Flexible composite negative electrode material and preparation method and application thereof

Abstract

The invention belongs to the technical field of preparation of negative electrode materials, and particularly relates to a flexible composite negative electrode material and a preparation method and application thereof. The flexible composite negative electrode material comprises three-dimensional foam and an active layer loaded on the three-dimensional foam, wherein the active layer comprises a silicon-based material layer, and a magnetic layer and a carbon nanotube which sequentially coat the surface of the silicon-based material layer; wherein in-situ growth of the carbon nanotubes is realized by performing heat treatment on a carbon source in an inert atmosphere, and a magnetic field is applied in the heat treatment process, so that directional arrangement of the carbon nanotubes is realized. The flexible three-dimensional network structure formed by the foam not only can be used as an independent current collector, but also has large gaps, and can be used as a ductile main body matrix, so that the flexible three-dimensional network structure adapts to the volume change of the silicon nanoparticles in the charge-discharge cycle process. The lithium ion transmission efficiency and the electrode stability are remarkably improved through in-situ oriented growth of carbon nanotubes, optimization of a three-dimensional conductive network and combination of doping regulation and control of the interfacial effect.

Description

Technical Field

[0001] The present invention belongs to the technical field of negative electrode material preparation, and in particular relates to a flexible composite negative electrode material and a preparation method and application thereof. Background Art

[0002] In recent years, the continuous development of power tools, mobile devices, and electric vehicles has placed higher demands on lithium-ion batteries in terms of energy density, charge and discharge rates, and cycle life. While traditional graphite anodes are widely used, their theoretical specific capacity is only 372 mAh / g, approaching the upper limit. Furthermore, they suffer from significant initial irreversible capacity loss and insufficient high-temperature stability, making them difficult to meet the demands of the next generation of high-performance energy storage systems.

[0003] Seeking breakthroughs, researchers have turned to exploring anode materials with higher theoretical capacities and more stable cycling performance. In recent years, foam materials with three-dimensional porous structures have been increasingly applied in new energy and environmental protection fields, moving beyond traditional packaging, insulation, and sound absorption applications due to their low density, high porosity, and excellent mechanical and thermal insulation properties. When used as electrode supports, foam materials can form a porous framework, mitigating structural damage caused by volume changes in the active material during charge and discharge, while also providing more effective interfaces for electrode reactions.

[0004] Among numerous foam materials, three-dimensional nickel foam is considered an ideal substrate for constructing composite anode structures due to its excellent conductivity and mechanical stability. Its unique three-dimensional network not only uniformly disperses and fixes active materials such as nano-silicon or tin-based alloys, but also accommodates over 300% volume expansion during lithiation, effectively reducing local stress concentration and the risk of electrode pulverization. Furthermore, the continuous metal skeleton forms an efficient conductive pathway, helping to reduce charge transfer impedance and improve overall electrochemical performance. The structure of nickel foam provides a buffer for active materials during charge and discharge cycles, slowing capacity decay and thus supporting the achievement of high energy density and long cycle life.

[0005] However, despite the significant advantages of three-dimensional nickel foam in improving the structural stability and electrochemical performance of negative electrodes, its industrial application still faces challenges such as complex preparation processes, high costs, and incompatibility with other battery components. Therefore, the development of a low-cost, high-performance, three-dimensional composite negative electrode material suitable for large-scale production has become an important issue in the current energy storage field, pointing the way to the realization of next-generation high-energy-density lithium-ion batteries. Summary of the invention

[0006] The purpose of the present invention is to provide a flexible composite negative electrode material and its preparation method and application, which can effectively improve the coulombic efficiency, cycle stability and energy density.

[0007] To achieve the above object, the present invention provides a flexible composite anode material, comprising a three-dimensional foam and an active layer supported on the three-dimensional foam. The active layer includes a silicon-based material layer, and a magnetic layer and carbon nanotubes sequentially coated on the surface of the silicon-based material layer. Among them, the carbon nanotubes are in-situ grown by heat-treating a carbon source in an inert atmosphere, and a magnetic field is applied during the heat treatment to enable the carbon nanotubes to be aligned directionally. The intensity of the magnetic field is 0.5-1.2T. The silicon-based material layer accounts for 20%-50% of the mass of the flexible composite anode material. The temperature of the heat treatment is 600°C to 900°C.

