CN110474025A - A kind of multi-stage buffering structure silicon-carbon cathode material and its preparation method and application - Google Patents

Abstract

The invention provides a method for preparing a silicon-carbon composite material for a lithium-ion battery. The silicon-carbon negative electrode material is formed by mixing nanoporous silicon, graphite, and pitch in a certain proportion, and is calcined to obtain a final product, wherein the silicon-carbon composite material is Microspheres with a multi-level buffer structure. The nanoporous silicon is obtained by magnesium thermal reduction of diatomite and nanometerization by sand milling, which is uniformly dispersed inside the silicon-carbon composite microspheres, wherein the particle size of silicon is less than 200 nm and there is a uniform coating layer on the surface. The silicon-carbon negative electrode material has high efficiency, large capacity and good cycle stability when used in lithium ion batteries, and the preparation method of the silicon-carbon negative electrode material is simple and low in cost, and is suitable for large-scale production.

Description

A multi-level buffer structure silicon carbon negative electrode material and its preparation method and application

technical field

The invention relates to a silicon-carbon negative electrode material with a multilevel buffer structure, a preparation method thereof and an application as a negative electrode material of a lithium ion battery.

Background technique

Lithium-ion batteries have been widely used in portable electronic devices and electric vehicles because of their outstanding advantages such as no pollution, long service life, small size, and fast charging and discharging. In recent years, with the increasing requirement for battery energy density, the current battery material system has gradually failed to meet the high energy density requirement. As far as negative electrode materials are concerned, graphite negative electrode materials have been widely used in commercial lithium-ion batteries, but the theoretical capacity of graphite negative electrode materials is only 372mAh/g, which cannot meet the requirements of small, light, long-term driving of lithium-ion batteries. development requirements. Therefore, the development of new anode material systems has always been the focus and hotspot of research and development.

Silicon-based anode materials are one of the most promising anode material systems in the future due to their high specific capacity and low lithium-extraction potential. However, during the process of lithium intercalation and deintercalation, silicon produces a huge volume change, which easily leads to the destruction of the electrode structure and the unstable SEI film, and finally causes the battery capacity to decay rapidly, which seriously limits the application of silicon-based negative electrode materials in lithium-ion batteries. application. In recent years, the cycle stability of silicon has been improved mainly through nanonization of silicon and silicon-based composite materials, but the preparation process is complicated and the yield is low, making it difficult to achieve commercial mass production.

Designing silicon-carbon composites with a multi-level buffer structure is one of the best ways to improve the silicon volume expansion problem. In the document Controlled synthesis of yolk-mesoporous shell Si@SiO ₂ nanohybrid designed for high performance Li ion battery. RSC Adv., 2014, 4(40): 20814-20820, Sun et al. used the template method to prepare a double carbon layer coating The silicon-carbon composite material with the egg yolk and eggshell structure uses the inner and outer double-layer carbon shells and the cavity between the silicon particles and the carbon layer to build a multi-level buffer structure to relieve the volume expansion of silicon. In the patent 201710232281.7, a silicon-carbon anode material with a core-shell coating structure was obtained. First, dopamine was used as a carbon source to obtain carbon-coated silicon particles through the template method, and then the carbon-coated silicon particles were obtained by graphene. Sub-coating and firing to obtain the final silicon-carbon anode material with a shell-coated structure. Most of the current silicon-carbon composite materials with multi-level buffer structure need to use more complicated preparation methods such as template method, which is extremely costly and difficult to meet the needs of commercial lithium-ion battery applications.

Contents of the invention

The purpose of the present invention is to provide a method for preparing a multi-level buffer structure silicon-carbon composite material and the silicon-carbon composite material prepared by the method and its application. The method of the present invention is simple to operate and lower in cost, and the obtained silicon-carbon composite material It has high electrochemical performance.

In order to achieve the above object, the present invention provides a silicon-carbon composite material with a multi-level buffer structure on the one hand. Secondary buffer structure, the skeleton structure constructed by flake graphite is used as the third buffer structure. The asphalt coating is further divided into a bulk coating and a surface coating. Based on the total weight of the negative electrode material, the silicon content is 10%-30%, and the carbon content is 70%-90%.

