CN108461723B - Silicon-based composite material for lithium ion battery and preparation method thereof - Google Patents

Silicon-based composite material for lithium ion battery and preparation method thereof Download PDF

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CN108461723B
CN108461723B CN201810140725.9A CN201810140725A CN108461723B CN 108461723 B CN108461723 B CN 108461723B CN 201810140725 A CN201810140725 A CN 201810140725A CN 108461723 B CN108461723 B CN 108461723B
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silicon
lithium
oxygen
particles
doping
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CN108461723A (en
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李喆
汪芳
王岑
赵志伟
叶兰
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Boselis Hefei Co ltd
Bosellis Nanjing Co ltd
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Amprius Nanjing Co ltd
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/36—Selection of substances as active materials, active masses, active liquids
    • H01M4/362—Composites
    • H01M4/366—Composites as layered products
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M10/00—Secondary cells; Manufacture thereof
    • H01M10/05—Accumulators with non-aqueous electrolyte
    • H01M10/052—Li-accumulators
    • H01M10/0525—Rocking-chair batteries, i.e. batteries with lithium insertion or intercalation in both electrodes; Lithium-ion batteries
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/36—Selection of substances as active materials, active masses, active liquids
    • H01M4/48—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/36—Selection of substances as active materials, active masses, active liquids
    • H01M4/48—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides
    • H01M4/485—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides of mixed oxides or hydroxides for inserting or intercalating light metals, e.g. LiTi2O4 or LiTi2OxFy
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/36—Selection of substances as active materials, active masses, active liquids
    • H01M4/58—Selection of substances as active materials, active masses, active liquids of inorganic compounds other than oxides or hydroxides, e.g. sulfides, selenides, tellurides, halogenides or LiCoFy; of polyanionic structures, e.g. phosphates, silicates or borates
    • H01M4/583—Carbonaceous material, e.g. graphite-intercalation compounds or CFx
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/62—Selection of inactive substances as ingredients for active masses, e.g. binders, fillers
    • H01M4/624—Electric conductive fillers
    • H01M4/625—Carbon or graphite
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/10—Energy storage using batteries

Abstract

The invention relates to a silicon-based composite material, which comprises silicon-oxygen-lithium compound particles, and a carbon film layer or a composite film layer of a carbon film and a conductive additive which are coated outside the silicon-oxygen-lithium compound particles; the silicon-oxygen-lithium compound particles have a core-shell structure, the shell is a compact lithium silicate compound, the inner core is a lithium silicate compound with the lithium content lower than that of the shell or a silicon-oxygen compound without lithium, the content of lithium elements is gradually reduced from the shell to the inner core, and no obvious interface exists; the silicon-oxygen-lithium compound particles also comprise uniformly dispersed elementary silicon nano particles. The silicon-based composite material can be applied to both oil systems and water system homogenate systems due to the carbon film layer or the carbon film/conductive additive composite film layer and the protection structure of the lithium silicate compound shell, so that the silicon-based composite material has excellent battery performance and good processing performance and is suitable for commercial use.

Description

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 silicon-based composite material and a preparation method thereof.
Background
Due to the rapid development and wide application of various portable electronic devices, electric vehicles, and energy storage systems in recent years, the 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 theoretical capacity is low (372mAh/g), so that the lithium ion battery is limitedFurther increase in cell energy density. Since silicon anode materials have the advantage of high capacity that other anode materials cannot compete, they have become a heat generation point in recent years and are gradually going from laboratory research and development to commercial application. The silicon cathode material mainly comprises two kinds, namely a composite material of simple substance silicon and the same carbon material thereof; the other is a composite material of a silicon oxide compound and a carbon material thereof. The simple substance silicon cathode material has high capacity advantage (the lithium-embedded state is Li at room temperature)15Si4The theoretical lithium storage capacity is about 3600mAh/g), which is about 10 times of the theoretical capacity of the current commercial graphite cathode material. However, the elemental silicon negative electrode material has a severe volume effect in the lithium intercalation and deintercalation process, and the volume change rate is about 300%, which may cause electrode material pulverization and separation of the electrode material from the current collector. In addition, the silicon negative electrode material is continuously expanded and contracted during the charging and discharging processes of the battery to continuously break, and a new SEI film can be formed when the produced fresh interface is exposed in the electrolyte, so that the electrolyte is continuously consumed, and the cycle performance of the electrode material is reduced. The above drawbacks severely limit their commercial applications. The theoretical capacity of the silicon-oxygen compound is about 1700mAh/g, although the capacity of the silicon-oxygen compound is lower than that of a simple substance silicon anode material, the expansion rate and the cycling stability of the silicon-oxygen compound have obvious advantages, and the silicon-oxygen compound is easier to realize industrial application compared with the simple substance silicon. However, silicon oxide compounds generate lithium silicate, lithium oxide and other substances when the lithium ion battery is charged for the first time, and lithium ions in the substances cannot be extracted when the lithium ion battery is discharged, so that the first coulombic efficiency of the battery is low (the theoretical efficiency is about 70%), thereby limiting the improvement of the energy density of the full battery. In addition, the ionic and electronic conductivities of the silicon-oxygen compound are low, and the lithium removal and lithium insertion reactions in the first round of charging and discharging of the lithium ion battery are not sufficient, so that the coulombic efficiency in the subsequent battery cycle process is low, and the cycle retention rate is not high. In response to the above problems of silicone compounds, researchers have made the following improvements.
Chinese patent documents CN103840136A and CN104471757A disclose a silicon-based negative electrode material for an electricity storage device, an electrode for an electricity storage device, and methods for producing the same. The method for producing the silicon-based negative electrode material is to mix and knead a silicon-based material capable of absorbing and releasing lithium ions and lithium metal in the presence of a solvent, wherein patent CN103840136A selects to form lithium silicate by heat treatment after the mixing and kneading, thereby producing a lithium pre-doped negative electrode material. The mixing of the materials needs to be carried out under an inert atmosphere and involves the use of organic solvents, disposal of emissions or recycling processes. The silicon-based negative electrode material is easy to react with water or partially dissolve in water whether subjected to heat treatment or not, so that the structure of the material is damaged and a homogenization system is unstable, and therefore the silicon-based negative electrode material cannot be applied to a water-based system commonly adopted for negative electrode homogenization in the lithium battery industry at present and can only be applied to a homogenization system of an organic solvent. Therefore, it is difficult to realize industrialization and commercialization both from the production process and the application process.