[0008] The geometric mass center of the carbon nanotubes shows an oriented arrangement in the vertical direction and preferably contains nitrogen or boron elements.

[0009] Further, the silicon-based material layer is nano-silicon or nano-silicon monoxide with a particle size of 20-200nm.

[0010] And / or, the magnetic layer contains one or more elements of iron, cobalt, and nickel.

[0011] And / or, the carbon source is one or more of melamine, cyanuric acid, dopamine hydrochloride, chitosan, and polyaniline.

[0012] Further, there is also a carbon layer between the silicon-based material layer and the magnetic layer.

[0013] Further, the magnetic layer also contains carbon. The magnetic layer is formed by a metal-organic framework material during the heat treatment process.

[0014] Further, the three-dimensional foam is formed by melamine foam during the heat treatment process. The three-dimensional foam has a mutually interconnected pore structure with a pore diameter of 10-200μm.

[0015] Further, the mass content of the active layer in the flexible composite anode material is 10%-90%. The silicon-based material layer accounts for 70-99% of the mass of the active layer.

[0016] The mass content of the carbon nanotubes in the flexible composite anode material is 2%-5%.

[0017] Further, the carbon nanotubes have a diameter of 1 to 5 nm and an aspect ratio of (2 - 70):1; the carbon nanotubes have a length of 1 - 5 mm and exhibit a specific spatial arrangement in situ, and the spatial arrangement has a strong orientation in the vertical direction. For example, the length can be 1 mm, 2 mm, … or a range composed of any two of the above values, and the orientation includes from small to large or from large to small. The carbon nanotubes are multi-walled carbon nanotubes, single-walled carbon nanotubes, carbon nanofibers, and / or mixtures thereof. The content of nitrogen or boron elements in the carbon nanotubes is 1% - 55%.

[0018] Further, the heat treatment time is 1 - 3 hours.

[0019] The present invention also provides a method for preparing a flexible composite negative electrode material, comprising the following steps:

[0020] S1. Prepare a mixed solution containing a silicon-based material, a metal salt, and an organic ligand;

[0021] S2. Immerse and adsorb the foam carrier in the mixed solution, and then take it out and freeze it to obtain a pretreated foam carrier;

[0022] S3. Heat-treat the pretreated foam carrier and a carbon source under an inert gas while applying a magnetic field to obtain a carbon foam with a silicon-carbon negative electrode material attached to the surface, and aligned carbon nanotubes grow on the surface of the silicon-carbon negative electrode material; wherein, the intensity of the magnetic field is 0.5 - 1.2 T; the mass content of the silicon-based material in the carbon foam with the silicon-carbon negative electrode material attached to the surface is 20% - 50%; the heat treatment temperature is 60°C to 850°C.

[0023] Further, the metal salt is one or more of cobalt salts, nickel salts, and iron salts; the organic ligand is dimethylimidazole or dicyandiamide; the foam carrier is melamine foam.

[0024] Further, the mass ratio of the silicon-based material to the organic ligand is (10 - 20):1, and the molar ratio of the organic ligand to the metal salt is (5 - 8):1; the concentration of the silicon-based material in the mixed solution is 50 - 120 g / L.

[0025] Further, the volume-mass ratio of the melamine foam to the silicon-based material is 1 cm 3 :(0.1 - 3.5) g; preferably 1 cm 3 :(0.1 - 2) g.

[0026] Further, the immersion and adsorption time is 6 - 36 h; the heat treatment holding time is 1 - 3 hours.

[0027] The obtained flexible composite negative electrode material is carbon foam with silicon-carbon negative electrode material containing carbon nanotubes attached to its surface. The mass percentage of cobalt element in the active material (silicon-carbon negative electrode material containing carbon nanotubes) is 0.001% to 2.3%, and the nitrogen element content in the carbon nanotubes is 0.1% to 43%.

