Negative electrode active material for secondary batteries, method for manufacturing the same, negative electrode for secondary batteries, and lithium secondary battery containing the same

Abstract

To provide a negative electrode active material having improved capacity characteristics by suppressing the production of silicon carbide (SiC), which is an inert material.SOLUTION: One embodiment of the present invention provides a negative electrode active material for a secondary battery, the material comprising: a silicon-carbon composite that includes a carbon-based material and a first silicon-based nano-coating layer formed on the carbon-based material; silicon carbide; silicon nitride; and second silicon-based particles.SELECTED DRAWING: Figure 1a

Description

Technical Field

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

Background Art

[0002] Recently, as the demand for electronic devices such as mobile devices has increased, the technological development of mobile devices has expanded. As a power source for driving such electronic devices, the demand for lithium secondary batteries such as lithium batteries, lithium ion batteries, and lithium ion polymer batteries has increased significantly. In addition, as global regulations related to automotive fuel efficiency and exhaust gas are being strengthened, the growth of the electric vehicle market is accelerating, and along with this, the demand for medium and large secondary batteries such as secondary batteries for electric vehicles (EVs) and secondary batteries for energy storage devices (ESSs) is expected to soar.

[0003] Recently, as the capacity of secondary batteries has been increasingly required, especially for medium and large secondary batteries, silicon-based negative electrode materials having excellent theoretical capacity as negative electrode materials for secondary batteries have been studied. However, silicon-based negative electrode materials cause large volume changes during the insertion / desorption of lithium, and there are problems such as electrical desorption due to pulverization of silicon particles and loss of reversible capacity under repeated cycles.

[0004] In order to solve the problems of mechanical damage to the electrode due to the volume expansion and contraction of silicon and the resulting rapid shortening of the life, performance improvement through a method of composite with a different material that can buffer the volume expansion of silicon and a method of producing silicon into nanoparticles has been pursued. In particular, the technical aspect for the composite process of silicon (nano) particles and graphite and the commercialization aspect of reducing the production cost of silicon nanoparticles have been actively developed.

[0005] This invention relates to a method for manufacturing a composite anode material composed of nano-sized silicon and carbon-based materials in a single step, and provides a silicon-carbon composite manufacturing method that forms silicon nanoparticles on the surface of carbon using microwaves. Silicon has a melting point of 1,414°C, and when melted, it exhibits a phenomenon in which its density increases even further than when it is solid, like water. Graphite (a carbon-based material) exhibits the characteristic of rapidly generating heat when the π electrons present in its structure are alternately induced to negative (-) and positive (+) using microwaves (wavelength 1m to 1mm; frequency 300MHz to 300GHz). By applying this phenomenon, silicon, which has a lower melting point than graphite, can be melted on the surface of graphite, and the entropy is rapidly increased by the rapid energy transfer, making it possible to manufacture silicon nanomaterials (graphite composited with nanosilicon) on the surface of graphite with an increased specific surface area.

[0006] The microwave-based silicon-carbon composite manufacturing method directly transfers energy to the carbon surface. Regardless of the medium supporting the carbon material, when microwaves are irradiated, only the temperature of the carbon itself rises rapidly, and when microwave irradiation is stopped, rapid cooling occurs. Compared to conventional heating methods, this method has the advantage of reducing manufacturing time and cost in terms of both time and energy.

[0007] However, since energy is transferred only to the area irradiated by microwaves, there is a problem in that silicon nanoparticles are not uniformly formed on the carbon (graphite) surface, or the impregnated silicon particles become coarser / thinner. In this case, it is difficult to adequately buffer the volume expansion of the silicon particles during lithium insertion / deinsertion.

[0008] Furthermore, on the surface of carbon (graphite) rapidly heated to around 1,400°C by microwaves, silicon carbide (SiC), which is synthesized at temperatures above 900°C, is produced as a byproduct. Such silicon carbide is inert to lithium ions, hindering the diffusion of lithium ions into silicon particles, leading to problems such as reduced capacity and decreased efficiency. [Prior art documents] [Patent Documents]

[0009] [Patent Document 1] Korean Patent Registration KR 2133961 B1 [Patent Document 2] China Patent Publication CN 113732013 A [Patent Document 3] Korean Patent Registration KR 10-1772113 B1 [Patent Document 4] Korean Patent Registration KR 10-1685776 B1 [Overview of the project] [Problems that the invention aims to solve]

[0010] The object of the present invention is to provide a method for manufacturing a silicon-carbon composite, which combines a composite anode material (silicon-carbon composite) and nanosilicon particles, which are nanosized silicon and carbon-based materials, through a microwave firing process, and a process for compounding silicon and carbon-based materials, into a single step. Furthermore, the present invention aims to provide a negative electrode active material with improved capacitance characteristics by suppressing the formation of silicon carbide (SiC), which is an inert substance.

[0011] Furthermore, the present invention aims to control the production ratio of silicon carbide and silicon nitride, which are reaction byproducts, within a specific range, thereby reducing the proportion of unreacted silicon.