The second aspect of the present invention provides a method for preparing a silicon-carbon composite material, the method comprising:

(1) Diatomite and metal magnesium powder are ball-milled and mixed evenly, and high-temperature reduction reaction is carried out under the protection of an inert gas.

(2) The reduction product obtained in step (1) is firstly acid-washed, then washed with water until the pH is 6 to 8, centrifuged, and vacuum-dried to obtain porous silicon.

(3) The porous silicon obtained in the step (2) is sand-milled and nano-sized, and then centrifuged and vacuum-dried to obtain nano-porous silicon.

(4) The nanoporous silicon obtained in the step (3), graphite flakes and pitch are wet-milled at a high speed to obtain a uniformly mixed slurry.

(5) After the slurry obtained in step (4) is granulated by a spray dryer, the prepared silicon-carbon material is roasted under an inert atmosphere to obtain a multi-level buffer structure silicon-carbon negative electrode material.

Wherein the mass ratio of nanoporous silicon, flake graphite and pitch is 1:2-10:0.5-3, and the roasting temperature is 600-1200 °C.

The second aspect of the present invention provides the silicon-carbon composite material prepared by the above method.

The third aspect of the present invention provides a negative electrode comprising the above-mentioned silicon-carbon composite material.

A fourth aspect of the present invention provides a lithium ion battery comprising the above negative electrode.

The method of the present invention has the following advantages:

(1) Using diatomite as raw material, porous silicon is obtained through magnesia thermal reduction, and finally nano-porous silicon is obtained by sand grinding and crushing, realizing the transformation from cheap ore raw materials to high value-added energy materials.

(2) The silicon-carbon anode material is synthesized by spray granulation technology. The method is simple. It only needs to spray and granulate the uniformly mixed slurry in one step, and after roasting, the silicon-carbon composite material with multi-level buffer structure can be obtained.

(3) Graphite in the prepared multi-level buffer structure silicon-carbon composite material is used as a supporting framework, which can improve the dispersion effect and conductivity of silicon particles; asphalt is used as a binder and a coated carbon layer to tightly combine silicon particles and graphite , and form a conductive network together with graphite, and the amorphous carbon formed by pitch carbonization can also improve the interface performance between silicon and electrolyte.

(4) Compared with the traditional coated silicon-carbon anode materials, the nano-sized porous silicon in the multi-level buffer structure silicon-carbon composite material is used as the first-level buffer structure, and the asphalt coating is used as the second-level buffer structure. The skeleton structure acts as a third-level buffer structure, and the multi-level buffer structure can better alleviate the volume expansion of silicon.

Description of drawings

The X-ray diffraction pattern of porous silicon in Fig. 1 embodiment 1

The X-ray diffraction pattern of Si/C-1 in Fig. 2 embodiment 1

The scanning electron micrograph of Si/C-1 in Fig. 3 embodiment 1

Relative to Fig. 3, the partially enlarged scanning electron microscope image of Si/C-1 in Fig. 4 embodiment 1

The transmission electron microscope figure of Si/C-1 in Fig. 5 embodiment 1

Partially enlarged transmission electron microscope diagram of Si/C-1 in Fig. 6 embodiment 1 relative to Fig. 5

The high-resolution transmission electron microscope picture of Si/C-1 in Fig. 7 embodiment 1

Detailed ways

Neither the endpoints nor any values of the ranges disclosed herein are limited to such precise ranges or values, and these ranges or values are understood to include values approaching these ranges or values. For numerical ranges, between the endpoints of each range, between the endpoints of each range and individual point values, and between individual point values can be combined with each other to obtain one or more new numerical ranges, these values Ranges should be considered as specifically disclosed herein.

One aspect of the present invention provides a silicon-carbon composite material with a multi-level buffer structure. The silicon-carbon composite material includes nano-sized porous silicon as the first-level buffer structure, an asphalt coating layer as the second-level buffer structure, and is constructed of graphite flakes. The skeleton structure serves as the third level buffer structure.