[0028] The present invention also provides an application of the flexible composite negative electrode material described in any one of the above, or the flexible composite negative electrode material obtained by the preparation method described in any one of the above in a lithium-ion battery. The lithium-ion battery includes a positive electrode plate, an electrolyte, a separator, and the flexible composite negative electrode material described in any one of the above.

[0029] The separator is disposed between the positive electrode plate and the negative electrode, and the electrolyte is filled between the positive electrode plate and the negative electrode and infiltrates the separator.

[0030] Optionally, the separator includes at least one of polyethylene, polypropylene, ceramic-coated separator, cellulose separator, polyimide, sodium polystyrene sulfonate, polyvinylidene fluoride, polytetrafluoroethylene, etc.

[0031] Optionally, the electrolyte includes at least one of lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide, silyl lithium salt, ethylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, silicon-based electrolyte additive, etc.

[0032] The positive electrode plate includes a positive electrode current collector and a positive electrode active material layer disposed on at least one surface thereof. For example, the positive electrode current collector has two opposite surfaces in its own thickness direction, and the positive electrode active material layer can be laminated on either one of the two surfaces of the positive electrode current collector, or laminated on both surfaces.

[0033] The positive electrode active material layer includes a positive electrode active material, a conductive agent, a binder, and a positive electrode current collector.

[0034] Optionally, the positive electrode active material includes lithium nickel oxide, lithium nickel cobalt aluminum oxide, lithium manganese oxide, lithium cobalt oxide, etc., and other traditional and recognized positive electrode active materials can also be selected, including LiCoO2, LiNiO2, LiMnO2, LiMn2O4, LiFePO4, LiMnPO4, LiNi 0.8 Fe 0.1 Mn 0.1 O2, lithium-rich manganese-based oxide, cobalt-free nickel-based oxide, etc.

[0035] Optionally, the conductive agent includes at least one of carbon black, conductive graphite, carbon nanotubes, graphene, reduced graphene oxide, liquid metal, conductive carbon fiber, graphene nanosheet, etc.

[0036] Optionally, the binder includes at least one of polyvinylidene fluoride, carboxymethyl cellulose, styrene-butadiene rubber, polyacrylic acid, polytetrafluoroethylene, polymethyl methacrylate, polyvinyl alcohol, sodium alginate, β-cyclodextrin polymer, polypropylene emulsion, etc.

[0037] Optionally, the positive current collector includes copper foil, aluminum foil, stainless steel current collector, titanium current collector, etc.

[0038] Generally speaking, compared with the prior art, the above technical solutions conceived by the present invention mainly have the following technical advantages:

[0039] 1. The flexible composite negative electrode material provided by the present invention includes a three-dimensional foam and an active layer supported on the three-dimensional foam. The flexible three-dimensional network structure formed by the foam can not only serve as an independent current collector but also has large voids, which can act as a ductile main matrix, thus adapting to the volume change of silicon nanoparticles during the charge and discharge cycle. By inducing the carbon nanotubes to arrange in a vertical direction gradient with a magnetic field, the three-dimensional conductive network can be optimized, significantly improving the lithium-ion transport efficiency and electrode stability.

[0040] 2. The present invention combines nitrogen doping to regulate the interface effect, further improving the lithium-ion transport efficiency and electrode stability. The lithium battery based on this negative electrode achieves breakthroughs in energy density, rate performance, and cycle life, with excellent capacity retention at high rates and excellent long-cycle stability. The flexible three-dimensional network effectively buffers the volume change of the silicon-based material, significantly reducing the capacity attenuation. This breakthrough design opens up a new path for the research and development of high-performance lithium battery negative electrode materials by enhancing conductivity and ion diffusion ability.

