Negative electrode active material, negative electrode comprising the negative electrode active material, and secondary battery comprising the negative electrode

Abstract

The present invention relates to a negative electrode active material, a negative electrode comprising the negative electrode active material, and a secondary battery comprising the negative electrode. The negative electrode active material comprises: a silicon-based compound comprising SiO x , wherein 0.5

Description

[0001] This patent application for invention is a divisional application of a Chinese patent application with application number 201880028840.5, filing date May 4, 2018, and invention title "Negative electrode active material, negative electrode comprising the same, secondary battery comprising the negative electrode, and method for preparing the negative electrode active material". Technical Field

[0002] Cross - reference to Related Applications

[0003] This application claims the benefit of Korean Patent Application No. 10 - 2017 - 0057050 filed on May 4, 2017, and Korean Patent Application No. 10 - 2018 - 0051920 filed on May 4, 2018 with the Korean Intellectual Property Office, the disclosures of which are incorporated herein by reference in their entireties. Technical Field

[0005] The present invention relates to a negative electrode active material, a negative electrode comprising the negative electrode active material, a secondary battery comprising the negative electrode, and a method for preparing the negative electrode active material. Specifically, the method for preparing the negative electrode active material includes: preparing a silicon - based compound containing SiO x (0.5 < x < 1.3); providing a polymer layer containing a polymer compound on the silicon - based compound; providing a metal catalyst layer on the polymer layer; and heat - treating the silicon - based compound provided with the polymer layer and the metal catalyst layer. Background Art

[0006] Due to the rapid growth in the use of fossil fuels, the demand for alternative or clean energy has increased. As part of this trend, power generation and storage using electrochemical reactions are the most active research fields.

[0007] Currently, a typical example of an electrochemical device using electrochemical energy can be a secondary battery, and its application fields have a tendency to expand more and more. In recent years, with the increase in the technology development and demand for portable devices (e.g., portable computers, mobile phones, and cameras), the demand for secondary batteries as an energy source has increased significantly. Among these secondary batteries, a considerable amount of research has been conducted on lithium secondary batteries with high energy density (i.e., large capacity), and they have been commercialized and widely used.

[0008] Generally, a secondary battery consists of a positive electrode, a negative electrode, an electrolyte, and a separator. The negative electrode contains a negative electrode active material from which lithium ions are inserted and extracted from the positive electrode, and silicon-based active materials with high discharge capacity can be used as the negative electrode active material. However, because silicon-based active materials undergo excessive contraction and expansion during battery charging and discharging, the conductive pathways in the active material are blocked, thus reducing the battery's cycle characteristics.

[0009] To address this limitation, a carbon coating is typically formed on the surface of silicon-based active materials (see Korean Patent Application Publication No. 10-2016-0149862). Furthermore, attempts have been made to further improve conductivity by incorporating graphene into the carbon coating. Chemical vapor deposition (CVD) is primarily used to deposit graphene on the surface of silicon-based active materials, but this process may not be simplified because a separate hydrocarbon source is required.

[0010] Therefore, there is a need to develop a simplified process for applying a carbon coating containing graphene to the surface of silicon-based active materials.

[0011] Existing technical documents

[0012] Patent documents

[0013] Korean Patent Application Publication No. 10-2016-0149862 Summary of the Invention

[0014] Technical issues

[0015] One aspect of the present invention provides a method for preparing a negative electrode active material, wherein the process of forming a carbon coating containing graphene on the surface of a silicon-based active material can be simplified.

[0016] Another aspect of the invention provides a negative electrode active material, a negative electrode, and a secondary battery, wherein excessive volume changes of the negative electrode active material can be controlled during battery charging and discharging.

[0017] Technical solution

[0018] According to one aspect of the present invention, a method for preparing a negative electrode active material is provided, the method comprising: preparing a material containing SiO2 xSilicon compounds where (0.5 < x < 1.3); a polymer layer containing a polymer compound is provided on the silicon compounds; a metal catalyst layer is provided on the polymer layer; heat treatment is performed on the silicon compounds having the polymer layer and the metal catalyst layer provided thereon; and the metal catalyst layer is removed, where the polymer compound includes any one selected from the group consisting of glucose, fructose, galactose, maltose, lactose, sucrose, phenolic resin, naphthalene resin, polyvinyl alcohol resin, polyurethane resin, polyimide, furan resin, cellulose resin, epoxy resin, polystyrene resin, resorcinol-based resins, phloroglucinol-based resins, coal-derived pitch, petroleum-derived pitch, and tar, or a mixture of two or more thereof.