Chinese patent publication No. CN104979524A discloses a negative electrode for a nonaqueous electrolyte secondary battery, which contains a negative electrode active material layer composed of two or more negative electrode active materials and a binder. The negative electrode active material includes a silicon-based active material and a carbon-based active material. The silicon-based active material is formed by covering at least a part of the surface layer of SiO with lithium carbonatexPreferably, the composition contains at least one of lithium metasilicate and lithium orthosilicate inside. The silicon-based active material electrochemically incorporates Li into SiO by a special devicexAnd lithium carbonate is formed on the surface. The method has the advantages of complex operation and low production efficiency, and is difficult to realize large-scale industrial production. In the water homogenizing process in the actual battery production, lithium carbonate and lithium orthosilicate coated by the silicon active material are partially dissolved in the water slurry, so that the pH value of the slurry is increased, the rheological property is deteriorated, and the stability and the coating of the slurry are not facilitated. The lithium carbonate coated by the silicon-based active material has no electrochemical activity, and the increase of the ineffective mass can reduce the specific capacity of the material, thereby reducing the energy density of the lithium ion battery.
Chinese patent publication No. CN101047234B discloses a silicon-silicon oxide-lithium composite material comprising silicon particles having a size within 0.5-50nm dispersed therein. The preparation method of the material comprises mechanically mixing silicon oxide with metallic lithium and/or an organolithium compound as a lithiating agent by a planetary ball mill in an inert gas atmosphere while controlling the temperature in the bowl of the ball mill at 40-120 ℃ during the reaction. The presence of the lithium orthosilicate phase is evident in the XRD structure diagram of the material shown in this patent. In the water-based homogenization process, lithium orthosilicate is soluble in water, so that the problems of damage to the structure of the material, increase of the pH value of slurry, deterioration of the stability of the slurry and the like are caused, and therefore the material is not suitable for the water-based homogenization process widely used in the production field of lithium ion batteries. The preparation method of the material adopts the technical scheme that metal lithium and/or organic lithium compounds and silicon-oxygen compounds are directly mixed and then are subjected to ball milling in a closed space for pre-lithiation reaction. The reaction is carried out spontaneously, is usually violent, the reaction degree is not easy to control, and if the cooling condition of the ball milling tank is not well controlled, explosion danger is easy to occur, so that industrial safety production is difficult to realize.
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 (generally larger than 100nm) selected by the method is far larger than that of nano silicon particles (generally smaller than 50nm) generated by disproportionation reaction in a silicon-oxygen compound, so that the volume expansion/contraction effect of the silicon particles in the battery charging and discharging process is obvious; 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 body generated after disproportionation and pre-lithiation reaction in a silicon oxygen compound, and the material particles are more easily cracked in the charging and discharging process of the battery. 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 cycle stability, complex and dangerous preparation process, incompatibility with the currently and 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
Aiming at the defects of the prior art, the invention provides a silicon-based composite material which is used for a lithium ion battery, has high capacity, high coulombic efficiency and long cycle life and can adopt a water system homogenate system, and a method for preparing the silicon-based composite material in a large scale.
In order to achieve the purpose, the technical scheme provided by the invention is as follows:
a silicon-based composite material comprises silicon-oxygen-lithium compound particles and a film layer coated outside the silicon-oxygen-lithium compound particles, wherein the film layer is a carbon film layer or a carbon film/conductive additive composite film layer formed by the carbon film layer and a conductive additive; the silicon-oxygen-lithium compound particles have a core-shell structure, the shell of the core-shell structure is a compact lithium silicate compound, the core of the core-shell structure is a lithium silicate compound with the lithium content lower than that of the shell or a silicon-oxygen compound without lithium, the content of lithium elements is gradually reduced from the shell to the core of the silicon-oxygen-lithium compound particles, and no obvious interface exists; the silicon-oxygen-lithium compound particles also comprise uniformly dispersed elementary silicon nano particles.
The median particle diameter of the silicon-oxygen-lithium compound particles is between 0.2 and 20 mu m, and the median particle diameter of the simple substance silicon nano particles dispersed in the silicon-oxygen-lithium compound particles is between 0.1 and 35 nm; the thickness of the carbon film layer or the carbon film/conductive additive composite film layer outside the silicon-oxygen-lithium compound particles is between 0.001 and 5 mu m; in the silicon-oxygen-lithium compound particles, the content of silicon element is 49.9-79.9 wt%, the content of oxygen element is 20-50 wt%, the content of lithium element is 0.1-20 wt%, and the sum of the three element contents is 100%; in the carbon film layer, the weight ratio of the carbon film to the silicon-oxygen-lithium compound particles is 0.01:100-20: 100; in the carbon film/conductive additive composite film layer, the weight ratio of the carbon film to the silicon-oxygen-lithium compound particles is 0.01:100-20:100, and the weight ratio of the conductive additive to the silicon-oxygen-lithium compound particles is 0:100-10: 100.
The silicon-oxygen-lithium compound particles also contain a small amount of doping elements, the content of the doping elements is gradually reduced from the outer shell to the inner core of the silicon-oxygen-lithium compound particles, and no obvious interface exists; the doping element is one or a combination of more of P, F, Mg, Al, Ca, Cu, B, Fe, Mn, Zn, Zr, Ti and Sn; in the silicon-oxygen-lithium compound particles, the content of silicon element is 49.89-79.89 wt%, the content of oxygen element is 20-50 wt%, the content of lithium element is 0.1-20 wt%, the content of doping element is 0.01-10%, and the sum of the content of silicon, oxygen, lithium and doping element is 100%;
the invention also discloses a preparation method of the silicon-based composite material, which comprises the following steps:
(1) coating a carbon film layer or a carbon film/conductive additive composite film layer on the surface of silicon oxide compound particles, and then crushing and screening;
(2) uniformly mixing the material obtained in the step (1) with lithium-containing compound powder; or uniformly mixing the material obtained in the step (1), lithium-containing compound powder and a doping substance at the same time; or uniformly mixing the material obtained in the step (1) with a doping substance, performing heat treatment doping in a non-oxidizing atmosphere, and then uniformly mixing the material with lithium-containing compound powder, wherein the doping substance is a simple substance or compound powder containing doping elements;
(3) and (3) heating the mixed material obtained in the step (2) in a non-oxidizing atmosphere to diffuse lithium element or lithium and doping elements into silicon oxide compound particles, and then crushing and screening to obtain the silicon-based composite material.
In step (1):
the stoichiometric ratio of silicon to oxygen in the silicon-oxygen compound particles is 1:0.5-1: 1.5;
the carbon film layer is directly obtained by a chemical vapor deposition mode, or is obtained by firstly coating a carbon precursor and then carrying out heat treatment carbonization in a non-oxidizing atmosphere;
the carbon film/conductive additive composite film layer is obtained by the following steps: after silicon oxide particles coated with a carbon film by chemical vapor deposition are mixed with a conductive additive and a carbon precursor, heat treatment carbonization is performed in a non-oxidizing atmosphere; or after mixing the silicon oxide particles with the conductive additive and the carbon precursor, carrying out heat treatment carbonization in a non-oxidizing atmosphere to obtain the silicon oxide/carbon composite material;
the coating method of the carbon precursor or the carbon precursor and the conductive additive adopts any one of a mechanical fusion machine, a VC mixer, a coating kettle, spray drying, a sand mill or a high-speed dispersion machine, and the solvent selected during coating 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 trichloromethane;
the carbon precursor is one or a combination of more of coal pitch, petroleum pitch, polyvinyl alcohol, epoxy resin, polyacrylonitrile, polymethyl methacrylate, glucose, sucrose, polyacrylic acid and polyvinylpyrrolidone;
the conductive additive is one or a combination of more of Super P, Ketjen black, vapor-grown carbon fiber, acetylene black, conductive graphite, carbon nanotubes and graphene;
the equipment for heat treatment and 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 heat treatment carbonization is 500-1200 ℃, and the heat preservation time is 0.5-24 hours;
the non-oxidizing atmosphere is provided by at least one of the following gases: nitrogen, argon, hydrogen or helium;
the crushing treatment adopts any one of an airflow crusher, a ball mill and a turbine type crusher.
