Double-layer carbon-coated silicon-based negative electrode material as well as preparation method and application thereof

Abstract

The invention provides a double-layer carbon-coated silicon-based negative electrode material as well as a preparation method and application thereof, and relates to the technical field of lithium ion batteries. The silicon-based negative electrode material sequentially comprises a silicon-carbon inner core, a loose silicon-carbon composite layer and a compact carbon layer from inside to outside, wherein the loose silicon-carbon composite layer is used for buffering material expansion; the compact carbon layer is used for improving the structural strength of the material. According to the invention, the technical problems of large volume expansion effect and poor structural stability of a silicon-carbon composite negative electrode material are solved, and the technical effects of effectively inhibiting material pulverization, reducing repeated rupture of an SEI film and improving the cycle performance of the material are achieved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of lithium ion batteries, in particular to a double-layer carbon-coated silicon-based negative electrode material and a preparation method and application thereof. BACKGROUND

[0002] As the current mainstream electrochemical energy storage device, the continuous improvement of the performance of lithium ion batteries is crucial for the development of electric vehicles, portable electronic devices and large-scale energy storage systems. As one of the core components of lithium ion batteries, the performance of the negative electrode material directly affects the energy density, cycle life and safety of the battery. Silicon (Si) is considered as the most potential candidate for the next generation of high-energy-density lithium ion battery negative electrode material due to its extremely high theoretical specific capacity (about 4200 mAh / g), which is much higher than that of commercial graphite negative electrode (372 mAh / g) and its low delithiation potential; however, its volume expansion rate during charging and discharging is >300%, which leads to particle pulverization, continuous rupture and regeneration of the SEI film, resulting in rapid cycle life decay. In order to overcome the above problems, carbon-coated silicon (Si / C) composite material has become the mainstream solution, and surface carbon coating of silicon nanoparticles is an effective strategy.

[0003] However, the traditional single-layer carbon coating or simple carbon composite structure still has significant deficiencies. Dense single-layer carbon coating: although it can provide good electrical conductivity and certain mechanical constraint, when silicon expands dramatically, the dense carbon layer cannot provide enough internal space to accommodate the expansion, which easily leads to the rupture of the coating layer, and this rupture makes the protective effect of the coating invalid. Loose / porous carbon layer: although its porous structure can provide space for silicon expansion and relieve stress, its structural strength is usually low and its mechanical stability is poor, and in the repeated charging and discharging process, the loose carbon skeleton is prone to collapse or deformation, which cannot effectively constrain the silicon particles and provide a long-term stable conductive network, and the overall structure of the particles may still collapse.

[0004] Therefore, the development of a new type of hybrid carbon layer or gradient structure of silicon-carbon composite negative electrode structure can synergistically solve the problems of large volume expansion effect and poor structural stability of silicon, which is crucial for realizing high capacity, long cycle life and practical application of silicon-based negative electrode material. The ideal coating structure needs to create a controllable space around the silicon particles to buffer the expansion stress, while building a solid and continuous skeleton to maintain the integrity of the particles and the stability of the conductive network, thereby effectively inhibiting pulverization, reducing the repeated rupture of the SEI film and improving the cycle performance.

[0005] CN107293700A discloses a lithium ion battery negative electrode active material with a three-layer composite structure, including an inner core, an intermediate layer and an outermost layer. The inner core is a silicon-carbon composite particle, the intermediate layer is a porous carbon layer, and the outermost layer is a dense carbon layer. The porosity of the porous carbon layer is 40% to 85%, the pore size distribution of the porous carbon layer is 30nm to 250nm, the thickness of the porous carbon layer is 10% to 50% of the inner core particle size, and the shell thickness of the dense carbon layer is 100nm to 500nm. The preparation method of the negative electrode active material includes: (1) mixing and dispersing silicon material and first organic carbon source, then performing first carbonization to obtain silicon-carbon composite particles A; (2) mixing and dispersing second organic carbon source and pore former to obtain a mixture, kneading the mixture with A obtained in step (1) to obtain silicon-carbon composite particles coated with the mixture, then performing second carbonization to obtain composite particles B; (3) mixing third organic carbon source with B obtained in step (2) in a solvent; after the solvent is volatilized, a precursor coated with the third organic carbon source is obtained, and then third carbonization is performed to obtain composite particles C; (4) removing the pore former in C with an acidic or basic solvent to obtain the negative electrode active material.

[0006] However, the three-layer composite negative electrode material preparation process described in CN107293700A has significant drawbacks. The first problem is that the process is extremely complex and costly. The process involves three high-temperature carbonizations and one chemical etching, involving multiple mixing, dispersing and coating, resulting in long production cycles, large equipment investment, high energy consumption and difficulty in industrialization. Secondly, the introduction of pore-forming agents (such as SiO2, CaCO3) in the process poses a risk of impurity residue. Although it is finally removed by acid / alkali etching, some of the pore-forming agents are wrapped by the carbon layer and cannot be completely removed, and trace amounts of residue may affect the long-term cycle performance and safety of the battery, and the use of dangerous chemicals such as hydrofluoric acid (HF) further increases the cost of operation and environment. Most importantly, the chemical etching process is performed from the outside in, making it difficult to uniformly remove the pore-forming agent inside the particles, which can lead to uneven distribution of pore size and porosity from the inside to the outside of the porous carbon layer. This inconsistency in structure can greatly reduce the effect of the buffer volume expansion, and even cause stress concentration in local areas, thereby damaging the integrity of the structure and ultimately affecting the life and consistency of the battery.

[0007] In view of the above, the present application is proposed. SUMMARY

[0008] The application aims to provide a double-layer carbon-coated silicon-based negative electrode material and a preparation method and application thereof.

[0009] In order to achieve the above-mentioned purpose of the application, the following technical scheme is adopted:

[0010] In a first aspect, the application provides a double-layer carbon-coated silicon-based negative electrode material, which comprises a silicon-carbon inner core, a loose silicon-carbon composite layer and a dense carbon layer from inside to outside.

[0011] The porosity of the loose silicon-carbon composite layer is 30-70%.

[0012] The porosity of the dense carbon layer is 1-30%.

[0013] Further, the pore size of the loose silicon-carbon composite layer is 1-300 nm, preferably 10-50 nm.

[0014] Further, the specific surface area of the loose silicon-carbon composite layer is 1-400 m 2 / g, preferably 90-150 m 2 / g.

[0015] Further, the thickness of the loose silicon-carbon composite layer is 0.001-10 μm.

[0016] Further, the Raman spectrum Id / Ig of the loose silicon-carbon composite layer ranges from 1.0 to 1.5.

[0017] Further, the pore volume of the loose silicon-carbon composite layer is 0.1-0.6 cm 3 / g.

[0018] Further, the true density of the loose silicon-carbon composite layer is 1.8-2.3 g / cm 3 .

[0019] Further, the mass ratio of silicon to carbon in the loose silicon-carbon composite layer is (10-30):(90-70).

[0020] Further, the pore size of the dense carbon layer is 1-50 nm, preferably 1-5 nm.

[0021] Further, the specific surface area of the dense carbon layer is 1-30 m 2 / g, preferably 5-20 m 2 / g.

[0022] Further, the thickness of the dense carbon layer is 0.001-10 μm.

[0023] Further, the Raman spectrum Id / Ig of the dense carbon layer ranges from 0.8 to 1.2.

[0024] Further, the pore volume of the dense carbon layer is 0.05-0.15 cm 3 / g.

[0025] Further, the true density of the dense carbon layer is 2.0-2.3 g / cm 3 .

[0026] Further, the particle size of the silicon-carbon core is 0.01-20 μm.

[0027] Further, the silicon-carbon core comprises silicon particles and carbon particles.

[0028] Further, the particle size of the silicon particles is 1-500 nm, and the particle size of the carbon particles is 1-300 nm.

[0029] Further, the mass ratio of silicon to carbon in the silicon-carbon core is (50-80):(50-20).

[0030] In a second aspect, the present application provides a preparation method of the double-layer carbon-coated silicon-based negative electrode material according to the first aspect, and the preparation method comprises:

[0031] (1) dispersing nano-silicon particles and a first carbon source in a solvent, so that the first carbon source is coated on the surface of the nano-silicon particles to obtain a mixed slurry;

[0032] (2) drying the mixed slurry and then performing a pre-carbonization treatment to obtain an intermediate product;

[0033] (3) mixing the intermediate product and a second carbon source and performing a carbonization treatment to obtain the double-layer carbon-coated silicon-based negative electrode material.

