Silicon composite for anode material, preparing method therefor, anode comprising same for secondary battery, and secondary battery comprising same

Abstract

A silicon composite according to various embodiments of the present invention may comprise: pure silicon particles; and a buffer layer thin film disposed on the surface of the pure silicon particles. A method for preparing a silicon composite according to various embodiments of the present invention may comprise the steps of: pulverizing metallic silicon particles; and forming a buffer layer thin film on the surface of the pulverized metallic silicon particles. An anode for a secondary battery according to various embodiments of the present invention may comprise the silicon composite. A secondary battery according to various embodiments of the present invention may comprise the anode.

Description

Silicon composite for cathode material, method for manufacturing same, cathode for secondary battery including same, and secondary battery including same Various embodiments of the present invention relate to a silicon composite for use as a negative electrode material, a method for producing the same, a negative electrode for a secondary battery comprising the same, and a secondary battery comprising the same. Specifically, the present invention relates to a silicon composite having a buffer layer thin film that can be used in a lithium secondary battery, a method for producing the same, a negative electrode for a secondary battery comprising the same, and a secondary battery comprising the same. One of the key issues in electrochemistry is improving the performance of electric vehicles, particularly their driving range and charging speed. To address the low battery capacity of graphite, the anode material commonly used in lithium secondary batteries, researchers have explored various options and concluded that pure silicon anode materials are a game-changer for secondary batteries. Silicon is one of the most abundant elements in the Earth's crust, making up over 25% of the total mass, second only to oxygen. It possesses a theoretical capacity nearly ten times greater than that of conventional graphite anode materials, and its specific gravity of 2.33 is similar to that of graphite, 2.25. This allows it to meet the price, capacity, and weight requirements of electric vehicle secondary batteries. Among silicon sources, metallic silicon is obtained by melting quartz sand with a carbon-based reducing agent at high temperatures, then solidifying and crushing the solidified mass. Metallic silicon is readily available, and is already widely used as a raw material in industries such as silicon ingots for semiconductors and solar cells, and in steel and aluminum alloys. However, when silicon anode materials in various shapes, such as particles, rods, flakes, and porous materials including hollow and porous materials, were applied to lithium secondary batteries and tested, a degradation problem arose in which the battery capacity rapidly decreased to a level lower than that of graphite after only a few charge / discharge cycles (hereinafter, in the present invention, particles, rods, flakes, and porous materials are used as expressions that are not limited to the shape of the material, meaning materials whose aspect ratios are equivalent to those of the silicon material in thickness and diameter). The cause of the decrease in battery capacity of silicon anode materials is being identified through the efforts of many researchers. While graphite expands up to 15% after lithiation, silicon anode materials expand up to 15% during lithiation. 1.7 Si, Li2Si, Li 2.3 Si, Li 2.8 Si, Li 3.25 Si, Li 3.75 Si, Li 4.4 Li such as Si x The Si phase is formed, and during charge and discharge, lithium and fluorine continue to combine, expanding up to three times or more. This volume change acts as stress on the surface of the silicon particles, pulverizing them. During this process, the newly formed silicon surface due to pulverizing reacts with the electrolyte to form an irreversible compound, so it is known that the anions of the electrolyte and lithium ions are continuously consumed, which reduces the battery capacity. In addition, it was revealed that during this process, lithium dendrites that were growing from the current collector surface fall off during discharge, and contact with surrounding particles is cut off, and the number of lithium that is isolated in the circuit (dead lithium) increases, which is also a cause of the decrease in battery capacity. To address these issues, electrolytes have been designed by combining organic solvents with low reactivity with silicon, and after injecting the electrolyte, sufficient time is provided for the formation of a stable solid electrolyte (SEI) through a chemical reaction that maintains the electrolyte at room temperature and aging at a low current, thereby suppressing excessive reaction with the electrolyte (Kong et al., Molecules 28, 2023, 2079; Toki et al., Ind. Chem. Mater., 2024, Advanced Article). However, despite these efforts, it has not been easy to prevent excessive consumption of the electrolyte by reacting with the silicon anode material during the electrochemical reaction. To improve this, efforts have been made to increase battery life by using silicon compounds such as silicon oxide, silicon carbide, and silicon nitride, which have low lithium storage properties, instead of pure silicon. However, these silicon compounds have the disadvantage of not only having low lithium storage properties but also low ionic and electronic conductivity compared to pure silicon, and increasing manufacturing costs. In the present invention, ionic conductivity refers to the conductivity of cations such as alkali metals or alkaline earth metals belonging to Groups 1 and 2 of the periodic table, such as lithium, sodium, or magnesium, within a material, and electronic conductivity is used in a similar sense to electrical conductivity, which refers to the flow of electrons or current. To improve lithium storage properties and conductivity, a process is being added to form a metal complex compound layer that is thermochemically stable and has excellent ionic conductivity by bonding a metal such as magnesium to the surface of the silicon compound. In addition, a method of mixing carbon nanotubes, activated carbon, or conductive polymer additives is being applied to supplement the low electronic conductivity of silicon compounds. In an effort to address the degradation phenomenon of silicon anode materials, it was found that silicon particles undergo pulverization due to volume expansion caused by the first lithiation that starts at the particle surface when lithiated by an electrochemical reaction until they reach a size of 200 nm, and that the volume change stress of silicon is significantly reduced when the initial silicon particle or wire diameter is less than 150 nm. Furthermore, as the particle size decreases to less than 30 nm, 4-6 nm, the pulverization phenomenon is reduced, and the battery capacity decrease is not severe during long-term cycling. In fact, a case of improving battery capacity by depositing nano-silicon on a nickel thin-plate current collector and using it as an anode has been reported (Ohara et al., Journal of Power Sources 136, 2004, pp. 303-306). Although silicon thin-plate anodes made in this way have excellent battery capacity retention ratios, it was found that the improvement effect disappears when the film thickness is increased. And as the silicon anode material particles become finer, the specific surface area increases, which not only lowers the initial Coulombic efficiency (ICE), but also significantly reduces the battery capacity to less than 40% of the theoretical capacity. In order to obtain these nano silicon particles, it is difficult to mass-produce them in terms of cost because it requires a high-cost process of reacting gaseous or liquid silicon precursors in a high-temperature vacuum device to precipitate nano silicon particles or etching silicon wafers with acid to obtain nanowires (Je et al., Accounts of chemical research 56, 2023, pp.2213-2224; Kong et al., Molecules 28, 2023, 2079; Keller et al., Nanomaterials 11, 2021, 307; Wang et al., Nano Energy 78, 2020, 105101; Meng et al., Green Energy & Environment 5, 2020, pp.22-36; Kim et al., Angewandte Chemie Int. Ed. 49, 2010, pp.2146 - 2149). In general, it is difficult to expect high electronic conductivity from silicon, and its ionic conductivity is known to be limited in the crystal structure of complex oxides combined with other metals or at grain boundaries. The electrical conductivity of silicon is about 6.7 x 10 -4 S / cm, and the electrical conductivity of silicon oxide is 1.65 x 10 -10 The electrical conductivity of silicon compounds in S / cm tends to be lower than that of pure silicon. The reason why the electrical conductivity of silicon compounds is lower than that of silicon is because their band gaps are larger. On the other hand, the ionic conductivity increases due to the tendency for the lattice constant to increase as the silicon compound becomes larger. The band gap varies depending on the crystal structure and the number of atomic sites, but generally, silicon has the smallest band gap, followed by silicon carbide, silicon nitride, silicon oxide, and silicon fluoride. Therefore, it is desirable to form a compound layer for the buffer layer that has a smaller band gap than the existing silicon compound and is less prone to forming the irreversible Li2SiF6 phase. Conventionally, silicon compounds are manufactured by methods such as vapor deposition, high-temperature solid-state synthesis, alloying, and electrostatic spinning. Among them, SiO is produced by reacting molten metal silicon with quartz at a high temperature of about 1,600℃ or higher, or by reacting quartz with a gas phase. x , SiN xBy synthesizing with the like, compounds with a narrower band gap could be obtained. For example, SiO negative electrode material has a smaller lattice constant and stronger metallicity due to fewer oxygen atoms than SiO2, so not only electrical conductivity but also ionic conductivity is higher than SiO2. However, SiO silicon oxide reacts with lithium ions during initial charging to generate irreversible Li2O, which expands in volume and reduces battery capacity, resulting in a lower initial Coulombic efficiency (ICE). In addition, the method of obtaining silicon oxide by reacting metallic silicon or quartz molten metal with gas at high temperatures has the disadvantage of a short lifespan of process equipment and requiring a lot of investment to increase productivity. Silicon nitride is obtained by heat-treating silicon particles in a nitrogen atmosphere at a high temperature of 1,000°C or higher to obtain silicon particles with a nitride layer, or by reacting a silicon gas precursor with ammonia decomposition gas under high power in a CVD device to obtain SiN. x Obtain the compound SiN x Silicon nitride reacts with lithium in an electrochemical reaction to form Si3N4 and Li3N or Li2SiN2, which is known to be a reversible reaction. However, SiN x When the ratio of x is as low as 0.32, it exhibits a high battery capacity of 2,300 mAh / g, but at 0.69, it drops sharply to less than 80 mAh / g. When the ratio of x is 0.92, if the film thickness is 200 nm, it maintains a battery capacity of 1,300 mAh / g after 100 cycles, but at a thickness of 500 nm, it exhibits a low battery capacity of 700 mAh / g, which is not stable (Ulvestad et al., Nature scientific reports 8, 2018, 8634). The method of synthesizing compounds at high temperatures using silicon gas precursors simplifies the process, but has low cost competitiveness due to the expensive gas precursor, and it is not easy to control the thickness of the surface compound layer of nanoparticles at high temperatures. In addition, the negative electrode material composed of nanoparticles has the problem that the battery capacity decreases as the electrode layer thickness increases. The method of alloying transition metals in a metal silicon melt at high temperatures and then squeeze casting them with fibrous wires or using an atomizing method to obtain fine