walled carbon nanotubes (fiber length of 80-100 μm, aspect ratio of 50-100:1). The mixture was then mixed with NMP to obtain the second mixed slurry (the solid content of the second mixed slurry was 60%). The mixture was stirred under vacuum until homogeneous to obtain the second active slurry.

[0059] The mass ratio of the first positive electrode active material, lithium iron phosphate, to the second positive electrode active material, is 5:5.

[0060] S2: Using a double-layer coating device, the first active slurry and the second active slurry are simultaneously and uniformly coated onto both surfaces of the aluminum foil. After air drying at room temperature, they are transferred to an oven for further drying. After drying in the oven, a positive electrode semi-finished product is obtained. Then, the positive electrode semi-finished product is cold-pressed and cut to obtain the positive electrode sheet to be assembled. See the structural diagram of the positive electrode sheet in this embodiment. Figure 1 .

[0061] 2. Preparation of negative electrode sheet

[0062] The negative electrode slurry was prepared as follows: artificial graphite, conductive agent acetylene black, thickener CMC, and binder SBR were added to a vacuum mixer in a mass ratio of 96.4%:1%:1.2%:1.4% and mixed. Then, deionized water was added to the resulting mixture, and the mixture was stirred in a vacuum mixer until it became homogeneous, thereby obtaining the negative electrode slurry of this embodiment.

[0063] The above-mentioned negative electrode slurry is uniformly coated on both surfaces of the negative electrode current collector copper foil. After drying at room temperature, it is transferred to an oven for further drying. After drying in the oven, a negative electrode semi-finished product is obtained. Then, the negative electrode semi-finished product is cold-pressed and cut to obtain the negative electrode sheet to be assembled.

[0064] 3. Assembly of lithium-ion batteries

[0065] Commercially available polyethylene film was used as the separator for the lithium-ion battery, and a commercially available electrolyte suitable for 4.2V (upper charging voltage) battery systems was used as the electrolyte. The positive and negative electrode sheets, along with the separator, were wound together to obtain a bare cell. The bare cell underwent encapsulation, electrolyte injection, settling, formation, and capacity testing to obtain the finished battery. The rate performance test of the lithium battery in this embodiment is as follows: Figure 2 As shown.

[0066] Example 2

[0067] 1. Preparation of positive electrode sheet

[0068] The positive electrode sheet is prepared using the following steps:

[0069] S1: Lithium manganese iron phosphate (the first active material with a particle size D50 of 0.8 μm), PVDF binder, and first conductive agent are mixed in a mass ratio of 98.5 wt% : 0.9 wt% : 0.6 wt%. The first conductive agent consists of 0.4 wt% carbon black (particle size of 45-60 nm) and 0.2 wt% multi-walled carbon nanotubes (fiber length of 40-60 μm, aspect ratio of 400-500:1). The mixture is then mixed with NMP to obtain a first mixed slurry (the solid content of the first mixed slurry is 60%). The mixture is stirred under vacuum until it becomes homogeneous, thereby obtaining the first active slurry.

[0070] A second active material, lithium manganese iron phosphate with a particle size D50 of 1 μm, a binder, PVDF, and a second conductive agent were mixed in a mass ratio of 97.2 wt%: 1 wt%: 1.8 wt% to obtain a second mixed slurry. The second conductive agent consisted of 1.08 wt% of vapor-grown carbon fibers (fiber length of 80-100 μm, aspect ratio of 500-800:1) and 0.72 wt% of single-walled carbon nanotubes (fiber length of 150-200 μm, aspect ratio of 600-1000:1). The mixture was then mixed with NMP to obtain the second mixed slurry (the solid content of the second mixed slurry was 60%). The mixture was stirred under vacuum until homogeneous to obtain the second active slurry.

[0071] The mass ratio of the first positive electrode active material, lithium manganese iron phosphate, to the second positive electrode active material, is 5:5.

[0072] S2: Using a double-layer coating device, the first active slurry and the second active slurry are simultaneously and uniformly coated onto both surfaces of the aluminum foil. After air drying at room temperature, they are transferred to an oven for further drying. After drying in the oven, a positive electrode semi-finished product is obtained. Then, the positive electrode semi-finished product is cold-pressed and cut to obtain the positive electrode sheet to be assembled. See the structural diagram of the positive electrode sheet in this embodiment. Figure 1 .

