Negative plate, preparation method thereof and battery

Abstract

The invention relates to a negative plate, a battery and electronic equipment. The negative plate comprises a negative current collector and a negative active material layer arranged on at least one side surface of the negative current collector; the mass percentage of the negative electrode active material layer is 100 wt%, and the negative electrode active material layer comprises 81 wt%-97 wt% of a carbon particle material, 1 wt%-15 wt% of a silicon particle material and 0.8 wt%-2 wt% of a binder; the binder is attached to at least part of particle surfaces of the silicon particle material to form a first adhesion layer, and the adhesion area of the first adhesion layer accounts for 0.5%-1.8% of the surface area of the silicon particle material; the binder is attached to at least part of the particle surface of the carbon particle material to form a second adhesive layer. According to the scheme provided by the invention, the cycle performance of the battery can be improved.

Description

Technical Field

[0001] The present application relates to the technical field of batteries, and particularly to a negative electrode sheet, a preparation method thereof, and a battery. Background Art

[0002] Lithium-ion batteries are widely used in fields such as consumer electronics, new energy vehicles, and energy storage. With the rapid development of the new energy vehicle industry, consumers have put forward higher requirements for the cruising range and application scenarios, that is, the battery has a higher energy density.

[0003] At present, silicon anodes have received extensive attention and research due to their advantages such as high theoretical capacity, low lithium insertion potential, rich raw materials, non-toxicity, and environmental protection. However, compared with graphite, silicon particle materials will undergo a large degree of expansion and contraction during charge and discharge, with a volume change of up to 300%. The large volume change is likely to cause electrode particle pulverization and active material shedding, resulting in the deterioration of the battery's cycle performance, and the deterioration of the battery performance is more serious under high-temperature conditions.

[0004] Therefore, it is urgent to develop a battery that still has high cycle performance under high-temperature conditions. Summary of the Invention

[0005] To solve or partially solve the problems existing in the related technologies, the present application provides a negative electrode sheet, a preparation method thereof, and a battery, which can improve the cycle performance of the battery.

[0006] The first aspect of the present application provides a negative electrode sheet, which includes a negative electrode current collector and a negative electrode active material layer provided on at least one surface of the negative electrode current collector; It is calculated that the mass percentage of the negative electrode active material layer is 100 wt%, and the negative electrode active material layer includes 81 wt% - 97 wt% of carbon particle material, 1 wt% - 15 wt% of silicon particle material, and 0.8 wt% - 2 wt% of binder; The binder adheres to at least part of the particle surfaces of the silicon particle material to form a first adhesion layer, and the percentage of the adhesion area of the first adhesion layer in the surface area of the silicon particle material is 0.5% - 1.8%; The binder adheres to at least part of the particle surfaces of the carbon particle material to form a second adhesion layer.

[0007] For the negative electrode sheet as described above, the negative electrode active material layer further includes 0.03 wt% - 0.2 wt% of conductive agent; The conductive agent coats at least part of the particle surfaces of the silicon particle material to form a first conductive layer, and the binder adheres to at least part of the particle surfaces of the silicon particle material coated with the first conductive layer to form the first adhesion layer; The conductive agent coats at least part of the surface of the carbon particle material to form a second conductive layer, and the binder adheres to at least part of the surface of the carbon particle material coated with the second conductive layer to form the second adhesion layer.

[0008] The negative electrode sheet as described above, wherein at least some of the silicon particle materials are connected to the carbon particle materials through the conductive agent.

[0009] The negative electrode sheet as described above, wherein the number of the conductive agent within a preset area in the first conductive layer is N, and 10 < N < 50.

[0010] The negative electrode sheet as described above, wherein the conductive agent includes carbon nanotubes; The carbon nanotubes include one or more of single-walled carbon nanotubes and multi-walled carbon nanotubes; and / or, The length of the carbon nanotubes is 1 μm to 20 μm; and / or, The diameter of the carbon nanotubes is 1 nm to 5 nm.

[0011] The negative electrode sheet as described above, wherein the average number of the carbon nanotubes connecting each silicon particle material and multiple surrounding carbon particle materials is not less than 5; and / or, Each carbon nanotube in the negative electrode active material layer intersects with at least 3 of the remaining carbon nanotubes.

[0012] The negative electrode sheet as described above, wherein the number of the silicon particle materials within an area of 10,000 square micrometers on the surface of the negative electrode active material layer is 1 to 15; and / or, The silicon particle materials include one or more of silicon oxide, prelithiated silicon oxide, premagnesiated silicon oxide material, and silicon-carbon composite material.

[0013] The negative electrode sheet as described above, wherein the binder includes styrene-butadiene rubber; and / or, The carbon particle materials include one or more of artificial graphite, natural graphite, hard carbon, and soft carbon; and / or, The negative electrode active material layer further includes 0.5 wt% to 1.8 wt% of a dispersant, and the dispersant includes one or more of lithium carboxymethyl cellulose and sodium carboxymethyl cellulose.

[0014] The second aspect of the present application provides a method for preparing a negative electrode sheet, which includes the following steps: S1. Dry-mix 1 wt% to 15 wt% of silicon particle materials and 81 wt% to 97 wt% of carbon particle materials to obtain a first mixture; S2. Mix 0.8 wt% - 2.0 wt% of the binder and the first mixture in an aqueous solvent to obtain a negative electrode slurry, such that the binder in the negative electrode slurry adheres to at least a part of the particle surfaces of the silicon particle material to form a first adhesion layer, the binder adheres to at least a part of the particle surfaces of the carbon particle material to form a second adhesion layer, and the percentage of the adhesion area of the first adhesion layer in the surface area of the silicon particle material is 0.5% - 1.8%; wherein, in step S2, stirring is used for mixing, the revolution speed of the stirring is 20 r / min - 50 r / min, the rotation speed is 200 r / min - 1000 r / min, and the stirring time is 30 min - 120 min; S3. Coating the negative electrode slurry onto at least one surface of a negative electrode current collector, and after drying and rolling, obtaining a negative electrode sheet.

[0015] The method for preparing a negative electrode sheet as described above, wherein step S1 includes: dry-mixing the silicon particle material, the carbon particle material, and the conductive agent to obtain a first mixture, such that the conductive agent in the first mixture coats at least a part of the particle surfaces of the silicon particle material to form a first conductive layer, and the conductive agent coats at least a part of the particle surfaces of the carbon particle material to form a second conductive layer; Wherein, the stirring speed of the dry-mixing in step S1 is 20 r / min - 50 r / min, and the stirring time is 10 min - 60 min.

[0016] The method for preparing a negative electrode sheet as described above, wherein before step S2 and after step S1, it further includes: Kneading the first mixture and a part of the dispersant in an aqueous solvent to obtain a second mixture; subsequently, stirring the second mixture and the remaining part of the dispersant in an aqueous solvent to obtain a third mixture; wherein, the revolution speed of the kneading process is 20 r / min - 50 r / min, the rotation speed is 0 - 500 r / min, and the stirring time is 10 min - 120 min; the revolution speed of the stirring process is 20 r / min - 50 r / min, the rotation speed is 1500 r / min - 3000 r / min, and the stirring time is 60 min - 240 min; Step S2 includes: mixing 0.8 wt% - 2.0 wt% of the binder and the third mixture in an aqueous solvent to obtain a negative electrode slurry, such that the binder in the negative electrode slurry adheres to at least a part of the surface of the silicon material coated with the first conductive layer to form the first adhesion layer, and the binder adheres to at least a part of the surface of the carbon particle material coated with the second conductive layer to form the second adhesion layer.