alloy powders has a somewhat lower process cost compared to using a gas precursor, but has the disadvantage of requiring particles or fibrous wires with a diameter of several tens of microns to be crushed again to prepare them in a size that can be used as a cathode material. Looking at this content, it can be seen that conventional silicon compound cathode materials have the disadvantage of requiring high process costs and making it difficult to control the compound layer on the surface. In Korean Patent No. 10-2309264, a method for manufacturing a pure silicon anode material using a simpler process compared to silicon compounds was proposed. Several micron metallic silicon was pulverized to a size of 100-200 nm, and this was heat-treated in air at 350-450°C for 2 hours to form a 5-10 nm thick SiO layer on the surface of the nano silicon powder. x(0.5 < x < 1.2) and milling it with about 25% weight of graphite powder to form a graphene layer on the surface. This method uses an atmospheric oxidation method to form an oxide film on the surface of silicon particles by controlling the temperature and time, which has the effect of significantly reducing the process cost. However, when pulverized silicon powder is heated in the atmosphere, the heating temperature and the exposure to oxygen inevitably differ depending on the location of the powder within the bulk container. Therefore, silicon powder exposed to the surface is easily exposed to the atmosphere, so it heats quickly and is easily oxidized, but silicon powder inside the bulk container not only has a large heating temperature depending on the location, but also has different exposure to atmospheric oxygen, so the degree of oxidation varies depending on the location of the powder. Looking more closely, even in a single powder particle, the temperature and oxygen concentration vary due to contact with surrounding particles, which causes the thickness and composition of the surface oxide layer to vary. The problem with the atmospheric oxidation method is that the non-uniformity in the composition and thickness of the silicon oxide film leads to non-uniformity in quality and performance. If particles with an uneven oxide layer are used as a cathode material, electrolyte consumption increases and it is difficult to maintain a constant battery capacity. Another example, U.S. Patent No. 9,077,029 discloses a silicon anode active material comprising oxygen atoms in a weight ratio of approximately 5% to 36%. The silicon powder is sputtered at high energy in an oxygen-mixed gas atmosphere, wet ball-milled with an ethanol solvent, or ball-milled in an oxygen-mixed gas atmosphere to form an oxide film about 200 nm thick. However, such a thick oxide film significantly reduces electrical conductivity and makes it difficult for the initial coulombic efficiency to exceed 80%. Chinese Patent Publication No. CN112366301A discloses a method of crushing silicon particles, oxidizing them, acid-cleaning them to remove impurities, and then coating an organic carbon source and coating the carbon layer using high-temperature pyrolysis. However, this method has the disadvantage that the buffer layer is damaged when impurities dissolve and escape during the acid-cleaning process, thereby offsetting the silicon particle protection effect, so a thick carbon protective layer must be applied afterwards. Korean Patent No. 10-2334001 and Patent Publication No. 10-2023-0023529 propose nano-silicon particles in which a polyacrylonitrile (PAN) polymer layer is thickly applied to the surface of silicon or silicon compound particles, with a weight ratio of approximately 1 / 3, and cross-linked through low-temperature heat treatment or carbonized at high temperatures of 500–1000°C. The carbonized layer on the surface of silicon particles produced by high-temperature pyrolysis makes silicon particles more stable than the oxide film formed by atmospheric oxidation. However, the process of applying the polymer layer to the silicon particles involves ultrasonic dispersion or multiple stages of stirring and centrifugation to disperse organic substances in a solvent, thereby forming a polymer layer on the surface of the particles. This is followed by heat treatment at 110–300°C to form cross-links on the surface of the polymer layer. Additionally, this is heat treated at 500–1,250°C to carbonize the polymer layer.In addition, instead of polyacrylonitrile polymers, there are those that carbonize by coating organic materials such as PVP, PAA, Li-PAA, PVDF, CMC, polypyrrole, PEDOT, polyaniline, polyethylene, PVA, PVAc, PMMA, etc. at a weight ratio of 30% or more on silicon particles. In this way, in conventional technologies, in order to form a stable buffer layer on silicon particles, expensive equipment is used to form a thick carbonized film, or multiple processes are required for a long time, which inevitably increases the manufacturing cost. The current industrial demand, on the contrary, requires an anode material that has both price competitiveness and performance, so there is a need to provide a material that can be used as a silicon anode material through a simple process with inexpensive raw materials. The purpose of the present invention is to provide a silicon anode material that improves the problem of pure silicon anode materials not being able to maintain battery capacity during repeated charge and discharge by forming a buffer layer thin film on the surface of pure silicon particles, thereby stably maintaining battery capacity and suppressing excessive reaction between silicon and electrolyte. Specifically, by forming a thin buffer layer thin film with an oxidizing agent on the surface of pure silicon particles, excessive lithium ion insertion is suppressed, thereby minimizing the pulverization phenomenon due to volume change of silicon particles, and suppressing the tendency of the silicon particle surface to form an irreversible compound when reacting with the electrolyte, thereby suppressing continuous electrolyte consumption during repeated charge and discharge processes. In particular, since electrolytes containing fluorine and lithium ions under electrochemical reactions have a strong tendency to form an irreversible Li2SiF6 phase on the silicon surface, suppressing this reaction and forming a more stable lithium silicate compound thin film as a buffer layer during pre-lithiation, initial formation, or initial charge and discharge is a way to utilize pure silicon as a stable anode material. A silicon composite according to various embodiments of the present invention may include silicon particles; and a buffer layer thin film disposed on the surface of the silicon particles. A method for manufacturing a silicon composite according to various embodiments of the present invention may include a step of crushing silicon particles; and a step of forming a buffer layer thin film on the surface of the silicon particles. A negative electrode for a secondary battery according to various embodiments of the present invention may include the silicon composite. A secondary battery according to various embodiments of the present invention may include the negative electrode. The silicon composite for anode material of the present invention forms a non-stoichiometric silicon oxide film as a buffer layer on the surface of pure silicon particles through a simple oxidation process, and forms a lithium silicate film prior to the formation of fluoride in the silicon anode material, thereby improving electrochemical stability. In addition, a stabilizing layer that increases conductivity and controls surface charge is applied on the buffer layer, thereby ensuring good dispersion of silicon in the anode material slurry and improving the movement speed of metal cations, thereby increasing the charge / discharge speed and improving the durability life. Figure 1 is a process flow diagram of a method for manufacturing a silicon composite negative electrode material according to various embodiments of the present invention. Figure 2 is a schematic diagram of a method for manufacturing a silicon composite anode material and a silicon composite anode material according to various embodiments of the present invention. FIG. 3 is a drawing for explaining the interaction between beads and metal silicon particles within a container during milling as an example of applying mechanical shear energy to silicon particles in the manufacturing method of the present invention. Fig. 4a is a photograph of Example 2 of the present invention observed at a magnification of 500, Fig. 4b is a photograph observed at a magnification of 1000, Fig. 4c is a photograph observed at a magnification of 5000, and Fig. 4d is a photograph observed at a magnification of 10000. FIG. 5a is a photograph of Example 7 of the present invention observed at a magnification of 500, FIG. 5b is a photograph observed at a magnification of 1000, FIG. 5c is a photograph observed at a magnification of 5000, and FIG. 5d is a photograph observed at a magnification of 10000. Figure 6a shows the results of XRD analysis of Example 1 of the present invention. Figure 6b shows the results of XRD analysis of Example 2 of the present invention. Figure 7a shows the results of a battery test using a negative electrode material mixed with 10% of Example 1 of the present invention. Figure 7b shows the results of a battery test using a negative electrode material mixed with 10% of Example 2 of the present invention. Figure 8a shows the results of a battery test using a negative electrode material mixed with 20% of Example 1 of the present invention. Figure 8b shows the results of a battery test using a negative electrode material mixed with 20% of Example 2 of the present invention. Hereinafter, various embodiments of this document are described with reference to the attached drawings. The embodiments and terminology used herein are not intended to limit the technology described in this document to specific embodiments, but should be understood to encompass various modifications, equivalents, and / or alternatives of the embodiments. silicone composite Various embodiments of the present invention relate to a silicon composite that can be used as a cathode material. The silicon composite of the present invention can be used as a pure silicon cathode material. In the present invention, the pure silicon anode material is not limited by shape, such as a zero-dimensional nanoparticle, a one-dimensional rod, a two-dimensional flake, or a three-dimensional polyhedron, and, unless specifically limited in meaning, can be used as a pure silicon anode material even if the modifier 'pure' is omitted. A silicon composite according to various embodiments of the present invention includes silicon particles and a buffer layer thin film. The silicon particles may be rod-shaped or flake-shaped with an average particle size of 10 to 1000 nm, or may have an average size of 10 to 1000 nm as calculated from the XRD peak using the Scherrer equation (see FIGS. 4, 5, and 6). Here, the size refers to the diameter or thickness. Through this size, the silicon composite of the present invention can exhibit high lithium ion storage capacity when used as an anode material for an energy storage device. Specifically, the silicon particles may have a Gaussian distribution of particles having a size of 5 to 2,000 nm. More specifically, silicon particles having an average particle size of 10 to 1,000 nm may account for 85% or more of the total silicon particles, silicon particles having a size of less than 10 nm may account for less than 5%, and silicon particles having a size of greater than 1,000 nm may account for less than 10%. Preferably, the particle distribution may include particles having a volume ratio of 1 to 10% of particles having a volume ratio of 10 to 35 nm, particles having a volume ratio of greater than 35 to 65 nm may account for 45 to 95%, particles having a volume ratio of greater than 65 to 100 nm may account for 3 to 45%, and particles having a volume ratio of greater than 100 nm may account for 0 to 15%. When the average particle size of the silicon particles is less than 10 nm, it does not significantly contribute to increasing battery capacity, and