[0019] According to another aspect of the present invention, there is provided a negative electrode active material, a negative electrode including the negative electrode active material, and a secondary battery including the negative electrode. The negative electrode active material includes: silicon compounds containing SiO x (0.5 < x < 1.3); an amorphous carbon layer provided on the silicon compounds; a graphene layer provided on the amorphous carbon layer; and a cavity layer corresponding to the interstitial space between the amorphous carbon layer and the graphene layer.

[0020] Advantageous effects

[0021] The method for preparing a negative electrode active material according to an embodiment of the present invention does not require a separate chemical vapor deposition (CVD) process for supplying a carbon raw material during the formation of the graphene layer. In addition, since the amorphous carbon layer and the graphene layer can be formed while heat-treating the polymer layer and the metal catalyst layer, the process can be simplified.

[0022] According to another embodiment of the present invention, the internal stress during battery charging and discharging can be alleviated by the cavity layer in the negative electrode active material. Thus, since the structural disintegration of the negative electrode can be suppressed and the conduction path in the negative electrode active material can be maintained, the cycle characteristics of the battery can be improved. Detailed embodiments

[0023] The present invention will be described in more detail below for a clearer understanding of the present invention.

[0024] It should be understood that words or terms used in the specification and claims should not be interpreted according to the meanings defined in an ordinary dictionary. It should also be understood that based on the principle that the inventor can appropriately define the meanings of the words or terms to best explain the present invention, the words or terms should be interpreted as having meanings consistent with their meanings in the relevant art and the context of the technical concept of the present invention.

[0025] The terms used herein are for the purpose of describing particular exemplary embodiments only and are not intended to limit the present invention. In this specification, the singular forms of terms may include the plural forms unless otherwise indicated.

[0026] It should also be understood that the terms "comprises," "comprising," or "having" used in this specification specify the presence of the stated features, numbers, steps, elements, or combinations thereof, but do not preclude the presence or addition of one or more other features, numbers, steps, elements, or combinations thereof.

[0027] In this specification, a transmission electron microscope (TEM) can be used to examine the thickness of the amorphous carbon layer, graphene layer, polymer layer, or metal catalyst layer.

[0028] A method for preparing a negative electrode active material according to an embodiment of the present invention includes: preparing a silicon-based compound containing SiO x (0.5 < x < 1.3); disposing a polymer layer containing a polymer compound on the silicon-based compound; disposing a metal catalyst layer on the polymer layer; heat-treating the silicon-based compound on which the polymer layer and the metal catalyst layer are disposed; and removing the metal catalyst layer, wherein the polymer compound may include any one selected from the group consisting of glucose, fructose, galactose, maltose, lactose, sucrose, phenolic resin, naphthalene resin, polyvinyl alcohol resin, polyurethane resin, polyimide, furan resin, cellulose resin, epoxy resin, polystyrene resin, resorcinol-based resin, phloroglucinol-based resin, coal-derived pitch, petroleum-derived pitch, and tar, or a mixture of two or more thereof.

[0029] The silicon-based compound may contain SiO x (0.5 < x < 1.3). The preparation of the silicon-based compound may include reacting SiO x1 (0 < x1 < 2) with a metal. The SiO x1 (0 < x1 < 2) may be in a form containing silicon (Si) and SiO2. That is, x and x1 respectively correspond to the ratio of the number of oxygen (O) atoms to Si atoms contained in SiO x (0.5 < x < 1.3) or SiO x1 (0 < x1 < 2).

[0030] The silicon-based compound may further contain metal silicate. Specifically, metal silicate can be doped into SiO x1 (0 < x1 < 2) by reacting with a metal, and the metal silicate can be disposed in the silicon-based compound.

[0031] ​​The metal silicate may exist in a state doped into SiO x (0.5 < x < 1.3). The metal silicate may include at least one selected from the group consisting of Li2Si2O5, Li3SiO3, Li4SiO4, Mg2SiO4, MgSiO3, Ca2SiO4, CaSiO3, and TiSiO4.

