Porous silicon structure, porous silicon-carbon composite containing the same, and negative electrode active material

Abstract

One embodiment of the present invention relates to a porous silicon structure, a porous silicon carbon composite containing the same, and a negative electrode active material. The porous silicon structure and the porous silicon carbon composite have a molar ratio (O / Si) of oxygen (O) atoms to silicon (Si) atoms that satisfies a specific range, and therefore, when used as a negative electrode active material, the porous silicon structure and the porous silicon carbon composite have an excellent capacity retention rate and can significantly improve the discharge capacity and initial efficiency.

Description

[Technical Field]

[0001] The present invention relates to a porous silicon structure, a porous silicon carbon composite, and an anode active material containing the same. [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 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] Patent No. 4393610 [Patent Document 2] Japanese Patent Application Laid-Open No. 2016-502253 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 structure in which the molar ratio (O / Si) of oxygen (O) atoms to silicon (Si) atoms in the porous silicon structure satisfies 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 structure 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 structures 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 structure containing silicon particles, wherein the molar ratio (O / Si) of oxygen (O) atoms to silicon (Si) atoms in the porous silicon structure is 0.01 to 0.35.

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

[0017] The present invention further provides a method for preparing a porous silicon structure, which method comprises: a first step of etching a silicon-based raw material powder using an etching solution containing a fluorine (F) atom-containing compound; and a second step of filtering and drying the product obtained by etching to prepare a porous silicon structure.

[0018] The present invention further provides a method for preparing a porous silicon carbon composite, which method comprises: a first step of etching a silicon-based raw material powder using an etching solution containing a fluorine (F) atom-containing compound; a second step of filtering and drying the product obtained by etching to prepare a porous silicon structure; and a third step of forming a carbon layer on the surface of the porous silicon structure using a chemical pyrolysis deposition method to prepare the 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 structure has a molar ratio (O / Si) of oxygen (O) atoms to silicon (Si) atoms in the porous silicon structure that satisfies a specific range. Therefore, when the porous silicon structure is used together with a binder and a conductive material as a negative electrode active material for a secondary battery to prepare a negative electrode active material composition, the porous silicon structure is easily dispersible and has excellent mechanical properties such as strength, and when used 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 structure and carbon, when applied to a negative electrode active material, significantly improves discharge capacity and initial efficiency, and has an excellent capacity retention rate.

[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 described as "comprising" an element, it should be understood that the part may include other elements as well, unless otherwise indicated.

[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 structure] A porous silicon structure according to one embodiment of the present invention contains silicon particles, and the molar ratio (O / Si) of oxygen (O) atoms to silicon (Si) atoms in the porous silicon structure is 0.01 to 0.35.

[0029] According to one embodiment of the present invention, the number of oxygen atoms present on the surface of a porous silicon structure can be reduced. That is, 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. When the porous silicon structure is used together with a binder and a conductive material as the negative electrode active material for a secondary battery to prepare a negative electrode active material composition, the porous silicon structure is easily dispersible and has excellent mechanical properties such as strength, and when used as a negative electrode active material, the performance of the secondary battery can be improved.

[0030] Specifically, the molar ratio (O / Si) of oxygen (O) atoms to silicon (Si) atoms in the porous silicon structure may be preferably 0.01 to 0.25, more preferably 0.01 to 0.10, and even more preferably 0.01 to 0.08.

[0031] Specifically, according to one embodiment of the present invention, most of the silicon dioxide contained in the porous silicon structure 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 the porous silicon structure 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.

[0032] Generally, a negative electrode active material containing silicon exhibits a higher charge / discharge capacity as the oxygen content decreases, but the volume expansion rate during charging may increase. On the other hand, an increase in the oxygen content suppresses the volume expansion rate, but the discharge capacity may decrease.

[0033] 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 structure is 0.01 or more, expansion and contraction due to charge and discharge can be suppressed. Therefore, when the porous silicon structure is used as a negative electrode active material, delamination 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 discharge capacity can be ensured and high charge and discharge characteristics can be maintained.

[0034] The porous silicon structure may also comprise silicon agglomerates in which the silicon particles are interconnected.

[0035] Specifically, the porous silicon structure may include 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 include interconnected silicon aggregates, excellent mechanical properties such as strength can be obtained. Furthermore, the porous silicon structure 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.

[0036] The silicon aggregates may be uniformly distributed within the porous silicon structure, 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 structure. In such cases, excellent electrochemical properties such as charge / discharge are achieved.

[0037] 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.

