CN113611843B

CN113611843B - Biomass-based porous silicon-carbon composite material and preparation method thereof

Info

Publication number: CN113611843B
Application number: CN202110862052.XA
Authority: CN
Inventors: 侯佼; 王兴蔚; 侯春平; 马勇; 杨丹; 贺超
Original assignee: Bolt New Materials Yinchuan Co ltd
Current assignee: Bolt New Materials Yinchuan Co ltd
Priority date: 2021-07-29
Filing date: 2021-07-29
Publication date: 2022-12-06
Anticipated expiration: 2041-07-29
Also published as: CN113611843A

Abstract

The invention provides a biomass-based porous silicon-carbon composite material and a preparation method thereof, wherein the preparation process comprises the steps of kudzu root residue pretreatment, spray granulation, high-temperature carbonization, hydrothermal-in-situ polymerization coating, pyrolysis treatment and the like. According to the invention, the kudzu root residue micro powder has excellent cohesiveness and coating performance, and the obtained material has a three-dimensional carbon fiber skeleton structure by controlling the swelling degree and viscosity of the micro powder, so that the structure effectively buffers the volume expansion of the nano silicon material, shortens the diffusion path of lithium ions, improves the diffusion rate of the lithium ions, and improves the capacity and electrochemical performance of the electrode material.

Description

Biomass-based porous silicon-carbon composite material and preparation method thereof

Technical Field

The invention belongs to the field of novel energy materials, relates to a porous silicon-carbon composite material and a preparation method thereof, and particularly relates to a biomass-based porous silicon-carbon composite negative electrode material and a preparation method thereof.

Background

The lithium ion battery has the advantages of high working voltage, high energy density, no pollution, small self-discharge, long cycle life and the like, and is widely applied to the fields of energy storage and power batteries. The theoretical capacity of the conventional graphite negative electrode carbon material is only 372mAh & g ^-1 The lithium iron phosphate lithium battery has low actual capacity and small tap density, so that the volume energy density of the electrode material is small, and the energy density of the commercial battery assembled by matching with the lithium iron phosphate, lithium manganate and other positive electrode materials is difficult to meet the increasing energy density requirement of the energy industry. In addition, it is easy to generate "lithium dendrite" during rapid charging and discharging process, resulting in potential safety hazard of the battery. And silicon-based materials due to their high 4200mAh g ^-1 Theoretical capacity of about 0.4V, low voltage plateau (Li/Li) ⁺ ) The lithium ion battery cathode material has the advantages of high safety, excellent low-temperature performance, abundant natural storage and the like, and becomes a new generation of commercial alternative cathode material.

However, problems in the application of silicon-based materials have prevented their commercial use. Firstly, in the process of charging and discharging, the silicon-based material can generate volume expansion and shrinkage of up to 300% when lithium is inserted and removed, so that silicon particles can be extruded and pulverized, effective active materials can fall off from a current collector, the electrode structure is damaged, further, the electric contact is lost, the initiation capacity is rapidly attenuated, and the cycle stability of the battery is poor. Secondly, the volume effect causes that a new and unstable SEI film is continuously formed on the fracture surface of the silicon-based material, thereby causing the problems of sudden increase of irreversible capacity, increase of internal resistance of the battery, reduction of coulombic efficiency, deterioration of conductivity and the like. Moreover, silicon is a semiconductor material, and its low conductivity and ionic diffusion coefficient reduce the diffusion kinetics of lithium ions. Therefore, the structure of the silicon-based material needs to be optimally designed to improve and enhance the performance thereof. The common modification method mainly reduces the volume change of silicon by nano-treatment of silicon, and constructs structures such as core-shell, yolk-shell, porous and the like by compounding the silicon and carbon-based materials with different structures to relieve the volume change and increase the conductivity of the materials so as to improve the cycle performance of the silicon-based materials.

In order to solve the problems of the existing silicon-carbon composite negative electrode material, it is necessary to develop a new biomass-based porous silicon-carbon composite material and a preparation method thereof.

Disclosure of Invention

The invention provides a biomass-based porous silicon-carbon composite material and a preparation method thereof, aiming at the problems.

The invention adopts the kudzu root residue as the biomass base, aims at the problems of large residual amount, single treatment method, low added value and the like of the residue (epidermis, root hair, residue and the like) in the kudzu root industry, utilizes the characteristic that the kudzu root residue powder swells and becomes sticky in water to form the three-dimensional porous carbon-based material, can be used as a binder, a thickening agent and a primary coating agent in the process of preparing the composite material, and provides a method for preparing the biomass-based porous silicon-carbon composite negative electrode material by reutilizing the kudzu root residue resource and the silicon-carbon composite negative electrode material. The method provides a brand new technical route for recycling the pueraria residues and solves the problem of the source of the novel power battery cathode electrode material.

According to the biomass-based porous silicon-carbon composite material, a three-dimensional porous silicon-carbon composite material prepared by uniformly dispersing and coating nano silicon powder on a partially graphitized thin coating carbon layer formed by carbohydrate and bridging a three-dimensional carbon fiber framework formed by plant fibers is used as a precursor, and a pyrolytic carbon layer subjected to hydrothermal in-situ secondary coating is coated outside the precursor. The pore diameter of the porous structure in the precursor is 50 nm-5 μm, the mass of the porous structure accounts for 50-60% of the mass of the precursor, and the mass of the secondary coating layer accounts for 5.0-15.0% of the mass of the precursor.

The biomass-based porous silicon-carbon composite material has the following structure: the core is nano silicon powder, a part of graphitized thin coating carbon layer formed by uniformly dispersing and coating carbohydrate outside the nano silicon powder, plant fibers are bridged between the nano silicon powder and the graphitized thin coating carbon layer to form a three-dimensional porous silicon-carbon composite material with a three-dimensional carbon fiber framework, and the outer layer of the three-dimensional porous silicon-carbon composite material is coated with a pyrolytic carbon layer secondarily.

Specifically, the size of the nano silicon particles is 30-100 nm, and the mass of the nano silicon particles accounts for 5-30% of the mass of the three-dimensional porous silicon-carbon composite material;

the pore diameter of the porous structure of the graphitized thin coating carbon layer is 50 nm-5 μm, and the mass of the porous structure accounts for 50% -60% of the mass of the precursor;

the thickness of the secondary coating pyrolytic carbon layer is 5 nm-15 nm, and the mass of the secondary coating pyrolytic carbon layer is 5.0% -15.0% of the mass of the precursor.