[0073] 2. Preparation of negative electrode sheet

[0074] The negative electrode slurry was prepared as follows: artificial graphite, conductive agent acetylene black, thickener CMC, and binder SBR were added to a vacuum mixer in a mass ratio of 96.4%:1%:1.2%:1.4% and mixed. Then, deionized water was added to the resulting mixture, and the mixture was stirred in a vacuum mixer until it became homogeneous, thereby obtaining the negative electrode slurry of this embodiment.

[0075] The above-mentioned negative electrode slurry is uniformly coated on both surfaces of the negative electrode current collector copper foil. After drying at room temperature, it is transferred to an oven for further drying. After drying in the oven, a negative electrode semi-finished product is obtained. Then, the negative electrode semi-finished product is cold-pressed and cut to obtain the negative electrode sheet to be assembled.

[0076] 3. Assembly of lithium-ion batteries

[0077] Commercially available polyethylene film is used as the separator for the lithium-ion battery, and a commercially available electrolyte suitable for 4.2V (upper charging voltage) battery system is used as the electrolyte. The above-mentioned positive electrode sheet, negative electrode sheet and separator are wound together to obtain bare cell. The bare cell is then packaged, injected with electrolyte, left to stand, formed and tested for capacity to obtain the finished battery.

[0078] Example 3

[0079] 1. Preparation of positive electrode sheet

[0080] The positive electrode sheet is prepared using the following steps:

[0081] S1: Lithium manganese iron phosphate (the first active material with a particle size D50 of 1.4 μm), PVDF binder, and first conductive agent are mixed in a mass ratio of 97.8 wt%: 1 wt%: 1.2 wt%. The first conductive agent consists of 0.77 wt% carbon black (particle size of 25-40 nm) and 0.43 wt% multi-walled carbon nanotubes (fiber length of 20-30 μm, aspect ratio of 50-300:1). The mixture is then mixed with NMP to obtain a first mixed slurry (the solid content of the first mixed slurry is 60%). The mixture is stirred under vacuum until homogeneous to obtain the first active slurry.

[0082] A second mixed slurry was obtained by mixing lithium iron phosphate (a second active material with a particle size D50 of 1.6 μm), PVDF binder, and a second conductive agent at a mass ratio of 97.3 wt% : 1.2 wt% : 1.5 wt%. The second conductive agent consisted of 0.85 wt% vapor-grown carbon fibers (fiber length of 45-75 μm, aspect ratio of 80-400:1) and 0.65 wt% single-walled carbon nanotubes (fiber length of 20-70 μm, aspect ratio of 200-500:1). The mixture was then mixed with NMP to obtain the second mixed slurry (the solid content of the second mixed slurry was 60%). The mixture was stirred under vacuum until homogeneous to obtain the second active slurry.

[0083] The mass ratio of the first positive electrode active material, lithium manganese iron phosphate, to the second positive electrode active material, lithium iron phosphate, is 5:5.

[0084] S2: Using a double-layer coating device, the first active slurry and the second active slurry are simultaneously and uniformly coated onto both surfaces of the aluminum foil. After air drying at room temperature, they are transferred to an oven for further drying. After drying in the oven, a positive electrode semi-finished product is obtained. Then, the positive electrode semi-finished product is cold-pressed and cut to obtain the positive electrode sheet to be assembled. See the structural diagram of the positive electrode sheet in this embodiment. Figure 1 .

[0085] 2. Preparation of negative electrode sheet

[0086] The negative electrode slurry was prepared as follows: artificial graphite, conductive agent acetylene black, thickener CMC, and binder SBR were added to a vacuum mixer in a mass ratio of 96.4%:1%:1.2%:1.4% and mixed. Then, deionized water was added to the resulting mixture, and the mixture was stirred in a vacuum mixer until it became homogeneous, thereby obtaining the negative electrode slurry of this embodiment.

[0087] The above-mentioned negative electrode slurry is uniformly coated on both surfaces of the negative electrode current collector copper foil. After drying at room temperature, it is transferred to an oven for further drying. After drying in the oven, a negative electrode semi-finished product is obtained. Then, the negative electrode semi-finished product is cold-pressed and cut to obtain the negative electrode sheet to be assembled.

[0088] 3. Assembly of lithium-ion batteries

[0089] Commercially available polyethylene film is used as the separator for the lithium-ion battery, and a commercially available electrolyte suitable for 4.2V (upper charging voltage) battery system is used as the electrolyte. The above-mentioned positive electrode sheet, negative electrode sheet and separator are wound together to obtain bare cell. The bare cell is then packaged, injected with electrolyte, left to stand, formed and tested for capacity to obtain the finished battery.

[0090] Example 4

[0091] The difference between this embodiment and Embodiment 1 is that, in this application, an equal weight of acid-treated single-walled carbon nanotubes is used instead of single-walled carbon nanotubes.