[0041] 3. By optimizing the loading amount of nanosilicon on the three-dimensional foam and regulating the three-dimensional conductive network with a hierarchical porous structure, the flexible negative electrode material of the present invention has better cycle stability and rate performance. Description of the Drawings

[0042] Figure 1 is the impedance test chart of the carbon foam negative electrode with in-situ grown CNTs without magnetic field and the carbon foam negative electrode with in-situ grown CNTs with magnetic field during button cell charging and discharging in Example 1.

[0043] Figure 2 is the rate test chart of the carbon foam negative electrode with in-situ grown CNTs without magnetic field and the carbon foam negative electrode with in-situ grown CNTs with magnetic field during button cell charging and discharging in Example 1. Detailed Embodiments

[0044] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention will be further described in detail below in conjunction with embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0045] For those not specifying specific experimental steps or conditions in the embodiments, the operations or conditions of the conventional experimental steps described in the literature in this field can be followed. For reagents or instruments without indicating the manufacturer, they are all conventional reagent products that can be obtained through commercial purchase.

[0046] Example 1

[0047] This example provides a negative electrode plate, which includes a negative electrode current collector and a negative electrode active layer provided on one side of the negative electrode current collector.

[0048] The preparation method of the negative electrode plate is as follows:

[0049] (1) 0.02 g of polyvinylpyrrolidone and 5 g of nano-silicon (100 nm) are placed in 50 mL of deionized water, and left to stand and disperse evenly to obtain a first solution;

[0050] (2) 0.325 g of dimethylimidazole is added to the first solution obtained in operation (1), and after stirring, a second solution is obtained;

[0051] (3) 0.145 g of cobalt (II) nitrate hexahydrate is added to the second solution obtained in operation (2), and after stirring for 3 min, a third solution is obtained;

[0052] (4) The melamine foam (3 cm × 3 cm × 0.5 cm) is immersed in the third solution. After being completely immersed and the foam has absorbed evenly and completely, it is left to stand and react for 24 hours to obtain a reacted melamine foam A;

[0053] (5) The foam A is taken out and placed in a clean beaker, and liquid nitrogen is poured into the beaker to quickly freeze the foam A to obtain foam B;

[0054] (6) The foam B and 0.3 g of melamine are placed in a porcelain boat, and the porcelain boat is placed in a tube furnace. In an argon atmosphere, it is heated to 800 °C at a heating rate of 5 °C / min and held for 2 h. A magnetic field with a magnitude of 0.8 T acts vertically on the foam B, and it is heated to a high temperature for carbonization at a certain heating rate. After holding for a period of time and the holding ends, it is cooled to room temperature in an argon atmosphere and then taken out to obtain a carbon foam (Carbon Foam@Si@C@CNT) with a carbon nanotube silicon-carbon negative electrode material attached to the surface, and the carbon nanotubes contain nitrogen elements.

[0055] The negative electrode material provided in this embodiment includes a foam with a pore diameter of 40 μm, silicon with a particle size of 100 nm, and a carbon coating layer with a thickness of 7 nm obtained by heat treatment. After calculation, the mass of silicon element in the composite carbon foam accounts for 33% of the total composite material. The diameter of the carbon nanotube layer is 1 - 5 nm, and the aspect ratio is 2:1 - 120:1.

[0056] Example 2

[0057] The negative electrode material was prepared according to the method of Example 1, except that in operation (4) of this example, the static reaction was carried out for 12 hours.

[0058] The negative electrode material of this embodiment includes a foam with a pore diameter of 40 μm, silicon with a particle size of 100 nm, and a carbon coating layer with a thickness of 5 nm obtained by heat treatment. After calculation, the mass of silicon element in the composite carbon foam accounts for 35% of the total composite material. The diameter of the carbon nanotube layer is 1 - 3 nm, and the aspect ratio is 8:1 - 80:1.

[0059] Example 3

[0060] The negative electrode material was prepared according to the method of Example 1, except that in operation (4) of this example, the static reaction was carried out for 20 hours.