The invention provides a method for preparing a porous silicon-carbon composite cathode material by forming a biomass-based porous carbon material from pueraria residues, which is characterized in that epidermis, root hairs and process residues remained during the production of pueraria powder are taken as raw materials, residue micro powder with high purity is obtained by cleaning, crushing, purifying and drying, the residue micro powder and nano silicon powder are uniformly dispersed under the action of an alcohol solvent, stirring and heating are carried out, the swelling degree and viscosity of the pueraria residue micro powder are controlled by regulating and controlling the temperature, then a porous silicon-carbon composite precursor is prepared by spray granulation, high-temperature pyrolysis and crushing screening treatment, and the in-vitro precursor is subjected to hydrothermal in-situ secondary organic coating and pyrolysis treatment to finally obtain the biomass-based porous silicon-carbon composite cathode material.

The invention provides a preparation method of a biomass-based porous silicon-carbon composite material, which comprises the following specific steps:

1. preparation of Biomass-based porous silicon-carbon composite precursor (p-C/Si)

S1, pre-treating kudzu vine root residues: cleaning the kudzu root residue at room temperature, then crushing the cleaned kudzu root residue into micro powder particles with the particle size of 3-5 mu m, further cleaning and purifying the crushed kudzu root residue powder by one step, and fully drying the powder to obtain pure kudzu root residue micro powder;

specifically, at room temperature, the kudzu root residue is stirred and cleaned by using a detergent as a cleaning agent to remove sand on the surface and residual impurities inside, then the kudzu root residue is crushed into micro powder particles with the particle size of 3-5 mu m, the crushed kudzu root residue powder is further cleaned and purified in aqueous solution by using a purifying device, and the kudzu root residue micro powder is fully dried at a certain drying temperature and drying time designed by the technical personnel in the field, for example, the kudzu root residue micro powder is dried in a blast oven at the temperature of 80-100 ℃ for 12-24 hours to obtain pure kudzu root residue micro powder;

in the step, the pueraria residue is one or more of epidermis, root hair and production residue which are remained when the pueraria product is produced;

the crushing mode is any one of mechanical crushing, air flow crushing and grinding crushing, and the equipment can be a mechanical crusher, a roller mill, an air flow crusher, a grinder and the like;

the purification mode is one of natural sedimentation, filtration and centrifugation, and the equipment can be a vacuum suction filter, a pressure filter, a centrifuge, a vortex separator, a belt press filter, a dehydrator and the like;

in the step, some surface layers and internal impurities in the pueraria residues are removed so as to prevent the impurities from influencing the performance of the electrode material. The residues are crushed into micro powder particles, so that not only can purification treatment be better carried out to obtain pure raw materials, but also the micro powder particles can be more fully heated, swollen and sticky in the later period, and a stable and effective three-dimensional porous structure carbon-based material is formed. The purification is to remove some suspended impurities in the micro powder in the grinding process to obtain pure residue micro powder particles with the impurity content of less than 1%.

S2, spray granulation: mixing the kudzu root residue micro powder and the nano silicon powder according to the mass ratio of 100: (5.0-30.0) dispersing in an alcohol solvent solution for liquid-phase mechanical mixing, then stirring and heating the mixed slurry in heating equipment, adjusting the temperature to control the swelling performance of the kudzu root residue powder to adjust the viscosity of the slurry, and then performing spray granulation to obtain a biomass-based porous silicon-carbon composite sample;

in the step, the nano silicon powder is silicon powder particles with the particle size of 30-100 nm;

the alcohol solvent is one or more of methanol, ethanol and isopropanol;

the mass fraction of the mixture in the slurry is 10-40%;

the liquid phase mixing speed is 200-1500 r/min, and the stirring time is 30-120 min;

the stirring and heating equipment is one of a magnetic stirrer, a constant-temperature oil bath kettle and a constant-temperature water bath kettle;

the heating temperature range is 50-120 ℃, and the stirring speed is 500-800 r/min;

heating and stirring the mixed slurry, and obtaining the mixed slurry with the viscosity of 1000-1500cP after the radix puerariae residue micro powder is fully swelled;

the inlet temperature of the spray dryer is 180-260 ℃, and the outlet temperature is 120-160 ℃;

the feeding speed of spray drying is 10 ml/min-50 ml/min;

the method utilizes the characteristic that the kudzu root micro powder can be used as a binder, a thickening agent and a primary coating agent in the process of preparing the composite material, omits the use of the binder, the thickening agent and the coating agent, directly uses the micro powder as the binder and the coating agent to coat and structurally treat the nano silicon powder, and obtains the biomass-based porous silicon-carbon composite sample with swelling holes through spray granulation.

S3, high-temperature carbonization: placing the biomass-based porous silicon-carbon composite sample in a high-temperature atmosphere furnace, carrying out pyrolysis treatment in a protective gas atmosphere, and crushing after the pyrolysis is finished and the material is cooled to obtain a biomass-based porous silicon-carbon composite precursor (p-C/Si); after being crushed, the biomass-based porous silicon-carbon composite precursor is screened by a fine screen, and the uniformity of the obtained biomass-based porous silicon-carbon composite precursor is better, such as a 150-mesh screen, a 200-mesh screen, a 250-mesh screen and the like;

in this step, the protective gas is one or more of nitrogen, helium, neon, argon, krypton, or xenon. The flow rate is 0.5L/min-10L/min;

the pyrolysis treatment is carried out, the temperature rising speed is 2 ℃/min to 5 ℃/min, the pyrolysis temperature is 600 ℃ to 1100 ℃, and the pyrolysis carbonization time is 2h to 5h;

in the step, under the conditions of protective atmosphere and high temperature, disordered fibers are distributed in the residue micro powder and are connected with each other to form a three-dimensional carbon fiber skeleton structure; and the carbohydrate forms a thin partially graphitized coating layer to dispersedly coat the nano silicon powder and bridge a three-dimensional carbon fiber skeleton structure to form the three-dimensional porous structure biomass silicon carbon composite precursor.

2. Preparation of Biomass-based porous silicon-carbon composite (p-C/Si/C)

S4, hydrothermal-in-situ polymerization coating: mixing the prepared biomass-based porous silicon-carbon composite precursor and an organic carbon source according to the mass ratio of 1: (1.0-3.0), uniformly mixing, ultrasonically dispersing in water, performing secondary carbon coating in a hydrothermal reaction kettle through hydrothermal-in-situ polymerization, centrifuging, washing and drying the mixture solution to obtain a secondary coating composite material;

in the step, the solid-liquid mass ratio of the mixture of the biomass-based porous silicon-carbon composite precursor and the organic carbon source to water is (1.0-2.0): (20-40);

the organic carbon source is one or more of glucose, sucrose, polyethylene glycol, carboxymethyl cellulose, water-soluble phenolic resin and polyvinyl alcohol;

the temperature of the hydrothermal-in-situ polymerization is 150-200 ℃, and the polymerization time is 12-24 h;

in the step, the organic carbon source is uniformly coated on the surface layer of the precursor for the second time by a hydrothermal-in-situ polymerization method, so that the secondary coated composite material is obtained.