[0092] Acid-treated single-walled carbon nanotubes were prepared by the following steps: single-walled carbon nanotubes were mixed with an acid solution (10 mol / L nitric acid and 10 mol / L sulfuric acid in a volume ratio of 3:1), stirred at room temperature for 1 h, heated at 90 °C for 2 h, and then the solid and liquid were separated. The obtained solid product was washed with deionized water until the pH value of the solid product was 7, and then dried to obtain acid-treated single-walled carbon nanotubes.

[0093] The rest of the parts are consistent with Example 1.

[0094] Example 5

[0095] The difference between this embodiment and Embodiment 1 is that, in this application, an equal weight of alkali-treated single-walled carbon nanotubes is used instead of single-walled carbon nanotubes.

[0096] Alkali-treated single-walled carbon nanotubes were prepared by the following steps: single-walled carbon nanotubes were mixed with a 5 mol / L sodium hydroxide solution and heated and stirred at 90°C for 2 hours. After solid-liquid separation, the obtained solid product was washed with deionized water until the pH value of the solid product was 7. After drying, alkali-treated single-walled carbon nanotubes were obtained; the rest of the process was the same as in Example 1.

[0097] Example 6

[0098] The difference between this embodiment and Embodiment 1 is that the aspect ratio of single-walled carbon nanotubes is smaller than that of multi-walled carbon nanotubes; specifically, the aspect ratio of single-walled carbon nanotubes is 30-100:1, and the aspect ratio of multi-walled carbon nanotubes is 50-150:1; the rest are consistent with Embodiment 1.

[0099] Example 7

[0100] The difference between this embodiment and Embodiment 1 is that the length of the single-walled carbon nanotube is shorter than that of the multi-walled carbon nanotube. Specifically, the length of the single-walled carbon nanotube is 10-30 μm, and the length of the multi-walled carbon nanotube is 15-50 μm; the rest is the same as in Embodiment 1.

[0101] Example 8

[0102] The difference between this embodiment and Embodiment 1 is that the mass ratio of carbon black to multi-walled carbon nanotubes in the first conductive agent is 1:1, and the mass ratio of vapor-grown carbon fiber to single-walled carbon nanotubes in the second conductive agent is 2:1. The rest are consistent with Embodiment 1.

[0103] Example 9

[0104] The difference between this embodiment and Embodiment 1 is that the mass ratio of carbon black to multi-walled carbon nanotubes in the first conductive agent is 2.5:1, and the mass ratio of vapor-grown carbon fiber to single-walled carbon nanotubes in the second conductive agent is 0.5:1. The rest are consistent with Embodiment 1.

[0105] Example 10

[0106] The difference between this embodiment and Embodiment 1 is that the second active material is a ternary cathode material NMC811, and the D50 of NMC811 is 8-12μm. The mass ratio of the first cathode active material to the second cathode active material is 9:1. The rest are the same as in Embodiment 1.

[0107] In this embodiment, the rate performance test of the lithium battery is as follows: Figure 2 As shown.

[0108] Example 11

[0109] The difference between this embodiment and Embodiment 1 lies in the following: During the preparation of the positive electrode, the specific steps of S2 are as follows:

[0110] S2: The first active slurry is uniformly coated on both surfaces of the aluminum foil and dried at room temperature to obtain the first slurry layer. The second active slurry is uniformly coated on the first slurry layer and dried at room temperature. Then, it is transferred to an oven for further drying. After drying in the oven, a positive electrode semi-finished product is obtained. The positive electrode semi-finished product is then cold-pressed and cut to obtain the positive electrode to be assembled.

[0111] Comparative Example 1

[0112] The difference between this comparative example and Example 1 is that the second conductive agent is replaced with an equal weight of the first conductive agent; the rest are the same as in Example 1.

[0113] The rate performance test of the lithium battery in this comparative example is as follows: Figure 2 As shown.

[0114] Comparative Example 2

[0115] The difference between this comparative example and Example 1 is that the first conductive agent includes single-walled carbon nanotubes and vapor-grown carbon fibers, and the second conductive agent includes carbon black and multi-walled carbon nanotubes; the rest are consistent with Example 1.

[0116] Comparative Example 3

[0117] The difference between this comparative example and Example 1 is that the first conductive agent includes carbon black and single-walled carbon nanotubes, and the second conductive agent includes multi-walled carbon nanotubes and vapor-grown carbon fibers; the rest are consistent with Example 1.

