CN109599551B

CN109599551B - Doped multilayer core-shell silicon-based composite material for lithium ion battery and preparation method thereof

Info

Publication number: CN109599551B
Application number: CN201811627828.4A
Authority: CN
Inventors: 查道松; 罗姝; 汪芳; 李喆; 王岑
Original assignee: Amprius Nanjing Co ltd
Current assignee: Boselis Hefei Co ltd; Bosellis Nanjing Co ltd
Priority date: 2018-12-28
Filing date: 2018-12-28
Publication date: 2021-08-24
Anticipated expiration: 2038-12-28
Also published as: CN109599551A

Abstract

The invention relates to a doped multilayer core-shell silicon-based composite material for a lithium ion battery and a preparation method thereof, wherein the material is at least doped with a non-metal element and a metal element besides Li element, and has the structure that silicon oxide compound particles with element doping are used as an inner core, and a multilayer composite film tightly coated on the surface of the inner core particles is used as a shell; the core particles contain uniformly dispersed simple substance silicon nano particles, wherein the content of doping elements is gradually reduced from outside to inside without obvious interfaces, and a layer of compact lithium silicate compound is formed on the surface of the core particles due to doping and embedding of lithium elements; the multilayer composite film is a carbon film layer and a doped composite film layer formed by compounding the carbon film layer and other element components. When used for the negative electrode of the lithium ion battery, the lithium ion battery has the electrochemical characteristics of high capacity, good rate capability, high coulombic efficiency, good cycle performance, low expansion rate and the like.

Description

Doped multilayer core-shell silicon-based 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 doped multilayer core-shell silicon-based composite material for a lithium ion battery and a preparation method thereof.

Background

As one of the most promising energy storage devices, lithium ion batteries have attracted attention due to their advantages of high voltage, low self-discharge rate, no memory effect, light weight, small size, etc., and are widely used in portable electronic products and electric vehicles. With the improvement of living standard and the continuous progress of technology, people also put higher demands on the performances of various aspects of lithium ion batteries, such as capacity, energy density, rate capability, cycle life and the like.

The most mature graphite cathode material is applied at present, and theoryThe limited capacity (372mAh/g) and near complete development of practical capacity have limited further increases in energy density of lithium ion batteries. Silicon anode materials have gradually become a research and development hot spot in the field in recent years by virtue of their remarkable high capacity advantage compared to graphite, and have started to gradually move from laboratory research and development to commercial application. In the silicon cathode material, the theoretical capacity of the simple substance silicon material is high (the lithium insertion state is Li at room temperature)₁₅Si₄Theoretical capacity of about 3600mAh/g) but its volume expansion is huge and the cycle performance is poor; although the theoretical capacity of the silicon oxide compound is relatively low (about 1700mAh/g), the advantages of expansion rate and cycle stability are obvious, and the silicon oxide compound is easier to realize industrial application compared with the simple substance silicon. However, when applied to lithium ion batteries, silicone compounds consume lithium to form lithium silicate, lithium oxide, and other substances when first charged, and lithium ions in the silicone compounds cannot be extracted again when discharged. This makes it generally faced with the bottleneck of first lower coulombic efficiency (theoretical efficiency of about 70%), which limits further improvement of the full-cell energy density. In addition, the ionic and electronic conductivities of the silicon-oxygen compound are generally low, so that the lithium removal and lithium insertion reactions of the silicon-oxygen compound in the first round charging and discharging process of the lithium ion battery are not sufficient, and the problems of low coulombic efficiency, low energy density under high multiplying power and poor cycle retention rate in the subsequent battery cycle process are caused. In order to solve the problems of the silicon-oxygen compound, researchers improve the silicon-oxygen compound through various means such as nanocrystallization, compounding with a carbon material, heteroatom doping, lithium pre-intercalation and the like.

Chinese patent publication No. CN105958036A discloses a double-layer carbon-coated silicon negative electrode material obtained by carbon-coating silicon powder twice. The preparation method of the material comprises the following steps: dispersing the silicon powder together with the first carbon coating layer and the dispersing agent after liquid phase dispersion; removing the solvent, and carrying out high-temperature carbonization treatment on the obtained solid material to obtain a primary carbon-coated silicon negative electrode material; and preparing dispersion liquid of a second carbon-coated material, dispersing the primary carbon-coated silicon material into the dispersion liquid of the second carbon-coated material, removing the solvent, and roasting for the second time to obtain the secondary carbon-coated silicon negative electrode material. The method adopts ball-milled broken silicon powder, and the size of the silicon particles is far larger than that of nano silicon particles generated in a silicon-oxygen compound through disproportionation reaction, so that the volume expansion/contraction effect of the silicon particles is obvious in the charging and discharging process of the battery; in addition, although the conductivity of the material is improved by two layers of carbon coating, the intrinsic conductivity of the core silicon particles is not improved, so that the high-speed electron transmission and the rate performance of the obtained material system are not obviously improved.

Chinese patent publication No. CN108172775A discloses a phosphorus-doped silicon-carbon negative electrode material, which is prepared by spraying, granulating and sintering phosphorus-doped nano-silicon material, graphite and an organic carbon source. The preparation method of the material comprises the following steps: uniformly dispersing the nano silicon material in a phosphoric acid solution, then carrying out spray drying, carrying out spray granulation, and carrying out primary sintering to obtain the phosphorus-doped nano silicon material; and carrying out spray drying and secondary sintering on the obtained phosphorus-doped nano silicon material, graphite and an organic carbon source to obtain the phosphorus-doped silicon-carbon negative electrode material. The size of the silicon particles (generally larger than 100nm) selected by the method is far larger than that of nano silicon particles (generally smaller than 30nm) generated by disproportionation reaction in a silicon-oxygen compound, so that the volume expansion/contraction effect of the silicon particles is obvious in the charging and discharging process of the battery; meanwhile, the silicon-carbon composite material is obtained by spray drying and later-stage sintering, the method is simple and convenient, but the strength of the obtained carbon coating layer and the firmness of the combination between the silicon particles and graphite are not high enough to fully cope with the huge volume expansion/contraction effect of the silicon particles in the charging and discharging processes. In conclusion, the long cycle stability of the silicon-based composite material is difficult to guarantee. In addition, phosphoric acid is adopted as a doping substance, strict control is needed in the later high-temperature heat treatment process, highly toxic and flammable byproducts such as pyrophosphoric acid, metaphosphoric acid or white phosphorus are easily generated, and the method is not suitable for large-scale popularization.

Chinese patent application publication No. CN107240693A discloses a phosphorus-doped silicon-graphite composite material, which includes phosphorus-doped N-type silicon and graphite. The preparation method of the material comprises the following steps: carrying out ball milling on the crystalline silicon powder and the red phosphorus powder in a protective atmosphere to obtain phosphorus-doped silicon; and ball-milling the phosphorus-doped silicon and graphite in a protective atmosphere to obtain the phosphorus-doped silicon-graphite composite material. Most of the silicon particles in the material have the size of 0.1-0.5 mu m and are far larger than nano silicon particles generated by disproportionation reaction in a silicon-oxygen compound, so the volume expansion/contraction effect of the silicon particles is obvious in the charge and discharge process of a battery; in addition, the silicon-carbon composite material obtained by the ball milling method has difficulty in realizing perfect coating of the silicon particles by the carbon material. Therefore, in the water homogenization process, the exposed silicon nanoparticles may contact with water to generate a gas generation reaction, thereby causing problems such as loss of active silicon material and poor coating quality. In addition, during the later cycle charge and discharge, silicon particles directly contacting the electrolyte undergo large directional expansion and cracking and continuously form an SEI film, resulting in poor cycle performance.

