Negative electrode material and preparation method thereof, negative electrode sheet and lithium ion battery

Abstract

The present invention discloses a lithium ion battery negative electrode material, the components and the mass fractions of the components are as follows: 88-105 parts of active material, 0.01%-0.1 parts of single-walled carbon nanotubes, 0.01%-0.5 parts of graphene nanoribbons, less than 1 part of super conductive carbon black, less than 1 part of few-walled or multi-walled carbon nanotubes, 0.05-0.5 parts of dispersant, 1-3 parts of thickener, 1-4 parts of binder, and 25-60 parts of solvent; the active material includes silicon oxide material SiO x and artificial graphite, of which 0

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium-ion batteries, and specifically relates to a negative electrode material for a lithium-ion battery and a preparation method thereof, and also relates to a negative electrode plate containing the negative electrode material and a lithium-ion battery using the negative electrode plate. Background Art

[0002] Human energy primarily comes from fossil fuels. However, with the depletion of fossil fuels and the resulting environmental pollution from their combustion, low-carbon, environmentally friendly, and sustainable development strategies are being proposed for sustainable human development. Low-carbon, environmentally friendly, green energy, and sustainable development are crucial global issues today. In particular, the development and reuse of clean energy is inextricably linked to human survival and development. Since the commercialization of lithium-ion batteries in the 1990s, lithium-ion battery technology has rapidly advanced. In the new century, lithium-ion battery technology has shifted from applications in mobile phones, cameras, and small electronic products to a strategic position in the development and utilization of new energy sources. Safe and reliable lithium-ion batteries will soon replace traditional energy sources in hybrid electric vehicles (HEVs) and electric vehicles (EVs). They can also be connected to solar and wind energy storage systems to build smart grids, propelling lithium-ion battery technology to a new and higher level. Lithium-ion batteries can convert and store discontinuous and unstable renewable energy, making them the most effective way to provide energy storage beyond power automation systems and an ideal green and environmentally friendly renewable energy source.

[0003] The parts of lithium-ion batteries that complete conversion and storage are the positive and negative electrodes. The positive electrode material is made of lithium ore raw materials that can reversibly insert and remove lithium, and the negative electrode is generally a graphite material that can insert lithium in a layered manner. With the development of lithium-ion batteries into large-scale application fields such as electric vehicles and renewable energy storage systems, the performance indicators of lithium-ion batteries such as energy density and power density need to be further improved. In terms of negative electrode materials, the specific capacity of traditional graphite carbon negative electrode materials is limited, and it is difficult to meet the needs of high energy density batteries. New high specific capacity negative electrode materials represented by Si-based materials have attracted widespread attention. Among them, SiO x While the material has a high specific capacity, it also has a smaller volume change than pure Si, and therefore has greater practical potential in terms of cycle life.

[0004] Silicon has poor conductivity, but SiO x The material's poor electronic conductivity, slow ion diffusion, high high-rate charge-discharge impedance, and rapid specific capacity decay have hindered its further development and application. To improve the low-temperature performance, high-rate charge-discharge performance, and charge-discharge cycling characteristics of silicon anode batteries, and broaden their application in electric vehicles, researchers have modified the silicon anode material.

[0005] In response to the above problems, on the one hand, scholars and scientists at home and abroad have made improvements from the perspective of nano-sizing of silicon materials, such as the synthesis of silicon nanowires. However, the synthesis process has remained in the pilot stage for a long time, and the process is complex, the growth control of nanowires is difficult, and mass production is impossible, resulting in the inability of scientific research results to be applied to actual production. On the other hand, negative electrode material manufacturers start from the material structure design and particle size distribution, such as using gas-phase carbon coating, doping metal elements, controlling silicon particle size and other methods to try to improve battery performance. Coating carbon materials on the outside of active particles can solve the problem of poor electrical conductivity of the bulk, but it still cannot solve the problem of maintaining effective connection of the conductive network between expanded particles after the active particles expand. Although the above two methods have improved the electrochemical performance of the battery to some extent, they cannot be converted into improvement measures for actual mass production services, so the effect is not significant and difficult to implement.

