CN111384373B

CN111384373B - Silicon-carbon composite material for lithium ion battery and preparation method thereof

Info

Publication number: CN111384373B
Application number: CN201811638954.XA
Authority: CN
Inventors: 张和宝; 李喆; 叶兰; 罗姝; 王岑
Original assignee: Amprius Nanjing Co ltd
Current assignee: Boselis Hefei Co ltd; Bosellis Nanjing Co ltd
Priority date: 2018-12-29
Filing date: 2018-12-29
Publication date: 2021-06-01
Anticipated expiration: 2038-12-29
Also published as: CN111384373A

Abstract

The invention relates to a silicon-carbon composite material for a lithium ion battery and a preparation method thereof, wherein the silicon-carbon composite material is spherical secondary particles; the secondary particles are formed by compounding a silicon material, a long-range conductive additive and carbon; in the secondary particles, the long-range conductive additive and the silicon material are uniformly dispersed; the median particle size of the primary particles of the silicon material is 1-10 mu m; the median diameter of the secondary particles is between 5 and 50 mu m; the secondary particle surface is coated with a carbon layer or not coated with carbon. The silicon-carbon composite material prepared by the invention has higher coulombic efficiency and capacity when being used for the lithium ion battery, the energy density, the rate capability and the cycle performance of the lithium ion battery are obviously improved, the process is simple, and the industrial production is easy to realize.

Description

Silicon-carbon composite material for lithium ion battery and preparation method thereof

Technical Field

The invention relates to the field of lithium ion batteries, in particular to a silicon-carbon composite material for a lithium ion battery and a preparation method thereof.

Background

Due to rapid development and wide application of various portable electronic devices and electric vehicles in recent years, demand for lithium ion batteries having high energy density and long cycle life is increasingly urgent. The negative electrode material of the lithium ion battery which is commercialized at present is mainly graphite, but the lithium ion battery is limited due to low theoretical capacity (372mAh/g)Further increase in energy density. Among many novel lithium ion battery cathode materials, silicon cathode materials have the advantage of high capacity (Li) that other cathode materials cannot match₂₂Si₅Theoretical lithium storage capacity of 4200mAh/g) which is more than 11 times of the theoretical capacity of the current commercial carbon negative electrode material. However, the silicon material has poor conductivity, and has a severe volume effect during lithium intercalation and deintercalation, and the volume change rate is about 400%, which may cause pulverization of the electrode material and separation of the electrode material from the current collector. In addition, due to the volume effect during charge and discharge, the silicon negative electrode material exposed to the electrolyte continuously forms a fresh surface, and thus the electrolyte is continuously consumed to generate an SEI film, reducing the cycle performance of the electrode material. The above-mentioned drawbacks of silicon-based materials severely limit their commercial applications.

In order to solve the above problems of silicon negative electrodes, the current domestic and foreign research on silicon negative electrode materials mainly focuses on the following aspects: (1) the size of the silicon particles is simply reduced, for example, by using silicon nanoparticles, so as to reduce the volume effect of the silicon particles. However, the nano silicon particles have a large specific surface area, so that the coulombic efficiency of the battery is very low, and in the circulation process, SEI on the surfaces of the silicon particles are repeatedly generated, so that SEI films on the surfaces are thick, the conduction of electrons is blocked, the particles are inactivated, and the circulation performance of the battery is limited. (2) The silicon material with special nano structure, such as silicon nano tube, silicon nano wire, porous silicon, etc. is prepared, but the method has higher cost and lower yield, and is only suitable for laboratory research at present. (3) Compounding silicon with carbon materials such as conductive additives, amorphous carbon, graphite and the like to prepare the silicon-carbon composite material. The composite material has attracted the attention of many researchers due to the combination of the high capacity of silicon and the cycle performance of graphite materials. However, when the content of graphite and amorphous carbon is too high and the content of silicon is low, the practical use capacity of the material is low. (4) The surface of the silicon material or the silicon-carbon composite material is coated, so that the material keeps stable SEI in the circulation of the lithium ion battery, and the occurrence of side reactions is reduced to improve the coulombic efficiency.

Chinese patent publication No. CN 108807861a discloses irregularly shaped secondary particles for lithium ion batteries and a method for preparing the same. And the secondary particles are subjected to secondary granulation by 0.01-5 mu m primary particles and then crushed to obtain secondary particles with irregular shapes, the conductive agent is uniformly dispersed in the secondary particles, and a layer of amorphous carbon is coated on the surfaces of the secondary particles. The synthesized irregular secondary particles are applied to lithium ion batteries, and the negative electrode has the advantages of high compaction density, difficult breakage of the secondary particles, more contact points among pole piece particles and lower polarization. Due to the technical limitation, the conductive additive is granular Super P, Ketjen black, acetylene black, or flaky conductive graphite, graphene, or short-distance carbon nanotubes or vapor grown carbon fibers, the number of silicon particles which can be covered and connected by the conductive additive is relatively small, small-particle silicon with the median particle size of 0.01-5 μm is selected in order to reduce silicon inactivation caused by contact as much as possible, however, the specific surface area of the small-particle silicon is relatively large, when the conductive additive is applied to a lithium ion battery, the first-round coulomb efficiency is only 81%, and the energy density of the battery is limited. Because of the adoption of small-particle silicon, the relative contact interface between the negative plates is increased, the resistance of lithium ion transmission is improved, and the rate performance of the battery is poor. Meanwhile, during circulation, the smaller silicon particles repeatedly generate new SEI due to expansion and contraction of silicon, and finally a thick SEI film is generated on the surface of each small particle, so that the SEI film blocks transmission of electrons, relative insulation between the silicon particles is caused, the silicon particles are inactivated, and further the cycle performance of the lithium ion battery is deteriorated. On the other hand, in the circulating process of silicon particles with the median diameter of 0.01-5 mu m, the silicon expands to cause cracking, submicron or even smaller nano particles are formed after the cracking, the pulverization of the particles is caused, the circulating performance of the battery is further deteriorated, the silicon-carbon composite material is only circulated in the full battery for about 200 times, and the application range is greatly limited.

Chinese patent publication No. CN105161695A discloses spherical active material particles for a negative electrode of a lithium ion battery, and a preparation method and application thereof. The spherical active substance particles are spherical composite particles prepared by spray drying active substance particles such as fibrous carbon, silicon with a micro-nano scale and the like. The spherical active material particles are not secondarily coated and have a porous structure having a larger specific surface area. Therefore, the first coulombic efficiency of the lithium ion battery made of the material is low, and the first round efficiency is only 60% as shown in the embodiment. In addition, the porous structure means that the material has a low density, which results in a low energy density of the lithium ion battery made of the material. Furthermore, the spherical active material particles contain up to 16.7% or more of fibrous carbon, which, in addition to a high specific surface area and a low density, also leads to a low content of active material in the material and thus to a low capacity of the composite material.

Chinese patent publication No. CN106207142A discloses a method for preparing a silicon-carbon composite negative electrode material for a power lithium ion battery. The preparation method of the silicon-carbon composite negative electrode material comprises the following steps: preparing polyimide coated nano silicon particle slurry; spray drying and granulating the slurry to prepare polyimide coated nano silicon particle powder; calcining the polyimide-coated nano silicon particle powder at high temperature, and then crushing and granulating the polyimide-coated nano silicon particle powder without damaging the coating structure; and mixing the crushed powder with graphite material powder to prepare the silicon-carbon composite negative electrode material. The polyimide in-situ polymerization coated nano-silicon has the advantages of complex process, high technical difficulty and difficulty in industrial production, and the polyimide is subjected to spray drying by using an organic solvent, so that explosion prevention and solvent recovery are involved, and the production risk and the cost are high. The silicon-carbon composite negative electrode material is a mixture of a silicon-carbon material and graphite, the proportion of the silicon-carbon material in the mixture is only 20 wt% at most, namely the actual silicon content of the silicon-carbon composite negative electrode material is lower than 20 wt%, so that the capacity of the silicon-carbon composite negative electrode material is far lower than the theoretical capacity of a silicon material.