According to the present invention, in the silicon-carbon composite material of the present invention, the first-level buffer structure is composed of nano-sized porous silicon, and the nano-sized porous silicon is an elemental silicon particle with a porous structure at the nanoscale level , preferably, the nanometerized porous silicon particles have a particle diameter of 80-150 nm. The second-level buffer structure is composed of amorphous carbon formed after pitch carbonization, wherein the amorphous carbon includes the interior of the bulk phase of the silicon-carbon composite material and the surface coating layer, and the thickness of the preferred surface amorphous carbon coating layer is between 10-20nm. The tertiary buffer structure is composed of a supporting framework composed of graphite flakes. The size of the flake graphite is 1-10 μm, and the size of the supporting framework composed of the flake graphite is 5-23 μm.

In the present invention, the nanoporous silicon is loaded on flake graphite, and between the silicon particles and the flake graphite, the flake graphite is covered by the amorphous form formed after pitch carbonization, and the flake graphite forms a supporting framework The combination between graphite and graphite is not completely tight, and it can be understood that the amorphous carbon formed by carbonization of the pitch forms a three-dimensional network structure.

Preferably, in the silicon-carbon composite material, the particle size of the silicon-carbon composite material with a multi-level buffer structure formed of porous nano-silicon-graphite-pitch is about 5-23 μm.

According to the present invention, preferably, in the silicon-carbon composite material, the content of silicon element is 10-30% by weight, and the content of carbon element is 70-90% by weight.

The second aspect of the present invention provides a method for preparing a multi-level buffer structure silicon-carbon composite material, the method comprising: according to the present invention, preferably, in the silicon-carbon composite material, the content of silicon element is 10-30% by weight , the content of carbon element is 70-90% by weight.

The second aspect of the present invention provides a method for preparing a multi-level buffer structure silicon-carbon composite material, the method comprising:

(1) Diatomite and metal magnesium powder are ball-milled and mixed evenly, and high-temperature reduction reaction is carried out under the protection of an inert gas.

(2) The reduction product obtained in step (1) is firstly acid-washed, then washed with water until the pH is 6 to 8, centrifuged, and vacuum-dried to obtain porous silicon.

(3) The porous silicon obtained in the step (2) is sand-milled and nano-sized, and then centrifuged and vacuum-dried to obtain nano-porous silicon.

(4) The nanoporous silicon obtained in the step (3), graphite flakes and pitch are wet-milled at a high speed to obtain a uniformly mixed slurry.

(5) After the slurry obtained in step (4) is granulated by a spray dryer, the prepared silicon-carbon material is roasted under an inert atmosphere to obtain a multi-level buffer structure silicon-carbon negative electrode material.

Wherein the mass ratio of nanoporous silicon, flake graphite and pitch is 1:2-10:0.5-3, and the roasting temperature is 600-1200 °C.

According to the present invention, in the step (1), the silicon element in the diatomaceous earth undergoes a magnesia thermal reaction with metal magnesium, so that the silicon is basically completely converted into elemental silicon, while the magnesium exists as magnesium oxide, and the remaining part may also be untreated. Fully reacted elemental magnesium. In step (2), pickling is used to remove magnesium oxide and the remaining incompletely reacted elemental magnesium to obtain elemental silicon with a porous structure. In step (3), a sand mill is used to perform nano-processing on the porous silicon to reduce its size to 80-150 nm. In step (4), nanoporous silicon, graphite flakes and asphalt are wet ball milled at a high speed to obtain a uniformly mixed slurry. In step (5), the nanoporous silicon-graphite-pitch precursor is obtained by spray granulation technology, and the silicon-carbon negative electrode material with multi-level buffer structure is obtained through high-temperature calcination.

Among them, in step (1), in order to fully convert the silicon in the silicon-containing raw material into elemental silicon, preferably, the molar ratio of diatomaceous earth to magnesium powder is 1:0.8-1.2, preferably 1:0.85-1 . The magnesium thermal reaction is preferably such that in the step (1), in the product of the magnesium thermal reaction, the silicon element exists in the form of elemental silicon, and the magnesium element exists in the form of magnesium oxide and optional metal magnesium. More preferably, the product of the magnesium thermal reaction is made to consist of elemental silicon, magnesium oxide and optionally metallic magnesium.

Wherein, the particle size of the silicon-containing raw material can vary within a wide range, preferably, the particle size of the silicon-containing raw material is 1-50 μm, preferably 1-30 μm, more preferably 1-20 μm.