[0012] Furthermore, the present invention provides a method for producing a negative electrode active material that can suppress the generation of reaction by-products and increase the reaction rate of silicon raw materials by controlling the gas atmosphere, reaction time, and average particle size of the raw materials within a specific range during the microwave firing process. [Means for solving the problem]

[0013] One embodiment of the present invention provides a negative electrode active material for a secondary battery, characterized by comprising: a silicon-carbon composite including a carbon-based material and a first silicon-based nanocoating layer formed on the carbon-based material; silicon carbide; silicon nitride; and second silicon-based particles.

[0014] The silicon carbide (SiC) and silicon nitride (SiNx) may be included in the silicon-based nanocoating layer or mixed with the silicon carbon composite.

[0015] The negative electrode active material may contain silicon carbide in an amount of 1 to 30% by weight relative to the total amount of silicon-containing compounds.

[0016] The negative electrode active material may contain 10 to 40% by weight of the silicon nitride relative to the total amount of the silicon-containing compound.

[0017] The weight ratio of silicon nitride (SiNx) to silicon carbide (SiC) (SiNx / SiC) may be 0.4 to 2.

[0018] The second silicon particles consist of silicon particles with an average particle size (D50) of 0.4 to 10 μm, and may be present in an amount of less than 40% by weight relative to the total amount of the silicon-containing compound.

[0019] The first silicon-based nanocoating layer may comprise at least one selected from: a silicon thin film with a thickness of 10 to 100 nm formed on the surface of the carbon-based material; and silicon particles with a particle size (D50) of 10 to 100 nm.

[0020] The carbon-based material may be at least one selected from the group consisting of natural graphite, artificial graphite, expanded graphite, graphene oxide, carbon nanotubes, carbon fibers, and hard carbon.

[0021] The negative electrode active material may contain the carbon-based material and the entire compound containing the silicon element in a weight ratio of 5:5 to 9.5:0.5.

[0022] Another embodiment of the present invention provides a method for manufacturing a negative electrode active material for a secondary battery, including a step of mixing a carbon-based material and a silicon-based material, and a firing step of irradiating the mixture with microwaves in a gas atmosphere exceeding N2 50% by volume.

[0023] In the mixing step, the silicon-based material may be silicon particles with an average particle size (D50) of 0.4 to 10 μm.

[0024] In the mixing step, the carbon-based material may be carbon-based particles with an average particle size (D50) of 5 to 30 μm.

[0025] The firing step may be performed in a gas atmosphere of N2 80 to 100% by volume. The firing step may maintain the exothermic temperature of the mixture at 1100 to 1400 °C and irradiate microwaves at an output of 1 to 3 kW for 10 seconds to 10 minutes. The manufacturing method may further include a step of cooling the fired product to -10 to 10 °C.

[0026] Another embodiment of the present invention provides a negative electrode for a secondary battery including the negative electrode active material, and a secondary battery including the negative electrode, a positive electrode, and an electrolyte.

Effects of the Invention

[0027] According to the present invention, when irradiating with microwaves, the carbon-based material is heated in a short time to melt silicon, and a negative electrode active material including silicon nanoparticles and thin films formed on the surface of the carbon-based material during the cooling process can be provided.

[0028] Furthermore, according to the present invention, specifically, by promoting microwave carbon thermal shock in an N2 atmosphere, the formation of silicon oxide, which lowers the surface exothermic temperature of the raw material to around 900°C, can be suppressed, and the surface exothermic temperature of the raw material can be raised to around the silicon melting point of 1200°C in a short time, thereby reducing unreacted Si or increasing the reaction rate of the Si raw material. In addition, the formation of SiCx is relatively suppressed by SiNx generated in an N2 atmosphere, and SiNx has the effect of improving lithium-ion electrical conductivity. [Brief explanation of the drawing]

[0029] [Figure 1a] This is a surface SEM image of the negative electrode active material produced in Example 1. [Figure 1b] This is a surface SEM image of the negative electrode active material produced in Example 2. [Figure 1c] This is a surface SEM image of the negative electrode active material produced in Example 3. [Figure 2] These are the XRD analysis results of the negative electrode active materials produced in Examples 1-3. [Figure 3a] This shows the SEM-EDS analysis results for the negative electrode active material produced in Comparative Example 1. [Figure 3b] This shows the SEM-EDS analysis results for the negative electrode active material produced in Example 1. [Modes for carrying out the invention]

[0030] The advantages and features of the present invention, and the methods for achieving them, will become clearer with reference to the embodiments described below in detail, along with the accompanying drawings. However, the present invention is not limited to the embodiments disclosed below, but can be embodied in a variety of different forms, and these embodiments are provided only to complete the disclosure of the present invention and to fully inform those who are ordinary skill in the art to which the present invention pertains, and the present invention is defined only by the scope of the claims.