Chinese patent publication No. CN104868107A discloses a spherical silicon-carbon composite material for lithium ion batteries, and a preparation method and application thereof. The spherical silicon-carbon composite material comprises a porous silicon-carbon composite material and an organic or inorganic carbon source filled in the porous silicon-carbon composite material. Chinese patent publication No. CN104716312B discloses a silicon-carbon composite material for lithium ion batteries, and a preparation method and an application thereof, and the patent is the same applicant and inventor as the patent with application publication No. CN 104868107A. The carbon-silicon composite materials described in the two patents are very similar in structure and preparation method, and the main difference is that in the patent with the publication number of CN104716312B, a step of coating aluminum-containing material on silicon powder by adopting a reduced pressure distillation method is added to the silicon-carbon composite material. The reduced pressure distillation method is only suitable for small-scale experiments in laboratories and cannot be used for industrial production. The silicon-carbon composite material described in both patents is spherical secondary particles prepared by spray drying equipment. After rolling, if the compaction density of a negative pole piece prepared by homogenizing and coating the spherical secondary particles is too high, the spherical secondary particles can be crushed, so that the internal conductive contact of the secondary particles is deteriorated, and the cycle performance of the battery can be deteriorated under the condition that silicon at a newly-broken interface is in direct contact with electrolyte; and if the pole piece compaction density is too low, poor conductive contact between secondary particles is caused, and the energy density of the battery is also low.

Chinese patent publication No. CN105720258A discloses a lithium ion battery negative electrode material, a preparation method and application thereof, and a lithium ion battery. The preparation method of the cathode material comprises the following steps: 1) uniformly mixing the silicon powder slurry and the binder, and performing spray drying to obtain primary particles A; 2) adding the primary particles A in the synthesis process of the asphalt resin to obtain the asphalt resin containing silicon powder, and sintering and crushing to obtain secondary particles B; 3) and uniformly mixing the secondary particles B with graphite, performing surface modification by using asphalt, and roasting to obtain tertiary finished product particles C. The silicon powder slurry is obtained by water-based wet grinding. Experiments show that silicon powder can react with water violently in the wet grinding process, and the products are silicon dioxide and hydrogen, so that the oxygen content in the ground product is increased by dozens of times compared with the raw material, and finally the problem of low coulombic efficiency of the material in a lithium ion battery is caused. And the smaller the particle size of the ground product, the more severe the oxidation. The negative electrode material is a mixture of a silicon-carbon material and graphite, and the actual silicon content of the silicon-carbon composite negative electrode material is calculated to be lower than 20 wt% according to the claims and the embodiment of the negative electrode material, so that the capacity of the silicon-carbon composite negative electrode material is far lower than the theoretical capacity of a silicon material.

Chinese patent publication No. CN105633387A discloses a method for preparing a silicon-based negative electrode material. The preparation method of the negative electrode material comprises the following steps: performing ball milling on the silicon monoxide, the carbon material and the binder in a solution, and performing spray drying on the mixture to form a precursor of the silicon-based negative electrode material; and sintering and cooling the silicon-based anode material precursor in an inert atmosphere to obtain the silicon-based anode material. The silicon-based negative electrode material is a porous structure with a larger specific surface without secondary coating, and the silicon monoxide primary particles are directly exposed to electrolyte. Therefore, the lithium ion battery made of the material has low initial coulombic efficiency, and the initial efficiency is only 65% as shown in the embodiment. In addition, the porous structure and spherical structure of the silicon-based negative electrode material mean that the packing density of the material is low, which results in low compaction density of the prepared pole piece and thus low energy density of the prepared lithium ion battery.

Chinese patent publication No. CN104362311B discloses a silicon-carbon composite microsphere negative electrode material and a preparation method thereof. The preparation method of the material comprises the steps of firstly mixing nano silicon particles and polyvinyl alcohol solution, and forming first composite microspheres after spray drying; then mixing the first composite microspheres with a polyacrylonitrile solution, coating the surfaces of the first composite microspheres, and volatilizing a solvent to form second composite microspheres with core-shell structures; and finally, carrying out oxidation and carbonization treatment on the second composite microspheres to form the silicon-carbon composite microsphere negative electrode material. The shell of the silicon-carbon composite microsphere negative electrode material is amorphous carbon obtained by carbonizing polyacrylonitrile, and the coulomb efficiency is low. The highest first coulombic efficiency in the six examples shown in this patent is only 65%. In addition, the negative pole piece made of the spherical particles can cause the crushing of the spherical secondary particles if the compaction density is too high after rolling; and if the pole piece compaction density is too low, poor conductive contact between secondary particles is caused, and the energy density of the battery is also low.

Chinese patent publication No. CN103346324B discloses a negative electrode material for lithium ion batteries and a preparation method thereof. The negative electrode material comprises an inner core and a shell wrapped outside the inner core, a hollow layer is arranged between the shell and the inner core, the inner core is a silicon-carbon composite material, the shell is a carbon composite material, and the carbon composite material is formed by a carbon material and a first amorphous carbon precursor. The preparation method of the negative electrode material comprises the following steps: A) mixing the silicon particles, the second amorphous carbon precursor and the first ball-milling medium, and then carrying out spray drying and granulation to obtain a first compound; B) mixing the first compound, the carbon material, the first amorphous carbon precursor and a second ball milling medium, performing ball milling in a protective atmosphere, and performing spray drying granulation to obtain a second compound; C) and roasting the second compound in a protective atmosphere to obtain the lithium ion battery cathode material. However, experiments show that in the step B, the first composite is damaged by the ball milling process, and is uniformly mixed with the carbon material and the first amorphous carbon precursor, so that the core-shell structure cannot be formed, and the hollow layer between the outer shell and the inner core cannot be formed.

Chinese patent publication No. CN102891297B discloses a silicon-carbon composite material for lithium ion batteries and a preparation method thereof. The preparation method of the cathode material is characterized in that silicon and graphite are mixed in a sodium carboxymethyl cellulose solution by ball milling, and are carbonized after being dried and granulated by a spray drying technology. Experiments show that silicon powder can react with water violently in the wet grinding process, and the products are silicon dioxide and hydrogen, so that the oxygen content in the ground product is increased by dozens of times compared with the raw material, and finally the problem of low coulombic efficiency of the material in a lithium ion battery is caused. And the smaller the particle size of the ground product, the more severe the oxidation. The silicon-carbon composite material is not subjected to secondary coating and has a porous structure with a larger specific surface, and primary particles of silicon and graphite are directly exposed to electrolyte. Therefore, the lithium ion battery made of the material has low coulombic efficiency for the first time. In addition, the porous structure and the spherical structure of the silicon-carbon composite material mean that the packing density of the material is low, so that the compaction density of the prepared pole piece is low, and the energy density of the prepared lithium ion battery is low.

The existing materials are mainly improved towards small particles, but the application of the materials in lithium ion batteries is limited due to low coulombic efficiency and poor cycle. Therefore, the problems that the coulombic efficiency and the capacity of the silicon-carbon composite material are low and the cycle performance is poor after the silicon-carbon composite material is applied to a battery are urgently needed to be solved.

Disclosure of Invention

Aiming at the defects of the prior art, the invention provides a silicon-carbon composite material which is used for a lithium ion battery and has high capacity, low polarization, high coulombic efficiency and long cycle life and a preparation method thereof.

In order to achieve the purpose, the technical scheme provided by the invention is as follows:

a silicon-carbon composite material for a lithium ion battery is provided, wherein the silicon-carbon composite material is spherical secondary particles; the secondary particles are formed by compounding a silicon material, a long-range conductive additive and carbon; in the secondary particles, the long-range conductive additive and the silicon material are uniformly dispersed; the median particle size of the primary particles of the silicon material is 1-10 mu m; the median diameter of the secondary particles is between 5 and 50 mu m; the secondary particle surface is coated with a carbon layer or not coated with carbon.