[0034] Further, in step (1), the first carbon source comprises any one or a combination of at least two of polyvinyl alcohol, polyethylene glycol, polyvinylpyrrolidone, carboxymethyl cellulose, polyacrylic acid, polyacrylamide, polyimide, polyaniline, polyethyleneimine or chitosan.

[0035] Further, in step (1), the mass ratio of the nanosilicon particles and the first carbon source is (0.01-100): 1.

[0036] Further, in step (1), the solvent includes any one or a combination of at least two of water, ethanol, ethylene glycol, N-methyl pyrrolidone, or tetrahydrofuran.

[0037] Further, in step (1), the solid content of the mixed slurry is 1-50%.

[0038] Further, in step (1), the dispersion includes any one or a combination of at least two of mechanical dispersion, stirring dispersion, or ultrasonic dispersion.

[0039] Further, in step (1), the mechanical dispersion has a rotation speed of 20-1500 rpm and a time of 0.1-36 h.

[0040] Further, in step (1), the stirring dispersion has a rotation speed of 20-1000 rpm and a time of 0.1-36 h.

[0041] Further, in step (1), the ultrasonic dispersion has a power of 100-1000 W and a time of 0.1-36 h.

[0042] Further, in step (2), the drying includes spray drying.

[0043] Further, in step (2), the spray drying has parameters including an inlet air temperature of 130-250°C, an outlet air temperature of 80-200°C, and a feeding rate of 5-90 mL / min.

[0044] Further, in step (2), the pre-carbonization treatment is performed in a protective gas atmosphere.

[0045] Further, the protective gas includes any one or a combination of at least two of argon, nitrogen, or helium.

[0046] Further, in step (2), the pre-carbonization treatment has a temperature increasing rate of 1-20°C / min, a temperature of 300-800°C, and a holding time of 0.5-10 h.

[0047] Further, in step (3), the second carbon source includes any one or a combination of at least two of glucose, sucrose, starch, phenol formaldehyde resin, or pitch.

[0048] Further, in step (3), the mass ratio of the intermediate product and the second carbon source is (0.01-100): 1.

[0049] Further, in step (3), the carbonization treatment is performed in a protective gas atmosphere.

[0050] Further, the protective gas comprises any one or a combination of at least two of argon, nitrogen or helium.

[0051] Further, in step (3), the heating rate of the carbonization treatment is 1-20℃ / min, the temperature of the carbonization treatment is 500-1500℃, and the holding time of the carbonization treatment is 0.5-10h.

[0052] Further, in step (3), the carbonization treatment further comprises the steps of cooling and sieving.

[0053] Further, in step (3), the sieving has a mesh size of 60-1000 mesh.

[0054] In a third aspect, the present application provides a use of the double-layer carbon-coated silicon-based negative electrode material as described in the first aspect in the preparation of a negative electrode of a lithium ion battery.

[0055] In a fourth aspect, the present application provides a lithium ion battery comprising the double-layer carbon-coated silicon-based negative electrode material as described in the first aspect.

[0056] Compared with the prior art, the present application has at least the following beneficial effects:

[0057] (1) The double-layer carbon-coated silicon-based negative electrode material provided by the present application can simultaneously meet the two crucial requirements of "reserving expansion space" and "providing a stable skeleton" through the design and spatial distribution of the carbon layer structure and function, and the synergistic cooperation between the porous structure carbon layer and the dense structure carbon layer, thereby solving the technical problems of large volume expansion effect and poor structural stability of the silicon-carbon composite negative electrode material in the prior art, and achieving the technical effects of effectively inhibiting the pulverization of the silicon-carbon material, reducing the repeated rupture of the SEI film, and improving the cycle performance of the material.

[0058] (2) The preparation method of the double-layer carbon-coated silicon-based negative electrode material provided by the present application has a gradient double-layer coating composite structure of inner loose and outer dense for the silicon-carbon material through twice coating of different polymer materials, and is simple, efficient, and has a high success rate, which is suitable for industrial production.

[0059] (3) The application of the double-layer carbon-coated silicon-based negative electrode material provided by the present application can achieve outstanding application effects. BRIEF DESCRIPTION OF DRAWINGS

[0060] In order to more clearly illustrate the technical solutions in the specific embodiments or the prior art of the present application, the drawings required to be used in the specific embodiments or the prior art description will be briefly introduced as follows. Obviously, the drawings described below are some embodiments of the present application, and all other drawings obtained by those skilled in the art without creative effort based on these drawings also belong to the protection scope of the present application.

[0061] Figure 1 The scanning electron microscope image of the silicon-based negative electrode material after the first layer of carbon coating in Example 1 of the present application.

[0062] Figure 2 The scanning electron microscope image of the silicon-based negative electrode material after the second layer of carbon coating in Example 1 of the present application.

[0063] Figure 3 The cycle curve graph of the silicon-based negative electrode material of Example 1 obtained in the test example of the present application at 100 cycles.

[0064] Figure 4 The cycle curve graph of the silicon-based negative electrode material of Comparative Example 1 obtained in the test example of the present application at 100 cycles.

[0065] Figure 5 The cycle curve graph of the silicon-based negative electrode material of Comparative Example 2 obtained in the test example of the present application at 100 cycles.

[0066] Figure 6 The cycle curve graph of the silicon-based negative electrode material of Comparative Example 3 obtained in the test example of the present application at 100 cycles.

[0067] Figure 7 The Raman spectrum of the silicon-based negative electrode material after the second layer of carbon coating in Example 1 of the present application. DETAILED DESCRIPTION

[0068] The technical solutions of the present application will be described below in conjunction with the embodiments, and obviously, the described embodiments are some of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative effort also belong to the protection scope of the present application.

[0069] In a first aspect, the present application provides a double-layer carbon-coated silicon-based negative electrode material, which comprises, from inside to outside, a silicon-carbon core, a loose silicon-carbon composite layer and a dense carbon layer.

[0070] The porosity of the loose silicon-carbon composite layer is 30-70%.

[0071] The porosity of the dense carbon layer is 1-30%.

[0072] In the present application, a gradient porous carbon-coated silicon-based negative electrode material is provided, taking a silicon-carbon negative electrode material as the core, and through twice coating with different polymer materials, a gradient double-layer coating composite structure is realized, in which the inner layer is loose and the outer layer is dense; the gradient can be indicated by the above parameters, and the carbon coating layer has a gradient decrease in pore size, porosity and specific surface area from inside to outside; the loose silicon-carbon composite layer is a nano-loose porous silicon-carbon composite layer, which provides sufficient space for the volume expansion of silicon particles during the charging and discharging process, effectively buffers the material expansion, and the dense carbon layer improves the structural strength of the material and reduces particle pulverization. Therefore, through the design and spatial distribution of the structure and function of the carbon layer, the porous structure of the first carbon layer and the dense and stable structure of the second carbon layer meet the needs of "reserving expansion space" and "providing a stable skeleton" at the same time, thereby effectively inhibiting the pulverization of the silicon-carbon material, reducing the repeated rupture of the SEI film, and improving the cycle performance of the material.

[0073] As an optional embodiment, the porosity of the loose silicon-carbon composite layer is 30-70%, for example, it can be 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70% and the like.

[0074] As an optional embodiment, the pore size of the loose silicon-carbon composite layer is 1-300 nm, for example, it can be 1 nm, 5 nm, 10 nm, 15 nm, 20 nm, 25 nm, 30 nm, 35 nm, 40 nm, 45 nm, 50 nm, 55 nm, 60 nm, 65 nm, 70 nm, 75 nm, 80 nm, 90 nm, 100 nm, 120 nm, 140 nm, 160 nm, 180 nm, 200 nm, 220 nm, 240 nm, 260 nm, 280 nm, 300 nm and the like.

[0075] As a preferred embodiment, the pore size of the loose silicon-carbon composite layer is 10-50 nm, for example, it can be 10 nm, 12 nm, 14 nm, 16 nm, 18 nm, 20 nm, 22 nm, 24 nm, 26 nm, 28 nm, 30 nm, 32 nm, 34 nm, 36 nm, 38 nm, 40 nm, 42 nm, 44 nm, 46 nm, 48 nm, 50 nm and the like.

[0076] As an optional embodiment, the specific surface area of the loose silicon-carbon composite layer is 1-400 m 2 / g, for example, it can be 1 m 2 / g, 5 m 2 / g, 10 m 2 / g, 20 m 2 / g, 40 m 2 / g, 50 m 2 / g, 60 m 2 / g, 80 m2 / g, 100 m 2 / g, 120 m 2 / g, 140 m 2 / g, 150 m 2 / g, 160 m 2 / g, 180 m 2 / g, 200 m 2 / g, 220 m 2 / g, 240 m 2 / g, 250 m 2 / g, 260 m 2 / g, 280 m 2 / g, 300 m 2 / g, 320 m 2 / g, 340 m 2 / g, 360 m 2 / g, 380 m 2 / g, 400 m 2 / g, etc.