[0006] Improving battery performance is strongly correlated with the integrity of the battery's internal conductive network. First, electrons are transferred from the external circuit through the foil to the coating layer. The conductive network then transports the electrons to the surface of the active particles to participate in lithium insertion and removal reactions. The extent and rate of lithium insertion and removal in the positive and negative electrode active materials are directly related to the speed and channel length of electron transport. Therefore, it is necessary to develop a conductive agent solution that can comprehensively improve the electron conductivity in the dead zone after the silicon anode expands due to conductivity. For Si-based anodes, the first step is to improve the contact resistance between the anode foil and the dressing to reduce polarization. Furthermore, the peel strength between the dressing and the foil should be improved to alleviate the problem of anode expansion and shedding after cycling. Secondly, it is necessary to build a more effective conductive network within the Si-based anode dressing layer to address the problem of electron transport between the anode active particles. Because the alloying and dealloying process during lithium insertion and removal in silicon anodes causes volume expansion several times greater than that of conventional graphite, this expansion disrupts the integrity of the conductive network within the anode material, resulting in the formation of dead zones. Due to the loss of conductive bridges, these dead zones cannot be used for further lithium insertion and removal, directly affecting battery capacity and consistency. Summary of the Invention

[0007] To address the deficiencies of the prior art, the present invention provides a negative electrode material for a lithium-ion battery comprising a silicon-oxygen material and a low-dimensional carbon conductive material. This negative electrode material for a lithium-ion battery solves the problems of capacity decay and increased internal resistance caused by the low electrical conductivity of the silicon-oxygen negative electrode active material and the loss of effective electron channels due to expansion during use, while also improving the cycle performance of lithium-ion batteries using this negative electrode material. The negative electrode material for a lithium-ion battery in the present invention has good dispersibility and a low amount of conductive agent added to its composition, which not only reduces costs but also helps to improve the energy density of the battery. Lithium-ion batteries using negative electrode sheets made of the negative electrode material in the present invention have lower DC internal resistance and more stable cycle performance than existing lithium-ion batteries.

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

[0009] The present invention provides a negative electrode material for a lithium-ion battery, and its components and mass parts of the components are as follows:

[0010] Active material: 88 - 105 parts

[0011] Single-walled carbon nanotubes: 0.01 - 0.1 part

[0012] Graphene nanoribbons: 0.01 - 0.5 part

[0013] Superconductive carbon black: less than 1 part

[0014] Few-walled or multi-walled carbon nanotubes: less than 1 part

[0015] Dispersant: 0.05 - 0.5 part

[0016] Thickener: 1 - 3 parts

[0017] Binder: 1 - 4 parts

[0018] Solvent: 25 - 60 parts

[0019] The active material includes silicon-oxygen material SiO x and artificial graphite, where 0 < x < 2, and the mass percentage content of silicon element in the active material is less than or equal to 3.5%;

[0020] The diameter of the single-walled carbon nanotubes, few-walled or multi-walled carbon nanotubes is 1 - 100 nm, the specific surface area is 200 - 2000 m 2 / g, the G / D ratio is 50 - 300, the aspect ratio is 50 - 2800, and the metal impurity content is lower than 5 ppm;

[0021] The number of layers of the graphene nanoribbons is less than 8 layers, the thickness is less than 2.35 nm, the specific surface area is 2000 - 3000 m 2 / g, the G / D ratio is 150 - 300, the monolayer rate is greater than 90%, and the aspect ratio is 1500 - 2500;

[0022] The viscosity of the negative electrode material for the lithium-ion battery is 2000 - 450 mPa·s.

[0023] The present invention utilizes single-walled carbon nanotubes (SWCNTs) and graphene nanoribbons with a larger aspect ratio to build a conductive network covering a wider area, reducing the polarization effect of electron transport between active particles in the applied active material layer. Compared to conventional conductive agents, such as point-type conductive agents (e.g., super conductive carbon black) and grape chain-like conductive agents (e.g., Ketjen Black), these agents conduct electricity based on point contact. After the silicon anode expands, the distance between particles increases, the original electrical contact bridges separate, and conductivity is lost. Therefore, point-type conductive agents do not help maintain capacity after silicon anode expansion. SWCNTs can be thought of as one-dimensional cylindrical tubes formed from rolled graphene sheets. Compared to multi-walled carbon nanotubes (MWCNTs), which have the disadvantages of less curvature and more agglomeration, SWCNTs have a larger aspect ratio and greater curvature, significantly reducing the risk of tube agglomeration. Graphene nanoribbons are a specific form of graphene material. Compared to conventional few-layer graphene materials, their flakes are larger, easily encapsulating active particles and limiting ion transport. The number of graphene nanoribbons is less than 8 and the aspect ratio is relatively large, so they will not wrap around the active particles. Moreover, with the advantage of their length direction, they can maintain the original good conductive bridge between the expanded particles.