[0031] Unless otherwise defined, all terms used herein (including technical and scientific terms) may be used in a way that is commonly understood by a person of ordinary skill in the art to which the invention pertains. Wherever any part of the specification “includes” a certain component, this means, unless otherwise stated, that it does not exclude other components, but rather that it may include other components. Furthermore, unless otherwise specified in the statement, singular forms also include plural forms.

[0032] One embodiment of the present invention provides a negative electrode active material comprising a silicon-carbon (Si / C) composite in which a silicon-based nanocoating layer is formed on a carbon-based material. The negative electrode active material is characterized by comprising a silicon-carbon composite including a carbon-based material and a first silicon-based nanocoating layer formed on the carbon-based material; silicon carbide; silicon nitride; and second silicon-based particles.

[0033] The first silicon-based nanocoating layer can improve the problem of mechanical damage to electrodes caused by the volume expansion and contraction of silicon, and the resulting rapid shortening of lifespan, by being compounded with a carbon-based material that can buffer the volume expansion of silicon.

[0034] Furthermore, silicon carbides can reduce cell capacity as a by-product that does not contribute to capacity, while silicon nitrides can improve lithium ion conductivity during cell charging and discharging, thereby increasing cell stability.

[0035] The second silicon-based particles, in this invention, refer to a substance in which the silicon raw material remains as unreacted Si in its raw material size (400 nm to 10 μm) even after the microwave firing process, without being nano-sized. Such unreacted Si can cause a change in the volume of the negative electrode because its volume expansion during lithium insertion / deinsertion is not sufficiently buffered.

[0036] The silicon carbide (SiC) and silicon nitride (SiNx) may be included in the first silicon-based nanocoating layer or mixed with the silicon carbon composite.

[0037] When the SiC and SiNx are included in the first silicon-based nanocoating layer, they can be mixed with silicon nanoparticles and silicon thin films in particle form to form a coating layer, or the coating layer can be formed on at least a portion of the silicon nanoparticles or silicon thin films.

[0038] The silicon carbide may be present in an amount of 1 to 30% by weight relative to the total amount of the silicon element compound, specifically in amounts of 1 to 25% by weight, 1 to 20% by weight, 5 to 20% by weight, or 5 to 15% by weight.

[0039] On the surface of a carbon-based material rapidly heated to around 1200°C by microwaves, silicon carbide (SiC), which is synthesized at high temperatures of 900°C or higher and is inert to lithium ions, may be impregnated. If the silicon carbide content exceeds the aforementioned design range, as the reaction proceeds at a temperature lower than the melting point of the Si raw material while lowering the surface exothermic temperature of the graphite raw material during the microwave calcination reaction, the Si reaction rate decreases, and SiC production can be increased. However, the generated SiC has the problem of reducing cell capacity as a by-product that does not contribute to capacity.

[0040] The silicon-containing compound is a silicon-containing compound included in the negative electrode active material produced by microwave calcination reaction, and includes the silicon nanocoating layer, unreacted silicon, silicon carbide, and silicon nitride.

[0041] The silicon nitride may be present in an amount of 10 to 40% by weight relative to the total amount of the silicon-containing compound, specifically in an amount of 10 to 35% by weight, 10 to 30% by weight, or 12 to 30% by weight.

[0042] When the silicon nitride content is within the aforementioned design range, the capacity is partially reduced, but an irreversible phase (Li2SiN2) with high lithium-ion conductivity is formed during charging and discharging, increasing lifespan stability.

[0043] The weight ratio of silicon nitride (SiNx) to silicon carbide (SiC) (SiNx / SiC) may be between 0.4 and 2.5, specifically between 0.4 and 2, 0.5 and 2, 0.5 and 1.5, or 0.5 and 1 by weight (SiNx / SiC). If the weight ratio (SiNx / SiC) exceeds the design range, the proportion of SiNx increases, and the total amount of SiNx and SiC produced tends to increase rapidly, which can lead to a deterioration in capacitance characteristics. Conversely, if the SiNx / SiC ratio is below the design range, the total amount of SiNx and SiC produced decreases, but the amount of unreacted Si (secondary silicon particles) increases, which can lead to a lower overall Si reaction rate (Si nano-atomization).

[0044] The second silicon particles may consist of silicon particles with an average particle size (D50) of 0.4 to 10 μm.

[0045] The second silicon particles may contain unreacted Si as particles remaining in a raw material size of 0.4 to 10 μm after the microwave firing process, without being nano-sized from the silicon raw material used in the mixing process. Since the second silicon particles are not included in the first silicon nanocoating layer, they exist in a simply mixed state with the silicon-carbon composite and can cause a volume change of the negative electrode during cell operation.

[0046] The second silicon particles may be present in an amount of 40% by weight or less relative to the total amount of the silicon element compound, for example, 5 to 40% by weight, 5 to 30% by weight, or 5 to 20% by weight.

[0047] If the content of the second silicon particles exceeds the upper limit, the proportion of unreacted Si increases, making it difficult to buffer the volume expansion of silicon during cell operation, which can lead to a decrease in the volume change of the negative electrode and a decline in cell life characteristics. Conversely, if it is below the lower limit, the specific gravity of graphite may become excessively high, or by-products such as SiN and SiC may increase, which can lead to a decline in capacity characteristics.