[0077] As a preferred embodiment, the specific surface area of the loose silicon-carbon composite layer is 90-150 m 2 / g, for example, it can be 90 m 2 / g, 95 m 2 / g, 100 m 2 / g, 105 m 2 / g, 110 m 2 / g, 115 m 2 / g, 120 m 2 / g, 125 m 2 / g, 130 m 2 / g, 135 m 2 / g, 140 m 2 / g, 145 m 2 / g, 150 m 2 / g, etc.

[0078] As an optional embodiment, the thickness of the loose silicon-carbon composite layer is 0.001-10 μm, for example, it can be 0.001 μm, 0.005 μm, 0.01 μm, 0.05 μm, 0.1 μm, 0.5 μm, 1 μm, 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, etc.

[0079] As an optional embodiment, the Raman spectrum Id / Ig of the loose silicon-carbon composite layer ranges from 1.0 to 1.5, for example, it can be 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, etc.

[0080] As an optional embodiment, the pore volume of the loose silicon-carbon composite layer is 0.1-0.6 cm 3 / g, for example, it can be 0.1 cm 3 / g, 0.2 cm 3 / g, 0.3 cm 3 / g, 0.4 cm 3 / g, 0.5 cm 3 / g, 0.6 cm 3 / g, etc.

[0081] As an optional embodiment, the true density of the loose silicon-carbon composite layer is 1.8-2.3 g / cm 3 , for example, it can be 1.8 g / cm 3 , 1.9 g / cm 3 , 2.0 g / cm 3 , 2.1 g / cm 3 , 2.2 g / cm 3 , 2.3 g / cm 3 , etc.

[0082] As an optional embodiment, the mass ratio of silicon and carbon in the loose silicon-carbon composite layer is (10-30):(90-70), for example, it can be 10:90, 12:88, 14:86, 15:85, 16:84, 18:82, 20:80, 22:78, 24:76, 25:75, 26:74, 28:72, 30:70, etc.

[0083] As an optional embodiment, the porosity of the dense carbon layer is 1-30%, for example, it can be 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 14%, 16%, 18%, 20%, 22%, 24%, 25%, 26%, 28%, 29%, 29.9% (not including the upper limit 30%).

[0084] As an optional embodiment, the pore size of the dense carbon layer is 1-50 nm, for example, it can be 1 nm, 2 nm, 4 nm, 6 nm, 8 nm, 10 nm, 12 nm, 14 nm, 16 nm, 18 nm, 20 nm, 22 nm, 25 nm, 30 nm, 35 nm, 40 nm, 45 nm, 50 nm, etc.

[0085] As a preferred embodiment, the pore size of the dense carbon layer is 1-5 nm, for example, it can be 1 nm, 1.5 nm, 2 nm, 2.5 nm, 3 nm, 3.5 nm, 4 nm, 4.5 nm, 5 nm, etc.

[0086] As an optional embodiment, the specific surface area of the dense carbon layer is 1-30 m2 / g, for example, can be 1 m 2 / g, 2 m 2 / g, 4 m 2 / g, 5 m 2 / g, 6 m 2 / g, 8 m 2 / g, 10 m 2 / g, 12 m 2 / g, 14 m 2 / g, 15 m 2 / g, 16 m 2 / g, 18 m 2 / g, 20 m 2 / g, 22 m 2 / g, 24 m 2 / g, 25 m 2 / g, 26 m 2 / g, 28 m 2 / g, 30 m 2 / g, etc.

[0087] As an optional embodiment, the specific surface area of the dense carbon layer is 5-20 m 2 / g, for example, can be 5 m 2 / g, 6 m 2 / g, 7 m 2 / g, 8 m 2 / g, 9 m 2 / g, 10 m 2 / g, 11 m 2 / g, 12 m 2 / g, 13 m 2 / g, 14 m 2 / g, 15 m 2 / g, 16 m 2 / g, 17 m 2 / g, 18 m 2 / g, 19 m 2 / g, 20 m 2 / g, etc.

[0088] As an optional embodiment, the thickness of the dense carbon layer is 0.001-10 μm, for example, can be 0.001 μm, 0.005 μm, 0.01 μm, 0.05 μm, 0.1 μm, 0.5 μm, 1 μm, 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, etc.

[0089] As an optional embodiment, the Raman spectrum Id / Ig of the dense carbon layer is in the range of 0.8-1.2, for example, can be 0.8, 0.85, 0.9, 0.95, 1.0, 1.05, 1.1, 1.15, 1.2, etc.

[0090] As an optional embodiment, the pore volume of the dense carbon layer is 0.05-0.15 cm 3 / g, for example, can be 0.05 cm 3 / g, 0.06 cm 3 / g, 0.07 cm 3 / g, 0.08 cm 3 / g, 0.09 cm 3 / g, 0.10 cm 3 / g, 0.11 cm 3 / g, 0.12 cm 3 / g, 0.13 cm 3 / g, 0.14 cm 3 / g, 0.15 cm 3 / g, etc.

[0091] As an optional embodiment, the true density of the dense carbon layer is 2.0-2.3 g / cm 3 , for example, can be 2.0 g / cm 3 , 2.05 g / cm 3 , 2.1 g / cm 3 , 2.15 g / cm 3 , 2 g / cm 3 , 2.25 g / cm 3 , 2.3 g / cm 3 , etc.

[0092] As an optional embodiment, the particle size of the silicon-carbon inner core is 0.01-20 μm, for example, can be 0.01 μm, 0.05 μm, 0.1 μm, 0.5 μm, 1 μm, 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, 15 μm, 16 μm, 17 μm, 18 μm, 19 μm, 20 μm, etc.

[0093] As an optional embodiment, the silicon-carbon inner core comprises silicon particles and carbon particles.

[0094] As an optional embodiment, the particle size of the silicon particles is 1-500 nm, for example, 1 nm, 5 nm, 10 nm, 15 nm, 20 nm, 25 nm, 30 nm, 35 nm, 40 nm, 45 nm, 50 nm, 60 nm, 70 nm, 80 nm, 90 nm, 100 nm, 150 nm, 200 nm, 250 nm, 300 nm, 350 nm, 400 nm, 450 nm, 500 nm, or the like.

[0095] As an optional embodiment, the particle size of the carbon particles is 1-300 nm, for example, 1 nm, 5 nm, 10 nm, 15 nm, 20 nm, 25 nm, 30 nm, 35 nm, 40 nm, 45 nm, 50 nm, 60 nm, 70 nm, 80 nm, 90 nm, 100 nm, 150 nm, 200 nm, 250 nm, 300 nm, or the like.

[0096] As an optional embodiment, the mass ratio of silicon and carbon in the silicon-carbon core is (50-80):(50-20), for example, 50:50, 55:45, 60:40, 65:35, 70:30, 75:25, 80:20, or the like.

[0097] In a second aspect, the present application provides a preparation method of the double-layer carbon-coated silicon-based negative electrode material according to the first aspect, and the preparation method comprises:

[0098] (1) dispersing the nano-silicon particles and the first carbon source in a solvent to coat the first carbon source on the surface of the nano-silicon particles, and obtaining a mixed slurry;

[0099] (2) drying the mixed slurry and performing a pre-carbonization treatment to obtain an intermediate product;

[0100] (3) mixing the intermediate product and the second carbon source and performing a carbonization treatment to obtain the double-layer carbon-coated silicon-based negative electrode material.

[0101] In the present application, the preparation method of the double-layer carbon-coated silicon-based negative electrode material is to dissolve the nano-silicon powder and the polymer in a solvent according to a certain ratio, to prepare a uniform slurry by a mechanical device, to use a spray drying method to make the polymer uniformly adsorbed on the nano-silicon material, to preliminarily decompose and carbonize the polymer on the surface of the nano-silicon material to remove part of volatile substances and to form more pore structures on the surface of the material by pre-carbonization at a controlled temperature in an inert gas. Then, the pre-carbonized product and the polymer are uniformly mixed at a certain ratio, and a uniform, dense and stable carbon layer is formed on the surface of the material by high-temperature carbonization at a controlled temperature in an inert gas, so that a gradient porous carbon-coated silicon-based negative electrode material is finally obtained.