[0038] Specifically, when a porous silicon structure 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 at around 2θ = 47.5°, the silicon particles preferably have a crystallite size of 1 nm to 10 nm, more preferably 1 nm to 6 nm. If the crystallite size of the silicon particles is less than 1 nm, it becomes difficult to form micropores within the porous silicon structure, which makes it difficult to suppress a decrease in Coulombic efficiency, which represents the ratio of charge capacity to discharge capacity. Furthermore, 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 difficult to suppress a decrease in Coulombic efficiency, which represents the ratio of charge capacity to discharge capacity, due to repeated charge and discharge.

[0039] 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.

[0040] Furthermore, the porous silicon structure may further comprise amorphous silicon or silicon in a similar phase. Silicon particles have high initial efficiency and battery capacity, but undergo very complex crystalline changes as they electrochemically absorb, store, and release lithium atoms.

[0041] Alternatively, the porous silicon structure may comprise a plurality of pores.

[0042] The silicon structure has a porous structure, and therefore can accommodate the volume expansion of silicon particles that occurs during charging and discharging of the secondary battery, thereby effectively mitigating and suppressing problems caused by the volume expansion.

[0043] The porous silicon structure may contain a plurality of pores within it, on its surface, or both, and the pores may be interconnected within the silicon structure to form open pores.

[0044] When the surface of the porous silicon structure 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.

[0045] The pore volume of micropores of 2 nm or less is 0.1 cm 3 / g~0.5cm 3 / g, preferably 0.15 cm 3 / g~0.45cm 3 / g, more preferably 0.2 cm 3 / g~0.4cm 3 / g.

[0046] The pore volume of mesopores with a diameter of more than 2 nm to 50 nm is 0.2 cm 3 / g~0.7cm 3It may be / g, preferably 0.2 cm 3 / g to 0.6 cm 3 / g, more preferably 0.2 cm 3 / g to 0.5 cm 3 It may be / g.

[0047] When the pores include micro-pores of 2 nm or less that fill the pore volume and meso-pores of more than 2 nm to 50 nm, the pores in the silicon structure may be uniformly and abundantly contained.

[0048] Furthermore, the pores may further include macro-pores of more than 50 nm to 250 nm. The pore volume of the macro-pores of more than 50 nm to 250 nm is 0.01 cm 3 / g to 0.3 cm 3 It may be / g, preferably 0.01 cm 3 / g to 0.2 cm 3 / g, more preferably 0.01 cm 3 / g to 0.15 cm 3 It may be / g.

[0049] In addition, in the porous silicon structure in which pores are formed, it is preferable that silicon particles and / or silicon aggregates in which silicon particles are connected are uniformly distributed inside the porous silicon structure. As a result, the porous silicon structure 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 volume expansion.

[0050] According to one embodiment of the present invention, the porous silicon structure may further contain a silicon oxide compound.

[0051] The silicon oxide compound may be a silicon-based oxide represented by the formula SiO x (0.5 ≦ x ≦ 2). The silicon oxide compound may specifically be SiO x (0.8 < x ≦ 1.5), and more specifically may be SiO x (0.9 < x ≦ 1.1). SiO xIn the formula, if the value of x is less than 0.5, the expansion or contraction during charge and discharge of the secondary battery becomes large, and the life characteristics may deteriorate. Further, if x exceeds 2, the amount of the inactive oxide increases, and there may occur a problem that the initial efficiency of the secondary battery decreases.

[0052] 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 structure.

[0053] When the content of the silicon oxide compound is less than 0.1% by weight, the volume of the secondary battery expands, 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 increases and the initial efficiency may decrease.

[0054] In the present 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.

[0055] The porous silicon structure according to an embodiment of the present invention may further contain silicon oxide (SiO x , 0.1 < x ≦ 2) formed on the surface of the silicon particles. The silicon oxide (SiO x , 0.1 < x ≦ 2) may be formed by oxidation of silicon.

[0056] The content of oxygen (O) in the porous silicon structure may be 0.1% by weight to 15% by weight, preferably 0.5% by weight to 10% by weight, more preferably 0.5% by weight to 5% by weight based on the total weight of the porous silicon structure. When the oxygen (O) content of the porous silicon structure is less than 0.1% by weight, the degree of expansion during charging of the secondary battery becomes large, and the cycle characteristics deteriorate, which is not preferable. When the oxygen (O) content of the porous silicon structure exceeds 15% by weight, when the porous silicon structure is used as a negative electrode active material, there may occur problems such as an increase in the irreversible reaction with lithium, a decrease in the initial charge-discharge efficiency, easy peeling from the negative electrode current collector, and a decrease in the charge-discharge cycle characteristics.