Preferably, the median particle diameter of the primary particles of the silicon material is between 1 and 8 μm; more preferably, the median particle diameter of the primary particles is between 2 and 7 μm; more preferably, the median particle diameter of the primary particles is between 3 and 8; most preferably, the median particle diameter of the primary particles is between 5 and 7 μm; most preferably, the primary particles have a median particle size of between 5.2 and 7.2 μm.

Preferably, the secondary particles have a median particle size of between 10 and 40 μm; more preferably, the secondary particles have a median particle size of between 15 and 30 μm.

In the silicon-carbon composite material, the content of silicon materials is 80-98 wt%, the content of conductive additives is 0.01-5 wt%, and the content of carbon is 20-1 wt%.

The long-range conductive additive is one or a combination of a plurality of vapor-grown carbon fibers, multi-walled carbon nanotubes and single-walled carbon nanotubes; the length of the long-range conductive additive is 10-100 mu m.

By adopting the long-range conductive additive, as a single conductive agent is longer and reaches 10-100 mu m, the conductive agent can cover and connect more silicon, the contact between silicon particles is enhanced by the long-range conductive agent, even if the silicon particles are cracked due to expansion, the silicon particles and the silicon particles still have more contact sites to ensure the connection, so that the silicon inactivation caused by the particle contact is reduced, and large-particle silicon with smaller specific surface area can be adopted.

The silicon primary particle material is crystalline silicon or amorphous silicon.

The invention also discloses a preparation method of the silicon-carbon composite material for the lithium ion battery, which comprises the following steps:

1) uniformly mixing the silicon primary particles, the conductive additive, the first carbon precursor, the dispersant and the solvent,

obtaining mixed slurry of silicon, conductive additive and first carbon precursor;

2) drying and granulating the mixed slurry, and then performing high-temperature carbonization in a non-oxidizing atmosphere;

3) screening the product obtained in the step 2) to obtain an uncoated silicon-carbon composite material;

4) coating the product obtained in the step 2) with a second carbon precursor, and then performing high-temperature carbonization in a non-oxidizing atmosphere;

5) screening the product obtained in the step 4) to obtain the silicon-carbon composite material.

In the step (1):

the first carbon precursor is one or a combination of more of glucose, sucrose, chitosan, starch, citric acid, gelatin, alginic acid, carboxymethyl cellulose, sodium carboxymethyl cellulose, coal pitch, petroleum pitch, phenolic resin, tar, naphthalene oil, anthracene oil, polyvinyl chloride, polystyrene, polyvinylidene fluoride, polyvinylpyrrolidone, polyethylene oxide, polyvinyl alcohol, epoxy resin, polyacrylonitrile and polymethyl methacrylate;

the solvent used for dispersing and dissolving is one or more of water, methanol, ethanol, isopropanol, N-butanol, ethylene glycol, diethyl ether, acetone, N-methylpyrrolidone, methyl butanone, tetrahydrofuran, benzene, toluene, xylene, N-dimethylformamide, N-dimethylacetamide and chloroform.

The dispersing agent used for dispersing is one or more of sodium tripolyphosphate, sodium hexametaphosphate, sodium pyrophosphate, cetyl trimethyl ammonium bromide, polyacrylate, polyvinylpyrrolidone and polyoxyethylene sorbitan monooleate.

The dispersant is capable of effectively dispersing the conductive additive. The solvent and the silicon can form a uniform fluid, and can completely dissolve the dispersing agent, the first carbon precursor or form a uniform fluid with the first carbon precursor.

The dispersion mode adopts a physical stirring mode. In a silicon system with larger particles, the primary particles of the silicon material, the long-range conductive additive and the first carbon precursor are dispersed and mixed in a solvent by a physical stirring mode in cooperation with the existence of a dispersing agent, so that the fracture of the long-range conductive agent caused by a sand grinding and wet grinding mode is avoided, and the coverage and connection of the long-range conductive agent on the silicon particles are ensured.

In the step (2):

the drying granulation is carried out in a spray drying mode;

the high-temperature carbonization adopts any one of a rotary furnace, a roller kiln, a pushed slab kiln, an atmosphere box furnace or a tubular furnace;

the temperature of the high-temperature carbonization reaction is 500-;

the non-oxidizing atmosphere is provided by at least one of the following gases: nitrogen, argon, hydrogen or helium.

In the step (4):

the coating process of the second carbon precursor is carried out by adopting any one of a mechanical fusion machine, a VC mixer or a high-speed dispersion machine;

the second carbon precursor is one or a combination of more of coal pitch, petroleum pitch, polyvinyl alcohol, epoxy resin, polyacrylonitrile and polymethyl methacrylate;

the equipment used for high-temperature carbonization is any one of a rotary furnace, a roller kiln, a pushed slab kiln, an atmosphere box furnace or a tubular furnace;

the temperature of the high-temperature carbonization reaction is 600-;

The invention also protects the lithium ion battery cathode containing the silicon-carbon composite material.

Furthermore, in the lithium ion battery cathode, the mass ratio of the silicon-carbon cathode material is 80-96%; the negative electrode also contains an organic polymer binder, wherein the organic polymer binder is at least one or a combination of more of carboxymethyl cellulose, lithium carboxymethyl cellulose, sodium carboxymethyl cellulose, styrene-butadiene rubber, polyacrylic acid, sodium polyacrylate, lithium polyacrylate, a polystyrene acrylic copolymer, a polyacrylate copolymer, a carboxymethyl cellulose-acrylic acid copolymer, polyimide, polyamide imide, polyacrylonitrile, a polyacrylonitrile acrylic acid copolymer, alginic acid, sodium alginate, lithium alginate, an ethylene acrylic acid copolymer, hydrogel, xanthan gum, polyethylene oxide, polyvinyl alcohol and a polyacrylic acid-polyvinyl alcohol cross-linked copolymer.

Preferably, the organic polymer binder in the negative electrode contains at least one binder having high tensile strength and high elastic deformation. By combining the organic polymer binders with high tensile strength and high elastic deformation characteristics, the surface of the silicon material is wrapped by the binders, so that on one hand, the expansion of particles can be inhibited to a certain extent, the damage to an SEI (solid electrolyte interphase) film is reduced, on the other hand, the particles can still be tightly connected with the particles and a current collector after the repeated expansion-contraction of the silicon material, the electrical activity of the material is kept, and the cycle performance of the battery is improved.

The invention also protects the lithium ion battery prepared by the lithium ion battery cathode.

In the application system of silicon, a traditional adhesive such as sodium carboxymethylcellulose and styrene butadiene rubber is generally adopted, the tensile strength of the traditional adhesive is relatively low, so that in practical application, the negative influence of cracking caused by expansion in circulation is smaller when the silicon particles are smaller, and when the particle size reaches 150nm, the critical cracking size is reached, and the silicon particles are not cracked any more. On the other hand, due to technical limitations, the conductive agent applied to the silicon material is generally short-range acetylene black, Super P, vapor-grown carbon fiber, conductive graphite, or graphene, and the short-range conductive agent covers and connects a relatively small amount of silicon, so that the nano-silicon is a main object for the application of the lithium ion battery in order to reduce the deactivation caused by poor contact as much as possible. However, small particles of silicon have a relatively large specific surface area and are less coulombic efficient when cycled. And a plurality of interfaces are arranged between the small-particle micron silicon and the nano silicon, and in the lithium intercalation process, the plurality of interfaces greatly inhibit the transmission power of lithium ions, and the lithium intercalation difficulty of silicon is increased to a certain extent, so that the rate performance of the lithium ion battery is reduced. During the circulation of small silicon particles, SEI is repeatedly formed along with the expansion of the silicon particles, and finally the surface of the silicon particles is completely covered by the SEI. SEI blocks the transport of electrons, resulting in the deactivation of silicon particles, further resulting in deterioration of the cycle of the lithium ion battery. The deactivation by the silicon particle expansion is not a bottleneck at this time, and the deactivation of silicon between particles by the SEI insulating layer becomes a major problem.

