Porous silicon-carbon composite, its manufacturing method, and negative electrode active material containing the same

Abstract

The present invention provides a porous silicon-carbon composite, a method for producing the same, and an anode active material comprising the same. The porous silicon-carbon composite of the present invention comprises silicon particles, magnesium fluoride, and carbon, and can therefore further improve the initial efficiency and capacity retention of secondary batteries, thereby increasing their discharge capacity.

Description

[Technical Field]

[0001] Technical Field The present invention relates to a porous silicon-carbon composite, a method for producing the same, and a negative electrode active material containing the same. [Background technology]

[0002] Background technology In recent years, with the development of the information and communications industry, electronic devices have become smaller, lighter, thinner, and more portable, which has led to an increasing demand for batteries with higher energy density to power these devices. Lithium secondary batteries are the type of battery that best meets this demand, and active research is being conducted into the application of small batteries to large electronic devices such as automobiles and energy storage systems.

[0003] Carbon materials are widely used as the negative electrode active material for such lithium secondary batteries. Research into silicon-based negative electrode active materials is underway to further increase the capacity of batteries. Silicon's theoretical capacity (4,199 mAh / g) is more than 10 times that of graphite (372 mAh / g), so a significant improvement in battery capacity is expected.

[0004] The reaction scheme for intercalating lithium into silicon is, for example, as follows: [ka]

[0005] The silicon-based anode active material produced by the above reaction scheme forms a high-capacity alloy containing up to 4.4 lithium atoms per silicon atom. However, many silicon-based anode active materials experience volume expansion of up to 300% upon lithium intercalation, destroying the anode, making it difficult to achieve high cycle performance.

[0006] In addition, this volume change may cause cracks on the surface of the negative electrode active material or the formation of ionic substances inside the negative electrode active material, which may cause the negative electrode active material to electrically peel off from the current collector. This electrical peeling phenomenon may significantly reduce the capacity retention rate of the battery.

[0007] To solve this problem, Japanese Patent No. 4393610 discloses a negative electrode active material in which silicon and carbon are mechanically processed to form a composite (or composite material or complex; composite), and the surfaces of the silicon particles are coated with a carbon layer by chemical vapor deposition (CVD).

[0008] Furthermore, Japanese Patent Application Laid-Open No. 2016-502253 discloses a negative electrode active material containing porous silicon particles and carbon particles, the carbon particles including fine carbon particles and coarse carbon particles having different average particle sizes.

[0009] However, these prior art documents relate to negative electrode active materials containing silicon and carbon, but there is a limit to the suppression of volume expansion and contraction during charge and discharge. Therefore, research to solve these problems is still required. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] Patent No. 4393610 [Patent Document 2] Japanese Patent Application Laid-Open No. 2016-502253 [Patent Document 3] Korean Patent Publication No. 2018-0106485 Summary of the Invention [Problem to be solved by the invention]

[0011] Detailed Description of the Invention Technical challenges An object of the present invention is to provide a porous silicon-carbon composite comprising silicon particles, a fluorine-containing magnesium compound, and carbon, which, when used as a negative electrode active material, exhibits improved capacity retention, charge-discharge capacity, and initial charge-discharge efficiency.

[0012] Another object of the present invention is to provide a method for producing porous silicon-carbon composites.

[0013] It is still another object of the present invention to provide a negative electrode active material comprising the porous silicon-carbon composite and a lithium secondary battery comprising the same. [Means for solving the problem]

[0014] Means to solve the problem The present invention provides a porous silicon-carbon composite comprising silicon particles, a fluorine-containing magnesium compound, and carbon.

[0015] In addition, the present invention provides a method for producing a porous silicon-carbon composite, comprising: a first step of obtaining a silicon composite oxide powder using a silicon-based raw material and a magnesium-based raw material; a second step of etching the silicon composite oxide powder using an etching solution containing a fluorine (F) atom-containing compound; a third step of filtering and drying the composite obtained by etching to obtain a porous silicon composite; and a fourth step of forming a carbon layer on the surface of the porous silicon composite using a chemical pyrolysis deposition method.

[0016] Additionally, the present invention provides a negative electrode active material comprising the porous silicon-carbon composite.

[0017] Furthermore, the present invention provides a lithium secondary battery comprising the above-mentioned negative electrode active material. [Effects of the Invention]

[0018] Effects of the present invention When the porous silicon-carbon composite according to this embodiment is used as a negative electrode active material, it contains silicon particles, a fluorine-containing magnesium compound, and carbon, and therefore can improve discharge capacity, initial efficiency, and capacity retention.

[0019] Additionally, the method according to one embodiment has the advantage that it allows for mass production through a continuous process with a minimum number of steps. [Brief explanation of the drawings]

[0020] BRIEF DESCRIPTION OF THE DRAWINGS The following drawings attached to this specification show preferred embodiments of the present invention, and together with the description of the present invention, are useful for further understanding of the technical idea of ​​the present invention. Therefore, the present invention should not be interpreted as being limited only to what is shown in the drawings. [Figure 1a] FIG. 1a is a scanning electron microscope (SEM) photograph of the porous silicon composite (composite B1) prepared in Example 1. [Figure 1b] FIG. 1b is an ion beam scanning electron microscope (FIB-SEM) photograph of the porous silicon composite (composite B1) produced in Example 1. [Figure 2a] Figure 2a is a field emission scanning electron microscope (FE-SEM) photograph of the surface of the porous silicon-carbon composite (composite C1) produced in Example 1. Figures 2a to 2d are shown, depending on the magnification. [Figure 2b] FIG. 2b is a field emission scanning electron microscope (FE-SEM) photograph of the surface of the porous silicon-carbon composite (composite C1) prepared in Example 1. [Figure 2c] FIG. 2c is a field emission scanning electron microscope (FE-SEM) photograph of the surface of the porous silicon-carbon composite (composite C1) prepared in Example 1. [Figure 2d]FIG. 2d is a field emission scanning electron microscope (FE-SEM) photograph of the surface of the porous silicon-carbon composite (composite C1) prepared in Example 1. [Figure 3] FIG. 3 is an ion beam scanning electron microscope (FIB-SEM) photograph of the porous silicon-carbon composite (composite C1) produced in Example 1. [Figure 4] FIG. 4 shows a FIB-SEM EDAX photograph of the porous silicon-carbon composite (composite C1) produced in Example 1 and a component analysis table of the composite. [Figure 5] FIG. 5 shows the results of transmission electron microscopy (TEM / EDS) analysis of the porous silicon-carbon composite (composite C1) prepared in Example 1. [Figure 6a] FIG. 6a shows the results of X-ray diffraction analysis of the silicon composite oxide (composite A1) (6a) of Example 1. [Figure 6b] FIG. 6b shows the results of X-ray diffraction analysis of the porous silicon composite of Example 1 (Composite B1) (6b). [Figure 6c] FIG. 6c shows the results of X-ray diffraction analysis of the porous silicon-carbon composite of Example 1 (composite C1) (6c). [Figure 7] FIG. 7 shows the results of X-ray diffraction analysis of the porous silicon-carbon composite of Example 2 (composite C2). [Figure 8] FIG. 8 shows the results of X-ray diffraction analysis of the porous silicon-carbon composite of Example 5 (Composite C5). [Figure 9] FIG. 9 shows the results of Raman analysis of the porous silicon-carbon composite of Example 1 (composite C1). DETAILED DESCRIPTION OF THE INVENTION

[0021] Best Mode for Carrying Out the Invention The present invention is not limited to the contents disclosed below, and various modifications can be made without departing from the spirit and scope of the present invention.

[0022] In this specification, when a part is referred to as "comprising" an element, unless otherwise indicated, it should be understood that the part may include other elements as well.

[0023] Additionally, all numbers and expressions relating to quantities of ingredients, reaction conditions, and the like used herein should be understood as being modified by the term "about" unless otherwise indicated.

[0024] The present invention will be described in detail below.

[0025] [Porous silicon-carbon composite] A porous silicon-carbon composite (or composite material) according to one embodiment of the present invention comprises silicon particles, a fluorine-containing magnesium compound (or a fluorine-containing magnesium compound), and carbon.

[0026] When the porous silicon-carbon composite according to one embodiment is used as a negative electrode active material, the porous silicon-carbon composite contains silicon particles, a fluorine-containing magnesium compound, and carbon, and therefore can improve discharge capacity, as well as initial efficiency and capacity retention.

[0027] Specifically, the porous silicon-carbon composite according to one embodiment contains both silicon particles and a fluorine-containing magnesium compound. Therefore, lithium does not react with the fluorine-containing magnesium compound during charging. Therefore, lithium ions are not rapidly charged during charging and are released into the silicon particles. This suppresses the volumetric expansion of silicon particles during charging of the secondary battery. The use of carbon provides excellent conductivity and suppresses side reactions in the electrolyte. Therefore, the porous silicon-carbon composite can further improve the performance of lithium secondary batteries.

[0028] The porous silicon-carbon composite has a silicon composite and a carbon layer on its surface, in which silicon particles and a fluorine-containing magnesium compound are present in the silicon composite, and carbon is present on some or all of the surfaces of the silicon particles and the fluorine-containing magnesium compound, forming a carbon layer.

[0029] Additionally, the porous structure of the porous silicon-carbon composite according to one embodiment of the present invention allows electrolytes to easily penetrate into the porous structure, improving output characteristics. Therefore, the porous silicon-carbon composite can be advantageously used in the manufacture of negative electrode active materials for lithium secondary batteries and lithium secondary batteries including the same.

[0030] Each component of the porous silicon-carbon composite will now be described in detail.

[0031] Silicon particles A porous silicon-carbon composite according to one embodiment of the present invention comprises silicon particles.

[0032] Because silicon particles charge (or fill) lithium, the capacity of secondary batteries may decrease if silicon particles are not used. Silicon particles may be crystalline or amorphous, specifically, amorphous or a similar phase. When silicon particles are crystalline, the small crystallite size results in a dense composite, strengthening the matrix and preventing cracks. This can further improve the initial efficiency or cycle life characteristics of secondary batteries. Additionally, when silicon particles are amorphous or a similar phase, the expansion and contraction during charging and discharging of lithium secondary batteries is small, further improving battery performance, such as capacity characteristics.