[0017] The third aspect of the present application provides a battery, including the negative electrode sheet as described above or the negative electrode sheet prepared according to the preparation method of the negative electrode sheet as described above.

[0018] The technical solution provided by the present application may include the following beneficial effects: when the ratio of the carbon particle material, the silicon particle material and the binder in the negative electrode active material layer is within the above range, and the percentage of the adhesion area of the binder on the surface of the silicon particle material to the surface area of the silicon particle material is within the above range, the binder can reasonably coat the silicon particle material. When the silicon particles undergo volume expansion, the binder can produce elastic deformation, absorb and buffer the stress generated by the expansion of the silicon particles, reduce the damage of the silicon particle expansion to the electrode structure, avoid the pulverization and shedding of the electrode material, thereby improving the stability of the electrode. At the same time, the adhesion layer formed by the binder on the surface of the silicon particle material can improve the interfacial performance between the silicon particle material and the electrolyte, prevent the direct contact between the electrolyte and the silicon particle material, reduce the erosion of the electrolyte on the silicon particle material and the occurrence of side reactions, thereby improving the cycle performance of the battery; in addition, the adhesion layer formed by the binder on the surface of the carbon material can enhance the adhesion between the carbon material, the silicon material and the negative electrode current collector, improve the stability of the negative electrode sheet, and thereby improve the electrochemical performance of the battery.

[0019] It should be understood that the above general description and the following detailed description are only exemplary and explanatory, and cannot limit the present application. Detailed embodiments

[0020] The terms used in the present application are only for the purpose of describing specific embodiments, and are not intended to limit the present application. The singular forms "a", "the" and "said" used in the present application and the appended claims are also intended to include the plural forms, unless the context clearly indicates otherwise. It should also be understood that the term "and / or" as used herein refers to and includes any or all possible combinations of one or more of the associated listed items.

[0021] It should be understood that although the terms "first", "second", "third", etc. may be used in the present application to describe various information, such information should not be limited to these terms. These terms are only used to distinguish the same type of information from each other. For example, without departing from the scope of the present application, the first information may also be referred to as the second information, and similarly, the second information may also be referred to as the first information. Thus, the features defined with "first" and "second" may explicitly or implicitly include one or more of such features. In the description of the present application, "a plurality" means two or more, unless otherwise specifically defined.

[0022] In the related art, during the charge and discharge process, the silicon particle material will expand and contract to a large extent. The large volume change is likely to cause the electrode particles to pulverize and the active material to fall off, resulting in the deterioration of the cycle performance of the battery. The deterioration of the battery performance is more serious under high-temperature conditions.

[0023] In view of the above problems, an embodiment of the present application provides a negative electrode sheet, which includes a negative electrode current collector and a negative electrode active material layer provided on at least one surface of the negative electrode current collector; the mass percentage of the negative electrode active material layer is 100 wt%, and the negative electrode active material layer includes 81 wt% - 97 wt% of carbon particle material, 1 wt% - 15 wt% of silicon particle material, and 0.8 wt% - 2 wt% of binder; the binder adheres to at least part of the particle surfaces of the silicon particle material to form a first adhesion layer, and the percentage of the adhesion area of the first adhesion layer in the surface area of the silicon particle material is 0.5% - 1.8%; the binder adheres to at least part of the particle surfaces of the carbon particle material to form a second adhesion layer.

[0024] The present application does not limit the selection of the negative electrode current collector, which can be selected according to actual needs. For example, copper foil can be selected. The negative electrode active material layer of the present application includes a negative electrode active material and a binder. The negative electrode active material of the present application includes a carbon particle material and a silicon particle material. The present application does not limit the selection of the carbon particle material, which can be selected according to actual needs. For example, graphite, carbon black, hard carbon, soft carbon, etc. can be selected; the present application does not limit the selection of the silicon particle material, which can be selected according to actual needs. For example, silicon oxide, prelithiated silicon oxide, premagnesiated silicon oxide material, silicon-carbon composite material, elemental silicon, etc. can be selected. The present application does not limit the selection of the binder, which can be selected according to actual needs. For example, styrene-butadiene rubber, polyvinylidene fluoride, polyacrylic acid, etc. can be selected.

[0025] The binder of the present application can be mixed with the silicon particle material and the carbon particle material, so that the binder can be loaded on at least part of the particle surfaces of the silicon particle material to form a first adhesion layer, and at the same time, the binder can also be loaded on at least part of the particle surfaces of the carbon particle material to form a second adhesion layer. In the active material layer of the negative electrode sheet of the present application, the silicon particle material and the carbon particle material are in a mixed state, rather than the silicon particle material coating the carbon particle material, or the carbon particle material coating the silicon particle material.

[0026] The mass percentage content of the carbon particle material in the negative electrode active material layer of the present application is 81 wt% - 97 wt%, such as 81 wt%, 82 wt%, 83 wt%, 84 wt%, 85 wt%, 86 wt%, 87 wt%, 88 wt%, 89 wt%, 90 wt%, 91 wt%, 92 wt%, 93 wt%, 94 wt%, 95 wt%, 96 wt% or 97 wt%, etc.

[0027] The mass percentage content of the silicon particle material in the negative electrode active material layer of the present application is 1 wt% to 15 wt%. For example, the mass percentage content of the silicon particle material in the negative electrode active material layer can be 1 wt%, 2 wt%, 3 wt%, 4 wt%, 5 wt%, 6 wt%, 7 wt%, 8 wt%, 9 wt%, 10 wt%, 11 wt%, 12 wt%, 13 wt%, 14 wt% or 15 wt%, etc.

[0028] The mass percentage content of the binder in the negative electrode active material layer of the present application is 0.8 wt% to 2.0 wt%. For example, the mass percentage content of the binder in the negative electrode active material layer can be 0.8 wt%, 0.9 wt%, 1 wt%, 1.1 wt%, 1.2 wt%, 1.3 wt%, 1.4 wt%, 1.5 wt%, 1.6 wt%, 1.7 wt%, 1.8 wt%, 1.9 wt% or 2 wt%, etc.

[0029] The percentage of the attachment area of the first attachment layer in the present application to the surface area of the silicon particle material is 0.5% to 1.8%. For example, the percentage of the attachment area of the first attachment layer to the surface area of the silicon particle material can be 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7% or 1.8%, etc.

[0030] According to the above solution provided by the present application, after applying the negative electrode sheet to a battery, the battery has excellent cycling performance. The applicant analyzed this principle and believes that the reason is that by regulating the ratio of the carbon particle material, silicon particle material and binder in the negative electrode active material within the above range, and controlling the percentage of the attachment area of the binder on the surface of the silicon particle material to the surface area of the silicon particle material within the above range, the binder can reasonably coat the silicon particle material. When the silicon particles undergo volume expansion, the binder can produce elastic deformation, absorb and buffer the stress generated by the expansion of the silicon particles, reduce the damage to the electrode structure caused by the expansion of the silicon particles, avoid the pulverization and shedding of the electrode material, thereby improving the stability of the electrode. At the same time, the attachment layer formed by the binder on the surface of the silicon particle material can improve the interfacial performance between the silicon particle material and the electrolyte, prevent the direct contact between the electrolyte and the silicon particle material, reduce the erosion of the electrolyte on the silicon particle material and the occurrence of side reactions, thereby improving the cycling performance of the battery; in addition, the attachment layer formed by the binder on the surface of the carbon material can enhance the adhesion between the carbon material, the silicon material and the negative electrode current collector, improve the stability of the negative electrode sheet, and thereby improve the electrochemical performance of the battery.