[0057] The average particle size of the porous silicon structure (D 50 The average particle size (D) of the porous silicon structure may be 1 μm to 15 μm, preferably 2 μm to 10 μm, and more preferably 3 μm to 8 μm. 50 If the average particle size (D) of the porous silicon structure exceeds 15 μm, the expansion of the porous silicon structure due to charging of lithium ions becomes intense, and the bonding strength between the particles in the structure and between the particles and the current collector decreases with repeated charging and discharging, which can significantly reduce the life characteristics. In addition, there is a concern that the activity will decrease due to a decrease in the specific surface area. 50 If the particle size is less than 1 μm, there is a concern that the dispersibility may decrease due to aggregation of the porous silicon structure when preparing a negative electrode slurry (negative electrode active material composition) using the porous silicon structure.

[0058] The specific gravity of the porous silicon structure is 1.5 g / cm 3 ~2.3g / cm 3 and preferably 1.6 g / cm 3 ~2.3g / cm 3 , more preferably 1.6 g / cm 3 ~2.2g / cm 3 may be.

[0059] 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.

[0060] The specific gravity of the porous silicon structure 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.

[0061] The porous silicon structure 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.

[0062] The specific surface area of ​​a porous silicon structure can be calculated from the sum of the surface area of ​​the particle surfaces of the porous silicon structure and the surface area of ​​the pores (micropores, macropores, mesopores) present on the surface and inside the porous silicon structure.

[0063] [Porous silicon carbon composite] According to an embodiment of the present invention, there is provided a porous silicon carbon composite comprising a porous silicon structure and carbon.

[0064] Specifically, the porous silicon carbon composite comprises a porous silicon structure and carbon, the porous silicon structure comprises silicon particles, and the molar ratio (O / Si) of oxygen (O) atoms to silicon (Si) atoms in the porous silicon structure is 0.01 to 0.35.

[0065] 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 structure. 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, it is possible to significantly improve the discharge capacity and initial efficiency while maintaining an excellent capacity retention rate.

[0066] Furthermore, since the porous silicon-carbon composite contains carbon, the porous silicon-carbon composite has sufficient electrical conductivity and the specific surface area can be appropriately adjusted, which can further improve the performance of the secondary battery when used as the negative electrode active material.

[0067] 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.

[0068] 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, sufficient discharge capacity can be ensured and high charge and discharge characteristics can be maintained.

[0069] 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. Carbon, more specifically, a carbon layer containing carbon, may have a single structure surrounding 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.

[0070] 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.

[0071] 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 formed in such a way that at least a portion of the pore walls are open, and may or may not be connected to other pores. They may also refer to pores located on the surface of a silicon structure and exposed to the outside.

[0072] 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.

[0073] Here, porosity refers to "(pore volume per unit mass) / {(specific volume + pore volume per unit mass)}" and can be measured by mercury intrusion or Brunauer-Emmett-Teller (BET) measurement.

[0074] 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.

[0075] 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.

[0076] The porous silicon carbon composite may contain a plurality of pores, each of which may have the same or different diameters.

[0077] In the porous silicon carbon composite, carbon may be present on the surfaces of silicon particles contained in the porous silicon structure, or on the surfaces of silicon aggregates contained in the porous silicon carbon composite.

[0078] Alternatively, carbon may serve as a matrix, with silicon particles and pores dispersed in the carbon matrix.

[0079] Specifically, the porous silicon carbon composite may have a sea-island structure in which silicon particles or closed pores (v) form islands and carbon forms a sea. The pores include open pores and closed pores. The closed pores may not be covered with carbon.

[0080] In the porous silicon-carbon composite, carbon may be present on the surface of the porous silicon structure or inside the open pores.

[0081] The state where silicon particles and carbon are uniformly dispersed is confirmed by observing images of dark-field images and bright-field images using a transmission electron microscope (TEM).

[0082] Also, carbon is present on the surface of the silicon particles. Carbon functions as a matrix, and the silicon particles and pores may be dispersed in the carbon matrix.

[0083] 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).

[0084] Carbon may further form a carbon layer on the surface of the porous silicon structure.

[0085] Since the porous silicon-carbon composite contains a carbon layer, this eliminates the difficulty of electrical contact between particles due to the presence of pores, and excellent conductivity can be obtained even after the electrode expands during charge and discharge, so the performance of the secondary battery can be further improved.

[0086] Furthermore, according to one embodiment of the present invention, by controlling the thickness and amount of carbon in the carbon layer, appropriate conductivity can be obtained, the degradation of life characteristics can be prevented, and a high-capacity negative electrode active material can be obtained.

