Porous silicon composite, porous silicon carbon composite containing the same, and anode active material

Abstract

One embodiment of the present invention relates to a porous silicon composite, a porous silicon carbon composite containing the same, and an anode active material, in which the porous silicon composite and the porous silicon carbon composite each contain both silicon particles and a magnesium compound, and have a molar ratio (O / Si) of oxygen (O) atoms to silicon (Si) atoms that satisfies a specific range, and by applying the porous silicon composite and the porous silicon carbon composite to the anode active material, it is possible to realize an excellent capacity retention rate, and to significantly improve the discharge capacity and initial efficiency.

Description

[Technical Field]

[0001] The present invention relates to a porous silicon composite, a porous silicon carbon composite, and an anode active material containing them. [Background technology]

[0002] 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 a growing demand for higher energy density batteries to power these devices. The battery that best meets this demand is the lithium secondary battery, and active research is being conducted into the application of these batteries to small batteries, 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. Silicon-based negative electrode active materials are being researched to further increase battery capacity. The theoretical capacity of silicon (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 when lithium is intercalated into silicon is, for example, as follows: [ka]

[0005] The silicon-based anode active material produced by the above reaction scheme can form high-capacity alloys containing up to 4.4 lithium atoms per silicon atom. However, in most silicon-based anode active materials, lithium intercalation induces a volume expansion of up to 300%, destroying the anode and making it difficult to demonstrate high cycleability.

[0006] Furthermore, this volume change may cause cracks on the surface of the negative electrode active material, forming ionic substances within the negative electrode active material, which may electrically separate the negative electrode active material from the current collector, resulting in a significant decrease in 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, and the surfaces of the silicon particles are coated with a carbon layer using a chemical vapor deposition (CVD) method.

[0008] Furthermore, Japanese Patent Application Laid-Open Publication No. 2016-502253 discloses a negative electrode active material containing porous silicon-based particles and carbon particles, and the carbon particles include 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 how much volume expansion and contraction can be suppressed during charging and discharging. Therefore, research to solve these problems is still needed. [Prior art documents] [Patent documents]

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

[0011] The present invention has been devised to solve the problems of the prior art. An object of the present invention is to provide a porous silicon composite that contains silicon particles and a magnesium compound, and has a molar ratio (O / Si) of oxygen (O) atoms to silicon (Si) atoms in the porous silicon composite controlled within a specific range, thereby improving the performance of secondary batteries when used as a negative electrode active material.

[0012] Another object of the present invention is to provide a porous silicon carbon composite comprising a porous silicon composite and carbon, which, when applied to a negative electrode active material, has a significantly improved discharge capacity and initial efficiency as well as an excellent capacity retention rate.

[0013] It is yet another object of the present invention to provide methods for preparing porous silicon composites and porous silicon carbon composites.

[0014] It is yet 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 porous silicon carbon composite. [Means for solving the problem]

[0015] The present invention provides a porous silicon composite comprising silicon particles and a magnesium compound, wherein the molar ratio (O / Si) of oxygen (O) atoms to silicon (Si) atoms in the porous silicon composite is 0.01 to 0.35.

[0016] Additionally, the present invention provides a porous silicon carbon composite comprising a porous silicon composite and carbon.

[0017] The present invention further provides a method for preparing a porous silicon composite, which method comprises: 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; and a third step of filtering and drying the composite obtained by etching to obtain a porous silicon composite.

[0018] The present invention further provides a method for preparing a porous silicon carbon composite, which includes 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 to obtain a porous silicon carbon composite.

[0019] Furthermore, the present invention provides a negative electrode active material for a lithium secondary battery, which comprises a porous silicon carbon composite.

[0020] The present invention further provides a lithium secondary battery including the negative electrode active material for lithium secondary batteries. [Effects of the Invention]

[0021] According to this embodiment, the porous silicon composite contains silicon particles and a magnesium compound, and the molar ratio (O / Si) of oxygen (O) atoms to silicon (Si) atoms in the porous silicon composite is controlled within a specific range. Therefore, when the porous silicon composite is used together with a binder and a conductive material as the negative electrode active material of a secondary battery to prepare a negative electrode active material composition, the dispersion stability is improved and the mechanical properties such as strength are excellent, and when applied as a negative electrode active material, the performance of the secondary battery can be improved.

[0022] According to another embodiment, a porous silicon carbon composite containing a porous silicon composite and carbon, when used as a negative electrode active material, has significantly improved discharge capacity and initial efficiency, as well as excellent capacity retention.

[0023] The method of this embodiment also has the advantage that mass production is possible through a continuous process with a minimum number of steps. [Brief explanation of the drawings]

[0024] The following drawings attached to this specification show preferred embodiments of the present invention, and are useful for further understanding the technical concept of the present invention together with the description of the present invention. Therefore, the present invention should not be interpreted as being limited to only those shown in the drawings.

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[0025] The present invention is not limited to the contents disclosed below, and may be implemented in various modifications within the scope of the present invention.

[0026] In this specification, when a part is said to "comprise" an element, unless otherwise indicated, it should be understood that the part may include other elements as well.

[0027] Furthermore, 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.

[0028] [Porous silicon composite] A porous silicon composite according to one embodiment of the present invention comprises silicon particles and a magnesium compound, and the molar ratio (O / Si) of oxygen (O) atoms to silicon (Si) atoms in the porous silicon composite is 0.01 to 0.35.

[0029] The porous silicon composite contains both silicon particles and a magnesium compound, which can further improve the performance of secondary batteries. Specifically, the silicon particles charge lithium, increasing the capacity of secondary batteries. Because the magnesium compound does not easily react with lithium ions, the degree of expansion and contraction of the electrode is reduced, suppressing the volume expansion of the silicon particles when lithium ions are absorbed, thereby improving cycle characteristics (capacity retention rate).

[0030] In addition, the magnesium compound strengthens the matrix, which is the continuous phase surrounding the silicon particles, and suppresses the volumetric expansion of the silicon particles, resulting in small volume changes during the absorption and desorption of lithium ions, minimizing the occurrence of cracks in the electrode active material even after repeated charge and discharge. Furthermore, because the magnesium compound is located adjacent to the silicon particles, contact between the silicon particles and the electrolyte solvent is minimized, minimizing the reaction between the silicon particles and the electrolyte solvent. This prevents a decrease in initial charge and discharge efficiency, suppresses the expansion of the silicon particles, and improves the capacity retention rate.

[0031] According to one embodiment of the present invention, the porous silicon composite may comprise silicon agglomerates in which the silicon particles are interconnected.

[0032] Specifically, the porous silicon composite may contain silicon aggregates having a three-dimensional (3D) structure in which two or more silicon particles are interconnected. Because silicon particles charge lithium, the capacity of a secondary battery may decrease if silicon particles are not used. In particular, when the silicon particles contain interconnected silicon aggregates, excellent mechanical properties such as strength can be obtained. Furthermore, the porous silicon composite can be easily dispersed when preparing a negative electrode active material composition using a binder and a conductive material. When the negative electrode active material composition is applied to a current collector, its workability is excellent.

[0033] The silicon aggregates may be uniformly distributed within the porous silicon composite, or may contain silicon particles that are not connected to each other. The silicon particles and / or silicon aggregates may be uniformly distributed within the porous silicon composite. In such cases, excellent electrochemical properties, such as charge / discharge, are achieved.

[0034] The silicon particles may contain crystalline particles, and may have a crystallite size in X-ray diffraction analysis (converted from the results of X-ray diffraction analysis) of 1 nm to 20 nm.

[0035] Specifically, when a porous silicon composite according to one embodiment of the present invention 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 Si(220) diffraction peak near 2θ=47.5°, the silicon particles may have a crystallite size of preferably 1 nm to 15 nm, more preferably 1 nm to 10 nm.

[0036] If the crystallite size of the silicon particles is less than 1 nm, it becomes difficult to form micropores within the porous silicon composite, which makes it impossible to suppress the decrease in Coulombic efficiency, which represents the ratio of charge capacity to discharge capacity, and the specific surface area is too large to prevent oxidation problems when handled in air.Furthermore, if the crystallite size exceeds 20 nm, the micropores cannot sufficiently suppress the volume expansion of the silicon particles that occurs during charge and discharge, making it impossible to suppress the decrease in Coulombic efficiency, which represents the ratio of charge capacity to discharge capacity, due to repeated charge and discharge.

[0037] The smaller the crystallite size of the silicon particles within the above range, the denser the composite can be obtained, and the stronger the matrix can be, which can further improve the performance of the secondary battery, such as discharge capacity, initial efficiency, and cycle life characteristics.

