Silicon-carbon composite, negative electrode active material, method for manufacturing negative electrode active material, and negative electrode comprising negative electrode active material

Abstract

According to exemplary embodiments of the present invention, provided is a silicon-carbon composite capable of improving lifespan characteristics of a secondary battery. The silicon carbon-composite is a silicon-carbon composite for a negative electrode active material, comprising a carbon matrix and silicon nanoparticles captured in the carbon matrix, wherein the surface roughness (Rq) of the silicon-carbon composite may be 4-50 nm.

Description

Silicon-carbon composite, negative electrode active material, method for producing negative electrode active material, and negative electrode including negative electrode active material

[0001] The present invention relates to a silicon-carbon composite, a negative electrode active material, a method for producing the negative electrode active material, and a negative electrode comprising the negative electrode active material.

[0002] Recently, the rapid advancement of portable and wireless electronic devices has led to a growing demand for compact, lightweight, and high-energy-density secondary batteries as power sources for these devices. Furthermore, high-output characteristics, long-term durability, and safety are required not only for small consumer applications but also for large-scale secondary batteries used in power storage devices and electric vehicles.

[0003] A secondary battery has a structure in which an electrode assembly including a positive electrode, a negative electrode, and a separator disposed between the positive and negative electrodes, and an electrolyte are accommodated in an outer body. The electrode includes an electrode current collector and an electrode active material layer positioned on at least one side of the electrode current collector. Specifically, the positive electrode includes a positive electrode current collector and a positive electrode active material layer positioned on at least one side of the positive electrode current collector. The negative electrode includes a negative electrode current collector and a negative electrode active material layer positioned on at least one side of the negative electrode current collector.

[0004] Secondary batteries can be categorized into lead-acid batteries, nickel-cadmium (Cd) batteries, nickel-metal hydride (MH) batteries, and lithium batteries, depending on the type of cathode and anode active materials. Among these, demand for lithium secondary batteries, with their superior energy density, output characteristics, and operating time, is growing.

[0005] The secondary battery reaction of a lithium secondary battery is carried out through a chemical reaction in which lithium ions (Li+) move between the positive electrode active material and the negative electrode active material.

[0006] In the field of secondary batteries, much research is ongoing because the lifespan characteristics of secondary batteries are directly related to the long-term performance of devices that use secondary batteries as power sources.

[0007] (Patent Document 1) Republic of Korea Patent Publication No. 10-1825919.

[0008] The technical idea of ​​the present invention aims to solve a problem by providing a silicon-carbon composite capable of improving the lifespan characteristics of a secondary battery.

[0009] Another technical idea of ​​the present invention is to provide a negative electrode active material capable of improving the life characteristics of a secondary battery.

[0010] Another technical idea of ​​the present invention is to provide a method for manufacturing a negative electrode active material capable of improving the life characteristics of a secondary battery.

[0011] Another technical idea of ​​the present invention is to provide a negative electrode including a negative electrode active material capable of improving the life characteristics of a secondary battery.

[0012]

[0013] The objectives of the present invention are not limited to the above-described content. Those skilled in the art will have no difficulty understanding the additional objectives of the present invention from the overall description of the present invention.

[0014] According to exemplary embodiments of the present invention, a silicon-carbon composite is provided. The silicon-carbon composite is a silicon-carbon composite for use as a negative electrode active material, comprising a carbon matrix and silicon nanoparticles captured in the carbon matrix, wherein the surface roughness (Rq) of the silicon-carbon composite may be 4 to 50 nm.

[0015] The D90 of the above silicon nanoparticles may be 180 nm or less.

[0016] The above silicon nanoparticles can be included in an amount of 35 to 60 wt%.

[0017] The above carbon matrix may be included in an amount of 35 to 55 wt%.

[0018] The above carbon matrix may include crystalline carbon and amorphous carbon.

[0019] The content of the above crystalline carbon may be 10 to 25 wt%. The content of the above amorphous carbon may be 25 to 40 wt%.

[0020] The crystalline carbon may include at least one of artificial graphite, flake graphite, earthy graphite, expanded graphite, graphene, and combinations thereof.

[0021] The D50 of the above crystalline carbon may be 5 to 10 μm.

[0022] The amorphous carbon may include any one of coal pitch, petroleum pitch, coal tar, polyacrylic acid (PAA), polyvinyl alcohol (PVA), and combinations thereof.

[0023]

[0024] According to other exemplary embodiments of the present invention, a negative electrode active material is provided. The negative electrode active material comprises a silicon-carbon composite including silicon nanoparticles and a carbon matrix; and a coating layer distributed on the surface of the silicon-carbon composite, wherein the coating layer may include carbon nanotubes distributed on the surface of the silicon-carbon composite and a residual binder.

[0025] The above carbon nanotubes may be included in an amount of 0.1 to 0.5 parts by weight based on 100 parts by weight of the silicon-carbon composite.

[0026] The above residual binder may be included in an amount of 0.1 to 1 part by weight based on 100 parts by weight of the silicon-carbon composite.

[0027] The moisture content of the above negative active material may be 0.6 to 2.0%.

[0028] The residual binder may include any one of carboxymethyl cellulose (CMC), hydroxypropyl cellulose (HPC), hydroxyethyl cellulose (HEC), polyvinylpyrrolidone (PVP), and polyethylene oxide (PEO), and combinations thereof.

[0029]

[0030] According to other exemplary embodiments of the present invention, a method for manufacturing an anode active material is provided. The method for manufacturing the anode active material includes the steps of: preparing silicon nanoparticles by pulverizing a silicon raw material; mixing and drying the silicon nanoparticles with crystalline carbon to obtain a silicon-crystalline carbon precursor; bonding the silicon-crystalline carbon precursor with an amorphous carbon precursor to obtain a mixture; heat-treating the mixture in an inert atmosphere at 700 to 1000°C to obtain a silicon-carbon composite; mixing a coating layer precursor solution including carbon nanotubes and a binder with the silicon-carbon composite to obtain a slurry; and drying the slurry in an air atmosphere at 20 to 100°C.