[0061] The negative electrode material of this embodiment includes a foam with a pore diameter of 40 μm, silicon with a particle size of 100 nm, and a carbon coating layer with a thickness of 6 nm obtained by heat treatment. After calculation, the mass of silicon element in the composite carbon foam accounts for 33% of the total composite material. The diameter of the carbon nanotube layer is 1 - 3 nm, and the aspect ratio is 12:1 - 92:1.

[0062] Example 4

[0063] The negative electrode material was prepared according to the method of Example 1, except that in operation (4) of this example, the static reaction was carried out for 28 hours.

[0064] The negative electrode material of this embodiment includes a foam with a pore diameter of 40 μm, silicon with a particle size of 100 nm, and a carbon coating layer with a thickness of 13 nm obtained by heat treatment. After calculation, the mass of silicon element in the composite carbon foam accounts for 33% of the total composite material. The diameter of the carbon nanotube layer is 1 - 3 nm, and the aspect ratio is 11:1 - 86:1.

[0065] Example 5

[0066] The negative electrode material was prepared according to the method of Example 1, except that in operation (6) of this example, the heat preservation temperature was 900 °C.

[0067] In the negative electrode material of this embodiment, it includes a foam with a pore diameter of 40 μm, silicon with a particle size of 100 nm, and the thickness of the carbon coating layer obtained by heat treatment is 8 nm. After calculation, the mass of silicon element in the composite carbon foam accounts for 36% of the total composite material, the tube diameter of the carbon nanotube layer is 1 - 3 nm, and the aspect ratio is 2:1 - 109:1.

[0068] Comparative Example 1

[0069] The negative electrode material was prepared according to the method of Example 1. The difference is that in operation (6) of this embodiment, the magnetic field strength was set to 0 T, that is, no magnetic field effect was set.

[0070] In the negative electrode material of this embodiment, it includes a foam with a pore diameter of 40 μm, silicon with a particle size of 100 nm, and the thickness of the carbon coating layer obtained by heat treatment is 10 nm. After calculation, the mass of silicon element in the composite carbon foam accounts for 35% of the total composite material, the tube diameter of the carbon nanotube layer is 1 - 3 nm, and the aspect ratio is 3:1 - 60:1.

[0071] Comparative Example 2

[0072] The negative electrode material was prepared according to the method of Example 1. The difference is that in operation (6) of this embodiment, the magnetic field strength was set to 0.4 T.

[0073] In the negative electrode material of this embodiment, it includes a foam with a pore diameter of 40 μm, silicon with a particle size of 100 nm, and the thickness of the carbon coating layer obtained by heat treatment is 8 nm. After calculation, the mass of silicon element in the composite carbon foam accounts for 33% of the total composite material, the tube diameter of the carbon nanotube layer is 1 - 3 nm, and the aspect ratio is 2:1 - 133:1.

[0074] Comparative Example 3

[0075] The negative electrode material was prepared according to the method of Example 1. The difference is that in operation (6) of this embodiment, the magnetic field strength was set to 1.3 T.

[0076] In the negative electrode material of this embodiment, it includes a foam with a pore diameter of 40 μm, silicon with a particle size of 100 nm, and the thickness of the carbon coating layer obtained by heat treatment is 8 nm. After calculation, the mass of silicon element in the composite carbon foam accounts for 33% of the total composite material, the tube diameter of the carbon nanotube layer is 1 - 3 nm, and the aspect ratio is 2:1 - 108:1.

[0077] Comparative Example 4

[0078] The negative electrode material was prepared according to the method of Example 1. The difference is that in operation (6) of this embodiment, the magnetic field strength was set to 1.8 T.

[0079] In the negative electrode material of this embodiment, it includes a foam with a pore size of 40 μm, silicon with a particle size of 100 nm, and the thickness of the carbon coating layer obtained by heat treatment is 7 nm. After calculation, the mass of silicon element in the composite carbon foam accounts for 32% of the total composite material, the diameter of the carbon nanotube layer is 1 - 3 nm, and the aspect ratio is 2:1 - 145:1.