[0031] Specifically, the percentage of the adhesion area of the first adhesion layer to the surface area of the silicon particle material can be measured by a scanning electron microscope (SEM), including the following steps: subject the negative electrode sheet to SEM measurement, maintain a magnification of 20,000 times, observe the binder particles on the surface of the silicon particle material, and select a set number of square micrometers S 设 (S 设 in the area where the proportion of the surface area of the silicon particle material is not less than 70%, such as 9 square micrometers, 3μm×3μm, or other empirical values), observe the number n of binder particles in the observation area, and at the same time measure the diameter D of the binder particles (the binder particles are spherical particles), and calculate the area S' of a single binder particle = 1 / 4×π×D 2 , therefore, the proportion of the adhesion area of the binder = nS' / S 设 ; repeat the above steps for 10 different silicon particles, and calculate the average value of the proportion of the adhesion area of the 10 binders obtained by the test, which is the percentage of the adhesion area of the first adhesion layer to the specific surface area of the silicon particle material.

[0032] In a specific embodiment, the negative electrode active material layer further includes 0.03wt% - 0.2wt% of a conductive agent; the conductive agent coats at least part of the surfaces of the silicon particle material to form a first conductive layer, and the binder adheres to at least part of the surfaces of the silicon particle material coated with the first conductive layer to form a first adhesion layer; the conductive agent coats at least part of the surfaces of the carbon particle material to form a second conductive layer, and the binder adheres to at least part of the surfaces of the carbon particle material coated with the second conductive layer to form a second adhesion layer.

[0033] A conductive agent is also added to the negative electrode active material layer of the present application. Adding a conductive agent to the negative electrode active material layer can improve the conductivity of the negative electrode active material, facilitate the rapid migration of lithium ions in the negative electrode active material layer, and contribute to improving the cycle performance of the battery. The mass percentage content of the conductive agent in the negative electrode active material layer is 0.03wt% - 0.2wt%, for example, the mass percentage content of the conductive agent in the negative electrode active material layer can be 0.03wt%, 0.05wt%, 0.1wt%, 0.15wt% or 0.2wt%, etc.

[0034] Among them, the silicon particle material can be first mixed with the conductive agent and then with the binder, so that at least part of the surface of the silicon particle material is first coated with the conductive agent to form a first conductive layer, and then the binder adheres to at least part of the surface of the silicon particle material coated with the first conductive layer to form a first adhesion layer; similarly, the carbon particle material can be first mixed with the conductive agent and then with the binder, so that at least part of the surface of the carbon particle material is first coated with the conductive agent to form a second conductive layer, and then the binder adheres to at least part of the surface of the carbon particle material coated with the second conductive layer to form a second adhesion layer.

[0035] In a specific embodiment, a conductive agent is connected between at least some of the silicon particle materials and the carbon particle materials, that is, a conductive channel is established between the silicon particle materials and the carbon particle materials, and this conductive channel is the conductive agent. When the silicon particle materials and the carbon particle materials are connected by the conductive agent, the conductive agent can provide a fast transmission channel for electrons, effectively improving the conductivity of the entire negative electrode material. At the same time, the conductive agent can play a buffering role when the volume of the silicon particle materials changes, inhibiting the expansion of the silicon particle materials, thereby improving the cycle performance of the battery.

[0036] In a specific embodiment, the number of conductive agents in a preset area of the first conductive layer is N, where 10 < N < 50.

[0037] The number of conductive agents in the preset area of the first conductive layer of the present application refers to the number of conductive agents in a certain area (the area selected accounts for no less than 70% of the surface area of the silicon particle material, such as 9 square micrometers, 3μm × 3μm) selected on the surface of the silicon particle material; if the conductive agent is a carbon nanotube, the number of conductive agents in the preset area of the first conductive layer refers to the number of carbon nanotube strips in the area of the preset area on the surface of the silicon particle material; if the conductive agent is conductive carbon black, the number of conductive agents in the preset area of the first conductive layer refers to the number of conductive carbon black particles in the area of the preset area on the surface of the silicon particle material.

[0038] The number of conductive agents in the preset area of the first conductive layer of the present application is less than 50 and greater than 10, which can enable the conductive agent to form a continuous conductive layer, avoid problems such as agglomeration and accumulation of the conductive agent, thereby improving the conduction rate of lithium ions in the negative electrode active material layer, improving the conductivity of the negative electrode active material layer, reducing the internal resistance of the battery, and thus improving the cycle performance of the battery. At the same time, the continuous conductive layer formed by the conductive agent can limit the expansion of the silicon particle materials, improve the stability of the negative electrode active material layer, and improve the cycle performance of the battery.

[0039] Specifically, the number of conductive agents in the preset area of the first conductive layer of the present application can be tested by a scanning electron microscope (SEM), including the following steps: performing SEM testing on the negative electrode sheet, maintaining a magnification of 20,000 times, observing the conductive agent on the surface of the silicon particle material, selecting a preset area (the preset area accounts for no less than 70% of the surface area of the silicon particle material, such as 9 square micrometers, 3μm × 3μm) on the surface of the silicon particle material, and observing the number N of conductive agents in the observation area; repeating the above steps for 10 different silicon particles, and calculating the average value of the 10 numbers of conductive agents obtained from the test, which is the number of conductive agents in the preset area on the surface of the silicon particle material.

[0040] The present application does not limit the specific selection of the conductive agent. For example, it can be carbon nanotubes, conductive carbon black, graphene, etc., which can be selected according to actual needs.

[0041] In a specific embodiment, the conductive agent includes carbon nanotubes; the carbon nanotubes include one or more of single-walled carbon nanotubes and multi-walled carbon nanotubes. When the conductive agent is carbon nanotubes, the carbon nanotubes have a large aspect ratio and can form an efficient conductive network on the surface of the silicon particle material or the carbon particle material, improving the electron transport efficiency of the electrode, thereby reducing the internal resistance of the battery and further enhancing the cycle performance of the battery. At the same time, the conductive network formed by the carbon nanotubes can also restrict the volume expansion of the silicon particle material and improve the cycle performance of the battery.

[0042] In a specific embodiment, the length of the carbon nanotubes is 1 μm to 20 μm. For example, the length of the carbon nanotubes can be 1 μm, 2 μm, 4 μm, 6 μm, 8 μm, 10 μm, 12 μm, 14 μm, 16 μm, 18 μm, or 20 μm, etc.

[0043] In a specific embodiment, the diameter of the carbon nanotubes is 1 nm to 5 nm. For example, the diameter of the carbon nanotubes can be 1 nm, 2 nm, 3 nm, 4 nm, or 5 nm, etc. When the length and diameter of the carbon nanotubes are respectively within the above ranges, the aspect ratio of the carbon nanotubes is large, and a better conductive network can be formed, which helps to further improve the electron transport rate of the electrode and makes the cycle performance of the battery better.

[0044] In a preferred embodiment, a conductive agent is connected between every two particles of the silicon particle material and the carbon particle material. One silicon particle material can be connected to multiple carbon particle materials, that is, one silicon particle material particle can be connected to multiple carbon particle material particles through the conductive agent, and one silicon particle material and one carbon particle material are connected by one conductive agent. When the silicon particle material and the carbon particle material are connected through the conductive agent, the conductive network formed by the conductive agent can connect the electron transport channels of the silicon particle material and the carbon particle material, resulting in more lithium ion transport channels, improving the lithium ion transport rate, reducing the resistance of the battery, and thus making the cycle performance of the battery better. At the same time, the conductive layer formed by the conductive agent has a higher inhibitory effect on the expansion of the silicon particle material, further enhancing the stability of the negative electrode active material layer and extending the cycle life of the battery.