[0032] Based on 100 parts by weight of SiO x (0.5 < x < 1.3), the metal of the metal silicate may be included in an amount of 1 part by weight to 30 parts by weight, for example, 2 parts by weight to 20 parts by weight. When the amount of the metal satisfies the above range, the growth of Si particles can be inhibited and the initial efficiency can be improved.

[0033] The reaction of SiO x1 (0 < x1 < 2) with the metal may include reacting SiO x1 (0 < x1 < 2) with a metal powder or a metal gas containing the metal.

[0034] The metal may include at least one selected from the group consisting of lithium (Li), magnesium (Mg), titanium (Ti), and calcium (Ca), and may specifically include Li and Mg. The reaction may be carried out at a temperature in the range of 300°C to 1,000°C for 1 hour to 24 hours.

[0035] The reaction may be carried out while flowing an inert gas. The inert gas may include at least one selected from the group consisting of argon (Ar), nitrogen (N2), neon (Ne), helium (He), and krypton (Kr).

[0036] The preparation of the silicon compound may further include removing a part of the metal silicate generated during the reaction with the metal. Specifically, the preparation of the silicon compound may include removing the metal silicate provided on the surface of the silicon compound among the metal silicate generated during the reaction with the metal. The metal silicate may be removed by using an HCl aqueous solution.

[0037] The average particle size (D 50 ) of the silicon compound may be in the range of 0.1 μm to 20 μm, for example, 0.5 μm to 10 μm. When the average particle size of the silicon compound satisfies the above range, the rate performance of the battery can be improved.

[0038] Forming a polymer layer comprising a polymer compound on a silicon-based compound can include general methods, but is not necessarily limited to them. For example, the polymer layer can be formed by coating the silicon-based compound with the polymer compound itself and then performing thermosetting, or by coating the silicon-based compound with a carbon-containing material and then performing heat treatment. Specifically, for example, if the polymer layer comprises polyimide, the polymer layer can be formed by coating the silicon-based compound with poly(acrylic acid) (PAA) and then performing heat treatment.

[0039] The polymeric compound may comprise any one selected from the group consisting of glucose, fructose, galactose, maltose, lactose, sucrose, phenolic resin, naphthalene resin, polyvinyl alcohol resin, polyurethane resin, polyimide, furan resin, cellulose resin, epoxy resin, polystyrene resin, resorcinol resin, phloroglucinol resin, coal-derived pitch, petroleum-derived pitch and tar, or a mixture of two or more thereof, and may specifically comprise polyimide.

[0040] The polymer layer can have a thickness of 0.001 μm to 10 μm, for example, 0.01 μm to 5 μm. Within this thickness range, sufficient carbon source can be supplied, allowing for the continuous and uniform formation of the graphene layer.

[0041] The method of setting a metal catalyst layer on a polymer layer may include the following, although it is not necessarily limited to these.

[0042] After a silicon-based compound with a polymer layer thereon is added to a solution containing a metal salt, a metal catalyst layer can be deposited on the polymer layer using a chemical plating method that involves adding a reducing agent and stirring.

[0043] The metal catalyst layer may contain at least one element selected from the group consisting of nickel (Li), copper (Cu), iron (Fe), and cobalt (Co), and may specifically contain Ni.

[0044] The metal catalyst layer may have a thickness of 0.001 μm to 10 μm, for example, 0.01 μm to 5 μm. Within this thickness range, a highly crystalline graphene layer can be formed continuously and uniformly.

[0045] When a metal catalyst layer is disposed on the polymer layer, the weight ratio of the polymer layer to the metal catalyst layer can be in the range of 1:1 to 20:1, for example, 2:1 to 10:1. When the above weight ratio range is met, the graphene layer can be formed continuously and uniformly.

[0046] The polymer layer on the silicon-based compound provided with the polymer layer and the metal catalyst layer can be carbonized by heat treatment. Thus, an amorphous carbon layer can be formed on the silicon-based compound. At the same time, since the carbon source generated from the polymer layer can be supplied to the metal catalyst layer, a graphene layer can be formed. The heat treatment can be carried out within a temperature range of 300 °C to 1,000 °C, for example, 450 °C to 900 °C. When the above heat treatment temperature range is satisfied, a highly crystalline graphene layer can be formed while suppressing the growth of silicon particles. The heat treatment can be carried out for 0.5 minutes to 1 hour.

[0047] The removal of the metal catalyst layer may include the following methods, although not necessarily limited thereto.