According to the present invention, the magnesium metal is preferably provided in the form of magnesium powder, especially magnesium powder with a particle size of 1-100 μm.

According to the present invention, in order to make the silicon-containing raw material more fully contact with the magnesium metal, the silicon-containing raw material and the metal magnesium can be mixed first, for example, by grinding and mixing, and then sent to the magnesium alloy for the magnesium thermal reaction. thermal reaction. The temperature of the magnesium thermal reaction is 650-750°C, preferably 650-700°C. Preferably, the time for the magnesium thermal reaction is 1-10 h, preferably 2-8 h. Wherein, the magnesothermic reaction can be carried out in any reactor capable of achieving the above conditions, for example, the silicon-containing raw material and metal magnesium can be sealed in the reactor first, and then the reactor can be placed in a tube furnace for heating.

Wherein, the inert atmosphere is one of the following: nitrogen, argon.

According to the present invention, the pickling process described in step (2) is to use one or more of hydrochloric acid, sulfuric acid, and hydrofluoric acid to carry out pickling in sequence, and the pickling process removes magnesium oxide, metal magnesium and some unreacted diatoms Soil, silicon particles with a porous structure are obtained. For example, the acid solution used in the pickling is hydrochloric acid, and its concentration is preferably 1-5 mol/L.

According to the present invention, the amount of the acid solution can be varied within a wide range, preferably, relative to 1 g of the roasted product, the amount of the acid solution is 50-100 mL.

According to the present invention, the sanding process described in step (3) can further reduce the size of the porous silicon obtained by the magnesia thermal reaction, making it nano-sized porous silicon. Wherein the speed of the sand mill is 1800-2400 r/min, preferably 2000-2300 r/min, the sand milling time is 0.1 h-3 h, preferably 0.5 h-2 h, the solvent used in the sand mill is water, ethanol, One or more mixtures of acetone.

According to the present invention, the purpose of the wet ball milling in step (4) is to mix nanoporous silicon, flake graphite, and asphalt into a uniform slurry to facilitate the operation of subsequent steps. Wherein the rotating speed of the ball mill is 300-1000 r/min, preferably 400-800 r/min; the solvent used in the wet ball milling is water, ethanol, acetone or a mixture of several. The mass ratio of nanoporous silicon, flake graphite and pitch is 1:2-10:0.5-3, preferably 1:2-8:0.5-2.

According to the present invention, the spray granulation process in step (5) is not only a process of drying the liquid slurry but also a process of granulating and sphericalizing the mixture of nanoporous silicon, flake graphite and asphalt.

The second aspect of the present invention provides the silicon-carbon composite material prepared by the above method.

As mentioned above, the method of the present invention can produce a silicon-carbon composite material with high electrochemical performance, especially high cycle performance, with simple operation, lower cost, and environmental protection. The silicon-carbon composite material is as described above. Among them, the carbon layer is mainly amorphous carbon.

The third aspect of the present invention provides a negative electrode comprising the above-mentioned silicon-carbon composite material.

According to the present invention, the above-mentioned silicon-carbon composite material is mainly distributed in the negative electrode material layer of the negative electrode as the negative electrode active material.

Wherein, the negative electrode mainly includes a negative electrode current collector and a negative electrode material layer formed thereon.

The anode material layer usually contains an anode active material, a conductive agent and a binder, wherein the anode material is the above-mentioned silicon-carbon composite material of the present invention, and the anode material commonly used in the art for both the conductor and the binder The conductive agent and binder used in the layer, for example, the conductive agent may be one or more of acetylene black, conductive graphite, conductive carbon black and the like. The binder can be, for example, one or more of PVDF, carboxymethyl cellulose, sodium carboxymethyl cellulose, styrene-butadiene rubber, and the like. Wherein, the weight ratio of the negative electrode active material, the conductive agent and the binder is preferably 70:5-15:15-25.

Wherein, the negative electrode current collector can be copper foil, copper mesh, etc., for example.

A fourth aspect of the present invention provides a lithium ion battery comprising the above negative electrode.

According to the present invention, the lithium-ion battery can be a conventional lithium-ion battery structure in the field, as long as it includes the above-mentioned negative electrode, and the present invention has no limitation on other components of the lithium-ion battery.