[0087] On the other hand, since the surface or the inside of the porous structure particles and the pores therein are covered with a carbon coating, the specific surface area of the porous silicon-carbon composite may change significantly.

[0088] The porous silicon-carbon composite has a specific surface area of 3 m 2 / g to 50 m 2 , preferably 3 m 2 / g to 40 m 2The 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.

[0089] 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 3 The 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.

[0090] 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, cracks in the negative electrode active material powder due to volume expansion and contraction of the negative electrode active material powder during charge and discharge can be minimized, and cycle deterioration 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.

[0091] 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.

[0092] 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.

[0093] 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.

[0094] 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.

[0095] 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.

[0096] 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.

[0097] The average thickness of the carbon layer may be measured, for example, by the following procedure.

[0098] 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.

[0099] 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 structure, but also on the surfaces of the silicon particles and in the carbon matrix.

[0100] 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. 50 If 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, 50Considering the significant improvement effect of optimizing the average particle size (D 50 ) is preferably 3 μm to 6 μm.

[0101] [Method for preparing porous silicon structures] A method for preparing a porous silicon structure according to one embodiment of the present invention comprises: a first step of etching a silicon-based raw material powder using an etching solution containing a fluorine (F) atom-containing compound; and a second step of filtering and drying the product obtained by etching to prepare a porous silicon structure.

[0102] The method according to one embodiment has the advantage that it allows for mass production using a continuous process with minimal steps.

[0103] Specifically, in the method for preparing a porous silicon carbon composite, the first step involves etching a silicon-based raw material powder with an etching solution containing a fluorine (F) atom-containing compound. The silicon-based raw material powder can be a powder containing silicon capable of reacting with lithium, such as a powder containing at least two of silicon, silicon oxide, and silicon dioxide.

[0104] The method may further include a step of forming a carbon layer on the surface of the silicon-based raw material powder by using a chemical pyrolysis deposition method, and therefore the silicon-based raw material powder may have a carbon layer formed thereon.

[0105] Specifically, the first etching step may be carried out after forming a carbon layer on the surface of a silicon-based raw material powder containing silicon particles, which has the advantage of enabling uniform etching and achieving a high yield.

[0106] 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 third step of the method of preparing a porous silicon carbon composite described below.

[0107] The etching process may include dry etching and wet etching.

[0108] Dry etching may allow selective etching.

[0109] The etching process dissolves and elutes silicon dioxide from the silicon-based raw material powder, forming pores.

[0110] The pores are also believed to be formed by the following reaction schemes 2 and 3.

[0111] [ka]

[0112] Through a reaction mechanism such as the above reaction scheme, silicon dioxide is dissolved and removed in the form of SiF4 and H2SiF6, which can result in the formation of pores (voids).

[0113] Furthermore, depending on the degree of etching, the silicon dioxide contained in the porous silicon structure may be removed, forming pores therein.

[0114] The degree of pore formation may vary depending on the degree of etching.

[0115] Furthermore, the O / Si ratio and specific surface area of ​​the porous silicon structure may change significantly during etching, and the specific surface area and specific gravity of the silicon structure with pores formed may change significantly before and after carbon coating.

[0116] By etching, it is possible to obtain a porous silicon structure powder in which a plurality of pores are formed on the surface of the porous silicon structure particles, or on the surface and inside thereof.

[0117] Here, etching refers to a process of treating a silicon-based raw material powder with an etching solution containing a fluorine (F) atom-containing compound.

[0118] 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.

[0119] 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 etching process can be carried out more quickly.

[0120] 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.

[0121] 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.

[0122] The porous silicon structure obtained by etching may comprise porous silicon particles.

[0123] Etching can result in a porous silicon structure having a plurality of pores formed on the surface, inside, or both of the porous silicon structure, where the porous silicon structure may have a three-dimensional (3D) structure in which two or more silicon particles are interconnected.

[0124] Another feature of the porous silicon structure is that the average grain size is hardly changed by etching.

[0125] That is, the average particle size of the silicon-based raw material powder before etching is approximately the same as the average particle size of the porous silicon structure obtained by etching. The difference (change) between the average particle size of the silicon-based raw material powder and the average particle size of the porous silicon structure may be within about 5%.

[0126] Furthermore, the number of oxygen atoms present on the surface of the porous silicon structure may be reduced by etching. That is, etching can significantly reduce the oxygen fraction on the surface of the porous silicon structure, thereby reducing the surface resistance. As a result, when the porous silicon structure is used as a negative electrode active material, the electrochemical characteristics, particularly the life characteristics, of lithium secondary batteries can be significantly improved.