S5, pyrolysis treatment: placing the obtained secondary coating composite material in a high-temperature atmosphere furnace, carrying out pyrolysis carbonization in protective gas atmosphere, and crushing after the material is cooled to obtain a biomass-based porous silicon-carbon composite material (p-C/Si/C); pulverizing, and sieving with fine sieve to obtain material with good uniformity, such as 150 mesh sieve, 200 mesh sieve, 250 mesh sieve, etc.;

in the step, the protective gas is one of nitrogen and argon, and the flow rate is 0.5L/min-10L/min;

carrying out pyrolysis treatment, wherein the heating rate is 2-5 ℃/min, the pyrolysis temperature is 500-1000 ℃, and the constant-temperature pyrolysis time is 2-4 h;

the method comprises the following steps of carbonizing an organic carbon source coating layer into a secondary carbon coating layer to coat the surface layer of a precursor at a high temperature in a protective atmosphere to obtain the final biomass-based porous silicon-carbon composite material.

Further, the biomass-based porous silicon-carbon composite material is a lithium ion battery cathode material.

The method takes the kudzu root residue and the nano silicon powder which are low in utilization rate, small in added value and low in price as raw materials, utilizes the characteristics that the kudzu root residue powder can swell and become sticky to generate a pore structure along with the rise of temperature in an aqueous solution and has excellent bonding and coating properties to uniformly disperse and coat the nano silicon powder, and prepares the biomass-based porous silicon-carbon composite material after spray granulation and hydrothermal in-situ secondary carbon coating. The three-dimensional carbon fiber skeleton formed by sintering the disorderly distributed plant fibers in the pueraria lobata residue not only provides a buffer space for volume expansion of the nano silicon material in the charge-discharge cycle process to protect the integrity of an electrode, but also shortens the diffusion path of lithium ions to improve the diffusion rate of the lithium ions, and improves the capacity and the electrochemical performance of the material through micropore lithium storage and excellent liquid absorption performance. The carbohydrate forms a thin partially graphitized coating layer to dispersedly coat the nano silicon powder and bridge a three-dimensional carbon fiber skeleton structure to form a three-dimensional porous structure precursor, so that the nano silicon in the precursor material is protected from being in contact with an electrolyte to generate a side reaction, and the conductivity of the electrode material is increased to improve the capacity and the rate cycle performance of the material. The carbon coating layer coated outside the precursor for the second time can not only further buffer the volume effect of the nano silicon material in the charge-discharge cycle process, prevent the stripping and falling of the electrode, but also reduce the electrochemical resistance and the specific surface area of the precursor to improve the electrochemical performance and the machining performance of the electrode material. The method not only provides a brand new technical route for recycling the pueraria residues, but also solves the problem of the source of the novel power battery silicon-carbon cathode electrode material.

The invention has the following beneficial effects:

1) The invention takes the residual epidermis, root hair and process residues in the production of the kudzu root powder as the raw materials for preparing the biomass-based porous silicon-carbon composite material, and the raw materials are low in price. The mechanical processing property and the electrochemical property of the prepared composite material can meet the requirements of electrode materials of power batteries.

2) The invention provides a brand new technical route for recycling the kudzu root residues, and explores a new raw material and a preparation method for a novel power battery silicon-carbon composite cathode electrode material.

3) The kudzu root residue micro powder has excellent cohesiveness and coating performance, the swelling of the micro powder can be controlled by adjusting the heating temperature, the viscosity of the slurry is further controlled, and finally the nano silicon is dispersed and coated to form a three-dimensional porous structure precursor. In the preparation process, the residue micropowder of the kudzuvine root can be comprehensively used as a binder, a thickening agent and a primary coating agent, and the addition of related auxiliary materials is omitted.

4) The method is characterized in that disordered fibers distributed in the pueraria lobata residue micropowder are connected with one another to form a three-dimensional carbon fiber skeleton structure, wherein carbohydrates form a thin partially graphitized coating layer to disperse and coat the nano silicon powder and bridge the three-dimensional carbon fiber skeleton to construct a three-dimensional porous structure. The structure effectively buffers the volume expansion of the nano silicon material, shortens the diffusion path of lithium ions, improves the diffusion rate of the lithium ions, and improves the capacity and the electrochemical performance of the electrode material through microporous lithium storage and excellent liquid absorption performance.

The preparation method provided by the invention is simple, the process is easy to control, and the material cost is low.

Drawings

FIG. 1 is an X-ray diffraction pattern of the composite material p-C/Si/C obtained in example 3,

FIG. 2 is an SEM image (1 μm) of a sample after the fine powder of the kudzu root residue is swollen and carbonized,

FIG. 3 is an SEM image (500 nm) of a sample after the fine powder of the kudzu root residue is swelled and carbonized,

FIG. 4 is an SEM photograph of the composite p-C/Si/C prepared in example 3,

FIG. 5 is a TEM image of the composite p-C/Si/C obtained in example 3.

Detailed Description

The above embodiments are further described in detail with reference to the following examples and the accompanying drawings:

for the characterization of the inventive examples, the prepared materials were characterized by their composition and structure by means of an X-ray diffractometer (SmartLab, japan science) under irradiation with Cu Ka at a scanning speed of 10 °/min. The morphology of the electrode material was observed with a scanning electron microscope (SEM; verios G4 UC, FEI, USA) and a transmission electron microscope (TEM; talos 200F, FEI, USA). The specific surface area of the electrode material was determined with a dynamic nitrogen adsorption surface analyzer (JW-DX, jing Wei Gao, china).

The electrode materials obtained in the examples and the comparative examples were used for half-cell tests, and a CR2025 button cell was prepared by preparing a slurry of the electrode material, conductive agent carbon black SP, and binder PVDF at a mass ratio of 85 to 7, coating the electrode sheet with a metal lithium sheet as a comparative electrode, using HR-8315 type electrolyte from shandong hai-co-generation power supply materials gmbh as electrolyte, and Celgard 2400 as a separator in a german braun MBRAUN glove box protected with high purity argon gas. Constant-current charging and discharging tests are carried out by using a CT2001A type blue-spot battery testing system of blue-electricity electronic corporation, wuhan City, wherein the charging and discharging voltage range is 0.003-2.0V (vs ⁺ /Li), measuring 0.1C first discharge capacity mAh/g, first efficiency%, respectively performing 1C charge-discharge cycle test after 3 weeks of charge-discharge activation at 0.1C, 0.2C and 0.5C, measuring discharge capacity mAh/g after 50 weeks of cycle, and calculating to obtain the capacity retention rate% after 50 weeks of cycle at 1C high magnification by using the ratio of 1C discharge capacity at 50 weeks to 1C discharge capacity at 1 week.