The present invention will be described in detail below by way of examples.

Example 1

This example is used to illustrate the multi-level buffer structure silicon-carbon composite material and its preparation method of the present invention.

(1) Mix 10g of diatomite (with a particle size of 1-20 μm, the same below) and 8.5 g of magnesium powder (with a particle size of 1-100 μm) by ball milling, the ball-to-material ratio is 1:10, and the protective gas is argon. Put the uniformly mixed powder into a closed reactor, seal it and place it in a tube furnace filled with argon with a purity of 99.9%, and raise the temperature from room temperature to 675°C at a rate of 5°C/min and keep it for 4 h to carry out Magnesium thermal reaction;

(2) After turning on the tube furnace, soak and wash the product of magnesia thermal reaction in 1mol/L dilute hydrochloric acid (the dosage of dilute hydrochloric acid is 100mL for 1g of the roasted product); then wash with water and use high-speed centrifugation The machine was centrifuged at 3900 rpm/min for 3 minutes, the supernatant was discarded and washed with water and centrifuged. This cycle was repeated three times until the pH of the supernatant was 7, and then washed with ethanol once and then centrifuged. The collected samples were placed in a vacuum-dried Dry in an oven at 70°C for 12 hours to obtain porous silicon particles.

(3) Disperse 5 g of porous silicon particles obtained in the appeal in 150 mL of ethanol solution and place them in a sand mill with a speed of 2200 rpm/min and a sanding time of 1.5 h. The product was centrifuged at 8000 rpm/ Centrifuge at a speed of min for 5 minutes, pour off the supernatant, and place the collected sample in a vacuum drying oven at 70° C. for 12 hours to obtain nanoporous silicon particles.

(4) Add 2 g of nanoporous silicon prepared by the appeal to 100 mL of deionized water, stir with a magnetic stirrer at a speed of 500 rpm/min, and stir for 0.5 h, then add 6 g of graphite and 2 g of asphalt powder at a speed of 800 rpm/min, stirring time 2 h. Transfer the stirred sample to a 300 mL stainless steel ball mill jar, and use a star ball mill for ball milling with a mass ratio of ball to material of 10:1, a rotational speed of 600 rpm, and a ball milling time of 8 h. The slurry obtained from the ball mill was placed in In a 250 mL beaker, the pellets were granulated using a spray dryer with an inlet temperature of 160 °C and an outlet temperature of 90 °C. The obtained material was calcined at 1000 °C for 2 h in an argon atmosphere with a heating rate of 5 °C/min. A multi-level buffer structure silicon-carbon negative electrode material Si/C-1 was obtained, and the content of silicon element was 23% by weight.

XRD identification results: Carry out X-ray diffraction to the product after magnesium thermal reaction and acid treatment, the result is shown in Figure 1, it can be seen that the XRD pattern 2θ of the product after magnesium thermal reaction and acid treatment is in the range of 10-80 ° There are clearly visible diffraction peaks, and all diffraction peaks can be indexed as cubic Si (JPCDS 77-2111). Carry out X-ray diffraction to Si/C-1 after spraying granulation, calcining, the result is shown in Figure 2, can see, the XRD spectrum 2θ of Si/C-1 composite material has clearly visible in the range of 10-80° The diffraction peaks are the characteristic peaks of graphite and silicon respectively.

SEM electron microscope image: Si/C-1 (see Figure 3 and Figure 4 for its SEM image) was analyzed using a scanning electron microscope; among them, the Si/C-1 composite material is a spherical particle with a relatively uniform particle size (Figure 3 ), the particle surface is relatively smooth and flat, and the particle size is about 20 μm (Fig. 4).

TEM electron microscope image: The Si/C-1 composite material (the TEM images are shown in Figure 5 and Figure 6) was analyzed by scanning electron microscope; among them, the Si/C-1 composite material is a dense solid particle (Figure 5) , Si/C-1 particles are uniformly dispersed with nanoporous silicon particles (Figure 6).

HRTEM electron microscope image: The Si/C-1 composite material was analyzed by high-resolution transmission electron microscope (see Figure 7 for its HRTEM image). There is a layer of 12 nm amorphous carbon coating on the surface of the Si/C-1 composite material, which is Amorphous carbon coating formed by carbonization of superficial pitch (Fig. 7).