[0127] Furthermore, because a large amount of silicon dioxide is removed by selective etching, the surface of the silicon particles 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 structure may be significantly reduced. In this case, a secondary battery with excellent capacity retention, high discharge capacity, and high initial efficiency can be obtained.

[0128] Furthermore, pores or voids may form where the silicon dioxide has been removed, which may result in an increase in the specific surface area of ​​the porous silicon structure compared to the specific surface area of ​​the silicon-based raw material before etching.

[0129] According to one embodiment of the present invention, physical properties such as element content and specific surface area may change before and after the etching process, i.e., the physical properties such as element content and specific surface area of ​​the silicon-based raw material before the etching process may be different from those of the silicon structure after the etching process.

[0130] In the method for producing a porous silicon structure, the second step may include filtering and drying the product obtained by etching to obtain a porous silicon structure. The filtering and drying steps may be carried out by conventional methods.

[0131] 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.

[0132] [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 structure.

[0133] 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 etching a silicon-based raw material powder using an etching solution containing a fluorine (F) atom-containing compound; a second step of filtering and drying the product obtained by etching to prepare a porous silicon structure; and a third step of forming a carbon layer on the surface of the porous silicon structure using a chemical pyrolysis deposition method to prepare a porous silicon carbon composite.

[0134] The carbon layer formation process can 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, improving the battery's output and cycle characteristics, and enhancing the stress relaxation effect during volumetric changes in the active material.

[0135] The method for preparing the porous silicon carbon composite will now be described in detail.

[0136] The first and second steps are the same as those described in the method for preparing the porous silicon structure.

[0137] In the method for preparing a porous silicon carbon composite, the third step may include forming a carbon layer on the surface of the porous silicon structure using a chemical pyrolysis deposition method to prepare the porous silicon carbon composite.

[0138] The carbon layer may include at least one selected from the group consisting of graphene, reduced graphene oxide, carbon nanotubes, carbon nanofibers, and graphite.

[0139] 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 structure 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.)

[0140] 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).

[0141] 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.

[0142] The carbon source gas may further include at least one inert gas selected from hydrogen, nitrogen, helium, and argon.

[0143] The reaction may be carried out at, for example, 400°C to 1200°C, specifically 500°C to 1100°C, and more specifically 600°C to 1000°C.

[0144] The reaction time (or heat treatment time) may be adjusted appropriately depending on the heat treatment temperature, the pressure during the heat treatment, the composition of the gas mixture, 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.

[0145] 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 structure by a gas-phase reaction of a carbon source gas, even at a relatively low temperature. Furthermore, desorption reactions in the carbon layer do not substantially occur.

[0146] Furthermore, because a carbon layer is formed uniformly over the entire surface of the porous silicon structure 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.

[0147] 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 structure, the reactive gas penetrates into the open pores of the silicon structure, 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 silicon structure. For example, the conductive carbon material deposited on the silicon surface of the silicon structure gradually grows over the course of the reaction time, resulting in a porous silicon-carbon composite.

[0148] The specific surface area of ​​the porous silicon carbon composite may decrease depending on the amount of carbon coating.

[0149] The structure of the graphene-containing material may be layered, nanosheet-like, or a mixture of multiple flakes.

[0150] When a carbon layer containing graphene-containing material is uniformly formed over the entire surface of a silicon structure, the highly conductive and flexible graphene-containing material is grown directly on the surface of the silicon particles, suppressing volume expansion. Furthermore, the carbon layer coating reduces the opportunities for direct contact between the silicon and the electrolyte, which may reduce the formation of a solid-electrolyte interface (SEI) layer.

[0151] Furthermore, according to one embodiment of the present invention, after the third step (after the formation of the carbon layer in the third step), one or more porous structure powders may be bonded to each other to form aggregates. This method may further include 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 solids 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 interparticle interference, particle shape, airflow turbulence, flow velocity distribution, static electricity, etc. Furthermore, the desired particle size distribution can be obtained by pulverizing or crushing and classifying the porous silicon carbon composite 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.

[0152] 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.

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

[0154] negative electrode active material A negative electrode active material according to one embodiment of the present invention may include a porous silicon structure.

[0155] Furthermore, the negative electrode active material according to one embodiment of the present invention may include a porous silicon carbon composite.

[0156] The negative electrode active material may further include a carbon-based negative electrode material, specifically, a graphite-based negative electrode material.

