Anode active material, method for manufacturing same, and lithium secondary battery comprising same

Abstract

The present invention relates to an anode active material, a method for manufacturing same, and a secondary battery including same. The anode active material includes: a silicon-carbon composite including crystalline carbon, silicon-based particles bound to the surface of the crystalline carbon, and amorphous carbon on the surfaces of the crystalline carbon and silicon-based particles; and a carbon coating layer covering the surface of the silicon-carbon composite, wherein the anode active material may include 0.05-0.25 wt% of sulfur based on the total weight thereof.

Description

Negative active material, method for producing same, and lithium secondary battery including same

[0001] The present invention relates to a negative electrode active material, a method for producing the same, and a lithium secondary battery including the same.

[0002] As the market for electronic devices such as mobile phones, laptops, and PCs grows, the market for lithium secondary batteries, the power sources for these devices, is also growing rapidly. Furthermore, growing concern over environmental issues has led to a surge in demand for eco-friendly vehicles like electric vehicles, leading to research into lithium secondary batteries that can meet a variety of applications.

[0003] Among the components that make up lithium secondary batteries, the anode active material stores lithium ions during charging and plays a crucial role in determining charging speed and battery capacity. Among these anode active materials, silicon-based active materials are attracting attention due to their superior electrochemical properties, such as capacity and energy output, compared to carbon-based materials.

[0004] However, silicon-based active materials typically suffer from significant volume expansion and contraction during charge and discharge, which reduces battery life. Furthermore, silicon-based active materials can break silicon particles or lose electrical contact with continuous charge and discharge.

[0005] To solve these problems, various attempts are being made to nanosize silicon particles and utilize silicon-carbon composites, which are composites of nano-sized silicon particles and carbon materials, as negative electrode active materials.

[0006] However, conventional silicon-carbon composites have a disadvantage in that the capturing effect by carbon-based materials decreases as the number of silicon-based particles with small crystal grain sizes increases, and the life characteristics decrease significantly as the specific surface area increases.

[0007] In order to overcome the above drawbacks, when manufacturing a silicon-carbon composite using amorphous carbon such as pitch, there was still a limitation that the initial charge / discharge efficiency and the life characteristics of the battery were lowered due to sulfur derived from the amorphous carbon.

[0008] Therefore, there is a need for research on silicon-carbon composites that have an excellent carbon-capturing effect on silicon while minimizing the content of sulfur generated from amorphous carbon.

[0009] One aspect of the present invention is to provide a negative electrode active material capable of improving initial efficiency and reducing volume expansion to improve life characteristics, a method for producing the same, and a lithium secondary battery including the same.

[0010] The objectives of the present invention are not limited to the above-described matters. Additional objectives of the present invention are described throughout the specification, and those skilled in the art will have no difficulty understanding the additional objectives of the present invention from the contents described in the specification.

[0011] According to one embodiment of the present invention, a negative active material comprises a silicon-carbon composite including crystalline carbon, silicon-based particles positioned on the surface and inside of the crystalline carbon, and amorphous carbon positioned on the surface of the crystalline carbon and the silicon-based particles; and a carbon coating layer covering the surface of the silicon-carbon composite.

[0012] In a negative active material according to one embodiment, the amorphous carbon may be derived from sulfur-containing pitch.

[0013] In a negative active material according to one embodiment, the sulfur-containing pitch may include beta resin in an amount of 20 wt% or more based on the total weight.

[0014] In a negative active material according to one embodiment, the sulfur-containing pitch may include at least one selected from the group consisting of coal-based pitch, petroleum-based pitch, natural pitch, and mesophase pitch.

[0015] In a negative active material according to one embodiment, the negative active material may contain 10 to 25 wt% of the crystalline carbon based on the total weight.

[0016] In a negative active material according to one embodiment, the crystalline carbon may include at least one selected from the group consisting of artificial graphite, flake graphite, natural graphite, and expanded graphite.

[0017] In a negative active material according to one embodiment, the negative active material may contain 20 to 45 wt% of the amorphous carbon based on the total weight.

[0018] In a negative active material according to one embodiment, the negative active material may include less than 3 wt% of the carbon coating layer based on the total weight.

[0019] In a negative active material according to one embodiment, the carbon of the carbon coating layer may be derived from at least one selected from the group consisting of coal pitch, petroleum pitch, coal tar, polylactic acid (PAA), and polyvinyl alcohol (PVA).

[0020] A method for manufacturing a negative electrode active material according to one embodiment of the present invention comprises the steps of: preparing silicon particles by pulverizing a silicon raw material; mixing the silicon particles and crystalline carbon; mixing the mixed silicon particles and crystalline carbon with sulfur-containing pitch to obtain a first mixture; first carbonizing the first mixture at 750 to 1100°C to form a silicon-carbon composite; mixing the silicon-carbon composite and a carbon precursor to obtain a second mixture; and second carbonizing the second mixture.

[0021] In a method for manufacturing a negative active material according to one embodiment of the present invention, after the step of obtaining the first mixture, a step of pressurizing the first mixture at a pressure of less than 1 ton / cm2 may be further included.

[0022] In a method for manufacturing a negative active material according to one embodiment of the present invention, the secondary carbonization step can be performed at 700 to 1100°C.

[0023] A lithium secondary battery according to one embodiment of the present invention includes a negative electrode; a positive electrode; and an electrolyte, wherein the negative electrode may include the above-described negative electrode active material.

[0024] In one embodiment, the negative active material may contain 0.05 to 0.25 wt% of sulfur based on the total weight.

[0025] According to the present invention, by controlling the content of sulfur derived from amorphous carbon, reversibility and capacity during the charge / discharge process can be improved, and electrochemical characteristics such as initial efficiency can be improved.

[0026] In addition, according to the present invention, the structural stability of the silicon-carbon composite can be improved and the life characteristics can also be increased.

[0027] The various advantageous and beneficial advantages and effects of the present invention are not limited to the above-described contents, and will be more easily understood in the course of explaining specific embodiments of the present invention.

[0028] Hereinafter, preferred embodiments of the present invention will be described. However, the embodiments of the present invention may be modified in various other forms, and the terminology used herein is for the purpose of describing the present invention and is not intended to limit the present invention. Furthermore, the singular forms used herein also include plural forms, unless the relevant definition clearly indicates a contrary meaning.

[0029] In this specification, the term "including" is used to indicate that other components may be included rather than excluding other components unless specifically stated to the contrary.

[0030] Terms including ordinal numbers, such as "first," "second," etc., may be used to describe various components, but the components are not limited by the terms. The terms are used solely to distinguish one component from another. For example, without departing from the scope of the present invention, a first component may be referred to as a second component, and similarly, a second component may also be referred to as a first component. The term "and / or" includes a combination of multiple related items or any one of multiple related items.