[0048] After adding the silicon-based compound provided with the metal catalyst layer to an acidic solution, the metal catalyst layer can be removed by etching for a predetermined time and drying.

[0049] A void layer corresponding to the spaced-apart space between the amorphous carbon layer and the graphene layer can be formed by removing the metal catalyst layer. Thus, since excessive volume change of the negative electrode active material can be prevented during charging and discharging of the battery, a conduction path in the negative electrode active material can be ensured, and thus the cycle characteristics can be improved.

[0050] The negative electrode active material according to another embodiment of the present invention may include: a silicon-based compound containing SiO x (0.5 < x < 1.3); an amorphous carbon layer provided on the silicon-based compound; a graphene layer provided on the amorphous carbon layer; and a void layer corresponding to the spaced-apart space between the amorphous carbon layer and the graphene layer. Here, since the silicon-based compound containing SiO x (0.5 < x < 1.3) is the same as described above, its description is omitted.

[0051] The amorphous carbon layer can be provided on the silicon-based compound. The amorphous carbon layer may contain amorphous carbon and may specifically be formed of amorphous carbon. The rate performance of the battery can be improved by the amorphous carbon layer.

[0052] The amorphous carbon layer may have a thickness of 0.001 μm to 10 μm, for example, 0.01 μm to 5 μm. When the above thickness range is satisfied, a battery having excellent rate performance can be prepared without reducing the initial efficiency.

[0053] The graphene layer may be disposed on the amorphous carbon layer. The graphene layer may contain graphene and may specifically be formed from graphene. In this invention, "graphene" refers to a carbonaceous structure with a thickness of less than 0.2 nm, containing carbon atoms constituting a hexagonal lattice, exhibiting flexibility, and existing in thin film form. Since the volume expansion of silicon compounds during charging and discharging can be controlled through the graphene layer, the cycle characteristics of the battery can be improved by preventing blockage of conductive paths.

[0054] The thickness of the graphene layer can be from 0.5 nm to 200 nm, for example, from 1 nm to 100 nm. Meeting this thickness range can improve the cycle characteristics of the battery.

[0055] The amorphous carbon layer and graphene layer can be formed by carbonizing the polymer layer described above.

[0056] The void layer can be disposed between the amorphous carbon layer and the graphene layer. Specifically, the void layer corresponds to the space between the amorphous carbon layer and the graphene layer. The void layer may contain a single space or two or more spaces. That is, the void layer may exist on at least a portion of the surface of the amorphous carbon layer, and if the void layer contains two or more spaces, these spaces may exist and be dispersed on the surface of the amorphous carbon layer. The void layer is not formed solely by removing the metal catalyst layer. Specifically, the void layer is achieved by the shrinkage of the polymer layer during carbonization (pyrolysis) through heat treatment. That is, the void layer is formed by heat treatment after sequentially setting the polymer layer and the metal catalyst layer according to the preparation method of the present invention. Since the void layer can mitigate the internal stress caused by the volume change of silicon compounds during battery charging and discharging, the conductive path of the negative electrode active material can be maintained.

[0057] The average thickness of the void layer can be from 0.5 nm to 200 nm, for example, from 100 nm to 200 nm. With this thickness, a sufficient area is ensured to mitigate the internal stress caused by volume changes in silicon compounds during battery charging and discharging, thus maintaining the conductive path of the negative electrode active material more smoothly.

[0058] According to another embodiment of the present invention, the negative electrode may comprise a negative electrode active material, which is the same as the negative electrode active material described above. Specifically, the negative electrode may comprise a current collector and a layer of negative electrode active material disposed on the current collector. The negative electrode active material layer may comprise the negative electrode active material. Furthermore, the negative electrode active material layer may also comprise a binder and / or a conductive agent. Additionally, the negative electrode may further comprise graphite particles, and the graphite particles may be contained within the negative electrode active material layer.

[0059] The adhesive may comprise at least one selected from the group consisting of: polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride (PVDF), polyacrylonitrile, polymethyl methacrylate, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, polyacrylate, ethylene-propylene-diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, polyacrylic acid, and polymers wherein hydrogen is replaced by Li, sodium (Na), or Ca, and the adhesive may also comprise various copolymers thereof.