when it exceeds 1,000 nm, the volume change during charge and discharge may be large, which may reduce the battery capacity retention rate. The buffer layer thin film of the present invention can be positioned on the surface of silicon particles. The buffer layer thin film can protect the silicon particles from rapid reactions with metal cations or electrolytes, thereby suppressing excessive volume changes. In addition, the buffer layer thin film can resolve the problem of uneven oxidation of the silicon particles and achieve excellent ionic conductivity. The buffer layer thin film may include any one of a silicon oxide thin film, a silicon nitride thin film, and a composite thin film thereof. The buffer layer thin film may be SiO x Non-stoichiometric silicon oxide thin film, SiN, having the chemical formula (0.5 < x < 1.2)x (0.1 ≤ x < 1) It may include any one of a non-stoichiometric silicon nitride thin film having a chemical formula and a composite thin film thereof. Preferably, the buffer layer thin film may include SiO silicon oxide. Since the buffer layer thin film includes a silicon compound, the electrochemical stability of the silicon anode material may be improved. Additionally, a stable SEI can be formed on a pure silicon surface through a buffer layer thin film. What is known about the SEI phase that is formed when the silicon anode material reacts with the electrolyte is lithium fluoride (LiF) that passivates the silicon surface, lithium oxide (Li2O) that is formed by reacting with the oxide film existing on the silicon surface, organic compounds that are formed by the decomposition of the electrolyte and organic additives, and silicon oxide (SiO) that is formed by free oxygen during the charging process. x ) exist. They are known to help form a stable SEI by suppressing the reaction between silicon and lithium ions or halogen ions of the electrolyte. However, in reality, pure silicon reacts with fluoride electrolytes, such as LiPF6, during the electrochemical reaction process to continuously form irreversible halides, especially irreversible and highly electrically insulating porous fluorides such as Li2SiF6, which passivate the surface of the silicon anode material, leading to a rapid decrease in battery capacity in only 1 to 3 cycles (Yu et al., Nano Letter, 2019, 19, pp. 5124-5132). For this reason, the development of pure silicon anode materials has been difficult, and anode materials using silicon compounds that form fewer irreversible compounds than pure silicon have been commercialized first. Equations (1) through (4) below are the results of confirming the reaction that generates irreversible fluoride between silicon-based anode materials and the electrolyte during the process of the present invention. Looking at the order of reaction energy, reaction (1) between pure silicon and the electrolyte occurs faster than reactions (2), (3), and (4) between silicon compounds and the electrolyte. In other words, because pure silicon has a stronger tendency to form irreversible fluoride than silicon compounds such as silicon carbide, silicon nitride, or silicon oxide, the development of anode materials utilizing silicon compounds instead of pure silicon could be achieved more quickly. Si + 2Li + 3F2 = Li2SiF6(1) SiC + 2Li + 3F2= Li2SiF6+ C (2) 1 / 3Si3N4+ 2Li + 3F2=Li2SiF6+ 2 / 3N2(3) SiO2+ 2Li + 3F2= Li2SiF6+ O2(4) The silicon compounds of equations (2) to (4) have a lower reactivity with fluorine than pure silicon, which reduces the tendency to form irreversible fluorides. However, the band gap (8.08 eV) and formation energy (-3.408 eV / atom) of lithium-silicon fluoride are higher than those of lithium silicates (4.66 to 5.35 eV) and formation energy (-2.63 to -3.01 eV / atom), and their electronic and ionic conductivities are low, so the formation of lithium-silicon fluoride lowers the initial coulombic efficiency, which results in a decrease in battery capacity. Therefore, since silicon compounds exhibit about half the battery capacity of pure silicon, twice the amount must be mixed to exhibit the same capacity as pure silicon. Also, when comparing the reaction (1) that produces lithium-silicon fluoride within the battery with the reactions (5) to (15) of lithium-only fluoride or carbon fluoride, the tendency for lithium-only fluoride, carbon fluoride, or lithium-silicon alloy to be produced before silicon oxide-derived lithium silicate is low. In addition, the reaction in which lithium ions diffused inside graphite and silicon during charging are separated again during discharge has a lower reaction energy than the reaction that forms oxides or fluorides within the battery. These reactions are listed in the order of their energy occurrence, assuming that the material size and crystallization energy are the same, and comparing only the reaction energies of pure materials. 2SiO + 2Li + 1.5O2= Li2Si2O5(5) SiO + 4Li + 1.5O2= Li4SiO4(6) SiO + 2Li + 3F2= Li2SiF6+ 0.5O2(7) SiO + 2Li + O2 = Li2SiO3(8) SiO2+ 4Li + O2= Li4SiO4(9) C + F2 = CF2(10) 2SiO2+ 2Li + 0.5O2=Li2Si2O5(11) SiO2+ 2Li + 0.5O2=Li2SiO3(12) Li + 0.5F2= LiF (13) C + 2F2 = CF4(14) Li x Si = xLi + + Li0Si + xe - (15) Li x C6= xLi + + Li0C6+ xe - (16) Comparing the above equations, it can be seen that when a SiO, SiO2 film is formed on the surface of a silicon particle, the lithium-silicon oxide acts as a buffer layer to protect it from the subsequent attack of fluorine that tries to continuously react. In addition, it can be seen that non-stoichiometric SiO has a stronger tendency to react with lithium than stoichiometric SiO2 oxide and form a stable lithium silicate film such as Li2Si2O5, Li2SiO3, or Li4SiO4 on the surface of the silicon particle, which can suppress the formation of Li2SiF6 fluoride. Against this backdrop, the present invention suppresses the formation of irreversible fluoride on the surface of pure silicon by forming a buffer layer thin film with one or a composite of non-stoichiometric silicon oxides as a means of forming a stable solid electrolyte (Soild Electrolyte Interface) on the surface of pure silicon. As a result, in the pre-lithiation, initial formation process, or initial charge / discharge, the composite oxides such as Li2Si2O5, Li2SiO3, Li4SiO4 react first with lithium to form a solid electrolyte (SEI) with a chemically stable lithium silicate composition on the surface of the silicon particles as a buffer layer thin film, thereby obtaining an inexpensive and high-performance anode material. The Li2Si2O5, Li2SiO3, Li4SiO4 layer has high ionic conductivity and does not significantly reduce electrical conductivity when in the form of a thin film, making it suitable as a buffer layer. When forming a buffer layer, depending on the type of oxidizing agent used in the chemical surface treatment, the buffer layer may be formed as a silicon oxide thin film, a silicon nitride thin film, or a composite thin film thereof. Therefore, excellent ionic conductivity and electrical conductivity can be secured through the buffer layer, and the reaction can occur preferentially in competition with the irreversible Li2SiF6 compound formation reaction during the electrochemical reaction, thereby suppressing irreversible fluoride formation. In the present invention, the term "ionic conductivity" can be understood as an expression of a wide range of ionic conductivity, such as "conductivity of alkali metal cations" or "diffusion in a cathode material" without any particular limitation. Meanwhile, the silicon composite of the present invention may additionally have a stabilizing layer disposed on the buffer layer thin film. The stabilizing layer improves the slurry dispersibility and conductivity of the silicon particles having the buffer layer, and promotes the silicon particles to react with the electrolyte to form a stable solid electrolyte (SEI), thereby enabling the silicon composite to exhibit excellent properties after chemical treatment as an anode material. Due to these properties, the silicon composite of the present invention achieves a higher initial coulombic efficiency than existing silicon compound anode materials, and by mixing it with existing graphite anode materials, the silicon content can be determined according to the driving range or required battery capacity. The stabilizing layer may include at least one selected from the group consisting of carbonaceous materials, fluorides, phosphides, hydroxides, iodides, nitrates, and organic acids. Cathode material using silicon composite In the present invention, the pure silicon anode material uses particles having a purity of 95% or higher and a size of 4 mm or less, preferably a purity of 98% or higher and a size of 100 mesh (approximately 150 microns) or less, for easy quality control in mass production. When the pure silicon is derived from silicon wafer scrap for semiconductors or solar cells, the purity is as high as 99.9999% or higher. In addition, the use of nano-silicon synthesized from gaseous or liquid silicon precursors may also be included in the scope of the present invention. In addition, the silicon raw material of the present invention is not greatly restricted in purity and size, and can be widely used from high-purity wafer scrap to relatively low-purity metallic silicon. Among silicon raw materials, the purity of metallic silicon, which is the cheapest, is approximately 95% or higher, and there are various grades up to 99% purity. The main impurities in metallic silicon are iron, aluminum, and calcium, which are elements with a higher electronegativity potential than lithium. Therefore, when these components are alloyed with metallic silicon, they do not interfere with the electrochemical reaction of lithium ions, making the electrochemical reaction insensitive to metallic impurities. Furthermore, since they can be supplied in a pulverized state from the supplier, the burden of the pulverization process can be minimized, which is another advantage. Hereinafter, the present invention will be described with reference to an example in which readily available metallic silicon is used as a raw material for the pure silicon anode material. As previously explained, even if the raw material is changed to high-purity wafer scrap, there is no significant difference in the appearance or performance of the pure silicon anode material. When the size is less than tens of nanometers, the volume expansion of the silicon particles is not large, but the fraction of SEI formed on the surface is large compared to the particle weight, so the decrease in battery capacity is large. In addition, when the size is greater than hundreds of nanometers, the fraction of SEI formed is small compared to the particle weight, but the amount of lithium ions that pass through the buffer layer thin film and penetrate into the silicon particles is less than the lithium amount of the maximum compound. In the present invention, the particles obtained by crushing raw silicon by applying various mechanical shear energies are a material having a Gaussian distribution with a size of about 5 to 2000 nm, and the material having an average diameter or thickness of 10 to 1000 nm is 85% or more, the remainder is composed of less than 5% of particles having a size of less than 10 nm, and less than 10% of particles having a size of more than 1000 nm, and is preferably characterized by an average diameter or thickness of 20 to 250 nm. The pure silicon anode material of the present invention can form a buffer layer thin film on the surface to reduce the extreme reactivity with metal cations or electrolytes, and a stabilizing layer to improve slurry dispersibility and conductivity can be applied thereon. Industrial uses of silicon anode materials According to the principles described above, the anode material of the present invention, which comprises a buffer layer thin film on pure silicon, can stably repeat charge and discharge in an electrochemical reaction, thereby improving initial coulombic efficiency and cation storage capacity. In particular, in lithium secondary batteries, it can exhibit the same level of performance with a smaller amount compared to conventional silicon compound anode materials such as silicon oxide, silicon carbide, and silicon nitride.