[0080] Comparative Example 5

[0081] The negative electrode material was prepared according to the method of Example 1, except that in operation (1) of this embodiment, 10 g of nano - silicon (100 nm) was added.

[0082] In the negative electrode material of this embodiment, it includes a foam with a pore size of 40 μm, silicon with a particle size of 100 nm, and the thickness of the carbon coating layer obtained by heat treatment is 7 nm. After calculation, the mass of silicon element in the composite carbon foam accounts for 59% of the total composite material, the diameter of the carbon nanotube layer is 1 - 3 nm, and the aspect ratio is 4:1 - 42:1.

[0083] Comparative Example 6

[0084] The negative electrode material was prepared according to the method of Example 1, except that in operation (1) of this embodiment, 15 g of nano - silicon (100 nm) was added.

[0085] In the negative electrode material of this embodiment, it includes a foam with a pore size of 40 μm, silicon with a particle size of 100 nm, and the thickness of the carbon coating layer obtained by heat treatment is 8 nm. After calculation, the mass of silicon element in the composite carbon foam accounts for 79% of the total composite material, the diameter of the carbon nanotube layer is 1 - 3 nm, and the aspect ratio is 4:1 - 47:1.

[0086] Comparative Example 7

[0087] The negative electrode material was prepared according to the method of Example 1, except that in operation (6) of this embodiment, the heat - preservation temperature was 550 °C.

[0088] In the negative electrode material of this embodiment, it includes a foam with a pore size of 40 μm, silicon with a particle size of 100 nm, and the thickness of the carbon coating layer obtained by heat treatment is 8 nm. After calculation, the mass of silicon element in the composite carbon foam accounts for 28% of the total composite material, the diameter of the carbon nanotube layer is 1 - 3 nm, and the aspect ratio is 3:1 - 63:1.

[0089] Experimental Example

[0090] Button cells and CR2032 and 3 Ah soft - pack batteries were respectively prepared using the negative electrode materials of Examples 1 - 5 and the negative electrode materials of Comparative Examples 1 - 7, and the electrical properties of each battery were detected.

[0091] Among them, the composition of the button cell is as follows:

[0092] Negative electrode: Directly use the obtained carbon foam (Carbon Foam@Si@C@CNT) containing the silicon-carbon negative electrode material with carbon nanotubes as the negative electrode material.

[0093] Positive electrode: Lithium metal.

[0094] Electrolyte: 1.2 mol / L LiPF6, and the solvent is EC / DMC with a molar ratio of 1:1 plus 10 wt% FEC (EC: ethylene carbonate, DMC: dimethyl carbonate, FEC: fluoroethylene carbonate).

[0095] Separator: Celgard 2500.

[0096] The composition of the soft-pack battery is as follows:

[0097] Negative electrode: Directly use the obtained carbon foam (Carbon Foam@Si@C@CNT) containing the silicon-carbon negative electrode material with carbon nanotubes as the negative electrode material.

[0098] Positive electrode: Use lithium nickel cobalt manganese oxide NCM811 as the positive electrode material, prepare a slurry according to the weight ratio of the positive electrode material, conductive agent SP and PVDF of 97:2:1, and coat it on the aluminum foil current collector to prepare the positive electrode.

[0099] Electrolyte: 1.2 mol / L LiPF6, and the solvent is EC / DMC with a molar ratio of 1:1 plus 10 wt% FEC (EC: ethylene carbonate, DMC: dimethyl carbonate, FEC: fluoroethylene carbonate).

[0100] Separator: Celgard 2500.