[0118] Comparative Example 4

[0119] The difference between this comparative example and Example 1 lies in the change of the composition of the second conductive agent. Specifically, in the second conductive agent, an equal weight of multi-walled carbon nanotubes is used to replace vapor-grown carbon fibers; the rest remains the same as in Example 1.

[0120] Comparative Example 5

[0121] The difference between this comparative example and Example 1 is that the hollow-structured vapor-grown carbon fiber in Example 1 is replaced with a solid-structured vapor-grown carbon fiber; the rest are the same as in Example 1.

[0122] Test methods

[0123] I. Energy Density Test

[0124] The energy density of the batteries in the above embodiments and comparative examples was tested. The specific test steps are as follows: the thickness of the battery (in mm) was measured using a 600g PPG thickness gauge, and the length and width (in mm) were determined according to the battery model and regarded as fixed values.

[0125] Energy Density (ED, unit Wh / L) = Sorting Discharge Energy Value (Wh) / Battery Thickness / Battery Length / Battery Width × 1000.

[0126] II. Ratio Performance Test

[0127] The batteries in the above embodiments and comparative examples were subjected to rate performance tests. The specific test steps are as follows: 3C rate charging test was performed at 25°C, and the constant current ratio was calculated. The constant current ratio was calculated by recording the charging capacity at the end of the constant current charging segment as C1 and the total charging capacity at the end of the charging segment as C0. Then the constant current ratio = C1 / C0.

[0128] III. Cyclic Performance Testing

[0129] The specific test steps for fast charging cycle performance testing of the batteries in the above embodiments and comparative examples are as follows:

[0130] The lithium battery is charged and discharged using a fast charging strategy. The temperature at the center of the battery before charging starts is recorded as T0, and the highest temperature at the center of the battery during charging is recorded as T. Then the charging temperature rise S = T - T0.

[0131] Table 1

[0132]

[0133]

[0134] Based on Examples 1-3, Examples 10-11, Comparative Examples 1-5, and Table 1, it can be seen that when the first and second conductive agents of this application are used in combination, a highly conductive network structure can be formed in both the first and second active material layers. Furthermore, the conductivity of the second active material layer, which is farther from the current collector, can be adjusted to maintain a relatively similar level to the conductivity of the first active material layer, which is closer to the current collector, significantly improving the rate performance of the lithium battery. In addition, the combination of vapor-grown carbon fibers and multi-walled carbon nanotubes can also significantly improve the wetting of the electrolyte into the first active material layer, which is farther from the electrolyte and closer to the current collector, thereby increasing the specific capacity of the lithium battery.

[0135] When the first conductive agent is used in all active material layers (Comparative Example 1), the adsorption capacity of the second active material layer for the electrolyte decreases, which is detrimental to the wetting of the active material layer by the electrolyte, and also hinders the reduction of ion transport impedance in the active material layer and the utilization of the specific capacity of the active material, thus reducing the energy density of the lithium battery to some extent. Similarly, when the first conductive agent is used in the active material layer far from the current collector and the second conductive agent is used in the active material layer close to the current collector (Comparative Example 2), both the kinetic performance and energy density of the lithium battery show a downward trend.

[0136] When the first conductive agent includes carbon black and single-walled carbon nanotubes, and the second conductive agent includes vapor-grown carbon fibers and multi-walled carbon nanotubes (Comparative Example 3), the continuity of the conductive network structure in the second conductive agent decreases because the surface defects of the multi-walled carbon nanotubes cannot be repaired. This is not conducive to the formation of point-to-point contact between active material particles, reduces the conductivity of the composite active material layer, and reduces the dynamic performance and rate performance of the lithium battery.

[0137] Vapor-grown carbon fibers with hollow structures can be combined with multi-walled carbon nanotubes to improve the electrolyte absorption performance of the second active material layer, while also enhancing the continuity of the conductive network structure in the second active material layer, thereby improving the conductivity and specific energy of the lithium battery.

[0138] Based on Examples 1, 4-5 and Table 1, it can be seen that single-walled carbon nanotubes can further improve the rate performance of lithium batteries after alkali or acid treatment. This is because the improved dispersion of acid-treated and alkali-treated carbon nanotubes can promote the formation of conductive network structures in the composite active material layer.

[0139] Based on Examples 1 and 6 and Table 1, it can be seen that when the aspect ratio of single-walled carbon nanotubes is less than that of multi-walled carbon nanotubes, the conductivity of the second active material layer, which is farther from the current collector, will be lower than that of the first active material layer, which is closer to the current collector. This is not conducive to improving the rate performance and cycle performance of lithium batteries.