[0031] Additionally, unless otherwise specifically provided in the specification of the present invention, the % unit means weight%.

[0032] Also, in this specification, "D n " means particle size distribution, and can mean particle size at n% point of cumulative particle number distribution according to particle size. For example, D 50is the particle size (average particle size) at the 50% point of the cumulative particle number distribution according to particle size, and D 90 is the particle size at the 90% point of the cumulative distribution of particle numbers according to particle size, and D 10 is the particle size at the 10% point of the cumulative particle number distribution according to particle size. Meanwhile, the particle size distribution can be measured using the laser diffraction method. Specifically, after the powder to be measured is dispersed in a dispersion medium, it is introduced into a laser diffraction particle size measuring device, and when the particles pass through the laser beam, the difference in diffraction pattern according to particle size can be measured to calculate the particle size distribution.

[0033] Unless otherwise defined, all terms, including technical and scientific terms, used herein have the same meaning as commonly understood by a person of ordinary skill in the art to which this invention pertains. Terms defined in the dictionary are to be interpreted to have meanings consistent with the relevant technical literature and the present disclosure.

[0034] Hereinafter, a negative electrode active material according to one embodiment of the present invention will be described. The negative electrode active material according to one embodiment of the present invention may include a silicon-carbon composite; and a carbon coating layer disposed on the surface of the silicon-carbon composite.

[0035] In order to increase the energy density of the negative electrode active material, it is advantageous to contain a large amount of silicon; however, the life characteristics deteriorate as the amount of silicon contained increases due to changes in the volume of silicon.

[0036] Since the above negative active material includes a silicon-carbon composite and a carbon composite covering the surface of the silicon-carbon composite, it can have excellent life characteristics despite having a high energy density.

[0037] Specifically, the silicon-carbon composite includes silicon particles, crystalline carbon, and amorphous carbon, and the silicon-carbon composite may have silicon particles positioned inside and on the surface of the crystalline carbon, and amorphous carbon positioned at least on the surface thereof. Specifically, fine silicon particles may be embedded or bonded to the surface of relatively coarse crystalline carbon, and amorphous carbon may cover the surface thereof. Such a silicon-carbon composite may have high electrical conductivity as the silicon particles are dispersed and positioned along the surface of the crystalline carbon, and the interfacial contact between the crystalline carbon and the silicon particles is maximized, and a continuous and uniform electron transfer network is formed in the surface direction of the crystalline carbon, thereby supplementing the low electrical conductivity of the silicon particles. Accordingly, high electrical conductivity can be maintained despite containing a high content of silicon. In addition, the silicon-carbon composite can maintain structural stability even when the volume of the silicon particles changes because the amorphous carbon is located on the surface of the crystalline carbon and silicon particles, and can have excellent capacity retention despite containing a high silicon content.

[0038] Silicon particles are substances whose main component is silicon atoms, and may include, for example, particles composed of pure silicon atoms (Si). However, the present invention is not limited thereto, and silicon particles may be SiOx(0 <x≤2)을 포함할 수도 있다.

[0039] The silicon particles may have a crystalline structure, an amorphous structure, or a composite structure in which crystalline and amorphous phases coexist. More specifically, the silicon particles may have a crystalline structure. Here, the crystalline structure refers to a solid material in which particles are regularly arranged in a lattice or are repeatedly arranged in a space forming the nanoparticle, and the crystalline structure may be used as a synonym, such as a crystal particle, a crystal grain, or a crystal particle.

[0040] In one embodiment, the silicon-based particles may have a half-width of a peak corresponding to the (111) plane of the silicon-based particles of 0.45 to 0.65°, more specifically, 0.50 to 0.65°, and even more specifically, 0.57 to 0.65°. Here, the half-width of the peak corresponding to the (111) plane of the silicon-based particles can be measured using X-ray diffraction analysis using CuKα rays. For example, the half-width of the silicon-based particles can be controlled by adjusting the particle size of the silicon-based particles or changing the silicon-based particle manufacturing process. When the half-width of the silicon-based particles satisfies the above range, the cracking phenomenon of the silicon-based particles can be minimized, and since reversible charge / discharge is possible, the electrochemical characteristics (e.g., cycle life characteristics, etc.) of the negative electrode active material can be improved.

[0041] In one embodiment, the silicon-based particles may have a grain size of 10 to 20 nm, 10 to 19 nm, or 10 to 17 nm. Here, the grain size may refer to an average silicon grain size. The crystallinity and grain size of silicon can be confirmed through X-ray diffraction analysis, and the X-ray diffraction analysis can be performed using an X-ray diffraction (XRD) analysis device. Silicon-based particles satisfying the grain size within the above range may have better output characteristics and excellent electrochemical performance.

[0042] In one embodiment, the D of the silicon-based particles 90 The silicon-based particles may have a particle size of 200 nm or less, 180 nm or less, 160 nm or less, or 150 nm or less, and may be, but is not limited to, 10 nm or more. Specifically, it may be 10 to 200 nm, 30 to 180 nm, or 50 to 150. The D of the silicon-based particles 90By satisfying this above range, the viscosity of the slurry can be maintained at a constant level. In addition, the D of the silicon particles 90 As the particle size decreases, the viscosity of the slurry containing the silicon particles may increase. An increase in the viscosity of the slurry may indicate an increase in the number of nano-sized silicon particles within the slurry.

[0043] In one embodiment, the silicon-based particles may be present in an amount of greater than 30 wt% and less than or equal to 70 wt%, specifically 35 to 65 wt%, and more specifically 35 to 60 wt%, based on the total weight of the silicon-carbon composite. Within this range, the crystalline carbon can act as a buffer more stably against changes in the volume of the silicon-based particles.

[0044] The crystalline carbon may include at least one selected from the group consisting of artificial graphite, flake graphite, natural graphite, expanded graphite, carbon black, fullerene soot, fluoroene, carbon nanotubes, and graphene. Specifically, the crystalline carbon may be graphite, a carbon body including a graphene layer structure. The graphite may include at least one selected from the group consisting of artificial graphite, flake graphite, natural graphite, and expanded graphite. Such crystalline carbon may capture silicon-based particles in the spaces between and on the surface of the graphene layers.

[0045] In one embodiment, the crystalline carbon may be graphite, and the average diameter (D) of the silicon-based particles Si ) for the average diameter (D) of the graphite G ) of the ratio (D) Si / D G ) may be 0.001 to 0.07, 0.005 to 0.05, 0.01 to 0.03, or 0.02 to 0.02. In the above range, silicon-based nanoparticles may be uniformly distributed and positioned in the plane direction of the graphene layer of graphite.