[0101] The method for testing the electrical performance is as follows:

[0102] (1) First-cycle negative electrode specific capacity utilization test: First-cycle discharge capacity mAh / mass of negative electrode active material g;

[0103] (2) First-cycle Coulombic efficiency: First-cycle charge capacity / first-cycle discharge capacity * 100%;

[0104] (3) First-cycle charge specific capacity: First-cycle charge capacity / mass of active material (mAh / g);

[0105] (4) 4C rate discharge capacity retention test: 4C discharge capacity / 1C discharge capacity;

[0106] (5) DC internal resistance DCR test: Divide the battery capacity and adjust it to 50% SOC, discharge at 5C for 10s, test the discharge resistance, and the resistance DCR = (V0 - V10) / I, where V0 is the potential before discharge, V10 is the potential at the 10th second of discharge, and I is the discharge current of 5C;

[0107] (6) Capacity retention rate test: ① Charging: Constant current charging at a current density of 1C until 4.2V and then standing for 10 min; ② Discharging: Constant current discharging at a current density of 1C until 2.5V, standing for 0 min, and the discharge capacity is recorded as Qn (n = 1, 2, 3... 400); ③ Repeat "①, ②" 400 cycles; The capacity retention rate of the soft-pack battery after 400 cycles is: Q400 / Q1;

[0108] (7) Volume expansion rate test: After 400 cycles, fully charged and disassembled, the thickness measured by a micrometer is d2, and the thickness of the fresh electrode roller-pressed is d1. The calculation of the 400-cycle full-charge expansion rate of the soft-pack battery is: (d2 - d1) / (d1 - 8), and the thickness of the copper foil is 8μm;

[0109] (8) Lithium deposition test: ① Charging: Constant current charging at a current density of 2.4C until 4.2V and then standing for 10 min; ② Discharging: Constant current discharging at a current density of 1C until 2.5V and then standing for 10 min; After 10 cycles, charge to 4.2V at a constant current density of 1.6C for full charge and disassemble to observe the negative electrode interface.

[0110] (9) Electrochemical impedance spectroscopy (EIS) test: The EIS measurement is carried out before the charge-discharge cycle, and the frequency range is from 1 MHz to 0.01 Hz.

[0111] The test results are shown in Table 1 and Table 2.

[0112] Table 1 Electrical performance detection results of each button cell

[0113]

[0114] It can be seen from Table 1 that for the negative electrode material of the CNT, C, and carbon foam composite conductive network generated under the magnetic field in the present invention, the first-cycle negative electrode specific capacity and Coulomb efficiency of the obtained button cell have higher capacity and first Coulomb efficiency compared with the negative electrode material of the CNT, C, and carbon foam composite conductive network generated without magnetic field in Comparative Example 1. Among them, Examples 2-6 show that the standing reaction time and heat treatment temperature also have a certain influence on the effect. However, under the same conditions, the effect of magnetic field treatment is better than that without magnetic field treatment, indicating that under the present invention's scheme, the directional arrangement of carbon nanotubes by magnetic field treatment has a promoting effect on the electrical performance of the negative electrode material.

[0115] Table 2 Electrical performance detection results of each soft-pack battery

[0116]

[0117] As can be seen from Table 2, too high heat treatment temperature will affect the aspect ratio after CNT growth, thus affecting the comprehensive effect of its conductive network. The overall conductivity and lithium ion transport ability of the material both decrease, and the volume expansion of the electrode increases. Therefore, the capacity retention rate during long cycling decreases. Within a certain magnetic field strength, the generated conductive network has electrochemical performance gains in terms of rate and long cycling compared to the original negative electrode material without magnetic field action. Under a certain Si loading, the conductive network with carbon foam as the main body has an obvious performance improvement effect. However, when the Si loading is too high, the carbon foam cannot well control the volume expansion and achieve high capacity retention at high rates. Generally speaking, using the conductive network of carbon foam, CNT and C layer can well inhibit volume expansion within a certain range.

[0118] As Figure 1 and 2 shown, the in-situ CNT growth-foam in the figure refers to the negative electrode material prepared in Comparative Example 1, and the magnetic field-assisted in-situ CNT growth-foam refers to the negative electrode material prepared in Example 1. As Figure 1 can be seen, the negative electrode material prepared in Example 1 has a smaller ohmic impedance and obvious advantages in terms of lithium ion transport rate.