[0102] As an optional implementation, in step (1), the first carbon source includes any one or a combination of at least two of polyvinyl alcohol (PVA), polyethylene glycol (PEG), polyvinylpyrrolidone (PVP), carboxymethyl cellulose (CMC), polyacrylic acid (PAA), polyacrylamide (PAM), polyimide (PI), polyaniline (PANI), polyethyleneimine (PEI), or chitosan (CS).

[0103] As an optional implementation, in step (1), the first carbon source has a viscosity-average molecular weight (Mv) of 5000-500000, for example, 5000, 10000, 15000, 20000, 25000, 30000, 35000, 40000, 45000, 50000, 60000, 70000, 80000, 90000, 100000, etc.

[0104] As an optional implementation, in step (1), the mass ratio of the nanosilicon particles and the first carbon source is (0.01-100):1, for example, 0.01:1, 0.05:1, 0.1:1, 0.5:1, 1:1, 5:1, 10:1, 15:1, 20:1, 25:1, 30:1, 35:1, 40:1, 45:1, 50:1, 55:1, 60:1, 70:1, 80:1, 90:1, 100:1, etc.

[0105] As an optional implementation, in step (1), the solvent includes any one or a combination of at least two of water, ethanol, ethylene glycol, N-methyl pyrrolidone (NMP), or tetrahydrofuran (THF).

[0106] As an optional implementation, in step (1), the mixed slurry has a solid content of 1-50%, for example, 1%, 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, etc.

[0107] As an optional implementation, in step (1), the dispersing includes any one or a combination of at least two of mechanical dispersion, stirring dispersion, or ultrasonic dispersion.

[0108] As an optional embodiment, in step (1), the rotation speed of mechanical dispersion is 20-1500 rpm, for example, it can be 20 rpm, 50 rpm, 100 rpm, 200 rpm, 400 rpm, 500 rpm, 600 rpm, 800 rpm, 1000 rpm, 1200 rpm, 1400 rpm, 1500 rpm, etc., and the time of mechanical dispersion is 0.1-36 h, for example, it can be 0.1 h, 0.5 h, 1 h, 2 h, 4 h, 6 h, 8 h, 10 h, 12 h, 14 h, 16 h, 18 h, 20 h, 22 h, 24 h, 26 h, 28 h, 30 h, 32 h, 34 h, 36 h, etc.

[0109] As an optional embodiment, in step (1), the rotation speed of mechanical dispersion is 20-1000 rpm, for example, it can be 20 rpm, 50 rpm, 100 rpm, 200 rpm, 400 rpm, 500 rpm, 600 rpm, 800 rpm, 1000 rpm, etc., and the time of mechanical dispersion is 0.1-36 h, for example, it can be 0.1 h, 0.5 h, 1 h, 2 h, 4 h, 6 h, 8 h, 10 h, 12 h, 14 h, 16 h, 18 h, 20 h, 22 h, 24 h, 26 h, 28 h, 30 h, 32 h, 34 h, 36 h, etc.

[0110] As an optional embodiment, in step (1), the power of ultrasonic dispersion is 100-1000 W, for example, it can be 100 W, 200 W, 300 W, 400 W, 500 W, 600 W, 700 W, 800 W, 900 W, 1000 W, etc., and the time of ultrasonic dispersion is 0.1-36 h, for example, it can be 0.1 h, 0.5 h, 1 h, 2 h, 4 h, 6 h, 8 h, 10 h, 12 h, 14 h, 16 h, 18 h, 20 h, 22 h, 24 h, 26 h, 28 h, 30 h, 32 h, 34 h, 36 h, etc.

[0111] As an optional embodiment, in step (2), the drying includes spray drying.

[0112] As an optional implementation, in step (2), the parameters of the spray drying include: the inlet air temperature is 130-250℃, for example, it can be 130℃, 140℃, 150℃, 160℃, 170℃, 180℃, 190℃, 200℃, 210℃, 220℃, 230℃, 240℃, 250℃, etc.; the outlet air temperature is 80-200℃, for example, it can be 80℃, 90℃, 100℃, 110℃, 120℃, 130℃, 140℃, 150℃, 160℃, 170℃, 180℃, 190℃, 200℃, etc.; the feeding rate is 5-90mL / min, for example, it can be 5mL / min, 10mL / min, 15mL / min, 20mL / min, 25mL / min, 30mL / min, 35mL / min, 40mL / min, 45mL / min, 50mL / min, 55mL / min, 60mL / min, 65mL / min, 70mL / min, 75mL / min, 80mL / min, 85mL / min, 90mL / min, etc.

[0113] As an optional implementation, in step (2), the pre-carbonization treatment is carried out in a protective gas atmosphere.

[0114] As an optional implementation, in step (2), the protective gas includes any one or a combination of at least two of argon, nitrogen or helium.

[0115] As an optional implementation, in step (2), the temperature increasing rate of the pre-carbonization treatment is 1-20℃ / min, for example, it can be 1℃ / min, 2℃ / min, 4℃ / min, 6℃ / min, 8℃ / min, 10℃ / min, 12℃ / min, 14℃ / min, 16℃ / min, 18℃ / min, 20℃ / min, etc.

[0116] As an optional implementation, in step (2), the temperature of the pre-carbonization treatment is 300-800℃, for example, it can be 300℃, 350℃, 400℃, 450℃, 500℃, 550℃, 600℃, 650℃, 700℃, 750℃, 800℃, etc.

[0117] As an optional implementation, in step (2), the holding time of the pre-carbonization treatment is 0.5-10h, for example, it can be 0.5h, 1h, 1.5h, 2h, 2.5h, 3h, 3.5h, 4h, 4.5h, 5h, 5.5h, 6h, 6.5h, 7h, 7.5h, 8h, 8.5h, 9h, 9.5h, 10h, etc.

[0118] As an optional embodiment, in step (3), the second carbon source includes any one or a combination of at least two of glucose, sucrose, starch, phenol formaldehyde resin, or pitch.

[0119] As an optional embodiment, in step (3), the mass ratio of the intermediate product to the second carbon source is (0.01-100): 1, which can be, for example, 0.01:1, 0.05:1, 0.1:1, 0.5:1, 1:1, 5:1, 10:1, 15:1, 20:1, 25:1, 30:1, 35:1, 40:1, 45:1, 50:1, 55:1, 60:1, 70:1, 80:1, 90:1, 100:1, etc.

[0120] As an optional embodiment, in step (3), the carbonization treatment is performed in a protective gas atmosphere.

[0121] As an optional embodiment, in step (3), the protective gas includes any one or a combination of at least two of argon, nitrogen, or helium.

[0122] As an optional embodiment, in step (3), the heating rate of the carbonization treatment is 1-20℃ / min, which can be, for example, 1℃ / min, 2℃ / min, 4℃ / min, 6℃ / min, 8℃ / min, 10℃ / min, 12℃ / min, 14℃ / min, 16℃ / min, 18℃ / min, 20℃ / min, etc.

[0123] As an optional embodiment, in step (3), the temperature of the carbonization treatment is 500-1500℃, which can be, for example, 500℃, 550℃, 600℃, 650℃, 700℃, 750℃, 800℃, 900℃, 950℃, 1000℃, 1050℃, 1100℃, 1150℃, 1200℃, 1250℃, 1300℃, 1350℃, 1400℃, 1450℃, 1500℃, etc.

[0124] As an optional embodiment, in step (3), the holding time of the carbonization treatment is 0.5-10h, which can be, for example, 0.5h, 1h, 1.5h, 2h, 2.5h, 3h, 3.5h, 4h, 4.5h, 5h, 5.5h, 6h, 6.5h, 7h, 7.5h, 8h, 8.5h, 9h, 9.5h, 10h, etc.

[0125] As an optional embodiment, in step (3), the carbonization treatment further includes the steps of cooling and sieving.

[0126] As an optional embodiment, in step (3), the sieving mesh number of the sieving is 60-1000 mesh, for example, it can be 60 mesh, 100 mesh, 200 mesh, 300 mesh, 400 mesh, 500 mesh, 600 mesh, 700 mesh, 800 mesh, 900 mesh, 1000 mesh, etc.

[0127] In a third aspect, the present application provides a use of the double-layer carbon-coated silicon-based anode material as described in the first aspect in the preparation of a negative electrode of a lithium ion battery.

[0128] In a fourth aspect, the present application provides a lithium ion battery comprising the double-layer carbon-coated silicon-based anode material as described in the first aspect.

[0129] The present application is further illustrated by the following examples. Unless otherwise specified, the materials in the examples are prepared according to the existing methods or directly purchased from the market.