[0046] In one embodiment, the crystalline carbon may be from 30 to 60 wt%, specifically from 35 to 60 wt%, and more specifically from 35 to 55 wt%, based on the total weight of the silicon-carbon composite.

[0047] In one embodiment, the crystalline carbon may be included in an amount of 1 to 40 wt%, 10 to 40 wt%, or 10 to 25 wt% based on the total weight of the negative electrode active material. Within this range, the conductivity of the negative electrode active material may be sufficiently secured, while the structural stability may be excellent.

[0048] In one embodiment, the average particle diameter (D) of the crystalline carbon 50 ) may be 15 ㎛ or less, 13 ㎛ or less, 10 ㎛ or less, 8 ㎛ or less, 6 ㎛ or less, or 4 ㎛ or less, and may be, but is not limited to, 1 ㎛ or more. Specifically, the average particle diameter of the crystalline carbon may be 1 to 15 ㎛, 1 to 10 ㎛, 1 to 8 ㎛ or 1 to 4 ㎛. The average particle diameter (D of the crystalline carbon 50 ) can compensate for the low conductivity of silicon particles and improve conductivity and charge / discharge reversibility as it satisfies the above range.

[0049] The amorphous carbon described above can enhance the capturing effect of silicon particles in silicon-carbon composites and further improve the volume expansion of the silicon-carbon composite due to continuous charge / discharge. Because the amorphous carbon is arranged to surround the silicon particles and crystalline carbon, the structural stability of the silicon-carbon composite can be enhanced.

[0050] In one embodiment, the amorphous carbon may be derived from sulfur-containing pitch. The amorphous carbon derived from sulfur-containing pitch reduces the internal porosity of the silicon-carbon composite, and thus, the negative active material containing the amorphous carbon may have reduced side reactions with the electrolyte, thereby increasing the initial efficiency of the battery.

[0051] Specifically, the sulfur-containing pitch may have a carbonization yield of 50% or more, specifically 50 to 80%, and more specifically 60 to 80%. Here, the carbonization yield may mean a value converted into a percentage by dividing the weight of carbon obtained after heat treatment of the carbon material by the carbon content before heat treatment. The amorphous carbon derived from the sulfur-containing pitch having a carbonization yield satisfying the above range may be more uniformly and densely coated on the surface of the crystalline carbon and silicon-based particles, thereby increasing the density and conductivity of the silicon-carbon composite, thereby minimizing side reactions with the electrolyte and lithium trapping during charge and discharge.

[0052] In one embodiment, the sulfur-containing pitch may include beta-resin in an amount of 20 wt% or more, specifically 20 to 40 wt%, and more specifically 25 to 40 wt%, based on the total weight. The content of the beta-resin is an indicator proportional to the cohesiveness, and may refer to the content of the portion remaining after extracting the toluene-insoluble content and excluding the quinoline-insoluble content. A silicon-carbon composite including amorphous carbon derived from a sulfur-containing pitch having a beta-resin content satisfying the above range may maintain a more stable structure. Accordingly, when using the negative active material according to one embodiment of the present invention, a lithium secondary battery having excellent cycle life characteristics and low expansion characteristics can be realized.

[0053] In one embodiment, the sulfur-containing pitch may include at least one selected from the group consisting of coal-based pitch, petroleum-based pitch, natural pitch, and mesophase pitch.

[0054] In one embodiment, the amorphous carbon may be derived from a carbon precursor comprising a carbide of sulfur-containing pitch and other organic materials. For example, the carbide of other organic materials may comprise a carbide of at least one organic material selected from the group consisting of sucrose, glucose, galactose, fructose, lactose, mannose, ribose, aldohexose, and kedohexose.

[0055] As a non-limiting example, the sulfur-containing pitch may be coal-based pitch. In this case, the coal-based pitch may function as a binder that stably supports the silicon-carbon composite, thereby enhancing the structural stability of the silicon-carbon composite.

[0056] In one embodiment, the amorphous carbon may be present in an amount of 20 to 45 wt%, more specifically 20 to 40 wt%, and even more specifically 25 to 40 wt%, based on the total weight of the negative active material. Within this range, the structural stability of the silicon-carbon composite can be maintained, and volume expansion due to continuous charge and discharge can be improved.

[0057] The carbon coating layer covers the surface of the aforementioned silicon-carbon composite, minimizing the exposure of silicon particles to the surface of the negative active material. Consequently, the volume expansion of silicon particles due to continuous charge / discharge can be improved.

[0058] The carbon coating layer may be disposed on the surface of the silicon-carbon composite. For example, the carbon coating layer may partially cover the surface of the silicon-carbon composite or may cover the entire surface of the silicon-carbon composite.

[0059] In one embodiment, the carbon coating layer is an amorphous carbon coating layer, and the carbon of the amorphous carbon coating layer may be derived from at least one selected from the group consisting of coal pitch, petroleum pitch, coal tar, polylactic acid (PAA), and polyvinyl alcohol (PVA).

[0060] In one embodiment, the carbon coating layer may be derived from a carbon precursor having a higher beta resin (β-resin) content than the sulfur-containing pitch. Specifically, the carbon precursor of the carbon coating layer may be coal-based pitch, petroleum-based pitch, or coal tar, and is not particularly limited as long as it has a higher beta resin (β-resin) content than the sulfur-containing pitch. Specifically, based on the beta resin content of the carbon precursor of the amorphous carbon, the beta resin content of the carbon precursor of the carbon coating layer may be 1.1 to 3 times, 1.2 to 2.5 times, or 1.3 to 2 times. A carbon precursor containing beta resin in the above range can form an amorphous carbon coating layer that is denser than the amorphous carbon. As a non-limiting example, the carbon precursor of the carbon coating layer may be petroleum-based pitch.

[0061] In one embodiment, the carbon coating layer may be present in an amount of less than 3 wt%, more specifically 0.3 wt% or more and less than 3 wt%, and even more specifically 0.5 wt% or more and less than 3 wt%, based on the total weight of the negative electrode active material. The capacity of the negative electrode active material can be maintained within the above range.

[0062] In one embodiment, the average thickness of the carbon coating layer may be 10 nm or less, more specifically, 1 to 10 nm. As the average thickness of the carbon coating layer is adjusted within the above range, the initial efficiency and capacity of the negative electrode active material may be improved.

[0063] Since the amorphous carbon of the silicon-carbon composite is derived from sulfur-containing pitch, it may contain a large amount of sulfur. However, the negative active material according to one embodiment has the characteristic of improving the above problem by controlling the sulfur content.

[0064] In one embodiment, the negative active material may contain 0.05 to 0.25 wt% of sulfur based on the total weight of the negative active material. Specifically, the sulfur content can be adjusted by controlling the carbonization temperature when forming amorphous carbon from sulfur-containing pitch.