[0119] As Figure 2 can be seen, the negative electrode material prepared in Example 1 has stronger conductivity, better specific capacity performance under battery rate charge and discharge, and higher capacity retention rate after high-rate current. The conductive network of in-situ grown CNT-foam significantly promotes the rate performance of the material.

[0120] It is easy for those skilled in the art to understand that the above are only preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A flexible composite anode material, characterized in that, It includes a three-dimensional foam and an active layer supported on the three-dimensional foam. The active layer includes a silicon-based material layer, a magnetic layer, and carbon nanotubes that are sequentially coated on the surface of the silicon-based material layer. Among them, the carbon nanotubes are in-situ grown by heat-treating a carbon source in an inert atmosphere, and a magnetic field is applied during the heat treatment to enable the carbon nanotubes to be aligned directionally. The intensity of the magnetic field is 0.5 - 1.2 T. The silicon-based material layer accounts for 20% - 50% of the mass of the flexible composite anode material. The temperature of the heat treatment is 600 °C - 900 °C.

2. The flexible composite anode material according to claim 1, wherein The silicon-based material layer is nano-silicon or nano-silicon monoxide with a particle size of 20 - 200 nm. And / or, the magnetic layer contains one or more elements of iron, cobalt, and nickel. And / or, the carbon source is one or more of melamine, cyanuric acid, dopamine hydrochloride, chitosan, and polyaniline.

3. The flexible composite anode material according to claim 2, wherein, There is also a carbon layer between the silicon-based material layer and the magnetic layer. And / or, the magnetic layer also contains carbon. The magnetic layer is formed from a metal-organic framework material during the heat treatment.

4. The flexible composite anode material according to any one of claims 1-3, characterized in that, The three-dimensional foam is formed from melamine foam during the heat treatment. The pore size of the three-dimensional foam is 10 - 200 μm.

5. The flexible composite anode material according to any one of claims 1-3, characterized in that The mass content of the active layer in the flexible composite anode material is 10% - 90%. The silicon-based material layer accounts for 70 - 99% of the mass of the active layer. The mass content of the carbon nanotubes in the flexible composite anode material is 2% - 5%.

6. The flexible composite anode material according to any one of claims 1-3, characterized in that, The diameter of the carbon nanotubes is 1 - 5 nm, and the aspect ratio is (2 - 70):

1. The length of the carbon nanotubes is 1 - 5 mm.

7. The flexible composite anode material according to any one of claims 1-3, characterized in that The time of the heat treatment is 1 - 3 hours.

8. A preparation method of a flexible composite anode material, characterized in that, It includes the following steps: S1. Prepare a mixed solution containing a silicon-based material, a metal salt, and an organic ligand. S2. Immerse and adsorb a foam carrier in the mixed solution, and then take it out and freeze it to obtain a pretreated foam carrier. S3. Heat-treat the pretreated foam carrier and a carbon source in an inert gas while applying a magnetic field to obtain a carbon foam with a silicon-carbon anode material attached to its surface, and the surface of the silicon-carbon anode material grows carbon nanotubes arranged directionally. Among them, the intensity of the magnetic field is 0.5 - 1.2 T. The mass content of the silicon-based material in the carbon foam with the silicon-carbon anode material attached to its surface is 20% - 50%. The temperature of the heat treatment is 600 °C - 900 °C.

9. The preparation method of the flexible composite anode material according to claim 8, wherein, The metal salt is one or more of cobalt salts, nickel salts, and iron salts. The organic ligand is dimethylimidazole or dicyandiamide. The foam carrier is melamine foam. And / or, the mass ratio of the silicon-based material to the organic ligand is (10 - 20):1, and the molar ratio of the organic ligand to the metal salt is (5 - 8):

1. The concentration of the silicon-based material in the mixed solution is 50 - 120 g / L. And / or, the time of the immersion and adsorption is 6 - 36 h. The time of the heat treatment is 1 - 3 hours.

10. Application of the flexible composite anode material according to any one of claims 1 - 7, or the flexible composite anode material obtained by the preparation method according to any one of claims 8 - 9 in a lithium-ion battery.

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