[0060] There are no particular limitations on the conductive agent, as long as it is conductive and does not cause adverse chemical changes in the battery. Conductive materials can be used, such as: graphite, such as natural graphite and artificial graphite; carbon black, such as acetylene black, Ketjen black, channel black, furnace black, lamp black and thermal cracking black; conductive fibers, such as carbon fibers or metal fibers; conductive tubes, such as carbon nanotubes; metal powders, such as fluorocarbon powders, aluminum powders and nickel powders; conductive whiskers, such as zinc oxide whiskers and potassium titanate whiskers; conductive metal oxides, such as titanium oxides; or polyphenylene derivatives.

[0061] The graphite-based active material particles may comprise at least one selected from the group consisting of artificial graphite, natural graphite, graphitized carbon fibers, and graphitized mesophase carbon microspheres. The charging and discharging characteristics of the battery can be improved by using the graphite-based active material particles in conjunction with secondary particles.

[0062] A secondary battery according to another embodiment of the present invention may include a negative electrode, a positive electrode, a separator disposed between the positive and negative electrodes, and an electrolyte, wherein the negative electrode is the same as the negative electrode described above. Since the negative electrode has already been described above, a detailed description thereof is omitted here.

[0063] The positive electrode may comprise a positive electrode active material. The positive electrode active material may be a commonly used positive electrode active material. Specifically, the positive electrode active material may comprise: layered compounds, such as lithium cobalt oxide (LiCoO2) or lithium nickel oxide (LiNiO2), or compounds replaced by one or more transition metals; lithium manganese oxide, such as Li... 1+y1 Mn 2-y1 O4 (0≤y1≤0.33), LiMnO3, LiMn2O3 and LiMnO2; lithium copper oxide (Li2CuO2); vanadium oxides, such as LiV3O8, V2O5 and Cu2V2O7; and LiNi 1-y2 M y2O2 represents nickel (Ni) site-type lithium nickel oxide (where M is cobalt (Co), manganese (Mn), aluminum (Al), copper (Cu), iron (Fe), magnesium (Mg), boron (B), or gallium (Ga), and y2 satisfies 0.01≤y2≤0.3); derived from the chemical formula LiMn 2-y3 M y3 Lithium-manganese composite oxides represented by O2 (where M is Co, Ni, Fe, chromium (Cr), zinc (Zn), or tantalum (Ta), and y3 satisfies 0.01≤y3≤0.1) or Li2Mn3MO8 (where M is Fe, Co, Ni, Cu, or Zn); and LiMn2O4 with a portion of Li replaced by alkaline earth metal ions, but the positive electrode active material is not limited to these. The positive electrode can be Li metal.

[0064] The separator isolates the negative and positive electrodes and provides a path for lithium ions to move. Any separator can be used without specific limitations, as long as it is generally used in secondary batteries. In particular, separators with high moisture-holding capacity for the electrolyte and low resistance to electrolyte ion transfer can be used. Specifically, porous polymer membranes can be used, such as porous polymer membranes prepared from polyolefin polymers, or laminated structures having two or more layers, such as ethylene homopolymers, propylene homopolymers, ethylene / butene copolymers, ethylene / hexene copolymers, and ethylene / methacrylate copolymers. Furthermore, typical porous nonwoven fabrics can be used, such as nonwoven fabrics formed from high-melting-point glass fibers or polyethylene terephthalate fibers. Additionally, coated separators containing ceramic or polymer components can be used to ensure heat resistance or mechanical strength, and separators with single-layer or multi-layer structures can be selectively used.

[0065] The electrolyte may include organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel polymer electrolytes, solid inorganic electrolytes, or molten inorganic electrolytes that can be used in the preparation of lithium secondary batteries, but the present invention is not limited thereto.

[0066] Specifically, the electrolyte may contain a non-aqueous organic solvent and a metal salt.

[0067] Examples of the non-aqueous organic solvents may be aprotic organic solvents, such as N-methyl-2-pyrrolidone, propylene carbonate, ethylene carbonate, butyl carbonate, dimethyl carbonate, diethyl carbonate, γ-butyrolactone, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, triphosphate, trimethoxymethane, dioxolane derivatives, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolium ketone, propylene carbonate derivatives, tetrahydrofuran derivatives, ethers, methyl propionate, and ethyl propionate.