[0157] The negative electrode active material may be a mixture of a porous silicon structure or a porous silicon-carbon composite with a carbon-based negative electrode material, such as a graphite-based negative electrode material. This reduces the electrical resistance of the negative electrode active material and alleviates expansion stress during charging. The carbon-based negative electrode material may include at least one material selected from the group consisting of natural graphite, artificial graphite, soft carbon, hard carbon, mesocarbon, carbon fiber, carbon nanotubes, pyrolytic carbon, coke, glass carbon fiber, sintered organic polymer compounds, and carbon black.

[0158] 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.

[0159] 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.

[0160] 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.

[0161] 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.

[0162] 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.

[0163] 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.

[0164] 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.

[0165] 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.

[0166] The secondary battery may include a non-aqueous secondary battery.

[0167] A negative electrode active material and a secondary battery using a porous silicon structure or a porous silicon-carbon composite can improve capacity, initial charge-discharge efficiency, and capacity retention rate.

[0168] 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]

[0169] Example 1 (1) Step 1: Silicon oxide (SiO ) having the physical properties shown in Table 1 below was prepared by vapor deposition using silicon powder and silicon dioxide powder. x 50 g of the powder (x=0.9) was dispersed in water and stirred at 300 rpm, and 50 ml of a 40 wt % aqueous HF solution was added as an etching solution, and the silicon oxide raw material powder was etched for 1 hour.

[0170] (2) Step 2: The product obtained by the above etching was filtered and dried at 150°C for 2 hours. Next, in order to control the particle size of the porous composite, it was crushed in a mortar to an average particle size of 5.1 μm, to prepare a porous silicon structure (B1).

[0171] (3) Step 3: 10 g of the porous silicon structure 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 structure was cooled to room temperature, resulting in a carbon coating on the surface of the porous silicon structure. A porous silicon-carbon composite was produced, with the component contents and physical properties shown in Table 3 below.

[0172] (4) Step 4: 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 5.1 μm, to produce a porous silicon carbon composite (C1).

[0173] 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.

[0174] <Examples 2 to 5> A porous silicon carbon composite was prepared in the same manner as in Example 1, except that a porous silicon structure having the element contents and physical properties shown in Table 2 below was used, and the content of each component and the physical properties of the composite were adjusted by changing the type and amount of carbon source gas, and a secondary battery was prepared using this composite.

[0175] <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 etching step using the etching solution in step (1) was not carried out.

[0176] <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 NaOH was used instead of the HF etching solution and etching was carried out at room temperature for 12 hours.

[0177] <Comparative Example 3> A porous silicon-carbon composite was produced in the same manner as in Example 1, except that 35 ml of a 40 wt % HF aqueous solution was used as the etching solution, and a secondary battery was produced using this.

[0178] Test Example <Test Example 1> Electron microscope analysis Figures 1 and 2 show ion beam scanning electron microscope (FIB-SEM, [FC-FI20]-FIB II instrument) photographs of the porous silicon structure (B1) and porous silicon carbon composite (C1) produced in Example 1, respectively. 1 and 2 show photographs of a porous silicon structure (B1) and a porous silicon carbon composite (C1) analyzed by ion beam scanning electron microscopy (FIB-SEM) at 10,000x and 30,000x magnifications, respectively. Figure 3 shows a TEM photograph of the surface of the porous silicon-carbon composite (C5) produced in Example 5. Here, the bright white dots represent Si crystals, and the dark dots represent carbon (C). As can be seen from Figure 3, the surface of the porous silicon-carbon composite (C5) particles contained a large amount of carbon, while the interior of the particles contained a large amount of Si.

[0179] <Test Example 2> X-ray diffraction analysis The crystal structure of the porous silicon carbon composites produced in the examples was analyzed using an X-ray diffractometer (Malvern Panalytical, X'Pert3). Specifically, the applied voltage was 40 kV and the applied current was 40 mA. The 2θ range was 10° to 80°, and measurements were performed by scanning at intervals of 0.05°. FIG. 4 shows the results of X-ray diffraction analysis of the porous silicon carbon composite (C1) of Example 1. 4, as can be seen from the X-ray diffraction pattern, the porous silicon carbon composite of Example 1 (composite C1) had peaks corresponding to Si crystals at diffraction angles (2θ) of 28.1°, 47.0°, 55.9°, 68.6°, and 76.1°. Furthermore, the diffraction angle (2θ) of carbon could not be confirmed because it overlapped with the peak of Si(111).

[0180] <Test Example 3> Analysis of the Content and Specific Gravity of Constituent Elements of Porous Silicon Structure or Porous Silicon Carbon Composite The porous silicon structures or porous silicon carbon composites produced in the examples and comparative examples were analyzed for the content of the constituent elements oxygen (O) and carbon (C). The oxygen (O) and carbon (C) contents were analyzed using an elemental analyzer. The silicon (Si) content was calculated based on the oxygen (O) content.