[0068] The silicon anode material of the present invention is manufactured by applying mechanical shear energy such as collision with beads, friction, stirring, and pressure to crush inexpensive metal silicon as a raw material, and chemically surface treating the silicon anode material with only an oxidizer without a separate heat treatment in a container, thereby forming a buffer layer thin film. Therefore, the process is simple, so that expensive high-temperature heat treatment equipment or expensive precursors are not required, and since the silicon particles deteriorate little, a high battery capacity can be obtained with a small amount. In addition, the silicon anode material of the present invention manufactured in this way can be an alternative that meets the demand for a price reduction of silicon anode materials to realize low-cost secondary batteries, which is an urgent task in the popularization of electric vehicles. Method for manufacturing silicone composites Referring to FIG. 1, a method for manufacturing a silicon composite according to various embodiments of the present invention may include a step of crushing silicon particles (S100) and a step of forming a buffer layer thin film (S200). After the step of forming the buffer layer thin film (S200), a step of forming a stabilizing layer (S300) may be further included. The method for manufacturing a silicon composite of the present invention can easily form a buffer layer thin film on the surface of pure silicon particles by chemical surface treatment, thereby suppressing a reaction that excessively forms an irreversible compound even when pure silicon particles are exposed to an electrochemical environment where cations and halogen ions coexist as an anode material, thereby preventing a phenomenon in which battery capacity decreases due to excessive consumption of electrolyte, and maintaining the battery capacity of the silicon anode material. Below, each step is explained in detail. (1) Step of crushing silicon particles (S100) First, in the step of crushing silicon particles (S100), hard beads made of alumina, zirconia, etc. are mixed with the metal silicon material and crushed by applying mechanical shear energy. Specifically, milling can be performed to mix hard beads with metallic silicon powder and apply mechanical shear energy to pulverize the particles. This can result in a silicon composite material with a micronized diameter or thickness of 10 to 1,000 nm on average. Referring to FIG. 2, the buffer layer thin film of the silicon composite according to various embodiments of the present invention is mainly composed of metallic silicon and can suppress continuous reaction with the electrolyte by surface-treating the particle surface with an oxidizing agent. The role of the oxidizing agent is to provide electrons to silicon to oxidize it, and contains at least one of active oxygen, nitrogen, boron, phosphorus, sulfur, or halogen elements or acid functional groups. In the XRD analysis of the silicon composite of the present invention, when the ratio of peak height to width is calculated by the Scherrer equation, the silicon particles that are pulverized to an average of 10 to 1000 nm are the main component. The present invention manufactures a silicon composite for an anode material by applying mechanical shear energy to pulverize silicon using inexpensive metallic silicon as a raw material and then performing a chemical surface treatment, so that the cost can be reduced because an expensive gaseous or liquid silicon precursor or advanced equipment is not required, and a high battery capacity can be obtained with a small amount. In the step of crushing silicon particles (S100), milling can be performed to crush metal silicon powder by mixing hard beads and applying mechanical shear energy. Any raw material that can be crushed as metal silicon powder can be used, but considering the milling time, particles having a size of preferably 20 ㎛ to 4 mm, or preferably 45 to 250 ㎛, can be used. However, since the particle size can be refined through the crushing process, particles of various sizes can be used as raw materials without being restricted by this size. In the step of crushing silicon particles (S100), milling may be selected as a method of applying mechanical shear energy to metal silicon particles. For milling, silicon particles and beads are mixed and placed in a milling container. The weight ratio of beads may be 50 to 95%, preferably 60 to 86%, of the total weight of the mixture of beads and silicon. The remainder is silicon particles. If the weight ratio of beads is less than 50%, the time required for crushing the silicon mixture may be excessively increased, and if the weight ratio of beads is more than 95%, the amount of silicon may be small, resulting in excessively low productivity. The amount of the beads and silicon mixture loaded into the milling vessel varies depending on the milling method, but is loaded in an amount no greater than 85% of the internal volume of the milling vessel, and preferably no greater than 75%. If the volume of the mixture exceeds 85% of the internal volume, it is difficult for the beads to roll inside the milling vessel when it rotates, which reduces the collision energy applied to the silicon particles between the beads, and may result in insufficient space to cushion the volume increase due to milling. This reduces the probability of shear energy acting between the beads and silicon particles, making milling difficult. Therefore, the amount of the mixture loaded is preferably no greater than 85% of the internal volume. In addition, if the amount of the beads and silicon mixture loaded is less than 25% of the internal volume of the milling vessel, the beads and silicon mixture may stick to the inner wall of the milling vessel and rotate as one body, preventing milling. Therefore, the amount is preferably 25% or more. More preferably, the beads and silicon mixture may be loaded in an amount no less than 30% and no more than 75% of the internal volume of the milling vessel. Table 1 below shows examples of particle sizes of beads and milled silicon. Bead diameter (mm)Bead force ratioMilled particle size (㎛)Size ratio of Bead / Particle0.42.0x10 -3 0.2416670.81.6x10 -2 0.988161.61.3x10 -1 3.914093.21.015.62056.48.062.510212.7642505125.4512100025 Meanwhile, in the milling step, grinding can be performed from beginning to end, but to increase efficiency, it can be divided into primary milling and secondary milling. The main purpose of primary milling is to uniformly disperse the silicon particles between the beads when the silicon and bead mixture is mixed. The optimal rotation speed for primary milling is determined by covering the milling container with a transparent temporary cover and observing with the naked eye or by the sound heard from the milling container. When the beads are seen rolling in an inclined manner or the sound of the beads hitting the wall of the milling container is heard, the maximum rotation speed for primary milling is that speed. When comparing the primary milling conditions with the rotation speed of the milling container, it varies depending on the size and method of the milling container, but for a planetary mill with a capacity of 250 ml to 500 ml, it is satisfied when it is about 150 to 340 rpm, preferably about 200 to 300 rpm. Primary milling can be performed for a time accounting for 5% to 30% of the total milling time. If the primary milling time is less than 5%, the silicon particle size distribution may become excessively broad due to insufficient dispersion, which may degrade the performance of the silicon anode material. Meanwhile, if the primary milling time exceeds 30%, production efficiency may decline. Preferably, the primary milling time should be between 10% and 25% of the total milling time. The main purpose of secondary milling is to pulverize silicon particles by colliding and friction with beads. The optimum rotation speed is determined by covering the milling container with a transparent temporary cover and visually observing or by the sound heard from the milling container. When the beads begin to rub against or hit the walls of the milling container, the minimum rotation speed for secondary milling is determined. Then, the speed is gradually increased until the sound of beads rubbing against each other or hitting the walls of the milling container is barely audible, which is the maximum rotation speed for secondary milling. When comparing the rotation speed of the milling container, the conditions for secondary milling vary depending on the size and method of the milling container, but for a planetary mill with a capacity of 250 ml to 500 ml, it is satisfied when it is about 300 to 550 rpm, preferably about 350 to 450 rpm. The secondary milling time varies depending on the loading amount, as the probability of collision between particles and beads decreases as the loading amount increases. Meanwhile, the process of crushing the charged material within the milling container is as follows. When silicon and beads are mixed and loaded into a milling container and the mixture is moved by rotating the milling container or rotating the bar, the beads, which are relatively large and heavy compared to the silicon particles, roll and rub against each other, applying a shear force to the silicon particles, and when they bounce up and fall and collide, the large and hard beads apply impact and shear force to the silicon particles. Depending on the method of the milling machine, a bar protruding in an arc, straight or inclined shape, or a rotating shaft with wings is inserted into the milling container to apply a shear force to the mixture at high speed, so that silicon can be pulverized by collision and friction between the beads and silicon particles. Meanwhile, referring to Fig. 3, the action of beads and silicon within a milling vessel during milling is described. Fig. 3 is a conceptual diagram illustrating the milling of beads and silicon within a rotating milling vessel, either horizontally or planetarily. The large circle represents the milling vessel, the gray spheres within it represent beads, and the brown spheres represent silicon particles. During milling, collisions between the charged particles can cause molten silicon powder to adhere to the walls of the milling vessel, thickening and forming a buffer layer, making effective grinding difficult. This phenomenon can occur due to an excessively high proportion of large beads, resulting in high impact energy, or excessive atmospheric moisture adsorbed on the raw silicon, causing fine silicon particles to adhere to the inner walls of the milling vessel. Therefore, it is necessary to adjust the size and weight ratios of the beads, or the rotational speed of the milling vessel or rotor, to ensure smooth movement of the beads within the milling vessel and their collision and rolling motions. At slow rotational speeds, the inclination angle of the charged particles within the milling vessel is low, allowing the beads to rub against each other and primarily crush and mix the silicon through shear force. At high rotational speeds, the rolling or collision motion of the beads causes the silicon to be crushed. The optimal rotational speed can be determined by visually observing the side of the milling vessel with a temporary cover equipped with a transparent window, or by audibly listening to the outer wall of the milling vessel. When checking the rotation speed according to the amount of charge, the minimum rotation speed is the speed at which the beads roll at an angle of about 25° or more. The grinding reaction is promoted as the rotation speed increases to the extent that the beads bounce and collide when the beads are inclined at an angle of about 40° or more. At an angle exceeding 60°, the beads do not collide with the silicon, the number of beads that bounce or roll decreases, and the charge begins to rotate together while being stuck to the inner wall of the milling container due to centrifugal force. This speed is the maximum rotation speed. Therefore, at angles below the lowest 25° and above the highest 60°, the collision or rolling motion between the beads and silicon decreases, making it difficult to grind the silicon particles. Depending on the size and method of the milling vessel, nano-sized silicon particles can be obtained in a short period of time by combining a high-speed motor system with a rotor rotation speed of up to 22,000 rpm, such as a small-diameter milling vessel or an attritor method, with beads of 3 mm or less. In the case of a general horizontal rotary or planetary type, it may be possible to grind silicon into the size targeted by the present invention even when milling with a motor system that satisfies about 60 to 650 rpm, preferably 150 to 450 rpm, depending on the vessel diameter. The milling vessel of the present invention can be equipped with a device to suppress temperature rise so that the heat generated during milling does not cause unintended oxidation of fine silicon particles. This can be achieved by contacting a line through which a coolant fluid circulates with the wall of the milling vessel and circulating it with a pump, or by using a PLC circuit or PID control device to provide a pause during milling for the motor that rotates the milling vessel. In addition, by utilizing the phenomenon of fine powders that are ground to float to the upper part of the milling vessel, the fine powders are guided to collect in an auxiliary tank installed on the outside of the milling vessel through the circulation line, and the fine powders that are not milled are continuously milled at the lower part of the vessel, thereby making the size of the silicon particles uniform. After milling, the silicon particles are recovered in a powder state. The silicon particles targeted in the present invention are composed of particles with a size of 5 to 2,000 nm having a Gaussian distribution, nano-sized particles with an average diameter of 10 to 1,000 nm accounting for 85% or more, particles less than 10 nm accounting for less than 5%, and particles greater than 1,000 nm accounting for less than 10%. When the particle size is less than 10 nm, the surface area to mass ratio is large, which does not significantly contribute to increasing battery capacity. When the particle size is greater than 1,000 nm, the volume change is large due to repeated charge and discharge, which reduces the battery capacity retention rate, and therefore is limited. (2) Step of forming a buffer layer thin film (S200) In the step of forming a buffer layer thin film (S200) after the step of crushing silicon particles (S100), the buffer layer thin film