[0031] In the step of obtaining the above silicon nanoparticles, the pulverization of the silicon raw material is performed through ball milling using beads, and the diameter of the beads may be less than twice the D99 of the silicon raw material.

[0032] In the step of obtaining the above silicon nanoparticles, the grinding of the silicon raw material may be wet grinding using an organic solvent including ethanol having a purity of 99.9%.

[0033] In the step of obtaining the above silicon nanoparticles, the D90 of the silicon nanoparticles may be 180 nm or less.

[0034] The D50 of the above crystalline carbon may be 5 to 10 μm.

[0035] The surface roughness (Rq) of the above silicon-carbon composite can be 4 to 50 nm.

[0036] The method for manufacturing the above negative active material may further include a pressurizing molding step of pressurizing a mixture of the silicon-crystalline carbon precursor and the amorphous carbon precursor.

[0037] The binder may include any one of carboxymethyl cellulose (CMC), hydroxypropyl cellulose (HPC), hydroxyethyl cellulose (HEC), polyvinylpyrrolidone (PVP), and polyethylene oxide (PEO), and combinations thereof.

[0038] The silicon-carbon composite according to exemplary embodiments of the present invention can improve the life characteristics of a secondary battery because it includes nano-scale silicon particles, and at the same time, can improve the characteristics of the secondary battery by including a carbon matrix.

[0039] According to other exemplary embodiments of the present invention, the negative active material has a structure in which carbon nanotubes and residual binder are distributed on the surface of the silicon-carbon composite described above. This can improve the lifespan characteristics of the secondary battery.

[0040] A method for manufacturing a negative electrode active material according to other exemplary embodiments of the present invention can manufacture a negative electrode active material at a relatively low temperature, thereby improving the manufacturing efficiency of the negative electrode active material.

[0041]

[0042] The various advantageous and beneficial 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.

[0043] Figure 1 is an SEM photograph of the surface of a negative active material according to Example 2.

[0044] Figure 2 is an SEM photograph of the surface of the negative active material according to Comparative Example 3.

[0045] Figure 2 is a graph showing the life characteristics of secondary batteries according to examples and comparative examples.

[0046] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the attached drawings. Prior to this, it should be noted that the terms and words used in this specification and claims should not be construed as limited to their conventional or dictionary meanings. Based on the principle that the inventor can appropriately define the concepts of terms to best explain his or her invention, they should be interpreted in a way that aligns with the technical spirit of the present invention.

[0047] Hereinafter, when explaining with reference to drawings, identical or corresponding components are given the same drawing reference numerals and redundant descriptions thereof are omitted.

[0048] In the examples below, the terms first, second, etc. are not used in a limiting sense, but are used for the purpose of distinguishing one component from another.

[0049] In the examples below, singular expressions include plural expressions unless the context clearly indicates otherwise.

[0050] In the examples below, terms such as “include” or “have” mean that a feature or component described in the specification is present, and do not preclude the possibility that one or more other features or components may be added.

[0051] For convenience of explanation, the sizes of components in the drawings may be exaggerated or reduced. For example, the sizes and thicknesses of each component shown in the drawings are arbitrarily indicated for convenience of explanation, and thus the present invention is not necessarily limited to what is shown.

[0052] In some embodiments, where implementations are otherwise feasible, specific process sequences may be performed in a different order than described. For example, two processes described in succession may be performed substantially simultaneously, or in a reverse order from the described order.

[0053] In addition, when describing the present invention, if it is determined that a detailed description of a related known configuration or function may obscure the gist of the present invention, the detailed description is omitted.

[0054]

[0055] In the present invention, having a nano-scale size means, with respect to the particle size distribution, D90 as the standard. That is, it means a case where the D90 of the particle is less than 1.0㎛.

[0056] In the present invention, D50, D10, and D90 can be defined as particle sizes corresponding to 50%, 10%, and 90% of the volume accumulation amount, respectively, in the particle size distribution curve of the particles. The D50, D10, and D90 can be measured using, for example, a laser diffraction method. The laser diffraction method can generally measure particle sizes from the submicron range to several mm, and can obtain results with high reproducibility and high resolution.

[0057]

[0058] Hereinafter, a negative electrode, a negative electrode active material, and a method for manufacturing a negative electrode according to exemplary embodiments of the present invention are specifically described.

[0059]

[0060] [cathode]

[0061] According to exemplary embodiments, the negative electrode may include a negative electrode current collector and a negative electrode active material layer.

[0062]

[0063] The negative electrode current collector is not particularly limited as long as it has high conductivity and does not induce chemical changes in the battery. As non-limiting examples, the negative electrode current collector may include one or more of copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface-treated with carbon, nickel, titanium, silver, etc., and an aluminum-cadmium alloy.

[0064] The thickness of the negative electrode current collector can be 3 to 500 μm.

[0065]

[0066] The negative electrode active material layer may be provided on one or both sides of the negative electrode current collector. The negative electrode active material layer may be obtained by coating and drying a negative electrode slurry containing a negative electrode binder, a conductive material, and a negative electrode active material on the negative electrode current collector.

[0067] The negative electrode binder is a component that assists in bonding between the negative electrode active material and the current collector. As a non-limiting example, the negative electrode binder may include at least one selected from polyvinyl alcohol, carboxymethyl cellulose / styrene-butadiene rubber, hydroxypropylene cellulose, diacetylene cellulose, polyvinyl chloride, polyvinylpyrrolidone, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, or polypropylene.

[0068] The content of the negative electrode binder is not particularly limited. As a non-limiting example, the negative electrode binder may be added in an amount of 30 wt% or less (including 0 wt%) within the negative electrode active material layer.

[0069]

[0070] A conductive agent is a component that further enhances the conductivity of the negative electrode active material. As non-limiting examples, the conductive agent may include at least one selected from the following conductive materials: graphite such as carbon nanotubes, natural graphite, and artificial graphite; carbon black such as acetylene black, Ketjen black, channel black, furnace black, lamp black, and summer black; conductive fibers such as carbon fibers and metal fibers; metal powders such as fluorocarbon, aluminum, and nickel powders; conductive whiskeys such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and conductive materials such as polyphenylene derivatives.