[0033] Silicon particles have high initial efficiency and battery capacity, but undergo very complex crystalline changes as they electrochemically absorb, store, and release lithium atoms. Silicon particles can be uniformly distributed inside porous silicon-carbon composites, resulting in excellent mechanical properties such as strength.

[0034] In addition, the porous silicon-carbon composite may have a structure in which silicon particles and a fluorine-containing magnesium compound are uniformly dispersed. In addition, by dispersing the fluorine-containing magnesium compound so as to surround the silicon particles, expansion and contraction of the silicon can be suppressed, thereby improving the performance of the secondary battery.

[0035] The silicon particles comprised in the porous silicon-carbon composite according to one embodiment of the present invention may be in an amorphous form, a crystalline form with a crystallite size of 2 to 20 nm, or a mixture thereof.

[0036] Specifically, in a porous silicon-carbon composite, when copper is used as a cathode target and silicon particles are subjected to X-ray diffraction (Cu-Kα) analysis, the crystallite size can be calculated using the Scherrer equation based on the full width at half maximum (FWHM) of the Si(220) diffraction peak at around 2θ=47.5°, and can be 2 nm to 20 nm, 2 nm to 15 nm, or 2 nm to 10 nm.

[0037] When silicon particles have a crystalline form, if the crystallite size of the silicon particles is less than 2 nm, it is difficult to prepare the silicon particles, and the yield after etching may be low. Also, if the crystallite size is more than 20 nm, the micropores cannot sufficiently suppress the volume expansion of the silicon particles that occurs during charging and discharging, and there are areas that do not contribute to discharging, and the decrease in Coulomb efficiency, which represents the ratio of charging capacity to discharging capacity, cannot be suppressed.

[0038] In the porous silicon-carbon composite, the silicon (Si) content may be 8 wt % to 95 wt %, 20 wt % to 80 wt %, or 30 wt % to 70 wt %, based on the total weight of the porous silicon-carbon composite.

[0039] If the silicon (Si) content is less than 8 wt%, the amount of active material that absorbs and releases lithium will be small, which may result in a decrease in the charge / discharge capacity of the lithium secondary battery. On the other hand, if the silicon content exceeds 95 wt%, while the charge / discharge capacity of the lithium secondary battery will be large, the electrode will expand and contract too much during charge / discharge, which may further atomize the negative electrode active material powder and deteriorate the cycle characteristics.

[0040] Fluorine-containing magnesium compounds A porous silicon-carbon composite according to one embodiment of the present invention comprises a fluorine-containing magnesium compound.

[0041] Preferred properties of a porous silicon-carbon composite containing a fluorine-containing magnesium compound according to one embodiment of the present invention will now be described.

[0042] Generally, during charging of a secondary battery, silicon particles absorb lithium ions to form an alloy, which may increase the lattice constant and thereby expand the volume, and during discharging of the secondary battery, lithium ions are released and return to the original metal nanoparticles, which reduces the lattice constant.

[0043] The fluorine-containing magnesium compound can be considered a zero-strain material, which does not undergo a change in the crystal lattice constant when lithium ions are absorbed and released. Silicon particles may be present between the fluorine-containing magnesium compound particles or may be surrounded by the fluorine-containing magnesium compound particles.

[0044] On the other hand, the fluorine-containing magnesium compound does not release lithium ions when a lithium secondary battery is charged, and is also an inactive material that does not absorb or release lithium ions when a lithium secondary battery is charged.

[0045] In other words, in the porous silicon-carbon composite, lithium ions are released from the silicon particles, whereas the fluorine-containing magnesium compound does not release the lithium ions that rapidly increase upon charging. Thus, the porous matrix containing the fluorine-containing magnesium compound is not involved in the chemical reactions of the secondary battery, but is expected to function as a body that suppresses the volume expansion of the silicon particles during charging of the secondary battery.

[0046] The content of the fluorine-containing magnesium compound may be 0.04 to 56.3 wt%, 0.5 to 25 wt%, or 1 to 19 wt%, based on the total weight of the porous silicon-carbon composite. When the content of the fluorine-containing magnesium compound satisfies the above range, the cycle characteristics and capacity characteristics of the secondary battery are further improved.

[0047] The fluorine-containing magnesium compound may comprise magnesium fluoride (MgF2), magnesium fluoride silicate (MgSiF6), or a mixture thereof, and may have a crystalline structure. For example, when the fluorine-containing magnesium compound is subjected to X-ray diffraction (Cu-Kα) analysis using copper as a cathode target and calculated using the Scherrer equation based on the full width at half maximum (FWHM) of the MgF2 (111) diffraction peak at around 2θ = 40°, the MgF2 may have a crystallite size of 2 nm to 35 nm, 5 nm to 25 nm, or 5 nm to 15 nm. When the MgF2 crystallite size is within the above range, it can function as a suppressor of the volume expansion of silicon particles during charge and discharge of a lithium secondary battery.

[0048] According to one embodiment of the present invention, in an X-ray diffraction (Cu-Kα) analysis using copper as a cathode target, IB / IA, which is the ratio of the diffraction peak intensity (IB) of the MgF2 (111) crystal near 2θ=40.4° to the diffraction peak intensity (IA) of the Si (220) crystal plane near 2θ=47.3°, may be greater than 0 and less than or equal to 1. If IB / IA exceeds 1, there is a possibility that a problem of reduced capacity of the secondary battery may occur.

[0049] The magnesium (Mg) content in the porous silicon-carbon composite may be 0.5 wt% to 20 wt%, 0.5 wt% to 15 wt%, or 0.5 wt% to 8 wt%, based on the total weight of the porous silicon-carbon composite material. If the magnesium (Mg) content in the porous silicon-carbon composite is less than 0.5 wt%, the cycle characteristics of the secondary battery may be reduced. If the magnesium (Mg) content exceeds 20 wt%, the charge capacity of the secondary battery may be reduced.

[0050] On the other hand, according to one embodiment of the present invention, the porous silicon-carbon composite may comprise a fluoride and / or silicate containing a metal other than magnesium. The other metal may be at least one selected from the group consisting of alkali metals, alkaline earth metals, Group 13 to Group 16 elements, transition metals, rare earth elements, and combinations thereof. Specific examples include Li, Ca, Sr, Ba, Y, Ti, Zr, Hf, V, Nb, Cr, Mo, W, Fe, Pb, Ru, Ir, Pd, Pt, Cu, Ag, Au, Zn, Cd, B, Al, Ga, Sn, In, Ti, Ge, P, As, Sb, Bi, S, and Se.

[0051] magnesium silicate The porous silicon-carbon composite may further comprise magnesium silicate.

[0052] The magnesium silicate may comprise MgSiO3 crystals, Mg2SiO4 crystals, or a mixture thereof.

[0053] In particular, when the porous silicon-carbon composite contains MgSiO3 crystals, the coulomb efficiency or capacity retention rate can be improved.

[0054] The magnesium silicate content may be 0 to 30 wt %, 0.5 to 25 wt %, or 0.5 to 20 wt %, based on the total weight of the porous silicon-carbon composite.

[0055] According to one embodiment of the present invention, in porous silicon-carbon composites, magnesium silicate can be converted to a fluorine-containing magnesium compound by etching.

[0056] For example, depending on the etching method or the extent of etching, some, most, or all of the magnesium silicate may be converted to fluorine-containing magnesium compounds. More specifically, most of the magnesium silicate may be converted to fluorine-containing magnesium compounds.

[0057] Silicon oxide compound The porous silicon-carbon composite may further comprise a silicon oxide compound (or silicon oxide compound).

[0058] The silicon oxide compound may be a silicon-based oxide represented by the formula SiO x (0.5 ≦ x ≦ 2). Specifically, the silicon oxide compound may be SiO x (0.8 < x ≦ 1.2), more specifically SiO x (0.9 < x ≦ 1.1). In the formula of SiO x When the value of x is less than 0.5, the expansion and contraction during charge and discharge of the secondary battery may increase, and the life characteristics may deteriorate. Also, when x exceeds 1.5, the amount of inert oxide increases, and there is a risk that the initial efficiency of the secondary battery may decrease.

[0059] The silicon oxide compound can be used in an amount of 0.1% to 45% by weight, preferably 0.1% to 35% by weight, more preferably 0.1% to 20% by weight based on the total weight of the porous silicon-carbon composite.

[0060] When the content of the silicon oxide compound is less than 0.1% by weight, the volume of the secondary battery may expand and the life characteristics may deteriorate. On the other hand, when the content of the silicon oxide compound exceeds 45% by weight, the initial irreversible reaction of the secondary battery may increase, thereby reducing the initial efficiency.

[0061] Pore In the porous silicon-carbon composite according to an embodiment of the present invention, the pores (or micropores or holes; pore) formed inside cause the volume expansion generated during charge and discharge to concentrate on the pores rather than outside the negative electrode active material. Thereby, the volume expansion is effectively controlled and the life characteristics of the lithium secondary battery are improved. In addition, the electrolyte can easily penetrate into the porous structure, improving the output characteristics and further improving the performance of the lithium secondary battery.

[0062] In this specification, pores may be used interchangeably with voids. In addition, pores may comprise open pores (or open pores), closed pores (or closed pores), or both. Closed pores refer to independent pores in which all pore walls have a closed structure and are not connected to other pores. In addition, open pores refer to pores in which at least a portion of the pore walls are formed in an open structure, and which may or may not be connected to other pores. In addition, open pores may refer to pores that are arranged on the surface of a silicon composite before carbon coating (before the carbon layer is formed) and are exposed to the outside.

[0063] The porosity of the pores in the porous silicon-carbon composite may be 0.1% to 40%, 10% to 35%, or 15% to 30% by volume, based on the volume of the porous silicon-carbon composite. The porosity may be the porosity of the closed pores of the porous silicon-carbon composite.