[0045] In a preferred embodiment, the average number of carbon nanotubes connected between each silicon particle material and no less than 5 carbon particle materials around it is no less than 5, and each carbon nanotube can be connected to one carbon particle material, so that the lithium ion transport rate in the negative electrode active material layer is higher and the volume of the silicon particle material is lower, thereby making the cycle performance of the battery better.

[0046] Specifically, the average number of carbon nanotubes connected to each silicon particle material and the multiple carbon particle materials around it in the present application can be tested by a scanning electron microscope (SEM), including the following steps: subjecting the negative electrode sheet to an SEM test, maintaining a magnification of 20,000 times, selecting a preset number, for example 10 silicon particle materials, and observing the number of carbon nanotubes connected to the surface of each silicon particle material and the multiple carbon particle materials around it; calculating the average value of the number of carbon nanotubes connected to the surface of the 10 silicon particle materials obtained from the test and the multiple carbon particle materials around them.

[0047] In a specific embodiment, each carbon nanotube in the negative electrode active material layer intersects with at least three carbon nanotubes in the remaining carbon nanotubes, that is, the carbon nanotubes in the negative electrode active material layer are intertwined with each other, and one carbon nanotube is staggered and connected with at least three other carbon nanotubes. In this case, the carbon nanotubes in the negative electrode active material layer are less agglomerated, more evenly dispersed, and the conductive network formed is denser and more stable, so that the electron transfer rate can be better improved and the volume expansion of the silicon particle material can be suppressed, thereby improving the cycle performance of the battery.

[0048] Specifically, the number of intersections between the carbon nanotubes of the present application and the remaining carbon nanotubes can be tested by a scanning electron microscope (SEM), including the following steps: subjecting the negative electrode sheet to an SEM test, maintaining a magnification of 20,000 times, and observing the number of nodes connecting a carbon nanotube to other carbon nanotubes; repeating the above steps 10 times for different carbon nanotubes, and calculating the average value of the number of intersections between the 10 carbon nanotubes obtained from the test and the remaining carbon nanotubes, which is the number of intersections between the carbon nanotube and the remaining carbon nanotubes.

[0049] In a specific embodiment, the number of silicon granules within an area of ​​10,000 square microns on the surface of the negative electrode active material layer is 1 to 15, for example, the number of silicon granules within an area of ​​10,000 square microns on the surface of the negative electrode active material layer may be 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14 or 15, etc. When the number of silicon granules on the surface of the negative electrode active material layer is within the above range, the silicon granules in the negative electrode active material layer are evenly distributed and have a reasonable silicon content, which can improve the energy density of the battery, and avoid the problem of pulverization and shedding of the negative electrode active material layer caused by the volume expansion of the silicon granules, so that the battery has excellent energy density and cycle performance.

[0050] Specifically, the number of silicon particle materials within an area of 10,000 square micrometers on the surface of the negative electrode active material layer of the present application can be tested by a scanning electron microscope (SEM), including the following steps: subject the negative electrode sheet to SEM testing, maintain a magnification of 1000 times, observe the silicon particle material particles on the surface of the negative electrode active material layer, select an area of 10,000 square micrometers on the surface of the negative electrode active material layer, and observe the number of silicon particle materials within the observed area; repeat the above steps for 10 different areas, and calculate the average value of the number of silicon particle materials obtained from the 10 tests, which is the number of silicon particle materials within an area of 10,000 square micrometers on the surface of the negative electrode active material layer.

[0051] In a specific embodiment, the silicon particle material includes one or more of silicon oxide, prelithiated silicon oxide, premagnesiated silicon oxide material, and silicon-carbon composite material.

[0052] In a specific embodiment, the binder includes styrene-butadiene rubber.

[0053] In a specific embodiment, the carbon particle material includes one or more of artificial graphite, natural graphite, hard carbon, and soft carbon.

[0054] In a specific embodiment, the negative electrode active material layer further includes a dispersant, and the dispersant includes one or more of lithium carboxymethyl cellulose and sodium carboxymethyl cellulose.

[0055] In a specific embodiment, the mass percentage content of the dispersant in the negative electrode active material layer is 0.5wt% - 1.8wt%. For example, the mass percentage content of the dispersant in the negative electrode active material layer is 0.5wt%, 0.6wt%, 0.7wt%, 0.8wt%, 0.9wt%, 1wt%, 1.1wt%, 1.2wt%, 1.3wt%, 1.4wt%, 1.5wt%, 1.6wt%, 1.7wt%, or 1.8wt%, etc. When the mass percentage content of the dispersant is within the above range, the dispersant can be dispersed on the surfaces of the silicon particle material and the carbon particle material, avoid the agglomeration of the silicon particle material and the carbon particle material, ensure the uniform distribution of the silicon particle material and the carbon particle material, and at the same time can enhance the stability of the negative electrode active material layer, reduce the sedimentation and agglomeration of the active substances during the cyclic charge and discharge process, and improve the cyclic performance of the battery.

[0056] The second aspect of the present application provides a method for preparing a negative electrode sheet, including the following steps: S1. Dry-mix 1wt% - 15wt% of silicon particle material and 81wt% - 97wt% of carbon particle material to obtain a first mixture; S2. Mix 0.8 wt% - 2.0 wt% of the binder and the first mixture in an aqueous solvent to obtain a negative electrode slurry, such that the binder in the negative electrode slurry adheres to at least a part of the particle surfaces of the silicon particle material to form a first adhesion layer, the binder adheres to at least a part of the particle surfaces of the carbon particle material to form a second adhesion layer, and the percentage of the adhesion area of the first adhesion layer in the surface area of the silicon particle material is 0.5% - 1.8%; wherein, in step S2, mixing is carried out by stirring, the revolution speed of the stirring is 20 r / min - 50 r / min, the rotation speed is 200 r / min - 1000 r / min, and the stirring time is 30 min - 120 min; S3. Coat the negative electrode slurry on at least one surface of the negative electrode current collector, and after drying and rolling, obtain a negative electrode sheet.

[0057] Specifically, in step S1, weigh 1 wt% - 15 wt% of the silicon particle material and 81 wt% - 97 wt% of the carbon particle material, and perform dry mixing on the silicon particle material and the carbon particle material to obtain a first mixture. In the first mixture, the silicon particle material and the carbon particle material are in a mixed state.

[0058] The present application does not limit the specific method of dry mixing in step S1, and it can be selected according to actual needs.

[0059] In step S2, weigh 0.8 wt% - 2.0 wt% of the binder, add the binder and the first mixture to the aqueous solvent, and perform mixing treatment to obtain a second mixture; during the mixing treatment, the binder adheres to the surfaces of the silicon particle material and the carbon particle material, forms a first adhesion layer on at least a part of the surface of the silicon particle material, forms a second adhesion layer on at least a part of the surface of the carbon particle material, and the percentage of the adhesion area of the first adhesion layer in the surface area of the silicon particle material is 0.5% - 1.8%.

[0060] In step S2, mixing is carried out by stirring, the revolution speed of the stirring is 20 r / min - 50 r / min, the rotation speed is 200 r / min - 1000 r / min, and the stirring time is 30 min - 120 min. For example, the revolution speed can be 20 r / min, 25 r / min, 30 r / min, 35 r / min, 40 r / min, 45 r / min or 50 r / min, etc.; for example, the rotation speed can be 200 r / min, 300 r / min, 400 r / min, 500 r / min, 600 r / min, 700 r / min, 800 r / min, 900 r / min or 1000 r / min, etc.; for example, the stirring time can be 30 min, 40 min, 50 min, 60 min, 70 min, 80 min, 90 min, 100 min, 110 min or 120 min, etc.