[0071] The content of the conductive agent is not particularly limited. As a non-limiting example, the conductive agent may be added in an amount of 20 wt% or less (including 0%) within the negative electrode active material layer.

[0072]

[0073] [Cathode active material]

[0074] According to exemplary embodiments, the negative active material may include a silicon-carbon composite and a coating layer.

[0075]

[0076] According to exemplary embodiments, the surface roughness (Rq) of the silicon-carbon composite may be 4 to 100 nm. More specifically, the surface roughness (Rq) of the silicon-carbon composite may be 4 to 90 nm. The surface roughness (Rq) of the silicon-carbon composite may be 4 to 80 nm. The surface roughness (Rq) of the silicon-carbon composite may be 4 to 60 nm. The surface roughness (Rq) of the silicon-carbon composite may be 4 to 50 nm. The surface roughness (Rq) of the silicon-carbon composite may be 4 to 30 nm. The surface roughness (Rq) of the silicon-carbon composite may be 6 to 30 nm. The surface roughness (Rq) of the silicon-carbon composite may be 9 to 30 nm.

[0077] If the surface roughness (Rq) of the silicon-carbon composite is less than 4 nm, the coating layer may not be properly fixed to the surface of the silicon-carbon composite during the coating process described below. Therefore, a higher surface roughness (Rq) of the silicon-carbon composite may be advantageous for the efficiency of the coating process. However, if the surface roughness (Rq) of the silicon-carbon composite is excessively high, the coating layer may not be formed uniformly but may be formed thickly in localized areas. This may increase the resistance of the negative active material. Alternatively, an increase in the specific surface area of ​​the negative active material may cause a side reaction with the electrolyte during use of the secondary battery.

[0078] A silicon-carbon composite may include silicon nanoparticles, a carbon matrix, and unavoidable impurities resulting from the manufacturing process. More specifically, the silicon-carbon composite may include a structure in which silicon nanoparticles are captured in a carbon matrix. In other words, the silicon-carbon composite may include a structure in which silicon nanoparticles are mechanically bonded to the carbon matrix while in contact with it.

[0079] Previously, graphite-based materials, known for their excellent capacity retention characteristics and efficiency, were used as anode active materials. However, graphite-based materials suffered from relatively low theoretical capacity values ​​and low discharge capacity ratios. Consequently, research into silicon (Si)-based materials as anode active materials to replace graphite-based materials has continued.

[0080] Silicon-based materials possess high theoretical capacity and low operating voltage characteristics, making them ideal anode active materials for secondary batteries. However, silicon-based materials have been plagued by particle cracking caused by the continuous charge-discharge cycle, which can shorten the battery's lifespan.

[0081] According to exemplary embodiments of the present invention, nanoscale silicon-based materials (silicon nanoparticles) can be used as negative electrode active materials for secondary batteries. By using nanoscale silicon-based materials, particle cracking can be minimized. This can improve the lifespan characteristics of secondary batteries.

[0082] More specifically, the silicon nanoparticles may have a D90 of 180 nm or less. More specifically, the silicon nanoparticles may have a D90 of 150 nm or less. The silicon nanoparticles may have a D90 of 100 to 180 nm. The silicon nanoparticles may have a D90 of 100 to 150 nm.

[0083] According to exemplary embodiments, the silicon nanoparticles may be included in an amount of 35 to 60 wt% based on the total weight of the silicon-carbon composite.

[0084] To improve the capacity of secondary batteries, the content of silicon nanoparticles needs to be increased. Furthermore, as the content of silicon nanoparticles increases, the silicon nanoparticles can be fully captured within the carbon matrix. This enhances the structural stability of the silicon-carbon composite. However, if the content of silicon nanoparticles is excessively high, the silicon nanoparticles may not be sufficiently captured within the carbon matrix, which may degrade the mechanical properties of the silicon-carbon composite. Furthermore, the silicon nanoparticles may detach from the carbon matrix.

[0085]

[0086] According to exemplary embodiments, the carbon matrix may be included in an amount of 35 to 55 wt% based on the total weight of the silicon-carbon composite.

[0087] The carbon matrix can be configured to support silicon nanoparticles. More specifically, the silicon nanoparticles can be mechanically bonded to the carbon matrix by direct contact. This allows the carbon matrix to support the silicon nanoparticles while maintaining contact with them. Furthermore, even when the silicon nanoparticles expand in response to the charge / discharge reaction of the secondary battery, the carbon matrix can maintain electrical contact with other internal components of the secondary battery (e.g., the negative electrode current collector).

[0088] The carbon matrix may include crystalline carbon and amorphous carbon.

[0089] Crystalline carbon can improve the reversibility of charge and discharge of secondary batteries by complementing the electrical properties of silicon nanoparticles.

[0090] According to exemplary embodiments, the crystalline carbon may be included in an amount of 10 to 25 wt% based on the total weight of the silicon-carbon composite. When the content of crystalline carbon is less than 10 wt%, the effect of complementing the electrical properties of the silicon nanoparticles may be minimal. However, when the content of crystalline carbon exceeds 25 wt%, the electrical properties may be improved, but the silicon nanoparticles may not be captured in the carbon matrix. This may excessively increase the specific surface area of ​​the silicon-carbon composite. In this case, excessive side reactions between the silicon-carbon composite and the electrolyte may occur during the charge / discharge process of the secondary battery, which may deteriorate the lifespan characteristics of the secondary battery. In addition, the capacity of the secondary battery may be deteriorated due to a decrease in the content of the silicon nanoparticles.

[0091] According to exemplary embodiments, the D50 of the crystalline carbon may be 5 to 10 μm.

[0092] As a non-limiting example, the crystalline carbon may include at least one of artificial graphite, flake graphite, earthy graphite, expanded graphite, graphene, and combinations thereof.

[0093] According to exemplary embodiments, the amorphous carbon may be included in an amount of 25 to 40 wt% based on the total weight of the silicon-carbon composite.

