Silicon dioxide / carbon coated silicon-based composite material as well as preparation method and application thereof

Abstract

The invention provides a silicon dioxide / carbon coated silicon-based composite material and a preparation method and application thereof, the silicon dioxide / carbon coated silicon-based composite material comprises a silicon core and a silicon dioxide / carbon composite coating layer coated on the surface of the silicon core, and the silicon dioxide / carbon composite coating layer comprises a carbon matrix and silicon dioxide particles uniformly embedded in the carbon matrix. The preparation method comprises the following steps: preparing a silicon / silicon dioxide / phenolic resin precursor; and sintering and carbonizing the silicon / silicon dioxide / phenolic resin precursor to obtain the silicon dioxide / carbon coated silicon-based composite material. The silicon dioxide / carbon coating layer is adopted to coat the silicon core, so that the electrochemical cycle stability of the silicon-based negative electrode material is greatly improved. The preparation method is simple, efficient and easy for large-scale production. And the prepared lithium ion battery has high initial coulombic efficiency, high energy density and excellent rate capability and cycle performance.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium ion batteries, and particularly relates to a silicon dioxide / carbon-coated silicon-based composite material and a preparation method and application thereof. Background Art

[0002] Lithium-ion batteries have become the main energy storage device for mobile electronic devices and electric vehicles due to their advantages such as long cycle life, good safety performance and no memory effect. However, the energy density of commercial lithium-ion batteries using graphite as the negative electrode is gradually approaching the theoretical limit and it is difficult to meet the needs of future social development. It is urgent to develop high-performance negative electrode materials and improve the energy density of lithium-ion batteries.

[0003] The silicon anode has a theoretical specific capacity of up to 3579 mAh / g, which is much higher than the theoretical specific capacity of the traditional graphite anode (372 mAh / g). In addition, the silicon anode has a low lithium insertion and extraction potential (~0.4 V vs. Li / Li + ), abundant reserves, and environmental friendliness, and are considered to be the ideal negative electrode material for the next generation of lithium-ion batteries. However, the practical application of silicon-based negative electrodes faces great challenges. Challenge 1: The lithiation process is accompanied by a large volume expansion (~300%), resulting in poor cycle performance; Challenge 2: The intrinsic electronic conductivity is low, making its rate performance less than ideal; Challenge 3: The continuous side reaction with the electrolyte will cause the continuous growth of the solid electrolyte interface (SEI), which consumes lithium sources and active materials, further accelerating the failure of the battery.

[0004] In the prior art, for example, patent number CN103730644A discloses a method for preparing a lithium-ion battery silicon-silicon oxide-carbon composite negative electrode material. This method has limited effect on improving the performance of the material, and involves the use of toxic solvents during the synthesis process, which is not conducive to industrial application. Patent number CN109713242A discloses a titanium silicon-carbon negative electrode material with a core-shell pomegranate structure and a preparation method thereof. In this scheme, the silicone gel coating effect is limited, the coating layer is prone to failure during the cycle, and increasing the thickness of the coating layer will increase the internal resistance or significantly increase the cost, and the actual application prospects are not good.

[0005] In the paper "Regulating lithium transfer pathway to avoid capacity fadingof nano Si through sub-nano scale interfused SiO x A method for constructing a sub-nanometer silicon oxide / carbon composite coating (SiO xThe specific steps include ultrasonically dispersing silicon nanoparticles in deionized water; adding an acidic solution of vinyl trimethoxysilane (VTMS) to the silicon solution to cause a hydrolysis reaction, thereby forming a polyvinyl silsesquioxane (PVSQ) coating layer on the surface of the silicon nanoparticles, thereby obtaining a Si@PVSQ precursor; annealing the precursor to finally obtain Si@SiO x / C material. Experimental data show that this sub-nanoscale silicon oxide / carbon coating has excellent electronic and ionic conductivity, which can simultaneously improve the rate performance and cycle stability of silicon-based negative electrode materials. However, the SiO obtained by this method x In the C coating layer, SiO x The proportion is relatively high and the distribution is uneven, which results in the low coulombic efficiency and unsatisfactory mass specific capacity of the prepared material. x After 350 cycles at a current density of 1 A / g, the specific capacity of the / C material is only 886 mAh / g, and the first coulombic efficiency is only 69.6%. These indicators are still far from the requirements of commercial applications, which seriously limits the application potential of this material in the field of high-performance lithium-ion batteries.

[0006] Therefore, to promote the practical application of silicon-based negative electrodes, it is also necessary to comprehensively consider the relationship between the various components, further optimize the structural design and develop a simple synthesis method to meet the requirements of high initial efficiency, high specific volume, high rate and long life. Summary of the invention

[0007] In order to solve the problems mentioned in the background technology, the present application provides a silicon dioxide / carbon-coated silicon-based composite material and a preparation method and application thereof.

[0008] In the first aspect of the present invention, a silicon-based composite material coated with silicon dioxide / carbon is proposed, which includes a silicon core and a silicon dioxide / carbon composite coating layer coated on the surface of the silicon core, and the silicon dioxide / carbon composite coating layer includes a carbon matrix and silicon dioxide particles uniformly embedded in the carbon matrix. The introduction of the coating layer can effectively reduce the volume expansion of silicon during the charging and discharging process, improve the ion and electron conductivity of the electrode, and maintain a high first coulomb efficiency, thereby significantly enhancing the electrochemical performance of the silicon-based negative electrode.