[0061] The equipment used for stirring and mixing in this application is not limited. For example, a planetary ball mill can be used for stirring and mixing in step S2.

[0062] The addition amounts of the binder and the first mixture in step S2 in this application are not limited and can be selected according to actual needs.

[0063] In step S3, a negative electrode paste is coated on at least one surface of the negative electrode current collector, and then the negative electrode current collector is dried and rolled, so that the negative electrode paste is transformed into a negative electrode active material layer, and a negative electrode sheet is obtained.

[0064] In this application, by first mixing the silicon particle material and the carbon particle material and then adding the binder, at least part of the surfaces of the silicon particle material and the carbon particle material are coated with the binder, so that the area of the binder coated on the surface of the silicon particle material can meet the aforementioned range, thereby ensuring the inhibitory effect of the binder on the expansion of the silicon particle material, and further making the cycle performance of the battery higher. In addition, this preparation method has simple process, low equipment requirements and wide raw material sources, which is conducive to the industrial application of this preparation method.

[0065] In a specific embodiment, step S1 includes: dry-mixing the silicon particle material, the carbon particle material and the conductive agent to obtain a first mixture, coating the conductive agent on at least part of the particle surfaces of the silicon particle material to form a first conductive layer, and coating the conductive agent on at least part of the particle surfaces of the carbon particle material to form a second conductive layer; Among them, the stirring speed for dry-mixing in step S1 is 20 r / min to 50 r / min, and the stirring time is 10 min to 60 min.

[0066] Step S1 also includes the mixing of the conductive agent. Specifically: the silicon particle material, the carbon particle material and the conductive agent are dry-mixed to obtain a first mixture; during the dry-mixing process, the conductive agent is coated on at least part of the particle surfaces of the silicon particle material to form a first conductive layer, and the conductive agent is coated on at least part of the particle surfaces of the carbon particle material to form a second conductive layer.

[0067] The stirring speed for dry-mixing in step S1 is 20 r / min to 50 r / min, and the stirring time is 10 min to 60 min. For example, the stirring speed for dry-mixing is 20 r / min, 30 r / min, 40 r / min or 50 r / min, etc., and for example, the stirring time is 10 min, 20 min, 30 min, 40 min, 50 min or 60 min, etc.

[0068] By dry-mixing the silicon particle material, carbon particle material and conductive agent, the conductive agent can be coated on the surfaces of the silicon particle material and carbon particle material to form a conductive layer. Subsequently, a binder is coated on the surface of the silicon particles or carbon particles coated with the conductive layer to form an attachment layer, so that the quantity, distribution mode of the conductive agent on the surface of the silicon particle material and the area of the coated binder can meet the aforementioned range, thereby ensuring the inhibitory effect of the binder on the expansion of the silicon particle material and the improvement of the conductivity of the negative electrode active material layer by the conductive agent, and further improving the cycle performance of the battery to a greater extent.

[0069] In a specific embodiment, after step S1 and before step S2, it further includes: Kneading the first mixture and a part of the dispersant in an aqueous solvent to obtain a second mixture; subsequently, stirring the second mixture and the remaining part of the dispersant in the aqueous solvent to obtain a third mixture; wherein, the revolution speed of the kneading process is 20 r / min to 50 r / min, the rotation speed is 0 to 500 r / min, and the stirring time is 10 min to 120 min; the revolution speed of the stirring process is 20 r / min to 50 r / min, the rotation speed is 1500 r / min to 3000 r / min, and the stirring time is 60 min to 240 min; Step S2 includes: mixing the binder and the third mixture in an aqueous solvent to obtain a negative electrode slurry, enabling the binder to adhere to at least part of the surface of the silicon particle material coated with the first conductive layer to form a first attachment layer, and enabling the binder to adhere to at least part of the surface of the carbon particle material coated with the second conductive layer to form a second attachment layer.

[0070] After step S1 and before step S2, it further includes: adding the first mixture and the first part of the dispersant to the aqueous solvent and performing a kneading process to obtain a second mixture, which is a paste-like mixture; subsequently, adding the second mixture and the second part of the dispersant to the aqueous solvent and performing a stirring process to obtain a third mixture.

[0071] The present application does not limit the mass ratio of the first part of the dispersant to the second part of the dispersant, which can be selected according to actual needs. For example, the first part of the dispersant can be 50% of the total amount of the dispersant, and the second part of the dispersant is 50% of the total amount of the dispersant.

[0072] The revolution speed of the kneading process is 20 r / min to 50 r / min, the rotation speed is 0 to 500 r / min, and the stirring time is 10 min to 120 min; for example, the revolution speed of the kneading process can be 20 r / min, 30 r / min, 40 r / min, or 50 r / min, etc., the rotation speed can be 0, 100 r / min, 200 r / min, 300 r / min, 400 r / min, or 500 r / min, etc., and the stirring time is 10 min, 20 min, 40 min, 60 min, 80 min, 100 min, or 120 min, etc. The revolution speed of the stirring process is 20 r / min to 50 r / min, the rotation speed is 1500 r / min to 3000 r / min, and the stirring time is 60 min to 240 min; for example, the revolution speed of the stirring process can be 20 r / min, 30 r / min, 40 r / min, or 50 r / min, etc., the rotation speed can be 1500 r / min, 2000 r / min, 2500 r / min, or 3000 r / min, etc., and the stirring time can be 60 min, 80 min, 100 min, 120 min, 140 min, 160 min, 180 min, 200 min, 220 min, or 240 min, etc.

[0073] In step S2, 0.8 wt% to 2.0 wt% of the binder and the third mixture are mixed in an aqueous solvent to obtain a negative electrode slurry. In this negative electrode slurry, the binder adheres to at least part of the surface of the silicon particle material coated with the first conductive layer to form a first adhesion layer, and the binder adheres to at least part of the surface of the carbon particle material coated with the second conductive layer to form a second adhesion layer.

[0074] The dispersant of the present application includes one or more of lithium carboxymethyl cellulose and sodium carboxymethyl cellulose.

[0075] In the present application, the dispersant is added in two steps, which can regulate the viscosity of the mixture, avoid the viscosity of the mixture being too high or too low, and avoid the difficulty of the finally prepared negative electrode slurry being loaded on the surface of the negative electrode current collector, which is beneficial to reducing the preparation difficulty of the negative electrode sheet.

[0076] In a specific embodiment, the mass percentage content of the silicon particle material in the negative electrode active material layer is 1 wt% to 15 wt%. For example, the mass percentage content of the silicon particle material in the negative electrode active material layer is 1 wt%, 2 wt%, 3 wt%, 4 wt%, 5 wt%, 6 wt%, 7 wt%, 8 wt%, 9 wt%, 10 wt%, 11 wt%, 12 wt%, 13 wt%, 14 wt%, or 15 wt%, etc.

[0077] In a specific embodiment, the mass percentage of the carbon particle material in the negative electrode active material layer is 81 wt% to 97 wt%, such as 81 wt%, 82 wt%, 83 wt%, 84 wt%, 85 wt%, 86 wt%, 87 wt%, 88 wt%, 89 wt%, 90 wt%, 91 wt%, 92 wt%, 93 wt%, 94 wt%, 95 wt%, 96 wt% or 97 wt%, etc.

[0078] In a specific embodiment, the mass percentage of the binder in the negative electrode active material layer is 0.8 wt% to 2.0 wt%. For example, the mass percentage of the binder in the negative electrode active material layer can be 0.8 wt%, 0.9 wt%, 1 wt%, 1.1 wt%, 1.2 wt%, 1.3 wt%, 1.4 wt%, 1.5 wt%, 1.6 wt%, 1.7 wt%, 1.8 wt%, 1.9 wt% or 2 wt%, etc.