[0065] If the sulfur content is adjusted to less than 0.05 wt%, it may result in an increase in manufacturing cost considering limitations in manufacturing equipment or manufacturing process. In addition, if the sulfur content exceeds 0.25 wt%, it may be difficult to sufficiently secure the above-described effect (i.e., improvement in electrochemical characteristics). That is, the sulfur content may be 0.05 to 0.25 wt%, more specifically, the sulfur content may be 0.05 to 0.23 wt%, and even more specifically, the sulfur content may be 0.05 to 0.20 wt%.

[0066] Hereinafter, a method for manufacturing a negative electrode active material according to an embodiment of the present invention will be described. However, this does not necessarily mean that the negative electrode active material according to an embodiment of the present invention must be manufactured using the manufacturing method described below.

[0067] [Preparation of silicon particles]

[0068] First, silicon raw material can be pulverized to prepare silicon particles. The description of the silicon particles prepared in this step is identical to the description of the negative active material described above.

[0069] For example, a silicon raw material is a material whose main component is silicon atoms, and may include, for example, particles made of pure silicon atoms (Si). However, the present invention is not limited thereto, and silicon particles may be SiOx(0 <x≤2)을 포함할 수도 있다. 또한, 예를 들어, 실리콘 원료 물질로서 Poly-Si를 이용할 수 있다.

[0070] For example, the step of preparing silicon particles can involve mechanically milling silicon raw materials into nanoparticles using beads. For example, zirconia beads can be used.

[0071] For example, the properties of silicon particles (e.g., grain size, half-width through X-ray diffraction analysis, etc.) can be controlled by controlling the BPR (Ball Per Ratio) of the silicon raw material and the beads. For example, the BPR of the silicon raw material and the beads can be 4 to 6:1.

[0072] For example, the particle size of the beads is D of the silicon raw material. 99 It may be less than twice the contrast. D of silicon raw material 99 If the size of the silicon raw material is excessively large or the particle size of the silicon raw material is excessively small, the nano-fication efficiency of the silicon raw material may be significantly reduced. Accordingly, the time required for nano-fication of the silicon raw material may increase, and thus side effects such as oxidation of the silicon particles may occur.

[0073] For example, the step of preparing silicon particles can be performed at 1500 to 2500 rpm.

[0074] For example, the step of preparing silicon-based particles can be performed using a wet grinding method using an organic solvent. In this case, oxidation of the silicon-based particles can be prevented. For example, the organic solvent can be ethanol, isopropyl alcohol, methanol, butanol, or propylene glycol. More specifically, ethanol can be used, and even more specifically, ethanol with a purity of 99.9% can be used.

[0075] For example, when using a wet grinding method, the solids content may be 8 to 15%. As the solids content is adjusted within the above range, the probability of collision between the silicon raw material and the beads can be increased, thereby effectively performing nano-grinding of the silicon raw material.

[0076] [Mixing stage of silicon particles and crystalline carbon]

[0077] Afterwards, silicon particles and crystalline carbon can be mixed. The description of crystalline carbon in this step is identical to the description of the negative active material described above. For example, silicon particles and crystalline carbon can be dispersed at high speed in this step to uniformly mix the two raw materials.

[0078] For example, the crystalline carbon may include at least one selected from the group consisting of artificial graphite, flaky graphite, natural graphite, expanded graphite, carbon black, fullerene soot, carbon nanotubes, and graphene. In one embodiment of the present invention, the crystalline carbon may include artificial graphite and / or flaky graphite. In this case, the conductivity and charge-discharge reversibility of the negative electrode active material may be improved.

[0079] As a non-limiting example, the average particle size (D) of crystalline carbon 50 ) can be 3 to 10 ㎛. The average particle diameter (D) of crystalline carbon 50) can complement the conductive properties of silicon particles and improve conductivity and charge / discharge reversibility as the above range is satisfied.

[0080] For example, when using a wet grinding method in the step of preparing silicon-based particles, crystalline carbon can be added to a dispersion containing silicon-based particles and an organic solvent, and then the silicon-based particles and crystalline carbon can be mixed through high-speed dispersion. In this case, the organic solvent used in the wet grinding can be dried after mixing. For example, a process such as spray drying can be used as a method for drying the organic solvent, and through spray drying, a silicon-carbon precursor in which silicon-based particles and crystalline carbon are condensed with each other to form a physical bond can be obtained.

[0081] [Obtaining the first mixture]

[0082] A first mixture can be obtained by mixing mixed silicon particles, crystalline carbon, and sulfur-containing pitch. The description of the sulfur-containing pitch in this step is the same as the description of the sulfur-containing pitch of the negative active material described above.

[0083] For example, the carbonization yield of the sulfur-containing pitch may be 50% or more, more specifically 50 to 80%, and even more specifically 60 to 80%. As the carbonization yield of the sulfur-containing pitch satisfies the above range, the internal pores of the negative active material ultimately manufactured are reduced, and thus side reactions with the electrolyte may also be reduced, thereby increasing the initial efficiency of the battery.

[0084] For example, based on the total weight of the sulfur-containing pitch, the content of beta resin may be 20 wt% or more, more specifically 20 to 40 wt%, and even more specifically 25 to 40 wt%. As the content of beta resin satisfies the above range, the structure of the silicon-carbon composite can be more stably maintained by the amorphous carbon formed in the first carbonization process described below.

[0085] For example, the sulfur-containing pitch may include at least one selected from the group consisting of coal-based pitch, petroleum-based pitch, natural pitch, and mesophase pitch. Without limitation, the amorphous carbon may be derived from a carbon precursor comprising a carbide of sulfur-containing pitch and other organic materials. For example, the carbide of other organic materials may include a carbide of at least one organic material selected from the group consisting of sucrose, glucose, galactose, fructose, lactose, mannose, ribose, aldohexose, and kedohexose. In one embodiment of the present invention, the sulfur-containing pitch may include coal-based pitch.

[0086] For example, the sulfur-containing pitch may be mixed in an amount of 20 to 45 wt% based on the total weight of the negative electrode active material to be ultimately manufactured.

[0087] In one embodiment, in the step of obtaining the first mixture, silicon-based particles, crystalline carbon, and sulfur-containing pitch can be uniformly mixed using equipment such as a mechanofusion unit. This minimizes the independent flow of each raw material and enhances the bonding strength between the two raw materials. However, this is not limited to this, and various types of equipment commonly used in the industry for uniform mixing of raw materials, such as a planetary mixer, can be utilized.