[0068] In particular, ethylene carbonate and propylene carbonate, which are cyclic carbonates in carbonate-based organic solvents, are high-viscosity organic solvents that effectively dissociate lithium salts in the electrolyte due to their high dielectric constants, thus cyclic carbonates are preferred. Since a highly conductive electrolyte can be prepared by mixing cyclic carbonates with low-viscosity, low-dielectric-constant linear carbonates (e.g., dimethyl carbonate and diethyl carbonate) in appropriate proportions, it is even more preferable to use cyclic carbonates in combination with linear carbonates.

[0069] Lithium salts can be used as the metal salts, and the lithium salts are materials that are readily soluble in the non-aqueous electrolyte, wherein, for example, any one selected from the group consisting of: F - Cl - I - NO3 - N(CN)2 - BF4 - ClO4 - PF6 - (CF3)2PF4 - (CF3)3PF3 - (CF3)4PF2 - (CF3)5PF - (CF3)6P - CF3SO3 - CF3CF2SO3 - (CF3SO2)2N - (FSO2)2N - CF3CF2(CF3)2CO - (CF3SO2)2CH - (SF5)3C - (CF3SO2)3C - CF3(CF2)7SO3 - CF3CO2 - CH3CO2- SCN - and (CF3CF2SO2)2N - .

[0070] In addition to the electrolyte components mentioned above, the electrolyte may also contain at least one additive, such as alkylene carbonate halides like difluoroethylene carbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, n-glycol dimethyl ether, hexamethylphosphoryltriamine, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted... Alzolidinediones, N,N-substituted imidazolidinedions, ethylene glycol dialkyl ethers, ammonium salts, pyrroles, 2-methoxyethanol, or aluminum trichloride are used to improve battery life characteristics, prevent battery capacity reduction, and improve battery discharge capacity.

[0071] According to another embodiment of the present invention, a battery module comprising the secondary battery as a unit cell and a battery pack comprising the battery module are provided. Because the battery module and battery pack comprise secondary batteries with high capacity, high rate performance, and high cycle characteristics, they can be used as power sources for medium to large-sized devices selected from the group consisting of electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, and energy storage systems.

[0072] The following preferred embodiments are provided to better understand the present invention. It will be apparent to those skilled in the art that these embodiments are provided merely to illustrate the invention, and various modifications and changes can be made within the scope and spirit of the invention. Such modifications and changes fall within the scope of the appended claims.

[0073] Examples and Comparative Examples

[0074] Preparation Example 1: Preparation of Silicon Compounds

[0075] 20g of SiO powder and 2g of lithium metal powder were mixed and placed in a chamber reaction vessel, which was then heated to 800°C. Ar was used as the inert gas in this process. After heat treatment for 2 hours, the temperature of the chamber was lowered to room temperature to collect the product from the reaction vessel. The collected product was acid-treated with HCl. Subsequently, the acid-treated product was ground to prepare an average particle size (D). 50 A silicon-based compound with a thickness of 5 μm was obtained. X-ray diffraction (XRD) measurements confirmed that the silicon-based compound contained Li₂Si₂O₅ and Li₂SiO₃ as metal silicates, and the total amount of lithium in the Li₂Si₂O₅ and Li₂SiO₃ contained in 100 parts by weight of the silicon-based compound was 5 parts by weight.

[0076] Example 1: Preparation of negative electrode active material

[0077] (1) Polymer layer formation

[0078] The silicon compound of Preparation Example 1 was added to a dimethylacetamide (DMAC) solution containing poly(acrylic acid) (PAA) and stirred for 1 hour in a reducing atmosphere. Ar gas was used to generate the reducing atmosphere in this case. Subsequently, the silicon compound was extracted by centrifugation and then dried under vacuum to obtain a silicon compound coated with PAA. The silicon compound coated with PAA was then imidized by heat treatment at 300°C for 60 minutes in a reducing atmosphere, thereby forming a 1 μm thick polyimide layer on the silicon compound.

[0079] (2) Formation of metal catalyst layer

[0080] A silicon compound with a polyimide layer formed on it was added to an aqueous potassium hydroxide (KOH) solution at 50°C and stirred for 10 minutes. The silicon compound was then extracted by centrifugation. The extracted silicon compound was added to an aqueous nickel sulfate (Ni₂SO₄) solution, stirred for 10 minutes, and then rinsed. Subsequently, the silicon compound was added to an aqueous borohydride solution at 50°C and stirred for 30 minutes to prepare particles in which a 100 nm thick nickel catalyst layer was formed on the polyimide layer.