[0009] In the above technical solution, the structural stability of the silica / carbon-coated silicon-based composite material is enhanced through the synergistic effect of silica particles and the carbon matrix. During the charge and discharge process, the lithium silicate produced after the silica is lithiated can effectively buffer the stress generated by the volume change of the silicon core, thereby preventing the material from structural damage and extending the service life of the battery. In addition, the presence of the carbon matrix can provide a fast electron transmission channel, and the presence of a large number of interfaces between the silica particles and the carbon matrix facilitates the rapid transmission of lithium ions, thereby improving the rate performance and energy density of the battery. At the same time, the product after the silica particles are lithiated has a wide electrochemical window, which effectively avoids direct contact between the electrolyte and the silicon core, inhibits the disordered generation of the solid electrolyte interface (SEI) film, maintains the stability of the battery performance, and improves the battery's cycle stability and first coulomb efficiency.

[0010] Furthermore, the silicon core is nano-silicon or micro-silicon, and the carbon matrix is ​​amorphous carbon. The silicon core is nano-silicon or micro-silicon, and the carbon matrix is ​​amorphous carbon, which helps to improve the conductivity and structural stability of the composite material.

[0011] Furthermore, the average diameter of the silicon dioxide particles is 1-10 nm, and the particle size of the silicon core is 30 nm-10 μm.

[0012] Furthermore, the thickness of the silicon dioxide / carbon composite coating layer is 10-40nm. When the silicon dioxide / carbon composite coating layer has a specific thickness, on the one hand, it can ensure that the inherent performance of the silicon core is not damaged, and on the other hand, it can significantly improve the overall cycle stability of the material. In addition, with the precise control of the thickness of the silicon dioxide / carbon composite coating layer, a coating process system suitable for silicon particles of different sizes is constructed to meet diversified production needs.

[0013] Furthermore, the mass fraction of the silicon dioxide particles in the silicon dioxide / carbon composite coating layer is 10%-95%, preferably 70%-95%. The silicon dioxide particles have a specific mass fraction in the composite coating layer, which can optimize the structure and electrochemical performance of the material.

[0014] Furthermore, the proportion of the silicon core in the silicon dioxide / carbon-coated silicon-based composite material is 20wt%-98wt%. The cycle capacity and cycle stability of the composite material can be adjusted by regulating the proportion of the silicon core.

[0015] In a second aspect of the present invention, a method for preparing a silicon dioxide / carbon-coated silicon-based composite material is provided, comprising:

[0016] S1, preparation of silicon / silicon dioxide / phenolic resin precursor;

[0017] S2, sintering and carbonizing the silicon / silicon dioxide / phenolic resin precursor to obtain a silicon dioxide / carbon-coated silicon-based composite material.

[0018] In the above technical scheme, the preparation method effectively synthesizes silica / carbon-coated silicon-based composite materials, providing a feasible approach for industrial production.

[0019] Further, the preparation steps of the silicon / silicon dioxide / phenolic resin precursor include:

[0020] S11, dispersing silicon particles in ethanol, deionized water and ammonia solution, stirring at room temperature for 1-60 minutes, to obtain a mixed solution 1;

[0021] In the above steps, ethanol and deionized water contribute to the initial dispersion of silicon particles, and the ammonia solution plays a role in adjusting the pH of the reaction system, providing a uniform reactant basis for subsequent reactions and ensuring the consistency and effectiveness of the subsequent coating process.

[0022] S12, dissolving resorcinol and formaldehyde in ethanol, adding mixed solution 1, and stirring at room temperature for 0.1-3h to obtain mixed solution 2;

[0023] In the above steps, resorcinol and formaldehyde undergo a condensation reaction in an ethanol solution to form oligomers of phenolic resin, laying the foundation for the subsequent formation of a stable composite structure.

[0024] S13, adding tetrapropoxysilane to the mixed solution 2, stirring at room temperature for 1-50 hours, washing the reactant after centrifugation with water and anhydrous ethanol, and drying the reactant to obtain a silicon / silicon dioxide / phenolic resin precursor;

[0025] The mass ratio of silicon particles, resorcinol, formaldehyde and tetrapropoxysilane is 100:(3-50):(5-80):(25-800).

[0026] In the above steps, tetrapropoxysilane is added to the mixed solution 2 and stirred for a long time, and the tetrapropoxysilane undergoes a hydrolysis reaction to generate small silica particles. In this process, the small silica particles and the phenolic resin prepolymer are intertwined and assembled together on the surface of the silicon particles into a composite coating. After the reaction is completed, the centrifuged reactants are washed with water and anhydrous ethanol to remove unreacted raw materials, by-products and impurities. The purpose of drying the reactants is to obtain a pure and stable silicon / silicon dioxide / phenolic resin precursor for subsequent carbonization treatment, and finally obtain the target silica / carbon-coated silicon-based composite material.

[0027] Further, step S2 includes placing the silicon / silicon dioxide / phenolic resin precursor in a tube furnace, and the carbonization process is processed according to the following procedure:

[0028] S21, heating to 150°C at a rate of 2°C / min and keeping at 150°C for 2 h;

[0029] In the above steps, the temperature is slowly increased from the initial temperature to 150°C at a rate of 2°C / min and kept at this temperature for 2 hours. During this stage, the water, low-boiling-point organic solvents and some volatile impurities in the precursor can be slowly evaporated, avoiding defects or stress concentration inside the material due to rapid heating, and at the same time making the material initially stable.

[0030] S22, heating to 350°C at a rate of 2°C / min and keeping at this temperature for 2h;

[0031] In the above steps, the temperature is continued to rise to 350°C at the same heating rate and kept for 2 hours. In this temperature range, the organic matter in the precursor (such as phenolic resin, etc.) begins to undergo carbonization reaction to form a preliminary carbon network structure. The control of the heating rate ensures the uniformity and stability of the carbonization process, which helps to improve the quality of the carbon matrix and its bonding strength with other components.