Any one of the sieving treatment vibrating screen machine and the air flow classifier is adopted.
In step (2):
the lithium-containing compound powder is a lithium-containing reducing compound;
the doping material is one or a combination of a plurality of simple substances or compound powder containing P, F, Mg, Al, Ca, Cu, B, Fe, Mn, Zn, Zr, Ti and Sn elements;
the maximum particle size of the lithium-containing compound powder and the doping material is less than or equal to 60 μm;
the lithium-containing compound powder and the dopant substance may be pulverized by any one of mortar grinding, ball milling, jet mill, and turbo 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.
In step (3):
the equipment used for the heat treatment 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 heat treatment is 400-950 ℃, the heat preservation time is 0.1-12 hours, and the temperature rise speed is more than 5 ℃ per minute and less than 100 ℃ per minute.
The non-oxidizing atmosphere is provided by at least one of the following gases: nitrogen, argon, hydrogen or helium.
The crushing treatment adopts any one of an airflow crusher, a ball mill and a turbine type crusher.
Any one of the sieving treatment vibrating screen machine and the air flow classifier is adopted.
The invention also protects the lithium ion battery cathode material prepared by the silicon-based composite material, the lithium ion battery cathode prepared by the lithium ion battery cathode material, and the lithium ion battery prepared by the lithium ion battery cathode.
Compared with the prior art, the invention has the following advantages:
1. the silicon nano particles in the silicon-oxygen-lithium compound particles are generated from bottom to top through disproportionation reaction, and the size of the silicon nano particles is far smaller than that of silicon nano particles obtained by other top to bottom crushing methods, so that the volume effect in the lithium extraction process is small. The silicon nano particles in the silicon-oxygen-lithium compound particles are uniformly dispersed and fixed in a lithium silicate compound or silicon-oxygen compound matrix, and the matrix can effectively inhibit and buffer the expansion of the silicon nano particles, prevent the silicon particles from gradually fusing into larger-size particles in the charging and discharging processes, and prevent the fused large-size silicon particles from causing larger expansion and partial failure of the silicon material.
2. Compared with the traditional silicon-oxygen compound cathode material, lithium, oxygen and silicon elements in silicon-oxygen-lithium compound particles exist in the form of lithium silicate compounds, and the oxygen elements do not continuously form lithium silicate or lithium oxide and other compounds in the process of lithium intercalation of the cathode, so that the irreversible loss of lithium ions caused by the oxygen elements during the first charge and discharge of the cathode is greatly reduced, and the first coulombic efficiency of the material in a lithium ion battery is improved.
3. Compared with the traditional silicon-oxygen compound material under the condition of the same lithium removal capacity, the silicon-oxygen lithium compound material has the advantages that a large number of lithium atoms are pre-inserted into the silicon-oxygen lithium compound particles, and the lithium ions required to be inserted into the silicon-oxygen lithium compound particles during the first round of charging and discharging of the battery are less than those of the silicon-oxygen lithium compound material, so that the expansion rate of the particles is lower, the expansion rates of a battery pole piece and the battery are lower, the negative electrode material particles, the negative electrode pole piece and the battery are beneficial to the stable structure.
4. In the preparation method of the material, the lithium-containing compound, the doping element simple substance or the maximum particle size of compound particles are controlled, the lithium-containing compound, the doping element simple substance or the compound particles are uniformly mixed with silicon oxide compound powder, and the subsequent heat treatment heating rate, temperature and time are precisely controlled, so that the uniform reaction of lithium and the doping element with the silicon oxide compound is facilitated, and a uniform and compact silicate compound shell is generated.
5. In the water system homogenizing process, the silicate system compound shell with a compact outer layer of the silicon-oxygen-lithium compound particles can completely isolate the silicon nano particles in the silicon-oxygen-lithium compound particles from external water system slurry, so that the loss of active silicon materials caused by gas production reaction generated by the contact of the silicon nano particles and water is avoided; the compact silicate compound shell has good water resistance, has limited effect on the pH rise of aqueous slurry, and cannot influence the rheological property and 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. In the lithium ion battery, the silicate compound shell with a compact outer layer of the silicon-oxygen-lithium compound particles can completely isolate the silicon nano particles in the silicon-oxygen-lithium compound particles from the external electrolyte, and a more stable SEI film can be formed on the surface of the silicon-oxygen-lithium compound particles, so that the coulombic efficiency and the capacity stability of the material in the charge-discharge cycle process of the battery are greatly improved.
7. For a material adopting doping elements codoped with lithium elements, the surface structure of the silicon-oxygen-lithium compound particles can be more water-resistant due to a small amount of doping elements enriched on the outer layer of the silicon-oxygen-lithium compound particles; the surface can also form a more stable and compact SEI film in the lithium ion battery.
8. When the silicon-oxygen compound particles coated with the carbon film on the outer surface or the composite film layer of the carbon film and the conductive additive are subjected to heat treatment and lithium doping reaction, lithium elements are prevented from being diffused into the silicon-oxygen compound particles to a certain extent by the carbon film, so that a lithium silicate compound shell with more enriched lithium elements than an inner core is easier to form on the outer layer.
9. The carbon film or the composite film layer of the carbon film and the conductive additive is coated outside the silicon-based composite material particles, so that the direct contact between the silicon-oxygen-lithium compound and the water-based slurry can be avoided to a certain extent, and the negative influence of the silicon-oxygen-lithium compound on the slurry is reduced. In addition, the carbon film can also prevent the electrolyte from directly contacting with the silicon-oxygen-lithium compound particles to a certain extent, can form a stable SEI film, improves the interface stability, and improves the coulombic efficiency and the cycle stability of the material.
10. The carbon film or the composite film layer of the carbon film and the conductive additive is coated outside the silicon-based composite material particles, so that the conductivity of the particles can be effectively improved, and the contact resistance among the particles in the negative pole piece, between the negative pole piece and a current collector is reduced, thereby improving the lithium extraction and insertion efficiency of the material, reducing the polarization of the lithium ion battery and promoting the cycle stability of the lithium ion battery.