[0038] Furthermore, the porous silicon composite may further contain amorphous silicon or silicon of a similar phase. The silicon particles may be uniformly distributed within the porous silicon composite. In such cases, excellent electrochemical properties, such as charge and discharge, can be achieved.

[0039] On the other hand, the porous silicon composite contains a magnesium compound.

[0040] Since magnesium compounds do not easily react with lithium ions during charge and discharge of secondary batteries, they can reduce the expansion and contraction of the electrode when lithium ions are absorbed into the electrode, improving the cycle characteristics of secondary batteries. Furthermore, the magnesium compounds strengthen the strength of the matrix, which is the continuous phase surrounding the silicon.

[0041] The magnesium compound may include a fluorine-containing magnesium compound.

[0042] Preferred properties of the porous silicon composite containing a fluorine-containing magnesium compound according to one embodiment of the present invention will now be described.

[0043] Generally, silicon particles absorb lithium ions to form an alloy during charging of a secondary battery, which increases the lattice constant and causes the volume to expand. When the secondary battery is discharged, the lithium ions are released and the silicon particles return to their original metal nanoparticle form, decreasing the lattice constant.

[0044] The fluorine-containing magnesium compound can be said to be a zero-strain material that does not undergo a change in the crystal lattice constant during the absorption and desorption of lithium ions. Silicon particles may be present between the fluorine-containing magnesium compound particles or may be surrounded by the fluorine-containing magnesium compound particles.

[0045] Furthermore, the fluorine-containing magnesium compound does not release lithium ions when a lithium secondary battery is charged, and is also an inactive substance that does not absorb or release lithium ions when a lithium secondary battery is charged.

[0046] Lithium ions are released from the silicon particles, but the fluorine-containing magnesium compound does not release the lithium ions that rapidly increase during charging. Thus, the porous matrix containing the fluorine-containing magnesium compound is not involved in the chemical reactions of the battery, but is expected to function as a body that suppresses the volume expansion of the silicon particles during charging of the secondary battery.

[0047] The fluorine-containing magnesium compound may include magnesium fluoride (MgF2), magnesium fluorosilicate (MgSiF6), or a mixture thereof. Furthermore, when X-ray diffraction (Cu-Kα) analysis is performed using copper as a cathode target on the fluorine-containing magnesium compound, and the MgF2 crystallite size is 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 may 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 corresponding to the Si(220) crystal plane of silicon particles as a cathode target, IB / IA, which is the ratio of the diffraction peak intensity (IB) corresponding to the MgF2(111) crystal plane around 2θ=40.4° to the diffraction peak intensity (IA) of Si(220) around 2θ=47.3°, may be greater than 0 and equal to or less than 1. If IB / IA exceeds 1, problems such as a decrease in the capacity of the secondary battery may occur.

[0049] The magnesium compound may further include magnesium silicate, which may be contained in a larger amount in the center of the porous silicon composite powder.

[0050] The magnesium silicate may include MgSiO3, Mg2SiO4, or a mixture thereof.

[0051] In particular, when the porous silicon composite contains MgSiO3, the coulombic efficiency or capacity retention rate may be improved.

[0052] The content of magnesium silicate may be 0 wt % to 30 wt %, 0.5 wt % to 25 wt %, or 0.5 wt % to 20 wt %, based on the total weight of the porous silicon composite.

[0053] In a porous silicon composite according to one embodiment of the present invention, the magnesium silicate may be converted to a fluorine-containing magnesium compound by etching.

[0054] 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.

[0055] On the other hand, the magnesium (Mg) content in the porous silicon composite may be 0.2 wt% to 20 wt%, preferably 0.2 wt% to 15 wt%, and more preferably 0.2 wt% to 10 wt%, based on the total weight of the porous silicon composite. When the magnesium (Mg) content in the porous silicon composite is 0.2 wt% or more, the initial efficiency of the secondary battery may be improved. When the magnesium (Mg) content is 20 wt% or less, the charge / discharge capacity, cycle characteristics, and operational stability may be improved.

[0056] On the other hand, according to one embodiment of the present invention, the number of oxygen atoms present on the surface of a porous silicon composite can be reduced, i.e., an important feature of the present invention is that the molar ratio of oxygen (O) atoms to silicon (Si) atoms (O / Si) can be significantly reduced.

[0057] The molar ratio (O / Si) of oxygen (O) atoms to silicon (Si) atoms in the porous silicon composite may be 0.01 to 0.35, preferably 0.01 to 0.25, more preferably 0.01 to 0.10, and even more preferably 0.01 to 0.08.

[0058] According to one embodiment of the present invention, the molar ratio (O / Si) of oxygen (O) atoms to silicon (Si) atoms is significantly reduced, thereby reducing the surface resistance. As a result, when the porous silicon composite is used as a negative electrode active material, the electrochemical properties of the secondary battery can be significantly improved, which is preferable.

[0059] Specifically, according to one embodiment of the present invention, most of the silicon dioxide contained in the porous silicon composite can be removed by etching. In such cases, the surface of the silicon particles may contain significantly more silicon (Si) atoms than oxygen (O) atoms. That is, the molar ratio O / Si may be significantly reduced. By using this porous silicon composite as a negative electrode active material, a secondary battery having excellent discharge capacity can be preferably obtained, and the initial efficiency of the secondary battery can be improved.

[0060] Generally, a negative electrode active material containing silicon has a higher discharge capacity as the oxygen content decreases, but the volume expansion rate upon charging increases. On the other hand, an increase in the oxygen content suppresses the volume expansion rate, which may result in a decrease in discharge capacity.

[0061] When the molar ratio (O / Si) of oxygen (O) atoms to silicon (Si) atoms satisfies the above range, the active phase originating from silicon increases, improving the initial discharge capacity and initial efficiency of the secondary battery.

[0062] Specifically, according to one embodiment of the present invention, when the molar ratio (O / Si) of oxygen (O) atoms to silicon (Si) atoms in the porous silicon composite is 0.01 or more, expansion and contraction due to charge and discharge can be suppressed. Therefore, when the porous silicon composite is used as a negative electrode active material, peeling of the negative electrode active material from the negative electrode current collector can be suppressed. Furthermore, when the molar ratio (O / Si) of oxygen (O) atoms to silicon (Si) atoms is 0.35 or less, sufficient charge and discharge capacity can be ensured, and high charge and discharge characteristics can be maintained.

[0063] The porous silicon composite may comprise a plurality of pores.

[0064] The porous silicon composite has a porous structure, which can accommodate the volume expansion of silicon particles that occurs during charging and discharging of a secondary battery, thereby effectively mitigating and suppressing problems caused by the volume expansion.

[0065] The porous silicon composite may contain a plurality of pores within it, on its surface, or both. The pores may be interconnected within the silicon composite to form open pores.

[0066] When the surface of the porous silicon composite is measured by a gas adsorption method (BET plot method), it may contain micropores of 2 nm or less and mesopores of more than 2 nm and not more than 50 nm.

[0067] The pore volume of micropores of 2 nm or less is 0.01 cm 3 / g~0.5cm 3 / g, preferably 0.05 cm 3 / g~0.45cm 3 / g, more preferably 0.1 cm 3 / g~0.4cm 3 / g.

[0068] The pore volume of mesopores with a diameter of more than 2 nm to 50 nm is 0.2 cm 3 / g~0.7cm 3 / g, preferably 0.2 cm 3 / g~0.6cm 3 / g, more preferably 0.2 cm 3 / g~0.5cm 3 / g.

[0069] When the pores include micropores of 2 nm or less that fill the pore volume and mesopores of more than 2 nm to 50 nm, the pores in the silicon composite may be contained uniformly and in large quantities.

[0070] Furthermore, the pores may further include macropores of greater than 50 nm to 250 nm. The pore volume of the macropores of greater than 50 nm to 250 nm is less than 0.01 cm 3 / g~0.3cm 3 / g, preferably 0.01 cm 3 / g~0.2cm 3 / g, more preferably 0.01 cm 3 / g~0.15cm 3 / g.

[0071] In the porous silicon composite, it is preferable that silicon particles and / or silicon aggregates in which the silicon particles are connected to each other are uniformly distributed inside the porous silicon composite. As a result, the porous silicon composite has excellent mechanical properties such as strength. Further, since it has a porous structure, it can accommodate (absorb) the volume expansion of silicon particles that occurs during charge and discharge of the secondary battery, and can effectively relieve and suppress problems caused by the volume expansion.

[0072] According to an embodiment of the present invention, the porous silicon composite may further contain a silicon oxide compound.