[0181] Test Example 4: Measurement of the average particle size of a porous silicon structure or porous silicon carbon composite 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.

[0182] <Test Example 5> Raman Spectroscopic Analysis The porous silicon carbon composite prepared in Example 1 was subjected to Raman spectroscopy. The Raman analysis was performed at 2.41 eV (514 nm) using a micro-Raman analyzer (Renishaw, RM1000-In Via). The results are shown in Figure 5. Referring to FIG. 5, 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 -1The 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, 0.20, and 0.75, respectively, and (I 2D +I G ) / I D was 0.95. The results of Raman spectroscopy show that the carbon layer is I D , I 2D , I G has the above values, the conductivity is good, and it can be seen that the characteristics of the secondary battery can be improved. In particular, (I 2D +I G ) / I D Since the value of 0.95 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.

[0183] <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.

[0184] [Formula 2] Initial efficiency (%) = discharge capacity / charge capacity x 100

[0185] The coin cells prepared in the examples and comparative examples were also 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.

[0186] [Formula 3] Capacity retention rate after 50 cycles (%) = 51st discharge capacity / 2nd discharge capacity × 100

[0187] The element contents and physical properties of the silicon-based raw material powders, porous silicon structures, and porous silicon carbon composites of the Examples and Comparative Examples are summarized in Tables 1 to 3. The characteristics of the secondary batteries using these are summarized in Table 4.

[0188] [Table 1]

[0189] [Table 2]

[0190] [Table 3]

[0191] [Table 4]

[0192] As can be seen from Table 4, the molar ratio (O / Si) of oxygen (O) atoms to silicon (Si) atoms in the porous silicon structures and porous silicon carbon composites of Examples 1 to 5 of the present invention satisfies a specific range. Therefore, the secondary batteries fabricated using these exhibited 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.

[0193] Specifically, the secondary batteries of Examples 1 to 5, which used porous silicon structures with a molar ratio (O / Si) of oxygen (O) atoms to silicon (Si) atoms of 0.01 to 0.2, had discharge capacities of 1,815 mAh / g to 2,032 mAh / g, initial efficiencies of 82.0% to 87.9%, and capacity retention rates of 82.7% to 85.4%, demonstrating that the performance of the secondary batteries was excellent overall.

[0194] On the other hand, when the secondary battery of Example 1 was compared with the secondary battery of Comparative Example 1 in which etching was not performed in the steps of Example 1, and the secondary battery of Comparative Example 2 in which etching was performed using NaOH instead of HF in the steps of Example 1, the discharge capacity of the secondary battery of Example 1 was 2,032 mAh / g and the initial efficiency was 87.9%, while the discharge capacities of the secondary batteries of Comparative Examples 1 and 2 were 1,720 mAh / g and 1,640 mAh / g and initial efficiencies of 75% and 74.1%, 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 1.

[0195] 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 0.38, it was found that the secondary battery of Comparative Example 3 had a discharge capacity of 1795 mAh / g and an initial efficiency of 80.4%, which was a significant decrease in performance compared to the secondary battery of Example 1 in which the discharge capacity was 2,032 mAh / g and the initial efficiency was 87.9%.

[0196] 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 porous silicon structure. Preferred embodiments of the present invention include the following. [1] A porous silicon structure containing silicon particles, wherein the molar ratio (O / Si) of oxygen (O) atoms to silicon (Si) atoms in the porous silicon structure is 0.01 to 0.35. [2] The porous silicon structure according to [1], which contains silicon aggregates in which silicon particles are interconnected. The porous silicon structure according to [1]. [3] The porous silicon structure according to [1], wherein the crystallite size of the silicon particles in X-ray diffraction analysis is 1 nm to 20 nm. The porous silicon structure according to [1]. [4] The porous silicon structure according to [1], further containing silicon oxide (SiOx, 0.1 < x ≤ 2) formed on the surface of the silicon particles. [5] The porous silicon structure according to [4], wherein the oxygen (O) content in the porous silicon structure is 0.1 wt% to 15 wt% based on the total weight of the porous silicon structure. [6] The porous silicon structure has an average particle size (D 50 ) of 1 μm to 15 μm and a specific gravity of 1.5 g / cm 3 to 2.3 g / cm 3 . The porous silicon structure according to [1]. [7] The porous silicon structure contains pores inside. When the surface of the porous silicon structure is measured by the gas adsorption method (BET plot method), it contains micropores with a pore volume of 0.1 cm 3 / g to 0.5 cm 3 / g and with a pore size of 2 nm or less, and mesopores with a pore volume of 0.2 cm 3 / g to 0.7 cm 3 / g and with a pore size of more than 2 nm to 50 nm. The specific surface area (Brunauer-Emmett-Teller method; BET) of the porous silicon structure is 100 m 2 / g to 1,600 m 2 / g. The porous silicon structure according to [1]. [8] A porous silicon-carbon composite containing the porous silicon structure according to [1] and carbon. [9] The porous silicon-carbon composite according to [8], 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.