[0032] S23, heating to 850°C at a heating rate of 2°C / min, keeping the temperature for 4 hours, and cooling to obtain a silicon dioxide / carbon-coated silicon-based composite material;

[0033] In the above steps, the temperature is raised to 850°C and maintained for 4 hours to fully carbonize the phenolic resin precursor and further improve the carbon network structure. At the same time, high temperature treatment helps to enhance the interaction between silica and carbon, improve the overall stability and electrochemical properties of the composite material, and finally obtain a silica / carbon-coated silicon-based composite material with good structure and performance.

[0034] or

[0035] S21', heating to 800°C at a heating rate of 5°C / min, and keeping at 800°C for 6h, and after cooling, obtaining a silicon dioxide / carbon-coated silicon-based composite material.

[0036] In the above steps, the temperature is directly heated to 800°C at a relatively fast heating rate of 5°C / min and kept at this temperature for 6 hours. This method shortens the heating time and can improve production efficiency.

[0037] In the third aspect of the present invention, an application of a silica / carbon-coated silicon-based composite material is proposed, wherein the silica / carbon-coated silicon-based composite material described in the first aspect or the silica / carbon-coated silicon-based composite material prepared by the method described in the second aspect is used as a negative electrode for a lithium-ion battery.

[0038] Compared with the prior art, the beneficial results of the present invention are:

[0039] (1) The present invention uses commercial silicon particles as the silicon source and phenolic resin as the carbon source. The raw materials involved are simple and easy to obtain, and the cost is relatively low. The synthesis process only requires a simple solvent method and a one-step carbonization method, avoiding the use of highly corrosive and highly dangerous reagents such as concentrated hydrochloric acid or concentrated sulfuric acid, and the target product can be prepared. Not only does it eliminate the safety hazards and environmental risks caused by the use of these hazardous chemicals from the source, but the entire synthesis process does not generate toxic by-products, and has the remarkable characteristics of simple preparation, energy saving, environmental protection and environmental friendliness.

[0040] (2) The present invention coats the silicon particle core with a silicon dioxide / carbon coating layer, which fundamentally strengthens the electrochemical stability of the silicon-based negative electrode material and lays a solid foundation for its long-term application in the battery field. At the same time, the preparation process of the silicon-based negative electrode material is simple, clear, efficient and convenient, showing excellent large-scale production potential, which can effectively meet the needs of industrial mass production.

[0041] (3) The silicon dioxide / carbon-coated silicon-based composite material for lithium-ion batteries of the present invention combines the advantages of high reversible capacity, stable cycle performance, good rate performance, etc., comprehensively improving the comprehensive electrochemical performance of lithium-ion batteries, providing a more competitive material choice for the lithium-ion battery industry, and effectively meeting the current urgent demand for high-performance battery materials. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] The accompanying drawings are included to provide a further understanding of the embodiments and are incorporated into and constitute a part of this specification. The accompanying drawings illustrate the embodiments and are used together with the description to explain the principles of the present invention. It will be easy to recognize other embodiments and many expected advantages of the embodiments because they become better understood by reference to the following detailed description. The elements of the drawings are not necessarily to scale with each other. The same reference numerals refer to corresponding similar parts.

[0043] Figure 1 is a flow chart of a method for preparing a silicon dioxide / carbon-coated silicon-based composite material according to an embodiment of the present invention;

[0044] Figure 2a-2b is a schematic structural diagram of a silicon dioxide / carbon-coated silicon-based composite material according to an embodiment of the present invention;

[0045] Figure 3 is a scanning electron microscope image (SEM) of the silicon dioxide / carbon-coated silicon-based composite material according to the present invention;

[0046] Figure 4 is an X-ray diffraction pattern (XRD) of the silicon dioxide / carbon-coated silicon-based composite material according to Example 1 of the present invention;

[0047] Figure 5It is a microscopic analysis image of the silicon dioxide / carbon-coated silicon-based composite material according to the present invention and a pore size distribution diagram before and after etching;

[0048] Figure 6 is a thermogravimetric analysis (TGA) of a silicon dioxide / carbon composite coating layer in a silicon dioxide / carbon-coated silicon-based composite material according to the present invention;

[0049] Figure 7 is a thermogravimetric analysis (TGA) of the silicon dioxide / carbon-coated silicon-based composite material according to the present invention;

[0050] Figure 8 These are XPS-C1s, XPS-Si2p and XPS-O1s spectra of the silicon dioxide / carbon-coated silicon-based composite material according to Example 1 of the present invention;

[0051] Fig. 9 This is a cycle performance diagram at a current density of 0.2 A / g when the silicon dioxide / carbon-coated silicon-based composite material according to Example 1 of the present invention is used as a negative electrode material for a lithium-ion battery;

[0052] Fig.10 This is a cycle performance diagram of the silicon dioxide / carbon-coated silicon-based composite material according to Example 1 of the present invention as a negative electrode material for a lithium-ion battery at a high current density of 2A / g;

[0053] Fig.11 The cyclic performance of the silicon-carbon composite material prepared in the comparative example at current densities of 1A / g and 2A / g;

[0054] Fig.12 This is a rate performance diagram when the silicon dioxide / carbon-coated silicon-based composite material according to Example 1 of the present invention is used as the negative electrode material of a lithium-ion battery. DETAILED DESCRIPTION

[0055] The technical solutions in the embodiments will be described clearly and completely below in conjunction with the drawings in the present invention, and similar component numbers in the drawings represent similar components. Obviously, the embodiments described below are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments in the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0056] It should be understood that when used in this specification and the appended claims, the terms "include" and "comprises" indicate the presence of described features, integers, steps, operations, elements and / or components, but do not exclude the presence or addition of one or more other features, integers, steps, operations, elements, components and / or combinations thereof.

[0057] It should also be understood that the terms used in this specification of the present invention are only for the purpose of describing specific embodiments and are not intended to limit the present invention. As used in the specification of the present invention and the appended claims, unless the context clearly indicates otherwise, the singular forms "a", "an" and "the" are intended to include plural forms.