[0073] 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), and more specifically, may be SiO x (0.9 < x ≦ 1.1). In the formula SiO x , when the value of x is less than 0.5, the expansion and contraction during charge and discharge of the secondary battery become large, and the life characteristics may deteriorate. Further, when x exceeds 2, there may be a problem that the amount of inactive oxide increases and the initial efficiency of the secondary battery decreases.

[0074] The silicon oxide compound may be used in an amount of 0.1 mol% to 5 mol% based on the total weight of the porous silicon composite.

[0075] When the content of the silicon oxide compound is less than 0.1% by weight, the volume of the secondary battery may expand and these life characteristics may deteriorate. On the other hand, when the content of the silicon oxide compound exceeds 5% by weight, the initial irreversible reaction of the secondary battery may increase and the initial efficiency may decrease.

[0076] In this specification, the properties of the silicon particles and the characteristics of their surfaces may include the properties of the silicon aggregates and the surfaces of the silicon aggregates as long as the effects of the present invention are not impaired.

[0077] The porous silicon composite according to one embodiment of the present invention may further contain silicon oxide (SiO x , 0.1 < x ≤ 2). The silicon oxide (SiO x , 0.1 < x ≤ 2) may be formed by oxidation of silicon.

[0078] The oxygen (O) content in the porous silicon composite may be 0.1 wt% to 15 wt%, preferably 0.5 wt% to 10 wt%, more preferably 0.5 wt% to 8 wt% based on the total weight of the porous silicon composite. If the oxygen (O) content of the porous silicon composite is less than 0.1 wt%, the swelling degree during charging of the secondary battery increases, and the cycle characteristics deteriorate, which is not preferable. If the oxygen (O) content of the porous silicon composite exceeds 15 wt%, when the porous silicon composite is used as a negative electrode active material, an irreversible reaction with lithium increases, the initial charge-discharge efficiency decreases, it becomes easy to peel off from the negative electrode current collector, and there may be a concern that the charge-discharge cycle deteriorates.

[0079] The average particle size (D 50 ) of the porous silicon composite may be 1 μm to 15 μm, preferably 2 μm to 10 μm, more preferably 3 μm to 8 μm. If the average particle size (D 50 ) of the porous silicon composite exceeds 15 μm, the expansion of the porous silicon composite due to the charging of lithium ions becomes intense, and during repeated charge-discharge, the binding force between particles in the composite and the binding force between particles and the current collector decrease, and the life characteristics may deteriorate significantly. In addition, there is a concern about a decrease in activity due to a decrease in specific surface area. When the average particle size (D 50 ) of the porous silicon composite is less than 1 μm, there is a concern that the dispersibility decreases due to aggregation of the porous silicon composite during the preparation of the negative electrode slurry (negative electrode active material composition).

[0080] The specific gravity of the porous silicon composite may be 1.5 g / cm 3 to 2.3 g / cm 3 , preferably 1.6 g / cm 3~2.3g / cm 3 , more preferably 1.6 g / cm 3 ~2.2g / cm 3 may be.

[0081] Here, specific gravity may refer to particle density, density, or true density. According to one embodiment of the present invention, specific gravity may be measured, for example, using a dry density meter, such as an Acupick II1340 dry density meter manufactured by Shimadzu Corporation. The purge gas used may be helium gas, and measurements may be performed after purging 200 times in a sample holder set at a temperature of 23°C.

[0082] The specific gravity of the porous silicon composite is 1.5 g / cm 3 In this case, the deterioration of the cycle characteristics of the secondary battery can be suppressed. 3 In the following cases, the impregnation of the electrolyte is improved, the utilization rate of the negative electrode active material is increased, and the initial charge / discharge capacity can be improved.

[0083] The porous silicon composite is 100m 2 / g~1,600m 2 / g, preferably 200m 2 / g~1,400m 2 / g, more preferably 300m 2 / g~1,200m 2 / g (Brunauer-Emmett-Teller method; BET). 2 If the ratio is less than 1,600m / 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 a 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.

[0084] The specific surface area of ​​a porous silicon composite can be calculated as the sum of the surface area of ​​the particle surfaces of the porous silicon composite and the surface area of ​​the pores (micropores, macropores, and mesopores) present on the surface and inside the porous silicon composite.

[0085] [Porous silicon carbon composite] According to an embodiment of the present invention, there is provided a porous silicon carbon composite comprising a porous silicon composite and carbon.

[0086] Specifically, the porous silicon carbon composite may comprise silicon particles and a magnesium compound, and may comprise a porous silicon composite having a molar ratio (O / Si) of oxygen (O) atoms to silicon (Si) atoms in the porous silicon composite of 0.01 to 0.35, and carbon.

[0087] The molar ratio (O / Si) of oxygen (O) atoms to silicon (Si) atoms in the porous silicon carbon composite may be the same as or slightly different from the molar ratio (O / Si) of oxygen (O) atoms to silicon (Si) atoms in the silicon composite. That is, the molar ratio (O / Si) of oxygen (O) atoms to silicon (Si) atoms in the porous silicon carbon composite may be 0.01 to 0.35. When a porous silicon carbon composite having a molar ratio (O / Si) of oxygen (O) atoms to silicon (Si) atoms that satisfies the above range is used as a negative electrode active material, the discharge capacity and initial efficiency can be significantly improved while maintaining an excellent capacity retention rate.

[0088] The molar ratio (O / Si) of oxygen (O) atoms to silicon (Si) atoms in the porous silicon carbon composite may be preferably 0.01 to 0.25, more preferably 0.01 to 0.10, and even more preferably 0.01 to 0.08.

[0089] According to one embodiment of the present invention, when the molar ratio (O / Si) of oxygen (O) atoms to silicon (Si) atoms in the porous silicon carbon composite is 0.01 or more, expansion and contraction due to charge and discharge can be suppressed. Therefore, when the porous silicon carbon composite is used as a negative electrode active material, peeling of the negative electrode active material from the negative electrode current collector can be suppressed. Furthermore, when the molar ratio (O / Si) of oxygen (O) atoms to silicon (Si) atoms is 0.35 or less, discharge capacity can be ensured and high charge and discharge characteristics can be maintained.

[0090] A porous silicon carbon composite is a composite in which silicon aggregates, in which multiple silicon particles are interconnected, are uniformly distributed in a composite having a single mass structure, such as a polyhedron, sphere, or similar shape. It may also be a single composite in which carbon, more specifically, a carbon layer containing carbon, surrounds part or all of the surface of one or more silicon particles, or the surface of a secondary silicon particle (silicon aggregate) formed by the aggregation of two or more silicon particles.

[0091] In porous silicon-carbon composites, volume expansion during charging and discharging of secondary batteries is concentrated in the pores rather than the outer part of the negative electrode active material, which effectively controls volume expansion and improves the life characteristics of lithium secondary batteries. Furthermore, the electrolyte can more easily penetrate the porous structure, improving output characteristics and further improving the performance of lithium secondary batteries.

[0092] In this specification, pores may be used interchangeably with voids. Furthermore, pores may include closed pores. Closed pores refer to independent pores in which the walls of the pores (pores) are all closed and not connected to other pores. Furthermore, pores may further include open pores. Open pores are pores in which at least a portion of the pore walls are formed in an open structure, and may or may not be connected to other pores. They may also refer to pores located on the surface of a silicon composite and exposed to the outside.

[0093] The porous silicon carbon composite may have a porosity of 0.5 to 40% 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. Specifically, the porosity of the porous silicon carbon composite may be preferably 0.5 to 30% by volume, more preferably 1 to 15% by volume, based on the volume of the porous silicon carbon composite.

[0094] Here, porosity refers to the pore volume per unit mass divided by the specific volume plus the pore volume per unit mass. It can be measured by mercury intrusion or the Brunauer-Emmett-Teller (BET) method.

[0095] In this specification, the specific volume is calculated as 1 / (particle density) of the sample. The pore volume per unit mass is measured by the BET method, and the porosity (%) is calculated from the above formula.

[0096] When the porosity of a porous silicon-carbon composite satisfies the above range, its application as a negative electrode active material in a secondary battery can provide a buffering effect against volume expansion while maintaining sufficient mechanical strength. This minimizes the problem of volume expansion due to the use of silicon particles, achieving a large capacity and improving life characteristics. If the porosity of a porous silicon-carbon composite is less than 0.5% by volume, it may be difficult to control the volume expansion of the negative electrode active material during charge and discharge. If the porosity exceeds 40% by volume, the presence of numerous pores in the negative electrode active material reduces mechanical strength, raising concerns about the negative electrode active material collapsing during the secondary battery manufacturing process, for example, during mixing of the negative electrode active material slurry or during the rolling process after coating.