Example 2

This example is used to illustrate the silicon-carbon composite material and its preparation method of the present invention.

According to the method described in Example 1, the difference is that the addition amount of flake graphite in step (4) is 7 g, and the addition amount of asphalt is 1 g; the silicon-carbon composite material Si/C-2 silicon content obtained finally It is 21% by weight.

Example 3

This example is used to illustrate the silicon-carbon composite material and its preparation method of the present invention.

According to the method described in Example 1, the difference is that the addition amount of flake graphite in step (4) is 6.5 g, and the addition amount of asphalt is 1.5 g; the content of the silicon-carbon composite Si/C-3 silicon element obtained finally is 20% by weight.

Example 4

This example is used to illustrate the silicon-carbon composite material and its preparation method of the present invention.

According to the method described in Example 1, the difference is that the addition amount of flake graphite in step (4) is 5.5 g, and the addition amount of asphalt is 2.5 g; the silicon-carbon composite material Si/C-4 silicon content finally obtained It is 23% by weight.

Comparative example 1

According to the method described in Example 1, the difference is that the amount of flake graphite added in step (4) is 8 g, and no pitch is added; the silicon-carbon composite material D1 finally obtained has a silicon content of 69% by weight.

Comparative example 2

According to the method described in Example 1, the difference is that the amount of flake graphite added in step (4) is 6 g, and 2 g of glucose is added instead of asphalt; the silicon-carbon composite material D2 finally obtained has a silicon content of 24 wt. %.

test case

Preparation of battery:

(1) Preparation of negative electrode: the silicon-carbon composite material obtained in the above example was used as the negative electrode active material, and acetylene black (purchased from Sichuan Kaiyuan Huineng New Material Technology Co., Ltd.), carboxymethyl cellulose (purchased from Sichuan Kaiyuan Huineng New Material Technology Co., Ltd.) Technology Co., Ltd.) and styrene-butadiene rubber (purchased from Sichuan Kaiyuan Huineng New Material Technology Co., Ltd.) were uniformly mixed in water at a weight ratio of 70:10:10:10 to prepare a negative electrode material slurry, which was then coated on the copper foil set On the fluid, it was dried in a vacuum oven at 80° C. for 10 h and then rolled into a negative electrode sheet.

(2) The negative pole piece is used as the working electrode, the lithium metal sheet is used as the counter electrode, the separator is Celgard 2400 separator (USA), and the electrolyte is LiPF ₆ (1mol/ L) mixed solution (the solvent is a mixed solvent of EC (ethylene carbonate), DMC (dimethyl carbonate) and DEC (diethyl carbonate) with a volume ratio of 1: 1: 1). Assemble the coin cells in a UN-Lab type glove box (O ₂ <1 ppm, H ₂ O <1 ppm). Thus, using silicon-carbon composite material Si/C-1 to silicon-carbon composite material Si/C-4 as negative electrode active materials, button cells C1-C4 will be produced, and silicon-carbon composite material D1 and elemental silicon material D2 will be used as negative electrode active materials. The negative active material will make button cells DC1 and DC2.

Charge-discharge specific capacity test: use the land battery test system to measure the first-time charge specific capacity and first-time Coulombic efficiency of the above-mentioned button batteries at a current density of 0.2 A/g, and the charge specific capacity after 100 cycles and the corresponding The Coulombic efficiency was measured, and the capacity retention after 100 cycles was calculated; the results are shown in Table 1.

Table 1

It can be seen from the results in Table 1 that the silicon-carbon composite material of the present invention has excellent electrochemical performance and can be used as a negative electrode active material.

The preferred embodiments of the present invention have been described in detail above, however, the present invention is not limited thereto. Within the scope of the technical concept of the present invention, various simple modifications can be made to the technical solution of the present invention, including the combination of various technical features in any other suitable manner, and these simple modifications and combinations should also be regarded as the disclosed content of the present invention. All belong to the protection scope of the present invention.