In the invention, the silicon cathode system adopts the adhesive combination with high tensile strength and high elastic deformation characteristics. Meanwhile, the long-range conductive agent with the length of 10-100 mu m is adopted in the conductive agent system, and the contact and connection particle size of the long-range conductive agent is larger. The silicon-carbon cathode for the lithium ion battery is synthesized by selecting micron silicon with the median particle size of 1-10 mu m as a raw material, and uniformly and tightly winding a long-range conductive agent and silicon particles in a spray drying mode to form spherical secondary particles, wherein each particle in the spherical secondary particles is wound by the conductive agent, and the silicon particles are tightly contacted in the spherical particles. The synthesized silicon-carbon cathode material has a lower specific surface area, has higher coulombic efficiency when being applied to a lithium ion battery cathode, and obviously improves the energy density of the battery. Meanwhile, in the circulation process, SEI generated by the reaction is relatively less, the conduction of electrons and ions can not be blocked, and meanwhile, the existence of the long-range conductive agent further improves the contact among particles. By adopting the silicon particles with the median particle size of 1-10 mu m, the total contact interface among the particles in the negative plate is less, the interface transmission resistance of lithium ions is greatly reduced, and the rate capability of the lithium ion battery is remarkably improved. On the other hand, in the circulation process, large silicon particles can expand to a certain degree and cause fracture, but the fractured silicon is still in a micron or submicron level, the contact among the particles is better, and meanwhile, the fractured silicon particles are connected in series by the uniformly dispersed conductive agent in the structure and still form a whole without inactivation, so that the circulation performance of the battery is greatly improved.

The invention provides a silicon-carbon composite material, which is spherical secondary particles, wherein silicon material primary particles and a conductive agent are uniformly dispersed in the secondary particles; the surface of the secondary particles can be coated with a layer of continuous amorphous carbon.

The invention adopts the long-range conductive agent and the silicon particles to be mutually and tightly wound to form spherical secondary particles, each silicon particle is coated and wound by the conductive agent inside the secondary particle, and the particles are tightly contacted, thereby ensuring that the inactivation caused by the contact of the particles is reduced to the maximum extent in the cycle process of the lithium ion battery and improving the cycle performance of the battery.

Compared with the prior art, the invention has the beneficial effects that:

1. according to the spherical secondary particles synthesized by the method, the primary silicon particles are tightly covered and connected by the long-range conductive agent, so that the contact among the silicon particles is enhanced, and the cracked silicon caused by expansion can still be connected by the conductive agent covered on the surface in the battery circulation process, so that the inactivation of the silicon is reduced, and the coulombic efficiency and the circulation performance of the battery are obviously improved.

2. Silicon particles with the median particle size of 1-10 mu m are adopted, expansion fracture occurs in the circulation process, the silicon particles are still in a micron or submicron level after the fracture, the particles are in good contact, the inactivation of the particles is greatly inhibited, and the coulomb efficiency and the circulation performance of the battery are obviously improved.

3. The silicon particles with the median particle size of 1-10 mu m are adopted, the specific surface area is small, and the synthesized silicon carbon material has high first effect when applied to the negative electrode of the lithium ion battery, so that the energy density of the battery is improved.

4. The silicon particles with the median particle size of 1-10 mu m are adopted, the specific surface area is small, an SEI film generated by reaction in the circulation process is thin, contact between the silicon particles is not isolated, conduction of electrons and ions is guaranteed, and the circulation performance of the battery is improved.

5. By adopting the silicon particles with the median particle size of 1-10 mu m, the contact interface in the negative electrode of the lithium ion battery is greatly reduced, and the rate capability of the battery is obviously improved.

6. The silicon particles are fixed in the amorphous carbon network or matrix, the amorphous carbon network obstructs the contact of electrolyte and the surface of the silicon particles, the coulombic efficiency of the battery is improved, meanwhile, the amorphous carbon network can effectively inhibit and buffer the expansion of the silicon particles, and the silicon particles are prevented from gradually fusing into particles with larger sizes in the charging and discharging processes, so that the larger expansion is caused, and part of silicon materials are failed.

Drawings

Fig. 1 is a Scanning Electron Microscope (SEM) photograph of the silicon carbon composite prepared in example 1.

Fig. 2 is an SEM photograph of the silicon carbon composite material prepared in example 1.

Fig. 3 is a full cell cycle curve prepared in example 1.

Fig. 4 is an SEM photograph of the silicon carbon composite prepared in example 1 after 200 cycles in a full cell.

Fig. 5 is an SEM photograph of the silicon carbon composite prepared in comparative example 1.

Detailed Description

The present invention will be further described with reference to the following specific examples.

Example 1

Taking 1000g of micron amorphous silicon powder with the median particle size of 2 mu m, adding 2000g of deionized water, adding 500g of cane sugar, stirring, dissolving and mixing, weighing 50g of single-walled carbon nanotube with the solid content of 0.4% and 50g of polyvinylpyrrolidone, fully stirring with the slurry, and uniformly mixing. And (3) carrying out spray drying granulation on the slurry to obtain secondary particles with the median particle size of 9.6 microns. And heating the spray-dried dry powder in an inert atmosphere of argon at 600 ℃ for 10 hours to carbonize the sucrose to obtain the silicon material tightly wound by the spherical single-walled carbon nanotube. And adding 800g of the spherical particles and 114g of the coal tar pitch into a mechanical fusion machine, and performing high-speed fusion treatment at 1500rpm for 30 minutes to obtain the coal tar pitch-coated spherical silicon composite particles. And (2) keeping the materials at 300 ℃ for 2 hours in an argon inert atmosphere, then heating to 850 ℃ for carbonization for 3 hours, naturally cooling to room temperature, and sieving to obtain the spherical silicon-carbon composite material coated with amorphous carbon.

Fig. 1 shows a Scanning Electron Microscope (SEM) photograph of the final silicon carbon composite product of example 1 at 2000 x magnification. The product was seen to be a regular spherical secondary particle. Fig. 2 is an SEM photograph of the silicon carbon composite product prepared in example 1, which is magnified 10000 times, and it is clear from the SEM photograph that the single-walled carbon nanotubes are tightly coated and wound around the surface of the silicon particles, and each silicon particle is tightly connected to ensure the contact between the silicon particles.

And (2) homogenizing, coating, drying and rolling 92 parts of the silicon-carbon composite material, 2 parts of a conductive additive and 6 parts of a binder in a water-based system to obtain the silicon-containing negative pole piece.

Half-cell evaluation: and (3) sequentially stacking the prepared silicon-containing negative pole piece, the diaphragm, the lithium piece and the stainless steel gasket, dripping 200 mu L of electrolyte, and sealing to prepare the 2016 type lithium ion half-cell. The capacity and discharge efficiency were tested using a small (micro) current range device from blue-electron, inc. The first reversible lithium removal specific capacity of the half-cell of the silicon-containing cathode is measured to be 1291.5mAh/g, and the first charge-discharge efficiency is 91.3%.

Full cell evaluation: the prepared silicon-containing negative pole piece is cut, vacuum-baked, wound together with a matched positive pole piece and a diaphragm, filled into an aluminum plastic shell with a corresponding size, injected with a certain amount of electrolyte, sealed and formed to obtain a complete silicon-containing negative pole lithium ion full battery. The capacity and the average voltage of the full battery at 0.2C and the capacity retention rate data of the full battery which is cycled for 200 times at the charge and discharge rate of 0.5C are tested by a battery tester of New Wille electronics Limited of Shenzhen. The initial constant current charging proportion of the full battery is 91.2%, the volume energy density is 806.5Wh/L, and the capacity retention rate after 200 charge-discharge cycles is 83.0%. Electrochemical data show that the inactivation of particles is reduced due to the lower specific surface area and the winding effect of the conductive agent of the larger micron silicon particles, the first irreversible capacity of the battery is reduced, the first coulombic efficiency of the battery is greatly improved, and the energy density of the battery is remarkably improved. Meanwhile, the larger micron silicon particles reduce the interface of lithium ion transmission, improve the rate capability of the battery and show that the battery has higher constant current charging proportion.

