Silicon negative electrode active material for secondary battery and preparation method thereof

Abstract

The present invention relates to a negative electrode active material for a secondary battery and a method for preparing the same. According to the present invention, a negative electrode active material for a secondary battery is provided, characterized in that it comprises: silicon particles containing a coating portion derived from a coupling agent; an amorphous carbon portion located on the silicon particles; and a carbon-based conductive material located on the amorphous carbon portion. In the present invention, chemical bonding and chemical interactions are introduced into the silicon particles by using a coupling agent and an amorphous carbon portion, etc. Therefore, the volume expansion problem of the negative electrode material can be reduced, and a lithium secondary battery using the negative electrode material can have improved life characteristics and cycle characteristics such as initial capacity.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to and the benefit of Korean Patent Application No. 10-2021-0121679, filed on September 13, 2021, the disclosure of which is incorporated herein by reference in its entirety. Technical Field

[0003] The invention relates to a silicon negative electrode active material for a secondary battery and a preparation method thereof. Background Art

[0004] With the technological advancement and growing demand for mobile devices, the demand for secondary batteries as energy sources has increased dramatically. In particular, the application of secondary batteries has expanded in recent years to include electric vehicle energy sources, leading to a growing demand for higher-capacity batteries. Active research is underway into methods for producing high-density electrodes with higher energy density per unit volume.

[0005] A lithium secondary battery is a secondary battery having a structure in which lithium ions dissolve from the positive electrode and move to the negative electrode during charging, and conversely, return from the negative electrode to the positive electrode during discharging. The graphite negative electrode used in existing lithium secondary batteries has limitations in designing batteries with both high energy density and light weight due to its low capacity.

[0006] As a material that can replace this graphite, there is silicon, which is an element in the same group as carbon that constitutes graphite. However, negative electrodes using silicon have problems in that their lifespan and electrical characteristics such as initial efficiency are not optimized. Summary of the Invention

[0007] Technical issues

[0008] An object of the present invention is to provide a silicon negative electrode active material for a secondary battery having improved lifespan characteristics and electrical characteristics such as initial efficiency.

[0009] Another object of the present invention is to provide a method for preparing the above-mentioned silicon negative electrode active material.

[0010] The inventors discovered that existing silicon negative electrode materials have the disadvantage of being insufficient to cope with the volume expansion of silicon negative electrode materials due to the introduction of only physical complexation between silicon and carbon. In order to solve the above problems, in addition to the physical complexation between silicon and carbon, chemical bonding and chemical interaction are introduced to improve the life characteristics and electrical characteristics, thereby achieving the present invention.

[0011] Solutions to the Problem

[0012] According to one aspect of the present invention, a negative electrode active material for a secondary battery is provided, comprising: silicon particles comprising a coating portion derived from a coupling agent; an amorphous carbon portion located on the silicon particles; and a carbon-based conductive material located on the amorphous carbon portion.

[0013] According to another aspect of the present invention, a method for preparing a negative electrode active material for a secondary battery is provided, and the method for preparing a negative electrode active material for a secondary battery includes: step a, mixing silicon particles with a coupling agent to prepare silicon particles including a coating portion derived from the coupling agent; step b, mixing the silicon particles including the coating portion derived from the coupling agent with an amorphous carbon portion supply source and a carbon-based conductive material; and step c, carbonizing the mixture of the step b.

[0014] According to yet another aspect of the present invention, there is provided a negative electrode for a lithium secondary battery including the negative electrode active material as described above.

[0015] According to another aspect of the present invention, a lithium secondary battery is provided, comprising a positive electrode, a negative electrode opposite to the positive electrode, and an electrolyte between the positive electrode and the negative electrode, wherein the negative electrode comprises the negative electrode active material described above.

[0016] According to yet another aspect of the present invention, there is provided a battery pack including the lithium secondary battery as described above.

[0017] Effects of the Invention

[0018] Silicon particles used as negative electrode active materials for secondary batteries are used in combination with carbon-based conductive materials such as graphene, rather than being used alone. As described above, in the case of silicon particles complexed with carbon-based conductive materials, since there is only physical bonding between the carbon-based conductive material and the silicon particles, problems such as volume expansion cannot be effectively solved, and there is also a problem of poor cycle characteristics. In the present invention, chemical bonding and chemical interaction are introduced into silicon particles by using a coupling agent and an amorphous carbon portion. Thus, the volume expansion problem of the negative electrode material can be reduced, and a lithium secondary battery using the negative electrode material can have improved life characteristics and electrical characteristics such as initial capacity. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 The chemical composition of the negative electrode active material according to the present invention is schematically shown.

[0020] Figure 2 FT-IR spectra of the starting materials (silicon nanoparticles, SiNPs), intermediate materials (Piranha-treated Si, Si-APTES, Si-APTES / rGO), and products (Si-APTES / rGO / suc) of the preparation examples are shown.

[0021] Figure 3 SEM images of the starting material (SiNP), intermediate materials (Si-APTES, p-Si-APTES / rGO), and product (Si-APTES / rGO / suc) of the preparation example are shown.

[0022] Figure 4 TEM image of Si-APTES / rGO / suc prepared in Preparation Example is shown.

[0023] Figure 5 The XRD patterns of the starting material (p-Si), intermediate material (p-Si-APTES / rGO), and product (p-Si-APTES / rGO / suc) of the preparation example are shown.

[0024] Figure 6 Raman spectra of the starting materials (Si, p-Si), intermediate materials (p-Si / APTES), and products (p-Si / APTES / suc / GO, p-Si / APTES / suc / rGO) of the preparation examples are shown.