can be formed on the surface of the silicon particles by chemical surface treatment. Specifically, the step (S200) of forming a buffer layer thin film can be performed by exposing the pulverized silicon particles to an oxidizing agent in a surface treatment vessel containing an oxidizing agent. At this time, since there is no high-temperature heat treatment, the surface of the silicon particles can chemically react with the oxidizing agent to uniformly form a non-stoichiometric compound layer. Through this, a non-stoichiometric compound layer that reduces the aggressive reactivity with metal cations or electrolytes can be uniformly formed. Through this, excessive formation of an irreversible compound layer in an electrochemical reaction can be suppressed, and preferably, the buffer layer can be formed as a thin film to improve electrical conductivity and ionic conductivity. Here, oxidation means that an atom, molecule, or ion loses electrons, and an oxidizer is a substance that has a higher electron affinity than silicon and reacts with silicon to cause silicon to lose electrons. In the step of forming a buffer layer thin film (S200), an oxidizing agent may be added alone or in combination. In general, among silicon compounds, silicon carbide has the highest electrical conductivity, followed by silicon nitride and silicon oxide. In addition, graphite generally has higher ionic conductivity than silicon, and ionic conductivity is improved when silicon carbide, silicon oxide, silicon nitride, or silicon fluoride is formed on the silicon surface. In addition, if the composition of the silicon compound layer is stoichiometric, the electrical conductivity is low. However, non-stoichiometric materials have higher ionic and electrical conductivities than stoichiometric compounds. In the present invention, the ratio of the oxidizer to the silicon particles is adjusted to a molar ratio of 0.05 to 0.20 mol, so that SiO is obtained rather than stoichiometric SiO2. x (0.5 < x < 1.2) or SiN other than Si3N4 x By forming a non-stoichiometric thin film of (0.1 ≤ x < 1) as a buffer layer on the surface of silicon particles, a layer with higher ionic conductivity and electrical conductivity than a stoichiometric compound can be obtained. The reason why the ionic conductivity and electrical conductivity of the non-stoichiometric buffer layer thin film are improved is that the buffer layer is formed as a thin film close to two-dimensional, so there is less electron confinement and electron movement is easy by hopping, and during the formation process and the initial charge / discharge process, the silicon compound layer reacts with lithium ions to form complex oxides such as Li2Si2O5, Li2SiO3, Li4SiO4 with excellent ionic conductivity or complex nitrides such as LiSi2N3, Li5SiN3, Li2SiN2, and when the functional group of the oxidizing agent is partially contained in the SEI, the thin film state is maintained more stably. As a result, the reaction that lowers ionic conductivity and electrical conductivity by continuously forming an irreversible compound layer in the silicon anode material and consumes the electrolyte is suppressed. Specifically, in the present invention, the oxidizing agent is a substance that oxidizes the surface of silicon and contains at least one of active oxygen, nitrogen, boron, phosphorus, sulfur, or halogen elements or acid functional groups. Specifically, oxygen, ozone, hydrogen peroxide, sodium peroxide, magnesium peroxide, calcium peroxide, nickel peroxide, zinc peroxide, urea-hydrogen peroxide, strontium peroxide, benzoyl peroxide, dicumyl peroxide, nitrogen, nitric acid, zinc nitrate, ammonium nitrate, ammonium nitrite, cerium ammonium nitrate, tetrapropylammonium perruthenate, potassium nitrate, potassium nitrite, sodium nitrite, lithium nitrate, lithium nitrite, aluminum nitrate, chloramine T, trimethylamine, TMAO (Trimethylamine N-oxide), 4-acetamido-2,2,6,6-tetramethyl-1-oxo-piperidine tetrafluoroboric acid, phenol nitroprusside, boric acid, tetrafluoroborate, nitrosyl tetrafluoroborate, sulfuric acid, sulfur trioxide, sodium sulfide, Sodium hydrogen sulfide, sodium persulfate, ammonium persulfate, potassium persulfate, lithium sulfide, silicon sulfide, aluminum sulfate, potassium nitrosodisulfonate, hydrochloric acid, sodium hypochlorite, calcium hypochlorite, 3-chlorobenzoic acid, perchloric acid, lithium perchlorate, sodium perchlorate, magnesium perchlorate, potassium perchlorate, calcium perchlorate, manganese perchlorate, iron perchlorate, nickel perchlorate, copper perchlorate, silver perchlorate, cesium perchlorate, barium perchlorate, tetrabutylammonium perchlorate, N-hydroxytetrachlorophthalimide, sodium isocyanurate dichloro, chloroglyoxylate ethyl ester, N-chlorobenzene Sodium sulfonamide, 2,3-dichloro-5,6-dicyano-p-benzoquinone, p-chloranil (tetrachloro-p-benzoquinone), PCC (Pyridinium chlorochromate), oxalyl chloride, methyloxalyl chloride, DPI (diphenyliodonium chloride), oxalyl bromide, iodine monobromide, sodium bromide, N-bromosuccinimide, manganese oxide, sodium permanganate,Potassium permanganate, periodic acid, sodium periodate, potassium periodate, potassium perruthenate, diacetoxyiodobenzene, bis-trifluoroacetoxyiodobenzene, 2-iodinated benzoic acid, HTIB (hydroxy-tosyloxy-iodo benzene), tert-butyl hydroxide, tert-butyl hydroperoxide, TEMPO (Tetramethylpiperidine 1-oxyl), phosphomolybdic acid, chromatic acid, potassium dichromate, pyridinium chlorochromate, pyridinium dichromate, ammonium dichromate, selenium dioxide, OXONE, cumene hydroperoxide, sodium metaperiodate, DMP (Dess-Martin periodinane), osmium tetroxide, potassium osmate, A compound containing one or a combination of sodium percarbonate, MMPP (magnesium monoperoxyphthalate), TCNE (tetracyanoethylene), NMMO (4-methylmorpholine-N-oxide), chromium trioxide-sulfuric acid solution, hydrofluoric acid, phosphoric acid, urea, oxalic acid, acetic acid, citric acid, tartaric acid, formic acid, malic acid, succinic acid, glycic acid, glycolic acid, nicotinic acid, lemon acid, malic acid, benzoic acid, butyric acid, salicylic acid, succinic acid, acetosalicylic acid, ascorbic acid, ascorbic acid, peracetic acid, carbonic acid, caffeic acid, couwen acid, palmitic acid, carboxylic acid, sulfinic acid, sulfonic acid, phenol, formic acid, fumaric acid, thiophenol, oxime, enol, amide, sulfonamide, and nitro compounds may be used. In the step (S200) of forming a buffer layer thin film of the present invention, a chemical surface treatment method of exposing silicon particles to a gaseous or liquid oxidizing agent is used. Since a liquid or gaseous oxidizing agent is used, it can form a compound layer by chemically reacting while uniformly contacting the surface of the silicon particles in a fluid state. Fig. 6a shows the results of XRD analysis of silicon particles coated with a buffer layer thin film and a stabilizing layer in Example 1 and Example 2 of the present invention, respectively. When compared with the XRD analysis results of pure silicon particles, it can be seen that the buffer layer and the stabilizing layer are thin films to the extent that there is almost no difference. In the present invention, the buffer layer thin film formed on the surface of the silicon particles is specifically SiO, SiO x (0.5 < x < 1.2), SiNx(0.1 ≤ x < 1) or at least one of nitrogen, boron, phosphorus, sulfur, or halogen elements or acid functional groups contained in the oxidizing agent, and a non-stoichiometric compound layer doped with the metal ion or functional group of the oxidizing agent. As a specific means for this, it is possible to oxidize the silicon particles by contacting them with the oxidizing agent through various methods, such as a method of immersing silicon particles in a surface treatment container containing a solution containing the oxidizing agent, a method of charging silicon particles in a sealed container, supplying a gaseous oxidizing agent and an inert gas, and oxidizing through fluidized bed treatment, mechanical stirring, plasma discharge, or ultraviolet irradiation, and a method of performing liquid plasma discharge in a tank containing a liquid oxidizing agent. Meanwhile, by adopting a chemical surface treatment in the step (S200) of forming the buffer layer thin film of the present invention, it is easy to obtain a non-stoichiometric compound layer by adjusting the oxidizing agent ratio in the solution or gas atmosphere. In the present invention, the oxidizing agent is added in the range of 0.05 mol to 0.20 mol per 1 mol of silicon. Considering the distribution of particles with an average size of 1,000 nm or less and a size of several tens of nm, if it is less than 0.05 mol, it is difficult for the buffer layer thin film to play a sufficient protective role, and if it exceeds 0.20 mol, an excessively thick buffer layer may be formed or a stoichiometric compound may be formed. Preferably, the oxidizing agent is added in the range of 0.08 mol to 0.15 mol per 1 mol of silicon. When a stoichiometric compound is formed, the volume expansion is alleviated, but the thickness of the irreversible compound formed on the particle surface increases, which lowers the initial Coulombic efficiency (ICE), and due to these, the electrical conductivity of the negative electrode material decreases, and the charge / discharge current (C rate) capacity decreases, which also reduces the charging speed. However, by controlling the ratio of the oxidizer supplied into the container depending on the amount of silicon particles, the buffer layer can be controlled to be formed as a non-stoichiometric compound, thereby increasing the electrical conductivity and ionic conductivity and enhancing the initial Coulombic efficiency (ICE) and charging speed. Since the buffer layer thin film formation process is a reaction with an oxidizing agent, promoting the reaction between the silicon particles and the oxidizing agent by heating may also be included in the means of the present invention. However, when heating, since the oxidation reaction of silicon is an exothermic reaction, the heating temperature is set to 200°C or lower, preferably 150°C or lower, only at the initial stage of the reaction. If the heated silicon particles are clumped together, they can be broken up again by milling for several minutes at low speed (less than 250 rpm in a 250-500 ml container). (3) Step of forming a stabilization layer (S300) After the step of forming a buffer layer thin film (S200), a step of forming a stabilizing layer (S300) may be further included. The step of forming a stabilizing layer (S300) may include applying a stabilizing layer forming material to the surface of a silicon particle on which a buffer layer is formed through a mechano-chemical reaction. Specifically, in the step of forming a stabilizing layer (S300), a carbonaceous substance such as activated carbon or graphite, or a fluoride, phosphide, hydroxide, iodide, nitrate, organic acid, etc. can be applied to the surface of the silicon anode material through a mechanical and chemical reaction. Through this, conductivity and slurry dispersibility can be improved, and stable SEI formation can be promoted. In the present invention, carbonaceous materials, fluorides, phosphides, hydroxides, iodides, nitrates, organic acids, etc. can be used as the stabilizing layer. In order to obtain high ionic conductivity and electrical conductivity of the stabilizing layer, it is characterized by being applied as thinly as possible. The simplest means of controlling the thickness of the stabilizing layer is to apply it by a milling method that enables mechanical bonding by adjusting the addition amount within the range of 4 to 45% by weight relative to the silicon particles, preferably 5 to 35% by weight, and more preferably 8 to 25% by weight. The thickness of the stabilizing layer can be minimized by applying it by a mechanical chemical reaction. If the weight ratio is less than 4% relative to the silicon particles, it is difficult to sufficiently form a stabilizing layer on the surface of the silicon particles, making it difficult to expect an effect. On the other hand, if the weight ratio exceeds 45%, the effective silicon amount decreases, which may result in a decrease in battery capacity. Carbonaceous materials applied as a stabilizing layer include activated carbon, graphite, fluorides, phosphides, hydroxides, iodides, nitrates, organic acids, etc. The most common is to apply graphite, which improves ionic conductivity, but as the application thickness increases, the electrical conductivity decreases, which lowers the initial coulombic efficiency and makes the cycling performance that maintains the battery capacity poor. On the other hand, applying activated carbon increases the initial coulombic efficiency and is also advantageous in cycling performance. However, activated carbon has the disadvantage of having somewhat lower chemical stability and heat resistance than graphite. The electrical conductivity of the activated carbon layer applied to stabilize the silicon anode material is 3.3 x 10 depending on the origin of the activated carbon. 2 From S / cm to 9.44 x 10 -11 It is known to vary from S / cm. However, since the electrical conductivity of silicon or activated carbon varies depending on the properties of the compound formed after the chemical reaction, the type of compound formed after the electrochemical reaction and the thickness of the stabilizing layer are important. In the present invention, the stabilization layer is mainly formed by applying highly hydrophobic activated carbon or iodide to silicon particles depending on the type of slurry to improve particle dispersibility in the slurry, and in the case of an aqueous solvent, relatively hydrophilic graphite or fluoride is applied to improve dispersibility. In addition, the stabilization layer can be applied by adjusting the ratio of activated carbon and graphite, applying activated carbon and graphite two or more times, or applying them alternately in a changed order. In the present invention, the thickness of the stabilization layer is minimized by applying it in a weight ratio of 4 to 45% or less with respect to silicon particles so as not to impair electronic conductivity and ionic conductivity. The activated carbon used in the present invention can be any of charcoal-derived activated carbon, anthracite-derived activated carbon, and hydrocarbon-derived activated carbon, regardless of origin, and has an internal surface area (BET) of 20 to 3,000 m depending on the origin of the raw material and the processing method. 