Chinese patent publication No. CN103400971A discloses a silicon-based composite material, which comprises: the silicon particles, the silicate and optionally carbon, the mixture of silicate and optionally carbon forming a mass, the silicon particles being dispersed in the mass. The preparation method comprises the following steps: dispersing silicon particles, silicate and optionally carbon in absolute ethanol and/or deionized water to form a suspension, heating and stirring until the paste is evaporated; and then drying, grinding and sieving, carrying out heat treatment in an inert atmosphere, and grinding and sieving to obtain the silicon-based composite material. The size of the silicon particles selected by the method is far larger than that of nano silicon particles generated by disproportionation reaction in a silicon-oxygen compound, so that the volume expansion/contraction effect of the silicon particles is obvious in the charge and discharge process of the battery; in addition, the dispersibility of the silicon particles in the method is also inferior to that of nano silicon generated by disproportionation reaction in a silicon oxide compound, and the uneven dispersion of the silicon particles can also cause uneven distribution of internal stress of the material in the charging and discharging process of the battery, thereby causing the breakage of the material particles; moreover, the interface stability of the silicon particles and lithium silicate in the material is not as good as that of a silicon simple substance/lithium silicate composite system generated after disproportionation and pre-lithiation reaction in a silicon oxygen compound, and the material particles are more easily cracked in the battery charging and discharging process. In conclusion, the cycling stability of the silicon-based composite material is difficult to guarantee.

Therefore, the existing silicon-based negative electrode material has the problems of low capacity, low coulombic efficiency, poor rate performance, poor cycle stability, complex and dangerous preparation process, incompatibility with the currently commonly used aqueous homogenate system and the like, is difficult to realize the commercial application in the lithium ion battery, and is a technical problem in the field.

Disclosure of Invention

The invention aims to provide a doped multilayer core-shell silicon-based composite material for a lithium ion battery and an efficient construction method for large-scale preparation of the doped multilayer core-shell silicon-based composite material, aiming at the technical shortages of low first coulomb efficiency, poor rate capability, short cycle life and the like existing when the conventional silicon-based negative electrode material is applied to the lithium ion battery.

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

a doping type multilayer core-shell silicon-based composite material for a lithium ion battery is at least doped with a non-metal element and a metal element besides a Li element; the doped multilayer core-shell silicon-based composite material has the structure that silicon oxide compound particles with element doping are used as an inner core, and a multilayer composite film tightly coated on the surfaces of the inner core particles is used as a shell; the core particles contain uniformly dispersed simple substance silicon nano particles, wherein the content of doping elements is gradually reduced from outside to inside without obvious interfaces, and a layer of compact lithium silicate compound is formed on the surface of the core particles due to doping and embedding of lithium elements; the multilayer composite film is a carbon film layer and a doped composite film layer formed by compounding the carbon film layer and other element components; the doping element is selected from one or more of N, S, P, B, Mg, Al, Cu, Mn, Ca and Zn, wherein the metal doping element exists in the shell film layer;

the median particle size of the doped multilayer core-shell silicon-based composite material is between 0.3 and 30 mu m, wherein the median particle size of core particles is between 0.3 and 25 mu m, and the median particle size of simple substance silicon nanoparticles distributed in the core silicon oxide compound particles is between 0.1 and 50 nm; the thickness of the multilayer composite film layer coated outside the core particles is between 0.005 and 10 mu m;

in the core particles, the content of silicon element is 49.9-79.9 wt%, the content of oxygen element is 20-50 wt%, the content of doping element is 0.01-10%, and the total content of silicon, oxygen, doping element and the like is 100%; the weight ratio of the multilayer composite film to the silicon oxide compound core particles is 0.01:100-25:100, and the doping amount of elements in the doped composite film layer is 0.01-5%;

the invention also discloses a preparation method of the doped multilayer core-shell silicon-based composite material for the lithium ion battery, which comprises the following steps:

(1) directly carrying out element doping on the silicon oxide particles to obtain doped silicon oxide particles;

(2) taking the doped silicon-oxygen compound obtained in the step (1) as an inner core, coating a carbon film layer on the surface of the inner core, and then crushing and screening;

(3) uniformly mixing the material obtained in the step (2) with a precursor substance containing doped metal elements, then carrying out heat treatment doping in a non-oxidizing atmosphere, and carrying out crushing and screening treatment;

(4) taking the material obtained in the step (3) as a kernel, uniformly coating a layer of element-doped carbon film on the surface of the kernel, and then crushing and screening;

(5) uniformly mixing the material obtained in the step (4) with lithium-containing compound powder, heating in a non-oxidizing atmosphere to further diffuse lithium elements and in-phase doping elements into silicon oxide compound particles, and then crushing and screening to finally obtain the doped multilayer silicon-based core-shell composite material;

in the above steps, the modulation sequence or synchronization of the steps (3), (4) and (5) can be performed.

In the step (1):

the stoichiometric ratio of silicon and oxygen elements in the silicon-oxygen compound particles is 1:0.5-1:1.5, and the median particle size range is 0.1-20 mu m;

the direct element doping of the silicon oxide particles is realized by one or a combination of a plurality of methods such as high-temperature vapor deposition, high-temperature solid phase sintering, spray drying or ball milling doping;

the doping method is realized by adopting one or a combination of a plurality of instruments of a CVD furnace, a tube furnace, an atmosphere box furnace, a ball mill or a spray dryer and the like;

the doping element is one or more of N, P, B, S, F and the like;

the doping material is gas or solid containing one or more doping elements, such as ammonia gas, hydrogen phosphide, red phosphorus, ammonium hypophosphite, ammonium dihydrogen phosphate, boron trioxide, boric acid, hydrogen sulfide, thiourea, thioacetamide and ammonium fluoride;

the atmosphere adopted in the doping is one or a combination of a plurality of nitrogen, argon, hydrogen, ammonia, phosphine and the like.

In the step (2):

the carbon film layer is directly obtained by a chemical vapor deposition mode, or is obtained by a mode of coating a carbon precursor in advance and then carrying out high-temperature heat treatment carbonization in a non-oxidizing atmosphere;

the coating method of the carbon precursor is a solid phase method or a liquid phase method, the adopted coating instrument is any one or combination of a plurality of mechanical fusion machines, mechanical stirrers, VC mixers, coating kettles, hydrothermal reaction kettles, spray drying, sand mills or high-speed dispersion machines, and the solvent selected during coating is one or combination of a plurality of water, methanol, ethanol, ethylene glycol, isopropanol, N-butanol, acetone, N-methylpyrrolidone, methyl butanone, tetrahydrofuran, benzene, toluene, xylene, N-dimethylformamide, N-dimethylacetamide and trichloromethane; the additive selected during coating is one or a combination of more of dilute hydrochloric acid, p-toluenesulfonic acid, ammonium persulfate, hydrogen peroxide, polyvinylpyrrolidone, sodium dodecyl sulfate, sodium dodecyl benzene sulfonate and the like;

the carbon precursor is one of petroleum asphalt, coal asphalt, polyvinyl alcohol, epoxy resin, polyacrylonitrile, polymethyl methacrylate, aniline, pyrrole, thiophene, glucose, sucrose, polyacrylic acid and the like;

the equipment for heat treatment and carbonization is any one of a rotary furnace, a roller kiln, an electric heating oven, a pushed slab kiln, a tubular furnace or an atmosphere box furnace and the like;

the temperature of the high-temperature heat treatment carbonization is 550-;

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

the crushing treatment adopts any one of a turbine type crusher, a ball mill and an airflow crusher;

the screening treatment adopts any one of a vibrating screen machine and an airflow classifier.