[0094] Amorphous carbon can function as a binder that stably supports crystalline carbon and silicon nanoparticles. Thus, amorphous carbon allows the crystalline carbon and silicon nanoparticles to be physically and electrically connected. This prevents the structural collapse of the silicon-carbon composite even after repeated charge-discharge cycles in a secondary battery, and more effectively compensates for the low electrical conductivity of silicon nanoparticles.

[0095] According to exemplary embodiments, the amorphous carbon may include any one of coal pitch, petroleum pitch, coal tar, polyacrylic acid (PAA), polyvinyl alcohol (PVA), and combinations thereof.

[0096] More specifically, the amorphous carbon may include coal-based pitch having a fixed carbon content of at least 50 wt%. As a non-limiting example, the fixed carbon content of the coal-based pitch may be 60 to 80 wt%.

[0097] As the fixed carbon content of coal-based pitch increases, it creates conductive paths that electrically connect with silicon nanoparticles, which have low electrical conductivity, leading to increased capacity and efficiency. Furthermore, when the fixed carbon content of coal-based pitch satisfies the above-described range, it can reduce the internal pores of the carbon matrix, thereby lowering the specific surface area. This can reduce side reactions between the negative active material and the electrolyte during use in secondary batteries, contributing to increased initial efficiency.

[0098] As a non-limiting example, the beta resin value of coal-based pitch may be greater than 20. More specifically, the beta resin value of coal-based pitch may be between 25 and 40. The beta resin value of coal-based pitch may be between 25 and 35.

[0099] The above β-resin value refers to the value obtained by subtracting the benzene-insoluble content from the quinoline content. This β-resin value is proportional to the cohesion. According to exemplary embodiments, when the β-resin value satisfies the above range, the internal pores of the silicon-carbon composite can be reduced to stably maintain the structure in which the carbon matrix and silicon nanoparticles are in contact (captured). Therefore, by using the negative active material according to exemplary embodiments, a secondary battery having excellent life characteristics and electrode plate expansion characteristics can be realized.

[0100] As a non-limiting example, a silicon-carbon composite may contain unavoidable impurities due to the manufacturing process. For example, the unavoidable impurity may be an oxide film formed on the surface of silicon nanoparticles. The remainder of the silicon-carbon composite, excluding the silicon nanoparticles and carbon matrix, may be comprised of these unavoidable impurities.

[0101]

[0102] The coating layer can be distributed on the surface of the silicon-carbon composite. This coating layer can suppress the volume expansion of the silicon-carbon composite during use of the secondary battery. This can improve the lifespan characteristics of the secondary battery. Furthermore, by improving the electrical conductivity of the negative electrode active material, the characteristics of the secondary battery utilizing the negative electrode active material can be improved.

[0103] The "coating layer" can refer to a layer provided with a predetermined thickness on the surface of a silicon-carbon composite. Alternatively, the materials distributed on the surface of the silicon-carbon composite can be collectively referred to as a "coating layer." Importantly, the distribution of the components of the coating layer, described below, on the surface of the silicon-carbon composite can ultimately improve the lifespan characteristics of the secondary battery.

[0104]

[0105] According to exemplary embodiments, the coating layer may include carbon nanotubes (CNTs) and a residual binder.

[0106] Carbon nanotubes may be distributed on the surface of the silicon-carbon composite. According to exemplary embodiments, the carbon nanotubes may be included in an amount of 0.1 to 0.5 parts by weight based on 100 parts by weight of the silicon-carbon composite.

[0107] Carbon nanotubes can improve the electrical properties of a silicon-carbon composite. Therefore, if the anode active material according to exemplary embodiments is employed, the electrical resistance of the anode can be reduced, thereby improving the characteristics of a secondary battery. To this end, the carbon nanotubes may be included in an amount of 0.1 parts by weight or more. However, if the carbon nanotubes exceed 0.5 parts by weight, the carbon nanotubes may aggregate during the manufacturing process. This may reduce the coating efficiency of the silicon-carbon composite. Furthermore, the volume expansion of the silicon-carbon composite cannot be sufficiently suppressed, which may deteriorate the lifespan characteristics of the secondary battery. Furthermore, the agglomerated carbon nanotubes may reduce the production efficiency during the classification process of the anode active material, which may increase the unit price of the anode active material.

[0108] As a non-limiting example, the carbon nanotube may be any one of a single-walled carbon nanotube (SWCNT), a thin-walled carbon nanotube (TWCNT), a multi-walled carbon nanotube (MWCNT), and a combination thereof. More specifically, the TWCNT may be a carbon nanotube having five or fewer layers. The MWCNT may be a carbon nanotube having more than five layers.

[0109] The residual binder may be distributed on the surface of the silicon-carbon composite. According to exemplary embodiments, the residual binder may be included in an amount of 0.1 to 1 part by weight based on 100 parts by weight of the silicon-carbon composite.

[0110] Residual binder can further enhance the contact efficiency between carbon nanotubes and silicon-carbon composites. This can further enhance the electrical conductivity of the negative electrode active material due to the carbon nanotubes. However, if the residual binder is distributed excessively, it can cause the carbon nanotubes to aggregate, reducing coating efficiency. Furthermore, it can increase the resistance of the negative electrode active material, thereby degrading its electrical properties.

[0111] According to exemplary embodiments, the residual binder may include a shear thinning aqueous binder. As a non-limiting example, the residual binder may include any one of carboxymethyl cellulose (CMC), hydroxypropyl cellulose (HPC), hydroxyethyl cellulose (HEC), polyvinylpyrrolidone (PVP), and polyethylene oxide (PEO), and combinations thereof.

[0112] According to exemplary embodiments, the moisture content of the negative electrode active material may be 0.6 to 2.0%. The moisture content of the negative electrode active material refers to the moisture content of the negative electrode active material. The moisture content of the negative electrode active material can be calculated by drying the negative electrode active material and comparing the weight before and after drying.

[0113] If the moisture content of the negative active material is less than 0.6%, residual binder may not be sufficiently distributed on the surface of the silicon-carbon composite. In this case, the aforementioned effects due to residual binder may not be fully achieved. However, if the moisture content of the negative active material is 2.0%, residual binder may be excessively distributed on the surface of the silicon-carbon composite. In this case, the residual binder may cause carbon nanotubes to aggregate, thereby degrading the coating quality of the silicon-carbon composite.