[0025] Figure 7 Shown are the differences in specific surface areas when the silicon (Si) content is fixed at 60 wt% and there is no amorphous carbon derived from sucrose (p-Si / APTES / rGO) and in the presence of amorphous carbon derived from sucrose (p-Si / APTES / suc / rGO), and the specific surface area when the silicon content is increased to 70 wt%.

[0026] Figure 8 The figures show the results of three cycle characteristics experiments using the negative electrode active material of Si-APTES / r-GO / suc (silicon content: 59.2 wt %) prepared in Preparation Example.

[0027] Figure 9 The TGA analysis results of the negative electrode active material according to the present invention (p-Si-APTES / suc / rGO) and the comparative negative electrode active material (p-Si-APTES / rGO) are shown.

[0028] Figure 10 The cycling characteristics of the comparative negative electrode active material (p-Si-APTES / rGO) are shown.

[0029] Figure 11 The figures show the results of measuring dQ / dV at 1 cycle, 5 cycles, 20 cycles and 100 cycles using the negative active material according to the present invention (Si-APTES / rGO / suc=6:2:2, silicon content of 59.2 wt %).

[0030] Figure 12 The figure shows the result of measuring rate capability using the negative electrode active material of the present invention (Si-APTES / rGO / suc=6:2:2, silicon content of 59.2 wt %).

[0031] Figure 13 The results of measuring dQ / dV according to the magnification are shown.

[0032] Figure 14 The results of relative irreversible capacity (RIC) analysis performed by changing the composition of the negative electrode active material are shown.

[0033] Figure 15 The zeta potential according to pH value in the solutions of the starting material, intermediate material, and final product in Preparation Example 2 is shown.

[0034] Figure 16 and Figure 17 FT-IR spectra and XPS spectra of the starting material, intermediate material, and final product of Preparation Example 2 are shown.

[0035] Figure 18 The results of measuring the cycle characteristics of the lithium secondary battery using the negative electrode active material B prepared in Preparation Example 2 twice are shown. DETAILED DESCRIPTION

[0036] Hereinafter, the present invention will be described in more detail.

[0037] The terms used in this application are only for the purpose of describing specific embodiments and are not intended to limit the present invention. Unless otherwise defined, all terms used herein, including technical or scientific terms, have the same meaning as commonly understood by those skilled in the art to which the present invention belongs.

[0038] Throughout the specification, when it is stated that a part “includes” or “comprising” a certain structural element, unless otherwise defined, it means that other structural elements may further be included.

[0039] When a part of a layer, film, etc. is referred to as being “on” or “above” another part, this includes not only the case where the part is directly “on” or “above” another part and in contact with the other part, but also the case where there is another part in between. Conversely, when a part is referred to as being “directly on” or “above” another part, this means that there is no other part in between.

[0040] Lithium secondary batteries typically include a positive electrode, a negative electrode positioned opposite the positive electrode, and an electrolyte interposed between the positive and negative electrodes. Alternatively, a lithium secondary battery may further include a battery container for housing the electrode assembly consisting of the positive electrode, negative electrode, and separator, and a sealing member for sealing the battery container.

[0041] The negative electrode is prepared by coating, drying and pressing the negative electrode active material on the negative electrode current collector. As needed, the negative electrode may further include the conductive material, binder and filler as described above.

[0042] According to one aspect of the present invention, as the above-mentioned negative electrode active material, a negative electrode active material for a secondary battery is provided, and the above-mentioned negative electrode active material for a secondary battery includes: silicon particles, including a coating portion derived from a coupling agent; an amorphous carbon portion located on the above-mentioned silicon particles; and a carbon-based conductive material located on the above-mentioned amorphous carbon portion.

[0043] In this specification, the coupling agent has two or more different reactive groups in the molecule, one of which is a reactive group chemically bonded to an inorganic material, and the other is a reactive group chemically bonded to an organic material. The coupling agent can be a silane coupling agent. The silane coupling agent is typically a vinyl group, an epoxy resin group, a styrene group, a methacrylic acid group, an acrylic acid group, an amino group, a urea group, an isocyanurate group, a sulfhydryl group, etc. According to one embodiment of the present invention, the coupling agent applied to the silicon particles of the present invention to form a coating portion is an aminosilane coupling agent. For example, the aminosilane coupling agent of the present invention can be at least one selected from 3-aminopropyltriethoxysilane (APTES), 3-aminopropyltrimethoxysilane (APTMS), 2-aminoethyl-3-aminopropylmethyldimethoxysilane and 2-aminoethyl-3-aminopropyltrimethoxysilane. According to one embodiment of the present invention, in the silicon particles including the coating portion derived from the above-mentioned coupling agent, the above-mentioned coupling agent may be at least one aminosilane coupling agent selected from 3-aminopropyltriethoxysilane, 3-aminopropyltrimethoxysilane, 2-aminoethyl-3-aminopropylmethyldimethoxysilane and 2-aminoethyl-3-aminopropyltrimethoxysilane.

[0044] According to one embodiment of the present invention, in the silicon particles, the coating portion derived from the coupling agent may be chemically bonded to the amorphous carbon portion. The chemical bonding may be a hydrogen bond, an ester bond, or a van der Waals bond. The chemical bonding between the coating portion derived from the coupling agent and the amorphous carbon portion may control volume expansion due to the use of the silicon negative electrode active material, thereby improving lifespan characteristics.