[0058] In the first aspect, the present invention provides a flow chart of a method for preparing a silicon dioxide / carbon-coated silicon-based composite material, as shown in FIG. Figure 1 As shown, the method includes,

[0059] S100, preparation of silicon / silicon dioxide / phenolic resin precursor.

[0060] S200, sintering and carbonizing the silicon / silicon dioxide / phenolic resin precursor to obtain a silicon dioxide / carbon-coated silicon-based composite material.

[0061] In some specific embodiments, the steps of preparing the silicon / silicon dioxide / phenolic resin precursor include:

[0062] S101, dispersing silicon particles in ethanol, deionized water and ammonia solution, stirring at room temperature for 1-60 minutes, to obtain a mixed solution 1.

[0063] S102, dissolving resorcinol and formaldehyde in ethanol, adding mixed solution 1, and stirring at room temperature for 0.1-3h to obtain mixed solution 2.

[0064] S103, adding tetrapropoxysilane to the mixed solution 2, stirring at room temperature for 1-50 hours, washing the reactants after centrifugation with water and anhydrous ethanol, and drying the reactants to obtain a silicon / silicon dioxide / phenolic resin precursor.

[0065] Specifically, first, silicon particles are dispersed in ethanol, deionized water and ammonia solution to obtain a mixed solution; then, the mixed solution is continuously magnetically stirred at room temperature for 10 minutes to obtain a mixed solution one; then, resorcinol and formaldehyde are dissolved in ethanol and added to the mixed solution one, and the obtained mixed solution is again continuously magnetically stirred at room temperature for 1 hour to obtain a mixed solution two; then, tetrapropoxysilane is added to the mixed solution two, and the new mixed solution is continuously stirred at room temperature for 25 hours; finally, the reactants obtained by the stirring reaction are washed with water and anhydrous ethanol, collected by centrifugation, and the collected reactants are dried to obtain a silicon / silicon dioxide / phenolic resin precursor.

[0066] Preferably, the mass ratio of silicon particles, resorcinol, formaldehyde and tetrapropoxysilane is 100:(3-50):(5-80):(25-800).

[0067] In some specific embodiments, step S200 includes placing the silicon / silicon dioxide / phenolic resin precursor in a tube furnace, and the carbonization process is processed according to the following procedure:

[0068] S201, heating to 150°C at a rate of 2°C / min and keeping at 150°C for 2h;

[0069] S202, heating to 350°C at a rate of 2°C / min and keeping at this temperature for 2h;

[0070] S203, heating to 850°C at a heating rate of 2°C / min, keeping the temperature for 4 hours, and cooling to obtain a silicon dioxide / carbon-coated silicon-based composite material;

[0071] or

[0072] S201', heating to 800°C at a heating rate of 5°C / min, and keeping at 800°C for 6h, and after cooling, obtaining a silicon dioxide / carbon-coated silicon-based composite material.

[0073] Specifically, in step S200, the sintering carbonization method adopted includes: placing the silicon / silicon dioxide / phenolic resin precursor obtained in step S100 in a tubular furnace, and starting heating at a heating rate of 2°C / min. When the temperature reaches 150°C, maintaining the temperature for 2 hours; then, continuing to increase the temperature to 350°C at a heating rate of 2°C / min, and keeping it at this temperature for 2 hours; then, still increasing the temperature to 850°C at a heating rate of 2°C / min, and keeping it for 4 hours; after the insulation is completed, allowing the material in the tubular furnace to cool naturally, and finally obtaining a silica / carbon-coated silicon-based composite material. Alternatively, the silicon / silicon dioxide / phenolic resin precursor obtained in step S100 is placed in a tubular furnace and heated at a heating rate of 5°C / min. When the temperature reaches 800°C, the temperature is maintained for 6 hours. After the insulation is completed, the material in the tubular furnace is allowed to cool naturally to finally obtain a silicon dioxide / carbon-coated silicon-based composite material.

[0074] In the second aspect, the present application proposes a silicon dioxide / carbon-coated silicon-based composite material, referring to Figure 2a-2b , Figure 2a-2bThe schematic diagram of the structure of a silicon dioxide / carbon-coated silicon-based composite material according to an embodiment of the present invention is shown. As shown in the figure, the composite material particles are tightly and evenly distributed, wherein the silicon dioxide / carbon-coated silicon-based composite material comprises a silicon core 1 and a silicon dioxide / carbon composite coating layer constructed on the surface of the silicon core 1, and the silicon dioxide / carbon composite coating layer comprises a carbon matrix 3 and silicon dioxide particles 2 uniformly embedded in the carbon matrix 3, and the carbon matrix 3 and the silicon dioxide particles 2 are in contact with the surface of the silicon core 1. Among them, in the silicon dioxide / carbon-coated silicon-based composite material, the silicon core 1 is a commercial silicon particle, which accounts for 20%-98% by mass and has a size of 30nm-10μm. The silicon dioxide / carbon composite coating layer is composed of ultrafine silicon dioxide particles 2 and a carbon matrix 3 of amorphous carbon, and the silicon dioxide particles 2 are uniformly embedded in the carbon matrix 3. The silicon core 1 is nano-silicon or micro-silicon; the diameter of the silicon dioxide particles 2 is 1-10 nm; the carbon matrix 3 is an amorphous structure, and the thickness of the silicon dioxide / carbon composite coating layer is 10-40 nm. Preferably, the diameter of the silicon dioxide particles 2 is about 7 nm, and the thickness of the silicon dioxide / carbon composite coating layer is 15 nm.