Fig. 3 is a graph of cycle performance of a silicon-containing negative electrode full cell prepared in example 1.

The full cell after 200 cycles is disassembled, the silicon-containing negative electrode plate is soaked in acetone, washed and dried, and then analyzed, and fig. 4 is an observed SEM photograph magnified 20000 times. As seen from the SEM photographs, although the silicon material was expanded after 200 cycles, the expanded silicon material still has more contact sites due to the surface having the coating of the dense single-walled carbon nanotubes. Meanwhile, SEM photos show that the silicon after expansion and fracture is still micron-sized silicon and can not fall off. Therefore, the inactivation of the material due to expansion of the micron silicon coated with the long-range conductive agent is greatly reduced, so that the cycle performance and the coulombic efficiency of the battery are improved.

Example 2

Taking 1000g of micron crystal silicon powder with the median particle size of 4.3 mu m, adding 2000g of ethanol, adding 25g of glucose, stirring, dissolving and mixing, weighing 25g of single-walled carbon nanotube with the solid content of 0.4% and 50g of polyvinylpyrrolidone, fully stirring with the slurry, and uniformly mixing. And (3) carrying out spray drying granulation on the slurry to obtain secondary particles with the median particle size of 14 mu m. And heating the spray-dried dry powder in an inert atmosphere of argon at 700 ℃ for 6 hours to carbonize the sucrose, thereby obtaining the spherical silicon composite material with the single-walled carbon nano tubes tightly wound. Taking 800g of spherical silicon composite material, taking 50g of petroleum asphalt which is sieved by a 200-mesh sieve, mechanically mixing the materials by a VC mixer for 10 minutes, heating the equipment to 300 ℃ while stirring in the atmosphere of nitrogen protection, continuously stirring the materials for 30 minutes, and then cooling the materials to room temperature. And (2) preserving the temperature of the materials at 300 ℃ for 2 hours in an argon inert atmosphere, then heating to 900 ℃ for carbonization for 2 hours, naturally cooling to room temperature, and sieving to obtain the spherical silicon-carbon composite material coated with amorphous carbon.

Taking 87 parts of the silicon-carbon composite material, 3 parts of a conductive additive and 10 parts of a binder, and homogenizing, coating, drying and rolling the mixture in a water-based system to obtain the silicon-containing negative pole piece.

The evaluation methods of the half cell and the full cell are the same as example 1, and the first reversible lithium removal specific capacity of the half cell containing the silicon negative electrode is measured to be 1542.7mAh/g, and the first charge-discharge efficiency is 92.3%. The volume energy density of the full battery is measured to reach 814.9Wh/L, the constant current charging proportion is 91.8%, and the capacity retention rate after 200 charge-discharge cycles is 83.5%.

Example 3

And taking 1000g of micron crystal silicon powder with the median particle size of 8 mu m, adding 2000g of water, adding 667g of sucrose, stirring, dissolving and mixing, weighing 400g of multi-walled carbon nanotubes with the solid content of 5% and 50g of polyvinylpyrrolidone, fully stirring with the slurry, and uniformly mixing. And carrying out spray drying granulation on the slurry to obtain secondary particles with the median particle size of 48 mu m. And heating the spray-dried dry powder in an inert atmosphere of argon at 800 ℃ for 2 hours to carbonize sucrose, thereby obtaining the silicon composite material tightly wound by the spherical multi-walled carbon nano-tubes. And screening the materials to obtain the spherical silicon-carbon composite material.

And (3) homogenizing, coating, drying and rolling 90 parts of the silicon-carbon composite material, 6 parts of the conductive additive and 4 parts of the binder in a water-based system to obtain the silicon-containing negative pole piece.

The evaluation methods of the half cell and the full cell are the same as example 1, and the first reversible lithium removal specific capacity of the half cell with the silicon-containing negative electrode is measured to be 1337.1mAh/g, and the first charge-discharge efficiency is 91.5%. The volume energy density of the full battery is measured to reach 807.2Wh/L, the constant current charging proportion is 90.8%, and the capacity retention rate after 200 charge-discharge cycles is 82.5%.

Example 4

Taking 1000g of micron crystal silicon powder with the median particle size of 5 microns, adding 2000g of water, adding 12.5g of sodium carboxymethylcellulose, stirring, dissolving and mixing, weighing 400g of multi-wall carbon nano-tube with the solid content of 5%, fully stirring with the slurry, and uniformly mixing. And (3) carrying out spray drying granulation on the slurry to obtain secondary particles with the median particle size of 12.5 microns. And heating the spray-dried dry powder in an inert atmosphere of argon at 500 ℃ for 6 hours to carbonize the sodium carboxymethyl cellulose, thereby obtaining the spherical silicon composite material with the multi-walled carbon nano-tubes tightly wound. And (2) taking 800g of spherical silicon composite material, taking 100g of petroleum asphalt which is sieved by a 200-mesh sieve, adding the petroleum asphalt into a mechanical fusion machine, and carrying out high-speed fusion treatment for 30 minutes at 1500rpm to obtain petroleum asphalt coated spherical silicon composite particles. And (2) preserving the temperature of the materials at 300 ℃ for 2 hours in an argon inert atmosphere, then heating to 1000 ℃ for carbonization for 2 hours, naturally cooling to room temperature, and sieving to obtain the spherical silicon-carbon composite material coated with amorphous carbon.

And (3) homogenizing, coating, drying and rolling 85 parts of the silicon-carbon composite material, 7 parts of a conductive additive and 8 parts of a binder in a water-based system to obtain the silicon-containing negative pole piece.

The evaluation methods of the half cell and the full cell are the same as example 1, and the first reversible lithium removal specific capacity of the half cell containing the silicon negative electrode is 1500mAh/g, and the first charge-discharge efficiency is 91.1%. The volume energy density of the full battery is measured to reach 817.7Wh/L, the constant current charging proportion is 91.1%, and the capacity retention rate after 200 charge-discharge cycles is 82.8%.

Example 5

Taking 1000g of micron amorphous silicon powder with the median particle size of 5.2 microns, adding 2000g of water, adding 167g of cane sugar, stirring, dissolving and mixing, weighing 50g of single-walled carbon nanotube with the solid content of 0.4% and 50g of polyvinylpyrrolidone, fully stirring with the slurry, and uniformly mixing. And (3) carrying out spray drying granulation on the slurry to obtain secondary particles with the median particle size of 26 mu m. And heating the spray-dried dry powder in an inert atmosphere of argon at 600 ℃ for 2 hours to carbonize the sucrose, thereby obtaining the spherical silicon composite material with the single-walled carbon nano tubes tightly wound. Taking 800g of spherical silicon composite material, taking 30g of petroleum asphalt which is sieved by a 200-mesh sieve, mechanically mixing the materials by a VC mixer for 10 minutes, heating the equipment to 300 ℃ while stirring in the atmosphere of nitrogen protection, continuously stirring the materials for 30 minutes, and then cooling the materials to room temperature. And (2) preserving the temperature of the materials at 300 ℃ for 2 hours in an argon inert atmosphere, then heating to 900 ℃ for carbonization for 2 hours, naturally cooling to room temperature, and sieving to obtain the spherical silicon-carbon composite material coated with amorphous carbon.

And (3) homogenizing, coating, drying and rolling 80 parts of the silicon-carbon composite material, 10 parts of the conductive additive and 10 parts of the binder in a water-based system to obtain the silicon-containing negative pole piece.