[0024] Single-walled carbon nanotubes and graphene nanoribbons, as conductive agents with low percolation thresholds, can also achieve the goal of reducing the amount of conductive agent added. When the volume fraction of conductive particles increases to a certain critical value, their conductivity suddenly increases sharply, transforming from an insulator to a conductor, with a change of 10 orders of magnitude. This phenomenon is called conductive percolation. The amount of conductive agent added at the conductivity mutation point is the percolation threshold of the conductive agent. Conductive agents with a low percolation threshold can achieve a conductive effect at extremely low addition amounts. The percolation threshold of conductive agent materials is negatively correlated with the aspect ratio of the carbon material. Single-walled carbon nanotubes and graphene nanoribbons with larger aspect ratios can achieve extremely low percolation thresholds.

[0025] The present invention significantly reduces the amount of conductive agent used in traditional negative electrode materials by combining single-walled carbon nanotubes and graphene nanoribbons as conductive agents. This reduction in conductive agent usage directly increases the active material content, thereby improving energy density. Auxiliary conductive agents can also be added to the negative electrode material formulation of the present invention based on actual needs. Since the conductive properties of the single-walled carbon nanotubes and graphene nanoribbons already meet the required performance, the amount of auxiliary conductive agent used is extremely low and can even be omitted if the required performance is met.

[0026] Preferably, the mass percentage of silicon in the active material is 2.5%-3.5%; the specific surface area of ​​the single-walled carbon nanotubes, few-walled carbon nanotubes or multi-walled carbon nanotubes is 1500-2000 m 2 / g, G / D ratio range is 100-300, and aspect ratio is 2000-2800.

[0027] Preferably, the dispersant is polyvinyl pyrrolidone with a molecular weight of 8000-700000, the thickener is sodium carboxymethyl cellulose, the binder is polyacrylate, and the solvent is one of N-methylpyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide or water. The dispersant selected in the present invention is compatible with the other components, and can effectively disperse the low-dimensional carbon material conductive agent uniformly and stably to form a uniformly distributed conductive network, shortening the lithium ion migration distance, thereby improving the capacity and cycle performance of the lithium ion battery, while reducing the DC internal resistance of the battery, reducing the polarization of the battery, and improving the overall battery performance. The present invention also improves the low-temperature discharge performance of the battery by preferably using a polyacrylate binder. The acid ester binder has a greater strength in conducting lithium ions. Even under low temperature conditions, the lithium ion migration speed of the battery cell using the acid ester binder is very rapid.

[0028] The present invention also provides a method for preparing the above-mentioned lithium ion battery negative electrode material, comprising the following steps:

[0029] S01: adding single-walled carbon nanotubes and a dispersant to a solvent according to a raw material component formula, and dispersing and mixing at high speed to obtain a single-walled carbon nanotube dispersion; wherein the mass of the dispersant is 0.2-1wt% of the total mass of the single-walled carbon nanotube dispersion, and the mass of the single-walled carbon nanotubes is 0.2-2wt% of the total mass of the single-walled carbon nanotube dispersion;

[0030] S02: According to the raw material component formula, large-scale expanded graphite is physically exfoliated in a solvent to obtain a graphene nanoribbon solution, a dispersant is added to the graphene nanoribbon solution, the solution is diluted, and the dispersion is uniformly mixed to obtain a graphene nanoribbon dispersion (i.e., a two-dimensional carbon material conductive agent dispersion); wherein the mass of the dispersant is 0.5-3wt% of the total mass of the graphene nanoribbon dispersion, and the mass of the graphene nanoribbons is 0.2-2wt% of the total mass of the graphene nanoribbon dispersion;

[0031] S03: adding a thickener to the remaining solvent and stirring for 0.5-1.5 hours to obtain a thickener glue solution; adding the single-walled carbon nanotube dispersion and graphene nanoribbon dispersion prepared in S01 and S02 to the thickener glue solution and mixing evenly to obtain a conductive glue;

[0032] S04: adding super conductive carbon black and few-walled or multi-walled carbon nanotubes to the negative electrode material, mixing and stirring uniformly to obtain a negative electrode dry powder;

[0033] S05: The conductive glue obtained in S03 is added to the negative electrode dry powder prepared in S04 in batches, and finally the binder is added. After stirring evenly, vacuum defoaming treatment is performed for 4-8 hours to obtain the desired lithium-ion battery negative electrode material.

[0034] The preparation method of the present invention is compatible with the composition formula of the negative electrode material of the present invention, so that the negative electrode material is organically combined with the dispersed components to form a uniform and stable negative electrode material slurry.

[0035] The present invention also provides a negative electrode sheet using the above-mentioned negative electrode material for a lithium ion battery, comprising a negative electrode current collector and an active material coated on the surface of the negative electrode current collector. The active material coated on the negative electrode sheet is the negative electrode material for a lithium ion battery in the present invention.