[0075] During the first lithiation process, the ultrafine silica particles in the material will be converted into inert substances, which effectively buffer the volume expansion of the silicon core during the charge and discharge process, while isolating the contact between the electrolyte and the silicon core, thereby preventing the growth of the solid electrolyte interface phase (SEI). In addition, since the content of silica particles is controlled within a reasonable range, the first coulomb efficiency will not be reduced due to excessive generation of inert substances. The coating layer uses carbon material as a matrix, which gives the coating layer good electronic conductivity. The rate performance of the silicon negative electrode material modified by this coating layer can be significantly improved. Moreover, the coating layer modifies various types of silicon particles, and the thickness of the coating layer can be precisely adjusted by extending the reaction time. With the above characteristics, the silicon dioxide / carbon coating layer significantly improves the cycle performance and rate performance of the silicon-based negative electrode.

[0076] Preferably, the carbon matrix in the silica / carbon coating layer is amorphous carbon, the modified silicon particles can be nano-silicon or micron-silicon, the thickness of the composite coating layer is 10-40nm, and the mass proportion of the silicon core in the silica / carbon-coated silicon-based composite material is 20%-98%, preferably, the proportion of silicon particles is 87%.

[0077] In a third aspect, the present application proposes a method for preparing a silicon dioxide / carbon-coated silicon-based composite material for use in lithium-ion batteries.

[0078] In some specific embodiments, the silicon dioxide / carbon-coated silicon-based composite material and the conductive carbon Super P are first fully ground to be uniformly mixed, and then sodium alginate and deionized water are added and stirred to form a uniform and fluid slurry. Then, the slurry is evenly coated on the copper foil using a scraper, and the electrode loading is strictly controlled to 0.8-1.2 mg / cm 2 After coating, the substrate is dried and cut into discs with a diameter of 12 mm, and finally assembled into button batteries. The preferred mass ratio of the negative electrode material, conductive carbon, sodium alginate, and deionized water is 8:1:1:90.

[0079] Example 1

[0080] A method for preparing a silicon dioxide / carbon-coated silicon-based composite material comprises the following steps:

[0081] Step 1: Add 200 mg of commercial nanosilicon to 70 mL of ethanol, 10 mL of deionized water and 3 mL of 25% ammonia water, stir at room temperature for 10 min, then dissolve 0.025 g of resorcinol and 0.035 mL of 37% formaldehyde in 2 mL of ethanol and add to the above solution, stir at room temperature for 1 hour. Then add 0.218 mL of tetrapropoxysilane to the above mixed solution, stir magnetically at 30°C for 25 hours, centrifuge three times with water and ethanol respectively, and dry the collected samples at 60°C overnight in an electric blast drying oven to obtain a silicon / silicon dioxide / phenolic resin precursor.

[0082] Step 2: The silicon / silicon dioxide / phenolic resin precursor obtained in step 1 is placed in a tubular furnace and first heated to 150°C at a heating rate of 2°C / min in a nitrogen atmosphere, and the holding time is 2h; secondly, heated to 350°C at a heating rate of 2°C / min, and kept warm for 2h; then, heated to 850°C at a heating rate of 2°C / min, and kept warm for 4h, and naturally cooled to obtain a silicon dioxide / carbon-coated silicon-based composite material, the coating thickness of which is 15nm. The mass fraction of silicon dioxide particles in the silicon dioxide / carbon composite coating layer is 82.9%. The carbon matrix accounts for 2.2% of the silicon dioxide / carbon-coated silicon-based composite material, from which it can be calculated that the mass ratio of the silicon dioxide / carbon composite coating layer is 12.9%, and the mass ratio of the silicon core is 87.1%.

[0083] Step 3: Assemble and characterize the prepared silica / carbon-coated silicon-based composite material as a negative electrode material for lithium-ion batteries: cycle 1000 and 1500 times at current densities of 1A / g and 2A / g, respectively, and perform rate performance tests at different current densities from 0.2A / g to 2A / g.

[0084] Example 2

[0085] The difference from Example 1 is that the mass ratio of commercial nano silicon, resorcinol, formaldehyde and tetrapropoxysilane in step 1 is 100:50:80:800, and the other steps are the same as Example 1, to prepare a silicon dioxide / carbon-coated silicon-based composite material, wherein the thickness of the composite coating layer is controlled to be 40nm, and the mass fraction of silicon dioxide particles in the silicon dioxide / carbon composite coating layer is 82.9%. The carbon matrix accounts for 10.6% of the silicon dioxide / carbon-coated silicon-based composite material, from which it can be calculated that the mass ratio of the silicon dioxide / carbon composite coating layer is 62.0%, and the mass ratio of the silicon core is 38.0%.

[0086] Example 3

[0087] The difference from Example 1 is that the mass ratio of commercial nano silicon, resorcinol, formaldehyde and tetrapropoxysilane in step 1 is 100:3:5:25, and a silicon dioxide / carbon-coated silicon-based composite material is prepared, and the thickness of the composite coating layer is 10nm. The mass fraction of silicon dioxide particles in the silicon dioxide / carbon composite coating layer is 82.9%. The mass fraction of carbon in the silicon dioxide / carbon-coated silicon-based composite material is 0.8%, and it can be calculated that the mass fraction of the silicon dioxide / carbon composite coating layer is 4.7%, and the mass fraction of the silicon core in this material is 95.3%

[0088] Example 4

[0089] The difference from Example 1 is that micron silicon is used as the silicon source in step 1, and the coating layer thickness of the obtained silicon dioxide / carbon-coated silicon-based composite material is 15 nm.

[0090] Example 5

[0091] The difference from Example 1 is that micron porous silicon is used as the silicon source in step 1, and the coating layer thickness of the obtained silicon dioxide / carbon-coated silicon-based composite material is 15 nm.