The evaluation methods of the half cell and the full cell are the same as example 1, and the first reversible lithium removal specific capacity of the half cell containing the silicon negative electrode is measured to be 1576.8mAh/g, and the first charge-discharge efficiency is 92.3%. The volume energy density of the full battery is measured to reach 831.6Wh/L, the constant current charging proportion is 92.5%, and the capacity retention rate after 200 charge-discharge cycles is 84.1%.

Example 6

And (2) taking 1000g of micron crystal silicon powder with the median particle size of 6 microns, adding 2000g of N, N-dimethylformamide, adding 100g of petroleum asphalt, stirring, dissolving and mixing, weighing 500g of multi-walled carbon nanotubes with the solid content of 5% and 50g of polyvinylpyrrolidone, fully stirring with the slurry, and uniformly mixing. And (3) carrying out spray drying granulation on the slurry to obtain secondary particles with the median particle size of 28.8 microns. And heating the spray-dried dry powder in an argon inert atmosphere at 900 ℃ for 5 hours to carbonize the petroleum asphalt, thereby obtaining the silicon composite material with the spherical multi-walled carbon nano-tubes tightly wound. And screening the materials to obtain the spherical silicon-carbon composite material.

And (3) homogenizing, coating, drying and rolling 96 parts of the silicon-carbon composite material, 1 part of a conductive additive and 3 parts of a binder in a water-based system to obtain the silicon-containing negative pole piece.

The evaluation methods of the half cell and the full cell are the same as example 1, and the first reversible lithium removal specific capacity of the half cell of the silicon-containing negative electrode is measured to be 1534.3mAh/g, and the first charge-discharge efficiency is 90.4%. The volume energy density of the full battery is 830Wh/L, the constant current charging proportion is 90.4%, and the capacity retention rate after 200 charge-discharge cycles is 82.2%.

Example 7

Taking 1000g of micron amorphous silicon powder with the median particle size of 5.5 mu m, adding 2000g of tetrahydrofuran, adding 29g of coal tar pitch, stirring, dissolving and mixing, weighing 50g of vapor grown carbon fiber, fully stirring with the slurry, and uniformly mixing. And (3) carrying out spray drying granulation on the slurry to obtain secondary particles with the median particle size of 26 mu m. And heating the spray-dried dry powder for 4 hours at 800 ℃ in an inert atmosphere of argon to carbonize the coal pitch, thereby obtaining the spherical silicon composite material with the conductive agent tightly wound. 800g of spherical silicon composite material and 46g of coal pitch are taken, mechanically mixed for 10 minutes by a VC mixer, the temperature of the equipment is raised to 300 ℃ while stirring in the atmosphere of nitrogen protection, the stirring is continued for 30 minutes, and then the temperature is cooled to room temperature. And (2) preserving the temperature of the materials at 300 ℃ for 2 hours in an argon inert atmosphere, then heating to 800 ℃ for carbonization for 4 hours, naturally cooling to room temperature, and sieving to obtain the spherical silicon-carbon composite material coated with amorphous carbon.

And (2) homogenizing, coating, drying and rolling 82 parts of the silicon-carbon composite material, 8 parts of a conductive additive and 10 parts of a binder in a water-based system to obtain the silicon-containing negative pole piece.

The evaluation methods of the half cell and the full cell are the same as example 1, and the first reversible lithium removal specific capacity of the half cell containing the silicon negative electrode is measured to be 1525.7mAh/g, and the first charge-discharge efficiency is 90.8%. The volume energy density of the full battery is measured to reach 827.7Wh/L, the constant current charging proportion is 91.2%, and the capacity retention rate after 200 charge-discharge cycles is 82.9%.

Example 8

Taking 1000g of micron crystal silicon powder with the median particle size of 10 mu m, adding 2000g of ethanol, adding 5g of glucose, stirring, dissolving and mixing, weighing 250g of single-walled carbon nanotube with the solid content of 0.4% and 50g of polyvinylpyrrolidone, fully stirring with the slurry, and uniformly mixing. And carrying out spray drying granulation on the slurry to obtain secondary particles with the median particle size of 48 mu m. And heating the spray-dried dry powder in an inert atmosphere of argon at 1000 ℃ for 1 hour to carbonize glucose, thereby obtaining the silicon composite material with the spherical multi-walled carbon nano-tubes tightly wound. And screening the materials to obtain the spherical silicon-carbon composite material.

And (3) homogenizing, coating, drying and rolling 91 parts of the silicon-carbon composite material, 0.5 part of conductive additive and 8.5 parts of binder in a water-based system to obtain the silicon-containing negative pole piece.

The evaluation methods of the half cell and the full cell are the same as example 1, and the first reversible lithium removal specific capacity of the half cell with the silicon-containing negative electrode is measured to be 1695.4mAh/g, and the first charge-discharge efficiency is 92.7%. The volume energy density of the full battery is measured to reach 812.6Wh/L, the constant current charging proportion is 90.9%, and the capacity retention rate after 200 charge-discharge cycles is 82.6%.

Example 9

Taking 1000g of micron crystal silicon powder with the median particle size of 7 mu m, adding 1800g of water, adding 286g of sodium polyacrylate glue solution with the solid content of 10%, stirring and mixing, weighing 600g of multi-walled carbon nano-tube with the solid content of 5%, fully stirring with the slurry, and uniformly mixing. And carrying out spray drying granulation on the slurry to obtain secondary particles with the median particle size of 18 mu m. And heating the spray-dried dry powder in an argon inert atmosphere at 600 ℃ for 2 hours to carbonize the sodium polyacrylate, thereby obtaining the silicon composite material tightly wound by the spherical multi-walled carbon nano tube. And taking 800g of spherical silicon composite material, taking 11.4g of coal tar pitch, adding into a mechanical fusion machine, and carrying out high-speed fusion treatment at 1500rpm for 30 minutes to obtain the coal tar pitch-coated spherical silicon composite particles. And (2) preserving the temperature of the materials at 300 ℃ for 2 hours in an argon inert atmosphere, then heating to 800 ℃ for carbonization for 2 hours, naturally cooling to room temperature, and sieving to obtain the spherical silicon-carbon composite material coated with amorphous carbon.

And (3) homogenizing, coating, drying and rolling 86 parts of the silicon-carbon composite material, 7 parts of the conductive additive and 7 parts of the binder in a water-based system to obtain the silicon-containing negative pole piece.

The evaluation methods of the half cell and the full cell are the same as example 1, and the first reversible lithium removal specific capacity of the half cell with the silicon-containing negative electrode is measured to be 1628.6mAh/g, and the first charge-discharge efficiency is 91.9%. The volume energy density of the full battery is measured to reach 806.7Wh/L, the constant current charging proportion is 92.2%, and the capacity retention rate after 200 charge-discharge cycles is 83.8%.

Example 10

Taking 1000g of micron crystal silicon powder with the median particle size of 1 mu m, adding 2000g of water, adding 50g of glucose, stirring and mixing, weighing 30g of vapor grown carbon fiber, fully stirring with the slurry, and uniformly mixing. And (3) carrying out spray drying granulation on the slurry to obtain secondary particles with the median particle size of 6.5 microns. And heating the spray-dried dry powder at 600 ℃ for 2 hours in an inert atmosphere of argon to carbonize glucose, thereby obtaining the spherical silicon-carbon composite material. And (3) taking 800g of spherical silicon composite material, taking 30g of petroleum asphalt which is sieved by a 200-mesh sieve, adding the petroleum asphalt into a mechanical fusion machine, and carrying out high-speed fusion treatment for 30 minutes at 1500rpm to obtain petroleum asphalt coated spherical silicon composite particles. And (2) preserving the temperature of the materials at 300 ℃ for 2 hours in an argon inert atmosphere, then heating to 900 ℃ for carbonization for 2 hours, naturally cooling to room temperature, and sieving to obtain the spherical silicon-carbon composite material coated with amorphous carbon.

And (3) homogenizing, coating, drying and rolling 93 parts of the silicon-carbon composite material, 5 parts of the conductive additive and 2 parts of the binder in a water-based system to obtain the silicon-containing negative pole piece.