[0036] Preferably, the negative electrode current collector is a conductive carbon-coated copper foil, which includes a copper foil and a conductive layer coated on the surface of the copper foil. The thickness of the copper foil is 8-12 μm, and the thickness of the conductive layer coated on one side is 20 nm-2 μm. The surface density of the conductive carbon-coated copper foil is 70-95 g / m 2 .

[0037] In the construction of lithium-ion batteries, the current collector is a key component for electron conduction. The positive and negative electrode slurries are evenly coated on the surface of the current collector. The electrons generated during the electrochemical reaction can be collected by the current collector and conducted to the external circuit through the tabs, thereby realizing the conversion of chemical energy into electrical energy. After long-term charge and discharge cycles, the active material layer and the current collector may expand and separate to varying degrees, and the conductive path is blocked, resulting in a further increase in the polarization internal resistance. Coating a conductive layer on the foil substrate can form an ultra-thin coating with good electrical properties on the surface of the current collector, which can significantly reduce the interface resistance and improve the conductivity while keeping the energy density basically unchanged. At the same time, the active material is partially embedded in the conductive coating, making the contact between the current collector and the foil more firm, effectively improving the expansion and separation problem between the active material layer and the current collector after the battery cycle, ensuring the integrity of the electrode sheet, and improving the cycle life and cycle performance of the battery. Due to the large expansion coefficient of silicon oxide negative electrode materials, they are rarely used in 3C electronics or smaller products. They are mostly used in power batteries, such as electric vehicle batteries. Precisely because of the large expansion coefficient of silicon oxide materials, the conductive layer on the collector coated with a conventional conductive layer is more likely to separate from the collector body during use. The conductive carbon-coated copper foil of the present invention has a thin conductive layer coating and high efficiency, which significantly reduces the interface resistance while improving the conductive performance and avoids the conductive layer coating from separating from the collector body.

[0038] Preferably, the conductive layer material is one or a combination of super conductive carbon black, conductive graphite microsheets, carbon nanotubes, few-layer graphene nanoribbons, Ketjen black, and acetylene black.

[0039] Preferably, the conductive carbon-coated copper foil is a carbon-coated copper foil coated with a conductive layer on both sides, and the conductive layers on both sides of the carbon-coated copper foil are single-walled carbon nanotube coatings with a thickness of 20nm-2μm, and the diameter of the single-walled carbon nanotubes is 1-3nm.

[0040] The silicon oxide material used in the present invention is used as the negative electrode active material

[0041] The present invention also provides a lithium-ion battery, which includes a positive electrode sheet, a negative electrode sheet, a battery separator and an electrolyte, and the negative electrode sheet is as described in the negative electrode sheet of the lithium battery in the present invention.

[0042] Preferably, the DC resistance of the lithium-ion full battery is less than 32 mΩ in the range of 30% to 80% SOC; the room temperature cycle performance is a cycle retention rate of more than 82% after 1000 cycles; and the charge transfer impedance is less than 160 Ω.

[0043] The present invention has the following advantages:

[0044] 1. The negative electrode material for lithium-ion batteries in the present invention solves the problems of capacity decay and internal resistance increase caused by the low conductivity of silicon-oxygen negative electrode active materials and the loss of effective electron channels due to expansion during use, while also improving the cycle performance of lithium-ion batteries using the negative electrode material.

[0045] 2. In the present invention, the use of single-walled carbon nanotubes and graphene nanoribbons as conductive agents greatly reduces the amount of conductive agents used in traditional negative electrode materials. The reduction in the amount of conductive agents used can directly increase the content of active materials, thereby increasing energy density.

[0046] 3. The conductive layer coating of the negative electrode sheet of the lithium-ion battery in the present invention is thin and efficient, which improves the conductive performance while significantly reducing the interface resistance and avoiding the separation of the conductive layer coating from the current collector body.

[0047] 4. The lithium-ion battery of the present invention has low DC resistance, good cycle performance at room temperature, and low charge transfer impedance. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] Figure 1 This is a scanning electron microscope image of the conductive adhesive in Example 3 of the present invention;

[0049] Figure 2 This is the DC internal resistance test process (current) for the 18650 cylindrical lithium-ion battery in the full SOC range in this embodiment;

[0050] Figure 3 This is the DC internal resistance test process (voltage) for the 18650 cylindrical lithium-ion battery in the full SOC range in this embodiment. DETAILED DESCRIPTION

[0051] The present invention will be described in detail below with reference to the accompanying drawings and embodiments.