[0081] (3) Heat treatment

[0082] The particles were placed in a tube furnace and heat-treated at 600°C for 10 minutes in a reducing atmosphere to form an amorphous carbon layer and a graphene layer containing multiple graphene atoms disposed on a nickel catalyst layer from the polyimide layer. In this case, Ar gas was used as the reducing atmosphere. The amorphous carbon layer had a thickness of 300 nm, and the graphene layer had a thickness of 50 nm.

[0083] (4) Removal of the metal catalyst layer

[0084] A silicon-based compound with an amorphous carbon layer and a graphene layer formed on its surface was added to a 1M FeCl3 aqueous solution and etched for 2 hours. The silicon-based compound was then dried with ethanol to remove the nickel catalyst layer. Simultaneously with the removal of the nickel catalyst layer, a void layer corresponding to the spacer between the amorphous carbon layer and the graphene layer was formed. The thickness of the void layer was 100 nm to 200 nm.

[0085] Example 2: Preparation of negative electrode active material

[0086] The negative electrode active material of Example 2 was prepared in the same manner as in Example 1, except that the nickel catalyst layer was formed to a thickness of 50 nm. In the prepared negative electrode active material, the thickness of the amorphous carbon layer was 150 nm, and the thickness of the graphene layer was 25 nm. Furthermore, the thickness of the void layer was 50 nm or more but less than 100 nm.

[0087] Comparative Example 1: Preparation of Negative Electrode Active Materials

[0088] (1) Formation of amorphous carbon layers

[0089] 100g of the silicon compound of Preparation Example 1 and 7g of coal tar pitch were mixed and then heat-treated at 950°C to prepare a negative electrode active material, wherein a 300nm thick amorphous carbon layer was disposed on the surface of the silicon compound.

[0090] Comparative Example 2: Preparation of Negative Electrode Active Materials

[0091] (1) Formation of amorphous carbon layers

[0092] 100g of the silicon compound of Preparation Example 1 was mixed with 7g of coal tar pitch and then heat-treated at 950°C to form a 300nm thick amorphous carbon layer on the surface of the silicon compound.

[0093] (2) Formation of crystalline carbon layer

[0094] Specifically, a silicon-based compound with an amorphous carbon layer is placed in a chamber reaction vessel, and the temperature of the chamber is raised to 950°C. In this case, the pressure in the chamber is maintained at 10 mTorr using a rotary pump. Subsequently, methane gas is injected for 5 minutes, and then the temperature of the chamber is lowered to room temperature. The product in the reaction vessel is then collected, forming a 50 nm thick crystalline carbon layer on the amorphous carbon layer. There is no space between the amorphous carbon layer and the crystalline carbon layer.

[0095] Comparative Example 3: Preparation of Negative Electrode Active Materials

[0096] (1) Formation of amorphous carbon layers

[0097] 100g of the silicon compound of Preparation Example 1 was mixed with 7g of coal tar pitch and then heat-treated at 950°C to form a 300nm thick amorphous carbon layer on the surface of the silicon compound.

[0098] (2) Formation of metal catalyst layer

[0099] A silicon-based compound with an amorphous carbon layer formed on it was added to an aqueous potassium hydroxide (KOH) solution at 50°C and stirred for 10 minutes. The silicon-based compound was then extracted by centrifugation. The extracted silicon-based compound was added to an aqueous nickel sulfate (Ni₂SO₄) solution, stirred for 10 minutes, and then rinsed. Subsequently, the silicon-based compound was added to an aqueous borohydride solution at 50°C and stirred for 30 minutes to prepare particles in which a 100 nm thick nickel catalyst layer was formed on the amorphous carbon layer.

[0100] (3) Formation of graphene layers

[0101] Specifically, a silicon-based compound with a nickel catalyst layer and an amorphous carbon layer disposed on its surface is placed in a chamber reaction vessel, and the temperature of the chamber is raised to 950°C. In this case, the pressure in the chamber is maintained at 10 mTorr using a rotary pump. Subsequently, methane gas is injected for 5 minutes, and then the temperature of the chamber is lowered to room temperature. The product in the reaction vessel is then collected, revealing a 50 nm thick graphene layer formed on the amorphous carbon layer. There is no space between the amorphous carbon layer and the graphene layer.