In conclusion, when the silicon-based composite material is used as a negative electrode of a lithium ion battery, the silicon-based composite material has the electrochemical characteristics of high capacity, high coulombic efficiency, good cycle performance and low expansion rate. The lithium ion battery prepared from the silicon-based composite material has the characteristics of high volume energy density, good cycle stability and low expansion. The preparation method of the silicon-based composite material is simple, low in cost, good in repeatability, simple in required equipment, capable of realizing large-scale industrial production, capable of being directly applied to a water system cathode homogenization process system commonly adopted in the industry and capable of really realizing 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 silicon-based composite material according to the present invention.
FIG. 2 shows the scanning electron micrograph of silicon-based composite material prepared in example 1 multiplied by 20000.
Fig. 3 is a graph of cycle performance of a silicon-containing negative electrode full cell prepared in example 1.
FIG. 4 is a scanning electron micrograph of the silicone-based composite material prepared in example 2 at 10000 times after being immersed in water for 72 hours.
FIG. 5 is an X-ray diffraction pattern of the silicon-based composite material prepared in example 2 after being soaked in water for 72 hours.
Detailed Description
The present invention will be further described with reference to the following specific examples.
The invention provides a silicon-based composite material, which comprises silicon-oxygen-lithium compound particles and a carbon film layer or a composite film layer of a carbon film and a conductive additive, wherein the carbon film layer or the composite film layer is coated on the surfaces of the silicon-oxygen-lithium compound particles. As shown in fig. 1, the shell 1 of the lithium siloxide compound particle is a dense lithium silicate compound, the core 2 is a silicon oxy compound or a lithium silicate compound, the elemental silicon nanoparticles 3 are uniformly dispersed in the silicon siloxide compound particle, the content of the lithium element is gradually reduced from the shell to the core, and no obvious interface exists. The carbon film layer 4 or the composite film layer of the carbon film and the conductive additive 5 is coated on the surface of the silica-lithium compound particles.
Example 1
1000g of silica compound particles having a median particle diameter of 6 μm (silica atom ratio: 1), 65g of low-temperature coal tar pitch powder, and 10g of Keqin black powder were mixed in a coating pot by a dry method, 2000g of dimethylformamide was added while stirring, and the mixed powder was dispersed in dimethylformamide. And then heating the coating kettle to 140 ℃ and keeping the constant temperature for stirring for 3 hours, and finally heating to 160 ℃ and keeping the constant temperature until the dimethylformamide is evaporated to dryness to obtain the silicon oxide compound material jointly coated by the coal tar pitch and the Ketjen black. The above materials were heated to 900 ℃ under nitrogen atmosphere and held for 3 hours to carbonize coal pitch while disproportionation of silica compound occurred. And crushing the cooled material and sieving the crushed material with a 500-mesh sieve to obtain silicon oxide compound powder coated by the carbon film/Ketjen black composite film layer.
Crushing the lithium hydride coarse powder in a drying room with a relative humidity of less than 30% by using a planetary ball mill, and sieving the crushed powder with a 600-mesh sieve to obtain lithium hydride fine powder with a maximum particle size of about 23 μm. The same method is adopted to prepare the magnesium hydride fine powder. 50g of the sieved lithium hydride fine powder and 5g of the magnesium hydride fine powder were mixed with 500g of the silicon oxide powder coated with the carbon film/ketjen black composite film layer in a VC mixer at a high speed for 20 minutes. And (3) putting the mixed powder into a tubular furnace, carrying out lithium doping and doping heat treatment in an argon atmosphere, heating to 750 ℃ at a heating rate of 10 ℃/min, keeping for 60 minutes, naturally cooling, taking the material out of the tubular furnace, and screening by using a 500-mesh screen to obtain the final silicon-based composite material product. Wherein the silicon-oxygen lithium magnesium particles comprise about 58 wt% of silicon element, about 33 wt% of oxygen element, about 8 wt% of lithium element and about 1 wt% of magnesium element; the weight ratios of the carbon film layer and the conductive additive to the lithium magnesium silicon oxide particles were about 4.1:100 and 0.9:100, respectively. When analyzed by X-ray diffraction, the silicon nanocrystal size, in which silicon lithium magnesium oxide particles are uniformly dispersed, is about 7 nm.
Fig. 2 is a scanning electron microscope photograph of the final silicon-based composite product of example 1, wherein the magnification is 20000 times, and it is evident that the carbon film and ketjen black are uniformly coated on the surface of the silica lithium magnesium particle.
And (2) homogenizing, coating, drying and rolling 10 parts of the silicon-based composite material, 85 parts of artificial graphite, 2.5 parts of a conductive additive and 2.5 parts of a binder in an aqueous system 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, 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 found to be 447mAh/g, and the first charge-discharge efficiency (lithium removal cut-off potential is 0.8V) is 91.6%.
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 are tested by using a battery tester of New Wille electronics Limited, 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 density of the full cell obtained in this way was 769Wh/L, and the capacity retention rate after 500 charge-discharge cycles was 85.3%. Fig. 3 is a graph of cycle performance of a silicon-containing negative electrode full cell prepared in example 1.
Example 2
In contrast to example 1, example 2 did not use ketjen black as a conductive additive and magnesium hydride as a dopant, and the other material preparation processes and evaluation methods were the same as those of example 1. The size of the silicon nano crystal particles uniformly dispersed in the silicon-based composite particles is about 6 nm. Finally, the first reversible lithium removal specific capacity of the half-cell of the silicon-containing negative electrode is 448mAh/g, and the first charge-discharge efficiency is 91.1%. The volume energy density of the full battery is 769Wh/L, and the capacity retention rate after 500 charge-discharge cycles is 84.3%.
Fig. 4 shows a scanning electron microscope photograph of the silicon-based composite product of example 2 after 72 hours of immersion in water, at a magnification of 10000 times. The surface appearance of the water-proof paint is not changed compared with that before soaking. Fig. 5 shows the X-ray diffraction pattern contrast of the silicon-based composite product of example 2 before and after 72 hours immersion in water, and it can be seen that there is no change in the crystal structure. Meanwhile, a water and gas drainage and collection method is adopted to measure the gas production rate possibly caused by the reaction with water in the process that the silicon-based composite material product in the embodiment 2 is soaked in water for 72 hours, and any gas production rate is not collected as a result. The above three points prove that the material has excellent water resistance, so that the material can completely endure the anode water system homogenate coating process which usually only needs 6 to 8 hours. In addition, in the slurry coating process of example 2, no problems such as slurry thinning, caking, gassing, uneven coating surface density, coating shrinkage and the like were observed.