In the step (3):

the doped metal element is one or a combination of more of Ca, Mg, Al, Zn, Cu, Mn, Zr and Fe, and the precursor substance is metal salt, metal oxide, metal hydroxide or metal hydride of the doped metal element;

the mixing method comprises solid phase mixing or liquid phase mixing, and is realized by one or more of mechanical stirring, sand mill, ball mill, spray drying, VC mixer, etc.;

the solvent adopted in the liquid phase mixing process comprises one or more of water, methanol, ethanol, isopropanol, acetone, N-methyl pyrrolidone, ethyl acetate and the like;

the additives selected in the mixing process comprise one or a combination of more of sucrose, glucose, polyacetimide, polyacrylonitrile, polyacrylic acid, polyvinylpyrrolidone, sodium dodecyl sulfate, sodium dodecyl benzene sulfonate and the like;

the equipment used for the heat treatment doping is any one of a roller kiln, a rotary furnace, a pushed slab kiln, an atmosphere box furnace or a tubular furnace and the like;

the doping temperature of the heat treatment is 700-;

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

In the step (4):

the element-doped carbon film is obtained by the following steps: coating an organic carbon precursor with a self framework containing heteroatoms, and then carrying out heat treatment in a non-oxidizing atmosphere, wherein the precursor is carbonized and the heteroatoms are doped in situ; or the doping substance and the carbon precursor are uniformly mixed and then coated together, and then the element doping of the carbon film layer is realized while the heat treatment carbonization is carried out in the non-oxidizing atmosphere; or after the carbon film layer precursor is coated and subjected to heat treatment carbonization, element doping is carried out;

the carbon precursor is one of petroleum asphalt, coal asphalt, polyvinyl alcohol, epoxy resin, polyacrylonitrile, polyacetyl imine, polymethyl methacrylate, aniline, pyrrole, thiophene, glucose, sucrose, polyacrylic acid and the like;

the doping substance is one of urea, melamine, dicyandiamide, red phosphorus, ammonium hypophosphite, boric acid, ammonia gas, phosphine, hydrogen sulfide and the like;

the temperature of the heat treatment carbonization is 600-;

In the step (5):

the lithium-containing compound powder is a lithium-containing reducing compound;

the maximum particle diameter of the lithium-containing compound powder is less than or equal to 60 [ mu ] m;

the lithium-containing compound powder is pulverized by any one of mortar grinding, a ball mill, a turbine pulverizer and a jet mill;

the mixing method adopts any one of a high-speed dispersion machine, a high-speed stirring mill, a ball mill, a conical mixer, a spiral mixer, a stirring mixer or a VC mixer;

the equipment used for the heat treatment is any one of a rotary furnace, a roller kiln, a pushed slab kiln, a tubular furnace or an atmosphere box furnace and the like;

the temperature of the heating treatment is 500-;

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

The invention also protects the lithium ion battery cathode containing the doped multilayer core-shell silicon-based composite material and a lithium ion battery prepared by adopting the lithium ion battery cathode.

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

1. unlike the conventional bulk material crushing method to obtain silicon particles, the silicon nanoparticles contained in the core silicon oxide particles of the present invention are formed "from bottom to top" by disproportionation reactions, and have significantly smaller sizes, and thus can significantly mitigate the volume effect generated during repeated lithium deintercalation. In addition, the nano silicon in the core particles is uniformly dispersed and fixed in the silicon oxide matrix, and the matrix can effectively inhibit and buffer the expansion of the silicon nano particles and simultaneously can effectively prevent the expansion acceleration and local failure of active silicon caused by gradual fusion of the silicon particles into larger particles in the charging and discharging processes.

2. Conventional silicon-oxygen compound materials are generally poor in conductivity and are not enough to support a corresponding electrode system to realize rapid electron transmission and large-current charge and discharge, i.e., the rate performance is poor. According to the invention, the intrinsic electronic conductivity of the silicon-oxygen compound particles can be effectively improved by directly doping elements into the silicon-oxygen compound particles, so that the rate capability of the obtained material is improved, and the structural stability of the carbon film coating layer can be effectively improved by the strong covalent bond formed between the doped heteroatom and the subsequent coated carbon film layer, so that the effects of enhancing the conductivity of the material and inhibiting the volume expansion of the silicon nanoparticles are achieved.

3. The common carbon-coated silicon-based negative electrode material at present is generally coated by a single-layer carbon film, and the problems of incomplete coating of the carbon film layer or weak bonding and a series of subsequent problems caused by the exposure of core particles are easy to occur. Therefore, the metal element doping and secondary doping type carbon film layer cladding are specifically and sequentially introduced in the invention. The doped metal elements are mostly uniformly dispersed on the surface of the first coated carbon film layer in the form of ultrafine oxide nanoparticles, can be used as anchor points to effectively inhibit the cracking of the carbon film coating layer caused by the expansion of inner silicon particles, and can also be used as active sites to promote the stable coating of the subsequent doped carbon layer. Subsequently, the doped carbon film layer is tightly coated, so that the volume effect of nano silicon in the core particles in the charging and discharging process can be further relieved and inhibited, the conductivity of the obtained silicon-based composite material can be remarkably improved, and the high-speed electron conductivity and rate capability of the obtained material when the material is applied to a lithium ion battery cathode are further improved.

4. In order to further relieve the volume expansion effect of the traditional silicon-oxygen compound material and effectively improve the first coulombic efficiency of the obtained material, a large number of lithium atoms are doped and embedded on the basis of constructing a multilayer coated core-shell structure. The lithium element content presents a concentration gradient which gradually decreases from outside to inside, and the lithium element and the oxygen element and the silicon element in the lithium element form a lithium silicate compound after entering the core silicon oxide compound particles, so that the oxygen element in the lithium element does not continuously form compounds such as lithium silicate or lithium oxide in the lithium intercalation process of the negative electrode, the irreversible loss of lithium ions generated in the first charge and discharge of the obtained material is greatly reduced, and the first coulomb efficiency of the material in a lithium ion battery is effectively improved. In addition, lithium atoms which are pre-intercalated in silicon oxide compound particles in the material enable fewer lithium ions to be intercalated under the same lithium removal capacity, so that the material has a lower particle expansion rate and a lower battery pole piece expansion rate and battery expansion rate, and is beneficial to the structural stability of the negative electrode material particles, the negative electrode pole piece and the battery, namely the cycling stability of the battery.

5. The conventional silicon nano material usually contacts with water in the water system homogenizing process to generate gas production reaction and cause loss of active silicon material, and the multi-layer core-shell structure constructed in the invention can fully protect nano silicon in core particles from contacting with external water system slurry, thereby effectively solving the problem of homogenizing gas production; the compact silicate compound formed on the surfaces of the outer-layer complete multilayer coating film and the inner core particles has good water resistance, can effectively inhibit the PH rise of the aqueous slurry, and simultaneously cannot influence the rheological property and the stability of the slurry, so that the quality problems of pole pieces such as pole piece pinholes, pits, uneven surface density, poor adhesion and the like caused by gas generation, slurry rheological property and stability deterioration in the coating process are effectively avoided.