[0048] Furthermore, the second silicon particles may be unreacted silicon particles with an average particle size (D50) of 0.4 to 10 μm, for example, unreacted silicon particles with an average particle size (D50) of 0.4 to 5 μm, 0.4 to 3 μm, 0.4 to 2 μm, 0.4 to 1 μm, 0.4 to 0.9 μm, 0.4 to 0.8 μm, 0.4 to 0.7 μm, 0.4 to 0.6 μm, or 0.4 to 0.5 μm.

[0049] The unreacted Si (secondary silicon) particles have an average particle size (D50) that falls within the range of the average particle size (D50) of the Si raw material. Therefore, a smaller average particle size (D50) of the secondary silicon particles is advantageous for increasing the Si reaction rate, but it can also promote the formation of SiC and SiN. Conversely, a larger average particle size may decrease the Si reaction rate.

[0050] The carbon-based material may be at least one selected from the group consisting of natural graphite, artificial graphite, expanded graphite, graphene oxide, carbon nanotubes, carbon fibers, and hard carbon, and is preferably, for example, natural graphite or artificial graphite. The carbon-based material may also be a single particle or a secondary particle formed by the aggregation of multiple particles.

[0051] Furthermore, the carbon-based material may be carbon-based particles with an average particle size (D50) of 5 to 30 μm, carbon-based particles with an average particle size (D10) of 5 to 20 μm, 3 to 15 μm, or 5 to 15 μm, and / or carbon-based particles with an average particle size (D90) of 15 to 35 μm, 15 to 30 μm, or 20 to 25 μm.

[0052] Furthermore, the carbon-based material may be spherical, plate-shaped, or needle-shaped particles. By adjusting the material structure (porous, hollow, core-shell, multi-layer, and hierarchical), such as by using plate-shaped or needle-shaped particles, the microwave propagation path can be expanded, further improving the absorption capacity of the microwave absorber.

[0053] Furthermore, the conditions for controlling the heating of carbon-based materials by passing microwaves include the type of carbon material, its content, and particle shape. When the type of carbon-based material, average particle size, and morphology fall within the scope of the present invention, the microwave absorption capability can be improved.

[0054] The first silicon-based nanocoating layer may include at least one selected from a silicon thin film with a thickness of 10 to 100 nm and silicon particles with a particle size (D50) of 10 to 100 nm, which are formed on the surface of the carbon-based material.

[0055] It is preferable to increase the proportion of molten silicon impregnating the surface of the carbon-based particles in the form of nanoparticles, thereby preventing coarsening of the silicon particles and bonding between particles (silicon thin film). If the impregnated silicon particles coarseen / become thin films, it may be difficult to adequately buffer the volume expansion of the silicon particles during lithium insertion / desorption. Therefore, it is preferable that the first silicon-based nanocoating layer contains silicon particles with a particle size (D50) of 10 to 100 nm in an even higher content (by weight or volume) than a silicon thin film with a thickness of 10 to 100 nm.

[0056] The silicon (Si) particles may have an average particle size (D50) of 10 to 100 nm or 20 to 100 nm, and are preferably spherical.

[0057] Furthermore, the size (average particle size) of the silicon nanoparticles, second silicon-based particles, carbon-based materials, etc., may refer to the size (D50) measured by cumulative volume, respectively, for silicon (Si) particles. Unless otherwise defined herein, D50 refers to the average size, which is the diameter of pores where the cumulative volume in the particle size distribution is 50% by volume. The average size (D50) can be measured by methods widely known to those skilled in the art, such as by transmission electron microscope (SEM) or scanning electron microscope (SEM) imaging. Alternatively, it can be measured using a dynamic light-scattering device, and after performing data analysis to count the number of pores for each pore size range, the average size (D50) value can be calculated from this count.

[0058] The first silicon-based nanocoating layer may be present in an amount of 1 to 50% by weight relative to the total weight of the silicon-carbon composite, for example, in amounts of 1 to 40% by weight, 1 to 30% by weight, 5 to 30% by weight, or 5 to 20% by weight. The first silicon-based nanocoating layer may also be formed with a thickness of 10 to 200 nm, for example, in thicknesses of 10 to 180 nm, 10 to 160 nm, 10 to 150 nm, or 10 to 130 nm.

[0059] The silicon-carbon composite may further include a carbon coating layer located on the first silicon-based nanocoating layer. This minimizes direct exposure of the silicon in the first silicon-based nanocoating layer to the electrolyte, thereby reducing electrolyte side reactions, further buffering the volume expansion of silicon particles, and improving the battery's lifespan characteristics. The carbon coating layer may be formed with a thickness of 0.01 μm or more, for example, with a thickness of 0.03 μm or more, or 0.05 μm or more and 10 μm or less, 5 μm or less, or 1 μm or less. This further improves the aforementioned effects.