[0088] For example, after the step of obtaining the first mixture, a step of pressurizing the first mixture at a pressure of less than 1 ton / cm2 may be further included. By pressurizing the first mixture, pores existing within the silicon-carbon composite can be minimized. Specifically, during the carbonization process, a structure in which sulfur-containing pitch easily fills the pores can be created, thereby improving the electrochemical properties of the negative electrode active material. For example, the pressurizing step can be performed by placing the first mixture in a mold and using a press machine.

[0089] [Formation steps of silicon-carbon composites]

[0090] The first mixture can be carbonized to form a silicon-carbon composite. Specifically, when the first mixture is heat-treated, the sulfur-containing pitch can surround the surface of the silicon-carbon composite and be introduced into the pores formed by the silicon-carbon composite. Accordingly, a capturing effect can be provided for the silicon particles, and since the sulfur-containing pitch is arranged in a form that surrounds the silicon particles, amorphous carbon can be bound between a plurality of silicon particles, thereby improving the structural stability of the silicon-carbon composite.

[0091] By controlling the primary carbonization temperature in the step of forming a silicon-carbon composite, the sulfur content derived from the sulfur-containing pitch can be easily controlled. The temperature of the primary carbonization may be 750 to 1100°C, more specifically 800 to 1000°C. Within this range, excellent electrochemical properties (e.g., initial efficiency, life characteristics, etc.) of the negative active material can be maintained, and the initial efficiency and life characteristics may not deteriorate.

[0092] In the step of forming the above silicon-carbon composite, the primary carbonization may be performed in an inert atmosphere. For example, the inert atmosphere may be at least one selected from the group consisting of a nitrogen atmosphere, an argon atmosphere, and a helium atmosphere.

[0093] For example, after the step of forming the silicon-carbon composite, a step of pulverizing the silicon-carbon composite may be further included. In one embodiment of the present invention, the pulverization of the silicon-carbon composite may be performed using physical impact. For example, the pulverization of the silicon-carbon composite may be performed using equipment such as a jet mill or a pin mill. However, the present invention is not limited thereto, and various types of equipment commonly used in the industry, such as an air classifier mill, a roller mill, a Raymond mill, a vertical roller mill, a jaw crusher, or a ball mill, may be used.

[0094] [Second mixture obtaining step]

[0095] A second mixture can be obtained by mixing a silicon-carbon composite and a carbon precursor. The carbon precursor can be at least one selected from the group consisting of petroleum pitch, coal pitch, coal tar, polylactic acid (PAA), and polyvinyl alcohol (PVA) having a softening point of 250°C or lower.

[0096] For example, the carbon precursor may be included in an amount of less than 3 wt% based on the total weight of the final manufactured negative electrode active material. More specifically, it may be 0.3 wt% or more and less than 3 wt%, and even more specifically, it may be 0.5 wt% or more and less than 3 wt%.

[0097] [Second carbonization stage]

[0098] The second mixture can be subjected to secondary carbonization. Specifically, when the second mixture is heat-treated, the carbon precursor is carbonized and liquefied, so that a carbon coating layer can be uniformly formed on the surface of the silicon-carbon composite.

[0099] For example, the temperature of the secondary carbonization may be 700 to 1100°C. When the temperature of the secondary carbonization exceeds 1100°C, a silicon carbide (SiC) compound is generated, and a side reaction may occur during the charge / discharge process due to the SiC compound, which may deteriorate the initial efficiency and life characteristics. That is, the temperature of the secondary carbonization may be 700 to 1100°C, more specifically, 750 to 1000°C, and even more specifically, 800 to 1000°C.

[0100] For example, the secondary carbonization step may be performed in an inert atmosphere. For example, the inert atmosphere may be at least one selected from the group consisting of a nitrogen atmosphere, an argon atmosphere, and a helium atmosphere.

[0101] Hereinafter, a lithium secondary battery according to an embodiment of the present invention will be described. A lithium secondary battery according to an embodiment of the present invention may include a negative electrode; a positive electrode; and an electrolyte.

[0102] The negative electrode may include a negative electrode active material layer including the above-described negative electrode active material; and a current collector.

[0103] Current collectors can serve to impart conductivity. There are no specific limitations on the materials that can be used as current collectors, and any material that possesses conductivity without causing chemical changes to the battery can be used. For example, current collectors can include copper, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel surface-treated with carbon, nickel, titanium, silver, etc.

[0104] The negative electrode active material layer may be disposed on at least one surface of the current collector, and specifically, may be disposed on one or both surfaces. For example, the negative electrode active material may be included in an amount of 80 to 98 wt% based on the total weight of the negative electrode active material layer.

[0105] The negative electrode active material layer may further include a binder and / or a conductive agent. The binder may serve to bond the particles constituting the negative electrode active material to each other and improve the adhesion performance of the negative electrode active material to the current collector. For example, the binder may be included in an amount of 1 to 5 wt% based on the total weight of the negative electrode active material layer.

[0106] For example, the binder may include a non-aqueous binder, an aqueous binder, or a combination thereof. For example, the non-aqueous binder may include at least one selected from the group consisting of an ethylene / propylene copolymer, polyacrylonitrile (PAN), polystyrene (PS), polyvinyl chloride (PVC), carboxylated polyvinyl chloride, poly(vinylidene fluoride; PVDF), polyurethane, polytetrafluoroethylene (PTFE), polyethylene (PE), polypropylene (PP), polyamide-imide (PAI), and polyimide (PI). However, it is not limited to this.

[0107] Water-based binders include polymers including carboxymethyl cellulose, styrene butadiene rubber (SBR), acrylated styrene butadiene rubber, nitrile butadiene rubber (NBR), acrylic rubber, butyl rubber, fluoro rubber, ethylene oxide, polyvinyl pyrrolidone (PVP), polyepichlorohydrine, polyphosphazenes, ethylene-propylene-diene copolymer (EPDM), poly(vinyl pyridine), and chlorosulfonated polyethylene. It may include at least one selected from the group consisting of polyethylene (CSM), latex, polyester resin, acrylic resin, phenol resin, epoxy resin, and polyvinyl alcohol (PVA), but is not limited thereto.

[0108] For example, when a water-based binder is used as a binder, the negative electrode active material layer may further include a thickener capable of providing viscosity. The thickener may include a cellulose-based compound. For example, the cellulose-based compound may include at least one selected from the group consisting of carboxymethyl cellulose, hydroxypropyl methylcellulose, methylcellulose, and alkali metal salts in which hydrogens of these are substituted with Na, K, or Li. For example, the thickener may be included in an amount of 0.1 to 3 wt% based on the total weight of the negative electrode active material layer.