[0064] When the porosity of a porous silicon-carbon composite satisfies the above range, its use as a negative electrode active material in a secondary battery can provide sufficient mechanical strength while buffering volume expansion. This minimizes the volume expansion problem associated with the use of silicon particles, achieving high capacity and improving life characteristics. If the porosity of a porous silicon-carbon composite is less than 0.1% by volume, it may be difficult to control the volume expansion of the negative electrode active material during charging and discharging. If the porosity exceeds 40% by volume, the presence of many pores in the negative electrode active material reduces mechanical strength, and the negative electrode active material may collapse during the secondary battery manufacturing process, for example, during the mixing of the negative electrode active material slurry and the rolling process after coating.

[0065] The porous silicon-carbon composite may have a plurality of pores, each of which may have the same or different diameters.

[0066] carbon A porous silicon-carbon composite according to one embodiment of the present invention comprises carbon.

[0067] According to one embodiment of the present invention, the porous silicon-carbon composite contains carbon, which ensures sufficient conductivity and allows the specific surface area to be appropriately adjusted, thereby improving the life and capacity of the secondary battery when used as an anode active material.

[0068] Generally, the electrical conductivity of a negative electrode active material is an important factor for promoting electron transfer during an electrochemical reaction. When a composite as a negative electrode active material does not contain carbon, for example, when a high-capacity negative electrode active material is prepared using silicon particles and a fluorine-containing magnesium compound, the electrical conductivity may not reach an appropriate level.

[0069] By forming a carbon layer on the surface of a silicon composite composed of silicon particles and a fluorine-containing magnesium compound, the inventors have been able to improve the charge / discharge capacity, initial charge efficiency, and capacity retention rate, enhance mechanical properties, impart excellent conductivity even after the electrode expands during charge / discharge, suppress side reactions in the electrolyte, and further improve the performance of lithium secondary batteries.

[0070] The porous silicon-carbon composite has a carbon layer on the surface of the silicon composite, and carbon is present on some or all of the surfaces of the silicon particles and the fluorine-containing magnesium compound to form the carbon layer. The carbon layer in the porous silicon-carbon composite eliminates the difficulty of electrical contact between particles due to the presence of pores, and provides excellent conductivity even after the electrode expands during charge and discharge, further improving the performance of secondary batteries.

[0071] In addition, according to one embodiment of the present invention, by controlling the thickness or carbon content of the carbon layer, it is possible to achieve appropriate conductivity and prevent a decrease in life characteristics, thereby realizing a high-capacity negative electrode active material.

[0072] The porous silicon-carbon composite with the carbon layer formed has an average particle diameter (D 50 In addition, the average particle size may be determined by measuring the particle size distribution by a laser diffraction method, and the volume average value D 50 , i.e., the particle size or median size at 50% of the cumulative volume. Specifically, the average particle size (D 50 ) may be 1 μm to 20 μm, 3 μm to 10 μm, or 3 μm to 8 μm. If the average particle size of the porous silicon-carbon composite is less than 1 μm, there is a risk that the dispersion will deteriorate due to aggregation of the composite particles when preparing a negative electrode slurry (negative electrode active material composition) using this composite. In addition, if the average particle size of the porous silicon-carbon composite is more than 20 μm, the composite particles will expand significantly upon charging with lithium ions, reducing the bonding strength between the composite particles and the bonding strength between the particles. This may cause deterioration of the current collector with repeated charge and discharge, resulting in a significant decrease in life characteristics. In addition, there is a concern that the activity will decrease due to a decrease in specific surface area.

[0073] According to one embodiment, the carbon (C) content may be 3 wt % to 80 wt %, 3 wt % to 50 wt %, or 10 wt % to 30 wt %, based on the total weight of the porous silicon-carbon composite.

[0074] If the carbon (C) content is less than 3 wt%, the effect of increasing the conductivity cannot be fully expected, and the electrode life of the lithium secondary battery may be shortened. In addition, if the carbon (C) content exceeds 80 wt%, the discharge capacity of the secondary battery may decrease, the bulk density may decrease, and the charge / discharge capacity per unit volume may decrease.

[0075] The carbon layer may have an average thickness of 1 nm to 300 nm, or 3 nm to 150 nm, more specifically 5 nm to 100 nm. When the carbon layer has a thickness of 1 nm or more, the conductivity can be improved. When the thickness is 300 nm or less, the decrease in the capacity of the secondary battery can be suppressed.

[0076] The average thickness of the carbon layer can be measured, for example, by the following procedure.

[0077] First, the negative electrode active material is observed under a transmission electron microscope (TEM) at an arbitrary magnification. The magnification is preferably such that it can be seen with the naked eye. Next, the thickness of the carbon layer is measured at 15 arbitrary points. In such cases, it is preferable to select the measurement positions randomly over as wide a range as possible, rather than concentrating them in a specific area. Finally, the average thickness of the carbon layer at the 15 points is calculated.

[0078] The carbon layer may comprise at least one selected from the group consisting of graphene, reduced graphene oxide, carbon nanotubes, carbon nanofibers, and graphite. For example, the carbon layer may comprise at least one selected from graphene, reduced graphene oxide, carbon nanotubes, and carbon nanofibers. Specifically, the carbon layer may comprise graphene. Additionally, the carbon layer may further comprise graphite.

[0079] Additionally, a porous silicon-carbon composite according to one embodiment of the present invention may be formed by heat-treating a porous silicon composite at high temperatures with a carbon source gas or vapor, and may contain pores within the composite. The pores may be present on the surface, the interior, or both of the silicon composite. The surface of the silicon composite may refer to the outermost portion of the silicon composite. The interior of the silicon composite may refer to a portion other than the outermost portion, i.e., the portion inside the outermost portion.

[0080] Additionally, a porous silicon-carbon composite is a composite in which multiple silicon particles are uniformly distributed in the composite and the structure thereof has the shape of a single mass, for example, a polyhedron, a sphere, etc. It may be a single structure in which carbon, more specifically, a carbon layer containing carbon, surrounds part or all of the surface of one or more silicon particles (primary silicon particles) or the surface of a secondary silicon particle (mass) by agglomeration of two or more silicon particles.

[0081] Porous silicon-carbon composites are 2m 2 / g~60m 2 / g, 3m 2 / g~50m 2 / g, or 3m 2 / g~40m 2 / g (Brunauer-Emmett-Teller method; BET). 2 If the content is less than 60m / g, the rate characteristics of the secondary battery may be reduced. 2 If the content exceeds 1 / g, it may be difficult to prepare a negative electrode slurry suitable for application to the negative electrode current collector of a secondary battery, and the contact area with the electrolyte may increase, accelerating the decomposition reaction of the electrolyte or causing a side reaction in the secondary battery.

[0082] The specific gravity of the porous silicon-carbon composite is 1.8 g / cm 3 ~2.6g / cm 3 , specifically 1.8 g / cm 3 ~2.5g / cm 3 , more specifically 2.0 g / cm 3 ~2.5g / cm 3 The specific gravity may vary depending on the amount of carbon layer applied. While the amount of carbon is constant, the higher the specific gravity within the above range, the fewer pores there will be in the composite. Therefore, when used as a negative electrode active material, the conductivity is improved, the matrix strength is strengthened, and the initial efficiency and cycle life characteristics are thereby improved. In this case, the specific gravity may refer to true specific gravity, density, or true density. According to an embodiment of the present invention, the specific gravity can be measured using, for example, a dry density meter, such as Shimadzu's Accupyk II 1340. Helium gas is used as the purge gas, and the measurement is performed after purging 200 times in a sample holder set at 23°C.

[0083] The specific gravity of the porous silicon-carbon composite is 1.8 g / cm 3If the specific gravity is 2.6 g / cm or more, dissociation between the negative electrode active material powder particles due to volume expansion of the negative electrode active material powder during charging can be prevented, and cycle deterioration can be suppressed. 3 If the ratio is equal to or less than this, the impregnation of the electrolyte solution is improved, the utilization rate of the negative electrode active material is increased, and the initial charge / discharge capacity can be increased.

[0084] [Method for manufacturing porous silicon-carbon composites] A method for producing a porous silicon-carbon composite according to one embodiment of the present invention comprises the following steps: a first step of obtaining a silicon composite oxide powder using a silicon-based raw material and a magnesium-based raw material; a second step of etching the silicon composite oxide powder using an etching solution containing a fluorine (F) atom-containing compound; a third step of filtering and drying the composite obtained by etching to obtain a porous silicon composite; and a fourth step of forming a carbon layer on the surface of the porous silicon composite using a chemical pyrolysis deposition method.

[0085] The method according to this embodiment has the advantage that mass production is possible through a continuous process with the number of steps minimized.

[0086] Specifically, in the method for producing a porous silicon-carbon composite, the first step may comprise obtaining a silicon composite oxide powder using a silicon-based raw material and a magnesium-based raw material.

[0087] The silicon-based raw material may comprise at least one selected from the group consisting of silicon powder, silicon oxide powder, and silicon dioxide powder.

[0088] The magnesium-based source may comprise metallic magnesium.

[0089] The first step can be carried out, for example, using the method described in Korean Patent Publication No. 10-2018-0106485.

[0090] The magnesium (Mg) content in the silicon composite oxide powder may be 0.5 to 20% by weight, 0.5 to 15% by weight, or 0.5 to 8% by weight, based on the total weight of the silicon composite oxide. If the magnesium (Mg) content in the silicon composite oxide is less than 0.5% by weight, the cycle characteristics of the secondary battery may be reduced, and if it exceeds 20% by weight, the charge capacity of the secondary battery may be reduced.