[0114]

[0115] [Method for manufacturing negative active material]

[0116] Hereinafter, a method for manufacturing a negative electrode active material according to exemplary embodiments will be described in detail.

[0117] According to exemplary embodiments, the negative active material can be manufactured in the following order: [preparation of silicon nanoparticles - formation of silicon-carbon composite - coating].

[0118]

[0119] Silicon nanoparticles can be prepared by crushing a silicon raw material. More specifically, the silicon raw material can be crushed mechanically. This allows for the production of silicon nanoparticles.

[0120] The silicon raw material can be pulverized through ball milling using beads. More specifically, the silicon raw material can be pulverized by rotating the silicon raw material and the beads together using a pulverizer. The diameter of the resulting silicon nanoparticles can be controlled by the diameter of the beads.

[0121] According to exemplary embodiments, the diameter of the beads may be less than twice the D99 of the silicon raw material. If the diameter of the beads is excessively large compared to the silicon raw material, the silicon grinding time may increase, which may lead to oxidation of the silicon particles.

[0122] The pulverization of these silicon raw materials can be performed using a wet pulverization method using an organic solvent. More specifically, a wet pulverization method using an organic solvent containing 99.9% pure ethanol can be used. As described above, as the surface area of ​​silicon particles increases during the pulverization process of the silicon raw material, the silicon particles may be oxidized. To prevent this side reaction, the silicon raw material can be mixed with anhydrous ethanol to prevent oxidation of the silicon nanoparticles. At this time, the solids ratio can be performed in the range of 8 to 15%. The solids content during silicon pulverization may refer to the silicon raw material.

[0123] As a non-limiting example, when grinding silicon, the ball-to-bead ratio (BPR) of silicon raw material to beads may be 5:1. The grinder's rotation speed may be 2,000 to 5,000 rpm. More specifically, the grinder's rotation speed may be 2,000 to 3,000 rpm.

[0124]

[0125] Formation of a silicon-carbon composite may include a step of obtaining a silicon-crystalline carbon precursor; an amorphous carbon bonding step; and a heat treatment step.

[0126] The step of obtaining a silicon-crystalline carbon precursor may be comprised of mixing and drying silicon nanoparticles and crystalline carbon particles.

[0127] Silicon nanoparticles and crystalline carbon can be mixed in an organic solvent to provide a silicon nanoparticle slurry.

[0128] As a non-limiting example, the obtained silicon nanoparticle powder and crystalline carbon particle powder may be mixed in separate organic solvents to provide a silicon nanoparticle slurry. Alternatively, when a wet grinding method is used for grinding the silicon raw material, the crystalline carbon particles may be mixed in an organic solvent containing the ground silicon raw material (silicon nanoparticles) to provide a silicon nanoparticle slurry.

[0129] According to exemplary embodiments, the D50 of the crystalline carbon may be 5 to 10 μm. This can complement the poor electrical properties of silicon nanoparticles and further improve the reversibility of charge and discharge of the secondary battery.

[0130] As a non-limiting example, the crystalline carbon may include at least one of artificial graphite, flake graphite, earthy graphite, expanded graphite, graphene, and combinations thereof.

[0131] Silicon-crystalline carbon precursor particles can be obtained by spray drying a silicon nanoparticle slurry using a spray dryer.

[0132] The amorphous carbon bonding step may be accomplished by mixing silicon-crystalline carbon precursor particles with an amorphous carbon precursor.

[0133] Mixing of the silicon-crystalline carbon precursor particles and the amorphous carbon precursor may be performed by a mechanical milling process. By way of non-limiting example, the mechanical milling process may be performed by any one of mechanofusion, ball milling, and combinations thereof.

[0134] As a non-limiting example, the amorphous carbon precursor may include any one of coal pitch, petroleum pitch, coal tar, polyacrylic acid (PAA), polyvinyl alcohol (PVA), and combinations thereof.

[0135] According to exemplary embodiments, the method may further include a pressurizing step of pressurizing a mixture of a silicon-crystalline carbon precursor and an amorphous carbon precursor. This allows for more intimate bonding of the amorphous carbon precursor, silicon nanoparticles, and crystalline nanoparticles. This allows for densification of the mixture of the silicon-crystalline carbon precursor and the amorphous carbon precursor by minimizing internal pores in the carbon matrix structure during a subsequent heat treatment process. Consequently, the electrochemical properties of the mixture of the silicon-carbon composite and the amorphous carbon precursor can be improved.

[0136] As a non-limiting example, the pressurizing pressure for the mixture of silicon-crystalline carbon precursor and amorphous carbon precursor may be less than 1 ton / cm2.

[0137] As a non-limiting example, after pressure molding, the mixture of the silicon-crystalline carbon precursor and the amorphous carbon precursor may be provided in the form of blocks, such as pellets. This may facilitate transport within the manufacturing process and minimize the surface area, thereby minimizing side reactions that oxidize the surface of the mixture of the silicon-crystalline carbon precursor and the amorphous carbon precursor.

[0138] The heat treatment step may be performed by heat treating a mixture of a silicon-crystalline carbon precursor and an amorphous carbon precursor at 700 to 1000°C in an inert atmosphere. More specifically, the mixture of a silicon-crystalline carbon precursor and an amorphous carbon precursor may be heat treated at 700 to 900°C in an inert atmosphere. As a result, amorphous carbon may be provided from the amorphous carbon precursor to the silicon-crystalline carbon precursor. At this time, the provided amorphous carbon may physically and electrically connect the silicon nanoparticles and the crystalline carbon, thereby obtaining a silicon-carbon composite having a structure in which the silicon nanoparticles are captured in a carbon matrix.

[0139] After heat treatment, the silicon-carbon composite may be dry milled and classified. By way of non-limiting example, the dry milling may be performed by any one of a JET mill, a pin mill, and a combination thereof.