Example 1

The preparation method of the biomass-based porous silicon-carbon composite material comprises the following specific steps:

S1, pre-treating kudzu vine root residues: at room temperature, the surface of the pueraria residue is stirred and cleaned by using a cleaning agent which is detergent to remove sand on the surface and residual impurities in the pueraria residue, then the pueraria residue is crushed into micropowder particles with the particle size of 3-5 mu m, the crushed pueraria residue powder is further cleaned and purified in aqueous solution by using a purification device, and the purified pueraria residue micropowder is obtained after drying for 12 hours in a blast oven with the temperature of 100 ℃.

S2, spray granulation: 100g of kudzu root residue micro powder and 5g of 30nm silicon powder are weighed and dispersed in an ethanol solution, the mass sum of the kudzu root residue micro powder and the nano silicon powder is 10% of the sum of the kudzu root residue micro powder, the nano silicon powder and the solvent, and the mixture is subjected to liquid-phase mechanical mixing for 120min at the rotating speed of 200 r/min. And transferring the mixed solution to a constant-temperature water bath kettle, stirring at the temperature of 50 ℃ and the rotating speed of 800r/min until the radix puerariae micropowder swells to obtain mixed slurry with the viscosity of 1000cP, spraying and granulating the uniformly mixed slurry at the inlet temperature of 260 ℃ and the outlet temperature of 120 ℃ at the feeding speed of 20ml/min to obtain the biomass-based porous silicon-carbon composite sample with swelling holes.

S3, high-temperature carbonization: placing the biomass-based porous silicon-carbon composite sample in a high-temperature atmosphere furnace, heating to 800 ℃ at a heating rate of 2 ℃/min under the condition that the nitrogen flow is 5L/min, carrying out pyrolysis treatment for 3h, naturally cooling to room temperature, crushing, and then screening by a 250-mesh screen to obtain a biomass-based porous silicon-carbon composite precursor (p-C/Si), wherein the biomass-based porous silicon-carbon composite precursor (p-C/Si) is a thin partially-graphitized coating layer, and is formed by dispersedly coating nano silicon powder and bridging a three-dimensional carbon fiber skeleton structure to form the three-dimensional porous biomass silicon-carbon composite precursor.

2. Preparation of Biomass-based porous silicon-carbon composite (p-C/Si/C)

S4, hydrothermal coating: mixing a biomass-based porous silicon-carbon composite precursor (p-C/Si) and sucrose according to a mass ratio of 1:1, adding the biomass-based porous silicon-carbon composite precursor (p-C/Si) and sucrose in water, wherein the solid-liquid mass ratio of the sucrose to the water is 1.

S5, pyrolysis treatment: and (3) placing the obtained secondary coating composite material in a high-temperature atmosphere furnace, heating to 800 ℃ at the heating rate of 2 ℃/min under the condition of nitrogen flow of 5L/min, carrying out pyrolysis treatment for 3h, naturally cooling to room temperature, crushing, and then sieving with a 250-mesh sieve to obtain the biomass-based porous silicon-carbon composite material (p-C/Si/C).

The size of the biomass-based porous silicon-carbon composite material (p-C/Si/C) nano silicon particles obtained in the embodiment is 30nm, and the mass of the nano silicon particles accounts for 5% of the mass of the three-dimensional porous silicon-carbon composite material;

the pore diameter of the porous structure of the graphitized thin coating carbon layer is 50 nm-2 mu m, and the mass of the porous structure accounts for 60% of the mass of the precursor;

the thickness of the secondary coating pyrolytic carbon layer is 5-10 nm, and the mass of the secondary coating pyrolytic carbon layer is 5% of that of the precursor.

The composite sample prepared in example 1 was subjected to physical and chemical property tests. The specific surface area of the composite material powder is 37.47m ² ·g ^-1 The tap density is 0.68g cm ^-1 And the first discharge capacity of 0.1C is 432.1mAh g ^-1 The first efficiency was 77.5%. Capacity retention 210.5mAh g after 50 weeks of cycling at 1C high rate ^-1 The capacity retention ratio was 70.7%. The test results are summarized in Table 1.

Example 2

S1, pre-treating kudzu vine root residues: stirring and cleaning the root hairs of the pueraria residues by using a cleaning agent which is detergent at room temperature to remove sand on the surface and residual impurities in the pueraria residues, then crushing the pueraria residues into micropowder particles with the particle size of 3-5 mu m, further cleaning and purifying the crushed pueraria residues in aqueous solution by using a purifying device, and physically purifying and drying the pueraria residues by using a vacuum pump filter to obtain pure pueraria residue micropowder.

S2, spray granulation: weighing 100g of radix Puerariae residue micropowder and 30g of 100nm silicon powder, dispersing in methanol solution, wherein the sum of the mass of the radix Puerariae residue micropowder and the mass of the nano silicon powder is 40% of the sum of the radix Puerariae residue micropowder, the nano silicon powder and the solvent, and performing liquid-phase mechanical mixing at the rotation speed of 600r/min for 110min. And transferring the mixed solution to a constant-temperature water bath kettle, stirring at the temperature of 80 ℃ and the rotating speed of 700r/min until the radix puerariae micropowder swells to obtain mixed slurry with the viscosity of 1300cP, spraying and granulating the uniformly mixed slurry at the inlet temperature of 240 ℃ and the outlet temperature of 130 ℃ at the feeding speed of 30ml/min to obtain the biomass-based porous silicon-carbon composite sample with swelling holes.

S3, high-temperature carbonization: and placing the biomass-based porous silicon-carbon composite sample in a high-temperature atmosphere furnace, heating to 850 ℃ at the argon flow rate of 6L/min and at the heating rate of 3 ℃/min, carrying out pyrolysis treatment for 3.5h, naturally cooling to room temperature, crushing, and then passing through a 150-mesh screen to obtain a biomass-based porous silicon-carbon composite precursor (p-C/Si), wherein the biomass-based porous silicon-carbon composite precursor (p-C/Si) is a thin partially graphitized coating layer, and nano silicon powder is dispersedly coated and is bridged with a three-dimensional carbon fiber skeleton structure to form the three-dimensional porous structure biomass-based silicon-carbon composite precursor.

2. Preparation of Biomass-based porous silicon-carbon composite (p-C/Si/C)

S4, hydrothermal coating: mixing a biomass-based porous silicon-carbon composite precursor (p-C/Si) and glucose according to a mass ratio of 1:1.5, adding the biomass-based porous silicon-carbon composite precursor (p-C/Si) and glucose into water, wherein the mass ratio of solid to liquid of the glucose to the water is 1.