2 / g. Among readily available charcoal-derived activated carbons, hard activated carbons manufactured at high temperatures of 600 to 1,100°C are more carbonized than soft activated carbons manufactured at low temperatures, resulting in larger pores and a greater number of pores, which are advantageous for the diffusion and storage of alkali metal cations. The internal surface area of ​​commonly used charcoal-derived activated carbons is 400 to 1,500 m 2 / g level, and depending on the form, granule or pellet type activated carbon has a larger internal surface area than powder. The larger the internal surface area of ​​activated carbon, the better the electrical conductivity and ionic conductivity. The activated carbon used in the present invention is a powder with an average diameter of 20 to 180 microns, and is approximately 700 to 1,500 m 2 / g internal surface area. When the activated carbon particles have an average diameter of less than 20 microns, the finer they become, the more often the pores capable of adsorption are damaged, which tends to decrease the specific surface area. In addition, even if the pores of the activated carbon are formed, it is unknown to what extent the pores will actually contribute to adsorption due to moisture or residual impurities. Therefore, in the present invention, the iron content is 0.3% or less as Fe2O3, chloride 0.25% or less, preferably 0.15% or less as Fe2O3, chloride 0.13% or less, and more preferably 0.03% or less as Fe2O3, chloride 0.05% or less, by weight ratio, is used. In addition, in order to minimize the effect on the electrolyte, the pH is 5.0 to 8.0, preferably 6.0 to 7.5, is used. In addition, the loss on drying is 10.0% or less, preferably 5.0% or less is used. When the drying loss exceeds 10.0%, the hydrophilic group increases due to oxidation by moisture adsorbed from the atmosphere, which may increase the possibility of changes in the slurry dispersibility and sedimentation. Graphite has many dangling bonds to which functional groups are easily bonded. When graphite is applied as a stabilizing layer to the silicon anode material of the present invention, cations dissolved in the electrolyte are induced to promote diffusion within the silicon particles, and functional groups adsorbed on the surface of the graphite particles and the dangling bonds can induce a smooth electrochemical reversible reaction in the silicon particles. In addition, silicon particles coated with graphite have improved dispersibility within the graphite anode material slurry. To improve the battery capacity of graphite anode materials by mixing silicon particles, it is necessary to understand the lithiation and delithiation processes of graphite and silicon. While lithiation of graphite is the process by which lithium ions are absorbed between graphite layers when lithium ions are inserted, lithiation of silicon is an alloying process that creates a compound, and their lithiation mechanisms are different. For example, according to a report by Jiang et al., when graphite and silicon are mixed, lithiation of graphite occurs at low voltage, but in silicon, alloying begins regardless of voltage, and in the electrochemical reaction, it reacts more quickly with the surrounding electrolyte. Delithiation occurs first in silicon and later in graphite (Jiang et al., J. Electrochem. Soc. 2022, 169, 020568). The graphite used to form the stabilizing layer in the present invention can be natural graphite or artificial graphite. Natural graphite is mined from minerals and has a slightly higher degree of graphitization and lithium-ion storage capacity than artificial graphite. Natural graphite, however, has a disadvantage in that its charge rate is lower than that of artificial graphite at high current (high C-rate) charging due to the layered nature of 2D graphene, which allows lithium ions to intercalate between the layers through the edges. Furthermore, the edges tend to peel or break during repeated volumetric expansion and contraction due to the electrolyte reaction, resulting in a shorter lifespan than artificial graphite. Artificial graphite is graphitized by heating needle coke, pitch, and other raw materials in an electric furnace to approximately 3,000°C. This results in a stable crystal structure even after repeated charge and discharge. Furthermore, its 3D structure allows for numerous lithium-ion migration paths, resulting in a faster charge rate and less volumetric expansion, resulting in a longer lifespan. However, its theoretical capacity is slightly lower than that of natural graphite. Graphite is known to have higher electrical conductivity than activated carbon, but this is a characteristic of its bulk form. When configured as an anode material, its conductivity is determined by the contact state between particles and particle size. That is, electrical conductivity changes depending on the density of the anode material, which is determined by the press pressure and compression ratio. Therefore, if the particle size is the same, the actual difference in electrical conductivity between graphite, which utilizes edges as the primary path for electron transfer, and activated carbon, which allows electron transfer in all directions of the particle surface, is considered to be small. In addition, fluorides, phosphides, hydroxides, iodides, nitrates, and organic acids such as acetates, citrates, and tartarates react with lithium in the electrolyte in the stabilization layer of the present invention to form lithium compounds, which have higher electrical conductivity than oxides and thus contribute to increasing the charging speed or initial coulombic efficiency of the stabilization layer. Specifically, the fluoride used in the present invention is one of sodium fluoride, potassium fluoride, lithium difluoride (LiHF2), magnesium fluoride (MgF2), calcium fluoride (CaF2), aluminum fluoride (AlF3), sodium hexafluorosilicate (Na2SiF6), potassium hexafluorosilicate (K2SiF6), and magnesium hexafluorosilicate (MgSiF6), or a complex thereof. Phosphides include phosphoric acid, pyrophosphates such as disodium pyrophosphate and sodium pyrophosphate, polyphosphates such as chloral salts and maderel salts, metaphosphates such as sodium metaphosphate, potassium metaphosphate or potassium glycerophosphate, hypophosphates such as potassium phosphate or calcium phosphate, hexafluorophosphoric acid, nickel phosphide (Ni2P), phosphorus pentasulfide (P2S5), trisodium sulfanylidene (Na3PS4), or combinations thereof. Hydroxides include sodium hydroxide, lithium hydroxide, magnesium hydroxide, barium hydroxide, and cesium hydroxide. Iodides include sodium iodide, potassium iodide, hydrogen iodide, iodine fluoride, and iodates. Nitrates include sodium nitrate, potassium nitrate, calcium nitrate, iron nitrate, magnesium nitrate, barium nitrate, triammonium nitrate, and silver nitrate. Organic acids include acetates such as sodium acetate, manganese acetate, cerium acetate, cellulose acetate, and vinyl acetate. Citrates include magnesium citrate, sodium citrate, and calcium citrate. And tartarates include tartaric acid, monopotassium tartrate, sodium tartrate, potassium sodium tartrate, ammonium tartrate, and potassium tartrate.Other organic acids that can be used include carboxylic acid, sulfinic acid, sulfonic acid, phenol, thiophenol, imide, oxime, enol, sulfonamide, nitro compounds, oxalic acid, fumaric acid, palmitic acid, lactic acid, malic acid, butyric acid, formic acid, succinic acid, glycylic acid, nicotinic acid, citric acid, benzoic acid, glacial acetic acid, salicylic acid, succinic acid, acetosalicylic acid, ascorbic acid, couwen acid, propionic acid, and sorbic acid. In order to improve the electrical conductivity of the stabilizing layer, it is necessary to apply the stabilizing layer as uniformly and thinly as possible, and by mixing one-dimensional conductive materials such as carbon fibers, carbon nanotubes, and boron nitride nanotubes into the zero-dimensional particle-type negative electrode material in terms of shape, or by adding two-dimensional network conductive materials such as graphene, MXene (transition metal carbide, carbonitride, or nitride), TMDs (transition metal dichalcogenides), silicene, germanene, stanine, phosphorene, and hexagonal boron nitride (h-BN), the contact state between particles is maintained even after the volume change, which is advantageous in improving the electrical conductivity. The negative electrode for a secondary battery of the present invention comprises the above-described silicon composite. Specifically, the negative electrode for a secondary battery of the present invention may comprise the above-described silicon composite, a conductive material, and an electrolyte. The conductive material may be a carbon nanotube. The conductive material may be included in an amount of 0.2 to 10% by weight relative to the silicon composite. The content of the conductive material may be proportional to the content of the silicon composite. The electrolyte additive may be a carbonate ester, specifically fluoroethylene carbonate (FEC), vinylene carbonate (VC), or polyethylene carbonate (PEC). The electrolyte additive may be included in an amount of 2 to 8% by weight relative to the entire electrolyte. Hereinafter, specific examples are provided. However, the following examples are intended only to illustrate the present invention and are not intended to limit or restrict the scope of the present invention. Furthermore, the features, structures, effects, etc. illustrated in each example can be combined or modified in other examples by those skilled in the art to which the examples pertain. Therefore, the contents related to such combinations and modifications should be construed as being included within the scope of the present invention. In addition, although the following description focuses on examples, these are merely examples and do not limit the present invention. Those skilled in the art to which the present invention pertains will appreciate that various modifications and applications not exemplified above are possible without departing from the essential characteristics of the present embodiments. For example, each component specifically shown in the embodiments can be modified and implemented. In addition, differences related to such modifications and applications should be interpreted as being included within the scope of the present invention defined in the appended claims. Example The metal silicon particles used in the battery test according to the embodiment of the present invention were reagent grade, 98% pure, and 100 mesh in size. The grinding was performed in a planetary mill with a capacity of 250 ml, with the first milling at 150 to 350 rpm for 10% to 25% of the total milling time, and the second milling at 300 to 550 rpm, thereby obtaining particles having a Gaussian distribution of sizes from 5 to 2,000 nm and an average diameter of 20 to 250 nm. Meanwhile, FIG. 4 is a photograph of Example 2 of the present invention observed at various magnifications, and FIG. 5 is a photograph of Example 7 of the present invention observed at various magnifications. The actual particle distribution of Examples 2 and 7 of the present invention, when calculated by the Scherrer equation as a ratio of the height and width of the XRD analysis peak, is such that particles of 8 to 15 nm are distributed at a volume ratio of about 1.3 to 2.8%, particles of 20 to 35 nm are distributed at a volume ratio of about 0.3 to 4.5%, particles of 40 to 65 nm are distributed at a volume ratio of about 50.1 to 91.6%, particles of 70 to 100 nm are distributed at a volume ratio of about 3.9 to 39.8%, and particles of 105 to 150 nm are distributed at a volume ratio of 0 to 8.8%. Through this, it can be seen that the silicon particle distribution includes 1 to 10% of particles with a volume ratio of 10 to 35 nm, 45 to 95% of particles with a volume ratio of more than 35 to 65 nm, 3 to 45% of particles with a volume ratio of more than 65 to 100 nm, 0 to 15% of particles with a volume ratio of more than 100 nm, and most of the particles are sized between 40 and 100 nm. The average particle size of Example 2 was calculated to be 45.4 nm. After placing these particles in a stainless steel container, adding 34.5% concentration aqueous solution of reagent-grade hydrogen peroxide or 60% concentration nitric acid for reagent as an oxidizing agent in an amount of 32 to 41% by weight relative to the silicon powder, immersing them in a paste state, and then performing secondary milling followed by additional milling or drying in an oven heated to 80 to 150°C to form a buffer layer thin film, the silicon paste is milled again in a planetary mill with a capacity of 250 ml to pulverize it. The activated carbon used to form the stabilizing layer was reagent-grade powder activated carbon with impurities of 0.3 ml or less of chlorine and 0.5 ml or less of sulfate (SO4). In addition, MCMB artificial graphite with a D50 average particle size of 17.6 microns and spherical shape was used as the graphite. The fluoride was reagent-grade sodium fluoride (NaF) with a purity of 97% or higher. Table 2 below compares the results of battery tests on conventional graphite, silicon compound, nano-silicon anode materials, and the silicon anode material of the present invention. The 5% converted discharge capacity is calculated based on the discharge capacity of graphite of 350 mAh / g when the silicon anode material content is 5%. This allows for a relative comparison of battery capacities according to silicon content, thereby determining the efficiency of silicon. In Example 1, the metallic silicon particles were first milled at 250 rpm for 30 minutes, secondarily milled at 450 rpm for 2 hours, and the hydrogen peroxide aqueous solution was added at a weight ratio of 40% (i.e., 0.12 mol) to silicon, and a paste was made, which was further milled at 450 rpm for 2 hours to obtain silicon particles coated with a buffer layer thin film. 8.6% of activated carbon was added thereto and milled at 400 rpm for 1 hour to obtain silicon particles coated with a stabilizing layer. The prepared silicon particles were mixed with 93% of spherical artificial graphite (MCMB) at 5%, 2% of acetylene black as a conductive material, 2.5% of CMC (carboxymethyl cellulose) and 2.5% of SBR (styrene butadiene rubber) as binders, and distilled water was added at the same weight to prepare a cathode slurry. The test results of a 2032 standard half-cell using lithium foil as the positive electrode and a slurry coated on copper foil and dried to manufacture a negative electrode, and a 1 mol LiPF6 (lithium hexafluorophosphate) solution dissolved in 1 liter of a solvent mixed with EC (ethylene carbonate), EMC (ethyl methyl carbonate), and DEC (diethyl carbonate) in a ratio of 3:4:3 was injected as the basic