[0130] Example 1

[0131] The present example provides a double-layer carbon-coated silicon-based anode material, which is prepared by the following steps:

[0132] (1) Nanosilicon powder with a particle size of 30 nm and PVA with a viscosity-average molecular weight of 45000 were dispersed in pure water at a mass ratio of 9:1, a magnetic stirrer was used to stir at a speed of 500 rpm for 6 h, and a mixed slurry with a solid content of 20% was obtained;

[0133] (2) The mixed slurry obtained in step (1) was subjected to spray drying, the feeding temperature was 170℃, the discharging temperature was 95℃, the feeding rate was controlled at 10 mL / min, after the spray drying was completed, the cooled material was collected, and product S1 was obtained; product S1 was added to a crucible, calcined in a tube furnace under a helium atmosphere, heated to 500℃ at a rate of 5℃ / min, and kept for 1 h, after the material was naturally cooled, it was passed through a 100 mesh sieve, and an intermediate product (as shown in Figure 1 ) was obtained;

[0134] (3) The intermediate product obtained in step (2) was uniformly dispersed and mixed with 5% sucrose in total mass using a three-dimensional mixer at a speed of 60 rpm for 2 h, and a mixed material was obtained; the mixed material was calcined in a tube furnace under a helium atmosphere, heated to 800℃ at a rate of 5℃ / min, and kept for 3 h, after the material was cooled, it was passed through a 300 mesh sieve, and the double-layer carbon-coated silicon-based anode material (as shown in Figure 2 ) was obtained.

[0135] As Figure 1As shown, the intermediate product in the application sequentially comprises a silicon-carbon inner core and a loose silicon-carbon composite layer from inside to outside, and a large number of pores exist between the surface particles of the silicon-carbon composite layer, forming a continuous three-dimensional interconnected porous network. These pores are uniformly distributed, and obviously, these pores can provide effective buffer space for the huge volume expansion of silicon particles in the subsequent charge and discharge process.

[0136] As shown, Figure 2 The outermost carbon layer of the final silicon-based negative electrode material obtained in the application is basically free of obvious open pores, has a flat, continuous and dense carbon layer structure morphology, and has fine wrinkle texture, which is usually caused by shrinkage stress of the carbon precursor during high-temperature carbonization. The silicon-carbon particles are completely "buried" under the carbon layer, indicating high coating completeness. Such a dense carbon shell provides a stable skeleton and provides a good electronic conduction path for the internal silicon-carbon inner core, effectively inhibits the penetration of electrolyte and the occurrence of side reactions, and thus improves the first coulombic efficiency and cycle stability of the material.

[0137] Figure 7 The Raman spectrum of the second layer carbon-coated silicon-based negative electrode material in Example 1 of the application is shown in Figure 2. As shown in Figure 2, Figure 7 A broad peak at 1340 cm-1 (D peak) and a narrow peak at 1585 cm-1 (G peak) can be seen. -1 The D peak mainly corresponds to amorphous carbon or defective graphite structure, and the G peak corresponds to the Sp 2 bond of graphitized carbon, so the intensity ratio of the D peak and the G peak can represent the amorphous degree of the carbon material. The I D / I G ratio of the first layer carbon-coated material in Example 1 is 1.35. This indicates that the material has a high defect density at this time. This is due to the low pre-carbonization temperature, incomplete decomposition of the carbon source, and a large number of structural distortions or very small grain sizes generated during carbonization. The I D / I G ratio of the second layer carbon-coated material is 0.92, and the I D / I G ratio is significantly reduced. This is because the second coating increases the temperature, and the decomposition products or intermediates of the carbon source during carbonization fill and repair part of the defects, vacancies and unsaturated bonds on the surface of the material after the first carbonization. This improves the integrity and order of the carbon structure.

[0138] Meanwhile, the second carbon coating is equivalent to an additional heat treatment for the material. Higher heat treatment temperature or longer heat treatment time provides more energy and opportunities for carbon atoms to rearrange, making small, disordered carbon crystallites fuse and grow, forming larger, more ordered graphitized regions. The conductivity of the material will be significantly enhanced. This is crucial for battery electrode materials, which can reduce internal resistance and improve rate performance. In addition, a more complete and thicker carbon coating layer can better protect the internal active material from side reactions with the electrolyte during the cycling process, thereby improving the cycle life of the material.

[0139] Example 2

[0140] The present embodiment provides a double-layer carbon-coated silicon-based negative electrode material, which is prepared by the following steps:

[0141] (1) Nanosilicon powder with a particle size of 100 nm and PEG with a viscosity average molecular weight of 85000 were dispersed in anhydrous ethanol at a mass ratio of 4:1. A magnetic stirrer was used to stir at a speed of 500 rpm at 60°C for 12 h to obtain a mixed slurry with a solid content of 10%;

[0142] (2) The mixed slurry obtained in step (1) was spray dried. The inlet temperature was 150°C, the outlet temperature was 85°C, and the feeding rate was controlled at 10 mL / min. After spray drying, the cooled material was collected to obtain product S1. The product S1 was added to a crucible and calcined in a tube furnace under a helium atmosphere. The temperature was raised to 350°C at a rate of 5°C / min and held for 1.5 h. After the material was naturally cooled, it was passed through a 100-mesh sieve to obtain an intermediate product;

[0143] (3) The intermediate product obtained in step (2) was uniformly mixed with 8% sucrose by total mass using a three-dimensional mixer at a speed of 60 rpm for 2 h to obtain a mixture. The mixture was calcined in a tube furnace under a helium atmosphere. The temperature was raised to 900°C at a rate of 10°C / min and held for 3 h. After the material was cooled, it was passed through a 300-mesh sieve to obtain the double-layer carbon-coated silicon-based negative electrode material.

[0144] Example 3

[0145] The present embodiment provides a double-layer carbon-coated silicon-based negative electrode material, which is prepared by the following steps:

[0146] (1) Nanosilicon powder with a particle size of 45 nm and PVA with a viscosity average molecular weight of 10000 were dispersed in pure water at a mass ratio of 0.5:1. An ultrasonic wave with a power of 500 W was used for dispersion for 6 h to obtain a mixed slurry with a solid content of 5%;

[0147] (2) the mixed slurry obtained in step (1) is subjected to spray drying, the feeding temperature is 160°C, the discharging temperature is 90°C, the feeding rate is controlled to be 10 mL / min, after the spray drying is completed, the cooled material is collected to obtain product S1; the product S1 is added into a crucible, calcination is carried out in a tube furnace under a helium atmosphere, the temperature is raised to 400°C at a rate of 5°C / min, and the temperature is kept for 2 h; after the material is naturally cooled, it is passed through a 100-mesh sieve to obtain an intermediate product;

[0148] (3) the intermediate product obtained in step (2) is uniformly mixed with 12% sucrose in total mass by using a three-dimensional mixer at a rotating speed of 60 rpm for 2 h to obtain a mixed material; the mixed material is calcined in a tube furnace under a helium atmosphere, the temperature is raised to 900°C at a rate of 10°C / min, and the temperature is kept for 3 h; after the material is cooled, it is passed through a 300-mesh sieve to obtain the double-layer carbon-coated silicon-based negative electrode material.

[0149] Example 4

[0150] The embodiment provides a double-layer carbon-coated silicon-based negative electrode material, which is prepared by the following steps.

[0151] (1) nano silicon powder with a particle size of 80 nm and PEG with a viscosity average molecular weight of 25000 are dispersed in THF at a mass ratio of 1:1, ultrasonic dispersion is carried out by using a power of 500 W for 0.2 h to obtain a mixed slurry with a solid content of 15%;

[0152] (2) the mixed slurry obtained in step (1) is subjected to spray drying, the feeding temperature is 145°C, the discharging temperature is 85°C, the feeding rate is controlled to be 10 mL / min, after the spray drying is completed, the cooled material is collected to obtain product S1; the product S1 is added into a crucible, calcination is carried out in a tube furnace under a helium atmosphere, the temperature is raised to 450°C at a rate of 5°C / min, and the temperature is kept for 2.5 h; after the material is naturally cooled, it is passed through a 100-mesh sieve to obtain an intermediate product;

[0153] (3) the intermediate product obtained in step (2) is uniformly mixed with 8% sucrose in total mass by using a three-dimensional mixer at a rotating speed of 60 rpm for 2 h to obtain a mixed material; the mixed material is calcined in a tube furnace under a helium atmosphere, the temperature is raised to 900°C at a rate of 10°C / min, and the temperature is kept for 3 h; after the material is cooled, it is passed through a 300-mesh sieve to obtain the double-layer carbon-coated silicon-based negative electrode material.

[0154] Example 5

[0155] The embodiment provides a double-layer carbon-coated silicon-based negative electrode material, which is prepared by the following steps.