[0045] Figure 1 The chemical composition of the negative electrode active material according to the present invention is schematically shown. Figure 1It is known that the reactive group of the silane coupling agent can react with the silicon particles to form SiOx bonds. In addition, another reactive group of the silane coupling agent ( Figure 1 The amino groups in the silane coupling agent can react with the organic material, that is, the amorphous carbon portion of the present invention, to form hydrogen bonds. Therefore, according to one embodiment of the present invention, the silicon particles coated with the silane coupling agent include a coating portion derived from the silane coupling agent (e.g., Figure 1 APTES bonded to silicon particles via SiOx bonds in the coating layer, and the coating layer can form hydrogen bonds with the amorphous carbon layer.

[0046] The amorphous carbon portion can be rendered amorphous by mixing silicon particles containing a coating portion derived from the coupling agent with a carbonizable material such as sucrose, which serves as a source of the amorphous carbon portion, and then applying the mixture. The carbonizable material is then carbonized by heat treatment at high temperature. The amorphous state reduces the specific surface area, allowing for smooth reversible insertion and extraction of lithium ions.

[0047] The carbonizable material is not particularly limited as long as it can be carbonized and has functional groups such as carboxyl and hydroxyl groups to form a bond with the coating portion of the silicon particles derived from the coupling agent. In the case of a chain-like material, since it may decompose during the carbonization process, a compound having a ring structure is generally used. According to one embodiment of the present invention, the amorphous carbon portion may be derived from at least one selected from the group consisting of dopamine, a monosaccharide, a polysaccharide, polyvinyl pyrrolidone (PVP), polyethylene glycol (PEG), polydioxythiophene (PEDOT), polyacrylonitrile (PAN), polylactic acid (PAA), polyvinyl alcohol (PVA), polymethyl methacrylate (PMMA), polyvinylidene fluoride (PVDF), polyvinyl acetate (PVac), polystyrene (PS), polyvinyl chloride (PVC), polyetherimide (PEI), polybenzimidazole (PBI), polyethylene oxide (PEO), polycaprolactone (PCL), polyamide-6 (PA-6), polytrimethylene terephthalate (PTT), poly D, L-lactic acid (PDLA), polycarbonate, and polydioxanone. According to another embodiment of the present invention, the amorphous carbon portion may be derived from at least one selected from the group consisting of dopamine, a monosaccharide, and a polysaccharide. According to another embodiment of the present invention, the amorphous carbon portion may be derived from at least one selected from the group consisting of dopamine, glucose, fructose, galactose, maltose, lactose, sucrose, dextran, and cellulose.

[0048] As the above-mentioned carbon-based conductive material, there is no particular limitation as long as it is a negative electrode active material for a secondary battery. According to one embodiment of the present invention, the above-mentioned carbon-based conductive material can be at least one selected from graphene, graphene derivatives, reduced graphene, reduced graphene derivatives, carbon-based zero-dimensional nanostructures, carbon-based one-dimensional nanostructures, carbon-based two-dimensional nanostructures and carbon-based three-dimensional nanostructures. Among them, graphene derivatives include structures with graphitic bonds partially introduced with heteroatoms such as oxygen or structural defects in the carbon lattice, such as graphene oxide. The above-mentioned carbon-based zero-dimensional nanostructures, carbon-based one-dimensional nanostructures, carbon-based two-dimensional nanostructures and carbon-based three-dimensional nanostructures refer to any known zero-dimensional, one-dimensional, two-dimensional and four-dimensional nanostructures that can be used as carbon-based conductive materials in addition to graphene and graphene derivatives. For example, they can include structures of graphite carbon in the form of nanotubes, nanobuds, fullerenes, nanopeapods, embedded fullerenes, nanoonions, carbide-derived carbon, carbon black, irregular carbon and other non-graphene forms that may include structural or chemical defects. According to one embodiment of the present invention, the carbon-based conductive material may be at least one selected from graphene, graphene oxide, reduced graphene, reduced graphene oxide, carbon nanotubes, carbon nanofibers, fullerenes, and graphene foam.

[0049] According to one embodiment of the present invention, the above-mentioned carbon-based conductive material may include a coating portion derived from a coupling agent. The above-mentioned coupling agent may be a silane coupling agent. The above-mentioned silane coupling agent is generally a vinyl group, an epoxy resin group, a styrene group, a methacrylic acid group, an acrylic acid group, an amino group, a urea group, an isocyanurate group, a thiol group, etc. According to one embodiment of the present invention, the above-mentioned coupling agent applied on the carbon-based conductive material of the present invention to form a coating portion is an aminosilane coupling agent. For example, the aminosilane coupling agent of the present invention may be at least one selected from 3-aminopropyltriethoxysilane (APTES), 3-aminopropyltrimethoxysilane (APTMS), 2-aminoethyl-3-aminopropylmethyldimethoxysilane and 2-aminoethyl-3-aminopropyltrimethoxysilane. The above-mentioned coupling agent of the above-mentioned carbon-based conductive material may be the same as or different from the coupling agent used in the above-mentioned silicon particles.

[0050] According to one embodiment of the present invention, the carbon-based conductive material may be chemically bonded to the amorphous carbon portion, or may chemically interact with the silicon particles. The chemical bond may be a hydrogen bond, an ester bond, or a van der Waals bond, and the chemical interaction may be an electrostatic interaction, such as electrostatic attraction. As described above, the carbon-based conductive material chemically bonds to or interacts with other components in the negative electrode active material, thereby controlling the volume expansion of the silicon negative electrode active material in use to improve the life characteristics. According to one embodiment of the present invention, as Figure 1As shown, the carbon-based conductive material can be firmly combined with the amorphous carbon portion by esterification. According to one embodiment of the present invention, in the chemical bonding between the carbon-based conductive material and the amorphous carbon portion, for example, as described above, the carbon-based conductive material includes a coating portion derived from a coupling agent, and the coating portion can form a chemical bond with the amorphous carbon portion. According to one embodiment of the present invention, the chemical interaction between the carbon-based conductive material and the silicon particles, such as electrostatic attraction, can be formed by making the zeta potential of the carbon-based conductive material and the zeta potential of the silicon particles positive (+) and the other zeta potential negative (-). For example, the zeta potential can be controlled by adjusting the pH value of each of the carbon-based conductive material and the silicon particles, wherein the pH value can be adjusted by controlling the concentration of the carbon-based conductive material, the silicon particles or both in the solution.