6. When the silicon nano-particles are applied to a lithium ion battery, under the synergistic effect of the multilayer coating and the compact silicate system compound formed on the surface of the core particle, the silicon nano-particles in the obtained material can be completely isolated from the external electrolyte, and meanwhile, a more stable SEI film can be formed on the surface of the material, so that the coulombic efficiency and the capacity stability of the obtained electrode material in the charge-discharge cycle process of the battery are remarkably improved.

In summary, the doped multilayer core-shell silicon-based composite material constructed in the invention has electrochemical characteristics of high capacity, good rate capability, high coulombic efficiency, good cycle performance, low expansion rate and the like when being used as a lithium ion battery cathode. The lithium ion battery prepared from the doped multilayer core-shell silicon-based composite material has the characteristics of high volume energy density, good rate capability, good cycle stability, low expansion and the like. The doped multilayer core-shell silicon-based composite material is high in preparation repeatability, is suitable for large-scale industrial production, can be directly applied to a water-based cathode homogenization process system commonly adopted in the industry, and can truly realize large-scale application of a silicon-containing cathode in the field of lithium ion batteries.

Drawings

Fig. 1 is a schematic structural diagram of a doped multilayer core-shell silicon-based composite material of the invention.

Fig. 2 is a graph of cycle performance of a silicon-containing negative electrode full cell prepared in example 1.

Detailed Description

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

The invention provides a doped multilayer core-shell silicon-based composite material which comprises silicon oxide particles doped with elements as a core and a multilayer composite film layer tightly coated on the surface of the core particles as a shell. As shown in fig. 1, the core 1 is an element-doped silicon oxide particle, in which a gradual lightening of the color from the outside to the inside indicates a gradual decrease in the content of the doping element including lithium without a distinct interface. The elemental silicon nanoparticles 2 are uniformly distributed in the bulk phase of the core particle, and a layer of compact lithium silicate compound 3 is formed on the surface of the core particle due to doping and intercalation of lithium element. The carbon film layer 4, the doped metal element 5 and the doped carbon film layer 6 are sequentially coated on the outer layer of the core particles.

Example 1

1000g of silicon oxide particles having a median particle diameter of 6 μm (silicon to oxygen atom ratio of 1:1) were weighed into a CVD furnace, and heated to 900 ℃ at a rate of 20 ℃/min under the protection of argon atmosphere. And after the temperature is increased to 900 ℃, introducing ammonia gas with the gas velocity of 300ccm into the furnace for 60min for carrying out nitrogen doping on the silicon oxide particles. Subsequently, the temperature of the furnace is kept at 900 ℃ for 30min under the argon atmosphere, and then acetylene gas with the gas speed of 300ccm is introduced into the furnace for 30 min. And then, keeping the temperature of 900 ℃ for 60min under the argon atmosphere, and then cooling to room temperature to obtain carbon-coated nitrogen-doped silicon oxide particles, wherein argon is introduced into the CVD furnace at the gas speed of 500ccm in the whole process. The obtained carbon-coated nitrogen-doped silica compound particles were passed through a 500-mesh screen, and then uniformly dispersed with aluminum nitrate nonahydrate and polyvinylpyrrolidone in a mass ratio of 5:1:0.1 in 2000ml of a mixed solvent of ethanol and water, wherein the volume ratio of ethanol to water was 9: 1. Stirring the uniformly dispersed suspension to be dry at a constant temperature of 50 ℃ by using a mechanical stirrer, then transferring the suspension into a tubular furnace, heating the suspension to 800 ℃ at a speed of 10 ℃/min in an argon atmosphere, and keeping the temperature for 3 hours to realize doping and coating of the aluminum element. Then, the mixture is naturally cooled to room temperature and then is screened by a 500-mesh screen for subsequent operation. Uniformly mixing the material obtained in the last step, 50% of polyimide solution and deionized water in a mass ratio of 50:1:100, and stirring at a constant temperature of 60 ℃ until the mixture is dry. And then transferring the mixture into a tubular furnace, heating the mixture to 800 ℃ at the speed of 5 ℃/min under the argon atmosphere, and keeping the mixture for 2 hours to realize the carbonization of the polyimide coating layer and the in-situ doping of nitrogen elements. After the tube furnace had cooled to room temperature, the material was removed and passed through a 500 mesh screen for subsequent operations.

Crushing the lithium hydride coarse powder in a drying room with the humidity of lower than 20 percent by adopting a planetary ball mill, and then sieving the crushed lithium hydride coarse powder by a 600-mesh sieve to obtain lithium hydride fine powder. And uniformly mixing the material obtained in the last step and lithium hydride powder in a mass ratio of 15:1 in a VC mixer for 20min, transferring into a tubular furnace, and then raising the temperature to 800 ℃ at a heating rate of 10 ℃/min under an argon atmosphere and keeping the temperature for 2 hours. Naturally cooling, taking out the material, and sieving with a 500-mesh sieve to obtain the final product. The obtained doped multilayer core-shell silicon-based composite material contains about 55 wt% of silicon element, about 35 wt% of oxygen element, about 5 wt% of lithium element, about 3 wt% of carbon element, about 1 wt% of aluminum element and about 1 wt% of nitrogen element; through X-ray diffraction analysis, the size of the uniformly dispersed silicon nano crystal grains in the obtained doped multilayer core-shell silicon-based composite material is about 8 nm.

The doped multilayer core-shell silicon-based composite material, natural graphite, a thickening agent and a binder are homogenized and coated on copper foil under a water system condition according to the mass ratio of 10:87:1.5:1.5, and then the copper foil is dried and rolled to obtain the silicon-containing negative pole piece.

Half-cell evaluation: and (3) sequentially stacking the silicon-containing negative pole piece, the diaphragm, the lithium piece and the stainless steel gasket, dropwise adding 200 mu L of electrolyte, and sealing to prepare the CR2016 type half-cell. The capacity and discharge efficiency of the half cell were tested using a test apparatus model CT2001A, wuhan blue electronics gmbh. The first reversible lithium removal specific capacity of the half-cell of the silicon-containing cathode is measured to be 461mAh/g, and the first charge-discharge efficiency (lithium removal cut-off potential is 0.8V) is 91.8 percent.

Full cell evaluation: the 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, degassed and sealed, and formed to obtain the silicon-containing negative pole lithium ion full battery with about 3.2 Ah. The capacity and the average voltage of the full battery under 0.2C and 1C are tested by using a battery tester of New Wille electronics Limited in Shenzhen, and the capacity retention rate data is obtained after 500 times of charge and discharge cycles under the multiplying power of 0.7C. The volume energy densities of the full cell at 0.2C and 1C were 764Wh/L and 739Wh/L, respectively, and the capacity retention ratio after 500 charge-discharge cycles was 86.4%. Fig. 2 is a graph of cycle performance of a silicon-containing negative electrode full cell prepared in example 1.

In the following examples, the negative electrode sheets were fabricated into half cells and full cells in the same manner as in example 1, and the specific capacities and the charge and discharge efficiencies of the half cells and the full cells were tested on the same equipment, unless otherwise specified.