[0109] Conductive materials can be used to impart conductivity to electrodes. There are no specific limitations on the materials that can be used as conductive materials, and any material that has conductivity without causing chemical changes in the battery can be used. For example, conductive materials can include carbon-based materials such as natural graphite, artificial graphite, carbon black, carbon nanotubes, graphene, acetylene black, Ketjen black, and carbon fibers; metal-based materials such as metal powders or metal fibers such as copper, nickel, aluminum, and silver; conductive polymers such as polyphenylene derivatives; or mixtures thereof.

[0110] The negative electrode can be obtained by mixing a binder and / or a conductive agent with a negative electrode active material in a solvent to prepare an active material composition, and then applying this active material composition to a current collector. For example, the solvent may include water. Since the method for preparing such a negative electrode can be applied using methods commonly used in the art, a detailed description thereof will be omitted herein.

[0111] The positive electrode comprises a current collector; and a positive electrode active material layer formed on at least one surface of the current collector and including a positive electrode active material. For example, the positive electrode active material may be included in an amount of 80 to 98 wt% based on the total weight of the positive electrode active material layer.

[0112] Current collectors can serve to impart conductivity. There are no specific limitations on the materials that can be used as current collectors, and any material that possesses conductivity without causing chemical changes to the battery can be used. For example, current collectors can include stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel surface-treated with carbon, nickel, titanium, silver, or other materials.

[0113] The cathode active material may include a compound capable of reversible intercalation and deintercalation of lithium (lithiated intercalation compound). The cathode active material may be a commonly used cathode active material, and may include, for example, at least one of a composite oxide of lithium and a metal selected from cobalt, manganese, nickel, and combinations thereof. Specifically, the cathode active material may include a layered compound such as lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), or a compound substituted with one or more transition metals; lithium iron oxide such as LiFe3O4; and a compound having the chemical formula Li. 1+c1 Mn 2-c1 Lithium manganese oxides such as O4(0≤c1≤0.33), LiMnO3, LiMn2O3, LiMnO2; lithium copper oxide (Li2CuO2); vanadium oxides such as LiV3O8, V2O5, Cu2V2O7; chemical formula LiNi 1-c2 M c2 Ni-site type lithium nickel oxide expressed as O2 (wherein, M is at least one selected from the group consisting of Co, Mn, Al, Cu, Fe, Mg, B, and Ga, and satisfies 0.01≤c2≤0.3); chemical formula LiMn 2-c3 Mc3 Lithium manganese composite oxide represented by O2 (wherein, M is at least one selected from the group consisting of Co, Ni, Fe, Cr, Zn, and Ta, and satisfies 0.01≤c3≤0.1) or Li2Mn3MO8 (wherein, M is at least one selected from the group consisting of Fe, Co, Ni, Cu, and Zn); LiMn2O4, etc., in which a portion of Li in the chemical formula is replaced with an alkaline earth metal ion, but is not limited thereto. The positive electrode may be Li-metal.

[0114] For example, a compound having a coating layer on the surface of the compound may be used, or a compound having the compound and a coating layer may be mixed and used. The coating layer may include at least one coating element compound selected from the group consisting of an oxide of the coating element, a hydroxide of the coating element, an oxyhydroxide of the coating element, an oxycarbonate of the coating element, and a hydroxycarbonate of the coating element. The compound forming the coating layer may be amorphous or crystalline. The coating element included in the coating layer may include at least one selected from the group consisting of Mg, Al, Co, K, Na, Ca, Si, Ti, V, Sn, Ge, Ga, B, As, and Zr. The coating layer forming process may use any coating method as long as it can coat the compound with these elements by a method (for example, spray coating, dipping, etc.) that does not adversely affect the properties of the positive electrode active material, and a method commonly used in the art may be applied, so a detailed description thereof will be omitted herein.

[0115] The positive electrode active material layer may further include a binder and / or a conductive agent. The binder may serve to bond the particles constituting the positive electrode active material to each other and improve the adhesion performance of the positive electrode active material to the current collector. For example, the binder may be included in an amount of 1 to 5 wt% based on the total weight of the positive electrode active material layer.

[0116] Binders include polyvinylidene fluoride (PVDF), vinylidene fluoride-co-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol (PVA), polyacrylonitrile (PAN), carboxymethyl cellulose, starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl chloride (PVC), carboxylated polyvinyl chloride, polyvinyl pyrrolidone (PVP), polytetrafluoroethylene (PTFE), polyethylene (PE), It may include at least one selected from the group consisting of polypropylene (PP), ethylene-propylene-diene copolymer (EPDM), sulfonated-EPDM, styrene-butadiene rubber (SBR), fluororubber, and various copolymers thereof.

[0117] A conductive material can be used to provide conductivity to an electrode. There are no specific limitations on the materials that can be used as a conductive material, and any material that has conductivity without causing a chemical change in the battery can be used. For example, conductive materials include graphite such as natural graphite or artificial graphite; carbon-based materials such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, summer black, and carbon fiber; metal powders or metal fibers such as copper, nickel, aluminum, and silver; conductive whiskeys such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; or conductive polymers such as polyphenylene derivatives. One of these may be used alone or a mixture of two or more may be used.

[0118] The positive electrode can be obtained by mixing a binder and / or a conductive agent with a positive electrode active material in a solvent to prepare an active material composition, and then applying this active material composition to a current collector. For example, the solvent may include water. Since methods commonly used in the art can be applied to prepare such a positive electrode, a detailed description thereof will be omitted herein.

[0119] The electrolyte may include, but is not limited to, an organic liquid electrolyte, an inorganic liquid electrolyte, a solid polymer electrolyte, a gel polymer electrolyte, a solid inorganic electrolyte, or a molten inorganic electrolyte that are commonly used in the industry when manufacturing lithium secondary batteries. Specifically, the electrolyte may include a non-aqueous organic solvent and / or a lithium salt.

[0120] Non-aqueous organic solvents can serve as a medium through which ions involved in the electrochemical reaction of a battery can move. For example, the non-aqueous organic solvent may include one or more selected from the group consisting of carbonate solvents, ester solvents, ether solvents, ketone solvents, alcohol solvents, and aprotic solvents.

[0121] Examples of the above carbonate solvent that can be used include dimethyl carbonate, diethyl carbonate, dicaprylyl carbonate, methylpropyl carbonate, ethylpropyl carbonate, methylethyl carbonate, ethylene carbonate, propylene carbonate, or butylene carbonate. Examples of the ester solvent that can be used include methyl acetate, ethyl acetate, n-propyl acetate, dimethyl acetate, methyl propionate, ethyl propionate, propyl propionate, decanolide, mevalonolactone, or caprolactone. Examples of the ether solvent that can be used include dibutyl ether, tetraglyme, diglyme, dimethoxyethane, 2-methyltetrahydrofuran, or tetrahydrofuran. Cyclohexanone, etc. can be used as the above ketone solvent. Ethanol or isopropyl alcohol, etc. can be used as the above alcohol solvent.As the aprotic solvent, nitriles such as R-CN (where R is a straight-chain, branched, or cyclic hydrocarbon group having 2 to 20 carbon atoms and may include a double-bonded aromatic ring or ether bond), amides such as dimethylformamide, dioxolanes such as 1,3-dioxolane, or sulfolanes may be used.