[0102] Examples 3 and 4 and Comparative Examples 4, 5 and 6: Preparation of Secondary Batteries

[0103] Secondary batteries of Examples 3 and 4 and Comparative Examples 4, 5 and 6 were prepared using the negative electrode active materials of Examples 1 and 2 and Comparative Examples 1, 2 and 3, respectively.

[0104] Specifically, the negative electrode active materials of Examples 1 and 2, and Comparative Examples 1, 2 and 3, were mixed in a weight ratio of 9.6:86.2:1.0:1.7:1.5, including natural graphite and an average particle size (D). 50 Carbon black (65 nm), CMC, and styrene-butadiene rubber (SBR) were added to distilled water as a solvent and mixed to prepare a negative electrode slurry with a mixture solids content of 45%.

[0105] A 20 μm thick copper (Cu) film, used as the negative electrode current collector, was subjected to a negative electrode mixture slurry at a concentration of 160 mg / 25 cm⁻¹. 2 The loading amount is coated and dried. In this case, the temperature of the circulating air is 70°C. Subsequently, the negative electrode current collector coated and dried with the slurry is calendered and dried in a vacuum oven at 130°C for 8 hours to prepare the negative electrode.

[0106] Each of the negative electrodes is cut into pieces with an area of ​​1.4875 cm². 2The electrode is a coin-shaped half-cell containing one piece of Li metal and one piece of negative electrode. A porous polyethylene membrane is placed between the positive and negative electrodes, and an electrolyte is injected to prepare the half-cell containing one piece of Li metal and one piece of negative electrode. In the electrolyte, 1M LiPF6 is dissolved in a mixed solution in which the volume ratio of ethyl methyl carbonate (EMC) to ethylene carbonate (EC) is 7:3.

[0107] Experimental Example 1: Evaluation of Capacity Retention

[0108] The capacity retention of the secondary batteries in Examples 3 and 4, and Comparative Examples 4, 5 and 6 was evaluated using the following methods, and the results are shown in Table 1.

[0109] The first cycle is performed by charging at 0.1C and discharging at 0.1C, and from the second to the 50th cycle, the cycle is performed by charging at 0.5C and discharging at 0.5C.

[0110] Charging conditions: CC (constant current) / CV (constant voltage) (5mV / 0.005C current cutoff)

[0111] Discharge conditions: CC (constant current) condition 1.0V

[0112] The capacity retention rate is derived using the following calculation.

[0113] Capacity retention (%) = (Discharge capacity at 50th cycle / Discharge capacity at 1.0V in the first cycle) × 100

[0114] [Table 1]

[0115] Example 3 Example 4 Comparative Example 4 Comparative Example 5 Comparative Example 6 Capacity retention 96 91 75 85 87

[0116] Referring to Table 1, the batteries of Examples 3 and 4, in which the negative electrode active material prepared according to the method of the present invention is used, exhibit significantly higher capacity retention than the battery of Comparative Example 4, in which a graphene-free negative electrode active material is used. Furthermore, it can be confirmed that the batteries of Examples 3 and 4 also have higher capacity retention compared to the batteries of Comparative Examples 5 and 6, which contain negative electrode active materials prepared by a general method in which an amorphous carbon layer and a crystalline carbon layer (or graphene layer) are sequentially formed. This is believed to be because, for the negative electrode active materials of Examples 1 and 2, the porous layer plays a role in suppressing the structural breakdown of the negative electrode active material and maintaining the conductive path.

Claims

1. A negative electrode active material, comprising: Contains SiO x Silicon compounds, of which 0.5 <x<1.3; An amorphous carbon layer disposed on the silicon-based compound; A continuous and uniform graphene layer is disposed on the amorphous carbon layer; and A void layer corresponding to the space between the amorphous carbon layer and the graphene layer, wherein the average thickness of the void layer is 0.5 nm to 200 nm.

2. The negative electrode active material according to claim 1, wherein the thickness of the amorphous carbon layer is from 0.001 μm to 10 μm.

3. The negative electrode active material according to claim 1, wherein the thickness of the graphene layer is from 0.5 nm to 200 nm.

4. A negative electrode comprising the negative electrode active material according to claim 1.

5. The negative electrode according to claim 4 further comprises graphite particles.

6. A secondary battery, comprising: The negative electrode according to claim 4; positive electrode; A diaphragm disposed between the positive electrode and the negative electrode; and Electrolytes.