The evaluation methods of the half cell and the full cell are the same as example 1, and the first reversible lithium removal specific capacity of the half cell containing the silicon negative electrode is 1440mAh/g, and the first charge-discharge efficiency is 91.0%. The volume energy density of the full battery is measured to reach 808.4Wh/L, the constant current charging proportion is 91.9%, and the capacity retention rate after 200 charge-discharge cycles is 83.5%.

Example 11

Taking 1000g of micron amorphous silicon powder with the median particle size of 1.7 mu m, adding 2000g of tetrahydrofuran, adding 13g of petroleum asphalt, stirring, dissolving and mixing, weighing 125g of single-wall carbon nano tube with the solid content of 0.4%, fully stirring with the slurry, and uniformly mixing. And (3) carrying out spray drying granulation on the slurry to obtain secondary particles with the median particle size of 8.5 microns. And heating the spray-dried dry powder in an inert atmosphere of argon at 850 ℃ for 2 hours to carbonize the petroleum asphalt, thereby obtaining the spherical silicon composite material with the single-walled carbon nano tubes tightly wound. 800g of spherical silicon composite material and 46g of coal pitch are taken, mechanically mixed for 10 minutes by a VC mixer, the temperature of the equipment is raised to 300 ℃ while stirring in the atmosphere of nitrogen protection, the stirring is continued for 30 minutes, and then the temperature is cooled to room temperature. And (2) preserving the temperature of the materials at 300 ℃ for 2 hours in an argon inert atmosphere, then heating to 800 ℃ for carbonization for 4 hours, naturally cooling to room temperature, and sieving to obtain the spherical silicon-carbon composite material coated with amorphous carbon.

And (3) homogenizing, coating, drying and rolling 88 parts of the silicon-carbon composite material, 6 parts of the conductive additive and 6 parts of the binder in a water-based system to obtain the silicon-containing negative pole piece.

The evaluation methods of the half cell and the full cell are the same as example 1, and the first reversible lithium removal specific capacity of the half cell of the silicon-containing negative electrode is measured to be 1627.7mAh/g, and the first charge-discharge efficiency is 91.4%. The volume energy density of the full battery is measured to reach 812.3Wh/L, the constant current charging proportion is 91.4%, and the capacity retention rate after 200 charge-discharge cycles is 83.1%.

Example 12

Taking 1000g of micron crystal silicon powder with the median particle size of 7.2 microns, adding 2000g of water, adding 167g of cane sugar, stirring, dissolving and mixing, weighing 50g of single-walled carbon nanotube with the solid content of 0.4% and 50g of polyvinylpyrrolidone, fully stirring with the slurry, and uniformly mixing. And (3) carrying out spray drying granulation on the slurry to obtain secondary particles with the median particle size of 38 mu m. And heating the spray-dried dry powder in an inert atmosphere of argon at 600 ℃ for 2 hours to carbonize the sucrose, thereby obtaining the spherical silicon composite material with the single-walled carbon nano tubes tightly wound. Taking 800g of spherical silicon composite material, taking 30g of petroleum asphalt which is sieved by a 200-mesh sieve, mechanically mixing the materials by a VC mixer for 10 minutes, heating the equipment to 300 ℃ while stirring in the atmosphere of nitrogen protection, continuously stirring the materials for 30 minutes, and then cooling the materials to room temperature. And (2) preserving the temperature of the materials at 300 ℃ for 2 hours in an argon inert atmosphere, then heating to 900 ℃ for carbonization for 2 hours, naturally cooling to room temperature, and sieving to obtain the spherical silicon-carbon composite material coated with amorphous carbon.

The evaluation methods of the half cell and the full cell are the same as example 1, and the first reversible lithium removal specific capacity of the half cell with the silicon-containing negative electrode is measured to be 1554.2mAh/g, and the first charge-discharge efficiency is 92.4%. The volume energy density of the full battery is measured to reach 827.6Wh/L, the constant current charging proportion is 92.2%, and the capacity retention rate after 200 charge-discharge cycles is 83.5%.

Comparative example 1

Taking 1000g of amorphous silicon nano-powder with the median particle size of 0.1 mu m, 1500g of ethanol and 10g of hexadecyl trimethyl ammonium bromide, and sanding and dispersing the amorphous silicon nano-powder with the median particle size of 0.1 mu m and the ethanol in a sand mill by using zirconia beads with the diameter of 0.3mm until silicon nano-particle slurry with the median particle size of 0.01 mu m is obtained. To the slurry was added 20g of ketjen black powder, and sanding was continued for 30 minutes. 250g of sucrose was dissolved in 2250g of deionized water to prepare an aqueous sucrose solution. The sucrose aqueous solution was poured into a sand mill and thoroughly mixed with the silicon nanoparticle slurry for 30 minutes. The uniformly mixed anhydrous ethanol/water slurry of silicon particles/ketjen black/sucrose was further diluted with deionized water to a solid content of 10%, followed by spray drying treatment. The resulting spherical secondary particles had a median particle diameter of about 28 μm. And heating the spray-dried dry powder at 700 ℃ for 2 hours in an inert atmosphere of argon gas to carbonize the sucrose, thereby obtaining amorphous carbon bonded and coated silicon particles/ketjen black composite particles. And (3) carrying out jet milling treatment on all the spherical composite particles to obtain irregular composite particles with the median diameter of 11 mu m. And (3) mixing 530g of the composite particles and 424g of 2000-mesh petroleum asphalt at a high speed for 10 minutes by using a VC mixer, adding a mechanical fusion machine, and performing high-speed fusion treatment at 1500rpm for 30 minutes to obtain the petroleum asphalt-coated silicon particle/Ketjen black/amorphous carbon composite particles. And (2) preserving the heat of the materials at 300 ℃ for 2 hours in an argon inert atmosphere, then heating to 900 ℃ for carbonization for 2 hours, naturally cooling to room temperature, crushing and sieving to obtain the silicon particles/Ketjen black/amorphous carbon composite particles with the amorphous carbon coating.

FIG. 5 is an SEM photograph of the product at 20000 magnification. As seen from SEM pictures, the primary particles of the silicon-carbon composite material are small, nano-scale and large in surface area. Meanwhile, after the air flow is broken, the spherical secondary particles are destroyed, and a silicon particle structure with the conductive agent tightly wound cannot be formed.

The evaluation methods of the half cell and the full cell are the same as those of the example 1, the first reversible lithium removal specific capacity of the half cell containing the silicon cathode is measured to be 1332mAh/g, and the first charge-discharge efficiency is only 81.8 percent due to the larger specific surface area. The volume energy density of the full battery is measured to reach 771Wh/L, the number of small particles is large, the number of lithium ion transmission interfaces is large, polarization is increased, the constant current charging ratio of the battery is 89.4%, and the capacity retention rate after 200 charge-discharge cycles is 84.2%.

Comparative example 2

The process is similar to that of example 1, and the difference from example 1 is that nano silicon of 0.2 μm is used as raw material.

The evaluation methods of the half cell and the full cell are the same as example 1, and the first reversible lithium removal specific capacity of the half cell of the silicon-containing negative electrode is measured to be 1285.4mAh/g, and the first charge-discharge efficiency is 83.6%. The volume energy density of the full cell of the silicon-containing cathode is 786.4Wh/L, the constant current charging ratio is 90.2%, and the capacity retention rate after 200 charge-discharge cycles is 80.5%.

By adopting the small-particle nano silicon, the specific surface area of the material is greatly increased, so that the first coulombic efficiency is reduced, and the energy density of the battery is reduced. Meanwhile, as the number of lithium ion transmission interfaces is increased, the constant current charging ratio of the battery is reduced.

Comparative example 3

The process is similar to that of example 3, and the difference from example 3 is that nano silicon of 0.8 μm is used as the raw material.