[0140] According to exemplary embodiments, the surface roughness (Rq) of the silicon-carbon composite may be 4 to 100 nm. More specifically, the surface roughness (Rq) of the silicon-carbon composite may be 4 to 90 nm. The surface roughness (Rq) of the silicon-carbon composite may be 4 to 80 nm. The surface roughness (Rq) of the silicon-carbon composite may be 4 to 60 nm. The surface roughness (Rq) of the silicon-carbon composite may be 4 to 50 nm. The surface roughness (Rq) of the silicon-carbon composite may be 4 to 30 nm. The surface roughness (Rq) of the silicon-carbon composite may be 6 to 30 nm. The surface roughness (Rq) of the silicon-carbon composite may be 9 to 30 nm.

[0141] If the surface roughness (Rq) of the silicon-carbon composite is less than 4 nm, the coating layer may not be properly fixed to the surface of the silicon-carbon composite during the coating process described below. Therefore, a higher surface roughness (Rq) of the silicon-carbon composite may be advantageous for the efficiency of the coating process. However, if the surface roughness (Rq) of the silicon-carbon composite is excessively high, the coating layer may not be formed uniformly but may be formed thickly in localized areas. This may increase the resistance of the negative active material. Alternatively, an increase in the specific surface area of ​​the negative active material may cause a side reaction with the electrolyte during use of the secondary battery.

[0142]

[0143] The coating can be achieved by mixing a silicon-carbon composite and a coating layer precursor solution to form a slurry, and drying the slurry. This allows the negative active material according to exemplary embodiments to be obtained.

[0144] The coating layer precursor solution may include carbon nanotubes and a binder.

[0145] As a non-limiting example, the carbon nanotube can be any one of a thin wall CNT, a single wall CNT, a multi wall CNT, and a combination thereof.

[0146] According to exemplary embodiments, the carbon nanotubes may be included in an amount of 0.1 to 0.5 parts by weight based on 100 parts by weight of the silicon-carbon composite.

[0147] According to exemplary embodiments, the binder may include a shear-thinning aqueous binder. A shear-thinning binder is a non-Newtonian fluid whose viscosity decreases as the applied shear stress increases.

[0148] More specifically, the solids content of the coating precursor solution containing the silicon-carbon composite may range from 50% to 70%. This allows the coating precursor solution to experience sufficient shear stress during the coating process due to its relatively high viscosity. However, if the viscosity of the coating precursor solution remains excessively high during the silicon-carbon composite coating process, an uneven coating layer may be formed. In this case, the coating effect is reduced, and the weight-based capacity of the negative electrode active material may be reduced. Consequently, the performance of the secondary battery may be degraded. The solids content may include the silicon-carbon composite.

[0149] Aqueous binders can exhibit excellent bonding properties because they come into point contact with the silicon-carbon composite. This allows carbon nanotubes to be fixed to the silicon-carbon composite even in relatively small quantities.

[0150] Therefore, by using the binder according to the exemplary embodiment, the thickness of the coating layer can be formed thinly, and the coating efficiency of carbon nanotubes can be improved.

[0151] According to exemplary embodiments, the binder may include any one of carboxymethyl cellulose (CMC), hydroxypropyl cellulose (HPC), hydroxyethyl cellulose (HEC), polyvinylpyrrolidone (PVP), and polyethylene oxide (PEO), and combinations thereof.

[0152] According to exemplary embodiments, the binder may be included in an amount of 0.1 to 1 part by weight based on 100 parts by weight of the silicon-carbon composite. More specifically, the binder content may be included in an amount of about 1.5 to 3 times the content of the carbon nanotubes. By controlling the content of the binder included in the coating layer precursor solution within the above-described range, the resistance of the negative electrode active material can be prevented from increasing excessively.

[0153]

[0154] According to exemplary embodiments, drying of the slurry can be performed at relatively low temperatures.

[0155] Conventionally, a slurry was heat-treated at temperatures exceeding 800°C to form a coating layer on the surface of the negative electrode active material. However, this process resulted in thermal deformation of the coating layer precursor materials and the particles constituting the negative electrode active material, resulting in unexpected changes in physical, chemical, and electrical properties.

[0156] However, according to exemplary embodiments of the present invention, by controlling the composition of the coating layer precursor solution as described above, slurry drying can be performed even at relatively low temperatures. This prevents changes in the properties of the negative electrode active material due to high-temperature environments. Furthermore, energy required for high-temperature heating can be saved, and the method for manufacturing the negative electrode active material according to exemplary embodiments can be easily applied to large-scale mass production processes.

[0157] Additionally, by drying the slurry at a relatively low temperature, the binder contained in the coating layer precursor solution remains on the surface of the silicon-carbon composite. This further enhances the electrode conductivity and electrode bonding strength of the negative active material, as described above.

[0158] According to exemplary embodiments, drying of the slurry may be performed in an air atmosphere of 20 to 100°C. More specifically, drying of the slurry may be performed in an air atmosphere of 20 to 95°C. Drying of the slurry may be performed in an air atmosphere of 20 to 80°C. Drying of the slurry may be performed in an air atmosphere of 20 to 70°C. Drying of the slurry may be performed in an air atmosphere of 20 to 60°C. Drying of the slurry may be performed in an air atmosphere of 30 to 100°C.

[0159] If the drying temperature of the slurry is below 20°C, excessively long drying times may be required. If the drying temperature of the slurry exceeds 100°C, oxidation of the silicon, carbonization or cyclization of the binder may occur, which may change the electrical properties of the coating layer or the negative electrode active material.

[0160] Hereinafter, embodiments of the present invention will be described in detail so that those skilled in the art can easily implement them. However, the present invention may be implemented in various different forms and is not limited to the embodiments described herein.

[0161]

[0162] (Example 1)

[0163] Poly-Si was used as the silicon raw material. The silicon raw material was pulverized using a 99.9% pure anhydrous ethanol solution and beads. More specifically, the BPR of the silicon raw material to the beads was 5:1, and the rotation speed of the pulverizer was maintained at 2,500 rpm. The size of the pulverized silicon raw material was measured using a nano particle sizer (Beckmann Coulter), and a silicon nanoparticle slurry containing silicon nanoparticles with a D90 of 180 nm or less was obtained.