Example 3
Compared with the embodiment 1, the embodiment 3 uses sucrose as the precursor for coating the carbon film layer, Super P as the conductive additive and copper as the doping element. The preparation method comprises the steps of dispersing 1000g of silicon oxide compound particles, 10g of Super P powder, 203g of copper citrate and 50g of cane sugar in 5000g of deionized water at a high speed, then carrying out spray drying treatment on the slurry, heating the obtained powder at 900 ℃ for 5 hours in a nitrogen atmosphere, crushing the powder and sieving the powder with a 500-mesh sieve. The lithium doping process and the evaluation method of the copper-doped silicon-oxygen compound particles coated with the carbon film layer/Super P composite film layer are the same as those of the embodiment 1. The size of the silicon nano crystal particles uniformly dispersed in the silicon-based composite particles is about 8 nm. Finally, the first reversible lithium removal specific capacity of the half-cell of the silicon-containing cathode is measured to be 437mAh/g, and the first charge-discharge efficiency is 92.1%. The volume energy density of the full battery is 767Wh/L, and the capacity retention rate after 500 charge-discharge cycles is 85.0%.
Example 4
Compared with example 1, the process of coating the carbon film with the silicon oxide particles in example 4 is performed at 900 ℃ for 3 hours by using methane as a carbon precursor by using a chemical vapor deposition method without adding a conductive additive. In addition, 55g of lithium borohydride fine powder was taken as a lithium-doped and doped substance instead of the lithium hydride and magnesium hydride fine powder. The other material preparation processes and evaluation methods were the same as in example 1. The size of the silicon nano crystal particles uniformly dispersed in the silicon-based composite particles is about 11 nm. Finally, the first reversible lithium removal specific capacity of the half-cell of the silicon-containing cathode is measured to be 436mAh/g, and the first charge-discharge efficiency is 91.9%. The volume energy density of the full cell is measured to be 764Wh/L, and the capacity retention rate after 500 charge-discharge cycles is 83.7%.
Example 5
In example 5, in comparison with example 2, petroleum asphalt was coated on silicon oxide powder by using a heated VC mixer, 50g of fine lithium hydride powder passed through a 500 mesh sieve was used as a lithium-doped compound, the temperature of lithium-doping heat treatment was raised to 820 ℃, and the preparation process and evaluation method of other materials were the same as those of example 2. The size of the silicon nano crystal particles uniformly dispersed in the silicon-based composite particles is about 10 nm. Finally, the first reversible lithium removal specific capacity of the half-cell of the silicon-containing cathode is measured to be 443mAh/g, and the first charge-discharge efficiency is measured to be 92.7%. The volume energy density of the whole battery is 774Wh/L, and the capacity retention rate after 500 charge-discharge cycles is 84.0%.
Example 6
In example 6, the heat treatment conditions after coating the silica compound particles with pitch and ketjen black were changed to 1000 ℃ for 2 hours, and 100g of lithium aluminum hydride fine powder passing through a 500 mesh screen was used as a lithium-doping/doping material instead of lithium hydride and magnesium hydride, as compared with example 1. The temperature of the lithium doping and the doping heat treatment is increased to 770 ℃. The other material preparation processes and evaluation methods were the same as in example 1. The size of the silicon nano crystal particles uniformly dispersed in the silicon-based composite particles is about 19 nm. Finally, the first reversible lithium removal specific capacity of the half-cell of the silicon-containing cathode is measured to be 432mAh/g, and the first charge-discharge efficiency is 92.2%. The volume energy density of the full cell is measured to be 764Wh/L, and the capacity retention rate after 500 charge-discharge cycles is 81.6%.
Example 7
Compared with example 5, the addition amount of the petroleum asphalt powder in example 7 was increased to 200g, the amount of the lithium hydride fine powder was increased to 75g, the lithium-doping heat treatment time was extended to 120 minutes, and the other material preparation processes and evaluation methods were the same as those of example 5. The size of the silicon nano crystal particles uniformly dispersed in the silicon-based composite particles is about 14 nm. Finally, the first reversible lithium removal specific capacity of the half-cell of the silicon-containing cathode is 440mAh/g, and the first charge-discharge efficiency is 90.8%. The volume energy density of the whole battery is determined to reach 761Wh/L, and the capacity retention rate after 500 charge-discharge cycles is 83.5%.
Example 8
1000g of silicon oxide powder with the median particle size of 3 mu m, 150g of polyvinyl alcohol and conductive slurry containing 10g of graphene are uniformly dispersed by a sand mill with 4000g of deionized water and then are subjected to spray drying treatment. And heating the obtained powder to 700 ℃ in a nitrogen atmosphere, keeping the temperature for 2 hours, cooling, crushing, and sieving by a 500-mesh sieve, wherein the silicon oxide compound powder coated by the carbon film/graphene composite film layer does not undergo disproportionation reaction at the heat treatment temperature. The heating rate of the lithium-doped heat treatment after the lithium-doped heat treatment is uniformly mixed with the lithium hydride fine powder is increased to 20 ℃/min. Other preparation processes and material evaluation methods were the same as in example 5. The size of the silicon nano crystal particles uniformly dispersed in the silicon-based composite particles is about 12 nm. Finally, the first reversible lithium removal specific capacity of the half-cell containing the silicon negative electrode is measured to be 441mAh/g, and the first charge-discharge efficiency is 92.4%. The volume energy density of the whole battery is determined to reach 772Wh/L, and the capacity retention rate after 500 charge-discharge cycles is 82.9%.
Example 9
In example 9, in comparison with example 5, 65g of coal pitch was used as a carbon precursor and 10g of ketjen black was used as a conductive additive, and the silica compound powder was liquid-phase-coated in a coating pot. The silicon oxide powder coated with the asphalt/ketjen black composite film was heat-treated at 1150 ℃ for 2 hours, and the other material preparation processes and evaluation methods were the same as those of example 5. The size of the silicon nano crystal particles uniformly dispersed in the silicon-based composite particles is about 10 nm. Finally, the first reversible lithium removal specific capacity of the half-cell of the silicon-containing cathode is measured to be 434mAh/g, and the first charge-discharge efficiency is 92.8%. The volume energy density of the full battery is 767Wh/L, and the capacity retention rate after 500 charge-discharge cycles is 82.7%.
Example 10
In example 10, lithium hydride fine powder passed through a 600 mesh screen was used in an amount reduced to 25g, and 5g of lithium fluoride fine powder passed through a 600 mesh screen was additionally added as a dopant substance, followed by heating at 450 ℃ for 480 minutes for lithium doping and doping heat treatment, as compared with example 5. The other material preparation processes and evaluation methods were the same as in example 5. The size of the silicon nano crystal particles uniformly dispersed in the silicon-based composite particles is about 5 nm. Finally, the first reversible lithium removal specific capacity of the half-cell of the silicon-containing cathode is measured to be 463mAh/g, and the first charge-discharge efficiency is 87.7%. The volume energy density of the full cell is measured to reach 762Wh/L, and the capacity retention rate after 500 charge-discharge cycles is 85.1%.
Example 11
Compared with example 5, the heat treatment carbonization conditions of the petroleum asphalt coated silica compound powder in example 11 are 1050 ℃ for 2 hours, the usage amount of the lithium hydride fine powder is reduced to 10g, the temperature of the lithium doping heat treatment is increased to 930 ℃, and the preparation process and the evaluation method of other materials are the same as those in example 5. The size of the silicon nano crystal particles uniformly dispersed in the silicon-based composite particles is about 26 nm. Finally, the first reversible lithium removal specific capacity of the half-cell of the silicon-containing cathode is determined to be 448mAh/g, and the first charge-discharge efficiency is 89.9%. The volume energy density of the full battery is measured to be 764Wh/L, and the capacity retention rate after 500 charge-discharge cycles is 80.8%.