[0091] The silicon composite oxide has a specific surface area (Brunauer-Emmett-Teller method; BET) of 2 m 2 / g~100m 2 / g, 3m 2 / g~80m 2 / g, or 3m 2 / g~50m 2 / g. The specific surface area of ​​the silicon composite oxide may be 2m 2 If the average particle size is less than 100m / g, the particles will have a too large average particle size. Therefore, when the material is applied to a current collector as a negative electrode active material for a secondary battery, irregularities may be formed on the electrode, possibly shortening the life of the secondary battery. 2 If the SiO2 content exceeds 1 / g, it becomes difficult to control the heat generated by the etching reaction in the second step, and the yield of the composite after etching may decrease.

[0092] According to one embodiment of the present invention, the method may further comprise the step of forming a carbon layer on the surface of the silicon composite oxide by chemical pyrolysis deposition.

[0093] Specifically, after forming a carbon layer on the surface of a silicon composite oxide powder containing silicon particles and a fluorine-containing magnesium compound, the second etching step may be carried out, which has the advantage of enabling uniform etching and achieving a high yield.

[0094] The step of forming the carbon layer can be carried out in the same manner as the step of forming the carbon layer in the fourth step described below.

[0095] In the method for producing a porous silicon-carbon composite, the second step may comprise etching the silicon composite oxide powder with an etching solution containing a fluorine (F) atom-containing compound.

[0096] The etching process may comprise dry etching and wet etching.

[0097] Dry etching allows selective etching.

[0098] The etching process dissolves and leaches the silicon dioxide of the silicon composite oxide powder, thereby forming pores.

[0099] Magnesium silicate can be converted to a fluorine-containing magnesium compound by an etching process to prepare a porous silicon composite comprising silicon particles and a fluorine-containing magnesium compound.

[0100] In the etching step, the silicon composite oxide powder is etched using an etching solution containing a compound containing fluorine (F) atoms to form pores.

[0101] The silicon composite oxide powder is etched with a fluorine (F) atom-containing compound (e.g., HF) to convert the magnesium silicate into a fluorine-containing magnesium compound, and simultaneously the silicon dioxide is dissolved to form pores in the removed area. As a result, a porous silicon composite containing silicon particles and a fluorine-containing magnesium compound can be prepared.

[0102] For example, in the etching step using HF, when dry etching is performed, the reaction can be represented by the following reaction schemes G1 and G2, and when wet etching is performed, the reaction can be represented by the following reaction schemes L1a to L2. MgSi3+ 6HF (gas) → SiF4(g) + MgF2+ 3H2O (G1) Mg2SiO4+ 8HF (gas) → SiF4(g) + 2MgF2+ 4H2O (G2) MgSiO3+ 6HF (aqueous solution) → MgSiF6+ 3H2O (L1a) MgSiF6+ 2HF (aqueous solution) → MgF2+ H2SiF6(L1b) MgSiO3+ 2HF → SiO2+ MgF2+ H2O (L1c) SiO2+ 6HF (l) → H2SiF6+ 2H2O (L1d) MgSiO3+ 8HF (aqueous solution) → MgF2+ H2SiF6+ 3H2O (L1) Mg2SiO4+ 8HF (aqueous solution) → MgSiF6+ MgF2+ 4H2O (L2a) MgSiF6+ 2HF (aqueous solution) → MgF2+ H2SiF6(L2b) Mg2SiO4+ 4HF (aqueous solution) → SiO2+ 2MgF2+ 2H2O (L2c) SiO2+ 6HF (aqueous solution) → H2SiF6+ 2H2O (L2d) Mg2SiO4+ 10HF (aqueous solution) → 2MgF2+ H2SiF6+ 4H2O (L2)

[0103] Additionally, the pores can be thought of as being formed by the following reaction schemes (3) and (4): SiO2+ 4HF (gas) → SiF4+ 2H2O (3) SiO2+ 6HF (aqueous solution) → H2SiF6+ 2H2O (4)

[0104] By a reaction mechanism such as the above reaction scheme, pores (voids) can be formed by dissolving and removing silicon dioxide in the form of SiF4 and H2SiF6.

[0105] In addition, depending on the extent of etching, the silicon dioxide contained in the porous silicon composite may be removed, forming pores.

[0106] The degree of pore formation may vary depending on the degree of etching. For example, almost no pores may be formed, or pores may be formed partially, specifically, pores may be formed only in the outer portion.

[0107] In addition, the O / Si ratio and specific surface area of ​​the porous silicon composite may change significantly during etching, and the specific surface area and specific gravity of the silicon composite with pores may change significantly before and after carbon coating.

[0108] According to one embodiment of the present invention, after etching, the surface may contain crystals of both fluorine-containing magnesium compounds and magnesium silicate.

[0109] Alternatively, the as-etched composite may comprise porous silicon particles, magnesium fluoride, and magnesium silicate. Additionally, even if the magnesium silicate pattern is barely detectable in the X-ray diffraction analysis, the as-etched composite may contain porous silicon particles and a fluorine-containing magnesium compound.

[0110] Etching can produce silicon composite powders with multiple pores formed on the surface or on the surface and inside of the composite particles. In addition, closed pores may be formed inside the porous silicon composite.

[0111] Additionally, according to one embodiment, after etching, crystals of both the fluorine-containing magnesium compound and magnesium silicate may be present. Here, etching refers to a process in which the silicon composite oxide powder is treated with an acidic aqueous solution, such as an etching solution containing a fluorine (F) atom-containing compound. Examples of a method for treating with an etching solution include adding the silicon composite oxide powder to the etching solution and stirring. Alternatively, the silicon composite oxide may be dispersed in a dispersion medium, and then the etching solution may be added to perform etching. The dispersion medium may comprise at least one selected from the group consisting of water, alcohol-based compounds, ketone-based compounds, ether-based compounds, hydrocarbon-based compounds, and fatty acids.

[0112] The stirring temperature (treatment temperature) is not particularly limited.

[0113] As the etching solution containing a fluorine (F) atom-containing compound, any commonly used etching solution can be used without limitation as long as it does not impair the effects of the present invention.

[0114] Specifically, the fluorine (F) atom-containing compound may include at least one selected from the group consisting of HF, NHF, NH, and HF. By using a fluorine (F) atom-containing compound, the porous silicon-carbon composite can include a fluorine-containing magnesium compound, which allows the etching process to be carried out more quickly.

[0115] In addition, the etching solution may further contain one or more selected from the group consisting of organic acids, sulfuric acid, hydrochloric acid, phosphoric acid, nitric acid, and chromic acid.

[0116] In silicon composite oxide powders, some silicon oxides may remain in addition to silicon dioxide, and the portions where silicon-containing oxides such as silicon dioxide and silicon oxide are removed by etching may form voids and pores inside the particles.In addition, most of the magnesium silicate reacts with fluorine (F) in the fluorine (F) atom-containing compound in the etching solution to form fluorine-containing magnesium compounds.

[0117] The composite obtained by etching is porous and may comprise silicon particles and fluorine-containing magnesium compounds.

[0118] Etching can result in porous composites with multiple pores formed on the surface, inside, or both of the composite particles. Additionally, the number of oxygen atoms present on the surface of the composite can be reduced by etching.

[0119] In other words, etching can significantly reduce the oxygen fraction on the composite surface, thereby reducing the surface resistance. As a result, when this composite is used as a negative electrode active material, the electrochemical properties, especially the life characteristics, of lithium secondary batteries can be significantly improved.

[0120] In addition, because a large amount of silicon dioxide is removed by selective etching, the silicon composite oxide can contain a very high fraction of silicon (Si) relative to oxygen (O). In other words, the molar ratio of oxygen (O) to silicon (Si) atoms (O / Si) present in the porous composite can be significantly reduced. In this case, a secondary battery with high capacity, excellent cycle characteristics, and improved initial charge / discharge efficiency can be obtained.

[0121] Additionally, pores or voids may be formed where the silicon dioxide was removed, which may increase the specific surface area of ​​the silicon composite compared to the specific surface area of ​​the silicon composite oxide before etching.

[0122] The molar ratio (O / Si) of oxygen (O) atoms to silicon (Si) atoms present in the porous composite can be 0.01 to 1, 0.03 to 0.7, or 0.03 to 0.6. If this ratio is outside the above range, it acts as a resistance during the lithium intercalation reaction, which can degrade the electrochemical characteristics of the secondary battery. As a result, the electrochemical characteristics, particularly the life characteristics, of the lithium secondary battery can be degraded.

[0123] In the porous silicon composite (precursor before carbon coating) obtained by etching, the silicon (Si) content may be 30 wt % to 99 wt %, 30 wt % to 80 wt %, or 30 wt % to 70 wt %, based on the total weight of the porous silicon composite.

[0124] If the silicon (Si) content is less than 30 wt%, the amount of active material that absorbs and releases lithium will be small, which may result in a decrease in the charge / discharge capacity of the lithium secondary battery.On the other hand, if the silicon (Si) content exceeds 99 wt%, while the charge / discharge capacity of the lithium secondary battery will be large, the electrode will expand and contract too much during charge / discharge, which may cause the negative electrode active material to deteriorate, the powder to become even finer, and the cycle characteristics may deteriorate.

[0125] The magnesium (Mg) content in the porous silicon composite can be 0.5 wt% to 20 wt%, 0.5 wt% to 10 wt%, or 0.5 wt% to 6 wt% based on the total weight of the porous silicon composite. If the magnesium (Mg) content in the porous silicon composite is less than 0.5 wt%, the cycle characteristics of the secondary battery may be reduced. If the magnesium (Mg) content exceeds 20 wt%, the charge capacity of the secondary battery may be reduced.

[0126] According to one embodiment of the present invention, physical properties such as element content and specific surface area may be changed before and after the etching process, i.e., the physical properties such as element content and specific surface area may be different between the silicon composite oxide before the etching process and the silicon composite after the etching process.

[0127] In addition, the silicon composite can be formed from silicon composite oxide (Mg x SiO y , 0 < x ≤ 0.2, 0.8 < y < 1.2). It has a plurality of silicon particles uniformly distributed in the composite, and its structure is a single mass, such as a polyhedron, a sphere, or a similar shape. It may include secondary silicon particles formed by bonding of two or more silicon particles (primary silicon particles). The fluorine-containing magnesium compound may be present on the surface of the silicon particles or between the silicon particles. In addition, the fluorine-containing magnesium compound may be present inside the silicon particles.