The evaluation methods of the half cell and the full cell are the same as example 1, and the first reversible lithium removal specific capacity of the half cell of the silicon-containing negative electrode is measured to be 1337.1mAh/g, and the first charge-discharge efficiency is 82.8%. The volume energy density of the full cell of the silicon-containing cathode is 790.3Wh/L, the constant current charging ratio is 89.4%, and the capacity retention rate after 200 charge-discharge cycles is 79.8%.

Comparative example 4

The process is similar to that of example 7, and the difference from example 7 is that 0.3 μm nano-silicon is used as the raw material.

The evaluation methods of the half cell and the full cell are the same as example 1, and the first reversible lithium removal specific capacity of the half cell of the silicon-containing negative electrode is measured to be 1527.7mAh/g, and the first charge-discharge efficiency is 83.0%. The volume energy density of the full cell of the silicon-containing cathode is 783.0Wh/L, the constant current charging ratio is 89.8%, and the capacity retention rate after 200 charge-discharge cycles is 80.2%.

Comparative example 5

The process is similar to that of example 7, and the difference from example 7 is that the composite spherical silicon carbon secondary particles are synthesized by spray drying, coal tar pitch is carbonized at 800 ℃, and then a step of airflow crushing process is added to crush the secondary particles into particles with the median diameter of 8 μm.

The evaluation methods of the half cell and the full cell are the same as example 1, and the first reversible lithium removal specific capacity of the half cell with the silicon-containing negative electrode is measured to be 1427.5mAh/g, and the first charge-discharge efficiency is 88.6%. The volume energy density of the full cell of the silicon-containing cathode is 784.2Wh/L, the constant current charging ratio is 89.6%, and the capacity retention rate after 200 charge-discharge cycles is 80.0%.

The long-range conductive agent formed by spray drying covers and is connected with the spherical structure of the silicon particles, when airflow crushing is carried out, the spherical secondary particles are crushed, so that the covering and connecting effects of the long-range conductive agent are damaged, the contact among the particles is not compact any more, after cyclic expansion, the contact sites of the silicon particles subjected to expansion and fracture are reduced, the inactivation of the silicon particles is increased, and the first coulomb efficiency, the energy density and the cyclic performance of the battery are further reduced.

Comparative example 6

The process is similar to that of example 8, and the difference from example 8 is that the composite spherical silicon-carbon secondary particles are synthesized by spray drying, after the glucose is carbonized, a step of airflow crushing process is added to crush the secondary particles into particles with the median diameter of 15 μm.

The evaluation methods of the half cell and the full cell are the same as example 1, and the first reversible lithium removal specific capacity of the half cell containing the silicon negative electrode is measured to be 1595.8mAh/g, and the first charge-discharge efficiency is 90.2%. The volume energy density of the full cell of the silicon-containing cathode is 773.9Wh/L, the constant current charging ratio is 90.5%, and the capacity retention rate after 200 charge-discharge cycles is 79.6%.

Comparative example 7

The process is similar to that of example 9, and the difference from example 9 is that nano-silicon with a median particle size of 0.05 μm is used as a raw material.

The evaluation methods of the half cell and the full cell are the same as example 1, and the first reversible lithium removal specific capacity of the half cell of the silicon-containing negative electrode is measured to be 1628.6mAh/g, and the first charge-discharge efficiency is 82.4%. The volume energy density of the full cell of the silicon-containing cathode is 728.5Wh/L, the constant current charging ratio is 90.0%, and the capacity retention rate after 200 charge-discharge cycles is 80.4%.

Comparative example 8

The process is similar to that of example 9, and the difference from example 9 is that nano-silicon with a median particle size of 0.01 μm is used as a raw material.

The evaluation methods of the half cell and the full cell are the same as example 1, and the first reversible lithium removal specific capacity of the half cell with the silicon-containing negative electrode is measured to be 1627.7mAh/g, and the first charge-discharge efficiency is 81.9%. The volume energy density of the full cell of the silicon-containing cathode is 792.5Wh/L, the constant current charging ratio is 89.3%, and the capacity retention rate after 200 charge-discharge cycles is 79.7%.

The above description is only a preferred embodiment of the present invention, and should not be taken as limiting the invention in any way, and any person skilled in the art can make any simple modification, equivalent replacement, and improvement on the above embodiment without departing from the technical spirit of the present invention, and still fall within the protection scope of the technical solution of the present invention.

Claims

1. A silicon carbon composite for a lithium ion battery, characterized by: the silicon-carbon composite material is spherical secondary particles; the secondary particles are formed by compounding a silicon material, a long-range conductive additive and carbon; in the secondary particles, the long-range conductive additive and the silicon material are uniformly dispersed; the median diameter of the primary particles of the silicon material is between 1 and 10 mu m; the median particle diameter of the secondary particles is between 5 and 50 μm; the surface of the secondary particle is coated with a carbon layer or not coated with carbon; the length of the long-range conductive agent is 10-100 mu m.

2. The silicon-carbon composite material for a lithium ion battery according to claim 1, characterized in that: in the silicon-carbon composite material, the content of a silicon material is 80-98 wt%, the content of a conductive additive is 0.01-5 wt%, the content of carbon is 20-1 wt%, and primary silicon particles are amorphous silicon or crystalline silicon.

3. The silicon-carbon composite material for a lithium ion battery according to claim 1, characterized in that: the long-range conductive additive is one or a combination of a plurality of vapor-grown carbon fibers, multi-walled carbon nanotubes and single-walled carbon nanotubes.

4. The method for preparing a silicon-carbon composite material for a lithium ion battery according to any one of claims 1 to 3, wherein: the method comprises the following steps:

1) uniformly mixing the silicon primary particles, the conductive additive, the first carbon precursor, the dispersing agent and the solvent to obtain mixed slurry of silicon/the conductive additive/the first carbon precursor;

3) screening and demagnetizing the product obtained in the step 2) to obtain a silicon-carbon composite material which is not coated by carbon;

5) and 4) screening and demagnetizing the product obtained in the step 4) to obtain the silicon-carbon composite material.

5. The method of preparing a silicon-carbon composite material for a lithium ion battery according to claim 4, wherein: in step 1):

the solvent used for dispersing and dissolving is one or a combination of more of water, methanol, ethanol, isopropanol, N-butanol, ethylene glycol, diethyl ether, acetone, N-methylpyrrolidone, methyl butanone, tetrahydrofuran, benzene, toluene, xylene, N-dimethylformamide, N-dimethylacetamide and chloroform;

6. The method of preparing a silicon-carbon composite material for a lithium ion battery according to claim 4, wherein: in step 2):

the drying granulation is carried out in a spray drying mode;

the temperature of the high-temperature carbonization reaction is 500-;

7. The method of preparing a silicon-carbon composite material for a lithium ion battery according to claim 4, wherein: in the step 4):

the temperature of the high-temperature carbonization reaction is 600-;

8. A lithium ion battery negative electrode, characterized in that: comprising the silicon-carbon composite material according to any one of claims 1 to 3.

9. The lithium ion battery negative electrode of claim 8, wherein: in the lithium ion battery cathode, the mass ratio of the silicon-carbon cathode material is 80-96%; the negative electrode also contains an organic polymer binder, wherein the organic polymer binder is at least one or a combination of more of carboxymethyl cellulose, lithium carboxymethyl cellulose, sodium carboxymethyl cellulose, styrene-butadiene rubber, polyacrylic acid, sodium polyacrylate, lithium polyacrylate, a polystyrene acrylic copolymer, a polyacrylate copolymer, a carboxymethyl cellulose-acrylic acid copolymer, polyimide, polyamide imide, polyacrylonitrile, a polyacrylonitrile acrylic acid copolymer, alginic acid, sodium alginate, lithium alginate, an ethylene acrylic acid copolymer, hydrogel, xanthan gum, polyethylene oxide, polyvinyl alcohol and a polyacrylic acid-polyvinyl alcohol cross-linked copolymer.

10. A lithium ion battery, characterized by: prepared using the lithium ion battery negative electrode of claim 8 or 9.