[0051] In the negative electrode active material of the present invention, in addition to the carbon-based conductive material on the silicon particles, other conductive materials may be mixed, wherein the other conductive materials may be additional carbon-based conductive materials or metal powders.

[0052] According to one embodiment of the present invention, the silicon (Si) content in the negative active material according to the present invention may be 80 wt % or less, for example, 40 wt % to 80 wt %, or 50 wt % to 65 wt %. Wherein, the above-mentioned silicon (Si) content is based on the measured value of the Si content remaining after burning the negative active material. In the case of a negative electrode using silicon as a substrate, the lower the silicon content, the better the life characteristics and the lower the capacity. Conversely, the higher the silicon content, the shorter the life and the higher the capacity. In the present invention, chemical bonding and chemical interaction are introduced into silicon particles and amorphous carbon portions or silicon particles and carbon-based conductive materials by using coupling agents and amorphous carbon portions. Specifically, in addition to the SiOx bond between the silicon particles and the coupling agent as described above, the chemical bonding (for example, hydrogen bonding) between the coupling agent and the amorphous carbon portion, a chemical bonding (for example, esterification) between the amorphous carbon portion and the carbon-based conductive material or a chemical interaction (for example, electrostatic interaction) between the carbon-based conductive material and the silicon particles is introduced, thereby improving the overall chemical bonding strength in the negative active material. As described above, by improving the overall chemical bonding strength, it is possible to design a negative electrode active material that has a reduced amount of carbon-based conductive material, an increased silicon content, excellent lifespan characteristics, and provides excellent high-density capacity.

[0053] In addition, according to one embodiment of the present invention, the weight ratio between the silicon particles coated with the silane coupling agent, the amorphous carbon portion, and the carbon-based conductive material can be 8-5: 4-0.5: 4-0.5, for example, 7-5: 3-1: 3-1, and more specifically, for example, 6.5-5.5: 2.5-1.5: 2.5-1.5. The above ratio is based on a measurement value after mixing the silicon particles with the amorphous carbon portion supply source and carbonizing them by heat treatment. By setting the weight ratio within the above range, when used as a negative electrode material for a lithium secondary battery, the electrochemical characteristics and life characteristics of the lithium secondary battery can be excellent.

[0054] According to one embodiment of the present invention, the negative electrode active material may include particles having a diameter of 0.5 μm to 10 μm, for example, particles having a diameter of 2 μm to 5 μm.

[0055] According to another aspect of the present invention, a method for preparing a negative electrode active material for a secondary battery is provided, and the method for preparing a negative electrode active material for a secondary battery includes: step a, mixing silicon particles with a coupling agent to prepare silicon particles including a coating portion derived from the coupling agent; step b, mixing the silicon particles including the coating portion derived from the coupling agent with an amorphous carbon portion supply source and a carbon-based conductive material; and step c, carbonizing the mixture of the step b.

[0056] According to one embodiment of the present invention, to improve reactivity with the coupling agent, step a may further include oxidizing the silicon particles, for example, using an RCA solution or a Piranha solution. Therefore, step a may include oxidizing the silicon particles and mixing the oxidized silicon particles with the coupling agent. The Piranha solution may be a mixture of sulfuric acid and hydrogen peroxide.

[0057] According to one embodiment of the present invention, the amorphous carbon portion supply source in the above step b can be various carbonizable materials as described above for the amorphous carbon portion. For example, the amorphous carbon portion supply source can be at least one selected from dopamine, monosaccharides and polysaccharides, specifically, it can be at least one selected from dopamine, glucose, fructose, galactose, maltose, lactose, sucrose, dextran and cellulose. The above carbon-based conductive material is the same as described above. The mixing in the above step b can be carried out in a suitable solvent, wherein, as the above solvent, solvents commonly used in the art such as distilled water, methanol, ethanol, tetrahydrofuran, etc. can be used. In addition, the above mixing can be carried out by stirring so that the reactants are smoothly dispersed in the solvent, and the above stirring can be carried out by ultrasonic treatment or a mechanical homogenizer. Wherein, for example, the above mixing can be carried out for 12 to 30 hours. According to one embodiment of the present invention, the above step b can include the steps of mixing the above silicon particles, the amorphous carbon supply source and the carbon-based conductive material and the step of drying the above mixture. For example, the above drying can be carried out by spray drying. When drying is performed by spray drying, the flow rate of the solution during spraying, the spray pressure, and the spray speed can be appropriately adjusted according to, for example, a desired average particle size.

[0058] According to one embodiment of the present invention, before the above-mentioned step b, the step of mixing the carbon-based conductive material with a coupling agent to prepare a carbon-based conductive material including a coating portion derived from the coupling agent is further included. The above-mentioned carbon-based conductive material in the above-mentioned step b can be a carbon-based conductive material including a coating portion derived from the above-mentioned coupling agent.