[10] The porous silicon-carbon composite according to [8], which has pores inside, and the porosity of the porous silicon-carbon composite is 0.5 vol% to 40 vol% based on the volume of the porous silicon-carbon composite.

[11] The porous silicon-carbon composite according to [8], wherein the content of silicon (Si) is 30 wt% to 90 wt% based on the total weight of the porous silicon-carbon composite.

[12] [8] A porous silicon carbon composite according to [8], wherein the carbon is present on the surface of the silicon particles, the carbon functions as a matrix and the silicon particles and pores are dispersed in the carbon matrix, or the carbon is present in both.

[13]

[12] The porous silicon carbon composite according to

[12] , wherein the carbon further forms a carbon layer on the surface of the porous silicon structure.

[14]

[13] The porous silicon carbon composite according to

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

[15] The porous silicon carbon composite according to

[13] , wherein the carbon (C) content is 10% by weight to 90% by weight based on the total weight of the porous silicon carbon composite.

[16] The porous silicon carbon composite according to

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

[17] The porous silicon carbon composite has an average particle size (D 50 ) is 2 μm to 15 μm.

[18] The specific gravity of the porous silicon carbon composite is 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 etching a silicon-based raw material powder using an etching solution containing a fluorine (F) atom-containing compound; and The second step is to filter and dry the etching product to prepare a porous silicon structure. A method for preparing the porous silicon structure according to [1], comprising:

[20] a first step of etching a silicon-based raw material powder using an etching solution containing a fluorine (F) atom-containing compound; A second step of filtering and drying the etching product to prepare a porous silicon structure; and The third step of preparing porous silicon carbon composites is to form a carbon layer on the surface of the porous silicon structure using chemical pyrolysis deposition. [8] A method for preparing a porous silicon carbon composite according to [8], comprising:

[21]

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

[20] , wherein in the first 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 third 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.

[23] The carbon layer is formed in the third step according to the following formulas 1 to 3: [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.) and reacting the mixture in a gaseous state at 400°C to 1200°C.

[24] [8] A negative electrode active material for a lithium secondary battery, comprising the porous silicon-carbon composite according to [8].

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

[24] , wherein the negative electrode active material further contains 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 structure containing silicon particles, wherein the molar ratio (O / Si) of oxygen (O) atoms to silicon (Si) atoms in the porous silicon structure is 0.01 to 0.35; A porous silicon structure containing pores therein, and when the surface of the porous silicon structure is measured by a gas adsorption method (BET plot method), the porous silicon structure contains micropores having a pore volume of 0.1 cm 3 / g to 0.5 cm 3 / g and a pore diameter of 2 nm or less, and mesopores having a pore volume of 0.2 cm 3 / g to 0.7 cm 3 / g and a pore diameter of more than 2 nm to 50 nm, and the porous silicon structure has a specific surface area (Brunauer-Emmett-Teller method; BET) of 100 m 2 / g to 1,600 m 2 / g.

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

3. 2. The porous silicon structure according to claim 1, wherein the silicon particles have a crystallite size of 1 nm to 20 nm as determined by X-ray diffraction analysis.

4. 2. The porous silicon structure of claim 1, further comprising silicon oxide (SiOx, 0.1<x≦2) formed on the surface of said silicon particles.

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

6. A porous silicon carbon composite comprising the porous silicon structure 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. The carbon is present on the surface of the silicon particles, or Carbon acts as a matrix, with silicon particles and pores dispersed in the carbon matrix, or 7. The porous silicon carbon composite of claim 6, wherein the carbon is present on the surface of the silicon particles, the carbon functions as a matrix, and the silicon particles 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 specific gravity is 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 etching a silicon-based raw material powder using an etching solution containing a fluorine (F) atom-containing compound; and The second step is to filter and dry the product obtained by etching to prepare a porous silicon structure.

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

12. a first step of etching a silicon-based raw material powder using an etching solution containing a fluorine (F) atom-containing compound; A second step of filtering and drying the product obtained by etching to prepare a porous silicon structure; and A third step of forming a carbon layer on the surface of the porous silicon structure using chemical pyrolysis deposition to prepare a porous silicon carbon composite.

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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