electrolyte. Example 2 is the result of a battery test in which 8.6% of activated carbon was added to silicon particles coated with the buffer layer thin film of Example 1, milled at 400 rpm for 1 hour, 14.2% of activated carbon was added, and milled at 450 rpm for 1 hour to apply a total of 22.8% of activated carbon as a stabilizing layer, and 5% of silicon particles were applied to the negative electrode material, and the rest was configured in the same manner as Example 1. Figure 6 shows the results of XRD analysis using different testers for Example 1 (Figure 6a) and Example 2 (Figure 6b) of the present invention, in which a buffer layer thin film and a stabilizing layer are applied. Example 3 is the result of a battery test in which 8.6% of activated carbon was added to silicon particles coated with the buffer layer thin film of Example 1, milled at 400 rpm for 1 hour, 9.3% of artificial graphite (MCMB) was added, and 17.9% of carbon was applied as a stabilizing layer in total at 350 rpm for 1 hour, and 5% of silicon particles were applied to the negative electrode material, and the rest was configured in the same manner as Example 1. Example 4 is a battery test result in which the metallic silicon particles were first milled at 300 rpm for 30 minutes, second milled at 450 rpm for 2 hours, and the hydrogen peroxide aqueous solution was added at 41% relative to silicon, i.e., 0.12 mol, to make a paste, which was further milled at 450 rpm for 2 hours to obtain silicon particles coated with a buffer layer film. 8.0% of activated carbon was added to the silicon particles formed with the buffer layer film, milled at 400 rpm for 1 hour, and 30.6% of sodium fluoride (NaF) was added, milled at 400 rpm for 30 minutes, and the silicon particles coated with a stabilizing layer were applied at 5% to the negative electrode material, and the rest was configured in the same manner as Example 1. Example 5 is a battery test result in which the metallic silicon particles were first milled at 300 rpm for 30 minutes, second milled at 450 rpm for 2 hours, and the hydrogen peroxide aqueous solution was added at 32% (i.e., 0.09 mol) based on silicon to make a paste, which was further milled at 450 rpm for 30 minutes, dried in an oven at 150°C for 1 hour, and then milled at 200 rpm for 2 minutes to obtain silicon particles coated with a buffer layer thin film. This is a result of a battery test in which 13.5% of activated carbon was added to the silicon particles formed with the buffer layer thin film, and the silicon particles coated with a stabilizing layer were applied at 5% to the negative electrode material by milling at 400 rpm for 30 minutes, and the rest was configured in the same manner as in Example 1. Example 6 is the result of a battery test in which silicon particles, which were formed as a buffer layer thin film in the same manner as in Example 5, were spread thinly on a glass plate, irradiated with a 405 nm ultraviolet lamp for 6 hours for additional oxidation, and then 13.7% of activated carbon was added, and a stabilization layer was applied by milling at 400 rpm for 1 hour to the negative electrode material at 5%, and the rest was configured in the same manner as in Example 1. Example 7 is a battery test result in which the metallic silicon particles were first milled at 300 rpm for 40 minutes, second milled at 450 rpm for 1 hour, and then 32% of the hydrogen peroxide aqueous solution and 3% of the nitric acid as oxidizing agents, that is, 0.12 mol and 0.04 mol in molar ratios, were added to form a paste, and the paste was further milled at 450 rpm for 1 hour, dried in an oven at 150°C for 1 hour, and then further milled at 350 rpm for 10 minutes to obtain silicon particles coated with a buffer layer thin film. This is a result of a battery test in which 18.8% of artificial graphite (MCMB) was added to the silicon particles formed with the buffer layer thin film, and the silicon particles coated with a stabilizing layer were applied at 5% to the negative electrode material by milling at 350 rpm for 1 hour, and the rest was configured in the same manner as in Example 1. Example 8 was prepared by mixing 10% of the buffer layer thin film and the stabilizing layer-coated silicon particles of Example 1 with 90% of artificial graphite (MCMB), and adding 2% of CMC (carboxymethyl cellulose) and 4% of SBR (styrene butadiene rubber) as binders. In addition, Example 8-1 added 4% of Super P, which is carbon black, alone as a conductive agent to the negative electrode material, and Example 8-2 added 3% of Super P and 1% of single-wall carbon nanotube as conductive agents to enhance conductivity. Then, slurries were prepared by mixing the mixture with distilled water of the same weight. After manufacturing the battery, Example 8-1 injected 1 mol of LiPF6 (lithium hexafluorophosphate) dissolved in 1 liter of a solvent containing 1:1 volume ratio of EC (ethylene carbonate) and DEC (diethyl carbonate) as the basic electrolyte, and the battery test results are indicated by a black dotted line in Fig. 7a. The electrolyte of Example 8-2 was made by adding 5% FEC to the basic electrolyte of Example 8-1 to enhance electrolyte stability, and the results of the battery test are indicated by the green dotted line in Fig. 7a. In the experiment, a 2032 standard half-cell was made using lithium foil as the positive electrode and tested. The purple dotted line in Fig. 7a is the capacity trend line of the graphite negative electrode material as a comparison material, and in contrast, the case where only carbon nanotubes were added to the conductive material is indicated by the blue dotted line in Fig. 7a, and the case where only FEC was added to the basic electrolyte is indicated by the red dotted line. In Example 8-1, the battery capacity retention rate decreased as the silicon particle content in the negative electrode material increased to 10%, which is different from the case where the silicon content was 5%. In Example 8-2, it was found that the battery capacity retention effect of the silicon particles was maintained even when the silicon content increased to 10% by improving both the conductivity by carbon nanotubes and the electrolyte stability by FEC. This shows that even if the performance of the silicon particle buffer layer is maintained, the connection between the silicon particles and the graphite particles and the electrolyte stability affect the battery capacity retention rate. Example 9 used artificial graphite (MCMB) mixed with 10% of the buffer layer thin film and stabilizing layer-coated silicon particles of Example 2, but made up 90% of the negative electrode raw material, and the binder and basic electrolyte were composed in the same manner as Example 8. In Example 9-1, 4% of Super P, a carbon black, was added as a conductive material, and the results of the battery test are indicated by a black dotted line in Figure 7b. Example 9-2 shows the results of a battery test in which 3% of Super P and 1% of single-walled carbon nanotubes were added as a conductive agent, and 5% of FEC was additionally added to the basic electrolyte, and is indicated by a green dotted line in Fig. 7b. In Figure 7b, the purple dotted line is the capacity trend line of the graphite anode material as a comparison material, and in contrast, the one with only 1% carbon nanotube addition is indicated by the blue dotted line. In the experiment, a 2032 standard half-cell was made and tested using lithium foil as the anode. Through this, it can be seen that in Example 9-1, the battery capacity retention rate decreased when only the silicon particle content increased, but in Example 9-2, by improving both the conductivity by carbon nanotubes and the electrolyte stability by FEC, there was a battery capacity retention effect even when the silicon particle content increased to 10%. Example 10-1 used 90% of the raw material for the anode material, which was artificial graphite (MCMB) mixed with 20% of the buffer layer thin film and stabilizing layer-coated silicon particles of Example 1, and the binder and electrolyte were composed in the same manner as in Example 8-2. 2% of Super P, a carbon black, and 2% of single-walled carbon nanotubes were added as conductive materials to enhance conductivity. The results of the battery experiment of Example 10-1 are indicated by a blue line in Fig. 8a, and the results of Example 8-2 with 10% of the silicon particles and single-walled carbon nanotubes, FEC, added are indicated by a red line, comparing that the battery capacity increases in proportion to the increase in silicon content. The black line indicates the change in battery capacity when the silicon particles of Example 8-1 were 10%. Example 10-2 used 90% of the raw material for the anode material, which was artificial graphite (MCMB) mixed with 20% of the buffer layer thin film and stabilizing layer-coated silicon particles of Example 2, and the binder and electrolyte were composed in the same manner as in Example 9-2. The results of a battery experiment in which 2% of Super P and 2% of single-walled carbon nanotubes were added as conductive materials and 5% of FEC was additionally added to the basic electrolyte are shown in blue line in Fig. 8b, and the red line shows the results of adding single-walled carbon nanotubes and FEC to 10% of the silicon particles of Example 9-2, comparing that the battery capacity increases in proportion to the increase in silicon content. The black line shows the change in battery capacity when 10% of the silicon particles of Example 9-1 were added. In the experiment, a lithium foil was used as the anode and a 2032 standard half-cell was made and tested. In the present invention, the carbon nanotubes added as a conductive material play a role in maintaining the connection between the silicon particles and the graphite particles when the silicon particle content is 10% or more, so the amount added increases according to the silicon content. The effect is minimal when the weight ratio of the negative electrode material is less than 0.2%, and the manufacturing cost increases significantly when it exceeds 10%, so it is limited. The fluoroethylene carbonate added as a carbonate ester to the electrolyte has a minimal effect when the weight ratio is less than 2% compared to the basic electrolyte, and when it exceeds 8%, the initial discharge capacity decreases, and the viscosity of the electrolyte increases, which causes a decrease in ionic conductivity, so it is limited. In addition to fluoroethylene carbonate, vinylene carbonate (VC) and vinylethylene carbonate (VEC) can be added as substitutes. Comparative Example 1 is the same as the embodiment of the present invention except that the composition is changed. This is the result of a battery test when the anode material is a mixture of 93% artificial graphite (MCMB), 2% acetylene black as a conductive agent, 2.5% CMC and 2.5% SBR as binders, and the rest is the same as Example 1. Comparative Example 1, which is an artificial graphite anode material, has an initial discharge capacity of 309-312 mAh / g, which is about 1 / 3 lower than the discharge capacity when 5% of the pure silicon anode material of the present invention is added. Comparative Example 2 is the battery test result of PCT International Patent WO2020256395, in which 10% magnesium silicate and silicon composite oxide were added to natural graphite. When converted to a 5% addition, this shows an initial discharge capacity of 217-232 mAh / g, which is only half that of the present invention. Comparative Example 3 is the battery test result reported by Sim et al. (Adv. Mat. 25, 2013) for a 70% silicon nanowire anode material. Converted to a 5% addition, this represents an initial discharge capacity of 46-163 mAh / g, which is 1 / 10-1 / 2 or less of that of the present invention. Comparative Example 4 is SiO reported by Xiong et al. x This is the result of a battery test when containing 15% of the present invention (RSC Adv. 11, 2021, 7801). When converted to a case where 5% is added, it shows an initial discharge capacity of 208-209 mAh / g, which is less than half of that of the present invention. Comparative Example 5 is the battery test result reported by Yang et al. when a 70% silicon anode material containing silicon oxide and carbon layers coated on 100 nm-sized nano-silicon particles was used (Nature Sci. Rep. 5, 2015, 10908). When converted to a 5% addition, this represents an initial discharge capacity of 45-150 mAh / g, which is 1 / 10-1 / 2 or less of that of the present invention. Comparative Example 6 is SiO that formed a carbonized layer reported by Wang et al. x This is the result of a battery test when the composite contains 75% of the material (Microporous and Mesoporous Materials 307, 2020, 110480). Converted to a 5% addition, this represents an initial discharge capacity of 45-53 mAh / g, 1 / 10 of that of the present invention. Comparative Example 7 is the result of a battery test containing 93% of artificial graphite (MCMB) mixed with 5% of SiO2 silicon oxide having an average diameter of about 12 microns according to the present invention, 2% of acetylene black as a conductive material, and 2.5% of CMC and 2.5% of SBR as binders. Although the initial discharge capacity is 400-412 mAh / g, which is slightly lower than that of the present invention, the manufacturing cost is more than twice as high as that of the embodiment of the present invention. Specimen Initial charge capacity (mAh / g) Initial discharge capacity (mAh / g) 5% converted discharge capacity (mAh / g) Initial coulombic efficiency (%) Capacity retention rate (%) Number of cycles Example 1 149343743788.68 380 2495 43743788.38 488 Example 2 1499 43643687.38 675 2487 43643689.58 780 Example 3 149343543588.28 290 2497 43543587.580 Example 4 1478 417 41787.28 190 247 342 142 189.083 Example 5153246446487.278100249143043087.578 Example 6152846546588.07890257750450487.381 Example 7151344544586.773100251644444486.080 Example 8153550742994.861100257656745998.489100 Example 9154852843996.450100258157246198.591100 Example 1011053103252198.091100294092149398.090100Comparative Example 11340312-91.81001002338309-91.4100100Comparative Example 2151543721984.98750251045422789.092350346323292.086450543421785.990550245022589.687Comparative Example 3Diameter 7nm2117227916392.98350Diameter 15nm7746454683.395Comparative Example 4SBR-1523.7624.920883.896.435SBR-2523.9626.520983.695.8Comparative Example 5Porous Si@Gr-120086-89.1200Si / CNT / C-2100150-95.5100Si-SiO2 nanowire-1910136-95.0100Si@C hollow core-62545-76.840Si / SiO2 / C-95668-83.0430Comparative Example 60@C-7925366.113.91502@C-7825265.228.16@C-7615163.570.310@C-6814556.654.3 Comparative Example 7140641241298.586100240940040097.880