S5, pyrolysis treatment: and (3) placing the obtained secondary coating composite material in a high-temperature atmosphere furnace, heating to 850 ℃ at the argon flow rate of 6L/min at the heating rate of 3 ℃/min, carrying out pyrolysis treatment for 3.5h, naturally cooling to room temperature, crushing, and screening by using a 150-mesh screen to obtain the biomass-based porous silicon-carbon composite material (p-C/Si/C).

The size of the biomass-based porous silicon-carbon composite material (p-C/Si/C) nano silicon particles obtained in the embodiment is 100nm, and the mass of the nano silicon particles accounts for 30% of the mass of the three-dimensional porous silicon-carbon composite material;

the pore diameter of the porous structure of the graphitized thin coating carbon layer is 100 nm-5 μm, and the mass of the porous structure accounts for 50% of the mass of the precursor;

the thickness of the secondary coating pyrolytic carbon layer is 10-15 nm, and the mass of the secondary coating pyrolytic carbon layer is 8% of that of the precursor.

The composite sample prepared in example 2 was subjected to physical and chemical property tests. The specific surface area of the composite material powder is 38.96m ² ·g ^-1 The tap density is 0.70g cm ^-1 And the first discharge capacity of 0.1C is 1073.2mAh g ^-1 The first efficiency was 73.3%. Capacity retention of 512.5mAh g after 50 weeks of cycling at 1C high rate ^-1 The capacity retention rate was 63.5%. The test results are summarized in Table 1.

Example 3

S1, pre-treating kudzu vine root residues: at room temperature, stirring and cleaning residues in the process of the pueraria lobata residues by using a detergent as a cleaning agent to remove surface sand and internal residual impurities, then crushing the residues into micropowder particles with the particle size of 3-5 mu m, further cleaning and purifying the crushed pueraria lobata residues in an aqueous solution by using a purifying device, and physically purifying and drying by using a centrifuge to obtain pure pueraria lobata residues micropowder.

S2, spray granulation: weighing 100g of radix Puerariae residue micropowder and 20g of 100nm silicon powder, dispersing in isopropanol solution, wherein the mass sum of radix Puerariae residue micropowder and nanometer silicon powder is 35% of the sum of radix Puerariae residue micropowder, nanometer silicon powder and solvent, and mechanically mixing with liquid-phase at 700r/min for 100min. And transferring the mixed solution to a magnetic stirrer, stirring at the temperature of 100 ℃ and the rotating speed of 800r/min until the radix puerariae micropowder swells to obtain mixed slurry with the viscosity of 1350cP, spraying and granulating the uniformly mixed slurry at the inlet temperature of 250 ℃ and the outlet temperature of 140 ℃ at the feeding speed of 40ml/min to obtain the biomass-based porous silicon-carbon composite sample with swelling holes.

S3, high-temperature carbonization: placing the biomass-based porous silicon-carbon composite sample in a high-temperature atmosphere furnace, heating to 900 ℃ at a nitrogen flow rate of 7L/min at a heating rate of 4 ℃/min, carrying out pyrolysis treatment for 4h, naturally cooling to room temperature, crushing, and then sieving with a 200-mesh screen to obtain a biomass-based porous silicon-carbon composite precursor (p-C/Si), wherein the biomass-based porous silicon-carbon composite precursor (p-C/Si) is a thin partially graphitized coating layer, and is formed by dispersing and coating nano silicon powder and bridging a three-dimensional carbon fiber skeleton structure to form the three-dimensional porous structure biomass-based silicon-carbon composite precursor.

2. Preparation of Biomass-based porous silicon-carbon composite (p-C/Si/C)

S4, hydrothermal coating: mixing a biomass-based porous silicon-carbon composite precursor (p-C/Si) and water-soluble phenolic resin according to a mass ratio of 1:2, adding the biomass-based porous silicon-carbon composite precursor (p-C/Si) and water-soluble phenolic resin into water, wherein the solid-liquid mass ratio of the water-soluble phenolic resin to the biomass-based porous silicon-carbon composite precursor (p-C/Si) is 1.

S5, pyrolysis treatment: and (3) placing the obtained secondary coating composite material in a high-temperature atmosphere furnace, heating to 900 ℃ at the heating rate of 4 ℃/min under the condition that the flow of nitrogen is 7L/min, carrying out pyrolysis treatment for 4h, naturally cooling to room temperature, crushing, and screening by using a 200-mesh screen to obtain the biomass-based porous silicon-carbon composite material (p-C/Si/C).

The size of the biomass-based porous silicon-carbon composite material (p-C/Si/C) nano silicon particles obtained in the embodiment is 100nm, and the mass of the nano silicon particles accounts for 20% of the mass of the three-dimensional porous silicon-carbon composite material;

the pore diameter of the porous structure of the graphitized thin coating carbon layer is 100 nm-5 mu m, and the mass of the porous structure accounts for 60% of the mass of the precursor;

the thickness of the secondary coating pyrolytic carbon layer is 12-14 nm, and the mass of the secondary coating pyrolytic carbon layer is 12% of that of the precursor.

The composite sample prepared in example 3 was subjected to physical and chemical property tests. The specific surface area of the composite material powder is 30.72m ² ·g ^-1 The tap density is 0.75g cm ^-1 0.1C first discharge capacity of 739.8mAh g ^-1 The first efficiency was 78.8%. Capacity retention 411.7mAh g after 50 weeks of cycling at a high rate of 1C ^-1 The capacity retention rate was 75.2%. The test results are summarized in Table 1.

As shown in FIG. 1, the X-ray diffraction pattern of the composite p-C/Si/C obtained in example 3, by comparison with the corresponding standard PDF card, revealed that the diffraction peaks observed at around 23.2 ℃ and 43.5 ℃ correspond to the (002) and (101) crystal planes of the biomass-based porous carbon material, respectively, and the diffraction peaks observed at 28.4 ℃,47.3 ℃,56.1 ℃,69.1 ℃ and 76.4 ℃ correspond to the (111), (220), (311), (400) and (331) crystal planes of crystalline silicon (PDF card No. 27-1402), respectively.

The main components of the prepared composite material are carbon material and silicon through XRD pattern analysis.

As shown in fig. 2 and fig. 3, in SEM images of the sample after swelling and carbonization of the pueraria lobata residue micropowder, after swelling and high-temperature sintering, the pueraria lobata residue micropowder has a three-dimensional carbon fiber skeleton structure formed by interconnecting randomly distributed fibers, and the carbohydrate forms a thin partially graphitized coating layer to wrap the three-dimensional carbon fiber skeleton in series to form a three-dimensional porous carbon material with a pore size of 100nm to 5 μm.