[0128] In addition, the porous silicon composite according to an embodiment of the present invention may include pores. Specifically, pores may be included on the surface, inside, or both of the silicon composite. In the method for producing a porous silicon-carbon composite, the third step may include filtering and drying the composite obtained by etching to obtain a porous silicon composite. The filtering and drying steps can be performed by commonly used methods.

[0129] In the method for producing a porous silicon-carbon composite, the fourth step may include forming a carbon layer on the surface of the porous silicon composite using a chemical vapor deposition method.

[0130] The electrical contact between the particles of the porous silicon-carbon composite can be enhanced by the carbon layer formation step. In addition, as charge and discharge proceed, excellent conductivity can be imparted even after the electrode expands, and the performance of the secondary battery can be further improved. Specifically, the carbon layer can increase the conductivity of the negative electrode active material, improve the output characteristics and cycle characteristics of the battery, and enhance the stress relaxation effect during volume change of the active material.

[0131] The carbon layer may include at least one selected from the group consisting of graphene, reduced graphene oxide, carbon nanotubes, carbon nanofibers, and graphite.

[0132] The step of forming the carbon layer can be carried out by injecting at least one carbon source gas selected from the compounds represented by the following formulas 1 to 3, and reacting the porous silicon composite obtained in the third step in a gaseous state at 400°C to 1,200°C. [Formula 1] C N H (2N + 2-A) [OH] A [In formula 1, N is an integer of 1 to 20, and A is 0 or 1] [Formula 2] C N H (2N-B) [In formula 2, N is an integer of 2 to 6, and B is an integer of 0 to 2], and [Formula 3] C x H y O z [In formula 3, x is an integer of 1 to 20, y is an integer of 0 to 25, and z is an integer of 0 to 5]

[0133] Additionally, in formula 3, x may be equal to or less than y.

[0134] Additionally, in formula 3, y is an integer greater than 0 and equal to or less than 25, or an integer from 1 to 25; and z is an integer greater than 0 and equal to or less than 5, or an integer from 1 to 5.

[0135] The compound represented by formula 1 may be at least one selected from the group consisting of methane, ethane, propane, butane, methanol, ethanol, propanol, propanediol, and butanediol. The compound represented by formula 2 may be at least one selected from the group consisting of ethylene, propylene, butylene, butadiene, and cyclopentene. The compound represented by formula 3 may be at least one selected from the group consisting of acetylene, benzene, toluene, xylene, ethylbenzene, naphthalene, anthracene, and dibutylhydroxytoluene (BHT).

[0136] The carbon source gas may further contain at least one inert gas selected from hydrogen, nitrogen, helium, and argon. The reaction may be carried out at a temperature of 400°C to 1200°C, specifically 500°C to 1100°C, and more specifically 600°C to 1000°C.

[0137] The reaction time (or heat treatment time) can be adjusted appropriately depending on the heat treatment temperature, the pressure during heat treatment, the composition of the mixed gas, and the desired carbon coating amount. For example, the reaction time may be, but is not limited to, 10 minutes to 100 hours, specifically 30 minutes to 90 hours, and more specifically 50 minutes to 40 hours. Without being bound by any particular theory, it is believed that a longer reaction time increases the thickness of the carbon layer formed, which may improve the electrical properties of the composite.

[0138] A method for producing a porous silicon-carbon composite according to one embodiment of the present invention can form a thin, uniform carbon layer containing at least one selected from graphene, reduced graphene oxide, carbon nanotubes, carbon nanofibers, and graphite. Specifically, at least one selected from graphene, reduced graphene oxide, carbon nanotubes, and carbon nanofibers is present on the surface of a porous silicon composite, even at relatively low temperatures, due to a gas-phase reaction of a carbon source gas. In addition, desorption reactions in the carbon layer do not substantially occur.

[0139] Furthermore, because a carbon layer is formed uniformly over the entire surface of the silicon composite by the gas-phase reaction, a highly crystalline carbon film (carbon layer) can be formed. Therefore, when the porous silicon-carbon composite is used as a negative electrode active material, the conductivity of the negative electrode active material can be improved without changing its structure.

[0140] According to one embodiment of the present invention, when a reactive gas containing a carbon source gas is supplied to the surface of a silicon composite, one or more graphene-containing materials selected from graphene, reduced graphene oxide, and graphene oxide, carbon nanotubes, or carbon nanofibers are formed on the surface of the silicon particles. Over the course of the reaction time, the graphene-containing materials are gradually dispersed and formed, resulting in a porous silicon-carbon composite.

[0141] The specific surface area of ​​the porous silicon-carbon composite can decrease depending on the amount of carbon coating.

[0142] The structure of the graphene-containing material may be layered, nanosheet-like, or a mixture of multiple flakes.

[0143] When a carbon layer containing a graphene-containing material is uniformly formed over the entire surface of a silicon composite, the highly conductive and flexible graphene-containing material grows directly on the surfaces of the silicon particles and the fluorine-containing magnesium compound, suppressing volume expansion. In addition, the carbon layer coating reduces the opportunities for direct contact between the silicon and the electrolyte, thereby reducing the formation of a solid electrolyte interface (SEI) layer.

[0144] Additionally, the porous silicon-carbon composites exhibited average particle sizes (D) of 1 μm to 20 μm, 3 μm to 10 μm, or 3 μm to 8 μm in the volumetric distribution as measured by laser diffraction. 50 ) D 50 If the diameter is less than 1 μm, the bulk density may be too small, resulting in a decrease in the charge / discharge capacity per unit volume. 50If the average particle diameter (D 50 ) is the particle size or median diameter at 50% cumulative volume in particle size distribution measurement by laser light diffraction method. 50 The value measured as:

[0145] Additionally, according to one embodiment of the present invention, the process may further comprise a crushing step or a pulverizing and classifying step of the porous silicon-carbon composite. Classification is performed to adjust the particle size distribution of the porous silicon-carbon composite, and dry classification, wet classification, and sieving may be used. In dry classification, the steps of dispersion, separation, recovery (separation of solids and gas), and discharge are performed sequentially or simultaneously using an airflow, with pretreatment (adjusting moisture, dispersibility, humidity, etc.) performed during this process. To prevent a decrease in classification efficiency due to interparticle interference, particle shape, airflow turbulence, velocity distribution, static electricity, etc., a classification process is performed before classification to adjust the moisture or oxygen concentration in the gas phase used. Additionally, crushing or pulverization and classification may be performed simultaneously to obtain the desired particle size distribution. After crushing or pulverization, it is effective to separate the coarse powder fraction from the granular fraction using a classifier or sieve.

[0146] By crushing or pulverization and classification, porous silicon-carbon composite powders with average particle sizes of 1 μm to 20 μm, 3 μm to 8 μm, or 3 μm to 6 μm can be obtained.

[0147] Porous silicon-carbon composite powder has a D of 0.3 μm or less min and D of 8 μm to 30 μm maxWithin this range, the specific surface area of ​​the composite can be reduced, improving the initial efficiency and cycle characteristics by approximately 10% to 20% compared to before classification. Because the composite powder after crushing or pulverization and classification has amorphous and crystalline grain boundaries, the stress relaxation effect of the amorphous and crystalline grain boundaries reduces particle collapse during charge-discharge cycles. When such silicon particles are used as the negative electrode active material of a secondary battery, the negative electrode active material can withstand the stress of volume expansion and changes associated with charge and discharge, demonstrating the characteristics of a high-capacity, long-life secondary battery. Furthermore, they can reduce lithium-containing compounds such as Li2O present in the SEI layer formed on the surface of a silicon-based negative electrode.

[0148] According to one embodiment of the present invention, physical properties such as element content and specific surface area may be different before and after the etching process, i.e., in the silicon composite oxide before etching and the silicon composite or silicon-carbon composite after etching.

[0149] For example, the molar ratio of oxygen (O) atoms to silicon (Si) atoms (O / Si) present in the porous silicon-carbon composite can be 0.01 or more and less than 1. The molar ratio of oxygen (O) atoms to silicon (Si) atoms present in the porous silicon-carbon composite (O / Si) can be 0.02 to 0.90 or 0.03 to 0.6.

[0150] In this case, the oxygen fraction of the porous silicon-carbon composite can be significantly reduced, resulting in reduced surface resistance. As a result, when this composite is used as an anode active material, the electrochemical properties, especially the initial efficiency, of lithium secondary batteries can be significantly improved.

[0151] Additionally, the oxygen (O) content in the porous silicon-carbon composite may be reduced by 5 wt % to 95 wt %, specifically 5 wt % to 75 wt %, compared to the oxygen (O) content in the silicon composite oxide.

[0152] The method according to one embodiment of the present invention has the advantage that it allows for mass production through a continuous process with a minimum number of steps.

[0153] A secondary battery using a porous silicon-carbon composite as a negative electrode can have improved capacity, capacity retention rate, and initial efficiency.

[0154] negative electrode active material A negative electrode active material according to one embodiment of the present invention may comprise a porous silicon-carbon composite, that is, the negative electrode active material may comprise a porous silicon-carbon composite containing silicon particles, a fluorine-containing magnesium compound, and carbon.

[0155] The negative electrode active material may further include a carbon-based negative electrode material, specifically, a graphite-based negative electrode material.

[0156] The negative electrode active material can be a mixture of a porous silicon-carbon composite and a carbon-based negative electrode material, such as a graphite-based negative electrode material. In this case, the electrical resistance of the negative electrode active material can be reduced, and the expansion stress associated with charging can be alleviated. The carbon-based negative electrode material can include at least one material selected from the group consisting of natural graphite, artificial graphite, soft carbon, hard carbon, mesocarbon, carbon fiber, carbon nanotubes, pyrolytic carbon, coke, glass carbon fiber, sintered organic polymer compounds, and carbon black.