[0097] The porous silicon carbon composite may contain a plurality of pores, each of which may have the same or different diameters.

[0098] In the porous silicon-carbon composite, carbon may be present on at least one surface selected from the group consisting of silicon particles and magnesium compounds contained in the porous silicon composite. Further, carbon may be present on the surface of silicon aggregates contained in the porous silicon-carbon composite.

[0099] Further, carbon may form a matrix, and silicon particles, magnesium compounds, and pores may be dispersed in the carbon matrix.

[0100] Specifically, the porous silicon-carbon composite may have an island-sea structure in which silicon particles or closed pores (v) form islands and carbon forms the sea. The pores include open pores and closed pores. The closed pores may be those in which the inside of the pores is not covered with carbon.

[0101] Also, carbon is present on at least one surface selected from the group consisting of silicon particles and magnesium compounds contained in the porous silicon composite. Carbon functions as a matrix, and silicon particles, magnesium compounds, and pores may be dispersed in the carbon matrix.

[0102] In the porous silicon-carbon composite, carbon may be present on the surface of the porous silicon composite or inside the open pores.

[0103] The state in which silicon particles and carbon are uniformly dispersed is confirmed by image observation of dark-field images and bright-field images using a transmission electron microscope (TEM). Also, the state in which pores are uniformly dispersed inside the porous silicon-carbon composite is also confirmed by the above-described image observation.

[0104] Also, when silicon oxide (SiO x , 0.1 < x ≦ 2) formed on the surface of the silicon particles is further contained, carbon may be present on the surface of the silicon oxide (SiO x , 0.1 < x ≦ 2).

[0105] The carbon may further form a carbon layer on the surface of the porous silicon composite.

[0106] Because the porous silicon-carbon composite contains a carbon layer, the difficulty in establishing electrical contact between particles due to the presence of pores is eliminated, and excellent conductivity is maintained even after the electrode expands during charge and discharge, further improving the performance of secondary batteries.

[0107] Furthermore, according to one embodiment of the present invention, by controlling the thickness and carbon content of the carbon layer, it is possible to obtain appropriate conductivity and prevent a decrease in life characteristics, thereby obtaining a high-capacity negative electrode active material.

[0108] On the other hand, since the surface of the porous silicon composite particle or the interior and pores therein are covered with a carbon coating, the specific surface area of ​​the porous silicon carbon composite may change significantly.

[0109] Porous silicon carbon composite is 3m 2 / g~50m 2 / g, preferably 3m 2 / g~40m 2 The porous silicon carbon composite may have a specific surface area of ​​3 m / g (Brunauer-Emmett-Teller method; BET). 2 If the content is less than 50m / 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 a 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.

[0110] The specific gravity of the porous silicon carbon composite is 1.8 g / cm 3 ~2.5g / cm 3 and preferably 2.0 g / cm 3 ~2.5g / cm 3 , more preferably 2.0 g / cm 3 ~2.4g / cm 3The specific gravity may vary depending on the coating amount of the carbon layer. While the amount of carbon is constant, the higher the specific gravity within the above range, the fewer pores there are in the composite. Therefore, when used as a negative electrode active material, the conductivity is improved, the strength of the matrix is ​​strengthened, and the initial efficiency and cycle life characteristics are improved. In such cases, the specific gravity may refer to particle density, density, or true density. The measurement method is as described above.

[0111] The specific gravity of the porous silicon carbon composite is 1.8 g / cm 3 When the specific gravity is 2.5 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 a decrease in cycle performance can be suppressed. 3 When the thickness 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.

[0112] The porous silicon carbon composite according to one embodiment of the present invention has a porous structure that allows electrolyte to easily penetrate into the porous structure, improving output characteristics. Therefore, the porous silicon carbon composite can be advantageously used in the manufacture of a negative electrode active material for a lithium secondary battery and a lithium secondary battery including the same.

[0113] On the other hand, in the porous silicon carbon composite, the silicon (Si) content may be 30 wt% to 90 wt%, 30 wt% to 80 wt%, or 30 wt% to 70 wt%, based on the total weight of the porous silicon carbon composite.

[0114] If the silicon (Si) content is less than 30% by weight, 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 90% by weight, the charge / discharge capacity of the lithium secondary battery will increase, but the expansion and contraction of the electrode during charge / discharge will become excessively large, further increasing the particle size of the negative electrode active material powder, which may result in a decrease in cycle characteristics.

[0115] The magnesium (Mg) content in the porous silicon carbon composite may be 0.2 wt % to 20 wt %, 0.2 wt % to 15 wt %, or 0.2 wt % to 6 wt % based on the total weight of the porous silicon carbon composite. If the magnesium (Mg) content in the porous silicon carbon composite is less than 0.2 wt %, the initial efficiency of the secondary battery may decrease. If it exceeds 20 wt %, the capacity of the secondary battery may decrease. If the magnesium (Mg) content in the porous silicon carbon composite satisfies the above range, the performance of the secondary battery can be further improved.

[0116] On the other hand, according to one embodiment of the present invention, the porous silicon carbon composite may contain 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 of these 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, Ge, P, As, Sb, Bi, S, and Se.

[0117] The carbon (C) content may be 10 to 90% by weight based on the total weight of the porous silicon carbon composite. Specifically, the carbon (C) content may be 10 to 70%, 15 to 60%, or 20 to 50% by weight based on the total weight of the porous silicon carbon composite.

[0118] If the carbon (C) content is less than 10% by weight, the effect of increasing the conductivity cannot be sufficiently expected, and the electrode life of the lithium secondary battery may be shortened. On the other hand, if the carbon (C) content exceeds 90% by weight, the discharge capacity of the secondary battery may decrease, the bulk density may decrease, and the charge / discharge capacity per unit volume may decrease.

[0119] The thickness of the carbon layer may be 1 nm to 300 nm. The thickness of the carbon layer is preferably 1 nm to 40 nm, and more preferably 1 nm to 30 nm. When the thickness of the carbon layer is 1 nm or more, improved conductivity may be achieved. When the thickness is 300 nm or less, a decrease in the capacity of the secondary battery may be suppressed.

[0120] The average thickness of the carbon layer may be measured, for example, by the following procedure.

[0121] 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.

[0122] The carbon layer may contain at least one selected from the group consisting of graphene, reduced graphene oxide, carbon nanotubes, carbon nanofibers, and graphite. Specifically, it may contain graphene. The at least one selected from the group consisting of graphene, reduced graphene oxide, carbon nanotubes, carbon nanofibers, and graphite may be contained not only in the carbon layer present on the surface of the porous silicon composite, but also on the surfaces of the silicon particles and in the carbon matrix.

[0123] The porous silicon carbon composite has an average particle size (D 50 ) and the average particle size (D 50 ) is the average diameter (D) at 50% cumulative volume in particle size distribution measurement by laser light diffraction method. 50 ) (i.e., particle size or median diameter). Specifically, the average particle size (D 50 The average particle size (D) is preferably 3 μm to 10 μm, and more preferably 3 μm to 8 μm. 50If the particle size is less than 2 μm, there is a concern that the dispersibility may decrease due to aggregation of the porous silicon-carbon composite particles when preparing a negative electrode slurry (i.e., a negative electrode active material composition) using the porous silicon-carbon composite. 50 If the average particle size (D) of the porous silicon-carbon composite is larger than 15 μm, the composite particles will expand significantly due to charging of lithium ions, and the bonding strength between the particles of the composite and between the particles and the current collector will decrease as charging and discharging is repeated, which may result in a significant decrease in the life characteristics. In addition, the activity may decrease due to a decrease in the specific surface area. More specifically, 50 Considering the significant improvement effect of optimizing the average particle size (D 50 ) is preferably 3 μm to 6 μm.

[0124] A secondary battery using the porous silicon carbon composite as the negative electrode can further improve its discharge capacity, initial efficiency and capacity retention rate.

[0125] [Method for preparing porous silicon composite] The method for preparing the porous silicon composite includes 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; and a third step of filtering and drying the composite obtained by etching to obtain a porous silicon composite.

[0126] The method according to one embodiment has the advantage that it allows for mass production using a continuous process with minimal steps.

[0127] Specifically, in the method for preparing a porous silicon composite, the first step may include obtaining a silicon composite oxide powder using a silicon-based raw material and a magnesium-based raw material. The first step may be carried out using, for example, the method described in Korean Patent Publication No. 2018-0106485.