[0156] (1) Nanometer silicon powder with a particle size of 60 nm and PVP with a viscosity average molecular weight of 300000 were dispersed in NMP at a mass ratio of 20:1, and stirred at a speed of 500 rpm for 1 h using a magnetic stirrer to obtain a mixed slurry with a solid content of 10%;

[0157] (2) The mixed slurry obtained in step (1) was spray dried, the feeding temperature was 180℃, the discharging temperature was 95℃, the feeding rate was controlled at 10 mL / min, and after the spray drying was completed, the cooled material was collected to obtain product S1; the product S1 was added into a crucible and calcined in a tube furnace under a helium atmosphere, the temperature was raised to 300℃ at a rate of 10℃ / min, and the material was kept at 300℃ for 1 h; after the material was naturally cooled, it was passed through a 200-mesh sieve to obtain an intermediate product;

[0158] (3) The intermediate product obtained in step (2) and a total mass of 5% phenolic resin (Mv of 5000±1000) were uniformly mixed at a speed of 60 rpm for 2 h using a three-dimensional mixer to obtain a mixed material; the mixed material was calcined in a tube furnace under a helium atmosphere, the temperature was raised to 900℃ at a rate of 10℃ / min, and the material was kept at 900℃ for 3 h; after the material was naturally cooled, it was passed through a 300-mesh sieve to obtain the double-layer carbon-coated silicon-based negative electrode material.

[0159] Example 6

[0160] The present embodiment provides a double-layer carbon-coated silicon-based negative electrode material, which is prepared by the following steps:

[0161] (1) Nanometer silicon powder with a particle size of 350 nm and PAM with a viscosity average molecular weight of 180000 were dispersed in pure water at a mass ratio of 2:1, and stirred at a speed of 500 rpm for 24 h using a magnetic stirrer to obtain a mixed slurry with a solid content of 30%;

[0162] (2) The mixed slurry obtained in step (1) was spray dried, the feeding temperature was 170℃, the discharging temperature was 85℃, the feeding rate was controlled at 10 mL / min, and after the spray drying was completed, the cooled material was collected to obtain product S1; the product S1 was added into a crucible and calcined in a tube furnace under a helium atmosphere, the temperature was raised to 350℃ at a rate of 10℃ / min, and the material was kept at 350℃ for 1.5 h; after the material was naturally cooled, it was passed through a 200-mesh sieve to obtain an intermediate product;

[0163] (3) The intermediate product obtained in step (2) and a total mass of 12% phenolic resin (Mv of 5000±1000) were uniformly mixed at a speed of 60 rpm for 2 h using a three-dimensional mixer to obtain a mixed material; the mixed material was calcined in a tube furnace under a helium atmosphere, the temperature was raised to 900℃ at a rate of 10℃ / min, and the material was kept at 900℃ for 3 h; after the material was cooled, it was passed through a 300-mesh sieve to obtain the double-layer carbon-coated silicon-based negative electrode material.

[0164] Example 7

[0165] The present example provides a double-layer carbon-coated silicon-based negative electrode material, which is prepared by the following steps:

[0166] (1) Nanosilicon powder with a particle size of 170 nm and PVP with a viscosity-average molecular weight of 90000 were dispersed in THF at a mass ratio of 0.1:1, and ultrasonic dispersion was performed at a power of 500 W for 36 h to obtain a mixed slurry with a solid content of 40%;

[0167] (2) The mixed slurry obtained in step (1) was subjected to spray drying, the feeding temperature was 190℃, the discharging temperature was 100℃, the feeding rate was controlled at 10 mL / min, after the spray drying was completed, the cooled material was collected to obtain product S1; the product S1 was added into a crucible, and calcination was performed in a tube furnace under a helium atmosphere, the temperature was raised to 400℃ at a rate of 10℃ / min, and the material was kept at this temperature for 2 h; after the material was naturally cooled, it was passed through a 200-mesh sieve to obtain an intermediate product;

[0168] (3) The intermediate product obtained in step (2) was uniformly mixed with 10% phenolic resin (Mv 5000±1000) in total mass using a three-dimensional mixer at a speed of 60 rpm for 2 h to obtain a mixture; the mixture was calcined in a tube furnace under a helium atmosphere, the temperature was raised to 900℃ at a rate of 10℃ / min, and the material was kept at this temperature for 3 h; after the material was cooled, it was passed through a 300-mesh sieve to obtain the double-layer carbon-coated silicon-based negative electrode material.

[0169] Example 8

[0170] The present example provides a double-layer carbon-coated silicon-based negative electrode material, which is prepared by the following steps:

[0171] (1) Nanosilicon powder with a particle size of 30 nm and PAM with a viscosity-average molecular weight of 8000 were dispersed in NMP at a mass ratio of 9:1, and ultrasonic dispersion was performed at a power of 500 W for 0.5 h to obtain a mixed slurry with a solid content of 20%;

[0172] (2) The mixed slurry obtained in step (1) was subjected to spray drying, the feeding temperature was 180℃, the discharging temperature was 90℃, the feeding rate was controlled at 10 mL / min, after the spray drying was completed, the cooled material was collected to obtain product S1; the product S1 was added into a crucible, and calcination was performed in a tube furnace under a helium atmosphere, the temperature was raised to 450℃ at a rate of 10℃ / min, and the material was kept at this temperature for 2.5 h; after the material was naturally cooled, it was passed through a 200-mesh sieve to obtain an intermediate product;

[0173] (3) The intermediate product obtained in step (2) is uniformly mixed with 8% phenolic resin (Mv is 5000±1000) in total mass using a three-dimensional mixer at a speed of 60 rpm for 2 h to obtain a mixture; the mixture is calcined in a tube furnace under a helium atmosphere, heated to 900°C at a rate of 10°C / min, and kept for 3 h; after the material cools, it is sieved through a 300-mesh sieve to obtain the double-layer carbon-coated silicon-based negative electrode material.

[0174] Example 9

[0175] The example provides a double-layer carbon-coated silicon-based negative electrode material, which is prepared by the following steps:

[0176] (1) Nanosilicon powder with a particle size of 400 nm and PAA with a viscosity-average molecular weight of 15000 are dispersed in ethanol at a mass ratio of 0.5:1, and a magnetic stirrer is used to stir at a speed of 500 rpm for 2 h to obtain a mixed slurry with a solid content of 10%.

[0177] (2) The mixed slurry obtained in step (1) is spray dried, the feeding temperature is 200°C, the discharging temperature is 100°C, the feeding rate is controlled at 10 mL / min, and after spray drying, the cooled material is collected to obtain product S1; product S1 is added to a crucible and calcined in a tube furnace under a helium atmosphere, heated to 500°C at a rate of 15°C / min, and kept for 1 h; after the material cools naturally, it is sieved through a 300-mesh sieve to obtain an intermediate product;

[0178] (3) The intermediate product obtained in step (2) is uniformly mixed with 5% pitch in total mass using a three-dimensional mixer at a speed of 60 rpm for 2 h to obtain a mixture; the mixture is calcined in a tube furnace under a helium atmosphere, heated to 900°C at a rate of 10°C / min, and kept for 3 h; after the material cools, it is sieved through a 300-mesh sieve to obtain the double-layer carbon-coated silicon-based negative electrode material.

[0179] Example 10

[0180] The example provides a double-layer carbon-coated silicon-based negative electrode material, which is prepared by the following steps:

[0181] (1) Nanosilicon powder with a particle size of 33 nm and PANI with a viscosity-average molecular weight of 40000 are dispersed in pure water at a mass ratio of 0.2:1, and a magnetic stirrer is used to stir at a speed of 500 rpm for 1 h to obtain a mixed slurry with a solid content of 40%;

[0182] (2) The mixed slurry obtained in step (1) is subjected to spray drying, the feeding temperature is 190°C, the discharging temperature is 85°C, the feeding rate is controlled at 10 mL / min, after the spray drying is completed, the cooled material is collected to obtain product S1; the product S1 is added into a crucible, calcination is carried out in a tube furnace under a helium atmosphere, the temperature is raised to 550°C at a rate of 15°C / min, and the temperature is kept for 1.5 h; after the material is naturally cooled, it is passed through a 300-mesh sieve to obtain an intermediate product;

[0183] (3) The intermediate product obtained in step (2) is uniformly mixed with pitch with a total mass of 12% for 2 h using a three-dimensional mixer at a rotating speed of 60 rpm to obtain a mixed material; the mixed material is calcined in a tube furnace under a helium atmosphere, the temperature is raised to 900°C at a rate of 10°C / min, and the temperature is kept for 3 h; after the material is cooled, it is passed through a 300-mesh sieve to obtain the double-layer carbon-coated silicon-based negative electrode material.