[0157] The content of the carbonaceous negative electrode material can be 30% to 90% by weight, specifically 30% to 80% by weight, and more specifically 50% to 80% by weight, based on the total weight of the negative electrode active material.

[0158] secondary battery According to one embodiment of the present invention, the present invention can provide a negative electrode comprising the above-described negative electrode active material and a secondary battery including the same.

[0159] The secondary battery may include a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte having a lithium salt dissolved therein. The negative electrode may include a negative electrode active material including a porous silicon-carbon composite.

[0160] The negative electrode may be composed of only a negative electrode mixture, or may be composed of a negative electrode current collector and a negative electrode mixture layer (negative electrode active material layer) supported thereon. Similarly, the positive electrode may be composed of only a positive electrode mixture, or may be composed of a positive electrode current collector and a positive electrode mixture layer (positive electrode active material layer) supported thereon. In addition, the negative electrode mixture and positive electrode mixture may further contain a conductive material and a binder.

[0161] The materials constituting the negative electrode current collector and the positive electrode current collector may be materials known in the art, and the binder and conductive material added to the negative electrode and the positive electrode may be materials known in the art.

[0162] When the negative electrode is composed of a current collector and an active material layer carried thereon, the negative electrode can be prepared by applying a negative electrode active material composition containing a porous silicon-carbon composite to the surface of the current collector and drying it.

[0163] Additionally, the secondary battery includes a non-aqueous liquid electrolyte, which may include a non-aqueous solvent and a lithium salt dissolved in the non-aqueous solvent. The non-aqueous solvent may be a solvent commonly used in the art. Specifically, an aprotic organic solvent may be used. Examples of aprotic organic solvents include cyclic carbonates such as ethylene carbonate, propylene carbonate, and butylene carbonate; cyclic carboxylic acid esters such as furanone; chain carbonates such as diethyl carbonate, ethyl methyl carbonate, and dimethyl carbonate; chain ethers such as 1,2-methoxyethane, 1,2-ethoxyethane, and ethoxymethoxyethane; and cyclic ethers such as tetrahydrofuran and 2-methyltetrahydrofuran. These may be used alone or in combination.

[0164] The secondary battery may comprise a non-aqueous secondary battery.

[0165] A negative electrode active material and a secondary battery using the porous silicon-carbon composite can improve capacity, initial charge-discharge efficiency, and capacity retention rate. [Example]

[0166] MODE FOR CARRYING OUT THE INVENTION The present invention will be described in more detail below with reference to examples. The following examples are provided to illustrate the present invention, and the scope of the present invention is not limited thereto.

[0167] Example Example 1 Preparation of porous silicon-carbon composites (1) Step 1: Using silicon powder, silicon dioxide powder, and metallic magnesium, a silicon composite oxide powder having the element contents and physical properties shown in Table 1 below was produced according to the method described in Example 1 of Korean Patent Publication No. 10-2018-0106485.

[0168] (2) Step 2: 50 g of silicon composite oxide powder was dispersed in water and stirred at a speed of 300 rpm, and 400 ml of a 30 wt % HF aqueous solution was added as an etching solution to etch the silicon composite oxide powder.

[0169] (3) Step 3: The porous composite obtained by the etching was filtered and dried for 2 hours at 150°C. Next, in order to control the particle size of the porous composite, it was crushed in a mortar to an average particle size of 5.8 μm, thereby preparing porous silicon composite (B1).

[0170] (4) Step 4: 10 g of the porous silicon composite was placed in a tubular electric furnace, and argon (Ar) gas and methane gas were passed through at a rate of 1 L / min. After holding at 900°C for 1 hour, the composite was cooled to room temperature, coating the surface of the porous silicon composite with carbon. This produced a porous silicon-carbon composite with the component contents and physical properties shown in Table 3 below.

[0171] (5) Step 5: To control the particle size of the porous silicon-carbon composite, the material was mechanically crushed and classified to an average particle size of 6.1 μm. A porous silicon-carbon composite (C1) was produced.

[0172] Secondary battery manufacturing A negative electrode and a battery (coin cell) containing the porous silicon-carbon composite as the negative electrode active material were fabricated. The negative electrode active material, Super-P as a conductive material, and polyacrylic acid were mixed with water in a weight ratio of 80:10:10 to prepare a negative electrode active material composition with a solid content of 45%.

[0173] This negative electrode active material composition was applied to a copper foil with a thickness of 18 μm and dried to prepare an electrode with a thickness of 70 μm. The copper foil with the electrode applied was punched out into a circle with a diameter of 14 mm to prepare a negative electrode plate for a coin cell.

[0174] The positive electrode plate was made of metallic lithium foil with a thickness of 0.3 mm.

[0175] A 25 μm thick porous polyethylene sheet was used as the separator. The electrolyte was a liquid electrolyte prepared by dissolving LiPF6 at a concentration of 1 M in a 1:1 volumetric mixture of ethylene carbonate (EC) and diethylene carbonate (DEC). Using the above components, a coin cell (battery) with a thickness of 3.2 mm and a diameter of 20 mm was fabricated.

[0176] <Examples 2 to 11> As shown in Tables 1 to 3 below, a silicon composite oxide powder having the element contents and physical properties shown in Table 1 was used, and the content of each component and the physical properties of the composite were adjusted by changing the type of dispersion medium, etching conditions, and the type and amount of carbon source gas. A porous silicon-carbon composite and a secondary battery using the same were produced in the same manner as in Example 1, except that the silicon composite oxide powder having the element contents and physical properties shown in Table 1 was used, and the content of each component and the physical properties of the composite were adjusted by changing the type of dispersion medium, etching conditions, and type and amount of carbon source gas.

[0177] <Comparative Example 1> As shown in Tables 1 to 3 below, a silicon-containing oxide having the element contents and physical properties shown in Table 1 was used, and a negative electrode active material and a secondary battery using the same were prepared in the same manner as in Example 1, except that the etching step in step (2) and the coating step in step (4) were not performed.

[0178] <Comparative Example 2> As shown in Tables 1 to 3 below, negative electrode active materials and secondary batteries using the same were produced in the same manner as in Example 1, except that the etching step of step (2) was not carried out.

[0179] <Comparative Example 3> As shown in Tables 1 to 3 below, negative electrode active materials and secondary batteries using the same were produced in the same manner as in Example 11, except that the etching step of step (2) in Example 11 was not carried out.

[0180] Test Example <Test Example 1> Electron microscope analysis The surfaces of the porous silicon composite (Composite B1) and the porous silicon-carbon composite (Composite C1) prepared in Example 1 were observed using a field emission scanning electron microscope (FE-SEM) (S-4700, Hitachi, Ltd.). The results are shown in Figure 1a and Figure 2, respectively.

[0181] Referring to FIG. 1a, pores were present on the surface of the porous silicon composite (composite B1) prepared in Example 1.

[0182] Furthermore, referring to Figure 2, field emission scanning electron microscope (FE-SEM) photographs of the surface of a porous silicon-carbon composite (Composite C1) containing a carbon layer on the surface of the porous silicon composite at various magnifications are shown in Figures 2a to 2d, respectively. As can be seen from Figures 2a to 2d, comparing Figures 1a and 2, there are differences in the surfaces of the composites, and it was confirmed that a carbon layer had formed on the surface of secondary silicon particles formed by the aggregation of silicon particles.

[0183] The interiors of the porous silicon composite (Composite B1) and the porous silicon-carbon composite (Composite C1) prepared in Example 1 were observed using ion beam scanning electron microscope (FIB-SEM) photographs (S-4700, Hitachi; QUANTA 3D FEG, FEI). The results are shown in Figure 1b and Figure 3, respectively.

[0184] Referring to Figure 1b, pores were present inside the porous silicon composite (Composite B1) produced in Example 1. From Figure 1b, it can be inferred that the pores were formed by the etching solution that permeated the porous silicon composite.

[0185] Furthermore, referring to Figure 3, when the interior of a porous silicon-carbon composite (composite C1) having a carbon layer on the surface of the porous silicon composite was observed, pores were observed inside the porous silicon composite even after a carbon coating layer was formed on the surface of the porous silicon composite.

[0186] On the other hand, Figure 4 shows (a) a FIB-SEM EDAX (S-4700, Hitachi; QUANTA 3D FEG, FEI; EDS system, EDAX) photograph of the porous silicon-carbon composite (Composite C1) produced in Example 1 and (b) a table of the analysis of the components in the composite.

[0187] Referring to Figure 4(b), when carbon coating with methane was performed after etching, a carbon content of approximately 15% was confirmed all the way to the interior of the porous silicon-carbon composite, suggesting that carbon was coated all the way to the inside of the pores after methane coating.

[0188] Furthermore, as shown in the component analysis table of FIG. 4(b), the porous silicon-carbon composite of Example 1 contained Mg, F, C, O, and Si components.

[0189] <Test Example 2> Carbon layer thickness analysis The thickness of the carbon-containing carbon layer of the porous silicon-carbon composite of Example 1 (composite C1) was analyzed.

[0190] FIG. 5 shows the results of a transmission electron microscope (TEM / EDS) analysis of the porous silicon-carbon composite (composite C1) prepared in Example 1.

[0191] To use TEM / EDS, milling was performed using a [FC-FI20]-FIB II instrument, and the thickness of the carbon layer was then analyzed using TEM.

[0192] As a result of the analysis in FIG. 5 and Table 3 below, the carbon content in the porous silicon-carbon composite of Example 1 was 34.9 wt %, and the thickness of the carbon layer when coated with methane gas was about 20 to 30 nm.

[0193] <Test Example 3> X-ray diffraction analysis The crystal structures of the silicon composite oxide (Composite A), porous silicon composite (Composite B), and porous silicon-carbon composite (Composite C) prepared in the examples were analyzed using an X-ray diffraction analyzer (Malvern Panalytical X'Pert3).

[0194] Specifically, the applied voltage was 40 kV and the applied current was 40 mA. The 2θ range was 10° to 90°, and measurements were performed by scanning at 0.05° intervals.