As shown in fig. 4 and 5, which are SEM images and TEM images of the composite material p-C/Si/C prepared in example 3, respectively, the pueraria lobata residue micropowder uniformly disperses and coats the silica nanoparticles and bridges the three-dimensional carbon fiber skeleton formed by the plant fiber by using its excellent swelling property and binding property to obtain a biomass-based porous silicon-carbon composite precursor, the precursor is uniformly secondarily coated by an amorphous carbon layer formed by an organic carbon source, and the thickness of the coating layer is about 12 to 14nm. This indicates that it is expected that nano-silicon is dispersed in a porous carbon material and an organic carbon layer is coated well on the surface layer.

Example 4

S1, radix puerariae residue pretreatment: mixing the epidermis, the root hair and the process residues of the kudzu root residues according to a mass ratio of 1.

S2, spray granulation: weighing 100g of radix Puerariae residue micropowder and 20g of 100nm silicon powder, dispersing in ethanol solution, wherein the sum of the mass of the radix Puerariae residue micropowder and the mass of the nano silicon powder is 40% of the sum of the radix Puerariae residue micropowder, the nano silicon powder and the solvent, and performing liquid-phase mechanical mixing for 80min at the rotation speed of 900 r/min. And transferring the mixed solution to a constant-temperature oil bath, stirring at the temperature of 110 ℃ and the rotating speed of 500r/min until the radix puerariae micro powder swells to obtain mixed slurry with the viscosity of 1300cP, and spraying and granulating the uniformly mixed slurry at the feeding speed of 45ml/min and the inlet temperature of 225 ℃ and the outlet temperature of 160 ℃ to obtain the biomass-based porous silicon-carbon composite sample with swelling holes.

S3, high-temperature carbonization: placing the biomass-based porous silicon-carbon composite sample in a high-temperature atmosphere furnace, heating to 950 ℃ at the flow rate of argon of 8L/min and at the heating rate of 5 ℃/min, carrying out pyrolysis treatment for 4.5h, naturally cooling to room temperature, crushing, and then sieving with a 200-mesh screen to obtain a biomass-based porous silicon-carbon composite precursor (p-C/Si), wherein the biomass-based porous silicon-carbon composite precursor (p-C/Si) is a thin partially graphitized coating layer, and nano silicon powder is dispersedly coated and is bridged with a three-dimensional carbon fiber skeleton structure to form the three-dimensional porous structure biomass-based silicon-carbon composite precursor.

2. Preparation of Biomass-based porous silicon-carbon composite (p-C/Si/C)

S4, hydrothermal coating: mixing a biomass-based porous silicon-carbon composite precursor (p-C/Si) and polyvinyl alcohol according to a mass ratio of 1:2.5, adding the biomass-based porous silicon-carbon composite precursor (p-C/Si) and polyvinyl alcohol into water, wherein the solid-liquid mass ratio of the polyvinyl alcohol to the water is 2.

S5, pyrolysis treatment: and (3) placing the obtained secondary coating composite material in a high-temperature atmosphere furnace, heating to 950 ℃ at the temperature rise speed of 5 ℃/min under the condition that the flow of argon is 8L/min, carrying out pyrolysis treatment for 4.5h, naturally cooling to room temperature, crushing, and screening by using a 200-mesh screen to obtain the biomass-based porous silicon-carbon composite material (p-C/Si/C).

the pore diameter of the porous structure of the graphitized thin coating carbon layer is 80 nm-3.5 μm, and the mass of the porous structure accounts for 55% of the mass of the precursor;

the thickness of the secondary coating pyrolytic carbon layer is 10-12 nm, and the mass of the secondary coating pyrolytic carbon layer is 10% of that of the precursor.

For the compound prepared in example 4And (4) carrying out physical property and chemical property tests on the sample. The specific surface area of the composite material powder is 34.35m ² ·g ^-1 The tap density is 0.72 g/cm ^-1 And the first discharge capacity of 0.1C is 728.4mAh g ^-1 The first efficiency was 75.9%. Capacity retention of 406.9mAh g after 50 weeks of cycling at 1C high rate ^-1 The capacity retention rate was 72.5%. The test results are summarized in Table 1.

Example 5

S1, radix puerariae residue pretreatment: mixing the epidermis, the root hair and the process residues of the pueraria residues at room temperature according to a mass ratio of 1.

S2, spray granulation: 100g of kudzu root residue micro powder and 20g of 100nm silicon powder are weighed and dispersed in a mixed solution of methanol and ethanol solution, the mass sum of the kudzu root residue micro powder and the nano silicon powder is 40% of the sum of the kudzu root residue micro powder, the nano silicon powder and the solvent, and the mixture is subjected to liquid-phase mechanical mixing for 60min at the rotating speed of 1000 r/min. And transferring the mixed solution to a magnetic stirrer, stirring at the temperature of 120 ℃ and the rotating speed of 700r/min until the radix puerariae micro powder swells to obtain mixed slurry with the viscosity of 1500cP, spraying and granulating the uniformly mixed slurry at the inlet temperature of 180 ℃ and the outlet temperature of 130 ℃ at the feeding speed of 50ml/min to obtain the biomass-based porous silicon-carbon composite sample with swelling holes.

S3, high-temperature carbonization: placing the biomass carbon-based silicon composite sample in a high-temperature atmosphere furnace, heating to 850 ℃ at a nitrogen flow rate of 9L/min at a heating rate of 5 ℃/min, carrying out pyrolysis treatment for 3.5h, naturally cooling to room temperature, crushing, and then sieving with a 200-mesh screen to obtain a biomass porous silicon-carbon composite precursor (p-C/Si), wherein the biomass porous silicon-carbon composite precursor (p-C/Si) is a thin partially graphitized coating layer, and nano silicon powder is dispersedly coated and bridged with a three-dimensional carbon fiber skeleton structure to form the three-dimensional porous biomass silicon-carbon composite precursor.

2. Preparation of Biomass-based porous silicon-carbon composite (p-C/Si/C)

S4, hydrothermal coating: mixing a biomass-based porous silicon-carbon composite precursor (p-C/Si) and polyethylene glycol according to a mass ratio of 1:3, adding the biomass-based porous silicon-carbon composite precursor (p-C/Si) and polyethylene glycol to water, wherein the solid-liquid mass ratio of the biomass-based porous silicon-carbon composite precursor to water is 2.