[0052] 1. Preparation of lithium-ion battery negative electrode materials

[0053] The components of the negative electrode material for the lithium ion battery in this embodiment are shown in the following table:

[0054] Example No. Active Materials Single-walled carbon nanotubes (dispersant) Graphite carbon nanoribbons (dispersant) Super conductive carbon black Few-walled or multi-walled carbon nanotubes 1 100 0.01 (0.05) 0 0 0 2 100 0.02 (0.2) 0 0 0 3 100 0.02 (0.2) 0 0 0 4 100 0.02 (0.2) 0 0 0 5 100 0.02 (0.2) 0 0 0 6 100 0.02 (0.1) 0 0 0 7 100 0.1(0.1) 0 0 0 8 100 0 0.01 (0.05) 0 0 9 100 0 0.1 (0.08) 0 0 10 100 0 0.1 (0.08) 0 0 11 100 0 0.1 (0.08) 0 0 12 100 0 0.1 (0.08) 0 0 13 100 0 0.1 (0.08) 0 0 14 100 0 0.1 (0.08) 0 0 15 100 0 0.1 (0.05) 0 0 16 100 0 0.5 (0.5) 0 0 17 100 0.01 (0.02) 0.01 (0.08) 0 0 18 100 0.02 (0.02) 0.1 (0.08) 0 0 19 100 0.02 (0.02) 0.1 (0.08) 0 0 20 100 0.02 (0.02) 0.1 (0.08) 0 0 21 100 0.02 (0.02) 0.1 (0.08) 0 0 22 100 0.02 (0.02) 0.1 (0.08) 0 0 23 100 0.02 (0.02) 0.1 (0.08) 0 0 24 100 0.1 (0.02) 0.5 (0.08) 0 0 25 100 0.02 (0.02) 0.1 (0.08) 1 0 26 100 0.02 (0.02) 0.1 (0.08) 0 0.5 27 100 0.02 (0.02) 0.1 (0.08) 0 0.5 28 100 0.02 (0.02) 0.1 (0.08) 1 0.5 Comparative Example 1 100 0 0 1.3 0 Comparative Example 2 100 0 0 0 0.8 Comparative Example 3 100 0 0 1.3 0.8

[0055] Example No. thickener binder solvent Viscosity 1 1.2 1.2 58 2520 2 1.2 1.2 52 2430 3 1.2 1.2 52 2430 4 1 1 52 2580 5 3 4 52 2600 6 1.2 1.2 54 2400 7 1.2 1.2 45 2360 8 1.2 1.2 56 2580 9 1.2 1.2 59 2500 10 1.2 1.2 59 2500 11 1.2 1.2 59 2590 12 1.2 1.2 59 2530 13 1 1 60 2480 14 3 4 55 2450 15 1.2 1.2 60 2480 16 1.2 1.2 55 2450 17 1.2 1.2 55 2630 18 1.2 1.2 50 2600 19 1.2 1.2 50 2650 20 1.2 1.2 50 2600 21 1.2 1.2 50 2650 22 1 1 50 2640 23 3 4 50 2640 24 1.2 1.2 49 2600 25 1.2 1.2 51 2620 26 1.2 1.2 50 2630 27 1.2 1.2 50 2620 28 1.2 1.2 51 2630 Comparative Example 1 1.2 1.2 75 3600 Comparative Example 2 1.2 1.2 69 3560 Comparative Example 3 1.2 1.2 70 3590

[0056] Among them, the active material, composite conductive agent components and negative electrode foil materials are selected as follows:

[0057]

[0058]

[0059]

[0060] In the embodiment, the dispersant is polyvinyl pyrrolidone with a molecular weight of 40,000, the thickener is sodium carboxymethyl cellulose, the binder is polyacrylate, and the solvent is water.

[0061] The preparation method of the negative electrode material in the embodiment is:

[0062] S01: adding single-walled carbon nanotubes and a dispersant to a solvent according to the raw material component formula, and dispersing and mixing at high speed to obtain a single-walled carbon nanotube dispersion; wherein the mass of the dispersant is 1wt% of the total mass of the single-walled carbon nanotube dispersion, and the mass of the single-walled carbon nanotubes is 1.8wt% of the total mass of the single-walled carbon nanotube dispersion;