[0059] Furthermore, according to one embodiment of the present invention, the step b may further include a step of adjusting the pH values ​​of the silicon particles and the carbon-based conductive material so that one of the zeta potential of the silicon particles and the zeta potential of the carbon-based conductive material is a negative (-) value and the other is a positive (+) value. The pH values ​​of the silicon particles and the carbon-based conductive material may be adjusted by controlling the concentrations of the carbon-based conductive material, the silicon particles, or both in the solution. According to one embodiment of the present invention, the step b may include: determining the pH values ​​of the silicon particles and the carbon-based conductive material required to make one of the zeta potentials of the silicon particles and the carbon-based conductive material a negative (-) value and the other a positive (+) value; determining the concentrations of the silicon particles and the carbon-based conductive material required to achieve the pH values; and mixing the silicon particles and the carbon-based conductive material having the determined concentrations with an amorphous carbon supply source.

[0060] According to one embodiment of the present invention, the carbonization in the above-mentioned step c can be carried out at a temperature of 500°C or above, for example, at a temperature of 500°C to 1000°C, at a temperature of 600°C to 900°C, or at a temperature of 800°C to 900°C for 1 hour to 24 hours, for example, 1 hour to 12 hours, or 1 hour to 4 hours. In addition, the above-mentioned carbonization can be carried out in an inert atmosphere, for example, under the conditions of one or more gases selected from argon, helium and nitrogen. In addition, the above-mentioned carbonization can also be carried out after preheating at a temperature of 100°C to 200°C before the above-mentioned carbonization. Through the above-mentioned carbonization, the above-mentioned amorphous carbon portion supply source can be in an amorphous state. In addition, the reduction of graphene can occur smoothly through the above-mentioned carbonization.

[0061] According to one aspect of the present invention, a lithium secondary battery is provided, comprising a positive electrode, a negative electrode opposite the positive electrode, and an electrolyte between the positive and negative electrodes, wherein the negative electrode comprises the negative electrode active material described above. The secondary battery may be a lithium ion battery, a lithium ion polymer battery, or a lithium polymer battery.

[0062] As the positive electrode and the electrolyte, those commonly used in the technical field to which the present invention pertains can be used.

[0063] For example, the positive electrode comprises a positive electrode active material, which may include layered compounds such as lithium cobalt oxide (LiCoO2) and lithium nickel oxide (LiNiO2), or compounds substituted by one or more transition metals; lithium manganese oxides such as Li1+xMn2-xO4 (wherein x is 0 to 0.33), LiMnO3, LiMn2O3, LiMnO2, etc.; lithium copper oxide (Li2CuO2); vanadium oxides such as LiV3O8, LiV3O4, V2O5 and Cu2V2O7; vanadium oxides such as LiNi1-xMxO2 (wherein M=Co, Mn, A l, Cu, Fe, Mg, B or Ga, and x = 0.01 to 0.3); a lithium manganese complex oxide represented by the chemical formula LiMn2-xMxO2 (wherein, M = Co, Ni, Fe, Cr, Zn or Ta, and x = 0.01 to 0.1) or Li2Mn3MO8 (wherein, M = Fe, Co, Ni, Cu or Zn); a lithium manganese complex oxide having a spinel structure represented by LiNixMn2-xO4; LiMn2O4, wherein part of the Li in the chemical formula is replaced by alkaline earth metal ions; disulfide; Fe2(MoO4)3, etc., but not limited to these.

[0064] Examples of the above-mentioned electrolytes may include organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel-type polymer electrolytes, solid inorganic electrolytes, molten inorganic electrolytes, and the like commonly used in the manufacture of lithium secondary batteries. Specifically, the above-mentioned electrolyte may include a non-aqueous organic solvent and a lithium salt. The above-mentioned lithium salt may be used without particular limitation as long as it is a compound capable of providing lithium ions used in lithium secondary batteries. Specifically, as the above-mentioned lithium salts, LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2)2, LiCl, LiI, or LiB(C2O4)2, etc. may be used. The concentration of the above-mentioned lithium salt is preferably in the range of 0.1M to 2.0M. When the concentration of the lithium salt is within the above-mentioned range, the electrolyte has appropriate conductivity and viscosity, and thus can exhibit excellent electrolyte performance, and lithium ions can be efficiently transferred. The electrolyte may further include at least one additive in addition to the electrolyte components in order to improve the life characteristics of the battery, suppress the reduction in battery capacity, and improve the discharge capacity of the battery.

[0065] According to another aspect of the present invention, a battery pack including the lithium secondary battery described above is provided. The battery pack can be used as a power source for at least one medium-sized or large-sized device such as a power tool; an electric vehicle including an electric vehicle (EV), a hybrid electric vehicle, and a plug-in hybrid electric vehicle (PHEV); or a power storage system.

[0066] Hereinafter, the present invention will be described in more detail with reference to the embodiments of the present invention. Since the embodiments are provided for the purpose of explaining the invention, the present invention is not limited thereto.

[0067] [Preparation Example 1] Preparation of negative electrode active material

[0068] 0.3 g of silicon nanoparticles were treated in 60 ml of piranha solution at 85°C for 2 hours. The piranha-treated silicon particles were filtered three times with distilled water. 0.3 g of the piranha-treated silicon particles were sonicated in 1,000 ml of distilled water for 1 hour. 3.75 ml of APTES (3.55 g, Si: 0.44 g) was added and stirred at 70°C for 4 hours. After filtering four times with distilled water, the mixture was dried in a vacuum oven at 100°C. 0.3 g of the APTES-coated silicon particles, 10 g of graphene oxide (GO) gel (0.2 g as GO), and 0.5 g of sucrose (the weight ratio of heat-treated APTES-coated silicon particles (Si-APTES): reduced graphene oxide (rGO): sucrose (sucrose) was 6:2:2) in distilled water. The mixture was then sonicated for 2 hours and spray-dried using a spray dryer. The resultant was heat treated at 180° C. for 1 hour in a tube furnace and then at 900° C. for 3 hours in a nitrogen atmosphere (heating rate: 10° C. / min).