[0128] According to one embodiment of the present invention, the silicon composite oxide powder may include magnesium silicate.

[0129] The magnesium (Mg) content in the silicon composite oxide powder may be 0.2 wt% to 20 wt%, 0.2 wt% to 15 wt%, or 0.2 wt% to 10 wt%, based on the total weight of the silicon composite oxide. When the magnesium (Mg) content in the silicon composite oxide satisfies the above range, the molar ratio of oxygen (O) atoms to silicon (Si) atoms (O / Si) in the porous silicon composite can satisfy the range to be achieved by one embodiment of the present invention. In this case, the performance of the secondary battery, such as the discharge capacity, initial charge / discharge efficiency, and capacity retention rate, can be further improved.

[0130] According to one embodiment of the present invention, the method may further include a step of forming a carbon layer on the surface of the silicon composite oxide using a chemical pyrolysis deposition method.

[0131] Specifically, after forming a carbon layer on the surface of silicon composite oxide powder containing silicon particles, the etching treatment of the second step may be carried out, which has the advantage of enabling uniform etching and achieving a high yield.

[0132] The step of forming the carbon layer may be carried out in a manner similar to or identical to the step of forming the carbon layer in the fourth step of the method of preparing a porous silicon carbon composite described below.

[0133] In the method for preparing a porous silicon composite, the second step may include a step of etching the silicon composite oxide powder with an etching solution containing a fluorine (F) atom-containing compound.

[0134] The etching process may include dry etching and wet etching.

[0135] Dry etching may allow selective etching.

[0136] The etching step dissolves and elutes silicon dioxide from the silicon composite oxide powder to form pores. That is, in the etching step, the silicon composite oxide powder is etched using an etching solution containing a fluorine (F) atom-containing compound to form pores.

[0137] Furthermore, the etching step converts magnesium silicate into a fluorine-containing magnesium compound, making it possible to prepare a porous silicon composite containing silicon particles and a fluorine-containing magnesium compound.

[0138] When silicon composite oxide powder is etched using a fluorine (F) atom-containing compound (e.g., HF), part of the magnesium silicate is converted to a fluorine-containing magnesium compound, and at the same time, pores are formed in the areas where the silicon dioxide has been dissolved and removed. As a result, a porous silicon composite containing silicon particles, magnesium silicate, and a fluorine-containing magnesium compound is prepared.

[0139] For example, in an etching process using HF, when dry etching is performed, the reactions may be represented by the following reaction formulas G1 and G2, and when wet etching is performed, the reactions may be represented by the following reaction formulas L1a to L2.

[0140] [ka]

[0141] It is also believed that the pores are formed by the following reaction schemes (2) and (3).

[0142] [ka]

[0143] Through a reaction mechanism such as the above reaction scheme, silicon dioxide is dissolved and removed in the form of SiF4 and H2SiF6, thereby forming pores (voids).

[0144] Furthermore, depending on the degree of etching, the silicon dioxide contained in the porous silicon composite may be removed, and pores may be formed inside the composite.

[0145] The degree of pore formation may vary depending on the degree of etching.

[0146] Furthermore, the O / Si ratio and specific surface area of ​​the porous silicon composite before and after etching may change significantly.

[0147] Furthermore, the specific surface area and specific gravity of the porous silicon composite in which pores are formed may change significantly before and after carbon coating.

[0148] By etching, it is possible to obtain a porous silicon composite powder in which a plurality of pores are formed on the surface of the porous silicon composite particle, or on the surface and inside the particle.

[0149] Here, etching refers to a process of treating the silicon composite oxide powder with an acidic aqueous solution containing an acid, for example, an etching solution containing a fluorine (F) atom-containing compound in the acidic aqueous solution.

[0150] As the etching solution containing a fluorine (F) atom-containing compound, any commonly used fluorine atom-containing etching solution can be used without limitation, as long as the effects of the present invention are not impaired.

[0151] Specifically, the fluorine (F) atom-containing compound may include at least one selected from the group consisting of HF, NHF, and HF. By using the fluorine (F) atom-containing compound, the porous silicon composite contains a fluorine-containing magnesium compound, and the etching process can be carried out more quickly.

[0152] The etching solution may further comprise one or more acids selected from the group consisting of organic acids, sulfuric acid, hydrochloric acid, phosphoric acid, nitric acid, and chromic acid.

[0153] As etching conditions, the stirring temperature (treatment temperature) may be, for example, 10°C to 90°C, preferably 10°C to 80°C, and more preferably 20°C to 70°C.

[0154] The porous silicon composite obtained by etching may comprise porous silicon particles.

[0155] By etching, a porous silicon composite can be obtained in which multiple pores are formed on the surface, inside, or both of the porous silicon composite. Here, the porous silicon composite may have a three-dimensional (3D) structure in which two or more silicon particles are interconnected. Furthermore, the porous silicon composite has the characteristic that the average particle size is hardly changed by etching.

[0156] That is, the average particle size of the silicon composite oxide powder before etching is approximately the same as the average particle size of the porous silicon composite obtained by etching. The difference (change) between the average particle size of the silicon composite oxide powder and the average particle size of the porous silicon composite may be within about 5%.

[0157] Furthermore, the number of oxygen atoms present on the surface of a porous silicon composite can be reduced by etching. That is, etching can significantly reduce the oxygen fraction on the surface of a porous silicon composite, thereby reducing surface resistance. As a result, when a porous silicon composite is used as a negative electrode active material, the electrochemical properties, particularly the life characteristics, of lithium secondary batteries can be significantly improved.

[0158] Furthermore, because a large amount of silicon dioxide is removed by selective etching, the surface of the silicon particles or silicon aggregates may contain significantly more silicon (Si) than oxygen (O). In other words, the molar ratio of oxygen (O) atoms to silicon (Si) atoms (O / Si) present in the porous silicon composite may be significantly reduced. In this case, a secondary battery with excellent capacity retention, high discharge capacity, and high initial efficiency can be obtained.

[0159] In the method for preparing a porous silicon composite, the third step may include filtering and drying the product obtained by etching to obtain a porous silicon composite. The filtering and drying steps may be carried out by conventional methods.

[0160] 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.

[0161] [Method for preparing porous silicon carbon composites] According to one embodiment of the present invention, there is provided a method for preparing a porous silicon carbon composite using a porous silicon composite.

[0162] A method for preparing a porous silicon carbon composite according to one embodiment of the present invention includes 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 to obtain a porous silicon carbon composite.

[0163] The carbon layer formation process may improve the electrical contact between particles in the porous silicon-carbon composite. Furthermore, excellent conductivity can be imparted even after the electrode expands during charge and discharge, further improving the performance of the secondary battery. Specifically, the carbon layer can increase the conductivity of the negative electrode active material, thereby improving the battery's output characteristics and cycle characteristics, and can enhance the stress relaxation effect during volumetric changes in the active material. A method for preparing a porous silicon-carbon composite is described in detail below.

[0164] Steps 1 to 3 are the same as those described in the method for preparing a porous silicon composite. In the method for preparing a porous silicon carbon composite, step 4 may include forming a carbon layer on the surface of the porous silicon composite by chemical pyrolysis deposition.

[0165] The carbon layer may include at least one selected from the group consisting of graphene, reduced graphene oxide, carbon nanotubes, carbon nanofibers, and graphite.

[0166] The carbon layer formation step may be carried out by injecting at least one carbon source gas selected from the compounds represented by the following chemical 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.) [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.)

[0167] The compound represented by Chemical 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 Chemical Formula 2 may be at least one selected from the group consisting of ethylene, acetylene, propylene, butylene, butadiene, and cyclopentene. The compound represented by Chemical Formula 3 may be at least one selected from the group consisting of benzene, toluene, xylene, ethylbenzene, naphthalene, anthracene, and dibutylhydroxytoluene (BHT).

[0168] The carbon coating may also be formed uniformly on the surfaces of the pores inside the porous silicon carbon composite, which is preferable because it further improves the cycle life.

[0169] 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, for example, 400°C to 1200°C, specifically 500°C to 1100°C, and more specifically 600°C to 1000°C.

[0170] The reaction time (or heat treatment time) may be adjusted appropriately depending on the desired amount of carbon coating, etc. 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.

[0171] In a method for preparing a porous silicon carbon composite according to one embodiment of the present invention, a thin and uniform carbon layer containing at least one material selected from graphene, reduced graphene oxide, carbon nanotubes, carbon nanofibers, and graphite as a main component can be formed on the surface of a silicon composite by a gas-phase reaction of a carbon source gas, even at a relatively low temperature, and a desorption reaction in the carbon layer does not substantially occur.