[0060] The following provides a method for producing a negative electrode active material for secondary batteries. The production method may be a method for producing a negative electrode active material according to one embodiment of the present invention.

[0061] The method for producing the negative electrode active material for the secondary battery includes a step of mixing a carbon-based material and a silicon-based material, and a firing step of irradiating the mixture with microwaves in a gaseous atmosphere with more than 250% by volume of N2.

[0062] The mixing step involves mixing a carbon-based material with a silicon-based material to obtain a mixture in which silicon-based particles are dispersed on the surface of the carbon-based material.

[0063] The mixing can be performed using a particle mixer at 500-2000 rpm or 700-1500 rpm for 1-10 minutes. The particle mixer can be a rotating agitator, a revolving agitator, a blade mixer, or a particle fusion machine, but is not limited to these.

[0064] Furthermore, the mixture can consist of carbon-based material and silicon-based particles in a weight ratio of 99:1 to 60:40, 99:1 to 70:30, 95:5 to 70:30, or 95:5 to 80:20.

[0065] The carbon-based material may be natural graphite, artificial graphite, expanded graphite, graphene oxide, carbon nanotubes, carbon fibers, hard carbon, or a combination thereof. The carbon-based material may also be carbon-based particles with an average particle size (D50) of 5 to 30 μm, or carbon-based particles with an average particle size of 5 to 20 μm or 10 to 20 μm.

[0066] The silicon-based material may be silicon particles with an average particle size (D50) of 0.4 to 10 μm, for example, with an average particle size (D50) of 0.4 to 5 μm, 0.4 to 3 μm, 0.4 to 2 μm, 0.4 to 1 μm, 0.4 to 0.9 μm, 0.4 to 0.8 μm, 0.4 to 0.7 μm, 0.4 to 0.6 μm, or 0.4 to 0.5 μm. A smaller average particle size (D50) of the Si raw material is advantageous for increasing the Si reaction rate, but it can promote the formation of SiC and SiN. Conversely, a larger average particle size may decrease the Si reaction rate.

[0067] The microwave firing process involves rapidly heating the surface of the carbon-based material in a mixture of carbon-based (raw material) material and silicon-based (raw material) material with microwaves, causing the silicon-based material to melt due to the microwave-heated carbon-based material, and forming a first silicon-based nanocoating layer on the surface of the carbon-based material.

[0068] The microwave firing process is carried out in a gaseous atmosphere with more than 250% by volume of N, for example, in a gaseous atmosphere with more than 290% by volume of N or a gaseous atmosphere with 2100% by volume of N.

[0069] By carrying out microwave firing in an N2 gas atmosphere, oxidation of silicon-based and carbon-based materials is prevented, and the surface exothermic temperature of the carbon-based material can be rapidly raised to the melting point level of silicon. This increases the Si reaction rate, promotes the generation of SiNx compared to SiC byproducts, improves capacity degradation, and enhances lithium-ion electrical conductivity.

[0070] Furthermore, N2 gas decomposes relatively easily upon microwave irradiation, readily producing SiNx compounds. On the other hand, using an inert gas such as Ar may reduce the amount of SiN and SiC produced and increase the amount of pure Si produced, as the gas does not contain oxygen and nitrogen. In this case, the contraction and expansion of Si particles become greater, which is unfavorable for the stability and lifetime characteristics of the cathode.

[0071] The microwave heating may involve irradiating with microwaves (wavelength 1m to 1mm; frequency 300MHz to 300GHz) at an output of 1 to 3kW for 10 seconds to 10 minutes, 30 seconds to 2 minutes, or 30 seconds to 60 seconds.

[0072] While increasing microwave power and irradiation time increases the Si reaction rate and SiNx production, which is advantageous for improving lifetime, it can actually decrease the silicon active material content and degrade the capacitance characteristics.

[0073] Furthermore, the microwave heating may be such that the surface of the carbon-based material is heated to 1200°C or higher within 1 minute, or for example, within 40 seconds or 10 seconds.

[0074] Therefore, when the silicon-based raw material reaches its thawing (melting) temperature, the interface temperature between the carbon-based material and Si reaches above the Si melting point, increasing the Si reaction rate, which in turn promotes SiNx formation.

[0075] The microwave firing step may further include a cooling step to prevent the crystallinity of the silicon-based material impregnated on the surface from increasing due to residual thermal energy in the carbon-based material. The cooling step can be carried out at room temperature or -10 to 10°C and under atmospheric pressure conditions, and preferably in a cooled reactor into which nitrogen gas is injected.

[0076] Another embodiment of the present invention provides a negative electrode for a secondary battery and a secondary battery, both containing the negative electrode active material. The negative electrode includes a negative electrode current collector and a negative electrode active material layer located on the negative electrode current collector, and the negative electrode active material according to one aspect of the present invention is present in the negative electrode active material layer.