[0195] 6a to 6c show the results of X-ray diffraction analysis of a silicon composite oxide (composite A1) (6a), a porous silicon composite (composite B1) (6b), and the porous silicon-carbon composite of Example 1 (composite C1) (6c).

[0196] Referring to FIG. 6a, as can be seen from the X-ray diffraction pattern, the silicon composite oxide of Example 1 (Composite A1) has a peak corresponding to SiO around a diffraction angle (2θ) of 21.7°, peaks corresponding to Si crystals around diffraction angles (2θ) of 28.1°, 47.0°, 55.8°, 68.6°, and 76.1°, and peaks corresponding to MgSiO crystals around diffraction angles (2θ) of 30.4° and 35.0°, confirming that the silicon composite oxide is composed of amorphous SiO, crystalline Si, and MgSiO.

[0197] 6b, the X-ray diffraction pattern of the porous silicon composite of Example 1 (Composite B1) shows peaks corresponding to MgF2 crystals at diffraction angles (2θ) of 27.1°, 35.2°, 40.4°, 43.5°, 53.3°, 60.9°, and 67.9°, as well as peaks corresponding to Si crystals at diffraction angles (2θ) of 28.1°, 47.0°, 55.8°, 68.6°, and 76.1°. Furthermore, the disappearance of the peaks corresponding to MgSiO3 and the appearance of peaks corresponding to MgF2 indicate that MgSiO3 was converted to MgF2 by etching.

[0198] Referring to Figure 6c, the X-ray diffraction pattern of the porous silicon-carbon composite of Example 1 (Composite C1) shows peaks corresponding to MgF2 crystals at diffraction angles (2θ) of 27.1°, 35.2°, 40.4°, 43.5°, 53.3°, 60.9°, 67.9°, and 76.4°, as well as peaks corresponding to Si crystals at diffraction angles (2θ) of 28.1°, 47.0°, 55.8°, 68.6°, and 76.1°. There were no significant changes other than intensity before and after carbon coating. The diffraction angle (2θ) of carbon could not be confirmed because it overlapped with the Si(111) peak.

[0199] FIG. 7 shows the results of X-ray diffraction analysis of the porous silicon-carbon composite of Example 2 (composite C2).

[0200] Referring to Figure 7, the X-ray diffraction pattern of the silicon-carbon composite oxide (Composite C2) of Example 2 showed a peak corresponding to SiO2 at a diffraction angle (2θ) of approximately 21.7°, peaks corresponding to Si crystals at diffraction angles (2θ) of 28.1°, 47.0°, 55.8°, 68.6°, and 76.1°, peaks corresponding to MgSiO3 crystals at diffraction angles (2θ) of approximately 30.4° and 35.0°, and peaks corresponding to MgF2 crystals at diffraction angles (2θ) of approximately 26.9°, 34.8°, 40.1°, 43.4°, 53.1°, 60.3°, and 67.8°. This confirmed that the composite contained SiO2, crystalline Si, MgSiO3, and MgF2. In addition, the diffraction angle (2θ) of carbon could not be identified because it overlapped with the Si(111) peak.

[0201] FIG. 8 shows the results of X-ray diffraction analysis of the porous silicon-carbon composite of Example 5 (Composite C5).

[0202] 8, as can be seen from the X-ray diffraction pattern, the porous silicon-carbon composite of Example 5 (Composite C5) had peaks corresponding to MgF2 crystals at diffraction angles (2θ) of approximately 28.0°, 34.9°, 40.1°, 43.4°, 53.0°, and 60.2°, as well as peaks corresponding to Si crystals at diffraction angles (2θ) of approximately 28.1°, 28.1°, 47.1°, 55.8°, 68.9°, and 76.4°. In addition, the diffraction angle (2θ) of carbon could not be confirmed because it overlapped with the Si(111) peak.

[0203] On the other hand, the crystallite size of Si in the obtained porous silicon-carbon composite was calculated based on the full width at half maximum (FWHM) of the peak corresponding to Si(220) in X-ray diffraction analysis, using the following formula 1:

[0204] [Formula 1] Crystallite size (nm) = Kλ / Bcosθ [In Equation 1, K is 0.9, λ is 0.154 nm, B is the full width at half maximum (FWHM), and θ is the peak position (angle)] was calculated using the Scherrer equation.

[0205] <Test Example 4> Analysis of the content and specific gravity of constituent elements of composite material

[0206] The composites produced in the examples and comparative examples were analyzed for the contents of the constituent elements magnesium (Mg), silicon (Si), oxygen (O) and carbon (C).

[0207] The magnesium (Mg) and silicon (Si) contents were analyzed by inductively coupled plasma (ICP) optical emission spectroscopy using a PerkinElmer Optima-5300. The oxygen (O) content was measured using a LECO O-836 and the average of three measurements was obtained. The carbon (C) content was analyzed using a LECO CS-744 elemental analyzer. The fluorine (F) content was calculated based on the silicon (Si), oxygen (O), and magnesium (Mg) contents.

[0208] In addition, the composite was filled to 2 / 3 of a 10 ml container using Micromeritics' Accupyc II 1340, and the specific gravity (true specific gravity) was measured five times.

[0209] Test Example 5: Measurement of average particle size of composite particles The average particle diameter (D 50 ) is the particle size at 50% cumulative volume or the median particle size average value D 50 was measured as.

[0210] <Test Example 6> Raman Spectroscopic Analysis The porous silicon-carbon composite prepared in Example 1 was subjected to Raman spectroscopy. Raman analysis was performed at 2.41 eV (514 nm) using a micro-Raman analyzer (Renishaw, RM1000-In Via). The results are shown in Figure 9.

[0211] Referring to FIG. 9, the Raman spectrum obtained by Raman spectroscopy shows a peak at 2600 cm -1 ~2760cm -1 and 2D band peaks in the range of 1500 cm -1 ~1660cm -1 and a G-band peak in the range of 1,300 cm -1 ~1,460cm -1 The intensity of the 2D band peak was calculated as I 2D and the intensity of the D band peak is I D and the intensity of the G-band peak is I G Then, I D , I 2D , I G are 536, 102, and 402, respectively, and (I 2D +I G ) / I D was 0.94.

[0212] From the results of Raman spectroscopy, the carbon layer has the above-mentioned I D , I 2D , and IG It can be seen that the conductive material has good conductivity and can improve the characteristics of secondary batteries. 2D +I G ) / I D Since the value of 0.94 was found to be sufficient to suppress side reactions during charging and discharging, it was possible to prevent a decrease in initial efficiency.

[0213] Therefore, the porous silicon-carbon composite produced in Example 1 has excellent electrical conductivity and can significantly improve the performance of lithium secondary batteries.

[0214] <Test Example 7> Analysis of specific surface area

[0215] The composites prepared in the examples and comparative examples were placed in tubes and heated for 10 minutes using a MicrotracBEL pretreatment device (BELPREP-vac2). -2 The treatment was carried out at 100°C and 100 kPa for 5 hours.

[0216] The pretreated tube was attached to the analysis port of an analyzer (BELSORP-max) and the specific surface area was analyzed.

[0217] <Test Example 8> Measurement of capacity, initial efficiency, and capacity retention rate of secondary batteries The coin cells (secondary batteries) prepared in the examples and comparative examples were charged at a constant current of 0.1 C until the voltage reached 0.005 V, and then discharged at a constant current of 0.1 C until the voltage reached 2.0 V, and the charge capacity (mAh / g), discharge capacity (mAh / g), and initial efficiency (%) were measured. The results are shown in Table 4 below. [Formula 2] Initial efficiency (%) = discharge capacity / charge capacity x 100

[0218] In addition, the coin cells prepared in the examples and comparative examples were similarly charged and discharged once, and from the second cycle onwards, they were charged at a constant current of 0.5 C until the voltage reached 0.005 V, and then discharged at a constant current of 0.5 C until the voltage reached 2.0 V, and the cycle characteristics (capacity retention rate at 50 cycles, %) were measured. The results are shown in Table 4 below.

[0219] [Formula 3] Capacity retention rate after 50 cycles (%) = 51st discharge capacity / 2nd discharge capacity × 100

[0220] The contents and properties of each element of the composites produced in the examples and comparative examples are summarized in Tables 1 to 3. The characteristics of the secondary batteries using these composites are summarized in Table 4 below.

[0221] [Table 1]

[0222] [Table 2]

[0223] [Table 3]

[0224] [Table 4]

[0225] As can be seen from Table 4 above, the secondary batteries fabricated using the porous silicon-carbon composites of Examples 1 to 11 of the present invention had significantly improved performance in terms of discharge capacity, initial efficiency, and cycle characteristics compared to Comparative Examples 1 to 3.

[0226] Specifically, the secondary batteries of Examples 1 to 11 had excellent overall discharge capacities of 1398 mAh / g to 2074 mAh / g, and in particular initial efficiencies of 80.8% to 91.4%, and capacity retention rates after 50 cycles of 82.1% to 89.5%.

[0227] In contrast, the secondary battery of Comparative Example 1, which used a silicon-containing oxide, had an initial efficiency of 62.2% and a capacity retention rate after 50 cycles of 59.2%, indicating that the initial efficiency and capacity retention rate were significantly lower than those of the secondary batteries of the Examples.

[0228] On the other hand, when the secondary battery of Example 1 was compared with the secondary battery of Comparative Example 2, which was produced by the process of Example 1 but without etching, the secondary battery of Example 1 had a discharge capacity of 1,702 mAh / g, an initial efficiency of 90%, and a capacity retention rate after 50 cycles of 84.1%, whereas the secondary battery of Comparative Example 2 had a discharge capacity of 1,443 mAh / g, an initial efficiency of 79.6%, and a capacity retention rate after 50 cycles of 81.5%. This indicated that the discharge capacity, initial efficiency, and capacity retention rate of the latter were significantly lower than those of the former of Example 1.