[0184] Example 11

[0185] The embodiment provides a double-layer carbon-coated silicon-based negative electrode material, which is prepared through the following steps:

[0186] (1) Nanometer silicon powder with a particle size of 50 nm and PAA with a viscosity-average molecular weight of 60,000 are dispersed in NMP at a mass ratio of 1:1, ultrasonic dispersion is carried out for 24 h using a power of 500 W to obtain a mixed slurry with a solid content of 3%;

[0187] (2) The mixed slurry obtained in step (1) is subjected to spray drying, the feeding temperature is 200°C, the discharging temperature is 95°C, the feeding rate is controlled at 10 mL / min, after the spray drying is completed, the cooled material is collected to obtain product S1; the product S1 is added into a crucible, calcination is carried out in a tube furnace under a helium atmosphere, the temperature is raised to 600°C at a rate of 15°C / min, and the temperature is kept for 2 h; after the material is naturally cooled, it is passed through a 300-mesh sieve to obtain an intermediate product;

[0188] (3) The intermediate product obtained in step (2) is uniformly mixed with pitch with a total mass of 10% for 2 h using a three-dimensional mixer at a rotating speed of 60 rpm to obtain a mixed material; the mixed material is calcined in a tube furnace under a helium atmosphere, the temperature is raised to 900°C at a rate of 10°C / min, and the temperature is kept for 3 h; after the material is cooled, it is passed through a 300-mesh sieve to obtain the double-layer carbon-coated silicon-based negative electrode material.

[0189] Example 12

[0190] The embodiment provides a double-layer carbon-coated silicon-based negative electrode material, which is prepared through the following steps:

[0191] (1) Nanometer silicon powder with a particle size of 90 nm and PANI with a viscosity average molecular weight of 100000 were dispersed in pure water at a mass ratio of 3:1, ultrasonic dispersion was performed for 2 h using a power of 500 W, and a mixed slurry with a solid content of 8% was obtained;

[0192] (2) The mixed slurry obtained in step (1) was spray dried, the feeding temperature was 175°C, the discharging temperature was 90°C, the feeding rate was controlled to be 10 mL / min, after the spray drying was completed, the cooled material was collected, and product S1 was obtained; product S1 was added into a crucible, calcination was performed in a tube furnace under a helium atmosphere, the temperature was increased to 450°C at a rate of 15°C / min, and the temperature was kept for 2.5 h; after the material was naturally cooled, it was passed through a 300-mesh sieve, and an intermediate product was obtained;

[0193] (3) The intermediate product obtained in step (2) was uniformly mixed with asphalt with a total mass of 5% using a three-dimensional mixer at a rotating speed of 60 rpm for 2 h, and a mixed material was obtained; the mixed material was calcined in a tube furnace under a helium atmosphere, the temperature was increased to 900°C at a rate of 10°C / min, and the temperature was kept for 3 h; after the material was cooled, it was passed through a 300-mesh sieve, and the double-layer carbon-coated silicon-based negative electrode material was obtained.

[0194] Comparative Example 1

[0195] This comparative example provides a silicon-based negative electrode material, which is prepared by the following steps:

[0196] (1) Nanometer silicon powder with a particle size of 50 nm was dispersed in pure water using a magnetic stirrer at a rotating speed of 500 rpm for 1 h, and a mixed slurry with a solid content of 8% was obtained;

[0197] (2) The mixed slurry obtained in step (1) was spray dried, the feeding temperature was 185°C, the discharging temperature was 95°C, the feeding rate was controlled to be 10 mL / min, after the spray drying was completed, the cooled material was collected, and product S1 was obtained; product S1 was added into a crucible, calcination was performed in a tube furnace under a helium atmosphere, the temperature was increased to 350°C at a rate of 10°C / min, and the temperature was kept for 2.5 h; after the material was naturally cooled, it was passed through a 300-mesh sieve, and an intermediate product was obtained;

[0198] (3) The intermediate product obtained in step (2) was calcined in a tube furnace under a helium atmosphere, the temperature was increased to 900°C at a rate of 10°C / min, and the temperature was kept for 3 h; after the material was cooled, it was passed through a 300-mesh sieve, and a silicon-based negative electrode material was obtained.

[0199] Comparative Example 2

[0200] This comparative example provides a silicon-based negative electrode material, which is prepared by the following steps:

[0201] (1) Nanometer silicon powder with a particle size of 200 nm and PAA with a viscosity average molecular weight of 100000 were dispersed in pure water at a mass ratio of 3:1, a magnetic stirrer was used to stir at a speed of 500 rpm for 2 h, and a mixed slurry with a solid content of 8% was obtained;

[0202] (2) The mixed slurry obtained in step (1) was spray dried, the feeding temperature was 185°C, the discharging temperature was 90°C, the feeding rate was controlled at 10 mL / min, after the spray drying was completed, the cooled material was collected, and product S1 was obtained; the product S1 was added into a crucible, calcined in a tube furnace under a helium atmosphere, heated to 550°C at a rate of 10°C / min, and kept for 2.5 h, after the material was naturally cooled, it was passed through a 300-mesh sieve, and an intermediate product was obtained;

[0203] (3) The intermediate product obtained in step (2) was calcined in a tube furnace under a helium atmosphere, heated to 900°C at a rate of 10°C / min, and kept for 3 h, after the material was cooled, it was passed through a 300-mesh sieve, and a silicon-based negative electrode material was obtained.

[0204] Comparative Example 3

[0205] This comparative example provides a silicon-based negative electrode material, which is prepared by the following steps:

[0206] (1) Micron silicon powder with a particle size of 100 μm and PVA with a viscosity average molecular weight of 50000 were dispersed in pure water at a mass ratio of 5:1, an ultrasonic wave with a power of 500 W was used for dispersion for 2 h, and a mixed slurry with a solid content of 10% was obtained;

[0207] (2) The mixed slurry obtained in step (1) was spray dried, the feeding temperature was 180°C, the discharging temperature was 85°C, the feeding rate was controlled at 10 mL / min, after the spray drying was completed, the cooled material was collected, and product S1 was obtained; the product S1 was added into a crucible, calcined in a tube furnace under a helium atmosphere, heated to 500°C at a rate of 15°C / min, and kept for 3 h, after the material was naturally cooled, it was passed through a 200-mesh sieve, and an intermediate product was obtained;

[0208] (3) The intermediate product obtained in step (2) was uniformly mixed with 5% pitch in total mass using a three-dimensional mixer at a speed of 60 rpm for 2 h, and a mixed material was obtained; the mixed material was calcined in a tube furnace under a helium atmosphere, heated to 900°C at a rate of 10°C / min, and kept for 3 h, after the material was cooled, it was passed through a 300-mesh sieve, and a silicon-based negative electrode material was obtained.

[0209] Comparative Example 4

[0210] The comparative example provides a silicon-based negative electrode material, which is different from example 1 only in that the PVA in step (1) is replaced by an equal mass and equal molecular weight phenolic resin, and other settings are completely consistent with example 1.

[0211] Comparative example 5

[0212] The comparative example provides a silicon-based negative electrode material, which is different from example 1 only in that the PVA in step (1) is replaced by an equal mass and equal molecular weight phenolic resin, and other settings are completely consistent with example 1.

[0213] Comparative example 6

[0214] The comparative example provides a silicon-based negative electrode material, which is different from example 1 only in that the PVA in step (1) is replaced by an equal mass and equal molecular weight phenolic resin, and other settings are completely consistent with example 1.

[0215] Test example 1

[0216] Test sample: double-layer carbon-coated silicon-based negative electrode materials provided by examples 1-12, and silicon-based negative electrode materials provided by comparative examples 1-6.

[0217] Test method: scanning electron microscopy, Raman spectroscopy area scanning.

[0218] The specific test results are shown in Tables 1 and 2 as follows:

[0219] Table 1

[0220]

[0221]

[0222] Table 2

[0223]

[0224]

[0225] As shown in Tables 1 and 2, the double-layer carbon-coated silicon-based negative electrode materials prepared in examples 1-12; wherein the pore size of the loose silicon-carbon composite layer is 12-40 nm, the porosity is 48-68%, the specific surface area is 85-135 m 2 / g, the Id / Ig range is 1.1-1.52, the pore volume is 0.3-0.58 cm 3 / g, the true density is 1.78-2.18 g / cm 3; thus, it is illustrated that a large number of pores exist between surface particles of the silicon-carbon composite layer, forming a continuous three-dimensional interconnected porous network, and the pores are uniformly distributed, and obviously, the pores can provide effective buffer space for the huge volume expansion of silicon particles in subsequent charge and discharge processes. The pore diameter of the dense carbon layer is 0.2-5 nm, the porosity is 4-15%, the specific surface area is 9-18 m 2 / g, the Id / Ig range is 0.82-1.15, the pore volume is 0.05-0.2 cm 3 / g, and the true density is 2.05-2.28 g / cm 3 ; thus, it is illustrated that the outermost carbon layer of the silicon-based negative electrode material finally obtained by the application basically presents no obvious open pores, has a flat, continuous and dense carbon layer structure morphology, the dense carbon shell provides a stable skeleton, and provides a good electronic conduction path for the internal silicon-carbon core, and effectively inhibits the penetration of electrolyte and the occurrence of side reactions, thereby improving the initial coulombic efficiency and cycle stability of the material.