[0122] The above lithium salt can be dissolved in an organic solvent and act as a source of lithium ions in the battery, enabling the basic operation of a lithium secondary battery and promoting the movement of lithium ions between the positive and negative electrodes. The anion of the lithium salt is F - , Cl - , I - , NO3 - , N(CN)2 - , BF4 - , ClO4 - , PF6 - , (CF3)2PF4 - , (CF3)3PF3 - , (CF3)4PF2 - , (CF3)5PF - , (CF3)6P - , CF3SO3 - , CF3CF2SO3 - , (CF3SO2)2N - , (FSO2)2N - , CF3CF2(CF3)2CO - , (CF3SO2)2CH - , (SF5)3C - , (CF3SO2)3C - , CF3(CF2)7SO3 - , CF3CO2 - , CH3CO2 - , SCN - and (CF3CF2SO2)2N - One or more selected from the group consisting of may be used.

[0123] For example, the concentration of the lithium salt may be 0.1 to 2.0 M. When the concentration of the lithium salt is within the above range, the electrolyte may exhibit excellent electrolyte performance because it has appropriate conductivity and viscosity, and thus lithium ions may move more effectively.

[0124] For example, depending on the type of lithium secondary battery, the lithium secondary battery may further include a separator formed between the positive and negative electrodes. The separator separates the negative and positive electrodes and provides a passage for lithium ions to move. Any separator used in the industry as a separator for secondary batteries may be used without particular limitations. For example, the separator may be a multilayer film made of polyethylene, polypropylene, polyvinylidene fluoride, or two or more layers thereof. However, the present invention is not limited thereto, and the separator may be a mixed multilayer film, such as a polyethylene / polypropylene two-layer separator, a polyethylene / polypropylene / polyethylene three-layer separator, or a polypropylene / polyethylene / polypropylene three-layer separator.

[0125] The above lithium secondary battery may be placed within a battery case. The shape of the battery case may be at least one selected from the group consisting of a cylindrical shape using a can, a square shape, a pouch shape, and a coin shape. However, the shape of the battery case is not limited thereto, and may have various shapes used in the relevant industry.

[0126] Hereinafter, the present invention will be described in more detail through examples. However, it should be noted that the following examples are intended only to illustrate and further illustrate the present invention and are not intended to limit the scope of the invention. This is because the scope of the invention is determined by the matters set forth in the patent claims and matters reasonably inferred therefrom.

[0127] (Example 1)

[0128] Poly-Si was mixed with 99.9% pure anhydrous ethanol, and then pulverized using zirconia beads (BPR of Poly-Si and zirconia beads = 5:1) at 2500 rpm to obtain silicon particles (D 90 : 150 nm) was prepared. After adding graphite to the dispersion, a high-speed disperser was used to disperse silicon particles and graphite (D 50 : 3 ㎛) were mixed and spray dried to form a silicon-carbon precursor.

[0129] A silicon-carbon precursor and pitch (sulfur: 7,500 ppm, beta resin: 22 wt%) were mixed to obtain a first mixture, which was then placed in a mold and pressurized. The pressurized first mixture was first carbonized in an inert atmosphere at 800°C to form a silicon-carbon composite.

[0130] After pulverizing the silicon-carbon composite, a petroleum pitch (sulfur: 6,800 ppm, beta resin: 35 wt%) having a softening point of less than 250°C was mixed with the silicon-carbon composite to obtain a second mixture. Thereafter, the second mixture was subjected to secondary carbonization in an inert atmosphere at 1000°C and then classified through a sieve to produce the negative active material of Example 1.

[0131] (Example 2)

[0132] In the manufacturing method of Example 1, a negative active material of Example 2 was manufactured in the same manner as in Example 1, except that the primary carbonization temperature was changed to 900°C.

[0133] (Example 3)

[0134] In the manufacturing method of Example 1, a negative active material of Example 3 was manufactured in the same manner as in Example 1, except that the primary carbonization temperature was changed to 1000°C.

[0135] (Example 4)

[0136] In the manufacturing method of Example 2, the negative active material of Example 4 was manufactured in the same manner as in Example 2, except that the secondary carbonization temperature was changed to 700°C.

[0137] (Example 5)

[0138] In the manufacturing method of Example 2, the negative active material of Example 5 was manufactured in the same manner as in Example 2, except that the secondary carbonization temperature was changed to 800°C.

[0139] (Example 6)

[0140] In the manufacturing method of Example 2, the negative active material of Example 6 was manufactured in the same manner as in Example 2, except that the secondary carbonization temperature was changed to 900°C.

[0141] (Comparative Example 1)

[0142] In the manufacturing method of Example 1, a negative active material of Comparative Example 1 was manufactured using the same method, except that the primary carbonization temperature was changed to 700°C.

[0143] (Experimental Example 1: Measurement of physical properties of negative electrode active material)

[0144] For the above examples and comparative examples, the sulfur content and the crystal grain size of silicon particles were measured, and the results are shown in Table 1.

[0145] Sulfur content was measured using a sulfur determinator. Specifically, the sulfur dioxide content generated by combusting a sample using high-frequency induction was measured, and the intensity of sulfur dioxide absorption by infrared light was analyzed.

[0146] For the crystal grain size, X-ray diffraction analysis was performed using CuKα rays on the (111) plane for silicon particles, the half width of the peak was measured, and the crystal grain size of the silicon particles was calculated using the Sherrer equation.

[0147] Content of sulfur (ppm)Crystal size of silicon particles (nm)Example 1 2000 15.9Example 2 1700 16.2Example 3 1500 16.6Example 4 1800 16.1Example 5 1700 16.1Example 6 1700 16.2Comparative example 12700 15.8

[0148] Referring to Table 1, it was found that Examples 1 to 6, which satisfied the manufacturing conditions proposed in the present invention, had the sulfur content and the crystal grain size of the silicon particles controlled to the desired levels.

[0149] Meanwhile, it was confirmed that Comparative Example 1, in which the primary carbonization temperature was controlled low, measured a high sulfur content of 2700 ppm.

[0150] (Experimental Example 2: Measurement of electrochemical properties of negative electrode active material)

[0151] In order to evaluate the electrochemical characteristics of the above examples and comparative examples, lithium half-cells were manufactured using each negative active material.