[0092] Example 6

[0093] The difference from Example 1 is that the silicon / silicon dioxide / phenolic resin precursor is carbonized in an argon atmosphere, and the coating layer thickness of the obtained silicon dioxide / carbon-coated silicon-based composite material is 15 nm.

[0094] Example 7

[0095] The difference from Example 1 is that in step 2, the temperature heating program of the carbonization process of the silicon / silicon dioxide / phenolic resin precursor is as follows: the precursor is heated to 800°C at 5°C / min and kept warm for 6 hours, followed by natural cooling, and the obtained silicon dioxide / carbon-coated silicon-based composite material has a coating layer thickness of 15nm.

[0096] Example 8

[0097] The difference from Example 1 is that in step 1, the stirring time of ethanol / deionized water / ammonia water / commercial nano-silicon is 30 minutes, and the above dispersion is added with 0.025g of resorcinol and 0.035mL of 37% formaldehyde dissolved in 2mL of ethanol, and the stirring time at room temperature is 2 hours. After adding 0.218mL of tetrapropoxysilane to the above solution, the magnetic stirring time is 10 hours, and the other steps are the same as in Example 1, and the coating layer thickness of the obtained silicon dioxide / carbon-coated silicon-based composite material is 15nm.

[0098] Comparative Example

[0099] This comparative example provides a preparation process and performance test of a traditional silicon-carbon composite material prepared by a solvent method. The specific preparation process is as follows.

[0100] S1: Disperse 200 mg of nano-silicon into 180 mL of deionized water and stir for 10 min.

[0101] S2: Dissolve 0.24226 g of tris(hydroxymethyl)aminomethane in 20 mL of deionized water, add to the above S1 solution and stir for 10 min.

[0102] S3: Add 200 mg of dopamine to the above S2 mixed solution and stir for 1 hour.

[0103] S4: Water and ethanol were centrifuged three times, and the collected samples were dried in an electric blast drying oven at 60°C overnight to obtain dopamine-coated nano-silicon. The nano-silicon powder sample with external dopamine was placed in a tubular furnace and carbonized under a nitrogen atmosphere. The carbonization method was to heat the sample to 750°C at a heating rate of 3°C / min, keep the temperature for 3h, and cool the sample to obtain a silicon-carbon composite material.

[0104] S5: The obtained material was then used as the negative electrode material for lithium-ion batteries to assemble button cells. After 1000 cycles at a current density of 1 A / g, the capacity was only 88 mAh / g, with a retention rate of 6.5%. After 100 cycles at a current density of 2 A / g, the capacity was only 2 mAh / g, with a retention rate of 0.2%. The performance was much lower than that of Example 1.

[0105] refer to Figure 3 , Figure 3 The scanning electron microscope (SEM) image of the silicon dioxide / carbon-coated silicon-based composite material according to the present invention is shown. Figure 3Figures A, B, C, and D in the figure correspond to the scanning electron microscope images of the silicon dioxide / carbon-coated silicon-based composite materials prepared in Example 1, Example 2, Example 3, and Example 5, respectively. It can be observed from the figure that the silicon dioxide / carbon-coated silicon-based composite material exhibits a granular structure under a scanning electron microscope. The sizes of these particles are relatively uniform, and there are certain gaps between the particles. This microstructure is conducive to the diffusion and storage of lithium ions inside the material, which in turn has a positive effect on the electrochemical properties of lithium-ion batteries.

[0106] Continue to refer Figure 4 , Figure 4 The X-ray diffraction pattern (XRD) of the silica / carbon-coated silicon-based composite material of Example 1 of the present invention is shown. As shown in the figure, obvious diffraction peaks appear at 28.3°, 47.4°, 56.1°, 56.3°, and 76.3°. According to the standard diffraction pattern of silicon found in the crystallographic database, they correspond to the (111), (220), (311), and (331) crystal planes of silicon, respectively, confirming the presence of silicon crystal structure in the silica / carbon-coated silicon-based composite material of Example 1.

[0107] Figure 5 The microscopic analysis image of the silicon dioxide / carbon-coated silicon-based composite material prepared according to Example 1 of the present invention and the pore size distribution diagram before and after etching are shown, wherein the microscopic analysis image includes a transmission electron microscope image (TEM), a TEM image before and after etching, and an X-ray energy spectrum analysis image (EDS). As shown in the figure, Figure 5 A shows that the coating thickness of the silicon dioxide / carbon-coated silicon-based composite material is about 15 nm, and the entire material is composed of Si element; Figure 5 B shows the TEM of the material before and after hydrofluoric acid etching. The appearance of pores after etching proves the presence of ultrafine silica nanoparticles in the coating layer, because hydrofluoric acid etches silica, leaving pores in the material. Figure 5 C is the X-ray energy spectrum analysis (EDS) of silicon dioxide / carbon-coated silicon-based composite materials, combined with Figure 3 The scanning electron microscope image shows that carbon, silicon and oxygen are evenly distributed on the surface of the material, indicating that ultrafine silica particles and carbon are evenly coated on the silicon particles, proving the feasibility of the coating strategy. Figure 5 D is the pore size distribution diagram of the material in Example 1 before and after hydrofluoric acid etching, wherein the abscissa represents the pore size and the ordinate represents the pore size distribution function. The red curve represents the pore size distribution of the material in Example 1 before etching, and the blue curve represents the pore size distribution of the material in Example 1 after etching. The results confirm that the material curve after etching has an obvious peak in the range of about 1-10 nm, indicating that a large number of mesoporous structures appear in the material after etching, and the size of these mesopores corresponds to the diameter size of the silica particles in the composite coating layer.