[0063] S02: According to the raw material component formula, large-scale expanded graphite is physically exfoliated in a solvent to obtain a graphene nanoribbon solution, a dispersant is added to the obtained graphene nanoribbon solution, and the solution is diluted and dispersed and mixed to obtain a graphene nanoribbon dispersion (i.e., a two-dimensional carbon material conductive agent dispersion); wherein the mass of the dispersant is 2wt% of the total mass of the graphene nanoribbon dispersion, and the mass of the graphene nanoribbons is 1wt% of the total mass of the graphene nanoribbon dispersion;

[0064] S03: adding a thickener to the remaining solvent and stirring for 1 hour to obtain a thickener glue solution; adding the single-walled carbon nanotube dispersion and graphene nanoribbon dispersion prepared in S01 and S02 to the thickener glue solution and mixing evenly to obtain a conductive glue;

[0065] S04: adding super conductive carbon black and few-walled or multi-walled carbon nanotubes to the negative electrode material, mixing and stirring uniformly to obtain a negative electrode dry powder;

[0066] S05: The conductive glue obtained in S03 is added to the negative electrode dry powder prepared in S04 in batches, and finally the binder is added. After stirring evenly, vacuum defoaming treatment is performed for 4 hours to obtain the desired lithium-ion battery negative electrode material.

[0067] 2. Preparation of lithium-ion battery negative electrode

[0068] The selection of negative electrode current collector (foil) is as shown in the table above. The prepared negative electrode material is coated on the current collector and then baked. The baking zone length is 12 meters, and the temperatures of the front, middle and rear baking zones are 80°C, 85°C and 90°C respectively. The coating conveying speed is 10 meters per minute.

[0069] 3. Preparation of lithium-ion batteries

[0070] The positive electrode materials, lithium nickel cobalt aluminum oxide (NCA), Ketjenblack, and polyvinylidene fluoride (PVdF), were dissolved in N-methylpyrrolidone (NMP) at a ratio of 100:1.0:1.6 to form a slurry, which was then coated on aluminum foil to form a positive electrode sheet. The positive electrode sheet was combined with the negative electrode sheets in this embodiment and the comparative example to form an 18650 cylindrical lithium-ion battery and a 2032 button battery for electrochemical performance testing.

[0071] 4. Test and test results

[0072] (1) The conductive adhesive in Example 3 was tested by scanning electron microscopy. The results are shown in the attached figure. Figure 1 As shown, the scanning electron microscope used is a Japanese JSM-6380 scanning electron microscope.

[0073] As shown in the figure is a SEM image of a composite conductive adhesive of single-walled carbon nanotubes and graphene nanoribbons. It can be seen that the curvature of the single-walled carbon nanotubes is large, there is no agglomeration, and the graphene sheets are spread out in ribbons and are not clustered. The composite conductive adhesive is well dispersed, and the two types of conductive agents are evenly dispersed. Because the percolation threshold of single-walled carbon nanotubes and graphene nanoribbons can reach an extremely low level, the single-walled carbon nanotube and graphene nanoribbon composite conductive adhesive is added in an extremely low amount in the negative electrode, making it difficult to identify in the electrode SEM electron microscope. Therefore, there is no need to perform SEM scanning observations on other embodiments one by one. Using extremely low amounts of single-walled carbon nanotubes and graphene nanoribbons to replace traditional conductive agents can build a more complete conductive network. The completeness of the conductive network directly affects the performance of the battery, including DC internal resistance performance, cycle performance, etc.

[0074] (2) DC internal resistance test

[0075] Figure 2 and Figure 3 This is the DC internal resistance test process for 18650 cylindrical lithium-ion batteries in the full SOC range in this embodiment. Figure 2 is the current-time diagram, Figure 3 is the voltage-time diagram;

[0076] Test steps: 1) 0.5C constant current and constant voltage charge to 100% SOC and then stand for 1 hour; 2) 1C discharge for 6 minutes to reach 90% SOC state point; 3) stand for 1 hour; 4) 5C discharge for 10 seconds, 4) stand for 5 minutes, 5) 5C charge for 10 seconds, 2) 1C discharge for 6 minutes to reach 80% SOC state point; 2) stand for 1 hour; 3) 5C discharge for 10 seconds, 4) stand for 5 minutes, 5) 5C charge for 10 seconds, repeat the above steps to complete the pulse discharge at each interval of 10% SOC state until the battery state of charge is 10% SOC.

[0077] The test material is the 18650 cylindrical lithium-ion battery prepared above.

[0078] (3) Normal temperature cycle test

[0079] Test method: Examples 1-3 and Comparative Examples 1-2 were charged at a constant current and constant voltage of 0.5C at a constant temperature of 25°C until the voltage reached 4.2V, and then discharged at a current of 1C until the voltage reached 2.75V. The above charge and discharge steps were repeated 1000 times to obtain the capacity retention rate after 1000 cycles at room temperature.