[0069] The above preparation process was repeated three times to obtain three negative electrode active materials with the same composition.

[0070] [Evaluation Example 1]

[0071] first, Figure 2 FT-IR spectra of the starting material (silicon nanoparticles, SiNPs), intermediate materials (Si, Si-APTES, Si-APTES / rGO treated with piranha), and product (Si-APTES / rGO / suc) of the above preparation examples are shown.

[0072] and, Figure 3 SEM images of the starting material (SiNP), intermediate materials (Si-APTES, p-Si-APTES / rGO), and product (p-Si-APTES / rGO / suc) of the above preparation example are shown.

[0073] and, Figure 4 TEM image of Si-APTES / rGO / suc prepared in the above preparation example is shown.

[0074] and, Figure 5 The XRD patterns of the starting material (p-Si), intermediate material (p-Si-APTES / rGO), and product (p-Si-APTES / rGO / suc) of the above preparation example are shown.

[0075] and, Figure 6The Raman spectra of the starting materials (Si, p-Si), intermediate materials (p-Si / APTES), and products (p-Si / APTES / suc / GO, p-Si / APTES / suc / rGO) of the above preparation examples are shown.

[0076] and, Figure 7 Shown are the differences in specific surface areas when the silicon (Si) content is fixed at 60 wt% and there is no amorphous carbon derived from sucrose (p-Si / APTES / rGO) and in the presence of amorphous carbon derived from sucrose (p-Si / APTES / suc / rGO), and the specific surface area when the silicon content is increased to 70 wt%.

[0077] From the above Figures 2 to 7 It can be confirmed that the amorphous carbon part is formed by the introduction of sucrose. Figure 3 It can be seen that the negative electrode active material includes microspherical particles. In Si-APTES / rGO and Si-APTES / rGO / Suc, the diameter of the microspherical particles is 2μm to 5μm. Figure 7 It can be seen that the BET specific surface area increases with the introduction of the amorphous carbon portion derived from sucrose, and the BET specific surface area decreases with an increase in the silicon content.

[0078] [Evaluation Example 2] Cycle Characteristics

[0079] The negative electrode active material of Si-APTES / r-GO / suc (silicon content: 59.2 wt%) prepared in the preparation example was used to conduct three cycle characteristics experiments. The results are shown in Figure 8 In the third experiment, the charge and discharge capacity decreased slightly, but the lifespan characteristics (cycle retention @ 100 cycles) were above 90%, and the ICE value was also above 85%. According to prior art reports, when using silicon negative electrode active materials, the lifespan characteristics are generally 80% and the ICE value is about 75%. This shows that the negative electrode active material according to the present invention provides significantly improved lifespan characteristics and ICE value.

[0080] On the other hand, p-Si-APTES / rGO without an amorphous carbon portion was prepared as a negative electrode active material for comparison. TGA analysis was performed to confirm the Si content, and the results are shown together with the TGA analysis results of the negative electrode active material according to the present invention (p-Si-APTES / suc / rGO). Figure 9 In. By Figure 9It can be seen that the Si content of the comparative negative electrode active material is 60.3 wt %. Compared with the negative electrode active material of the present invention (Si content: 59.2 wt %), the Si content is slightly different, but the characteristics are similar. Therefore, it can be fully used as a comparative negative electrode active material for comparing capacity, life and output characteristics.

[0081] Figure 10 The cycle characteristics of the comparative negative electrode active material (p-Si-APTES / rGO=1:1 weight ratio, Si content of 60.3 wt%) are shown. As the comparative negative electrode active material, three different types of heat treatment (carbonization step) times of 4 hours, 12 hours, and 24 hours were used. Figure 10 It can be seen that as the bonding time of APTES and GO (i.e., heat treatment time) increases, many chemical bonds are formed in the particles, thereby improving the electrochemical performance. Figure 8 When compared with the negative electrode active material according to the present invention (heat treated for 2 hours), in the case of the present invention, even if the heat treatment time is significantly shorter than the heat treatment time of the comparative negative electrode active material, it has more excellent electrochemical properties, especially more excellent life characteristics (cycle retention) and ICE value.

[0082] [Evaluation Example 3] Differential Capacity

[0083] The negative electrode active material according to the present invention (Si-APTES / rGO / suc=6:2:2, silicon content of 59.2 wt%) was used to measure dQ / dV at 1 cycle, 5 cycles, 20 cycles, and 100 cycles. The same experiment using the same material was repeated 3 times, and the results are shown in FIG. Figure 11 In. By Figure 11 It can be seen that the sharpness of the reaction peak does not decrease with the progress of charge and discharge, and the overpotential of the reaction also decreases. This confirms that the insertion and extraction of lithium ions proceed smoothly even during the evaluation of life characteristics.

[0084] [Evaluation Example 4] Rate Performance

[0085] The rate capability was measured using the negative electrode active material of the present invention (Si-APTES / rGO / suc=6:2:2, silicon content of 59.2 wt%), and the results are shown in Figure 12 Compared with 0.2C, the efficiency at 6C is 98.5% (discharge) and 98.3% (charge), which means that the material has excellent conductivity and shows excellent high-rate performance even in electrodes without conductive materials.

[0086] The dQ / dV was measured according to the magnification change as described above, and the results were as follows: Figure 13 As the rate changes from 6C to 1C, the overpotential decreases, confirming that lithium ion insertion and extraction proceed smoothly even in the evaluation of high-rate charge and discharge characteristics.