Example 2

In comparison with example 1, in example 2, phosphine gas was used to replace ammonia gas when silicon oxide particles were doped by a high temperature CVD method, and the remaining parameter conditions during the entire CVD process were the same as in example 1, and carbon-coated phosphorus-doped silicon oxide particles were obtained after the reaction was completed. 1000g of carbon-coated phosphorus-doped silica compound particles were uniformly dispersed with aluminum nitrate nonahydrate, sucrose and polyvinylpyrrolidone in a mass ratio of 10:2:1:0.1 in 4000ml of deionized water by high-speed stirring followed by spray drying. The inlet air temperature is 150 ℃, the outlet temperature is 105 ℃, the rotating speed of the rotary atomizing nozzle is 350Hz, and the feeding speed is 100 g/min. The spray dried product was transferred to a box furnace and heated to 800 ℃ at a rate of 10 ℃/min under a high purity nitrogen atmosphere, followed by 4 hours at 800 ℃. During the high-temperature treatment, the doping of aluminum element and the carbonization of cane sugar to form film are simultaneously realized to coat on the surface of the material. After the treatment, the resulting material was screened through a 500 mesh screen for further processing. Subsequently, the nitrogen-doped carbon film layer was coated again by coating carbonization with polyimide in the same manner as in example 1 and passed through a 500-mesh screen for subsequent operations.

Uniformly mixing the material obtained in the last step and lithium hydride fine powder in a drying room with the humidity of lower than 20% in a VC mixer for 20min at a mass ratio of 15:1, transferring the mixture into a tube furnace, and then raising the temperature to 800 ℃ at a heating rate of 10 ℃/min under an argon atmosphere and keeping the temperature for 2 hours. Naturally cooling, taking out the material, and sieving with a 500-mesh sieve to obtain the final product. The obtained doped multilayer core-shell silicon-based composite material contains about 54 wt% of silicon element, about 35 wt% of oxygen element, about 6 wt% of lithium element, about 2.5 wt% of carbon element, about 1 wt% of aluminum element, about 1 wt% of nitrogen element and about 0.5 wt% of phosphorus element. Through X-ray diffraction analysis, the size of the uniformly dispersed silicon nano crystal grains in the obtained doped multilayer core-shell silicon-based composite material is about 9.2 nm.

The first reversible lithium removal specific capacity of the half-cell with the silicon-containing negative electrode prepared and measured by the same process as in example 1 was 463mAh/g, and the first charge-discharge efficiency was 91.6%. The volumetric energy densities of the full cell at 0.2C and 1C were measured to be 767Wh/L and 743Wh/L, respectively, and the capacity retention rate after 500 charge-discharge cycles was 85.9%.

Example 3

1000g of silicon oxide particles having a median particle diameter of 6 μm (silicon to oxygen atomic ratio of 1:1) were weighed out and mixed homogeneously with 200g of ammonium hypophosphite in a dry room having a humidity of less than 20%. And ball-milling the obtained mixture in a ball mill by using zirconium beads with the diameter of 0.8mm under a protective atmosphere for 2 hours, transferring the ball-milled mixture into a tube furnace, heating the ball-milled mixture to 900 ℃ at the speed of 10 ℃/min under the atmosphere of argon, keeping the temperature for 4 hours, and naturally cooling the ball-milled mixture to room temperature to obtain the silicon oxide compound particles doped with the phosphorus element. Uniformly mixing the obtained particles and petroleum asphalt in a heating VC mixer according to the mass ratio of 10:1 to realize asphalt coating, then heating to 900 ℃ at the speed of 10 ℃/min in a box furnace surrounded by high-purity nitrogen atmosphere and keeping for 2 hours, then naturally cooling to room temperature and sieving by a 500-mesh sieve to obtain the carbon-coated phosphorus-doped silicon oxide compound particles. Subsequently, aluminum element doping was performed by using spray drying and high-temperature treatment in the same manner as in example 2. Then dispersing the obtained material and ammonium persulfate in 0.1mol/L dilute hydrochloric acid by mechanical stirring at a mass ratio of 30:1, continuously stirring for 30min under an ice bath system, then dropwise adding aniline monomer solution with the mass being twice of that of the ammonium persulfate into the mixed system during stirring, and then continuing stirring for 8 hours under the ice bath condition. After the reaction is finished, the obtained product is alternately and repeatedly filtered and washed by deionized water and ethanol until the product is neutral, and then the product is dried in vacuum. And (3) heating the product obtained by suction filtration to 850 ℃ at the speed of 5 ℃/min in a box furnace enclosed by a high-purity nitrogen atmosphere, keeping the temperature for 3 hours, naturally cooling to room temperature, and sieving by a 500-mesh sieve for subsequent operation.

Subsequently, the process of doping the material obtained in the previous step with lithium was the same as in example 1. The finally obtained doped multilayer core-shell silicon-based composite material contains about 54 wt% of silicon element, about 34 wt% of oxygen element, about 6 wt% of lithium element, about 3 wt% of carbon element, about 1 wt% of aluminum element, about 1 wt% of nitrogen element and about 0.5 wt% of phosphorus element; through X-ray diffraction analysis, the size of the uniformly dispersed silicon nano crystal grains in the obtained doped multilayer core-shell silicon-based composite material is about 9.8 nm. The first reversible lithium removal specific capacity of the half-cell with the silicon-containing negative electrode prepared by the same process as the example 1 is 459mAh/g, and the first charge-discharge efficiency is 91.7%. The volumetric energy densities of the full cell at 0.2C and 1C were measured to be 762Wh/L and 738Wh/L, respectively, and the capacity retention rate after 500 charge-discharge cycles was 85.7%.

Example 4

Carbon-coated nitrogen-doped silicon oxide compound particles were first prepared using the same process as the CVD doping method in example 1. And (3) screening the obtained particles by a 500-mesh screen, uniformly dispersing the particles, copper acetate monohydrate, sucrose and polyvinylpyrrolidone in 4000ml of deionized water by high-speed stirring according to the mass ratio of 10:2:1:0.1, and then carrying out spray drying treatment. The inlet air temperature is 150 ℃, the outlet temperature is 105 ℃, the rotating speed of the rotary atomizing nozzle is 350Hz, and the feeding speed is 100 g/min. The spray dried product was heated to 800 ℃ at a rate of 10 ℃/min under a high purity nitrogen atmosphere and then held at 800 ℃ for 4 hours. During the high-temperature treatment, the doping of copper element and the carbonization and film-forming coating of cane sugar on the surface of the material are simultaneously realized. The obtained material is screened by a 500-mesh screen, and is uniformly mixed and coated with petroleum asphalt in a mass ratio of 10:1 by a heating type VC mixer. And then heating to 900 ℃ in a tubular furnace in an argon atmosphere at the speed of 10 ℃/min, introducing ammonia gas into the tubular furnace at the gas speed of 100ccm for 30min after keeping for 1 hour for doping nitrogen elements on the carbon coating layer, then keeping for 1 hour at 900 ℃, naturally cooling to room temperature, and screening by a 500-mesh screen for subsequent operation.

Compared with example 1, in the subsequent lithium doping process, the parameters are the same except that the mass ratio of the material to the lithium hydride fine powder is changed from 15:1 to 12: 1. The finally obtained doped multilayer core-shell silicon-based composite material contains about 53 wt% of silicon element, about 33 wt% of oxygen element, about 8 wt% of lithium element, about 3 wt% of carbon element, about 0.5 wt% of copper element and about 2 wt% of nitrogen element; through X-ray diffraction analysis, the size of the uniformly dispersed silicon nano crystal grains in the obtained doped multilayer core-shell silicon-based composite material is about 8.8 nm. The first reversible lithium removal specific capacity of the half-cell with the silicon-containing cathode is 466mAh/g and the first charge-discharge efficiency is 92.3% which are obtained by adopting the process the same as the process in the embodiment 1. The volume energy density of the full cell at 0.2C and 1C was determined to be 768Wh/L and 744Wh/L, respectively, and the capacity retention rate after 500 charge-discharge cycles was 86.2%.