[0079] In a specific embodiment, the mass percentage of the conductive agent in the negative electrode active material layer is 0.03 wt% to 0.2 wt%. For example, the mass percentage of the conductive agent in the negative electrode active material layer can be 0.03 wt%, 0.05 wt%, 0.1 wt%, 0.15 wt% or 0.2 wt%, etc.

[0080] In a specific embodiment, the mass percentage of the dispersant in the negative electrode active material layer is 0.5 wt% to 1.8 wt%. For example, the mass percentage of the dispersant in the negative electrode active material layer is 0.5 wt%, 0.6 wt%, 0.7 wt%, 0.8 wt%, 0.9 wt%, 1 wt%, 1.1 wt%, 1.2 wt%, 1.3 wt%, 1.4 wt%, 1.5 wt%, 1.6 wt%, 1.7 wt% or 1.8 wt%, etc.

[0081] The third aspect of the present application provides a battery, including the above-mentioned negative electrode sheet or the negative electrode sheet prepared according to the preparation method of the above-mentioned negative electrode sheet.

[0082] In a specific embodiment, the battery of the present application further includes a positive electrode sheet. The positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer coated on the positive electrode current collector. The positive electrode active material layer includes a positive electrode active material, and the positive electrode active material includes at least one of lithium cobaltate, lithium nickel cobalt manganate, and lithium iron phosphate. When the above compounds are selected as the positive electrode active material, the positive electrode active material can fully exert its performance and improve the electrochemical performance of the lithium-ion battery.

[0083] In the embodiments of the present application, there is no particular limitation on the type of the positive current collector, and it can be any material known to be suitable for use as a positive current collector. In one embodiment, the positive current collector includes metal materials such as aluminum, stainless steel, nickel plating, titanium, tantalum, etc., and carbon materials such as carbon cloth and carbon paper. Preferably, the positive current collector is a metal material.

[0084] In a specific embodiment, the positive active material layer further includes a conductive agent and a binder. The conductive agent includes at least one of carbon materials such as natural graphite, artificial graphite, acetylene black, needle coke, carbon nanotubes, graphene, etc. The binder includes at least one of polyethylene, polypropylene, polyethylene terephthalate, polymethyl methacrylate, polyimide, aromatic polyamide, cellulose, and nitrocellulose.

[0085] In a specific embodiment, the battery of the present application further includes an electrolyte, which is an electrolyte well-known in the art that can be used in a battery and has excellent electrochemical performance of the battery, including a lithium salt and an organic solvent, and can be specifically set according to needs.

[0086] In a specific embodiment, the battery further includes a separator. There is no particular limitation on the material and shape of the separator in the embodiments of the present application, as long as the effects of the present application are not significantly damaged. It can include substances in the form of porous sheets or non-woven fabrics with excellent liquid retention properties, etc. The materials of the resin or glass fiber separator include, but are not limited to, polyolefins, aromatic polyamides, polytetrafluoroethylene, polyethersulfone, etc., and can be specifically set according to needs.

[0087] In a specific embodiment, the battery may include an outer package, which can be used to encapsulate the above electrode assembly and electrolyte.

[0088] In a specific embodiment, the outer package of the battery can be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. The outer package of the secondary battery can also be a soft package, such as a pouch soft package. The material of the soft package can be plastic. As plastics, polypropylene, polybutylene terephthalate, and polybutylene succinate can be listed, etc.

[0089] The present application has no particular limitation on the shape of the secondary battery, and it can be cylindrical, square, or any other shape.

[0090] Hereinafter, the present application will be further described in detail through specific examples.

[0091] Example 1 1. Preparation of the negative electrode sheet Silicon-carbon particles, graphite, carbon nanotubes, styrene-butadiene rubber (SBR), and sodium carboxymethyl cellulose (CMC-Na) were proportioned according to a solid mass ratio of 4.8:92.3:0.1:1.6:1.2 with reference to the following batching process: Step 1: Dry-mix silicon carbide particles, graphite, and carbon nanotubes at a stirring speed of 25 r / min for 30 min to obtain a first mixture; Step 2: Add 50% CMC-Na and 5% of the total solid content of deionized water to the first mixture for kneading. The revolution speed is 25 r / min, the rotation speed is 200 r / min, and the stirring time is 60 min to obtain a second mixture; Step 3: Add 50% CMC-Na and 5% of the total solid content of deionized water to the second mixture for the first stirring. The revolution speed is 30 r / min, the rotation speed is 2000 r / min, and the stirring time is 120 min to obtain a third mixture; Step 4: Add SBR to the third mixture for the second stirring. The revolution speed is 25 r / min, the rotation speed is 500 r / min, and the stirring time is 45 min. Then perform vacuum degassing to obtain the negative electrode slurry; Step 5: Uniformly coat the negative electrode slurry on one surface of a negative electrode current collector copper foil with a thickness of 10 μm and dry it at 110 °C to obtain a negative electrode sheet with a negative electrode material layer coated on one side and a coating thickness of 150 μm. Repeat the above steps on the other surface of this negative electrode sheet to obtain a negative electrode sheet with negative electrode active material layers coated on both sides. Then, cut the negative electrode sheet into a specification of 74 mm × 867 mm for standby.

[0092] 2. Preparation of the positive electrode sheet Mix the positive electrode active material lithium cobalt oxide, conductive carbon black, and polyvinylidene fluoride (PVDF) binder evenly according to a mass ratio of 95:2.5:2.5, and fully stir and mix in N-methylpyrrolidone solvent to prepare a slurry with a solid content of 75%. Stir it evenly to obtain the positive electrode slurry. Uniformly coat the positive electrode slurry on one surface of an aluminum foil with a thickness of 12 μm and dry it at 90 °C. After cold pressing, obtain a positive electrode sheet with a positive electrode active material layer thickness of 110 μm. Then repeat the above steps on the other surface of this positive electrode sheet to obtain a positive electrode sheet with positive electrode active material layers coated on both sides. Cut the positive electrode sheet into a specification of 76 mm × 851 mm, weld the tab, and set it aside for use.

[0093] 3. Preparation of the electrolyte In an environment with a water content of less than 10 ppm, mix propylene carbonate (PC), ethylene carbonate (EC), and diethyl carbonate (DEC) according to a mass ratio of 1:3:6. Based on the total mass of the electrolyte, add 8% of butyl butyrate; then add lithium hexafluorophosphate (LiPF 6 ), dissolve and mix it evenly, and then add fluoroethylene carbonate (FEC) to obtain the electrolyte. Among them, LiPF 6The molar concentration in the electrolyte is 1.15 mol / L, and the mass concentration of FEC in the electrolyte is 10.1%.

[0094] 4. Fabrication of Lithium-Ion Batteries A polyethylene porous polymer film is used as the separator. The positive electrode sheet, the separator, and the negative electrode sheet are stacked in sequence, with the separator placed in the middle of the positive and negative electrodes to play a separating role. Then, the stacked electrode sheets and the separator are wound to obtain an electrode assembly. The electrode assembly is placed in a pre-formed aluminum-plastic film shell, dehydrated at 80 °C, injected with the prepared electrolyte, and subjected to processes such as vacuum packaging, standing, formation, and shaping to obtain a lithium-ion battery.

[0095] The main differences between Examples 2 - 12 and Example 1 are the parameters of the stirring treatment and the components of the negative electrode active material layer during the preparation of the negative electrode sheet. Refer to Table 1.