[0087] [Evaluation Example 5] RIC Analysis

[0088] Relative irreversible capacity (RIC) analysis was performed by changing the composition of the negative electrode active material, and the results are shown in Figure 14 middle.

[0089] In brief, RIC analysis is a method for modeling and analyzing the mechanism of the phenomenon of life characteristic degradation when evaluating the life characteristics of Si negative electrode materials. Examples include 1) SEI formation and 2) electrochemical pathway disconnection. SEI formation involves the formation of an SEI layer when primary particles that have not yet displayed capacity are replaced by new capacity during the repeated charge and discharge of lithium ions. Since the newly formed SEI layer is also irreversible lithium ion consumption, it may cause a decrease in capacity due to its new SEI formation. In the case of electrochemical pathway disconnection, the volume expansion and contraction of Si particles during repeated charge and discharge may cause the structure that can serve as a pathway for electrons and ions, such as graphene with low elasticity, to be destroyed. Therefore, if the analysis is based on the size of the Si particles, the degradation of life characteristics can be conveniently measured through the SEI formation model with different surface areas per volume. If the analysis is based on the complex carbon structure, the effect of disconnection can be easily achieved. The RIC value is a value related to the SEI and the shutdown value. It can be considered that the smaller the value, the less the life degradation and the better the life characteristics.

[0090] Depend on Figure 14 It can be seen that the negative electrode active material according to the present invention (Si-APTES / suc / rGO = 6:2:2) has a smaller value for SEI formation and disconnection. In other words, it can be confirmed that the negative electrode active material according to the present invention has a lower degree of degradation in life characteristics than the comparative material, that is, the degradation of life characteristics is less.

[0091] [Preparation Example 2] Preparation of negative electrode active material B

[0092] Treat 0.3 g of silicon nanoparticles in 60 ml of piranha solution at 85°C for 2 hours. Repeat the piranha-treated silicon particles with distilled water three times. Ultrasonicate 0.3 g of the piranha-treated silicon particles in 1,000 ml of distilled water for 1 hour. Add 3.75 ml of APTES (3.55 g, Si: 0.44 g) and stir at 70°C for 4 hours. Repeat the filtration with distilled water four times and then dry in an oven at 100°C under vacuum.

[0093] 0.4 g of graphene oxide was sonicated in 1,000 ml of distilled water for 1 hour, 4.24 ml of APTES (4 g, GO: 0.4 g) was added, and the mixture was stirred at 80° C. for 24 hours.

[0094] 0.3 g of APTES-coated silicon particles, 0.2 g of APTES-coated graphene oxide gel, and 0.5 g of sucrose (the weight ratio of heat-treated APTES-coated silicon particles (Si-APTES): amorphous carbon part derived from sucrose (suc): reduced graphene oxide coated with APTES (APTES-rGO) was 6:2:2) were ultrasonically treated in distilled water for 2 hours and then spray-dried using a spray dryer.

[0095] The resultant was heat treated in a tube furnace at 180° C. for 30 hours and then at 900° C. for 3 hours in a nitrogen atmosphere (heating rate: 10° C. / min).

[0096] [Experimental Example] Zeta Potential Control

[0097] On the other hand, the pH value and zeta potential of each starting material, intermediate product, and final product in the above preparation process were analyzed, and the results are shown in the following Table 1. The zeta potential results are compiled based on the pH value of the solution in the experimental process.

[0098] Table 1

[0099]

[0100] p-Si-APTES and graphene oxide (GO) have opposite zeta potential signs. However, in the case of p-Si-APTES and APTES-GO, both zeta potentials were measured as positive at the corresponding pH values, indicating the need to control pH in complexation experiments.

[0101] In order to control the pH value, the zeta potential of each material according to the pH value is shown in Figure 15 . Figure 15The circled portion in the figure represents the zeta potential according to the pH value in the solution used in the usual experimental process. In order to complex p-Si-APTES and APTES-GO, their zeta potentials should have opposite signs, so the pH value of the two materials needs to be adjusted to Figure 15 Therefore, by adjusting the concentration of each material to the same amount as described in Preparation Example 2 above, the zeta potential of p-Si-APTES is negative (-) and the zeta potential of APTES-GO is positive (+), resulting in an electrostatic interaction between the two.

[0102] [Evaluation Example 6] FT-IR and XPS spectral measurements

[0103] Figure 16 and Figure 17 FT-IR spectra and XPS spectra of the starting materials, intermediate materials, and final products of Preparation Example 2 are shown.

[0104] Depend on Figure 16 It can be seen that GO is complexed by APTES to form an APTES-GO complex, and compared with GO, amino groups (-NH2) and Si functional groups are observed.

[0105] like Figure 17 As shown, when APTES-GO is complexed with p-Si-APTES and suc to form p-Si-APTES / suc / APTES-GO, the -OH ratio on the Si surface increases, but the amine groups are consumed during chemical bond formation, so their ratio is partially reduced. The Si-OC bond at 1240 cm-1 derived from the amorphous carbon portion of sucrose and reduced graphene oxide (rGO) is well maintained.

[0106] [Evaluation Example 7] Cycle Characteristics

[0107] Preparation Example 2 was repeated twice to prepare two identical negative electrode active material B samples (p-Si-APTES / amorphous carbon portion derived from sucrose / APTES-rGO = 6:2:2 weight ratio). The cycle characteristics of each of the two samples were measured and the results are shown in FIG. Figure 18 middle.