[0077] The negative electrode current collector is not particularly limited as long as it is conductive without inducing chemical changes in the battery. For example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel surfaces treated with carbon, nickel, titanium, silver, etc., can be used. The negative electrode current collector can also typically have a thickness of 3 to 500 μm, and fine irregularities can be formed on the surface of the current collector to enhance the adhesion of the negative electrode active material. Such negative electrode current collectors can be provided in various forms such as films, sheets, foils, nets, porous materials, foams, and nonwoven fabrics.

[0078] The negative electrode active material layer may also be a layer containing a conductive material and a binder along with the aforementioned negative electrode active material.

[0079] The conductive material is used to impart conductivity to the electrode and can be used without special limitations as long as it is conductive without causing a chemical change in the negative electrode active material. Non-limiting examples of conductive materials include graphite such as natural graphite and artificial graphite; carbon-based materials such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal black, and carbon fiber; metal powders or metal fibers such as copper, nickel, aluminum, and silver; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and conductive polymers such as polyphenylene derivatives. The conductive material may usually be included in an amount of 1% to 30% by weight based on the total weight of the negative electrode active material layer.

[0080] The binder is a substance that improves adhesion between negative electrode active material particles and adhesion between the negative electrode active material and the current collector. Non-limiting examples of binders include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated-EPDM, styrene-butadiene rubber (SBR), fluororubber, or various copolymers thereof. The binder may typically be present in an amount of 1% to 30% by weight based on the total weight of the negative electrode active material layer.

[0081] A negative electrode according to one embodiment of the present invention can be manufactured by a conventional method for manufacturing a negative electrode for a secondary battery, except that the negative electrode active material described above is used. For example, a negative electrode can be manufactured by selectively coating a slurry for forming a negative electrode active material layer, which includes a negative electrode active material, a binder, and a conductive material, onto a negative electrode current collector, followed by drying and rolling. In another example, a negative electrode can be manufactured by casting the slurry for forming a negative electrode active material layer onto a separate support, peeling the negative electrode active material layer from the support, and then laminating the resulting film onto a negative electrode current collector.

[0082] According to yet another aspect of the present invention, an electrochemical element including the aforementioned negative electrode is provided. Here, the electrochemical element may be a battery, a capacitor, or more specifically, a lithium secondary battery.

[0083] A secondary battery includes a negative electrode, a positive electrode positioned opposite the negative electrode, a separator interposed between the negative and positive electrodes, and an electrolyte. A lithium secondary battery may also include a battery container (case) that houses the electrode assembly including the positive electrode, negative electrode, and separator, and a sealing member that seals the battery container.

[0084] In this context, lithium secondary batteries can be classified into two types based on the shape of the battery container (case): can-type lithium secondary batteries in which the electrode assembly is housed in a metal can, and pouch-type lithium secondary batteries in which the electrode assembly is housed in a pouch made of a sheet such as aluminum laminate. The present invention will be described in detail below based on examples, but these are for the purpose of explaining the present invention in more detail, and the scope of the rights of the present invention is not limited by the following examples.

[0085] Examples (Example 1) Natural graphite and plate-shaped Si particle (D50: 400 nm) powder were placed in a super mixer in a weight ratio of 9:1 and mixed at 1000 rpm for 10 minutes. The mixture was irradiated with microwaves (2450 MHz, 1 kW output, irradiation time: 40 seconds) while maintaining the surface temperature at 1200°C in an N21 atmosphere. Next, a negative electrode active material containing a silicon-carbon composite with a silicon nanocoating layer formed on natural graphite was fabricated by cooling at -5°C / min.

[0086] A negative electrode slurry was prepared by dispersing 30 g of 96 wt% silicon-carbon composite, 1 wt% carbon black, 1.5 wt% SBR binder, and 1.5 wt% carboxymethylcellulose in distilled water. The prepared negative electrode slurry was uniformly coated onto a Cu thin film and vacuum-dried at 135°C to fabricate a negative electrode for a lithium secondary battery.

[0087] LiRing 0.9 Co 0.1 Mn 0.1 A cathode slurry was prepared by mixing 94 wt% O2 (NCM911, EcoPro BM), 3 wt% carbon black, and 3 wt% PVdF binder with N-methyl-2-pyrrolidone. The prepared cathode slurry was uniformly coated onto an Al thin film, vacuum-dried at 135°C, and then rolled to produce a cathode for a lithium secondary battery.

[0088] The manufactured positive and negative electrodes and a porous polyethylene membrane (Celgard 2300, thickness: 25 μm) separation membrane were placed in a battery container, and a lithium secondary battery was manufactured using an electrolyte solution containing EC:EMC (ethylene carbonate:ethyl methyl carbonate) mixed in a volume ratio of 3:7 and LiPF6 at a concentration of 1.15 M.

[0089] (Examples 2 and 3) Except for using plate-shaped Si particle powder with D50:2μm (Example 2) and D50:10μm (Example 3) as shown in Table 3 below, the negative electrode active material, negative electrode, and lithium secondary battery were manufactured in the same manner as in Example 1.