[0152] (1) Manufacturing of lithium half-cells to evaluate electrode characteristics manufactured solely with the negative active material of the example

[0153] Specifically, a negative electrode was manufactured by coating a negative electrode active material on a Cu current collector so that the loading amount was 5 mg / cm2 and the electrode density was 1.2 to 1.3 g / cc, and then rolling. At this time, the binder for electrode manufacturing used was a polyacrylic acid (PAA) system.

[0154] Lithium metal (Li-metal) was used as a counter electrode, and as an electrolyte, a 1.0 M LiPF6 solution was dissolved in a mixed solvent of EC:EMC = 3:7 without any additives, and 1.5 wt% VC was added. Using each of the above components, a half-cell of the 2032 coin cell type was manufactured according to a conventional manufacturing method.

[0155] (2) Manufacturing of lithium half-cells to evaluate the characteristics of electrodes manufactured with negative active materials mixed with graphite.

[0156] After mixing the negative active material and natural graphite, a coin cell type half-battery was manufactured while maintaining the negative electrode capacity at 600 mAh / g. Specifically, when manufacturing the negative electrode, the negative electrode slurry was composed of negative active material + natural graphite : conductive material : CMC : SBR = 95.8 : 1 : 1.7 : 1.5, and the rolling density was maintained at 1.4 g / cc.

[0157] Lithium metal (Li-metal) was used as a counter electrode, and as an electrolyte, a 1.0 M LiPF6 solution was dissolved in a mixed solvent of EC:EMC = 3:7 without any additives, and 1.5 wt% of VC was added. Using each of the above components, a half-cell of the 2032 coin cell type was manufactured according to a conventional manufacturing method.

[0158] (3) Evaluation method of electrochemical properties

[0159] For the half-cells manufactured solely with the above examples and comparative examples, the initial capacity was measured, and for the half-cells manufactured by mixing natural graphite with the above examples and comparative examples, the initial capacity, initial efficiency, and capacity retention rate at 50 cycles were measured, and the results are shown in Table 2.

[0160] Specifically, the current during charge and discharge was applied at 0.1 C in the initial cycle. At this time, the initial discharge capacity was measured, and the initial efficiency was measured as the percentage of the initial discharge capacity to the initial charge capacity.

[0161] Afterwards, a current of 0.5 C was applied during charge and discharge based on the first 1C capacity, and the charge and discharge cycle was repeated 50 times, and the charge cut-off current was set to 0.005 C. After 50 cycles, the capacity retention rate at 50 cycles was calculated using the following equation.

[0162] Capacity retention rate (%) = [(discharge capacity at the 50th cycle) / (discharge capacity at the 1st cycle)] × 100

[0163] Characteristics of electrodes alone Characteristics of electrodes mixed with graphite Initial capacity (mAh / g) Initial capacity (mAh / g) Initial efficiency (%) Capacity retention (%) Example 1 1598.76 13.68 9.155 7.5 Example 2 1608.16 14.88 9.28 63.2 Example 3 1609.46 15.889 29 61.2 Example 4 1605.76 13.889 2162.7 Example 5 1607.76 14.789 26 63.6 Example 6 1608.06 14.689 28 63.1 Comparative example 11591.86 11.48 8.72 5 1.5

[0164] Referring to Table 2, it was found that Examples 1 to 6, which satisfied the manufacturing conditions proposed in the present invention, had both the characteristics of the single electrode and the characteristics of the electrode mixed with graphite controlled to the desired level.

[0165] Meanwhile, in the case of Comparative Example 1, where the primary carbonization temperature was controlled low, the initial capacity was measured to be low when the electrode was manufactured using only the negative active material. Furthermore, when the electrode was manufactured using the negative active material mixed with Comparative Example 1 and graphite, the initial capacity, initial efficiency, and capacity retention rate were all measured to be low. In other words, it was found that the electrochemical characteristics of Comparative Example 1 were deteriorated because the sulfur content was not controlled to an appropriate level.

[0166] The above embodiments are merely examples and the present invention is not limited thereto. Anything that has substantially the same configuration and achieves the same operational effects as the technical concepts described in the claims of the present invention is included within the technical scope of the present invention.

Claims

1. A silicon-carbon composite comprising crystalline carbon, silicon particles located on the surface and inside of the crystalline carbon, and amorphous carbon located on the surface of the crystalline carbon and the silicon particles; and A negative electrode active material comprising a carbon coating layer covering the surface of the silicon-carbon composite.

2. In paragraph 1, The above amorphous carbon is a negative electrode active material derived from sulfur-containing pitch.

3. In paragraph 2, The above sulfur-containing pitch is a negative active material comprising 20 wt% or more of beta resin based on the total weight.

4. In paragraph 2, A negative electrode active material, wherein the sulfur-containing pitch comprises at least one selected from the group consisting of coal-based pitch, petroleum-based pitch, natural pitch, and mesophase pitch.

5. In paragraph 1, The negative electrode active material comprises 10 to 25 wt% of the crystalline carbon based on the total weight.

6. In paragraph 1, A negative electrode active material, wherein the above crystalline carbon comprises at least one selected from the group consisting of artificial graphite, flaky graphite, natural graphite, and expanded graphite.

7. In paragraph 1, The negative electrode active material comprises 20 to 45 wt% of the amorphous carbon based on the total weight.

8. In paragraph 1, The negative electrode active material comprises less than 3 wt% of the carbon coating layer based on the total weight.

9. In paragraph 1, A negative electrode active material, wherein the carbon of the carbon coating layer is derived from at least one selected from the group consisting of coal pitch, petroleum pitch, coal tar, polylactic acid (PAA), and polyvinyl alcohol (PVA).

10. A step of preparing silicon particles by crushing silicon raw material; A step of mixing the above silicon particles and crystalline carbon; A step of obtaining a first mixture by mixing the above mixed silicon particles and crystalline carbon and sulfur-containing pitch; A step of first carbonizing the first mixture at 750 to 1100° C. to form a silicon-carbon composite; A step of mixing the silicon-carbon composite and the carbon precursor to obtain a second mixture; and A method for producing a negative active material, comprising the step of secondary carbonization of the second mixture.

11. In Article 10, A method for producing a negative electrode active material, further comprising, after the step of obtaining the first mixture, a step of pressurizing the first mixture at a pressure of less than 1 ton / cm2.

12. In paragraph 10, A method for producing a negative electrode active material, wherein the above-mentioned secondary carbonization step is performed at 700 to 1100°C.

13. Cathode; Bipolar; and Contains electrolyte, A lithium secondary battery comprising a negative electrode active material according to any one of claims 1 to 9.

14. In paragraph 1, A negative electrode active material comprising 0.05 to 0.25 wt% of sulfur based on the total weight.