[0108] Figure 6 The thermogravimetric analysis (TGA) of the silicon dioxide / carbon composite coating according to the present invention is shown. The difference between the preparation method of the silicon dioxide / carbon composite coating and Example 1 is that no silicon core is added. As can be seen from the figure, the mass of the sample decreases significantly from nearly 100% and finally stabilizes at about 82.9%, that is, the mass fraction of silicon dioxide particles in the silicon dioxide / carbon composite coating is 82.9%. The results show that in the silicon dioxide / carbon composite coating, the mass fraction of silicon dioxide is 82.9% and the mass fraction of carbon is 17.1%.

[0109] Further references Figure 7 , Figure 7 Thermogravimetric analysis (TGA) of silicon dioxide / carbon-coated silicon-based composite materials prepared according to Examples 1-3 of the present invention is shown. As shown in the figure, by measuring the carbon ratio in the independent coating layer, the silicon content of the material can be calculated. This is because during the thermogravimetric analysis process, carbon and other volatile components will decompose or volatilize at high temperatures, while silicon and silicon dioxide are relatively stable, so the silicon content can be inferred by mass loss. When the silicon dioxide / carbon-coated silicon-based composite material is used as the negative electrode of a lithium-ion battery, the lithium storage function is mainly achieved through silicon. If the mass fraction of silicon is reduced, the battery capacity will be reduced; conversely, if the silicon content is too high, the overall expansion rate of the material will increase, causing it to rupture during the cycle, resulting in battery failure.

[0110] like Figure 7As shown, Example 1 (red curve): During the temperature increase process, the mass fraction of the silicon dioxide / carbon-coated silicon-based composite material prepared in Example 1 gradually decreases from nearly 100%, and finally stabilizes at about 97.8%, that is, the mass fraction of the silicon core and silicon dioxide particles in the silicon dioxide / carbon-coated silicon-based composite material prepared in Example 1 is 97.8%, and the mass fraction of carbon is 2.2%. Further combined with the composition information of the coating layer, it can be calculated that the mass fraction of the silicon dioxide / carbon composite coating layer is 12.9%, so the mass fraction of the silicon core in this material is 87.1%. Example 2 (blue curve): The mass fraction of the silicon dioxide / carbon-coated silicon-based composite material prepared in Example 2 begins to decrease from nearly 100%, and finally stabilizes at about 89.4%, that is, the mass fraction of the silicon core and silicon dioxide particles in the silicon dioxide / carbon-coated silicon-based composite material prepared in Example 2 is 89.4%. Further, it can be calculated that the mass fraction of the silicon dioxide / carbon composite coating layer is 62.0%, and the mass fraction of the silicon core is 38.0%. Example 3 (purple curve): The mass fraction of the silicon dioxide / carbon-coated silicon-based composite material prepared in Example 3 begins to decrease from nearly 100% and finally stabilizes at about 99.2%, that is, the mass fraction of the silicon core and silicon dioxide particles in the silicon dioxide / carbon-coated silicon-based composite material prepared in Example 3 is 99.2%. Using a similar method, the mass fraction of the silicon dioxide / carbon composite coating layer can be calculated to be 4.7%, and the mass fraction of the silicon core is 95.3%. .

[0111] Figure 8 The XPS-C1s, XPS-Si2p and XPS-O1s spectra of the silicon dioxide / carbon-coated silicon-based composite material according to Example 1 of the present invention are shown. As shown in the figure, the C 1s spectrum ( Figure 8 A) The horizontal axis binding energy ranges from 282eV to 292eV, and the vertical axis is intensity (au). The positions at 284.8eV, 286.3eV, and 288.6eV correspond to CC bond, CO bond, and O=C-OH bond, respectively; Si 2p spectrum ( Figure 8 C) The horizontal axis binding energy ranges from 98eV to 108eV, with 99.8eV and 103.2eV corresponding to Si (silicon) and Si 4 + (oxidized silicon); O 1s spectrum ( Figure 8 B) The horizontal axis binding energy ranges from 528eV to 536eV, with 532.8eV and 533.5eV corresponding to CO bonds and Si-O bonds, respectively. These fine spectra clearly show the bonding modes of each element.

[0112] Continue to refer Figure 9-11 , Fig. 9 and Fig.10The cycle performance diagrams at 1 A / g and 1 A / g current density when the silicon dioxide / carbon-coated silicon-based composite material according to Example 1 of the present invention is used as the negative electrode material of a lithium-ion battery are shown respectively, Fig.11 The cycle performance of the silicon-carbon composite material prepared according to the comparative example at current densities of 1A / g and 2A / g is shown. Fig. 9 As shown in the figure, the battery was activated with a low current of 0.2A / g in the first three cycles, and the first coulombic efficiency was 84.22%. From the fourth cycle, the current density increased to 1A / g, and the discharge capacity of the fourth cycle was 1252mAh / g. After 1000 cycles, the capacity was 928mAh / g, and the capacity retention rate was as high as 74.1%, fully demonstrating the excellent cycle stability of the material. Fig. 9 It can be seen that the first three cycles were activated with a small current of 0.5A / g, and the current density was adjusted to 2A / g from the fourth cycle. Among them, the discharge specific capacity of the fourth cycle was 813mAh / g. After 1500 cycles, its specific capacity can still be maintained at 529mAh / g, and the capacity retention rate reaches 65%, which shows that the material has excellent cycle stability. The silicon-carbon composite material prepared in the comparative example has a high initial specific capacity at a current density of 1A / g and 2A / g, but poor cycle stability. With the increase of the number of cycles, the specific capacity decreases rapidly, showing poor cycle performance (reference Fig.11 A and Fig.11 B).