Claims

1. Silicon particles; and A non-stoichiometric buffer layer thin film disposed on the surface of the silicon particle, The above buffer layer thin film is placed on the surface of the silicon particle by chemical surface treatment by adding 0.05 to 0.2 mol of an oxidizer to 1 mol of silicon, The above buffer layer thin film includes a solid electrolyte interphase (SEI) layer including one of Li2Si2O5, Li4SiO4, Li2SiO3 or a composite oxide thereof formed after prelithiation, or an initial formation process, or an initial charge / discharge process, The oxide of the above solid electrolyte (SEI) layer is a silicon complex formed prior to fluoride.

2. In paragraph 1, A silicon composite, characterized in that the buffer layer thin film comprises any one of a silicon oxide thin film, a silicon nitride thin film, and a composite thereof.

3. In paragraph 1, The above buffer layer thin film is SiO x Non-stoichiometric silicon oxide thin film, SiN, having the chemical formula (0.5 < x < 1.2) x A silicon composite characterized by comprising any one of a non-stoichiometric silicon nitride thin film having a chemical formula of (0.1 ≤ x < 1) and a composite thereof.

4. In paragraph 1, A silicon composite, characterized in that the above buffer layer thin film comprises SiO silicon oxide.

5. In paragraph 1, A silicon composite characterized in that silicon particles having an average particle size of 10 to 1000 nm account for more than 85% of the total silicon particles and an average diameter or thickness of 20 to 250 nm.

6. In paragraph 1, A silicon composite characterized in that the silicon particle distribution comprises 1 to 10% of particles having a volume ratio of 10 to 35 nm, 45 to 95% of particles having a volume ratio of more than 35 to 65 nm, 3 to 45% of particles having a volume ratio of more than 65 to 100 nm, and 0 to 15% of particles having a volume ratio of more than 100 nm.

7. In paragraph 1, A silicon composite, characterized in that a stabilizing layer is further disposed on the above buffer layer thin film.

8. In paragraph 7, A silicon composite, characterized in that the stabilizing layer comprises at least one selected from the group consisting of carbonaceous materials, fluorides, phosphides, hydroxides, iodides, nitrates, and organic acids.

9. Step of crushing silicon particles; and A step of forming a buffer layer thin film on the surface of the silicon particle; and A step of forming a stabilizing layer on the above buffer layer thin film is included, The step of forming the above buffer layer thin film is to form a non-stoichiometric silicon oxide thin film by adding an oxidizer to silicon particles, A method for manufacturing a silicon composite, characterized in that the step of forming the stabilizing layer comprises adding a stabilizing layer forming material after the step of forming the buffer layer thin film.

10. In paragraph 9, A method for producing a silicon composite, characterized in that, in the step of crushing the silicon particles, milling is performed.

11. In paragraph 9, The step of forming the above buffer layer thin film is: A method for producing a silicon composite, characterized by chemically treating the surface of the silicon particles.

12. In paragraph 11, A method for producing a silicon composite, characterized in that the above chemical surface treatment is performed by adding 0.05 to 0.20 mol of an oxidizing agent per mol of silicon.

13. In paragraph 12, The above oxidizing agent is, A method for producing a silicon composite, characterized in that the material comprises at least one selected from the group consisting of active oxygen, nitrogen, boron, phosphorus, sulfur, halogen elements, and acid functional groups.

14. In paragraph 9, The above stabilizing layer forming material comprises at least one selected from the group consisting of carbonaceous materials, fluorides, phosphides, hydroxides, iodides, nitrates, and organic acids, A method for manufacturing a silicon composite, characterized in that the stabilizing layer forming material is added in an amount of 4 to 45% by weight relative to the silicon particles and mechanically bonds with the buffer layer thin film.

15. A negative electrode for a secondary battery comprising a silicon composite according to any one of paragraphs 1 and 4 to 7.

16. A silicone composite according to any one of paragraphs 1 and 4 to 7; A conductive material comprising carbon nanotubes; and A negative electrode for a secondary battery, comprising an electrolyte containing a carbonate ester.

17. A secondary battery comprising a negative electrode according to any one of claims 15 and 16.

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