[0090] (Example 4 and Comparative Example 1) Except for performing microwave firing in the gaseous atmosphere described in Table 2 below, the negative electrode active material, negative electrode, and lithium secondary battery were manufactured in the same manner as in Example 1.

[0091] (Examples 5-8) Except for performing microwave firing for the microwave (MW) time (s) listed in Table 4 below, the negative electrode active material, negative electrode, and lithium secondary battery were manufactured in the same manner as in Example 1.

[0092] Experimental example Experimental Example 1: Confirmation of particle morphology and composition of negative electrode active material by SEM imaging and XRD analysis Figures 1a to 1c are surface SEM images of the negative electrode active materials produced in Example 1, Example 2, and Example 3, respectively. Referring to Figures 1a to 1c, we were able to confirm silicon-carbon composites in which a silicon nanocoating layer with a mixture of thin film and particle forms was formed on the surface of natural graphite, as well as unreacted Si that was not impregnated into the graphite surface.

[0093] Figure 2 shows the XRD analysis results of the negative electrode active material produced in Example 1. XRD analysis was performed using a Bruker D8 ADVANCE A25, with a voltage of 45kV and a current of 30mA applied. From the analysis results in Figure 2, peaks corresponding to all material compositions, including Si, SiC, Si3N4, and graphite crystalline phases, were identified.

[0094] Experimental Example 2: Analysis of the surface exothermic temperature of raw materials depending on the type of atmospheric gas used in the firing process. Table 1 shows the surface exothermic temperatures of the natural graphite raw materials measured during the microwave calcination process in Example 1 and Comparative Example 1. The exothermic temperature is the temperature measured in real time on the sample surface at the point where microwave irradiation time reached 40 seconds. [Table 1]

[0095] When microwave carbon thermal shock is carried out in an oxygen-containing air atmosphere as in Comparative Example 1, it is analyzed that SiC is formed through the following reaction equation in the process of increasing the reaction rate (nano-Si formation) of the Si raw material by adjusting the microwave processing time / power, etc. SiO2(s)+C(s)→SiO(g)+CO(g)……(1) SiO(g)+2C(s)→SiC(s)+CO(g)……(2) SiO(g)+3CO(g)→SiC(s)+2CO2(g)……(3)

[0096] Referring to reaction equation (1), silicon oxide and graphite react at a high temperature of approximately 900°C or higher to form SiO and CO gas. Referring to reaction equation (2), the SiO gas reacts with graphite to generate SiC nuclei. Referring to reaction equation (3), the SiO gas formed in reactions (1) and (2) reacts with the CO gas to grow SiC crystals.

[0097] The silicon oxide formed in Comparative Example 1 has a low microwave absorption rate, which lowers the surface exothermic temperature of the natural graphite and silicon raw materials, and acts as a factor that increases unreacted Si. In addition, the silicon carbide produced from the silicon oxide reduces the cell capacity as a by-product that does not contribute to the capacity.

[0098] On the other hand, when microwave carbon thermal shock is carried out in the N2 atmosphere of Example 1, the formation of silicon oxide is suppressed, and the surface heating temperature of the raw material can be raised to around the silicon melting point of 1200°C in a short time, thereby reducing unreacted Si. In addition, silicon nitride produced in the N2 atmosphere forms an irreversible phase (Li2SiN2) with high lithium ion conductivity during cell charging and discharging, which has the effect of increasing lifespan stability.

[0099] Experimental Example 3. Analysis of the composition of negative electrode active material particles: SiN, SiC, and Si content and Si reaction rate. (Method for evaluating SiN, SiC, and Si content) The SiN content is expressed as a value (wt%) obtained by converting the mass ratio of N measured through ONH analysis to Si3N4, while the SiC and Si content is expressed as a value (wt%) obtained by converting the XRD peak area (XRD peak deconvolution) to mass ratio. The results are summarized in Tables 2 to 4 below.

[0100] (4. Method for evaluating response rates) Unreacted Si refers to the silicon raw material used in the mixing process that remains in a raw material size of 400 nm to 10 μm even after the microwave firing process without being nano-sized. Nano Si refers to silicon that has reacted with the silicon raw material used in the mixing process and deposited on the carbon surface in spherical and thin film forms of 50 to 100 nm size.

[0101] SEM-EDS analysis was performed on the negative electrode active materials produced in Comparative Example 1 and Example 1, and the results are shown in Figures 3a and 3b. The silicon particle size distribution before and after the microwave firing process was analyzed in Examples 1-8 and Comparative Example 1. The increase or decrease in the area of ​​the particle size peak corresponding to the unreacted Si size (D50 = 400 nm to 10 μm) was calculated, and the Si reaction rate (%) was determined. The results are summarized in Tables 2-4 below.

[0102] [Table 2] Referring to Table 2, the SiNx content can be adjusted by adjusting the partial pressure of the N2 gas. Comparing Examples 1 and 4, it was confirmed that the higher the partial pressure of nitrogen in the atmospheric gas, the higher the proportion of Si3N4 content and the higher the Si reaction rate.