Example 5

1000g of silicon oxide particles having a median particle diameter of 6 μm (silicon to oxygen atom ratio of 1:1) and ammonium dihydrogen phosphate were weighed out and uniformly dispersed in 3000ml of deionized water by mechanical stirring at a mass ratio of 10:3, followed by spray-drying. The inlet air temperature is 150 ℃, the outlet temperature is 105 ℃, the rotating speed of the rotary atomizing nozzle is 350Hz, and the feeding speed is 100 g/min. The spray dried product was heated to 950 ℃ at a rate of 10 ℃/min under a high purity nitrogen atmosphere and then held at 950 ℃ for 3 hours. During this high-temperature treatment, disproportionation of silicon oxide and doping of phosphorus element are simultaneously achieved, resulting in phosphorus-doped silicon oxide compound particles. Uniformly mixing the obtained phosphorus-doped silicon oxide particles and petroleum asphalt in a heating VC mixer according to the mass ratio of 10:1, coating the asphalt, heating to 900 ℃ at the speed of 10 ℃/min in a box furnace surrounded by high-purity nitrogen atmosphere, keeping for 2 hours, naturally cooling to room temperature, and sieving with a 500-mesh sieve to obtain the carbon-coated phosphorus-doped silicon oxide particles. The obtained carbon-coated nitrogen-doped silica compound particles were passed through a 500-mesh screen, and then uniformly dispersed with copper acetate monohydrate and polyvinylpyrrolidone in a mass ratio of 5:1:0.1 in 2000ml of a mixed solvent of ethanol and water, wherein the volume ratio of ethanol to water was 9: 1. Stirring the uniformly dispersed suspension to be dry at a constant temperature of 50 ℃ by using a mechanical stirrer, then transferring the suspension into a tubular furnace, heating the suspension to 800 ℃ at a speed of 10 ℃/min in an argon atmosphere, and keeping the temperature for 3 hours to realize doping and coating of the copper element. Then, the mixture is naturally cooled to room temperature and then is screened by a 500-mesh screen for subsequent operation. The subsequent coating with a doped carbon film and the subsequent lithium doping process were the same as in example 4. The finally obtained doped multilayer core-shell silicon-based composite material contains about 54 wt% of silicon element, about 33 wt% of oxygen element, about 8 wt% of lithium element, about 3 wt% of carbon element, about 0.5 wt% of copper element, about 1 wt% of nitrogen element and about 0.5 wt% of phosphorus element; through X-ray diffraction analysis, the size of the uniformly dispersed silicon nano crystal grains in the obtained doped multilayer core-shell silicon-based composite material is about 9.1 nm. The first reversible lithium removal specific capacity of the half-cell with the silicon-containing negative electrode prepared and measured by the same process as in example 1 is 460mAh/g, and the first charge-discharge efficiency is 91.7%. The volumetric energy densities of the full cell at 0.2C and 1C were measured to be 763Wh/L and 739Wh/L, respectively, and the capacity retention rate after 500 charge-discharge cycles was 85.1%.

Example 6

Carbon-coated phosphorus-doped silica compound particles were first prepared using the same process as the CVD doping method of example 2. The obtained particles are screened by a 500-mesh screen, uniformly dispersed with manganese acetate tetrahydrate and polyvinylpyrrolidone in a mass ratio of 10:1:0.1 in 2000ml of mixed solvent of ethanol and water, wherein the volume ratio of the ethanol to the water is 9: 1. Stirring the uniformly dispersed suspension to be dry at a constant temperature of 50 ℃ by using a mechanical stirrer, then transferring the suspension into a tubular furnace, heating the suspension to 800 ℃ at a speed of 10 ℃/min in an argon atmosphere, and keeping the temperature for 3 hours to realize doping and coating of the manganese element. Then, the mixture is naturally cooled to room temperature and then is screened by a 500-mesh screen for subsequent operation.

The obtained material is uniformly mixed and coated with petroleum asphalt by a heating type VC mixer in a mass ratio of 10: 1. Then, the temperature is increased to 900 ℃ in a tubular furnace with argon atmosphere at the speed of 10 ℃/min, phosphine gas is simultaneously introduced into the tubular furnace at the air speed of 100ccm for 30min after the temperature is maintained for 1 hour for doping the phosphorus element on the carbon coating layer, then the temperature is maintained for 1 hour at 900 ℃, and the mixture is naturally cooled to the room temperature and screened by a 500-mesh screen for subsequent operation.

The subsequent coating with a doped carbon film and the subsequent lithium doping process were the same as in example 4. The finally obtained doped multilayer core-shell silicon-based composite material contains about 54 wt% of silicon element, about 33 wt% of oxygen element, about 8 wt% of lithium element, about 3 wt% of carbon element, about 1 wt% of manganese element and about 1 wt% of phosphorus element; through X-ray diffraction analysis, the size of the uniformly dispersed silicon nano crystal grains in the obtained doped multilayer core-shell silicon-based composite material is about 8.6 nm. The first reversible lithium removal specific capacity of the half-cell with the silicon-containing negative electrode prepared and measured by the same process as in example 1 was 457mAh/g, and the first charge-discharge efficiency was 91.9%. The volumetric energy densities of the full cell at 0.2C and 1C were respectively 760Wh/L and 737Wh/L, and the capacity retention rate after 500 charge-discharge cycles was 84.2%.

Example 7

First, carbon-coated nitrogen-doped silica compound particles were prepared by the same process as the CVD doping method in example 1, and were screened through a 500-mesh screen for further use. Crushing the coarse magnesium hydride powder in a drying room with the humidity of lower than 20 percent by adopting a planetary ball mill, and then sieving the crushed magnesium hydride powder by a 600-mesh sieve to obtain fine magnesium hydride powder. Uniformly mixing the material obtained in the last step and the fine powder of the magnesium hydride in a VC mixer for 20min according to the mass ratio of 70:1, then transferring the mixture into a tube furnace, then raising the temperature to 850 ℃ at the heating rate of 10 ℃/min under the atmosphere of argon, keeping the temperature for 2 hours for doping magnesium, then naturally cooling the mixture to room temperature, and sieving the mixture by a 500-mesh sieve for subsequent operation. The subsequent coating of the doped carbon film and the lithium doping process were the same as in example 1. The finally obtained doped multilayer core-shell silicon-based composite material contains about 55 wt% of silicon element, about 33 wt% of oxygen element, about 7 wt% of lithium element, about 3 wt% of carbon element, about 1 wt% of magnesium element and about 1 wt% of nitrogen element; through X-ray diffraction analysis, the size of the uniformly dispersed silicon nano crystal grains in the obtained doped multilayer core-shell silicon-based composite material is about 8.6 nm. The first reversible lithium removal specific capacity of the half-cell with the silicon-containing negative electrode prepared and measured by the same process as in example 1 was 453mAh/g, and the first charge-discharge efficiency was 91.6%. The volumetric energy densities of the full cell at 0.2C and 1C were measured to be 756Wh/L and 733Wh/L, respectively, and the capacity retention rate after 500 charge-discharge cycles was 83.9%.