Example 12
Taking 1000g of silicon oxide powder, 150g of polyvinyl alcohol, conductive paste containing 10g of multi-wall carbon nano tubes and 10g of amphiphilic Ketjen black powder, uniformly dispersing the four materials by using 4000g of deionized water through a sand mill, and then carrying out spray drying treatment to obtain the silicon oxide material jointly coated by the polyvinyl alcohol, the multi-wall carbon nano tubes and the Ketjen black. The above material was heated to 900 ℃ under nitrogen atmosphere and held for 2 hours, after cooling the resulting material powder was crushed and sieved through a 500 mesh sieve. The amount of lithium hydride fine powder used in the lithium doping process was increased to 65 g. The other material preparation processes and evaluation methods were the same as in example 5. The size of the silicon nano crystal particles uniformly dispersed in the silicon-based composite particles is about 13 nm. Finally, the first reversible lithium removal specific capacity of the half-cell of the silicon-containing cathode is measured to be 447mAh/g, and the first charge-discharge efficiency is 91.1%. The volume energy density of the whole battery is 766Wh/L, and the capacity retention rate after 500 charge-discharge cycles is 83.7%.
Example 13
Taking 1000g of silicon oxide powder with the median particle size of 1 mu m, 200g of cane sugar and slurry containing 1g of single-walled carbon nanotubes, uniformly dispersing the silicon oxide powder, the cane sugar and the single-walled carbon nanotubes by a sand mill through 4000g of deionized water, and then carrying out spray drying to obtain the silicon oxide material jointly coated by the cane sugar and the single-walled carbon nanotubes. The above material was heated to 900 ℃ under nitrogen atmosphere and held for 2 hours, after cooling the resulting material powder was crushed and sieved through a 500 mesh sieve. In the lithium doping process, 25g of lithium hydride fine powder passing through a 600-mesh screen is used, the temperature rise rate of lithium doping heat treatment is increased to 20 ℃/min, and the heat preservation time is shortened to 10 minutes. The other material preparation processes and evaluation methods were the same as in example 5. The size of the silicon nano crystal particles uniformly dispersed in the silicon-based composite particles is about 9 nm. Finally, the first reversible lithium removal specific capacity of the half-cell of the silicon-containing cathode is measured to be 447mAh/g, and the first charge-discharge efficiency is measured to be 90.1%. The volume energy density of the whole battery is 765Wh/L, and the capacity retention rate after 500 charge-discharge cycles is 82.1%.
Example 14
Compared with example 2, the process of coating the carbon film with the silicon oxide particles in example 14 is performed by using a chemical vapor deposition method at 1000 ℃ for 2 hours by using ethylene as a carbon precursor, without adding a conductive additive. The temperature of the lithium-doping heat treatment is reduced to 600 ℃. The other material preparation processes and evaluation methods were the same as in example 2. The size of the silicon nano crystal particles uniformly dispersed in the silicon-based composite particles is about 5 nm. Finally, the first reversible lithium removal specific capacity of the half-cell of the silicon-containing cathode is measured to be 457mAh/g, and the first charge-discharge efficiency is 89.4%. The volume energy density of the full battery is 763Wh/L, and the capacity retention rate after 500 charge-discharge cycles is 85.7%.
Example 15
In example 15, 15g of calcium hydride fine powder passing through a 600 mesh screen was additionally added as a doping material in the lithium doping process, compared to example 14, and the other material preparation processes and evaluation methods were the same as those of example 14. The size of the silicon nano crystal particles uniformly dispersed in the silicon-based composite particles is about 6 nm. Finally, the first reversible lithium removal specific capacity of the half-cell of the silicon-containing cathode is measured to be 449mAh/g, and the first charge-discharge efficiency is measured to be 90.1%. The volume energy density of the whole battery is 765Wh/L, and the capacity retention rate after 500 charge-discharge cycles is 86.0%.
Comparative example 1
The process was similar to example 2 except that lithium hydride fine powder was not used as a lithium-doped substance, and thus the product was carbon film-coated silicon oxide compound particles containing no lithium element. The size of the silicon nano crystal particles uniformly dispersed in the silicon-based composite particles is about 4 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 with the silicon-containing cathode is measured to be 463mAh/g, and the first charge-discharge efficiency is 84.5%. The volume energy density of the whole battery is determined to reach 745Wh/L, and the capacity retention rate after 500 charge-discharge cycles is 84.2%. Since the material of comparative example 1 does not contain lithium element, the negative electrode material forms a lithium silicate compound or lithium oxide when lithium intercalation is performed in the lithium ion battery for the first charge, resulting in a high irreversible loss of lithium ions, resulting in a low first charge-discharge efficiency, and thus resulting in a low energy density of the entire battery.
Comparative example 2
The process was similar to that of example 2, except that the silicon oxide powder was not subjected to the pitch coating treatment, but was subjected to the disproportionation reaction heat treatment. The size of the uniformly dispersed silicon nano crystal particles in the obtained final silicon-based composite particles is about 7 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 measured to be 430mAh/g, and the first charge-discharge efficiency is 89.2%. The volumetric energy density of the full cell of the silicon-containing negative electrode was 740Wh/L, and the capacity retention rate after 500 charge-discharge cycles was 78.3%. The material of comparative example 2 is not coated with a carbon film layer or a carbon film/conductive agent composite film layer, so that lithium element is more likely to diffuse into the inside of silica compound particles during the lithium doping process and is less likely to concentrate on the outer layer of the particles to form a dense silicate compound shell. And the silicon-oxygen-lithium compound particles without the protection of the carbon film layer or the composite film layer are more unstable in the water system homogenizing process. In addition, the material lacks a carbon film layer or a carbon film/conductive agent composite film layer, so that the conductivity of the material is poor, and the polarization of the battery in the charging and discharging process also causes the problems of low charging and discharging efficiency, low capacity, low cycle retention rate and the like. Finally, stable SEI is not easily formed on the surface of the coated negative electrode particles which lack the carbon film layer or the carbon film/conductive agent composite film layer, so that the cycle retention rate is poor.