[0103] [Table 3] Referring to Table 3, it was found that the Si3N4 / SiC ratio is maintained regardless of the Si raw material size, and it was confirmed that the Si reaction rate decreases as the Si raw material size increases.

[0104] [Table 4]

[0105] We were able to confirm that SiC formation was suppressed and the Si reaction rate increased with increasing microwave irradiation time. As mentioned above, these results indicate that when silicon oxide formation is suppressed, the surface exothermic temperature of natural graphite can be raised to around the silicon melting point of 1200°C in a short time, resulting in a higher Si reaction rate and a decrease in the unreacted Si content.

[0106] However, if the MW irradiation time is increased excessively, the Si3N4 / SiC ratio will increase, the total amount of Si3N4 and SiC will increase, and the rate of capacity reduction may become larger.

[0107] As described above, the present invention has been illustrated and explained in relation to specific embodiments, but it will be obvious to those ordinary in the art that the present invention can be improved and modified in various ways without departing from the technical spirit of the invention provided by the following claims.

Claims

1. A silicon-carbon composite comprising a carbon-based material and a first silicon-based nanocoating layer formed on the carbon-based material; With silicon carbide; With silicon nitride; Includes; second silicon-based particles; The weight ratio of silicon nitride (SiNx) to silicon carbide (SiC) (SiNx / SiC) is 0.4 to 2. The negative electrode active material for a secondary battery is characterized in that the second silicon-based particles are mixed with the silicon-carbon composite.

2. The negative electrode active material for a secondary battery according to claim 1, characterized in that the silicon carbide (SiC) and silicon nitride (SiNx) are included in the first silicon-based nanocoating layer or mixed with the silicon carbon composite.

3. The silicon carbide is contained in an amount of 1 to 30% by weight relative to the total amount of the silicon element compound. The negative electrode active material for a secondary battery according to claim 1, characterized in that the silicon element-containing compound comprises the first silicon-based nanocoating layer, the silicon carbide, the silicon nitride, and the second silicon-based particles.

4. The silicon nitride is contained in an amount of 10 to 40% by weight relative to the total amount of the silicon element compound. The negative electrode active material for a secondary battery according to claim 1, characterized in that the silicon element-containing compound comprises the first silicon-based nanocoating layer, the silicon carbide, the silicon nitride, and the second silicon-based particles.

5. The second silicon-based particle is, It is composed of silicon particles with an average particle size (D50) of 0.4 to 10 μm and contains silicon element. It is contained in less than 40% by weight of the total amount of the compound. The negative electrode active material for a secondary battery according to claim 1, characterized in that the silicon element-containing compound comprises the first silicon-based nanocoating layer, the silicon carbide, the silicon nitride, and the second silicon-based particles.

6. The first silicon-based nanocoating layer is The negative electrode active material for a secondary battery according to claim 1, characterized in that it comprises at least one selected from: a silicon thin film with a thickness of 10 to 100 nm formed on the surface of the carbon-based material; and silicon particles with a particle size (D50) of 10 to 100 nm.

7. The negative electrode active material for a secondary battery according to claim 1, characterized in that the carbon-based material is at least one selected from the group consisting of natural graphite, artificial graphite, expanded graphite, graphene oxide, carbon nanotubes, carbon fibers, and hard carbon.

8. The carbon-based material and the entire compound containing silicon elements are contained in a weight ratio of 5:5 to 9.5:0.

5. The negative electrode active material for a secondary battery according to claim 1, characterized in that the silicon element-containing compound comprises the first silicon-based nanocoating layer, the silicon carbide, the silicon nitride, and the second silicon-based particles.

9. A process of mixing carbon-based materials and silicon-based materials to produce a mixture, N ２ The process includes a calcination step of irradiating the mixture with microwaves in a gaseous atmosphere exceeding 50% by volume, The aforementioned firing process is A method for producing a negative electrode active material for a secondary battery, characterized by maintaining the exothermic temperature of the mixture at 1100 to 1400°C and irradiating it with microwaves at an output of 1 to 3 kW for 10 seconds to 10 minutes.

10. The method for producing a negative electrode active material for a secondary battery according to claim 9, characterized in that, in the step of producing the mixture, the silicon-based material is silicon particles having an average particle size (D50) of 0.4 to 10 μm.

11. The method for producing a negative electrode active material for a secondary battery according to claim 9, characterized in that, in the step of producing the mixture, the carbon-based material is carbon-based particles having an average particle size (D50) of 5 to 30 μm.

12. The aforementioned firing process is N ２ A method for producing a negative electrode active material for a secondary battery according to claim 9, characterized in that the process is carried out in a gaseous atmosphere of 80 to 100 volume percent.

13. The method for producing a negative electrode active material for a secondary battery according to claim 9, further comprising a step of cooling the product of the calcination step to -10 to 10°C.

14. A negative electrode for a secondary battery comprising the negative electrode active material described in claim 1.

15. A secondary battery comprising the negative electrode, positive electrode, and electrolyte described in claim 14.

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