[0172] Furthermore, because a carbon layer is formed uniformly over the entire surface of the porous 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.

[0173] According to one embodiment of the present invention, when a reactive gas containing a carbon source gas and an inert gas is supplied to the surface of a silicon composite, the reactive gas penetrates into the open pores of the silicon composite, and one or more graphene-containing materials selected from graphene, reduced graphene oxide, and graphene oxide, and conductive carbon materials such as carbon nanotubes and carbon nanofibers grow on the surface of the porous silicon composite. For example, the conductive carbon material deposited on the silicon surface of the porous silicon composite gradually grows over the course of the reaction time, resulting in a porous silicon carbon composite.

[0174] The specific surface area of ​​the porous silicon carbon composite may decrease depending on the amount of carbon coating.

[0175] The structure of the graphene-containing material may be layered, nanosheet-like, or a mixture of multiple flakes.

[0176] When a carbon layer containing graphene-containing material is uniformly formed over the entire surface of a silicon composite, the volume expansion can be suppressed because the highly conductive and flexible graphene-containing material is grown directly on the surface of silicon aggregates, silicon particles, and / or fluorine-containing magnesium compounds. Furthermore, the carbon layer coating reduces the opportunity for direct contact of silicon with the electrolyte, thereby potentially reducing the formation of a solid-electrolyte interface (SEI) layer.

[0177] Furthermore, according to one embodiment of the present invention, the method may further include, after the fourth step (after the formation of the carbon layer in the fourth step), a step of pulverizing or crushing the porous silicon carbon composite and classifying it so that the average particle size of the porous silicon carbon composite is 2 μm to 15 μm. Classification is performed to adjust the particle size distribution of the porous silicon carbon composite, and examples of such methods include dry classification, wet classification, and sieving. In dry classification, the steps of dispersion, separation, recovery (separation of solid and gas), and discharge are performed sequentially or simultaneously using an airflow. In this case, pretreatment (adjusting moisture, dispersibility, humidity, etc.) may be performed before classification to adjust the moisture or oxygen concentration in the airflow used, so as to prevent a decrease in classification efficiency due to interference between particles, particle shape, airflow turbulence, flow velocity distribution, static electricity, etc. Furthermore, the desired particle size distribution can be obtained by pulverizing or crushing the porous silicon carbon composite and classifying it simultaneously. After pulverization or crushing, it is effective to separate the material into a coarse powder fraction and a granular fraction using a classifier or sieve.

[0178] The method of the present invention has the advantage that it allows for mass production through a continuous process with a minimum number of steps.

[0179] A secondary battery using this porous silicon-carbon composite as the negative electrode can have improved capacity, capacity retention rate and initial efficiency.

[0180] negative electrode active material A negative electrode active material according to one embodiment of the present invention may include a porous silicon composite.

[0181] Furthermore, the negative electrode active material according to one embodiment of the present invention may include a porous silicon carbon composite.

[0182] The negative electrode active material may further include a carbon-based negative electrode material, specifically, a graphite-based negative electrode material.

[0183] The negative electrode active material may be a mixture of a porous silicon composite or a porous silicon-carbon composite with 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 at the same time, expansion stress during charging can be alleviated. The carbon-based negative electrode material may include at least one 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.

[0184] The content of the carbonaceous negative electrode material may be 5% to 95% by weight, preferably 10% to 70% by weight, and more preferably 10% to 60% by weight, based on the total weight of the negative electrode active material.

[0185] Furthermore, when silicon particles with a crystallite size of 20 nm or less are mixed with graphite-based materials, which generally have a small volume expansion, the silicon particles alone do not cause a large volume expansion. Because there is little separation between the graphite-based material and the silicon particles, a secondary battery with excellent cycle characteristics can be obtained.

[0186] secondary battery According to one embodiment of the present invention, the present invention may provide a negative electrode including the negative electrode active material and a secondary battery including the same.

[0187] The secondary battery may include a positive electrode, a negative electrode, a separator sandwiched between the positive electrode and the negative electrode, and a non-aqueous liquid electrolyte having a lithium salt dissolved therein. The negative electrode may include a negative electrode active material including a porous silicon carbon composite.

[0188] The negative electrode may be composed of only the 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 the 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. Furthermore, the negative electrode mixture and the positive electrode mixture may further contain a conductive agent and a binder.

[0189] 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.

[0190] When the negative electrode is composed of a current collector and an active material layer carried thereon, the negative electrode may be produced by coating the surface of the current collector with a negative electrode active material composition containing a porous silicon carbon composite and drying it.

[0191] Furthermore, 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 this field. Specifically, an aprotic organic solvent may be used. Examples of the aprotic organic solvent 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 of two or more.

[0192] The secondary battery may include a non-aqueous secondary battery.

[0193] A negative electrode active material and a secondary battery using a porous silicon composite or a porous silicon-carbon composite can improve capacity, initial charge-discharge efficiency, and capacity retention rate.

[0194] MODE FOR CARRYING OUT THE INVENTION The present invention will be described in more detail below with reference to examples. The following examples are for illustrative purposes only and are not intended to limit the scope of the present invention. [Example]

[0195] Example 1 Preparation of porous silicon composites and 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 property values ​​shown in Table 1 below was prepared by the method described in Example 1 of Korean Patent Publication No. 2018-0106485.

[0196] (2) Step 2: 50 g of silicon composite oxide powder was dispersed in water and stirred at 300 rpm. Then, 650 ml of a 30 wt % aqueous HF solution was added as an etching solution, and the silicon composite oxide powder was etched at room temperature for 1 hour.

[0197] (3) Step 3: The product obtained by the above 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 to produce a porous silicon composite (B1).

[0198] (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 flow rate of 1 L / min. After holding at 900°C for 1 hour, the porous silicon composite was cooled to room temperature, resulting in a carbon coating on the surface of the porous silicon composite. A porous silicon-carbon composite was produced, with the component contents and physical properties shown in Table 3 below.

[0199] (5) Step 5: In order to control the particle size of the porous silicon carbon composite, it was pulverized and classified by a mechanical method so that the average particle size was 6.1 μm, to produce a porous silicon carbon composite (C1).

[0200] Secondary battery manufacturing A battery (coin cell) containing a negative electrode and a porous silicon-carbon composite as a negative electrode active material was prepared. 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%. The negative electrode active material composition was applied to a copper foil with a thickness of 18 μm and dried to prepare a 70 μm thick electrode. The copper foil with the electrode applied was punched into a circle with a diameter of 14 mm to prepare a negative electrode plate for a coin cell. On the other hand, a 0.3 mm thick metallic lithium foil was used for the positive electrode plate. 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 1M 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.

[0201] <Examples 2 to 8> A porous silicon-carbon composite was prepared in the same manner as in Example 1, except that a silicon composite oxide powder having the element contents and physical properties shown in Table 1 below was used, and the etching conditions and the type and amount of carbon source gas were changed to adjust the contents of each component and the physical properties of the composite as shown in Tables 1 to 3 below. A secondary battery was then prepared using this porous silicon-carbon composite.

[0202] <Comparative Example 1> A negative electrode active material and a secondary battery using the same were produced in the same manner as in Example 1, except that the silicon composite oxide of Example 6 was used, the etching step using the etching solution in step (2) was not performed, and the contents of each component and the physical properties of the composite were adjusted as shown in Tables 1 to 3 below.

[0203] <Comparative Example 2> A negative electrode active material and a secondary battery using the same were produced in the same manner as in Example 1, except that the silicon composite oxide of Example 6 was used, that nitric acid was used as the etching solution instead of HF, that etching was carried out at 70°C for 12 hours, and that the amount of carbon source gas was changed to adjust the contents of each component and the physical properties of the composite as shown in Tables 1 to 3 below.

[0204] <Comparative Example 3> A porous silicon-carbon composite was prepared in the same manner as in Example 1, except that 410 ml of a 30 wt % aqueous HF solution was used as the etching solution and the contents of each component and the physical properties of the composite were adjusted as shown in Tables 1 to 3 below, and a secondary battery was manufactured using the porous silicon-carbon composite.

[0205] Test Example <Test Example 1> Electron microscope analysis The surfaces of the porous silicon composite and porous silicon carbon composite prepared in Example 6 were observed using a scanning electron microscope (FE-SEM) (S-4700, Hitachi). The results are shown in Figures 1 and 3, respectively.

[0206] Referring to FIG. 1, the porous silicon composite prepared in Example 6 had pores on the surface.