[0096] Comparative Example 1 The difference between this comparative example and Example 1 lies in the preparation of the negative electrode sheet: Silicon-carbon particles, graphite, carbon nanotubes, styrene-butadiene rubber (SBR), and sodium carboxymethyl cellulose (CMC-Na) are proportioned according to a mass ratio of 4.8:92.3:0.1:1.6:1.2 and prepared according to the following batching process: Step 1: Stir silicon particles, graphite, carbon nanotubes, CMC-Na, and SBR. The revolution speed is 30 r / min, the rotation speed is 500 r / min, and the stirring time is 240 min to obtain a negative electrode slurry. Step 2: Uniformly coat the negative electrode slurry on one surface of a negative electrode current collector copper foil with a thickness of 10 μm and dry it at 110 °C to obtain a negative electrode sheet with a negative electrode material layer coated on one side and a coating thickness of 150 μm. Repeat the above steps on the other surface of this negative electrode sheet to obtain a negative electrode sheet with a negative electrode active material layer coated on both sides. Then, cut the negative electrode sheet into a size of 74 mm × 867 mm for use.

[0097] The main differences between Comparative Examples 2 - 10 and Example 1 are the parameters of the stirring treatment and the component ratios in the negative electrode active material layer during the preparation of the negative electrode sheet. Refer to Table 1.

[0098] Comparative Example 11: The basic content is the same as that of Example 1, except that polyvinylpyrrolidone (PVP) is used instead of SBR.

[0099] Comparative Example 12: The basic content is the same as that of Example 1, except that SBR is not added.

[0100] Table 1

[0101] Test Example 1 Testing the percentage of the adhesion area of the first adhesion layer in the surface area of the silicon particle material includes the following steps: Perform SEM testing on the negative electrode sheet, maintain a magnification of 20,000 times, observe the binder particles on the surface of the silicon particle material, select a 9-square-micron area on the surface of the silicon particle material, observe the number n of binder particles within the observation area, and simultaneously measure the diameter D of the binder particles (the binder particles are spherical-like particles). Calculate the area of a single binder particle S’ = 1 / 4×π×D 2 , therefore, the adhesion area ratio of the binder = nS’ / 9; Repeat the above steps for 10 different silicon particles, and calculate the average value of the adhesion area ratios of the 10 tested binders, which is the percentage of the adhesion area of the first adhesion layer in the specific surface area of the silicon particle material, as shown in Table 2.

[0102] Testing the number of conductive agents within a 9-square-micron area in the first conductive layer includes the following steps: Perform SEM testing on the negative electrode sheet, maintain a magnification of 20,000 times, observe the conductive agents on the surface of the silicon particle material, select a 9-square-micron area on the surface of the silicon particle material, and observe the number n of conductive agents within the observation area; Repeat the above steps for 10 different areas, and calculate the average value of the 10 tested numbers of conductive agents, which is the number of conductive agents within a 9-square-micron area on the surface of the silicon particle material, as shown in Table 2.

[0103] Testing the number of intersections of a carbon nanotube with the remaining carbon nanotubes includes the following steps: Perform SEM testing on the negative electrode sheet, maintain a magnification of 20,000 times, and observe the number of nodes where a carbon nanotube is connected to other carbon nanotubes; Repeat the above steps for 10 different carbon nanotubes, and calculate the average value of the 10 tested numbers of intersections of the carbon nanotube with the remaining carbon nanotubes, which is the number of intersections of the carbon nanotube with the remaining carbon nanotubes, as shown in Table 2.

[0104] The average number of carbon nanotubes connecting each silicon particle material to multiple surrounding carbon particle materials can be tested by scanning electron microscopy (SEM), including the following steps: Perform SEM testing on the negative electrode sheet, maintain a magnification of 20,000 times, select a preset number, for example, 10 silicon particle materials, and observe the number of carbon nanotubes connecting each silicon particle material to multiple surrounding carbon particle materials on the surface of each silicon particle material; Calculate the average value of the numbers of carbon nanotubes connecting the 10 tested silicon particle materials to multiple surrounding carbon particle materials on their surfaces, as shown in Table 2.

[0105] Testing the number of silicon particle materials within an area of 10,000 square micrometers on the surface of the negative electrode active material layer includes the following steps: Conduct SEM testing on the negative electrode sheet, maintaining a magnification of 1000 times, observe the silicon particle material particles on the surface of the negative electrode active material layer, select an area of 10,000 square micrometers on the surface of the negative electrode active material layer, and observe the number of silicon particle material particles within the observation area; Repeat the above steps for 10 different areas, calculate the average value of the number of silicon particle material particles obtained from the 10 tests, which is the number of silicon particle materials within an area of 10,000 square micrometers on the surface of the negative electrode active material layer, as shown in Table 2.

[0106] Table 2

[0107] Test Example 2 Perform the following performance tests on the batteries prepared in the examples and comparative examples: 1. Cycle performance test In a constant temperature oven at (45 ± 2)°C and (25 ± 2)°C respectively, charge the lithium-ion battery at a constant current and voltage of 0.7C until 4.53V, charge at a constant voltage until 0.05C, let it stand for 5 minutes, and then discharge at 0.5C to 3V. The capacity obtained from this step is the initial capacity. Conduct a cycle test using 0.7C charging / 0.5C discharging, and calculate the ratio of the capacity at 600 cycles to the initial capacity to obtain the capacity retention rate.

[0108] For each group of 5 batteries, calculate the average value of the room temperature cycle capacity retention rate and the high temperature cycle capacity retention rate and record it in Table 3.

[0109] 2. Swelling rate test Use a micrometer to measure the thickness of the lithium-ion battery when it is half-charged, that is, at a 50% state of charge (SOC), as the initial thickness. After 600 cycles of charging to full charge, that is, at a 100% SOC state, use a micrometer to measure the thickness of the lithium-ion battery at this time, and compare it with the thickness of the lithium-ion battery when it was initially half-charged to obtain the swelling rate of the fully charged lithium-ion battery at this time.

[0110] For each group of 5 batteries, calculate the average value of the room temperature cycle swelling rate and the high temperature cycle swelling rate and record it in Table 3.

[0111] Table 3

[0112] As can be seen from Table 3: According to Examples 1 - 12, the room temperature cycle capacity retention rate of the battery cells is higher than 88%, the thickness swelling rate is lower than 13%, the 45°C cycle capacity retention rate is higher than 80%, and the thickness swelling rate is lower than 16%; In Comparative Example 1, the stirring process was changed, resulting in a poorer dispersion effect of the conductive agent, a decrease in the number of CNT connections and intersections, a deterioration of the conductive network, and a significant deterioration of the capacity retention rate; In Comparative Example 2, the proportion of the binder was relatively low, and the swelling of the negative electrode sheet could not be effectively suppressed, resulting in an increase in the thickness swelling of the battery cell; In Comparative Example 3, the second stirring speed was relatively low, and the SBR binder could not be evenly dispersed, resulting in an increase in the swelling of the battery cell; In Comparative Example 4, the stirring rates of the dry mixing and the first stirring treatments were relatively low, resulting in a poorer dispersion effect of the conductive agent, a decrease in the number of CNT connections and intersections, a relatively poor conductive network, and a deterioration of the cycling performance; In Comparative Example 5, an excessive amount of binder was added. The excessive binder wrapped around the surface of the negative electrode material, inhibiting the lithium ion transmission, resulting in a deterioration of the kinetics, a deterioration of the room temperature performance, and lithium deposition in the arc area of the battery cell, leading to an increase in the battery swelling; In Comparative Example 6, an excessive amount of the conductive agent CNT was added, making the slurry dispersion difficult, with a high viscosity, and fish scale patterns appeared on the coating surface. Lithium deposition at the cycling interface led to an increase in the cycling thickness. Therefore, an excessive amount of CNT would cause a deterioration of the performance; In Comparative Example 7, the amount of the conductive agent CNT added was too little, and a good conductive network could not be formed on the negative electrode sheet, resulting in a poor decline of the capacity retention rate of the battery cell; In Comparative Example 8, the stirring speed of the kneading treatment was relatively low, and the CNT could not be evenly wrapped on the surface of the silicon particles, and some CNTs showed agglomeration phenomena, and the overall electrical performance also deteriorated; In Comparative Example 9, the addition amount of the silicon-carbon particles was relatively small, and the performance of the battery cell was relatively better, but the energy density of the battery cell with a lower silicon content would be relatively lower; In Comparative Example 10, the addition amount of the silicon-carbon particles exceeded too much. An excessively high silicon-carbon content would cause large-volume swelling of the battery cell and rapid capacity decay, and it was difficult for the performance to meet the application requirements; In Comparative Example 11, PVP was used instead of SBR. The binding force of PVP was relatively poor, and it could not effectively suppress the swelling, and there was also a problem of insufficient kinetics; In Comparative Example 12, the binder SBR was not added, and the powder of the electrode sheet could not be effectively bonded directly, and serious powder dropping occurred during the rolling of the electrode sheet, and the performance of the battery cell was very poor.