[0108] In the first test (Si-APTES / suc / rGO 2nd), the discharge and charge capacities decreased slightly, but the lifespan characteristics (cycle retention @ 100 cycles) were above 88%, and the ICE value was also above 83%. This shows that the negative electrode active material according to the present invention provides significantly improved lifespan characteristics and ICE value.

[0109] Although the present invention has been described above with reference to preferred embodiments, it will be understood by those skilled in the art that various modifications and variations can be made to the present invention without departing from the spirit and scope of the invention as described in the appended claims.

Claims

1. A negative electrode active material for a secondary battery, characterized in that include: Silicon particles including a coating portion derived from a coupling agent; an amorphous carbon portion located on the silicon particles; as well as a carbon-based conductive material located on the amorphous carbon portion; The carbon-based conductive material includes a coating portion derived from a coupling agent, and the carbon-based conductive material is chemically bonded to the amorphous carbon portion.

2. The negative electrode active material for secondary batteries according to claim 1, characterized in that The coupling agent is a silane coupling agent.

3. The negative electrode active material for secondary batteries according to claim 2, characterized in that The silane coupling agent is at least one aminosilane coupling agent selected from 3-aminopropyltriethoxysilane, 3-aminopropyltrimethoxysilane, 2-aminoethyl-3-aminopropylmethyldimethoxysilane and 2-aminoethyl-3-aminopropyltrimethoxysilane.

4. The negative electrode active material for secondary batteries according to claim 1, characterized in that The amorphous carbon portion is derived from at least one selected from dopamine, monosaccharides, polysaccharides, polyvinyl pyrrolidone, polyethylene glycol, polydioxythiophene, polyacrylonitrile, polylactic acid, polyvinyl alcohol, polymethyl methacrylate, polyvinylidene fluoride, polyvinyl acetate, polystyrene, polyvinyl chloride, polyetherimide, polybenzimidazole, polyethylene oxide, polycaprolactone, polyamide-6, polytrimethylene terephthalate, poly D, L-lactic acid, polycarbonate and polydioxanone.

5. The negative electrode active material for secondary batteries according to claim 4, characterized in that The amorphous carbon portion is derived from at least one selected from the group consisting of dopamine, glucose, fructose, galactose, maltose, lactose, sucrose, dextran, and cellulose.

6. The negative electrode active material for secondary batteries according to claim 1, characterized in that In the silicon particles, the coating portion derived from the coupling agent is chemically bonded to the amorphous carbon portion.

7. The negative electrode active material for secondary batteries according to claim 1, characterized in that The carbon-based conductive material is at least one selected from graphene, graphene derivatives, reduced graphene, reduced graphene derivatives, carbon-based zero-dimensional nanostructures, carbon-based one-dimensional nanostructures, carbon-based two-dimensional nanostructures and carbon-based three-dimensional nanostructures.

8. The negative electrode active material for secondary batteries according to claim 7, characterized in that The carbon-based conductive material is at least one selected from graphene, graphene oxide, reduced graphene, reduced graphene oxide, carbon nanotubes, carbon nanofibers, fullerene and graphene foam.

9. The negative electrode active material for secondary batteries according to claim 1, characterized in that The coupling agent of the carbon-based conductive material is at least one aminosilane coupling agent selected from 3-aminopropyltriethoxysilane, 3-aminopropyltrimethoxysilane, 2-aminoethyl-3-aminopropylmethyldimethoxysilane and 2-aminoethyl-3-aminopropyltrimethoxysilane.

10. The negative electrode active material for secondary batteries according to claim 1, characterized in that The carbon-based conductive material reacts chemically with the silicon particles.

11. The negative electrode active material for secondary batteries according to claim 1, characterized in that The silicon content in the negative electrode active material is 80 wt % or less.

12. The negative electrode active material for secondary batteries according to claim 1, characterized in that The weight ratio of the silicon particles, the coating portion derived from the coupling agent, the amorphous carbon portion, and the carbon-based conductive material is 8-5:4-0.5:4-0.

5.

13. The negative electrode active material for secondary batteries according to claim 1, characterized in that: The negative electrode active material includes fine particles having a diameter of 0.5 μm to 10 μm.

14. A method for preparing a negative electrode active material for a secondary battery, characterized in that: include: Step a, mixing silicon particles with a coupling agent to prepare silicon particles including a coating portion derived from the coupling agent; Step b, mixing the silicon particles including the coating portion derived from the coupling agent with an amorphous carbon portion supply source and a carbon-based conductive material; and Step c, carbonizing the mixture of step b; Before the above step b, the method further comprises mixing the carbon-based conductive material with a coupling agent to prepare a carbon-based conductive material including a coating portion derived from the coupling agent. Wherein, the carbon-based conductive material in the step b is a carbon-based conductive material including a coating portion derived from the coupling agent.

15. The method for preparing a negative electrode active material for a secondary battery according to claim 14, characterized in that: The step b further includes adjusting the pH values ​​of the silicon particles and the carbon-based conductive material so that one of the zeta potentials of the silicon particles and the carbon-based conductive material is negative and the other is positive.

16. The method for preparing a negative electrode active material for a secondary battery according to claim 14, characterized in that: The carbonization in the above step c is performed at a temperature above 500° C. for 1 to 24 hours.

17. A negative electrode for a lithium secondary battery, characterized in that: The negative electrode active material according to claim 1 is included.

18. A lithium secondary battery, characterized in that: The invention comprises a positive electrode, a negative electrode opposite to the positive electrode, and an electrolyte between the positive electrode and the negative electrode, wherein the negative electrode comprises the negative electrode active material according to claim 1.

19. A battery pack, characterized in that: Including the lithium secondary battery according to claim 18.

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