[0207] Furthermore, referring to Figure 3, field emission scanning electron microscope (FE-SEM) photographs of the surface of a porous silicon carbon composite containing a carbon layer on the surface of the porous silicon composite are shown in Figures 3(a) to 3(d) at different magnifications. As can be seen from Figures 3(a) to 3(d), when comparing Figures 1 and 2, there are differences in the surfaces of the composites, and it was confirmed that a carbon layer was formed on the surface of secondary silicon particles (silicon aggregates) formed by agglomeration of silicon particles.

[0208] The interiors of the porous silicon composite and porous silicon carbon composite prepared in Example 6 were observed using ion beam scanning electron microscope (FIB-SEM) photographs (S-4700, Hitachi; QUANTA 3D FEG, FEI). The results are shown in Figures 2 and 4, respectively.

[0209] Referring to Figure 2, pores were present inside the porous silicon composite produced in Example 6. From Figure 2, it is presumed that the pores were formed by the etching solution that penetrated into the porous silicon composite.

[0210] Furthermore, referring to Figure 4, when the interior of a porous silicon carbon composite containing 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.

[0211] On the other hand, Figure 5 shows (a) an FIB-SEM EDAX (S-4700, Hitachi; QUANTA 3D FEG, FEI; EDS System, EDAX) photograph of the porous silicon-carbon composite prepared in Example 6, and (b) an analysis table of the components in the composite.

[0212] Referring to FIG. 5(b), a carbon content of about 15% was confirmed inside the porous silicon carbon composite, suggesting that the inside of the pores was coated with carbon.

[0213] Furthermore, as shown in the component analysis table of FIG. 5(b), the porous silicon carbon composite of Example 6 contained Mg, F, C, O and Si components.

[0214] 8a and 8b show photographs at different magnifications of the porous silicon composite (B1) prepared in Example 1 analyzed by scanning electron microscope (SEM).

[0215] As can be seen from FIGS. 8a and 8b, silicon agglomerates were present in which silicon particles having the crystallite size according to the embodiment of the present invention were connected to each other, and voids were formed between them.

[0216] <Test Example 2> X-ray diffraction analysis The crystal structures of the silicon composite oxide (A), porous silicon composite (B) and porous silicon carbon composite (C) prepared in the examples were analyzed using an X-ray diffractometer (Malvern Panalytical, X'Pert3).

[0217] 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 intervals of 0.05°.

[0218] FIG. 6 shows the results of X-ray diffraction analysis of the silicon composite oxide (a), porous silicon composite (b), and porous silicon carbon composite (c) of Example 6.

[0219] Referring to FIG. 6(a), as can be seen from the X-ray diffraction pattern, the silicon composite oxide of Example 6 has a peak corresponding to SiO2 at a diffraction angle (2θ) of approximately 21.7°; peaks corresponding to Si crystals at diffraction angles (2θ) of approximately 28.1°, 47.0°, 55.8°, 68.6°, and 76.1°; and peaks corresponding to MgSiO3 at diffraction angles (2θ) of approximately 30.4° and 35.0°, confirming that the silicon composite oxide contains amorphous SiO2, crystalline Si, and MgSiO3.

[0220] 6(b), as can be seen from the X-ray diffraction pattern, the porous silicon composite of Example 6 had peaks corresponding to MgF2 crystals at diffraction angles (2θ) of approximately 27.1°, 35.2°, 40.4°, 43.5°, 53.3°, 60.9°, and 67.9°, and peaks corresponding to Si crystals at diffraction angles (2θ) of approximately 28.1°, 47.0°, 55.8°, 68.6°, and 76.1°. Furthermore, the disappearance of the peak corresponding to MgSiO3 and the appearance of the peak corresponding to MgF2 indicate that MgSiO3 was converted to MgF2 by etching.

[0221] Referring to Figure 6(c), as can be seen from the X-ray diffraction pattern, the porous silicon carbon composite of Example 6 had 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°; and 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.

[0222] FIG. 7 shows the results of X-ray diffraction analysis of the porous silicon carbon composite of Example 3.

[0223] 7, as can be seen from the X-ray diffraction pattern, the porous silicon carbon composite of Example 3 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°, and peaks corresponding to Si crystals at diffraction angles (2θ) of approximately 28.1°, 47.1°, 55.8°, 68.9°, and 76.4°. Furthermore, the diffraction angle (2θ) of carbon could not be confirmed because it overlapped with the Si(111) peak.

[0224] The crystal size of Si in the obtained porous silicon carbon composite was calculated using the Scherrer equation (Equation 1) below, based on the full width at half maximum (FWHM) of the peak corresponding to Si(220) in X-ray diffraction analysis.

[0225] [Formula 1] Crystal 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).)

[0226] <Test Example 3> Analysis of the content and specific gravity of the constituent elements of the composite The contents of the constituent elements magnesium (Mg), oxygen (O) and carbon (C) in the composites produced in the examples and comparative examples were analyzed.

[0227] The magnesium (Mg) content was analyzed by inductively coupled plasma (ICP) emission spectroscopy. The oxygen (O) and carbon (C) contents were measured using an elemental analyzer. The silicon (Si) content was calculated based on the oxygen (O) and magnesium (Mg) contents.

[0228] Test Example 4: Measurement of average particle size of composite particles The average particle size (D 50 ) is the particle size or median diameter at 50% cumulative volume measured by the laser diffraction method. 50 was measured as.

[0229] <Test Example 5> Raman analysis The porous silicon carbon composite prepared in Example 1 was subjected to Raman spectroscopy analysis. The Raman analysis was carried out at 2.41 eV (514 nm) using a micro-Raman analyzer (Renishaw, RM1000-In Via). As a result, the Raman spectrum obtained by Raman spectroscopy shows a peak at 2,600 cm -1 ~2,760cm -1 2D band peak in the range of 1,500 cm -1 ~1,660cm -1 G-band peak in the range of 1,300 cm -1 ~1,460cm -1 The D band peak was in the range of I 2D , the intensity of the D band peak is I D , the intensity of the G band peak is I G Then, I D , I 2D , I G are 1.0, 0.1, and 0.82, respectively, and (I 2D +I G ) / I D was 0.92. The results of Raman spectroscopy show that the carbon layer is I D , I 2D , I G It was found that the above values ​​were obtained, the conductivity was good, and the characteristics of the secondary battery could be improved. 2D +I G ) / I D Since the value of 0.92 is small, side reactions during charging and discharging are suppressed, and the decrease in initial efficiency is suppressed. Therefore, the porous silicon carbon composite produced in Example 1 has excellent electrical conductivity and can significantly improve the performance of lithium secondary batteries.

[0230] <Test Example 6> 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.

[0231] [Formula 2] Initial efficiency (%) = discharge capacity / charge capacity x 100

[0232] The coin cells prepared in the examples and comparative examples were charged and discharged once in the same manner as above, 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 after 50 cycles, %) were measured. The results are shown in Table 4 below.

[0233] [Formula 3] Capacity retention rate after 50 cycles (%) = 51st discharge capacity / 2nd discharge capacity × 100

[0234] The contents of each element and physical properties of the composites produced in the examples and comparative examples are summarized in the following Tables 1 to 3. The characteristics of the secondary batteries using these composites are summarized in the following Table 4.

[0235] [Table 1]

[0236] [Table 2]

[0237] [Table 3]

[0238] [Table 4]

[0239] As can be seen from Table 4, the porous silicon composites and porous silicon carbon composites of Examples 1 to 8 of the present invention contained silicon particles and a magnesium compound, and the molar ratio (O / Si) of oxygen (O) atoms to silicon (Si) atoms satisfied a specific range. As a result, the secondary batteries fabricated using these composites showed significantly improved performance in terms of discharge capacity, initial efficiency, and capacity retention rate compared to the secondary batteries of Comparative Examples 1 to 3.

[0240] Specifically, the secondary batteries of Examples 1 to 8, which used porous silicon composites with a molar ratio (O / Si) of oxygen (O) atoms to silicon (Si) atoms of 0.03 to 0.11, had discharge capacities of 1,445 mAh / g to 2,074 mAh / g, initial efficiencies of 86.1% to 94.1%, and capacity retention rates of 84.1% to 89.5%, demonstrating that the performance of the secondary batteries was excellent overall.