Claims (7)

1. A multi-level buffer structure silicon-carbon negative electrode material, which includes nano-sized porous silicon as the first-level buffer structure, asphalt coating as the second-level buffer structure, and a skeleton structure constructed of flake graphite as the third-level Buffer structure, wherein the asphalt coating layer is further divided into a bulk phase coating layer and a surface phase coating layer, calculated based on the total weight of the negative electrode material, wherein the silicon content is 10%-30%, and the carbon content is 70% -90%. 2. According to claim 1, the silicon-carbon negative electrode material is prepared from nanoporous silicon powder, graphite and pitch as raw materials, wherein the average particle diameter of nanoporous silicon powder is 10 nm-500 nm, preferably 40 nm-300 nm Graphite is flake graphite and spherical graphite, preferably flake graphite with an average particle size between 500 nm-30 μm, more preferably flake graphite with an average particle size between 1 μm-10 μm. 3. according to the preparation method of the multilevel buffer structure silicon carbon negative electrode material described in any one of claim 1 to 2, comprise the step of preparing nanoporous silicon, the step of preparing silicon carbon negative electrode precursor, and calcining silicon carbon negative electrode precursor A step of. 4. according to the preparation method of the silicon-carbon anode material of multi-level buffer structure described in any one of claim 1 to 3, concrete steps are as follows: (1) diatomite is mixed with metal magnesium powder ball mill, under the protection of inert gas (2) the reduction product obtained in step (1) is acid-washed first, then washed with water until the pH is 6 to 8, centrifuged, and vacuum-dried to obtain porous silicon; (3) the porous silicon obtained in step (2) is Silicon is subjected to sand milling and nano-processing, then centrifuged, and vacuum-dried to obtain nanoporous silicon. (4) The nanoporous silicon obtained in step (3), flake graphite and pitch are wet ball milled at high speed to obtain a uniformly mixed slurry. (5) After the slurry obtained in step (4) is granulated by a spray dryer, the prepared silicon-carbon material is roasted under an inert atmosphere to obtain a multi-level buffer structure silicon-carbon negative electrode material. 5. The preparation method according to claim 4, characterized in that: in the step (1), the mol ratio of diatomite to magnesium powder is 1:0.8-1.2, preferably 1:0.85-1; the rotating speed of the ball mill is 100 -400 r/min, preferably 200-300 r/min; Ball milling process and high temperature reduction process all carry out under inert atmosphere, and described inert atmosphere is a kind of in following: nitrogen, argon; High temperature reduction temperature is 650 °C-750 °C, preferably 650 °C-700 °C, the pickling process described in step (2) is to use one or more of hydrochloric acid, sulfuric acid, hydrofluoric acid to carry out pickling, the centrifugal The rotating speed of separation is 3000-4000 r/min, the temperature of described vacuum drying is 80 ℃-100 ℃, the time is 2 h-5 h, and the described sand mill rotating speed of step (3) is 1800-2400r/min , preferably 2000-2300 r/min, the sanding time is 0.1 h-3 h, preferably 0.5 h-2 h, the solvent used in the sanding is water, ethanol, acetone or a mixture of several, the The rotating speed of centrifugal separation is 8000-10000 r/min, the temperature of described vacuum drying is 80 ° C-100 ° C, the time is 2 h-5 h, the rotating speed of step (4) ball mill is 300-1000 r/min, Preferably 400-800r/min; the solvent used for wet ball milling is water, ethanol, acetone or a combination of several, the mass ratio of nanoporous silicon, flake graphite and pitch is 1:2-10:0.5-3 , preferably 1:2-8:0.5-2, the inlet temperature of the spray dryer in step (5) is 120-200°C, the outlet temperature is 50°C-130°C; the atomizer is a two-fluid atomizer , the inlet speed is 1-10 L/min, and the gas required for the spray dryer is one of the following: air, nitrogen, argon; the feed speed is 10-50 r/min, and the roasting temperature is 600 -1200°C, the heating rate is 1-10°C/min, preferably 2-8°C/min; the sintering time is 1-10h, preferably 1-8h, and the inert atmosphere is one of the following: nitrogen, Argon. 6. The silicon-carbon anode material with a multi-level buffer structure prepared by the preparation method according to any one of claims 3-5. 7. The application of the multi-level buffer structure silicon-carbon negative electrode material as claimed in claim 6 as the negative electrode material for lithium ion batteries.