S5, pyrolysis treatment: and (3) placing the obtained secondary coating composite material in a high-temperature atmosphere furnace, heating to 850 ℃ at the heating rate of 5 ℃/min under the condition that the flow of nitrogen is 9L/min, carrying out pyrolysis treatment for 3.5h, naturally cooling to room temperature, crushing, and screening by using a 200-mesh screen to obtain the biomass-based porous silicon-carbon composite material (p-C/Si/C).

the pore diameter of the porous structure of the graphitized thin coating carbon layer is 70 nm-3.0 μm, and the mass of the porous structure accounts for 53% of the mass of the precursor;

the thickness of the secondary coating pyrolytic carbon layer is 10-15 nm, and the mass of the secondary coating pyrolytic carbon layer is 15% of that of the precursor.

The composite sample prepared in example 5 was subjected to physical and chemical property tests. The specific surface area of the composite material powder is 36.28m ² ·g ^-1 The tap density was 0.71g · cm ^-1 0.1C first discharge capacity 716.5mAh g ^-1 The first efficiency was 76.8%. The circulation capacity is kept at 397.6mAh g under the large multiplying power of 1C ^-1 The capacity retention ratio was 71.2%. The test results are summarized in Table 1.

Comparative example 1

The preparation method of the silicon-carbon composite material by using the common carbon material as the raw material comprises the following specific steps:

1. preparation of silicon-carbon composite precursor (C/Si)

S1, spray granulation: weighing 100g of natural spherical graphite with the average particle size D50 of 8.0 and 20g of 100nm silicon powder, dispersing the natural spherical graphite and the 100nm silicon powder into an isopropanol solution, adding 50g of coating agent polyethylene glycol and 5g of binding agent carboxymethyl cellulose, wherein the sum of the mass of the natural graphite, the nano silicon powder, the polyethylene glycol and the carboxymethyl cellulose is 35% of the sum of all solutes and solvents, and mechanically mixing the materials for 100min at the rotating speed of 700 r/min. And (3) obtaining mixed slurry with the viscosity of 1350cP, and performing spray granulation on the obtained uniform mixed slurry at the feeding speed of 40ml/min and the inlet temperature of 250 ℃ and the outlet temperature of 140 ℃ to obtain the silicon-carbon composite sample.

S2, high-temperature carbonization: and (3) placing the silicon-carbon composite sample in a high-temperature atmosphere furnace, heating to 900 ℃ at the heating rate of 4 ℃/min under the condition that the flow of nitrogen is 7L/min, carrying out pyrolysis treatment for 4h, naturally cooling to room temperature, crushing, and screening by using a 200-mesh screen to obtain a silicon-carbon composite precursor (C/Si).

2. Preparation of silicon-carbon composite (C/Si/C)

S3, hydrothermal coating: mixing a silicon-carbon composite precursor (C/Si) and water-soluble phenolic resin according to a mass ratio of 1:2, adding the silicon-carbon composite precursor (C/Si) and the water-soluble phenolic resin into water, wherein the solid-liquid mass ratio of the silicon-carbon composite precursor (C/Si) to the water is 1.

S4, pyrolysis treatment: and (3) placing the obtained secondary coating composite material in a high-temperature atmosphere furnace, heating to 900 ℃ at the heating rate of 4 ℃/min under the condition that the flow of nitrogen is 7L/min, carrying out pyrolysis treatment for 4h, naturally cooling to room temperature, crushing, and then sieving with a 200-mesh sieve to obtain the silicon-carbon composite material (C/Si/C).

The size of the silicon-carbon composite material (C/Si/C) nano-silicon particles obtained in the comparative example is 100nm, and the mass of the nano-silicon particles accounts for 20% of the mass of the silicon-carbon composite material;

no microporous structure was detected on the surface of the composite.

The thickness of the secondary coating pyrolytic carbon layer is 10-12 nm, and the mass of the secondary coating pyrolytic carbon layer is 12% of that of the precursor.

The composite sample prepared in comparative example 1 was subjected to physical and chemical property tests. The specific surface area of the composite material powder is 4.65m ² ·g ^-1 The tap density was 0.91g · cm ^-1 And the first discharge capacity at 0.1C was 786.5mAh g ^-1 The initial efficiency is 80.6%. Circulation capacity retention at 1C high rate of 340.6mAh g ^-1 The capacity retention rate was 65.3%. The results are summarized in Table 1.

Comparative example 2

The preparation method selects corncob micropowder as a biomass raw material to prepare the biomass-based porous silicon-carbon composite material, and comprises the following specific steps:

1. preparation of Biomass-based porous silicon-carbon composite precursor (p' -C/Si)

S1, pretreatment of corncobs: at room temperature, the corn cobs after threshing the corn cobs are stirred and cleaned by using detergent as cleaning agent to remove sand gravel on the surface and residual impurities in the corn cobs, then the corn cobs are crushed into micro powder particles with the particle size of 3-5 mu m, the crushed corn cob powder is further cleaned and purified in aqueous solution by using purification equipment, and the pure corn cob micro powder is obtained after physical purification and drying treatment by a centrifuge.

S2, spray granulation: 100g of corncob micro powder with the particle size of 3-5 mu m and 20g of 100nm silicon powder are weighed and dispersed in isopropanol solution, the mass sum of the kudzu root residue micro powder and the nano silicon powder is 35 percent of the sum of the kudzu root residue micro powder, the nano silicon powder and the solvent, and the mixture is subjected to liquid-phase mechanical mixing for 100min at the rotating speed of 700 r/min. And transferring the mixed solution to a magnetic stirrer, stirring at the temperature of 100 ℃ and the rotating speed of 800r/min, adding binder carboxymethyl cellulose to adjust the viscosity of the slurry to obtain a mixed slurry with the viscosity of 1350cP, and performing spray granulation on the obtained uniformly mixed slurry at the inlet temperature of 250 ℃ and the outlet temperature of 140 ℃ at the feeding speed of 40ml/min to obtain the biomass silicon-carbon composite sample.

S3, high-temperature carbonization: and (2) placing the biomass silicon-carbon composite sample in a high-temperature atmosphere furnace, heating to 900 ℃ at the nitrogen flow rate of 7L/min at the heating rate of 4 ℃/min, carrying out pyrolysis treatment for 4h, naturally cooling to room temperature, crushing, and screening with a 200-mesh screen to obtain a biomass-based porous silicon-carbon composite precursor (p' -C/Si).

2. Preparation of Biomass-based porous silicon-carbon composite (p' -C/Si/C)

S4, hydrothermal coating: mixing a biomass-based porous silicon-carbon composite precursor (p' -C/Si) and water-soluble phenolic resin according to a mass ratio of 1:2, adding the biomass-based porous silicon-carbon composite precursor (p '-C/Si) and water-soluble phenolic resin into water, wherein the solid-liquid mass ratio of the water-soluble phenolic resin to the biomass-based porous silicon-carbon composite precursor (p' -C/Si) is 1.