Comparative example 3
The procedure is analogous to example 2, with the difference that the amount of lithium hydride used is increased to 150 g. The size of the silicon nano crystal particles uniformly dispersed in the silicon-based composite particles is about 13 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 407mAh/g, and the first charge-discharge efficiency is 84.0%. The volume energy density of the full cell of the silicon-containing negative electrode was measured to be 652Wh/L, and the capacity retention rate after 500 charge-discharge cycles was 70.3%. Too high an amount of lithium hydride results in a large amount of lithium orthosilicate or even lithium oxide being formed in the product. Lithium orthosilicate and lithium oxide react with water during aqueous homogenization, thereby making the slurry strongly alkaline and causing the destruction of the silicon-oxygen lithium particle structure. The slurry is extremely unstable under alkaline conditions, such as agglomeration of the conductive agent, failure of the thickening agent and the adhesive and the like. In addition, the silicon nano-crystal particles in the silicon-oxygen lithium particles with the damaged structures can be violently reacted when exposed to an alkaline solution, a large number of bubbles are generated in the slurry, the coating quality of the slurry is affected, and a large number of pits and pinholes are caused. The above problems ultimately lead to the disadvantages of low capacity, low efficiency, low energy density, poor cycle retention, etc. of the battery.
Comparative example 4
The process is similar to that of example 5, except that in the lithium-doped heat treatment process, the temperature rise rate is reduced to 5 ℃/min, and the constant temperature time of 820 ℃ is prolonged to 1440 minutes. The size of the silicon nano crystal particles uniformly dispersed in the silicon-based composite particles is about 15 nm. The evaluation methods of the half cell and the full cell are the same as example 5, and the first reversible lithium removal specific capacity of the half cell containing the silicon negative electrode is measured to be 420mAh/g, and the first charge-discharge efficiency is 89.1%. The volumetric energy density of the full cell of the silicon-containing negative electrode was 739Wh/L, and the capacity retention rate after 500 charge-discharge cycles was 75.9%. In comparative example 4, the lower temperature rise rate and the excessively long lithium-doping heat treatment time resulted in sufficient time for lithium ions to uniformly diffuse throughout the silicon oxide compound particles, and the formed lithium silicate-based compound matrix was too high in crystalline quality and excessively dense in growth, reducing the lithium ion and electron mobilities throughout the lithium silicate-based compound particles, thereby resulting in a severe decrease in the electrochemical activity of the material.
Comparative example 5
The procedure was analogous to example 5, except that lithium carbonate was used as the lithium-doping compound instead of lithium hydride. The size of the silicon nano crystal particles uniformly dispersed in the silicon-based composite particles is about 5 nm. The evaluation methods of the half cell and the full cell are the same as example 5, and the first reversible lithium removal specific capacity of the half cell containing the silicon negative electrode is measured to be 415mAh/g, and the first charge-discharge efficiency is 87.2%. The volume energy density of the full cell of the silicon-containing cathode is 688Wh/L, and the capacity retention rate after 500 charge-discharge cycles is 76.3%. The lithium carbonate used in comparative example 5 was low in reactivity and low in efficiency of doping lithium to the silicon oxide compound, and a sufficient amount of the lithium silicate-based compound was not formed. In addition, the lithium doping reaction of lithium carbonate and silicon oxide compound can not reduce the simple substance silicon like the reductive lithium compound adopted in the embodiment, so that the electrochemical active silicon content is less, and the specific capacity of the material is lower. Finally, the residual lithium carbonate which cannot react with the silicon oxide is dissolved in the slurry in the homogenizing process to be alkaline, so that the slurry state is deteriorated, the coating quality is deteriorated, and the final electrochemical performance of the whole battery is influenced.
Comparative example 6
The process was similar to example 5 except that the temperature of the lithium doping heat treatment was increased to 980 ℃. The size of the silicon nano crystal particles uniformly dispersed in the silicon-based composite particles is about 36 nm. The evaluation methods of the half cell and the full cell are the same as example 5, and the first reversible lithium removal specific capacity of the half cell containing the silicon negative electrode is 407mAh/g, and the first charge-discharge efficiency is 89.1%. The volumetric energy density of the full cell of the silicon-containing negative electrode was found to be 731Wh/L, and the capacity retention rate after 500 charge-discharge cycles was found to be 53.2%. The excessively high lithium-doped sintering temperature in comparative example 6 causes the uniformly dispersed nano silicon grains in the silicon-based composite particles to grow to an excessively large size, thereby causing the material to have excessively large volume expansion and shrinkage during charge and discharge, and the particles are continuously broken to form a new interface, thereby continuously and repeatedly forming SEI to consume lithium ions; excessive expansion and contraction of the particles can also disrupt the electrical contact with surrounding particles and even the electrical contact of the pole pieces and current collectors, causing the cycle capacity retention of the battery to decay.
Comparative example 7
The process was similar to that of example 14 except that the lithium hydride coarse powder was not subjected to ball milling and sieving treatment, and was mixed with the carbon-film-coated silicon oxy compound powder only after grinding in a mortar, and the temperature rise rate in the lithium-doping heat treatment process was 5 ℃/min. The size of the silicon nano crystal particles uniformly dispersed in the silicon-based composite particles is about 6 nm. The evaluation methods of the half cell and the full cell are the same as those in example 14, and the first reversible lithium removal specific capacity of the half cell containing the silicon negative electrode is 436mAh/g, and the first charge-discharge efficiency is 87.5%. The volume energy density of the full cell of the silicon-containing negative electrode was 705Wh/L, and the capacity retention rate after 500 charge-discharge cycles was 79.1%. The lithium hydride powder in comparative example 7 was not particle-size-controlled, and there were a large number of lithium hydride particles having a particle size much larger than that of the silicon oxide particles. More lithium hydride particles with too large a particle size will result in too high a doping of the silicon oxide particles surrounding them with lithium, so that lithium orthosilicate and even lithium oxide is formed. The lithium ions can be diffused into the silicon-oxygen compound particles at a low temperature rise rate, and the obtained silicon-oxygen lithium compound particles are uniform in element distribution and have no compact water-resistant shell. The material causes the slurry to be alkaline during aqueous homogenization, resulting in an unstable slurry state. Meanwhile, the material also reacts with water-based slurry to generate more bubbles, so that the loss of active silicon materials and the deterioration of coating quality are caused. The above problems eventually lead to deterioration of electrochemical performance of the full cell.
Examples summary of electrochemical data:
Figure BDA0001577517940000151
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 (10)

1. A silicon-based composite material, characterized by: the film layer is a carbon film layer or a carbon film/conductive additive composite film layer formed by the carbon film layer and a conductive additive; the silicon-oxygen-lithium compound particles have a core-shell structure, the shell of the core-shell structure is a compact lithium silicate compound, the core of the core-shell structure is a lithium silicate compound with the lithium content lower than that of the shell or a silicon-oxygen compound without lithium, the content of lithium elements is gradually reduced from the shell to the core of the silicon-oxygen-lithium compound particles, and no obvious interface exists; the silicon-oxygen-lithium compound particles also comprise uniformly dispersed elementary silicon nano particles;
the preparation method of the silicon-based composite material comprises the following steps:
(1) coating a carbon film layer or a carbon film/conductive additive composite film layer on the surface of silicon oxide compound particles, and then crushing and screening;
(2) uniformly mixing the material obtained in the step (1) with lithium-containing compound powder; or uniformly mixing the material obtained in the step (1), lithium-containing compound powder and a doping substance at the same time; or uniformly mixing the material obtained in the step (1) with a doping substance, performing heat treatment doping in a non-oxidizing atmosphere, and then uniformly mixing the material with lithium-containing compound powder, wherein the doping substance is a simple substance or compound powder containing doping elements, and the maximum particle size of the lithium-containing compound powder is less than or equal to 60 mu m;
(3) and (3) heating the mixed material obtained in the step (2) in a non-oxidizing atmosphere to diffuse lithium element or lithium and doping elements into silicon oxide compound particles, and then crushing and screening to obtain the silicon-based composite material, wherein the heat treatment temperature is 450-820 ℃, the heat preservation time is 0.1-8 hours, and the heating rate is more than 5 ℃ per minute and less than 100 ℃ per minute.