[0113] The above has described the embodiments of the present application. The above description is exemplary and not exhaustive, and is not limited to the disclosed embodiments. Many modifications and variations are obvious to those of ordinary skill in the art in the technical field without departing from the scope and spirit of the described embodiments. The selection of the terms used herein is intended to best explain the principles of the embodiments, the practical applications, or the improvements to the technologies in the market, or to enable other ordinary technical personnel in the technical field to understand the embodiments disclosed herein.

Claims

1. A negative electrode sheet, characterized in that: It includes a negative electrode current collector and a negative electrode active material layer disposed on at least one side surface of the negative electrode current collector; The mass percentage of the negative electrode active material layer is 100%, and the negative electrode active material layer includes 81wt% to 97wt% of carbon particle material, 1wt% to 15wt% of silicon particle material, and 0.8wt% to 2wt% of binder; The binder is attached to at least part of the surface of the silicon granule material to form a first attachment layer, and the attachment area of ​​the first attachment layer accounts for 0.5% to 1.8% of the surface area of ​​the silicon granule material; The binder is attached to at least a portion of the surface of the carbon particle material to form a second attachment layer.

2. The negative electrode sheet according to claim 1, characterized in that: The negative electrode active material layer further includes 0.03wt% to 0.2wt% of a conductive agent; The conductive agent is coated on at least part of the surface of the silicon granule material to form a first conductive layer, and the binder is attached to at least part of the surface of the silicon granule material coated with the first conductive layer to form the first adhesion layer; The conductive agent is coated on at least part of the surface of the carbon particle material to form a second conductive layer, and the binder is attached to at least part of the surface of the carbon particle material coated with the second conductive layer to form the second adhesion layer.

3. The negative electrode sheet according to claim 2, characterized in that: At least some of the silicon particle material are connected to the carbon particle material through the conductive agent.

4. The negative electrode sheet according to claim 3, characterized in that: The number of the conductive agents within a preset area in the first conductive layer is N, where 10<N<50.

5. The negative electrode sheet according to any one of claims 1 to 4, characterized in that: The conductive agent includes carbon nanotubes; The carbon nanotubes include one or more of single-walled carbon nanotubes and multi-walled carbon nanotubes; and / or, The length of the carbon nanotubes is 1 μm to 20 μm; and / or, The diameter of the carbon nanotube is 1nm-5nm.

6. The negative electrode sheet according to claim 5, characterized in that: The average number of carbon nanotubes connected between each silicon particle material and the surrounding carbon particle materials is not less than 5; and / or, Each of the carbon nanotubes in the negative electrode active material layer intersects with at least three of the remaining carbon nanotubes.

7. The negative electrode sheet according to claim 1, characterized in that: The number of silicon granular materials within an area of ​​10,000 square micrometers on the surface of the negative electrode active material layer is 1 to 15; and / or, The silicon particle material includes one or more of silicon oxide, pre-lithiation silicon oxide, pre-magnesiation silicon oxide material, and silicon-carbon composite material.

8. The negative electrode sheet according to claim 1, characterized in that: The binder comprises styrene-butadiene rubber; and / or, The carbon particle material includes one or more of artificial graphite, natural graphite, hard carbon, and soft carbon; and / or, The negative electrode active material layer further includes 0.5 wt % to 1.8 wt % of a dispersant, and the dispersant includes one or more of lithium carboxymethyl cellulose and sodium carboxymethyl cellulose.

9. A method for preparing a negative electrode sheet, characterized in that: The following steps are involved: S1, dry-mixing 1 wt% to 15 wt% of silicon granular material and 81 wt% to 97 wt% of carbon granular material to obtain a first mixture; S2, mixing 0.8wt%~2wt% of the binder and the first mixture in an aqueous solvent to obtain a negative electrode slurry, so that the binder in the negative electrode slurry is attached to at least part of the particle surface of the silicon granule material to form a first attachment layer, and the binder is attached to at least part of the particle surface of the carbon granule material to form a second attachment layer, and the attachment area of ​​the first attachment layer accounts for 0.5%~1.8% of the surface area of ​​the silicon granule material; wherein, stirring is used for mixing in step S2, the revolution speed of stirring is 20r / min~50r / min, the rotation speed is 200r / min~1000r / min, and the stirring time is 30min~120min; S3, coating the negative electrode slurry onto at least one side of the surface of the negative electrode current collector, and obtaining a negative electrode sheet after drying and rolling.

10. The method for preparing a negative electrode sheet according to claim 9, characterized in that: The step S1 comprises: dry-mixing 1wt% to 15wt% of silicon granular material, 81wt% to 97wt% of carbon granular material and 0.03wt% to 0.2wt% of a conductive agent to obtain a first mixture, coating the conductive agent in the first mixture on at least a portion of the surface of the silicon granular material to form a first conductive layer, and coating the conductive agent on at least a portion of the surface of the carbon granular material to form a second conductive layer; Wherein, the stirring speed of the dry mixing in step S1 is 20 r / min~50 r / min, and the stirring time is 10 min~60 min.

11. The method for preparing a negative electrode sheet according to claim 9, characterized in that: After step S1 and before step S2, the following further includes: The first mixture and a part of the dispersant are kneaded in an aqueous solvent to obtain a second mixture; the second mixture and the remaining part of the dispersant are then stirred in an aqueous solvent to obtain a third mixture; wherein the revolution speed of the kneading process is 20 r / min~50 r / min, the rotation speed is 0~500 r / min, and the stirring time is 10 min~120 min; the revolution speed of the stirring process is 20 r / min~50 r / min, the rotation speed is 1500 r / min~3000 r / min, and the stirring time is 60 min~240 min; The step S2 comprises: mixing 0.8wt% to 2.0wt% of the binder and the third mixture in an aqueous solvent to obtain a negative electrode slurry, allowing the binder in the negative electrode slurry to adhere to at least a portion of the surface of the silicon particle material coated with the first conductive layer to form the first adhesion layer, and allowing the binder to adhere to at least a portion of the surface of the carbon particle material coated with the second conductive layer to form the second adhesion layer.

12. A battery, characterized in that: The battery comprises the negative electrode sheet according to any one of claims 1 to 8 or a negative electrode sheet prepared by the method for preparing a negative electrode sheet according to any one of claims 9 to 11.

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