Example 8

Phosphorus-doped silica compound particles were first prepared using the same process as in example 3, and then the resulting product was transferred to a CVD furnace and carbon-coated using a process similar to that of example 1 to obtain carbon-coated phosphorus-doped silica compound particles. Subsequently, the resultant material was uniformly dispersed with copper acetate monohydrate, sucrose, a 50% polyimide solution, and polyvinylpyrrolidone in a mass ratio of 100:2:1:2:0.1 in 4000ml of deionized water, followed by spray drying and subsequent heat treatment by the same process as in example 2. And simultaneously doping copper element and coating the nitrogen-doped carbon film layer during the heat treatment. Subsequently, the resulting material was screened through a 500 mesh screen and doped by the same lithium doping process as in example 4. The final doped multilayer core-shell silicon-based composite material contains about 54 wt% of silicon element, about 34 wt% of oxygen element, about 7 wt% of lithium element, about 3 wt% of carbon element, about 1 wt% of copper element, about 1 wt% of nitrogen element and about 0.5 wt% of phosphorus element. Through X-ray diffraction analysis, the size of the uniformly dispersed silicon nano crystal grains in the obtained doped multilayer core-shell silicon-based composite material is about 8.2 nm. The first reversible lithium removal specific capacity of the half-cell with the silicon-containing negative electrode prepared and measured by the same process as the example 1 is 456mAh/g, and the first charge-discharge efficiency is 91.5%. The volumetric energy densities of the full cell at 0.2C and 1C were respectively 760Wh/L and 738Wh/L, and the capacity retention rate after 500 charge-discharge cycles was found to be 84.5%.

Example 9

1000g of silicon oxide particles having a median particle diameter of 6 μm (silicon to oxygen atom ratio 1:1) were weighed out and mixed homogeneously with 200g of boron trioxide in a drying cabinet having a humidity of less than 20%, and subsequently ball-milled in a ball mill for 3 hours under a protective atmosphere with zirconium beads having a diameter of 0.8 mm. And transferring the obtained product into a tubular furnace, heating to 950 ℃ at the speed of 3 ℃/min under the argon atmosphere, keeping the temperature for 4 hours, and naturally cooling to room temperature to obtain the silicon oxide compound particles doped with the boron element. Subsequently, coating of a carbon film layer was performed by the same process as in example 3 using a heating type VC mixer and subsequent heat treatment, and aluminum element doping was performed by the same process as in example 1 using spray drying and high-temperature treatment.

The subsequent coating with a doped carbon film and the subsequent lithium doping process were the same as in example 4. The finally obtained doped multilayer core-shell silicon-based composite material contains about 55 wt% of silicon element, about 33 wt% of oxygen element, about 8 wt% of lithium element, about 3 wt% of carbon element, about 1 wt% of aluminum element and about 0.5 wt% of boron element; through X-ray diffraction analysis, the size of the uniformly dispersed silicon nano crystal grains in the obtained doped multilayer core-shell silicon-based composite material is about 8.8 nm. The first reversible lithium removal specific capacity of the half-cell with the silicon-containing negative electrode prepared and measured by the same process as in example 1 was 453mAh/g, and the first charge-discharge efficiency was 92.1%. The volumetric energy densities of the full cell at 0.2C and 1C were measured to be 759Wh/L and 735Wh/L, respectively, and the capacity retention rate after 500 charge-discharge cycles was 84.4%.

Example 10

Example 10 adjusts the order of doping cladding compared to the previous examples. Carbon-coated nitrogen-doped silicon oxide compound particles were first prepared using the same process as the CVD doping method in example 1. Subsequently, the same process as that in example 4 is adopted, and the material obtained in the previous step is coated with the nitrogen-doped carbon film layer in a manner of combining heating type VC mixing with thermal treatment doping, so that the core-shell structure coated with the double-layer carbon film layer is obtained.

Then, the obtained material is uniformly mixed with lithium aluminum hydride fine powder and lithium hydride fine powder in a VC mixer for 20min in a drying room with the humidity of lower than 20% in a mass ratio of 100:3:8, and then transferred into a tube furnace, and then the temperature is increased to 800 ℃ at a heating rate of 10 ℃/min under an argon atmosphere and is kept for 2 hours, wherein the doping infiltration of aluminum elements and lithium elements is simultaneously realized. Naturally cooling, taking out the material, and sieving with a 500-mesh sieve to obtain the final product. The obtained doped multilayer core-shell silicon-based composite material contains about 54 wt% of silicon element, about 33 wt% of oxygen element, about 7 wt% of lithium element, about 3 wt% of carbon element, about 1.5 wt% of aluminum element and about 1.5 wt% of nitrogen element. Through X-ray diffraction analysis, the size of the uniformly dispersed silicon nano crystal grains in the obtained doped multilayer core-shell silicon-based composite material is about 9.6 nm. The first reversible lithium removal specific capacity of the half-cell with the silicon-containing negative electrode prepared by the same process as in example 1 was 449mAh/g, and the first charge-discharge efficiency was 91.8%. The volumetric energy densities of the full cell at 0.2C and 1C were found to be 753Wh/L and 731Wh/L, respectively, and the capacity retention rate after 500 charge-discharge cycles was found to be 83.7%.

Comparative example 1

The process was similar to example 4 except that the silicon oxide particles were not doped with nitrogen during the CVD process. The size of the silicon nano crystal particles uniformly dispersed in the silicon-based composite material particles is about 8.5 nm. The evaluation methods of the half cell and the full cell are the same as example 4, and the first reversible lithium removal specific capacity of the half cell containing the silicon negative electrode is measured to be 461mAh/g, and the first charge-discharge efficiency is 91.9%. The volumetric energy densities of the full cell at 0.2C and 1C were measured to be 763Wh/L and 721Wh/L, respectively, and the capacity retention rate after 500 charge-discharge cycles was 84.7%. It can be seen that whether the silicon oxide core particles are doped with nitrogen or not has no obvious influence on the capacity and energy density of the obtained material at a low magnification of 0.2C, but the energy density difference between the two is obvious at a high magnification of 1C. This is because the nitrogen doping of the core silicon oxide particles in example 4 can provide lone pair electrons in the silicon oxide system, thereby promoting the rapid transmission of electrons in the system, increasing the conductivity of the silicon oxide particles, and significantly improving the rate capability of the obtained material.

Comparative example 2

The process is similar to example 4 except that the carbon coating is not nitrogen doped during the carbonization process after the post-VC coating of the pitch. The size of the silicon nano crystal particles uniformly dispersed in the silicon-based composite material particles is about 8.6 nm. The evaluation methods of the half cell and the full cell are the same as example 4, and the first reversible lithium removal specific capacity of the half cell containing the silicon negative electrode is measured to be 458mAh/g, and the first charge-discharge efficiency is measured to be 91.7%. The volumetric energy densities of the full cell at 0.2C and 1C were respectively 760Wh/L and 732Wh/L, and the capacity retention rate after 500 charge-discharge cycles was 84.5%. It can also be seen that the capacity and energy density of the resulting material at 0.2C low power is not significantly different from that of example 4. However, compared with the difference of nitrogen doping of the silicon oxide core particles in the comparative example 1, whether the nitrogen element of the secondary coating carbon film layer is doped or not has relatively small influence on the difference of energy density of the obtained material under a large magnification of 1C. This is because although the two nitrogen dopings can provide lone-pair electrons to improve the conductivity and rate capability of the system, the conductivity of the core silicon-oxygen compound particle is significantly weaker than that of the outer carbon film layer, and the corresponding degree of the conductivity improvement is also significant, so that the heteroatom doping of the core silicon-oxygen compound particle is more significant for the rate capability improvement of the material.