[0080] The test material is the 18650 cylindrical lithium-ion battery prepared above.

[0081] (4) AC impedance test

[0082] Alternating current impedance (AC), also known as electrochemical impedance spectroscopy (EIS), is a method of applying a small-amplitude alternating current (usually a sinusoidal wave) voltage (or current) as a disturbance signal to an electrode system. After reaching a stable n-state, the impedance spectrum is obtained from the relationship between the response of the electrode system and the disturbance signal. Since small-amplitude alternating signals basically do not change the state of the measured system, this method can accurately and non-destructively study the relationship between the kinetic parameters of each electrode process and the electrode state. The smaller the arc fitting radius in the EIS spectrum, the smaller the resistance to electron and ion transmission in the electrode, which is more conducive to improving the speed and extent of electrochemical reactions in the battery electrode.

[0083] The test material is the 2032 button battery prepared above.

[0084] The results of the DC internal resistance test, room temperature cycle test, and AC impedance test are:

[0085]

[0086] The test results show that the group using the low-dimensional carbon material conductive agent in this embodiment significantly reduces the DC internal resistance across the entire SOC range. Taking Examples 1-3 as an example, compared to the lithium-ion batteries assembled using the super conductive carbon black in Comparative Example 1 and the multi-walled carbon nanotube conductive agent in Comparative Example 2, the groups using the low-dimensional carbon material conductive agent in Examples 1, 2, and 3 significantly reduce the DC internal resistance across the entire SOC range. The single-walled carbon nanotube conductive agent in Example 1 and the graphene nanoribbons in Example 2 leverage their high aspect ratio to form an electron transport network with a larger coverage area. The graphene used in this invention exhibits a nanoribbon structure, avoiding the shortcomings of traditional sheet-like graphene encapsulating active particles and preventing ion transport channels from being blocked. The lithium-ion battery prepared in Example 3 using a one- and two-dimensional carbon material composite conductive agent (i.e., a single-walled carbon nanotube-graphene nanoribbon composite conductive agent) exhibits the lowest DC internal resistance. Single-walled carbon nanotubes are only a few microns long, but graphene nanoribbon conductive agents often contain ribbons of graphene over ten microns long, thanks to the mechanical exfoliation process used to prepare the graphene nanoribbons. When designing composite conductive agents for silicon anode systems, the goal is to use single-walled carbon nanotubes and graphene nanoribbon conductive agents together when silicon anode particles expand and segregate. This approach compensates for the weakness of single-walled carbon nanotubes, which are only a few microns long, in creating a sufficiently long conductive path, and further increases the active material content.

[0087] The lithium-ion battery in this embodiment achieved a cycle performance of 82%. This demonstrates that the lithium-ion battery negative electrode material in this embodiment effectively and rapidly improved the electrical contact between the active particles in the silicon negative electrode after expansion, significantly enhancing the battery's cycle performance. Furthermore, the use of the lithium-ion battery negative electrode material in this embodiment effectively reduced charge transfer impedance.

[0088] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the embodiments of the present invention rather than to limit them. Although the embodiments of the present invention are described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the embodiments of the present invention can still be modified or replaced by equivalents, and these modifications or equivalent replacements cannot cause the modified technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A negative electrode material for a lithium-ion battery, characterized in that: The components and their mass parts are as follows: Active material: 88 - 105 parts Single-walled carbon nanotubes: 0.01% - 0.1 part Graphene nanoribbons: 0.01% - 0.5 part Superconductive carbon black: less than 1 part Few-walled or multi-walled carbon nanotubes: less than 1 part Dispersant: 0.05 - 0.5 part Thickener: 1 - 3 parts Binder: 1 - 4 parts Solvent: 25 - 60 parts The active material includes silicon oxide material SiOx and artificial graphite, where 0 < x < 2, and the mass percentage of silicon element in the negative electrode material is less than or equal to 3.5%; the diameters of the single-walled carbon nanotubes, few-walled or multi-walled carbon nanotubes are 1 - 100 nm, the specific surface area is 200 - 2000 m2 / g, the G / D ratio is 50 - 300, the aspect ratio is 50 - 2800, and the metal impurity content is less than 5 ppm; The number of layers of the graphene nanoribbons is less than 8 layers, the thickness is less than 2.35 nm, the specific surface area is 2000 - 3000 m2 / g, the G / D ratio is 150 - 300, the monolayer rate is greater than 90%, and the aspect ratio is 1500 - 2500; The viscosity of the negative electrode material of the lithium-ion battery is 2000 - 4500 mPa·s; The negative electrode material of the lithium-ion battery is coated on the surface of the negative electrode current collector as an active coating. The negative electrode current collector is a conductive carbon-coated copper foil, and the conductive carbon-coated copper foil is a copper foil coated with conductive layers on both sides. The conductive layers on both sides of the copper foil coated with carbon are single-walled carbon nanotube coatings with a thickness of 20 nm - 2 μm, and the diameter of the single-walled carbon nanotubes is 1 - 3 nm.