[0164] A silicon-crystalline carbon precursor was obtained by mixing flake graphite as crystalline carbon into a silicon nanoparticle slurry and spray-drying them. The silicon-crystalline carbon precursor and coal-based pitch were mixed using a milling process. The mixture of the silicon-crystalline carbon precursor and amorphous carbon was placed in a mold and uniaxially pressed at a pressure of approximately 50 tons. The molded body was then carbonized in an inert atmosphere at 700-1000°C, followed by pulverization and classification to obtain a silicon-carbon composite.

[0165] A silicon-carbon composite was mixed with a coating precursor solution, and a binder and carbon nanotubes were coated on the silicon-carbon composite. At this time, a coating precursor solution containing approximately 0.125 parts by weight of carbon nanotubes and approximately 0.375 parts by weight of binder based on 100 parts by weight of the silicon-carbon composite was used. Carboxymethyl cellulose was used as the binder. Single-walled carbon nanotubes were used as the carbon nanotubes. The coating process was performed using a twisted blade mixer, mixing at 25 rpm for 5 minutes, and then at 100 rpm for 30 minutes. Finally, the coating precursor solution mixed with the silicon-carbon composite was dried at 100°C in an air atmosphere to obtain a negative electrode active material.

[0166]

[0167] (Example 2)

[0168] A negative active material was manufactured in the same manner as in Example 1, except that a coating layer precursor solution containing 0.25 parts by weight of carbon nanotubes and 0.375 parts by weight of binder based on 100 parts by weight of silicon-carbon composite was used.

[0169]

[0170] (Example 3)

[0171] A negative electrode active material was manufactured in the same manner as in Example 1, except that a coating layer precursor solution containing 0.5 parts by weight of carbon nanotubes and 0.75 parts by weight of binder based on 100 parts by weight of silicon-carbon composite was used.

[0172]

[0173] (Comparative Example 1)

[0174] It was manufactured in the same manner as Example 1, except that coal tar was coated on the silicon-carbon composite and heat-treated in an inert atmosphere at 800°C or higher.

[0175]

[0176] (Comparative Example 2)

[0177] A silicon-carbon composite was used as the negative electrode active material without coating.

[0178]

[0179] Experimental Example 1: Confirming the coating level according to surface roughness

[0180] The surface roughness (Rq) of the silicon-carbon composite according to Example 2 was measured three times through AFM (Atomic Force Microscope) analysis and is shown in Table 1 below. In addition, as Comparative Example 3, a silicon-carbon composite having the surface roughness (Rq) shown in Table 1 below was prepared.

[0181] Silicon-carbon composite Rq (nm) Example 227.4 9.27 29.8 Comparative example 33.1 51.6 92.91

[0182] Thereafter, the silicon-carbon composite according to Example 2 and the silicon-carbon composite according to Comparative Example 3 were coated in the same manner as the coating conditions according to Example 2 described above.

[0183] After coating, the final sample was obtained through sieving, and the surfaces of the negative active material according to Example 2 and the negative active material according to Example 3 were observed using a scanning electron microscope (SEM).

[0184] Figure 1 is an SEM photograph of the surface of a negative electrode active material according to Example 2 and Comparative Example 3.

[0185] Referring to Fig. 1, it was confirmed that the surface of Comparative Example 3 was not properly coated with carbon nanotubes. This is believed to be because the surface roughness of Comparative Example 3 did not satisfy the range suggested by the present invention. In contrast, it was confirmed that the negative active material according to Example 2 had carbon nanotubes and residual binder distributed. This is believed to be because the surface roughness of the silicon-carbon composite according to Example 2 satisfies the range according to the exemplary embodiments of the present invention.

[0186]

[0187] Experimental Example 2: Confirming the Moisture Content of the Negative Active Material

[0188] The negative active material according to Example 2 and the negative active material according to Comparative Example 1 were dried in a drying oven at 120°C. Thereafter, the moisture content was calculated by comparing the mass difference before and after drying, and is shown in Table 2 below.

[0189] The function rate was calculated as follows.

[0190] [Relationship 1]

[0191] Moisture content = (Weight before drying - Weight after drying) / Weight before drying

[0192] Distinctive function rate (%)Average (%)Example 21.331.351.361.36Comparative example 10.550.550.530.56

[0193] Thus, Comparative Example 1, which was carbonized in a high-temperature atmosphere, confirmed that the moisture content was outside the range proposed by the present invention. That is, it is interpreted that the binder remaining on the surface of the silicon-carbon composite was not properly distributed in the negative active material according to Comparative Example 1. However, Example 2, which was dried at a low temperature, confirmed that it satisfied the moisture content range proposed by the present invention. That is, it is interpreted that the binder remaining on the surface of the silicon-carbon composite was properly distributed.

[0194]

[0195] Experimental Example 3: Confirming the Lifespan Characteristics of Secondary Batteries

[0196] A secondary battery was manufactured using the above-described Examples 1 to 3 and Comparative Examples 1 and 2.

[0197] More specifically, an anode slurry was prepared by mixing an active material, a cathode binder, and a conductive material. The active material used was an active material mixture containing the cathode active material according to Example 1 and commercial natural graphite. The anode binder used was carboxymethyl cellulose (CMC) and styrene-butadiene rubber (SBR). The prepared cathode slurry was coated on a copper thin film and dried to obtain an anode. Thereafter, a cathode, a separator, and a cathode were sequentially laminated, and the laminated electrodes were impregnated with an electrolyte to manufacture a CR2032 type secondary battery (coin half cell).

[0198] Coin half cells of the CR2032 type were manufactured using the negative active materials according to Example 2, Example 3, Comparative Example 1, and Comparative Example 2 using the above-described method.

[0199] Afterwards, charge-discharge tests were conducted on the manufactured secondary batteries in the operating voltage range of 0.005 V to 1.0 V. During charge-discharge, a current of 0.5 C was applied based on the first 1 C capacity, and the lifespan of 50 cycles was measured. At this time, the charge cut-off current was set to 0.005 C. The charge capacity and discharge capacity of the secondary batteries were measured for each cycle (charge-discharge) to verify the charge-discharge efficiency.