[0241] On the other hand, when the secondary battery of Example 6 was compared with the secondary battery of Comparative Example 1 in which etching was not performed in the steps of Example 6, and the secondary battery of Comparative Example 2 in which etching was performed using nitric acid instead of HF in the steps of Example 6, the discharge capacity of the secondary battery of Example 6 was 1,702 mAh / g and the initial efficiency was 90%, while the secondary batteries of Comparative Examples 1 and 2 had discharge capacities of 1,453 mAh / g and 1,410 mAh / g and initial efficiencies of 79.6% and 77.5%, respectively, indicating that the discharge capacity and initial efficiency of the secondary batteries of Comparative Examples 1 and 2 were significantly lower than that of the secondary battery of Example 6.

[0242] Furthermore, when the secondary battery of Example 1 was compared with the secondary battery of Comparative Example 3 in which the molar ratio (O / Si) of oxygen (O) atoms to silicon (Si) atoms was 0.37, it was found that the secondary battery of Comparative Example 3 had a discharge capacity of 1,505 mAh / g and an initial efficiency of 84.2%, which was a significant decrease in performance compared to the secondary battery of Example 1 in which the discharge capacity was 1,922 mAh / g and the initial efficiency was 91.4%.

[0243] Therefore, it was confirmed that the performance of secondary batteries can be improved by adjusting the molar ratio (O / Si) of oxygen (O) atoms to silicon (Si) atoms in the composite. Preferred embodiments of the present invention include the following. 〔1〕 A porous silicon composite containing silicon particles and a magnesium compound, wherein the molar ratio (O / Si) of oxygen (O) atoms to silicon (Si) atoms in the porous silicon composite is 0.01 to 0.35. 〔2〕 The porous silicon composite according to [1], wherein the porous silicon composite contains silicon aggregates in which the silicon particles are interconnected. 〔3〕 The magnesium compound includes a fluorine-containing magnesium compound, and the fluorine-containing magnesium compound includes magnesium fluoride (MgF 2 ) and magnesium fluorosilicate (MgSiF 6 ), or a mixture thereof. 〔4〕 The magnesium compound according to [3], including MgSiO 3 [[ID= 2 ​ 4 ​ 〔5〕 ​ 〔6〕 ​ 〔7〕 ​ x ​ 〔8〕 ​ 〔9〕 ​ 50 ​ 3 ​ 3 ​ 〔10〕 ​ 3 ​ 3 ​ 3 ​ 3 ​ 2 ​ 2 ​ 〔11〕 ​ 〔12〕 The porous silicon carbon composite according to

[11] , wherein the molar ratio (O / Si) of oxygen (O) atoms to silicon (Si) atoms in the porous silicon carbon composite is 0.01 to 0.35. 〔13〕 The porous silicon carbon composite according to

[11] , wherein the porous silicon carbon composite has pores therein and the porosity of the porous silicon carbon composite is 0.5 to 40% by volume based on the volume of the porous silicon carbon composite. 〔14〕 The porous silicon carbon composite according to

[11] , wherein the silicon (Si) content is 30% to 90% by weight based on the total weight of the porous silicon carbon composite. 〔15〕 The porous silicon carbon composite according to

[11] , wherein carbon is present on the surface of at least one selected from the group consisting of silicon particles and magnesium compounds, carbon functions as a matrix and silicon particles, magnesium compounds and pores are dispersed in the carbon matrix, or carbon is present in both ways. 〔16〕 The porous silicon carbon composite according to

[15] , further comprising a carbon layer formed on the surface of the porous silicon composite, the carbon layer comprising at least one selected from the group consisting of graphene, reduced graphene oxide, carbon nanotubes, carbon nanofibers and graphite, and the carbon layer having a thickness of 1 nm to 300 nm. 〔17〕 The porous silicon carbon composite according to

[11] , wherein the carbon (C) content is 10% by weight to 90% by weight based on the total weight of the porous silicon carbon composite. 〔18〕 The porous silicon carbon composite has an average particle size (D 50 ) is 2 μm to 15 μm, and the porous silicon carbon composite has a specific gravity of 1.8 g / cm 3 ~2.5g / cm 3 and the specific surface area (Brunauer-Emmett-Teller method; BET) is 3 m 2 / g~50m 2 / g. 〔19〕 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; and The third step is to filter and dry the composite obtained by etching to obtain a porous silicon composite. A method for preparing the porous silicon composite according to [1], comprising: 〔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; and The fourth step in preparing porous silicon carbon composites is to form a carbon layer on the surface of the porous silicon composite using chemical pyrolysis deposition. The method for preparing the porous silicon carbon composite according to

[11] , comprising: 〔21〕 The method for preparing a silicon carbon composite according to

[20] , wherein in the second step, the etching solution further contains one or more acids selected from the group consisting of organic acids, sulfuric acid, hydrochloric acid, phosphoric acid, nitric acid, and chromic acid. 〔22〕 The method for preparing a porous silicon carbon composite according to

[20] , further comprising, after the fourth step, a step of pulverizing or crushing and classifying the porous silicon carbon composite so that the average particle size of the porous silicon carbon composite is 2 μm to 15 μm. 〔23〕 The carbon layer is formed in the fourth step according to the following formulas 1 to 3: [Formula 1] CN 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.) [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.) and reacting the compound in a gaseous state at 400°C to 1200°C. 〔24〕

[11] A negative electrode active material for a lithium secondary battery, comprising the porous silicon-carbon composite according to

[11] . 〔25〕

[25] The negative electrode active material for a lithium secondary battery according to

[24] , further comprising a carbon-based negative electrode material. 〔26〕 The negative electrode active material for a lithium secondary battery according to

[25] , wherein the content of the carbon-based negative electrode material is 5% by weight to 95% by weight based on the total weight of the negative electrode active material. 〔27〕

[24] A lithium secondary battery comprising the negative electrode active material for lithium secondary batteries according to

[24] .

Claims

1. A porous silicon composite comprising silicon particles and a magnesium compound, wherein the molar ratio (O / Si) of oxygen (O) atoms to silicon (Si) atoms in the porous silicon composite is 0.01 to 0.35; A porous silicon composite, wherein the magnesium compound comprises a fluorine-containing magnesium compound, said fluorine-containing magnesium compound comprising magnesium fluoride (MgF 2 ), magnesium fluorosilicate (MgSiF 6 ), or a mixture thereof.

2. 10. The porous silicon composite of claim 1, wherein the porous silicon composite comprises silicon agglomerates in which the silicon particles are interconnected.

3. The magnesium compound is MgSiO ３ , Mg ２ SiO ４ or a mixture thereof, wherein the content of magnesium (Mg) in the porous silicon composite is 0.2 wt % to 20 wt %, based on the total weight of the porous silicon composite.

4. The silicon oxide (SiO ｘ , 0.1<x≦2).

5. 5. The porous silicon composite according to claim 4, wherein the content of oxygen (O) in the porous silicon composite is 0.1 wt % to 15 wt %, based on the total weight of the porous silicon composite.

6. A porous silicon carbon composite comprising the porous silicon composite of claim 1 and carbon.

7. 7. The porous silicon carbon composite according to claim 6, wherein the molar ratio of oxygen (O) atoms to silicon (Si) atoms (O / Si) in the porous silicon carbon composite is 0.01 to 0.

35.

8. 7. The porous silicon carbon composite of claim 6, wherein the porous silicon carbon composite has pores therein and the porosity of the porous silicon carbon composite is 0.5% to 40% by volume, based on the volume of the porous silicon carbon composite.

9. Carbon is present on the surface of at least one selected from the group consisting of silicon particles and magnesium compounds, or Carbon serves as a matrix, and silicon particles, magnesium compounds, and pores are dispersed in the carbon matrix; or 7. The porous silicon carbon composite of claim 6, wherein carbon is present on the surface of at least one selected from the group consisting of silicon particles and magnesium compounds, the carbon functions as a matrix, and the silicon particles, magnesium compounds, and pores are dispersed in the carbon matrix.

10. The porous silicon carbon composite has an average particle size (D ５０ ) is 2 μm to 15 μm, and the porous silicon carbon composite has a specific gravity of 1.8 g / cm ３ ~2.5g / cm ３ and the specific surface area (Brunauer-Emmett-Teller method; BET) is 3 m ２ / g to 50m ２ / g.

11. 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; and The third step is to filter and dry the composite obtained by etching to obtain a porous silicon composite.

2. A method for preparing the porous silicon composite of claim 1, comprising:

12. 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 preparing a porous silicon carbon composite by forming a carbon layer on the surface of the porous silicon composite using a chemical pyrolysis deposition method.

7. A method for preparing the porous silicon carbon composite of claim 6, comprising:

13. A negative electrode active material for a lithium secondary battery, comprising the porous silicon carbon composite of claim 6 .

14. A lithium secondary battery comprising the negative electrode active material for lithium secondary batteries according to claim 13.

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