Comparative example 3

The process is similar to that of example 4, except that lithium hydride coarse powder which is not subjected to ball milling and sieving treatment is directly used for doping lithium, and the lithium hydride coarse powder is mixed with the material obtained in the previous step after being ground in a mortar for 20min, and then the heat treatment process is the same as that of example 4. The size of the silicon nano crystal particles uniformly dispersed in the silicon-based composite material is about 5.8 nm. The evaluation methods of the half cell and the full cell are the same as example 4, and the first reversible lithium removal specific capacity of the half cell with the silicon-containing cathode is found to be 438mAh/g, and the first charge-discharge efficiency is 88.9%. The volumetric energy densities of the full cell at 0.2C and 1C were respectively determined to be 722Wh/L and 677Wh/L, and the capacity retention rate after 500 charge-discharge cycles was 79.8%. The lithium hydride powder in comparative example 3 was not subjected to particle size control, and there were a large number of lithium hydride particles having a particle size much larger than that of the silicon oxide particles. An excessively large particle size of the lithium hydride particles leads to an excessively high amount of lithium finally doped into the silicon oxide particles bound when they diffuse into the core, so that lithium orthosilicate and even lithium oxide are formed. Therefore, the material obtained in comparative example 3 has a higher slurry alkalinity during the aqueous homogenization process compared to example 4, resulting in a slightly unstable slurry state thereof, resulting in loss of active silicon material and deterioration of coating quality, and finally resulting in overall deterioration of electrochemical performance of the full cell.

Comparative example 4

The process is similar to example 2, except that no doping with metallic elements is carried out by spray drying and subsequent heat treatment. The size of the silicon nano crystal particles uniformly dispersed in the silicon-based composite material is about 7.3 nm. The evaluation methods of the half cell and the full cell are the same as example 2, and the first reversible lithium removal specific capacity of the half cell containing the silicon negative electrode is 454mAh/g, and the first charge-discharge efficiency is 91.1%. The volumetric energy densities of the full cell at 0.2C and 1C were measured to be 759Wh/L and 733Wh/L, respectively, and the capacity retention rate after 500 charge-discharge cycles was 81.7%. It can be seen that the small rate capacity and first charge-discharge efficiency gap of the product obtained in comparative example 4 are not significant, but are slightly decreased in both large rate capacity and cycle stability, compared to example 2. This is because in the doped multilayer core-shell silicon-based composite material obtained in example 2, the doped metal element generally exists stably in the form of ultrafine nanoparticles, which can be used as an anchor point to effectively inhibit the cracking of a carbon film coating layer caused by the expansion of inner silicon nanoparticles, and can also be used as an active growth site to promote the stable coating of a doped carbon layer in subsequent steps, thereby improving the structural stability of the obtained material and the high-speed electron conductivity when the material is applied to a negative electrode of a lithium ion battery, and thus the material has significantly improved rate capability and cycle stability.

Comparative example 5

The process was similar to example 4 except that no carbon film coating was applied during the CVD treatment and no doped carbon film coating was applied after the spray drying heat treatment. The size of the silicon nano crystal particles uniformly dispersed in the silicon-based composite material is about 6.6 nm. The evaluation methods of the half cell and the full cell are the same as example 4, and the first reversible lithium removal specific capacity of the half cell containing the silicon negative electrode is 455mAh/g, and the first charge-discharge efficiency is 91.4%. The volume energy density of the full cell at 0.2C and 1C was determined to be 761Wh/L and 705Wh/L, respectively, and the capacity retention rate after 500 charge-discharge cycles was 72.7%. It can be seen that the comparative example 5 has a relatively small difference between the capacity and the energy density at a small magnification compared to example 4, but the difference between the energy density at a large magnification and the capacity after 500 cycles of charge and discharge remains large. This can be attributed to the fact that without the carbon protective layer, the conductivity of the resulting material is significantly reduced, making its rate capability worse; meanwhile, without the protection of the carbon film layer, the volume expansion of the silicon nanoparticles in the core particles in the repeated charge and discharge process cannot be effectively inhibited, and therefore material pulverization and obvious weakening of battery cycle performance are caused.

Examples summary of electrochemical data:

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 doped multilayer core-shell silicon-based composite material for a lithium ion battery is characterized in that: the doped multilayer core-shell silicon-based composite material is at least doped with a non-metal element and a metal element besides the Li element; the doped multilayer core-shell silicon-based composite material has the structure that silicon oxide compound particles with element doping are used as an inner core, and a multilayer composite film tightly coated on the surfaces of the inner core particles is used as a shell; the core particles contain uniformly dispersed simple substance silicon nano particles, wherein the content of doping elements is gradually reduced from outside to inside without obvious interfaces, and a layer of compact lithium silicate compound is formed on the surface of the core particles due to doping and embedding of lithium elements; the multilayer composite film is a carbon film layer and a doped composite film layer formed by compounding the carbon film layer and other element components; the doping element is selected from one or more of N, S, P, B, Mg, Al, Cu, Mn, Ca and Zn, wherein the metal doping element exists in the shell film layer; the median particle size of the doped multilayer core-shell silicon-based composite material is between 0.3 and 30 mu m, wherein the median particle size of core particles is between 0.3 and 25 mu m, and the median particle size of simple substance silicon nanoparticles distributed in the core silicon oxide compound particles is between 0.1 and 50 nm; the thickness of the multilayer composite film layer coated outside the core particles is between 0.005 and 10 mu m;

in the core particles, the content of silicon element is 49.9-79.9 wt%, the content of oxygen element is 20-50 wt%, the content of doping element is 0.01-10%, and the total content of silicon, oxygen, doping element and the like is 100%; the weight ratio of the multilayer composite film to the silicon oxide compound core particles is 0.01:100-25:100, and the doping amount of elements in the doped composite film layer is 0.01-5%; the preparation method comprises the following steps:

(5) uniformly mixing the material obtained in the step (4) with lithium-containing compound powder, heating in a non-oxidizing atmosphere to further diffuse lithium elements and in-phase doping elements into silicon oxide compound particles, and then crushing and screening to finally obtain the doped multilayer core-shell silicon-based composite material;

2. The preparation method of the doped multilayer core-shell silicon-based composite material for the lithium ion battery according to claim 1, wherein the preparation method comprises the following steps: in the step (1):

the doping element is one or more of N, P, B, S, F and the like;

3. The preparation method of the doped multilayer core-shell silicon-based composite material for the lithium ion battery according to claim 1, wherein the preparation method comprises the following steps: in the step (2):

the temperature of the high-temperature heat treatment carbonization is 550-;

4. The preparation method of the doped multilayer core-shell silicon-based composite material for the lithium ion battery according to claim 1, wherein the preparation method comprises the following steps: in the step (3):

the doping temperature of the heat treatment is 700-;

5. The preparation method of the doped multilayer core-shell silicon-based composite material for the lithium ion battery according to claim 1, wherein the preparation method comprises the following steps: in the step (4):

the temperature of the heat treatment carbonization is 600-;

6. The preparation method of the doped multilayer core-shell silicon-based composite material for the lithium ion battery according to claim 1, wherein the preparation method comprises the following steps: in the step (5):

the temperature of the heating treatment is 500-;

7. A lithium ion battery negative electrode, characterized in that: comprising the doped multilayer core-shell silicon-based composite of claim 1.

8. A lithium ion battery, characterized by: prepared using the lithium ion battery negative electrode of claim 7.