[0226] Test Example 1

[0227] Test sample: double-layer carbon-coated silicon-based negative electrode material provided by Examples 1-12, and silicon-based negative electrode material provided by Comparative Examples 1-6.

[0228] Test method: the above samples are respectively made into negative electrode sheets, and assembled into button-type half batteries to test the electrochemical performance.

[0229] The specific test results are shown in Table 3.

[0230] Table 3

[0231]

[0232]

[0233] As shown in Table 2, the lithium ion battery assembled from the double-layer carbon-coated silicon-based negative electrode material prepared by Examples 1-12 has a 0.1C charge specific capacity of 1854.91-1947.22 mAh / g, a 0.1C discharge specific capacity of 2078.63-2199.50 mAh / g, a 0.1C initial coulombic efficiency of 88.24-89.70%, and a 0.8V initial coulombic efficiency of 83.67-84.97%. This fully illustrates that the lithium ion battery assembled from the double-layer carbon-coated silicon-based negative electrode material of the application has excellent charge specific capacity, discharge specific capacity and initial coulombic efficiency.

[0234] In addition, Figure 3 , Figure 4 , Figure 5 and Figure 6The cycle curves of the silicon-based negative electrode materials prepared in example 1, comparative example 1, comparative example 2 and comparative example 3 respectively at 100 cycles, the cycle stability of the silicon-based negative electrode material prepared in example 1 is obviously superior to that of the silicon-based negative electrode materials prepared in comparative examples 1-3, which indicates that the silicon-based negative electrode material prepared in example 1 has obvious effect on relieving volume expansion.

[0235] Finally, it should be noted that: the above examples are only used to illustrate the technical solutions of the present application, but not to limit them; although the present application has been described in detail with reference to the foregoing examples, those skilled in the art should understand that: it can still modify the technical solutions recorded in the foregoing examples, or make equivalent replacement for part or all of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present application.

Claims

1. A silicon-based anode material with double-layer carbon coating, characterized in that, The silicon-based anode material comprises, from the inside out, a silicon-carbon core, a loose silicon-carbon composite layer, and a dense carbon layer. The porosity of the loose silicon-carbon composite layer is 30-70%. The porosity of the dense carbon layer is 1-30%.

2. The double-layer carbon-coated silicon-based anode material according to claim 1, characterized in that, The pore size of the loose silicon-carbon composite layer is 1–300 nm, preferably 10–50 nm; Preferably, the specific surface area of ​​the porous silicon-carbon composite layer is 1–400 m². 2 / g, preferably 90-150m 2 / g; Preferably, the thickness of the porous silicon-carbon composite layer is 0.001–10 μm; Preferably, the Raman spectrum Id / Ig range of the porous silicon-carbon composite layer is 1.0 to 1.5; Preferably, the pore volume of the porous silicon-carbon composite layer is 0.1–0.6 cm. 3 / g; Preferably, the true density of the porous silicon-carbon composite layer is 1.8–2.3 g / cm³. 3 ; Preferably, the mass ratio of silicon to carbon in the porous silicon-carbon composite layer is (10-30):(90-70).

3. The silicon-based anode material with double-layer carbon coating according to claim 1, characterized in that, The pore size of the dense carbon layer is 1–50 nm, preferably 1–5 nm; Preferably, the specific surface area of ​​the dense carbon layer is 1–30 m². 2 / g, preferably 5-20m 2 / g; Preferably, the thickness of the dense carbon layer is 0.001–10 μm; Preferably, the Raman spectrum Id / Ig range of the dense carbon layer is 0.8 to 1.2; Preferably, the pore volume of the dense carbon layer is 0.05–0.15 cm³. 3 / g; Preferably, the true density of the dense carbon layer is 2.0–2.3 g / cm³. 3 .

4. The silicon-based anode material with double-layer carbon coating according to claim 1, characterized in that, The particle size of the silicon-carbon core is 0.01–20 μm; Preferably, the silicon-carbon core comprises silicon particles and carbon particles; Preferably, the silicon particles have a particle size of 1–500 nm, and the carbon particles have a particle size of 1–300 nm. Preferably, the mass ratio of silicon to carbon in the silicon-carbon core is (50-80):(50-20).

5. A method for preparing a silicon-based anode material with double-layer carbon coating according to any one of claims 1 to 4, characterized in that, The preparation method includes: (1) Disperse the nano-silicon particles and the first carbon source in a solvent, so that the first carbon source coats the surface of the nano-silicon particles to obtain a mixed slurry; (2) After drying the mixed slurry, it is subjected to pre-carbonization treatment to obtain an intermediate product; (3) The intermediate product and the second carbon source are mixed and carbonized to obtain the double-layer carbon-coated silicon-based anode material.

6. The method for preparing the double-layer carbon-coated silicon-based anode material according to claim 5, characterized in that, In step (1), the first carbon source includes any one or a combination of at least two of polyvinyl alcohol, polyethylene glycol, polyvinylpyrrolidone, carboxymethyl cellulose, polyacrylic acid, polyacrylamide, polyimide, polyaniline, polyethyleneimine or chitosan; Preferably, in step (1), the mass ratio of the nano-silicon particles to the first carbon source is (0.01~100):1; Preferably, in step (1), the solvent includes any one or a combination of at least two of water, ethanol, ethylene glycol, N-methylpyrrolidone or tetrahydrofuran; Preferably, in step (1), the solid content of the mixed slurry is 1-50%; Preferably, in step (1), the dispersion includes any one or a combination of at least two of mechanical dispersion, stirring dispersion, or ultrasonic dispersion; Preferably, in step (1), the rotation speed of the mechanical dispersion is 20 to 1500 rpm, and the mechanical dispersion time is 0.1 to 36 h; Preferably, in step (1), the stirring speed is 20 to 1000 rpm, and the mechanical dispersion time is 0.1 to 36 h; Preferably, in step (1), the ultrasonic dispersion power is 100-1000W and the ultrasonic dispersion time is 0.1-36h.

7. The method for preparing the double-layer carbon-coated silicon-based anode material according to claim 5, characterized in that, In step (2), the drying includes spray drying; Preferably, in step (2), the parameters of the spray drying include: an inlet air temperature of 130-250°C, an outlet air temperature of 80-200°C, and a feed rate of 5-90 mL / min; Preferably, in step (2), the pre-carbonization treatment is carried out in a protective gas atmosphere; Preferably, the protective gas includes any one or a combination of at least two of argon, nitrogen, or helium; Preferably, in step (2), the heating rate of the pre-carbonization treatment is 1 to 20 °C / min, the temperature of the pre-carbonization treatment is 300 to 800 °C, and the holding time of the pre-carbonization treatment is 0.5 to 10 h.

8. The method for preparing the double-layer carbon-coated silicon-based anode material according to claim 5, characterized in that, In step (3), the second carbon source includes any one or a combination of at least two of glucose, sucrose, starch, phenolic resin or asphalt; Preferably, in step (3), the mass ratio of the intermediate product to the second carbon source is (0.01 to 100):1; Preferably, in step (3), the carbonization process is carried out in a protective gas atmosphere; Preferably, the protective gas includes any one or a combination of at least two of argon, nitrogen, or helium; Preferably, in step (3), the heating rate of the carbonization treatment is 1 to 20 °C / min, the carbonization temperature is 500 to 1500 °C, and the holding time of the carbonization treatment is 0.5 to 10 h. Preferably, in step (3), the carbonization process further includes cooling and sieving steps; Preferably, the sieve mesh size is 60 to 1000 mesh.

9. The application of a silicon-based anode material with double-layer carbon coating according to any one of claims 1 to 4 in the preparation of anodes for lithium-ion batteries.

10. A lithium-ion battery, characterized in that, The lithium-ion battery includes the silicon-based anode material with double-layer carbon coating as described in any one of claims 1 to 4.

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