S5, pyrolysis treatment: and (3) placing the obtained secondary coating composite material in a high-temperature atmosphere furnace, heating to 900 ℃ at the heating rate of 4 ℃/min under the condition that the flow of nitrogen is 7L/min, carrying out pyrolysis treatment for 4h, naturally cooling to room temperature, crushing, and screening by using a 200-mesh screen to obtain the biomass-based porous silicon-carbon composite material (p' -C/Si/C).

The size of the biomass-based porous silicon-carbon composite material (p' -C/Si/C) nano silicon particles obtained by the comparative example is 100nm, and the mass of the nano silicon particles accounts for 20% of the mass of the porous silicon-carbon composite material;

the pore diameter of the porous structure of the graphitized thin coating carbon layer is 30-200 nm, the pore diameter of the porous structure is smaller, and the mass of the pore diameter accounts for 40% of the mass of the precursor;

the thickness of the secondary coating pyrolytic carbon layer is 10-13 nm, and the mass of the secondary coating pyrolytic carbon layer is 12% of that of the precursor.

The composite sample prepared in comparative example 2 was subjected to physical and chemical property tests. The specific surface area of the composite material powder was 33.54m ² ·g ^-1 The tap density was 0.65 g/cm ^-1 And the first discharge capacity at 0.1C was 719.7mAh g ^-1 The first efficiency was 70.5%. Capacity retention of 330.9mAh g after 50 weeks of cycling at 1C high rate ^-1 The capacity retention rate was 60.1%. The test results are summarized in Table 1.

TABLE 1 test results of physical and chemical properties of examples and comparative examples

Finally, it should be noted that: the above examples are only intended to illustrate the technical solution of the present invention, but not to limit it; although the present invention has been described in detail with reference to the foregoing embodiments, it will be understood by those of ordinary skill in the art that: the technical solutions described in the foregoing embodiments may still be modified, or some or all of the technical features may be equivalently replaced; and the modifications or the substitutions do not make the essence of the corresponding technical solutions depart from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A preparation method of a biomass-based porous silicon-carbon composite material comprises the steps of preparing a biomass-based porous silicon-carbon composite precursor, preparing a biomass-based porous silicon-carbon composite material,

the method is characterized in that the specific steps for preparing the biomass-based porous silicon-carbon composite precursor are as follows:

s1, pre-treating kudzu vine root residues: cleaning the kudzu root residue at room temperature, crushing the cleaned kudzu root residue into micro powder particles with the particle size of 3-5 mu m, further cleaning and purifying the crushed kudzu root residue powder, and fully drying to obtain pure kudzu root residue micro powder;

s2, spray granulation: mixing the kudzu root residue micro powder and the nano silicon powder according to the mass ratio of 100: (5.0-30.0) dispersing in an alcohol solvent solution to perform liquid phase mechanical mixing, then stirring and heating the mixed slurry in heating equipment, adjusting the temperature to control the swelling performance of the radix puerariae residue powder to adjust the viscosity of the slurry, after the radix puerariae residue micropowder is fully swelled, obtaining the mixed slurry with the viscosity of 1000 cP-1500 cP, and performing spray granulation to obtain a biomass-based porous silicon-carbon composite sample;

s3, high-temperature carbonization: placing the biomass-based porous silicon-carbon composite sample in a high-temperature atmosphere furnace, performing pyrolysis treatment in a protective gas atmosphere, and crushing after the pyrolysis is finished and the material is cooled to obtain a biomass-based porous silicon-carbon composite precursor;

2. the preparation method of the biomass-based porous silicon-carbon composite material comprises the following steps: s4, coating by hydrothermal-in-situ polymerization, and S5, carrying out pyrolysis treatment.

2. The preparation method of the biomass-based porous silicon-carbon composite material according to claim 1, wherein the two specific steps for preparing the biomass-based porous silicon-carbon composite material are as follows:

s4, hydrothermal-in-situ polymerization coating: mixing the prepared biomass-based porous silicon-carbon composite precursor and an organic carbon source according to the mass ratio of 1: (1.0-3.0), uniformly mixing, ultrasonically dispersing in water, performing secondary carbon coating in a hydrothermal reaction kettle through hydrothermal-in-situ polymerization, centrifuging, washing and drying the mixture solution to obtain a secondary coated composite material;

s5, pyrolysis treatment: and (3) placing the obtained secondary coating composite material in a high-temperature atmosphere furnace, carrying out pyrolysis carbonization in protective gas atmosphere, and crushing after the material is cooled to obtain the biomass-based porous silicon-carbon composite material.

3. The method of preparing the biomass-based porous silicon-carbon composite material according to claim 1, wherein the biomass is kudzu root residue, and the kudzu root residue is one or more of epidermis, root hair and process residue remained in the production of kudzu root products.

4. The method for preparing the biomass-based porous silicon-carbon composite material according to claim 1, wherein in the step S2 of spray granulation, the temperature of stirring and heating the mixed slurry in a heating device is 50-120 ℃.

5. The method for preparing the biomass-based porous silicon-carbon composite material according to claim 1, wherein the biomass-based porous silicon-carbon composite precursor is a three-dimensional porous biomass-based silicon-carbon composite precursor formed by dispersing and coating nano silicon powder with a thin partially-graphitized coating layer and bridging a three-dimensional carbon fiber skeleton structure.

6. The method for preparing the biomass-based porous silicon-carbon composite material according to claim 2, wherein the solid-liquid mass ratio of the mixture of the biomass-based porous silicon-carbon composite precursor and the organic carbon source to water in the step S4 is (1.0-2.0): (20 to 40).

7. The biomass-based porous silicon-carbon composite material prepared by the preparation method of the biomass-based porous silicon-carbon composite material as claimed in any one of claims 1 to 6 has the following structure: the core is nano silicon powder, a part of graphitized thin coated carbon layer formed by uniformly dispersing and coating carbohydrate outside the nano silicon powder, plant fibers are bridged between the nano silicon powder and the graphitized thin coated carbon layer to form a three-dimensional porous silicon-carbon composite material with a three-dimensional carbon fiber framework, the outer layer of the three-dimensional porous silicon-carbon composite material is coated with a pyrolytic carbon layer for the second time, and the biomass matrix is kudzu root residue.

8. The biomass-based porous silicon-carbon composite material of claim 7, wherein the size of the nano silicon particles in the nano silicon powder is 30 nm-100 nm, and the mass of the nano silicon powder accounts for 5% -30% of the mass of the three-dimensional porous silicon-carbon composite material;

9. The use of the biomass-based porous silicon-carbon composite material according to any one of claims 7 to 8, as a negative electrode material for lithium ion batteries.