2. The silicon-based composite material of claim 1, wherein: the median particle diameter of the silicon-oxygen-lithium compound particles is between 0.2 and 20 mu m, and the median particle diameter of the simple substance silicon nano particles dispersed in the silicon-oxygen-lithium compound particles is between 0.1 and 35 nm; the thickness of the carbon film layer or the carbon film/conductive additive composite film layer outside the silicon-oxygen-lithium compound particles is between 0.001 and 5 mu m; in the silicon-oxygen-lithium compound particles, the content of silicon element is 49.9-79.9 wt%, the content of oxygen element is 20-50 wt%, the content of lithium element is 0.1-20 wt%, and the sum of the three element contents is 100%; in the carbon film layer, the weight ratio of the carbon film to the silicon-oxygen-lithium compound particles is 0.01:100-20: 100; in the carbon film/conductive additive composite film layer, the weight ratio of the carbon film to the silicon-oxygen-lithium compound particles is 0.01:100-20:100, and the weight ratio of the conductive additive to the silicon-oxygen-lithium compound particles is 0:100-10: 100.
3. The silicon-based composite material of claim 1, wherein: the silicon-oxygen-lithium compound particles also contain a small amount of doping elements, the content of the doping elements is gradually reduced from the outer shell to the inner core of the silicon-oxygen-lithium compound particles, and no obvious interface exists; the doping element is one or a combination of more of P, F, Mg, Al, Ca, Cu, B, Fe, Mn, Zn, Zr, Ti and Sn; in the silicon-oxygen-lithium compound particles, the content of silicon element is 49.89-79.89 wt%, the content of oxygen element is 20-50 wt%, the content of lithium element is 0.1-20 wt%, the content of doping element is 0.01-10%, and the sum of the content of silicon, oxygen, lithium and doping element is 100%.
4. A process for the preparation of a silicon-based composite material according to any one of claims 1 to 3, characterized in that: the method comprises the following steps:
(1) coating a carbon film layer or a carbon film/conductive additive composite film layer on the surface of silicon oxide compound particles, and then crushing and screening;
(2) uniformly mixing the material obtained in the step (1) with lithium-containing compound powder; or uniformly mixing the material obtained in the step (1), lithium-containing compound powder and a doping substance at the same time; or uniformly mixing the material obtained in the step (1) with a doping substance, performing heat treatment doping in a non-oxidizing atmosphere, and then uniformly mixing the material with lithium-containing compound powder, wherein the doping substance is a simple substance or compound powder containing doping elements;
(3) and (3) heating the mixed material obtained in the step (2) in a non-oxidizing atmosphere to diffuse lithium element or lithium and doping elements into silicon oxide compound particles, and then crushing and screening to obtain the silicon-based composite material.
5. The method for preparing a silicon-based composite material according to claim 4, wherein in the step (1):
the stoichiometric ratio of silicon to oxygen in the silicon-oxygen compound particles is 1:0.5-1: 1.5;
the carbon film layer is directly obtained by a chemical vapor deposition mode, or is obtained by firstly coating a carbon precursor and then carrying out heat treatment carbonization in a non-oxidizing atmosphere;
the carbon film/conductive additive composite film layer is obtained by the following steps: after silicon oxide particles coated with a carbon film by chemical vapor deposition are mixed with a conductive additive and a carbon precursor, heat treatment carbonization is performed in a non-oxidizing atmosphere; or after mixing the silicon oxide particles with the conductive additive and the carbon precursor, carrying out heat treatment carbonization in a non-oxidizing atmosphere to obtain the silicon oxide/carbon composite material;
the coating method of the carbon precursor or the carbon precursor and the conductive additive adopts any one of a mechanical fusion machine, a VC mixer, a coating kettle, spray drying, a sand mill or a high-speed dispersion machine, and the solvent selected during coating 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 trichloromethane;
the carbon precursor is one or a combination of more of coal pitch, petroleum pitch, polyvinyl alcohol, epoxy resin, polyacrylonitrile, polymethyl methacrylate, glucose, sucrose, polyacrylic acid and polyvinylpyrrolidone;
the conductive additive is one or a combination of more of Super P, Ketjen black, vapor-grown carbon fiber, acetylene black, conductive graphite, carbon nanotubes and graphene;
the equipment for heat treatment and 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 heat treatment carbonization is 500-1200 ℃, and the heat preservation time is 0.5-24 hours;
the non-oxidizing atmosphere is provided by at least one of the following gases: nitrogen, argon, hydrogen or helium;
the crushing treatment adopts any one of an airflow crusher, a ball mill and a turbine crusher;
any one of the sieving treatment vibrating screen machine and the air flow classifier is adopted.
6. The method for preparing a silicon-based composite material according to claim 4, wherein in the step (2):
the lithium-containing compound powder is a lithium-containing reducing compound;
the doping material is one or a combination of a plurality of simple substances or compound powder containing P, F, Mg, Al, Ca, Cu, B, Fe, Mn, Zn, Zr, Ti and Sn elements;
the maximum particle size of the lithium-containing compound powder and the doping material is less than or equal to 60 μm;
the lithium-containing compound powder and the doping material are pulverized by any one of mortar grinding, a ball mill, a jet mill and a turbine pulverizer;
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.
7. The method for preparing a silicon-based composite material according to claim 4, wherein in the step (3):
the equipment used for the heat treatment 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 heat treatment is 450-820 ℃, the heat preservation time is 0.1-8 hours, and the temperature rise speed is more than 5 ℃ per minute and less than 100 ℃ per minute;
the non-oxidizing atmosphere is provided by at least one of the following gases: nitrogen, argon, hydrogen or helium;
the crushing treatment adopts any one of an airflow crusher, a ball mill and a turbine crusher;
any one of the sieving treatment vibrating screen machine and the air flow classifier is adopted.
8. A lithium ion battery negative electrode material is characterized in that: a lithium ion battery negative electrode material comprising the silicon-based composite material according to any one of claims 1 to 3.
9. A lithium ion battery negative electrode, characterized in that: a lithium ion battery negative electrode prepared by using the lithium ion battery negative electrode material of claim 8.
10. A lithium ion battery, characterized by: a lithium ion battery prepared using the lithium ion battery negative electrode of claim 9.
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