2. The negative electrode material for a lithium-ion battery according to claim 1, wherein: The mass percentage of silicon element in the active material is 2.5% - 3.5%; the specific surface area of the single-walled carbon nanotubes, few-walled or multi-walled carbon nanotubes is 1500 - 2000 m2 / g, the G / D ratio range is 100 - 300, and the aspect ratio is 20OO - 2800.

3. The negative electrode material for a lithium-ion battery according to claim 1, wherein: The dispersant is polyvinylpyrrolidone with a molecular weight of 8000 - 700000, the thickener is sodium carboxymethylcellulose, the binder is polyacrylate, and the solvent is one of N-methylpyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide or water.

4. A method for preparing a negative electrode material for a lithium-ion battery according to any one of claims 1 to 3, characterized in that It includes the following steps: S01: Add single-walled carbon nanotubes and a dispersant to the solvent according to the raw material formula, and obtain a single-walled carbon nanotube dispersion liquid after high-speed dispersion and mixing evenly; among them, the mass of the dispersant is 0.2 - 1 wt% of the total mass of the single-walled carbon nanotube dispersion liquid, and the mass of the single-walled carbon nanotubes is 0.2 - 2 wt% of the total mass of the single-walled carbon nanotube dispersion liquid; S02: Physically exfoliate large expanded graphite in the solvent to obtain a graphene nanoribbon solution according to the raw material formula, add a dispersant to the graphene nanoribbon solution and dilute the solution, and obtain a graphene nanoribbon dispersion liquid (i.e., a two-dimensional carbon material conductive agent dispersion liquid) after dispersion and mixing evenly; among them, the mass of the dispersant is 0.5 - 3 wt% of the total mass of the graphene nanoribbon dispersion liquid, and the mass of the graphene nanoribbons is 0.2 - 2 wt% of the total mass of the graphene nanoribbon dispersion liquid; S03: adding a thickener to the remaining solvent and stirring for 0.5-1.5 hours to obtain a thickener glue solution; adding the single-walled carbon nanotube dispersion and graphene nanoribbon dispersion prepared in S01 and S02 to the thickener glue solution and mixing evenly to obtain a conductive glue; S04: adding super conductive carbon black and few-walled or multi-walled carbon nanotubes to the negative electrode material, mixing and stirring uniformly to obtain a negative electrode dry powder; S05: The conductive glue obtained in S03 is added to the negative electrode dry powder prepared in S04 in batches, and finally the binder is added. After stirring evenly, vacuum defoaming treatment is performed for 4-8 hours to obtain the desired lithium-ion battery negative electrode material.

5. A lithium-ion battery negative electrode plate, comprising a negative electrode current collector and an active coating applied to the surface of the negative electrode current collector, characterized in that: The active coating is the lithium-ion battery negative electrode material according to any one of claims 1 to 3.

6. The negative electrode plate for a lithium-ion battery according to claim 5, wherein: The negative electrode current collector is a conductive carbon-coated copper foil, which includes a copper foil and a conductive layer coated on the surface of the copper foil. The thickness of the copper foil is 8-12 μm, and the single-sided coating thickness of the conductive layer is 20 nm-2 μm; the surface density of the conductive carbon-coated copper foil is 70-95 g / m2.

7. The negative electrode plate for a lithium-ion battery according to claim 6, wherein: The conductive layer material is one or a combination of super conductive carbon black, conductive graphite microsheets, carbon nanotubes, few-layer graphene nanoribbons, Ketjen black, and acetylene black.

8. A lithium-ion battery comprising a positive electrode sheet, a negative electrode sheet, a battery separator and an electrolyte, characterized in that: The negative electrode plate is the negative electrode plate for a lithium-ion battery as claimed in any one of claims 5 to 7.

9. The lithium-ion battery according to claim 8, wherein: The DC resistance of the lithium-ion full battery is less than 32 mΩ in the range of 30% to 80% SOC; the room temperature cycle performance has a cycle retention rate of more than 82% after 1000 cycles; and the charge transfer impedance is less than 160 Ω.

Images