[0200] FIG. 2 is a graph showing the life characteristics of secondary batteries according to examples and comparative examples. Among the graphs in FIG. 2, Example 1 shows data for a secondary battery including an anode active material according to Example 1. Example 2 shows data for a secondary battery including an anode active material according to Example 2. Example 3 shows data for a secondary battery including an anode active material according to Example 3. Comparative Example 1 shows data for a secondary battery including an anode active material according to Comparative Example 1. Comparative Example 2 shows data for a secondary battery including an anode active material according to Comparative Example 2.

[0201] Referring to FIG. 2, it was confirmed that the secondary batteries including the negative electrode active materials according to Examples 1, 2, and 3 maintained the charge-discharge efficiency even when the charge-discharge cycle was repeated. That is, it was confirmed that the life characteristics of the secondary battery can be improved by using the negative electrode active material including a silicon-carbon composite and a coating layer including carbon nanotubes and a residual binder. In contrast, in the case of the secondary batteries including the negative electrode active materials according to Comparative Examples 1 and 2, it was confirmed that the charge-discharge capacity began to decline from the beginning of the cycle (less than 10 times) and the charge-discharge capacity could not be maintained. That is, it was confirmed that the life characteristics were inferior. In particular, Comparative Example 2, which was heat treated in a high-temperature environment, exhibited the poorest life characteristics.

[0202]

[0203] The present invention has been described in more detail through drawings and examples. However, the configurations described in the drawings or examples described in this specification are merely embodiments of the present invention and do not represent all of the technical ideas of the present invention. Therefore, it should be understood that various equivalents and modified examples may exist as of the time of this application.

Claims

1. Carbon matrix; A silicon-carbon composite for a negative electrode active material comprising silicon nanoparticles captured in the carbon matrix, The above silicon-carbon composite has a surface roughness (Rq) of 4 to 50 nm.

2. In paragraph 1, A silicon-carbon composite having a D90 of the above silicon nanoparticles of 180 nm or less.

3. In paragraph 1, The above silicon nanoparticles are a silicon-carbon composite containing 35 to 60 wt%.

4. In paragraph 1, The above carbon matrix is ​​a silicon-carbon composite containing 35 to 55 wt%.

5. In paragraph 1, The above carbon matrix is ​​a silicon-carbon composite including crystalline carbon and amorphous carbon.

6. In paragraph 5, A silicon-carbon composite having a content of crystalline carbon of 10 to 25 wt% and a content of amorphous carbon of 25 to 40 wt%.

7. In paragraph 5, The above crystalline carbon is a silicon-carbon composite including at least one of artificial graphite, flaky graphite, earthy graphite, expanded graphite, graphene, and combinations thereof.

8. In paragraph 5, A silicon-carbon composite having a D50 of 5 to 10 μm of the above crystalline carbon.

9. In paragraph 5, The above amorphous carbon is a silicon-carbon composite including any one of coal pitch, petroleum pitch, coal tar, polyacrylic acid (PAA), polyvinyl alcohol (PVA), and combinations thereof.

10. A silicon-carbon composite comprising silicon nanoparticles and a carbon matrix; and Comprising a coating layer distributed on the surface of the above silicon-carbon composite, The above coating layer is, A negative active material comprising carbon nanotubes and residual binder distributed on the surface of the silicon-carbon composite.

11. In paragraph 10, The above carbon nanotubes are a negative electrode active material contained in an amount of 0.1 to 0.5 parts by weight based on 100 parts by weight of the silicon-carbon composite.

12. In paragraph 10, The above residual binder is a negative electrode active material contained in an amount of 0.1 to 1 part by weight based on 100 parts by weight of the silicon-carbon composite.

13. In paragraph 10, The above residual binder, A negative active material comprising any one of carboxymethyl cellulose (CMC), hydroxypropyl cellulose (HPC), hydroxyethyl cellulose (HEC), polyvinylpyrrolidone (PVP), and polyethylene oxide (PEO), and combinations thereof.

14. In paragraph 10, A negative electrode active material having a moisture content of 0.6 to 2.0%.

15. A step of preparing silicon nanoparticles by crushing silicon raw materials; A step of mixing and drying the above silicon nanoparticles and crystalline carbon to obtain a silicon-crystalline carbon precursor; A step of obtaining a mixture by bonding the silicon-crystalline carbon precursor with an amorphous carbon precursor; A step of heat-treating the above mixture in an inert atmosphere at 700 to 1000°C to obtain a silicon-carbon composite; A step of obtaining a slurry by mixing a coating layer precursor solution containing carbon nanotubes and a binder and the silicon-carbon composite; and A method for manufacturing a negative electrode active material, comprising a drying step of drying the above slurry in an air atmosphere of 20 to 100°C.

16. In paragraph 15, In the step of obtaining the above silicon nanoparticles, The above silicon raw material is crushed through ball milling using beads. A method for manufacturing a negative active material, wherein the diameter of the above beads is less than twice the D99 of the above silicon raw material.

17. In paragraph 15, In the step of obtaining the above silicon nanoparticles, A method for manufacturing a negative electrode active material in which the above silicon raw material is ground by wet grinding using an organic solvent containing ethanol having a purity of 99.9%.

18. In paragraph 15, In the step of obtaining the above silicon nanoparticles, A method for manufacturing a negative electrode active material wherein the D90 of the above silicon nanoparticles is 180 nm or less.

19. In Article 15, A method for manufacturing a negative electrode active material having a D50 of 5 to 10 μm of the above crystalline carbon.

20. In paragraph 15, A method for manufacturing a negative electrode active material having a surface roughness (Rq) of 4 to 50 nm of the above silicon-carbon composite.

21. In paragraph 15, A method for manufacturing the above negative active material is as follows: A method for manufacturing a negative electrode active material, further comprising a pressurizing molding step of pressurizing a mixture of the silicon-crystalline carbon precursor and the amorphous carbon precursor.

22. In paragraph 15, The above binder, A method for producing a negative active material comprising any one of carboxymethyl cellulose (CMC), hydroxypropyl cellulose (HPC), hydroxyethyl cellulose (HEC), polyvinylpyrrolidone (PVP), and polyethylene oxide (PEO), and combinations thereof.

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