[0113] Fig.12 The rate performance diagram of the silicon dioxide / carbon-coated silicon-based composite material as the negative electrode material of a lithium ion battery according to Example 1 of the present invention is shown. As shown in the figure, when the current density rises from 0.2A / g (discharge specific capacity is 2058mAh / g) to 2A / g (discharge specific capacity is 717mAh / g), at current densities of 0.2A / g, 0.5A / g, 1.0A / g, 1.5A / g and 2A / g, the material can show 2058, 1457, 1093, 860, 717mAh / g capacity respectively. This shows that the material has excellent rate performance and has broad application prospects. At the same time, although the specific capacity decreases with the increase of the number of cycles, a relatively stable specific capacity can be maintained at each current density, which further proves the cycle stability of the material.

[0114] This application innovatively uses nano or micro silicon particles as the core, and carefully constructs a composite coating layer composed of silicon dioxide particles with a diameter of 1-10nm and amorphous carbon on its surface. The composite coating layer plays a key role. On the one hand, it can effectively buffer the volume expansion of the silicon-based negative electrode during charging and discharging, effectively improve the cycle performance, greatly extend the service life of lithium-ion batteries compared to the prior art, and greatly reduce the battery performance degradation caused by volume changes; on the other hand, it also significantly enhances the ionic and electronic conductivity of silicon-based materials, improves the rate performance, and allows the battery to maintain excellent charge and discharge performance at different current densities, changing the drawbacks of similar materials in the prior art that the capacity drops sharply at high current density. In summary, the silicon dioxide / carbon-coated silicon-based composite material for lithium-ion batteries of the present invention successfully integrates the advantages of high reversible capacity, stable cycle performance, good rate performance, etc., comprehensively improves the comprehensive electrochemical performance of lithium-ion batteries, provides the industry with highly competitive material options, and accurately meets the current urgent demand for high-performance battery materials.

[0115] Obviously, those skilled in the art can make various modifications and changes to the embodiments of the present invention without departing from the spirit and scope of the present invention. In this way, if these modifications and changes are within the scope of the claims of the present invention and their equivalents, the present invention is also intended to cover these modifications and changes. The word "comprising" does not exclude the presence of other elements or steps not listed in the claims. The simple fact that certain measures are recorded in mutually different dependent claims does not indicate that the combination of these measures cannot be used to profit. Any reference numerals in the claims should not be considered to limit the scope.

Claims

1. A silicon dioxide / carbon-coated silicon-based composite material, characterized in that: The invention comprises a silicon core and a silicon dioxide / carbon composite coating layer coated on the surface of the silicon core. The silicon dioxide / carbon composite coating layer comprises a carbon matrix and silicon dioxide particles uniformly embedded in the carbon matrix.

2. The silicon dioxide / carbon-coated silicon-based composite material according to claim 1, characterized in that: The silicon core is nano-silicon or micro-silicon, and the carbon matrix is ​​amorphous carbon.

3. The silicon dioxide / carbon-coated silicon-based composite material according to claim 1, characterized in that: The average diameter of the silicon dioxide particles is 1-10 nm, and the particle size of the silicon core is 30 nm-10 μm.

4. The silicon dioxide / carbon-coated silicon-based composite material according to claim 1, characterized in that: The thickness of the silicon dioxide / carbon composite coating layer is 10-40 nm.

5. The silicon dioxide / carbon-coated silicon-based composite material according to claim 1, characterized in that: The mass fraction of the silicon dioxide particles in the silicon dioxide / carbon composite coating layer is 10%-95%, preferably 70%-95%.

6. The silicon dioxide / carbon-coated silicon-based composite material according to claim 1, characterized in that: The mass proportion of the silicon core in the silicon dioxide / carbon-coated silicon-based composite material is 20%-98%.

7. A method for preparing a silicon dioxide / carbon-coated silicon-based composite material, characterized in that: The preparation method comprises: S1, preparation of silicon / silicon dioxide / phenolic resin precursor; S2, sintering and carbonizing the silicon / silicon dioxide / phenolic resin precursor to obtain the silicon dioxide / carbon-coated silicon-based composite material.

8. The method for preparing the silicon dioxide / carbon-coated silicon-based composite material according to claim 7, characterized in that: The preparation steps of the silicon / silicon dioxide / phenolic resin precursor include: S11, dispersing silicon particles in ethanol, deionized water and ammonia solution, stirring at room temperature for 1-60 minutes to obtain a mixed solution 1; S12, dissolving resorcinol and formaldehyde in ethanol, adding the mixed solution 1, and stirring at room temperature for 0.1-3h to obtain a mixed solution 2; S13, adding tetrapropoxysilane to the mixed solution 2, stirring at room temperature for 1-50 hours, washing the reactant after centrifugation with water and anhydrous ethanol, and drying the reactant to obtain the silicon / silicon dioxide / phenolic resin precursor; Wherein, the mass ratio of the silicon particles, the resorcinol, the formaldehyde and the tetrapropoxysilane is 100:(3-50):(5-80):(25-800).

9. The method for preparing the silicon dioxide / carbon-coated silicon-based composite material according to claim 7, characterized in that: The step S2 includes placing the silicon / silicon dioxide / phenolic resin precursor in a tube furnace, and the carbonization process is processed according to the following procedures: S21, heating to 150°C at a rate of 2°C / min and keeping at 150°C for 2 h; S22, heating to 350°C at a rate of 2°C / min and keeping at this temperature for 2h; S23, heating to 850° C. at a heating rate of 2° C. / min, keeping the temperature for 4 hours, and cooling to obtain the silicon dioxide / carbon-coated silicon-based composite material; or S21', heating to 800°C at a heating rate of 5°C / min, and keeping the temperature at 800°C for 6 hours, and after cooling, obtaining the silicon dioxide / carbon-coated silicon-based composite material.

10. Use of the silica / carbon-coated silicon-based composite material according to any one of claims 1 to 6 or the silica / carbon-coated silicon-based composite material prepared by the method for preparing the silica / carbon-coated silicon-based composite material according to any one of claims 7 to 9 as a negative electrode material for lithium-ion batteries.

Images