[0229] In addition, when the secondary battery of Example 11 was compared with the secondary battery of Comparative Example 3, which was obtained by the process of Example 11 but without etching, the secondary battery of Example 11 had a discharge capacity of 1,398 mAh / g, an initial efficiency of 87.4%, and a capacity retention rate after 50 cycles of 83.7%, while the secondary battery of Comparative Example 3 had a discharge capacity of 1,219 mAh / g, an initial efficiency of 83.9%, and a capacity retention rate after 50 cycles of 80.1%. This indicated that the latter had significantly lower discharge capacity, initial efficiency, and capacity retention rate than the former of Example 11.

[0230] Therefore, it was confirmed that the secondary battery of Comparative Example 1, which did not use magnesium fluoride and carbon, and the secondary batteries of Comparative Examples 2 and 3, which did not use magnesium fluoride, showed overall reduced performance compared to the secondary batteries of the Examples. The preferred embodiments of the present invention are as follows. [1] A porous silicon-carbon composite comprising silicon particles, a fluorine-containing magnesium compound, and carbon. [2] The porous silicon-carbon composite according to [1], wherein the porous silicon-carbon composite has pores therein, and the porosity of the pores in the porous silicon-carbon composite is 0.1% by volume to 40% by volume, based on the volume of the porous silicon-carbon composite. [3] [1] The porous silicon-carbon composite according to [1], wherein the fluorine-containing magnesium compound comprises magnesium fluoride (MgF2), magnesium silicate fluoride (MgSiF6), or a mixture thereof. [4] The porous silicon-carbon composite according to [3], wherein the crystallite size of the magnesium fluoride (MgF2) measured by X-ray diffraction analysis is 2 nm to 35 nm. [5] The porous silicon-carbon composite according to [1], further comprising magnesium silicate. [6] [5] The porous silicon-carbon composite according to [5], wherein the magnesium silicate comprises MgSiO3 crystals, Mg2SiO4 crystals, or a mixture thereof. [7] The porous silicon-carbon composite according to [1], wherein the content of magnesium (Mg) in the porous silicon-carbon composite is 0.5 wt % to 20 wt % based on the total weight of the porous silicon-carbon composite. [8] [3] A porous silicon-carbon composite according to [3], which has an IB / IA ratio greater than 0 and less than 1 in X-ray diffraction analysis, where IB / IA is the ratio of the diffraction peak intensity (IB) of the MgF2(111) crystal plane to the diffraction peak intensity (IA) of the Si(220) crystal plane. [9] The porous silicon-carbon composite according to [1], further comprising a silicon oxide compound.

[10] The silicon oxide compound is SiO x The porous silicon-carbon composite according to [9], wherein x is 0.5≦x≦2.

[11] The porous silicon-carbon composite according to [1], wherein the silicon (Si) content is 8% by weight to 95% by weight based on the total weight of the porous silicon-carbon composite.

[12] The porous silicon-carbon composite according to [1], wherein the silicon particles are in an amorphous form, a crystalline form having a crystallite size of 2 nm to 20 nm, or a mixture thereof.

[13] The porous silicon-carbon composite according to [1], wherein the porous silicon-carbon composite has a silicon composite and a carbon layer on its surface, the silicon particles and the fluorine-containing magnesium compound are present in the silicon composite, and the carbon is present on part or all of the surfaces of the silicon particles and the fluorine-containing magnesium compound to form a carbon layer.

[14] The porous silicon-carbon composite according to

[13] , wherein the molar ratio of oxygen atoms to silicon atoms (O / Si) present in the porous silicon-carbon composite is 0.01 or more and less than 1.

[15]

[13] The porous silicon-carbon composite according to

[13] , wherein the carbon layer comprises at least one selected from the group consisting of graphene, reduced graphene oxide, carbon nanotubes, carbon nanofibers, and graphite.

[16] The porous silicon-carbon composite according to [1], wherein the carbon (C) content is 3% by weight to 80% by weight based on the total weight of the porous silicon-carbon composite.

[17] The porous silicon-carbon composite according to

[13] , wherein the carbon layer has a thickness of 1 nm to 300 nm.

[18] Average particle size (D 50 ) The porous silicon-carbon composite according to [1].

[19] 1.8g / cm 3 ~2.6g / cm 3 It has a specific gravity of 2m 2 / g~60m 2 The porous silicon-carbon composite described in [1] has a specific surface area (Brunauer-Emmett-Teller method; BET) of 1000 s / g.

[20] A first step of obtaining a silicon composite oxide powder using a silicon-based raw material and a magnesium-based raw material; a second step of etching the silicon composite oxide powder using an etching solution containing a fluorine (F) atom-containing compound; a third step of filtering and drying the composite obtained by etching to obtain a porous silicon composite; a fourth step of forming a carbon layer on the surface of the porous silicon composite using chemical pyrolysis deposition; 1. A method for producing a porous silicon-carbon composite, comprising: [twenty one]

[20] The method for producing a porous silicon-carbon composite according to

[20] , further comprising crushing or pulverizing and classifying the porous silicon-carbon composite after forming the carbon layer in the fourth step so that the composite has an average particle size of 1 μm to 20 μm. [twenty two]

[20] The method for producing a porous silicon-carbon composite according to

[20] , wherein in the second step, the etching solution further comprises one or more acids selected from the group consisting of organic acids, sulfuric acid, hydrochloric acid, phosphoric acid, nitric acid, and chromic acid. [twenty three] The formation of the carbon layer in the fourth step is carried out by reacting the carbon layer with the carbon nanotube material in accordance with the following formulas 1 to 3: [Formula 1] C N H (2N+2-A) [OH] A [In the formula 1, N is an integer of 1 to 20, and A is 0 or 1] [Formula 2] C N H (2N-B) [In the formula 2, N is an integer of 2 to 6, and B is an integer of 0 to 2], and [Formula 3] C x H y O z [In the formula 3, x is an integer of 1 to 20, y is an integer of 0 to 25, and z is an integer of 0 to 5] and reacting the mixture in a gaseous state at 400°C to 1200°C. [twenty four]

[20] The method for producing a porous silicon-carbon composite according to

[20] , further comprising forming a carbon layer on the surface of the silicon composite oxide using a chemical pyrolysis deposition method in the first step. [twenty five] A negative electrode active material comprising the porous silicon-carbon composite according to [1].

[26] The negative electrode active material according to

[25] , further comprising a carbon-based negative electrode material.

[27] The negative electrode active material according to

[26] , wherein the content of the carbon-based negative electrode material is 30% by weight to 90% by weight based on the total weight of the negative electrode active material.

[28] A lithium secondary battery comprising the negative electrode active material according to

[25] .

Claims

1. A porous silicon-carbon composite comprising silicon particles, a fluorine-containing magnesium compound, and carbon, The fluorine-containing magnesium compound is magnesium fluoride (MgF ２ ), magnesium fluoride silicate (MgSiF ６ ), or mixtures thereof; The magnesium fluoride (MgF) measured by X-ray diffraction analysis ２ ) has a crystallite size of 2 nm to 35 nm, the porous silicon-carbon composite further comprises magnesium silicate; the magnesium silicate comprises MgSiO 3 crystals, Mg 2 SiO 4 crystals, or a mixture thereof; The porous silicon-carbon composite has a silicon composite and a carbon layer on the surface thereof, the silicon particles and the fluorine-containing magnesium compound are present in the silicon composite, and the carbon is present on part or all of the surfaces of the silicon particles and the fluorine-containing magnesium compound to form a carbon layer; A porous silicon-carbon composite in which the molar ratio of oxygen atoms to silicon atoms (O / Si) present is 0.01 or more and less than 1.

2. 2. The porous silicon-carbon composite of claim 1, wherein the porous silicon-carbon composite has pores therein, the porosity of the pores in the porous silicon-carbon composite being 0.1% to 40% by volume, based on the volume of the porous silicon-carbon composite.

3. 2. The porous silicon-carbon composite of claim 1, wherein the magnesium (Mg) content in the porous silicon-carbon composite is 0.5 wt % to 20 wt %, based on the total weight of the porous silicon-carbon composite.

4. In X-ray diffraction analysis, the IB / IA is greater than 0 and less than 1, and the IB / IA is the ratio of the diffraction peak intensity (IA) of the Si (220) crystal plane to the MgF ２ 2. The porous silicon-carbon composite of claim 1, wherein the ratio is the ratio of diffraction peak intensities (IB) of the (111) crystal plane.

5. 10. The porous silicon-carbon composite of claim 1, further comprising a silicon oxide compound.

6. 2. The porous silicon-carbon composite of claim 1, wherein the carbon (C) content is 3% to 80% by weight, based on the total weight of the porous silicon-carbon composite.

7. A first step of obtaining a silicon composite oxide powder using a silicon-based raw material and a magnesium-based raw material; a second step of etching the silicon composite oxide powder using an etching solution containing a fluorine (F) atom-containing compound; a third step of filtering and drying the composite obtained by etching to obtain a porous silicon composite; a fourth step of forming a carbon layer on the surface of the porous silicon composite using chemical pyrolysis deposition; A method for producing the porous silicon-carbon composite of any one of claims 1 to 6, comprising:

8. The formation of the carbon layer in the fourth step is carried out by reacting the carbon layer with the carbon nanotube in the presence of ... [Formula 1] C Ｎ H （２Ｎ＋２－Ａ） [OH] Ａ [In the formula 1, N is an integer of 1 to 20, and A is 0 or 1], [Formula 2] C Ｎ H （２Ｎ－Ｂ） [In the formula 2, N is an integer of 2 to 6, and B is an integer of 0 to 2], and [Formula 3] C ｘ H ｙ O ｚ [In the formula 3, x is an integer of 1 to 20, y is an integer of 0 to 25, and z is an integer of 0 to 5] and reacting the mixture in a gaseous state at 400°C to 1200°C.

9. A negative electrode active material comprising the porous silicon-carbon composite of claim 1.

10. A lithium secondary battery comprising the negative electrode active material according to claim 9.

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