[0140] Based on Examples 1 and 7 and Table 1, it can be seen that when the length of single-walled carbon nanotubes is less than that of multi-walled carbon nanotubes, although the energy density of the lithium battery does not change significantly, the rate performance of the lithium battery decreases. This is mainly because when the aspect ratio of the single-walled carbon nanotubes in the second active material layer is less than that of the multi-walled carbon nanotubes in the first active material layer, it is not conducive to the formation of well-connected conductive pathways in the second active material layer, resulting in a decrease in the electronic conductivity between the second negative electrode active material layer and the first active material layer, leading to a slight decrease in the rate performance of the lithium battery.

[0141] Based on Examples 1, 8-9, and Table 1, it can be seen that when the mass ratio of carbon black to multi-walled carbon nanotubes in the first conductive agent is too small, or the mass ratio of vapor-grown carbon fibers to single-walled carbon nanotubes in the second conductive agent is too large, or when the mass ratio of carbon black to multi-walled carbon nanotubes in the first conductive agent is too large, or the mass ratio of vapor-grown carbon fibers to single-walled carbon nanotubes in the second conductive agent is too small, it is not conducive to constructing a first active material layer and a second active material layer with similar conductivity. This leads to a deterioration in the electron transport performance in the composite active material layer, reducing the rate performance and specific energy of the lithium battery.

[0142] The above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention, but such modifications or substitutions are all within the scope of protection of the present invention.

Claims

1. A positive electrode sheet, characterized by: The device includes a current collector and a composite active material layer disposed on at least one surface of the current collector. The composite active material layer includes a first active material layer and a second active material layer stacked sequentially, with the second active material layer disposed on the surface of the first active material layer away from the current collector. The first active material layer includes a first conductive agent, which includes carbon black and multi-walled carbon nanotubes; the second active material layer includes a second conductive agent, which includes single-walled carbon nanotubes and vapor-grown carbon fibers, wherein the vapor-grown carbon fibers have a hollow structure. The aspect ratio of the single-walled carbon nanotube is greater than that of the multi-walled carbon nanotube; the aspect ratio of the multi-walled carbon nanotube is 3-500:1; and the aspect ratio of the single-walled carbon nanotube is 30-1000:

1.

2. The positive electrode sheet according to claim 1, characterized by: The single-walled carbon nanotubes include at least one of acid-treated single-walled carbon nanotubes and alkali-treated single-walled carbon nanotubes.

3. The positive electrode sheet according to claim 1, characterized by: The length of the single-walled carbon nanotube is greater than that of the multi-walled carbon nanotube; the length of the multi-walled carbon nanotube is 5-60 μm, and the length of the single-walled carbon nanotube is 10-200 μm.

4. A positive electrode sheet according to claim 1, characterized in that: The carbon black has a particle size of 10-60 nm.

5. A positive electrode sheet according to claim 1, characterized in that: The length of the vapor-grown carbon fiber is 10-100 μm, and the aspect ratio of the vapor-grown carbon fiber is 300-800:

1.

6. A positive electrode sheet according to claim 1, characterized in that: The first conductive agent has a content of 0.5wt%-1.8wt% in the first active material layer; and / or, the second conductive agent has a content of 0.8wt%-2wt% in the second active material layer.

7. A positive electrode sheet according to claim 1, characterized in that: The mass ratio of carbon black to multi-walled carbon nanotubes in the first conductive agent is 1.5-2:1; the mass ratio of vapor-grown carbon fiber to single-walled carbon nanotubes in the second conductive agent is 1-1.5:

1.

8. A positive electrode sheet according to claim 1, characterized in that: The first active material layer further includes a first active material, and the second active material layer further includes a second active material, wherein the mass ratio of the first active material to the second active material is 1:9-9:

1.

9. A method for preparing a positive electrode sheet according to any one of claims 1-8, characterized in that: Includes the following steps: S1: Prepare a first active slurry from raw materials including a first active material and a first conductive agent; prepare a second active slurry from raw materials including a second active material and a second conductive agent; S2: The first active slurry is coated onto at least one surface of the current collector and then dried, followed by coating with the second active slurry and drying to obtain the positive electrode sheet; or, the first active slurry and the second active slurry are simultaneously coated onto at least one surface of the current collector and dried to obtain the positive electrode sheet.

10. A method for preparing a positive electrode sheet according to claim 9, characterized in that, The coating process in S2 is carried out using a double-layer coating device.

11. A lithium-ion battery, characterized in that: The lithium-ion battery includes a positive electrode, which is the positive electrode according to any one of claims 1-8, or prepared by the method according to any one of claims 9-10.

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